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Synthesis of polystyrene / acrolein latexes and their surface characterization Le Dissez, Corinne 1994

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SYNTHESIS OF POLYSTYRENE I ACROLEIN LATEXES AN])THEIR SURFACE CHARACTERIZATIONbyCORINNE LE DISSEZB.Sc.Hons., Robert Gordon’s University, Aberdeen, 1991A THESIS SUBMITthD IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinTHE FACULTY OF GRADUATE STUDIESDepartment of ChemistryTHE UNWERSITY OF BRITISH COLUMBIAApril 1994Cormne Le Dissez, 1994We accept this thesis as conformingto the standardIn presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)__________________________________Department of____________________The University of British ColumbiaVancouver, CanadaDate 2 4.DE-6 (2/88)ABSTRACTA semi-continuous synthesis of polystyrene/acrolein latex was carried out. The optimumaddition time of acrolein monomer was determined to be 10 hours after addition of the initiator,giving a monodisperse polystyrene latex with aldehyde on the surface. A DNPH assay was usedto assay the aldehyde group in the latex suspension; it was specifically a surface assay. Itsuccessfully detected aldehydes at concentrations above 1 x 10 mol/g but was limited at highsurface concentrations of aldehyde probably due to the steric hindrance. Quantitative agreementwith respect to surface concentration of aldehyde groups was obtained using freshly solubilizedNaBT4 reduction in a radiochemical assay.NMR was used to assay aldehydes present in dissolved latex and the same result as forthe DNPH assay was obtained. NMR provided information on the environment of the surfacealdehydes, the chemical shift showing the possible presence of polyacrolein chains on the latexwith the seeded reaction. This was an important observation as the polyacrolein blocks are notfavorable for the grafting reactions to be carried out later in this project.XPS provided strong evidence that aldehyde functions were present on the latex surface,were reactive with DNPH and were reduced by NaBH4.The XPS spectra obtained were thoseexpected based on the surface chemistry anticipated and no unexpected groups were detected.iiTABLE OF CONTENTSPageAbstract iiListoftables VListoffigures viAcknowledgements viiiDedication ixCHAPTER 1 INTRODUCTION 1CHAPTER 2 BACKGROUND AND THEORY 32.1 Emulsion polymerization 32.1.1 General background and history 32.1.2 Mechanism of reaction 42.1.3 Surfacestabilization 72.1.4 Copolymertheory 92.1.5 Copolymerization techniques 102.1.6 Surface characterization of latex 112.1.7 Aldehyde functionalization of latex 122.2 X-ray photoelectron spectroscopy 12CHAPTER 3 EXPERIMENTAL 143.1 Polystyrene latex synthesis 143.1.1 Materials 143.1.2 Methods 143.1.3 Monitoring the reaction 153.2 Surface functionalization with acrolein 173.2.1 Materials 173.2.2 MethOdS 17Two stage reaction 17Semi continuous reaction 18m3.3 General latex chcon. 183.3.1 Size distribution 183.3.2 Concentration determination 193.4 Surface analysis 193.4.1 Dinitrophenylhydrazine assay 19Materials 19Standardcurve 19Assay on the latex suspension 213.4.2 X-ray photoelectron spectroscopy 223.4.3 Radiolabelled assay using tritiated NaBT4 22Malerials 22Methods 233.5 Nuclear magnetic resonance 25CHAPTER 4 RESULTS AN]) DISCUSSION 264.1 Polystyrene latex synthesis 264.2 Surface modification with acrolein 304.2.1 Time dependence of acrolein addition 324.2.2 Concentration dependence of acrolein addition 374.2.3 Two stage reaction 404.3 Developing an alternate aldehyde assay 414.3.1 A model for the latex surface 414.3.2 NaBH4assay 424.3.3 Nuclear magnetic resonance 514.3.4 X-ray photoelectron spectroscopy 59CHAPTER 5 CONCLUSION 67References 69ivLIST OF TABLESTable 1: Experimental conditions for the emulsion polymerization ofpolystyrene latex with persuiphate initiation 28Table 2: The effect of acrolein addition at the times indicated 32Table 3: Study of the effect of variation of the monomer ratio on thecopolymerization; acrolein addition after 10 hours 38Table 4: DNPH assay results from latex synthesized by seeded reaction 40Table 5: Characteristics of batch 68 (blank) and 78 (aldehyde grafted) 42Table 6: Reaction of the polystyrene/aldehyde latex (batch 78) withNABH4 ; verification of the efficiency of reduction 43Table 7: Radiolabel assays on batches 63, 64, 65, 66, 67, 68 using the 0.1 Msolution of NaBT4 45Table 8: Radiolabel assay on batch 68 and 78 using the solid stock oftritiatedNABT4 46Table 9: Results of the radiolabel assay of aldehydes using theScatchard plot 46Table 10: Quantitative analysis of the NMR peaks in the 8 = 9 ppmregion and 8 = 10.8 ppm region 58Table 11: Results of the curve fitting of the Cis peak 65VLIST OF FIGuRESFigure 1: Mechanism of particle formation according to the coagulationtheory for a surfactant free emulsion polymerization 6Figure 2 : The electrical double layer around a latex particle.Reproduced from reference (28) 8Figure 3 Electron ejected from the sample surface by XPS 13Figure 4 Styrene monomer calibration curve. Measured OD at 247 nmvs styrene monomer concentration in methanol molIl.c(247 nm) = 15,287 1 / (mol cm) 16Figure 5 : DNPH assay calibration curve. Measured OD at 360 nmvs concentration of hydrazone in THF.c(360nm) = 26,760 1 I (mol cm) 20Figure 6 : Scanning electromicrograph of batch 68, blankpolystyrene I sulphate latex 27Figure 7 : Scanning electromicrograph of batch 65, polystyrene!acroleinlsulphate latex. Acrolein added after 10 hours 27Figure 8 Monitoring of the emulsion polymerization of polystyrene 29Figure 9 Theoretical composition of styrene/acrolein copolymer. r,=0.034andr=0.32 31Figure 10: Effect of acrolein addition at the times indicated. Resultsobtained using the DNPH assay 34Figure 11: Scanning electromicrograph of batch 69, polystyrene!acrolein/sulphate latex .Acrolein added before the initiator 36Figure 12: Scanning electromicrograph of batch 67, polystyrene!acrolein/suiphate latex. Acrolein added after 2 hours 36Figure 13: Effect of variation of the monomer ratio on the graftingofacrolein 39Figure 14: Result of the DPNH assay after reduction of polystyrene/acroleinlatex (batch 78) with NaBH4 44Figure 15: Radiolabel assays of the aldehyde concentrations with NaBT4. Bindingviisotherms of batches 63, 64, 65, 66, 67 and68.47Figure 16: Radiolabel assays of the aldehydes concentrations with NaBT4. Bindingisotherms of batch 68 and batch 78 48Figure 17: Resulting Scatchard plots from Figure 16, batches63, 64, 65, 66, 67 and 68 49Figure 18: Resulting Scatchard plot from Figure 17, batch 78 50Figure 19: 1 H NMR spectrum of batch 78, polystyrene/acrolein/suiphatelatex in d8-THF 54Figure 20: 1 H NMR spectrum of IDC latex, polystyrene! acroleinlatex in d8- THE 55Figure 21: 1 H NMR spectrum of batch 85, polystyrene/acrolein/suiphatelatex (seeded reaction) in d8 TifF 56Figure 22: 1 H NMR spectrum of batch 86, polystyrene/acrolein/suiphate latex(seeded reaction swollen latex with styrene monomer) in d8 THF 57Figure 23: XPS spectra of batch 68, blank polystyrene/acrolein/suiphatelatex. Al kc 1486.6 eV was used 60Figure24: Clspeakofbatch68,78,ll)C 60Figure 25: Olspeakofbatch68andlDC 61Figure 26: Nis peak of IDC latex after reaction with DNPH 61Figure 27: Cis peak of IDC latex peak after reaction with DNPH 63Figure 28: Cis peak of batch 78 after reaction with DNPH 63Figure 29: Curve fitting of the Cis peak from batch 78 64Figure 30: Curve fitting of the Cis peak from JDC latex 64Figure 31: Cis peak of batch 78 after reaction with NABH4 65ViiACKNOWLEDGEMENTSI would like to thank everyone in the Donald Brooks laboratory for their invaluable help,constructive criticisms and patience when trying to comprehend my accent.Special thanks to Don Brooks for his expert guidance and stimulating ideas.I would like to acknowlege the staff of the NMR and XPS labs and the library for theirinvaluable services.Finally, but by no means least, I would like to thank Bruce for his helpful views bothwithin and without the field of chemistry.viiiA mes parentsixCIIAYTER 1 INTRODUCTIONPolymer latexes obtained from surfactant free emulsion polymerization are composedof clean particles with large surface areas which are of great interest in the newly developingareas of drug delivery and immunoassays (1-4). Polystyrene latex is the most commonlyused, because it possesses the required mechanical properties (5-7). However thehydrophobic character of the surface tends to be a problem in blood compatibility (6).Modification to achieve a more hydrophillic surface, leading to better blood compatibilityis necessary. It is achieved by copolymerization or surface grafting (8). The effects of suchsurface modification on protein adsorption are under wide investigation (9-12).It is known that polyacrylamide gives appropriate hydrophillic properties (6).Although polystyrene/acrylamide latexes have been synthesized by emulsioncopolymerization (11), surface grafting has been the prefered approach to surfacemodification. Grafting of polyacrylamide on surfaces has been reported in the literatureusing several different methods e.g. peroxide initiation (13), oxidation by corona discharge(14) or glow discharge (15). It is also