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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 CHARACTERIZATION by CORINNE LE DISSEZ B.Sc.Hons., Robert Gordon’s University, Aberdeen, 1991 A THESIS SUBMITthD IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Chemistry We accept this thesis as conforming standard to the  THE UNWERSITY OF BRITISH COLUMBIA April 1994 Cormne Le Dissez, 1994  In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  (Signature)  Department  of  The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  2  4.  ABSTRACT  A semi-continuous synthesis of polystyrene/acrolein latex was carried out. The optimum addition 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 used to assay the aldehyde group in the latex suspension; it was specifically a surface assay. It successfully detected aldehydes at concentrations above 1  x  10 mol/g but was limited at high  surface concentrations of aldehyde probably due to the steric hindrance. Quantitative agreement with respect to surface concentration of aldehyde groups was obtained using freshly solubilized 4 reduction in a radiochemical assay. NaBT  NMR was used to assay aldehydes present in dissolved latex and the same result as for the DNPH assay was obtained. NMR provided information on the environment of the surface aldehydes, the chemical shift showing the possible presence of polyacrolein chains on the latex with the seeded reaction. This was an important observation as the polyacrolein blocks are not favorable 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 NaBH . The XPS spectra obtained were those 4 expected based on the surface chemistry anticipated and no unexpected groups were detected.  ii  TABLE OF CONTENTS  Page Abstract  ii  Listoftables  V  Listoffigures  vi  Acknowledgements  viii  Dedication  ix  CHAPTER 1 INTRODUCTION  1  CHAPTER 2  3  BACKGROUND AND THEORY  2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5 2.1.6 2.1.7  Emulsion polymerization General background and history Mechanism of reaction Surfacestabilization Copolymertheory Copolymerization techniques Surface characterization of latex Aldehyde functionalization of latex  3 3 4 7 9 10 11 12  2.2  X-ray photoelectron spectroscopy  12  CHAPTER 3 EXPERIMENTAL  3.1 3.1.1 3.1.2 3.1.3  14  Polystyrene latex synthesis Materials Methods Monitoring the reaction  14 14 14 15  Surface functionalization with acrolein 3.2 3.2.1 Materials 3.2.2 MethOdS Two stage reaction Semi continuous reaction  m  17 17 17 17 18  General latex chcon 3.3 3.3.1 Size distribution 3.3.2 Concentration determination  .  Surface analysis 3.4 3.4.1 Dinitrophenylhydrazine assay Materials Standardcurve Assay on the latex suspension 3.4.2 X-ray photoelectron spectroscopy 3.4.3 Radiolabelled assay using tritiated NaBT 4 Malerials Methods 3.5  Nuclear magnetic resonance  19 19 19 19 21 22 22 22 23 25  CHAPTER 4 RESULTS AN]) DISCUSSION  4.1  18 18 19  26  Polystyrene latex synthesis  26  4.2 4.2.1 4.2.2 4.2.3  Surface modification with acrolein Time dependence of acrolein addition Concentration dependence of acrolein addition Two stage reaction  30 32 37 40  4.3 4.3.1 4.3.2 4.3.3 4.3.4  Developing an alternate aldehyde assay A model for the latex surface 4 assay NaBH Nuclear magnetic resonance X-ray photoelectron spectroscopy  41 41 42 51 59  CHAPTER 5 CONCLUSION  67  References  69  iv  LIST OF TABLES  Table 1:  Experimental conditions for the emulsion polymerization of polystyrene latex with persuiphate initiation  28  Table 2:  The effect of acrolein addition at the times indicated  32  Table 3:  Study of the effect of variation of the monomer ratio on the copolymerization; acrolein addition after 10 hours  38  Table 4:  DNPH assay results from latex synthesized by seeded reaction  Table 5:  Characteristics of batch 68 (blank) and 78 (aldehyde grafted)  42  Table 6:  Reaction of the polystyrene/aldehyde latex (batch 78) with 4 ; verification of the efficiency of reduction NABH  43  Radiolabel assays on batches 63, 64, 65, 66, 67, 68 using the 0.1 M solution of NaBT 4  45  Radiolabel assay on batch 68 and 78 using the solid stock of 4 tritiatedNABT  46  Results of the radiolabel assay of aldehydes using the Scatchard plot  46  Quantitative analysis of the NMR peaks in the 8 region and 8 = 10.8 ppm region  58  Table 7:  Table 8: Table 9:  Table 10:  Table 11:  Results of the curve fitting of the Cis peak  V  =  40  9 ppm  65  LIST OF FIGuRES Figure 1:  Figure 2 :  Mechanism of particle formation according to the coagulation theory for a surfactant free emulsion polymerization  6  The electrical double layer around a latex particle. Reproduced from reference (28)  8  Figure 3  Electron ejected from the sample surface by XPS  13  Figure 4  Styrene monomer calibration curve. Measured OD at 247 nm vs styrene monomer concentration in methanol molIl. c(247 nm) = 15,287 1 / (mol cm)  16  DNPH assay calibration curve. Measured OD at 360 nm vs concentration of hydrazone in THF. c(360nm) = 26,760 1 I (mol cm)  20  Scanning electromicrograph of batch 68, blank polystyrene I sulphate latex  27  Scanning electromicrograph of batch 65, polystyrene! acroleinlsulphate latex. Acrolein added after 10 hours  27  Figure 8  Monitoring of the emulsion polymerization of polystyrene  29  Figure 9  Theoretical composition of styrene/acrolein copolymer. r,=0.034 andr=0.32  31  Effect of acrolein addition at the times indicated. Results obtained using the DNPH assay  34  Scanning electromicrograph of batch 69, polystyrene! acrolein/sulphate latex .Acrolein added before the initiator  36  Figure 5 :  Figure 6 : Figure 7 :  Figure 10:  Figure 11:  Figure 12:  Figure 13: Figure 14:  Figure 15:  Scanning electromicrograph of batch 67, polystyrene! acrolein/suiphate latex. Acrolein added after 2 hours  36  Effect of variation of the monomer ratio on the grafting ofacrolein  39  Result of the DPNH assay after reduction of polystyrene/acrolein latex (batch 78) with NaBH 4  44  Radiolabel assays of the aldehyde concentrations with NaBT . Binding 4 vi  isotherms of batches 63, 64, 65, 66, 67 and 68.47 Figure 16:  Radiolabel assays of the aldehydes concentrations with NaBT . Binding 4 isotherms of batch 68 and batch 78  48  Resulting Scatchard plots from Figure 16, batches 63, 64, 65, 66, 67 and 68  49  Figure 18:  Resulting Scatchard plot from Figure 17, batch 78  50  Figure 19:  1  Figure 17:  Figure 20:  Figure 21:  H NMR spectrum of batch 78, polystyrene/acrolein/suiphate latex in d8-THF  54  1  H NMR spectrum of IDC latex, polystyrene! acrolein latex in d8- THE  55  1  H NMR spectrum of batch 85, polystyrene/acrolein/suiphate latex (seeded reaction) in d8 TifF  56  1  H NMR spectrum of batch 86, polystyrene/acrolein/suiphate latex (seeded reaction swollen latex with styrene monomer) in d8 THF  57  XPS spectra of batch 68, blank polystyrene/acrolein/suiphate latex. Al kc 1486.6 eV was used  60  Figure24:  Clspeakofbatch68,78,ll)C  60  Figure 25:  Olspeakofbatch68andlDC  61  Figure 26:  Nis peak of IDC latex after reaction with DNPH  61  Figure 27:  Cis peak of IDC latex peak after reaction with DNPH  63  Figure 28:  Cis peak of batch 78 after reaction with DNPH  63  Figure 29:  Curve fitting of the Cis peak from batch 78  64  Figure 30:  Curve fitting of the Cis peak from JDC latex  64  Figure 31:  Cis peak of batch 78 after reaction with NABH 4  65  Figure 22:  Figure 23:  Vii  ACKNOWLEDGEMENTS  I 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 their invaluable services. Finally, but by no means least, I would like to thank Bruce for his helpful views both within and without the field of chemistry.  viii  A mes parents  ix  CIIAYTER 1 INTRODUCTION  Polymer latexes obtained from surfactant free emulsion polymerization are composed of clean particles with large surface areas which are of great interest in the newly developing areas of drug delivery and immunoassays (1-4). Polystyrene latex is the most commonly used, because it possesses the required mechanical properties (5-7). However the hydrophobic character of the surface tends to be a problem in blood compatibility (6). Modification to achieve a more hydrophillic surface, leading to better blood compatibility is necessary. It is achieved by copolymerization or surface grafting (8). The effects of such surface 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  emulsion  copolymerization (11), surface grafting has been the prefered approach to surface modification. Grafting of polyacrylamide on surfaces has been reported in the literature using 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 surfaces containing alcohols, aldehydes and amines using cerium salts (16). The oxidation by ceric ion of those functional group proceeds with the formation of a free radical on the reducing agent (17,18). Polyacrylamide can be grafted on surfaces containing such functional groups e.g. modified polyethylene films (19) or granular starch (20). W. Muller used cerium ammonium nitrate to graft acrylamide on the surface of beads used as chromatographic supports (20-40  m diameter) carrying hydroxyl groups on their surfaces (21). He reported  no polymerization of acrylamide in solution. The overall aim of the project is to graft polyacrylamide onto polystyrene particles 1  carrying aldehyde groups on the surface. These functional groups can be added to the surface through copolymerization with acrolein in such a way that the grafted layer has quantifiable 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 surface layers 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 aldehyde functions are explored. Since it should be possible to count the number of polyacrylamide chains 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-ray photoelectron spectroscopy which proves very useful in this application. Application of the assays developed combined with nitrogen analysis of the polyacrylamide-derivatized latex should provide values for the average molecular weight of the grafted layer.  