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One and two-dimensional NMR characterization of organofunctionalized silica gels Aroca, Patricia Paulina 1995-06-04

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ONE AND TWO-DIMENSIONAL NMRCHARACTERIZATIONOF ORGANOFUNCTIONALIZED SILICA GELSbyPATRICIA PAULINA AROCAB.Sc. (Honours) Windsor University, 1989A THESIS SUEMIITED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF CHEMISTRYWe accept this thesis as conformingto the required standardTHE UNWERS1TY OF BRITISH COLUMBIAApril 1995© Patricia Paulina Aroca(authorization)In presenting this thesis in partial fulfilmentof the requirements for an advanceddegree at the University of British Columbia,I agree that the Library shall make itfreely available for reference andstudy. I further agree that permissionfor extensivecopying of this thesis for scholarlypurposes may be grantedby the head of mydepartment or by his or her representatives.it is understood that copying orpublication of this thesis for financial gain shallnot be allowed without my writtenpermission.(Signature)_________________________________Department of_________________The University of BritishColumbiaVancouver, CanadaDateDE.6 (2188)ABSTRACTAn alternative preparation is proposed in this thesis for the synthesis oforganofunctionalized silica gels based on the copolymerization of tetraethoxysilane (TEOS)and an organofunctionalized triethoxysilane. The principal analytical techniquesused todetermine the integrity and disthbution of the functionality in the copolymer matrix weresolution and solid state NMR.The model copolymer chosen for these studies is methyltriethoxysilane (MTES)/TEOS.One-dimensional 29Si solid state NMR was used to establish the integrity of the system andrelative proportions of the different silicon environments present. Two-dimensional 1H-29Siheteronuclear correlation NMR experiments unambiguously demonstrated that the monomersin the copolymer were “mixed and not phase-separated.To quantify the extent of mixing of the two monomers in the copolymer, kineticstudies were undertaken with the objective of determining reactivity ratios for each monomer.The acid catalyzed hydrolysis and dimer formation reactions of the TEOS and MTESmonomers were characterized at a number of catalyst concentrations in order to determine thepH independent kinetic rate constants. With both homopolymers well characterized, thehydrolysis and dimer formation kinetic rate constants were determined for the copolymer.From these kinetic data reactivity ratios were calculated which suggest that theMTES/TEOSsystem tends to form a random copolymer.One interesting possible application of organofunctionalized silica gels is in themanufacturing of low temperature functionalized glasses. The synthesis of large pieces ofglasses from the copolymerization of TEOS and MTES requires the usage of a dryingchemical control agent (DCCA) to prevent cracking. The most commonly used DCCA isformamide. Solution NMR was used to determine the integrity and interaction of fonnamide11in the polymerization process. Another step taken to characterize the suitability of thesecompounds for the manufacturing of low temperature functionalized glasses was tocharacterize the thermal stability of numerous organic functionalities by thermal analysistechniques and NMR. Of all the functionalities studied the methyl group had the highestthermal stability, up to 700°C when thermally treated under nitrogen, showing promise for thesynthesis of methyl functionalized glasses.The cross polarization (CP) dynamics of a number ofMTES/TEOS copolymers werethen studied, to determine the distance range between the methyl group and the fullycondensed unfunctionalized silane (Si(OSi)4). These results further support the conclusionthat the functionality in the MTES/TEOS copolymer is quite evenly distributed throughout thecopolymer matrix.111TABLE OF CONTENTSAbstract.iiTable of ContentsivList of FiguresxList of Tables xviiList of Symbols xixAcknowledgement xxChapter 1: Introduction 11.1 Immobilized Reagents 11.2 Characterization 41.3 Nuclear Magnetic Resonance 61.4 Basic Interaction Tenns 81.4.1 Zeeman Interaction 81.4.2 Dipole-Dipole Interaction 91.4.3 Chemical-Shift Interaction 131.4.4 Indirect Spin-Spin or J-Coupling Interaction 151.5 High Resolution Solid State NMR Spectroscopy 181.5.1 High Power Dipolar Decoupling 181.5.2 Magic Angle Spinning 181.5.3 Cross Polarization 211.5.4 Distance Determination from CP curves 241.6 Two-Dimensional NMR Spectroscopy 28iv1.7 Objectives of the Thesis 31Chapter 2: Characterization of Silica Gel and Functionalized Silica Gel by 1Dand 2DSolid State NMR Experiments 342.1 Introduction 342.2 Introduction to the Alternative Synthesis 382.3 Experimental 392.3.1 Silica Gel 392.3.1.1 Silica Gel - Fast Gel Preparation 392.3.1.2 Silica Gel - Preparation Used in the Kinetic Studies 392.3.2 Methyltriethoxysilane Homopolymer392.3.3 Methyl Substituted Silica Gel392.3.3.1 Reaction with Preformed Silica Gel392.3.3.2 Copolymerization - Fast Gel Preparation402.3.3.3 Copolymerization - Under Kinetic Conditions402.3.4 Solid State NMR Spectra402.4 Results and Discussion412.4.1 Characterization of the Functionalized Silica Gel by 1D NMRExperiments412.4.2 Two-Dimensional Heteronuclear Correlation NMR Experiments472.5 Conclusions54vChapter 3: High Resolution 29Si Solution NMR Investigations of the Hydrolysis andDimer Formation Reactions of Tetraethoxysilane (TEOS)553.1 Introduction553.2 Experimental593.2.1 Chemicals and Reaction Mixtures593.2.2 NMR Measurements 593.3 Results and Discussion 613.3.1 Experimental Results 613.3.2 Kinetic Analysis 613.3.3 Justification for Not Including Other Reactions 753.4 General Discussion 793.5 Conclusions 85Chapter 4: High Resolution 29Si Solution NMR Investigation of the Hydrolysis andDimer Formation Reactions of Methyltriethoxysilane (MTES)Homopolymerization and MTES/TEOS Copolymerization 864.1 Introduction 864.2 Experimental 884.2.1 MTES Homopolymerization Reaction Conditions 884.2.2 MTES/TEOS Copolymer Reaction Conditions 884.3 Results and Discussion 914.3.1 Kinetic Analysis for the MTES Hydrolysis and Dimer FormationReactions 914.3.2 MTES Hydrolysis and Dimer Formation Kinetic Rate Constants 100vi4.3.3 MTES/TEOS Copolymer Kinetic Analysis .1054.3.4 MTESII’EOS Copolymer Dimer Formation 1254.4 Conclusions129Chapter 5: The Effect of Formamide, a Drying Chemical Control Agent (DCCA) on theKinetics of the TEOS, MTES and MTES/TEOS polymerizations 1305.1 Introduction 1305.2 Experimental 1335.2.1 Kinetics .1335.2.2 NMR 1335.3 Results and Discussion 1355.3.1 The Effect of Formamide on the Kinetics of the Hydrolysis and DimerFormation Reactions 1355.3.2 High Resolution ‘H Solution NMR Investigation 1475.3.3 High Resolution 15N Solution NMR Investigation 1475.3.4 High Resolution 13C Solution NMR Investigation 1535.4 Conclusions 155Chapter 6: Solid State NMR and Thermal Analysis Studies of the Thermal Stabilities ofFunctionalized Silica Gels Prepared by the Copolymerization Method 1566.1 Introduction 1566.2 Experimental 1586.2.1 Sample Preparation 1586.2.2 Solid State NMR Experiments 158vii6.2.3 DSC and TGA Measurements .1606.3 Results and Discussion 1616.3.1 Tetraethoxysilane (TEOS) Homopolymer - Pure Silica Gel 1616.3.2 Methylthethoxysilane (MTES)/TEOS Copolymer 1646.3.3 Phenyltriethoxysilane (PTES)ITEOS Copolymer 1766.3.4 Ethyltriethoxysilane (ETES)/TEOS Copolymer 1836.3.5 Phenethyltriethoxysilane (PETES)/TEOS Copolymer 1936.4 Conclusions 199Chapter 7: Further Solid State NMR Studies to Investigate the Extent of Mixing in theMTES/TEOS Copolymer Gel 2007.1 Introduction 2007.2 Experimental 2027.2.1 Samples Used in the ‘H-29Si Contact Time Experiments 2027.2.2 Samples Used in the 19F-29Si Contact Time Experiments .2027.2.3 Variable Contact Time Experiments 2037.3 Results and Discussion 2047.3.1 1H-29Si Contact Time Experiments 2047.3.1.1 Analysis of the Contact Time Results 2047.3.1.2 Pure Silica Gel Contact Time Experiments .2087.3.1.3 MTES/TEOS Copolymer Contact Time Experiments 2087.3.1.4 MTES/TEOS Copolymer: Series of 2D HeteronuclearCorrelation Experiments 2157.3.1.5 Discussion 219viii7.3.2‘9F-29Si Contact Time Expements . 2227.3.2.1 Simplification of the Expression for the TFPTMSiTEOSCopolymer 2227.3.2. 1A 19F Second Moment Considerations 2227.3.2. lB 29Si Second Moment Considerations 2257.3.2.1C Geometrical Factor in Equation (1.18) 2277.3.2.2 Analysis of the‘9F-29Si Contact Time Experimental Results....2297.3.2.3 Discussion 2357.4 Summary of Conclusions 238Chapter 88.1 Summary 2398.2 Suggestions for Future Investigations 243References 245Appendix 1 256Appendix 2 257Appendix 3 258Appendix 4 259ixLIST OF FIGURESFigure 1.1 - An organofunctionalized silane anchored onto silica gel 2Figure 1.2 - Schematic representation of the effect of the heteronuclear dipolar interaction ona solid state NMR spectrum 11Figure 1.3 - (A-C) Schematic representation of the effect of the chemical shift anisotropy ona solid state NMR spectrum. D) The isotropic chemical shift observed in solution NMR 14Figure 1.4 - High resolution 29Si solution NMR spectrum for (CH3)SiCN,si-H= 7.3 Hz.[1.13] 17Figure 1.5 - A schematic representation of the relevant angles involved in the rotation of amacroscopic sample at an angle1to the applied magnetic field B0 20Figure 1.6 - The cross polarization (CP) pulse sequence illustrated for ‘H-29Si CP NMRexperiments 22Figure 1.7 - The geometric relationship of two spins I and S in an external magnetic field.. 27Figure 1.8 - The pulse sequence used for a 2D ‘H-29Si heteronuclear correlation NMRexperiment with ‘H decoupling 30Figure 2.1 - Industrial synthesis of functionalized silica gels 35Figure 2.2 - 29Si CP/MAS NMR spectrum of silica gel with the three different siliconenvironments labelled according to Maciel and co-workers.[1.20] 36Figure 2.3 - The ‘3C and 29Si CP/MAS solid state NMR spectra of an MTES/TEOScopolymer sample 43Figure 2.4 - 29Si CP/MAS NMR spectra of different types of functionalized silica gels andmixtures 45Figure 2.5 - Schematic diagram illustrating the possible local silicon environments in anMTES homopolymers and an MTES/TEOS copolymer 46Figure 2.6 - 2D ‘H-29Si heteronuclear correlation NMR experiments for silica gel (preparation2.3.1.1) 49Figure 2.7 - 2D ‘H-29Si heteronuclear correlation NMR experiments for a mixture of silicagel and polymethylsiloxane 50Figure 2.8 - 2D ‘H-29Si heteronuclear correlation NMR experiment, for a D20 washed 25/75MTESITEOS copolymer (preparation 2.3.3.2) 52xFigure 2.9 - 2D 1H-29Si heteronuclear coffelation NMR experiment, fora D20 washed 10/90MTES/TEOS copolymer (preparation 2.3.3.3) 53Figure 3.1 - A typical 29Si spectrum obtained during the hydrolysis ofTEOS with the peaksassigned as indicated62Figure 3.2 - Stacked plot of the one-dimensional 29Si NMR spectra obtained during thepolymerization of tetraethoxysilane 62Figure 3.3 - The time dependence of the relative concentrations of all the intermediatespecies involved in the hydrolysis of TEOS63Figure 3.4 - Calculated curves from the least square fitting of equations (3.13-3.16) for thetime dependence of the relative concentrations of the intermediate species formed during theTEOS hydrolysis assuming a constant water concentration67Figure 3.5 - Comparison of different calculated curves for the TEOS system (pH=2.55) whenthe water concentration is held constant and when its change in concentration as a function oftime is taken into account69Figure 3.6 - Calculated curves of the relative concentrations of Si(OH)3(OEt) and Si(OH)4intermediates (A) excluding and (B) including the equilibrium back reaction together with theexperimental data. (C) is a comparison of the curves without the experimental data 71Figure 3.7 - The time dependence of the relative concentrations of all the intermediatespecies involved in the hydrolysis of TEOS together with the final calculated curves 73Figure 3.8 - ‘3C solution NMR spectrum for the TEOS polymerization reaction with naturalabundance ‘3C ethanol 77Figure 3.9 - Series of ‘3C solution NMR spectra for the TEOS polymerization reaction with(A) natural abundance and (B) with 13C enriched ethanol 78Figure 3.10 - Plots of the pH dependent kinetic constants versus acid concentration for thehydrolysis and dimer formation reactions of the TEOS homopolymerization 81Figure 4.1A - A typical 295i spectrum obtained during the hydrolysis of MTES with the peaksassigned as indicated 92Figure 4.1B - Stacked plot of the one-dimensional 29Si NMR spectra obtained during thepolymerization of methyltriethoxysilane 92Figure 4.2 - The time dependence of the relative concentrations of all the intermediatespecies involved in the hydrolysis of MTES 93Figure 4.3 - Calculated curves for the time dependence of the MTES hydrolysis intermediateconcentrations, equations (4.9)-(4.l 1), assuming constant water concentration 96xiFigure 4.4 - Calculated curves of the relative concentrations of theCH3Si(OH)2(OEt) andCH3Si(OH) intermediates (A) excluding and (B) including theequilibrium back reactiontogether with the experimental data. (C) is a comparison of the curveswithout theexperimental data98Figure 4.5 - The time dependence of the relative concentrationsof all intermediate speciesinvolved in the hydrolysis of MTES together with the final calculatedcurves 99Figure 4.6 - Plots of the pH dependent kinetic constants versus acidconcentration for thehydrolysis and dimer formation reactions of the MTES homopolymerization 102Figure 4.7 - Series of 1D 29Si solution NIvIR spectra as functions of time acquired duringtheMTES,TEOS copolymerization106Figure 4.8 - Experimental relative concentration/time curves for the MTES hydrolysisintermediates in the MTES/TEOS copolymerization107Figure 4.9 - Experimental relative concentration/time curves for the TEOS hydrolysisintermediates in the MTESJTEOS copolymerization 108Figure 4.10 - Experimental data for the TEOS hydrolysis intermediate Si(OEt)4relativeconcentrations as functions of time, during the MTES/TEOS copolymerization, together withdifferent calculated curves 112Figure 4.11 - Experimental data for the TEOS hydrolysis intermediate Si(OH)(OEt)3relativeconcentrations as functions of time, during the MTES/TEOS copolymerization, together withdifferent calculated curves 114Figure 4.12 - Experimental data for the TEOS hydrolysis intermediates Si(OH)2(OEt) andSi(OH)3(OEt) relative concentrations as functions of time, during the MTES/TEOScopolymerization, together with the calculated curves 115Figure 4.13 - Experimental data for the MTES hydrolysis intermediates CH3Si(OEt) andCH3Si(OH)(OEt)2relative concentrations as functions of time, during the MTES/TEOScopolymerization, together with the calculated curves 116Figure 4.14 - Experimental data for the fully hydrolyzed monomers CH3Si(OH) and Si(OH)4relative concentrations as functions of time, during theMTESITEOS copolymerization,together with the calculated curves 118Figure 4.15 - Experimental data for the TEOS hydrolysis intermediates, Si(OH)3(OEt) andSi(OH)4,relative concentrations as functions of time, during the MTES/TEOScopolymerization, together with the calculated curves 119Figure 4.16 - Experimental data for the MTES hydrolysis intermediates, CH3Si(OH)2(OEt)and CH3Si(OH) relative concentrations as functions of time, during the MTESITEOScopolymerization, together with the calculated curves 120xiiFigure 4.17 - 29Si solution NMR spectrum, 76 minutes into theMTES/TEOScopolymerization (water acidified to pH=2.55) 123Figure 5.1 - Experimental data and calculated relative concentrationcurves for the hydrolysisintermediates as functions of reaction time for two TEOS homopolymerizations, involving0 mole % and 10 mole % formamide136Figure 5.2 - Relative concentrations of the hydrolysis intermediatesas functions of reactiontime for two MTES homopolymerizations, involving 0 mole% and 10 mole %formamide 137Figure 5.3 - Experimental data and calculated relative concentration curves for the hydrolysisintermediates as functions of time for two TEOS homopolymerizations, involving0 mole % and 20 mole % formamide 138Figure 5.4 - Experimental data and calculated relative concentration curves for the hydrolysisintermediates as functions of time for three MTES homopolymerizations, involving 0 mole %,20 mole % and 30 mole % formamide 139Figure 5.5 - The (A) TEOS and (B) MTES gelation time versus formamideconcentration .143Figure 5.6 - The TEOS regions of the 29Si NMR spectra of50150 MTESITEOScopolymerizations at 130 minutes for reactions involving 0 mole %, 10 mole% and20 mole % formamide 145Figure 5.7 - The MTES regions of the 29Si NMR spectra of50/50 MTESiTEOScopolymerizations at 130 minutes for reactions involving 0 mole %, 10 mole% and20 mole % formamide 146Figure 5.8 - 1H solution NMR spectra of solutions containing:Formamide/H2OIEthanol andFormamide/H20/EthanoljTEOS 148Figure 5.9 - ‘5N solution NMR spectra for neat formamide in deuterated acetone and neatformamide in H20/D0together with the molecular structure of formamide 149Figure 5.10 - Analysis of the coupling patterns in the ‘5N NMR spectrum of fonnamide inthe TEOS reaction mixture 150Figure 5.11 - ‘5N solution NMR spectra of 15N labelled formamide (20 mole %) in a TEOSpolymerization mixture 152Figure 5.12 - ‘H coupled ‘3C solution NMR spectrum of a TEOS polymerization mixture with20 mole % formamide after three days 154Figure 6.1 - (A) DSC and (B) TGA analysis curves for silica gel obtained under nitrogen..162x’ilFigure 6.2 - TG/MS data for silica gel with the identifyingfragment mass numbers for eachcurve163Figure 6.3 - 29Si solid state MAS NMR spectra for the four25/75 organofunctionalizedcopolymers investigated165Figure 6.4 - 29Si CP/MAS NMR spectra of the25/75 MTES/TEOS copolymer sample heatedfor two hours at the temperatures indicated167Figure 6.5 - ‘3C CP/MAS NMR spectra of the25175 MTES1UEOS copolymer sample heatedfor two hours at the temperatures indicated.169Figure 6.6 - ‘H MAS NMR spectra of the 25/75 MTESII’EOS copolymer sample heated fortwo hours at the temperatures indicated .170Figure 6.7 - 2D ‘H-29Si heteronuclear correlation contour plot ofthe thermally treatedMTESiTEOS copolymer 171Figure 6.8 - DSC thermal analysis curves obtained under nitrogen for (A)50/50 MTES/TEOScopolymer and (B) silica gel 173Figure 6.9 - TGA thermal analysis curves obtained under nitrogen for (A)50/50MTES/TEOS copolymer and (B) silica gel 174Figure 6.10 - TG/MS data for the25/75 MTESITEOS copolymer with the identifyingfragment mass numbers for each curve 175Figure 6.11 - 29Si CP/MAS NIVIR spectra of the25/75 PTESITEOS copolymer sample heatedfor two hours at the temperatures indicated 177Figure 6.12 - ‘3C CP/MAS NMR spectra of the25/75 PTES/TEOS copolymer sample heatedfor two hours at the temperatures indicated .178Figure 6.13 - DSC thermal analysis curves of the25/75 PTES/TEOS under (A) nitrogen and(B) air 179Figure 6.14 - TGA thermal analysis curves of the25/75 PTES/TEOS copolymer undernitrogen 181Figure 6.15 - TG/MS data for the 25/75 PTESITEOS copolymer with the identifyingfragment mass numbers for each curve 182Figure 6.16 - 29Si CP/MAS NMR spectra of the 25/75ETES/TEOS copolymer sample heatedfor two hours at the temperatures indicated .184Figure 6.17 - 29Si CP/MAS NMR spectra of the50/50 ETES/TEOS copolymer sample heatedunder nitrogen for two hours at the temperatures indicated 185xivFigure 6.18 - TG/MS data for the50/50 ETESiTEOS copolymer with the identifyingfragment mass numbers for each curve 187Figure 6.19 - ‘3C CP/MAS NMR spectra of the 25/75 ETES/TEOS copolymer sample heatedfor two hours at the temperatures indicated188Figure 6.20 - ‘H MAS NMR spectra of the50/50 ETES/TEOS copolymer sample heated fortwo hours under nitrogen at the temperatures indicated 190Figure 6.21 - DSC thermal analysis curves of the 25,75ETES/TEOS copolymer sampleobtained under (A) nitrogen and (B) air191Figure 6.22 - TGA thermal analysis curves obtained undernitrogen for (A) 25/75ETESITEOS copolymer and (B) 25/75 PTES/TEOS copolymer 192Figure 6.23 - 29Si MAS NMR spectra of the 25/75PETES/TEOS copolymer sample heatedfor two hours at the temperatures indicated .194Figure 6.24 - ‘3C CP/MAS NMR spectra of the 25175PETES/TEOS copolymer sampleheated for two hours at the temperatures indicated 195Figure 6.25 - DSC and TGA thermal analysis curves for the25/75 PETESiTEOS copolymerobtained under nitrogen 196Figure 6.26 - TG/MS data for the 25/75 PETES/TEOS copolymer with the identifyingfragment mass numbers for each curve 198Figure 7.1 - The1H-29Si CP/MAS NMR spectra as a function of the contact time for thethermally treated 25/75 MTES/TEOS copolymer sample 205Figure 7.2 - The definition ofSa, Sib,Si and the distances calculated using literaturedata [7.6-7.8] 207Figure 7.3 - Contact time curves for theCH3Sa(OS)andSi(OSi)4silicons in the thermallytreated 25/75 MTES/TEOS copolymer together with the calculated curves 212Figure 7.4 - Contact time curves for theCH3Sa(OSj)andSi(OSi)4silicons in the D20washed 25/75 MTES/TEOS copolymer together with their calculated curves 213Figure 7.5 - Contact time curves for theCH3Sa(OS)andSi(OSi)4silicons in the D20washed 10/90 MTESITEOS copolymer together with their calculated curves 214Figure 7.6 - 2D 1H-29Si heteronuclear correlation contour plot of the25/75 MTES/TEOScopolymer with the rectangle indicating the “volume” of the cross peaks used in the analysisof the set of 2D data 217xvFigure 7.7 - Contact time curves for theCH3Sa(OSj) and Si(OSi)4silicons inthe 25/75MTES/TEOS copolymer sample, from the 15 2D 1H-29Si heteronuclear correlationexperiments together with the calculated curves 218Figure 7.8 - The definition ofSaand Si for the TFPTMS/TEOS copolymer samples 223Figure 7.9 -‘9F-29Si CP/MAS spectra for the thermally treated25/75CF3CH2Si(OMe)jFEOS copolymer 231Figure 7.10 - Contact time curves for theCF3CH2Sja(OS)andSi(OSi)4silicons in thethermally treated 25/75 TFPTMS/TEOS copolymer, together with the calculated curves 232xviLIST OF TABLESTable 2.1 - A summary of the deconvoluted peak areas fromthe quantitative 1D 29Si MASNMR spectra42Table 3.1 - Average pH dependent kinetic rate constants (M’min’) determinedfor the TEOShydrolysis and dimer formation reactions80Table 3.2 - pH independent hydrolysis and dimer formationkinetic rate constants forTEOS82Table 4.1 - Compositions of different samples used in the copolymer study89Table 4.2 - The average pH dependent kinetic rateconstants (M’min1)determined for theMTES homopolymerization hydrolysis and dimer formationreactions 101Table 4.3 - pH independent hydrolysis and dimer formation kinetic rateconstants for theMTES and TEOS homopolymers103Table 4.4 - Comparison of the pH dependent kinetic rate constants determined for the TEOSand MTES homopolymers and the MTES/TEOS copolymer 121Table 4.5 - Dimer percentages for the different copolymer samples after 101 minutes 126Table 4.6 - Dimer percentages for the different copolymer samples after 131 minutes 127Table 5.1 - The pH values and water/formamide ratios of the samplesused in the kineticinvestigations 134Table 5.2 - TEOS hydrolysis and dimerization kinetic rate constants, as defined in equations(3.4)-(3.8), determined for different formamide concentrations 140Table 5.3 - MTES hydrolysis and dimerization kinetic rate constants, as defined in equations(4. 1)-(4.4), determined for different formamide concentrations 141Table 6.1 - Sample compositions used in the thermal analysis investigations 159Table 6.2 - Observed 29Si chemical shifts for the different copolymers shown in Figure 6.3together with their assignments 166Table 6.3 - The minimum limits of the thermal stabilities of different functionalizedcopolymers for thermal treatments in air and under nitrogen 199Table 7.1 - The values from the non-linear least squares fitting of the contact time curvesof the indicated silicons for the pure silica gel sample 209Table 7.2 - The T1 and T0 values from the non-linear least squares fitting of theCH3Si(OSi) contact time curves for the samples indicated 209xviiTable 7.3-values derived from fitting theSi(OSi)4contact time curves for the differentsamples indicated, using equation (1.17) and fixing the T, value (Table 7.2) 211Table 7.4 - ‘H toSidistances calculated for the MTES/TEOS copolymer samples 216Table 7.5 - The percentages ofSi(OSi)4andCH3Sja(OSj)which are cross polarized togetherwith the actual percentages that exist in the thermally treated 25/7 5 MTES/TEOS copolymersample 221Table 7.6 - The T, and values for the 29Si resonances in the different TFPTMSITEOScopolymer samples 233Table 7.7 - Distance betweenSi(OSi)4and the CF3 functionality (A) obtained using equation(7.14) 234Table 7.8 - The percentages ofSi(OSi)4andCF3”Sja(OS)which are cross polarized and theactual percentages that exist in the TFPTMS/TEOS samples 236xviiiLIST OF SYMBOLSgyromagnetic ratioa chemical shiftLarmor frequency<2>second momento, angular velocity.0external magnetic fieldI nuclear spin vectorT1 spin lattice relaxation timePlanck’s constant/ 2itr distance vector between I and S spinsCsCurie constant for the S nucleusC15 geometrical constant in equation (1.18)kTl-kT5kinetic constants referring to TEOS monomer polymerization reactionskMl-kM4kinetic constants referring to the MTES monomer polymerization reactionsk kinetic constant referring to MTES-TEOS codimer formation reactionencoding period in a 2D experimentt detection period in a 2D experimentTLtemperature of the latticeTtemperature in the spin locking fieldcross polarization time constantT1 relaxation time constantFrequently used AbbreviationsCP cross polarizationDCCA drying chemical control agentDSC differential scanning calorimetryETES ethyltriethoxysilaneFID free induction decayMAS magic angle spinningMTES methyltriethoxysilanePTES phenyltriethoxysilanePETES phenethyltriethoxysilaneTFPTMS 3,3,3-.trifluoropropyltrimethoxysilaneTEOS tetraethoxysilaneTMOS tetramethoxysilaneTGA thermogravimetric analysisTG/MS thermogravimetric analysis with mass spectroscopyT-T TEOS-TEOS homodimerT-M, M-T TEOS-MTES codimerM-M MTES-MTES homodimerxixACKNOWLEDGEMENTI would like to thank Dr. Cohn Fyfe for his insight and guidance, and for giving methe opportunity to learn more about NMR.There are a number of other people, whose support, expertise and friendship has beeninvaluable to me throughout this thesis. Dr. Hiltrud Grondey who has not only helped meimprove my writing skills but who has had the time and patience to provide hope andencouragement when things looked a little bleak. Tom Markus whose patience and pricelessknowledge concerning Bruker spectrometers has made parts of this thesis possible. I wouldalso like to thank Dr. Ken MacKenzie who did theTG/MS work and Bruker Canada who lentme the 10 mm broad-band probe which was used in the kinetic studies.I wish to thank my husband Patric, who supported all my decisions and whoseunconditional love and confidence are my pillar of strength. I would like to thank myparents, whose encouragement, moral support, model and love give me the strength to strivefor my dreams. Gracias por todo! To my brother, Ricardo (Pumpkin), and my sister,Marcella, who are always there for me, thank you for your love, support and example.Finally, I want to thank my sons, Miguel André and Stéphane Tomás who put life intoperspective and who have endured the pains of having a mom in a doctorate program.xxCHAPTER 1INTRODUCTION1.1 IMMOBILIZED REAGENTSReagents immobilized onto inert surfaces are of great technical and industrialimportance. One of the most common substrates onto which reagents are immobilized issilica gel because it is inexpensive, non-swelling and possesses a very high surface area.Organofunctionalized silanes (R’-Si(OR)3),where R’ is the organic functionality determinedby the intended application, can be anchored onto silica gel to create immobilized reagentswhich are usually represented as illustrated in Figure 1.1.In industry, these immobilized reagents are used to obtain a higher degree of controlover product formation. The immobilization of chemical species facilitates the handling oftoxic compounds, such as (Si-3-pyrrole), as well as the conservation of expensive catalysts,for example (Si-O-Si(CH2)PPhRh(acac)CO) which is used for hydroformylationreactions.[l.16] Industrial applications involve quantities between 100 to 1000 tonsworldwide per year of organofunctionalized silanes in: glass-fiber reinforced thermosettingand thermoplastic resins, mineral-filled rubbers (elastomers), sealants and foundry sandmolds.[1.1]Chromatography columns are often silica gel supports with immobilized functionalitieswhich control the characteristics of the column.[1.6]Organofunctionalized silanes can act as adhesion promoters between two incompatiblesurfaces because of the coexistence of both inorganic and organic moieties in the samemolecule. For example, dental varnishes and filling materials involve organic polymersadhering to an inorganic surface.In the textile fiber industry, treatment with organofunctionalized silanes, such as1.OH:O%%%O—r--Si—R’‘0-I I ISilica gel I Immobilized reagent(Organofunctionalized silane)Figure 1.1 - An organofunctionalized silanè anchored onto silica gel.On the immobilized reagent, R’ is an organofunctionalitydictated by the application.0H23-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride, producesmaterials whichexhibit bactericidal and algicidal activity essentialfor surgical drapes and other suchmaterials.Tailoring electrode characteristics, via immobilization of an organofunctionalizedsilane, presents opportunities to create specific electrocatalyticproperties on the surface andprovides a fundamentally new approach to thestudy of electrochemical reactions.[1.17JThe immobilization of highly selective biological speciessuch as enzymes on thesurface of a polymer matrix is used commerciallyto isomerize glucose to fructose and toproduce optically pure amino acids.[1.81In summary, organofunctionalized silanes play an important role in themanufacturingof many products, in the extraction of chemicals, in theanalysis of mixtures and incontrolling reactions via immobilized catalysts. The overall objective of the presentwork isto contribute to the understanding of the formation and structure of these immobilizedreagents synthesized by an alternative preparative route.31.2 CHARACTERIZATIONSilica gel and organofunctionalized siloxane polymersconsist of long chains that arecoiled up, intertwined and cross-linked with eachother. Techniques commonly used instructural studies of ordered crystalline materials, for instanceX-ray diffraction, cannot beused to characterize these disordered systems.[1.3] Consequently,their solid state structuresin general are poorly characterized.A number of techniques have been used to gather generalstructural information aboutthese systems. For example, infrared (IR) and Ramanvibrational spectroscopies have beenused to study silica gel and organofunctionalized silica gels, both in solution and in the solidstate, to obtain information about the relative numbers and types of bondspresent. Theapplication of JR and Raman vibrational spectroscopies to these systems is limitedby the factthat they cannot differentiate between silicons in two very similar structural units, for exampleSi(OSi)3(OH) and Si(OSi)4.Size exclusion chromatography/Fourier transform infrared(SEC/FTIR) spectroscopy has also been used to obtain molecular weight and functional groupinformation about species in solution. However, SEC does not resolve the different silanolspecies formed during the synthesis of functionalized or unfunctionalized silica gels.