possible to polymerize vinyl polymers on surfacescontaining alcohols, aldehydes and amines using cerium salts (16). The oxidation by cericion of those functional group proceeds with the formation of a free radical on the reducingagent (17,18). Polyacrylamide can be grafted on surfaces containing such functional groupse.g. modified polyethylene films (19) or granular starch (20). W. Muller used ceriumammonium nitrate to graft acrylamide on the surface of beads used as chromatographicsupports (20-40 m diameter) carrying hydroxyl groups on their surfaces (21). He reportedno polymerization of acrylamide in solution.The overall aim of the project is to graft polyacrylamide onto polystyrene particles1carrying aldehyde groups on the surface. These functional groups can be added to thesurface through copolymerization with acrolein in such a way that the grafted layer hasquantifiable characteristics. The plan is to assay the amount of acrylamide on the surface:The number of chains and their molecular weight. With such well characterized surfacelayers it ought to be possible to test theories of surface-attached polymer layer behaviour(22,23).In the present project, routes to synthesize polystyrene latex with surface aldehydefunctions are explored. Since it should be possible to count the number of polyacrylamidechains if the surface concentration of aldehyde before and after grafting is determined,methods for assaying surface-exposed aldehydes are developed using both wet chemical,radiochemical and NMR approaches. The latex surfaces are also characterized by X-rayphotoelectron spectroscopy which proves very useful in this application. Application of theassays developed combined with nitrogen analysis of the polyacrylamide-derivatized latexshould provide values for the average molecular weight of the grafted layer.2CHAPTER 2 BACKGROUND AND THEORY2.1 Emulsion polymerization2.1.1 General background and historyEmulsion polymerization is a reaction technique to produce latexes which are stabledispersions of polymeric materials in aqueous media. They can be designated as colloidalsystems as their particle size ranges from 1 nm to 1 u m implying that the particles willremain suspended in solution for a long period of time as single entities in Brownian motion.A latex is composed of a large number of polymer chains of molecular weight (MW) 1Oto 1O. The particle is stabilized by surface active components which arise either from theinitiator or from surfactants added to the polymerization.The first commercial emulsion polymerization was the development of butadienestyrene and polybutadiene latexes for synthetic rubber in the mid 1930s. This marked thebeginning of an enormous industrial interest in polymer colloids. The industrial developmentlead to some theoretical studies and to the first scientific publications in the 1940s . Theemulsion polymerization of styrene with surfactant was described by Harkins in 1947 (24)and the first quantitative theory was published by Smith & Ewart in 1948 (25).Vast amounts of latex are produced every day for a wide variety of applications suchas synthetic rubber, paints, coating, adhesives, latex foam, carpet backing or additives inconstruction materials and the more recent area of biological applications (26). Latexes canbe applied in three different ways (27):- Particles can be coagulated and the solid polymer is recovered3- Upon drying continuous films can be formed and used as coatings- Latexes can be used as particlesThe use of latexes in different applications depends on their stability and their surfacecharacteristics. Latexes have been used in industry empirically but the need for theory arisesfor more understanding. Today the area is still developing and although much has beenachieved in the understanding of the reaction kinetics many areas are unclear and the fieldremains active.2.1.2 Mechanism of reactionEmulsion polymerization proceeds via free radical polymerization and involves thethree following steps:- initiation-propagation-terminationContemporary theories of mechanism and reaction kinetics are still based on thework of Smith and Ewart (25). Most theoretical work has been done using styrene emulsionin the presence of surfactant (28,29). The surfactant forms micelles in solution which arebelieved to be sites for particle formation as they give stability to the particle due to theirsurface active properties. However there is still some debate on the exact mechanism ofnucleation. Three theories are proposed for particle nucleation: Entry of free radical into themicelle (28,30), homogeneous nucleation due to the precipitation of growing chains andcoagulative nucleation of precursor particles (31). Polymerization utilizing surfactant ismainly used and heavily documented in the literature (32,33). For some applications4however, the surfactant may be undesirable on the surface of the latex, for instance wherethe particle is used as an adsorbent. It is very difficult to remove surfactant completely fromthese systems.It was shown in 1965 (34) that latexes could be formed from some monomer e.g.styrene in a surfactant free system, using persuiphate initiation. Monodisperse latices havebeen synthesized since (35-37) and the mechanism of reaction investigated (38), whichhelped in the theoretical discussion of nucleation. The coagulative nucleation proposed byOttewill (39) seems to be the best accepted theory and has been further discussed forsurfactant free emulsion polymerization (38,40,41). This theory is described below and willbe used as the basis of discussion.In the case of persulphate initiated styrene emulsion polymerization, since styreneis only slightly soluble in water monomer droplets are formed in aqueous suspension. Onlya very low concentration of styrene is present dissolved in solution in the aqueous phase.Addition of the water soluble persuiphate initiator forms free radicals in solution by thermaldecomposition (42)S2O8 -‘ 2 S04which react in the aqueous phase with monomers (M)M + S04. - . MSO1M + . MS04 -. MMSO4M + . MMSO4 - . MMMSO1to form growing radicals. These are surface active due to their charged end group. Initiallythe monomer solubility increases due to the polar group (35). These individually growingoligomers become insoluble polymer chains at the point in their growth where they exceed5the critical degree of polymerization required for solubility in the aqueous phase. They arebelieved to precipitate out, described as a homogeneous nucleation, to form small precursorparticles about 5 nm in diameter which are colloidally unstable and are slightly swollen bymonomer. Coagulative nucleation occurs as these precursor particles coagulate to formmature growing latex particles. This process allows them to reach colloidal stabffity with thepolar groups remaining on the surface (36), swollen with enough monomer to allowpropagation to occur and increase particle size. The particles continue to grow until all themonomer has been consumed and termination occurs. Kinetic models have been establishedfor this coagulative nucleation theory and it is the most widely supported although competingtheories still carry some weight (39,4 1). Figure 1 shows a schematic outhne of the proposedreaction mechanism.-SO4.-. .MSO4-Formationof oligomers•44W S04soiOligomers S04144Jprecipitate-so4 so4-50 nm 0 colloidallyinstableCoagulationQ stable latex’GmwthFinal latexFigure 1: Mechanism of particle formation according to the coagulation theory fora surfactant free emulsion polymerization.62.1.3 Surface stabilizationSurfactant-free latex particles form colloid systems thanks to the surface chargesarising from the initiator used. Monodisperse latexes are the result of their colloid stabilitywhich is achieved at a given size depending on the stabilizing charge. Different chargegroups have been reported: a weak acid originating from hydrogen peroxide, a base fromazo compounds and a strong acid from persuiphate. Both cationic (43) and anionic latexesare possible. The stabilizing group on the surface exists either in the ionized or unionizedform depending on its plc and the pH of the solution. Where the surface of the particlebecomes electrically charged, electrical neutrality is maintained by balancing surface chargeswith counter ions from solution. This surface arrangement is called the electrical doublelayer and is represented schematically in Figure 2. The electrical double layer controls thedecay of the electrostatic surface potential V() [1], which is responsible for the repulsiveforces between particles, i.e., for their stability in solution (28). The thickness of the doublelayer, that is the extension of electrostatic potential away from the surface, depends on theionic strength, being greater at lower ionic strengths according to [ 2 ] (44). Hence,increasing ionic strength reduces stability.a= ‘0 --exp[ic(a-r)J [1]rwhere ‘F = the potential in the solution phase at a distance r from the center of the particle‘P0 = the surface potentiala = radius of the particler = distance from the center of the particle7ic =[ (8 it N1e2) / (1,000 ckT)] ½ [21I = ½ Ec1z?c, = molar concentration of ionic species izj = valence of th speciesN1 = Avogadro