2  CHAPTER 2 BACKGROUND AND THEORY  2.1  Emulsion polymerization  2.1.1 General background and history  Emulsion polymerization is a reaction technique to produce latexes which are stable dispersions of polymeric materials in aqueous media. They can be designated as colloidal systems as their particle size ranges from 1 nm to 1 u m implying that the particles will remain 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) 1O to 1O. The particle is stabilized by surface active components which arise either from the initiator or from surfactants added to the polymerization. The first commercial emulsion polymerization was the development of butadiene styrene and polybutadiene latexes for synthetic rubber in the mid 1930s. This marked the beginning of an enormous industrial interest in polymer colloids. The industrial development lead to some theoretical studies and to the first scientific publications in the 1940s  .  The  emulsion 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 such as synthetic rubber, paints, coating, adhesives, latex foam, carpet backing or additives in construction materials and the more recent area of biological applications (26). Latexes can be applied in three different ways (27): -  Particles can be coagulated and the solid polymer is recovered 3  -  -  Upon drying continuous films can be formed and used as coatings Latexes can be used as particles  The use of latexes in different applications depends on their stability and their surface characteristics. Latexes have been used in industry empirically but the need for theory arises for more understanding. Today the area is still developing and although much has been achieved in the understanding of the reaction kinetics many areas are unclear and the field remains active.  2.1.2 Mechanism of reaction  Emulsion polymerization proceeds via free radical polymerization and involves the three following steps: -  initiation  -propagation -termination Contemporary theories of mechanism and reaction kinetics are still based on the work of Smith and Ewart (25). Most theoretical work has been done using styrene emulsion in the presence of surfactant (28,29). The surfactant forms micelles in solution which are believed to be sites for particle formation as they give stability to the particle due to their surface active properties. However there is still some debate on the exact mechanism of nucleation. Three theories are proposed for particle nucleation: Entry of free radical into the micelle (28,30), homogeneous nucleation due to the precipitation of growing chains and coagulative nucleation of precursor particles (31). Polymerization utilizing surfactant is mainly used and heavily documented in the literature (32,33). For some applications 4  however, the surfactant may be undesirable on the surface of the latex, for instance where the particle is used as an adsorbent. It is very difficult to remove surfactant completely from these 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 have been synthesized since (35-37) and the mechanism of reaction investigated (38), which helped in the theoretical discussion of nucleation. The coagulative nucleation proposed by Ottewill (39) seems to be the best accepted theory and has been further discussed for surfactant free emulsion polymerization (38,40,41). This theory is described below and will be used as the basis of discussion. In the case of persulphate initiated styrene emulsion polymerization, since styrene is only slightly soluble in water monomer droplets are formed in aqueous suspension. Only a 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 thermal decomposition (42) 8 O 2 S  -‘  2 S0 4  which react in the aqueous phase with monomers (M) M  +  . 4 S0  -  .  1 MSO  M +  .  4 MS0  -  .  4 MMSO  M +  .  4 MMSO  -  .  1 MMMSO  to form growing radicals. These are surface active due to their charged end group. Initially the monomer solubility increases due to the polar group (35). These individually growing oligomers become insoluble polymer chains at the point in their growth where they exceed 5  the critical degree of polymerization required for solubility in the aqueous phase. They are believed to precipitate out, described as a homogeneous nucleation, to form small precursor particles about 5 nm in diameter which are colloidally unstable and are slightly swollen by monomer. Coagulative nucleation occurs as these precursor particles coagulate to form mature growing latex particles. This process allows them to reach colloidal stabffity with the  polar groups remaining on the surface (36), swollen with enough monomer to allow propagation to occur and increase particle size. The particles continue to grow until all the monomer has been consumed and termination occurs. Kinetic models have been established for this coagulative nucleation theory and it is the most widely supported although competing theories still carry some weight (39,4 1). Figure 1 shows a schematic outhne of the proposed reaction mechanism.  . 4 -SO  -.  4 .MSO  Formation of oligomers  4 •44W S0  Oligomers precipitate  soi  4 144J S0  4 -so 50 nm  4 so 0  colloidally instable  Q  stable latex’  Coagulation  Gmwth Final latex  Figure 1: Mechanism of particle formation according to the coagulation theory for a surfactant free emulsion polymerization.  6  2.1.3 Surface stabilization  Surfactant-free latex particles form colloid systems thanks to the surface charges arising from the initiator used. Monodisperse latexes are the result of their colloid stability which is achieved at a given size depending on the stabilizing charge. Different charge groups have been reported: a weak acid originating from hydrogen peroxide, a base from azo compounds and a strong acid from persuiphate. Both cationic (43) and anionic latexes are possible. The stabilizing group on the surface exists either in the ionized or unionized form depending on its plc and the pH of the solution. Where the surface of the particle becomes electrically charged, electrical neutrality is maintained by balancing surface charges with counter ions from solution. This surface arrangement is called the electrical double layer and is represented schematically in Figure 2. The electrical double layer controls the decay of the electrostatic surface potential V() [1], which is responsible for the repulsive forces between particles, i.e., for their stability in solution (28). The thickness of the double layer, that is the extension of electrostatic potential away from the surface, depends on the ionic strength, being greater at lower ionic strengths according to [ 2 ] (44). Hence, increasing ionic strength reduces stability.  =  where ‘F  =  0 ‘P  =  ‘0  a --exp[ic(a-r)J r  [1]  the potential in the solution phase at a distance r from the center of the particle the surface potential  a  =  radius of the particle  r  =  distance from the center of the particle  7  ic =[ (8 I  it  N 1 2 e 1 ) / (1,000 ckT)]  ½  [21  = ½ Ec z? 1  c, = molar concentration of ionic species i zj = valence of th species 1 = Avogadro number N e = electron charge  = dielectric constant k = Boltzman constant T = absolute temperature  ———--  +  / +  —  _+  / -  / Ill  +  /  FIgure 2: The electrical double layer around a latex particle. Reproduced from reference (28) In persulphate-initiated latex -0S0 3 are the main groups on the surface. However reaction of persuiphate in solution is believed to also produce hydroxyl groups on the surface according to the following reaction (42): . + H 1 SO 0 2  -  4 + HO. HSO  Furthermore, oxidizing conditions during polymerization may oxidize hydroxyls to 8  carboxylic acids (36). Hydrolysis could be decreased by using buffers such as NaHCO 3 or CO (45) as the reaction is minimized at higher pH (42). 