[1.42]The formation of silica gel has been studied invasively using gas chromatography[1.43] and chemical analysis [1.41]. General macroscopic information about the gelationprocess of silica gel has been obtained using viscosity measurements. [1.43]However, none of the above mentioned techniques provides a comprehensive structuralcharacterization of amorphous silica gel or related materials.The pioneering work of Maciel and co-workers in 1980 [1.20] showed that silica geland functionalized silica gels could be characterized and studied non-invasively in the solidstate by nuclear magnetic resonance (NMR) spectroscopy. High resolution NMR4spectroscopy, unlike the above mentioned techniques, clearly differentiates between thedifferent structural units present in silica gel and organofunctionalized silica gel. Highresolution solid state NMR spectra providing structural information for amorphousmaterialsnot otherwise obtainable have only been possible with recent major advancesin solid stateNMR spectroscopy. During gel synthesis, 29Si solution NMR is invaluablefor studying thekinetic processes since it can resolve all the hydrolysis species and some of the smalleroligomers. Thus, information about both the formation and structure of the final solid productof both native and derivatized silica gels can be obtained using NMR spectroscopy. NMR isthe major experimental technique that will be used in this work, and therefore a discussion ofsolution and solid state NMR spectroscopy is presented in the following section.51.3 NUCLEAR MAGNETIC RESONANCENuclear magnetic resonance (NMR) spectroscopy deals with the interaction betweennuclear magnetic moments and magnetic fields. Nuclei with a spin greater than zero haveanuclear magnetic moment(p).In an external magnetic field (B0) a torque is exerted on thenuclear magnetic moment causing it to precessabout B0. The frequency of precession isknown as the Larmor frequency (1)) and is proportional to the gyromagnetic ratio(y, acharacteristic of each nucleus) and to the external magnetic field as shown in equation (1.1),= 2itv0 = yB0(1.1)where Co0 is the corresponding angular velocity.All the nuclei of interest in this research, such as ‘H and 29Si, have a spin quantumnumber (I) of ½, and since the multiplicity of eigenstates is given by 21+ 1, they both havetwo energy levels. The placement of a sample containing such nuclei in an external magneticfield removes the degeneracy of the eigenstates (energy levels) resulting in a small majorityof the magnetic moments being in the more energetically favorable state, i.e. aligned parallelor antiparallel with the external magnetic field depending on whetheryis positive or negative,respectively. The transition between the two energy levels is stimulated by an appliedradiofrequency (if) field (B,) whose direction is perpendicular to the external magnetic fieldand whose frequency matches the resonance frequency of the nucleus of interest. Theresonance frequency for a nucleus at a specific field strength is dependent on its gyromagneticratio. These transitions disturb the equilibrium distribution(Mi)of the spin orientations. There-establishment of the equilibrium distribution depends on the spin-lattice interactions.The signal recorded after application of an on-resonance if pulse as a function of timeS(t), is called the free induction decay (FID) and is Fourier transformed to obtain the6frequency spectrum S(u).The nucleus most commonly observed is ‘H because of its high natural abundance,nearly 100%, and its relatively short spin-lattice relaxation time (T,). These two attributesmake it possible to acquire a spectrum with a good signal-to-noise ratio in a relatively shorttime period. For nuclei other than protons, for example 29Si, it is often more challenging toacquire an NMR spectrum due to the lower sensitivity, which can be a consequence of alower natural abundance, lowery,longer T, or in the worst case a combination of all three.The special techniques developed to overcome these experimental difficulties will bediscussed in section 1.5. The next section discusses the interactions present for spin ½ nucleiin both solution and solid state NMR.71.4 BASIC INTERACTION TERMSAt high magnetic fields, the dominant interaction thata nucleus experiences is thatwith the external magnetic field (Zeeman interaction). In additionto this, the nuclearmagnetic moment experiences several other interactionsthat perturb the energy levels. For aspin ½ nucleus, these include:1) the through space interaction with other nuclear magneticdipoles (directdipole-dipole interactions),2) the interaction with the electrons of the atom (chemical shielding),3) the interaction with other nuclear magnetic dipoles through bonds (indirectspin-spin interactions or J couplings).1.4.1 Zeeman InteractionThe interaction with the external magnetic field (B0) is called the Zeeman interaction.It represents, at high magnetic fields, the largest term in the Hamiltonian, and it removes thedegeneracy of the energy levels.H=—yIiB0. (1.2)In equation (1.2),His the Zeeman term of the Hamiltonian, is the gyromagnetic ratio, andI is the nuclear spin vector.[1.181In many cases, the remaining interactions can be considered perturbations to theZeeman interaction. The relative importance of the different perturbation terms mentionedabove varies for different experimental situations; for example at low magnetic fields theZeeman interaction may no longer be dominant.81.4.2 Dipole-Dipole InteractionsThe direct dipole-dipole interaction is a through space interaction between two nuclei.The dipolar Hamiltonian term(HD)between two spins, S and I, is [1.18, 1.22]H=JJ—___________(1.3)D5jr1 r1wherei refers to the distance vector between the I and S spins; the subscripts I and S of thegyromagnetic ratio(y)and spin vector (1) indicates which spin it refers to. Equation (1.3)shows that the dipolar-dipolar interaction is independent of the external magnetic field and isproportional to the gyromagnetic ratio of both nuclei.[1.22]The interaction that will be of concern in this research is the heteronuclear dipolarinteraction, i.e. in equation (1.3) the S spin refers to the sensitive or abundant nuclei and the Ispin refers to the insensitive or dilute nuclei.Equation (1.3) can be expanded by re-expressing the spin operators in terms of raisingand lowering operators and in terms of spherical coordinates.[1.22] The final expression isshown below. [1.18]H= oYJ7S[A +B +C +D +E +F]DA=—II1(3cos2e— 1)B=41+11+II1](3cos2e-1)C=4u9’1+D = - . [1 I + I I] sine cose e(i42Sz I- S- Iz9E = _4’s+’+smn20e2i4F = —2_i I sinOe4S I-For most practical purposes the resulting expression of the dipoiar Haniiltonian canbesimplified by omitting the last four terms which describe the weak absorptionsat zero and2co0 and since the case of interest is the heteronuclear dipolar interaction, where the I andSspins have different Larmor frequencies, the second term can also be omitted.[1.22,1.30]The simplified form of the heteronuclear Hamiltonian, equation (1.4) illustrates thatthedipoiar-dipolar interaction has a strong dependence on the distance(1/r31)between the twonuclei and depends on the angle(O)between the internuclear vector F) and the externalmagnetic field. [1.18, 1.1 9C, 1.30] The significance of these dependencies wifi be expandedon in the discussions of magic angle spinning (MAS) and cross polarization in section 1.5.HD=______(1-3cos2O1)(‘IzSz)(1.4)(13)4 itr3Of great practical importance is the case of two systems of spins; the heteronucleardipolar Hamiltonian is now the summation over all the pair-wise interactions, equation (1.5),[1.22]yyNNm(1-3cos2e.)H1, =S 0 im(‘ ‘m)(1.5)i mr1where N refers to the number of S (sensitive) spins andNmrefers to the number of [(insensitive) spins.In solution the fast molecular motion averages the dipole-dipole interactions to zeroand therefore they are not directly observed. This is not the case in solid state NMR. For10B0Figure 1.2 -ABCDSchematic representation of the effect of the heteronuclear dipolarinteraction on a solid state 29Si NMR spectrum.A) The dipolar interaction between a spin pair of 29Si and 1H nuclei.pxare the z components of the nuclear magnetic moments.B) The dipolar interaction of an isolated Si-H pair at one angle to theexternal magnetic field.C) The Pake pattern expected for isolated Si-H spin pairs with adistribution of angles to the external magnetic field in a powder sample.D) An approximate Gaussian lineshape observed for non-isolated Si-Hspin pairs considering all the dipolar interactions.[1.23]II11example, a 29Si solid state NMR spectrum of an isolated ‘H-29Si pair at a fixed angle to theexternal magnetic field (Figure 1.2A), will consist of two signals, from the effect of the twopossible proton spin states. This is illustrated in Figure 1 .2B. Isolated ‘H-29Si pairs with adistribution of angles (9) will produce a Pake pattern which is shown in Figure 1 .2C. This isthe case in a powder sample where all orientations are possible. When the ‘H-29Si pairs arenot isolated and more than a single pair of dipoiar interactions occur, a broad featureless linewill be observed (Figure 1.2D). In order to obtain high resolution NMR spectra of solids,magic angle spinning and high power decoupling are used to narrow the broadening producedby dipole-dipole interactions.[1.111121.4.3 Chemical Shift InteractionThe induced movement of the electronssurrounding a nucleus by an externalmagnetic field produces a current which results ina secondary magnetic field. Thisadditional magnetic field adds to the external magneticfield and changes the local fieldexperienced by the nucleus. Different resonance frequencies(“chemical shifts”) are observedfor nuclei in different local magnetic fields. Therefore thechemical shifts reflect thechemical surrounding of a nucleus, giving a very sensitive probeof the immediate chemicalenvironment of the nucleus.The chemical shift Harniltonian(H)term is shown in equation (1.6).HyIB0(1.6)where the chemical shift tensor()describes the orientation and magnitude of this three-dimensional shielding. It is evident from equation (1.6) that the size of the chemical shifts isdirectly proportional to the external magnetic field (B0). In orderto facilitate comparison ofdata from different B0 values, chemical shifts are usually given in ppm (parts per million) ofthe applied field, and measured with respect to a convenient reference standard.In terms of the chemical shielding, a single signal will be observed for a nucleus in asingle crystal (in the absence of other interactions). The position of this signal will dependon the orientation of the crystal with respect to B0, as shown in Figure 1.3A. For a powdersample, a characteristic chemical shift anisotropy pattern will be observed depending on thesymmetry of the chemical shift tensor and the amount of molecular motion present. Figure1.3 shows two schematic chemical shift anisotropy patterns for axially symmetric (Figure1.3B) and nonsymmetric (Figure 1 .3C) shieldings. Any molecular motion will cause at leastapartial narrowing of the chemical shift tensor powder pattern and possibly a change in its130c0 HIABCI)Figure 1.3 - Schematic representation of theeffects of the chemical shift anisotropy onsolid state NMR spectra(A-C).A) The chemical shifts observed for twoorientations of a carbon monoxidesingle crystal relative tothe external magnetic field.B) This axially symmetrical chemical shift anisotropypowder pattern isobserved for axially symmetrical moleculessuch as polycrystallinecarbon monoxide.C) A general anisotropic chemical shift anisotropy powder patternfornonsymmetrical molecules.D) The isotropic chemical shift observed insolution NMR. [1.25114symmetry.The chemical shift anisotropy, like the dipolar interaction hasa spatialdependency that can be seen in equation (1.7).[1.45]Gcsa= ½ (a — c) (3cos2O — 1) + ½(a5 —a150)sin2O cos2phir= aa(1.7)a -a.zz isowherea>,3,and are the diagonal elements of the chemical shift tensor and e and arepolar coordinates.[1.45]In solution NMR, the chemical shift tensor is averaged to its trace, since the moleculestumble freely and isotropically. This “isotropic chemical shift” (Figure 1.3D) can becalculated from the principal elements of the diagonalized chemical shift tensor:a. =..1_(a+y+a). (1.8)iSO3XX yy ZZSilicon-29 has a chemical shift range of about 500 ppm, with iodine-containing siliconhalogenides at one end (for example Si14 at -351.71 ppm) and some organic silicon chlorides(for exampleCl2Si[Fe(CO)(C5H)]at 146.65 ppm) at the other.[1.31 The chemical shifts for295i NMR spectra are usually given with respect to that of tetramethylsilane (TMS) which isassigned a value of 0.0 ppm.1.4.4 Indirect Spin-Spin or J-Coupling InteractionThe Hamiltonian (Hi) shown in equation (1.9) describes the indirect interaction of twospins (I and S) via their intervening bonding electrons.[1.10]H—hIfI(1.9)15where i is the indirect coupling tensor.[1.18] This spin-spin interaction is independent of themagnetic field strength. The orientational dependency of the .1 tensor is usually not verystrong and this coupling is often referred to as a scalar coupling.If two inequivalent spins I and S (with a spin ½) are coupled to each other, the Iresonance will be split into N+1 lines (where N is the number of equivalent attached Snuclei). The S resonance will also be split, in this case into M+1 lines (where M is thenumber of equivalent attached I nuclei). The separation of the lines is referred to as thescalar coupling constant and since its value does not depend on the external magnetic field itis usually given directly in Hertz.In solution NMR, the scalar coupling is often an important feature of the NMRspectrum. For example, in the 29Si solution NMR spectrum shown in Figure 1.4 the nine 1Hnuclei (S) in the three equivalent methyl groups couple to the 29Si nucleus (I) and split theresonance line into 10 lines separated by 7.3 Hertz.In most cases, the coupling constant is similar in magnitude in both solution and solidstate NMR, however, the broad lines in solid state NMR spectroscopy of static samples due tothe direct dipole-dipole interactions and the chemical shift anisotropy usually conceal thescalar couplings. The next section discusses three techniques that improve the resolution andsignal to noise ratio in solid state NMR spectroscopy, namely high-power dipolar decoupling,magic angle spinning and cross-polarization.1631H.29Sj4-4, IF’’Ji)s.:...- •‘ -‘ — ,,‘— 5—10—15 ppmFigure 1.4 - Highresolution 29Sisolution NMR spectrumfor (CH3)SiCN,3Si-H= 7.3Hz.[1.13]1715 HIGH RESOLUTION SOLID STATE NMRSPECTROSCOPY1.5.1 High-Power Dipolar DecouplingFor nuclei of low natural abundance, such as 29Si, the dominant dipolar interactionisheteronuclear, usually involving 1H and thus it canbe removed by the application of apowerful decoupling field applied at the S nucleus (‘H) frequency while observingthe Inucleus.High power proton decoupling is achieved by applying an additional radio frequency(rf) field at the proton Larmor frequency (400 MHz in a 9.4 Tesla field). The irradiation ofthe protons induces rapid transitions, or spin flips, between the two allowed energy levels.This results in the abundant nuclei generating a heteronuclear dipolar field that is in the timeaverage zero and thus “decouples” the abundant spins (‘H) from those of interest(29Si).[1.28]The difference between solid state and solution NMR proton decoupling is themagnitude of the rf field necessary for decoupling. In solid state NMR spectroscopy thedipolar coupling is tens of kilohertz which requires a decoupling power up to approximatelyone kilowatt. In solution NMR spectroscopy, the J-coupling interaction is usually of the orderof hundreds of Hertz requiring relatively a low power level (around 5 watts).[1.29]1.5.2 Magic Angle SpinningAll interactions with an angular dependence of (3cos2O-1), such as the dipolarinteraction and the chemical shift anisotropy, can be averaged using a technique proposed andimplemented independently by Lowe [l.12B] and Andrew et al [1.12A} in 1958-1959. Thistechnique, called magic angle spinning (MAS), involves imposing an external motion on asolid sample. This external motion is a fast rotation around an axis forming an angle13withthe magnetic field (Figure 1.5). For the rotation, equation (1.10) can be written:18<3cos2ø — 1>= .._(3cos2a— 1)(3cos2f3 — 1)(1.10)where x,13and 0 are as defined in Figure 1.5. When1= 54.74°, which is known as the“Magic Angle”, the whole angular dependence term becomes zero. Inthe time average thedipolar interactions are reduced to zero and the chemical shift (and J-coupling interactions)totheir isotropic values. In other words, magic angle spinning (MAS) hasa similar effect torapid random motion in solution NMR.[1.18, 1.22]In order for MAS experiments to succeed in narrowing the chemical shift anisotropyproperly, the spinning rate has to be greater than the frequency spread causedby the spininteractions. If the spinning frequency is not larger than the chemical shift anisotropy,spinning side bands are observed on either side of the isotropic peak, separated bythespinning frequency. The sideband intensities approximately reflect the profile of the chemicalshift anisotropy powder pattern.Linebroadening resulting from dipolar interactions is narrowed by magic anglespinning. The spinning speed determines the extent of this reduction in the linewidth. Thecombination of magic angle spinning (MAS) and high-power decoupling, makes theelimination of the heteronuclear dipolar interactions technically feasible.In summary, the combination of magic angle spinning with high-power decouplingmakes it possible for high resolution NMR spectra of solids to be obtained that arecomparable to those in solution.[l.10, 1.27]19B0RFigure 1.5 - A schematic representation of the relevant angles involved in therotation of a macroscopic sample at an angle(13)to the appliedmagnetic field B0. R is the rotation axis about which the sampleis spun. A typical intemuclearivector r is inclined at the angle awith respect to R.[1.18, 1.22]201.5.3 Cross PolarizationIn solid state NMR, a direct one-pulse 29Si NMR experiment will often be veryinefficient due to the low sensitivity of 29Si and its long longitudinal relaxation time (T,).The technique of cross polarization (CP) was introduced by Pines,Gibby and Waughto overcome the sensitivity problem in the observation of dilute nuclei.[1.191It involvesenhancing the signal of the dilute nucleus such as 29Si by taking advantage of theheteronuclear dipolar coupling between the dilute (I for insensitive) and abundant (S forsensitive) spins.The equilibrium magnetization (M50) in a static magnetic field (B0) is given byCurie’s law, equation (1.11) (shown for the S spins).[1.19C]M= C5B0(1.11)SOTLwhereTLis the temperature of the lattice and C is the Curie constant of the S nucleus.The transfer of magnetization in a cross polarization experiment (Figure 1.6) is carriedout in three steps. First, a900radiofrequency pulse is applied to the S spins (for example,‘H) along the x’ axis in the rotating frame, rotating the magnetization onto the y’ axis. Thesecond step involves spin locking the S spins (‘H or 19F) in the xy plane with aB,fieldalong y’ which is static in the rotating frame. The B,5 field forces the proton spins to precessabout y’ with a frequency o15 which depends on the magnitude of the B,5 field. For any nonequilibrium state of the magnetization it is possible to define a “spin temperature”Tsuch thatCurie’s law is still fulfilled, provided that the magnetization does not change too quickly.211H(spin 1ock) decoupleallow protons tore-equilibratecontact time_____.eFIDrecycle delayI II II I)TimeStep 1 Step 2 Step 3Figure 1.6 - The cross polarization (CP) pulse sequence illustrated for1H-29SiCP NMR experiments.22The magnetization of the S spins in the spin locking field(B1) is given byequation(1.12). [1.1 9C1M=(1.12)ST5where T5 is the spin temperature in the spin locking field,in the rotating frame. Theamplitude of M has not changed; therefore the magnetization M5 is equaltoM0(theequilibrium magnetization). The applied magnetic field (B15) issignificantly smaller than theexternal static magnetic field (B0); therefore the relationship (1.13)is true.T5<<< TL(1.13)This implies that spin locking the S (‘H or ‘9F) magnetization along the B15 field correspondsto a “cooling of the spins.Simultaneous with the spin locking of the S spins, another long if pulse with anamplitude B1 is applied to the I spins(295i) causing the I spinsto precess with a frequencyo around B11. If the two spin reservoirs are coupled via the Hartmann-Hahn condition bychoosing appropriate values for B11 and B15, [1.19C10)s=1(1.14)=their momentary precession frequencies 0,, and o become equal and energy exchangebetween them is possible. The I spins(29Si) have a lower spin temperature in the spinlocking field than the S spins (‘H or‘9F).[1.26] Therefore energy flows from the S spins tothe I spins until both the S and I spins have the same final spin temperature, equation(1.15).[1.5, 1.26]23B= T/= TL(...i.) 103TL(1.15)B0This energy flow to the I spins is manifested asa growth of the I magnetization along thespin locking field B11.Step three involves observing the free induction decay ofthe 29Si signal. TheB1fieldmay be kept on during the acquisition period if decouplingis needed. This pulse sequence isrepeated and theFIDSadded until a sufficiently good signal to noise ratio is attained.A cross polarization experiment has two advantageswith respect to the signal to noiseratio (S/N) over a simple900pulse experiment. Firstly, there is the direct enhancementfactor,‘Ys/.of the I magnetization [1.5, 1.19C], which in the case of 29Si can be upto afactor of 5, even though in practice this maximum enhancement israrely achieved. Secondly,the pulse sequence can be repeated after a delay determinedby the T, of the S nuclei (‘H)which is normally much shorter than that of the I nuclei(29Si). The shorter recycle periodand the enhanced signal to noise of each scan result in a higher signalto noise ratio in a CPversus a simple 90° pulse experiment in a fixed period of time.Cross polarization is transmitted via the heteronuclear dipolar interactions between thedilute and abundant nuclei. Therefore this technique, like the dipolar interaction itself, has avery strong inverse dependence on the distance between the two nuclei [1.18, 1.22] providinga possible tool to measure internuclear distances.1.5.4 Distance determination from CP curvesSpin locked magnetization decays exponentially with a relaxation time constant(spin-lattice relaxation in the rotating frame), equation (1.16).[1.31]24M(t) = M(O) e(1.16)The T1 values of the S and I magnetizations limit the time for polarization transfer(step two in the pulse program, Figure 1.6). Tn this thesis, the T,(29Si) was large enough forits effect to be neglected in this regard. The T1(1H), however, represents an importantcharacteristic of the polarization transfer and has to be taken into account in all quantitativeexperiments. In this case, the change in the magnetization of the I spins being observed isgiven by equation (1.17).[1.5, 1.19C]M1(t) = M10(TcP)(e e).(1.17)cpipThe rate of cross-polarizationTC;’can thus be determined by fitting the change in intensity ofthe 29Si signals as a function of contact time using equation (1.17).[1.51The relationship between the cross polarization rate(T’)and the internuclear distancebetween the S spins and the I spins is given approximately in equation (1.18).[1.19C]1_=c*‘S(1.18)COC1 is here a geometrical factor given by expression (1.19):[1.19C]b.2C — 3z (1.19)NDa2[(2b.+b.)2+(b+2b.)2Iwhere25,2 2(320—Pa.. =— IS “(l.19A)2r.ii.,,,, 2( 2n —b= IiIS “‘‘ “im(1 19B)imr.and i throughj refer to the S nuclei and m to the I nuclei. As well, r and r are theinternuclear vectors and and 0 are the angles that the internuclear vectors r1 and r makewith the external magnetic field, (Figure 1.7).[1.18JThe expressions for the second moment, equation (1.20) and (1.21), were firstpublished by J.H. Van Vleck.[1.34] He developed the method of moments to compute theproperties of a resonance line without knowing explicitly the energy eigenstates andeigenvalues.[1.34] <Av2>55 is the homonuclear second moment for abundant nuclei (1H or‘9F) in a rigid lattice: [1.18,1.2213y2 -‘ (3cos2O..—1)= —(S 0)2S(S+1) L1v(1.20)4 4ti r..6Iiwhere p0 is the permeability constant and S is the spin quantum number.[1. 18] If twodifferent types of spins are involved, the heteronuclear second moment</iv2>1is given byequation (1.21).[1.18, 1.22] This equation is only valid in the absence of motion.1 n’n’v-.. (3cos2O..1)2= —(‘ 0)21(1+1) L1 (1.21)3 4it r.6I is the spin quantum number.[1.18] In both cases, equations (1.20) and (1.21), the sum isover all the relevant nuclei i, in relation to the considered nucleus j or m, respectively.[1. 18,1.22] The effect of motion on these second moment calculations is discussed in Chapter 7.26B0S spinI spinrim= internuclear vectorFigure 1.7 - The geometric relationship of two spins I and S in anexternal magnetic field. [1.18, 1.22]°im271.6 TWO-DIMENSIONAL NMR SPECTROSCOPYThe concept of two-dimensional NMR experiments was firstsuggested by Jeener[1.36] in 1971. There was a rapid growth in the application and development of highresolution two-dimensional (2D) NMR after Ernst and co-workers demonstrated that a largevariety of 2D NMR experiments were possible.[1.37-1.39]All multidimensional NMR pulse sequences consist of three parts: preparation,evolution and detection periods. The preparation period can include oneor more pulses. Thespin system then evolves under the influence of different interactions depending on theexperiment during the evolution period (t1). Then the signal (FID) is acquired during thedetection period (t2).In a 2D NMR experiment the second time domain is introducedby acquiring a seriesof 1D experiments where the evolution time, t1 is incremented. The incrementation of t1creates a second frequency domain reflecting the interactions within thespin system duringthis time.The data are arranged in a two-dimensional matrix, S(t1,t2)with n rows (number ofFIDs gathered, i.e. number of increments of t1) and k columns (number ofdata pointscollected in t2). After Fourier transformation of these data in one dimension, S(t1,t2)—*S(t1,F2), a series of spectra is obtained that are phase modulated. A second Fouriertransformation of corresponding columns in the data set yields a 2D data matrix, S(F1,F2).A 2D NMR experiment is always possible if a systematic change in the evolutionperiod results in a periodic change of the phase and/or amplitude of the spectra. The secondFourier transform detennines the frequency of these modulations and provides the seconddimension for the spectrum. This dimension reflects the effect of the interactions underwhich the spin system evolves during t1.28A 2D experiment can be carried out to obtain heteronuclearcorrelations in the solidstate. Ernst and coworkers were the first to describea 2D heteronuclear correlationexperiment in solution.[1.381 The pulse sequence foran analogous 2D heteronuclearcorrelation solid state NMR experiment is shown in Figure 1.8. This experimenttakesadvantage of the cross polarization between a dilute and abundant nuclei.In a1H-29Siheteronuclear correlation experiment for example, the proton frequency is encodedduring aperiod (t1), following the900proton pulse. The remaining component of the protonmagnetization is spin-locked and cross polarized to silicon. Thesilicon magnetization is thenobserved during the period t2. Thus, the proton chemical shiftwill be on one axis and thesilicon chemical shift on the other. Examples of the application of thistechnique will bepresented in Chapter 2.29allow toFigure 1.8 - The pulse sequence used for a 2D 1H - 29Si heteronuclearcorrelation NMR experiment with 1H decoupling.The corresponding 1D cross polarization pulse sequenceis illustrated for1H-29Si in Figure 1.6.recycle delayt2301.7 OBJECTIVES OF THE THESISTraditionally, the preparation of organofunctionalized silica gels has involved thecondensation of a functionalized polychloro- or polyalkoxy- silane ontothe surface of solidsilica gel. The general reaction is usually representedas0Si02 + (R0)3Si-R’ —> SiO -o -Si-R’o(1.22)where RO = alkoxy ligandR’ = organic functionalityThe establishment of the anhydrous conditions requiredto obtain a monolayercoverage of the functionalized silane on silica gel is hinderedby the hydrophilic nature ofsilica gel. The presence of water favours both surface and self-condensationof thefunctionalized silane resulting in the formation of siloxane oligomers and polymers. Thesepolymers may or may not bind to the surface and if they do there is no guaranteethat theirfunctionalities will be accessible for further reactions. Consequently, irreproducible productsresult. Generally, operational success determines the concentrations used industrially.Silica gel can be synthesized by hydrolyzing SiCl4 or tetraethoxysilane (TEOS) toform Si(OH)4which undergoes repeated condensation reactions. In this thesis an alternativepreparation is proposed for the synthesis of these organofunctionalized silica gels via thecopolymerization of the silica gel monomer (TEOS) with the organofunctionalizedtrialkoxysilane as shown in reaction (1.23).Si(OH)4 + R’ -Si(OH)3 —> R’ -Si-O-Si- + 3H20(1.23)It is hypothesized that the final organofunctionalized product is produced directly during theformation of the gel.31The objectives of this thesis are to determine the integrity and distribution of thefunctionalities in organofunctionalized silica gel samples prepared using the alternativesynthesis described above. Solution and solid state NMR are the principal analytical toolsused in these investigations.The chosen model for a functionality was a methyl group. Functionalized silica gelwas prepared by the copolymerization of methyltriethoxysilane (MTES) and tetraethoxysilane(TEOS). Chapter 2 describes the sample preparation and the experiments carried out tocharacterize the functionalized silica gel in its solid form. The first step was to determinewhether the functionality remained intact in the final copolymer product. The next task wasto determine whether the two siloxane components produced a phase separated or a mixedcopolymer.The copolymerization kinetics were studied in order to estimate the reactivity ratioswhich determine the extent of mixing in the copolymer formation. In order to study the morecomplex kinetics of the copolymerization, the homopolymerization processes of bothmonomers have to be well known. Therefore, the hydrolysis and dimer formations of thetetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) monomers were characterized indetail as described in Chapters 3 and 4. In Chapter 3 the kinetic constants for the TEOShydrolysis intermediate reactions and dimer formation were experimentally determined. Toobtain the kinetic constants independent of the acid concentration, the polymerization ofTEOS was studied and analyzed over a range of pH values. Chapter 4 discusses the kineticconstants obtained for the MTES hydrolysis reactions and dimer formation. This is followedby a kinetic analysis of the copolymerization reaction. In addition, in Chapter 4 the ratios ofthe different dimers formed in a variety of MTES/TEOS copolymerizations are determined by295i solution NMR.32One of the potential applications of functionalized silica gels is inthe manufacturingof low temperature organofunctionalized glasses where thefunctionality is distributedthroughout the glass matrix. Chapter 5 deals with the effect of formamide,a commonly useddrying agent, on the hydrolysis and dimer formationkinetics of TEOS and MTEShomopolymers and the MTES/TEOS copolymer. The integrity of the drying agent over aperiod of a few months was probed by 29Si, ‘H, ‘5N and13C solution NMR. The focus ofChapter 6 is to determine the suitability of different functionalized silicagels to make lowtemperature glasses by establishing the thermal stability of these compounds using differentialscanning calorimetry (DSC), thermogravimetric analysis (TGA), thermogravimethc analysisaccompanied by mass spectroscopy (TGIMS), as well as 29Si, ‘3C and ‘H solid state NMR.Chapter 7 presents the results of an additional study using solid state NMR thatattempts to further characterize the distribution of the functionalities in functionalized silicagel samples. The distance range between the functionality and the fully condensedunfunctionalized silicons is deduced and the percentage of condensed unfunctionalizedsilicons within this distance range determined.A summary of all the conclusions and suggestions for future research are presented inChapter 8.33CHAPTER 2CHARACTERIZATION OF SILICA GEL AND FUNCTIONALIZED SILICAGEL BY 1D & 2D SOLifi STATE NMR EXPERIMENTS2.1 INTRODUCTIONIndustrially, immobilized reagents are synthesized by reacting a trichioro- or trialkoxysubstituted silane with hydroxyl groups on an activated silica gel surface, as illustrated inFigure 2.1.[1.1O] Trichioro- and trialkoxy- organofunctionalized silanes are used because theycan be synthesized directly and cheaply by the reaction of trichiorosilane with an appropriatealkene as in equation (2.1).C13Si-H + CH2=CH-R’ —* C13Si-CH2CH-R’(2.1)These trialkoxy- and trichioro- substituted silanes are potential monomers; therefore,besides reacting with the silica gel surface, they can undergo self-polymerization.Consequently, the preparation of immobilized reagents by reacting a trichloro- or trialkoxysubstituted silane with silica gel results in organofunctionalized silanes whose probablestructure is much more complicated than that shown in Figure 2.1. In order to prevent theoligomers of the functionalized silane from attaching to the silica gel surface, monomers suchas X-Si(CH3)2R’ (where X is either an alkoxide or a halide and R’ is the functionality) can beused instead of the trichloro- or trialkoxy- substituted silane. However, this is moreexpensive and therefore seldom used.Maciel and co-workers were the first group to use solid state NMR to study silica gel.Using 29Si CP/MAS NMR they identified the different silicon environments found in silicagel, i.e. (Si(OH)2(OSi),Si(OH)3(OSi) and Si(OSi)4)shown in Figure 2.2.[1.201 They went on34AOHSiO2 O—Si-R’ + 3HC12 OH+ C13Si-R’ IOHBOHSiO2 OSi-R’+ 3EtOH2 OH+ (OEt)3Si - R’OHFigure 2.1 - Industrial synthesis of functionalized silica gels.35Si(OSi)3(OH)Si(OSi)4-100 -1S0 PPMFigure 2.2 - 29Si CP/MAS NMR spectrum of silica gel withthe three different silicon environmentslabelled according to Maciel and co-workers.[1.201Si(OSi)2(OH)1I‘ I TT150 0 -5036to determine the T1(’H) and the cross polarization time constants(THsI)for all three siliconresonances. [1.20]In this chapter an alternative approach for the preparation of functionalizedsilica gelsof the general type shown in Figure 2.1 is proposed. Theresulting products are characterizedby one and two-dimensional solid state NMR spectroscopy to determine the integrity, quantityand distribution of the functionalities in the functionalized silica gel. The completecharacterization involves establishing the chemical integrity of the organic functionality inthegel, then the relative proportions of the different silicon environments in thesample andfinally, the distribution of the functionality, with respectto the question of a phase separatedor mixed copolymer.372.2 INTRODUCTION TO THE ALTERNATIVE SYNTHESISThe alternative preparation for functionalized silica gel is based on considering silicagel as a crosslinked polymer made via a condensation polymerization of SiX4 where X can beeither an alkoxy group or a halide. A functionalized silica gel with the functionality spreadthroughout the silica gel surface could therefore be considered a crosslinked copolymersynthesized in a one-step reaction by copolymerizing the functionalized silane monomerconcurrently with tetraethoxysilane (used to synthesize silica gel), (2.2).0 0n Si(OI-[)4 + n R’ -Si(OI-J)3—> [R’ -Si -0-Si -O-],+ n H20(2.2)A random distribution of the functionality throughout the silica gel matrix would beideal, since this gives minimal interactions between the functionalities.A functionalized silica gel which consists of tetraethoxysilane (TEOS) andmethyltriethoxysilane (MTES) with the methyl group representing the immobilizedfunctionality was chosen for the present work.382.3 EXPERIMENTAL2.3.1 Silica Gel2.3.1.1 Silica Gel - Fast Gel PreparationSilica gel was synthesized by the acid hydrolysis of tetraethoxysilane (TEOS) usingthe method described by Pen and Hensley.