numbere = electron charge= dielectric constantk = Boltzman constantT = absolute temperature———--+/+— _+/-/+Ill/FIgure 2: The electrical double layer around a latex particle.Reproduced from reference (28)In persulphate-initiated latex -0S03 are the main groups on the surface. Howeverreaction of persuiphate in solution is believed to also produce hydroxyl groups on the surfaceaccording to the following reaction (42):SO1. + H20 - HSO4 + HO.Furthermore, oxidizing conditions during polymerization may oxidize hydroxyls to8carboxylic acids (36). Hydrolysis could be decreased by using buffers such as NaHCO3orNa2CO3 (45) as the reaction is minimized at higher pH (42).The surface arrangement of the particles consistof the described hydrophilic chargesand a large hydrophobic (polystyrene) surface. It is possible to modify the surface to controlthe properties. Modification of the surface is possible to achieve other properties. Forinstance adsorption of macromolecules or surfactants is often used (46-49), but theseapproaches will not be discussed here. Instead this work will concentrate on the modificationof the bulk of the latex or its surface by copolymerization to achieve the desired propertiesby methods for which theory is expected to be applicable (50,5 1).2.1.4 Copolymer theoryIf the free-radical polymerization of a mixture of two monomers Mi and M2 isconsidered the composition of the copolymer can be estimated knowing the monomercomposition of the feedstock and the monomer reactivity ratios (52).The polymerization consist of four established propagation steps.-Mi.+Mi - -MiMi.-M1.+M2 - -M1M2.-M2.+M1-, -M2M1.-M2. + M2 -‘ -M2M2.giving the four rate laws Rp,ij corresponding to the four propagation steps:Rp,ii = ku {Mi .][Mi]Rp,12 = k12 [Mi .][M2]9Rp,21 = k21 [M2 .][Mi]Rp,22 = k22 [M2 .][M2]From those equations, by applying the state approximation for free radicalpolymerization the following expression can be written:d[Mi] 1 + ri [Mi] I [M2] where ri = ku I k12d[M2] 1 + r2 [M2] / [Mi] & 2 = k21 / k22If F i is the mole fraction of component i in the polymer and f i the mole fractionof monomer i in the feedstock, it can be shown that (52):ri . f12 + ft . ftFi=—ri . fi2 + 2.fi.f2 + r2.f22Values of ri and r for many pairs of monomers are available in the literature (53)and allow prediction of the copolymerization product composition. This theory can beapplied to predict copolymerization in the case of latexes.2.1.5 Copolymerization techniquesTwo different approaches can be applied in latex copolymerization. In the firstapproach are grouped the batch and semi-continuous processes, respectively where all themonomers are either added at the beginning of the reaction, for example the use ofisothiuronium salt (e.g. 2-[4-(4-vinylphenyl)butylj isothiuronium bromide) as comonomer(45,54), or where one monomer or both are added during the polymerization, for example10the synthesis of styrene p-formylstyrene copolymer latex (55). The second approach isgenerally termed a seed reaction, where particles previously synthesized are used as seedsfor growing the second polymer to form bigger particles with core shell structures, forexample the two stage emulsion polymerization of acrylonitrile and butadiene (core) (56).The morphology of two stage reaction depends on the properties of each monomer. Thisarea is extensively discussed in the literature (57-60) as the properties of the resultingparticle have wide potential technological application.2.1.6 Surface characterization of latex.A comprehensive survey of the different techniques used to characterize latexes hasbeen published by El-Aasser and Fitch (27). The particle size distribution as established byelectron microscopy or light scattering and the surface charge density obtained byelectrophoretic mobility measurements or conductometric titration are the two maincharacteristics of a latex batch. Many other analytical technique such as X-ray photoelectronspectroscopy (XPS)(60), static secondary ion mass spectrometry (SSIMS)(61), nuclearmagnetic resonance (NMR) (62), Fourier transform infra-red spectroscopy (FTIR)(63),differential thermal analysis (64) and ultra-violet spectroscopy (UV) have also been used.Staining of styrene with Ru04 for electron microscope (EM) analysis has beenused to studythe morphology of core-shell polymethylmethacrylate/polystyrene (PMIvIA/PS) latexes (59).Latex characterization is an active area of investigation as more information is required forthe understanding of latex formation and the development of theories.112.1.7 Aldehyde functionalization of latexPolyacrolein -[CH2çH1- latex has been synthesized by surfactant emulsionCHOpolymerization (65,66) and its use as a drug carrier discussed (67). The high concentrationof aldehyde groups on the surface allows covalent binding with amino groups on biologicalmaterials. However some problems are encountered in the polydispersity as well as somesedimentation due to the specific gravity (68).A seeded copolymerization of styrene I glutaraldehyde using aldol condensation wasachieved by Okubo et al. (63) and batch copolymerization of styrene/acrolein was reportedby Slomkowski et al. (7,68). The advantage of the seeded copolymerization is to keep thepolystyrene particle as a well defmed hydrophobic core. Those two techniques will be thebasis of the present work and will be further developed.2.2 X-ray photoelectron spectroscopyXPS is an electron spectroscopic technique used for qualitative and semiquantitativesurface analysis. In an ultra high vacuum (UHV) (2 x 10_la mbar) an X-ray photon whichis directed onto the sample is absorbed by surface atoms. As a result of the higher energyof the X-ray photon compared to the binding energies of electrons in the atoms, electronsare ejected,( Figure 3). These electrons possess kinetic energies characteristic of theelectronic configuration of the atom configuration in the sample. The electron kinetic energyfollows the equation:12Eke =hv-Eb-4)where Eke- kinetic energy of the ejected photoelectronhv- characteristic energy of the X-ray photonEb - binding energy of core level4) - work function termEjected electronX-ray energy E kehv\__Sampling depthapproximately, 30 AIigure 3 : Electron ejected from the sample surface by XPS.The ejected electron binding energy depends on the original molecular environmentand its oxidation state, giving a chemical shift characteristic of the molecule in a givenenvironment. Only photoelectrons from the outermost atom layers have sufficient energyto escape the sample and reach the detector. Clark and coworkers showed the escape depthfor photoelectrons from polymers is relatively short and give a sampling depth of 30 A (69).XPS has been used here to study the modification of the latex surface with acrolein bylooking at the Cis, and Nis signals (7,63).13CHAPTER 3 EXPERIMENTAL3.1 Polystyrene latex3.1.1 MaterialsStyrene monomer, ACS grade obtained from Fisher, New Jersey, was distilled underreduced pressure and stored under nitrogen. Potassium persulphate, K2S208 from B.D.H.,Toronto, and sodium chloride (NaC1) from Fisher chemical, New Jersey, were used withoutfurther treatment. Absorbance measurements were made on a IIEWLET-PACKARDUV/VIS spectrophotometer, model 8450A , using 1 cm path length quartz cuvettes. Doublydistilled water was used for all experiments.3.1.2 MethodsThe synthesis was carried out according to the method of J. W. Goodwin et al. (36)to achieve a polystyrene /sulphate latex of approximately 650nm. The emulsionpolymerizations were carried out in a 500 cm3 pyrex round bottomed four necked flaskequipped with an internal overhead stirrer. The T-shaped teflon stirring blades were onecentimetre from the bottom of the flask allowing a stirring range of 60 to 500 rpm. A watercooled condenser connected to the atmosphere via a wash bottle and an addition funnelconnected to a nitrogen inlet were fitted to the second and third outlet. The fourth outlet wasleft available for sampling. The flask was immersed in a temperature controlled water bath(+1- 1°C).14The total reaction volume was 180 cm3. NaC1, 0.109 g dissolved in 132 ml of waterwas added to 16.9 g of styrene monomer in the reaction flask with stirring at 350 rpm.Nitrogen was bubbled for 10 minutes through the solution to remove oxygen. The additionfunnel was set in place with 0.130 g of persulphate initiator dissolved in 10 ml of water andwas left to sit for another 15 mm with the nitrogen flow set to minimum to avoidevaporation. The reaction flask was equilibrated at 70 °C. The initiator was added to thesolution and the funnel washed with 20 cm3 of degassed water. The reaction was allowedto proceed for 24 hours. The preparation was cooled down before being filtered through afilter pack of glass wool to remove any major agglomerates. The latexes were dialysedagainst 12 liters of distilled water for a week, changing water every 24 hours, to removemost of the unreacted monomer and salts. Finally, they were washed 3 times with water andrecovered by centrifugation. All latex preparations were stored at 4 °C in 50 mlpolypropylene tubes at a concentration of 4 % w/w in water.3.1.3 Monitoring the reactionThe reaction was monitored by taking suspension samples which were diluted inmethanol, as styrene monomer is soluble in methanol. The sample was centrifuged and thesupernatant optical density (OD) measured at 247 nm, the styrene Amax for the it -,transition. Figure 4 shows the standard curve obained by dilution of pure monomer inmethanol. The extinction coefficient was found to be E(247 nm) = 15,287 1 / (mol cm).15C’)G)CuC.).1-I00Figure 4: Styrene monomer calibration curve: Measured OD at 247 nm vs styrenemonomer concentration in methanol molIl. c (247 nm) = 15,287 1 I (mol cm).1 E-05 3E-05 5E-05[STYRENE] in mol/I163.2 Surface functionalization with acrolein3.2.1 MaterialsAcrolein was obtained from Aldrich, Milwaukee, distilled at reduced pressure beforeuse and stored under nitrogen at 4 °C.Polystyrene/aldehyde latex was used as supplied by Interfacial DynamicsCorporation, Portland, Oregon. The mean diameter was 604 nm with a charge density ofone charge per 280 A2. The synthesis technique was also essentially that of Goodwin et al.