Model prediction, experimental determination, and control of emulsion copolymer microstructure
Citation for published version (APA): van Doremaele, G. H. J. (1990). Model prediction, experimental determination, and control of emulsion copolymer microstructure. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR340343
DOI: 10.6100/IR340343
Document status and date: Published: 01/01/1990
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Download date: 08. Oct. 2021 MODEL PREDICTION, E ERIMENTAL DETERMINATION, ND CONTROL OF EMULSION POLYMER MICROSTRUCTURE
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G.H.J. VAN DOREMAELE MODEL PREDICTION, EXPERIMENTAL DETERMINATION, AND CONTROL OF EMULSION COPOLYMER MICROSTRUCTURE CIP • Gegevens Koninklijke Bibliotheek, Den Haag
van Doremaek; Gerardus Hemicus 1osephus
Model Prediction, Experimental Detennination, and Control of Emulsion Copolymer Microstructure / Gerardus Henricus 1osephus van Doremaele. [S.1. : s.n.]. • 111. Proefschrift Eindhoven. Met lit. opg. Met samenvatting in het Nederlands.
ISBN 90-9003718-7
SISO 542 UDC 541.64(043.3) Trel'w.: emulsiecopolymerisatie.
@1990 G.HJ. van Doremaele, Berlicum
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Cover
Experimentally determined Molar Mass Chemical Composition Distribution of a styrene - methyl acrylate emulsion copolymer. MODEL PREDICTION, EXPERIMENTAL DETERMINATION, AND CONTROL OF EMULSION COPOLYMER MICROSTRUCTURE
PROEFSCHRIFf
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof. ir. M. Tels, voor een commissie aangewezen door het College van Dekanen in het openbaar te verdedigen op vrijdag 9 november 1990 te 16.00 uur door GERARDUS HENRICUS JOSEPHUS VAN OOREMAELE geboren te Berlicum
druk: wlbro dlssertaliedrukkerij, helmond. Dit proefschrift is goedgekeurd door
de promotoren: prof. dr. ir. A.L German prof. dr. J.M. Asua en de copromotor: dr. A.M. van Herk
Het in dit proefschrift beschreven onderzoek werk uitgevoerd onder auspiciën van de Stichting Scheikundig onderzoek in Nederland (SON) met financiële steun van de Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). Aan mijn ouders
quo quis doctior, eo modestior est. Hoe geleerder iemand is, des te bescheidener is hij.
Latijns spreekwoord Contents
Contents
Chapter 1 Introduetion
1.1 Short historie overview 1 1.2 Background of the investigation 3 1.3 Choice of model systems 5 1.4 Aim of this investigation 7 1.5 Outline of this thesis 8 References 10
Chapter 2 Theoretieal Background
2.1 Emulsion polymerization 13 2.2 The ultimate and the penultimate model for copolymerization 18 References 21
Chapter 3 Experimental Copolymerization Procedures and Development of Experimental Methods of Copolymer Analysis
3.1 Procedures foliowed in solution and emulsion copolymerization 24 3.1.1 Purification of chemieals 3.1.2 Preparation of reference copolymers by solution copolymerization 3.1.3 Emulsion copolymerization equipment and procedures 25 3.2 Copolymer analysis 29 3.2.1 Cross-fractionation metbod for determining copolymer MMCCD 29 3.2.1.1 SEC 31 3.2.1.2 Gradient 1LC/FID 32 3.2.1.3 Cross-fractionation data treatment 36 3.2.1.4 Accuracy and reliability of gradient 1LC/FID 36 3.2.1.5 Gradient HPLC 41 3.2.2 1H and 13C NMR investigation of the intramolecular structure of solution S-MA copolymers and determination of reactivity ratios 43 Contents
3.2.2.1 Introduetion 43 3.2.2.2 Experimental section 46 3.2.2.3 Results and discussion 47 3.2.3 Determination of reactivity ratiosof MA-BA solution copolymerization and a feastbility study on the determination of the sequence distribution in MA-BA copolymers 62 3.2.3.1 Introduetion 62 3.2.3.2 Experimental section 63 3.2.3.3 Results and discussion 64 3.3 Conclusion 66 Relerences 67
Chapter 4 Model Evalustion of Emulsion Copolymerization Klnetics and Copolymer Microstructure
4.1 Introduetion 71 4.2 Model description 74 4.2.1 Physical and chemical outline 74 4.2.2 Basic principles of the theoretica! model 77 4.3 Model development 78 4.4 Model calculations 92 45 Conclusions 97 Appendix 4.A: MMO calculation 98 Appendix 4.B: discussion 101 Glossary of symbols 102 References 107
Chapter 5 Monomer Partitioning
5.1 Introduetion; theoretica! aspeets of monomer partitioning 111 5.2 Experimental seetion 117 5.3 Results; monomer partitioning 118 5.4 Results; bulk copolymerizations 125 55 Conclusions 127 References 128
Chapter 6 The Effect of Composition Drift on Copolymerization Rate
6.1 Introduetion 129 6.2 Experimental 131 6.3 Results and discussion 132 6.4 Condusion 143 References 144 Contents
Chapter 7 Mierostruetural lnvestlgatlon of Batch Emulslon Styrene-Ac:ryUc Copolymers
7.1 Introduetion 146 7.2 Experimental section 148 7.3 Model ealculations: monomer reactivity ratios and monomer partitioning 149 7.4 Experimentally determined emulsion eopolymer microstructure in eomparison with model ealculations 151 7.4.1 (Molar mass) chemieal eomposition distribution 151 7.4.2 Sequence distribution of S-MA emulsion eopolymers 162 7.5 Conclusions 166 References 167
Chapter 8 Copolymer Composition Control by means of Semi-Continuons Emulsion Copolymerization
8.1 Introduetion 169 8.2 Experimental section 172 8.3 Results 174 8.3.1 Semi-eontinuous emulsion copolymerization with constant addition rates 175 8.3.2 Semi-eontinuous emulsion copolymerization with optima! addition profile 177 8.4 Discussion 186 8.5 Conclusion 187 References 188
Appendix A Partiele Morphology of Composite and Copolymer Latices 189
Appendix B lnvestigation of the Methoxy Proton Region in 1H NMR Spectra of 8-MA Copolymers by means of COLOC and NOESY NMR 201
Summary 211
Samenvatting 213
Dankwoord 216
Curriculum Vitae 218 Introduetion 1
Chapter 1 Introduetion
1.1 Short historie overview
Conventional radical emulsion polymerization involves the dispersion of a monomer, an unsaturated organic molecule, in a continuous aqueous phase stabilized by an oil-in-water emulsifier, foliowed by free radical addition polymerization started with usually a water soluble initiator. This results in a reaction medium consisting .of submicron polymer particles swollen with the monomer and dispersed in an aqueous phase. The final product is called a latex and consists of a colloidal dispersion of polymer particles in water. The first attempts for emulsion polymerization were started during World War I in order to provide an artificial product as a substitute of natural rubber. Luther and Heuck presented the first viabie emulsion polymerization methodl). Synthetic latices were first produced in the mid-1930s in Germany. The commercial use of heterogeneaus emulsion (co )polymerization started in the United States during the period around World War 11 with the production of styrene-butadiene copolymer, developed under the guidance of the Office of the Rubber Reserve Program. Since then a huge number of papers bas appeared on emulsion polymerization, which is still growing in industrial importance. A detailed bistorical survey bas been given by Blackler>. Industrial and scientific interest in homogeneaus radical copolymerization in salution and bulk dates back to the 1920s3,4.S>. Copolymerization offers the possibility of modifying the properties of homopolymers into tailor made products. Nowadays emulsion copolymerization is a widely used process. Modem 2 Chapter 1 synthetic latlees find a braad range of applications in the coating. ink, plastic and adhesive industry. For application as elastomers, commodity and engineering plastics, prior to use, polymer isolation processes (i.e., separation from the aqueous phase) are required, because emulsifier (and initiator) residues may affect polymer properties adversely. Despite many complexities, the emulsion (co )polymerization process allows the synthesis of high molecular weight (molar mass in this thesis) (co)polymers at high polymerization rates. Emulsion (co )polymerization displays the following advantageous properties, making it a very attractive process for the cor:nnlercial production of polymerie materials: excellent heat transfer as a result of its relatively low viscosity and therefore good temperature control; toxic and flammable organic solveuts do not have to be used and reaction can proceed to high conversion; molar mass can easily be controlled by the use of chain transfer agents; the produced latex can be formulated directly into many final products. As disadvantages must be mentioned the relatively poor film formation properties as compared with solvent based systems, partially due to the presence of surfactants. In industry many emulsion polymerization processes are empirically developed, often resulting in process conditions far from optimal. Emulsion polymerization is a very complex process due to its beterogeneaus and colloidal nature. Although emulsion polymerization is most abundantly applied these days, it is still not well understood. Fortbelast three decades, enormous efforts have been made by researchers towards the elucidation of the complex chemical and pbysical pbenomena goveming tbe various mechanisms operative in an emulsion polymerization system. Modelling of emulsion (co )polymerization is necessary for a better understanding of the process that enables a better control of the polymer synthesis and optimal product quality. The variables that determine product quality include latex stability, partiele size distribution (PSD), partiele morpbology, gel content, chemical composition distribution (CCD) and molecular weight distribution (molar mass distribution, MMD) of the copolymer formed, etc. Introduetion 3
Harkins6> provided the first theory (based on styrene ), which adequately explains the important charactistics of emulsion polymerization. Based on this concept Smith and Ewart developed a mathematical kinetic treatment7). These theories provided the starting point of the overwhelming number of papers dealing with modelling emulsion polymerization8>. Since 1970 the volume of fundamental scientific work publisbed in the various areas of emulsion polymerization is still expanding considerably. Relevant in this field are the influences of different aspects of the process conditions (recipe, temperature, reactor design, mode of operation) on reaction rate, partiele size distribution and partiele morphology, latex stability and (co )polymer properties. Several excellent review artiel es on the kinetics of 9 10 11 12 13 emulsion (co )polymerization have appeared • • • • >.
1.2 Background of the investigation
Although much attention bas been paid to the influence of process conditions on product properties, it is still poorly understood. It is generally recognized that detailed revelation of copolymer microstructure is a major factor, contributing to a better onderstanding of both process and polymer properties. On the one hand, the copolymer microstructure directly reflects the fundamental 14 15 mechanistic chemical processes occurring in the reaction loci • >. On the other 16 17 18 19 20 21 22 hand, copolymer microstructure controls copolymer properties • • • • • • >. The essence of modelling emulsion copolymerization is the proper description of the heterogeneity of the emulsion in combination with the different properties of the monomers, e.g., reactivity, water solubility and swellability of latex particles by monomer. Several authors have already worked in this field. Basic theories valid for emulsion homopolymerization were mathematically extended to emulsion copolymerization. Significant contributions to this approach were made by investigators such as Nomura and 23 25 26 2 28 29 30 31 2 33 34 35 36 Fujita .24· • • 7), Guillot • • • >, Doughercyl • >, Poehlein • >, Hamielec >, 37 38 39 40 41 Asua • >, Gilbert >, Storti >and Ray >. 4 Chapter 1
In most of t_hese descriptions, only reaction rate, composition drift and average copolymer composition were predicted and measured, whereas copolymer sequence distribution, ·CCD or even MMCCD might have given far more detailed and useful information, because the copolymer microstructure directly reflects the history of the chemical kinetic events on a molecular scale. In the very few cases where methods of determining the latter are available, model calculations are qualitative in character and only roughly camparabie to ex:perimentally determined emulsion copolymer microstructure.
Molecular microstructure of emulsion copolymers in terms of (molar inass) chemical composition distribution (MMCCD) were studied mathematically by 42 43 Giannetti et aL • >. However, they did not oompare the MMCCD model calculations with ex:perimental distribution measurements. Tacx ex:perimentally measured (MM)CCDs of emulsion copolymers of styrene-ethyl methacrylate, but did not make the comparison with applicable model calculations44.S2>. A model study of tbe sequence distribution of emulsion copolymers was performed by Ballard45>. As one of the very few, Guillot et aL compared model calculations of sequence distribution with ex:perimental data, at the triad level, determined by means of 13C NMR46>. Furthermore, there exists a lack of reliable and comprehensive studies on monomer partitioning parameters, probably because it calls for very laborious and time consuming ex:periments. The chemical heterogeneity of the copolymer formed is due to the well known composition drift of the monomers during reaction in combination with the statistica! character of the monomer addition process. The chemical composition distribution (CCD) of random copolymers, in which the monomeric units moderately differ in polarity, can he measured by thin layer 47 48 49 chromatography / flame ionisation detection (TLC/FID) • • ,so,st,sz>, or by high performance liquid chromatography (HPLC)53.54.84>. These CCD data can be combined with the molar mass distribution (MMD) obtained by size exclusion chromatography (SEC), to derive the complete two-dimensional MMCCD. TLC of random copolymers is based on adsorption differences and is useful for the elucidation of the CCDs of copolymers with monomeric units that moderately Introduetion 5
differ in polarity, e.g., styrene-(meth)acrylates. The CCDs of these kinds of 5 1 copolymers have been studied by e.g., Teremachiso,s.s,st~>, Inagak:i 7) and Tacr .52>. However, these techniques cannot yet be regarded as standard and are still being developed, mostly using bulk or solution copolymers. Unfortunately, hardly any attention bas so far been paid to the experimental CCD determination of emulsion copolymers51.s2>. Nowadays, many research efforts are focussed on developing methods of controlling emulsion copolymer composition37.SS.S9>. Surprisingly, however, they are carried out without performing detailed copolymer analysis (i.e., MMCCD). From the above considerations, it is evident that for the purpose of optimizing latex properties (in strong relation with emulsion copolymer microstructure ), further research should aim at combining model development and experimental verification of copolymer microstructure.
1.3 Choiee of model systems
Important factors in the choice of a suitable model system are the monomer properties such as its capability to swell the polymer and its water solubility, and the inherent possibility of analysing the copolymer formed with respect to the intra- and intermolecular microstructure. Styrene (S) and methyl acrylate (MA) are known to swell each others (co)polymers very well and also moderately differ in water solubility. Moreover, sequence distributions can be studied by means of 13C NMR46>. Copolymer CCD can be determined by TLC/FID or HP~·84>. The molar mass distribution of S-MA copolymers can be analysed by size exclusion chromatography (SEC) providing a separation exclusively according to molar mass without interference witb copolymer chemica} composition over a large range of compositions61 >. All these properties make S MA a suitable model system for emulsion copolymerization model studies. ' Chapter 1
Only a very _limited number of studies have been publisbed on styrene methyl acrylate emulsion copolymerization. Styrene is the most widely stuclied monomer in emulsion polymerization. Only a limited number of studies deal with the emulsion polymerization of methyl acrylate. Methyl acrylate is well known for its gel effect62>. Studying emulsifier-free MA polymerization in water above and below the saturation concentration. Gerrens63>démonstrated that the MA emulsion polymerization is strongly affected by the gel effect. Final partiele number of MA emulsion polymerization in dependenee of emulsifier 64 concentradon was studied by Sütterlin ). Gerrens et aL reported the kinetic 6 results of continuous and semi-continous emulsion polymerizations of MA6S;66, 7). Radiation induced emulsion polymerization of MA, studied by Hummel et 69 10 aL 68, • >, also indicated the presence of astrong gel effect. The effect of initiator and emulsifier on the methyl acrylate batch emulsion bomopolymerization bas 71 72 been studied by Banerjee • >. Only the group of Guillot, bas publisbed papers 73 74 75 on the batch emulsion copolymerization of styrene and methyl acrylate • • >. They showed that emulsion copolymerizations of styrene and methyl acrylate under certain circumstances exhibit a strong composition drift due to the reactivity ratios and the different water solubility of the monomers. They determined also the dependenee of monomer ratio, initiator and emulsifier 34 35 concentration on polymerization rate. Recently, Mead et aL • > studied the emulsion copolymerization of styrene and methyl acrylate in continous stirred tank reactors. During this investigation, besides S-MA, several reference model systems were also investigated. Considering monomer partitioning and reactivity ratios, the folowing systerns were chosen: styrene (S) - n-butyl acrylate (BA): with comparable reactivity ratios and different monomer partitioning methyl acrylate (MA) - n-butyl acrylate (BA): comparable monomer partitioning behaviour and its expected tendency to form perfectly random copolymers (i.e., both reactivity ratios equal to one) styrene (S) - acrylic acid (AA): extreme different water solubility of the monomers. Introduetion 7
1.4 Aim or this investigation
After the above general introduetion into the field of emulsion copolymerization, it will be clear that there is a lack of knowledge that justifies a structured investigation into the typical aspects of emulsion mpolymerization as compared with emulsion hrunQpolymerization. This research airns at a better and quantitative understanding of the effect of the process parameters determining the chemical microstructure of the copolymers. The importance of this kind of investigations is well illustrated by the enormous amount of papers dealing with emulsion copolymerization that have appeared during the last few years. As a result of the composition drift, often occurring during copolymerization, heterogeneous copolymers are obtained, sometimes limiting their practical application as commercial products. The present lack of detailed and complete investigations of emulsion copolymer microstructure retards this kind of fundamental research. Important aim of this study is therefore the development of experimental techniques to analyse copolymer microstructure. Final aim of this investigation is to develop monomer addition strategies in semi-batch processes to control composition drift and copolymer microstructure. The choice of the main model system (S-MA) bas been suggested by the moderate difference in water solubility, resulting in a more pronounced composition drift, and by the expected possibilities of analysing the copolymers. 8 Chapter 1
1.5 Outline of this thesis
In chapter 2 the basic theoretica! aspects of the emulsion copolymerization mechanisms relevant to this investigation are briefly discussed, viz., emulsion homopolymerization, the monomer partitioning during emulsion copolymerization and the ultimate and the penultimate- models for radical copolymerization. Chapter 3 contains all information on the experimental methods used in copolymerization as well as in copolymer characterization, viz., copolymer cross fractionation by SEC-TLC/FID and SEC-HPLC, and determination of the intramolecular microstructure by 1H NMR and 13C NMR. Furthermore, experimental results are given of preliminary investigations, viz., determination of the monomer reactivity ratios of the monomeric pairs used (i.e., S-MA, MA BA and S-BA). In chapter 4 the emulsion copolymerization model developed bere is presented. The model can be used to quantitatively describe kinetics (polymerization rate and composition drift), and copolymer microstructure (sequence distribution and MMCCD). It is demonstrated that apparent reactivity ratios, which are frequently used, are inadequate to describe composition drift. Instead, reactivity ratios measured in solution or bulk have to be used in combination with the monomer partitioning data, that depend on monomer ratio and monomer water ratio. In chapter 5 the theoretica! backgrounds and experimental results of the monomer partitioning are discussed. In chapter 6 the results are given of a detailed study of the effect of composition drift during emulsion copolymerization on reaction kinetics. In chapter 7 experimentally determined chemica! microstructures of emulsion copolymers are compared with model calculations. The effect of reactivity ratios, monomer water solubility, initial monomer ratio, monomer to water ratio and chain transfer agent concentration, on both intra- and intermolecular microstructure, are discussed. Introduetion 9
Finally, in chapter 8 a study is presented on how to control copolymer microstructure, using both experimental evidence and model calculations of semi-continuous emulsion copolymerization under starved conditions and semi continuous processess applying optimal monomer addition strategies. In the appendix A the results are given of attempts to prepare structured latex particles (core-shells) by means of multi-stage emulsion copolymerization of styrene with methyl acrylate or n-butyl acrylate, in the presence or absence of crosslinking agents. Moreover, the morphology of S-MA latex particles prepared under non-azeotropic conditions bas been investigated. In appendix B the results are given of attempts to verify the peak assignment at triad level of the methoxy proton region in 1H NMR spectra of S-MA copolymers by means of COWC and NOESY 20 NMR experiments.
Parts of this work have been presented at the IUPAC International Symposium on Free Radical Polymerization (Santa Margherita Ligure, Italy, May
1987), the 2nd International Symposium on Copolymerization and Copolymers Prepared in Dispersed Media (Lyon, France, April 1989), the Rolduc Polymer Meeting-4 (Kerkrade, The Netherlands, April 1989) and the IUPAC International Symposium on Macromolecules (Montréal, Canada, July 1990).
Parts of this thesis have been publisbed or will be publisbed soon : the 76 78 79 80 (MM)CCD part of chapter 7 ·n· • >, the NMR parts of chapters 3 and 7 >, chapter 481>, chapter 6 and the monomer partitioning study of chapter 582> and chapter 883>. Furthermore, related HPLC work, not extensively described in this 84 85 thesis, bas been publisbed or will be publisbed soon • >. 10 References
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64. N. Sütterlin, in "Polymer Colloids 11", Ed. R.M. Fitch, Plenum Press., New York and London, 583, 1980 65. H. Gerrens, Polymer Preprints, 2 699 (1966) 66. H. Gerrens, J. Polym. Sci., Part C, 27, 77 (1969) 67. H. Gerrens, Br. Polym. J., 2, 18 (1970) 68. D. Hummel, G. Ley, C. Schneider, Adv. Chem. Ser., 34, 60 (1962) 69. G.J.M. Ley, O.O. Hummel, C. Schneider, Adv. Chem. Ser., 66, 184 (1967) 70. G.J.M. Ley, C. Schneider, O.O. Hummel, J. Polym. Sci., Part C, 27, 119 {1969) 71. M. Banerjee, U. Sathpathy, T.K. Paul, R.S. Konar,Polymer, 22, 1729 (1981) 72. M. Banerjee, R.S. Konar, Polymer, 27, 147 (1986) 73. L Rios, C. Pichot, J. Guillot, MakromoL Chem., 181, 677 (1980) 74. W. Ramirez-Marquez, J. Guillot, MakromoL Chem., 189, 361 (1988) 75. W. Ramirez-Marquez, J. Guillot, MakromoL Chem., 189, 379 (1988) 76. G.HJ. van Doremaele, AM. van Herk, J.L Ammerdorffer, AL German, Polymer Commun., 29, 299 (1988) n. G.HJ. van Doremaele, AM. van Herk, AL German, MakTornoL Chem., MacromoL Symp., 35/36, 231 (1990) 78. G.HJ. van Doremaele, R.L Adolphs, R.W. Sparidans, AL German, in prep. 79. G.H.J. van Doremaele, F. Geerts, AL German, in prep. 80. G.H.J. van Doremaele, AL German, N.K. de Vries, G.P.M. van der Velden, Macromolecules, accepted 81. G.H.J. van Doremaele, AM. van Herk, AL German, in prep. 82. G.H.J. van Doremaele, F. Geerts, H. Schoonbrood, J. Kurja, A.L German, in prep. 83. G.HJ. van Doremaele, H. Schoonbrood, J. Kurja, AL. German, in prep. 84. R.W. Sparidans, H.A aaessens, G.HJ. van Doremaele, AM. van Herk, J. Chromatogr., 508, 319 (1990) 85. G.HJ. van Doremaele, J. Kurja, H.A Claessens, AL. German, in prep. Theoretieal Baekground 13
Chapter 2 Theoretical Background
SUMMAR Y: In this chapter the basic theoretical aspects of emulsion copolymerization mechanisms relevant to tbis investigation are briefly discussed. The kinetics of emulsion copolymerization is essentially different from that in bomogeneaus systems (bulk, solution). Important features are the kinetics of emulsion polymerization, tbe monoroer partitioning bebaviour and tbe ultimate and penultimate copolymerization model.
2.1 Emulsion polymerization
Emulsion polymerization camprises tbe emulsification of a monomer in a continuous aqueous medium using an oU-in-water emulsifier, followed by free radiCat polymerization usually initiated by a water soluble initiator to give a colloidal sol of polymer particles called a latex. According to tbe Harkins-Smith Ewart theory1.2> the process can he divided into tbree distinct intervals. Interval I camprises partiele nucleation. A third pbase is formed consisting of monoroer swollen polymer particles (20-200 nm). In intervals II and III ideally the partiele number remains constant. During interval II the polymerization proceeds in the presence of monomer droplets. The monoroer dropiets are totally consumed at the end of interval U, and the monoroer remaining in the particles is polymerized during interval lil. Therefore, intervals 11 and lil are called the partiele growth stages. 14 Chapter 2
Partiele nucleation
In literature, many mechanisrns have been proposed for the partiele nueleation. These theories can be divided into three main categones according 3 toa different locus of nucleation: (a) monoroerswollen rnicelles \ (b) monoroer droplets4.S> and (c) aqueous phase6.7). According to the first mechanism (a), micellar nucleation, small oligomeric radicals generated in the aqueous phase enter monoroer swollen micelles and initiate polymerization to form mature monoroer swollen polymer particles. Because the total surface area of the micelles is orders of magnitude larger than that of the monoroer droplets, normally no nueleation occurs in the monoroer droplets, except under eertaio extreme circurnstances, e.g., the monomer droplet size is decreased significantly as in mini·emulsion4>(mechanism (b)). As the micelles disappear the partiele nueleation period ends. According to the initiation·in-the-aqueous·phase mechanism ((c), homogeneaus nucleation), that is believed to be dominant when the emulsifier concentration is below the critical micelle concentration (cmc) or in the case of polar monomers, radicals generated in the aqueous phase add monoroer until the oligomeric radicals so formed reach a critica! degree of polymerization. This crideal length is suggested to be the point at which they exceed their solubility or the length at which they become surface active, and (co)precipitate. The precipitated oligomeric ebains absorb monoroer and adsorb emulsifier to form primary particles. These primary particles are colloidally unstable and can flocculate with each other or with already existing mature latex particles. In the beginning of the emulsion polymerization, however, partiele nucleation and partiele growth concurrently occur. For convenience, however, the two processes are considered separately. The end of the partiele nucleation period (i.e., in case of hydrapbobic monoroers the moment at which the free emulsifier concentration drops below the crideal micelle concentradon (cmc)) is taken as the beginning of the partiele growth period in the ideal· case characterized by a constancy of the partiele number. Once the particles have been formed by some appropriate mechanism, they will continue to grow until the supply of monomer is exhausted. Theoretleal Background 15
Partiele growth
The partiele growth period can be divided into intervals n and m. In interval n the monomer dropiets are still present and as a result a constant monomer concentradon in the particles leads to a constant polymerization rate. In interval 111 the monomer dropiets have disappeared and a continuons decrease in monomer concentradon results in a decreasing polymerization rate. For several systems an increase in polymerization rate is observed due to the Trommsdorff effect. Aqueous phase polymerization is usually negligible. The rate 1 1 of polymerization during interval n and m (~) (mol·Lw" ·s" ) is given by:
~ = kpii[M]N/Nav (2.1)
1 1 where kp is the propagation rate constant (L·mol" ·s- ), ii is the average number of radicals per particle, [M] is the monomer concentradon in the latex particles 1 (mol· L" ), N is the number of latex particles per liter water and Nav is Avogadro's number. N and ii are determined by the experimental conditions, where N is mainly determined by the kind and amount of emulsifier and initiator, and kind of monomer. According to recent theories (see chapter 4) the radical absorption (entry) rate, the radical desorption (exit) rate from the particles and the bimolecular terminalion inside the particles determine n. In contrast to the partiele nucleation period, the partiele growth period is much more reproducible. In emulsion polymerization high polymerization rate can be combined with high molecular mass of the polymer formed, because the free radicals present in the latex particles are segregated and hence are unable to terminate with those in another partiele due to the intervening aqueous phase. 16 Cbapter 2
Emulsion copolymerization
Like emulsion homopolymerization emulsion copolymerization can also be carried out using batch, semi-continuons (sometimes called semi-batch) or continuons processes. The chemical microstructure of the copolymers formed depends upon the reactivity ratios of the monomers, the monomer partitioning behaviour (phase equilibria) between the various phases in the emulsion system, and the type of process used. Batch processes are known to give two-peaked distributions of copolymer composition, when a strong composition drift occurs during the course of the emulsion copolymerization. (Semi)-continuous processes (i.e., addition of monomer during polymerization) can be used toprepare more homogeneons copolymers. Dynamic mechanica! spectroscopy or differential scanning calorimetry and transmission electron microscopy combined with preferendal staining techniques, have been used to determine the possible occurrence of phase separation due to double-peaked CCDs.
The characteristics of emulsion polymerization of polar monomers are somewhat different from those of apolar monomers like styrene that follow the Harkins-Smith-Ewart theory. The power dependenee of emulsifier concentradon on the number of particles (i.e., 0.6 above cmc for hydrapbobic monomers), generally deviates strongly from 0.68>. The monomers are distributed between the particles, aqueous phase and monomer dropiets (if present). The knowledge of (co)monomer concentradon in the partieles, where (co )polymerization takes place, is essential for the correct use of kinede models. As a result of the monomer partitioning, during emulsion copolymerization the local monomer ratio inside the particles can be quite different from the overall monomer ratio. The schematic diagram given in Figure (2.1) demonstrales for the styrene- methyl acrylate emulsion copolymerization the influence of the initial monomer to water ratio on the local monomer ratio inside the latex particles with the same overall monomer ratio. The aqueous phase acts as a reservoir only for methyl acrylate, because methyl acrylate is Theoretieal Background 17
much more water soluble than styrene. This results in a relatively higher concentration of styrene and a lower concentration of methyl acrylate in the latex particles, being more pronounced at low monomer to water ratios. In chapters 4, 6, and 7 it will be demonstrated that the initial monomer to water ratio is an important factor determining the emulsion copolymerization behaviour and copolymer microstructure.
MONOMER PARTITIONING
high mon/water
monomer dropiets [Mon) l latex particles Ma~
s· water
volume monomer dropJets low mon/water [Mon] r latex partieles Ma~
water
volume
Figure 2.1. Schematic qualitative diagram of the injluence of monomer to water ratio on the monomer partitioning during interval ll of the emulsion copolymerization of styrene and methyl acrylate. 18 Chapter 2
2.2 The ultimate _and the penultimate model for copolymerlzation
The most frequently used model to describe copolymerization kinetics. and sequence distribution and chemical composition of copolymers (for homogeneons copolymerization processes. viz., bulk and solution copolymerization), is the ultimate model, also known as the terminal model. In 1944' this ultimate model was independently introduced by Alfrey and Goldfinger'>, and Mayo and Lewis10>. In this model the monomer addition rate only depends on the nature of the terminal group and therefore obeys first order Markov statistics11>.
Table 21. Copolymerization scheme according to the u/timate modeL
terminal group added monomer rate final
-A* [A] ky[A*)[A] -AA* -A* [B] ~(A*](B] -AB* -B* [A] koa[B*][A] -BA* -B* [B] kt,.,[B *][B] -BB*
The reactivity ratlos of monoroers A and Bare defined by r. = kul~ and r11 = k""/~8, respectively, where JGJ is the propagation rate constant of the propagation reaction between radical i and monomer j. CombiDing the equations for monomer consumption and the steady-state principle, the instantaneous copolymerization equation giving the copolymer composition in mol fraction monomer a units (F.) as a function of reactivity ratios and monomer feed, can easily be derived:
Fa = (2.2)
where fa ( = 1-f11) is the mol fraction of monomer a. Theoretical Background 19
The average propagation rate constant ( k p) cao be calculated using equation (2.3).
r.f.2+ 2fafb +rb42 kp= ------(2.3) ( r .f./k,.) + (rb4/~b)
In systems wbere tbe nature of tbe penultimate unit bas a significant effect on tbe absolute rate constauts in copolymerization, tbe penultimate model bas to be used, in wbicb in principle eight different reactions have to be considered 12) (see Table 2.2).