[2.lJ Ten ml of concentrated HC1 were addeddropwise while stirring to 0.2242 mole of TEOS in 75 ml of ethanol (40% by volume). Thesolution gelled in a few hours. The gel was broken into small pieces after standing overnightand placed in 200 ml ethanol/distilled water (50% by volume). After standing for 12 hours,the gel was washed with distilled water several times until the eluate had a pH of 7. Thesample was dried at 100°C for at least one day and ground to a fine powder.2.3.1.2 Silica Gel - Preparation Used in the Kinetic Studies0.08967 mole of TEOS was mixed with 0.5171 mole of ethanol and then 0.9928 moleof acidified water (pH=2.76) was added dropwise to the solution. The sample was left to gelat room temperature and normal pressure, a process which took several days. These are theconditions used in the kinetic studies discussed in later chapters. The gel was not washedwith water since in this preparation; the eluate was found to be neutral. The sample wascrushed and dried in the oven at 100°C for 24 hours and then ground to a fine powder.2.3.2 Methyltriethoxysilane HomopolymerMethyltriethoxysilane homopolymer samples were prepared by the same procedures asthose used for the silica gels (preparation 2.3.1.1 and 2.3.1.2) above, except that the startingmaterial was methyltriethoxysilane (MTES) instead of TEOS.2.3.3 Methyl Substituted Silica Gel2.3.3.1 Reaction with Preformed Silica GelThe methyl substituted silane was anchored onto preformed silica gel using the39following procedure. Silica gel (Fisher s-157; approximately 5 g) was pre-dried in vacuo at-150°C overnight, and was then refluxed with an excess of MTES in dry toluene for 12hours. After filtration, the solid was soxhiet-extracted with dry toluene for 12 hours and thendried in vacuo at —150°C for 12 hours.2.3.3.2 Copolymerization - Fast Gel PreparationMTES and TEOS were mixed in various mole ratios in ethanol. The preparation thenproceeded as in 2.3.1.1 for silica gel.2.3.3.3 Copolymerization - Under Kinetic ConditionsMTES and TEOS were mixed in various mole ratios in ethanol. The preparation thenproceeded as in 2.3.1.2 for silica gel.2.3.4 Solid State NMR SpectraQuantitative 295i solid state MAS NMR spectra were acquired on a Bruker AM 400spectrometer at 79.495 MHz with a home-built narrow bore MAS probe with a 9 mm spinnersystem. For the experiments a 45° 29Si pulse and 180 seconds(3*T1)recycle delay were used.The cross polarization experiments, both 1D and 2D, were performed on a BrukerMSL 400 spectrometer at a spectral frequency of 79.495 MHz. All the 2D spectra consist of64 experiments, and unless otherwise stated, they were acquired with a 22 millisecondscontact time, 3 seconds recycle delay and a 4-4.3 kHz spinning speed. The spinning rate forall the experiments on the MTES/TEOS mixed and copolymer samples was carefully chosento prevent overlap of spinning sidebands and the 29Si resonances. Specific experimentalconditions are given in the figure captions.The Bruker programttGlinfit” program was used for the deconvolution of the spectra.It was found that the spectra were best fitted using a Gaussian lineshape.402.4 RESULTS AND DISCUSSION2.4.1 Characterization of the FunctionalizedSilica Gel by 1D NMR ExperimentsOne-dimensional high resolution ‘3C and 29Si solidstate CP/MAS NMR spectroscopywas used to determine if the functionality was intactin the MTES/TEOS copolymer. The 13Cand 29Si CP/MAS NMR spectra of silica gelincorporating approximately 25%methyltriethoxysilane (preparation 2.3.3.3) are shownin Figure 2.3. The assignments aremade in accordance to those previously reportedin the literature. [1.10, 1.20] The ‘3C and29Si spectra clearly show that the functionalityis intact and attached to silicon.The relative concentrations of the different types ofsilanes were determined viadeconvolution of the overlapping peaks in the quantitative29Si MAS NMR spectra. Theprinciple difference between the preparations (2.3.1.1 and 2.3.1.2)and (2.3.3.1 and 2.3.3.3) isthat one set is synthesized with no additional water and concentrated HC1,while the otheruses excess water and a much smaller concentration of HC1.The quantitative 29Si MASNMR spectra of the samples synthesized under either preparations(2.3.1.1 and 2.3.1.2) or(2.3.3.2 and 2.3.3.3) show that they have essentially the same composition. InTable 2.1,when the code name starts with A it refers to samples prepared with either preparation2.3.1.1or 2.3.3.2 and when it starts with Bit refers to preparations 2.3.1.2 or 2.3.3.3. The next 3 or4 numbers in the code refer to the percentage of the monomersin the copolymer.Figure 2.4 shows the 29Si CP/MAS NMR spectra of two functionalized silica gelsamples (preparation 2.3.3.1 and 2.3.3.2) and a physical mixture of the MTESand TEOShomopolymers. The amorphous nature of these samples creates a distribution of isotropicshifts resulting in broad lines in the spectra. All the spectra have the same number ofpeaksat approximately the same chemical shifts. The differences in intensities come fromcomposition differences and different cross polarization characteristics of the samples.41Table 2.1 - A summary of the deconvoluted peak areas (i.e. concentration) from thequantitative 1D 29Si MAS NMR spectra.Concentration(%)**CODE*CH3Si(OSi)(OH) Si(OSi)4(OH)n=1 n=0 n=2 n=1 n=0A100 - - 4 39 57B100 4 36 60A2575 6 17 5 29 44B2575 5 19 4 30 41A5050 8 38 4 20 29B5050 8 40 2 17 33A7525 9 59 4 10 19B7525 15 61 - 11 13Code names starting with A are samples prepared with either preparation 2.3.1.1 or 2.3.3.2.Code names starting with B are samples prepared with either preparation 2.3.1.2 or 2.3.3.3.The next 3 to 4 numbers refer to the percentage of the monomers in the copolymer. Forexample A2575 was a copolymer synthesized using preparation 2.3.3.2 using 25% MTESand 75% TEOS.**Estimated error 10%.42-CH3‘3c chemical shift (ppm)CH3Si(OSi)—øø29Si chemical shift (ppm)Figure 2.3 - The 13C and 29Si CP/MAS solid state NMR spectra of anMTES/TEOScopolymer sample obtained at 100.3 and 79.5MHz,respectively.A - 13C CP/MAS NMR spectrum; 296 scans were acquired usinga 5msec contact time and 4s recycle delay.B - 29Si CP/MAS NMR spectrum; 100 scans were acquired usinga lOmsec contact time and lOs recycle delay.432Z 0Si(OSi)3(OH)Si(OSi)4Si(OSi)2(OH)—4g —60 —80 —120Therefore the intensity differences cannot be used conveniently to distinguish between thethree different preparations. No information describing the nature of the incorporation of thefunctionalized silane onto the silica gel can be obtained from the 1D CP/MAS NMR spectrain Figure 2.4.Since there is no significant difference in the chemical shift to differentiate between aphase separated and a mixed copolymer, another way to distinguish between a-a and a-b pairs(shown in Figure 2.5) must be found.The possible sources for cross polarization in a 1H-29Si CP/MAS experiment are themethyl and hydroxyl protons. In a phase separated sample (in the extreme case, MTES andTEOS homopolymers are only physically mixed), the 29Si magnetization originating from theMTES homopolymer should cross polarize mainly from the methyl protons while the 29Simagnetization within the TEOS homopolymer should only cross polarize from hydroxylprotons. In a random copolymer however, the TEOS silicons should also cross polarize fromthe methyl protons.The presence of hydroxyl groups poses a problem. They can cross polarize to all the29Si since they are present throughout the copolymer. In order to prove that the methylprotons of the MTES monomer cross polarize to the silicons in the TEOS monomer andtherefore the existence of a mixed copolymer, any cross polarization from hydroxyls has to beruled out. To completely eliminate the hydroxyl protons is extremely difficult due to thehydrophilic nature of silica gel. A more viable and less ambiguous option is to identify thesource of cross-polarization for each 295i signal using a 2D NMR experiment.44CH3Si(OSi)________________i • I ••1C -12C —iieFigure 2.4 - 29Si CP/MAS NMR spectra of differenttypes of functionalized silicagels and mixtures. The spectra were obtainedat 79.5MHz, 320 scanswere accumulated using a 22 ms contact time and 5s repetition time.The samples were spun at 3.2kHz.A - Methylsubstituted silica gel prepared by method 2.3.3.1B - Methylsubstituted silica gel preparedby method 2.3.3.3C - A mechanical mixture of silica gel and polymethylsiloxane.ABCI I • I • •-2C — —6r —sepp’I45MTES homopolymerCH3 CH3O—Si—O—Si 0I I0 0a aMTESiTEOS copolymerCH30b0— Si—O—Si—OI I0 0a bFigure 2.5 - Schematic diagram illustrating the possible localsilicon environments in an MTES homopolymer andan MTESII’EOS copolymer.462.4.2 Two-Dimensional Heteronuclear Correlation NMR ExperimentsOne approach to determine the extent of mixing of the two monomers is to integratecross polarization into a 2D experiment to identify the proton polarization sources.A 2D heteronuclear correlation ‘H-29Si NMR experiment (pulse sequence Figure 1.8)identifies the source of polarization. In this 2D experiment, one dimension is the 29Sichemical shift and the other is the ‘H chemical shift. Cross peaks appear between each 29Sipeak and those proton signals that are sources of magnetization transfer. In the proton MASspectrum of a mixture or copolymer of the TEOS and MTES monomers the -OH and -CH3signals are resolved. However, the lines are fairly broad and if the -OH peak is too dominantit overlaps with the -CH3 signal. To obtain a clear correlation of the cross polarizationsources and the different silicons, the contribution of the hydroxyl groups must be minimized.Sometimes the experimental parameters can be chosen to discriminate against somecomponents of the spectrum. One possibility to differentiate the two proton sources would bevia T, or T2 weighing. The T, values for hydroxyl and methyl protons in a MTES/TEOScopolymer sample were measured to be 358 msec and 364 msec, respectively. The T2 valuesof the hydroxyl and methyl protons are 1.2 msec and 1.4 msec, respectively. Thesemeasurements show that both types of protons have equal relaxation times (within theexperimental error) which suggests that a common spin-temperature exists. Thus T1 and T2discrimination cannot be used to differentiate between the hydroxyl and methyl proton peaks.Another option is to physically replace the hydroxyl groups by -OD groups bywashing with D20. This proved to be a more viable approach. All the samples were filtered,dried and washed repeatedly with D20. After the last washing the samples were quicklydried and packed under nitrogen, into the spinner.The observation of a cross peak in the 2D spectrum correlating the methyl protons and47the TEOS silicon peaks would signify that cross polarization occurs between the MTESmethyl group and the silicon nuclei of the TEOS monomer. This would prove that the MTESmonomers are spatially close to the TEOS monomers, providing evidence that a “mixed”copolymer has been formed.The reliability of the 2D ‘H-29Si heteronuclear correlation experiment was establishedby investigating two systems of known structure; a TEOS homopolymer and a physicalmixture of the MTES and TEOS homopolymers. The two 2D heteronuclear correlationspectra for the TEOS homopolymer before and after washing with D20 are shown in Figures2.6A and 2.6B, respectively. The 1D spectra shown along the Fl and F2 dimensions are theprojections of the 2D experiment.The 2D spectra in Figure 2.6 clearly demonstrate that the hydroxyls are a good sourceof cross polarization to all three types of silicons in the unfunctionalized silica gel. Asalready mentioned, this is a problem and therefore the challenge is to minimize theconcentration of hydroxyls in the samples. As is shown in the bottom 2D spectrum, washingthe sample with D20 twice, drying it at 100°C and avoiding subsequent exposure to air(packed in a dry N2 atmosphere) substantially decreases the intensity of the hydroxyl crosspeaks but does not eliminate them.Two-dimensional spectra from the other test case, the physically mixed MTES andTEOS homopolymers, before and after washing with D20, are shown in Figure 2.7. In thecase of the physical mixture no cross peaks should exist between the methyl protons and theTEOS silicon nuclei, since this is clearly a phase separated structure. In the mixture that wasnot washed with D20, there is significant cross polarization from the hydroxyl protons to allthe different silicon environments. In addition, cross polarization from the methyl protons tothe MTES silicons is observed, showing a characteristic extensive sideband pattern.48Ic)SUC.)Si(OSi)3(OH)Si(OSi)2(OH)Si(OSi)4-OHC0aC.)aUC-)-1o0•-OHFigure 2.6 - 2D 1H - 29Si heteronuclear correlation NMR experiments for silicagel(preparation 2.3.1.1). The top spectrum is of the unwashed samplewith80 scans per experiment. The bottom spectrum is of the same samplewashed with D20 several times with 200 scans per experiment.0049I0E0Si(OSi)3(OH)00CH3Si(OSi)IC)EVC)- Io-50 -100Figure 2.7 - 2D 1H - 29Si heteronuclearcorrelation NMR experimentsfor a mixtureof silica gel and polymethylsiloxane.The top spectrum is of theunwashed sample with 80 scans perexperiment. The bottomspectrum is of the same samplewashed with D20 severaltimeswith 200 scans per experiment.0050In Figure 2.7 the bottom 2D spectrum is of the same physical mixture of MTES andTEOS homopolymers after several washings with D20. As in the TEOS homopolymer, thehydroxyl cross peak is reduced remarkably, to the extent that the methyl cross peak is by farthe dominant interaction observed. These spectra of the MTES/TEOS physical mixture allowfor the unambiguous identification of the cross polarization sources: the methyl and hydroxylgroups for the MTES silicons and hydroxyl groups for the TEOS silicons. It should be notedthat the efficiency with which the hydroxyl groups were removed in this sample was notattainable for all the preparations, presumably due to differences in their microstructures.These results show that the 2D heteronuclear correlation experiments are reliableenough to be used on samples where the extent of mixing is not known, such as theMTESII’EOS copolymer (preparation 2.3.3.2 and 2.3.3.3). These gels were washed with D20and dried to enhance the contribution from the methyl group relative to those from thehydroxyls. The 2D spectra for the MTES/TEOS copolymers are shown in Figures 2.8 and 2.9(preparation 2.3.3.2 in Figure 2.8 and preparation 2.3.3.3 in Figure 2.9). In both cases, theTEOS silicons show small connectivities to the residual hydroxyl protons, as well as veryclear connectivities to the methyl protons. The observed sidebands in the connectivities areconsistent with the origin of the polarization being the methyl groups. In both cases, there isclear evidence for the incorporation of the methyl-substituted silicons throughout the matrix,i.e. the absence of phase separation, and the presence of substantial numbers of localstructures of the type a-b (Figure 2.5).51I:1Si(OSi)4Si(OSi)3(OH)CH3Si(OSi)CH3Si(OSi)2(OH)0000 -50 -10029Si chemical shift(ppm)C00Figure 2.8 - 2D 1H - 29Si heteronuclear correlationNMR experiment, for aD20washed 25/75 MTESI1’EOS copolymer(preparation 2.3.3.2).with 280 scans per experiment using a contact timeof 22 msec.52Si(OSi)4Si(OSi)3(OH)Si(OSi)2(OH)\ICH3Si(OSi)40z.0C.)I-‘a)C.),. QQI I I -. I • I I-50 -100 -15029Si chemical shift(ppm)Figure 2.9 - 2D 1H - 29Si heteronuclear correlation NMR experiment,for aD20 washed 10/90 MTES/TEOS copolymer (preparation 2.3.3.3).with 320 scans per experiment using a contact time of 10msec.532.5 CONCLUSIONSUsing ‘3C and 29Si solid state CP/MAS NMR, it has been shown that thefunctionalities in the MTESITEOS copolymer prepared by the alternative copolymerizationpreparations are intact. One-dimensional NMR experiments were not able to provideunambiguous information on the extent of mixing of the two monomers due to the linewidthsof the 29Si resonances and the presence of hydroxyl groups. Therefore, two-dimensional1H-29Si heteronuclear correlation experiments were used to identify the sources of crosspolarization to the different silicon peaks. These experiments demonstrated that the MTESand TEOS monomers in the MTES/TEOS copolymer are “mixed” and not “phase separated”.However, these 2D NMR experiments do not provide quantitative information on the degreeof mixing. It was anticipated that this information could be obtained from kineticinvestigations of the hydrolysis and the initial stages of condensation in solution. Thesestudies for the TEOS and MTES homopolymers, and MTES/TEOS copolymer are describedin Chapters 3 and 4.54CHAPTER 3HIGH RESOLUTION 29Si SOLUTION NMR INVESTIGATION OF THEHYDROLYSIS AND DIMER FORMATION REACTIONS OFTETRAETHOXYSILANE (TEOS)3.1 INTRODUCTIONThe results presented in the previous chapter demonstrate that themethyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) monomers are “mixed” in thesolid MTESiTEOS copolymer gel. To determine the extent of mixing a kinetic study of theformation of the copolymer dimer was undertaken, but first the hydrolysis and dimerformation kinetics of both monomers had to be well understood.Despite the versatility and wide usage of silica gel, the details of the individualsequential reactions during the hydrolysis and condensation of TEOS are not well described inthe literature. In early literature on the hydrolysis and condensation of TEOS, different typesof chromatography (GC- [3.12], GLPC-[3. 11], GPC-[3. 10]), and IR and Raman spectroscopy[3.91were used to obtain global kinetic rate constants of the hydrolysis and condensationstages, i.e. the hydrolysis and condensation are described by two reactions. The generalassumption in these works was that only the ligand involved in the reaction had an effect onthe hydrolysis and condensation rate constants. Some NMR investigations (principally 1HNMR) were done during this time period but at best global kinetic constants wereexperimentally determined.[3. 13, 3.10, 3.14] In general, the majority of investigations carriedout to date have dealt with simplified kinetic models which only permit the evaluation ofglobal hydrolysis and condensation rate constants. [3.10, 3.15-3.23] The increase in global55reaction rates with acid concentration was observed indirectly fromhydrolysis time versuscondensation time curves. [3.10, 3.17, 3.24, 3.25]More recently, Assink and Kay have used 1H and 29Si NMRspectroscopy to study thehydrolysis and polymerization of a related system, tetramethoxysilane(TMOS), undersubstoichiometric water conditions. In their work, they developed a statistical reaction modelassuming that both hydrolysis and condensation reactions depended solely on the functionalgroup reactivity, not on the local silicon environments or structures.[3.13,3.22, 3.23] Fromtheir global kinetic hydrolysis constants they deduced kinetic constants for each sequentialhydrolysis reaction assuming that the rate coefficient of a given species is simply the productof a statistical factor and the appropriate functional group rate coefficient. [3.28] From theirstatistical model they concluded that the relative magnitudes of the sequential hydrolysiskinetic rate constantsk1:k234were 4:3:2:1.Turner et al. studied the hydrolysis of TEOS with varying concentrations ofwater. [3.26] By qualitative arguments based on the information obtained from their 1H NMRdata they found that the concentration of water did not affect the kinetics over the rangestudied and concluded that k1 is predominant.[3.26]Contradicting the above results, Pouxviel’s group proposed that for TEOSpolymerization at water/silane ratios of 4 and 10 the relative magnitudes of the kineticconstants increased with each sequential hydrolysis, i.e.k1<k234.[3.27] Pouxviel’s groupnumerically simulated the concentration versus time graphs, obtained from 29Si NMR data, fora number of species over a four hour period. In their simulation they considered reactionsbetween sites, not molecules, and the water concentration was kept constant. Unfortunately,further details about their analysis were not presented in their paper. They concludedhowever, that the hydrolysis rate constants increased as the number of hydroxyls56increased. [3.27]Hui’s group found the same trend as Pouxviel et a!. for the kinetic constants. Theyestimated their individual kinetic constants using the data obtained from ‘H NMRspectroscopy and gel permeation chromatography (GPC), and a Linear Free EnergyRelationship (the ratios used were not stated) to give a systematic change in the k.values. [3.10]Chojnowski et al. studied TEOS at substoichiometric concentrations of water in aneutral solvent (dioxane).[3.24A] They quenched the reaction at different times,trimethylsilated the intermediates and determined their relative concentrations by gaschromatography/mass spectrometry (GC/MS). The individual kinetic constants were deducedby numerically solving the kinetic equations using the Runge-Kutta-Fehlberg method [3.24B],and optimized by the Rosenbrock procedure [3.24C].At the present time Chojnowski et al. and Pouxviel et al. are the only two groups thathave published experimentally determined kinetic constants for the sequential hydrolysisreactions of TEOS, although in both cases their results contain a pH dependence. These willbe discussed in more detail subsequently in terms of the results from the present work.Unfortunately, several literature results contradict each other, possibly due to the factthat some studies have been done at substoichiometric concentrations of water [3.13, 3.24A,3.26] where a good separation of the hydrolysis and condensation processes was notestablished; some have used peak heights not areas from the NMR spectra for quantitativeanalysis of the components [3.10, 3.15, 3.26], and in some cases there was no control of thetemperature at which the kinetics were studied [3.10, 3.15]. Of the few studies that dopresent kinetic rate constants for the individual hydrolysis reactions, most use eitherrelationships between the kinetic constants that may not be valid[3.101or assume unproven57generalizations about the reactivity of groups [3.13, 3.22, 3.23]. Consequently, a detailedcharacterization of the hydrolysis and dimer formation reactions of TEOS was undertaken.This study will serve as a reference for investigations of the effects of functionality,formamide and copolymerization in later chapters.The polymerization of TEOS involves three general reactions; [3.19]1) hydrolysis:Si(OEt)4 + H20 -÷ Si(OEt)3(OH) + EtOH(3.1)Si(OEt)3(OH) + H20 —* Si(OEt)2(OH) + EtOHetc. until the TEOS monomer is completely hydrolyzed2) condensation with the elimination of water:2 Si(OH)4 — =—SiOSi— + H2 0(3.2)and 3) condensation with the elimination of an alcohol:Si(OI-f)4 + Si(OI-I)3(OEt) —> —Si0Si_= + EtOH. (3.3)This chapter presents the kinetic model used to determine individual kinetic constants.The pH dependent kinetic constants were determined experimentally for the sequential TEOShydrolysis reactions and for the first time the pH independent kinetic constants for thehydrolysis and dimer formation reactions were calculated.583.2 EXPERIMENTAL3.2.1 Chemicals and Reaction MixturesThe experimental reaction conditions of TEOS were chosen to simplify the kineticsstudy. Acid catalysis and high water/silane ratios were used so that a good separation of thehydrolysis and condensation reactions was achieved. [3.1,3.111Tetraethoxysilane (TEOS) was studied instead of tetramethoxysilane (TMOS) becauseit is less hazardous and there is a greater chemical shift difference between the resonances ofthe intermediates.[3.2} The composition of the TEOS reaction mixture was: 4 ml (0.0 179mole) TEOS, 6.07 ml (0.1034 mole) ethanol, 3.58 ml (0.1981 mole) water acidified withconcentrated HC1 (various acid concentrations) and 0.075 g (0.0002 15 mole) of chromiumacetylacetonate (Cr(acac)3). Chromium acetylacetonate is a paramagnetic compound added toincrease the nuclear relaxation rates, i.e. decreaseT1.[3.8JDeuterium oxide needed for the NMR lock signal was added to the water. At leasttwo independent experiments were done at each pH value (3.35, 3.04, 2.88, 2.76, 2.55, 2.45and 2.33). pH values between 2.33 and 3.35 were used since outside these limits the reactionproceeded either too quickly or too slowly to accurately characterize the hydrolysis and dimerformation rates by 29Si solution NMR spectroscopy. Time zero in the experimentscorresponds to the time when the acidified water was added to the reaction mixture.3.2.2 NMR Measurements29Si solution NIVIR spectroscopy was used to quantitatively characterize each step ofthe hydrolysis and dimer formation of the acid catalyzed homopolymerization of TEOS inwater/ethanol. The resonances of the intermediate species in the TEOS hydrolysis can beclearly resolved in a 1D 29Si solution NMR spectrum.59A Bruker AMX 500 MHz spectrometer with a 10 mm broad-band probe was used anda temperature control unit maintained a constant sample temperature of 300.0 ± 0.1 K. Arelaxation agent, chromium acetylacetonate (Cr(acac)3)was added to reduce the 29Si spin-lattice relaxation time (T1) allowing for a faster repetition time between scans. The 29Si spin-lattice relaxation times (T1) of TEOS, in water and inwater/ethanol, were determined fordifferent concentrations of Cr(acac)3(0.0 1575 to 0.002711 M). The water/silane,ethanol/silane and water/ethanol ratios were kept the same as those used in the kinetic studies.The Cr(acac)3concentration of 0.0 16 M was chosen since this concentration approaches thelimit of solubility in the solvent system. No significant line broadening due to the Cr(acac)3was observed in the 29Si spectrum. Previous studies have shown that the presence of arelaxation agent has no effect on the polymerization process. [3.2, 3.3]In the presence of 0.0 16 M Cr(acac)3,the 29Si resonance of TEOS has a T1 value of1.7 sec. Two kinetic runs with different recycle delays (1*T1and2*T1)gave identicalconcentration profiles (within the ± 2% experimental scatter), implying that the 29Si T1 valuesfor TEOS and all the hydrolyzed intermediate species are virtually the same. Consequently,no bias exists in the measured concentrations. The Ernst angle equation [3.4] was thereforeused to optimize the signal/noise ratio. In both of these kinetic experiments and in all furtherones, the total 29Si signal intensity observed was constant within ± 2%.As a compromise between signal/noise ratio and a disproportionate averaging of thefast reacting species eight scans were accumulated per spectrum with a repetition time of onesecond and a 60° pulse angle on the 29Si channel. Proton gated decoupling was used toensure there were no negative NOE effects [3.4], even though previous data indicate that thenegative NOE effect is negligible.[3.22, 3.23] A complete kinetic experiment consisted ofacquiring 64 spectra at two minute intervals with a spectral accumulation time of 10 seconds.603.3 RESULTS AND DISCUSSION3.3.1 Experimental ResultsA typical 29Si spectrum obtained during the hydrolysis of tetraethoxysilane (TEOS) isshown in Figure 3.1. The assignment of the different 29Si environments is in agreement withthe literature.[3.61 At 500 MHz, under these reaction conditions, the resonances of all themonomeric species are clearly separated while those of the condensation products are lesswell-resolved, limiting the study of the formation of oligomers more complex than the dimer.A typical example of the time dependence of the spectra during the reaction is presented inFigure 3.2.The concentration versus time curves for the hydrolysis intermediates were determinedby integration over fixed individual frequency ranges in the spectra. These integrated areaswere used to calculate the percentages of the total concentration. The total integrated area forthe initial 29Si spectrum was taken to be 100%. The relative concentration versus time graphsfor all the intermediate species from one typical experiment (pH=2.55) are shown in Figure3.3.3.3.2 Kinetic AnalysisThe data obtained from the kinetic experiments was analyzed by modifying existingsoftware to deal with the specific set of equations which were dictated by the kinetic model.From the analysis of the data, kinetic constants for each of the hydrolysis steps and for thefirst condensation step have been determined. The TEOS hydrolysis rate constants that wereindependent of the catalyst concentration (HC1) were calculated from the pH dependence ofthe kinetic constants.The simplest model describing the hydrolysis of TEOS involves a series of sequentialreactions that proceed from TEOS to the formation of the fully hydrolyzed dimer. These five61Si(OEt)4Si(OH)4Si(OH)3(OEt)Si(OH)(OEt)3(OH) Si(OSi(OH))29Si chemical shift (ppm)Figure 3.1 - A typical 29Si spectrum obtained during the hydrolysis ofTEOS with the peaks assigned as indicated.Si(OH)4(OH)3Si(OSi(OH))Si(OH)3(OEt)122 mm41r90mm..58mm-72 -76St)426mm29Si chemical shift (ppm)Figure 3.2 - Stacked plot of the one-dimensional 29Si NMR spectra obtained duringthe polymerization of tetraethoxysilane at the times indicated: thewater was acidified to pH=2.55, and 8 scans/spectrum were acquiredusing a 1 second recycle delay and a600pulse.62100 2080. l5oSi(OEt)4 o Si(OH)(OEt)360a. 10.. •.40 “._ *3•) ......II.. •20Q • •••• ••••.• . .0 0••. •._•_•p•___0 20 40 60 80 100 120 0 20 40 60 80 100 120Time (mm)Time min)N5 100..Si(OH)2(OEt).a.aa060a.20400 0cb&0 Cm000QO •0 0000000 o00 (ftO________________________0ci’ o 000 20 40 60 80 100 120 0 20 40 60 80 100 120Time (mm)Time min)/////20 100-8015Si(OH)3(OEt)Si(OH)4060L0.xox54020xxxxxxxxxxxxxxxxxxxlxx___________________________________U 00 20 40 60 80 100 120 0 20 40 60 80 100 120Time (mm)Time (mm)Figure 3.3 - The time dependence of the relative concentrations of all of the intermediatespecies involved in the hydrolysis of TEOS. TEOS hydrolysis data atpH=2.55. The experimental error is ±2%.63reactions are:kTJ34Si(OEt)4+ H20 —* Si(OH)(OEt)3+ EtOHkT235Si(OH)(OEt)3+ H20 —* Si(OH)2(OEt) + EtOHkT336Si(OI-f)2(OEt) + H20 —* Si(OI-I)3(OEt) + EtOHkT437Si(OI-f)3(OEt) + H20 —÷ Si(OH)4+ EtOHkTs382Si(OH)4 —> Si(OH)3OSi(OI-I) + H20where OEt represents the ethoxy ligand.In the literature, the determination of a global hydrolysis rate constant(kH)involvesthe assumption that reactions (3.4)-(3.7) can be described by a single reaction:k11Si(OEt) + H20 —ESi(OH) + EtOHUnder the conditions chosen there is a good separation of the hydrolysis reactions andthe condensation product formation, which justifies that the kinetic analysis include reactionsup to the fully hydrolyzed dimer formation. For example, at pH=2.55, the first indications ofcondensed species in the spectra occur after 25 minutes, and after 40 minutes, only 9%conversion to dimer species has occurred. The separation is even clearer at higher pH values.64The kinetic curves, such as those shown in Figure 3.3, were fitted sequentiallyover afour stage process which is outlined in the following paragraphs. Successivestages of thefitting process involved the controlled introduction of additional variables.The initial step in the fitting procedure involved a non-linear least squares fitconsidering only the first four kinetic reactions (3.4)-(3.7) as forward (irreversible)reactions.The corresponding kinetic equations for the reactions (3.4)-(3.7) are (3.9)-(3.12).d[TEOS]= —kTl[TEOS][H20]0 (3.9)dtd[Si(OI-[)(OEt)______________= (kTJ[TEO8Jdt(3.10)—kT2[Si(OII)(OEt)3])[H20j0d[Si(OH) (OEt)]di’2= (kT[Si(OH)(OEt)3]j(3.11)—kT3[Si(OH)2(OEt)2] )[H20]0d[Si(OH) (OEt)]di’= (kT3[Si(OH’)2(OEt)]l(3.12)—k4[Si(OJ-I)3(OEt)])[H20]0Explicit expressions (3.13)-(3.16) for each monomeric species can be obtained uponintegration of the derivatives (3.9)-(3. 12).[TEOS1= e(3.13)[TEOSI0[Si(OI-I)(OEt)3]k1(ek1[H2O],t— e“°)(3.14)[TEOS]0(kT2 - kTl)65[Si(OH2(OEt)]= kTIkT2F e — e-k[H2O],i[TEOS]0(kT2 — kTl) (kT3 — kTJ) (kT3 — kT2)(3.15)+1 — 1)ek3[H2O],t]1(kT3-kT2) (kT3 - kTJ)[Si(OH)3(OEt)] = kT32kTJ F e[TEOSIQ(kT2 - kTJ)I.(kT3 — — k1)-k [HO]te 1 1 —k73[1120])(k3 - kT2)(kT4 - kT2) (kT3 - kT2)(kT4 - kT3) (k3 - kTI)(kT4 - k3)e(3.16)1 — 1 1 — 1+(k.4— k3) (kT4 — k1) + (k4 — kT2) (kT4 — kT3)__)e-k4[H2O]t1(k3 — kTJ) (kT3kT2)The expressions (3. 13)-(3. 16) were entered into a non-linear least squares fitting program toprovide the first approximations to the kinetic constants. In this first approximate analysis, allcondensation and possible equilibrium reactions are ignored and the water concentration wasassumed to be constant ([H20]0=initial concentration of water). Inspection of theexperimental results in Figure 3.3 reveals that the intermediate species Si(OH)2(OEt) andSi(OH)3(OEt) never reach concentrations greater than 9±1%. In fact, the Si(OH)2(OEt)concentration is never greater than 2 ± 1%. This limits the accuracy with whichkT3andsubsequent rate constants can be determined. Figure 3.4 presents the experimental andcalculated curves from this process for pH=2.55.To minimize the errors in the assumptions made in this approach, large water/silaneand ethanol/silane ratios (11.05 and 5.8 respectively) were chosen for the reaction mixture.These high ratios reduce the variation in both the water concentration and the solventcomposition as the reaction progresses. In addition, a high water/silane ratio favours thehydrolysis process, producing a better separation between the hydrolysis and condensation66100 20Si(OEt)480 -15060-110Si(OH)(OEt)340.0•••’20-0 20 40 60 80 100 1200 20 40 60 80 100 120Time (mm) Time (mm)20Si(OH)2(OEt) Si(OH)3(OEt)1503-0ioo KC XXX xxo 5xxx0XXKX1o% 0 o:a0 20 40 60 80 100 120Time (mm)Time (mm)Figure 3.4 - Calculated curves from the least squares fitting of equations (3.13-3.16)for the time dependence of the relative concentrations of the intermediatespecies fomied during the TEOS hydrolysis assuming a constant waterconcentration. The experimental error is ±2%.67reactions, and minimizes condensation between partially hydrolyzed species.