(36), but the surface modification technique to produce the aldehydes was unknown. Theywere used as a reference to test the aldehyde groups assays studied.3.2.2 MethodsTwo stage reaction- Synthesis of batch 85A batch of pure sulphate latex was synthesized according to the technique decribedin section 3.1.2. A dialysed and characterized polystyrene sulphate latex (batch 68) wasused as the seed latex. The same experimental set-up as for the synthesis of the polystyrenesulphate latex was used. Nitrogen was bubbled through 50 g of 3.3 % w/w of seed latexsuspension and 3 g of acrolein monomer. The flask was left to reach an equilibriumtemperature of 50 °C, with constant stirring at 350 rpm. After 15 minutes 38 mg ofpotassium persulphate initiator in 8 ml of water was added and the system allowed to reactfor 12 hours.- Synthesis of batch 86The same above technique was used in this synthesis, but the seed latex were swollen17for 15 minutes with 1 g of styrene monomer before the addition of 3 g of acrolein, and thereaction was left to reactSemi-continuous reactionThe semi-continuous reaction was the main focus of this project. The experimentalset up was the same as for the polystyrene latex synthesis but the acrolein monomer,dissolved in 10 ml of water, was added during the latex synthesis and rinsed with 5 ml ofwater . In this work the effect of addition time and the amount of acrolein added wereinvestigated.3.3 General latex characterization3.3.1 Size distributionInitial observations were made on a light microscope. The size distributionobservations were made from photographs from a I{ITACH model S2300 scanning electronmicroscope. The average diameter of 20 latex particles gave a satisfactory estimation of 650± 50 nm for all batches (only reasonably distributed batches were sized). It is important tonote that this is not the standard procedure for full latex characterization; more time wouldbe required for a good assay on 1000 latex particles. However it was sufficient as thepresent work did not require an accurate diameter or size distribution. The sample wasprepared as follows: A drop of diluted latex suspension was dried out on the graphite surfaceof the sample holder and plated with 100 A thick film of Au-Pd (60/40 %) using a Hummer4 gold coater, (source power 600-700 v, 2OmA, in an Argon atmosphere).183.3.2 Concentration determinationThe concentration of the latex suspension was determined by drying a sample in anoven at 65 °C. This allowed the preparation of latex suspensions in water from known dryweight concentrations.3.4 Surface analysis3.4.1 Dinitrophenyihydrazine assayMaterialsDinitrophenythydrazine (DNPH;(02N)C63fINH ),a moist solid containing 30% water was purchased from Aldrich, Milwaukee, Wis, USA. Ethanol was ACS grade.Tetrahydrofuran ([I’HF) was obtained from BDH, Vancouver, Canada spectroscopic grade.Standard curveDinitrophenythydrazine reacts specifically with aldehydes to form a hydrazonecomplex [3] which absorbs in the UV range making the reaction a valuable spectroscopicassay for aldehydes. These compounds absorb at 350/360 nm due to their * transition,a useful absorbance range in the presence of dissolved latex solution because styrene absorbsat 200 to 280 nm191.2>C,)Cci)-D(U00Figure 5 : DNPH assay calibration curve. Measured OD at 360 nm vsconcentration of hydrazone in THF. c (360 nm) = 26,760 1 / (mol cm)1.00.80.60.40.20.0•0.OE+005.OE-06I I I I I1 .OE-05 2.OE-05 3.OE-051 .5E-05 2.5E-05[HYDRAZONE] mol/I4.OE-053.5E-0520H HRH+H2NNO2_________N NO2+ H20NO2 R NO2[3]DNPH HYDRAZONE derivativeAmax = 350 nm Amax 360 nmThe extinction coefficient at 360nm for the hydrazone derivative in T.I{F wasobtained using 3-phenyl propanaihydrazone, a closely related compound to thepolystyrene/acrolein copolymer. It is reported in the literature that the extinction coefficientof hydrazone complexes do not vary much with the type of aldehyde (70). It was synthesizedin the following way: 0.25 g of DNPH was dissolved in 5 ml of methanol and 0.5 ml ofsulphuric acid was added carefully. The solution was filtered and 0.2 g of 3-phenyl propanaladded, filtered again and washed with methanol, before crystallization from ethanol (71)• Figure 5 shows the standard plot obtained by serial dilution of hydrazone in THF. Thecalculated extinction coefficient is€(360) = 26,760 IJ(mol.cm), comparable to value foundin the literature (70).Assay on the latex suspensionThe hydrazine and hydrazone absorb in the UV at 350nm and 360nm respectivelymaking it difficult to quantify a mixture. The hydrazine therefore was separated from thehydrazone by centrifugation (7).Ten nil of a saturated solution of DNPH in ethanol was added to a 2m1 sample of 1 %w/w latex suspension in a 15 ml polyethylene tube and mixed for 24 hours. Latexes areswollen in ethanol which can facilitate the access of the DNPH molecule. The solution was21washed with ethanol by centrifugation to remove the unreacted DNPH until the supernatantwas clear and did not absorb in the UV. The latex was dried under reduced pressure forthree hours. A known amount was then dissolved in TI]F, the OD measured at 360 nm andthe concentration of hydrazone associated with the latex calculated. This reaction allows anestimation of the concentration of aldehyde specifically on the surface of the latex, i.e.,accessible to DNPH in solution3.4.2 X-ray photoelectron spectroscopyLatex suspensions were freeze dried and were put on one side of a double sidedadhesive (39). It was then transferred under continuous U1{V (2 x 10-10 mbar) into theanalysis chamber of a Leybold MAX200 spectrophotometer. An unmonochromatized Al koc(1486.6 eV) excitation source was operated at a potential of 10 kV and x-ray current of 20mA. The emitted photoelectrons were collected from a 2 mm2 area.3.4.3 Radiolabelled assay using NaBT4MaterialsSodium boro[3Hlhydride (NaBT4) was purchased from Amersham Canada, Ltd,Oakville, Ontario. Batch 101 was in 0.1 M NaOH with a specific activity of 440 GBqmmol1.Batch 183 was 1 mg of solid with a specific activity of 220 GBq mmol1.Atomlightscintillation cocktail was purchased from Dupont, Boston, Massachussets. A Phillips PW4700 liquid scintillation counter was used for all scintillation counting.22MethodsNaBH4is an effective reducing agent which converts aldehydes to their correspondingalcohols. (72,73). The mechanism of this reaction is shown below [4] (unlike standardsolution chemistry borohydride cannot react with 4 aldehyde moieties because of theirsurface immobilization). A hydrogen from the NaBH4 attaches to the carbon of thealdehyde. Using tritiated NaBT4 a tritium bounds to this carbon allowing quantification ofthe reduced aldehyde.0 — +S.L+[H- BH3] Na- R— CH2-OBH3Na+ [4]R—CH2-OH + B(OH)3The advantage of a radiochemical assay is the very low limit of detection ( - l0mol) (74). Counts determine number of tritium atoms and thus the number of aldehydegroups present.In a 1.5 ml Eppendorf centrifuge tube, cold NaBH4 and NaBT4 in 0.1 M NaOHsolution were mixed with a latex suspension. A 200 i 1 aliquot of ethanol was added toswell the latex and favor the reaction. The tubes were mixed for 24 hours. The NaOHsolution kept the pH alkaline and limited hydrolysis (75). The suspension was centrifugedto separate the latex from the unreacted NaBH4.The latex was then resuspended in 0.8 mlof water to which 100 1 of 0.1 M HCL was added to hydrolyse the complex. After 1 hourthe latex was washed three more times before dissolving it in imi of THF. Ten ml ofscintillation cocktail was added and the samples counted.The concentrations of NaBT4 and latex were kept constant and the concentration23of NABH4varied. The equilibrium concentration of thtium dissolved in THF solution wascalculated from the disintegration per minutes (DPM) results. Knowing the specific activityof the radiolabel the concentration of aldehyde on the surface for a given total equilibriumNaBH4 concentration was obtained which gave the binding isotherm: [NaBH4J vs [RCH2OH]. A Scatchard plot was calculated to extrapolate graphically the number of aldehydegroups on the latex at saturation. The Scatchard analysis is derived the following way (76):Take the following reaction at equilibrium:-BR4 + R-CHO + H + 3 HO R -CH2O + H3B0 + 3 H2the equilibrium constant ka ie equal to:[R-CH2OH]. [H3BO] [R-CH2OH] . [H3BO]ic=[NaBH4]. [R-CHO]. [I{O] [ffl{][I{O]([ ALDOJ - [R-CH2OH])since [ALDJ = [R-CH2OH] + [R-CHOJwhich can be rearranged to give[RCH2OH1q. ([H3B0]eq + Ka.