2 Na 3 The surface arrangement of the particles consistof the described hydrophilic charges and a large hydrophobic (polystyrene) surface. It is possible to modify the surface to control the properties. Modification of the surface is possible to achieve other properties. For instance adsorption of macromolecules or surfactants is often used (46-49), but  these  approaches will not be discussed here. Instead this work will concentrate on the modification of the bulk of the latex or its surface by copolymerization to achieve the desired properties by methods for which theory is expected to be applicable (50,5 1).  2.1.4 Copolymer theory  If the free-radical polymerization of a mixture of two monomers Mi and M2 is considered the composition of the copolymer can be estimated knowing the monomer composition of the feedstock and the monomer reactivity ratios (52). The polymerization consist of four established propagation steps. -Mi.+Mi -M1.+M2 -M2.+M1 -M2. + M2  -  -  -,  -‘  -MiMi. -M1M2. -M2M1. -M2M2.  giving the four rate laws Rp,ij corresponding to the four propagation steps: Rp,ii  =  ku {Mi .][Mi]  Rp,12  =  k12 [Mi .][M2]  9  Rp,21  =  k21 [M2 .][Mi]  Rp,22  =  k22 [M2 .][M2]  From those equations, by applying the state approximation for free radical polymerization the following expression can be written:  d[Mi]  1 + ri [Mi] I [M2]  d[M2]  1 + r2 [M2] / [Mi]  where &  ri  =  ku I k12  2  =  k21 / k22  If F i is the mole fraction of component i in the polymer and f i the mole fraction of monomer i in the feedstock, it can be shown that (52):  ri f12 + ft ft Fi=— ri fi2 + + r2.f22 .  .  .  Values 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 be applied to predict copolymerization in the case of latexes.  2.1.5 Copolymerization techniques  Two different approaches can be applied in latex copolymerization. In the first approach are grouped the batch and semi-continuous processes, respectively where all the monomers are either added at the beginning of the reaction, for example the use of isothiuronium 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 example 10  the synthesis of styrene p-formylstyrene copolymer latex (55). The second approach is generally termed a seed reaction, where particles previously synthesized are used as seeds for growing the second polymer to form bigger particles with core shell structures, for example the two stage emulsion polymerization of acrylonitrile and butadiene (core) (56). The morphology of two stage reaction depends on the properties of each monomer. This area is extensively discussed in the literature (57-60) as the properties of the resulting particle have wide potential technological application.  2.1.6 Surface characterization of latex.  A comprehensive survey of the different techniques used to characterize latexes has been published by El-Aasser and Fitch (27). The particle size distribution as established by electron microscopy or light scattering and the surface charge density obtained by electrophoretic mobility measurements or conductometric titration are the two main characteristics of a latex batch. Many other analytical technique such as X-ray photoelectron spectroscopy (XPS)(60), static secondary ion mass spectrometry (SSIMS)(61), nuclear magnetic 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 Ru0 4 for electron microscope (EM) analysis has beenused to study the morphology of core-shell polymethylmethacrylate/polystyrene (PMIvIA/PS) latexes (59). Latex characterization is an active area of investigation as more information is required for the understanding of latex formation and the development of theories.  11  2.1.7 Aldehyde functionalization of latex  Polyacrolein 2 -[CH çH1- latex has been synthesized by surfactant emulsion CHO polymerization (65,66) and its use as a drug carrier discussed (67). The high concentration of aldehyde groups on the surface allows covalent binding with amino groups on biological materials. However some problems are encountered in the polydispersity as well as some sedimentation due to the specific gravity (68). A seeded copolymerization of styrene I glutaraldehyde using aldol condensation was achieved by Okubo et al. (63) and batch copolymerization of styrene/acrolein was reported by Slomkowski et al. (7,68). The advantage of the seeded copolymerization is to keep the polystyrene particle as a well defmed hydrophobic core. Those two techniques will be the basis of the present work and will be further developed.  2.2  X-ray photoelectron spectroscopy  XPS is an electron spectroscopic technique used for qualitative and semiquantitative surface analysis. In an ultra high vacuum (UHV) (2 x 10_la mbar) an X-ray photon which is directed onto the sample is absorbed by surface atoms. As a result of the higher energy of the X-ray photon compared to the binding energies of electrons in the atoms, electrons are ejected,( Figure 3). These electrons possess kinetic energies characteristic of the electronic configuration of the atom configuration in the sample. The electron kinetic energy follows the equation:  12  Eke =hv-Eb-4)  where Eke hv Eb  4)  -  -  -  -  kinetic energy of the ejected photoelectron characteristic energy of the X-ray photon binding energy of core level work function term  X-ray hv\  Ejected electron energy E ke  Sampling depth  approximately, 30  A  Iigure 3 : Electron ejected from the sample surface by XPS.  The ejected electron binding energy depends on the original molecular environment and its oxidation state, giving a chemical shift characteristic of the molecule in a given environment. Only photoelectrons from the outermost atom layers have sufficient energy to escape the sample and reach the detector. Clark and coworkers showed the escape depth for 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 by looking at the Cis, and Nis signals (7,63).  13  CHAPTER 3 EXPERIMENTAL  3.1  Polystyrene latex  3.1.1 Materials  Styrene monomer, ACS grade obtained from Fisher, New Jersey, was distilled under reduced pressure and stored under nitrogen. Potassium persulphate, K2S208 from B.D.H., Toronto, and sodium chloride (NaC1) from Fisher chemical, New Jersey, were used without further treatment. Absorbance measurements were made on a IIEWLET-PACKARD UV/VIS spectrophotometer, model 8450A , using 1 cm path length quartz cuvettes. Doubly distilled water was used for all experiments.  3.1.2  Methods  The 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 emulsion polymerizations were carried out in a 500 cm 3 pyrex round bottomed four necked flask equipped with an internal overhead stirrer. The T-shaped teflon stirring blades were one centimetre from the bottom of the flask allowing a stirring range of 60 to 500 rpm. A water cooled condenser connected to the atmosphere via a wash bottle and an addition funnel connected to a nitrogen inlet were fitted to the second and third outlet. The fourth outlet was left available for sampling. The flask was immersed in a temperature controlled water bath (+1- 1°C). 14  The total reaction volume was 180 cm . NaC1, 0.109 g dissolved in 132 ml of water 3 was 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 addition funnel was set in place with 0.130 g of persulphate initiator dissolved in 10 ml of water and was left to sit for another 15 mm  with the nitrogen flow set to minimum to avoid  evaporation. The reaction flask was equilibrated at 70 °C. The initiator was added to the solution and the funnel washed with 20 cm 3 of degassed water. The reaction was allowed to proceed for 24 hours. The preparation was cooled down before being filtered through a filter pack of glass wool to remove any major agglomerates. The latexes were dialysed against 12 liters of distilled water for a week, changing water every 24 hours, to remove most of the unreacted monomer and salts. Finally, they were washed 3 times with water and recovered by centrifugation. All latex preparations were stored at 4 °C in 50 ml polypropylene tubes at a concentration of 4 % w/w in water.  3.1.3 Monitoring the reaction  The reaction was monitored by taking suspension samples which were diluted in methanol, as styrene monomer is soluble in methanol. The sample was centrifuged and the supernatant 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 in methanol. The extinction coefficient was found to be E(247 nm) = 15,287 1 / (mol cm).  15  C’) G) Cu C.) .1-I  0  0  1 E-05  3E-05 [STYRENE] in mol/I  5E-05  Figure 4: Styrene monomer calibration curve: Measured OD at 247 nm vs styrene monomer concentration in methanol molIl. c (247 nm) = 15,287 1 I (mol cm).  