[1.43] Thisapproach is limited mainly by the sensitivity of the NMR experiments. A relatively highethanol/silane ratio was needed to prevent phase separation.The calculated curves shown for the hydrolysis intermediate species in Figure 3.4describe to a good approximation what is happening in the reaction vessel. Only the fit for[Si(OH)3(OEt)] does not match well with the experimental data. The next step in the fittingprocess was aimed at improving the fit of this curve by taking into account the change inwater concentration. Consequently, [H20]0is now replaced bym(3.17) in equations (3.9)-(3.12):= [HO]0— [Si(OEt)3(OI-T)]— 2*[Si(OEt)2(OI-1)](3 17)— 3 *[Sj(OEt)(OJ-J)]— 4 *[Sj(OJ-J)jThe resulting interdependent differential equations are now not explicitly solvable and thesolutions must be obtained numerically.A new set of subroutines was written in order to call a program called LSODE whichnumerically solves interdependent differential equations. [3.7] It uses a backward differentialformula approach which is a multi-step method first implemented by C.W. Gear.[3.7] Theresults from the first fit (Appendix 1) were refined in this step and these numerical solutionswere the starting points for the next step in the fitting procedure.In general, including the water concentration variation had a very small effect on thecurves, as seen in Figure 3.5. Thus it is not considered to be a critical factor in the fitting ofthe kinetic curves for these reactions, at least not under the conditions used. Nevertheless, itwas included in all the final calculated curves for completeness.The next addition to the fitting procedure was to include the first condensation68100•60C)0400 20 40 60 80 100 120Time (mm)3.2Si(OH)2(OEt)10 20 40 60 80 100 120Time (mm)0 20 40 60 80 100 120Time (mm)80Si(OEt)420Si(OH)(OEt)35.0 20 40 60 80 100 120Time (mm)I20110Si(OH)3(OEt)5,UFigure 3.5- Comparison of different calculated curves for the TEOS system (pH2.55).The fits obtained when the water concentration is held constant (dashed lines)and when the change in water concentration is taken into accountas the reactions progresses (solid lines) are shown.The experimental error ±2%.69reaction to the kinetic scheme by adding equation (3.18) to the series of kinetic equations.d[Si(OI-1)Idt‘(kT4[Si(OI-I)3(OEt)] 1i — kTs[Si(OI-I)4])(3.18)By including the effect of the first condensation reaction, the Si(OH)4speciesconcentration asa function of time could be fitted.The net effect was to reproduce reasonably well the time dependence of the Si(OH)4product but no significant change was detected for the calculated curve of the lowconcentration Si(OH)3(OEt) intermediate (Figure 3.6A). Thus the fit for the Si(OH)3(OEt)species was still inadequate.In order to satisfactorily fit the concentration profile of the Si(OH)3(OEt) intermediatespecies, an additional process had to be included in the model. The model used up to thispoint predicts a curve for the Si(OH)3(OEt) concentration that reaches a maximum and thendecays to zero, which obviously does not agree with the experimental data. The experimentaldata are characterized by a plateau after the maximum which suffers minimal decay as thereaction proceeds, suggesting that an equilibrium is established between Si(OH)(OEt)3andSi(OH)4.Therefore, in order to reproduce the curve shapes for the decay of Si(OH)3(OEt)and the formation of Si(OH)4,an equilibrium between these two species was considered. Thismeans that equations (3.12) and (3.18) must be modified leading to a new set of differentialequations. Equations (3.20)-(3.22) correspond to equation (3.9)-(3. 11), taking the change inthe water concentration into account. Equations (3.23) and (3.24) incorporate the backreaction just discussed resulting in the inclusion of the change in ethanol concentration duringthe reaction.7020 50Si(OH)3(OEt)40Si(OH)40A1530/‘,10.C20 IV5</o1’10g0“L)/C_I————_.,_C0 20 40 60 80 100 120 0 20 40 60 80 100 120Time (mm) Time (mm)20 50Si(OH)4Si(OH)3(OEt)_____15o•oB3010 SCV20r7:..‘:_____________0 20 40 60 80 100 120 0 20 40 60 80 100 120Time (mm) Time (mm)20 50____Si(OH)4Si(OH)3(OEt)4010 0‘4__ •C30/,10.e20I)100 20 40 60 80 100 1200 20 40 60 80 100 120Time (mm) Time (mm)Figure 3.6 - Calculated curves of the relative concentrations of Si(OH)3(OEt) andSi(OH)4intermediates (acidified water was at pH=2.55).The experimental error is ±2%.A - is the shape of the curve when not considering the equilibriumback reaction.B - is the shape of the curve when considering the equilibrium backreaction. The experimental data is also shown in both A and B.C - is a comparison of these two curves without the experimental data.71d[TEOS]= -kTj[TEOS],(3.19)dtd[Si(OH)(OEt)]______________= (kTI[TEOSjdt(3.20)—k2[Si(OH)(OEt)3j)‘d[Si(OH) (OEt)]dt2= kT2[Si(OJ-I)(OEt)3](3.21)— kT3[Si(OI-f’)2(OEt)J)i,d[Si(OH) (OEt)]dt= kT3[Si(OJ-[)2(OEt)],(3.22)— kT4[Si(OH)3(OEt)] )T1, +k4[Si(OFf)]1e,d{Si(Off)Idt= kT4[Si(OI-r)3(OEt)I,-q— k4 [Si(O1-I)]e(3.23)— kT5[Si(OH’)4Jwherec,=[EtOI-I]0+ [Si(OI-f)(OEt)3]+ 2 [Si(OH)(OEt)]1(3.24)+ 3 [Si(OI-I)(OEt)j + 4 [Si(OI-I)jThe inclusion of the back reaction,condensation reaction and thechange in water andethanol concentrationsagain precludes the possibility ofexplicitly integrating this finalset ofdifferential equations.Consequently, these interdependent differentialequations werenumerically solved usingLSODE with the previouskT1T5values as starting values. Theeffect of this modificationon the Si(OH)3(OEt) and Si(OH)4curves is illustrated in Figure3.6B and C. The results fromthis fourth fitting procedure arepresented in Figure 3.7 for72100 20‘—‘ 80040Q 20150.00c;)100‘—‘ 80C0oL) 20Figure 3.7 - The time dependence of the relative concentrations of all the intermediate speciesinvolved in the hydrolysis of TEOS together with the calculated curves. Thedata are from the kinetic run of TEOS at pH=2.55.The experimental error is ±2%.Si(OEt)4 Si(OH)(OEt)3“—‘15C10C.)N0 20 40 60 80 100 120Time (mm)00 20 40 60 80 100 120Time (mm)Si(OH)2(OEt)0020/00 20 40 60 80 100 120Time (mm)At.100Time (mm)Si(OH)3(OEt)100 20 40 60 80 100 120Time (mm)Si(OH)4400C200 20 40 60 80 100 120Time (mm)73all of the intermediate hydrolysis species of TEOS at pH=2.55.The final analysis procedure was done in terms of five sequential chemical reactions(3.4)-(3.8) and their kinetic equations (3.19)-(3.23). This set of equations considers reaction(3.7) as an equilibrium process and includes the condensation reaction between two fullyhydrolyzed monomers, reaction (3.8). In addition, it assumes that the dimer formationreaction is the only reaction depleting the monomer concentration.743.3.3 Justification for Not Including Other ReactionsIn the time frame used to study the kinetic reactions, dimers but no significantamounts of higher oligomers were detected. There are three possible types of condensationreactions that could form a dimer, (3.25)-(3.27): [3.24]2 Si(OI-1)4 — SiOSi + H20(3.25)Si(OI-f)4 + Si(OEt)(OI-f)3—* SiOSi + H20(3.26)Si(OI-I)4 + Si(OEt)(OH)3—> SiOSi + EtOH(3.27)Condensation between partially hydrolyzed species can be ignored because theirconcentrations are not significant compared to Si(OH)4(see Figure 3.3), making reaction(3.25) the predominant dimer formation reaction under these experimental conditions.The polymerization of TEOS could involve other reactions, such as re-esterification.One such reaction was included in equation (3.23) for the Si(OH)4fully hydrolyzed species.In order to rule out other re-esterification reactions for TEOS and the other intermediatehydrolysis species, an experiment was designed using 13C labelled ethanol (CH313CH2OH), todetermine directly the total extent of trans- and re-esterification. Figure 3.8 shows a naturalabundance 13C solution NMR spectrum obtained during the hydrolysis of TEOS. Figure 3.9shows the SiOCH2-region of the spectra obtained from two kinetic runs, one with 13Cnatural abundance ethanol (Figure 3.9A) and the other with 99.99% ‘3C enriched ethanol(CH313CH2OH) (Figure 3.9B).The total area of the -CH3 chemical shift region is proportional to the ethoxy ligandconcentration, including those in the form of ethanol and those attached to silicon. Therefore,the integral of the SiOCH2-region divided by the total -CH3 area gives the percent of ethoxy75ligand attached to silicon. After 100 minutes of the natural abundance kinetic run, theconcentration of SiOCH2CH3is 54% of its initial concentration. This decrease is due to thehydrolysis, compensated by any trans- and re-esterification reactions which may occur. Theenriched ‘3C ethanol kinetic run shows a growth to 163% of the initial ESiOCH2CH3concentration. The natural abundance of ‘3C is 1.11% and 99.99% enriched ethanol was used,therefore the trans- and re-esterification reactions appear magnified by a factor of 99. Withthis information, it was calculated that only about 1% of the ESiOH and of the SiOCH2CH3ligands are involved in trans- or re-esterification reactions. This result shows that the extentof trans- and re-esterification reactions under the chosen experimental conditions is small andtherefore justifies the assumption in the kinetic model that the only re-esterification reactionoccurring to any extent involves Si(OH)4.76H-OCH2CH3H-OCH2CH3Si-OCH2CR34013C chemical shift (ppm)Figure 3.8 - 13C solution NMR spectrum for the TEOS polymerizationreaction with natural abundance 13C ethanol. The spectrum wasobtained after 131 minutes with 32 scans, using a recycle delayof 3.5s and a600pulse.77153 mmA154mmB13C chemical shift (ppm)Figure 3.9 - Series of 13C solution NMR spectra for the TEOS polymerizationreaction (A) with natural abundance and (B) with 13C enrichedethanol. The spectra were obtained with 32 scans, a recycle delayof 3.5s and a600pulse. A spectrum was acquired every 2.5 minutes,but only every fourth spectrum is shown.Si-OCH2CH3-Natural Abundance5 mm13C Enriched783.4 GENERAL DISCUSSIONThe average kinetic rate constants for all the pH values studied are presented inTable3.1. These kinetic constants contain the acid catalyst concentrations. Forthe first time pHindependent rate constants can now be obtained for the individual reactions. Four kineticconstants showed a linear dependency on the acid concentration while those derived fromintermediate species with very small maximum concentrations are less well defineddue to themuch larger errors, Figure 3.10. The linear relationships confirm that the acid is onlyacatalyst, in agreement with previous findings.[3.10, 3.17, 3.24, 3.25, 3.29] The pHindependent rate constants are presented in Table 3.2.The general trend observed is that rate constants(kTj-kT3)increase as the number ofhydroxyls increase, i.e. hydrolysis becomes easier as n increases in Si(OEt)4(OH). This isin agreement with some previous observations.[3. 10, 3.24, 3.27] The notable exception is thelast hydrolysis which occurs at about the same rate as the previous reaction.It is clear from the data in Table 3.2 that the kinetic constants for the differentreactions are neither equal nor related in some simple incremental manner as has beenassumed in previous investigations.[3.10, 3.22] The present research shows that the approachtaken by Hui et al. is not a valid one and that it is impossible to deduce individual rateconstants from the experimentally determined global hydrolysis rate constant.[3.10]Assink and Kay considered 15 distinguishable local silicon chemical environments buttheir kinetic constants were proportionally related to the number of directly attachedhydroxyls. [3.22] The present results suggest that this nearest neighbour statistical model isnot valid for the TEOS hydrolysis and dimer formation since no evident relationship existsbetween the kinetic constants.Previously, two groups [3.24, 3.27] have determined experimentally the individual79Table 3.1 - Average pH dependent kinetic rate constants (M1min’) determined for theTEOS hydrolysis and dimer formation reactions, equations (3.4)-(3.8).pHkTlk.kT3 kT4f kT4b kT53.35 0.00054 0.0036 0.015 0.07 0.01 0.0123.04 0.00041 0.0035 0.033 0.083 0.011 0.0232.88 0.0008 0.0043 0.017 0.06 0.014 0.0162.76 0.0009 0.0065 0.034 0.08 0.016 0.022.55 0.0018 0.011 0.06 0.05 0.009 0.0332.45 0.0027 0.014 0.06 0.05 0.013 0.0422.33 0.0037 0.024 0.134 0.11 0.01 0.062.13 0.0072 0.038 0.36 0.085 0.016 0.1800.0100.05kTl=3.3 mm40.0005 0.0010 0.0015Concentration H (M)E0.0005 0.0010 0.0015 0.0020Concentration H (M)0.150.100.050.000.0000 0.0005 0.0010 0.0015Concentration H (M)Figure 3.10 - Plots of the pH dependent kinetic constants versus acid concentrationfor the hydrolysis and dimer formation reactions of the TEOShomopolymerization. The fits were constrained such that the interceptswere at zero.0.00830.006!0.0040.OOC0.0000.‘0.04E0.00200.0000 0.0005 0.0010 0.0015 0.0020Concentration W (M)0.150.10•.0.050.000.0000 0.0005 0.0010 0.0015 0.0020Concentration H (M)0.0000 0.0005 0.00 10 0.00 15 0.0020Concentration W (M)0.0200.0150.0100.005j0.0000kTs= 51 min’0.002081Table 3.2 - pH independent hydrolysis and dimer formationkinetic rate constants for TEOS(minj with the standard error. The fits were constrained with the interceptat 0.The data for the forward and backward reactions for reaction (3.7) were notconsidered accurate enough to include in this table.Kinetic rate constant Value (min1) Reaction no.kTl 3.3± 0.1 3.4kT218±1 3.5kT372±5 3.6kT551±2 3.882kinetic rate constants for the sequential hydrolysis and condensationreactions of two systemssimilar to that used in the present study. Chojnowskiet al[3.241studied the hydrolysis ofTEOS at a substoichiometric water/silane ratio of 2, in dioxane.The relative concentrationsof the individual species were determined byGC/MS after quenching the reaction andtrimethylsilating the intermediates at different times. They numerically solved the kineticdatato obtain the individual kinetic rate constants. Their results differ from those obtained in thepresent study when pH dependent values are compared. This is notunexpected given thelarge differences in the reaction conditions, i.e. they used substoichiometric concentrations ofwater and dioxane as the solvent. However, a general trend common to both experiments isan increase in the magnitude of the kinetic rate constants with increasing numbers of attachedhydroxyl groups. Chojnowski suggested that the large differences in k1 through to k4, in theircase 1000, is due to the strong hydrogen bonding of the silane hydroxyl groups to thedioxane. This was postulated to give additional polarization of the silane, increasing itsreactivity.Pouxviel et al. [3.27] is the only other group to have obtained experimental individualkinetic rate constants for a specific pH. The system studied was similar to that investigatedin the present work with respect to the reactant and solvent concentrations. They determinedone set of pH dependent kinetic constants from simulations of their data and their results arein part similar to those presented in this work. In particular, the ratios of the pH dependentkinetic constantskTlfkT2 , kT2/kandkT4/kTsare all very similar to those presented here. Amajor difference is inkT3/kT4,where Pouxviel et al. determined a value twice as large as thatfound in this study.The kinetic constants summarized in Tables 3.1 suggest that the dimer formation isslower than the formation of the fully hydrolyzed species. This is confirmed by the build-up83of the fully hydrolyzed species and the constant presence ofa small quantity of Si(OH)3(OEt)in the reaction mixture. It is this build-up of the fully hydrolyzedspecies that necessitates theinclusion of the equilibrium reaction for the last hydrolysis.843.5 CONCLUSIONSIt was possible to fit the quantitative kinetic data for the TEOShydrolysis and dimerformation well in terms of a very simple kinetic model, which consideredonly the lasthydrolysis as an equilibrium reaction. The TEOS kinetic constants for allthe sequentialhydrolysis steps and the dimer formation were experimentally determined for a series of pHvalues (Table 3.1). From these data, pH independent kinetic constants for the hydrolysisstepsand dimer formation were derived for the first time (Table 3.2).It is clear from the data obtained that the hydrolysis kinetic constants(kT1T3)are notthe same or related in a simple incremental fashion although their magnitudes increase withthe number of hydroxyls, It was also found that the main re-esterification reaction is fromSi(OH)4to Si(OH)3(OEt) which are the only two monomeric species left when condensationstarts under the reaction conditions chosen. In addition, the dimer formation for the TEOSsystem is slower than the hydrolysis, resulting in a build-up, to some extent, of the fullyhydrolyzed species.The pH dependent and independent kinetic constants for the hydrolysis and dimerformation of TEOS have been determined. These values will be the reference data whenstudying the effect of organic functionality (Chapter 4), organic drying agents (Chapter 5) andthe formation of a functionalized copolymer (Chapter 4).85CHAPTER 4HIGH RESOLUTION29SOLUTION NMR INVESTIGATION OFTHE HYDROLYSIS AND DIMER FORMATION OF METHYLTRIETHOXYSILANE(MTES) HOMOPOLYMERIZATION AND MTES/TEOSCOPOLYMERIZATIONREACTIONS4.1 INTRODUCTIONThe previous chapter presented a detailed kinetic analysis of the tetraethoxysilane(TEOS) monomer. In order to do a kinetic analysis of the methyithethoxysilane(MTES)/TEOS copolymer a detailed kinetic analysis for the MTES monomer is still needed.Van Bommel and co-workers using a two step process (acid and then base catalysis)studied qualitatively the effect of substituents (methyl-, dimethyl- and phenyltriethoxysilanes)on the hydrolysis and condensation. They found that the allcyl-substituted triethoxysilaneshave higher hydrolysis and condensation rates than TEOS.[4.5] This supports the generalstatement in the literature, that in an acid catalyzed reaction the presence of a non-hydrolyzable electron donating ligand on the silane results in higher hydrolysis rates.[3.29]Yet no literature data pertaining to the analysis of the hydrolysis and dimerization offunctionalized triethoxysilanes could be found. Thus this chapter presents the firstcharacterization (via kinetic rate constants) of the methylthethoxysilane (MTES) hydrolysisand dimer formation.The investigation of the MTESITEOS copolymer focusses on the kinetics of theintermediate hydrolysis species and on the nature and concentration of the dimers formed, toprovide insight into the extent and nature of the copolymerization process. The MTESITEOScopolymer kinetic rate constants were determined using the TEOS and MTES homopolymer86pH dependent kinetic rate constants for the hydrolysis and dimer formationas a starting point.Reactivity ratios were calculated from the copolymer dimer formation rate constants. Thisinformation provides another indication of the extent of mixing of the MTES and TEOSmonomer in the copolymer. No literature data are available on the extent of co-condensationbetween two different silane species.874.2 EXPERIMENTAL4.2.1 MTES Homopolymerization Reaction ConditionsThe MTES homopolymerization reaction conditions in general follow thosealreadyoutlined for the TEOS homopolymerization in the previous chapter. The samplesused consistof: 3.57 ml (0.0179 mole) MTES, 6.07 ml (0.1034 mole) ethanol, 3.58 ml (0.1981 mole)water acidified with HC1 and 0.075g(0.000215 mole) chromium acetylacetonate (Cr(acac)3).As previously, chromium acetylacetonate was added to reduce the spin-lattice relaxationtimes. The 29Si T1 value for MTES inethanol/water with 0.016 M of Cr(acac)3was found tobe 1.6 seconds.All the spectra were acquired on a Bruker AMX 500 spectrometer using a 10 mmbroadband probe maintaining the sample temperature at 300.0±0.1 K. For each spectrum,eight scans were acquired with a 0.8 s recycle delay and using a 60° pulse angle. To followthe concentrations of the different species as functions of time a new spectrum was obtainedevery 1.5-2 minutes. Two kinetic experiments were done at each pH value (2.45, 2.55, 2.76,2.88, 3.04 and 3.35).4.2.2 MTES/TEOS Copolymer Reaction ConditionsThe MTES/TEOS copolymerization reaction was carried out at a pH of 2.55. Theconcentrations of the reagents in the control system were: 2 ml (0.0090 mole) TEOS, 1.79 ml(0.0090 mole) MTES, 6.07 ml (0.1034 mole) ethanol, 3.58 ml (0.1981 mole) water acidifiedwith HC1 and 0.075g (0.0002 15 mole) chromium acetylacetonate (Cr(acac)3). Each spectrumwas acquired with eight scans using a 60° pulse angle and a 1 second recycle delay.Copolymers were synthesized under a number of different conditions, Table 4.1, inorder to determine the effect of different parameters on the relative dimer ratios. Allreactions were carried out in duplicate.88Table 4.1 - Compositions of different samples used inthe copolymer study.TEOS MTES Ethanol H20 (moles) Ref.(moles) (moles) (moles) pH = 2.55 Name0.0090 0.0090 0.1034 0.1981 COill0.0134 0.0045 0.1034 0.1981 C02710.0045 0.0134 0.1034 0.1981 C07210.0090 0.0090 0.1034 0.3962 C01120.0179 0.0179 0.1034 0.1981 C02210.0179 0.0179 0.1034 0.3962 C022289Homo and co-dimers resonances are not very well resolved in the spectra andtherefore the analysis of the dimer region required that the signals be deconvoluted todetermine the extent of homo versus co-dimers present in the reaction mixture. Thedeconvolution used gaussian lineshapes and was performed using a program available withthe Bruker UXNMR software on the Bruker AMX 500 spectrometer.904.3 RESULTS AND DISCUSSION4.3.1 Kinetic Analysis of the MTES Hydrolysis and Dimer Formation ReactionsA typical MTES spectrum with the intermediate hydrolysis and dimer productslabelled, is shown in Figure 4. 1A. The dimer resonance is well separated and shiftedsignificantly upfield from all the resonances of the hydrolysis products.An experimental data set for the hydrolysis and dimer formation of MTES is shown inFigure 4.1B, which illustrates the good separation between the hydrolysis reactions (4.1)-(4.3),and condensation reaction (4.4). The peaks corresponding to the different intermediatespecies were integrated and their relative concentrations plotted as functions of time (asdescribed in Chapter 3 for the TEOS polymerization). An example of the experimental dataobtained from this is shown in Figure 4.2. After 20 minutes for the MTES polymerization atpH=2.55, there is still a clear differentiation between the hydrolysis and condensationreactions. At this time approximately 95% of the silicons are present in eitherCH3Si(OH)2(OEt) or CH3Si(OH) which means that there are only approximately 5% of thesilicons in the dimer form. Thus the important time period for the hydrolysis intermediates isup to 20 minutes, and these parts of the graphs were considered first in the analysis.The model for the kinetic analysis is analogous to that outlined in the previouschapter, but one less hydrolysis reaction has to be considered. Four reactions (4. 1)-(4.4) wereused to fit the intermediate hydrolysis concentration curves for the MTES polymerization.kMj(4 1)CH3Si(OEt) + H20 _ CH3Si(OH)(OEt)2+ EtOH91CH3Si(OH)/CH3Si(OH)2(OCH)ICHSi(OH)(OCH__j._________29Si chemical shift (ppm)Figure 4.1 A - A typical 29Sispectrum obtained during the hydrolysis ofMTES with the peaks assignedas indicated.29Si chemical shift (ppm)Figure 4. lB - Stacked plot of the one-dimensional29Si NMR spectra obtained duringthe polymerization of methyithethoxysilane (water was acidifiedtopH=2.55). The acquisition parameters used were: 8 scans/spectrumwith a recycle delay of 0.8 sec and a600pulse angle.92___.J .-ppm -38 —40 —42CH3Si(OSicH(OH)2)(OH)—44•1CH3Si(OH)/CH3Si(OH)2(OCH)CH3Si(OS1CH(OH)2)(OH)1’pp —38 —40 —42 —44 —46 —4810020CH3Si(OEt) CH3Si(OH)(OEt)2801506O._40-10a C.)C2005• L) •a. ..•..e0—-—• •.__•__t_e_••_pe_.p_p-0 20 40 60 800 20 40 60 80Time (mm)Time min)20- 100CH3Si(OH)2(OEt)801 5 .•.•CC•ê.• 0XXXXX0•. •6010 •••.•• •e..% 40• ac3200U•..r•.•%..............•.....•...•....e.....•.•0n0 20 40 60 800 20 40 60 80Time (mm)Time (mm)100CH3Si(OH)80C XX0• 60,(XX)CXXXXXxXXXX40XxXXXXC2000 20 40 60 80Time (mm)Figure 4.2 - The time dependence of the relative concentrations of all intermediate speciesinvolved in the hydrolysis of MTES. MTES hydrolysis data at pH=2.55.The experimental error is±2%.93kM2(42)CH3Si(OI-I)(OEt)2+ H20 _> CH3Si(OH)(OEt) +EtOHkM3(43)CH3Si(OH)2(OEt) + H20 _ CH3Si(OH)+ EtOHkM4(44)2CH3Si(OH) _> CH3Si(OH)2(OSICH(OH))+The kinetic constants for reactions (4. 1)-(4.4)can be obtained by expressing the concentrationchanges of the hydrolysis species as functionsof time, equations (4.5)-(4.8).d[MTESJ= -kMJ[MTES] [H2O1(4.5)dtd[CH Si(Ol-I)(OEt)Idt2= (kMl[MTES]((4.6)-kM2[CH3Si(OH)(OEt)],)[H20],d[CH Si(OH) (OEt)]dt2= (kM2[CH3Si(OH)(OEt)]l-kM[CHSi(OI])(OEt)J()[H201,d[CH3Si(OI-f)]= kM3[CHSi(OI-1)2(OEt)]1[H20]1(4.8)- kM4[CH3Si(OH)]Equations (4.5)-(4.7) can be solved explicitly, giving equations (4.9)-(4.11) assuming thewater concentration to be constant.[MTESI,= e(4.9)[MTESI094[CH3Si(OH)(OEt)21= kMJ(e-kMl[H2O],t—ekJHZO]t)(4.10)[MTESI0(kM2-kMJ)[CH3Si(OF[)2(OEt)j,= kM2kMlF e — e-k,jH2O],t[MTESIQ(kM2 - kMI) (kM3 - kMj) (kM3 - kM2)(4.11)+1 — 1)e-1cM3[HzO]t1(kM3 - kM2) (kM3 - kMl)The calculated curves using equations (4.9)-(4. 11) are shown in Figure 4.3 and the kinetic rateconstants determined by the non-linear least squares fit are summarized in Appendix 2. Ascan be seen, the curves except the one for CH3Si(OH)2(OEt) fit the experimental data pointswell.The next step to improve the fit ofCH3Si(OH)2(OEt) was to include the change inwater concentration and the dimer formation in the differential equations. The concentrationof water at any time t can be expressed by equation (4.12).=[H2O]0—[CH3Si(OH)(OEt)](4.12)- 2[CHSi(OH)(OEt)]- 3[CHSi(OH)]1As in the TEOS case, the inclusion of the change in water concentration did not havea significant effect on the shape of the curves. To fit the decay of the fully hydrolyzedspecies, a dimer formation reaction was included. The differential equations (4.5)-(4.8) arenow re-expressed, equations (4. 13)-(4. 16), taking into account the change in waterconcentration and the dlimer formation.d[MTESI= -kMJ[MTES](4.13)9510020CH3Si(OH)(OEt)2CH3Si(OEt)80C60•10..__.‘_e• -40820•05C.)(1 --..-- •_______________________________0 20 40 60 800 20 40 60 - - 80Time (mm) Time (mm)130 10080CH3Si(OH)2(OEt)—b.•60ooo201040C.)n0 20 40 60 80 0 20 40 60 80Time (mm) Time (mm)Figure 4.3 - Calculated curves for the time dependence of the MTES hydrolysisintermediate concentrations, equations (4.9)-(4. 11), assuming aconstant water concentration.The experimental error is ± 2%.96d[CH Si(OH)(OEt)]3 2= (kMJ[MTES],dt(4.14)-kM2[CH3Si(OH)(OEt)]f)d[CH Si(OH) (OEt)J3 2= kM2[CH3Si(OH)(OEt)z],dt(4.15)— kM[CHSi(OI-I)(OEt)] )TL’d[CH Si(OI-f)]_____________=kM3[CHSi(OIf)2(OEt)JTdt(4.16)- kM4[CH3Si(OI1)JThese interdependent equations (4. 13)-(4.16) were solved numerically, as previously,using the LSODE program. For the same reasons discussed previously for the TEOS case(section 3.3.2), it was found necessary to include a re-esterification reaction for the lasthydrolysis species in order to respect the curve shape forCH3Si(OH)2(OEt) (Figures 4.4Acompared to Figure 4.4B). With this additional modification, equations (4.15) and (4.16)become equations (4.17) and (4.18) whereemis the concentration of ethanol at any time tgiven by equation (4.19).d[CH Si(OH) (OEt)]dt2= kM2[CH3Si(OI1)(OEt)]((4.17)— kM3f[CHSi(OH)2(OEt)])r+ kMb[CHSi(OI-I)]ed[CH Si(OH)J3dt=kM3CHSi(OI-f)2(OEt)]1i(4,18)- kMb[CHSi(OH)]le- kM4[CH3Si(OIi)]97,_ 30Cci)C.)CCH3Si(OH)fl:0 20 40 60 80Time (mm)CClci)C.)0(-)Time (mm) Time (mm)Figure 4.4 - Calculated curves of the relative concentrations ofCH3Si(OH)2(OEt)andCH3Si(OH) intermediates (A) excluding and (B) including theequilibrium back reaction, together with the experimental data.(C) is a comparison of these two curves without the experimental data.The experimental error is ± 2%.CH3Si(OH)2(OEt)100806040C)0c-)AB,— 30!20ci) 10C)0U00 20 40 60 80Time (mm)CH3Si(OH)2(OEt)2010I0 20 40 60 80Time (mm)Time (mm)CH3Si(OH)(OEt)CH3Si(OH)0 20 40 60 80 0 20 40 60 8098100‘8060C1)Cc)402000 20 40 60 80Time (mm)Figure 4.5 - The time dependence of the relative concentrations of all intermediatespecies involved in the hydrolysis of MTES together with the finalcalculated curves. The data are from the kinetic run of MTES at pH=2.55.The experimental error is ± 2%.CH3Si(OEt)1(CCC)CCCH3Si(OH)(OEt)220 40 60 8010030CH3Si(OH)2(OEt)‘ 80CUUC0 20 40 60 80 0 20 40 60 80Time (mm)zzx100o0‘—. 0CC,40C)C52060Time (min)CH3Si(OH)0 20 40 60 80Time (miii)99where= [EtOI-I]0 + [CH3Si(OI-I)(OEt)2]1(4.19)+ 2[CH3Si(OI-I)2(OEt)]+ 3[CH3Si(OHj1The fitting of the experimental data was done using equations (4.13), (4.14), (4.17)and (4.18). An example of the results is shown in Figure 4.5. Experimentally, small signalsindicative of trimer formation start to appear in the spectrum at longer reaction times (greaterthan 60 minutes). It is therefore expected that the calculated curve overestimates the dimerconcentration at longer reaction times since it does not consider any depletion reactions forthe dimer.4.3.2 MTES Hydrolysis and Dimer Formation Kinetic Rate ConstantsThe average pH dependent kinetic constants obtained from this study are listed inTable 4.2. The pH independent rate constants were calculated from the slope of the pHdependent kinetic rate constants versus the acid concentrations as shown in Figure 4.6. Theseresults are listed in Table 4.3 together with those previously obtained for the TEOS system.The kinetic rate constants describing the hydrolysis of MTES are greater than thecorresponding ones for TEOS. This result is in agreement with the qualitative conclusion inthe literature regarding the complete hydrolysis process, that alkyl-substituted ailcoxysilaneshydrolyze faster than the corresponding tetraallcoxysilane. [4.1, 4.2]The data in Table 4.3 show that the rate of hydrolysis increases with the number ofhydroxy ligands. This contradicts what one would expect from an inductive effect: apositively charged transition state would be further stabilized by an increasing number ofalkoxy ligands since ailcoxy groups are more electron donating than hydroxy groups [3.19,3.29] implying that k1 > k2 > k3.100Table 4.2 - The average pH dependent kinetic rate constants (M1min’) determined fortheMTES homopolymerization hydrolysis and dimer formation reactions.pHkMl kM2 kM3f kM3b kM43.35 0.0015 0.0059 0.0085 0.0005 0.0013.04 0.0052 0.019 0.023 0.001 0.0052.88 0.006 0.024 0.035 0.00350.0062.76 0.013 0.045 0.05 0.004 0.0072.55 0.0205 0.0655 0.07 0.008 0.0172.45 0.025 0.075 0.085 0.01 0.02151010.06jcn—0.0000Figure 4.6 - Plots of the pH dependent kinetic constants versus acid concentrationfor the hydrolysis and dimer formation reactions of the MTEShomopolymerization. The fits were constrained such that the interceptswere at zero.0.040.02kMl=25 mind0.0005Concentration H (M)0.00100.0005Concentration H (M)0.0010E0.0005Concentration W (M)0.00100.0005Concentration H (M)0.00100.0005Concentration H (M)0.0010102Table 4.3 - pH independent hydrolysis and dimer formationkinetic rate constants (min1)forthe MTES and TEOS homopolymers together withthe estimated error (taken asthe standard error of all the pH dependent values).Kinetic MTES homopolymer TEOS homopolymerConstants (min1) (min1)kMl, kTl 25± 1 3.3 ± 0.1kM2,kT 81±3 18±1kM3f, kT3 91± 2 72 ± 5kM3b 9.9 ± 0.7kT4f 59 ± 10kT4b 11±2kM4,kTs21±1 51±2103Interestingly, the dimer formation kinetic constant isgreater for TEOS than MTES.There are two possible reasons for this.It is possible that in the dimer formation involvingthe reaction of two CH3Si(OH),the methylgroups’ inductive effect retards the reaction. Inaddition there maybe more steric hinderance involvedin the MTES dimerization, since bothmonomers contain a methyl group, in comparison to theTEOS homodimerization. The resultis that TEOS dimerizes faster thanMTES.Even though the relative rates of the reactions differsignificantly, there are a numberof similarities between the MTES and TEOS reactions.Both systems display the same trendfor the first three sequential hydrolysis rate constants,k1 <k2 <k3 . Both require theinclusion of a re-esterification reaction for the lasthydrolysis in order to respect the generalshape of the second to last hydrolysisproduct curve. This re-esterification reaction is in bothcases smaller than the forward hydrolysis reactionby at least a factor of five, i.e. TEOS:kT4f>> kT4band MTES:kM3f>> kM3b.In both cases, the dimer formation is slower than thelast hydrolysis,kT4f > k.r5 (TEOS) and kM3f> kM4 (MTES), so that at all pH values studied,there is an accumulation of the fully hydrolyzed monomers.1044.3.3 MTES/TEOS Copolymer Kinetic AnalysisThe hydrolysis and dimer formation of a50/50 MTESiTEOS copolymer was analyzedin detail to determine how the kinetic constants comparedto those from the MTES and TEOShomopolymerizations. Reactivity ratios for the MTES and TEOS monomers weredeterminedusing the kinetic constants for dimerization. The different dimerconcentrations were thenestimated from the reactivity ratios.A typical collection of spectra obtained during the copolymerization reaction is shownin Figure 4.7. As expected, the intermediates createdby the hydrolysis during theMTES/TEOS copolymerization are the same as those observed in the homopolymer cases(Figures 3.1 and 4.1). The concentration curves for the intermediate hydrolysis speciesasfunctions of time in the MTES/TEOS copolymerization are shown in Figures 4.8 and 4.9.Seven hydrolysis reactions are considered in modelling the MTES/TEOScopolymerization:kTl(420)Si(OEt)4 + H20—* Si(OI-f)(OEt)3+ EtOH(421)Si(OH)(OEt)3+ H20 - Si(OH)2(OEt) + EtOHkT3(422)Si(OH)2(OEt) + H20 —> Si(OI-f)(OEt) + EtOHkT4(423)Si(OH)3(OEt) + H20 —> Si(OH)4 + EtOH105CH3Si(OH)M-TA\M-M_____ ________________b-.:”Z _-z— -. r- -%r--fl J-..J r—fl________________________ _______________________- i— - - —‘—- ——-----.