[H3O].[NaBH4])= Ka .[H304]. [NaBH4]. [ALDO]dividing by ( [NaBH4] . [H3BO])[R-CH2OH] - Ic. [H3O].[ALDJ- K,. [H3O]. [R-CH2OH][NaBH4] [H3BO] [H3BO]if [R-CH2OH1 I [NaBH4]is plotted vs [R-CH2OH1 the x-intercept is equal to [ALD]where: Ic equilibrium constant assuming no isotope effect[RCH2OHJ = equilibrium concentration of reduced aldehydes,24obtained from the counts on the latex.[R-CHOJ = equilibrium concentration of non -reacted aldehydes[NaBH4J = equilibrium concentration of NaBH4 in solution,obtained by difference between initial counts andcounts on the surface.[ALD] = total concentration of aldehydes from the surfaceThe reaction was tested using NaBH4 (unlabelled) at higher concentration. Afterhydrolysis and wash of the latex the DNPH assay was performed to verify if the aldehydeswere effectively reduced to alcohols.3.5 Nuclear magnetic resonanced8-Tetrahydrofuran 99.5 atom % D (THF) was obtained from Aldrich, Milwaukee,WI, USA. Freeze dried latexes were dissolved in d8-THF and the ‘H NMR spectra recordedon a BRUKER WH 400 spectrometer at a proton frequency of 400.13 MHz.25CHAPTER 4 RESULTS AND DISCUSSION4.1 Polystyrene latex synthesisAll batches of polystyrene latex were synthesized using the experimental conditionsgiven in Table 1. This ensured reproducible batch characteristics of charge density,concentration and diameter (approximately 650nm), allowing comparison among batcheswithout having to fully characterize them after each synthesis.Thirty minutes after addition of the initiator the typical opaque appearance of a latexsuspension was observed. Some aggregates were formed around the stirring paddle but didnot interfere with the polymerization and were removed in the filtration step.At 400x magnification uniform well dispersed latex particles in Brownian motionwere observed. Figures 6 & 7 are scanning electronmicrographs of latex particles withdiameters of approximately 650nm. This value compares to Goodwin’s (36) reporteddiameter of 678 nm for the same polymerization conditions. Slight variations can beexpected as the reproducibility of emulsion polymerization from lab. to lab, is very difficult.Goodwin reported other characteristics: A total charge density of 1 charge per 377 A2 anda number average molecular weight of Mn = 1(Y, which were used as references.Figure 8 shows the styrene monomer concentration in solution vs reaction timeobtained from monitoring the reaction by U.V. spectroscopy. The graph shows 4 distincttypes of behaviour (A-D) as a function of time.A: 30-50mm C: 250- 600 mliiB: 50-250 miii D: 600- end26Figure 6: Scanning electromicrograph of batch 68, polystyrene! sulphate latex.Figure 7: Scanning electromicrograph ofbatch 65, polystyrene/acrolein/suiphatelatex. Acrolein added after 10 hours.27Table 1: Experimental conditions for the emulsion polymerization of polystyrene latexwith persuiphate initiation.UNDERIVATIZED Amount in Concentration inPOLYSTYRENE grams molllLAThXStyreneMonomer 16.9 0.90Water 132NaC1 0.109 1.04 10-2InitiatorK2S208 0.139 2.86 10-2BATCHNUMBER 68Mixing 350 rpmTemperature 700 CReactionTime 24 hoursTotalVolume 180 cm3It is possible to explain the general features of Figure 8 in terms of the coagulativenucleation theory of emulsion polymerization discussed in the introduction. The initialconcentration is slightly lower than the bulk concentration of 0.9 mol/l. The oligomersformed in solution at the beginning of the reaction have increased solubility in water due tothe polar sulphate group (35). As a result the droplet size reduces as monomer goes intosolution for propagation. It leads to a nearly total solubility of the styrene: 0.88 molJl.280ECa)CG)Figure 8: Monitoring of the emulsion polymerization of polystyrene. Styrenemonomer concentration vs reaction time; the four distinct regions(A-D) are indicatedReaction time in hours29In region A the sudden decrease in concentration would correspond to homogeneousnucleation, when the chain length of the oligomers exceeds the critical degree ofpolymerization required for solubffity. They precipitate to form precursor particles, smallapproximately 5 nm diameter particles which are not soluble, leading to the sudden decreasein monomer concentration observed. It is expected that by the end of region A all theprecursor particles have coagulated to form polymer particles which carry a sufficient chargeto be stable and which are swollen with monomer. In region B a more or less constantconcentration of monomer in solution is observed. The propagation reaction continues insidethe particle which would account for this roughly constant monomer concentration insolution. As the monomer inside the particle is consumed, monomer from solution migratesto the particle and partitions into it to polymerize (region C), until no more styrenemonomer is available in solution (region D). Region D marks the termination step.It was unexpected that this simple monitoring technique by U.V. spectroscopy wouldallow us to distinguish the different stages of the emulsion as outlined for the reactionmechanism. However, the monitoring experiment was repeated and the results wereconfirmed.4.2 Surface modification with acroleinFigure 8 can now be used as a basis for discussing the surface copolymerization withacrolein. In the one stage polymerization, since no more initiator is used, acrolein needs tobe added when radicals are present, which from Figure 8 excludes any addition in regionE. Figure 9 shows the theoretical copolymer composition of the styrene/acrolein system asa function of the feedstock composition in monomer with r,=O.O34 andr0=O.32 (53).30w00-J0C-)zzw-J00z0I—0U-Figure 9:racro =0.32.Theoretical composition of styrene/acrolein copolymer. r, = 0.034 and0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9FRACTION OF STYRENE IN FEEDSTOCK31The probability of integrating acrolein in the polymer is higher when the styreneconcentration is low but as long as styrene is not in large excess copolymerization shouldoccur.4.2.1 Time dependence of acrolein additionIt was decided to vary the time of acrolein addition in the single step polymerizationto gain more information on the reaction. Table 2 shows the different batches synthesizedfor this study. The DNPH assay was performed on the suspension to quantify the aldehydegroups on the surface and demonstrate that copolymerization had occured.Table 2 : The effect of acrolein addition at the times indicated following initiation of thesynthesis of polystyrene/sulphate latex; 1.0 x l0 mol of acrolein was added per gram ofstyrene for each synthesis, total volume = 195 cm3BATCH TIME OF DNPH ASSAYNUMBER ADDITION AMOUNTin hours ALDEHYDEin mollg dry latex68 Blank 1.8 1069 0 1.6 1067 2 1.1 1066 4 1.21064 6 8.710663 8 6.3 10.665 10 5.4 10673 12 6.4 10670 14 5.0 1072 16 2.0 1071 18 2.3 1062 20 3.0 1032Figure 10 shows the concentration of aldehyde in mol/g (dry latex) vs time ofaddition obtained from the DNPH assay. After 12 hours no aldehyde was measured on thesurface. This time corresponds to the region E where termination has already occurred andno radicals are present as interpreted from Figure 8. This result implies that the acrolein iscopolymerized with the polystyrene chains. It suggest polyacrolein is not polymerized insolution and adsorbed onto the latex (from left over initiator or via thermal initiation).Radicals are necessary for copolymerization.Addition of acrolein at any time during the propagation step producedcopolymerization. The amount associated with the latex did not vary dramatically throughthe reaction period but two different regions can be discussed : region 1= 2 to 4 hours,region 2 = 6 to 12 hours (the initial addition will be discussed separately).Region 1:After 2 hours (region C), polymerization is occurring in the soft swollen particle.The rate of propagation is high compared to later in the reaction. The lower viscosity inthe swollen particle should lead to an increased rate of appearance of radicals at the surface.Apparently this favours copolymerization with acrolein monomer present at the interfaceRegion 2:The particle is now growing by polymerization of monomer which is partitioning intothe particle (region D), as the monomer is compatible with its own polymer. Acrolein isnot expected to partition into the particle significantly because of noncompatibility and itshydrophiffic character.331.8E-051.6E-05I 1.4E-051.2E-051E-058E-06U]6E-06U]4E-062E-06U I I , I I I I I I IblO 2 4 6 8101214161820ACROLEIN ADDITION in hoursFigure 10: Effect of acrolein addition at the times indicated. Results obtained usingthe DNPH assay (under conditions where surface groups are detected).34The random movement of the radical on the end of the growing polymer chain isreduced as the chain molecular weight grows and the internal viscosity and entanglementincreases. The frequency of the radical appearing near the surface is thus reduced as is thecopolymerization rate. When an acrolein