16  3.2  Surface functionalization with acrolein  3.2.1 Materials Acrolein was obtained from Aldrich, Milwaukee, distilled at reduced pressure before use and stored under nitrogen at 4 °C. Polystyrene/aldehyde latex was used as supplied by  Interfacial Dynamics  Corporation, Portland, Oregon. The mean diameter was 604 nm with a charge density of one 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. They were used as a reference to test the aldehyde groups assays studied.  3.2.2 Methods  Two stage reaction -  Synthesis of batch 85  A batch of pure sulphate latex was synthesized according to the technique decribed in section 3.1.2. A dialysed and characterized polystyrene sulphate latex (batch 68) was used as the seed latex. The same experimental set-up as for the synthesis of the polystyrene sulphate latex was used. Nitrogen was bubbled through 50 g of 3.3 % w/w of seed latex suspension and 3 g of acrolein monomer. The flask was left to reach an equilibrium temperature of 50 °C, with constant stirring at 350 rpm. After 15 minutes 38 mg of potassium persulphate initiator in 8 ml of water was added and the system allowed to react for 12 hours. -  Synthesis of batch 86  The same above technique was used in this synthesis, but the seed latex were swollen 17  for 15 minutes with 1 g of styrene monomer before the addition of 3 g of acrolein, and the reaction was left to react  Semi-continuous reaction The semi-continuous reaction was the main focus of this project. The experimental set 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 of water  .  In this work the effect of addition time and the amount of acrolein added were  investigated.  3.3  General latex characterization  3.3.1 Size distribution  Initial observations were made on a light microscope. The size distribution observations were made from photographs from a I{ITACH model S2300 scanning electron microscope. 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 to note that this is not the standard procedure for full latex characterization; more time would be required for a good assay on 1000 latex particles. However it was sufficient as the present work did not require an accurate diameter or size distribution. The sample was prepared as follows: A drop of diluted latex suspension was dried out on the graphite surface of the sample holder and plated with 100 A thick film of Au-Pd (60/40  %) using a Hummer  4 gold coater, (source power 600-700 v, 2OmA, in an Argon atmosphere). 18  3.3.2 Concentration determination  The concentration of the latex suspension was determined by drying a sample in an oven at 65 °C. This allowed the preparation of latex suspensions in water from known dry weight concentrations.  3.4  Surface analysis  3.4.1 Dinitrophenyihydrazine assay  Materials Dinitrophenythydrazine (DNPH; 2 N (0 N 3 H 6 C fINH ) ),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 curve Dinitrophenythydrazine reacts specifically with aldehydes to form a hydrazone complex [3] which absorbs in the UV range making the reaction a valuable spectroscopic assay 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 absorbs at 200 to 280 nm  19  1.2  1.0  0.8 > C,)  C  ci)  -D (U 0  0.6  0 0.4  0.2  I I 0.0• I I I 4.OE-05 3.OE-05 2.OE-05 0.OE+00 1 .OE-05 3.5E-05 2.5E-05 5.OE-06 1 .5E-05  [HYDRAZONE] mol/I  Figure 5 : DNPH assay calibration curve. Measured OD at 360 nm vs concentration of hydrazone in THF. c (360 nm) = 26,760 1 / (mol cm)  20  H  H 2 NO  +  RH  N  N 2 H  R  2 NO  2 NO  0 2 + H  2 NO [3]  DNPH Amax  =  HYDRAZONE derivative 350 nm  The extinction coefficient  Amax  360 nm  at 360nm for the hydrazone derivative in T.I{F was  obtained using 3-phenyl propanaihydrazone, a closely related compound to the polystyrene/acrolein copolymer. It is reported in the literature that the extinction coefficient of hydrazone complexes do not vary much with the type of aldehyde (70). It was synthesized in the following way: 0.25 g of DNPH was dissolved in 5 ml of methanol and 0.5 ml of sulphuric acid was added carefully. The solution was filtered and 0.2 g of 3-phenyl propanal added, 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. The calculated extinction coefficient is €(360)  =  26,760 IJ(, comparable to value found  in the literature (70).  Assay on the latex suspension The hydrazine and hydrazone absorb in the UV at 350nm and 360nm respectively making it difficult to quantify a mixture. The hydrazine therefore was separated from the hydrazone 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 are swollen in ethanol which can facilitate the access of the DNPH molecule. The solution was 21  washed with ethanol by centrifugation to remove the unreacted DNPH until the supernatant was clear and did not absorb in the UV. The latex was dried under reduced pressure for three hours. A known amount was then dissolved in TI]F, the OD measured at 360 nm and the concentration of hydrazone associated with the latex calculated. This reaction allows an estimation of the concentration of aldehyde specifically on the surface of the latex, i.e., accessible to DNPH in solution  3.4.2 X-ray photoelectron spectroscopy  Latex suspensions were freeze dried and were put on one side of a double sided adhesive (39). It was then transferred under continuous U1{V (2 x 10-10 mbar) into the analysis 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 20 mA. The emitted photoelectrons were collected from a 2 mm 2 area.  3.4.3 Radiolabelled assay using NaBT 4  Materials Sodium 3 boro[ H lhydride (NaBT ) was purchased from Amersham Canada, Ltd, 4 Oakville, Ontario. Batch 101 was in 0.1 M NaOH with a specific activity of 440 GBq . Batch 183 was 1 mg of solid with a specific activity of 220 GBq mmol 1 mmol . Atomlight 1 scintillation cocktail was purchased from Dupont, Boston, Massachussets. A Phillips PW 4700 liquid scintillation counter was used for all scintillation counting.  22  Methods 4 is an effective reducing agent which converts aldehydes to their corresponding NaBH alcohols. (72,73). The mechanism of this reaction is shown below [4] (unlike standard solution chemistry borohydride cannot react with 4 aldehyde moieties because of their surface immobilization). A hydrogen from the NaBH 4 attaches to  the carbon of the  aldehyde. Using tritiated NaBT4 a tritium bounds to this carbon allowing quantification of the reduced aldehyde.  0  +  —  ] 3 S.L+[H- BH  -OH 2 R—CH  +  Na  -  R  —  - OBH 2 CH 3 Na+  [4]  3 B(OH)  The advantage of a radiochemical assay is the very low limit of detection  (  -  l0  mol) (74). Counts determine number of tritium atoms and thus the number of aldehyde groups present. In a 1.5 ml Eppendorf centrifuge tube, cold NaBH 4 and NaBT 4 in 0.1 M NaOH solution were mixed with a latex suspension. A 200  i  1 aliquot of ethanol was added to  swell the latex and favor the reaction. The tubes were mixed for 24 hours. The NaOH solution kept the pH alkaline and limited hydrolysis (75). The suspension was centrifuged to separate the latex from the unreacted NaBH . The latex was then resuspended in 0.8 ml 4 of water to which 100  1 of 0.1 M HCL was added to hydrolyse the complex. After 1 hour  the latex was washed three more times before dissolving it in imi of THF. Ten ml of scintillation cocktail was added and the samples counted. The concentrations of NaBT 4 and latex were kept constant and the concentration 23  4 varied. The equilibrium concentration of thtium dissolved in THF solution was of NABH  calculated from the disintegration per minutes (DPM) results. Knowing the specific activity of the radiolabel the concentration of aldehyde on the surface for a given total equilibrium 4 concentration was obtained which gave the binding isotherm: 4 NaBH [NaBH vs [R J OH]. A Scatchard plot was calculated to extrapolate graphically the number of aldehyde 2 CH groups on the latex at saturation. The Scatchard analysis is derived the following way (76):  Take the following reaction at equilibrium:  + R-CHO + H + 3 HO  4 -BR  R2 -CH O H  +  B0 + 3 H 3 H 2  the equilibrium constant ka ie equal to: BO [H ] [R-CH O 2 H]. 3  [R-CH O 2 H]  .  BO 3 [H ]  ic= [NaBH ] 4 . [R-CHO]. since [ALDJ  =  [I{O]  [ffl{][I{O]([ ALDOJ  -  [R-CH O 2 H])  [R-CH O 2 H] + [R-CHOJ  which can be rearranged to give O].[NaBH 3 .[H ] ) [RCH O 2 H1q. ] B 3 ([H eq 0 + Ka 4 dividing by  (  ] 4 [NaBH  [R-CH O 2 H] ] 4 [NaBH  .  ]. [ALDO] 4 ]. [NaBH 4 0 3 Ka .[H  =  BO 3 [H ] ) -  Ic. [H O].[ALDJ 3 BO 3 [H ]  -  O]. 2 3 K,. [H [R-CH O H] BO 3 [H ]  if 2 [NaBH is plotted vs 2 ] [R-CH O H1 I 4 [R-CH O H1 the x-intercept is equal to [ALD]  where:  equilibrium constant assuming no isotope effect  Ic [RCH O 2 HJ  =  equilibrium concentration of reduced aldehydes, 24  obtained from the counts on the latex. [R-CHOJ  =  equilibrium concentration of non -reacted aldehydes  J 4 [NaBH  =  equilibrium concentration of NaBH 4 in solution, obtained by difference between initial counts and counts on the surface.  [ALD]  =  total concentration of aldehydes from the surface  The reaction was tested using NaBH 4 (unlabelled) at higher concentration. After hydrolysis and wash of the latex the DNPH assay was performed to verify if the aldehydes were effectively reduced to alcohols.  