- -——- 21 mm____________-— fl- —Ippm —35 —40 —45 —55295i chemical shift (ppm)BSi(OH)4S0 OCHCHT-Ti( H)3(23) T-MI41 mmSi(OCH2CH3)4I I I 1ppm —72 —74 —76 —76 —60 —8229Si chemical shift (ppm)Figure 4.7 - Series of 1D 29Si solution NMR spectra, as functions of time,acquired during the MTES1TEOS copolymerization (water wasacidified to pH=2.55). The acquisition parameters used were:8 scans per spectrum, a 1 second recycle delay and a600pulse.A) MTES region and B) TEOS regionT-T = TEOS-TEOS homodimer,T-M and M-T = TEOS-MTES codimer andM-M = MTES-MTES homodimer.1065015CH3Si(OEt) CH3Si(OH)(OEt)24010.3020c1o.•.00--.0 20 40 60 800 20 40 60 80Time (mm)Time (mm)20- 50CH3Si(OH)2(OEt)15 xx03000 020• x000000000x5.OOQ 0o000000 100.000000000 0 • —0 20 40 60 80 0 20 40 60 80Time (miii)///Time (mm)5040CH3Si(OH)030xxxx2010.0I I I0 20 40 60 80Time (mm)Figure 4.8 - Experimental relative concentration versus time curves for theMTES hydrolysis intermediates in the MTESITEOScopolymerization. The experimental error is ± 2%.1075015‘—4O.oSi(OEt)4010Si(OH)(OEt)330 .20- •I. 5(10 I.0U0 20 40 60 800 20 40 60 80Time (miii)Time (mm)5 504Si(OH)2(OEt)40‘.— ..302 20V) 00C.) C) ••0100 0 00 000 C1O•L) 00000 c:.)O 00o,-O-——0049O00fl8Qp&_J0 20 40 60 80 0 20 40 6080Time (miii)Time (mm)5010 -8- Si(OH)3(OEt) Si(OH)4400 06- 30••••••••••ci)4,“ X ‘ X ‘ci.20 •••..•o 0c10‘cx0(1—0 20 40 60 80 0 20 40 6080Time (miii) Time (miii)Figure 4.9 - Experimental relative concentration/time curves for the TEOShydrolysis intermediates in the MTES/TEOS copolymerization.The experimental error is ± 2%.108kMI424CH3Si(OEt) + H20 —* CH3Si(OH)(OEt)2+ EtOHkM2425CH3Si(OH)(OEt)2+ H20—* CH3Si(OH)2(OEt) + EtOHkMg426CH3Si(OH)2(OEt) + H20 —* CH3Si(OH) + EtOH.In addition three dimer formation reactions have to be considered:kM44272CH3Si(OI-I) —> CH3SI(OH)2(0 Si(CH3)(0I-I)2)+ H20kT54282Si(OH)4 —> Si(OI-f)3(OSi(OH))+ H20k10(4 29)Si(0I-f)4 + CH3Si(0I-f) — CH3S1(OH)2(OSi(OH))+ HpThe only reaction that has not been encountered in dealing with the homopolymers is the onedescribed by equation (4.29). Attempting to solve this set of differential equationsnumerically showed that the re-esterification reactions needed to fit the TEOS and MTEShomopolymerizations had to be included. The final differential equations used to describe thehydrolysis kinetics of the copolymer are (4.30)-(4.40).d [TEOS]= -kTI[TEOS]((4.30)109d [Si(OH)(OEt)3]= (k1[TEOS](4.31)dt—kT2[Si(OH)(OEt)3]!)Tgd [Si(OI-])2(OEt)]= (kT2[Si(OH)(OEt)3](4.32)dt—k3[Si(OI-I)2(OEt)j1)nd [Si(OH)3(OEt)]= (kT3[Si(OH)2(OEt)1t(4.33)dt—kT4?S1(OH)3(OEt)1:)11 + kT4b[Si(OIi)]ed [Si(OI-I)4]=k41[Si(OH)3(OEt)]T — kT4b[Si(OH)](edt— kT5[Si(OII)4]—10[CHSi(OI-I)][Si(OI-I)4]1d[MTESI= -kMJ[MTES1jr1________(4.35)dtd[CH3Si(OH)(OEt)2]= (kMl[MTESj(4.36)dt-kM2[CH3Si(OI-f)(OEt)]l)TId[CH3Si(OI-I)2(OEt)J= (kM2[CH3Si(OH)(OEt)1t(4.37)dt— kMCHSi(OI[)(OEt)]f)T+ kMb[CHSi(OI1)](ed [CH3Si(OH)1- kf[CH3Si(OH)2(OEt)]rj___________________— M3dt2 (4.38)— kM3b[CHSi(OI-l)]e — kM4[CHSi(OH)1:—k10[CHSi(OI-])][Si(OH)4]110where[HO]0-[CH3Si(OI-f)(OEt)2]1— 2[CH3Si(OI-f)2(OEt)],— 3[CH3Si(OI-f)]— [Si(OH)(OEt)3](4.39)—2[Si(OI-f)2(OEt)J—3[Si(OI-J)(OEt)]— 4[Si(OI-f)4]= [EtOIl]0+ [CH3Si(OR)(OEt)2]+2[CH3Si(OH)2(OEt)J+ 3[CH3Si(OH)J+ [Si(OJ-J’)(OEt)3](4.40)+ 2[Si(OI-I)2(OEt)]+ 3[Si(OI-I)3(OEt)] +4[Si(OI-I)4]The composition of the copolymerization reaction mixture was chosen such that it wasmost comparable to the homopolymer reaction mixtures previously studied. The water/silaneand acid/silane ratios affect the kinetic process. The total silane concentration of bothmonomers and total acid concentration was chosen to be equal to the total silane and acidconcentration in the homopolymer reactions, which means that the water/TEOS andwater/MTES ratios in the copolymer reaction are double those used in the correspondinghomopolymer reactions.Thirteen hydrolysis and dimer species can be resolved in the spectra and theirconcentrations can be monitored simultaneously as functions of time. The kinetic curves forthe copolymer system were fitted by making use of the kinetic constants determined for thehomopolymers. This means that for eleven out of twelve kinetic constants reasonable startingvalues are available although it is anticipated that these may change as the concentrationratios of the reaction mixtures are different. The only kinetic rate constant in the system notpreviously determined is that of the co-dimerization reaction, equation (4.29). A briefdiscussion of the logic used in the variation of the rate constants from their starting values, isgiven in the following paragraphs.111500IAC.)0c-)403’2000 20 40 60Time (mm)80504030B10020 40 60Time (mm)80Figure 4.10 - Experimental data for the Si(OEt)4 relativeconcentrationas a function of time, during theMTESJTEOS copolymerizationtogether with the calculated curves assuming:A- kTl= 0.0025 B- kTl0.006.The experimental error is ± 2%.112In Figure 4.1OA, the calculated curve obtained using the homopolymerizationkTl valueis superimposed on the experimental TEOS concentration data for the MTES/TEOScopolymerization. As is illustrated in Figure 4.10 the initial hydrolysis rate of TEOS in thehomopolymer is slower than in the copolymer by about a factor of two.The data for the second hydrolysis reaction of the TEOS monomer in thecopolymerization are shown in Figure 4.11. The calculated curve shown in Figure 4.11 A wasobtained with the k from the homopolymerization of TEOS. It is clear that this rateconstant underestimates the decay of the Si(OH)(OEt)3by about a factor of two. The curvein Figure 4.1 lB was obtained by optimizingkT2for this experimental data set.The experimental data and the calculated curves for the next two hydrolysisintermediates of the TEOS monomer are shown in Figure 4.12. Both of these intermediatespecies are present at very low concentrations and larger errors are associated with both themeasurement and the fitting of these curves. The kinetic constantkT3used to calculate thecurves shown in Figure 4. 12A is the same as in the homopolymerization. In this case, thecurve fits the experimental data within the error limits and therefore, further variation was notnecessary. In Figure 4. 12B, thekT4fvalue from the homopolymerization of TEOS was usedto calculate the curve for Si(OH)3(OEt) which obviously underestimates the decay of thisspecies. The calculated curve with an optimized value forkT4fis shown in Figure 4. 12C.For the MTES hydrolysis products in the copolymerization, the concentration versustime curves for CH3Si(OEt) and CH3Si(OH)(OEt)2are shown in Figure 4.13. Again, thecalculated curves using the hydrolysis rate constants obtained from the homopolymerizationare compared to those optimized for these experimental data.Up to this stage, the codimerization reaction (4.29) has not been included in theanalysis. The next step is to focus on the fully hydrolyzed species for both the TEOS and113.— 12C.,Bc)CAI1512963001520 40 60Time (mm)809620 40 60300 80Time (mm)Figure 4.11 - Experimental data for the TEOS hydrolysis intermediateSi(OH)(OEt)3relative concentration as a function of time,during the MTESITEOS copolymerization, togetherwith the calculated curves assuming:kTl=O.006and A-kT2=O.0l3 B-kT2=O.028.The experimental error is ± 2%.1145.ACa)C)C000 60 80B80.•C -00 80Figure 4.12 - Experimental data for the TEOS hydrolysis intermediatesSi(OH)2(OEt) and Si(OH)3(OEt) relative concentrationsas functions of time, during the MTES/TEOS copolymerization,together with the calculated curves assuming for A, B and C:kTl= 0.006kT2= 0.028kT3= 0.063,and for B)kT4f = 0.053; kT4b= 0.02and for C)kT4f= 0.1;kT4b= 0.02.The experimental error is ± 2%.Si(OH)2(OEt)20 40Time (mm)•••. •• •••Si(OH)3(OEt)••20 40 60Time (miii)1086 ••••. •Si(OH)3(OEt)••••. ••220 40 60Time (mm)115505OAB‘—4030302020C.) .CH3Si(OEt) CH3Si(OEt)dOc_)(1 - - r-?- - - • - .- -.0 20 40 60 800 20 40 60 80Time (mm)Time (mm)15 ,15CDTime (mm)Time (mm)Figure 4.13 - Experimental data forCH3Si(OEt) and the MTES hydrolysisintermediateCH3Si(OH)(OEt)2relative concentrations as functionsof time, in the MTESITEOS copolymerization, together with thecurves calculated assuming:A- kMl= 0.0203 (homopolymerization value)B-kMl= 0.034C- kMl= 0.034 k = 0.068 (homopolymerization values)D-kMl=0.034k=0.085.The experimental error is ±2%.116MTES monomer in order to determine the dimer formation constants.The approach taken to determine the kinetic constant for the codimer (k<)was tostart with the homodimer kinetic constants(kT5andkM4)and a very small value for thecodimer kinetic constant(lc). The variationofk10clearly showed that this parameter has astrong effect on the maxima of the calculated curves for the fully hydrolyzedspecies. Themaximum value of k was determined to be 0.05-0.06 M’min’. For largervalues, themaximum of the Si(OH)4curve is too low to fit the experimental data; consequently, k wasset to 0.05 M1min’ andkT5andkM4were adjusted in order to best fit the experimental curves.The initial conditions and the final set of calculated curves for Si(OH)4and CH3Si(OH) areshown in Figure 4.14 together with experimental data.The two back reactions in the copolymerization reaction scheme were addressed next.As is shown in Figures 4.15 and 4.16 the calculated curves for Si(OH)4and CH3Si(OH) arenot very sensitive tokT4bandkM3brespectively; however the Si(OH)3(OEt) andCH3Si(OH)2(OEt) curves prove to be very sensitive. These parameters were thereforeoptimized by evaluating their effect on the concentration curves of Si(OH)3(OEt) andCH3Si(OH)2(OEt), respectively. Figure 4.15 compares the calculated curves obtained whenthe homopolymerization rate constants are used with those using the optimized value forkT4b.The experimental data for the remaining two hydrolysis species of the MTESmonomer are shown in Figure 4.16. Again, two calculated curves are shown for each species,one calculated withkM3bfrom the homopolymerization reaction and the other with theoptimized value.All the kinetic rate constants determined for the copolymerization and thehomopolymerization cases are summarized in Table 4.4, and they show that the hydrolysisrate constants for the TEOS monomer have been significantly altered. A preliminary study on11750S.-., 400 60 8040DI___I_20 40 60 80Time (mm) Time (mm)Figure 4.14 - Experimental data for the fully hydrolyzed monomers CH3Si(OH)(A, B) and Si(OH)4(C, D) relative concentrations as functions of time,during the MTESiTEOS copolymerization, together with the curvescalculated assuming:kT5= 0.035kM4= 0.015kclo= 0.001 forA and CkT5O.O5 kM4O.O4 kcioO.05forBandD.The experimental error is ± 2%.B20 40Time (mm)40 C0 20 40 60 80Time (mm)500CC.)C0L)0 20 40 60 8011810A Bo. 0. Li ...•. .Ct. .S4 ..•..00406O80Time (mm)10CS..-, 0o0.— I; .—U•S1)0 20 40 60 80Time (mm) Time (mm)Figure 4.15 - Experimental data for the TEOS hydrolysis intermediates,Si(OH)3(OEt) and Si(OH)4,relative concentrations as functionsof time, during the MTES/TEOS copolymerization, togetherwith the calculated curves assuming:kTl= 0.006, k-1-2 = 0.028, k-1-3 = 0.063,kT4f=0.1,kcio= 0.05A) Si(OH)3(OEt) and B) Si(OH)4withkT4b= 0.0093,kT5= 0.05C) Si(OH)(OEt) and D) Si(OH)4withkT4b= 0.02,kT5= 0.05.The experimental error is t2%20 40 60 80Time (miii)504030D11950150• —C)0c-)Figure 4.16 - Experimental data for the MTES hydrolysis intermediates,CH3Si(OH)2(OEt) and CH3Si(OH),relative concentrationsasfunctions of time, during the MTESITEOS copolymerization,together with the calculated curves assuming:kMl= 0.034 k = 0.085kM3f= 0.1kM4= 0.041ci= 0.05A)CH3Si(OH)2(OEt) and B) CH3Si(OH) withkM3b= 0.02C) Si(OH)(OEt) and D) CHSi(OH) withkM3b= 0.025.The experimental error is ±2%.A5..30I20C)0c-)B1020 40 60 80Time (mm)IC20 40 60 80Time (mm)ID0 20 40 60 80Time (mm)0 20 40 60 80Time (mm)120Table 4.4 - Comparison of the pH dependent kinetic rate constantsdetermined for the TEOSand MTES homopolymers and the MTES/TEOS copolymerat a pH=2.55.Homopolymer results Copolymer resultsTEOSkTl 0.0018 0.006kT2 0.011 0.028kT30.06* 0.06*kT4f 0.05 0.1kT4b 0.009 0.02kT5 0.033 0.05MTESkMl0.02 1 0.034kM20.066 0.085kM3f0.07 0.10kM3b0.01 0.025kM40.02 1 0.04CODIMERk 0.05*no change in this case121the effect of the water/silane ratio on the hydrolysis of TEOS suggests thatas the water/TEOSratio increases the rate of the first hydrolysis decreases. This implies that the higherwaterfT’EOS ratio compared to the TEOS homopolymerization cannot cause the increase inkTl.However, the difference between the homopolymer and copolymer hydrolysis rateconstants for the TEOS monomer may be explained by the presence of the MTES monomer.The presence of the MTES monomer in the copolymer reaction alters the solution properties,such as viscosity, polarity, etc, inevitably affecting the TEOS-water interactions. The MTEShydrolysis is also affected by the presence of TEOS, but to a lesser extent.The kinetic constants obtained for the dimer formations are not unique solutions to thefitting of the experimental data. Other solutions were possible, but eliminated because theydid not reflect the proper ratios of the different dimers present in the reaction medium at anymoment in time.Concentrations as functions of time can be now calculated for both homo- andcodimers with the three dimer formation rate constants determined above. These calculatedcurves overestimate the dimer concentrations in all cases since no dimer depletion reactions,i.e. further condensation reactions, were included. The spectra indicate that such species areformed. The copolymerization spectrum acquired at 76 minutes is shown in Figure 4.17.After this reaction time, there are a number of small peaks in the regions of -84 ppm and of -91.5 ppm. The peaks at —-84 ppm have been assigned to dimers or trimer end groups wherethere is an ethoxy ligand directly attached to the silicon, such as: Si(OH)2(OEt)(OSi).[3.1,3.15] After 76 minutes, only 5% of silicons contribute to the signals in the -84 ppm regionwhich supports the statement that few non-fully hydrolyzed monomers take part incondensation reactions up to this point in the reaction.The signals at -91.5 ppm have been assigned to the center silicon in trimers,122CH3Si(OH)DimerItrimer end group regionCH3Si(OEt)(OH)2Tppm —40.0 —42.5I I I—45.0 —47.5 —50.0 —52.5 —55.0Dimer/trimer end group region298i chemical shiftDimer region forSi(OSi*(OH)2OEt)Figure 4.17 - 29Si solution NMR spectrum,76 minutes into the MTES/TEOScopolymerization (water acidified topH=2.55). The spectrumwas acquired with 8 scans and a 1 secondrecycle delay.Top: MTES region.Bottom: TEOS region.Si(OH)4Si(OEt)(OH)31 1 1—---1ppm —75.0 —77.5 —B0.0 —82.5—85.0 —87.5 —90.0Linear trimerregion123Si(OSi)2(OH).[3.1, 3.15] The NMR spectrum shows no evidence for theexistence of cyclictrimers whose signals appear at -90.6 ppm, -92.9 ppm and -95.2 ppm,[3.15] At76 minutes,the total concentration of central silicons in trimers is9% (the signals from silicons at theends of the chain will appear in the dimer region). Thus, the total percentageof trimers is—9%. The total concentration of silicons in dimers (excluding the trimer end group siliconsignals that appear in this region) is approximately50% which implies that the totalconcentration of dimers is 25%. Therefore, from the NMR intensities, the totaldimerpercentage including those that were consumed to form trimers is 34%. Using thekineticconstants for dimer formation, the total dimer concentration is predictedto be 37%.Considering the complexity of the problem when taking the dimer and trimercondensationproducts into account, the data in general support the kinetic rateconstants obtained in theanalyses for the formation of the dimer species.Having determined the kinetic rate constants for the dimer formation, reactivityratioscan now be calculated for both MTES and TEOS monomers. A reactivity ratio [4.3]for theMTES and TEOS monomers is defined in equation (4.41).p — MTES-MTES— M4P— TEOS-TEOS — T5(4411’MTES— k —‘ 1’TEOS— k —MTES-TEOS dO MTES-TEOS ClOThe reactivity ratio for CH3Si(OH) or Si(OH)4is the homodimer kinetic constant over thecodimer kinetic rate constant(k10).Using the data from Table 4.4, the reactivity ratios forMTES and TEOS are 0.8 and 1, respectively. This suggests that the copolymerization ofMTES and TEOS monomers will tend to produce a random copolymer if subsequentcondensation reactions follow the same trend. In fact, they are close enough to the ideal casethat the copolymer composition should also approximate the reaction mixture composition atany given time.124In agreement with the reactivity ratios discussed above, the dimerintensities in theNMR spectrum, Figure 4.17, suggest that the TEOS homodimer concentration isapproximately half that of the codimer concentration. This implies that the TEOS monomerhas little preference whether it reacts with another TEOS monomer or witha MTESmonomer. The same is true for the MTES monomer. Therefore, it isto be expected that in aMTES/T’EOS copolymer the functionalized monomer will be randomly distributed in the silicagel matrix.4.3.4 MTES/TEOS Copolymer Dimer FormationTo prove that the results obtained above are valid for other MTESTEOS copolymercompositions, their dimer ratios were compared at specific reaction times. These differentcopolymers involved varying either the relative monomer ratio or thewater/silane ratio. Therelative dimer concentrations in a copolymer (homodlimers versus codimers) provide a markerof the polymer composition, because silicon-oxygen bonds are not broken to any appreciableextent.[1.41, 4.4jA variety of MTESTEOS copolymer compositions (summarized in Table 4.1) werefollowed by high resolution 29Si solution NMR. Within each data set two spectra weredeconvoluted, those obtained after 101 and 131 minutes. The experimentally determineddimer concentrations are summarized in Table 4.5 and 4.6.The MTES/TEOS copolymerization samples COl 11, C0271 and C0721 have thesame water/(total silane) ratio as in the homopolymerization studies but the relative ratios ofthe monomers were50/50, 25/75 and 75/25, respectively. Assuming a random dimerizationprocess, the theoretically expected ratios are also listed in Table 4.5 and 4.6. The measuredconcentrations of the dimers approximate those predicted for a random copolymer.125Table 4.5 - Dimer concentrations for the different copolymer samples after 101 minutes.The relative concentrations presented are in percent with respect to the total dimerconcentration in each spectrum. The theoretical values are calculated foracompletely random process. Sample composition codes are summarized in Table4.1 and T-T = TEOS-TEOS homodimer, M-M = MTES-MTES homodimer andT-M = TEOS-MTES codimer.DIMER C0271 C0721 COlli(25% MTES) (75% MTES) (50% MTES)T-T 50±2 18±2 36±2THEORY 56 6 25T-M 41±2 26±3 43±3THEORY 38 38 50M-M 8±2 56±3 21±1THEORY 6 56 25DIMER CO112 CO221 CO222(50% MTES) (50% MTES) (50% MTES)T-T 31±4 22±1 26±2THEORY 25 25 25T-M 46±7 53±2 47±4THEORY 50 50 50M-M 23±3 25±1 28±2THEORY 25 25 25126Table 4.6 - Dimer concentrations for the different copolymer samples after 131 minutes.Therelative concentrations are in percent with respect to the total dimerconcentration in each spectrum. The theoretical values are calculated for acompletely random process. Sample composition codes are summarized in Table4.1 and T-T = TEOS-TEOS homodimer, M-M = MTES-MTES homodimer andT-M = TEOS-MTES codimer.DIMER C0271 C0721 COlli(25% MTES) (75% MTES) (50% MTES)T-T 53±1 13±1 27±1THEORY 56 6 25T-M 37±2 34±4 51±2THEORY 38 38 50M-M 11±2 54±3 23±1THEORY 6 56 25DIMER CO112 CO221 CO222(50% MTES) (50% MTES) (50% MTES)T-T 31±3 23±2 27±3THEORY 25 25 25T-M 47±5 51 49±5THEORY 50 50 50M-M 23±2 26±2 25±2THEORY 25 25 25127In order to investigate the possible effect of thewater/silane ratio the waterconcentration was doubled. The total number of moles of silane (COl 12)was kept equal tothe total number of moles of silane in the homopolymercase. There was no significant effecton the ratio of dimers formed. These results are summarized in Table4.5-4.6 and support thefact that the MTESiTEOS copolymer tends to be a random copolymer under the conditionsused in the kinetic study (COl 11).The other approach to varying the water/(total silane) ratio would be to keep the waterconcentration the same as in the homopolymerization and manipulate the total silaneconcentration. If the water/(total silane) ratio is half that in the homopolymerization (C0221versus COil 1) no significant effect is detected on the relative ratios of the codimer tohomodimer ratios (Table 4.5-4.6). The difference in the NMR spectra for C0221 comparedto con i, is that in the C022 1 case after a significant amount of dimer formation hasoccurred (131 minutes) there is still some unreacted Si(OEt)4present in the reaction vessel, asituation not encountered before. Interestingly, no unreacted CH3Si(OH) monomer is present.Now, if the water/(total silane) ratio is the same as in the homopolymer case but theamount of water and total silane is doubled with respect to the homopolymer, C0222, theratio of codimer versus homodimers formed is unaffected but again some unhydrolyzed TEOSmonomer is detected.1284.4 CONCLUSIONSFor the first time, the pH dependent kinetic rate constants were determined forthehydrolysis and dimerization reactions of the MTES homopolymerization.pH independentkinetic rate constants were calculated from plots of the pH dependent kinetic constantsasfunctions of acid concentration. These kinetic rate constants provide the first quantitativeevidence that the MTES monomer hydrolyzes faster than the TEOS monomer at eachsequential hydrolysis reaction. This implies that the methyl group stabilizes the transitionstate involved in the hydrolysis reactions.The dimerization rate constant for the MTES homopolymerization is slower than thatfor the TEOS monomer, suggesting that the methyl ligand destabilizes the transitionstateinvolved in the MTES-MTES homodimerization reaction. As in the TEOS polymerization, anequilibrium reaction was required in the reaction model in order to respect the general curveshape of the second to last hydrolysis intermediate, CH3Si(OH)2(OEt).The determination of the MTES and TEOS homopolymerization hydrolysis andcondensation rate constants provided a starting point for the determination of theMTES/TEOS copolymer hydrolysis and dimer formation rate constants. The hydrolysiskinetic rate constants obtained for the copolymerization suggest that in comparison with thehomopolymerizations, the reaction kinetics of the TEOS monomer are significantly moreaffected than that of the MTES monomer. Reactivity ratios of 0.8 and 1 for the MTES andTEOS monomers, respectively, were calculated from the dimer formation kinetic rateconstants determined for the MTESITEOS copolymer. These reactivity ratios suggest that theMTESITEOS copolymer tends to approximate a random copolymer. The effect of altering therelative monomer proportions, and the water/(total silane) ratio, further confirms that theMTESITEOS copolymer has a strong tendency to form a random copolymer.129CHAPTER 5THE EFFECT OF FORMAMIDE, A DRYING CHEMICAL CONTROL AGENT(DCCA) ON THE KINETICS OF THE TEOS, MTES ANDMTES/TEOS POLYMERIZATIONS5.1 INTRODUCTIONThe copolymerization of TEOS and a functionalized silane provides a uniqueopportunity to make novel glasses, ceramics and composites with otherwise unattainableproperties. To take full advantage of this new synthetic approach to form macroscopic pieces,the problem of cracking which generally occurs from the non-uniform shrinkage of the gelduring the drying process, must be solved.[5.8] In order to prevent cracking, the drying stressmust be minimized by either controlling the pore size distribution or rate of evaporation. Toaddress this problem, Wallace and Hench suggested that drying chemical control agents(DCCAS),such as formamide (HCONH2),be introduced into the solvent mixture.[5.7]Formamide decreases the vapor pressure of the solvent; consequently the rate of evaporationdecreases, resulting in a reduction in the drying stress and therefore the cracking.[5.7] In aseries of subsequent publications, 29Si NMR, SAXS, Raman, FTIR and different forms ofchemical analysis [5.8-5.12] were used to further investigate what chemical processes mightoccur when formamide is present in the conversion of tetramethoxysilane (TMOS) to silicagel.Wallace and Hench found that as the formarnide concentration increased, the hardnessand density of the sample decreased, the pore size distribution narrowed, and the average poresize increased.[5.8J Horiuchi studied the effect of DCCAs on the polymerization of siicicacid prepared from TEOS in an acid environment (HNO3).[5.13] His data, in contrast to130those of Hench et al, suggest that the pore size distributionincreases, as well as the averagepore size, with increasing formamide concentration.Hench et al. concluded, from an FTIR study, that formamideforms a protective layeron the gel surface facilitating the removal of water and poreliquor.[5.10] Orcel et alhypothesized, because of the differences observed between base and acid catalyzed gels, thatformamide hydrogen bonds to the surface via the amide functionalityto form a protectivecoating.[5.10] A study on amides with bulky substituents suggested that the amides formhydrogen bonds to silanol groups via the carbonyl group.[5.14]The only study on the effect of formamide on the kinetics of TMOS was carried outby Orcel and Hench using 29Si solution NMR.[5.9] The polymerization of TMOS was studiedat high water/silane ratios (10:1), with no added acid and very high formamide concentrations(25-50 volume % of the TMOS volume used). From the NMR data, global hydrolysis rateconstants were determined assuming that the reaction rates were independent of thesubstitution/polymerization of the silicons. These results suggested that as the formamideconcentration increased, the global hydrolysis rate decreased.[5.9,5.111Artaki et al [1.42] studied the effect of formamide on the polymerization of TMOS athigh water/silane ratios with no added acid. Unfortunately the formamide concentrations usedwere not stated. Their 29Si solution NMR spectra indicated that fewer hydrolyzed monomerswere present in the sample containing foimamide than in the sample without formamide,supporting the findings of Hench et al. that formamide inhibits hydrolysis. In addition, the29Si solution NMR chemical shifts and the Raman absorptions were unaffected by thepresence of formamide which they interpreted as evidence that formamide is not chemicallybonded to the silicon polymer. [1.42]The studies of Boonstra et al. [5.15] and Horiuchi [5.15], are the only published131investigations to date, on the effect of formamide on the polymerization ofTEOS. Boonstraet al. used 29Si solution NMR to investigate the effect of formamideon a two step (first acidthen base catalyzed) polymerization of TEOS.[5.15] Theystudied the reaction under differentstoichiometric and substoichiomethc water/silane ratios.[5.151 Boonstra et al. ‘ s studyconcluded that when the reaction was acid catalyzed,the hydrolysis and dimerization rateswere reduced as a function of the formamide concentration.In addition, they found that inthe presence of formamide, as the water concentration increased, the total concentration ofhydrolyzed species increased. They also concluded that the DCCA favors condensation ofincompletely hydrolyzed species resulting in a larger percentage of ethoxy groups in the finalgel.[5. 15]Supporting the findings of Hench et al. and Horiuchi [5.8,5.13], Boonstra et al. foundthat as the formamide concentration increased, the mean pore size increased.[5. 15] Inagreement with Artaki’s study, they found no evidence in the ‘H and 29Si solution NMRdatafor chemical bond formation between formamide and water or any 29Si functionalities.[5.l5]In this chapter, the effect of formamide on the hydrolysis kinetics of tetraethoxysilane(TEOS), methyltriethoxysilane (MTES) and the MTES/TEOS copolymer was investigatedusing high resolution 29Si solution NMR spectroscopy. The kinetic analysis for all threepolymerizations presented in Chapters 3 and 4 provide the reference data for the effect offormamide on the hydrolysis reactions. For the first time, it is possible to isolate the effect offormamide on the individual hydrolysis reactions and dimer formation of the MTES, TEOShomopolymerizations and the MTESJTEOS copolymerization.High resolution ‘H, ‘3C, ‘5N and 295i solution NMR spectroscopy were used toinvestigate the possibility of formamide bonding to the silica gel surface during the TEOSpolymerization.1325.2 EXPERIMENTAL5.2.1 KineticsThe reaction mixture had the same composition as described in Chapter3 (4 ml(0.018 mole) TEOS, 6.07 ml (0.1034 mole) ethanol,3.58 ml (0.1981 mole) water acidified topH=2.55, 0.075 g (0.000215 mole) Cr(acac)3)with thedifference that formamide was addedto the reaction mixture. For the kinetic studies by 29Si solution NMR theconcentrations ofthe other reagents were kept constant and only the concentrationof formamide was variedbetween 0 and 30 mole %. In the case of the MTESJTEOS copolymer 0.009 moleof TEOSand 0.009 mole of MTES were used.The TEOS and MTES homopolymerizations, andMTESiTEOS copolymerization wereacid catalyzed (pH=2.55) with a formamide/acid ratio of 400:1. The pH of the system isaffected by the addition of formamide but concentrations were chosen such that the mediumremained acidic (Table 5.1). Maintaining the acidic conditions is the principaldifferencebetween the present study and those of Orcel and Hench [5.9] and Artaki et al [1.42].The reaction mixture used in the 1H, ‘3C and 15N solution NMR study, contained 0.16ml (0.004 mole) of ‘3C or ‘5N enriched formamide (20 mole%) and the water was acidifiedto pH=1. 15. All the other concentrations were unchanged.5.2.2 NMRHigh resolution ‘H solution NMR spectra were obtained at a frequency of 500.13 MHzon a Bruker AMX 500 spectrometer and ‘3C solution NMR spectra were obtained using thesame spectrometer at a frequency of 125.76 MHz. A Varian 300 MHz spectrometer wasusedto acquire ‘5N NMR spectra at a frequency of 30.406 MHz. In order to observe the15N/’Hcouplings the data were collected using gated decoupling. The temperature was 298 K for allexperiments. Specific experimental details are given in the figure captions.133Table 5.1 - The pH values andwater/formamide ratios of the samples used in the kineticinvestigations.pH Formamide (mole %)2.55 02.83 53.13 103.63 203.94 301345.3 RESULTS AND DISCUSSION5.3.1 The Effect of Formamide on the Kinetics of the Hydrolysis andDimer Formation ReactionsHigh resolution 29Si solution NMR spectra were acquired as functions of reactiontimeduring the TEOS and MTES homopolymerizations and MTES/TEOS copolymerizationcontaining formamide. The spectra show that the presence of formamide does not affect the29Si chemical shifts of any of the hydrolyzed species.A formamide concentration of 10 mole % or less has a small to negligible effect onthe hydrolyses and dimer formation rates of the TEOS and MTES homopolymerizations, InFigures 5.1 and 5.2, the concentration curves are compared to those from the formamide freereaction.Observable effects on the hydrolysis rates are detected when the concentration offormamide exceeds 10 mole %. The relative concentration versus time curves of the TEOSand MTES hydrolysis products, for reactions containing various amounts of formamide areshown in Figures 5.3 and 5.4, respectively. The concentration of the intermediateSi(OH)2(OEt) in the TEOS homopolymerization is so small that the effect of the formamideon the rate of decay is lost within the scatter of the data. It is obvious that formamideconcentrations exceeding 10 mole % result in a dramatic retardation of the hydrolysisreactions. The kinetic constants are summarized in Tables 5.2 and 5.3.The qualitative 29Si NMR study of Boonstra et al. deals with TEOS and formamideconcentrations of 0 to 42 mole %; the difference between their system and the reactionconsidered here is that they used a two step acid/base synthesis with substoichiometric tostoichiometric water concentrations.[5.15J At a water/silane ratio of 1, it is obvious fromtheir figures that during the acid step just 8 mole % formamide affects the hydrolysis rates.135CC)CCc-)0 20 40 60 80 100Time (mm)Figure 5.1 - Experimental data and calculated relative concentration curves forthehydrolysis intermediates as functions of reaction time for two TEOShomopolymerizations, involving 0 mole % formamide (circles andsolid lines) and 10 mole % formamide (open circles and dashed lines).The experimental error is ± 2%.100Si(OEt)4IISi(OH)(OEt)30 20 40 60 80 100Time (mm)ICC)CCL)Time (mm)10080Si(OH)46020 40 60 80 100Si(OH)2(OEt)Time (mm)000o0Si(OH)3(OEt)00 o0o00 c 000 20 40 60 80 100 0 20 4060 80 100Time (mm)13610020‘—‘ 80CH3Si(OEt)‘ 15CH3Si(OH)(OEt)206O4010820°5• c_)o- 0-°--‘---.-‘---i?°0 20 40 60 80 0 20 40 60 80Time (mm)Time (mm)20 10015- 00000CH3Si(OH)2(OEt)80CH3Si(OH)• .•••° 00. 0 00 0010-600 ••.0o’-’40C.)CC.)C- 0,,L).00 U0 20 40 60 80 0 20 40 60 80Time (mm)Time (mm)Figure 5.2 - Relative concentration curves of the hydrolysis intermediates asfunctions of reaction time, for two MTES homopolymerizationsinvolving 0 mole % formamide (circles) and 10 mole %formamide (open circles).The experimental error is ±2%.137100,2080 Si(OEt)4 Si(OH)(OEt)315000 00_______020I)•0 20 40 60 80 10000 20 40 60 80 100Time (mm)Time (mm)10 208Si(OH)2(OEt)Si(OH)3(OEt)1500 10oc30)4•0 0• 0 t.