Table 2.2. Copolymerization scheme according to the penu/timate model.
penultimate group added monomer rate fin al
-AA* [A] k888[AA *)[A] -AAA* -AA* [B] k,.b[AA *][B] -AAB* -BA* [A] ~[BA*][A] -BAA* -BA* [B] ~b[BA*][B] -BAB* -AB* [A] k.ba[AB*][A] -ABA* -AB* [B] k.bb(AB*][B] -ABB* -BB* [AJ kbba[BB*](A] -BBA* -BB* [B] ~[BB*][B] -BBB*
The six reactivity ratios of monomer i and j are defined by: r; = km/k;;i• r'; = ~;/kiii and s; = ~;;/k;;; (with i = a,b and j = b,a, successivily) 20 Chapter 2
The average prol?agation rate constant kP and copolymer composition can be calculated using equations (2.4-7):
r .f.2 + 2f.4, + r J.., 2 k p = (2.4) (r.f./ k ..)+(r bfb/kbb) and
r .r.2 +f.fb Fa = (2.5) r.f.2+2fa4,+ rbfb2 where
~~(rli + ~) kii = (2.6) ·r~~ + (2.7) As recently shown for several styrene-(meth)acrylate monomeric systems, the kinetic behaviour may exhibit significant deviations from the ultimate model, whereas composition drift and sequence distribution apparently follow the more simpte ultimate model13>. Numerical values of average propagation rate constants 14 15 of those systems have to be described by the penultimate model • >. The apparently contradictory situation that polymerization rate can be described by the penultimate model, while the compostion drift and sequence distribution still can he described by the ultimate model, can be easily verified from equations (2.4-7) by substituting r1 = r1' with s1 # 1. Keferences Zl 1. W.D. Harkins, J. Am. Chem. Soc., 6,, 1428 (1947) 2. W.V. Smith, R.H. Ewart, J. Chem. Phys., 16, 592 (1948) 3. W.D. Harldns, J. Am. Chem. Soc., 6,, 1428 (1947) 4. J. Ugelstad, M.S. El-Aasser, J.W. Vanderhoff, J. Pol;ym. Sci., Polym. Lett. Ed., 11, 503 (1973) 5. J. Ugelstad, F.K. Hansen, S. Lange, MakromoL Chem., 175, 507 (1974) 6. · WJ. Prlest, J. Phys. Chem., 56, 1077 (1952) 7. G. lichti, R.G. Gilbert, D.H. Napper, J. Polym. Chem., Polym. Chem. Ed., ll, 269 (1983) 8. R.M. Fitch, Br. Polym. J., 5, 467 (1973) 9. T. Alfrey, G. Goldfinger, J. Chem. Phys., U, 205 (1944) 10. F.R. Mayo, F.M. Lewis, J. Am. Chem. Soc., 66, 1594 (1944) 11. J.L Koenig, "Chemica/ Microstructure of Polymer Chains", Jobn Wiley and Sons, New York, 1980 12. E. Merz, T. Alfrey, G. Goldfinger, J. Polym. Sci., 1, 75 (1946) 13. This thesis: Chapter 3 14. T.P. Davis, K.F. O'Driscoll, M.C. Piton, M.A. Winnik, Br. Polym. J., submitted 15. T. Fukuda, Y. Ma, H. lnagald, Macromolecules, 18, 17 (1985) 22 Experimental 23 Chapter 3 Experimental Copolymerization Procedures and Development of Experimental Methods of Copolymer Analysis SUMMAR Y: In this chapter the experimental procedures foliowed for preparing and purifying the reference solution copolymers as well as the emulsion copolymers are outlined in detail. The emulsion copolymerization reactor equipped with on line GLC is described. Latex partiele size was detennined by means of dynamic light scattering. TEM was used to check the partiele sizes and for the determination of partiele morphology. The glass transition temperatures were measured by DSC. A feasibility study on the experimental determination of copolymer microstructure, using well- defined solution copolymers, is presented in two major parts. The first part describes a study on the detennination of the intermolecular microstructure. The second part deals with the analysis of the intramolecular microstructure. The intermolecular microstructure in terms of the molar-mass-chemical-composition distribution (MMCCD) of styrene (S) - methyl acrylate (MA) copolymers cao be detennined by means of SEC-TLC/FID or alternatively by means of SEC-HPLC. Reliable analysis of the intramolecular microstructure in terms of sequence distribution of S-MA copolymers appeared possible oot only by means of high resolution 13C NMR but also by means of high resolution 1H NMR. The sequence . distribution of MA-BA copolymers, however, could not be determined. The reactivity ratiosof S-MA copolymerization have been determined from low conversion solution copolymerization data: r5 = 0.73 ± 0.05 and rma = 0.19 ± 0.05. The copolymerization of methyl acrylate with butyl acrylate appeared to follow Bernoullian statistics: rba = 1.0 ± 0.05 and rma = 1.0 ± 0.05. 24 Chapter 3 3.1 Procedures f~llowed in solution and emulsion copolymerization 3.1.1 Purification of chemieals Styrene (S) (Merck), n-butyl acrylate (BA) (Merck) ánd methyl acrylate (MA) (Merck) were distilled under nitrogen at reduced pressure, and subsequently storedat 277 Kunder nitrogen. After distillation, acrylic acid (AA) (Union Carbide, industrial grade) wasstoredat 263 K. The initiator potassium persulfate (~Sp8) (Merck p.a.), the chain transfer agent n-dodecyl mercaptan (NDM) (Fluka for synthesis), the emulsifier sodium dodecylsulfate (SDS) (Fluka purity = 99%), the buffersodium hydrogencarbonate (NaHC03) (Merck p.a.), and the solvent toluene were used as received without further purification. The water was distilled twice and stored under nitrogen. The initiator a,a'-azobis(isobutyronitrile) (AIBN) (Fluka p.a.) was recrystallized once from methanol. 3.1.2 Preparation of reference copolymers by solution copolymerization Reference copolymers were required for calibration of the TLC/FID metbod to determine the CCD of the (emulsion) copolymers. Furthermore, these copolymers were used for the accurate determination of the reactivity ratios. These copolymers were prepared under nitrogen atmosphere by low-conversion ( < 10%) solution copolymerization in toluene in a 1-L glass vessel, thermostated at 335 K for the S-MA and S-BA, and 323 K for the MA-BA copolymerizations. The total monomer concentration was 3 mol· e 1 and the AIBN concentration 1 was 8 mmol· e • Conversion and monomer feed ratio were measured by means of GLC using toluene as intemal standard. The solution copolymers were isolated and purified by pouring out the reaction mixture in a 15 fold excess of cold (273 K) hexane. The final products were dried at 328 K in a vacuum stove for at least 6 h at 10-1 Torr and finally for 8 h at 10-5 Torr. Experimental 25 3.1.3 Emulsion copolymerization equipment and procèdures The emulsion copolymerizations were carried out under nitrogen in a 1-L glass or 1.3-L stainless-steel vessel normally tbermostated at 323 K. In case of MA-BA emulsion copolymerizations tbe reaelions were carried out at 308 K becáuse of the extremely high reactivity of tbe acrylates at higher temperatures. In Figure (3.1) the configuration of tbe (semi-)batch reactor system is given. The principle of on-line GLC analysis bas been described by e.g., Guillot et aL •>, and German et aL 2>. Altematively, it would have been also possible to use head space analysis3>. We have chosen tbe following modification. The reaction mixture is pumped in a sample loop using a pulsating membrane pump (Orlita, type 1W 1515/MKOO). In order to prevent phase separation the reaction mixture is circulated continuously tbrooghout the entire course of tbe emulsion polymerization. Up to 33 wt% latices could be pumped without pump or sampling valve fouling which would cause analysis problems. By means of the sampling valve4> present in the sample loop, a constant volume of reaction mixture can be analysed at any time during reaction. As a consequence of the heterogeneity of tbe emulsion, relatively large sample volumes had to be taken (5 pL). This 'large' sample appeared to be representative of tbe heterogeneons emulsion polymerization system as a whole and gave significantly less scatter of GLC data, as compared with a smaller sample volume (for instanee 1 pL). Normally, the scatter of peak areas of each monomer, due to variadons in sample volume and due to the heterogeneity of the emulsion, was less than 20% during intervals I and 11 of the emulsion (co)polymerization and less than 10% during interval III. No internat standard was used in order not to influence copolymerization behaviour in this heterogeneons system. The scatter of the Iil1i2 of peak areas of both monomers was less than 5% during the entire course of tbe reaction. The monomers and the water present in the sample mixture are evaporated in an injection block (placed directly .after the sample chamber) thermostated at about 473 K. and as a result the copolymer present in the sample is retained. The gas phase is subsequently split at a variabie ratio (but ftxed within each 16 Chapter 3 single experimen9 in order to prevent overtoading of the column (HP-1 Methyl Silicone Gum wide-bore column (length • intemal diameter • film thickness = 5 · m • 0.53 mm • 2.65 /MD., thertnostated at 378 K) and overloadiog of the detector (FID, 473 K). The GLC used was a Carlo-Erba Instruments GC 6000 Vega Series 2. Helium was used as carrier gas. A personal computer (Atari 1040 ST) operated the pneumatic sampling valve, integrated the chromatograms, calculated partial conversions ofboth monomers and (in case of semi-continuous operation) controlled the dosage of monomer via an automatic buret (Metrohm, type 645 Multi-Dosimat, refillable 10 mL buret). Usually, every 2 minutes a sample was taken for GLC analysis. Reactions were commonly followed during 4 hours, thus 120 samples were taken, in addition to the 10 or 20 GLC reference samples taken prior to the beginning of the polymerization. The amount of copolymer residue that remained in the injection block generally did not cause any probieros to sample and carrier gas passage, provided no more than 120 samples were taken during reaction. At the end of every experiment the sample valve and injection block were disconnected and cleaned. No fouling inside the sample chamber was ever observed. This may be attributed to the continuous circulation of the reaction mixture. Before use the reactor was purged with nitrogen (or argon: in case of MA BA copolymerization) in order to remove oxygen. The monomers, in which n dodecyl mercaptan (NDM) was dissolved, were added dropwise to the SDS solution in water. Theemulsion was stirred with a twelve-bladed turbine stirrer at 250 rpm. After the emulsion had reached reaction temperature and the reference GLC determinations were performed, the reaction was started by adding a potassium persulfate solution in water to the reaction mixture. No induction time was observed. Experimental 17 ------1 I I ~----0 -"'---Lf> 1-EL/Uvl Figw'e 3.1. Schematic diagram ofreactor configuration equipped with on-line GLC; reactor (A), Junnel (B), valve (C), personal computer (D), GLC (E), sampling valve (F), stirrer (G), pump (H), monomerstock (/). The overall ratio of monomers during the entire course of the reaction was monitored by means of on-line GLC. Total weight conversion was determined by means of dry solid analysis. Polymerization in the dry solid samples was stopped by adding hydroquinone (Merck). Mter curve fitting of the GLC data, partial conversions of both monomers were calculated combining both data sets. The emulsion copolymers were purified by removing sodium dodecylsulfate and unreacted potassium persulfate, monomers and n-dodecyl mercaptau by dialysis foliowed by carefut coagulation with an aluminium nitrate · (Merck cryst. pure) solution in water (0.001 molfL). Subsequently, the copolymers were thoroughly wasbed at least ten times with hot (ca. 350 K) water foliowed by filtration. Finally, the copolymers were dried at 10·5 Torr for at least 24 b. 28 Cbapter 3 Partiele size meOSI;U'ements and morphology detennination Partiele sizes were measured by means of dynamic light scattering using a Malvem Autosizer 2C apparatus. In principle, the mean-volume diameter (<1.,} bas to be used to calculate the number of particles. However, the average diameter (<1., or d0 LS) used is defined somewhat differently (see eq. (3.1}). As checked by 1EM, the use of <1., instead of <~., did result in a small but negligible error in the calculated number of particles. and Besides partiele size, morphology determinations were also carried out with transmission electron microscopy (1EM) (Jeol model JEM 2000 FX). Various staining techniques were investigated and usually more than one technique was applied. Phosphotungstic acid (PTA, H3P04 ·12W03 ·xHzO) was used as a negative staining agent (staining of the background). Positive (selective for styrene units) staining was performed with Ru04 or Os04 using a standard procedure {see Appendix A). Prior to drying the latices on 200 Mesh grids, for acrylate rich particles hardening appeared to be necessary. Otherwise the particles would be too soft and as a consequence would flow during drying on the grids. HardeDing was achieved by UV cross-linking. A latex was diluted in water to 10-50 ppm and exposed to UV radiation in a quartz tube of a 0.5 cm diameter for 24 hours 5>. Measuring glass transition temperature by means of DSC Differential scanning calorimetrie {DSC) measurements were performed in air using a Perkin-Elmer (DSC-7) differential scanning calorimeter. The samples were prepared alter dialyses of the latices. Only second run measurement data were used. A standard heating rate of 10 •cjmin was adopted. Experimental 19 3.2 Copolym~ analysis 3.2.1 Cross-fraetionation metbod tor determining eopolymer MMCCD One of the fundamental issues in the analysis of copolymers is the correlation between CCD and MMD. To enable evaluation of the two dimensional distribution (MMCCD), cross-fractionation techniques are required. The mutual dependency of chemical composition and rnalar mass analyses is one of the main experimental problems to deal with. As often erroneously believed, SEC equipped with a dual detector system (e.g., UV and refractive index) alone is incapable of giving complete information on the cbemical beterogeneity. lt merely gives information about the average cbemical composition at different rnalar masses. The cross-fractionation metbod described bere, is capable of measuring tbe full molar-mass-cbemical composition-distribution (MMCCD ). The intermolecular microstructure of the S-MA copolymers in terms of the MMCCD was determined by means of the cross-fractionation metbod developed earlier in our laboratory'> and based on SEC-TLC/FID. First, the copolymer is separated according to molar mass by means of SEC. Althougb the MMD of homo.po)ymers can be determined accurately by SEC, correct information on the MMD of co.polymers cannot always be obtained in a straightforward manner by SEC. The main reason is that separation by SEC is achieved according to the hydrodynamic volume of the molecules in solution. In the case of copolymers this volume generally depends not only upon the molar mass, but also upon the chemical composition. As was already shown by Teremachi7) within the range between 45 and 80 mol% styrene of S-MA copolymers, the SEC separation is performed nearly exclusively according to molar mass without a disturbing influence of chemical composition. This accords with a recent study by Davis et aL 8>. Subsequently, each SEC fraction of the S-MA copolymer is analysed by 9 10 11 12 13 14 means of TLC/FID • • • • • >, exclusively according to chemical composition as is demonstrated in this chapter (§ 3.2.1.2). 30 Chapter 3 Alternatively! if desired, it is also possible to detennine merely the CCD of the copolymer without a preceding separation according to molar mass, because the influence of molar mass on the retention factor (R,) is negligible as will be demonstrated later(§ 3.2.1.2). The TLC/FID appeared to be not useful forS. BA and S-AA copolymers (§ 3.2.1.4). Gradient high performance liquid chromatography (HPLC) with UV detection was used as an alternative to TLC/FID for determining the CCDs of S-MA. S-BA and S-AA copolymers. HPLC bas an even higher resolution and reproducibility and is less laborious. As a serious disadvantage of HPLC with UV detection must be mentioned that quantitative detection of styrene (meth)ácrylate copolymers having a very low styrene content becomes impossible since UV detection at 254 nm is sensitive only to pbenyl (styrene) units and bardly to carbonyl (acrylate) units. In this case of coarse, a differentlal refractometer çannot be used because the HPLC separation requires a gradient elution tecbnique. Very recently, however, it bas been demonstrated that it is possible to use a "moving wire FID" as an alternative detection metbod for polyacrylates and acrylate rich copolymers15>. The HPLC metbod bas been described elsewhere16>, and will not be discussed in detail in this thesis. Equipment and experimental conditions are given in §32.1.5. Expertmental 31 3.2.1.1. SEC Size exclusion chromatography (SEC) of s-MA copolymers was performeel on a chromatographic system (Waters Associates) equipped with both a differential refractometer and an ultraviolet (UV) detector (254 nm). Using both signals, SEC chromatograms with peaks directly proportional to the relevant amounts of copolymer were calculated using an experimentally determined relation between copolymer composition on the one band, and refractometer and UV signals on the other hand. This calibration of the SEC chromatographic 1 signal was performed without accounting for non-linearity of UV response 7) and without accounting for any possible differences in intramolecular structure. However, Garcia18>showed that the intramolecular microstructure can have some effect on UV absorption of copolymers. Because all copolymers are random copolymers, the relative small differences in intramolecular structure are expected to have a negligible effect on the ratio of UV absorption and refractometer signal. A series of three or four p-Styragel columns with nominal 1 pore sizes of the pacldngs of (10 ), 1~. l«Y, Hf nm thermostated at 313 K. were used. The SEC columns were calibrated using 18 polystyrene samples, with narrow molar mass distributions. The 1HF flow rate was set at 0.9 mL/min in case of four columns and set at 0.6 mL/min in case of three columns. All emulsion copolymers were dissolved in 1HF after purifying the latex, except for the styrene-acrylic acid emulsion copolymers. In order to dissolve these copolymers complete S-AA latices were dissolved in THF. In case of the conventional SEC analysis for the determination of the MMD, a 100 pL (0.1% wfv) sample was injected. However, a 1000 pL (0.15% w/v) sample was injected in case of the SEC fractionations in hebalf of subsequent TLC/FID or HPLC analyses. Typically, the copolymer was fractionated into 6, 8 or 10 fractions. The 1HF was evaporated from each fraction at 313 K under a nitrogen flow. Subsequently, the remaining copolymer was redissolved in 10-40 pL toluene. These very small samples contain sufficient copolymer to perform the subsequent TLC/FID or HPLC analyses. The other (S-BA, MA-BA and S-AA) copolymers were analysed under the same experimental conditions. 32 Chapter 3 3.2.1.2 Gradlent TLC/FID The chromarods (Iatron Lab .. type S) were aètivated in a vaeuum oven at 393 K and subsequently scanned twice. In order to obtain accurate copolymer CCDs by means ofTI..C/FID it is.extremely important that the sample (0.2 I'L 1% w/v) is spotted on the rod very meticulously with a syringe resulting in little spot broadening. Also spatting should be carried out in a solvent-saturated atrnosphere. Furthermore, and in contrast to common practice in TI..C when low molecular weight samples are analysed, the rods were not dried after spatting and prior to elution. This procedure prevents precipitation of the copolymer on the rods and as a consequence the copolymer will stay in dynamic equilibrium with solvent and ~dsorbent. Without these precautions; precipitation of the copolymer occurs prior to elution and this leads to slow redissolution during elution giving tailing and thus resulting in an apparent CCD. A few (2 to 4) rods of each set of 10 rods, hold by a roetal frame, were not spotted with samples from SEC. Instead these rods were spotted with a mixture of well-defined reference copolymers, i.e., homogeneons copolymers prepared by low-conversion solution copolymerization (§ 3.1.2). The R, valnes of the reference copolymers were used to calibrate the chromatograms of the unknown samples. A correction was made accounting for small differences in the elution front distances observed among the ten different rods. Howevér, these differences were minimized ( < 3%) by selecting rods that show similar elution rate behaviour. For MA rich copolymer samples, reference copolymers were taken with copolymer compositions, in termsof S molar fractions, of 0, 0.12, 0.33, 0.46, 0.57, and 0.76. In case of S rich samples, the chemical composition of the reference copolymers were 0.46, 0.57, 0.76, 0.81, and 1. All reference copolymers had molar masses of approximately Mn = 40000 (g/mol). A gradient elution technique was applied by adding polar liquids to a rather apolar starting eluent during elution. In order to prevent precipitation of the copolymer during elution, leading to a molar mass dependenee of the retardation factor (Rr), and thus in an apparent CCD, the copolymer was eluted permanently under saturated solvent conditions. This appeared to be a prerequisite for a Experbnental 33 molar mass independent retardation factor. The absence of a possible and unwanted molar mass dependency was checked by oomparing the Rr values of several copolymers of the same chemical composition but with different molar masses. In Figure (3.2) it is shown that the molar mass dependency on Rr is negligible in a wide range of molar mass. In order to minimize peak broadening and to obtain an optimal separation, the elution procedure was adapted to the average copolymer composition involved. For methyl aCJ)'late rich S-MA copolymers 25 mL toluene was introduced into the special development tank as described by Tacx19>. After an equilibration time of 15 minutes, elution was started by adding 75 mL toluene. After 1, 2, 3, and 4 cm elution front position 5 mL acetone was added to the eluent. At 5 and 6 cm 10 mL acetone and finally at 7 and 8 cm 5 mL methanol was added. Blution was allowed to continue until the elution front position had reached a level of 9 cm above the spotting point. This procedure resulted in an excellent separation in tbe MA rich area. Pure PMA also migrated under these conditions (Rr= 0.05- 0.1). In Figure (3.3) a typical chromalogram demonstrales the resolving power of this technique. 1.00 r------, c{ o.so - 0.00 .______.__ ___,__ __...... ,. ____.______, 0 100 300 500 Molar mass (kg/mol) Figure 3.2. R values as a function of molar mass at constant copolymer 1 composition (33 mol% S) 34 Chapter 3 0.33 0.46 Fs 0.12 = 0.57 0.76 > distance on the rod Figure 3.3. TLC/FID chromalogram showing the separation of a mixture of 5 reference copolymers each having a narrow distribution according to chemica/ composition, but having a different average chemica/ composition. For styrene ricb S-MA copolymers a different elution procedure was used, starting witb 85 mL CC14 (by adding 60 mL to the 25 mL CCl4 used for 15 minutes equilibration) and adding 10 mL toluene at 1 and 2 cm elution front positions, foliowed by adding each time 5 mL acetone at 4, 5, 6, and 7 cm. After 8 cm tbe elution was stopped. After elution the rods were dried in a vacuum stove at 393 K for ca. 30 min. The TLC/FID scanning apparatus Iatroscan TH-10 was used to detect tbe separation pattem of tbe (co )polymers. The electronk FID amplifier of tbe Iatroscan was replaced by a Carlo Erba amplifier type EL480 for improved linearity. The optimal conditions for complete detection and minimal rod 3 damage were 1 atm. H2 pressure, an air flow rate of 1800 cm /min and a scanning speed of 0.42 cm/sec. The effect of copolymer composition on tbe FID response was investigated and appeared to be very small (Figure (3.4)). Therefore, it was neglected in the calculation of all CCDs of the S-MA copolymers. Experim.ental 35 2.00 1.50 iQ)... 1.00 ....b ....g 2: 0.50 0.00 0.00 0.50 1.00 F. Figure 3.4. Effect of chemica/ compostion of S-MA copolymers on the FID response in arbitrary units. · The small peak corresponding to copolymer material tbat remained on tbe spatting place, was always less tban 5% of tbe total peak areas. This little peak was neglected in tbe CCD calculations. The average copolymer compositions, as deterrnined from tbe measured CCDs, were verified by means of 1H NMR. Tacx19>bas sbown tbat tbe 1LC/FID metbod failed for emulsion copolymers prepared in the presence of eertaio emulsifiers (Antarox CQ...880 and RE-610), presumably due to cbemical bonding of emulsifier to tbe polymer ebains during polymerization. However, emulsion copolymers prepared in the presence of SDS could be analysed very well, provided tbey were well purified from SDS by dialysis. Generally, the difference between tbe average S-MA copolymer composition determined by means of 1LC and that deterrnined by 1H NMR was less than 3 mol% styrene. Cbapter 3 3.2.1.3 Cross-fraetionation data treatment SEC and TI..C/FID chromatograms were digitiZed and the distributions were calculated on a personal computer using both eaUbration curves. In those cases where pure PMA or pure PS is present in the sample, direct use of the calculated 1LC calibration curves would .result in physically impossible negative styrene fractions (for PMA) or physically impossible styrene fractions larger than 1 (for PS) due to chromatographical peak broadening. To deal with this problem in MMCCDs, the homopolymer peaks were corrected using the convention that, in the distribution plot, the pure components have a peak broadening of ·1 %. Because the distribution is normalized to unity and the composition is given in mol% styrene, pure homopolymer (PS or PMA) will have a height of 100 in CCD plots. Altematively, insome plots the area under the distribution curve was normalized to the molar conversion. 3.2.1.4 Aceuracy and reliability of gradient TLC/FID In Figure (3.5) the experimentally determined CCD of a low-conversion solution S-MA copolymer ( (S/MA)0 = 0.11 (mol/mol); conversion = 3 (mol%); M., = 62700 (g/mol)) is compared with the model CCD, calculated using the modified Stockmayer equation proposed by Tacro>. The original chromalogram of this copolymer is given in Figure (3.6). Figure (3.5) demonstrales that a low conversion solution copolymer indeed bas a narrow chemical distribution. In Figure (3.7) an experimental CCD of a high-conversion solution S-MA copolymer ( (S/MA)0 = 0.85 (molfmol); conversion = 97 (mol%); M., = 48800 (gfmol)) is compared with the model CCD. The original chromatagram of this copolymer is given in Figure (3.8). The model CCDs have been calculated accounting for both the composition drift and the instantaneous heterogencity due to the statistica! character of the monoroer addition process. The model also takes into account the difference in molar mass between the two monomers. The Experimental 37 excellent agreement between theory and 1LC results gives confidence in both tbe reliability of the 1LC metbod of determining the CCD and the validity of the model calculations. 30r------~ 20 ~ ,, (( I I I I I I 10 I I' I I 1 I ,/) 1\\. 0 0.00 0.50 1.00 Fs Figure 3.5. Experimental (- ~-) and model (--) CCD of a low-conversion so/Uûon S-MA copolymer. (S/MA)0 = 0.11 (mol/mol); conversion = 3 (mol%); Mw = 62700 (g/mol) Copolymer Spotting point 1 Distance Figure 3.6. Original TLC/FID chromatagram of copolymer of Figure (3.5). 38 Chapter 3 10~------~------. ) a: !S 0~,~~~~~~--~~~~--~~~ 0.00 0.!50 1.00 Figure 3. 7. Experimental (- - -) and model (--) CCD of a high-conversion solution S-MA copolymer. (S/MA)0 = 0.85 (mol/mol); conversion = 97 (mol%); Mw = 48800 (g/mol). Copolymer Spotting point Distance Figure 3.8. Original TLC/FID chromalogram of copolymer of Figure (3. 7). E:xperimental 39 Figure (3.9a) shows the cross-fractionation result of a low-conversion (3 mol%) solution S-MA copolymer having an average composition of styrene of 33 (mol%) and a ~ = 62000 (g/mol). All SEC fractions exhibit symmetrically shaped CCDs. lt is clearly shown that the calibration by means of the low conversion solution reference copolymers with ~ = 40000 (g/mol) is quite satisfactory for all SEC fractions, indicating once again the independency of Re in TLC of copolymer molar mass. Finally, in Figure (3.9b) an experimentally determined MMCCD of a high-conversion copolymer prepared with q0 = 5.7 (mol/mol) at 99 (mol%) and with ~ = 46700 (g/mol) is given, demonstraling the occurrence of a composition drift towards the styrene rich side for all SEC fractions: all SEC fractions exhibit asymmetrically shaped CCDs. From these results it can be concluded that the TLC/FID technique is useful for the CCD determination of S-MA copolymers provided extreme care is taken during the sample spotting and during development of the rods. Although it appeared to be possible to separate MA-BA copolymers according to chemica} composition using TLC/FID, the TLC/FID metbod cannot be used for reliable CCD analysis because of a lack of reproducibility of the retention factor (Rr)· For S-BA copolymers the above described TLC/FID metbod could not be used due to the small difference in polarity between the S and the BA monomer units. S-AA copolymers also could not be analysed in this manner, because the acrylic acid monomerunits are too polar. As aresult S-AA copolymers remain at the spotting points on the rods during elution regardless of their chemical composition. Therefore, in these cases HPLC was applied. 40 Cbapter 3 20 20 16 16 ~ (a) 12 12 8 8 4 4 0 0 '1 lo& s ~OI· ~~ ·IJ..) 10 10 8 8 ~ (b) 6 6 4 4 2 2 0 0 '1 lo& s ~Of ·IJ..) ~ .. Figure 3.9. Typical experimental MMCCDs of solution copolymers (a) ((S/MA)0 = 0.11 (mol/mol); conversion = 3 (mol%); Mw = 62000 (g/mol), (b) ((S/MA)0 = 5.7 (mol/mol); conversion = 99 (mol%); Mw = 46700 (g/mol). Experimental 41 3.2.1.5. Gradient HPLC HPLC was used as an alternative to TI..C/FID of CCD analyses of S-MA copolymers. The advantages of HPLC over TI..C/FID are an improved reproducibility and an increased separaûon efficiency. Furthermore, in contrast to TI..C/FID, with HPLC it appeared to be possible to analyse S-BA copolymers and S-AA copolymers according to chemical composition. Solvent gradients were created with a system controller (Model 720) and two HPLC pumps (Model 510, Millipare-Waters Corp., Milford, MA, USA). A Waters Intelligent Sample Processor (Wisp) (Model 710) was used to inject 10 or 20 pL of the samples (0.2 wt% copolymer solution in TIIF). The measurements of S-MA and S-AA copolymers were performed on a home made reversed phase column (Uchrosorb RP-C18 with a partiele size of 5 pm; L= 10 cm, ID = 4.0 mm, Knauer, Berlin, FRG) at 313 K With this packing no gel 21 permeation effect was noticed as reported by Teramachi et aL ) when using a ( normal phase) packing with much smaller particles (diameter of 10 and 20 nm). In the case of S-MA copolymers a linear elution gradient was applied on going from a mixture of 80% TIIF, 15% methanol and 5% water, to pure TIIF in 16 minutes with a flow rate of 0.8 mL/min. Under these conditions the separation is totally govemed by precipitation. The molar mass dependenee on retention time (R1) of S-MA copolymers bas been investigated and is negligible for molar masses between 5000 and 300000 (g/mol)22>. Figure (3.10) shows a typical HPLC chromatogram, exhibiting the separation of a mixture of low conversion salution S-MA copolymers. In the case of S-AA copolymers a linear gradient was applied on going from methanol containing 25% water to pure TIIF in 20 minutes with a flow rate of 0.8 mL/min. The S-BA copolymers were analysed at 313 K on a normal phase column (Chromosphere SI with a partiele size of 5 pm; L= 10 cm, ID = 3.0 mm. Chrompack, Middelburg, The Netherlands). A gradient ranging from a mixture ',, of 10 % dichloromethane (with 1.2% methanol) and 90% n-heptane, to 42 Chapter 3 dichloromethane containing 1.2% methanol in 18 min with a flow rate of 0.4 mL/min was applied. A personal computer was used for the acquisition of UV data from a Waters Multiwavelength Detector Model490 (set on 260 nm) and to calculate the final (MM)CCDs from the chromatograms and the calibration curves. All chromatograms were recorded after UV baseline correction. A more detailed describtion of the HPLC metbod bas been publisbed separately16>. l 0.46 c; =öl) ·c;; 0.31 ;::::,> (b) 5 10 15 20 min Retention time Figure 3.10. HPLC chromalogram showing the separation of a mixture of 7 reference (co)polymers each having a narrow distribution according to chemica/ composition, but having a different average chemica/ composition; (a) without, (b) with base linë correction. Experlmental 43 3.2.2. 1H and 13C NMR investigation of the intramolecular strueture of solution styrene-methyl aerylate eopolymers and detennination of reaetivity ratlos Abstract: The intramolecular stmcture (triad distribution and tacticity parameter o.... ) of well defined homogeneons styrene (S) • methyl acrylate (MA) oopoTymers, obtained by low-and high~onversion solution polymerization, bas been studied by 1H and 13C NMR. With the set of reactivity ratios r, = 0.73 and rm = 0.19 and elassical formulas, basedon Alfrey-Mayo (AM) kineties, it was possible to verify the experimentally observed triad distributions. 