3.5  Nuclear magnetic resonance  d8-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 recorded on a BRUKER WH 400 spectrometer at a proton frequency of 400.13 MHz.  25  CHAPTER 4 RESULTS AND DISCUSSION  4.1  Polystyrene latex synthesis  All batches of polystyrene latex were synthesized using the experimental conditions given in Table 1. This ensured reproducible batch characteristics of charge density, concentration and diameter (approximately 650nm), allowing comparison among batches without having to fully characterize them after each synthesis. Thirty minutes after addition of the initiator the typical opaque appearance of a latex suspension was observed. Some aggregates were formed around the stirring paddle but did not interfere with the polymerization and were removed in the filtration step. At 400x magnification uniform well dispersed latex particles in Brownian motion were observed. Figures 6 & 7 are scanning electronmicrographs of latex particles with diameters  of approximately 650nm. This value compares to Goodwin’s (36) reported  diameter of 678  nm for the same polymerization conditions. Slight variations can be  expected 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 a number average molecular weight of Mn  =  2 and A  1(Y, which were used as references.  Figure 8 shows the styrene monomer concentration in solution vs reaction time obtained from monitoring the reaction by U.V. spectroscopy. The graph shows 4 distinct types of behaviour (A-D) as a function of time. A: 30-50mm  C: 250- 600 mlii  B: 50-250 miii  D: 600- end  26  Figure 6:  Scanning electromicrograph of batch 68, polystyrene! sulphate latex.  Figure 7: Scanning electromicrograph ofbatch 65, polystyrene/acrolein/suiphate latex. Acrolein added after 10 hours.  27  Table 1: Experimental conditions for the emulsion polymerization of polystyrene latex with persuiphate initiation. UNDERIVATIZED POLYSTYRENE LAThX  Amount in grams  Concentration in molll  Styrene Monomer  16.9  Water  132  NaC1  0.109  1.04 10-2  Initiator K2S208  0.139  2.86 10-2  BATCH NUMBER  68  Mixing  350 rpm  Temperature  700  Reaction Time  24 hours  Total Volume  180 cm3  0.90  C  It is possible to explain the general features of Figure 8 in terms of the coagulative nucleation theory of emulsion polymerization discussed in the introduction. The initial concentration is slightly lower than the bulk concentration of 0.9 mol/l. The oligomers formed in solution at the beginning of the reaction have increased solubility in water due to  the polar sulphate group (35). As a result the droplet size reduces as monomer goes into solution for propagation. It leads to a nearly total solubility of the styrene: 0.88 molJl.  28  0  E  C  a)  C G)  Reaction time in hours  Figure 8: Monitoring of the emulsion polymerization of polystyrene. Styrene monomer concentration vs reaction time; the four distinct regions(A-D) are indicated 29  In region A the sudden decrease in concentration would correspond to homogeneous  nucleation,  when the chain length of the oligomers exceeds the critical degree of  polymerization required for solubffity. They precipitate to form precursor particles, small approximately 5 nm diameter particles which are not soluble, leading to the sudden decrease in monomer concentration observed. It is expected that by the end of region A all the precursor particles have coagulated to form polymer particles which carry a sufficient charge to be stable and which are swollen with monomer. In region B a more or less constant concentration of monomer in solution is observed. The propagation reaction continues inside the particle which would account for this roughly constant monomer concentration in solution. As the monomer inside the particle is consumed, monomer from solution migrates to the particle and partitions into it to polymerize (region C), until no more styrene monomer is available in solution (region D). Region D marks the termination step. It was unexpected that this simple monitoring technique by U.V. spectroscopy would allow us to distinguish the different stages of the emulsion as outlined for the reaction mechanism. However, the monitoring  experiment was repeated and the results were  confirmed.  4.2  Surface modification with acrolein  Figure 8 can now be used as a basis for discussing the surface copolymerization with acrolein. In the one stage polymerization, since no more initiator is used, acrolein needs to be added when radicals are present, which from Figure 8 excludes any addition in region E. Figure 9 shows the theoretical copolymer composition of the styrene/acrolein system as a function of the feedstock composition in monomer with r,=O.O34 and r =O.32 (53). 0 30  w 0 0  -J  0  C-)  z z  w -J 0 0  z  0 I— 0 U-  0.1  0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9  FRACTION OF STYRENE IN FEEDSTOCK  Figure 9: racro =0.32.  Theoretical composition of styrene/acrolein copolymer. r, = 0.034 and  31  The probability of integrating acrolein in the polymer is higher when the styrene concentration is low but as long as styrene is not in large excess copolymerization should occur.  4.2.1 Time dependence of acrolein addition It was decided to vary the time of acrolein addition in the single step polymerization to gain more information on the reaction. Table 2 shows the different batches synthesized for this study. The DNPH assay was performed on the suspension to quantify the aldehyde groups on the surface and demonstrate that copolymerization had occured. Table 2 : The effect of acrolein addition at the times indicated following initiation of the synthesis of polystyrene/sulphate latex; 1.0 x l0 mol of acrolein was added per gram of styrene for each synthesis, total volume = 195 cm 3 BATCH NUMBER  TIME OF ADDITION in hours  DNPH ASSAY AMOUNT ALDEHYDE in mollg dry latex  68  Blank  1.8 10  69  0  1.6 10  67  2  1.1 10  66  4  1.210  64  6  8.7106  63  8  6.3 10.6  65  10  5.4 106  73  12  6.4 106  70  14  5.0 10  72  16  2.0 10  71  18  2.3 10  62  20  3.0 10 32  Figure 10 shows the concentration of aldehyde in mol/g (dry latex) vs time of addition obtained from the DNPH assay. After 12 hours no aldehyde was measured on the surface. This time corresponds to the region E where termination has already occurred and no radicals are present as interpreted from Figure 8. This result implies that the acrolein is copolymerized with the polystyrene chains. It suggest polyacrolein is not polymerized in solution 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 produced copolymerization. The amount associated with the latex did not vary dramatically through the 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 in the 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 interface  Region 2: The particle is now growing by polymerization of monomer which is partitioning into the particle (region D), as the monomer is compatible with its own polymer. Acrolein is not expected to partition into the particle significantly because of noncompatibility and its hydrophiffic character. 33  1.8E-05 1.6E-05  I  1.4E-05 1.2E-05 1E-05 8E-06  U]  6E-06 U]  4E-06 2E-06 U  I  I  ,  I  I  I  I  I  I  I  blO 2 4 6 8101214161820 ACROLEIN ADDITION in hours  Figure 10: Effect of acrolein addition at the times indicated. Results obtained using the DNPH assay (under conditions where surface groups are detected).  34  The random movement of the radical on the end of the growing polymer chain is reduced as the chain molecular weight grows and the internal viscosity and entanglement increases. The frequency of the radical appearing near the surface is thus reduced as is the copolymerization 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 or acrolein monomers. It seems reasonable to expect that the polymer chain formed after copolymerization of acrolein remains at or near the surface, depending on the hydrophobic styrene and hydrophillic acrolein sequence in the copolymer. The chains terminate with another polymer chain either in the particle or on the surface.  Initial addition When acrolein is added with styrene and the initiator, copolymerization occurs in solution before particle formation. Thus because of its relative hydrophilicity, the reaction initially occurs preferentially with acrolein. These copolymers will then precipitate to form the particle. Figure 11 shows the EM of batch 69. The latex shows major aggregation, as was also seen in suspensions observed with an optical microscope. Evidently the presence of acrolein-rich oligomers results in a less uniform charge distribution on the surface of the growing particle. The result is particle aggregation and a non-uniform size distribution. It is believed that by changing the initiator and electrolyte concentrations, better stability can be achieved, as suggested by the results described by S. Slomkowski (7). The amount of aldehyde shown in Figure 10 might have been expected to be higher, but since the latex agglutinated all the surface may not be available for reaction with DNPH.  35  Figure 11 : Scanning electromicrograph of batch 69 , polystyrene/acroleinlsulphate latex. Acrolein added before the initiator.  