monomer has reacted, because of its hydrophilicity,it will tend to stay at the surface to carry on the copolymerization with further styrene oracrolein monomers. It seems reasonable to expect that the polymer chain formed aftercopolymerization of acrolein remains at or near the surface, depending on the hydrophobicstyrene and hydrophillic acrolein sequence in the copolymer. The chains terminate withanother polymer chain either in the particle or on the surface.Initial additionWhen acrolein is added with styrene and the initiator, copolymerization occurs insolution before particle formation. Thus because of its relative hydrophilicity, the reactioninitially occurs preferentially with acrolein. These copolymers will then precipitate to formthe particle. Figure 11 shows the EM of batch 69. The latex shows major aggregation, aswas also seen in suspensions observed with an optical microscope. Evidently the presenceof acrolein-rich oligomers results in a less uniform charge distribution on the surface of thegrowing particle. The result is particle aggregation and a non-uniform size distribution. Itis believed that by changing the initiator and electrolyte concentrations, better stability canbe achieved, as suggested by the results described by S. Slomkowski (7). The amount ofaldehyde shown in Figure 10 might have been expected to be higher, but since the latexagglutinated all the surface may not be available for reaction with DNPH.35Figure 11 : Scanning electromicrograph of batch 69 , polystyrene/acroleinlsulphatelatex. Acrolein added before the initiator.Figure 12: Scanning electromicrograph of batch 67, polystyrene/acrolein/suiphatelatex. Acrolein added after 2 hours.36The electronmicrographs obtained for the batches with acrolein addition in the first6 hours of the reaction showed a non-uniform size distribution. Figure 12 shows the resultof addition at 2 hours (batch 67). Again the hydrophillic character of acrolein appears todisturb the crucial stage of particle formation. We can conclude therefore that it is preferableto add the acrolein after particle formation and during the particle growth period, to obtaina good size distribution.Figures 8 & 10 are in accordance with each other and can both be explained usingthe model for emulsion polymerization discussed in the introduction. The monitoring of thereaction allowed us to determine the preferable time for acrolein addition to maximize thesurface concentration of aldehyde groups, i.e., 10 hours.4.2.2 Concentration dependence of acrolein additionTo achieve the aim of this project aldehyde groups must be available on the surfacefor grafting. For this reason latex resulting from the addition of acrolein at 10 hours waskept for further study as these conditions ensure surface copolymerization and a good sizedistribution (Figure 7). The styrene monomer concentration in solution is 1.2 x 102 mol/1giving a copolymer ratio in acrolein of 0.5 for an addition of 1 g of acrolein. Increasingthe acrolein monomer concentration is expected to allow greater copolymerization withacrolein, i.e., to give a higher aldehyde concentration on the surface.Different amounts of acrolein were added to study the effect of the monomer ratioon the copolymerization. Table 3 describes the different latex synthesized as well as resultsof the DNPH assay. Figure 13 gives the surface aldehyde concentration as a function of theacrolein concentration.37Table 3: Study of the effect of variation of the monomer ratio on the copolymerization;acrolein addition at 10 hours.BATCH AMOUNT OF DNPH ASSAY %NUMBER ACROLEIN ADDED in aldehyde conversionmollg of styrene added mol/g dry latex of acrolein68 0 0 —79 6.3 10 1.1 10 1.765 1.0 10 5.4 10 0.5478 2.1 10 2.2 10 1.077 3.1 10 1.8 10 0.058100 4.2 10 1.6 10 0.3880 9.0 10 7.3 106 0.081mc 9.0 10The data show no obvious relation exists between the surface aldehyde concentrationand the acrolein concentration and very low ratios of conversion. This result wasunexpected. The temperature, 70 °C, was not believed to be the main factor as the seededreaction (next page) gave similar low ratios although it was done at 50 °C (as the acroleinboiling point is 52.5 °C). The result is probably due to other parameters in the reactionwhich dictate a saturation point of hydrophilic acrolein on the hydrophobic surface. Noattempt to determine or quantify these unknowns was undertaken. Since the amount obtainedon the latex was sufficient for the present application (as will be seen later) this aspect wasnot investigated further.382.5E-0)0ECl)LJI0>-zw0-J2E-051 .5E-051 E-055E-06O.OE+OO 1.OE-03 3.1E-03 9.OE-036.3E-04 2.1 E-03 4.2E-03ACROLEIN ADDED mol/gFigure 13 : Effect of variation of the monomer ratio on the grafting of acrolein394.2.3 Two stage reactionThe seed reaction was investigated briefly to allow a comparison of the amount ofaldehyde obtained by this technique with that from the previous syntheses. The synthesesof batches 85 and 86 are described in the experimental section. Table 4 shows the resultsof the DNPH assay.Table 4: DNPH assay results from latex synthesized by the seeded reaction.Batch number amount acrolein DNPH assay % ofadded in mol/g of dry mollg of dry conversionlatex latex85 0.031 8.8 106 0.02886 0.031 1.1 10 0.035Although a higher acrolein concentration was added the results are of the same order,with an even lower conversion ratio. Since more initiator is added, polymerization insolution is expected and oligomers formed should then adsorb on the surface of the latexparticle. In the case of batch 86 where styrene monomer was present to swell the particle,it would be expected, after initiation in solution, that the oligomer radical wouldcopolymerize with styrene monomer. For batch 85 the same phenomenon could occur assome monomer remains in the particle but it is more likely that some oligomers from theaqueous phase simply adsorb on the surface. Because of its hydrophillic character most ofthe polyacrolein formed apparently stayed in the aqueous phase; this would explain the lowconversion ratio calculated for the acrolein associated with the latex..It is important to note that after extensive washing or after leaving the particles to40equilibrate for 2-3 weeks the DNPH assay gave the same result. It shows that oligomeradsorption if present, does not seem to be reversible. The seeded polymerization procedurewas not utilized further as it was felt it could produce local high densities of acrolein inoligomers but a non-uniform distribution over the surface as a whole.The amount of aldehyde on the surface was expected to be higher in the seededreaction yet it gave the same order of coverage as in the earlier procedure. It is possible thatbecause the DNPH molecule is relatively bulky, it is hindered from fully reacting if thecoverage is too high. It was therefore important to investigate this possibility by developinganother technique to assay the aldehydes to prove the validity of the DNPH assay.4.3 Developing an alternate aldehyde assay4.3.1 A model for the latex surfaceIt is helpful to estimate the surface concentrations of the groups of interest to providean average picture of the surface. Batch 68 (blank) and 78 (aldehyde) were used asexamples. Table 5 gives some of the characteristics of those batches.The number of molecules of acrolein per latex particle calculated from these resultsis 2.01 x 106 for a surface area of 1.32 x 108 A2. If it is assumed that the aldehydes areuniformly distributed over the surface and that an aldehyde site at the surface occupies 10A2, the percentage coverage would be 12.1% of the surface for batch 78. If we take thecharge density as one per 377 A2 as given by Goodwin, assuming an area 10 A2 perhydrophiffic group41Table 5 : Characteristics of batch 68 (blank) and batch 78 (aldehyde grafted)BATCH NUMBER 68 blank 78 aldehydediameter estimation 650 650volume/latex (cm3) 1.44 1013 1.44 l0’surface/latex ( A2) 1.32 108 1.32 108number particle/g 6.58 1012 6.58 1012specific surface area 8.7 8.7(m2/g)aldehyde mol/g 0 2.2 l0aldehyde molecule/latex 0 2.01 106** density of polystyrene 1.055 g/cm3(sulphate, weak acid or hydroxyl) the calculated coverage is 2.6 % of the surface. Thisleaves a hydrophobic polystyrene occupying 85.4 % of the surface. From the perspectiveof grafting acrylamide on the surface of the particle a lower coverage would be of interest.The assay used has limited sensitivity however. The DNPH assay allowed us to detect 1.8106 mol/g (batch 77) which would give approximately a 1.2 % coverage; any loweramount would not be assayed with any accuracy.4.3.2 NaBH4 assayA radiochemical assay was investigated to improve the detection sensitivity. Thereduction of aldehyde by radiolabelled NaBH4 was thought to be an appropriate reaction.The reaction with non-tritiated NaBH4 is summarized in Table 6. The completeness of thereaction was checked using the DNPH assay on the reduced latex. Figure 14 shows theamount of aldehyde on the latex vs the amount of NaBH4 added.42batch number 78a 78b 78c 78d 78edry weight of latex 0.02 0.02 0.02 0.02 0.02ingNaBH4 in mollg of 4 10 2 10 1 10 4 10 1 10-2dry latex in 2m1waterinmi 2 2 2 2 2ethanolinmi 3 3 3 3 3reaction time in 24 24 24 24 24hours0.1MHCLinm1 1 1 1 1 1reactiontimein 1 1 1 1 1hours* the aldehyde concemration for batch 78 is 2.2 10 mol/g by the DINPH assaySome aldehyde groups were reduced but even with a large excess of NaBH4 thereaction did not go to completion. The result perhaps is not surprising as NaBH4is reportednot to be very reactive (77) and it is possible that the double layer is going to affect theapproach of BR4 - ion towards the surface. The use of the Scatchard plot for theradiolabelled reaction does not require the reaction to go to completion, however, as thetotal number of reactive sites is determined by extrapolation. It was therefore decided tocarry out an analogous reaction with NaBT4.Tables 7 and 8 summarize the data from the radiolabelled reaction. Figures 15 & 16show the binding isotherms obtained. The DPM values on the surface of batch 68 (blank)were not expected as no groups reactive to NaBR4 should be present on the surface. Thelinearity of 68 compared to the other batches implies an adsorption process and will beconsidered as background DPM.Table 6: Reaction of the polystyrene/acrolein latex (batch 78) with NaBH4 ; verificationof the efficiency of reduction.43[ALDEHYDE]surfmol/g01m0001001+1n00000000)01010101I 0 :1::::.:...:...........:.........:..::..::..:.:....p 0 m + 0.0 1”• 0 0 .. 