—:XY 000• 000 0005.0:•0•.• •.•______________________________________•.0 20 40 60 80 100 0 20 40 60 80 100Time (mm) Time (mm)100’80Si(OH)4• 6040’200 20 40 60 80 100Time (mm)Figure 5.3 - Experimental data and calculated relative concentration curves for thehydrolysis intermediates as functions of time for two TEOShomopolymerizations, involving 0 mole % formamide (circlesand solid lines) and 20 mole % formarnide (open circles and dashedlines). The experimental error is ±2%.138100‘—‘ 8006040U 20Figure 5.4 - Experimental data andcalculated relative concentration curves for thehydrolysis intermediates as functions of time forthree MTEShomopolymerizations, involving0 mole % formamide (circles),20 mole % formamide (diamonds) and30 mole % formamide (crosses).The experimental error is ±2%.CH3Si(OEt)201510ICH3Si(OH)(OEt)2x0 20 40 60 80Time (mm)0 20 40 60 80Time (mm)CH3Si(OH)2(OEt).‘.-.- 15CL)CU100i80CH3Si(OH)0 20 40 60 80Time (mm)20 40 60 80Time (mm)139Table 5.2 - TEOS hydrolysis and dimerization kinetic rate constants,as defined inequations (3.4)-(3.8) (page 65), determined for different formamideconcentrations. The formamide mole % is with respect to the total silaneconcentration. The water used in the reaction was acidified with HC1 to pH=2.55.The effect of formamide on the pH is summarized in Table 5.1.Kinetic Constants(M*min)4Formamide (mole %) 0% 10% 20% 30%kTl0.0018 0.0016 0.0011 0.000034k 0.011 0.008 0.0065 0.00021kT3 0.07 0.050.072kT4f0.07 0.07 0.05lcT4b 0.0090.01 0.012kT50.03 0.015 0.025140Table 5.3 - MTES hydrolysis and dimerization kinetic rate constants,as defined inequations (4.l)-(4.4) (pages 9 1-92), determined for different formamideconcentrations. The formamide mole % is with respect to the total silaneconcentration. The water used in the reaction was acidified with HC1 to pHt2.55.The effect of formamide on the pH is summarized in Table 5.1.Kinetic Constants(M*min)4Formamide (mole %) 0% 10% 20% 30%kMl0.0205 0.0205 0.012 0.0014kM2 0.066 0.066 0.043 0.0065kM3f0.096 0.096 0.086 0.017kM3b0.018 0.020 0.019 0.001kM40.011 0.011 0.007 0.0001141In addition, their results suggest that the higher the water concentration the greater theconcentration of hydrolyzed species, even when formamide was present. All thepolymerizations studied have a very high water concentration and the minimum formamideconcentration influencing the hydrolysis rate was found to be higher. A comparison betweenBoonstra et al. ‘5 work and the present results suggests that the water/formamide ratio isimportant in determining at what concentration the formamide influences the hydrolysis rate.All the studies by Hench, Orcel et al. [5.8-5.10] focused on TMOS systems with highwater ratios but involved very high formamide concentrations (76 to 385 mole % relative toTMOS). Their results are in accordance with our study which shows that the effect offormamide on the hydrolysis is significant at concentrations exceeding 10 mole %.At much higher formamide concentrations (40 mole %) the concentration of hydrolysisproducts is reduced to the extent that after 27 hours only the TEOS and the first hydrolysisproduct peaks are detected in the 29Si NMR spectra even though the system started to gel 24hours later. A possible explanation for why the other intermediate species are not observed isthat there is no build-up of any species except Si(OH)(OEt)3in the reaction sequence,consequently the concentration of any one intermediate species was not large enough to givea detectable NMR signal. Another possibility which has been suggested by Boonstra [5.15] isthat condensation between partially hydrolyzed species is favoured. The 13C CP/MAS NMRspectra of two TEOS polymer samples made with 0 % and 20 mole % formamide show thatthe ethoxy concentration is 1.6 ± 0.3 times greater in the sample synthesized with formamide.These findings support Boonstra’ s hypothesis.The gelation time was taken as the time when the TEOS polymer has cross-linked tothe extent that it no longer flows like a solution when the vial is tipped. The gelation timesfor the TEOS and MTES homopolymerizations as functions of formamide concentration are14230 -20AF. 10 -‘.0I0 10 20 30 40Formamide (mole %)60 -.‘‘40-s -- -FD0—0I I0 10 20 30 40Formamide (mole %)Figure 5.5 - The (A) TEOS and (B) MTES gelation times (days)versus formamide concentrations (mole %).143shown graphically in Figure 5.5. The largest observed change in the TEOS gelation time(from 22 to 6 days) occurs at low formamide concentrations (below 10 mole %) at pH=2.55.Formamide concentrations higher than 10 mole % affect the concentration distribution of theintermediate hydrolysis species but do not have a dramatic effect on the gelation time whichonly changes from 6 to 4 days. This suggests that the reduction of gelation time is not aconsequence of a change in the hydrolysis mechanism of TEOS (Table 5.2). The presentobservations suggest that the formamide plays only an indirect role in the reactions betweenhigher TEOS oligomers. At low concentrations, formamide may alter solution properties suchas polarity and pH, favouring the cross-linking of higher oligomers which would result in asignificant decrease of the gelation time. Horiuchi’s small angle X-ray scattering (SAXS)data also suggest that the decrease in the gelation time results from an acceleration of thenetwork formation. [5.13]The electron donating capacity of the methyl group in the MTES monomer hinders theMTES-MTES dimer formation rate (relative to TEOS-TEOS dimer formation (Chapter 4)). Inthis case, the presence of formamide on the MTES homopolymerization reaction has littleeffect on the network formation process as reflected in the gelation times (Figure 5.5).The 29Si NMR spectra of the MTES/TEOS copolymer system are shown in Figures 5.6and 5.7 for reactions in the presence of 0, 10 and 20 mole % formamide. These resultsclearly show that the concentrations of hydrolyzed monomers and dimers are not significantlyaffected at formamide concentrations of less than 10 mole %. However, as with thehomopolymer systems the MTES/TEOS copolymerization is affected when formamideconcentrations around 20 mole % are used (under these experimental conditions).144ABCSi(OH)4Figure 5.6 - The TEOS regions of the 29Si NMR spectra of 50/50 MTES/TEOScopolymerizations at 130 minutes for reactions, involvingA - 0 mole % formamideB - 10 mole % formamideC - 20 mole % formamide.Si(OH)3(OEt)T-MT-TVSi(OEt)4I Ippn —74 —76 —78—80 —82295i chemical shift (ppm)145ABCCH3Si(OH)Figure 5.7 - The MTES regions of the 29Si NMR spectra of 50/50 MTES/TEOScopolymerizations at 130 minutes for reactions, involvingA -0 mole % formamideB - 10 mole % formamideC - 20 mole % formamide.CH3Si(OEt)(OH)2M-MM-Tppm -3B—4Cm29Si chemical shift (ppm)—46—481465.3.2 High Resolution 1H Solution NMR InvestigationA typical 1H NMR spectrum taken during the reaction of TEOS in the presence of 20mole % formamide is shown in Figure 5.8B, where the upfield peaks are due to the ethanoland water protons as indicated. The downfield peaks have been magnified (by a factor of100) in order to clearly observe the formamide proton resonances. An identical spectrum wasacquired of a similar sample which did not contain TEOS since the TEOS proton peaksoverlap with the ethanol signals. In particular, all of the formamide signals were identical inthe two spectra.The 1H NMR spectra did not change over a period of 90 days suggesting that nocovalent chemical bonding had occurred between the formamide protons and other functionalgroups present in the system. However, these results do not eliminate the possibility that asmall percentage of the formamide protons might hydrogen bond to the silica gel in a fastexchange process. If this is the case, the concentration of such species must be very smallsince no chemical shift difference was observed.5.3.3 High Resolution 15N Solution NMR InvestigationHigh resolution 15N solution NMR was used to follow 15N enriched formamide duringa TEOS hydrolysis to determine if the formamide molecule covalently bonds to or interactsstrongly with the silanol groups via the amide functionality. ‘5N has a very large chemicalshift range [5.5] (hundreds of ppm compared to 10 ppm for 1H) and therefore it wasanticipated that even small interactions would be more easily detected than with thecorresponding 1H solution NMR data.The high resolution ‘5N solution NMR of neat formamide in deuterated acetone isshown in Figure 5.9B and consists of four doublets. The doubletacof 90 Hz (for labelling14790 00 00 8 02 B0 7!S 7.5Xl 00CH32OHI IiiiIIIiIIIIIIIjI”IllojIllIllIll0.0 0.5 0.5 5. 0.2 0.0 7.0 7.5 P5Xl 001H chemical shift (ppm)Figure 5.8 - 1H solution NMR spectra of solutions containing:A - Formanijde/H20/Ethanol in the ratiosused for the TEOSpolymerization reactionB - Formamide/H0/Ethanol/TEOS in the ratios used for the TEOSpolymerization reaction. The spectra were acquired with2000 scans, a 0.1 second recycle delay and100pulse angle.AflCONH2H2O/H3OCHOHCH32OHHCONfl210 9 8coNH24BCH32OHSi(OCHCH)CH32OHSi(OCHCH)H2O/H3OCHOHSi(OH)HCONiI220 9 8 7 6 4 3 2 IPPM 0148I IuwU50HzFigure 5.9 A - Formamide molecularstructure, labelled according to [5.6]B - 15N solution NMR spectrum of neatformamide in deuterated acetoneC - solution NMR spectrum of neat formamide inH20/D0.The 15N solution NMR spectra were acquired with2000 scans, a1 second recycling delay, a450pulse angle and no decoupling.1490II /AHdCNa\HFac__a-dBCJII I IFigure 5.10 - Analysis of the coupling patterns in the 15N NMR spectrumof formamide in the TEOS reaction mixture.ia-b-1.78 ppmia-cII ‘III ‘‘3a-bFL]a-dII II‘‘ II III Ii iiI I I III IIIII-2.35 ppmI II I}Ja.2Hth position b= 14 Hz}a2H = 14 HzII IIII IIII III I I’IIII IIII IIIIij2I0 -2 -4-615N chemical shift (ppm)150refer to Figure 5.9A) is due to the coupling between ‘5N and‘H.[5.6]The second doubletof 88 Hz is due to the coupling between 15N and‘Hb.[5.6]The third doubleta-d(14 Hz) isassigned to the coupling between ‘5N and‘Hd.[5.6]The presence of two different N-Hcoupling constantsabandia-cindicates restricted rotation about the C-NH2 bond.If the solvent is a mixture of water and deuterated water the ‘5N solution NMRspectrum of formamide shows additional peaks (arrows), as indicated in Figure 5.9C, whichincrease in intensity as theD20/H0ratio is increased. This suggests that the amidehydrogens in formamide exchange with the deuterium in the D20 and that the additional linescan be interpreted as originating from partially deuterated formamide. The replacement ofeither H,, orH bydeuterium does not lead to equivalent structures due to the resthctedrotation around the C-N amide bond. Therefore, two additional groups of lines are expected,each showing the coupling pattern of the ‘5N with two protons and one deuterium. Thereplacement of a ‘H by 2H results in three different chemical environments and hencethree different isotopic shifts assigned as; CHONH2-1.78 ppm, CHON’H2H-2.30 ppm andCHON2H’H -2.35 ppm. The chemical shifts are referenced with respect to formamide indeuterated acetone. An analysis of the coupling pattern is given in Figure 5.10.Figure 5.1 1A shows the ‘5N spectrum when the TEOS polymerization was initiated.The sample gelled after two days at pH=l.15. Figure 5.1 lB presents the ‘5N NMR spectrumof the same sample after 30 days. Even after gelation, and aging for 90 days at roomtemperature, no significant changes in the ‘5N NMR spectrum were observed.The ‘5N NMR data clearly show that no chemical shifts or intensity changes areobserved in the ‘5N spectra of formamide during the hydrolysis, condensation, gelation andaging of TEOS. It is concluded therefore that the formamide does not form covalent bonds orstrong hydrogen bonding interactions via the amide group to either TEOS or to the151-415N chemical shift(ppm)Figure 5.11- 15N solution NMR spectra of 15N labelled formamide (20mole %)in a TEOS polymerization mixture. The spectra wereacquiredat 30.4 MHz with 600 scans, a 1 second recycle delay anda 45° pulse angle,A - at time zero andB - after 30 days.AB20 --6152intermediate species formed during the hydrolysis, condensation, gelation and aging of thesample.5.3.4 High Resolution 13C Solution NMR InvestigationIn a similar manner, 13C solution NMR and ‘3C enriched formamide were used tofollow the behaviour of formamide during the reaction. An example of the‘3C NMR spectraobtained is shown in Figure 5.12. The sample gelled after 2days, and the 13C NMR spectrumshown is taken on the third day. The signal at 165 ppm is split intoa doublet of 15 Hz dueto the carbon-proton coupling. A carbon coupling to the amide protons is not observed.However, if this region is magnified (see insert) two smaller doublets of nearly equal intensitywith the same splitting as the main signal are detected. These signals are due to isotopeshifts from the exchange of an amide proton by deuterium. In agreement with the 15N NMRspectra, no chemical shifts or intensity changes were observed in the ‘3C NMR spectra takenover a period of 50 days.The fact that no change is observed in the ‘3C solution NMR spectra during the TEOSpolymerization process again suggests that formamide does not chemically bond to anyspecies in the reaction mixture, in this case, via the carbonyl functionality.153Ii 11111111111i1 111111 11 111111111II 11111 111111T 11TlI 1I rjIT111111TIIIppm 150 10050Figure 5.12 - 125.8 MHz 1H coupled 13C solutionNMR spectrum of aTEOS polymerization mixturewith 20 mole % formamideafter three days. The spectrum was acquiredwith 384 scans,a 5 seconds recycle delayand a900pulse angle.HCONH2CH32OHppmCH32OH164 16213C chemicalshift (ppm)1545.4 CONCLUSIONSThe hydrolysis and dimer formation reactions of the TEOS and MTES homopolymersand MTES/TEOS copolymer are affected dramatically by the presence offormamide whenthe formamide concentration exceeds 10 mole % when excess water isused in the reaction.No significant effect is observed on the hydrolysis and dimer formation reactions when theformamide concentration is below 10 mole %.In the homopolymerization of TEOS in the presence of formamide, the hydrolysisrates are not significantly affected when formamide concentrations below 10 mole% areused. Yet the greatest reduction in gelation time is acquired when 2.5 mole % formamide isused. This is the first evidence that the decrease in gelation time caused by the presence offormamide is not a consequence of a change in the mechanisms or rates of the hydrolysis anddlimer formation reactions.At formamide concentrations of 20 mole %, the high resolution ‘H, ‘3C and ‘5Nsolution NMR study clearly demonstrate that the formamide remains intact during the TEOSpolymerization over a period of 90 days with no change in the spectra. Therefore, chemicalbonding to the polymer or one of its precursors via either the carbonyl or the amide groupcan be ruled out.155CHAPTER 6SOLID STATE NMR AND THERMALANALYSIS STUDIES OF THE THERMALSTABILITIES OF FUNCTIONALIZEDSILICA GELS PREPARED BYTHE COPOLYMERIZATIONMETHOD6.1 INTRODUCTIONGlasses are conventionally made bymelting powders of crystalline mixed oxidesattemperatures greater than 1000°C, followedby undercooling the sample below the glasstransition temperature (Tg). The conventionalfabrication of glasses thus involves such hightemperatures that all organic functionalitiesdecompose.Glasses could also be manufactured from thermallytreated ailcoxide derived gels. Theadvantage of this latter process is that lower temperaturesare required to convert the gel intoa glass.[6.8] Yamane et al. found that glasses madefrom silica gel heated to 900°C wereidentical to glasses made from molten silica indensity, refractive index and hardness.[6.4]Fabrication of glasses at temperatures lower than1000°C from gels raises the possibility ofmaking functionalized low-temperatureglasses where the functionality could be distributedwithin the glass structure. The limit to thesynthesis of low-temperature functionalizedglasses is the thermal stability of the functionalityto be incorporated.Extensive literature is available onthe thermal behaviour of silica gel.[6.4,6.5, 6.6,6.9] Yet only one study was found concerningorganofunctionalized silanes, Kamiya et a![6.111 investigated the thermal evolution of gels derived from CH3Si(OEt) with IR andthermal analysis techniques. They foundthat the methyl functionality was stable up toaround 700°C under nitrogen while above this temperaturethey observed the formation ofSi-H bonds. In the present work,the thermal stabilities of the organic functionalities ina156number of different functionalized copolymers were measured. The thermalstabilities of themethyl, ethyl, phenyl and phenethyl functionalities were characterized in orderto determinewhat is the weakest point in an organofunctionalizedsilane/TEOS copolymer. The thermalstabilities were characterized by solid state 29Si, ‘3C and 1H NMR, differentialscanningcalorimetry (DSC), thermogravimetric analysis (TGA), and TGA-.mass spectroscopy(TGIMS).DSC experiments measure the power difference needed to keep a zero temperaturedifferential between the sample and a reference sample.[6. 1] This experimental techniquedetects endo- and exothermic processes but is limited to temperatures up to approximately600°C. The thermogravimetric analysis (TGA) technique will also be used to characterize thesamples since it reaches temperatures of up to 1200°C. TGA characterizes thermaldecomposition events by measuring weight changes as a function of temperature.[6.1,6.21The TGJMS data complement this information and provide insight into the chemical nature ofthe volatile decomposition products.1576.2 EXPERIMENTAL6.2.1 Sample PreparationSeven different copolymers were synthesized using the procedure ofthe “kineticpreparation” described in Chapter 2. This preparation involves mixing ethanol,theorganofunctionalized triethoxysilane and tetraethoxysilane (TEOS) and thenslowly adding theacidified water. After the sample gelled it was crushed, washedand dried at 100°C. Thedifferent gel compositions are summarized in Table 6.1.Each sample was divided into 12 small portions of which 10 were individuallythermally treated for two hours in a quartz tube furnaceat temperatures of either 200, 300,400, 500 or 600°C, under either nitrogen or air. Nitrogen wasused to provide an inertatmosphere during the heating so that degradation due to oxidation could be minimized.Nitrogen (purity 99.9%) was flowed at a constant rate through the sample before heating, inorder to expel any remaining air, and during the actual heating. One untreated portion of thesample was used as a reference for all the NIvIR studies and another untreated portionwasused for the TGA, DSC and TGJMS measurements.6.2.2 Solid State NMR ExperimentsAll the NMR spectra were acquired at room temperature. The 295i CPIMAS NMRspectra were acquired on a Bruker MSL 400 spectrometer using a Bruker double tuned MASprobe with a 7 mm ceramic spinner. The 29Si MAS NMR spectra were acquired on a BrukerAM 400 spectrometer using a home built single tuned probe with a 9 mm vespel spinner.Quantitative 29Si MAS NMR spectra were acquired using a recycle delay of 180 seconds(3*T1)and a 45° pulse angle. The ‘3C CPIMAS NMR spectra were acquired on either aBruker MSL 400 or a CXP 100 spectrometer using a double tuned MAS probe with a 7 mmceramic spinner. The 1H MAS NMR spectra were acquired on a Bruker MSL 400158Table 6.1 - Sample compositions used in the thermal analysis investigations.SAMPLE R-Si(OEt)3 Si(OEt)4 EthanolWater*(moles) (moles) (moles) (moles)TEOS 0 1.791 1.0343 1.9825/75 MTES/TEOS 0.0448 0.1340 1.0343 1.98R=CH350/50 MTES/TEOS 0.0895 0.0897 1.0343 1.98R=CH325/75 ETES/TEOS 0.0448 0.1340 1.0343 1.98R = CH2350/50 ETES/TEOS 0.0896 0.0897 1.0343 1.98R = CH2325/75 PTES/TEOS 0.0448 0.1340 1.0343 1.98R=C6H525/75 PETES/EOS 0.0448 0.1340 1.0343 1.98R=C6H5CH2TEOS = tetraethoxysilaneMTES = methyltriethoxysilaneETES = ethyltriethoxysilanePTES = phenykriethoxysilanePETES= phenethyithethoxysilane*Thewater was acidified to pH=1.15 with HC1.159spectrometer. The experimental conditions for the different spectra are given in theappropriate figure captions. The 29Si, 13C and 1H chemical shifts are all referenced to theappropriate resonances of TMS.6.2.3 DSC and TGA MeasurementsThe thermal analyses (TGA and DSC) were carried out using a TA InstrumentsThermal Analyst 2000 system. Both the DSC and TGA studies were carried out undernitrogen gas (purity 99.9%) unless otherwise indicated. During the DSC experiments anitrogen flow rate of 40 cc/mm and a temperature increase of 10 °C/min were maintained.The sample size for the DSC studies was kept under 10 mg. For the TGA experiments,anitrogen flow rate of 50 cc/mm and a temperature increase of 10°C/mm were used. Thesample size for the TGA studies was kept between 10 and 20 mg. All the samples wereground to fine powders with a mortar and pestle.Independent TG/MS measurements on all the copolymers and pure silica gel werecarried out by Dr. K. MacKenzie at the New Zealand Institute for Industrial Research andDevelopment. For these experiments, a Stanton Redcroft TG770 Thermobalance connected toa quadrupole mass spectrometer (Extranuclear Laboratories Inc.) was used. The partialpressure of each species in the TG/MS graphs given is the fraction of the total vacuum in thespectrometer which is contributed by that species. It is related to the ion current for eachspecies by an instrumental calibration factor which pertains to the conditions inside the massspectrometer. The calibration factor was determined by the manufacturer. All of the TG/MSexperiments were run under similar conditions, i.e. roughly the same amount of sample, flowrate, etc. Consequently, the results for any fragment mass number of one sample can bereasonably compared to those of another.1606.3 RESULTS AND DISCUSSIONIn a TGA experiment, the temperature ranges observed for different processes arepartiy dependent on operational and instrumental factors. For example,some of the observeddiscrepancies in the temperature ranges of thermal events may be accounted forby the sampleparticle size which would affect the transfer of heat and the diffusion of volatile compounds,such as water through the sample.6.3.1 Tetraethoxysilane (TEOS) Homopolymer - Pure Silica GelThe DSC and TGA data for silica gel are useful as references for the DSC and TGAresults on the different functionalized silica gels. The silica gel DSC thermal analysis curve,Figure 6.1A, shows two endothermic processes: one occurs over a temperature range ofapproximately 100°C centered around 140°C, and another throughout the whole temperaturerange studied (20°C to 600°C) since the signal never reaches a clear plateau.The TGA thermal analysis curve for silica gel is shown in Figure 6.1B and confirmsthe above observations. In the temperature range of 45°C to 172°C there is a weight loss of14%. This is followed by a further weight loss of 6% between 200°C and 1000°C. A plateaucorresponding to a constant chemical composition, is not reached below 1000°C.In order to further characterize these thermal events the TGA exhaust was fed into amass spectrometer (work done by Dr. K. MacKenzie). These data are shown in Figure 6.2.Hydroxyl groups (fragment mass number 17) are detected throughout the temperature range asshown by the slow but linear rise in the partial pressure, indicative of the removal ofhydroxyl groups and water from the surface. The initial detection of hydroxyl groupscorresponds to the first endothermic thermal event around 140°C in the DSC, and to the initialthermal event in the TGA resulting in a mass loss of 14%. The initial loss of water isprobably due to the loss of physisorbed water molecules. This is followed by the elimination16110—1.— -2,— -3C-4-5-6-7-810095858075Temperature (°C)Figure 6.1 - (A) DSC and (B) TGA analysis curves for silicagelobtained under nitrogen.600100 200300 400500TemperatureCc)B100 200 300 400 500 600 700800 900 10001621.8E-07a— —V 1.4E-07 - - - - - - -- - -. - - - -F1’__1.OE-07-ni/z 15----m/z17I-m/z266.OE-08 - .. - . . - m/z 29..2.OE-08 -0—0 100 200 300 400 500 600 700 800 900Temperature (°C)Figure 6.2 - TG/MS data for the silica gel sample with the identifyingmass number for each curve.163of water presumably produced by the condensation of neighbouring hydroxyl groups to formsiloxane bonds. Smaller concentrations of fragments with mass numbers such as 26 (C2H),28 (C2H4)and 29 (C2H5)are observed due the elimination and degradation of residual ethoxygroups on the surface. The curve for mass number 28 is not taken into account because herethe major contribution is not from ethoxy fragments but fromthe carrier N2 gas. Theelimination of ethoxy and hydroxy groups via surface condensation are not resolved asdiscrete events in the TGA and DSC thermal analysis curves but are presumably responsiblefor the absence of a plateau in both data sets.Previous studies [6.9] on the hydration/dehydration characteristics of silica gel suggestthat the dehydration of silica gel occurs in three steps: 1) the physisorbed water is eliminated(25 to 170°C), 2) surface silanol groups start to condense (above 170°C),3) the dehydrationprocess becomes irreversible above 400°C so that above 800°C only isolated silanol groupsremain.[6.6, 6.9] The present thermal data are in excellent agreement with these conclusions.Boonstra and Mulder observed that their silica gel samples which were made by atwo-step reaction process turned blackish-grey when heated to 400°C in air.[6.5] Theysuggested that this was due to the pyrolysis of remaining ethoxy ligands on the surface. Thesamples of silica gel used in the present work all remained white powders, even when heatedto 1000°C suggesting that very few alkoxy ligands remained on the surface with thispreparation in agreement with the TG/MS data.6.3.2 Methyltriethoxysilane (MTES)/TEOS CopolymerA typical 29Si CP/MAS NMR spectrum for the methyltriethoxysilane/ tetraethoxysilane(MTES/TEOS) copolymer is shown ii Figure 6.3D. As described in Chapter 2, the downfieldpeaks are due to silicons with directly attached organic functionalities, while the group ofupfield peaks originate from the unfunctionalized silicons. An overview of the 29Si NMR164CSi(OSi)3(OH) Si(OSi)4—80 —tOO —i20 —i4029Si chemical shift (ppm)Figure 6.3 - 29Si solid state MAS NMR spectra for the four25/75 organofunctionalizedcopolymers investigated. The different silicon chemical shifts aresummarized in Table 6.2.C6H5Si(OSi)2(OH?...J/ABDI I • I—40 —60165chemical shifts for the different copolymers studied is given in Figure6.3 and Table 6.2.Table 6.2 - Observed 29Si chemical shifts for the different copolymers shownin Figure 6.3together with their assignments.CH3-Si(OH)(OSi)2 -60 ppm C6H5CH2-Si(OH)(OSi)-58 ppmCH3-Si(OSi) -64 ppm CH-Si(OSi)3-66 ppmCH32-Si(OH)(OSi) -58 ppm Si(OH)2(OSi)-93 ppmCH-Si(OSi) -65 ppm Si(OH)(OSi)3 -102 ppmSi(OSi)4 -110 ppmC6H5-Si(OH)(OSi)2 -70 ppmC6H5-Si(OSi)3 -78 ppmThe 29Si CP/MAS NMR spectra for the series MTES/TEOS copolymer samplesthermally treated for two hours in either air or nitrogen at the temperatures indicated, areshown in Figure 6.4. A comparison of the two series of spectra clearly shows that the methylfunctionality is stable to higher temperatures (600°C vs. 300°C) when the MTES/TEOScopolymer is heated under nitrogen rather than air.The broadening of the 29Si CP/MAS NMR lines indicates that structural changes occuras the temperature increases (Figure 6.4). These could be due to a number of reactions suchas: 1) loss of water, 2) elimination of metastable three-membered rings, 3) condensation ofadjacent hydroxyls and ethoxy ligands, 4) the loss of organic groups, and/or 5) relaxation ofthe Si02 framework.[6.5, 6.9] The elimination of the hydroxy and ethoxy ligands on the166ABUntreated200°C300°C ‘I______400°C_____________J’JJ -_ __J’J’J v_________________- —jeøPPMPPMFigure 6.4 - Room temperature 79.5 MHz 29Si CP/MASNMR spectra of the25/75 MTES/TEOS copolymersample heated for two hours atthe temperatures indicated. These spectra were obtainedwith 400 scans,lOms contact time, 4s recycle delay and3.2 kHz sample spinning rate.Two series of experiments were performed, thespectra acquired for thosesamples thermally treated under nitrogenare shown in series Aand the spectra for those samples thermally treated inair are shownin series B.167surface of the silica gel, as a consequence of thermal treatment forms distorted Si-O-Si bondangles and changes in sioxane lengths.[6.3, 6.6] This results in a larger disthbution of bondangles and lengths, i.e. silicon environments, increasing the width of the 29Si NMRresonances. In general, the elimination of the ‘H polarization source, i.e. the removal ofhydroxyl groups and the oxidation of methyl groups, as the temperature increases, results inweaker 29Si CP/MAS signals (Figure 6.4).The ‘3C CPIMAS NMR spectra of the same samples, Figure 6.5, confirm that themajority of the methyl groups are intact at 600°C when the sample is thermally treated undernitrogen but are oxidized when thermally treated in air above 300°C. These results are inagreement with Kamiya et al.[6. 11]The ‘H MAS NMR spectra presented in Figure 6.6 show that the methyl group isoxidized in air at approximately300°C; however, even for the sample heated at 600°C asignificant concentration of hydroxyl groups remain on the sample surface at least aftercooling and running the spectra at room temperature. In contrast, the spectra of the samplesthermally treated under nitrogen at 600°C, Figure 6.6A, show a methyl signal but no hydroxylresonance. This might be due to the more hydrophobic nature of the sample. The 2D‘H-29Si heteronuclear correlation NMR spectrum shown in Figure 6.7 confirms that the onlycross polarization source in this sample is the methyl protons. This sample will be furtherinvestigated in Chapter 7.Quantitative 29Si NMR spectra were acquired for the untreated sample and the samplethermally treated at 600°C for two hours. The samples were calculated to have a compositionof 24% and 17% functionalized silicons, respectively, confirming the good stability of themethyl functionality under nitrogen at temperatures up to 600°C. The small loss of methylgroups may be due to oxidation by oxygen impurity (about 0.1%) in the nitrogen gas over the168ABUntreated200°C300°C400°C500°C30 20 £0 0 —iO —20 —30PPNFigure 6.5 - Room temperature 100.6 MHz 13CCP/MAS NMR spectra of the25/75 MTES/TEOS copolymer sample heated for two hours atthe temperatures indicated. These spectra were obtained with 296scans, 5ms contact time, 4s recycle delay and 3.2 kHz sample spinningrate. Two series of experiments were performed, the spectra acquiredfor those samples thermally treated under nitrogen are shown inseries A and the spectra for those samples thermally treated in airare shown in series B.600°CI I I30 20 £0 0 —0 —20 —30PPM169A B- CH3 -OH- 600°C --OH500°C400°C -CH3— 300°C - — —- 200°CUntreated -I____to 0tO CPPMPPMFigure 6.6 - Room temperature 400.1 MHz 1H MAS NMR spectra of the25/75 MTES/TEOS copolymer sample heated for two hours atthe temperatures indicated. These spectra were obtained with4 scans, 4s recycle delay, a900pulse and 3.2 kHz sample spinningrate. Two series of experiments were performed; the spectra acquiredfor those samples thermally treated under nitrogen are shown inseries A and the spectra for those samples thermally treated in airare shown in series B.170Si(OH)(OSi)30-=c)E- 20C)Figure 6.7 - 2D1HJ29Si heteronuclear correlation contour plotof thethermally treated 25/75 MTES[FEOScopolymer. Thespectrum consists of 64 experiments. Eachexperimentwas acquired with 320 scans, a 10 millisecond contacttime,a 3 second recycle delay and a4 kHz spinning speed.CH3Si(OSi)Si(OSi)4CH3Si(OH)(OSi)2c)• —20—1005020PPM0 —50 —10029Si chemical shift (ppm)—150171two hour heating period. A similar study was carried out on a50/50 MTES/TEOScopolymer. The series of 29Si and ‘3C CP/MAS NMR spectra for these samples heated undernitrogen and air indicate that the thermal stability is the same as that of the25/75MTESITEOS copolymer.The DSC and TGA thermal analysis curves for the50/50 MTES/TEOS copolymer andthe unfunctionalized silica gel are shown in Figures 6.8 and 6.9, respectively. The generalform of the DSC curve under nitrogen for theMTES/TEOS copolymer is similar to that ofthe unfunctionalized silica gel, Figure 6.8. This implies that no endo- or exothermic processcorresponding to a decomposition of the methyl groups is detected up to 600°C. The slightshift of the temperature range where the physisorbed water is released may be explained bythe difference in macroscopic structure due to the presence of the methyl groups in theMTES/TEOS copolymer and the less hydrophilic nature of this system. Both the DSC andNMR data indicate that the methyl functionality in the MTESITEOS copolymer is stable up to600°C, when thermally treated under nitrogen.The first inflection in the TGA curves shown in Figure 6.9 (20°C to 175°C) is due tothe loss of physisorbed water. The TGA for the MTESITEOS copolymer suggests that theloss of hydroxyl and remaining ethoxy groups overlaps with the decomposition of the methylgroups in the temperature range above 600°C.A comparison of the fragment mass number 15 curve for silica gel (Figure 6.2) andthe MTES/TEOS copolymer (Figure 6.10) in the TG/MS data clearly shows a rise above600°C indicating the decomposition of the methyl functionality in the MTESITEOScopolymer. The curves for mass numbers 26 and 29 for the MTESR’EOS copolymer showno significant variation suggesting that the ethoxy ligand concentration is negligible, as in thecase of silica gel. The mass number 17 is indicative of the elimination of hydroxyl groups.1721EC0—1-2-3-4-5-6-7-8B100 200 300 400 500 600Temperature (°C)Figure 6.8 - DSC thermal analysis curves obtained undernitrogen forA - 50/50 MTESITEOS copolymer andB - silica gel173100Temperature (°C)Figure 6.9 - TGA thermal analysis curves obtained under nitrogen forA - 50/50 MTESR’EOS copolymer andB - silica gel100095908580A75200 400 600 800174I .8E-07‘1 .4E-07 -1.0E07—m/z15----m/z17I-m/z266.0E08 -- .