Figure 12: Scanning electromicrograph of batch 67, polystyrene/acrolein/suiphate latex. Acrolein added after 2 hours. 36  The electronmicrographs obtained for the batches with acrolein addition in the first 6 hours of the reaction showed a non-uniform size distribution. Figure 12 shows the result of addition at 2 hours (batch 67). Again the hydrophillic character of acrolein appears to disturb the crucial stage of particle formation. We can conclude therefore that it is preferable to add the acrolein after particle formation and during the particle growth period, to obtain a good size distribution. Figures 8 & 10 are in accordance with each other and can both be explained using the model for emulsion polymerization discussed in the introduction. The monitoring of the reaction allowed us to determine the preferable time for acrolein addition to maximize the surface concentration of aldehyde groups, i.e., 10 hours.  4.2.2 Concentration dependence of acrolein addition  To achieve the aim of this project aldehyde groups must be available on the surface for grafting. For this reason latex resulting from the addition of acrolein at 10 hours was kept for further study as these conditions ensure surface copolymerization and a good size distribution (Figure 7). The styrene monomer concentration in solution is 1.2  x  102 mol/1  giving a copolymer ratio in acrolein of 0.5 for an addition of 1 g of acrolein. Increasing the acrolein monomer concentration is expected to allow greater copolymerization with acrolein, i.e., to give a higher aldehyde concentration on the surface. Different amounts of acrolein were added to study the effect of the monomer ratio on the copolymerization. Table 3 describes the different latex synthesized as well as results of the DNPH assay. Figure 13 gives the surface aldehyde concentration as a function of the acrolein concentration. 37  Table 3: Study of the effect of variation of the monomer ratio on the copolymerization; acrolein addition at 10 hours. BATCH NUMBER  AMOUNT OF ACROLEIN ADDED in mollg of styrene added  DNPH ASSAY aldehyde mol/g dry latex  % conversion of acrolein  68  0  0  79  6.3 10  1.1 10  1.7  65  1.0 10  5.4 10  0.54  78  2.1 10  2.2 10  1.0  77  3.1 10  1.8 10  0.058  100  4.2 10  1.6 10  0.38  80  9.0 10  7.3 106  0.081  mc  —  9.0 10  The data show no obvious relation exists between the surface aldehyde concentration and the acrolein concentration and  very low ratios of conversion. This result was  unexpected. The temperature, 70 °C, was not believed to be the main factor as the seeded reaction (next page) gave similar low ratios although it was done at 50 °C (as the acrolein boiling point is 52.5 °C). The result is probably due to other parameters in the reaction which dictate a saturation point of hydrophilic acrolein on the hydrophobic surface. No attempt to determine or quantify these unknowns was undertaken. Since the amount obtained on the latex was sufficient for the present application (as will be seen later) this aspect was not investigated further.  38  2.5E-  2E-05 0) 0  E  1 .5E-05 Cl)  LJI 0 >-  z w  1 E-05  0  -J  5E-06  O.OE+OO 1.OE-03 3.1E-03 9.OE-03 6.3E-04 2.1 E-03 4.2E-03 ACROLEIN ADDED mol/g  Figure 13 : Effect of variation of the monomer ratio on the grafting of acrolein 39  4.2.3 Two stage reaction  The seed reaction was investigated briefly to allow a comparison of the amount of aldehyde obtained by this technique with that from the previous syntheses. The syntheses of batches 85 and 86 are described in the experimental section. Table 4 shows the results of the DNPH assay.  Table 4: DNPH assay results from latex synthesized by the seeded reaction.  Batch number  amount acrolein added in mol/g of dry latex  DNPH assay mollg of dry latex  % of conversion  85  0.031  8.8 106  0.028  86  0.031  1.1 10  0.035  Although 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 in solution is expected and oligomers formed should then adsorb on the surface of the latex particle. 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 would copolymerize with styrene monomer. For batch 85 the same phenomenon could occur as some monomer remains in the particle but it is more likely that some oligomers from the aqueous phase simply adsorb on the surface. Because of its hydrophillic character most of the polyacrolein formed apparently stayed in the aqueous phase; this would explain the low conversion ratio calculated for the acrolein associated with the latex.. It is important to note that after extensive washing or after leaving the particles to 40  equilibrate for 2-3 weeks the DNPH assay gave the same result. It shows that oligomer adsorption if present, does not seem to be reversible. The seeded polymerization procedure was not utilized further as it was felt it could produce local high densities of acrolein in oligomers 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 seeded reaction yet it gave the same order of coverage as in the earlier procedure. It is possible that because the DNPH molecule is relatively bulky, it is hindered from fully reacting if the coverage is too high. It was therefore important to investigate this possibility by developing another technique to assay the aldehydes to prove the validity of the DNPH assay.  4.3  Developing an alternate aldehyde assay  4.3.1 A model for the latex surface  It is helpful to estimate the surface concentrations of the groups of interest to provide an average picture of the surface. Batch 68 (blank) and 78 (aldehyde) were used as examples. Table 5 gives some of the characteristics of those batches. The number of molecules of acrolein per latex particle calculated from these results is 2.01  x 106  for a surface area of 1.32  x 108  A2. If it is assumed that the aldehydes are  uniformly distributed over the surface and that an aldehyde site at the surface occupies 10 A2, the percentage coverage would be 12.1% of the surface for batch 78. If we take the charge density as one per 377 A2 as given by Goodwin, assuming an area 10 hydrophiffic group  41  2 A  per  Table 5 : Characteristics of batch 68 (blank) and batch 78 (aldehyde grafted)  BATCH NUMBER diameter estimation volume/latex (cm ) 3 surface/latex  **  ( A2)  68 blank  78 aldehyde  650  650  1.44 1013  1.44 l0’  1.32 108  1.32 108  1012  1012  number particle/g  6.58  specific surface area /g) 2 (m  8.7  8.7  aldehyde mol/g  0  2.2 l0  aldehyde molecule/latex  0  2.01 106  6.58  density of polystyrene 1.055 g/cm 3  (sulphate, weak acid or hydroxyl) the calculated coverage is 2.6 % of the surface. This leaves a hydrophobic polystyrene occupying 85.4 % of the surface. From the perspective of 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 1.8 106 mol/g (batch 77) which would give approximately a 1.2 % coverage; any lower amount would not be assayed with any accuracy.  4.3.2 NaBH 4 assay  A radiochemical assay was investigated to improve the detection sensitivity. The reduction of aldehyde by radiolabelled NaBH 4 was thought to be an appropriate reaction. The reaction with non-tritiated NaBH 4 is summarized in Table 6. The completeness of the reaction was checked using the DNPH assay on the reduced latex. Figure 14 shows the amount of aldehyde on the latex vs the amount of NaBH 4 added. 42  Table 6: Reaction of the polystyrene/acrolein latex (batch 78) with NaBH 4 ; verification of the efficiency of reduction. batch number  78a  78b  78c  78d  78e  dry weight of latex ing  0.02  0.02  0.02  0.02  0.02  NaBH in mollg of 4 dry latex in 2m1  4 10  2 10  1 10  4 10  1 10-2  waterinmi  2  2  2  2  2  ethanolinmi  3  3  3  3  3  reaction time in hours  24  24  24  24  24  0.1MHCLinm1  1  1  1  1  1  reactiontimein 1 1 1 1 1 hours * the aldehyde concemration for batch 78 is 2.2 10 mol/g by the DINPH assay  Some aldehyde groups were reduced but even with a large excess of NaBH 4 the reaction did not go to completion. The result perhaps is not surprising as NaBH 4 is reported not to be very reactive (77) and it is possible that the double layer is going to affect the approach of BR 4  -  ion towards the surface. The use of the Scatchard plot for the  radiolabelled reaction does not require the reaction to go to completion, however, as the total number of reactive sites is determined by extrapolation. It was therefore decided to  carry out an analogous reaction with NaBT . 