3.2.2.1 Introduetton Determination of the intramolecular (triad distribution, tacticity) and intermolecular (chemical composition and molar mass distribution) copolymer microstmcture is generally reoognized as a prerequisite, since reveà.ling the mol~lar microstructure may supply information about the monomer addition process, e.g., about the preferenee of monomers to add in a (co }iso- or oosyndiotactic oonfiguration23.24.2S>, Moreover, knowledge about the intermolecular and intramolecular stmctures is of paramount importance for the onderstanding of relations between molecular stmcture and polymer properties26.27.28>. Furthermore, the microstructure depends upon the polymerization processes and provides information about the reaction 29 mechanisms occurring during polymerization ,30). NMR methods provide information about the average copolymer composition as well as about the intramolecular stmcture23.2S>. However, the majority of publications concerning the elucidation of intramolecular microstmcture of copolymers deal with well defined low-conversion ( < 15%) bulkor solution oopolymers. Several examples in the field of styrene-(meth)acrylate copolymers can be found in the 31 literature ,32,33,34,3S). Par less attention bas been paid to the determination of the 36 microstructure of oopolymers obtained by high-eonversion solution processes ·37) 39 40 41 or high-eonversion emulsion processes38, • • \ even though high conversion is of great technological importance. 44 Chapter 3 In the case of high-conversion solution copolymeriza:tion of styrene (S) and methyl methacrylate (MMA), it bas also been shown that the inter- and intramolecular structure could be successfully predicted, on the basis of the integrated Alfrey-Mayo (AM) model, by using kinetic parameters obtained from 36 2 43 low~nversion data .4 • >. Similar results were obtained by our group on the styrene (S) - ethyl 35 methacrylate (EMA) system .37). However, due to the overlap of OCH3 or OCH2 resonances (in the S-MMA and S-EMA systems, respectively) and the methine main chain ( = backbone) resonances in the 1H NMR spectra of tbe above mentioned systems33,3S), as bas been already put forward by Harwood25>, still problems may occur with the correct quantitative analysis of the sequence assignment of tbe acrylate-centred triads, see, e.g., recent publication of Kale et aL on tbe S-MMA system44>. We have studied the styrene (S) -methyl acrylate (MA) system, where (i) no overlap occurs between the OCH3 resonances and the other main-chain resonances45>, (ii) mixed configurational and compositional 1 45 46 4 sequence effects are reported to occur for the OCH3 H NMR resonances • • 7), and (iü) the polymer statistics can be rigorously tested via an analysis of the 13C 48 49 35 13 carbonyl carbons • .so>. Contrary to S-MMA or S-EMA systems > the C chemica! shifts of these carbons in the S-MA system are only influenced by compositional sequence effectsSO). We have measured the chemical composition of low-conversion ( <20%) solution S-MA copolymers and we have calculated the reactivity ratios. This set of reactivity ratios, tagether with the assumed AM model bas been used to test the validity of this model (via an interpretation of the 13C NMR data). Moreover, as described in appendix B, in order to distinguish between on the one hand Ito's earlier peak assignment45> (tentatively based on the same assignment order for S-MMA copolymers31>), whicb results in an extremely high 1 coisotacticity (u,m = 0.8 - 0.9) and on the other hand an alternative assignmenF ) apparently resulting in a much lower usm, three 2D COLOC NMR (correlation long range coupling) experiments52> were performed on well-defined homogeneaus low~nversion copolymers in an attempt to correlate the 1H NMR Experimental 45 13 OCH3 region and the C NMR c:arbonyl region. Related 2D NMR techniques (2D 1H NOESY: Nuclear Overhauser Effect Spectroscopy) have been performed on altemating S-MMA copolymers53>, indicating in fact the correctnessof the old lto assignment for the S-MMA system. 2D NOESY is an NMR tecbnique, specially tailored to detect spatial interactions over short distances (generally < 5 Á). By this technique, the close proximity of the phenyl rings of the styrene · units and methoxy protons of the (meth)acrylate units in case of co-isotactic configuration can be sbown through NOEs. Very recently using 2D NOESY NMR on statistical S-MMA copolymers Aerdts et al. S4) reassigned the methoxy 1H NMR signals based on pentad sequence distributions. In appendix B 2D NOESY NMR experiments are also used in order to fmd a correct peak assignment for statistical S-MA copolymers. The experiments described in appendix B, however, indicate that no unambiguous peak assignment can be made at a triad level and that pentads have to be taken into account. Because the methoxy proton area is only moderately resolved into three distinct resonances, no attempt was undertaken to find a suitable peak assignment on pentad level. We have used the assignment of Ito45> in order to be compatible with earlier assignments in the literature, giving reliable triad fractions as was verified by means of 1~ NMR, but unfortunately an uureliabie tacticity parameter (a,m). Finally, we analysed the microstructure up to a triad level of some high conversion salution S-MA copolymers and compared it to model calculations using the integrated AM model37). Chapter 3 3.2.2.2 Experimental sedion 1H NMR spectra were recorded with a 400-MHz (Bruker AM 400) spectrometer at 298 K, by using CDC13 as the solvent and locking agent. Generally, the spectra were obtained by using a speetral width of 6024 Hz, an acquisition time of 1.4 s, a flip angle of 45• and a pulse delay of 5 s. Spectra were obtained after accumulating 64 scans, using a sample concentradon of 1% (wfv). The digital resolution amounted to 0.18 Hz, corresponding to a data length of 32K. 13C NMR were recorded at 100 MHz (Bruker AM 400) at 298 K. The sample concentradon was 9% (wfv) CDC13• Spectra were obtained by using W ALTZ-16 decoupling and a pulse delay of 5 s, accumulating 2000 scans with a digital resolution of 0.4 Hz/point, corresponding to a speetral width of 25000 Hz and a data lengtb of 64K. The flip angle and acquisition time were 90• and 1.3 s, respectively. Monomer sequence placements were determined by oomparing the relative peak areas of the proton or carbon atoms involved. In performing quantitative NMR measurements via compositional or configurational sequence placements one must take into account difference in nuclear Overhauser effects (NOB) and spin-lattice relaxation times (T1). No NOB or T1 values have initially been determined (see Appendix B for T1 values), but an additional 13C NMR experiment was performed on one sample with a much longer delay (15 s) and gating off the dècoupler to remove the NOB. The results are identical with those obtained via 13C NMR with the standard method. lmplicitly we assumed that no differential T1 values are present for different stereoisomerie ( coiso, cohetero and cosyndio) triads or compositional triads (MMM, MMS, SMS, SSS, etc.) in the 1H or 13C NMR spectra. No differential 1H NOEs were considered to occur. Within these limits relative peak areas are proportional to the numbers of proton and carbon atoms involved. Peak areas were determined via electtonic integration methods or planimeter methods ( after truncation of overlapping speetral regions). Experimental 47 3.2.2.3 Results and discussJon Low-conversion solution copolymers 1H NMR spectra: determination of co.polymer composition and of the reactivity ratios. Figure (3.11) depiets as a typical example the 400-MHz 1H NMR spectrum of a low-conversion solution S..MA copolymer dissolved in CDC13 at 25*C, whereas in Figure (3.12) expanded 400-MHz spectra are shown for four copolymers and poly(methyl acrylate) (PMA). Expansions of the methoxy region are shown since in particular this region displays additional fine splittings due to combined configurational ( =tacticity) and compositional sequence effects45>. The average copolymer composition (mol fraction styrene: F.) can be readily obtained by using = (3.2) where A1 and A2 represent the total peak areas of the aromatic and methoxy proton resonances, respectively. The initial feed (q0 = (S/MA)0), the average copolymer composition and the conversion are summarized in Table (3.1). With the use of the well-known Kelen-Tüdös low-conversion metbod and the data, presented in Table (3.1), the reactivity ratios were determined for an AIBN-initiated system at so·c: r. = 0.73 ± 0.05 , rm = 0.19 ± 0.05. These values were used in all model calculations forS-MA emulsion copolymerizations where KzSp8 was applied as initiator at a temperature of 5o•c. The values are in reasonable agreement with literature data (6o•c, benzoyl peroxide) r. = 0.64, rm = 0.1642>, or r. = 0.75 and rm = 0.18 (55•c, benzoyl peroxide))48>. 48 Chapter 3 H H Ar -CH,-CH- -c-c- H I c~o I @ 0 OCH, I CH 3 M s M 7.50 7.00 6.50 6.00 5.50 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 .50 0.0 -.50 Figure 3.11. 400-MHz 1H NMR spectnun of a low-conversion solution styrene (S) - melbyl acrylate (M) copolymer in CDCI3 at 25•c The mol fraction styrene (Fs) is 0.46. Table 3.1. Initia/monomerratio q0 = (S/MA)0 an4..fraction styrene ifs.J; observed cumulative average copolymer composition ( F5, mol fraction styrene) and final conversions of some low-conversion solution S-MA copolymers. Qo ~.o Fs conversion mol% 0 0 0 20.0 0.038 0.037 0.12 7.0 0.11 0.099 0.33 5.0 0.36 0.26 0.46 12.6 0.85 0.46 0.57 14.3 1.98 0.66 0.67 16.1 2.97 0.75 0.77 10.2 4.78 0.83 0.81 9.7 Experimental 49 3.74 3.54 lL-Me 3.65 3.45 3.25 K···~ 3.60 3.40 3.20 FM = 0.43 3.70 3.50 3.30 3.10 X : _z_ FM = 0.19 3.so 3.3o 3.1o -oppm Figure 3.12. Expanded 400-MHz 1H NMR spectra of low-conversion salution S-MA copolymers and poly(methyl acrylate) showing the methoxy region only. Spectra were recorded in CDC13 at 25°C. Copolymer compositions are ûtdicated for each copolymer. Area measurements have been performed for the regions X, Y and Z using the ûtdicated areas. 50 Chapter 3 Sequence ana}ysis by 1H NMR and 13C NMR The 10 theoretically possible MA-centred triads are not resolved in 10 distinct resonances and up to now the methoxy peak region bas been analysed in terros of the lto-Yamashita (1-Y) assignment, developed for styrene-methyl methacrylate copolymers (S-MMA)31>. According to Ito and Yamashita45>, the three different groups of peaks (X, Y, Z) can be assigned to the following combination of MA-centred triads: (3.3a) (3.3b) (3.3c) where Fx> FY and Fz are normalized peak areas. In equation (3.3), the parameter usm ( = ums or u in shorthand notation) is defined as a measure of the probability that altemating MA and S units adopt a coisotactic configuration. MMM, MMS, and SMS denote the three kinds of compositionally different MA-centred triads. More details can be found in the 25 31 45 literature • • >. Experimental 51 Assuming the Alfrey-Mayo (AM) model ( = first-order Markovf5>tobevalid at any moment of the reaction for these low-conversion copolymers, the number fraction (F) of methyl acrylate (MA or M)- and styrene (S)-centred triads can be predicted. The relation between intramolecular structure (number fraction of triads) andreaction kinetics is given by the following set of equations (3.4): . FMMM = (1-P(S/M))2 (3.4a) FMMS = FsMM = 2P(S/M)(1-P(S/M)) (3.4b) FSMS = (P(S/M)i (3.4c) Fsss = (1-P(M/S))2 (3.4d) FSSM = FMss = 2P(M/S)(l-P(M/S)) (3.4e) FMSM = (P(M/S)i (3.4f) where P(S/M) = 1/(1 +rm/q) , P(M/S) = 1/(1 +r,q) and q = [S]/[MA] is the instantaneous feed ratio. F represents the number fraction of triads normalized to unity, and P(MtfM2) the probability of a growing chain having an M2-type chain-end, to add monomer M1• All these triad fractions are predicted using initial feed ratios ( In the 1-Y framework various equations cao be derived: (3.5a) (3.5b) 2 Fy = 4Fl<~y (3.5c) 1 + (3.Sd) Equations (3.3) aod (3.5) cao be solved numerically for a by using the experimental values of ao MA-centred resonance (vide infra, 13C NMR C=O resonaoce) for each copolymer. A graphical analysis cao also be used (eq. (3.5)), assuming the values for a to be constant over the whole series of copolymers. If both rm and a are unknown, the second part of equation (3.5a) aod (3.5b) cao be used, platting (1- Fx*Y1 and Fz"*) vs. 1/q. If rm is known, the fustpart of equations (3.5a) aod (3.5b) cao be used, by platting 1-Fx~ vs qf(q+rm) or Fz 2 versus (q/(q+rm)) • From the slopes, the values of a and a2 cao be evaluated. The rearraogement of equation (3.3) put forward by Harwood and Ritebey (H-R 25 approach) .56) leads to the following set: FMMs =(1-a) + (1-a)2 (3.6a) FSMS + 2o(1-a) (3.6b) (3.6c) Experimental 53 The left-hand part of equations (3.6a-c) either can be plotted versus FMMsfFSMSt assuming for the whole range of copolymers a to be constant. or alternatively equations (3.6) can be numerically solved giving a for each copolymer, by using experimental valnes for F xt FY' and Fz and theoretically calculated or experimentally observed valnes for FMMM, FMMs, and FSMs e3CNMR). lf the interpretation of the OCH3 resonances is correct, the graph shonld be linear and slopes and intercepts shonld be consistent with a single value of a. Figure (3.13) depiets the 100-MHz 13C NMR spectra ofpoly(methyl acrylate) and 4 copolymers recorded at room temperature in coq~. These spectra only show the carbonyl region and the C1 ipso region ( quarternary aromatic carbon resonances), because sequence splittings are observed and have been assigned tentatively by other workers41,48,50>. Remarkably, the methoxy carbon resonance for all copolymers ( contrary to the methoxy proton resonance) is observed as a singlet. Although the pattem of the C1 ipso resonances is not well understood and the signals are not as resolved as the C =0 signal, we tentatively have divided this speetral region in three areas, which have been assigned to SSS, SSM, and MSM triad sequence placements in a manoer analogous with the truncation, executed by Ramirez-Marqnez41 >. In Table (3.2) the three theoretically calculated triad fractions (using AM statistics) and the experimentally observed values (both M- and S-centred triads) are presented. There is a good agreement between the relative resonance areas and the calculated triad fractions. This reconfirms the findings of Tanabe et aL 4S.SO), that the carbonyl carbon patterns are only sensitive to seqnence effects and that AM statistles hold to describe the seqnence distribution of S-MA copolymers. Similar arguments, althongh with considerably less certainty, hold for the S-centred triads. As demonstrated by Davis et a/. 8> the nltimate model (AM statistics) is inappropriate in descrihing the kinetic behaviour of S-MA copolymerization. In this case the penultimate model provides a significantly better description. 54 Chapter 3 (a) (b) Fy = 1.0 C=O C, ipso region region MMM FM = 0.88 MSM bppm-- 177 176 175 174 147 145 143 141 Figure 3.13. Expanded 1()().MHz 13C NMR spectra offour low-conversion solution S-MA copolymers and PMA, showing only the carbonyl region (174-177 ppm) and the quatemary aromatic carbon region (C1 ipso, 141-147 ppm). Spectra were recorded in CDC/3 at 25•c. Copolymer compositions are indicated on the left. Area measurements have been performed on the indicated regions. Experlmental ss Table 3.2 Initial monomer feed ratio q0 = (S/MA)0 ; theoretica/ and experimental cumulative number fractions (*100) of both S· and MA-centred triads of some low-conversion solution copolymers. C=O region theoretical fractions experimental fractions MMM MMS SMS MMM MMS SMS 0 100 0 0 100 0 0 . 0.038 70 28 2 74 24 2 0.36 12 46 42 13 47 40 0.85 3 30 67 0 28 72 4.78 0 7 93 0 10 90 cl region theoretical fractions experimental fractions sss SSM MSM sss SSM MSM 0.038 0 5 95 0 0 100 0.36 4 33 63 5 35 60 0.85 15 47 38 16 47 37 4.78 60 35 5 66 34 0 Considering the 1H NMR methoxy resonances in greater detail for our statistica! copolymers (see Figure (3.12)), it is evident that no tacticity-induced splittings appear for PMA, i.e., (coiso, co hetero, cosyndio) MMM all resonate at 3.64 ppm. However, a three-fold splitting can be observed for the OCH3 resonance for altemating S-MA-copolymers48.so>. The resonance assignments are 3.54 (cosyndio), 3.39 (cohetero) and 3.21 (coiso) ppm. The calculated coisotacticity parameter for these copolymers amounts to O'sm = 0.5. In Figure (3.14) are depicted two 400-MHz 1H NMR spectra of an alternating S-MA copolymer (synthesized in our Iabaratory according to a procedure described by Tanabe48,5°)) and a statistica! copolymer, containing a large amount of SMS triads (in our case Cio = 20, F. = 0.92, FsMs = 0.98). From this Figure it is evident that similar chemical shifts and a similar a.m parameter 48 (= 0.5) are found for the alternating copolymer, as observed by Tanabe et aL ). 56 Chapter 3 Nevertheless. the spectrum of the statistical copolymer is much more complicated than that of the alternating copolymer. However, it is immediately clear, that the o-value for this copolymer is much larger than 0.5, because the fractional area intensity of the rr SMS triad (at~ 3.50 ppm) is vel}' small. The methoxy proton region is broken into three composite peak groupings, designated X, Y, Z, repectively. The chemical shift regions betonging to X, Y and Z are somewhat dependent on the copolymer composition and increase e.g., for X from 3.66-3.55 for high Fm copolymers to X = 3.62-3.48 for low Fm copolymers (see Figure (3.12)). The divisions, used for the determination of the experimentalFX' F,. and Fz values are indicated in Figure (3.12). In Figure (3.15) two examples of 1-Y plots are shown, by using the experimentally observed values for Fx and Fz, collected in Table (3.3) and the experimentally observed values for rm and (Mayo-Lewis model is valid) and experimentally observed values for F X' F,.. i.md F. have been used. E:xperimental 57 V A) Allemaling çopo1ymer x z B) S1allotlcal copolymer 3.2 3.0 Figure 3.14. 400-MHz 1H NMR spectra oftwo low-conversion S-M copolymers showing only the methoxy region. (a) altemating copolymer, (b) statistica/ copolymer. q0 = 20, F, = 0.92, FsMS = 0.98. 1 Fz 1 r~ (a) t (b) 0.5 0.5 • q2 --P- ---i"- ...... 1 + 'l q (q + ri 0 0.5 1.0 0 0.5 Figure 3.15. Ito-Yamashita (1-Y) plots for a series of seven S-M low conversion solution copolymers, according to eq. (3.5a) and (3.5b) (see text). 58 Chapter 3 From these plots it is immediately clear tbat extremely high values of o are found (o 9 L.H.P. Harwood - Aitchey Plot (H-A) 8 t 7 6 . Fx • FsMs 5 Fy 0 FsMs 4 6 3 FsMs 2 1 0.5 0 0 2 3 4 5 6 7 8 9 10 Figure 3.16. Harwood-Ritchey (H·R) plot of the left hand part of equation.s (3.6a-3.6c) vs. FMMMI'F SMs for a series of seven low-conversion solution copolymers. Experimental 59 Table 33. Initia/monomerratio q0 = (S/MA)0 ; observed and normalized intensities (*100) ofpeaks X, Y and Z representing the methoxy proton region and the calculated values of X, Y and Z using Ito~ assignment (equation (3.3), see text) and an alternative assignment (equation (3.7)) (see text) of some low-conversion salution copolymers. experimental Ito's assignment alternative assignment 'la a= 0.92 a = 0.32 x y z x y z x y z 0 100 0 0 100 0 0 100 0 0 0.038 70 27 3 73 25 2 69 28 3 0.11 38 49 13 45 44 10 40 48 12 0.36 19 47 34 16 48 35 12 50 38 0.85 10 40 50 7 39 54 3 37 60 1.98 4 25 71 3 29 68 1 24 75 2.97 2 21 77 2 26 72 0 20 79 4.78 1 15 84 2 23 77 0 17 83 From the H-R plot (Figure (3.16)) it is immediately clear that there hardly exists any intensity betonging to the SMM resonance in the low field area (Fx). These considerations, hearing in mind that (1-a)2 SMS ( =rr SMS) resonates at 48 3.54 ppm·(Tanabe et a/. )), may support the following tentative assignment as reported by Nikman and Harwood51>, where the resonance assignments for (1· a)2SMS and alSMS can be interchanged (resulting in asm: = 1-asm). If asm equals 0.5 (e.g., in the case of alternating copolymers48>), these values are identical. (3.7a) (3.7b) (3.7c) Chapter 3 Within the 1-Y approach (the H-R approach is not presented bere), two equations similar to equation (3.Sa) and (3.5b) can be derived, assuming the set of equations (3.7) to be valid: (3.8a) (3.8b) From equation (3.8) and Figure (3.15b), a value of a can be estimated, being a = 0.30. This value is more realistic in relation to the a found for the alternating copolymer. In Table (3.3) the best fit values for the Ito assignment and the possibly new assignment have been compared with the experimental values. From an inspeetion of this table, it becomes evident that, for the triad fractions alone, a discrimination between these two roodels hardly can be made. In appendix B a detailed study, applying COLOC and NOESY NMR, is presented on the discrimination between the two tentative assignments. The result from those experiments is that both assignments are incorrect. In fact no suitable peak assignment at triad level can be made and pentads have to be taken into account. Because the methoxy proton area is only moderately resolved into three distinct resonances, no attempt was ondertaken to find a suitable peak assignment on pentad level. Therefore, the I-Y assignment will be used in the following, in order to remaio compatible with earlier literature. This assignment of Ito gives an unrealistic tacticity parameters but reliable MA-centred triads as was verified by means of 13C NMR. Experhnental 61 High-conversion solution copolymers In Table (3.4) a comparison is made between on one side the ex:perimentally observed valnes of the copolymer composition. triad fractions, and the fractional intensities of peak X, Y, and Z representing the metboxy proton region. and on the other side the predicted valnes using the integrated form of the Alfrey-Mayo · (AM) model37), o = 0.9, and reactivity ratins obtained from the low-conversion compositional data (r1 = 0.73, rm = 0.19). Both calculated and experimentally determined valnes are therefore average (cumulative) number fractions of the high-conversion copolymers. Taking into account the poor peak resolution of the <; ipso region. it can be concluded from the data in Table (3.4), that the solution copolymer microstructure can be descnbed according to tbe integrated AM model until high conversions. Table 3.4. Experlmentally observed cumulative triad fractions (*100), molar ratio (qo), chemica! composition (FJ and [unctional area intensities (X,Y,Z) for two high-conversion solution copolymers. Theoretically calculated values for all experimentally observed values are also depicted. Q.o = 0.85, conv. = 92 mol% q0 = 5.7, conv. = 99 mol% Experimental Model Experimental Model 13CNMR MMM 10.1 9.5 0 0 MMS 38.6 40.5 7 7 SMS 51.3 50.0 93 93 sss 13.0 12.0 78 68 SSM 41.8 38.0 22 28 MSM 45.2 50.0 0 4 1HNMR Fs 0.53 0.50 0.88 0.85 X (o"" 0.9) 14.4 14.5 0 2 y 44.5 44.0 14 23 z 41.1 41.5 86 75 62 Chapter 3 3.2.3 Determination ofreadivity ratlos of methyl acrylate-butyl acrylale solution copolymerization and a feasibllity study on the determination of the sequence distributton in MA-BA copolymers Abstract: The set of reactivity ratios of methyl acrylate - n-butyl acrylate solution copolymerization at 323 K was determined by using 1H NMR to measure the chemical composition of low-conversion solution copolymers. Copolymer compositional data were verified by means of elemental analysis. Both reactivity ratios were found to be equal to 1.0 ± 0.05. 1H NMR and 1l(; NMR appeared to be unsuitable techniques of determining the intramolecular microstructure in terros of sequence distribution of MA· BA copolymers. 3.2.3.1 Introduetion Composition drift in emulsion copolymerization is determined by the reactivity ratios of both monoroers and by the monomer partitioning behaviour. In an attempt to separate these two important aspects, we looked for a pair of monoroers which copolymerize in a perfectly random fashion according to Bemoullian statistles (i.e., r1 = r2 = 1) in a homogeneaus process (e.g., solution polymerization). In such a system the composition drift during the emulsion copolymerization exclusively exhibits the effect of monoroer partitioning, without the disturbing effect of differences in radical and monomer reactivity. We noticed that the systems S-MA (r. = 0.73 and rm = 0.19) and S-BA (r, =0.7 and rb =0.2) have similar sets of reactivity ratios58>. On these ground we expected that MA-BA will form random copolymers according to the Bemoullian statistics. To our knowledge no investigations of the copolymerization of MA with BA resulting in the determination of reactivity ratios have been reported in the open literature. Therefore, we determined the reactivity ratios of MA BAby means of low-conversion solution copolymerization. Copolymer chemical compositions were measured using elemental analysis and 1H NMR. Because the intramolecular microstructure is one of the most important factors affecting the Experimental solution. bulk and chemica! properties of copolymers59>, a feasibility study was performed on the use of 1H NMR and 13C NMR in determining the sequence distribution. No studies have been reported on co-acrylate copolymer sequence distribution analysis, in contrast to sequence distribution analysis of copolymers 60 61 62 of co-methacrylates or co-acrylate-methacrylates by means of NMR • • >. 3.2.3.2 Experimental section The preparation and purification of the low-conversion solution copolymers bas been described in § 3.1.2. The 1H NMR and 13C NMR spectra were recorded on a Varian XL-200 NMR spectrometer'>. In order to achieve optima! resolution of the 1H NMR spectra of the MA-BA copolymers, various combinations of solvents and temperatures were tested (Table (3.5)). For 1H 13 NMR highest resolution was found in case of CDC13 at 6o•c. For C NMR best results were achieved using c;o2Cl4/1,2,4 trichlorobenzene {1/3 v/v) at 12o·c. Table 3.5. Various combinations of solvents and temperatures tested for optima/ recording of 1H NMR spectra solvent temperature •c 1,2,4 trichlorobenzene 100 pyridine-d5 90 benzene-d6 20 benzene-d6 90 chloroform-d 20 chloroform-d 60 a courtesy of DSM Research BV, Geleen, The Netherlands Chapter 3 3.2.3.3 Results and discussion In Figure (3.17) the 1H NMR spectra of three MA-BA copolymers have been plotted and the peak assignment is indicated. Copolymer composition in terms of the BA molar fraction (Fb) can be calculated from the ratio of the -OCH2- (BA) and the -OCH3 (MA) peak areas using equation (3.9): Fb = 3A.J I (3A.J + 2") (3.9) where A.J and " are the peak areas of the -OCH2- and the -OCH3 protons, respectively. No sequence effects could be noticed in these signals. Table 3.6. Initia/ monomer composition, and experimentally determined copolymer composition by means of 200-MHz 1H NMR and elemental analysis (mol fraction butyl acrylate). 4 Fb (NMR) Fb (C,O) 0.102 0.11 0.071 0.206 0.22 0.201 0.408 0.40 0.434 0.526 0.51 0.529 0.592 0.585 0.701 0.69 0.708 0.767 0.76 0.782 0.899 0.89 0.901 From the NMR and elemental analysis data in Table (3.6) the reactivity ratios have been determined: rm = rb = 1.0 ± 0.05. In Figure (3.18) a typical 13C NMR spectrum is given. The observed splittings of the -CH2- resonances in the 13C NMR spectra (35 - 36 ppm) of theMA-BA copolymers are dominated by tacticity effects over possible sequence effects. This bas also been observed for PBA63>. Unfortunately, the other resonances are not affected by configurational or compositional sequence effects. Therefore, also 13C NMR cannot be used to determine sequence distributions in MA-BA copolymers. Experimental 65 -OCH3 chain -CH2- (a) 'Y·CH2 (b) (c) 5 4 3 2 1 0 ppm Figure 3.17. 200 MHz 1H NMR spectra of three MA-BA copolymers. Fb = 0.11 (a), 0.51 (b) and 0.89 (c). 66 Chapter 3 (BA) (BA) CDCI, /l-CH, "(-CH, (BA) chain (BA) (MA} ,_....,._._.., -CH, -OCH,- C=O -O<:'H, -CH- -cH2- (MA) 1 (BA) 11 L }\ ~ 180 170 160 80 70 60 50 40 30 20 10 ppm Figure 3.18. 13C NMR spectrum of a MA-BA copolymer (Fb = 0.51). 3.3 Conclusion The experimental characterization possibilities described in this chapter can be applied in the investigation of emulsion styréne - acrylic copolymer microstructure. Cross-fractionation techniques such as SEC-1LC/FID and SEC HPLC can be applied to obtain the complete MMCCD of the styrene-acrylate copolymers studied in this thesis. Both 13C NMR and 1H NMR can be used to obtain the sequence distribution of S-MA copolymers at the triad level. The MA-BA solution copolymerization follows Bemoullian stadstics (rm = rb = 1.0 ± 0.05). 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Avail. through Univ. Microfilms, Int. Order No. DA 85 13339 52. H. Kessler, C. Griesinger, J. Zarbock, H.R. Loosli, J. Magn. Reson., 51, 331 (1984) 53. S.A Heffner, F.A Bovey, LA Verge, P.A Mirau, AE. Tonelli, Macromolecules, 19, 1628 (1986) 54. AM. Aerdts, J.W. de Haan, AL. German, G.P.M. van der Velden, submitted to Macromolecules 55. This thesis, Chapter 2 Keferences '' 56. HJ. Harwood, W.M. Ritchey, l Polym. Sci.: Part B: Polym. Lett. Ed., 3, 419 (1965) 57. C.H. Bamford, "Alternating Copolymerization in the Presence of LewisAcids" in "Aiternating Copolymers", Ed. hy J.M.G. Cowie, Plenum Press, New York, 1985 58. This thesis: Chapter 3 59. J.C. Randall, "Polymer Sequence Determinationn, Academie Press, London, 19n 60. N. Grassie, BJ.D. Torrance, J.D. Fortune, J.D. Gemrnel, Polymer, 6, 653 (1965) 61. J.C. Brosse, J.M. Gauthier, J.C. Lenain,MakromoL Chem.,l84, 1379 (1983) 62. AS. Brar, A.K. Saini, J. AppL Polym. Sd., 32, 4607 (1986) 63. C. Pichot, M.F. Uauro, Q.T. Pham, J. Polym. Sci., Polym. Chem. Ed., 19, 2619 (1981) 70 Model 71 Chapter 4 Model Evalaation of Emulsion Copolymerization Kinetics and Copolymer Microstmcture SUMMAR Y: A model bas been developed to describe seeded batch emulsion copolymerlzation, and its predictive capabilities have been investigated by appllcation to the system styrene-methyl acrylate. The main reaction site is considered to be the monomer-swollen polymer particle. Copolymerlzation rate and copolymer microstructure (MMCCD and sequence distrlbution at the trlad level) are controlled by the local concentradons of monomers and free radicals inside the particles. The model accounts for radical absarpdon and desorption processes, bimolecular terminadon within the particles, and transfer to monomer and chain transfer agent. Monomer partitioning is described using experlmentally determined relations. The results include rate of (co)polymerlzation, composition drift and copolymer microstructure. lt is demonstrated that "apparent" reactivity ratios are generally incapable of descrihing the course of emulsion copolymerlzations in an adequate manner. 4.1 Introduetion Modelling of the industrially important emulsion copolymerization process, is not only essendal for reliable prediedons (descriptions) of the kinetic behaviour, but also for the possibility of predicting the molecular mi crostroeture of the copolymer formed. The molecular microstructure can be characterized in terms of sequence distribution, tacticity and molar-mass-chemical-composition distrlbution (MMCCD). lts importance lies in the fact that in addition to the intramolecular sequence distribution not only the averages of molar mass and chemica! composition, but also the complete distributions as a whole (MMCCD), may determine the (bulk) product properties, tagether with for instanee the latex 1 2 3 partiele size distribution (PSD) and partiele morphology • • >. n Chapter 4 Copolymer microstructure directly reflects the microscopie elementary steps involved in emulsion copolymerization. Therefore, detailed modelling of the emulsion copolymer microstructure necessitates a modelling of the process. Thus the copolymer MMCCD can be considered as a fmgerprint of all the molecular events that contribute to polymer growth. Harkins4>, and Smith and Ew~> were the pioneers in modeHing the emulsion process. Stockmayer>>, O'Toole7) and Ugelstad8> gave general solutions for the calculation of the average radical number per partiele in the steady~state situation. Major contributions to the model development of the emulsion polymerization process (particle nucleation, PSD and kinetics) were made by 9 10 11 12 13 14 15 1 17 1 19 Ray • • >, Hansen and Ugelstad >, Gilbert and Napper • • • 6. • 8, > and Chen 20.21>. Regarding the emulsion polymer microstructure, Lichti, Gilbert and Napper22> and also Lin23> have developed theories to descrlbe the MMD of homopolymers. Matbemadeal extensions to emulsion copolymerization kinetics 24 1 32 were made by Nomura and Fujita .2.'i.26.27), Guillot211.29,30), Doughertf • >, Poehle~».l4>, Storti et al.3S). Gilbert et a/.36,31), and Forcada38>. 36 Ballard et al. >and also Guillot et al. l9,40) studied the sequence distribution 41 42 of emulsion copolymers. Storti et al. • > developed a model to describe the (MM)CCD of the (instantaneously) formed emulsion copolymer, but did not oomparetheir model results with experimental MMCCD data. However, detailed experimental elucidation of the copolymer microstructure is a prerequisite in these kind of studies. Several experimental techniques now have become available to determine not only the average copolymer composition, but also its complete MMCCD43>. Together with experimental sequence distribution information by means of NMR44>, this kind of detailed information about copolymer microstructure can provide the necessary experimental proof of the reliability of the model calculations. In this chapter a comprehensive model is presented, "SIEMCO" : Simuiadon Emulsion Q!polymerization. SIEMCO describes tbe (seeded) emulsion copolymerization process including the emulsion copolymer microstructure. 