- mlz 292.0E08i0 100 200 300 400 500600 700 800 900Temperature (°C)Figure 6.10 - TG/MS data for the 25175 MTES/TEOS copolymer samplewith the identifying mass number for each curve.1756.3.3 Phenyltriethoxysilane (PTES)/TEOS CopolymerThe 29Si CPIMAS NMR spectra for the thermally treated25/75 PTES/TEOScopolymer samples are shown in Figure 6.11. They show that heating the sample in airoxidizes the phenyl group at temperatures above 400°C, but under nitrogen the phenyl groupis stable up to 600°C. Increasing the temperature of the thermal treatment broadens the 29Siresonances. As discussed for the MTESITEOS copolymer, this is indicative of structuralchange resulting from internal condensations.The large chemical shift anisotropy of the phenyl group results in a large number ofspinning side bands in the ‘3C CPIMAS NMR spectra acquired at 400 MHz (9.4 Tesla) unlessvery high spinning speeds are used.[1.10] The ‘3C CP/MAS spectra were therefore obtainedon a Bruker CXP 100 spectrometer where spinning speeds around 2-3 kHz are adequate toremove the spinning side band pattern. The series of 13C CP/MAS NMR spectra shown inFigure 6.12 prove that the majority of the phenyl groups are still intact at 500°C; however,decomposition of a small proportion of the phenyl groups has occurred as shown by theappearance of an additional resonance at 143 ppm. If the phenyl functionality is oxidized,phenolic groups could be formed. In solution, the phenolic group shows a ‘3C resonancearound 155 ppm for the carbon directly attached to the oxygen.[6.7] Therefore, the new low-field resonance can be interpreted as a result from the oxidation of the phenyl group to aC6H50- group by the oxygen impurity in the nitrogen gas.The DSC data acquired under nitrogen and in air for the PTES/TEOS copolymer areshown in Figure 6.13. The curve obtained under nitrogen, Figure 6.13A, is similar to those ofthe MTESJTEOS copolymer and silica gel samples, suggesting that the phenyl group whenthermally treated under nitrogen for short time periods is stable up to temperatures of 600°C.In air, the thermal treatments result in the oxidation of the phenyl functionality at176A BUntreated200°C300°C400°C500°C_____________________________________I I • • I 1—40 —60 —80 —100 —120 —140PPNFigure 6.11 - Room temperature 79.5 MHz 29Si CP/MASNMR spectra of the25/75 PTES/TEOS copolymer sampleheated for two hours atthe temperatures indicated. These spectra were acquired with 800 scans,lOms contact time, 2s recycle delay and 2.5-3 kHz sample spinning rate.Two series of experiments were performed, the spectra acquired for thosesamples thermally treated under nitrogen are shown in series A andthe spectra for those samples thermally treated in air areshown in series B.600°CI • I I •—40 —60 —80 —100 —120 —140PPM177AB_ _Uf_500°C—600°C —II I160 140 120100 160 140 120100PPMFigure 6.12 - Room temperature 25.2MHz 13C CP/MAS NMR spectra of the25/75 PTES/TEOS copolymer sample heated for two hoursatthe temperatures indicated. These spectra were acquiredusing13000 scans, 5ms contact time,3s recycle delay and 3 kHz samplespinning rate. Two series of experiments wereperformed thespectra acquired for those samples thermally treatedunder nitrogenare shown in series A and the spectra forthose samples thermallytreated in air are shown in series B.178‘31050C100 200 300 400500 600Temperature (°C)Figure 6.13 - DSC thermal analysis curves for the 25175 PTESiTEOS under(A) nitrogen and (B) air, with all the other experimentalparameters kept constant.179temperatures greater than 400°C. The 13C NMR spectra suggest that the decomposition of thephenyl groups under nitrogen starts at 500°C, this is not reflected in the DSC curve. Thedynamic heating in the thermal analysis versus the long-term (two hours) isothermal heatingin the NMR studies may well account for this discrepancy. The PTESITEOS copolymer TGAdata acquired under nitrogen are presented in Figure 6.14. A comparison of the TGA curvefor the PTESiTEOS copolymer with that of silica gel indicates that the phenyl functionality inthe PTES/TEOS copolymer decomposes between 500°C and 700°C. The TGA data show nodecomposition of the functionality below 500°C when acquired under nitrogen.The TGIMS data acquired under nitrogen for the PTES/TEOS copolymer are shown inFigure 6.15. There are three groups of fragment mass numbers, as illustrated in Figure 6.15.The fragment mass number 79 is identifiable as a six carbon species indicative of a phenylgroup. The mass number 79 curve maximum is detected before that of the other fragmentsimplying that the weakest link is the C-Si bond. The maxima for the four, two and onecarbon species, fragment mass numbers 50-53; 26,29 and 15, respectively, are reached athigher temperatures. Thus, they are probably products of the decomposition of the phenylgroup after the cleavage of the C-Si bond. Another possible explanation for the 26, 29 and15 curves is that there are still some ethoxy ligands present in this copolymer, which arecleaved under the thermal treatment.Quantitative 29Si MAS spectra indicate that, out of the possible 23%, 18% of the PTESmonomer is still intact after thermal treatment under nitrogen at 500°C. The 5% loss ispossibly due to oxidation by impurities in the nitrogen gas during the two hour time period ofthe heating process.1801009590858075Temperature (°C)Figure 6.14 - TGA thermal analysiscurves for 25175 PTES/TEOS undernitrogen.1000200 400 600 8001811.6E-07- —m/z15----m/z17• m1z261.2E-07mIz 29-8.OE-084.oEo:o 100 200 300 400 500 600 700800 900Temperature (°C)1.8E-06—1.4E-06—____________0 100 200 300 400500 600 700 800 900Temperature (°C)Figure 6.15 - TG/MS data for the25/75 PTES/TEOS copolymer samplewith the identifying mass number for each curve.1826.3.4 Ethyltriethoxysilane (ETES)/TEOS CopolymerThe 29Si CP/MAS NIvIR spectra of the thermally treated25/75 ETES/TEOS copolymersamples, are shown in Figure 6.16. These two series of spectra are similar to those presentedfor the MTESiTEOS and PTESiTEOS copolymers (Figures 6.4 and 6.11), As the temperatureof the thermal treatment is increased, the resonances of the resulting material broadens. Thisis indicative of a larger distribution of silicon environments, again a consequence of internalcondensations during the thermal treatment.The 29Si CP/MAS NIvIR spectra suggest that the ethyl functionality has almostcompletely decomposed after thermal treatment in air at temperatures greater than 200°C. A50/50 ETESTEOS copolymer was synthesized in order to increase the contribution from thefunctionalized silane in the 29Si NMR spectra. It is evident from Figure 6.17 that the ethylfunctionalized silane starts to decompose above 400°C under nitrogen. In comparison to themethyl and phenyl functionalities, the ethyl functionality decomposes more readily. The largedecrease in intensity observed in the spectra of the samples thermally treated at highertemperatures is due to the decomposition of the ethyl groups resulting in a loss of protonswhich are the polarization source in this CP/MAS experiment.The spectra of the ETES/TEOS copolymer sample thermally treated under an inertatmosphere, i.e. nitrogen, at temperatures above 400°C contain an additional resonance at—-85 ppm. This resonance suggests the formation of ESi-H bonds.[6.10J This is inagreement with the conclusions of Kamiya et al, in their study of the thermal decompositionof the MTES homopolymer.[6. 11] As expected, the resonance at —-85 ppm is absent in theseries of spectra acquired for the ETESITEOS sample thermally treated in air.The TG/MS data shown in Figure 6.18 provide some additional information. The183ABFigure 6.16 - Room temperature 79.5 MHz 29SiCP/MAS NMR spectra of the25/75 ETESITEOS copolymer sample heated for two hoursat the temperatures indicated. These spectra were acquired using400 scans, lOms contact time, 4s recycle delay and 3.2 kHz samplespinning rate. Two series of experiments were performed, thespectra acquired for those samples thermally treated under nitrogenare shown in series A and the spectra for those samples thermallytreated in air are shown in series B.Untreated200°C300°C400°C—500°C___________ ___600°CPPMPPM184Figure 6.17 - Room temperature 79.5 MHz 29Si CP/MAS NMR spectra ofthe50/50 ETES/TEOS copolymersample heated for two hoursunder nitrogen ‘at the temperatures indicated. These spectra wereacquired using 400 scans, lOms contact time, 4s recycle delayand a 3.2 kHz sample spinning rate.Untreated200°C300°C400°C500°C600°C/_\-sePPM185curves representing the fragment mass numbers 26-31 (ethyl fragments) reacha maximum at610°C (Figure 6.18B) before the one for the fragment mass number 15 (correspondingto amethyl group) at 640°C. Therefore, the decomposition of the ethyl functionality,as was thecase for the phenyl, involves the cleavage of the C-Si bond before the C-C bond. The curveof mass number 29 possesses two maxima: one maximum coincides with the maximum of themass number 31 at 540°C, and the other with the maximum observed for the mass number 26and 30 at 610°C. The fact that ethyl fragments are released at two different temperaturescould be explained considering that ethyl functionalities in the bulk and those on or near thesurface of the sample may contribute differently to the curve. It is possible that thedecomposition products of the ethyl functionalities near or on the surface are detected as soonas the Si-C bond is broken, but those in the bulk may be trapped within the copolymer matrixuntil further changes in the Si02 framework allow these trapped fragments to escape.The fragments with mass numbers 77-79 (Figure 6.1 8C) can be explained byrecombination of ethyl fragments. The maxima of the ethyl fragment curves, mass numbers26-31 (610°C), precede the curve maxima of mass numbers 77-79 (660°C) which is alsocompatible with the notion of ethyl fragments trapped in the copolymer matrix, encouragingrecombination. Again, the fragment mass number 17 (Figure 6.18A) indicative of theelimination of hydroxyl groups is observed throughout the whole temperature range.Figure 6.19 presents a series of ‘3C CPIMAS NMR spectra for the 25/75 ETES/TEOScopolymer thermally treated under nitrogen and air. A single resonance for both carbons ofthe ethyl functionality is observed due to the small chemical shift difference between them.The results of the 13C CP/MAS spectra confirm that the ethyl group decomposes in air above200°C, and above 400°C when thermal treatment is carried out in nitrogen. With thedecomposition of the ethyl functionality, an additional resonance appears at 0 ppm in the ‘3C186I .8E-07____________1 .4E07LOE-076.OE-08 -2.OE-08 -3.OE-08 -2.5E-08 -2.OE-08-___________1.5E-08 -1.OE-08 -5.OE-09 -3.5E-08 —3.OE-08 —I.. 2.5E-08 —2.OE-08 —1.5E-08 —1.OE-08 -5.OE-09 -Figure 6.18 - TO/MS datafor the 50/50 ETES/TEOScopolymer samplewith the identifying massnumber for each curve.—mhz 15----m/z17m)z26m/z29I.-..-z::::7,:.0 100 200 300 400 500 600700Temperature (°C)800 900----mhz 29mJz3O—mhz 31I’IIV0 100 200 300400 500 600 700 800900Temperature (°C)—m/z77----m/z78m)z79I—’I•.•I.$ .• SI •. S• .. . VVI. . SU0I II100 200 300 400 500600 700 800 900Temperature (°C)187A BUntreated200°C300°C400°C_________ ________500°C600°C40 20 -20 40 20 0—20PPM PPMFigure 6.19 - Room temperature 100.6 MHz 13CCP/MAS NMR spectra of the25/75 ETES/TEOS copolymer samples heated for two hoursat the temperatures indicated. These spectra were acquired using296 scans, 5ms contact time, 4s recycle delay and 3.2 kHz samplespinning rate. Two series of experiments were performed, the spectraacquired for those samples thermally treated under nitrogen areshown in series A and the spectra for those samples thermallytreated in air are shown in series B.-_188CPIMAS NMR spectra. This peak may be due to a methyl functionality derived from thedecomposition of the ethyl functionality. No peaks appear downfield from the ethyl ‘3Cresonance indicating that no conjugated functionalities are formed.The ‘H MAS NMR spectra for the thermally treated50/50 ETES/TEOS copolymersamples under nitrogen are shown in Figure 6.20. There is a narrow resonance due to waterand hydroxyl groups, and a broader resonance due to the five ethyl protons (with a width athalf height of approximately 3 ppm in the untreated sample). The ‘H spectrum changesdramatically after the sample is thermally treated at 500°C (under nitrogen). The protonsignals narrow and three distinguishable resonances are resolved, which remain even after athermal treatment at 600°C. The chemical shifts are in accordance with protons in thefollowing different chemical environments: Si-H, -OH and -CH2CH3,as indicated in Figure6.20.The DSC curve of the ETES/TEOS copolymer is shown in Figure 6.21. The removalof physisorbed water is the first endothermic process, as seen previously in Figures 6. 1A,6.8A and 6.13. Above 540°C, the second endothermic process is the decomposition of theethyl group. In the DSC data acquired in air, the second thermal event is exothermic andstarts at 240°C. Since the NMR results indicate that the ethyl functionality is stable up to200°C in air this thermal event can be understood as the decomposition of the ethylfunctionality.The TGA curve for the ETESII’EOS copolymer, Figure 6.22, is very similar to thatobtained for the PTESITEOS copolymer. Initially there is a loss of water, then a loss ofsurface hydroxy and ethoxy ligands which overlaps with the decomposition of the ethylfunctionality. Interestingly, a plateau exists above 650°C for the next 200°C in theETES/TEOS copolymer TGA curve signifying a constant chemical composition.1890H2H3Untreated200°C300°C400°C500°CSi-H-CR3600°C£—J 1 IiS iO 5 0 —5 —iO —iSPPMFigure 6.20 - Room temperature 1HMAS NMR spectra of the 50/50 ETES/TEOScopolymer sample heated for twohours under nitrogen atthetemperatures indicated. These spectrawere acquired using 4 scans,a 4s recycle delay, a 90° pulseand 3.3 kHz sample spinning rate.19025 -B20—,/‘‘I15I II:10I)5 I0 — —-5 -I I I I I I I I I I I100 200 300 400 500 600Temperature (°C)Figure 6.21 - DSC thermal analysis curvesfor the 25/75 ETES/TEOScopolymer sample obtained under(A) nitrogen and(B) air, with all the otherexperimental parameterskept constant.191Temperature (°C)Figure 6.22 - TGA thermal analysis curves obtained undernitrogen forA - 25/75 ETES/TEOS copolymer andB - 25/75 PTES/TEOS copolymer.10001009590AB858075200 400 600 8001926.3.5 Phenethyltriethoxysilane (PETES)ITEOS CopolymerThe previous results suggest that the weakest point in an organofunctionalizedsilanefTEOS copolymer is the initial ESi-C bond not the initial C- bond. In order toconfirm this, the thermal stability of the phenethyl functionality wasinvestigated. Thisfunctionalized silane serves as a model for many organofunctionalizedsilanes with thestructure X-CH2-CH-SiY3(where X is any organic or inorganic ligand andY is a halide oralkoxide ligand). Organofunctionalized silanes with the structure X-CH2CH-SiY3are verycommon since they can be synthesized in a single step reaction between the appropriatealkene and a thhalo-silane as shown in equation (6.1).X-CH=CH2+ HSIY3 —* X-CH2-CH-SiY3(6.1)The 29Si MAS NMR spectra for the series of thermally treated PETES/TEOScopolymer samples, Figure 6.23, show that the phenethyl functionality also decomposes fasterin air than in nitrogen. After treatment at 600°C under nitrogen the 29Si spectrum possessesan additional resonance at —-85 ppm similar to that observed in theETES/TEOS case (Figure6.17). Again, the formation of Si-H is postulated. The13C CP/MAS NMR spectra (Figure6.24) confirm that the phenethyl functionality decomposes faster in air than in nitrogen.The DSC curve (Figure 6.25A) shows the three anticipated endothermic events,adominant one around 100°C due to the removal of water, a second one over the wholetemperature range due to the removal of hydroxy and ethoxy groups from the sample, and athird one due to the decomposition of the phenethyl functionality above 430°C. The TGAdata (Figure 6.25B) are in agreement with the DSC data.The TG/MS data, Figure 6.26, suggest that a C-C bond is cleaved before the first193ABUntreated200°C300°C400°C500°C_1100PPM PPMFigure 6.23 - Room temperature 79.5 MHz 29Si MAS NMR spectra of the25115 PETES/TEOS copolymer sample heatedfor two hoursat the temperatures indicated. These spectra were acquired using856 scans, a 42 s recycle time, and a600pulse angle. Two series ofexperiments were performed, the spectra acquiredfor those samplesthermally treated under nitrogen are shown in series A and thespectra for those samples thermally treated in air are shown in series B.600°C-50194AB—•iQO 50 0PPM PPMFigure 6.24 - Room temperature 25.2 MHz 13C CP/MAS NMR spectraof the25/75 PETES/TEOS copolymersample heated for two hoursat the temperatures indicated. These spectra wereacquired using14000 scans, a 5 msec contact time, a 3 s recycle delay and2kHzspinning speed. Two series of experiments were performed;the spectra acquired for those samplesthermally treated under nitrogenare shown in series A and the spectra for those samples thermallytreated in air are shown in series B.Untreated200°C300°C400°C500°C600°C100 50• 019510—1Cr-4-6-7-81009590fJcL)807570100 200 300 400 500 600 700800 900 1000Temperature (°C)Figure 6.25 - (A) DSC and (B) TGA analysiscurves for the25/75 PETES/TEOS copolymerobtained under nitrogen.ATemperature (°C)B196Si-C bond. Phenyl fragments are observed first, followed veryclosely by ethyl fragments,but the phenethyl fragments reach a maximum much later (at temperatures greater than800°C). The TG/MS data therefore implies that in thePETES/TEOS copolymer the weakestbond in the phenethyl functionality is the SC-CE bond between the phenyl group and theethyl chain.1971 .8E-07 9.OE-081 .4E-076OE-08a..aa.17.OE-085.OE-083.OE.081 .OE-089.0EO77.0E-075.OE-073.OE-07I .OE-07Figure 6.26 - TO/MS data for the 25/75 PETESIFEOS copolymer samplewith the identifying mass number for each curve.—m/z15----m/z17mJz26niJz291.OE-07a.2.OE.08.__;:/i0 100 200 300 400 500 600 700 800Temperature (°C)3.5E-08&2.5E-08a1.5E-085.OE-090Temperature (SC)0 100 200 300 400 500 600 700 800Temperature (SC)) 100 200 300 400 500 600 700 800Temperature (°C)1986.4 CONCLUSIONSFour different organofunctionalized copolymers were studied. These organicfunctionalities showed different temperature stabilities (Table 6.3). In all cases the stabilitywas much higher in nitrogen than in air. Since “hot pressing” gels into glasses requiresthermal treatment for much shorter periods of time then used in the NMRstudy, thetemperatures summarized in Table 6.3 are, realistically, the lower limits to the highesttemperatures that might be used.Table 6.3 - The minimum limits of the thermal stabilities of different functionalizedcopolymers for thermally treatments in air and under nitrogen.SAMPLE AIR N2MTESIfEOS 300°C > 600°CPTESITEOS 400°C > 500°CETES/TEOS 200°C > 400°CPETES/TEOS 200°C > 300°COf the four functionalities studied, the methyl group was stable to the highest temperatureunder nitrogen. The least thermally stable was the phenethyl functionality which decomposedunder nitrogen at temperatures above 300°C. From the results obtained no completely generalstatement can be made about which bond is the least thermally stable.199CHAPTER 7FURTHER SOLID STATE NMR STUDIES TO INVESTIGATETHE EXTENT OF MIXING IN THE MTES/TEOS COPOLYMER GEL7.1 INTRODUCTIONThe solid state NMR data presented in Chapter 2 showed that the MTES/TEOScopolymer was “mixed. In Chapter 4, the 29Si solution NMRdata provided evidence that theMTESiTEOS copolymer synthesized under acidic conditions with excess water tended towarda random copolymer, confirming the findings of Chapter 2.This chapter provides additional structural information, from ‘H-29Si variable contacttime experiments on silica gel and MTESJTEOS copolymer samples. These experiments wereconsidered earlier but not pursued since the initial solid state NMR experiments (Chapter 2)demonstrated the difficulty of completely removing the hydroxyl functionalities thus makingthe source of ‘H polarization ambiguous for quantitative investigations of the polarizationtransfer. Variable contact time experiments were carried out on a number of MTES/TEOScopolymer gels, which had been treated with deuterated water to minimize the -OH contents,after the ‘H MAS spectra of the thermally treated sample heated to 600°C for two hours undernitrogen exhibited no observable hydroxyl proton resonance (Chapter 6). Thus it was feltthat, at least for the thermally treated sample, the results should be ambiguous and could beused as benchmark data for the other samples which were treated with deuterated water. Thecross polarization rateTCP1describes the magnetization transfer between the abundant nuclei(‘H or ‘9F) to the dilutee9Si).[1.19C] As discussed in Chapter 1,T can be determined fromthe 29Si signal intensity measured as a function of the contact time. These values areanalyzed with the goal of determining a distance range between the methyl group protons andthe non-functionalized silicons. These data provide further evidence for the extent of mixing200in the MTES/TEOS copolymer.The complications from the possible presence of hydroxyls in theMTES/TEOScopolymers, are absent when ‘9F is the polarization source anda system containing ‘9F in theorganic functionality was studied. TheCF3CH2Si(OMe)/TEOS copolymerwas chosenbecause it contained fluorine and it had a single probe group, i.e. -CF3,comparable instructure to that in the MTESITEOS copolymer, i.e.-CH3. Together, these twocomplementary studies provide sufficient evidence fora conclusive statement to be madeabout the extent of mixing in these copolymer gel samples.2017.2 EXPERIMENTAL7.2.1 Samples Used in the ‘H-295i Contact Time ExperimentsA control sample of unfunctionalized silica gel was synthesizedas described inpreparation 2.3.1.2, and dried at 120°C for several hours in orderto remove any adsorbedwater.Four copolymer samples were synthesized as described in preparation 2.3.3.3 with thefollowing treatments aimed at minimizing the concentration of hydroxyl groups in order toobtain informative data from the ‘H-29Si contact time experiments. A10/90 MTES/TEOScopolymer sample was washed several times with 99% D20 and stored in a D20 saturatedatmosphere. The other three samples were 25/75 MTESITEOS copolymers. The 25/75MTESITEOS copolymer subsequently used in the series of 2D experiments was soaked inD20 six times, for several days each time, and subsequently dried at 120°C under vacuum.The second 25/75 MTES/TEOS copolymer was synthesized using D20 rather then water,washed several times with D20, dried at 120°C under vacuum and stored in a D20environment. The third 25/75 MTES/TEOS copolymer sample was soaked in D20 and driedseveral times, after which it was thermally treated at 600°C under nitrogen for four hours.This procedure was based on the results of Chapter 6 which demonstrated that a very efficientway to reduce the hydroxyl group concentration was through high temperature treatment(600°C under nitrogen).7.2.2 Samples Used in the 19F-29Si Contact Time ExperimentsTheCF3CH2Si(OMe)/TEOS copolymer was synthesized following the procedureoutlined in preparation 2.3.3.3. Two 25/75CF3CH2Si(OMe)ITEOS copolymer sampleswere studied. One of these was thermally treated under nitrogen for 2.5 hours at 300°C. ThethirdCF3CH2Si(OMe)/TEOS copolymer sample contained a smaller percentage of the202functionalized monomer (approximately 10%).7.2.3 Variable Contact Time ExperimentsThe cross polarization (CP) experiment pulse sequence wasillustrated in Figure 1.6and discussed in detail in section 1.5.3. A ‘contact time’ experimentinvolves acquiring aseries of CP spectra as a function of the contact time. An exampleof the 29Si signalintensities versus contact time is shown in Figure 7.3. The curves grow exponentially withthe time constant This exponential growth overlaps with an exponential decay of thespin-locked magnetization T1 (‘H or 19F depending which is the sourcenucleus for the crosspolarization experiment). Therefore the fitting of each curve depends on three variables, M0(the maximum signal intensity which would be obtained with an infinitely long T1(29Si)andT, (1H or ‘9F)),T and T, as described by equation (1.17).One of the 25/75 MTES/TEOS copolymer samples was used to acquire a series of 2D‘H-29Si heteronuclear (CP) correlation experiments at different contact times. The pulsesequence for the 2D ‘H-29Si heteronuclear correlation experiment was given in Figure 1.8.The advantage of the set of 2D experiments is that the interactions from the hydroxyl andmethyl protons can be clearly distinguished. The disadvantage is that they require substantialspectrometer time, one 2D experiment took about 16 hours. Thus, the complete set of datapoints shown in Figure 7.7 took 10 days in total to acquire.All the ‘H-29Si and 19F-29Si contact time experiments were carried out on a BrukerMSL 400 MHz spectrometer. In order to synthesize the ‘9F frequency some hardwaremodifications had to be carried out. The schematic for these modifications is given inAppendix 3. Further experimental details of the acquisition parameters are given in the figurecaptions.2037.3 RESULTS and DISCUSSION7.3.11H-29Si Contact Time Experiments7.3.1.1 Analysis of the Contact Time ResultsA typical set of spectra obtained from a‘H/29Si variable contact time experiment areshown in Figure 7.1, for the thermally treatedMTES/TEOS copolymer. To obtain the relativepeak intensities for such a data set, the Bruker deconvolution program “Glinfit”was used.This program does not allow for referencing between spectra, therefore all theintensities werereferenced to the total area(AOJ of each spectrum:I=____*A01 (7.1)where I is the intensity of the deconvoluted signal referenced to the spectra with themaximum intensity, L, is the deconvoluted area obtained from “GlinfiC’ and El1 is thesum ofthe deconvoluted areas for all of the peaks in one single spectrum.The decay, of the Si(OH)(OSi)3curve for the silica gel sample and the CH3Si(OSi)curve for the MTES/TEOS copolymer samples, was fitted to a single T, (‘H). These werechosen because there were clear decays for these resonances which covered a substantial timeinterval.To determine the 29Si peak intensities as functions of contact time were fitted toequation (1.17), using a non-linear least squares fitting program available in the“Mathematica” software, keeping the T, (1H) value fixed.204Si(OSi)490Figure 7.1 - The ‘H-29Si CP/MAS NMR spectra asa function of the contact timefor the thermally treated 25/75MTES/TEOS copolymer sample.CH3Si(OSi)6650352014105I II I —.——60 —eo—o —120PPM31 Contact time (msec)2051— ttTTM(t) = M0*1CO1*(e“— eP)(1.17)- T1 (‘B)As discussed in section 1.5.3, the cross polarization time constant(T)is proportionalto the inverse of the sixth power of the distance (r6) between the 1H (the polarization source)and the dilute 29Si nuclei. In the case of pure silica gel and theMTESITEOS copolymersamples, the calculation of the distance is simplified since there are silicons within thesample, such as ESi-OH and Si-CH3,that are connected to a polarization source over twocovalent bonds. The distance to these protons can be calculated using literature data on bondlengths and bond angles [7.6-7.8]. Therefore, instead of solving equation (1.18) explicitly, thedistance for1H-29Si (Sitis a non-functionalized fully condensed silicon, refer to Figure 7.2)can be calculated from a ratio of two cross polarization rates, equation (7.2), one of which isdetermined by this known distance.6r=(7.2)6cp8BThe subscripts A and B refer to the two different silicon environments in the sample, one ofwhich is the silicon with an already defined distance to the methyl or hydroxyl protons. TheSaCH3(Sais a methyl functionalized fully condensed silicon, refer to Figure 7.2) andSib-OH (Sibis a non-functionalized silicon with one hydroxyl group attached, refer to Figure7.2) distances, calculated using literature values for bond angles and lengths, are shown inFigure 7.2. The values were evaluated from fitting of the contact time curves and the 1H-29Si, distances were then determined from equation (7.2). For future comparison, Figure 7.2gives the calculated1H-Sidistances using literature values for bond lengths and angles.206H‘ISi(OSi)40‘“ 2.2-2.4 AI ,Sib(OSi)3H02.3-4.4 AI”0HSiI ISa0o /\4_.5.0-5.3 A0SibCSCStrans-trans03.0-5.0 ALH4.SicC,’cII“2.6-2.7 AI, “Si(ISia(OSi)3+4.85.3 Acis-cisSictrans-transFigure 7.2 - The definition ofSia, Siband Si and the distances calculatedusing literature data [7.6-7.8].2077.3.1.2 Pure Silica Gel Contact Time ExperimentsThe‘HO-Sib(OSi)3distance was calculated from the literature data to be 2.2-2.4 A(Figure 7.2). The values for Si(OH)(OSi)3and Si(OSi)4were determined from fitting therespective contact time curves and are summarized in Table 7.1. Usingthe above informationand equation (7.2), the ‘H to29Si(OSi)4distance for silica gel was calculated to be 2.7-2.9 A.These results are in good agreement with those previously publishedby Maciel et al.[l.20]Using literature bond lengths and angles[7.6-7.81, the shortest calculated distancebetween a hydroxyl proton to a neighbouringSiDresults from a cis-cis configuration, 2.3-4.4A (Figure 7.2). Distances calculated from cross polarization experiments are heavilyweighted in favour of the shortest distances to the polarization source due to the r6 distancedependence. As expected, the determined1H-Sidistance lies at the lower end of the range ofdistances defined by the possible configurations.In conclusion, when the hydroxyl groups are the main source of polarization transfer,the calculated ‘H-Si, distance should lie in the range of 2.7-2.9 A.7.3.1.3 MTESITEOS Copolymer Contact Time ExperimentsThe MTES/TEOS copolymer contains principallyCH3Sa(OS)andSi(OSi)4silicons.The methyl protons are the closest and therefore the dominant source of cross polarization toSa.Therefore it is reasonable to fit theCH3Sa(OS)contact time data considering only asingle value, equation (1.17).The T, (‘H) and values for CH3Si(OSi),for the different MTESiTEOS copolymersamples studied, determined by fitting the variable contact time data as described above areconsistent with one another, Table 7.2.208Table 7.1 - The values from the non-linear least squares fitting of the contact timecurvesfor the indicated silicons for the pure silica gel sample.Silicon signal (msec)Si(OH)(OSi)3 8Si(OSi)4 27Table 7.2 - The T1 and values from the non-linear least squares fitting of theCH3Si(OSi) contact time curves for the samples indicated.Sample T1(1H) (msec)(msec)*25/75 MTES/TEOS copolymer thermally treated 136 3-4for four hours at 600°C25/75 MTESII’EOS copolymer washed with D20 153 4-510/90 MTES/TEOS copolymer washed with D20 283 5-6*The range in the values results from fitting the data with and without fixing the M0parameter in equation (1.17).209Each of theSi(OSi)4contact time curves for all the MTES/TEOS copolymer samplesshowed only one maximum with a relatively slow growth of the magnetization (longTherefore they were initially fitted with a single value, while fixing the T1 value to thatdetermined from the decays of the respectiveCH3Sa(OS)curves. The results aresummarized in Table 7.3 and the experimental data together with the calculated curvesobtained from the fitting procedure are given in Figures 7.3-7.5. Figures 7.3-7.5 clearly showthat the calculated curves (solid line) obtained with a single value are in very goodagreement with the experimental data. Since the sample is amorphous a range of valuesmight be expected. However, the average distance range between the probe group (-CH3)andSiis narrow enough that the contact time curve is well characterized by a singlevalue, implying that the methyl groups are evenly distributed throughout the copolymermatrix.For comparison purposes and for completeness, two T0 values with different weightedcontributions, equation (7.3), were considered.1____-7LM(t) = M*[X*COl*(eT1 (H)— e)l_ 1T T1 (1H)(73)1_tI-1L-+ (1 —X)*_____________________*(eT1, (H)— ep2)]1_ 1T2 T1(111)It is expected that the fit to the experimental data should improve since the number ofparameters in the fitting process has increased from two(Tand M0) to four(T1,therelative proportions (X) and M0). The weighted contributions considered were 10/90, 20/80,30/70, 40/60 and 50/50. The best results from these weighted fittings are plotted in Figures210Table 7.3- values derived from fitting the Si(OSi)4contact time curves for thedifferent samples indicated, using equation (1.17) and fixing the T1value (Table 7.2).(msec)*weightedsingleSample20% 80%25/75 MTES/TEOS copolymerthermally treated 23-30 6 38-41D20 washed 19-21 3 2710/90 MTES/TEOS copolymer 30-41 4 50-51*The range is obtained from fitting the contact time curve with and without M0 as avariable in the fit. In the cases when M0 was not a variable, it was fixed to the average M0value obtained from the previous fits.21115>.I.)