4 Tables 7 and 8 summarize the data from the radiolabelled reaction. Figures 15 & 16 show 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. The linearity of 68 compared to the other batches implies an adsorption process and will be considered as background DPM. 43  :1  0  I c)  r’)  0  a  0 0  B  -  I  w  r  c?.  0  0  ..  0  1”•  0. 0 0 cii 0 01 0  +  m  0  p  :I  ::::.:...  :.::.:.:::]  I  0 0)  0 0  1  0  m +  0  :..........  .:.........:..::..::..:.  0 01  0  :....  0 01  01  [ALDEHYDE]surf mol/g  0 01  1n  0  0 01  01  Counting of further washes of the latex showed that some DPM continued to be released into the supernatant. It is likely that these counts are associated with water trapped in the wet latex before being dissolved in THF. However, extensive washing to remove these DPM was 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 the blank to obtain the Scatchard plot. The binding isotherm of the aldehyde latex showed some curvature due to saturation. Figures 17 & 18 are the Scatchard plots. Table 9 shows the amount 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 of 4 NaBT batch number 63,64,65,66,67  1  2  3  4  dry latex in g  0.001  0.001  0.001  0.001  NaBT in molig of 4 latex  1.3 108  1.3 108  1.3 108  1.3 10.8  cold NaBH 4 in mollg latex  1.0 10  2.0 10  4.0 10.6  8.0 10  water in  200  200  200  200  200  200  200  200  100  100  100  100  ethanol in  1 1  0.1 M HC1 in 1 to stop the reaction  45  Table 8 : Radiolabel assay on batch 68 and 78 using the solid stock of tritiated NaBT 4  SAMPLEfor batch 68 and 78  1  2  3  4  5  dry latex in g  0.001  0.001  0.001  0.001  0.001  4 mol/g NABT  1 1O  1 10  1 10  1 10  1 10  4 mollg NABH  0  5 10  1 10  5 10  1 10-2  water in p 1  200  200  200  200  200  ethanol  1  0.1 M HCL p. 1 to stop the reaction  200  200  200  200  200  100  100  100  100  100  Table 9 : Results of the radiolabel assay of the aldehydes using the Scatchard plot.  batch number  aldehyde in mollg  result from DNPH  68  blank  blank  67  1.9 10  1.1 10  66  2.5 106  1.2 10  64  1.6 10.6  8.7 10.6  63  9.3 10  6.3 10.6  65  1.2 10  5.4 106  78  2.3 i0  2.2 10  The Scatchard plots give lower values compared to the DNPH assay results for the first experiment. The second experiment with batch 78 gave the same aldehyde surface concentration. It was performed using a fresh preparation of NaBT 4 , received as a solid and diluted just before the experiment. In the other experiment the NaBT4 was received in solution in 0.1 M NaOH. In either case the results were not reproducible. 46  1.8E-06  . ci  1.6E-06 +  64  1.4E-06  *  65  ? 1.2E-06 E 1E-06 *  C,, -  8E-07  66 x 67  ci  68  6E-07 4E-07  A  2E-07 A I I I I I I 0• O.OE+OO 1 .OE-04 2.OE-04 3.OE-04 4.OE-04 5.OE-05 1 .5E-04 2.5E-04 3.5E-04 4.5E-04 [hydride]eq mol/g  Figure 15 : Radiolabel assays of the a.tdehyde concentrations with NaBT . Binding 4 isotherms of batches 63, 64, 65, 66, 67 and 68. 47  9E-05 +  68  8E-05  + +  7E-05  78  ? GE-OS 0 E D U)  0  4E-05  + +  2E-05 1E-05 ;_J.  a  a  O.OE+OO 1 .OE-02 2.OE-02 3.OE-02 4.OE-02 5.OE-03 1 .5E-02 2.5E-02 3.5E-02 4.5E-02 [hydride]eq mol/g  Figure 16: Radiolabel assays of the aldehydes with NaBT 4 ; binding isotherm of batch 68 and batch 78 48  0.06 63 D  0.05  +  64  a  *  65  0.04 C  D  -  > -c X  0.03  -  D  U)  ci) -D  -I  -a -c  0  >1  x  0.01  *  +  A I I I I I I I 0 2E-06 1 E-06 8E-07 4E-07 OE+00 2E-06 1 E-06 1 E-06 6E-07 2E-07 [hydride]surf mol/g  Figure 17: Resulting Scatchard plots from Figure 16, batches 63, 64, 65, 66, 67 and  68. 49  a G)  G)  -a >1  I 6E-05  1 E-05  5E-05 3E-05 [hydride]surf mol/g  Figure 18 : Resulting Scatchard plot from Figure 17, batch 78. 50  Experiments repeated with either of the batches gave only background DPM. It is possible that NaBT 4 hydrolysed in solution. This would explain the nonreproducibility of the experiments and the lower value obtained from the experiment carried out with the radiolabel stored in solution. As seen from the unlabelled reaction a large excess of NaBH 4 is needed to reduce all the aldehyde groups. In the reaction more than 700 000 DPM were used for each experiment yet only 200 to 1300 DPM were recovered on the latex. It is not possible to work with higher activities for disposal reasons. The few results obtained, especially for batch 78 which gave the same result as with the DNPH assay, indicated some potential for a good aldehyde group assay as the limit of detection would have been very low. The nonreproducibility and the high cost of radiolabelled sodium borohydride makes this methodology unattractive. However a more reactive radiolabeled assay would be of great interest and perhaps a non-ionic reagent will facilitate the access through the electrical double layer on the latex surface.  4.3.3 Nuclear magnetic resonance  It was not possible to fully validate the DNPH assay with the NaBH 4 reaction nor did it give more information on the configuration of the aldehydes on the latex. NMR spectroscopy was therefore examined as it would seem to be an appropriate technique since the aldehyde present should resonate in a relatively clear window at 8 =9 ppm. It allows a quantitative analysis by integrating the singlet and the broad peaks in the aromatic region resulting from the benzene ring of the polystyrene at 8= 5-6 ppm. Broad peaks are expected in NMR spectra of polymer solutions because of the reduced motion of the 51  polymer chains (78). Figures 19 & 20 show the NMR spectra of batch 78 suiphate/aldehyde latex). The spectra of the dissolved latex gave aliphatic region 8  =  and IDC  (  both  broad peaks in the  1-2 ppm due to the aliphatic backbone of the polymer chains (79); this  region 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). All batches 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 aromatic  protons of the benzene ring. The multiple peaks are due to the configurational arrangements of the polymer usually observed by NMR (80). A peak at 8  =  10.8 ppm was observed in  all spectra. It corresponds to the acid region and is believed to be due to the sulphate end group or carboxyl groups resulting from the sulphate hydrolysis. The aldehyde peak is observed in the region from  a  =  8.9 ppm to 8  =  9.4 ppm, the exact value depending on  the batch.  Both acid and aldehyde peaks can be quantified. Table 10 shows the results of the  quantitative analysis. It was done as follows:  laromatic  =  Integration of the signals from the five aromatic H from styrene  laldehyde  =  Integration of the signal from the one H from the aldehyde group  lacid  =  Integration of the signal due to the acid proton.  Considering the aldehyde quantification: Raid  =  (laromatic  /5) / I aidehyde  where Raid is the mole ratio of styrene/aldehyde. 52  One gram of latex is composed of polymerized styrene, acrolein, and sulphate and acid end groups. The mole ratio of styrene is high enough that we can assume: 1 (g of latex)  =  flsty  .  MWsty  where n sty is the number of moles of styrene in 1 gram of latex. Therefore where  flacro  flacro  =  1 I (Raid  .  MWsty)  is the number of moles of acrolein per g of latex  The 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 be  expected if its occurrence is due to the initiator. Using the result from the DNPH assay for batch 78 of 2.9  x  10 mollg and IDC of 3.9  x  10 mollg and assuming that one acid  group gives one charge: The area per charge would be 50 A2 /charge and 40 A2 /charge respectively. Goodwin gives the area per charge as 377 A2 /charge, however, and IDC quoted 280 A2 /charge. Hence the NMR estimation gives much larger charge densities than those implied from the values measured by titration, values that could not be attributable to experimental error. Hence the peak near 10.8 ppm cannot be due to the acid. It is possible the peak is due to protonated THF (plc  =  H plC 4 4 -3.5 (81,82), c.f. RSO  —  -9). It could be  due to a weak acid although this would not be consistent with the titration results (discussed later in XPS results). Aldehydes were observed as expected on batch 78 and not 68 (blank), but the limit of detection can be seen as batch 77 did not give a signal.  53  .Oppm 7  11.1 ppm  x  128  8.9 ppm .Sppm 6  I  =  0.0342  x128  I  IL 0  =  0.0267  9.0  I  =  31.76  I  =  23.53  7. 0 PPM  Figure 19: 1 H NMR spectrum of batch 78, polystyrene/acrolein/suiphate latex in d8-THF 54  7.0 ppm  10.8 ppm  6.5 ppm  16  x  16  1=0.25 I  =  0.18  10.8 PPM  9.2  I  =  I  131.8  I  I  =  9.0 Fr H  86.3  I  6•6.4 PPM  Figure 20 :  ‘H NMR spectrum of DC latex, polystyrene/acrolejn latex in d8-  55  8.3  9.4 ppm  I  =  0.035  9.4 PPM  7.0 ppm  10.8 ppm  6.5 ppm x64  I  =  0.055  i. 9  PPM  1=32.84  I=22.  •  0  6.3  I  6.6  PPM  Figure 21  1  H NMR spectrum of batch 85, polystyrenefacrolein/suiphate latex (seeded reaction) in d8-THF. 56  -9. 1 ppm 9.4 ppm 10.8 ppm x64  I  =  1 0.05J  1=0.265  x64 94  I  =  9.2 PPM  0.1418 6 p 5 . pm  PPM  1. 