'Ibis Model 73 model is partially based on the models of Nomura24,25.26) (kinetics), Lichti, Gilbert and Napperl2> (MMD), Stockmayer45>and Tacx46>(instantaneous CCD), 40 and Guillof9· ) (sequence distribution). The new aspect of this model is the emulsion copolymer MMCCD prediedon that takes both conversion heterogencity and instantaneous heterogencity into account In the partiele nucleation interval, the number of particles formed depends · upon the type and concentradon of emulsifier, the radical generation rate, the type and concentradon of electrolyte, the temperature, and the reactor type, as wellas upon other parameters, which are often not completely clear. Therefore, no attempts were made to include partiele formation in SIEMCO. Furthermore, SIEMCO does nottake into account the Trommsdorff-Norrish effect at high 1 7 48 conversion 6.4 • >). For the emulsion copolymerization of styrene and methyl acrylate (the main model system in this investigation) this effect is not important, because ïi < 05. If the dependenee of the volume fraction polymer on the several rate constants were known, this dependenee could, however, easily be implemènted. The studied systems are styrene (S) · methyl acrylate (MA), styrene (S) - n-butyl acrylate (BA) and methyl acrylate (MA) - n-butyl acrylate (BA), with most emphasis on the S-MA system. Although MA is known for its gel effect in bulk polymerizations49>, this chosen S-MA system appeared to be an ideal model ' system for three reasons. First of all, the monoroers of the system swell each others (homo )polymer well but differ strongly in water solubility resulting in the so-called "apparent" emulsion reactivity ratios29>. Secondly, it was shown by Teremachi50>, that within a wide range the retention time in size exclusion chromatography (SEC) is independent of the S-MA copolymer composition, as can be deduced from the fact that the intrinsic viscosity of S-MA copolymers is independent of the chemica! composition and depends exclusively on molar mass. The third reason is that the CCD of S-MA copolymers can be determined by means of TI..C/FID (chapter 3) or HP~l). In chapter 7 the model prediedons of the microstructure of S-MA batch emulsion copolymers are compared to experimentally obtained data. 74 Chapter 4 Furthermore, by modeDing the process, important information is provided enabling the calculation of the optimal monomer addition strategies in semi continuons emulsion copolymerization. Such an attempt may lead to tailor-made products with very weD-controlled microstructure resulting in optimization and fine-tuning of material properties (chapter 8)S2.S3>. 4.2 Model description 4.2.1 Physlcal and chemical outline The physical and chemica! aspects of emulsion copolymerization are extremely complex, due to the presence of several phases (monomer droplets, aqueous phase and latex particles), different monomers, different radicals and other ingredients, and also due to a very complex partiele formation mechanism that is not completely understood. When the emulsion copolymerization starts, the monomers are present at an overall concentradon higher than their saturation concentration in water, resulting in monomer dropiets dispersed in water and stabilized by surfactant. Usually chain transfer agents are added to reduce the (co )polymer molar mass. Electrolytes are used to control the partiele size. The initiator dissolved in the water phase, dissociates into free radicals, that will add several monomers to form radical oligomeric chains. The number of added monomeric units depends on the length at which the oligomers beoome surface active and/or insoluble in water. These free radical oligomeric ebains either may terminate in the water phase, or enter micelles ( micellar nucleation), or (co )precipitate to form primary colloidally unstable precursor particles that subsequently can coagulate (homogeneous and coagulative nucleation), or enter (coagulate with) an existing mature latex particle. SIEMCO starts from the following strongly simpUfled model. Regardless of the nucleation mechanism involved and the parameters determining the number of latex particles and partiele size distribution (PSD), in this model as a first Model 75 appro:ximation it bas been assumed that the number of latex particles is known as a function of conversion and that the latex is monodisperse. The number of particles during interval I in case of ab initio (batch) emulsion (co)polymerization is not reproducible and difficult to measure. Therefore, in principle this model is only valid for seeded reactions in the absence of coagulation or secondary nucleation. However, the amount of copolymer formed during interval I of emulsion (co)polymerizations with water soluble acrylic monomers is very small, due to fast homogeneons nucleation. Baneljee et aL 54> showed that relatively water soluble monomers such as methyl acrylate can polymerize in the aqueous phase, hut the amount of this aqueous phase polymerization of methyl acrylate is much smaller than the amount of polymerization taking place in the particles. In several publications it bas been confirmed that the amount of polymer formed in the aqueous phase is usually negligible ( < 1%) at moderate conversion, even in case of monomers with moderate water solubilit:y'3.34.S.S>. Therefore, these model assumptions do not seem to form a severe threat to the reliability of the copolymer MMCCD and sequence distribution predictions, even when calculated for copolymers formed under ab initio batch process conditions. The monomers are distributed among water phase, latex particles and monomer dropiets if present. As generally accepted, in the present model it has been also assumed that the monomers in the latex particles are in equilibrium with the monomersin the water phase and (if present) with the monomersin the monomer droplets. By comprehensive equilibria experiments described in chapter 5 the monomer concentradon in the latex particles bas been measured as a function of temperature, the volume of the particles, and the copolymer composition and molar mass. In addition, it is assumed that the latex particles do not have a core-shell morphology but are homogeneous. The monomer dropiets are supposed to act as a monomer reservoir only. In the mature latex particles, swollen with monomer, the free radicals then will continue to grow according to the laws of ultimate or penultimate kinetics until chain transfer to chain tranfer agent or monomer, or bimolecular terminadon ( either by combination or disproportionation) occurs. After chain transfer, the low Chapter 4 molecular free radicals may start to grow or desorb from the latex particles and subsequently either terminate in the water phase or re-enter a particle. In SIEMCO for S-MA emulsion copolymerization a 100% radical (re)entry efficiency is assumed. The latest theories on entry of free radicals demonstrate that water phase propagation up to critical chain length, whereupon capture of the oligomeric free radical occurs instantaneously, is the rate determining step 1 in the entry processes 6,56,S7,58,S9). According to Gilbert et al. 56) the minimum length of surface activity (zmiii,SIIrf) and the minimum length for water insolubility (zmin,;nsot) of a oligomeric radical containing a SO,[ · group can be estimated from the water solubility [Maq,sat1 of the monomer according to the semi-empirical eqs. (4.1) and (4.2). The length is given in monomer units. Zmïn.surt = 1 + int(-23 kJ ·mol"1/(RT ln([Maq,sat])) (4.1) Zmin,;nsot = 1 + int(-55 kJ ·mor1j(RT ln([Maq,sat])) (4.2) Because MA is far more water soluble than S, MA will dominate the processes in the water phase. The water solubility of MA [Maq,sa1] is 0.6 (molfL) and by applying eqs. (4.1) and (4.2) Zmin.surt is circa 18 and Zmïn,insol is circa 41. Using the following estimated values of the several kinetic parameters: k., = 10-6 s"1 for the dissociation rate constant of persulfate at 5o·c, the propagation and termination rate constauts of MA~ = 3400 L·mol"1 ·s·1 60>, 9 1 1 56 kt,w = 10 L· mol" · s· ( diffusion controlled), the entry theory of Gilbert et al. >, leads to a confirmadon of the 100% radical capture efficiency for S-MA emulsion copolymerization. This can be explained as follows. Due to the high water solubility of MA in combination with its high propagation rate constant, the propagation rate in the water phase will be relatively fast. This results in a very short time required for the growing oligomeric ebains to reach their crideal lengthof surface activity. This time is too short for (significant) termination to occur. A radical capture efficiency of 100% and thus the absence of water phase terminadon is therefore a very reasonable assumption provided sufficient MA is present in the aqueous phase. Model 77 4.2.2 Basie prlnciples of tbe tbeoretieal model Our model for predicting emulsion copolymerization and copolymer mi crostroeture contains suitable combined elements of several otber modeIs, that have been described in literature. For more detailed information, tbe reader is · referred to tbe original papers. The basis of tbe present comprehensive model is tbe tbeory of Nomura. This tbeory is capable of descrihing tbe copolymerization rate of both monomers in Smitb-Ewart intervals ll and m of the emulsion copolymerization by an extention of emulsion homopolymerization theories about the average number of radicals per particle. We used a similar metbod of extension to adapt the model of Uchti et aL :m, originally developed to predict the MMD of emulsion b.mnQpolymers, to the MMD of emulsion mpolymers. From tbe individual polymeri~tion rates of both monomers, one can calculate the average instantanous copolymer composition at each conversion. Integration over conversion provides the conversion part of the total CCD. The instantaneous statistical part of the total CCD of the copolymer is calculated by a model developed by Stockmayer45>for bulk and solution copolymers. A correction was made to account for the difference in molar mass of both monoroers by applying the extension proposed by Tacx46>. The sequence distribution is calculated using the well-known equations assuming first order Markov kinetics using the reactivity ratios determined in solution (chapter 3), in combination with the local monomer feed ratios inside the swollen latex particles. As mentioned in chapter 2, although the composition drift of S-MA copolymerization and the sequence distribution of the S-MA copolymers can be described by the ultimate model, recent measurements have clearly demonstrated that the kinetic behaviour bas to be descibed by the penultimate modet60>. 78 Chapter 4 4.3 Model development Monomer partitioning In order to correctly predict the rate of (co )polymerization it is essential to be able to predict the monoroer concentrations inside the swollen latex particles. For this purpose in literature two approaches have been proposed. Tbe fi.rst one 61 is the thermadynamie approach developed by Morton ) for emulsion homopolymerization and later extended by Guillot30> to emulsion copolymerization. Tbis approach is very fundamental and promising. At present the lack of knowledge beyond the rudimentary principles currently available tagether with the fact that a thermadynamie approach involves many parameters of which the valnes are aften not known, holds back a proper prediction of monoroer partitioning (see chapter 5). Tberefore, we apply the second approach. Tbis is an empirical method. Very often, experimentally determined monoroer partitioning coefficients (ratio of monoroer concentrations in organic and aqueous phase) are used, but also other empirical relations we re used to describe the monoroer partitioning21>. The monoroer partitioning is determined by means of static swelling experiments ( chapter 5), yielding relations between monoroer concentration in the latex particles and in the water phase on the one hand and the monoroer ratio inside the monoroer dropiets on the other hand. Assuming no water is dissolved in the monoroer dropiets and in the swollen latex particles, at each conversion the monoroer partitioning can then be calculated from the empirical equilibrium equations (4.3) through (4.6), the molar mass balance equations (4.7) and (4.8) or (4.9) and (4.10), and equation (4.9). Alternatively, for other systems other empirical relations may have to be applied. In the case of for instanee methyl acrylate the solubility of the monomer in water is definitely not negligible. As long as there are monoroer dropiets present in the system, the monoroer concentration in the water phase is given by experimentally determined equations (4.3) and (4.4). Model 79 (MaJw = Kw.f. (4.3) [MbJw = KwJt, (4.4) where (MaJw and [Mb1w are the molar concentradons of monomer a and b in the aqueous phase (mol/L), respectively, f8 ( = 1-fb) is tbe molar fraction of monomer a inside the monomer dropiets and Kw. and Kwb are experimentally determined parameters. The monomer concentration inside tbe swollen latex particles is given by equations (4.5) and (4.6) as a function of tbe mol fractions of the monomersin the monomer droplets. (4.5) (4.6) where [Mi]p : monomer concentration of monomer i in tbe particles (mol/L) and Kp1,a, Kp1.b, Kp2,a and Kp2,b are experimentally determined fitting parameters. In principle all equilibrium parameters may also be dependent on partiele size, molar mass and chemical composition of the copolymer, temperature and ionic strength ( surface tension). In model calculations these effectscan be taken into account, but as wil be demonstrated in cbapter 5 these parameters have only minor effects on these equilibrium parameters. The equilibrium equations must be solved simultaneously with the molar mass balance equations by means of iterative computational methods: (4.7) (4.8) 80 Chapter 4 where Mad : amount of monomer a in monomer dropiets per liter water (mol/L) vP :volume of a singleswollen latex partiele (L) x. : mol ar conversion of monomer a (-) M3o : initial monomer a in reactor per liter water (mol(!--) NT : total number of latex particles per liter water (L* ) The swollen partiele volume is calculated assuming that the volumes of all componentsin the particles, i.e., monomers and (co)polymer, are additive. (4.9) where ~a> : density of monomer a,b (g/L) Mfo = Mao + Mb0 : total monomer concentradon at the beginning of the reaction per liter water (moljL) x. : total molar conversion (-) ! J' : average copolymer density (g/L) M.p: molar mass of monomer a,b (g/mol) MP = F.·Ma + Fb-Mb: average molar mass of the monomerunits of the cumulative copolymer (g/mol) where F. = (1- Fb) is the mol fraction monomer a in the copolymer (-) During interval m of the emulsion (co)polymerization, when there are no monomer dropiets present, the same equations ( 4.3-4.6) can be used. However, it must be mentioned that then f. and fb are hypothetical monomer fractions, because there is no Jonger separate monomer phase present. In that case, the molar mass balance equations are reduced to: (4.10) (4.11) Model 81 The monoroers in the systems styrene-methyl acrylate, styrene - n-butyl acrylate and methyl acrylate - n-butyl acrylate all swell their (co)polymers well, although strong differences exist between the water solubility of the monomers. As is experimentally shown in chapter 5, ~e monoroer ratio inside the latex particles appears to be equal to the monomerratio inside the monomer droplets, and is almost independent of copolymer composition. Radical production 1 1 The radical production rate (Ri) per liter water (mol- L- -s" ) is calculated using equation (4.12). (4.12) 1 where k.t is the initiator decomposition rate constant (s- ), f is the initiator decomposition efficiency (.) and [Milw is the initiator concentradon in the aqueous phase {moi/L). Chain transfer The average chain transfer rate constant is calculated by taking all possible chain transfer reactions into account (viz., transfer to monoroer and chain transfer agent). ~r = {(knlu[Ma]p + kiDab[Mb]p)na + (kmbb[Mb]p + (4.13) klllt,a[Ma]p)~ + kcta,a[CfA]pna + kcta,b[CfA]~ }/ ~ where [CfA]p is the chain transfer concentration in the latex particles (mol/L), 1 1 ~j and kcta,i are chain transfer rate constants (L · mot· • s· ) and na and nb ( average number of a and b radicals per particle) are calculated using equations (4.20) and (4.25). 82 Cbapter 4 Desorption of radicals It is generally recognized that free radicals can desorb from the polymer particles. Exit can play a crucial role, even for relatively water insoluble monoroers such as styrene58>. According to tbe transfer/diffusion theory of Nomura and co-workers 24,62,63>, radical desorption can be regarded as a four step process. (1) A propagating polymer chain transfers its free radical activity to a monoroer molecule or to a (low molar mass) chain transfer agent. (2) This small molecule (radical) may then diffuse towards tbe surface of the partiele and (3) subsequently desorb from the partiele provided this radical bas not undergone significant propagation. (4) The exit process is completed by diffusion away from the partiele into the bulk of the aqueous phase. In case of homopolymerization the average desorption rate constant (k,) 1 (s" ) is given by the equation (4.14) under the restrietion of a uniform concentradon of free radicals within the particles. Extension of this theory to monoroer radical transport in the particles bas been given by Mead and Poehlein64>. Asua included aqueous phase reactions65l. Both these extensions have not been included in SIBMCO. (4.14) where k; is the propagation rate coefficient of a monomeric free radical, ~ is the diameter of the swollen particle, and Dw and Dp are the monomer diffusion constants in the water pbase and in tbe particles, respectively. Implicitly, it is assumed that the partitioning coefficient (between patticles and aqueous phase) of the exiting radical species is equal to the partitioning coefficient of the monomer. Model 83 According to the theory of Nomura for copolymerization the two monomeric radical species may desorb at different rates depending on the monomer concentradon ratlos witbiD the particles and the rates of chain transfer to both 1 ) monomers. The desorption rate constant for monomer a radicals (Ko4 (s- ) can be calculated according to equation (4.15). 12 Dw.'· z (4.15) Ko4 = ~.. C\,l with '· = { 1 + 6DwJ(~Dp.) r\ and (4.16) ~.. = [M. *]p/[M. *).,. (4.17) where ~.. is the partitioning coefficient for the a·radicals between water phase and polyîner particle, respectively, and &. is the ratio of film mass transfer resistance to overall mass transfer resistance of monomer a. Dp. and Dw. are the diffusion coefficients for the exiting species (free a-radicals) in the partiele and the water phase, respectively, and z is the average degree of polymerization of the exiting free radicals. In this model it is assumed that only monomeric radicals can desorb (z= 1). When n-dodecyl mercaptan (NDM) is used as transfer agent, the desorption of chain transfer free radicals can be neglected due to the extremely low water solubility of ND~>. Exit is only possible after transfer to monomer and when the monomer radical neither initiates (propagates), nor transfers or tenninates its radical activity. Equation (4.18) gives an expression for the net desorption rate fora radicals: kra = Ko•. { } (4.18) 84 Chapter 4 A simHar equation can be deduced for ~- The average rate coefficient for 1 radical desorption from a polymer partiele (s" ) then can be calculated by means of equation (4.19). (4.19) Radical entry and number of radicals per partiele The average number of a and b ràdicals in the latex particles is calculated according to the metbod of Nomura 26>, based on a proper averaging. The steady state assumption (propagation is faster than change in number of radicals) is used to calculate the ratio (A) of a and b radicals. Assuming that propagation is much faster than chain transfèr it can easily be shown that: iib ~,aar.[Mb]p A=-=---- (4.20) ii8 ~,bbrb[Ma]p In the well-known Smith-Ewart equations, that provide the fraction of latex particles having i radicals in the steady-state situation, the parameters have their usual meaning: (p/Ny)Nn_1 + kc(n+1)Nn+ 1 + c(n+2)(n+1)Nn+2 (4.21) dt 1 1 where the termination rate constant (c) (s- ) can be calculated from ~vP· Nav-t, 1 1 ~Pis the terminadon rate constant (L-mol" ·s- ), vP is the volume of a single swollen latex partiele and Nav is the Avogadro's constant. Equation (4.21) is valid for homopolymerization. However, by applying the pseudo-homopolymerization approach by using average rate constants, as Model 85 26 introduced by Nomura ) and further developed by Storti et aJ.lSl, this equation cao also be used for copolymerization. By using pseudo-homopolymerization constauts the radical entry rate (pJ is expressed in the same form as in emulsion homopolymerization by solving the mass balance equadon on the radicals in the aqueous phase. 00 Pe = k.[R*]~1 = R; + Z kc n Nn- 2 ktw [R*]w2 (4.22) n=l where n is n. + Dt, , [R *1w is the concentradon of all free radicals in tbe aqueous phase (mol/L), ktw is the average free radical terminadon rate constant in the 1 1 aqueous phase (L·mor ·s" ), ke is the average rate constant for radical 1 desorpdon (mol·s" ) and k 8 is tbe average radical absorption constant for latex 1 particles (L· s" ). Equations (4.21) and (4.22) cao also be writen in dimensionless parameters: «r·Nn-1 +m·(n+l)·Nn+l + (n+2)·(n+l)·Nn+:Z = cr.N11 + m·n·N0 + n·(n·l)·N0 (4.23) and o = o' + m· ii · Y .r} (4.24) with PeV~8} kevpNav2 2ktwktp a=-- a'= m=-_- Y= ktpNT k,P ka2NTvpNav2 Chapter 4 If there is no water phase terminadon (thus 100% re-entry or 100% radical capture efficiency is assumed), Y is equal to zero. The general salution of the equation ( 4.24) gives the average number of radicals per partiele : X D·Nn a iit = -- = (4.25) N1 4 where ~ and Im-1 represent the modified Bessel functions of the first kind, defined by the following recursive equations : Because Pc• a and thèrefore also a are unknown, a bas to be eliminated. Ugelstad et a/. 8> solved equations (4.23) and (4.24) simultaneously, thereby eliminating the parameter a that contains Pc· Assuming a fixed value for Y (in our case for S· MA Y = 0, i.e., negligible water phase terminadon compared with the entry rate) ii 1 can be expressedas a function of the predictabie parameters a' and m only. The copolymerizaton rate As long as the ultimate model can be employed, the polymerizaûon rate of each monomer (Rpi) is expressed in terms of monomer concentradon ([Mi]p) in the particles, average number of i radicals (iii), propagation rate constants (~ij) and number of latex particles NT , and is given by equations (4.26) and (4.27). (4.26) (4.27) Model 87 However, for many copolymer systems tbe penultimate model bas to be used. Intbat case tbe average propagation rate constant (kp) and the average instantaneous copolymer composition (F.) can be calculated using equations (4.28) and (4.29), respectively (see chapter 2). 1 .f.2 +2fafb + 1 bfb2 kp=------(4.28) (1 .fJk ..)+(1 b4/kbb) 1 .r.2+f.4 Fa = ------(4.29) 1 .f.2 +2f.4 + 1 bfb2 where r/(rifi+9 (with i f' j) (4.30) r1'~+G ~(rifi+G) (4.31) rlfl + (G/s;) with the reactivity ratios r1= ~m/~üi• r1' = ~iikro 1i and s1= ~ii~m· where kklm is tbe propagation rate constant for the radicals with a terminal unit I and a penultimate unit k adding a monomer m (k,~m = a or b ). Although several styrene (meth)acrylate monomeric systems exhibita kinetic behaviour significantly deviating from the ultimate model, composition drift and 60 6 sequence distribution still appear to follow the ultimate model • 7) (see chapter 2). 88 Chapter 4 Terminalion rate constant in the particles In general, terminadon processes in emulsion polymerization will be different from those operative in low viscosity non-compartmentalized systems. If the terminadon would be solely determined by intrinsic kinetics26> then: (4.32) North and co-workers68.69>, however, have pointed out that terminadon is strongly diffusion controlled. (4.33) It is to be expected that the 'effective' k.a and ktb dramatically decrease at 47 16 70 72 higher. volume fractions of polymer (wp) in the latex particles • • • >. Termination rate coefficients may also be a function of the molar mass of the (co )polymer matrix, and the degree of polymerization of the two mutually terminadng chains. Because at present the dependenee ofwP on kt is not known forS-MA copolymerizadon system, this parameter bas to be estimated within certain realistic limits. Whenever the latex particles are small ( < 100 nm) and radical exit is important, the average number of radicals per partiele (ii) will be low, and as a result the value of kt is no longer important, since the bimolecular terminalion of the growing radical chain with the entering species will be instantaneous and not rate determining. Model 89 The instantaneous copolymer composition The instantaneous copolymer composition (F.) is calculated using eq.(4.34): F. = RpJ(Rp.+RP.,) (4.34) The instantaneous sequence distribution in terms of triad fractions can be calculated assuming first order Markovian kinetics71>. The a-centred triad fractions are given by: Faaa = (1-P(B/A))2 (4.35) Faab = Fbaa = 2 P(B/A) (1-P(B/A)) (4.36) Fbab = (P(B/A)) 2 (4.37) P(B/A) = 1/ (l+r.[A]/[B]) (438) P(B/A) is the conditionat probability of an a-radical propagating with a b-monomer. For the b-centred triads simHar equations can be deduced. Calculation of the instantaneous MMD 22 The theory of developed by Lichti, Gilbert and Napper (LGN) ) is used for the calculation of the instantaneous MMD of linear chains. This theory developed for homopolymerization, can be used for copolymerization after the introduetion of the pseudo-homopolymerization approach35.26>. The pseudo-homopolymerization constants are calculated by averaging of the separate homopolymerization constants and taking into account the local monomer ratio, the ratio of radicals andfor the instantaneous copo1ymer composition (see chapter 2). Chapter 4 The use of the pseudo-homopolymerization approach necessarily results in the toss of part of the information35>, because it is implicitly assumed, that the MMD is equal for the instantaneous copolymer fractions with different chemical compositions. Of course, this will not be completely true. However, the resulting error can he neglected, because the instantaneous CCD is small compared with the overall CCD. Furthermore, the parameters that determine the MMD (p, Ie,., k.r, c), may change during conversion as a result of partiele growth and composition drift and therefore have a far more pronounced effect on the MMCCD. In Appendix 4.A are given the actual equations employed in this study. lmplementation of the instantaneous chemica/ heterogeneity In order to take into account the instantaneous chemical heterogencity due to the stastistical character of the monomer addition process and the finite length of the (co )polymer chains, the modified Stockmayer equations, originally derived for bulk and solution radical copolymerization4S), are now used in emulsion copolymerization. In principle this should be possible, because the stadstics that determine the monomer addition on molecular level are independent of the process (i.e., bulk, solution or emulsion copolymerization). In the calculation of the instantaneous CCD, instead of the original Stockmayer equations (wr(y)), extended equations (w/ (y)) are used, as derived by Tacx to account for monomers differing in molar mass46>. -ll.l wr(y) = K·exp ( ) (4.39) 2 y.(l-y.)k y (1-D) wz'(y) = wr(y)· { 1 + } (4.40) D + y.(l-D) Model 91 where 2 05 K = exp(-1/l) l/! {l/(2wy 8 (1-y .)k) } (4.41) 05 with k = { 1 - 4 y a (1- y J (1-r.rb) } (4.42) D = Mb/M. :ratio of the molar masses of bath monomers (·) y = ya - y• : difference in chemical composition between a single copolymer molecule and the average chemical composition (-) l : degree of polymerization (-) >. : number average degree of polymerization (-) Numerical integration over conversion At each point of conversion the above quantities can be calculated. In order to describe the microstructure of the copolymer formed during a eertaio conversion interval, copolymer microstructural parameters will have to be integrated. The cumulative triad fraction AAA from the start to the end of the copolymerization is given by: (4.43) x. : molar conversion of monomer a Xa.e : molar conversion of monomer a at the end of conversion interval A similar numerical inlegration is nessessary to calculate the conversion part of the MMCCD. Using these valnes the next (equilibrium) situation can be calculated. Semi-continuous processes Extension of the model to semi-continuons operadon is very straightforward. A similar procedure is foliowed which accounts for the addition of monomer (and other ingredients) during the emulsion copolymerization (see chapter 8). Chapter 4 4.4 Model calculations From the above it appears that MMCCDs depend on a large number of variables. As examples some results of model calculations of styrene-methyl acrylate emulsion copolymerization are given. In all these simuiadons the base case parameters given in Table ( 4.1) have been used. In order to illustrate the effect of varying the initial monomer to water ratio, in Figure (4.1) model calculations are presented of high conversion CCDs of styrene-methyl acrylate emulsion copolymers with the same overall chemical composition (25 % styrene ), but prepared at different initial monomer to water ratins (M/W)0• Under these conditions composition drift occurs. As an important result all these copolymers have bimodal or two-peaked CCDs. Furthermore, it is demonstraled that the (M/W)0 bas a pronounced effect on the CCD of the copolymer formed. Lower (M/W)0 results in a braader and more heterogeneons CCD, as can be explained by the relatively high water solubility of MA In Figure (4.2) the shift of apparent low-conversion reactivity ratins (r') as a function of initia! water to monomerratio is shown. These apparent reactivity ratins have been calculated from monomer composition-copolymer composition data points at low conversion, thereby intentionally neglecting the effect of conversion on monomer partitioning. The shortcornings of this approach, frequently used in literature, are clearly demonstraled in Figure (4.3) showing that although the overall monomerratio (S/MA) decreases, the styrene fraction in the loci and thus in the instantaneously formed copolymer increases. This phenomenon can never be described by simple apparent reactivity ratios, but can only be explained on the basis of more detailed monomer partitioning considerations. It unambiguously demonstrales that the ~real" solution reactivity ratins have to be used in combination with the ~ monomer ratio (concentrations) at the locus of polymerization, i.e., the latex particle. Model 93 a! 5 0~~~~~~~--~~--~--~~~ 0.0 0.5 1.0 Figure 4.1. CCD model calculations (relative weight versus mol fraction styrene) of several S-MA high conversion (95 mol%) batch emulsion (or bulk) copolymerizations with the same initial monomerratio (viz. (S/MA)0 = 0.33 (mol/mol)), but with various monomer to water ratios (g/g): (M/W) 0 = 0.05 (- ·- ·-), 0.2 (---), 0.5 ( · · · ) and bulk copolymer (-). 2.00 0.20 1.60 0.16 1.20 0.12 -,_111 ·,_l 0.80 0.08 0.40 0.04 0.00 0.00 0 10 20 WIM (g/g) Figure 4.2. Effect of water to monomer ratio on apparent reactivity ratios of S-MA emulsion copolymerization. 94 Chapter 4 10 ~------. 1 5 ------.... ' ' 0 0.5 1.0 Conversion (·) Figure 4.3. The overall monomerratio (· - -) and the monomerratio in the latex particles (--) as a fuction of total molar conversion of a S-MA batch emulsion copolymerization with (S/MA)0 = 5 (mol/mol) and (M/W)0 = 0.2 (g/g). The arrow irtdieales the disappearance of the monomer drop/ets. In Figure (4.4) are given two copolymer MMCCDs calculated at different conversions (50% and 95%) of a 8-MA emulsion copolymerization exhibiting a strong composition drift. This figure clearly shows that at low conversion the copolymer formed is still homogeneous with respect to chemica! composition, whereas at high conversion the chemical distribution bas become bimodal. The results of these kind of model calculations will be compared with the experimentally determined MMCCDs in chapter 7. This detailed information on the complete two-dimensional distribution of ( emulsion) copolymers is essential in order to gain a better insight in the control of relations between emulsion copolymerization process conditions and final product properties. Model 95 15 Figure 4.4. MMCCD model calculation of a S-MA batch emulsion copolymerization with (S/MA)0 = 0.33 (mol/mol) and (M/W)0 = 0.2 (g/g) at 50 mol% conversion (a) and 95 mol% conversion (b). Chapter 4 Table 4.1. Base case parameters for S·MA emulsion copolymerization simulations (50"C). Parameter unit Styrene Methyl acrylate M gfmol 104.16 86.09 Pmon g/L 878 950 p g/L 1100 oJ;•> cm2/s 1.4 ·10-6 2·10-6 Dw cm2/s 1.2·10"5 27) 2.4. w-s 33) L-mor1-s·1 25872) 340d'.60) r~ 73) (·) 0.73 0.19 s 60) (-) 0.94 0.11 1 1 6 ~ L-mol" ·s· 3-ld' U) 3.5·10 7S) kum L-mol"1·s·1 2.5-1d' %combination (·) variabie Cm (·) 5 .10-s 74) 3.2-10"5 em_c,33,7.S) (·) 0.7·1~ Monomer partitioning76> Kpt (-) 5 5 Kp2 (-) 0 0 Kw (·) 0.0025 0.6 n-Dodecyl mercaptan (NDM) M gfmol 202.42 k.rd) L·mol"1-s"1 200 200 Potassium Persu1fate M g/mol 270.23 s·t e,77) ~ 7.8-10-6 f (-) 0.5 70 a Dp may decrease at high volume fraction of polymer \ especially if Tg of monomer swollen copolymer is higher than the reaction temperature; adjustable parameter b estimation; exact value unknown c 60 oe d estimated 1 e in the presence of 50 mmoi·L" MAand at pH=7 Model 97 4.5 Condusloos The model "SIEMCO" bas been developed to descibe the {seeded) batch emulsion copolymerization. Copolymerization rate, composition drift and copolymer microstructure (MMCCD and triad fractions) cao be described. On the system styrene - methyl acrylate it bas been demonstrated that "apparent" · reactivity ratios are iocapabie of descrihing the course of emulsion copolymerizations in a adequate manner. Instead monomer partitioning effects have to be taken into account. Model calculations show that under eertaio conditions the copolymer formed exhibits bimodal (MM)CCD distributions. 