0 0 c?.r w I - B 00 cii0 01 0 c) a 0 r’)I:.::.:.:::]:ICounting of further washes of the latex showed that some DPM continued to be released intothe supernatant. It is likely that these counts are associated with water trapped in the wetlatex before being dissolved in THF. However, extensive washing to remove these DPMwas not carried out each time as loss of some latex in the supematant occurs.It was decided to wash the latex only 3 times, and to subtract the counts from theblank to obtain the Scatchard plot. The binding isotherm of the aldehyde latex showed somecurvature due to saturation. Figures 17 & 18 are the Scatchard plots. Table 9 shows theamount of aldehyde on the surface obtained from extrapolation of the Scatchard plot.Table 7: Radiollabel assays on batches 63, 64, 65, 66, 67, 68 using the 0.1 M solution ofNaBT4batch number 1 2 3 463,64,65,66,67dry latex in g 0.001 0.001 0.001 0.001NaBT4 in molig of 1.3 108 1.3 108 1.3 108 1.3 10.8latexcold NaBH4 in mollg 1.0 10 2.0 10 4.0 10.6 8.0 10latexwater in 1 200 200 200 200ethanol in 1 200 200 200 2000.1 M HC1 in 1 to 100 100 100 100stop the reaction45Table 8 : Radiolabel assay on batch 68 and 78 using the solid stock of tritiated NaBT4SAMPLEfor 1 2 3 4 5batch 68 and 78dry latex in g 0.001 0.001 0.001 0.001 0.001NABT4 mol/g 1 1O 1 10 1 10 1 10 1 10NABH4mollg 0 5 10 1 10 5 10 1 10-2water in p 1 200 200 200 200 200ethanol 1 200 200 200 200 2000.1 M HCL p. 1 to stop 100 100 100 100 100the reactionTable 9 : Results of the radiolabel assay of the aldehydes using the Scatchard plot.batch number aldehyde in result frommollg DNPH68 blank blank67 1.9 10 1.1 1066 2.5 106 1.2 1064 1.6 10.6 8.7 10.663 9.3 10 6.3 10.665 1.2 10 5.4 10678 2.3 i0 2.2 10The Scatchard plots give lower values compared to the DNPH assay results for the firstexperiment. The second experiment with batch 78 gave the same aldehyde surfaceconcentration. It was performed using a fresh preparation of NaBT4 , received as a solid anddiluted just before the experiment. In the other experiment the NaBT4 was received insolution in 0.1 M NaOH. In either case the results were not reproducible.461.8E-06.ci1.6E-06+1.4E-06 64*? 1.2E-06 65E1E-06 66C,, * x- 8E-07ci676E-07 684E-07 A2E-07A0• I I I I I IO.OE+OO 1 .OE-04 2.OE-04 3.OE-04 4.OE-045.OE-05 1 .5E-04 2.5E-04 3.5E-04 4.5E-04[hydride]eq mol/gFigure 15 : Radiolabel assays of the a.tdehyde concentrations with NaBT4. Bindingisotherms of batches 63, 64, 65, 66, 67 and 68.479E-05+8E-05 68+7E-05 + 78? GE-OS0EDU)0 4E-05 + +2E-051E-05;_J. a aO.OE+OO 1 .OE-02 2.OE-02 3.OE-02 4.OE-025.OE-03 1 .5E-02 2.5E-02 3.5E-02 4.5E-02[hydride]eq mol/gFigure 16: Radiolabel assays of the aldehydes with NaBT4 ; binding isotherm ofbatch 68 and batch 78480.0663D0.05 +64a*0.04 65-C D>-c- 0.03 XDU)ci)-D-I-a>1 0-cx0.01 * +A0 I I I I I I IOE+00 4E-07 8E-07 1 E-06 2E-062E-07 6E-07 1 E-06 1 E-06 2E-06[hydride]surf mol/gFigure 17: Resulting Scatchard plots from Figure 16, batches 63, 64, 65, 66, 67 and68.49aG)G)-a>1I[hydride]surf mol/gFigure 18 : Resulting Scatchard plot from Figure 17, batch 78.6E-051 E-05 3E-05 5E-0550Experiments repeated with either of the batches gave only background DPM. It is possiblethat NaBT4 hydrolysed in solution. This would explain the nonreproducibility of theexperiments and the lower value obtained from the experiment carried out with theradiolabel stored in solution.As seen from the unlabelled reaction a large excess of NaBH4 is needed to reduceall the aldehyde groups. In the reaction more than 700 000 DPM were used for eachexperiment yet only 200 to 1300 DPM were recovered on the latex. It is not possible towork with higher activities for disposal reasons. The few results obtained, especially forbatch 78 which gave the same result as with the DNPH assay, indicated some potential fora good aldehyde group assay as the limit of detection would have been very low. Thenonreproducibility and the high cost of radiolabelled sodium borohydride makes thismethodology unattractive. However a more reactive radiolabeled assay would be of greatinterest and perhaps a non-ionic reagent will facilitate the access through the electricaldouble layer on the latex surface.4.3.3 Nuclear magnetic resonanceIt was not possible to fully validate the DNPH assay with the NaBH4reaction nor didit give more information on the configuration of the aldehydes on the latex. NMRspectroscopy was therefore examined as it would seem to be an appropriate technique sincethe aldehyde present should resonate in a relatively clear window at 8 =9 ppm. It allowsa quantitative analysis by integrating the singlet and the broad peaks in the aromaticregion resulting from the benzene ring of the polystyrene at 8= 5-6 ppm. Broad peaks areexpected in NMR spectra of polymer solutions because of the reduced motion of the51polymer chains (78).Figures 19 & 20 show the NMR spectra of batch 78 and IDC ( bothsuiphate/aldehyde latex). The spectra of the dissolved latex gave broad peaks in thealiphatic region 8 = 1-2 ppm due to the aliphatic backbone of the polymer chains (79); thisregion was not used in the analysis. This region also gave additional peaks due to the solvent(THF peaks: ö = 0.75 and 8= 3.6 ppm) and the water trapped in the sample (79). Allbatches of latex showed two further groups of three peaks: first groups at 8=6.40, 6.51,6.61 ppm and the second at 8 = 6.89, 7.01, 7.O7ppm . They are due to the 5 aromaticprotons of the benzene ring. The multiple peaks are due to the configurational arrangementsof the polymer usually observed by NMR (80). A peak at 8 = 10.8 ppm was observed inall spectra. It corresponds to the acid region and is believed to be due to the sulphate endgroup or carboxyl groups resulting from the sulphate hydrolysis. The aldehyde peak isobserved in the region from a = 8.9 ppm to 8 = 9.4 ppm, the exact value depending onthe batch.Both acid and aldehyde peaks can be quantified. Table 10 shows the results of thequantitative analysis. It was done as follows:laromatic = Integration of the signals from the five aromatic H from styrenelaldehyde = Integration of the signal from the one H from the aldehyde grouplacid = Integration of the signal due to the acid proton.Considering the aldehyde quantification:Raid = (laromatic /5) / I aidehydewhere Raid is the mole ratio of styrene/aldehyde.52One gram of latex is composed of polymerized styrene, acrolein, and sulphate and acid endgroups. The mole ratio of styrene is high enough that we can assume:1 (g of latex) = flsty . MWstywhere n sty is the number of moles of styrene in 1 gram of latex.Therefore flacro = 1 I (Raid . MWsty)where flacro is the number of moles of acrolein per g of latexThe same analysis can be used for the acid peak.The peak at ö = 11.8 ppm does not vary much from batch to batch as would beexpected if its occurrence is due to the initiator. Using the result from the DNPH assayfor batch 78 of 2.9 x 10 mollg and IDC of 3.9 x 10 mollg and assuming that one acidgroup gives one charge: The area per charge would be 50 A2 /charge and 40 A2 /chargerespectively. Goodwin gives the area per charge as 377 A2 /charge, however, and IDCquoted 280 A2 /charge. Hence the NMR estimation gives much larger charge densities thanthose implied from the values measured by titration, values that could not be attributable toexperimental error. Hence the peak near 10.8 ppm cannot be due to the acid. It is possiblethe peak is due to protonated THF (plc = -3.5 (81,82), c.f. RSO4HplC4— -9). It could bedue to a weak acid although this would not be consistent with the titration results (discussedlater in XPS results).Aldehydes were observed as expected on batch 78 and not 68 (blank), but the limitof detection can be seen as batch 77 did not give a signal.5311.1 ppmIL 08.9 ppmI = 0.0267I = 31.767. 