8  I  7.6 PPM  Figure 22 :  7I1  7.2  =  77.45  7.0  1= 50.33  6.6  6.6  H NMR spectrum of batch 86 , polystyrene/acrolein/suiphate latex (seeded reaction swollen, latex with styrene monomer) in d8-THF.  57  Table 10: Quantitative analyses of the NMR peaks in the 8 the 8 = 10.8 ppm region. Batch number  Aldehyde peak 8 (ppm)  8  mole (mollg)  8 (ppm)  =  =  9 ppm region and  10-11 ppm region mole (mol/g)  68  none  0  10.8  4.5 10  78  8.9  2.3 10  11.1  2.9 l0  78e NaBH 4  none  0  10.8  3.3 10  77  none  0  10.8  3.6 10  85  9.4  3.0 10  10.8  4.7 10  86  9.1 9.4  9.8 10 1.9 10  10.8  5.3 10  mc  9.0  Analysis of batch 78 gave 2.3  5.4 10  x  10.8  3.9 10-s  iO mollg which compares very well with 2.2 x 10  moL’g found by the DNPH assay and 2.2  x  10 mollg by the NaBH 4 assay. It is interesting  to 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 the peaks at 9.1 ppm are likely due to acrolein copolymerized n the styrene chain. In batches 85 and 86 (Figure 21 & 22 respectively) the acrolein can polymerize in solution as more initiator is added. In the case of batch 86 copolymerization also occurs since the particle is swollen by styrene monomer, which would explain the peak at 9.1 ppm observed for the other batches. This supports the idea that in the semi-continuous reaction the acrolein actually copolymerizes on the surface and only on the surface, as the NMR and DNPH assays agree for batch 78. Batches 85, 86 and the mc latex show a higher concentration of aldehyde with the 58  NMR than with the DNPH assay. This suggests the aldehyde groups are not only on the surface. This is improbable since the synthetic technique of 85 and 86 should only lead to surface functionalization. It is more likely that the DNPH cannot react with all the aldehydes when their local concentration is too high, the reaction being stericaly hindered. This conclusion is supported by the result from batch 85 where the DNPH concentration was found to be 8.8 the 2.3  x  x 106  mollg, lower than the 3.0 x 10 mol/g found by NMR and lower than  l0 mollg of batch 78. In batch 85 the NMR evidence suggests acrolein is present  partly as an oligomer on the surface, not distributed randomly on the surface as expected for the one step reaction. The DNPH reaction is going to be hindered as access to the aldehyde is limited on the surface.  4.3.4  X-ray photoelectron Spectroscopy  Since we are trying to achieve a surface fuctionalization, XPS should be of great help 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. Carbon is the major detectable element on the latex which leads to the Cis peak having the highest intensity. The Ols and S2s,S2p peaks are due to the sulphate group on the surface. The Nals is thought to come from the salt added in the reaction. The Si2s and Si2p peaks are from impurities on the surface expected from handling and storage of the samples in glass containers. Figure 24 shows the XPS spectra of batches 68 (blank), 78 and IDC (both aldehydes). The peak at 292 eV is due to the typical aromatic  -,  it  shake up (69). All  the carbons of the polystyrene chains have a very similar environment and give the 59  ,c (B)  Intensity [cps]  + C is  +521  +S2p  *S2p  A  At  eo  160  OO  40  bind. energy [eVJ 4’C (a)  + 0 is  INais  4.Na (a)  +0 (a)  area ebown in detail  r 1000  1200  600  800  400 bind,  200 energy  l1gure 23: XPS spectxa of batch 68, blank polystyrene/sulphate latex. Al koc 1486.6 eV was used.  Intensity [cpsJ  —  —6.7  285 eV  eV  Cis  IDC  -b  296  FIgure 24:  294  292  288  Cis peak of batch 68,78, IDC 60  286  284 282 bind, energy  280 LeVI  -  fe V)  Intensity [cps] IDC  Ols 532.6  538  eV  534  536  532  530  528 bind,  526 energy  Figure 25: 0 is peak of batch 68 and JDC  intensity [cps]  Nis  400.4 eV  406.2 eV  412  410  408  406  404  402 400 bind. energy  Figure 26 : N 1 s peaks of IDC latex, after reaction with DNPH.  61  398 [eV]  524 [eV]  symmetrical Cis peak at 285.0 eV observed for batch 68 (83). The symmetry of the peak is lost for 78 and  mc. The carbon  -  oxygen double bond shifts the binding energy of the  Cis peak by 3 eV (84) to 288.0 eV due to the aldehyde groups. Figure 25 shows the increase 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 the DNPH is present on the surface of the latex. The two Nis peaks at 400.4 eV and 406.2 eV are due to the two different nitrogens of the hydrazone complex, respectively the nitrogens from the carbon-nitrogen double bond and from the nitro groups  .  Batch 68 following the  DNPH reaction was run as well and as expected did not show any nitrogen. Figure 27 shows the spectrum of 68,  mc  and  broadening is parallel to the  mc  mc  + DNPH. The DNPH broadens the Cis peak. The curve giving a similar assymmetry in the peak. It is  expected as the DNPH layer on the surface is going to mask to some extent signal originating beneath it and contribute to its own characteristics. Figure 28 shows the same experiment but with batch 78 which has a lower aldehyde coverage, implying the signal is going to be less masked. A broadening is observed on the surface due to the new carbon present, especially the C-NO 2 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 with 3 peaks  ( see Table 11):  285.0 eV due to the core polystyrene, 286.5 eV with the 1.5 eV  shift due to a C-O bond and 288.0 eV, the shift due to a C=O bond. The shift of 1.5 eV is usually considered to be due to a hydroxyl group but could be due in this case to the 3 (84) which would explain the high percentage obtained. 0S0  62  c  Intensity [cps]  8 Ci  IDC  294  292  290  288  286  284 282 bind, energy  Figure 27: C is peak of IDC latex after reaction with DNPH  Intensity [cps]  Cis  78+ DNPH  296  294  292  290  288  286  284 282 bind, energy  Figure 28: Cis peak of batch 78 after reaction with DNPH  63  280 [eV]  [eV)  Intensity [cps] Cia  78  286.5 eV 288.0 eV  296  294  FIgure 29:  292  290  288  286  284 282 bind, energy  280 EeV]  Curve fitting of the Cis peak from batch 78  intensity  [cpsll  Cia  IDC  285.0 eV  288.0 eV  296  292  290  288  286  284 282 bind. energy  Figure 30: Curve fitting of the Cis peak from ]DC latex  64  280 [eVI  Intensity [cps  Cis  78 78+  296  Figure 31:  284 bind, energy  288  292  [eV)  Cis spectra of batch 78 after reaction with NaBH 4  Table 11 : Results of curve fitting of the Cis peak batch number 78  285.eeV polystyrene 90.1 %  mc  88.3 %  286.5.0eV alcohol/SO 4  288.0eV aldehyde  6.6 %  3.3 %  7.9 %  3.7 %  Since XPS is a semi-quantitative technique, the percentages cannot be taken as  absolute. It is surprising, for instance that the  mc latex shows the same concentration as  batch 78, while from the DNPH assay or from NMR the  mc  latex bears more aldehyde  groups.. Figure 31 shows the reduction in the aldehyde peak after reaction with NaBH , using 4 sample 78e from the NaBH 4 reduction experiment. It confirms that all the aldehydes are not 65  reduced. XPS showed no evidence of acid groups in the Cis peak, which would have been  expected at 289.5 eV (84). Although this is not absolute proof of the absence of acid it is consistent with the explanation that the NIv1R peak at ö = 10.8 ppm is due to the protonation of the solvent. XPS confirmed the presence of aldehydes on the surface through the Cis peak and particularly 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 interpreted to provide the surface concentrations of the species.  66  CHAPTER 5 CONCLUSION  The semi-continuous synthesis of polystyrene/acrolein latex was succesfully carried out. The optimum addition time of the acrolein monomer was determined to be 10 hours after addition of the initiator. It gave monodisperse polystyrene latex with aldehyde on the surface. A DNPH assay was used to assay the aldehyde group in the latex suspension; it was specifically a surface assay. It successfully detected aldthydes at concentrations above 1 x10 6 mollg and was determined to be limited at high surface concentrations of aldehyde probably due to the steric hindrance. Quantitative agreement with respect to surface concentration of aldehyde groups was obtained using freshly solubilized NaBT 4 reduction in a radiochemical assay. NMR was used as a valuable technique to assay the aldehydes present in dissolved latex and the same result as for the DNPH assay was obtained. It was found to be very useful as it provided information on the environment of the aldehydes on the surface, the chemical shift showing the possible presence of polyacrolein chain on the latex with the seeded reaction. This was an important observation as the polyacrolein blocks are not favourable 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 NaBH . The XPS spectra obtained 4 were those expected based on the surface chemistry anticipated and no unexpected groups were detected.  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