98 Chapter 4 Appendix 4.A: MMO caleulation Assuming a steady-state situation (i.e., p, c, Jc,. and Icuare eonstant during the growth time of a polymer chain) in a zero-one-two (0:1:2) system (ii <0.7; Ni= 0 if i>2), the classical Smith-Ewart equations are valid and can be used to calculate the number fraction of latex particles having i radicals at timet (Ni(t)). 2 N0(t) = (2pc + pk, + 2Jc,. + 2ck,)/à'' (4.a.l) N1(t) = P(P + 2Jc,. + 2c)fa'' (4.a.2) (4.a.3) where a" is the normalizing factor defined by eq. (4.a.4). a'' = p(2p + 3Jc,. + 4c) + 2Jc,.(Jc,. + c) (4.a.4) 1 and where p is the radical entry rate coefficient (s" ) , Jc,. is the radical 1 desorption rate eoefficient (s" ) and cis the pseudo-first-order terminadon rate 1 eonstant (s" ). With the introduetion of the 'singly-and doubly-distinguished particles' Uchti et aL 22> have developed equations providing the growth time distribution of polymer ebains as a lunetion of the type of terminadon reaction mainly occurring. Here, only the final equations used will be given. For the denvation of these equations the reader is referred to the original paper of Lichtiet al. 22>. The molar mass of a single copolymer chain can be calculated by means of equation (4.a.5). This implies that the growth time (t') of a single polymer chain is much smaller than the time scale on which the concentrations vary. M = lmm<~·t' (4.a.5) Model 99 1 1 where a...md is the average growth rate (g · mol" · s" ). In the case of validity of the ultimate model: ammd = ~[Ma].,M8 ii 8/ÏÏ 1 + kp,b[Mb].,Mbiib/iit + ~[Mb].,Mbiia/ii 1 + kp,_[Ma].,M.iib/iit (4.a.6) When the penultimate model bas to be used : (4.a.7) where ii 1 = ÏÏ8 + iib and [Mt]P = [Ma]P + [Mb]P Mab: molar mass monomer a,b (g/mol) M ' : average molar mass of monomer units in copolymer (g/mol) [Mt]p : total monomer concentrations inside the particles (molfL) S"(t*,t') = t*{ (F + kr)[B1exp(-À+t') + E1exp(-ÀJ')] (4.a.8) + (F + kc+ pf2)[B2exp(-À+t') + E2exp(-À.t')]} S11(t*,t') : Distribution of dead chains, stopped by transferorentry of a radical in a latex partiele with 2 radicals Sbc(t*,t') = 2cct*{ (F B2 + pB1)[exp(-À+M/a)- exp(-OM/2a)]/(0-2).+) + (F E2 + pE1)[exp(-À_M/a)- exp(-OM/(2a))]/(0- 2)._) } (4.a.9) Sbc(t*,t') : Distribution of dead chains, stopped by bimolecular termination by combination S1bd(t*,t') = 2cdt*{ (F B2 + pB1)[exp(-À+M/a)- exp(-OM/a)]/(0-À+) (4.a10) + (F E2 + pE1)[exp(-À_M/a)- exp(-OM/a)]/(0- )._) } 100 Chapter 4 Sbd(t*,t') : Distribution of dead chains, stopped by bimolecular termination by disproportionation (subscript I : "long ebains ; subscribt s : "short chains") with Q=p+2c+2F+2k, (4.a.12) 1 where F : transfer without exit = ~. - k, (s- ) (4.a.13) where a11 = -p - F - k, (4.a.14) ~1 = p {4.a.l5) a12 = P/2 + k, (4.a.16) ~ = -p - 2k, - F - 2c (4.a.l7) B1 = N' 1{t,t' =0) - E1 (4.a.18) B2 = N' 2(t,t' =0) - E2 (4.a.19) N' 1{t,t' =0) = pN0{t) + F N1(t) (4.a.20) N' 2{t,t' =0) = pN1(t) + 2F N2(t) (4.a.21) E1 = [ (a11 + .>.+)N' 1(t,t' =0) + a12N' 2(t,t' =0)]/(.>.+ - .>._) (4.a22) E2 = [ ~ 1 N' 1(t,t' =0) + (~ + .>.+)N' 2(t,t' =0)]/(.>.+ - .>._) (4.a.23) For calculating the bimolecular termination also eq. (4.a.24) bas to be used : Nt(t' ,t") = [pN1'(t) + F N' 2(t')]exp(-Qt") (4.a.24) Model 101 Appendix 4.8: discussion The heterogeneaus and complex emulsion copolymerization processcan only be adequately described by models containing a large number of parameters. It is extremely important to minimize as much as possible the number of parameters and to determine the sensitivity of copolymerization rate and · copolymer microstructure to the value of the different parameters. In chapter 6 the kinetic aspects including composition drift predicted by the model are submitted to a comparison with experimental data of the S-MA emulsion copolymerization. The validity of the model SlEMCO to predict intra and intermolecular microstructure of S-MA copolymers is tested in chapter 7 and its validity in semi-continuous processes is tested in chapter 8. As far as rate prediedons are involved, the predictive power of the model SIEMCO is limited, since the values of the radical entry, radical exit and bimolecular terminadon rate in particles and aqueous phase are not known accurately enough, and can only be determined from special seeded kinetic experiments ?8). The values of the different rate parameters (e.g., ~ and k,) and diffusion coefficients of monomeric species are strongly dependent on the weight fraction 79 of polymer (wp) in the latex particles at high wP (say wP> 0.8) >. So far detailed measurements of~ and ~ as a function of wP only have been reported for methyl methacrylate16.so.&t). The lack of reliable data forS-MA copolymerization given in literature holds back an accurate and reliable prediction of the MMO at high conversions. lOl Chapter 4 Glossary of symbols capitals A : ratio of monoroer radicals a and b in the latex particles (-) Bt,z : constant used in the MMD calculation (eqs. 4.a.18, 4.a.19) (-) Cm...,bb,ab,ba : ratio of transfer- and propagation rate constants (-) [CfA]p : chain transfer concentradon in the (swollen) particles (molfL) D : Mb/M.: ratio of the molar masses of both monoroers Dw,pa,b : diffusion coefficient of monoroer a, b radicals, respectively, in polymer, water phase (cm2/s) :constant defined by eqs 4.a.22 and 4.a.23 (-) : rate constant of transfer that does not result in desorption (-) : instantaneous mol fraction monoroer a units in copolymer (-) : cumuialive mol fraction monoroer a units in copolymer (-) : instantaneous AAA triad fraction (-) : parameter in Stockmayer equations defined by eq. 4.41. (-) 1 : desorption rate constant for monoroer radicals (s' ) : constant descrihing the partitioning of monoroer a,b between the water phase (w) or polymer particles (p), respectively, and the monoroer dropiets (mol/L) : modified Besset function of the first kind : molar mass of a polymer chain (g/mol) : amount of monoroer a in monoroer dropiets per liter water (mol/L) Mao : initial amount of monoroer a per liter water (mol/L) [Ma,bJw,p : monoroer a,b concentration in water, particles (mol/L) Ma,b : molar mass of monoroer a,b (g/mol) MP : average molar mass monoroer units in copolymer (g/mol) [Mercp]p : mercaptan concentradon in the particles (mol/L) 1 Nav : Avogadro's number (mol" ) NT : number of latex particles per liter emulsion (1/L) Model 103 : fraction loci with n radicals (-) : normalized distribution of latex particles containing i radicals at time t : normalized distribution of "singly distinguished" latex particles containing i growing radicals, with one radical, that started to grow at time t and stil grows at time t + t 1 · N 1 'i(t,t' =0) : normalized distribution of "doubly distinguished" latex particles with i growing radicals, of which two radicals have a defined growth time of t' and t' + t' ', respectively Q : parameter used in the MMD calculation defined according 1 eq. 4.a.12 (s' ) 1 1 Ri : radical production rate (mol· L' . s' ) 1 1 Rp.,b : polymerization rate of monomer a,b in the loci (mol· L' · s' ) [R *1w : radical concentration in the water phase (molfL) 11 1 S (t* ,1 ) : distribution of non growing chains, stopped by transfer and entry of a radical in a partiele containing two radicals Sbc(t*,t') : distribution of non growing chains, stopped by bimolecular terminadon by combination : distribution of non growing chains, stopped by bimolecular termination by disproportionation (subscribt l : "long ebains ; subscribt s : "short chains") y : termination/absorption (-) 104 Chapter 4 small letters a : parameter in the ii calculation (eq. 4.25) : average growth rate of a chain (g. mort. s·t) 1 au,t2,2t,n : constantsin the MMD calculation (eq. 4.a.14-17) (s" ) c : average terminadon rate constant (s-t) 1 : average terminadon rate constant by combination (s" ) 1 : average tetmination rate 'constant by disproportionation (s" ) f : initiator decomposition efficiency factor ( ·) : mol fraction monomer a,b in monomer dropiets (-) : constant in the CCD calculation (eq. 4.42) : average radical absorption constant by latex particles {L/s) 1 1 :parameter in penultimate model (L·mol" ·s- ) 1 : initiator decomposition rate constant (s" ) 1 1 : average desorption rate constant (mor . s" ) 1 : average desorption rate constant (s- ) kcta, a,b : transfer rate constant of radical a,b to mercaptan 1 1 (L · mot· · s· ) 1 1 :transfer rate constant of radical a to monomer b (L·mol" ·s- ) 1 1 : propagation ra te constant (L. moJ" • s· ) 1 1 : terminalion rate constant (L· mol" ·s- ) 1 : average transfer rate constant (s" ) : degree of polymerization (-) m : exit rate/termination rate (-) Ill:P.a : partitioning coefficient of monomer a radicals (eq. 4.17) (-) n,.,b,t : number of radicals {a,b or total) {-) : monomerratio (mol mon. a/mol mon. b) in the loci(-) : reactivity ratios (-) : reactivity ratio ( = ~ju/~m) (-) : time (s) Model 105 t' : growth time of a radical (s) t" : total simultaneons growth time of the distinguishing ebains in a "doubly distinguished" partiele (s) t• : experimental time (MMD calculation) (s) : volume of a singleswollen latex partiele (L) : Stockmayer distribution function of copolymer molecules with degree of polymerization I over the chemical composition : w1(y), modified for different monomer rnalar masses : weight fraction of polymer inside the latex particles (-) : rnalar conversion of monomer a,b (-) : end conversion of monomer a,b (-) : chemical composition (fraction) of a copolymer molecule (-) : average fraction a in the copolymer material (-) : difference in chemical composition between a single copolymer molecule and the mean chemical composition (-) z : average degree of polymerization of desorbing radicals Greek §Y111bols a : radical absorptionftermination (-) a' : radical productionftermination (-) 2 a'' : parameter defined according to eq. 4.a.4 (s- ) ó : ratio of the water-side film monomer transfer resistance and the total transfer monomer resistance (-} À+,- :parameter in the MMD calculation (eq. 4.a.13) À : number average degree of polymerization (-) 1 p : absorption rate of radicals to the loci (s' ) Pe : absorption rate of radicals to the loci per liter water 1 1 (mol· L- • s· ) Pa,b,p : density of monomer a,b or polymer (g/L) 106 Chapter 4 subscripts a : monoroer a b : monoroer b or bimolecular c : combination d : disproportionation or decomposition e : radical entry or end conversion : initiator or number of radicals : locus or "long chains" I : degree of polymerization p : latex partiele or polymer or propagation s : "short chains" : total or termination T : in NT number of latex particles w :water various symbols : average * : radical : several different meanings References 107 1. 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Akatsuka, A Yamashita, N. Takemoto, Macromolecules, 11, 1206 (1978) 51. R.W. Sparidans, H.A. Claessens, G.H.J. van Doremaele, A.M. van Herk, J. Chromatogr., 508, 319 (1990) 52. G. Arzamendi, J.M. Asua, J. AppL Polym. Sci., 38, 2019 (1989) 53. G. Arzamendi, J.M. Asua, Makromol. Chem., Macromol. Symp., 35/36, 249 (1990) 54. M. Banerjee, U. Sathpathy, T.K. Paul, R.S. Konar,Polymer, 22, 1729 (1981) 55. M. Nomura, U.S. Satpathy, Y. Kouno, K. Fujita, J. Polym. Sci. : Part C: Polym. Lett. Ed., 26, 385 (1988) 56. I.A Maxwell, B. R. Morrison, D.H. Napper, R.G. Gilbert, Macromolecules, in press. 57. B.S. Hawkett, D.H. Napper, R.G. Gilbert, 1 Polym. Sci., Polym. Chem. Ed., 19, 3173 (1981) 58. M.E. Adams, M. Trau, R.G. Gilbert, D.H. Napper, D.F. Sangster, Aust. J. Chem., 41, 1799 (1988) 59. I.A. Maxwell, B.R. Morrison, R.G. Gilbert, D.H. Napper, in ''Integralion of Fundamental Polymer Science and Teclmology", Ed. P. Lemstra and L Kleintjes, p. 116, Elsevier Science Publishers Limited, London, 1990 60. T.P. Davis, K.F. O'Driscoll, M.C. Piton, M.A Winnik, Br. Polym. J., submitted 61. M. Morton, S. Kaizermann, M.W. Altier, J. Colloid Sci., 9, 300 (1954) Keferences 109 62. M. Nomura, K. V amamoto, I. Horie, K. Fujita, M. Harada, l AppL Polym. Sd., 27, 2483 (1982) 63. M. Nomura, in "Emulsion Polymerization", Ed. I. Piirma, Academie Press, New Vork, 1982 64. R.N. Mead, G.W. Poehlein, l AppL Polym. Sci., 38, 105 (1989) 65. J.M. Asua, E.D. Sudol, M.S. El-Aasser,l Polym. Sci.: Part A: Polym. Chem. Ed., 27, 3903 (1989) 66. M. Nomura, V. Minamino, K. Fujita, M. Harada, l Polym. Sd., Polym. Chem. Ed., 20, 1261 (1982) 67. T. Fukuda, V. Ma, H. lnagaki, Macromolecules, 18, 17 (1985) 68. AM. North, Polymer, 4, 134 (1963) 69. J.N. Atherton, AM. North, Trans. Faraday Soc., 58, 2049 (1962) 70. M.E. Adams, B.S. Casey, M.F. Mills, G.T. Russel, D.H. Napper, R.G. Gilbert, Makromol. Chem., Macromol. Symp., 35/36, 1 (1990) 71. J.L Koenig, "Chemica/ Microstructure of Polymer Chains", John Wiley and Sons, New Vork 1980 72. M. Buback, LH. Garcia-Rubio, R.G. Gilbert, D.H. Napper, J. Guillot, AE. Hamielec, D. Hili, K.F. O'Driscoll, O.F. Olaj, J. Shen, D. Solomon, G. Moad, M. Stickler, M. Tirrell, MA Winnik, l Polym. Sci.: Part C : Polym. Lett. Ed., 2" 293 (1988) 73. This thesis: Chapter 3 74. G. Odian, "Principles of Polymerization", Me. Graw-Hill Inc., New Vork, 1970 75. J. Brandrop and E.H. Immergut, "Polymer Handbook", John Wiley & Sons, New Vork, 1989 76. This thesis: Chapter 5 77. C.E.M. Morris, AG. Parts, MakromoL Chem., 119, 212 (1968) 78. R.G. Gilbert, D.H. Napper, l MacromoL Sci. Rev., MacromoL Chem. Phys., C23, 127 (1983) 79. B.S. Casey, lA Maxwell, B.R. Morrison, R.G. Gilbert, MakromoL Chem., MacromoL Symp., 31, 1 (1990) 80. G.L Leslie, I.A Maxwell, M.J. Ballard, R.G. Gilbert, D.H. Napper, Aust. l Chem., 41, 279 (1988) 81. M. J. Ballard, R.G. Gilbert, D.H. Napper, P.J. Pomery, P.W. O'Sullivan, J.H. O'Donnel, Macromolecules, 19, 1303 (1986) 110 Monomer Partitioning 111 Chapter S Monomer Partitioning SUMMAR Y: In this chapter some important theoretica! aspects given in literature are discussed that describe the monoroer partitioning behaviour during emulsion copolymerization. Experimental results are given for several monomeric (styrene, acrylic) systems at equilibrium. It appears that in all cases the monoroer ratio. in the latex pf!rticles is equal to the monoroer ratio in the droplets, although the total monoroer swellability of the (co )polymer latex particles .depends upon copolymer composition and monoroer droplet composition. In the absence of monoroer dropiets the equilibrium monoroer concentration in the aqueous phase is closer to the saturation value (i.e., water solubility) than the monoroer concentradon in the latex particles. In a comparison of high conversion bulk and solution copolymerization it was demonstrated that the reactivity ratios are not affected by viscosity, thus permitting their use in emulsion copolymerization. 5.1 Introduction; theoretical aspects of monomer partitioning In order to accurately describe emulsion copolymerization by means of a model, it is of paramount importance to imptement a reliable prediction of the monoroer concentrations within the latex particles. The monoroers are distributed between the particles, the aqueous phase and, if present, the monoroer droplets. In emulsion (co )polymerization it is generally recognized that the monoroer partitioning is determined by thermodynamic equilibrium. Equilibrium requires the chemica! potendal (p.) of the monoroers to be equal in all phases present. Morton et al. I) developed a model that describes the monoroer partitioning in case of homopolymers. This model is based on the classical Flory-Huggins lattice theory for monomer-(homo)polymer mixtures and includes an interfacial energy term. Morton stated that even in those cases where 112 Cbapter 5 the monomer is a good solvent for the polymer only a lirnited amount of monomer is absorbed by the latex particles, because the increase of surface energy on swelling compensates for the free energy gain of mixing. Moreover, tbe particles are assumed to be homogeneous. Morton's equation, descrihing the monomer partitioning during emulsion bomopolymerization of monomers with low water solubility like for instanee .styrene, reads: (5.1) with 1 interfacial tension between aqueous phase and the polymer partiele swollen with monomer [N/m] ~l' volume fraction polymer in polymer partiele [-] 3 V m rnalar volume of monomer [m /mol] 1 1 R gas constant [J · K" - mor ] T temperature [K] 1 3 r radius of swollen polymer partiele [m] (r = roon.t&WOUen/tp / ) x (temperature dependent) Flory-Huggins interaction parameter [-] Illj,p ratio of molar volumes of monomer i and the polymer [-] It directly follows from equation (5.1) that the monomer concentradon inside the particles will increase with decreasing interfacial tension. An increase in tempersture would result in a increase in monomer concentration in the particles. The complexity of this matter is demonstraled by experimentally determined data for styrene (ranging from 5.9 mol/Lat 25"C to 4.8 mol/Lat 70"C) reported by van der Hoff2> exhibiting the npposite behaviour. This · unexpected behaviour bas to be explained by the temperature dependenee of x. Monomer Partitioning 113 An excellent paper bas been writen by Gardon3> (as long ago as 1968), where he arrived at the following conclusions. (a) The interfacial tension (1) mainly depends on kind and concentration of emulsifier, ionic strength, type of polymer and polar (sulfate) chain ends at the partiele-water interface. (h) Latices prepared under different process conditions often have a similar swelling behaviour due to the self-compensating effects of some of these parameters. For example, a higher emulsifier concentration during emulsion polymerization reduces interfacial tension between the particles and the aqueous phase which would lead to a higher extent of swelling, but it also results in the formation of more and smaller particles that have the tendency to swellless with monomer. In most cases these two opposite tendencies appear to compensate to a large extent. (c) Furthermore, Gardon noticed that (among the monomees that are good solvents for their polymers in bulk) the monomees exhibiting a better water solubility swell their polymer particles to a larger extent than the poorly water soluble monomers. This was attnbuted to the fact that monomer dissolved in the aqueous phase reduces the interfacial tension between the swollen particles and the aqueous phase. (d) For styrene and also for methyl methacrylate, below saturation (i.e., in absence of monomer dropiets) the monomer partitioning was found to adjust itself in such a way that the aqueous phase is closer to monomer saturation conditions than the latex particles. For MMA this unexpected behaviour, together with the insensitivity of the monomer concentradon in the particles to surfactant and ionic strength, was later also observed by Ballard et al 4>. Guillot S) extended the thermadynamie monomer partitioning treatment of Morton, in an effort to describe the monomer partitioning during emulsion mpolymerization by introducing interaction terms for the monomers. Guillot gave equations to calculate the monomer and polymer volume fractions and (partial molar) free energies of both monomers in the three phases. The monomer chemical potendals were defined as the differences between the chemical potential of a monomer in a given particular phase and the chemical 114 Chapter S potendal of pure monomer at a reference state (ÄJ.'i.plwc = l'ï,p~wc • p.1} He assumed that the chemical potendals of the monomers are the same in each phase (i.e., equilibrium). The polymerization within the particles tends to change the chemical potendal of the monomers in the particles (p.n continuously. The resulting infinitely smalt di:fferences in l'p of the monomers lead to very rapid transfer of monomer towards the particles from the aqueous phase and, if present, from the monomer droplets, in order to re-establish the equality of the chemica! potential in all phases (phase equilibrium). If necessary, additional terms accounting for crosslinks in the particles and surface electric charge can be implemented. The chemica! potendal (Ap.1) of monomer i in the particles, dropiets and aqueous phase are given by equations (5.2), (5.3) and (5.4), respectively. 2 2 Ap.1.p = RT { ln(ti,p) + (1·~;)tj,p + (1-~p)tP,p + X1j+j,p + X1ptP,p (5.2) + tj,ptp,p{X1i + XiP • Xi~i) } + 2-yVïfrP Ap.1,. = RT { ln(t1,.) + (1-~i)ti.a + (1-lll;w)t.,.,. + (5.4) X;;+j,a2 + x~,.,.tw} + ti.a+w,.(Xij + Xïw • X~i)} with vi rnalar volume of monomer i [m3/mol] P,ij polymer, monomer i and monomer j, respectively p polymer particles +i,p volume fraction monomer i in swollen polymer partiele H tP,p volume fraction polymer inswollen polymer partiele [·] ~j ratio of mo~ar yolume~ (VJVi) [·] x Flory-Huggms mteract1on parameter where (x1i = ~i· X;1) rP radius of swollen polymer partiele [m], 1 interfacial tension between polymer partiele and aqueous phase [N /m] 1 1 R gas constant [J · K- • mor ] T temperature [K] d monomer dropiets 1d interfacial tension between rnonomer dropiets and aqueous phase [N/m] radius of rnonorner dropiets aqueous phase water Monomer Partitionlng 115 Ugelstad6.7) studied the effect of low molecular mass additives on the swelling of seed latex particles to be used for the preparation of large monodisperse polymer particles. Tseng et al. S) described the effect of the presence of water in the latex particles and in the monomer dropiets on the partitioning of relatively hydrophilic monomers. In the case of emulsion copolymerization Guillot9>and also de la Cal et al. 10>implemented the existence of monomer concentration gradients within the particles, due to varying thermadynamie interactions between the monomers and copolymer molecules differing in chemical composition at various locations within the heterogeneaus (co )polymer partiel es. In the last few years the thermadynamie treatment bas been applied by 11 1 13 14 15 several investigators • z. • • >. However, at present no reliable metbod of prediedog the free energy of a monomer swollen latex partiele is available. The use of the above-mentioned simplified models to describe the very complicated phenomena of monomer partitioning and also the lack of sufficient and accurate experimental monoroer partitioning data will frequently result in an unreliable or unrealistic estimation of interaction parameters. For example this metbod was 11 used by Mead et al. >, and the interaction parameter (XMA,PSMA) for the MA and copolymer was claimed to be 1.2. However, this is impossible for a monoroer that is a good solvent for the (co)polymer. In that case x bas to be less than 0.5. This result demonstrates that one bas to be extremely carefut when using only a limited number of experimental partitioning data to derive a large number of thermadynamie parameters. Moreover, the surface tension between the monoroer swollen polymer particles and the aqueous phase is very difficult to predict and to measure. In the field of monoroer partitioning experiments there exists inconsistency between several publications. For instanee for S-MMA it has been stated that the total monoroer concentration in the particles [Mt]P is independent of the composition of the monoroer droplets16>, whereas other data from Nomura et al. indicate that there is a small, but significant, increase in [Mt]p with increasing MMA content in tbe monoroer droplets1n. 116 Chapter 5 Among all emulsion copolymerization systems the monoroer partitioning behaviour of S-MMA bas been most extensively described. Nomura performed numerous monoroer equilibrium experiments on S-MMA, determining the effect of monoroer ratio, partiele size, copolymer composition, interfacial tension and ionic strength on the latex partiele swellability. For other copolymer systems such detailed data are not availabe. Unfortunately, the experiments on the S-MMA system were carried out only in the presence of monoroer dropiets and the water solubility of the monoroers was neglected in the monoroer mass balances. Therefore, the results obtained can only be used for intervals I and 11 during emulsion copolymerization. As for the systems studied in this wor1c, only lirnited 11 12 1 19 data have been reported for styrene (S) - methyl acrylate (MA) • • 8, > and for styrene - n-butyl acrylate (BA)20>, whereas no data at all were found for MA - BA The above considerations provided sufficient motives to perform a set of equilibrium experiments to study the effect of several parameters (monomer ratio, copolymer composition and molecular mass, cross-linie density and partiele size) on the monoroer partitioning of the main monoroer pairs studied in this thesis. Experiments were carried out with or without the presence of a separate monoroer layer. The experiments with a monoroer layer are representative for intervals I and 11 of the emulsion copolymerization, whereas the experiments without a separate monoroer layer are representative for interval m of the emulsion copolymerization. In modelling emulsion copolymerization not only monoroer partitioning is an important factor but also the accurate values of the reactivity ratios. A re-evaluation of the reactivity ratios of the S-MA system under well defined conditions in low conversion solution and bulk copolymerization was considered necessary. Some bulk copolymerizations were carried out in the presence of additional polystyrene in order to study a possible effect of viscosity on reactivity ratios. Monomer Partltioning 117 5.2 Expertmental section A latex of known solid content ( determined by means of standard dry solid content analysis) and copolymer composition was mixed with known amounts of the two monoroers in the absence of initiator. Prior to use the latex had been heated (90•C) for 24 hours in order to remove last traces of the initiator. The system was allowed to reach equilibrium by shaking (for at least 24 hours) while thermostated at the chosen temperature. The phases (swollen polymer particles. aqueous phase and monomer layer) were separated using an ultracentrifuge (380000 g, Centrikon, T-2060, 1-3 hours) thermostated at a temperature of max. 45•c. The swollen particles could not be analysed without including a small part of the adhering aqueous phase. Monomer concentradons in the partiele phase were determined by means of GLC after dissolving the monoroer swollen (co)polymer phase (with minor aqeous phase content) in acetone with some toluene as an internat standard or, alternatively in the case of PS latices. after dissolving in toluene with some 2-propanol (IPA) as internal standard. Determination of dry solid content of the sample gave the copolymer content. The concentradon of methyl acrylate in the water phase was determined after adding a standard IPA solution in water to the aqueous layer. The presence of styrene ( < 3 mmol/L) and n-butyl acrylate ( < 11 mmol/L) in the water phase was neglected. For the determination of the monomer concentrations in the particles, appropriate corrections were performed for the MA content in the sample, dissolved in the aqueous phase. If present, the monomer layer was analysed by means of GLC in terms of molar monomer ratio. This monomer ratio was always in very good agreement with the value calculated from the molar mass balance equations. To calculate the monomer concentradon inside the particles, the volumes of all component (monomers and (co)polymer) present in the monomer swollen latex particles were assumed to be additive. Copolymer density was calculated by the appropriate averaging of the densities of the homopolymers (Table 5.1 ). 118 Cbapter S The reliability of the above mentioned metbod was confumed by means of additional monomer equilibrium experiments (without a separate monomer phase present) using common dialysis tubing for the separation of the aqueous phase serum from the latex particles. Phase equilibrium was always achieved within 30 min. Table 5.1. Densities of monomos and polymers used. 3 monomer density (g · cm· ) temp. re) styrene 0.9060 20 methyl acrylate 0.9535 20 n-butyl acrylate 0.899 35 PS 1.05 20 PMA 1.2 20 PBA 1.026 35 5.3 Results; monomer partitioning Methyl acrylare concentration in the aqueous phase as a function of composition of monomer phase, temperature and SDS concentradon In the presence of a separate monomer phase the aqueous phase concentration of S was always less than 3 mmol/L and could be neglected in the emulsion copolymerization model calculations (see chapter 4). The concentradon of MA in the aqueous phase linearly increases when the mol fraction of MA in the monomer phase increases (Figure (5.1)). The partitioning coefficient I<_= [MA]w/[MA]d is ca. 0.05. As already pointed out by Emelie21> for BA and MMA the type and concentration of surfactants can affect the water solubility of a monomer. Above CMC (= 2.2 g/L for SOS) this effect is reinforeed by the monomer solubilization within micelles. In Figure (5.2) it is demonstrated that the effect of sodium dodecylsulfate (SOS) concentration ( <4 g/L) and temperature (35-5o•q on the water solubility of MA is negligible. Monomer Partitioning 119 0.80 0.70 :J ' 0.60 ö 0.50 ...... E 0.40 ,....,~ 0.30 <( 6 0.20 0.10 0.00 0.00 0.20 0.40 0.60 0.80 1.00 fMI\ in dropiets (-) Figure 5.1. MA concentration in the aqueous phase as a function of monomer phase composition at 5o·c. 1.00 --....I 'ö + ~ ~+ ~ ~ • + s + 0.50 .... IA~ ':? 6 0.00 0 2 3 4 [SDS] (g/U Figure 5.2. Water solubility of MA depending upon SDS concentration and temperature: 45°C (o), 5o·c ( +). 120 Chapter 5 Methyl acrylale partitioning between aqueous phase and polymer (PSMA) particles in the absence of a separate monomer phase During S-MA batch emulsion copolymerization, especially in the case of the very interesting MA rich recipes ( chapter 6) ), at moderate total eonversion the major part of the styrene already bas been depleted while a major part of the methyl acrylate is still present. Therefore, the methyl acrylate concentradon inside the particles ([MA]p) versus the methyl acrylate concentradon in the aqueous phase ([MAJw) was determined in the absence of styrene. All these experiments were carried out at 45•c. In Figure (5.3a) it is demonstrated that in the absence of a separate monomer phase, [MAJw is closer to the saturation value (i.e., water solubility) than [MA]p in PMA latex particles {i.e., 11111 11111 [MAJw/[MAlw > [MA]p/[MA]p ). This swelling behaviour of MA is similar to that of styrene in polystyrene particles and methyl methacrylate in polymethyl methacrylate particles3>. Furthermore, it is shown that the presence of cross-links in the PMA partiele on the swellability is negligible. Apparently, the extent of cross-linking is too ·low to restriet the monomer swelling of the latex particles. The crosslinked PMA latex was prepared in the presence of 5 mol% ethylene diacrylate (EDA) (see appendix A). In Figures (5.3b,c,d) the effect of copolymer composition (of S-MA {co)polymer), partiele size and molar mass on MA partitioning is shown. At MA concentradons close to saturation a very small influence is noticed. This effect, however, is negligible in practical simulations. Lower molar mass, increasing MA content in the (co )polymer and larger partiele size increase the equilibrium methyl acrylate concentradon within the particles ([MA]p) as a function of MA concentradon in the aqueous phase ([MA]w). Combining all data of Figure (5.3) [MA]p can he expressed as a function of [MA]w by the empirical equation (5.5): [MA]p = 5.l[MAJw + ll.O[MA]w3 + ([MAJw + (1-[MA]w581))Y (5.5) where y = 30. Monomer Partitioning 121 10 10 (a) (b) 5 5 -;f -;f ~ ~ 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.1 0.2 0.3 9.4 0.5 0.6 0.7 [MA]w (mol/U [MA)w (mol/U 10 10 ~ (c) ~ (d) .s0 ] 5 5 ~ <( -;f ~ ~ 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.1 0.2 0.3' 0.4 0.5 0.6 0.7 [MA]w (mol/U [MA]w (mol/U Figure 5.3. MA concentration in the particles as a function of the MA concentration in the aqueous phase. Lines are calculated according to eq. (5.5). (a) PMA latex particles; cross-linked ( +) and non-cross-linked PMA (A) (b) effect of copolymer composition; Fs = 0 (a), 0.25 (9), 0.5 (o), 0. 