0PPMFigure 19: 1H NMR spectrum of batch 78, polystyrene/acrolein/suiphate latex ind8-THF7.Oppmx 128I = 0.03426.Sppmx1289.0I = 23.5354I I I6•6.4PPMFigure 20 : ‘H NMR spectrum of DC latex, polystyrene/acrolejn latex in d8-7.0 ppm10.8 ppm166.5 ppmI = 0.1810.8x 16PPM I = 131.81=0.259.2 9.0 8.3Fr HI = 86.3559.4 ppmFigure 21 1 H NMR spectrum of batch 85, polystyrenefacrolein/suiphate latex(seeded reaction) in d8-THF.I = 0.035PPM9.47.0 ppm10.8 ppmx646.5 ppmI = 0.055i. 9PPM1=32.84 I=22.0PPM• I6.3 6.65610.8 ppm9.4 ppm-9. 1 ppmx64x64I = 0.05J1 1=0.26594 9.2PPMI = 0.14181. 8PPM6.5ppm7.6PPMI = 77.45 1= 50.337I1 7.2 7.0 6.6 6.6Figure 22 : H NMR spectrum of batch 86 , polystyrene/acrolein/suiphate latex(seeded reaction swollen, latex with styrene monomer) in d8-THF.57Table 10: Quantitative analyses of the NMR peaks in the 8 = 9 ppm region andthe 8 = 10.8 ppm region.Batch number Aldehyde peak 8 = 10-11 ppm region8 (ppm) mole (mollg) 8 (ppm) mole (mol/g)68 none 0 10.8 4.5 1078 8.9 2.3 10 11.1 2.9 l078e NaBH4 none 0 10.8 3.3 1077 none 0 10.8 3.6 1085 9.4 3.0 10 10.8 4.7 1086 9.1 9.8 10 10.8 5.3 109.4 1.9 10mc 9.0 5.4 10 10.8 3.9 10-sAnalysis of batch 78 gave 2.3 x iO mollg which compares very well with 2.2 x 10moL’g found by the DNPH assay and 2.2 x 10 mollg by the NaBH4assay. It is interestingto note the change in chemical shift for batch 85 and especially the two peaks for batch 86.The peaks in the 9.1 - 9.4 ppm region are due to differences in the aldehyde environment.It is believed they originate from a polyacrolein oligomer from the surface, whereas thepeaks at 9.1 ppm are likely due to acrolein copolymerized n the styrene chain. In batches85 and 86 (Figure 21 & 22 respectively) the acrolein can polymerize in solution as moreinitiator is added. In the case of batch 86 copolymerization also occurs since the particle isswollen by styrene monomer, which would explain the peak at 9.1 ppm observed for theother batches. This supports the idea that in the semi-continuous reaction the acroleinactually copolymerizes on the surface and only on the surface, as the NMR and DNPHassays agree for batch 78.Batches 85, 86 and the mc latex show a higher concentration of aldehyde with the58NMR than with the DNPH assay. This suggests the aldehyde groups are not only on thesurface. This is improbable since the synthetic technique of 85 and 86 should only lead tosurface functionalization. It is more likely that the DNPH cannot react with all the aldehydeswhen their local concentration is too high, the reaction being stericaly hindered. Thisconclusion is supported by the result from batch 85 where the DNPH concentration wasfound to be 8.8 x 106 mollg, lower than the 3.0 x 10 mol/g found by NMR and lower thanthe 2.3 x l0 mollg of batch 78. In batch 85 the NMR evidence suggests acrolein is presentpartly as an oligomer on the surface, not distributed randomly on the surface as expectedfor the one step reaction. The DNPH reaction is going to be hindered as access to thealdehyde is limited on the surface.4.3.4 X-ray photoelectron SpectroscopySince we are trying to achieve a surface fuctionalization, XPS should be of greathelp to prove if the aldehydes are in fact present on the surface and are reduced by DNPH.Figure 23 shows the XPS spectrum for a polystyrene latex initiated with persuiphate. Carbonis the major detectable element on the latex which leads to the Cis peak having the highestintensity. The Ols and S2s,S2p peaks are due to the sulphate group on the surface. TheNals is thought to come from the salt added in the reaction. The Si2s and Si2p peaks arefrom impurities on the surface expected from handling and storage of the samples in glasscontainers.Figure 24 shows the XPS spectra of batches 68 (blank), 78 and IDC (bothaldehydes). The peak at 292 eV is due to the typical aromatic -, it shake up (69). Allthe carbons of the polystyrene chains have a very similar environment and give the59,c (B)400 200bind, energyl1gure 23: XPS spectxa of batch 68, blank polystyrene/sulphate latex. Al koc1486.6 eV was used.FIgure 24: Cis peak of batch 68,78, IDCIntensity[cps]+521*S2p+ C is+S2pAt AOO 1604’C (a)eo 40bind. energy [eVJINais+0 (a)+ 0 is4.Na (a)1200 1000 800 600area ebown indetailr -fe V)285 eVIntensity[cpsJ——6.7Cis-beVIDC296 294 292 288 286 284 282 280bind, energy LeVI60Figure 25: 0 is peak of batch 68 and JDCFigure 26 : N 1 s peaks of IDC latex, after reaction with DNPH.Intensity[cps]OlsIDC532.6 eV538 536 534 532 530 528 526 524bind, energy [eV]intensity Nis[cps]400.4 eV406.2 eV412 410 408 406 404 402 400 398bind. energy [eV]61symmetrical Cis peak at 285.0 eV observed for batch 68 (83). The symmetry of the peakis lost for 78 and mc. The carbon - oxygen double bond shifts the binding energy of theCis peak by 3 eV (84) to 288.0 eV due to the aldehyde groups. Figure 25 shows theincrease in the Ols peak due to the aldehyde as more oxygen is present on the surface.Figure 26 is the Nis spectrum of batch 78 after reaction with DNPH and confirms that theDNPH is present on the surface of the latex. The two Nis peaks at 400.4 eV and 406.2 eVare due to the two different nitrogens of the hydrazone complex, respectively the nitrogensfrom the carbon-nitrogen double bond and from the nitro groups . Batch 68 following theDNPH reaction was run as well and as expected did not show any nitrogen. Figure 27 showsthe spectrum of 68, mc and mc + DNPH. The DNPH broadens the Cis peak. Thebroadening is parallel to the mc curve giving a similar assymmetry in the peak. It isexpected as the DNPH layer on the surface is going to mask to some extent signaloriginating beneath it and contribute to its own characteristics. Figure 28 shows the sameexperiment but with batch 78 which has a lower aldehyde coverage, implying the signal isgoing to be less masked. A broadening is observed on the surface due to the new carbonpresent, especially the C-NO2 carbon which gives a shift of 1.8 eV to 286.8 eV (84).Figures 29 & 30 show the results of the Cis curve fitting. The best fit was obtained with3 peaks ( see Table 11): 285.0 eV due to the core polystyrene, 286.5 eV with the 1.5 eVshift due to a C-O bond and 288.0 eV, the shift due to a C=O bond. The shift of 1.5 eVis usually considered to be due to a hydroxyl group but could be due in this case to the c0S03 (84) which would explain the high percentage obtained.62Intensity[cps]Figure 27: C is peak of IDC latex after reaction with DNPHFigure 28: Cis peak of batch 78 after reaction with DNPH[eV)Ci8IDC294 292 290 288 286 284 282bind, energyIntensity[cps] Cis78+ DNPH296 294 292 290 288 286 284 282 280bind, energy [eV]63intensity[cpsllFIgure 29: Curve fitting of the Cis peak from batch 78280EeV]Figure 30: Curve fitting of the Cis peak from ]DC latexIntensity[cps]Cia 78286.5 eV288.0 eV296 294 292 290 288 286 284 282bind, energyCia IDC285.0 eV288.0 eV296 292 290 288 286 284 282bind. energy280[eVI64Figure 31: Cis spectra of batch 78 after reaction with NaBH4Table 11 : Results of curve fitting of the Cis peakbatch 285.eeV 286.5.0eV 288.0eVnumber polystyrene alcohol/SO4 aldehyde78 90.1 % 6.6 % 3.3 %mc 88.3 % 7.9 % 3.7 %Since XPS is a semi-quantitative technique, the percentages cannot be taken asabsolute. It is surprising, for instance that the mc latex shows the same concentration asbatch 78, while from the DNPH assay or from NMR the mc latex bears more aldehydegroups..Figure 31 shows the reduction in the aldehyde peak after reaction with NaBH4,usingsample 78e from the NaBH4reduction experiment. It confirms that all the aldehydes are notIntensity[cpsCis7878+296 292 288 284bind, energy [eV)65reduced.XPS showed no evidence of acid groups in the Cis peak, which would have beenexpected at 289.5 eV (84). Although this is not absolute proof of the absence of acid it isconsistent with the explanation that the NIv1R peak at ö = 10.8 ppm is due to the protonationof the solvent.XPS confirmed the presence of aldehydes on the surface through the Cis peak andparticularly by the effect of the reaction of DNPH with the appearance of a new Nis peak.Because of the inherent limitations of the technique, however, the data cannot be interpretedto provide the surface concentrations of the species.66CHAPTER 5 CONCLUSIONThe semi-continuous synthesis of polystyrene/acrolein latex was succesfully carriedout. The optimum addition time of the acrolein monomer was determined to be 10 hoursafter addition of the initiator. It gave monodisperse polystyrene latex with aldehyde on thesurface. A DNPH assay was used to assay the aldehyde group in the latex suspension; it wasspecifically a surface assay. It successfully detected aldthydes at concentrations above 1 x106mollg and was determined to be limited at high surface concentrations of aldehyde probablydue to the steric hindrance. Quantitative agreement with respect to surface concentration ofaldehyde groups was obtained using freshly solubilized NaBT4 reduction in a radiochemicalassay.NMR was used as a valuable technique to assay the aldehydes present in dissolvedlatex and the same result as for the DNPH assay was obtained. It was found to be veryuseful as it provided information on the environment of the aldehydes on the surface, thechemical shift showing the possible presence of polyacrolein chain on the latex with theseeded reaction. 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