..Contact time (msec)Figure 7.3 - Contact time curves for theCH3Sia(OSi)afldSi(OSi)4silicons in the thermally treated 25/75 MTESR’EOScopolymer together with the calculated curves obtained withequation (1.17) (solid lines) and equation (7.3) (dashed lines)and the parameters given in Tables 7.2 & 7.3.CH3Sia(OSi)I50 20 40 60 80 100Contact time (msec)100%-20%Ti& 80%T2100%Si(OSi)40 20 40 60 80 10021220 -CH3Sia(OSi).:100%0 20 40 60 80 100Contact time (msec)40Si(OSi)4• 3020100%T: 1O2O%Tc:1&8O%TcP:Contact time (msec)Figure 7.4 - Contact time curves for theCH3Sia(OSi)afldSi(OSi)4silicons in the D20washed 25/75 MTES/TEOS copolymersample together with their calculated curves obtained usingequation (1.17) (solid lines) and equation (7.3) (dashed line)and the parameters given in Tables 7.2 & 7.3.213cl• —Contact time (msec)Figure 7.5 - Contact time curves for theCH3Sia(OSi)andSi(OSi)4silicons in the D20washed 10/90 MTESII’EOS copolymersample, together with their calculated curves obtainedusing equation (1.17) (solid lines) and equation (7.3)(dashed line) and the parameters in Tables 7.2 & 7.3.86CH3Sia(OSi)4200 20 40 60 80Contact time (msec)10040• 100%To100%TSi(OSi)4201000 20 40 60 80 1002147.3-7.5 (dashed lines). A comparison of the calculated curves (solid and dashed lines) inFigures 7.3-7.5 demonstrates that the consideration of two values instead of one results inonly a marginal improvement to the fits.Substituting the data in Tables 7.1-7.3 and using theCH3Sja(OSj)calculated distanceinto equation (7.2), the methyl ‘H toSi(OSi)4distance was calculated for the differentsamples. These results are presented in Table 7.4.7.3.1.4 MTESITEOS Copolymer: Series of 2D Heteronuclear Correlation ExperimentsThe series of fifteen 2D experiments were processed slightly differently from theprevious data sets discussed. In this case, the 2D data was processed with respect to acommon reference so that comparable intensities were obtained. The “volumes” of the crosspeaks between the methyl protons and the different silicon resonances were determined andplotted as functions of the contact time. An example of the 2D 1H-29Si heteronuclearcorrelation contour plot obtained (which corresponds to one data point in Figure 7.7) is shownin Figure 7.6. The T, (‘H) was determined to be 67 msec from theSacontact time curve asdiscussed previously. The for this curve was then determined to be 9-10 msec and that oftheSi(OSi)4cross peak was 18 msec, Figure 7.7. Using these data and equation (7.2), the ‘HtoSidistance was calculated to be 2.9 A for this sample.215Table 7.4 - ‘H toSi distances calculated for the MTES/TEOS copolymer samples, usingequation (7.2), a‘HSa distance of 2.6-2.7 A, and the CH3Sa(OSj) and Si(OSi)4values given in Tables 7.2 and 7.3, respectively.Distances in (A)Calculated ‘H toSi(OSi)4distances (A) forweightedthe copolymer samples indicated:single20% 80%25/75 MTES/TEOS thermally treated 3.5-4.0 2.8-3.0 3.8-4.225/75 MTES/TEOS washed with D20 3.3-3.6 2.4-2.6 3.4-3.710/90 MTESiTEOS washed with D20 3.4-3.8 2.4-2.6 3.7-4.0216S i(OH)(OSi)3Si(OSi)4CH3Si(OSi)___2040• •PPH0 —50 —iOO29Si chemical shift (ppm)Figure 7.6 - 2D 1H - 29Si heteronuclea correlation contour plot of the25/75 MTES/TEOS copolymer,with the rectangle indicatingthe “volume” of the cross peaks usedin the analysis of the set of2D data. There are 300 scans per experiment acquiredusing a 10 msec contact time, a 3 second recycle delayand 4 kHzspinning speed. This spectrum consists of 64 experiments.Theprojections are shown along the Fl and F2 axis.21750CFigure 7.7 - Contact time curves for theCH3Sia(OSi)afldSi(OSi)4silicons in the 25/75 MTESiTEOS copolymer samplefrom 15 2D1H-29Si heteronuclear correlation experiments,together with the calculated curves.CC—CH3Sia(OSi)T1 = 67 msec0 20 40 60 80 100Contact time (msec)100Si(OSi)460402000 20 40 60 80 100Contact time (msec)2187.3.1.5 DiscussionTheSi(OSi)4contacttime data for the different MTESITEOS copolymers were fittedslightly better when two weighted cross polarization rates wereused (Figures 7.3-7.5). Tn theweighted fitting of the contact time curvesthe larger value was in all cases very similar tothat obtained from fitting the curve with a single value. Theimplication is that although arange of cross polarization rates would be expected due tothe amorphous nature of thecopolymers, the methyl groups are distributed evenly enough throughout the whole polymermatrix that the average distance range between a probing group (-CH3)and the fullycondensed quatemary silicons(Si(OSi)4),is narrow, providing further evidence that themonomers in the MTES/TEOS copolymers are randomly mixed.The ‘H to Si distance was determined using: equation (7.2), aSaCH3distance of2.6-2.7 A,CH3Sa(OSj)values in Table 7.2 and theSic(OSi)4dominant values listed inTable 7.4. The distance range determined for each sample is:1)25/75 MTES/TEOS copolymer thermally treated 3.5-4.2 A2)25/75 MTES/TEOS copolymer - washed with D20 3.3-3.7 A3)10/90 MTESIrEOS copolymer 3.4-4.0 A.All these distances fall within the range calculated for a methyl proton to Si (3.0-5.3 A) andthey exclude the distance range of 2.7-2.9 A which the study of pure silica gel demonstratedwas the distance when hydroxyl groups are the principal source of polarization. Therefore itis concluded that the methyl groups are the principal source of polarization transfer toSi(OSi)4in the MTES/TEOS copolymer samples.In addition, if the hydroxyl protons were the principal source for cross polarizationthen the calculated distances should be dramatically different from sample to sample due tothe different hydroxyl concentrations. The distances determined from the contact time data of219all three samples are all within the same range.Therefore the principal source of cross-polarization to Si must be the methyl groups which are evenlydistributed throughout thecopolymer. An implication of this is that at theseconcentrations of ESaCH3,all the Sinuclei are close to one or more -CH3 groups. Thereforein a cross polarization experiment alltheSinuclei should be cross polarized.Little ambiguity exists in the data from theset of 2D experiments since the seconddimension differentiates the two cross polarization sources. The fact thatthese experimentsalso gave a narrow‘H-Sitdistance range, of approximately the same magnitude, providesfurther evidence that the methyl groups are distributed throughoutthe copolymer matrix.All the ‘H toSi(OSi)4distances strongly suggest that the two monomers areintimately mixed at a molecular level, confirming the previous conclusionsof Chapter 4 thatthe methyl functionality is distributed throughout theMTESITEOS copolymer matrix, i.e. themonomers do not form a phase separated copolymer.The 29Si intensities (M0 values) obtained from fitting the contact time data provideinformation with respect to the proportion of silicons in different environments that are crosspolarized. These data compare well with the quantitative 1D spectrum in which the relativesignal intensities are indicative of the actual proportion of different silicons present, Table 7.5.These results suggest that essentially all of theSi(OSi)4are cross polarized by the methylprotons, supporting the above conclusion that the two monomers are intimately mixed in thecopolymer.220Table 7.5 - The percentage ofSi(OSi)4andCH3Sa(OS)which are cross polarized(determined from the fitting of the variable contact time curves) together with theactual percentages that exist in the thermally treated 25/75 MTES/TEOScopolymer sample (determined from a quantitative 1D 29Si NMR spectrum). Notethat the quantitative 1D 29Si NMR spectrum contains a small signal forSi(OH)(OSi)3which is not included in this table, so the concentrations do notsum to 100%.29Si resonances M0 (fromfit)*29Si NMRspectrum**CH3Sa(OSj)(18 ± 2)% (19 ± 2)%Si(OSi)4(73 ± 2)% (75 ± 7)%*The errors given are estimates obtained by fitting the contact time data with a range of T1values.**error estimated to be 10%.2217.3.2 19F-29Si Contact Time ExperimentsTo complement the ‘H-29Si cross polarization studies,‘9F-29Si contact time experimentswere carried out on another functionalized silane copolymer containing ‘9F. Theseexperiments provide complementary data that are not compromised by the possible presenceof hydroxyl groups.7.32.1 Simplification of the Expression for the TFPTMSITEOS CopolymerTheCF3CH2Si(OMe)/TEOS copolymer (TFPTMS/TEOS), unlike theMTES/TEOS copolymer, does not contain an internal reference, i.e. an unambiguous ‘9F toSladistance, that can be used for the calculations. Therefore the internuclear distancebetween 19F andSi(OSi)4(Figure 7.8) cannot be calculated using equation (7.2), as was donepreviously. Instead, the 19F to Sit, distance must be calculated explicitly using equation (1.18),which means that the homo- and heteronuclear second moments for the ‘9F and 29Si nucleimust be evaluated.7.32.1A 19F Second Moment ConsiderationsThere are three possible contributions to the total second moment of 19F: dipolarinteractions between the ‘9F nuclei in a single -CF3 group, dipolar interactions between the ‘9Fnuclei in two different -CF3 groups and dipolar interactions between the ‘9F in the -CF3 groupand the 29Si nuclei. The dipolar interaction has a hr6 dependence and therefore the furtherapart the two dipoles are, the weaker the interaction. Consequently, the conthbutions to thetotal second moment of ‘9F can be simplified. The dipolar interaction between two ‘9F in twodifferent -CF3 groups is expected to be significantly smaller interaction than that of two ‘9F inthe same -CF3 group for two reasons. First, the two ‘9F are much further apart when locatedon two different -CF3 groups and secondly, the motion of both -CF3 groups will furtherreduce the dipolar interactions between them.[1.22] Thus the interaction between ‘9F in two222= CF3TFPTMS/CH2CH2Sia(OSi)3_.Si(OSi)4I“ Distance being determined.CH2Sia(OSi)3Figure 7.8 - The definition ofSiaand Si for the TFPTMS/TEOS copolymersamples.223different -CF3 groups will not be considered in the homonuclear second moment calculations.The magnitude of the dipoiar interaction between the ‘9F ina -CF3 group and a 29Si nucleus isinsignificant compared to the dipoiar interaction between two 19F in the same CF3 group,principally due to the low natural abundance of 29Si (4.7%), the low 29Si magnetogyric ratioand the large distances involved. The above arguments reduce the defining interactions of thetotal second moment for 19F to only one term, the dipoiar interaction between the 19F in asingle -CF3 group averaged over the motion of the group. The total second moment is in factinvariant to the presence of motion, but the measured second moment is not because part ofthe resonance is shifted far from the center of the resonance where it is hidden in the noiseand consequently not measured.[1.lO, 1.35, 7.5]The TFPTMS/TEOS copolymer is a powder sample, with motion around the -CF3three-fold axis and possible motion within the chains of the polymer. In the second momentcalculation, equation (1.20), the term (3e1-1), is therefore first averaged over the motion toobtain the second moment for a single orientation of the group. Then the average over allpossible orientations is calculated. [1.221Equation (7.4) is the general equation for the homonuclear second moment whenconsidering motion. [1.22]= 3 (YS__O)2S(S + 1)*SSrot(7.4)[3cos2 — 112[3cos29— 1122r..The -CF3 group in the TFPTMS1TEOS copolymer rotates about its C3 axis. It hasbeen shown by Pake that for the special cases where the rotation axis is perpendicular to theinternuclear vector (ct = 90°), as is the case for the -CF3 group, the intramolecular second224moment for a powder is reduced to one-fourth that of the rigidlattice yielding equation(7.5).[1 .22]= 32S(S + 1) (3cos2O— 1)22562iiThe sum is over all relevant nuclei i when consideringthe jth nucleus. In the examplebeing studied, S refers to ‘9F (spin of ½) and the three19F atoms forming an equilateraltriangle, giving= 2rF6.The -CF3 groups can have any orientation with respectto the external magnetic field.The second moment is thus averaged over all possibleorientations [1.22],i.e. <(3cos2O-1)>=4/5 (where <..> means average value),simplifying the homonuclearsecond moment equation when considering motionto equation (7.6).9y42 2= F(7.6)6402ror in terms of field:<j2>= 9(F2(7.6B)160rFF7.32.1B 29Si Second Moment ConsiderationsThe other second moment to be calculated is that of the 29Si nuclei. There are twodifferent clipolar interactions which contribute to the total second moment of 29Si: dipolarinteractions between two 29Si and that between 19F and 29Si. The dipolar interaction betweentwo 295i is negligible in comparison to the latter term due to the low natural abundance of 29Si225(4.7%), with correspondingly large distancesbetween two 29Si nuclei. Consequently, the total29Si second moment is calculated considering only the heteronucleardipoiar interactionbetween the abundant ‘9F nuclei in a -CF3 group and the 29Si nuclei,equation (1.21).The poiar coordinates, i.e. 9 and r (Figure 1.7) in theheteronuclear second momentfor a rigid lattice, equation (1.21), must be averagedover all space when considering motion,equation (7.7).—y221121(1 + 1),(3cos2ejm— 1)2\IS rot— 48n2r.,?6/To evaluate equation (7.7), <(3cos28-1)2r6>must be evaluated. The <(3cos2e-l)2r6>value is:/(3COS2Oim— 1)2\ = 1P_:lFr-6(3cos29— 1)2r,,,,/(1,q)2i,mim+ 4(3sinejm cosejmcosjm)2 + 4(3sineim cosOjmsin4jm)2(7.8)15 15+ (3sin2ejm sin2jm)2+(29jmC0S2Ø.)215 15iiThe derivation of equation (7.8) is given in Appendix 4. In equation (7.8),prepresents thenumber of equivalent sites that the S (19F) nucleus can occupy over the motion and qrepresents the number of equivalent sites that the I(29Si) nucleus can occupy over the motion.The evaluation of equation (7.8) for the amorphous TFPTMS/TEOS copolymer willconsider the -CF3 group at the center of a sphere whereSi(OSi)4can be located anywhere onthe surface of this sphere. Therefore the angles, O andØ..,in equation (7.8) are averagedover all space. The distance calculated is well approximated by the distance fromSi(OSi)4to226the center of the -CF3 group, thusEr16 = 3rSI6. With additional substitutions of I=½, p=3and q=1, the final form for the heteronuclear second moment includingmotion is equation(7.9).= YF YSI2l2IS240i2r.6Si-F(79)<2>= 2 2Is60rS.F7.32.1C Geometrical Factor in Equation (1.18)The last term to be evaluated in equation (1.18) is the geometrical factor C1, equation(1.19). Consider once again the 29Si nucleus and the three ‘9F nuclei in the CF3 group. Theterm in equation (1.19), defined by equation (1.19A), is independent of the relative 19F toSipositions. The terms b andbjmin equation (1.19), defined by equation (1.19B), areaffected by the relative orientation of the 19F to the 29Si.In the case thatSiis located anywhere on the shell of a sphere where the -CF3 groupis in the center, the terms in equation (1.19) involving b and b must be averaged over allspace:=y2 W<(13cos29jm)>(7.10)24’y22<bim> =________227andF2(1—3cos2O.)(1—3cos29.) 12<(2b.+ 12)2>=2 2h4____________+jmim ‘jm2 2 4[<(1—3cos2eim)>6(7.11)jTim2<(1 —3C0S2O. )(l—3cos2e.m)><(13cos2Om)> 1+r•3 r•3+r.6tm jrn jmAs already discussed, the ‘9F to Si distance being calculated is that to the middle of the -CF3group; thereforer6=rjmande=ejm,and since <(3cos201111-1)>=4/5, equation (7.11)simplifies to:28y2 y2<(2b. +b)2>=I Sim jm65 .rjm(7.12)567272 4+ b.)2+ (bim+ 2b.)2>= S5 rEquation (1.19) can be re-expressed in terms of the average values of the individualterms. Then by substituting equations (7.10) and (7.12), and N5=3 into equation (1.19),equation (7.13) is obtained for C1.228=___________________________________(7.13)..C=3itcIs=The equation to determine the 19F to Sit, distance(rSIF),equation (7.14), in terms ofknown parameters:rFF(the‘9F-’Fdistance within the CF3 group) and is obtained, bysubstituting equations (7.13), (7.9) and (7.6) into equation (1.18)= 6 s(7.14)rS.FI6/1057.3.2.2 Analysis of the‘9F-29Si Contact Time Experimental ResultsA typical set of spectra obtained from the 19F-29Si contact time experiment is shown inFigure 7.9. As before, this series of 1D spectra was Fourier transformed, integrated relativeto each other, and the relative peak intensities calculated using equation (7.1).The‘9F-’F distance in a -CF3 group was determined to be 2.16 A using atrigonometric calculation and literature values [7.8] for the F-C-F bond angle and SC-F bondlength. The contact time curves and the fits obtained for one sample are shown in Figure7.10. The T1 (‘9F) value for each sample was determined from the decay of theCF3CH2Sja(OS)silicon intensity as a function of contact time, as described previouslyfor the MTES/TEOS copolymer since this resonance shows the clearest decay. The values2 2<a><bim>N8<a><(2bjm+12)2+(b + 2b)2>N8<(212jm+b.)2+(b+2b.)2>229were determined from the non-linear least squares fitting of theSicontact time curve withequation (1.17), keeping T1(19F) fixed. A summary of the T1(‘9F) and values are givenin Table 7.6. TherSIF distances for the different 29Si environments for each sample given inTable 7.7 were calculated using equation (7.14), therFFdistance and the data given inTable 7.6.230Si(OSi)3(OH)Si(OSi)4CF3CH21 —- -6 -85 -11029Si chemical shift (ppm)‘60403025201510-35 -60I5Contact time (msec)-110-8529Si chemical shift (ppm)Figure 7.9 - 19F-29Si CP/MAS spectra for the thermallytreated 25/75CF3CH2Si(OMe)/TEOScopolymer sample.2315a).-a)a)a)>.a)Contact time (msec)= 17 msec= 17 msecContact time (msec)Figure 7.10 - Contact time curves for theCF3CH2Sia(OSi)andSi(OSi)4silicons in the thermally treated TFPTMSITEOScopolymer sample, together with the calculated curvesobtained using equation (1.17) and the data in Table 7.6.432CF3CH2Sia(OSi)I= 13 msec0 20 40 60 800 20 40 60 80232Table 7.6 - The T1 and values for the 29Si resonances in differentTFPTMS/TEOScopolymer samples. The T1 values were determined by fitting the decayof the CF3”Si(OSi) contact time curve and the values by fitting the indicatedSi signal contact time curve. The T1 was fixed when the values weredetermined.Samples 29Si resonance T1(msec)* (msec)*25/75 TFPTMS/TEOS CF3Sia(OSj)313 ± 147 ± 5Si(OSi)417 ± 2Thermally treated 25/75TFPTMSIrEOS**CF3Sia(OS)310 ± 165 ± 7Si(OSi)411 ± 110/90 TFPTMSITEOS CF3Sia(OS)315 ± 245 ± 55ja(05j)430 ± 3error estimated to be 10%.**Thermally treated at 300°C under nitrogen for 2.5 hours.233Table 7.7 - Distance between29Si(OSi)4and the -CF3 functionality (A) obtained usingequation (7.14),rFF= 2.16 ± 0.01 A and the data presented inTable 7.6.Samples r ± 0.1 A25/75 TFPTMS/TEOS 3.2Thermally treated 25/75 TFPTMS/TEOS 3.010/90 TFPTMS/TEOS 3.5*The T1 value was obtained from fitting the decay of theCF3”Sa(OS)contact time curve.2347.3.2.3 DiscussionUsing equation (7.14), therFFdistance and the values given in Table 7.6, the 19F-Sidistance was determined for each TFPTMSITEOS copolymer sample. As expected, thecalculated distances are similar for all three samples. TheSi(OSi)4contact time curves arefitted well with only a single value. This implies that although the copolymer isamorphous, the probe group (-CF3)is well distributed throughout the copolymer matrix suchthat the average distance from a -CF3 group to aSi(OSi)4lies within a narrow range. This isa similar situation to that observed for the MTES/TEOS copolymer samples investigated.To determine the relative concentrations of the silicons in different environments andto confirm the validity of the assumed approximations, quantitative 1D 29Si NMR spectra forthe 25/75 and 10/90 TFPTMS/TEOS copolymers were acquired. The relative concentrationsofSaandSiwere compared to the M0 values from the contact time curves, Table 7.8. Therelative concentrations obtained by the latter method are an indication of the percentage of theSi(OSi)4taking part in the cross polarization process. In both TFPTMS/TEOS copolymers,most of theSi(OSi)4arecross polarized, supporting the conclusion that functionalized silicagels synthesized using the copolymerization preparation have the functionality evenlydistributed throughout the copolymer matrix.The 1H to Sic, distances calculated for the different MTES/TEOS copolymers were inthe range of 3.3-4.2 A. The ‘9F toSidistance calculated for the different TFPTMSITEOScopolymers is in the range of 3.0-3.5 A. The calculated distances for both sets of variablecontact time experiments are consistent with the fact that the ‘probe’ groups in bothcopolymers (-CH3 in the MTES/TEOS copolymers and -CF3 in the TFPTMS/TEOScopolymers) are well distributed throughout the copolymer matrix. These probe” groups areevenly enough distributed that the average distance from any probe group toSi(OSi)4lies235Table 7.8 - The percentages ofSi(OSi)4andCF3Sla(OSj) which are cross polarized(determined from the fitting of the contact time curves) and the actualpercentages that exist in the samples indicated (as determined from quantitative1D 29Si NMR spectra).Sample 29Si resonances M0 (from fit)*29Si NMR spectrum**25/75 TFPTMS/TEOSCF3”Sa(OS) (23 ± 1)% (21 ± 2)%Si(OSi)4(44 ± 1)% (48 ± 5)%10/90 TFPTMS/TEOSCF3Sa(OS)(13 ± 3)% (8 ± l)%Si(OSi)4 (56± 3)% (65 ± 7)%*estimated errors obtained by fitting the experimental contact time data with a range of T1values.**error estimated to be 10%236within a relatively narrow distance range.2377.4 SUMMARY OF CONCLUSIONS‘H-29Si contact time measurements were perfonned on fourMTES/TEOS copolymers.The fitting of the silicon intensities as functionsof contact time for the different samplesprovided values. The methyl proton to Sit, distances were determined usingthesevalues and the knownSaCH3distance.The ‘H to Si distances for all the samples were inthe range of 3.3-4.2 A.‘9F29Si contact time measurements were performed on threeTFPTMSIFEOScopolymer samples. values were determined for Si,as described above. The -CF3 to Si,distances were calculated for all of the samples using the Si values and the knownrFFdistance in a -CF3 group. The 19F toSidistance for all the samples was in the distance rangeof 3.0-3.5 A.The estimated distance range for -CH3 to Si and -CF3 toSiare relatively narrow andthe results are very similar for all seven copolymer samples studied. The determineddistances, their narrow distributions and the fact that essentially all theSi(OSi)4nuclei werecross polarized in the samples, provide conclusive evidence that in all cases the functionalityis evenly distributed throughout the copolymer matrices.238CHAPTER 88.1 SUMMARYThe principal goal of this thesis was to structurally characterize a particular class offunctionalized silica gels using NMR spectroscopy. An alternative synthesis for thefunctionalized silica gel was introduced that results in a reproducible product and leaves thefunctionalities evenly distributed and accessible for future reactions.The integiity of the functionalities was established utilizing ‘3C and 29Si CP/MASNMR experiments. Deconvoluted quantitative 29Si MAS NMR spectra provide informationabout the relative proportions of the different silicon environments in the samples. Two-dimensional ‘H-29Si heteronuclear correlation experiments, which identify the 1H sources ofcross polarization for each silicon resonance, were used to distinguish between domainstructured and “mixed” copolymers. The 2D NMR results unambiguously demonstrate thatthe MTES and TEOS monomers in a MTES/TEOS copolymer are “mixed” and not “phaseseparated”.To quantify the extent of mixing in the MTES/TEOS copolymer the kinetic constantsfor the hydrolysis and dimer formation were determined. Both homopolymer systems, MTESand TEOS, were first studied in order to obtain reference data for the more complex kineticsof the MTES/TEOS copolymerization.The TEOS hydrolysis and dimer formation reactions were investigated first to providereference data, so that the effect of such variables as: an organic functionality (Chapter 4), thepresence of two different monomers (Chapter 4), and the presence of organic drying agents(Chapter 5) on the hydrolysis and dimer formation rate constants could be determined. Thekinetic constants for all the sequential hydrolysis and dimer formation of the TEOSpolymerization were calculated from the experimental data for a series of pH values, using a239simple kinetic model. The kinetic model considered only onere-esterification reaction, theone between Si(OH)3(OEt) and Si(OH)4,which provedto be essential to respect the curveshape of Si(OH)3(OEt). From the series of pH dependent kineticconstants, pH independentkinetic constants for the hydrolysis and dimer formation reactions were derived for the firsttime. The magnitudes of the hydrolysis kinetic constants(kTl)increase with the number ofhydroxyls, but they are not related in any simple incremental fashion.In the study of the MTES homopolymerization, pH dependent kineticrate constantsfor the hydrolysis and dimerization reactions were also determined.As in the TEOS case, pHindependent kinetic rate constants were also calculated for the first time. These kinetic rateconstants provide the first quantitative evidence that the MTES monomer hydrolyzes fasterthan the TEOS monomer in each sequential hydrolysis step. The homodimerization rateconstant, however, was found to be slower for the MTES than for the TEOS monomer. As inthe TEOS case, the kinetic model had to include the last hydrolysis asan equilibrium reactionin order to respect the shape of the CH3Si(OH)2(OEt) relative concentration curve.The kinetic constants determined for the MTES and TEOS hydrolysis and dimerformation reactions were used as the starting point for the analysis of the MTESITEOScopolymer kinetic data. The kinetic constants for theMTES/TEOS copolymer hydrolysis anddimer formation reactions were found to be somewhat different from those of thehomopolymerizations, due at least in part to the unavoidable differences in reaction mixturecompositions. The TEOS monomer, in comparison to the MTES monomer, was moreaffected by the presence of the other type of monomer. Reactivity ratios of 0.8 and 1 werecalculated from the dimer formation rate constants for the MTES and TEOS monomers,respectively. These reactivity ratios indicate that the MTESITEOS reaction mixture tendstoward forming a random copolymer. Altering the relative monomer proportions and the240water/(total silane) ratio had little effect on the relative dimer ratios, suggesting that theMTES/TEOS copolymer has a very strong tendency to form a random copolymer.The final step in the characterization of theMTES/TEOS copolymer involved adetailed analysis of the ‘H-29Si CP/MAS contact time curves for a number of differentsamples with minimal -OH content in order to determine average -CH3toSi(OSi)4distances.These calculated distances confirmed that the methyl functionality was the principal source ofcross polarization and is distributed throughout the copolymer matrix. These results weresupported by a similar‘9F-29Si CP/MAS contact time study, onCF3CH2Si(OMe)jfEOScopolymer samples.One possible application of these functionalized silica gels is the production oforganofunctionalized glasses where the functionality would be evenly distributed throughoutthe copolymer matrix. Formamide is a commonly used drying agent, which prevents crackingso that large pieces of glass may be formed. The kinetic study of its effect on the hydrolysisand dimer formation of the TEOS, MTES homopolymerizations and MTESITEOScopolymerization at a water/silane ratio of 11, found that only when the formamideconcentration exceeds 20 mole % of the silane does it have a significant effect on thehydrolysis and dimer formation reactions. However, the largest reduction in the gelation timefor TEOS occurred at lower formamide concentrations. This is the first evidence that thedecrease in gelation time caused by the presence of formamide is not a consequence of achange in the hydrolysis and dimer formation rates. High resolution ‘H, ‘3C and ‘5N solutionNMR studies over a 90 days period clearly demonstrated that the majority of the formamideremained intact during the TEOS homopolymerization, i.e. no chemical bonds were formedneither to the carbonyl nor to the amide functional group. Since the conversion offunctionalized silica gels to glasses involves “hot pressing” the gel, the thermal stabilities of a241number of different functionalized copolymers were determined. Of the copolymers studied,the methyl functionalized copolymer has the highest thermal stability.242Suggestions for Future InvestigationsThe present research has established a simple kinetic model and supplied referencedata for the hydrolysis and dimer formation reactions of the TEOS and MTEShomopolymerizations, as well as that of the MTES/TEOS copolymerization. Furtherinvestigations on the effect of the functionality’s structure on the hydrolysis and dimerformation rate constants could provide further insight into the reaction mechanism.The results presented in Chapter 5 clearly show that formamide significantly reducesthe gelation time of TEOS, even at concentrations that do not affect the hydrolysis rate.Further investigations are required to determine if the reduction in the gelation time is due tothe interference of formamide with the cross-linking mechanism.Attempts at directly measuring the T, (‘H) yielded results that were not in all cases inagreement with the values obtained from the contact time curve fits. The effect of themethod of detection and of other experimental parameters, such as the spinning rate and thepulse power, should be investigated, since in some cases especially when no significant decayof the contact time curve is observed, reliable direct measurements of the T, (‘H) may benecessary.Preliminary results suggest that thermal treatment removes differences in the extent ofcrosslinking which seems to occur in samples synthesized with an acid versus thosesynthezised with a base catalyst. Further work is required to determine the effect of thermaltreatment on the final polymeric structure.Overviewing the thermal stabilities of the different functionalitized copolymers, Ipropose that for saturated alkanes the weakest bond is the initial C-Si bond and for largefunctionalities involving unsaturated groups, such as the phenyl group in the phenethylfunctionality, the weakest bond is the C-C bond connecting the unsaturated group to the rest243of the functionality. 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SpiegelMathematical Handbook ofFormulas and TablesSchaum’s Outline SeriesMcGraw-Hill Book Company,Toronto, (1968)254Appendix 1 : First approximation of the TEOS kinetic constantsAverage pH dependent kinetic rate constants (M’min’) for the reaction of TEOS determinedfrom the non-linear least squares fitting of equations (3.13)-(3.16).pHkTl kT2 kT3 kT43.35 0.00049±0.00005 0.002±0.0005 0.0091±0.0005 0.003±0.0013.04 0.00039±0.00008 0.0034±0.0004 0.022±0.001 0.03±0.022.88 0.00072±0.00002 0.004±0.0001 0.016±0.001 0.007±0.0012.76 0.00082±0.00002 0.0065±0.0005 0.04±0.005 0.0073±0.00052.55 0.0016±0.0001 0.011±0.001 0.07±0.03 0.014±0.0052.45 0.0022±0.0002 0.012±0.001 0.05±0.007 0.013±0.0012.33 0.003±0.0001 0.02±0.004 0.12±0.03 0.023±0.0022.13 0.0052±0.0002 0.028±0.003 0.1±0.02 0.025±0.003255Appendix 2 : First approximation of the MTES kinetic constantsAverage pH dependent kinetic rate constants (M’mint)for the reaction of MTES determinedfrom the non-linear least squares fitting of equations (4.9)-(4. 11).pHkMl kM2 kM3f3.35 0.0014±0.0003 0.006±0.001 0.0048±0.00023.04 0.0037±0.0004 0.015±0.002 0.009±0.00022.88 0.005±0.001 0.019±0.003 0.01±0.012.76 0.04±0.03 0.1±0.07 0.025±0.0052.55 0.03±0.01 0. 10±0.05 0.029±0.0052.45 0.0205±0.0005 0.07±0.01 0.037±0.005256Appendix 3 : Schematic of the modifications made to the Bruker MSL400SynthesizerF3Output,160.3125FREQUENCYDOUBLERRFOUT#52FlOUTPUT(6)CONSOLEspectrometer for 19F’ to 29Si cross polarization.400.13MHzF2 inSynth.RFSignal_JFILTER Attenuator]80B.P. 36dBInAMT M3205APulse AmplifierINPUTOUTPUTINOUTReceiver Console# 131HP Selective Am70-90 MHZVPREAMP400 MHzFILTER80MHzL.P.257Appendix 4 : Deduction of Equation (7.8)To solve the heteronuclear second moment in the presence of motion, equation (7.7)must be evaluated. Therefore the term shown in (1) must be evaluated over all the3COS2O — 1 12orientations of the vector r over the motion. The angle between the external magnetic field,B0, and the the internuclear vector, r11, is em,. Consider that B0 is along the z-axis of thecoordinate system.Equation (1) can be re-expressed in terms of Legendre Polynomials (P2) [7.10]:(3C0520im— 1)12— [2P2(COSO.)2(2)r. rHimFrom the addition theorem [7.4] of spherical harmonics, the following is true:— 2rDf â\,2 rn—2IkCOS)I 8it2 im=Y2,(9j,Øj,;)*(ØØ)(3)rim 5rimn—2where Y2,is an orthonormal function called a spherical harmonic, e & Ø1,and e2 &02arethe spherical coordinates (Figure A4. 1) of the internuclear vector before and after motion,respectively. Expressions for the spherical harmonics can be found in references [7.3, 7.4].i i can occupy p sites and m can occupy q sites equation (3) becomes,13 cos8 - 1 12[82y(e2,0)[_L * Y2 (9,0.)]12(4)rimn=—2pqi=1 m=1rimExpanding the Associated Legendre Polynomials one obtains:258ABaxis of rotationaxis of rotationFigure A4. 1 - Model for a nuclear pair in motion about an axis with the appropriateangles labelled forA - the homonuclear second moment equation andB - the heteronuclear second moment equation.B0eu259/[3cos2eim— 112\ —\[ 3 jIrimq1 [[8it 5qi=1 m=1[ [_31.(3cos29. — 1) (3cos28 — 1)im[8it 512+(3sin9.coseimcosOim)(3sinecosecosO)(5)[8it(3sinecoseimsinOim)(3sinO2cosocosØ)I+im[8it 5+ [....5—— (3sin2eimsin2O)(3sin2esin292)im[8it.3sin2ecos2ø)(3sin2ecos2e]21548itUn tifljThe internuclear vector orientation is described by the spherical coordinates O andØ.Aftersome motion these internuclear vectors can have all orientations, so e2 and02must beaveraged over all space4<(3cos29— 1)2> = —5<(3srne2cosecosØ)> = <(3sin92cosOsinØ)>=12<3sin2Ocos2O)>= <(3sin2Osin2Ø)>= —5Substituting these values into equation (5), the final form for <(3cos2e-1)2/r61>is:260/(3 COS2OI — 1)2r.6/1q1 [2(3coseim— 1)2226L5pqi=1 m=1rim(7.8)+ 4(3sineimcoseimcosø.)2+ 4 (3sine.cose.sinØ.)215 15(3Sifl2O. S1n29.)2(3sin2eimcos2eim) 1+ 15 + 15i261

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