75 (A), I ( +) (c) effect of partiele size (Fs = 0.25); Dw = 32 ( + ), 61 (o) and 97 (A) (nm) (d) effect of copolymers molar mass (Fs = 0.25); Mw = 7500 (o), 48000 (A), 1300000 ( +) (g/mol). U2 Chapter 5 MolWmer partitioning in the presence of a separate moMmer phase In Figures (5.4a-c) the monomer concentrations inside the particles in mol monomerper liter swollen latex partiele volume are given, with (co)polymer composition as parameter, as a function of the composition of the monomer phase, representing the systems 8-MA (20•C), 8-BA (2o•c) and MA-BA (3s•C), respectively. The lines represent 2nd order polynomals obtained by linear least square fitting of the data and used in the model calculation in chapters 6, 7 and 8. Por comparison, the concentrations of the pure monomers are: S 8.7 mol/I.., MA 11.1 mol/L and BA 7.0 molfL. In Figures (S.Sa-c) the monomer compositions in the particles are compared with the monomer compositions of the monomer phase. From Figures (5.4a-c) it is concluded that monomer swellability of polymer particles increases with increasing content of the most water soluble monomer (S and BA < MA). The monomer ratio in the dropiets is equal to the monomer ratio in the swollen particles for all monomer systems and all copolymer chemical compositions studied (Fig. (S.Sa-c)). Copolymer composition only affects the total monomer concentradon inside the latex particles. The curves of the monomer concentradons in the particles versus the mol fraction of the same monomer in the monomer phase are curved in such a manner that, independent of the monomer ratio in the monomer phase ( dropiets ), the monomer ratio in the particles is equal to the monomer ratio in the monomer phase, although the total monomer concentradon may still depend on the monomer ratio in the monomer phase. Nomura found a simHar behaviour for S-MMA22>. It is known that solution copolymerizations of monoroers strongly differing in polarity (e.g., styrene and acrylic acid) only can be described by "apparent" reactivity ratios that depend on the solvent used. This can be explained by the "bootstrap" model, presented by Harwood23>, that accounts for local monomer concentradons in the environment of the growing radical ebains differing from the overall monomer concentrations. In the three systems (S-MA, S-BA and MA- Monomer Partitioning 123 BA) the result of Figure (5.5) suggests the absence of a "bootstrap" effect. This is in agreement with the findings that reactivity ratios of these systems are only weakly dependent on solvent24>. 10 (MAp " PS :J...... À [$TYp 0 PS (MAp .5 • 50 mol% cop. 5 0 [STYp 50 mol% cop. !MAP • PMA 0 [STYp PMA 0.50 1.00 f ..... in dropiets (-) 10 [STYp " PS :J (b) ...... À [SAp 0 PS .5 • [STYp 50 mol% ·cop. 5 0 (SAp ~ 50 mol% cop. [STYp ö " P8A ~ V (SAp P8A 0.50 1.00 fi!A in dropiets (-) 10 (MAp " PMA A (SAp :J...... 0 PMA • (MAi> 50 mol% cop. .5 5 Q. 0 (SAi> c 50 mol% cop. 0 !MA i> L • P8A 0 (SAp P8A 0.50 1.00 fMA in dropiets (-) Figure 5.4. Monomer concentration inside the latex particles as a function of composition of the monomer phase; (a) S-MA, (b) S-BA, (c) MA-BA. 124 Chapter 5 1.00 I (a) (/) 0 Q) PMA 0 0 -.;;.... 0.50 50 mol% eop. m a. .a. PS ,!;;;; ....i 0.00 0.00 0.50 1.00 f"... in dropiets (-) 1.00 I (b) (/) Q) V PSA 0 :;::; .... 0 50 mol% eop. co 0.50 a. .a. PS ,!;;;; ..... " 0.00 0.00 0.50 1.00 t .... in dropiets (-) 1.00 I (c) (/) Q) V P8A :'Q...., .... 0 50 mol% cQP. co 0.50 a. c PMA .~ ....i 0.00 0.00 0.50 1.00 f"... in dropiets (-) Figure 5.5. Monomer composition of the particles compared with the monomercomposition ofthe monomer phase; (a) S-MA, (b) S-BA, (c) MA- BA. Monomer Partitioning 125 5.4 Results; bulk eopolymerizations In the S-MA emulsion copolymerization model calculations described in this thesis, it is assumed that solution reactivity ratios (r5 = 0.73, rma = 0.19) (§ 3.2.2.3) cao be used to describe the copolymerization behaviour inside the latex particles. In the latex particles the copolymer concentration is high and in principle viscosity effects on reactivity ratios caooot be ruled out beforehand. Deviations of the (apparent) reactivity ratios caused by diffusion eontrol of the process performed under starved conditions was reported by Snuparek et al. 25>. In order to verify whether the reactivity ratios remaio unaffected, two bulk copolymerization experiments wére carried out to deternline the sensitivity of the reactivity ratios to viscosity, i.e., to any possible diffusion controlled propagation steps. Such an effect might result in a drift of both reactivity ratios towards unity as viscosity increases. Styrene and methyl acrylate · were ·cöpolymerized .at low conversion ( < 1%) in the presence of polystyrene (PS) (Mw = 42000 g/mol) at 323 K. The recipes are given in Table (5.2). No chain transfer agent was added in order to prepare a high molecular weight copolymer. The copolymer formed and the PS were isolated, purified, and finally analysed by means of calibrated TLC/FID. This technique appeared to be extremely useful for this purpose. As indicated by the two well separated peaks in Figure (5.6) the chemical composition of the very small amount of copolymer could be analysed very well even in the presence of an excess of pure PS. The results shown in Table (5.3) indicate unambiguously that the reactivity ratios remaio unaffected in the latex particles at least during intervals I and 11 of the emulsion copolymerization. 126 Chapter 5 PS Spotting point Copolymer Distance- Figure 5.6. TLC/FID chromalogram of a very smal/ amount of copolymer formed in the presence of an excess of PS. Table 5.2. Recipe (in grams) of preliminary bulk polymerizations. exp. A B s 1.40 0.55 MA 2.93 1.88 PS 4.31 2.02 AIBN 0.02 0.02 Table 5.3. Experimentally determined average chemica/ composition of bulk S-MA copolymers prepared in the presence of PS, compared to model calculations. F, model F, model F, TLC/FID with r, = 0.73 with r, = 1 ( experimental) and rma = 0.19 and rma = 1 A 0.395 0.465 0.28 0.44 B 0.242 0.40 0.20 0.45 q0 = molar monoroer feed ratio (S/MA)0 F5 = average molar fraction styrene units in copolymer Monomer Partitioning 127 5.5 Conclusions From the equilibrium monoroer partitioning experiments in the absence of a separate monoroer layer it can be concluded that the methyl acrylate concentradon in the aqueous phase is closer to the saturation value than the methyl acrylate concentradon in the latex particles. The monoroer partitioning in the systems investigated is only marginally affected by partiele size, copolymer composition, and molar mass. The monoroer partitioning experiments in the presence of a separate monoroer layer revealed that the moriomer ratio in the particles is equal to the monoroer ratio in the monoroer droplets, although the total monoroer concentration intheswollen latex particles depends upon copolymer composition and upon monoroer ratio in the droplets. In principle possible ''bootstrap effects" can be ruled out for the systems investigated. Bulk copolymerization experiments when compared with solution copolymerizations proved that at least during intèrvals I and II of the emulsion copolymerization, the reactivity ratios remaio unaffected by the high viscosity. These results lead to the important inference that in emulsion copolymerization modelling, at least for the present systems, solution reactivity ratios can be used in combination with monoroer partitioning data. Keferences 1. M. Morton, S. Kaizerman, M.W. Altier, J. CoUoid Sci., 9, 300 (1954) 2. B.M.E. van der Hoff, J. Polym. Sci., 44, 241 (1960) 3. J.L Gardon, J. Polym. Sci.: Part A, 6, 2859 (1968) 4. MJ. Ballard, D.H. Napper, R.G. Gilbert, J. Polym. Sci., Polym. Chem. Ed., 22, 3225 (1984) 5. J. Guillot, Acta Polym., 32, 593 (1981) 6. J. Ugelstad, H.R. Mfutakamba, P.C. M0rk, T. Ellingsen, A Berge, R. Schmid, L Holm, A J0rgedal, F.K. Hansen, K. Nustad, J. Polym. Sci., Polym. Symp., Tl, 225 (1985) 7. J. Ugelstad, P.C. M0rk, I. Nordhuus, H.R. Mfutakamba, E. Soleimany, MacromoL Chem., SuppL, 10/11, 215 (1985) 8. C.M. Tseng. M.S. EI-Aasser, J.W. Vanderhoff, ACS, Div. of Org. Coatings Plastic Chem. Preprint., 373 (1981) 9. J. Guil1ot, in "Future Directions in Polymer Coloids", Ed. M.S. El-Aasser, R.M. Fitch, Martinus Nijhoff Publishers, Amsterdam, 65, 1987 10. J.C. de la Cal, R. Urzay, A Zamora, J. Forcada, J.M. Asua, J. Polym. Sci.: Part A: Polym. Chem. Ed., 28, 1011 (1990) 11. R.N. Mead, G.W. Poehlein, /nd. Eng. Chem. Res., 27, 2283 (1988) 12. R.N. Mead, G.W. Poehlein, /nd. Eng. Chem. Res., 28, 51 (1989) 13. G. Arzamendi, J.M. Asua, J. Appl. Polym. Sci., 38, 2019 (1989) 14. J. Forcada, J.M. Asua, J. Polym. Sci.: Part A: Polym. Chem. Ed., 28, 987 (1990) 15. J. Dimitratos, C. Georgakis, M.S. El-Aasser, A. Klein, Comp. Chem. Eng., 13, 21 (1989) 16. J.M. Goldwasser, AL Rudin, J. Polym. Sci., Polym. Chem. Ed., 20, 1993 (1982) 17. M. Nomura, K. Y amamoto, I. Horie, K. Fujita, M. Harada, J. AppL Polym. Sci., 27, 2483 (1982) 18. W. Ramirez-Marquez, J. Guillot, MakromoL Chem., 189, 361 (1988) 19. W. Ramirez-Marquez, J. Guillot, Makromot Chem., 189, 379 (1988) 20. J.K. Siiupárek, F. Kdka, J. AppL Polym. Sci., 20, 1753 (1976) 21. B. Emelie, C. Pichot, J. Guillot, MakromoL Chem., Suppl., 10/11, 43 (1985) 22. M. Nomura, K. Fujita, MakromoL Chem., Suppl., 10/11, 25 (1985) · 23. HJ. Harwood, Makromol Chem., Symp., 10/11, 331 (1987) 24. J. Bandrup, E.H. Immergut, ''Polymer Handbook", 3tdEd., Wiley Interscience, New-York, 1989 25. J. Stlupárek, K. ~<$par, J. AppL Polym. Sci., 26, 4081 (1981) Copolymerization Rate Chapter6 The EtTect of Composition Drift on Copolymerization Rate SUMMAR Y: With potassium persulfate as initiator and sodium dodecyl sulfate as emulsifier, the batch emulsion copolymerization rate behaviour of styrene (S) with methyl acrylate (MA) was investigated at so•c, varying the monomer ratio and the monomer to water ratio. As a consequence of the composition drift usually occurring during copolymerization, the kinetic behaviour differs widely from the homopolymerizations depending upon the initia! monomer ratio. The copolymerization rate strongly varles during copolymerization resulting in a shape of the conversion time plots differing from the sigmoidal shape usually observed in emulsion bomopolymerization. The penultimate effect in styrene - methyl acrylate and styrene - n-butyl acrylate copolymerizations ,bas be~n proved to be responsible, as seen clearly from conversion dependenee of the copolymerization rate durlog batch emulsion copolymerizations starting from an methyl acrylate or n-butyl acrylate rich monomer feed, teading to a strong composition drift. 6.llntroduction Batch (emulsion) copolymerization processes produce often bighly beterogeneaus copolymers with respect to chemica! composition1>. This heterogeneity may go along with partiele structuration2> affecting the product properties either beneficially or adversely. If required, this can be avoided by using more sophisticated processes such as semi-continuons (sometimes called semi-batch) processes3.4.S,6,7) or controlled composition processes8>. Before studying semi-continuons processes9> it is necessary to obtain a good knowledge of the kinetics in the conventional batch process. 130 Chapter 6 Bmulsion polymerization involves many complicated chem.ical and pbysical rate processes. Generally~ the rate of emulsion polymerization strongly depends on temperature and the kind and amount of monomers, initiator and surfactants. A very important factor is the number of latex particles. The overall polymerization rateis proportional to the partiele number, average number of radicals per partiele and the monomer concentradon inside the particles, the latter being the main loci of polymerization. Normally, a conversion-time plot of an emulsion polymerization exhibits the well-known characteristic sigmoidal shape. Sametimes a sudden increase in polymerization rate at high conversion cao be observed due to the gel effect. For instance, this effect was reported by Nomura for the styrene (S)-methyl methacrylate (MMA) emulsion copolymerization10>. As compared with homopolymerizations, in copolymerizations the monoroer feed ratio is an additional parameter significaotly affecting copolymerization rate. This may result in an even more complex rate behaviour. Due to the occurrence of composition. drift during emulsion copolymerization the conversion time plot may exhibit different shapes. For the emulsion copolymerizations of vinyl acetate 1 1 13 (VAc) with methyl methacrylate 1), vinyl acetate with n-butyl acrylate (BA) 2. > and vinyl acetate with metbyl acrylate (MA)14>it bas been demonstraled that the conversion time plots cao exhibit a double bend. This was attributed to the fact that in these cases there is a large difference between the values of the reactivity ratios (rvAI: <0.1 and r(metb)acrylate >5) in combination with a large difference between the propagation rate constants. The large difference in reactivity ratios results in a strong composition drift. Vinyl acetate is polymerized in two stages. The first stage camprises a real copolymerization with the (meth)acrylate and during the second stage, after the (meth)acrylate bas been depleted, VAc practically homopolymerizes. According to the well-known ultimate model (Alfrey-Mayo kinetics) the average kP is only a function of the reactivity ratios and the (local) monomer feed ratio, and is dependent on the monomer ratio. In these systems the average propagation rate constant ( kp) strongly depends upon the monomer ratio, because the propagation rate constants of the homopolymerizations are very different. Copolymerization Rate 131 The aim of the current investigation is to furnish forther fundamental inforrnation about the kinetic mechanism of S-MA emulsion copolymerization. It might be expected that the emulsion copolymerization of S and MA exhibits a similar kind of kinetic behaviour, because the reactivity (ty of methyl acrylate is high as compared with that of styrene and the values of the reactivity ratios in combination with the higher water solubility of MA ( chapter 5) may result in a significant composition drift. Moreover, Davis et aL 15) recently reported a penultimate effect for the copolymerizations of S-MA and S-BA. wbere kP is strongly dependent on monomer ratio. Ramirez-Marquez et aL t6,t7) investigated the effect of initiator concentration, emulsifier concentration and monomer to water ratio on the (co )polymerization rate of S-MA emulsion copolymerization. Although they observed the occurrence of a strong composition drift in MA-rich recipes, from their limited conversion time data they did not notice a sudden increase in polymerization rate at tbe moment styrene is almost totally depleted. As a result they were able to describe the S-MA emulsion copolymerization rate by means of tbe ultimate model. This required us to perfarm very accurate kinede measurements in order to investigate whether tbe occurrence of composition drift would affect the average propagation rate constant, and whether this phenomenon is reflected in the conversion time curves. U Experimental In chapter 3 the purification of tbe reactants, the equipment used, and the emulsion copolymer procedures followed, have been described in detail. The polymerizations were carried out at so·c with a sodium dodecyl sulfate (SDS) 1 concentradon of 0.0116 mol· L" water and witb a potassium persulfate (K~z08) concentradon of 1.233 mmol· L"1 water. The monomer ratio, the monomer to water ratio, and tbe n-dodecyl mercaptan (NDM) concentradon (usually 1 wt% 1 on monomer basis) were varied. NaHC03 was added (1.223 mmol·L- ) 132 Chapter 6 maintaining a sufficlent pH level. Partiele size measurements were performed with TEM after UV hardening of the latex and by means of dynamic light scattering (Dl.S, chapter 3). The monomer partitioning experiment& have been described in chapter 5. The size of the monomer dropiets under the experimental conditions was measured applying a metbod described by Hoedemakers18>. The volume-mean diameter of the monomer dropiets was circa 15 pm As is generally recognized, in this range of monomer droplet sizes there is no significant dilfusion limitation of monomer transport from the dropiets to the aqueous phase. Furtheimore, polymerization inside the monomer dropiets is negligible. 6.3 Results and diseusslon A typical conversion time curve of a batch emulsion copolymerization of S with MA is given in Figure (6.1a). The initial monomerratio (S/MA)0 was 0.33 (molfmol) with a monomer to water ratio (M/W)0 of 0.2 (g/g). From this plot it is obvious that the S-MA copolymerization passes through several stages. Polymerization starts with the partiele nucleation stage. After 10% conversion an almost constant copolymerization rate is observed until 40 % conversion. Copolymerization rate then decreases between 40 and 55% conversion. This decrease in copolymerization rate is attributed to a decrease in monomer concentradons in the latex particles. At the point at which styrene is (almost) totally depleted, the polymerization rate is at a minimum. From this point on the reaction rate suddenly increases and as a result methyl acrylate homopolymerizes. Depletion of methyl acrylate, the preferential presence of MA in the aqueous phase ( chapter 5), and dilfusion controlled propagation, result in a final decrease of polymerization rate at high conversions. Thus composition drift strongly affects the S-MA copolymerization rate. Copolymerization Rate 133 1.00 .. .t • • • (a) .. •"' .. 0.80 ... ·:... . . + I 0.60 0 .tiP·,. Q "'"" ... • 0 4 4 +"' + x 0.40 .. 0 • .+ + . ..• .+ 020 ... :t•.. 0.00 0 3000 6000 9000 12000 Time (sl 5e-04 4e-04 (b) lil _j ..... :le-04 g0 2&--04 +++ ++ o!flloo ct + 0 <>o + 1e-04 "o o'o Time (s) Figure 6.1. Batch emulsion S-MA copolymerization at 50°C with a (SjMA)0 = 0.33 (mol/mol), (M/W)0 = 0.2 (g/g), a SDS concentration of 0.0116 1 1 (mol·L- ), a KzSPs concentration of 1.233 (mmol·L- ), and 1 wt% NDM. (a) conversion-time plot: MA conversion ( +), S conversion (o), total mol conversion (A). (b) polymerization rate-time plot: MA ( +), S (o). In order to obtain further supporting evidence the initial monomer ratio (Figure (6.2)) and tbe initial monomer to water ratio (Figure (6.3)) have been changed. All experiments with a styrene content of less than 50% were found to exhibit a sudden increase in polymerization rate at the moment wbere styrene bas almost totally been consumed. No sudden increase in polymerization rate was noticed in case of higher styrene contents. As has already been noted by Ramirez16>, higher methyl acrylate fractions in the initial monomer feed result in a higher polymerization rate. 134 Chapter 6 As depicted in Figure (6.2) at lower (S/MA)o ratios acceleration and pure polymethyl acrylate formation occurs at a lower conversion. As depicted in Figures (6.3) and (6.4) a decrease of monomer to water ratio results in a higher fractional (co)polymerization rate. Because of the stronger composition drift due to the buffer capacity of water for methyl acrylate, this also results in a lower critical conversion at wbich the acceleration of polymerization rate occurs. 1.00 0.80 r 0.60 DI I D D Time (s} Figure 6.2 Conversion time plots of batch emulsion S-MA copolymerizations at 5o•c with a (M/W)0 = 0.2 (g/g), a SDS concentration of 0.0116 1 1 (mol·L' ), a K~P8 concentration of I.233 (mmoi·L. ), I wt% NDM, and a variabie monomer feed ratio: (S/MA)0 = 0 (D), I/19 ( +), 1/11 (A), I/1 (o) (mol/mol). 0.8 I o.e x 0.4 OD~----~----~------._----~ 0 7200 10800 14400 Time (s) Figure 6.3. Conversion time plots of batch emulsion S-MA copolymerizations at 50"C with a (S/MA)0 = 0.33 (mol/mol), a SDS concentration of 0.0116 1 1 (mol·L. ), a K~p8 concentration of I.233 (mmol·D ), I wt% NDM, and variabie initia/ monomer to water ratios; (M/W)0 = 0.05 (o), 0.2 (o), 0.5 ( +) (g/g). Copolymerization Rate 135 0~8 I c 0.7 0 ëii 0 l!; 0 ~ 0.6 0 0 ïö 0 :.-:; 0.5 ·;: 0 0 0.4 0.00 0.25 0.50 0.75 (M/W) 0 Figure 6.4. Critical conversion as a function of (M/W)0 of batch emulsion S MA copolymerizations at so•c with a (S/MA)0 = 0.33 (mol/mol), a SDS 1 concentration of0.0116 (mol·L· ), 1 wt% NDM and aK~p concentration 1 8 of 1.233 (mmol-L" ). The line represents the model calculation by applying ''SIEMCO" (see mechanism d) From these experiments it can be concluded that composition drift is an important factor determining anomalous copolymerization rate behaviour. In principle this behaviour could be attributed to several possible mechanisms. since according to equation (2.1) given in chapter 2 polymerization rate is proportional to (a) the number of latex partiel es, (b) the monoroer concentradon inside the particles, (c) the number of radicals per partiele (influenced by a gel effect) and (d) the average propagation rate constant. Mechanism (a): The acceleration observed could be attributed to a sudden increase in partiele number (secundary nucleation) at the moment at which the homopolymerization of the more water soluble monoroer (in this case MA) starts. More water soluble (hydrophilic) monoroers have the tendency to form more latex particles in emulsion polymerization as compared with less water Chapter 6 soluble monomers. However, DLS measurements and, as reported in appendix A. transmission electron micrograpbs (after UV hardeDing of the latex) did not indic:ate a significant amount of small PMA particles at high conversion (see Figure (6.5)). Only a very small, gradual increase in partiele number was observed sometimes16>. The finallatex particles appeared to exhibita core-sbell type of morphology, as can be explained by the composition drift and the incompatibility of the (co )polymers formed. Furthermore, a significant increase in latex partiele number is unlikely to occur, since the monomer dropiets already disappeared below 50% conversion and, as a consequence, at 50 % conversion the major part of the unreacted monomer MA is already inside tbe latex partieles. Only a small amount of MA ( <0.6 mol/L) is dissolved in the aqueous phase. 5~17r------~ 0 0 0 4e+17 0 0 0 0 ...... 0 'i~ 3e+17 d z 2~17 1~17 o~~--~~--~~--~~----~~~ 0.0 0.5 1.0 Figure 6.5. Partiele number calculated from DLS data versus conversion of a S·MA batch emulsion copolymerization at so·c; (S/MA)0 = 0.33 1 (mol/mol), (M/W) = 0.2 (g/g), a SDS concentra/ion of0.0116 (mol·L" ), 0 1 I wt% NDM, and a K;PP8 concentration of 1.233 (mmol·L' ) Copolymerization Rate 137 Mechanism (b): During interval lil of the emulsion polymerization an increase in (average) MA concentration in the particles ([MA]p) is of course impossible. However, an increase in local MA concentration in the shell of the polymer particles cannot be completely ruled out. In chapter 5 it is shown that the equilibrium · concentratioos of MA is higher in polymethyl acrylate (PMA) latex particles than in polystyrene (PS) latex particles, 8.5 and 6 mol· L·t, respectively. Given these data, however, this phenomenon cannot be a major factor causing the sudden strong increase in rate. Therefore, a possible monoroer concentration gradient inside the particles, due to differences between the thermodynamic interactloos of MA with PMA and MA with copolymer (PSMA), could only be slightly respoosible for the observed increase in polymerization rate. Mechanism (c): At first sight the gel effect, causing an increase of ii, might cause the observed acceleration. MA is well-known for its gel effect. However, for several reasoos the gel effect must be ruled out as a main cause in suddenly increasing the polymerization rate, although it will probably affect the rate to some extenL This is because, when varying initial monoroer ratios, the increase in polymerization rate was found to occur at different conversioos, and thus at different monoroer concentratioos, different volume fractioos and different chemical compositioos of the copolymer in the monoroer swollen latex particles (see Table (6.1)). Invariably, in all cases the increase in rate was found to occur just at the moment styrene was exhausted. 138 Chapter 6 Table 6.1. Initia/ overall monomerratio (S/MA)0 and composition ifm.oJ of various batch emulsion copolymerizations, logether with the critica/ mol fraction methyl acrylale of the monomer inside the swollen latex particles (f J, the critica/ volume fraction polymer (vcp) in the swollen latex particles ;;,:J the critica/ copolymer composition (P",) at which the acceleration occurs. (S/MA)0 fm,O rm,l yp Fm 1/3 0.75 0.93 0.74 0.62 1/7 0.88 0.99 0.53 0.72 1/11 0.92 0.98 0.38 0.75 1/19 0.95 0.97 0.26 0.81 1.0 o.e I 0.6 -.~~ > 0.4 0.2 0.0 0.7 0.8 0.9 1.0 f (-) MA.() Figure 6.6. Critica/ volume fraction of polymer versus initia/ methyl acrylate fraction ofS-MA batch emulsion copolymerizations at so·c; (S/MA)0 = 0.33 1 (mol/mol), (M/W) = 0.2 (g/g), a SDS concentration of0.0116 (mol·L- ), 0 1 1 wt% NDM and a K;PP8 concentration of 1.233 (mmol·L- ). Moreover, it was found that the presence and the amount (ranging from 0 to 10 wt% on monomer basis) of n-dodecyl mercaptan, having a paramount effect on molar mass of the copolymer formed, did oot affect polymerization rate. As a consequence, the occurrence of an important gel effect can be ruled out, because, as reported by Matheson et aL 19>, the gel effect occurring during Copolymerization Rate 139 bulk polymerization of MA is eliminated by the presence of a small amount of chain transfer agent. The average number of radicals inside latex particles was calculated at any moment from polymerization rate data, the average propagation rate constant kP (see Figure (6.7)), the monomer concentradons inside the latex particles and the number of latex particles. For the experiment given in Figure (6.1) the · calculated values of ii were approximately 0.3. However, due to errors and uncertainties in all parameters the values of ii must be regarded as an approximation and oot as absolute values, and therefore should be prudently used. Mechanism ( d): The cause of the suddet_: increase in rate may also be found in the average propagation rate constant (kp). The composition and sequence distribution of the copolymer formed and the composition drift durlog a copolymerization of S and MA cao be adequately described by the ultimate model20>. However, recent measurements of kP as a function of monomer ratio by Davis et al. IS) using the laser-flash technique ( oomparabie to the well-known rotating sector method) in low-conversion bulk and solution systems revealed that the kinetie behaviour of the S-MA system cannot be adequately described by the ultimate model. Instead the penultimate model proved to be appropriate in this case. Given the fact that composition drift is well described by the ultimate model, it can easily be demonstrated that from the six reactivity ratios in the penultimate model, two pairs of reactivity ratios must be equal (see chapter 2). Given the homopropagation rate constants of MA being larger than 340015>, and 1 1 21 of S being 258 (L·mol" ·s' ) > at so•c, the average propagation rate constant (kp) can be calculated from the reactivity ratios (rs = r5' = 0.73, rma = rma' = 0.19, Ss = 0.94 and s_ = 0.11). 140 Chapter 6 In Figure (6.7) values of k, have been plotted versus MA mol fraction in tbe local monoroer feed, according to (1) tbe ultimate model (inadequate intbis case) and (2) tbe penultimate model (adequate after recent findingslS)). As can beseen from Figure (6.7), k, strongly increases going from a monoroer feed witb 10 % S to pure MA. Thus tbe sudden increase in polymerization rate can be attributed to tbe penultimate effect of tbe copolymerization of S and MA. o~--~~~~--~----~ 0.0 0.5 1.0 Figure 6. 7. Average propagation rate constant for S·MA copolymerizations at so·c as a function of the fraction MA at the locus of reaction calculated according to the ultimate model : ------; and the penu/timate model : • • • • •. 15J In Figure (6.8) the experimentally determined partial conversions versus total conversion are compared with a model calculation (SffiMCO, cbapter 4). From tbe satisfying agreement it can be concluded tbat, using independently determined monoroer partitioning data and reactivity ratios, SIEMCO also correctly prediets the point at which the sudden acceleration occurs (see Fig. (6.4)). Copolymerization Rate 141 1.00 I 5 ïii ~ > 5 0.50 0 'jij ·.;::;.... a.10 Total conversion (-) Figure 6.8 Model calculation of partial conversions of S (A) and MA ( +) versus overall mol conversion compared to experimental data of an emulsion copolymerization with (S/MA)0 = 0.33 (mol/mol) and (M/W)0 = 0.2 (g/g). As additional proof of validity of our hypothesis we investigated the emulsion copolymerization of styrene (S) and n-butyl accylate (BA) (Fig. (6.9)). Davis et al. IS) found that the kinetic behaviour of S-BA copolymerizations aiso bas to bedescribed by the penultimate model. At 50"C the reactivity ratios are r, = r's = 0.95, rb = r'b = 0.18, s, = 0.90 and ~ = 0.11, and ~.b > = 280015) 1 1 21 while ~.s = 258 (L· mor • s- ) >. In the area of the high BA fractions the average kP strongly increases as the monomerratio S/BA decreases. In the case of S-BA emulsion copolymerizations generally a smaller composition drift is noticed as compared with S-MA. This can be attributed to the smaller difference between water solubilities of S and BA as compared with the system S-MA As a consequence, S-BA batch emulsion copolymerization with 25% S in the monomer feed does oot exhibit the double bend in the conversion time plot, since when S is just depleted the conversion of BA is already high (Figure (6.9a). However, when the styrene content is further lowered (till ca. 142 Chapter 6 10%) the sudden increase is also noticed (Figure 6.9b) at the moment styrene is depleted. Earlier, whlle studyinc emulsifier free S.BA emulsion copolymerization Guülaume et al 22) erroneously attributed this effect to an increase of the average number of radicals per partiele assuming the ultimate model to be valid. Now, with the knowledge that the ultimate model is inadequate to describe S-BA copolymerization kinetics, it bas beoome very clear that a penultimate effect causes a strong increase of the average propagation rate constant resulting (in combination with the composition drift) in the acceleration of copolymerization rate. 1.00 0 0 ++A~ (a) +:AA 0 +A 0 +A 0 + ::c A 0.50 + 0 + A x + A A +A +A A 0.00 0 3600 7200 10800 14400 Time lsl 1.00 0 (b) '!#"' ...... 0 !~ + 0 A + ::c +A 0.50 0 ... x +A 0 " • A• +A... ~A 0 0.00 0 2000 4000 6000 8000 Time (s) Figure 6. 9. Total convenion ( +) and partial conver.sions of S (o) and BA (A) of two S-BA batch emulsion copolymerizations at so•c with (M/W) 0 = 0.2 1 (g/g), a SDS concentration of 0.0116 (mol·L- ), a KzSP8 concentration of 1 1.233 (mmoi·L- ), (a) (S/BA)0 = 0.33 (mol/mol), (b) (S/BA)0 = 0.098 (mol/mol). Copolymerlzatlon Rate 143 6.4 Conciasion In the acrylate rich batch emulsion copolymerizaûons of styrene with methyl acrylate or n-butyl acrylate a strong composition drift is observed. 1be occurrence of a penultimate effect is reflected in the conversion-time curve by a sudden increase in polymerizaûon rate at the moment all styrene is depleted. · This moment can be predicted by SIEMCO by taking into account the readivity ratios and the monomer partitioning, the latter depending upon initial monoroer ratio and monoroer to water ratio. 144 References 1. This thesis: Chapter 7 2. S.C. Misra, C. Pichot, M.S. El-Aasser, J. W. Vanderhoff, J. Polym. Sc~ Polym. Lett. Ed., 17, 567 (1979) 3. M.S. El-Aasser, T. Makgawinata, J.W. Vanderhoff, C. Pichot, J. Polym. Sci., Polym. Chem. Ed., 21, 2363 (1983) 4. S.C. Misra, C. Pichot, M.S. El-Aasser, J.W. Vanderhoff, J. Polym. Sci., Polym. Chem. Ed., 21, 2383 (1983) 5. A Garcia-Rejon, C. Guzman, J.C. Mendez, L. Rios, Chem. Eng. Commun., 24, 71 (1983) 6. G. Arzamendi, J.M. Asua, J. AppL Polym. Sci., 38, 2019 (1989) 7. G. Arzamendi, J.M. Asua, MakromoL Chem., MacromoL Symp., 35/36, 249 (1990) 8. A Guyot, J. Guillot, C. Pichot, L Rios-Guerrero, in "Emu/sion Polymer and Emu/sion Polymerization~ Am. Chem. Soc., Symp. Ser., No 165, 415 (1981) 9. This thesis: Chapter 8 10. M. Nomura, I. Horie, M. Kubo, K. Fujita, J. AppL Polym. Sci., 37, 1029 (1989) 11. M. Nomura, K. Fujita, MakromoL Chem., SuppL, 10/11, 25 (1985) 12. X.Z. Kong, C. Pichot, J. Guillot, Eur. Polym. J., 24, 485 (1988) 13. J. Delgado, M.S. El-Aasser, C.A Silebi, J.W. Vanderhoff, J. Polym. ScL: Part A: Polym. Chem. Ed., 28, 777 (1990) 14. J. Leiza, University of San Sebastián, Spain, personal communication 15. T.P. Davis, K.F. O'Driscoll, M.C. Piton, M.A Winnik, Br. Polym. J., submitted 16. W. Ramirez-Marquez, J. Guillot, MakromoL Chem., 189, 361 (1988) 17. W. Ramirez-Marquez, J. Guillot, MakromoL Chem., 189, 379 (1988) 18. G. Hoedemakers, PhD. Thesis, Eindhoven University of Technology, Eindhoven, The Netherlands, 1990 19. M.S. Matheson, E.E. Auer, E.B. Bevilacqua, EJ. Hart,!. Am. Chem. Soc., 73, 5395 (1951) 20. G.HJ. van Doremaele, AL German, N.K. de Vries, G.P.M. van der Velden, Macromolecules, accepted; Chapter 3 of this thesis 21. M. Buback, L.H. Garcia-Rubio, R.G. Gilbert, D.H. Napper, J. Guillot, AE. Hamielec, D. Hili, K.F. O'Driscoll, O.F. Olaj, J. Shen, D. Solomon, G. Moad, M. Stickler, M. Tirrell, M. A Winnik, J. Polym. Sci.: Part C: Polym. Lett. Ed., 26, 293 (1988) 22. J.L Guillaume, C. Pichot, J. Guillot, J. Polym. Sci.: Part A: Polym. Chem. Ed., 28, 119 (1990) Mlcrostructural Investlgation 145 Chapter 7 Microstructural Investigation of Batch Emulsion Styrene--Acrylic Copolymers Experimental verification of model calculations SUMMARY: Tbe macromolecular mi crostroeture of copolymers cao be characterized in terros of triad fractions and tacticity parameters ·(i.e., intramolecular structure ), and in terros of a three dimensional Molar Mass Cbemical Composition Distribution (MMCCD) (i.e., intermolecular structure). Tbe mi crostcueture obtained is controlled by tbe copolymerization conditions, for instanee by the choice of the reaction system (homogeneous solution or heterogeneous emulsion), the degree of conversion, and the recipe. Computer simulations. of emulsion copolymerization (SIEMCO), accounting for the main chemical and physical processes occurring, provide predictions of the sequence length distributions and MMCCDs of the copolymers. These (MM)CCDs are calculated by considering the conversion heterogeneity (composition drift) as well as the instantaneous (statistical) composition distribution of the copolymers formed. Cross-fractionation (2D chromatography) was used to verify the predicted MMCCDs of the copolymer products. Tbe copolymers are separated according to molar mass by means of Size Exclusion Cbromatography (SEC), and each SEC fraction is subsequendy analysed according to chemical composition by means of either gradient elution quantitative Tbin Layer Chromatography (TLC/FID) or gradient High Performance Uquid Chromatography (HPLC). The difference in water solubility of the two monoroers (styrene (S) and methyl acrylate (MA)) appears to be one of the major factors determining the mi crostmeture of the copolymers. Depending on conversion, monoroer ratio and monoroer to water ratio, the model prediets either single- or double-peaked (MM)CCDs, in full agreement with the experimentally obtained distributions. Furthermore, the sequence length distributions of S-MA copolymers have been determined by means of NMR and are also correcdy predicted by the model calculations. In order to determine the effect of monoroer reactivity ratios and monoroer partitioning on copolymer microstructure, several other emulsion copolymers have been analysed, viz. styrene - n-butyl acrylate, methyl acrylate - n-butyl acrylate, and styrene acrylic acid copolymers. 146 Chapter 7 7.1 Introduetion Copolymer microstructure is one of the key factors determining the final product properties. It is well known that copolymers with the same average chemical composition and molar mass may exhibit different chemical and physical properties depending on the way they have been prepared1>. This may be attributed to differences in sequence distribution and differences in· molar mass chemical composition distribution (MMCCD). The emulsion copolymer microstructure (MMCCD, sequence distribution and tacticity) depends on the recipe and on the process conditions. Important parameters are reactivity ratios and monomer partitioning. The importance of studying emulsion copolymer microstructure is generally recognized. On the one hand, the copolymer microstructure directly reflects the microscopie kinetic events taking place during emulsion copolymerization. On the other hand, the microstructure determines the final product properties. Some progress bas been reported on the development of models descrihing emulsion copolymerization and the molecular 2 microstructure of emulsion copolymers in terms of sequence distribution ,3) and MMCCD4.s>. From existing theories it appears that in general the MMCCD of emulsion copolymers significantly deviates from the one that would be expected on the grounds of the classical copolymerization kinetics in {homogeneous) bulk or solution processes, due to the heterogeneity of the emulsion copolymerization system. Reliable experimental determination of emulsion copolymer microstructure is a prerequisite in any effort to verify the applicability of newly developed models. Recent progressin orthogonal (two dimensional) chromatography (SEC 10 11 HPLC)6·7·8·9> and SEC-TLC/FID • >) bas considerably contributed to an improved analysis of copolymer (MM)CCDs. Unfortunately, in the open literature hardly any attention bas been paid to the experimental MMCCD determination of emulsion copolymers. Practically all papers descrihing the experimental analysis of intra- and intermolecular Mierostructural lovestiption 147 copolymer mierostructuret are dealing with solution or bulk copolymers. However, besides measurements of for instanee monomer conversion and partiele number durlog emulsion copolymerization. the determination of copolymer intermolecular mierostructure (MMCCD) and intramoleeular mierostructure (sequence distribution) will provide information important to a better understanding of the emulsion copolymerization process. This information · is much more detailed and useful than the information merely obtained on the basis of determination of average chemical composition and molar mass. In the literature there are hardly any investigations of the experimental determination of the (MM)CCDs of styrene (S) • methyl acrylate (MA) (batch) emulsion copolymers. As one of the few, Ramirez12> investigated the S.MA emulsion copolymerization using differential scanning calorimetry (DSC) in an attempt to determine the copolymer CCD. Ramirez also used 13C NMR to determine the sequence distribution in terms of triads. However, as will be demonstraled in chapter 8, DSC is not a very powerfut tooi in the accurate determination of CCDs. The resolution obtained is very low and intermolecular interactions may affect the DSC curves. Ramirez performed model calculations of the sequence distribution (in terms of triads) of S.MA batch emulsion copolymers. Unfortunately, these prediedons were compared with a limited set of experimental data (obtained by 13C NMR) only. In this chapter an investigation is presented of the intra- and intermolecular mierostructure of batch emulsion S-MA copolymers. The experimentally determined copolymer microstructures of the emulsion copolymers were compared with the predictions generated by the simulation model "SIEMCO", presented in chapter 4, and considered to be a proof of the validity of several model assumptions. In addition, the mierostructures of several other emulsion copolymers have been investigated, viz. styrene (S) - n-butyl acrylate (BA), methyl acrylate (MA)- n-butyl acrylate (BA) and styrene (S)- acrylic acid (AA). The monomerpairs studied differ in monomer partitioning behaviour and/or in monomer reactivity. The order of the water solubility of the monomers used is 148 Cbapter 7 S < BA < < MA < < AA The effect of different reactivity ratios could be examined separately from any partitioning effects by oomparing the ernulsion copolymer rnicrostructure of the rnonomer pairsS-MA (r, = 0.73, rm = 0.19), and MA-BA (rm = rb = 1.0), having almost identical monomer partitioning behaviour. 7.2 Experimental section Ernulsion copolyrnerizations were carried out at so•c according to the procedures described in detail in chapter 3. ~5208 was used as initiator (1.233 1 1 mmol· L- ), sodiurn dodecylsulfate was used as emulsifier (0.0116 mol· L" ), and 1 NaHC03 was used as buffer (1.223 mmol· L" ). Monomerratio and monomer to water ratio were varied. Normally, n-dodecyl mercaptan was used as chain transfer agent at a concentration of 1 wt% on monomer basis. Prior to (MM)CCD and sequence distibution analyses, the latices were dialysed in order to remove impurities. NMR experiments, however, were carried out on unpurified samples taken frorn the reactor during polyrnerization, which also seiVed for the purpose of dry solid content analyses. The equipment used and experimental procedures foliowed to deterrnine the molar rnass distribution (viz., SEC) and/or the chemical cornposition distribution (viz., 1LC/FID and 13 HPLC )) have been described in chapter 3. 100-MHz 13C NMR was used todetermine the sequence distribution of S· MA emulsion copolyrners at the a triad level. Alternatively, for MA-centred triads it appeared possible to use 400-MHz 1H NMR by applying the rnethoxy proton peak assignment given by Ito14>, assumin~ that the coisotacticity parameter (asm) for emulsion copolyrners is equal to a,m for solution copolyrners (see chapter 3, § 3.2.2). Microstruetural Investigation 149 7.3 Model ealenlations: monomer reaetivity ratlos and monomer partitioning All model predictions were calculated using tbe simulation program SIEMCO described in chapter 4. Comparison of experimental data and model calculations on composition drift, average copolymer composition and sequence distribution, provides a rigorons test of SIEMCO. The composition drift is exclusively determined by tbe recipe, reactivity ratios and monomer partitioning. All these parameters were independently determined and none were adjustable (i.e., determined by some kind of iterative model fitting procedure). The reactivity ratios (chapter 3) and tbe experimentally determined parameters (Kp and Kw) (chapter 5) in behalf of the relations descrihing monomer partitioning, are given in Table (7.1). 1 The concentradon of monomer i in the latex particles [~]p (mol· e ) and 1 its concentradon in tbe aqueous phase [~lw (mol· L" ) are given as functions of tbe mol fraction (Q of monomer i in the monomer dropiets (see chapter 5): (7.1) (7.2) In the absence of monomer dropiets tbe monomer partitioning behaviour is calculated by equation (7.3) obtained by combiDing eq. (7.1) and (7.2): (7.3) The experimentally determined valnes valid for the monomer partitioning in copolymer latices of composition 50/50 (mol/mol) are given in Table (7.1). The valnes of these equilibrium parameters fulfill tbe condition of equal monomer ratios in both organic phases. For other copolymer compositions tbe val u es of tbe equilibrium parameters are different, because (co )polymer composition affects tbe equilibrium totai monomer concentradon in the particles lSO Chapter 7 (see chapter 5). The systems studied fulfill the condition of equal monomer ratios in both organic phases, and also for latices with different copolymer compositions. From model calculations it appeared that the chemical composition and sequence distribution of the copolymer formed is not very sensitive to the exact total monomer swellability of the polymer particles, provided the monomer ratio inside the polymer particles is equal to the monomerratio in the monomer dropiets (chapter 3). Therefore, in Table (7.1) only a limited number of monomer partitioning data are given. Model calculations given in this chapter, however, were performed with the complete data from in chapter 5. The kinetic behaviour of the systems S-MA and S-BA was found to obey the penultimate model15>. Due to a lack of kinetic data the system MA-BA is described by the less complex ultimate model. Table 7.1. Reactivity ratios and monomer partitioni.ng parameters of the emulsion copolymerizations of several monomer pairs. coup Ie monomer r s Kpt> Kp2a) Kw S-MA s 0.73 0.94 8.94 -2.83 0.003 MA 0.19 0.11 6.89 1.18 0.61 S-BA s 0.95 0.90 5.44 0.08 0.003 BA 0.18 0.11 5.17 -0.20 0.011 MA-BA MA 1.0 5.41 3.31 0.61 BA 1.0 7.91 -2.93 0.011 •> values valid for copolymer latices with a composition of 50/50 (moljmol) Microstructural lnvestigation 151 7.4 Experimentally determined emulsion copolymer microstructure in comparison with model ealculations 7.4.1 (Molar mass) chemical composition distrlbution S-MA emulsion copolymers As a result of the difference between the water solubilities of MA and S, in case of emulsion copolymerization, the occurrence and composition of the azeotropic monomer feed depends on the overall monomer to water ratio. But using an overall initial monomer feed ratio of (S/MA)0 = 3 (mol/mol) (this is the azeotropic composition in S-MA solution copolymerization), at an initial monomer to water ratio of (M/W)0 = 0.2 (g/g), little composition drift is observed during emulsion copolymerization. Hence it might be expected that the copolymer formed is homogeneous. In Figure (7.1) the experimental CCD (determined by means of TLC/FID) and the model CCD of this particular emulsion S-MA copolymer are depicted From this figure it can he conc1uded that under these conditions at least up to 90 mol% conversion, the copolymer formed is homogeneons with respect to the chemical composition. In Figure (7.2) the MMCCD of the same high conversion copolymer is given. As expected in this case, the average chemical compositions of all SEC fractions are almost identical. The narrow composition distribution indicates that the polymer particles are the main site of polymerization. However, under different conditions (e.g., higher temperature and higher initiator concentrations) we have found that non-negligible polymerization in the aqueous phase and polymerization inside the very small, precursor particles during the early stages of emulsion polymerization (interval I) may lead to anomalous CCDs, because the monomer ratio then will he different at the various sites of 16 (co )polymerization ). 152 Chapter 7 ~~------· ~ 0: 10 0 a...______.___...::;,_) .J..:...... J 0.0 0.5 1.0 Figure 7.1. CCD of a styrene·methyl acrylate emulsion copolymer prepared Uilder a/most azeotropic conditions. (S/MA)0 = 3 (mol/mol); (M/W)0 = 0.2 (g/g); conversion = 90 mol%; Mw = 160000 (g/mol) TLC\FID: · · · · · , model: --~ R,. (b) 15 (a) 22 Figure 7.2. MMCCD ofa styrene·methyl acrylate emulsion copolymer prepared under a/most azeotropic conditions. (S/MA)0 = 3 (mol/mol); (M/W) 0 = 0.2 (g/g); conversion = 90 mol%; Mw = 160000 (g/mol) (a) experimentally determined by means of SEC-TLC/FID (b) model prediction Microstructural Investigation 153 Under the same reaction conditions, but by applying a different recipe (non azeotropic conditions), asymmetrically shaped and even bimodal CCDscan be obtained. In Figure (7.3) three experimentally determined CCDs are shown of an emulsion copolymer prepared at successively increasing conversions, starting from (S/MA)0 = 0.33 (mol/mol) and (M/W)0 = 0.5 (g/g). The area under the distribution curve bas been taken proportional.to the conversion. The suggestion of bimodality in the chemical composition distribution at 36% conversion is probably due to an artefact (the experimental inaccuracy). As already pointed out in chapter 3, the peak broadening, due to spotting and elution, results in calculated, but physically impossible, negative styrene fractions in the PMA peaks. 10r----r------~ .. ' ::·.~, ., cl 5 I ..;~r: I : :I : ~ : I i . Ij \ OL-~~--~~~~~~--~~~~.· ···--·-·-·..,/< r.• 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Figure 7.3. Experimental CCDs of SMA emulsion copolymers cumulatively obtained at successive total conversion levels: 36 (mol%) : --, 64 (mol%) : ----- and 95 (mol%) : · · · · ·; (S/MA)0 = 0.33 (mol/mol); (M/W)0 = 0.5 (g/g); Mw = 100000 (g/mol). 154 Chapter 7 Neglecting aqueous phase polymemation, which generally comprises less 17 1 19 than 1% of the total polymer formed • s. >, the monomer ratio inside the latex particles, together with the reactivity ratios, governs the instantaneous copolymer composition. The local monomer ratio inside the latex particles is equal to the monomer ratio in the dropiets but differs from the overall monomer feed ratio in the latex ( chapter 5). In the present case, the latex particles will contain more styrene as compared with the overall monomer ratio. Therefore, also tbe copolymer initially formed will be richer in styrene as compared with expectations based homogeneons systems, thus neglecting monomer partitioning. This results in a strong composition drift during polymerization towards compositions richer in the less reactive and more water soluble monomer, i.e., MA, eventually teading to a considerable amount of pure PMA formation at high conversion. This bimodality in copolymer CCD is reflected in the occurrence of two glass transition temperatures due to phase separation: one at circa t5•c characteristic of the. PMA rich domains, and one at a temperature between 15 and 1oo•c depending on the chemica! composition of the (mixed) copolymer peak domain (see chapter 8). By varying the monomer to water ratio one expects a behaviour qualitatively similar to the upper case but quantitatively different due to a changing buffer capacity of the aqueous phase for MA. This effect is very clearly demonstrated in Figure (7.4), wbere the experimental CCDs are shown of two different S-MA copolymers, prepared using the same initia! overall monomer feed ratio but starting from different monomer to water ratios. It is clearly shown tbat a lower monomer to water ratio leads to a 'copolymer peak' more rich in styrene. For conservation of mass, the average composition of the high conversion copolymer must beF, = 0.25 (equal to the initia! monomer feed). As a consequence, a lower monomer to water ratio is bound to result in the formation of more pure PMA at the end of the emulsion copolymerization, because the copolymer formed at low conversion is more styrene rich. Microstructural Investlgation 155 The predicted and observed CCDs are in favourable agreement. The model calculations also predict homopolymerization at high conversion (Figure (7 5)). In Figure (7.5) the PMA peak of the model calculation is represented by a bar of an arbitrarily chosen width of 10% in styrene units, and an area conesponding with the weight fraction PMA. This is done because the originally calculated PMA peaks are too narrow (see chapter 4) to allow a comparison of the model predictions with the experimentally observed PMA peak areas. Several emulsion copolymers were prepared using (S/MA)0 = 0.33 (moljmol) and (M/W)0 = 0.2 (g/g), but applying different n·dodecyl mercaptan (NDM) contents varying from 1 to 8 wt% on the monomer. As was expected, the use of higher NDM concentrations results in a lower molar mass of the copolymer formed. In Table (7.2) tbe SEC results of the emulsion and solution S.MA copolymers are given in order to show the results obtained with UV and refractive index detection. The UV (254 nm) detector is only sensitive to styrene units and not to methyl acrylate units, whereas refractive index decteetor is sensitive to both monomeric units. Therefore, the ratio of both SEC decteetor signals is a measure of the average cbemical composition at each molar mass. Compared with UV detection, refractive index detection results in higher calculated molar masses of the emulsion copolymers given in Table (7 .2). This indicates an increase of average MA content on increasing molar mass of the copolymer molecules. Without any further knowledge this could be attributed to a gradual drift in copolymer composition along molar mass. However, it is clear from Figure (7.6a) that the microstructure is more complex. The emulsion copolymer bas a bimodal (MM)CCD, wbere PMA bas a significant higher molar mass than the copolymer. This could be attributed to an earlier depletion of NDM. This result emphasizes tbe validity of the cross·fractionation method. 156 Cbapter 7 cl 5 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 F s (-) Figure 7.4. Experimental (TLC/FID) CCDs of two S-MA emulsion copolymers (conversion = 95 (mol%)) preparedat the same monomerratio (S/MA)0 = 0.33 (mol/mol), but at different monomer to water ratios: (M/W)0 = 0.05: -- - - -, and 0.5: (g/g). rr~ 5 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Fs Figure 7.5. CCD of a high conversion (97 mol%) S-MA emulsion copolymer. (S/MA)0 = 0.33 (mol/mol), (M/W)0 = 0.05 (g/g) and Mw = 330000 (g/mol). Model(······-) and experimental (TLC/FID) (--). Microstruetural Investigation 157 Table 7.2 Resu/ts of UV and refractive index deleetion in SEC meosurements on several emulsion S-MA copolymers, all prepared with (S/MA)0 = 0.33 (mol/mol) and (M/W}0 = 0.2 (g/g), but with various NDM concentrations. M,. (gfmol) Mw (g/mol) %NMD uv Refr. uv Refr. 1 27000 32400 70384 79800 4 9500 10200 18600 23200 8 7500 7900 12200 14500 R., IS (a) Figure 7.6. Experimental (a) and model (b) MMCCD of a high conversion (95 mol%) S-MA emulsion copolymer. (S/MA)0 = 0.33 (mol/mol), (M/W}0 = 0.5 (g/g), 1 wt% NDM, 5o•c, and Mw = 110000 (g/mol). 158 Chapter 7 The reactivity ratios of S-BA copolymerization are comparable with those of S-MA. but not exactly equal (see Table (7.1)). In contrast to the system S MA. where MA is moderately water soluble, both S and BA have a very small water solubility (see chapter 3). Hence, the monomer to water ratio will have a negligible effect on the monomer partitioning. Figure (7.7) shows the CCD as determined by means of HPLC, and the CCD model calculation of a high conversion (98 mol%) batch emulsion S-BA copolymer with (S/BA)0 = 3 (mol/mol) and (M/W)0 = 0.2 (g/g). 15 I' 10 ~~ I ~ I a: I I I 5 I I \ 0 l 0.0 0.5 1.0 Fa Figure (7.7). The CCD detennined by means of HPLC and a model caJcu/ation of the CCD of a batch emulsion S-BA copolymer. (S/BA)e =3 (mol/mol), (M/WJe = 0.2 (g/g), conv = 98 mol%, and Mw = 496000 (g/mol); = experimenta~ • • • • = model. Microstructural Investigation 159 As already demonstrated in chapter 6, when applying oomparabie recipes (as for monomer ratio and monomer to water ratio) the composition drift durlog S BA emulsion copolymerization is generally less strong as compared with the S MA emulsion copolymerization. As a consequence, S-BA emulsion copolymers generally will be somewhat more homogeneous thanS-MA emulsion copolymers. · Nevertheless, the in Figure (7.7) plottedCCDof the S-BA (25/75) copolymer is somewhat broader than the CCD of the "azeotropic" S-MA (25/75) copolymer given in Figure (7.1). This can be explained by the fact that on the basis of reactivity ratios this monomerratio (S/BA)0 of 3 (mol/mol), is not exactly the azeotropic composition forS-BA, in contrast to (S/MA)0 = 3 (molfmol) forS MA Related work bas been recently publisbed by Guillotzo>, who demonstrated that S-BA emulsion copolymers with 70 mol% BA are heterogeneous and exhibit two glass transition temperatures. S-AA emulsion copolymers Batch emulsion copolymers of S-AA are expected to be very heterogeneous. The S-AA batch emulsion copolymer microstructure is almost completely govemed by the extreme difference in water solubility. As a consequence, at the beginning of the reaction the latex particles will be relatively S rich as compared with the overall monomer ratio. Depending on monomer ratio and monomer to water ratio, a strong composition drift is therefore expected to occur until at the end, after depletion of S, AA will homopolymerize. Moreover, aqueous phase polymerization of AA cannot be ruled out bere. Figure (7.8) shows the expected heterogencity in the CCD determined by means of HPLC of a S-AA batch emulsion copolymer. Because polyacrylic acid (PAA) cannot be quantitativily detected by UV (260 nm) durlog HPLC analysis, the amount of P AA (equivalent to all AA rich copolymer) bas been calculated from the S rich part ( > 30 mol% S) ofthe CCD and the average chemical composition (i.e., F, =0.75) determined by means of a titration method. 160 Chapter 7 No chain transfer agents were used, because oligo,mers in the copolymer would strongly disturb the CCD determination by means of HPLC (molar mass dependent retention time). A more detailed investigation of S-AA emulsion copolymer microstructure will be given in a separate paper1>. 15 .-.------~ 10 a: ll 5 0 rh 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Figure 7.8. CCD detennined by means of HPLC and titration of a S-AA batch emulsion copolymer. (S/AA)0 = 3 (mol/mol), (M/W)0 = 0.2 (g/g), conversion = 98 mol%. MA-BA emulsion copolymers Copolymers of methyl acrylate and n-butyl acrylate could oot be analysed according to chemical composition, neither by TLC/FID, due to a lack of reproducibility, nor by HPLC equipped with merely UV detection. This obliged us to merely determine the average chemical composition of these MA-BA copolymers by means of 1H NMR. Figures (7.9a-c) depiet the effect of initial monomer ratio and monomer to water ratio on the cumulative copolymer composition as a function of conversion. Experimentally determined cumulative average copolymer compositions are compared with model calculations. Mierostruetural lovestiption 1,1 Monomer to water ratio was found to strongly affect monomer partitioning. Because bath reactivity ratios of MA and BA are equal to one, eomposition drift is only caused by the monomer partitioning and not by the intrinsic reactivity of the monomers. These results exhibit a satisfying agreement between model prediedons and experimental values. This is an additional proof for the reservoir behaviour for MA of the aqueous phase. The small discrepancy at very low monomer to water ratios eould indicate the existence of a smalt extent of aqueous phase polymerization. 1.00,------, 1.00,------~ -----:----.------+------.------.-----~.--- I oso ------t~.----~~:------·- I -----.------ET-----~- u.' . u.,""" ·"----"------"'- (a) (b) o.ooL-..~--~---...... J 0.00 1.00 T ota1 mol conversion (-) Figure 7.9. Effect of monomer to water ratio on the average ... + + --;-- cumulative MA-BA emulsion 1 '"'"r------: ------:---:--~6.----·-- copolymer composition 1 determined by means of H NMR u.~"""~ r Q o as a function of conversion. l (a) (M/W)0 = 0.5, (b) (M/W)0 (e) = 0.2, (c) (M/W)0 = 0.05 (g/g). Initia/ monomer composition: Total me:» conversicn (-) 25% BA (o), 50% BA (A), 75% BA ( +). The drawn lines represent the model predictions. 162 Chapter 7 7.4.2 Sequence distribution of 8-MA emulsion copo~rs The intramolecular microstructure of S-MA batch emulsion copolymers was studied in terms of triad fractions. In Figure (7.10) the predicted and experimentally observed cumulative copolymer compositions have been plotted versus conversion for an emulsion copolymerization with (S/MA)0 = 0.33 (mol/mol) and (M/W)0 = 0.5 (g/g). In Figures (7.11a,b) the theoretically predicted triad fractions are shown (lines) tagether with the measured triad fractions applying the 1H and 13C NMR peak assignments as described in chapter 3. The excellent agreement beween experimental results and model calculations (without any adjustable parameters!) supports the validity of some assumptions made in the emulsion oopolymerization model with respect to copolymer sequence distribution. It indicates that the individual propagation rate constauts are the same as in bulk or solution copolymerization. Intbis particular experiment, at circa 60 mol% conversion all styrene already bas beèn depleted, and after this moment almost pure polymethyl acrylate (PMA) was formed, thus resulting in a strong increase of the cumulative MMM triad fractions, at the expense of the cumuialive MMS and SMS triad fractions. Because almost no free styrene was present after 60 mol% oonversion, the cumulative styrene-centred triad fractions remain constant. In Figure (7.12) the cumulative MA-centred triads are given as a function of conversion for two batch emulsion S-MA copolymers prepared using different monomer feed ratios. Also under these oonditions the experimentally observed MA-centred triads calculated from 1H NMR data are in excellent agreement with the theoretical values. Mierostruetural lnvestigation 1'-3 1.0 ..------. I 0.5 ro'----"o..__".:__:o~o~2-.Q~-..n0 u.'" 0 0 0.0 L---~-~____. _ _,__..___,__...... _---''--~----1 0.0 0.5 1.0 Total mol conversion (-) Figure 7.10. Cumulative emulsion copolymer composition as a function of conveTSion. (S/MA)0 = 0.33 (mol/mol), (M/W) 0 = 0.5 (g/g). 1H NMR (o), the line represents the model calculation. I 1.o ,------, (b) ~ - _._---. "' ---·--- Eo.s" I ------·------~------· ~ 0.0 1::::==:::±::===:::><==-...J o.o 0.5 1.0 Total mol converston (-1 Total mol conversion {-) Figure 7.11. Cumulative emulsion copolymer triad fractions as a function of conversion. (S/MA)0 = 0.33 (mol/mol), (M/W)0 = 0.5 (g/g) (a) MA-centred triads: MMM (A), MMS ( +), SMS (o) (b) S-centred triads: SSS (!), SSM (+), MSM (•) open symbols: 1H NMR; closed symbols: 13C NMR; lines: model calculations. 164 Chapter 7 I 0 1.0.------., g (b) ~ Lo _=__.è"!:_=--='~±=_ .. _.!bcL=-~cL=-~-~=-~~-==-·~: _ _j 6 16 1.0 ::; 0.0 0.5 1.0 Total mol conversion (-) Total mol conversion (-) Figure 7.12 Cumuialive emulsion copolymer MA-centred triad fractions (MMM (A), MMS(+), SMS (o)) determined by means of 1H NMR as a fuilction of conversion. (a) (S/MA)0 = 0.33 (mol/mol), (M/W)0 = 0.2 (gjg); (b) (S/MA)0 = 1 (mol/mol), (M/W)0 = 0.2 (g/g). The lines represent the model calculations. In Table (7.3) the effect of initial monomer ratio, and in Table (7.4) the effect of monomer to water ratio on the triad fractions of high conversion S MA batch emulsion copolymers are demonstrated. From these data it appears that an increase in the initial S/MA ratio increases SSS and SMS and decreases MMM and MSM triad fractions. For (S/MA)0 = 0.33 (mol/mol), at decreasing initial monomer water ratio (M/W)0 a stronger composition drift results in a more heterogeneons (high conversion) copolymer, indicated by higher SSS, SSM and MMM fractions and lower MMS and MSM fractions. The SMS triad fraction, however, appears to be almost independent of the monomer to water ratio. For all copolymers there is excellent agreement between the model calculations and the experimentally determined triad fractions. Mierostruetural lnvestlption 165 Table 7.3. Experimental and model values of the cumulative average chemica/ composition and triad fractions of S-MA high conversion batch emulsion copolymers all prepared at (M/W)0 = 0.2 (g/g), but at various (S/MA)0 ratios. (S/MA)0 conv. F, MMM MMS SMS sss SSM MSM (mol/mol) mol% % % % % % % % 1/3 93 exp. 25.2 60.6 23.7 15.7 5.4 30.0 64.6 mod. 26.7 55.6 27.6 16.7 3.2 26.8 70.0 1 78 exp. 62.2 3.2 28.1 68.7 19.4 48.4 32.2 mod. 58.5 3.5 28.8 67.7 17.9 47.5 34.6 3 90 exp. 78 0 15.0 85.0 50.7 40.4 8.9 mod. 76.9 0.3 10.2 89.5 51.5 40.3 8.2 Table 7.4. Experimental and model values of the cumulative average chemica/ composition and triad fractions of S-MA high conversion batch emulsion copolymers all prepared at (S/MA)0 = 0.33 (mol/mol), but at various (M/W)0 ratiosin comparison with a high conversion solution copolymer. (M/W)o conv. F, MMM MMS SMS sss SSM MSM (g/g) mol% % % % % % % % soluûon 95 exp. 28.3 46.2 38.1 15.7 5.0 24.2 70.8 mod. 26.2 52.7 32.4 14.9 1.9 21.6 76.5 0.5 95 exp. 25.0 57.3 28.6 14.1 2.8 23.5 73.7 mod. 26.2 54.4 30.1 15.5 2.3 23.8 73.9 0.2 93 exp. 25.2 60.6 23.7 15.7 5.4 30.0 64.6 mod. 26.7 55.7 27.6 16.7 3.2 26.8 70.0 0.05 97 exp. 25.7 71.4 12.1 16.5 13.7 42.8 43.5 mod. 25.5 69.5 14.0 16.5 11.9 40.6 47.5 166 Chapter 7 7.S CoDCiusions Copolymer analysis by means of SEC-TLC/FID is a powerful tooi in the experimental determination of the (MM)CCD of copolymers in which the two monomer units moderately differ in polarity. This metbod was successfully applied to styrene-methyl acrylate batch emulsion copolymers. The effects on copolymer microstructure of different water solubilities of the monomers in combination with varying monomer to water ratios, were correctly predicted by the model. The model predictions of sequence distrlbutions in terms of triads of S-MA batch emulsion copolymers were experimentally verified using 13C NMR. Moreover, a comparison between 1H NMR data on the one hand and 13C NMR data and model calculations of MA-centred trlad fractions, on the other hand, demonstrales that altematively 1H NMR can be used for the determination of MA-ceritred trlads provided the Ito-Yamashita assignment is employed under the assumption that the coisotacticity parameter is independent of the (radical) polymerization process used (solution versus emulsion). The effects of monomer reactivity and monomer partitioning on copolymer microstructure can be very well predicted by the model and have been verified by microstructural analyses of S-BA, MA-BA and S-AA emulsion copolymers using SEC-HPLC and 1H NMR. When modeHing the emulsion copolymerization process of water soluble monomers it must be taken into account that the aqueous phase is a monomer reservoir strongly affecting copolymer microstructure. Relerences 167 1. M. Lambla, "Proceedings of the CoUoque International sur les Copolymères en Emu/sion", Lyon. 156, 1984 2. MJ. Ballard, D.H. Napper, R.G. Gilbert, l Polym. Sci, Polym. Chem. Ed., 19, 939 (1981) 3. M.F. Uauro, C. Pichot, W. Ramirez, J. Guillot, Polym. Mat. Sci Eng., 54, 613 (1986) 4. E. Giannett~ G. Storti, M. Morbide~J. Polym. Sci.: Part A: Polym. Chem. Ed., 26, 1835 (1988) 5. E. Giannetti, G. Storti, M. Morbide~ l Polym. Sci.: Part A: Polym. Chem. Ed., 26, 2307 (1988) 6. S. Teremachi, A Hasegawa, Y. Shima, M. Akatsuka, M. Nakajima, Macromolecules, 12, 992 (1979) 7. G. Glöckner, in ~dvances in Polymer Science", Springer-Verslag, Heidelberg, 79, 159 (1986) 8. G. Glöckner, J.H.M. v.d. Berg, J. Chromatogr., 384, 135 (1987) 9. S. Mori, AnaL Chem., 60, 1125 (1988) 10. J.CJ.F. Tacx, J.L. Ammerdorffer, AL. German. Polymer, 29, 2087 (1988) 11. J.CJ.F. Tacx, AL. German.J. Polym. Sci.: Part A: Polym. Chem. Ed., 27, 817 (1989) 12. W. Ramirez-Marquez, Ph.D. Thesis, University Oaude Bernard, Lyon. France (1987) 13. R.W. Sparidans, H.A Claessens, G.H.J. van Doremaele, AM. van Herk, l Chromatogr., 508, 319 (1990) ' 14. K. Ito, Y. Yamashita, J. Polym. Sci.: Part B: Polym. Lett. Ed., 3, 637 (1965) 15. T.P. Davis, K.F. O'Driscoll, M.C. Piton. M.A Winnik, Br. Polym. J., submitted 16. G.HJ. van Doremaele, AM. van Herk, J.L. Ammerdorffer, AL German. Polym. Commun., 29, 299 (1988) 17. G. Arzamen~ J.M. Asua, MakromoL Chem., MacromoL Symp., 35/36, 249 (1990) 18. M. Nomura, U.S. Satpathy, Y. Kouno, K. Fujita, l Polym. Sci.: Part C: Polym. Lett. Ed., 26, 385 (1988) 19. R.N. Mead, O.W. Poehlein. /nd. Eng. Chem. Res., 28, 51 (1989) 20. J. Guillot, Makromol. Chem., MacromoL Symp., 35/36, 269 (1990) 21. G.HJ. van Doremaele, R.L. Adolphs, R.W. Sparidans, AL. German. in preparati on 168 Copolymer Composition Control Chapter 8 Copolymer Composition Control by means of Semi-Continuons Emulsion Copolymerization Comparison of various monomer addition strategies SUMMARY: The effect of two different monomer addition strategies on the copolymer chemical composition distribution in the (seeded) semi continuous emulsion copolymerization of styrene and methyl acrylate bas been investigated. (1) The addition of a mixture of the monomers at a constant feed rate results in a homogeneons copolymer, provided that extremely long process times are used (starved conditions). When not starting from a seed latex, aqueous phase polymerization was observed at low feeding rates. Using a seed latex, aqueous phase polymerization is negligible. (2) In order to achieve an optimal monomer addition pattern, a pragmatic approach is presented. An optimal monomer addition profile is be calculated from accurate monomer partitioning data and by application of a short iteration procedure. Gradient High Performance Uquid Chromatography (HPLC) was successfully applied as a rigorous test of the homogeneity of the copolymer formed. The information obtained contains much more detail than in case of using differential scanning calorimetry or 1H NMR. In contrast to 1H NMR, that provides only average cumulative chemical compositions, the complete chemical composition distribution can be obtained by means of HPLC. 8.1 Introduetion The semi-continuous (sometimes called semi-batch) emulsion copolymerization process is widely used in industry. Tbe main advantages of this process as compared with conventional emulsion batch processes include a convenient control of emulsion polymerization rate in relation with heat removal, 170 Chapter 8 and control of partiele morphology and chemical composition of tbe copolymer. The latter is important in tbe preparation of specialty or high performance polymer latices. Semi-continuous emulsion copolymerization processes can be performed by applying various monoroer addition strategies. The most widely investigated and described procedure is tbe addition of a given mixture of the monoroers at a constant rate (sometimes pre-emuJsified monoroers are added)W.4.SA?,8.9). For instance, this procedure is foliowed in many papers dealing witb tbe semi-continuous emulsion copolymerization of vinyl acetate and 1 4 n-butyl acrylate o,n,u,t3,t ,1S). Witb respect to the monoroer addition rate two main situations can be distinguished. (1) Flooded conditions: the addition rate is higher tban the polymerization rate. (2) Starved conditions: tbe monoroers are added at a rate lower tban tbe maximum attainable polymerization rate (if more monoroer would be present). The latter process (starved conditions) is aften applied for tbe preparadon of homogeneaus copolymers. Sametimes semi continuous processes witb a variabie feed rate (power feed) are used to prepare latex particles witb a core-sheU morphologyl6). One of the main problems of tbe semi-continuous process performed under starved conditions is tbe extremely long reaction time required for tbe preparadon of homogeneaus copolymers. A more advanced metbod is tbe semi 14 17 1 19 continous process performed in a controlled composition reactor • • a, .20>. The overall monoroer concentradons then have to be monitored by means of e.g., on line GLC. The monoroer concentradons are kept at the constant level required to obtain a desired copolymer composition by controlled feeding of the separate monoroers into tbe reactor. Due to scatter of the GLC data it is usually difficult to maintain constant levels of the monoroer concentrations in tbe reactor. Furthermore, tbe required optimal overall monoroer concentradons may change, because the volume ratio of organic to aqueous phase continuously changes and subsequently tbe monoroer partitioning changes:m. A metbod described in Uterature by Arzamendi and Asua%2,23> is tbe so-called optimal monoroer addition strategy. By using this metbod Asua demonstrated that witbin a relatively short period of time homogeneaus vinyl acetate (V Ac) - methyl acrylate (MA) emulsion copolymers can be prepared in spite of the large Copolymer Composition Control 171 difference between tbe pertaining reactivity ratios. Tbe reactor was initially cbarged witb all of tbe less reactive monomer (viz. VAc) plus tbe amount of tbe more reactive monomer (viz., MA) needed to initially form a copolymer of tbe desired composiûon. Subsequently, tbe more reactive monomer (MA) was added at a (time variable) flow rate in sucb a way as to eiiSUI'e tbe formation of a bomogeneaus copolymer~ However, it sbould be realized tbat despite all efforts · genuinely predictive modeDing of emulsion copolymerizaûon is not yet possible and it is also unlikely tbat sucb models will beoome available in tbe near future. However, modeDing witb the use of a comparaûvely small number of strategie measurements is currently possible. Tbe time dependent average number of radicals per partiele ( ii ( t)) and tbus polymerlzation rate and optimal monomer addiûon profiles still cannot be predicted in advance. Generally, ii(t) is determined by tbe entry rate and exit rate and tbe bimolecular termmation rate of free radicals. These values cannot be predicted witbout several strategie experiments. Arzamendi22.23> tided over tbis difficulty (unknown values of ii(t)) by applying a semi-empirical metbod tbat oomprises a series of semi-continuous emulsion oopolymerizations, used to oorrelate ii with tbe volume fraction of polymer in tbe latex particles (;~ tbus finding tbe time-dependenee of ii (t ). In case of a high ii system (viz. ii > 0.5), ii ( t) strongly depends on ;P, because an increase in ;P results in a higher viscosity and a lower terminadon ra te ( dirfusion oontrolled). In low ii systems (viz. ii < 0.5), in the limit, wbere dilfusion witbin tbe partiele is rate determining, the exit rate and tbus ii will be proportional to (Dp/;p'')'24>. Intbis chapter a more pragmatic approach is presented. Tbis metbod can be applied without actually calculating ii(t) or ii(;p). Tbe emulsion oopolymerization ofstyrene (S) and methyl acrylate (MA) is known to often produce bigbly heterogeneaus copolymers25> and is generally a low ii system (ii <0.5)26>.In tbis chapter an investigation is presented on tbe preparadon of homogeneaus S-MA emulsion copolymer by applying seeded semi-continuous emulsion oopolymerization. Rathertban a large difference between reactivity ratios (VAc-MA), the large difference between tbe water solubilities of S and MA is the main problem bere (chapter 7). In order to evaluate the results, tbe copolymer formed was not 172 Chapter s merely analysed by means of 1H NMR according to average cbemical composition. In addition high performance Uquid cbromatograpby (HPLC) provided more detailed microstructural information (viz. cbemical composition distnbution, CCD) of the copolymer formed. Such .detailed information is a necessity, because the chemical composition measured by means of 1H NMR is only an average value of the cumuialive copolymer composition, and the composition of the newly formed copolymer is blurred by the copolymer formed earlier in the process and by the copolymer of the seed latex particles. The latter is the main reason wby in this investigation very small seed latex particles have been used. 8.2 Experlmental seetion Monomer partitioning, reactor equipment and emulsion copolymerization procedures have been comprehensively described elsewhere%7). The seed latex was preparedat 92"C under starved conditions according to the recipe given in Table (8.1). After flitration in order to remove coagulum, the seed latex was used without further puriftcation. The seed latex partiele size was measured by means of dynamic light scattering. The obtained z-average diameter was ca. 30 nm and the weight-average diameter was ca. 17 nm. Final solids content of the latex amounted to 7.8 wt%. The recipes of the constant addition rate experiments and the optimal addition rate experiment are given in Tables (8.2) and (8.3), respectively. All semi-continuous experiments were carried out at so•c ± 0.2. In the case of the experiments leading to the optimal addition rate proftle, all ingredients, except for part of the styrene, were initially cbarged into the reactor. The remaining styrene was continuously added into the reactor according to one of the calculated monomer addition proftles. The overall monomer ratio was monitored by means of on-line GLC and the conversion was determined by means of dry solid content analysis. The total volume of all samples taken during the entire course of the reaction was always less than 4% of the total volume. The appropriate corrections were made in the calculation Copo)Jmer Composition Control 173 of the conversion. The volume of the samples taken from the reactor awses a sHght error in the monomer addition profile. Although it is possible to adjust the monomer addition profile for the deercase in volume of the emulsion (wben samples are taken with constant volume at fixed intervals), this minor effect was neglected in all experiments. Partiele size measurements were carried out by means of dynamic light scattering and TEM. The determination of the copolymer composition distribution by means of gradient HPLC bas been described elsewhere27). The intramolecular microstructure of the copolymer was analysed by means of 1QO-MHz 13C NMR27). Table 8.1. Recipe used for preparing the seed latex. initial charge discontinuons continuons (g) addition (g) addition (g/h) every lh hour during 4lh hours H20 990 MA 17.638 s 7.112 NDM 0.25 SOS 4.20 KzSzOs 2.31 0.15 NaHC03 0.77 0.05 Table 8.2. Recipe used in the unseeded starved reactions with constant feed rate. initial charge (g) feed (g) during 4, 8 or 32 bours H20 600 MA 85.5 s 34.5 NOM 1.2 SOS 2.0 ~:Ps 0.20 NaHC03 0.067 174 Chapter 8 Table 8.3. Redpe used in the seeded semi-continUous experiments with calculated alldition profile. initial charge (g) feed (g) according to calculated addition profile H20 900 MA 127.4 s 8.0 43.3 NDM 1.4 0.43 SDS 3.0 ~~Os 0.30 NaHC03 0.10 Seed latex-> 16.7 •> see Table (8.1) 8.3 Results The possibility bas been investigated to prepare homógeneous copolymers by means of a convenient semi-continuous process. The primary aim is to prepare a homogeneous high conversion emulsion copolymer (25 mol% S and (M/W)0 = 0.2 (g/g), i.e., ca. 17 wt% solid in finallatex). As demonstrated in chapter 7 a latex with the same average chemical composition and solids content prepared in batch, is very heterogeneaus in chemical composition. A seed latex is used for reproducibility reasons. As can beseen in Table (8.3), the amount of copolymer, charged as seed copolymer latex particles into the reactor, is negligible. Copolymer Composition Control 175 8.3.1 Semi-continuons emulslon copolymerizations with constant addition rates 1.00 o.ao 0.60 ...... Î x 0..40 0.20 0.00 0.00 0.20 0.40 0.60 o.ao 1.00 t/t* (-) Figure 8.1. Effect of monomer addition rate on total conversion (xJ venus normalized time (!l't*) of unseeded semi-continuous S-MA emulsion copolymerizations. t = addition time (reaction time): 4 hours (A), 8 hours (o), 32 hours ( +); the dotted line ( · · · · · ·) represents 100% instantaneous conversion. In Figure (8.1) it is demonstrated that lower monomer addition rates result in more starved conditio.ns (i.e., higher instantaneous conversions). Homogeneons copolymers can only be prepared under extremely starved conditions. Too fast addition rates lead to inhomogeneons copolymers as can clearly beseen in Figures (8.2a and b). In this Figure the fractional conversions of S and MA are given as a function of time of the 8 and 32 hours starved reactions. Moreover, the instantaneous average cumulative copolymer composition (FJ bas been plotted versus time. It is clear that the initial copolymer is relatively MA rich. This can be explained by a non-negligible amount of polymerization (of MA) in the aqueous phase in the very early part of the reaction when no latex particles have been formed yet. As soon as particles have been formed, polymerization in the particles will dominate aqueous phase polymerization. In the particles relatively S rich copolymer is 176 Chapter 8 formed due to the fact that the latex particles fotmed are relatively S rich, because a significant part of MA added is dissolved in the aqueous phase, whereas the styrene added is almost completely absorbed by the parûcles. One additional 8 hours experiment was carried out in the presence of seed latex parûcles. In that case no initial MA rich copolymer was observed indicating the absence of a significant aqueous phase polymerization. 1.00 I 6 I ïn 0.150 Qj ~ 2 4 & Time (h) 1.00 I 6 '(ij I j 0.150 8 16 24 Time (h) Figure 8.2 Fractional conversions of S (o) and MA ( +), and the average copolymer composition (FJ (A) calculated from GLC data of an unseeded semi-continuous emulsion copolymerization with a constant feed rate. (S/MA)ft!ttd = 0.33 (mol/mol) and (M/W) = 0.2 (s/g) process time: (a) 8 hours; (b) 32 hours. As expected under starved conditions homogeneons copolymers can only be prepared when applying extremely long feeding times. In §8.3.2 a strategy bas been described by which this problem can be overcome. Copolymer Composltion Control 177 8.3.2 Semi-continuons emulsion eopolymerization with optimal addition prome Strategy foUowed to achieve optimal addition profile All ingredients are charged into the reactor, except for a part of the most reactive monomer (i.e., styrene). lbe model SIEMajS>, the reactivity ratios · given in chapter 3, and the monomer partitioning data given in chapter 5 are used to calculate the amount of the most reactive monomer to be charged initially into the reactor in order to obtain the desired local monomer ratio qP = (S/MA)p inside the latex particles. This particular ratio is related to the copolymer of the desired chemical composition (FJ according to the instantaneous copolymer equation (eq. (8.1)). In this case F, = 0.25, corresponding with qP = 0.087 (mol/mol). (8.1) A first estimation of the semi·continuous (co )polymerization rate is made by SIEMCO assuming an arbitrarily chosen value of ii(t) (e.g., 0.5) throughout the reaction, while the most reactive monomer (S) is added at a time dependent rate in order to maintain the optimal monomer ratio inside the particles. Three functions are calculated (fitted as polynomials); the subscripts indicate the number of iteration steps. (1) Instantaneous molar conversion (Xmst) versus time (dependent on ii(t)) : Xmst = fo(t) (8.2) the first estimation of the conversion-time plot bas subscript 0, the first experimentally obtained conversion·time plot bas subscript 1. 178 Chapter 8 (2) Calculated ideal amount of added styrene (As)versus instantaneous molar conversion : (8.3) This tunetion is only dependent on reactivity ratios and monomer partitioning and is depicted in Figure (8.3). The bend corresponds to the disappearance of the monomer droplets. The offset at 0% conversion represents the initially charged styrene. 1.00 0.80 Î 0.60 -i (/) <{ 0.40 '(/) <{ 0.20 di~ of droplets 0.00 0.00 020 0.40 0.60 0.80 1.00 xinat (-) Figure 8.3. Normalized optima/ addition profile of styrene versus the instantaneous conversion calculated from the monomer partitioning parameters gi.ven in chapter 5 and allowing to prepare the desired homogeneous emulsion copolymer (25 mol% S, soüd content of latex • 17 wt%). As101 = total amount of styrene to be added (g). (3) Estimated amount of styrene to be added during the ftrst experiment as a function of time : (8.4) Copolymer Composltion Control 179 It should be noticed that the semi-continuous emulsion copolymerization bas an optimal addition profile when styrene is added according to A. = A.· = g(Xfut). However, at this stage of the procedure polymerization rate is not known and cannot be reliably calculated beforehand. First of all. the time dependent polymerization rate under the experimental conditions must be determined in order to obtain this optimal addition rate profile. This is accomplished by · applying an iteration procedure (Fig. (8.4)). Model curve of amou'lt of estimated conversion-tima styrene versus conversion plot with e.g. n(t) = 0.5 A. = g(xl x = f 0(t) i:=i+1 NO YES OPTIMAL MONOII.ER AODITION PROFILE Figure 8.4. Outline of the semi-empirica/ iteration procedure to develop the optima/ monomer addition profile. 180 Chapter 8 The first semi-continuous experiment is carried out by applying the calculatèd styrene addition profile based on an estimated (constant) value of ii(t) = O.S. Generally speaking, it would be highly fortuitous if this first estimatèd addition profile would be optimal, because the average number of radicals will generally deviate from its first estimation (i.e., ii = 0.5), ;md may also depend on conversion (monomer concentradons in the particles). Furthermore, partiele number and average propagation rate constant may deviate from the values assumed to be valid in the calculations. From the experimental conversion-time data of this first experiment a new conversion time curve is fitted: x. = f1(t). Subsequently, for the next experiment an addition profile eloser to (generally not exactly equal to) the optimal one is calculatèd by taking A. = g(~ns~) = g(f1(t)) = ~(t). In this manner x. is eliminated by combiDing the experimental conversion-time curve with the ideal curve of monoroer addition versus conversion. So in fact, a number of parameters of whicb the valnes are uncertain (including ii, partiele number and average propagation rate constant as a function of time) do not have to be known. Thus, by including these effects the calculated value of ii no longer purely represents the average number of radicals per partiele (as a function of conversion), but becomes a more complex curve-fitting parameter. The next · semi-continuons emulsion copolymerization then is carried out by applying this second addition profile of S. The entire procedure is continued until the last monoroer addition proflle is equal to the subsequently calculated monoroer addition proflle within a chosen toleranee region to be applied in the following experiment(~ = ~. 1 ± tolerance). 'Ibis condition implies that during the last experiment S was optimally added according to A. = g(~nst) and consequently also the experimental conversion time curves must be equal (fi = ~+t ± tolerance). Due to the fact that ii depends on the local monoroer ratio inside the particles and that the monoroer ratios may differ at equal conversions of the successive iteration steps, the number of required iteration steps is generally larger than one. The present system converges rapidly, only four iterations being required in S-MA emulsion copolymerization to arrive at (visibly) indistinguishable monoroer addition versus time curves. Copolymer Composltlon Control 181 Experimental application of the developed procedure The very rapid convergence of the iterative procedure is depicted in Figure (8.5) showing the conversion time plots at successive iteration steps. Only four steps were necessary to achieve the criterion that the monomer addition profile is essentially constant with increasing iteration steps. i.e., the optimal monomer addition profile bas been reached. According to the optimal addition profile styrene bas to be added very slowly toward the very end of the polymerization due to the buffering behaviour of water for methyl acrylate. However, in order to reduce process time, this phenomenon had to be neglected and after 4 hours addition time all remaining styrene was added at once and polymerization was allowed to continue during one more hour, resulting in a 5 hours reaction time. The obtained optimal addition profile of styrene is given in Figure (8.6). Applying this optimal monomer addition profûe, two experiments have been carried out: one experiment without and another one with an intentionally created induction time. In Figure (8.7) the conversions and average copolymer composition of both experiments have been plotted versus time. 1.00 0.80 ...... l ().60 x 0.40 0.20 0.00 0 3600 7200 10800 14400 Time {s) Figure 8.5. Experimental conversion time CUJVes of jour seeded sem.i continuous emulsion S-MA copolymerizations at jour successive iteration steps. 11 1 :--, zul: · · · · · ·, ytJ: -·-····, .fh and optimal: -----. 182 Chapter 8 1.00 0.80 Î...... 0.60 § Cl) ...... 0.00 0 3600 7200 10800 14400 Time (s) Figure 8.6. Optimal monomer addition profile, for the described semi continuous emulsion copolymerization, obtained after 4 iteration steps. I u. .. u.'" Time {sl Time lsl Figure 8. 7. Experimentaltotal molar conversion (A), partial conversions ofMA ( +) and S (o) and average cumulative chemica/ composition of the copolymer determined from GLC data (<>), and by means of 1H NMR (a), prepared according to the optimal addition profile. (a) no induction time; (b) induction time of 600 s (indicated by tliTow). Copolymer Composition Control 183 Finally, as a draconician test of the homogeneity of the copolymer formeel, the copolymer CCDs of both reaction products were measured by means of gradient HPLC. As can be clearly seen in Figure (8.8), the copolymer formed in the optimal experiment is very homogeneous up to high conversion, whereas the copolymer formed in the experiment with the induction time was oot completely homogeneous. Instead, this copolymer exhibited bimodality. At low conversion a copolymer bas been formed more rich in styrene than that at high conversions. This must be attnbuted to too high a concentration of styrene in the reactor at the beginning of the experiment due to the delay in the start of the polymerization, in turn resulting in a too low styrene concentradon at high conversions. This bimodality in the CCD cannot be easily observed from average cumulative chemica! composition data obtained by means of for instanee 1H NMR (Figure (8.7)). HPLC, on the other hand, provides high resolution information about copolymer CCDs and proves to be a powerfut tooi in this type of investigations. 15 15 (a) (b) 10 10 ...... 94 ...... J._ ...... I ~ ~ (( 62 (( 5 5 97 26 67 39 0 0 0.00 0.00 F (-) F • (-) • Figure 8.8. CCDs (normalized according to conversion and determined by means of HPLC) of S-MA copolymer prepared, at successively iru:reming conversions, according to the optima/ addition profile. (a) no induction time, (b) induction time. The total mol% conversion is indicated. 184 Chapter 8 Althougb, the difference in ebemical eomposition of the two parts of the CCD (fig. (8.8b)) signifieantly shows up in the powerfut HPLC method. it is too small to result in phase separation of the produet as indicated by the oeeurrence of only one glass transition temperature (Figure (8.9b,c)). 025 0.20 ~ ! 0.15 :;::~ .... 0.10 1'0 f 0.05 0.00 0 50 100 Tefll)efat~.re ("C) Figure 8.9. Differential calorimetrie thermograms (JO•Cjmin) of someS-MA emulsion copolymers wilh Fs = 0.25. (a) conventional batch process, preparedat (M/W)0 = 0.2 (g/g) (b) semi-continuous, optimal monomer addition profile (c) semi-continuous, non-optimal addition profile due to induction. In Figure (8.10) tbe CCDs are depicted of tbree high eonversion eopolymers prepared by different processes. The one prepared by the eonventional batch process (Fig 8.10a) exhibits bimodality, bas two glass transition temperatures (Fig. 8.9a) and bas a minimum film formation temperature of 11·c. Botb tbe one prepared in a semi-batch process under starved eonditions (32 bours) (Fig 8.10b), and one obtained in a semi-continuons process while applying the optimal monomer addition strategy (5 hours) (Fig 8.10c), are homogeneons with Copolymer Composition Control 185 respect to cbemical composition and have a minimum film formation temperature of 27"C. The optimal addition strategy allows the preparatien of homogeneaus copolymer within a considerably shorte~ period of time than the strategy of monomer mixture addition at constant rate. Figure &1 0. Experimental CCDsof three different S-MA emulsion copolymers, all aiming at Fs = 0.25 with (M/W)0 = 0.2 (g/g). (a) conventional batch process (3 hours) (b) semi-continuous, starved conditions (32 hours) (c) semi-continuous, optimal addition profile (5 hours). Intramolecular structure In Table (8.4) are given the experimentally determined MA-centred triad fractions of the two high conversion S-MA emulsion copolymers. Comparison with the theoretical values of a bomogeneaus copolymer, baving the same average chemical composition, also indicates that the optimal metbod of monomer addition gives a homogeneaus copolymer in contrast to the batch copolymer. 186 Chapter 8 Table 8.4• .By means of 40fJ..MHz 1H NMR detennined mol fraction styrene (FJ (•IOO) and by means of 13C NMR detennined trlad [ractions (•JOO) of various S-MA emulsion copolymers compared with the theoretical values of a homogeneow copolymer. metbod F. MMM MMS SMS sss SSM MSM tbeoretieal 25.0 47.3 42.9 9.8 0.4 11.1 88.5 homogeneons copolymer batch 25.2 60.6 23.7 15.7 5.4 30.0 64.6 optimal 27.5 45.1 43.7 11.2 0.0 18.5 81.5 addition SA Diseossion An iterative procedure aiming at optimal addition profiles bas been successfully applied to semi-continuons S-MA emulsion copolymerization. The applicabllity (in terms of tbe required convergence) to other systems with different reactivity ratios and different monomer partitioning behaviour bas to be investigated. However, it can be argued tbat tbis iterative procedure cannot converge to addition profiles forming inhomogeneons copolymers. Finally ii during tbe semi-continuons experiment witb the optimal addition profile bas been ealculated. By applying a suitable penultimate model21.26>, ii as a function of conversion bas been calculated from experimental conversion time 17 1 data, the experimentally determined partiele number (ca. 6·10 L' ), and tbe monomer partitioning data given in chapter 5 (see Figure (8.11)). lts reliability is limited since tbe values of tbe parameters used to ealculate ii, such as tbe number of latex particles, k P and tbe monomer concentradons in the panieles, can only be determined with limited accuracy. However, it is obvious tbat ii < 0.5 (i.e., low ii system). The apparent decrease of ii ealculated at high conversion is unlikely and could be attributed to tbe diffusion controlled decrease of kP. Copolymer Composltion Control 187 It bas to be noticed tbat the application of semi-continuous emulsion copolymerizations with optimal monomer addition profiles can be a powerfut tooi in ldnetic investigations. It enables to obtain rate data at various local monomer ratios inside tbe particle, but with a local monomer ratio inside the particles tbat is constant during a single experiment. IC ODO~~--~----~------~----~----~ o.óo OAO 0.60 o.eo 1.00 xm (-) Figure 8.11. Calculated ii as a function of instantaneous conver.sion from the semi-continuous S-MA copolymerization with the optima/ monomer addition profile. 8.5 Condusion Homogeneaus S-MA emulsion copolymers can be prepared by applying a constant addition rate of tbe monomer mixture, provided extremely low addition rates are being used. Unfortunately, this results in very long process times. A considerably sborter process time can be obtained by calculating the optimal S (i.e., the most reactive monomer) addition profile, wben all MA bas initially been cbarged to the reactor. The time dependent mte of addition of Scan be computed by iterative procedures involving reactivity ratios, accurate monomer partitioning data, and a limited series of stmtegic experiments. The bomogencity of tbe copolymer formed was confll1lled by means of Hhigb resolution" gradient HPLC. 188 Reterences 1. J. Si'iupárek, Angew. MakromoL Chem., 25, 105 (1972) 2. R.A. Wessling, D.S. Gibbs, l MacromoL Sci., Chem., A-7, 647 (1973) 3. J. Si'iupárek, F. Kräka, l Appl. Polym. Sci., 20, 1753 (1976) 4. J. Snupárek, F. Kr~ka, l Appl. Polym. Sci., 21, 2253 (1977) 5. J. Sflupárek, K. IWpar, l Appl. Polym. Sci., 26, 4081 (1981) 6. A. Garcia-Rejon. C. Guzman. J.C. Mendez, L Rios, Chem. Eng. Commun., 24 71 (1983) 7. J. ~nupárek, MakromoL Chem., SuppL, 10/11, 129 (1985) 8. L Rios, M.A. Cruz, J. Palacios, LM. Ruiz, A Garcia-Rejon. MakromoL Chem., Suppl., 10/11, 4n (1985) 9. S. Omi, M. Negishi, K. Kushibiki, M. lso, MakromoL Chem., Suppl., 10/11, 149 (1985) . 10. K. Chujo, Y. Harada, S. Tokuhara, K. Tanaka, l Polym. Sci., Part C, 27, 321 (1969) 11. M.S. EI-Aasser, T. Makgawinata, J.W. Vanderhoff, C. Pichot, l Polym. Sci., Polym. Chem. Ed., 21, 2363 (1983) 12. S.C. Misra, C. Pichot, M.S. Bl-Aasser, J.W. Vanderhoff, J. Polym. Sci., Polym. Chem. Ed., 21, 2383 (1983) 13. T. Makgawinata, M.S. Bl-Aasser, A. Klein, J.W. Vanderhoff,l Dispersion Sci. Technol., 5, 301 (1984) 14. J. Dimitratos, C. Georgakis, M.S. Bl-Aasser, A Klein, Comput. Chem. Eng., 13, 21 (1989) 15. J. Dimitratos, M.S. Bl-Aasser, C. Georgakis, A Klein, l Appl. Polym. Sci., 40, 1005 (1990) 16. D.R. Basset, in HScience and Techno/Ogy of Polymer CoUoids", Vol. I, Ed. O.W. Poehlein, R.H. Ottewil, J.W. Goodwin, Martinus NiJhoff Publishers, The Hague, 220, 1983 17. J. Guillot, Acta Polym., 32, 593 (1981) 18. L Rios, J. Guillot, MakromoL Chem., 183, 531 (1982) 19. J. Guillot, C. Rios-Guerrero, MakromoL Chem., 183, 1979 (1982) 20. A Guyot, J. Guillot, C. Graillat, M.F. Uauro, l Makromol. Sci., Chem., All, 683 (1984) 21. This thesis: Chapter 2 22. G. Arzamendi, J.M. Asua, l Appl. Polym. Sci., 38, 2019 (1989) 23. G. Arzamendi, J.M. Asua, Makromol. Chem., MakromoL Symp., 35/36, 249 (1990) 24. M. Nomura, M. Harada, l Appl. Polym. Sci., 26, 17 (1981) 25. This thesis: Chapter 7 26. This thesis: Chapter 6 27. This thesis: Chapter 3 28. This thesis: Chapter 4 Partiele Morphology 189 Appendix A Partiele Morphology of Composite and Copolymer Latices SUMMAR Y: The partiele morphology of composite styrene-acrylie latices, prepared by means of two-stage emulsion polymerization. has been. investigated. Depending on the polymerization sequence and process parameters, various morpbologies have been observed: 'void' or 'partially localized', 'snowman', and 'raspberry' or 'oil in oil' strueture. Furthermore, it has been demonstraled that emulsion mpolymer latices of styrene and methyl acrylate exhibit a 'core-shell' type of morphology, if a strong composition drift has occurred during preparation. A.l Introduetion Multipbase polymerie matenals exhibit properties often superior to those of the homopolymers due to the morphological structure, i.e., the existence of 1 4 phase separated domains ,2,3, >. Seeded emulsion polymerization resulting in structured latex particles is a valuable procedure in preparing multipbase polymerie materials. Control of partiele morphology in emulsion (co )polymerization processes forms the key in optimizing the properties and performance of a latex system in a given applications,6,7.S.9>. Therefore, during the last decade there has been a great industrial and academie interest in two stage emulsion (co )polymerization. In principle, a large number of factors can affect the morphology of composite latex particles. Some of these are inherent to the ehoice of the monomers and others arise from the way in which the synthesis was conducted. 190 Appendix A Partiele morpbology is known to be affected by temperature10>, type, amount and 11 4 15 16 mode of monomer addition •12,13.t >, initiator • > and emulsifier17), and the presence of chain transfer agents or cross-linking agents10.13.16.1S.19>. Many of the parameters affecting the structure of the multistage composite particles are not well understood. At present only a limited qualitative theory 9 1 bas been published •12,20,2 ), taking into account not only the interfacial tension between the various polymers and the water phase, but also the interfaclal tension between the mutual polymer phases. There is no unified model for the mechanism of core-shell partiele formation available yet. Apparently COntradietory results are somelimes reported in the literature for similar systems, because in a composite latex partiele a wide range of morpbologies is possible depending on small differences in process ·conditions. The morphology of composite latex particles of polystyrene and poly(meth)acrylates is knowil to vary from 'void' structure or partially localized (i.e., 'half moon-like')1o,ts,n) to 'raspberry-like' 19> depending on the process conditions9>. For S.MA composite particles inverted core/shell morpbologies have been reported in great contrast with the generally mentioned tendency of the more hydrophilic polymer (i.e., polymethyl acrylate (PMA)) to form the shell phase12>. During conventional batch emulsion copolymerization of S.MA a strong composition drift may occur. For example, under certain conditions ((S/MA)0 = 0.33 (mol/mol) and (M/W)0 = 0.2 (gfg)) after 60 mol% conversion almost pure homopolymer PMA is formed (see chapters 6 and 7). This migbt result in the formation of core-shell structured latex particles, whose morphology is expected to strongly affect copolymerization rate. The soap titration method, described by Maron et aL 23>, bas been used by Okubo9>, and Ramirez and 24 Guillot ,25) as a tooi to determine whether the latex particles are homogeneaus or have a core-shell type of morphology. However, this metbod alone camtot distinguish between core-shell morphology and the formation of new PMA latex particles (secondary nucleation), which can be expected to occur due to the fact that MA is much more polar than styrene. Partiele Morphology 191 The purpose of tbe current investigaûon was fourfold. (1) Study of tbe parameters tbat affect partiele morpbology (polymerization sequence, presence of cross1inldng agents, monomer polarity, temperature). (2) Development of stainin~ tecbniques in hebalf of tbe determination of tbe morpbology of S~MA (co)polymer latex particles witb TEM. (3) Investigation of possible secondary nueleation in tbose cases wbere astrong composition drift occu.rs. (4) Testing tbe assumption made in the model caculations tbat all particles have a uniform composition. Witbout hardening, tbe polyacrylic latex particles in general cannot be observed by using TEM. TEM sample preparadon includes drying on a grid. During drying, tbe soft particles beoome flattened on tbe grid due to tbe enormous pressure caused by tbe surface tension. Therefore, all experiment& were carried out twice, once in tbe presence and once in tbe absence of cross linking agents. Tbe reason for using cross-linkers is tbat the presence of cross links in tbe latex particles can prevent flow during sample preparation for TEM. The optionally added cross-linkers, however, may also have an effect anpartiele swellability and may counteract possible phase inversion26>. Therefore, attention bas been paid to tbe effect of cross-links intbeseed latex particles and/or in tbe second stage polymer phase on tbe final composite latex partiele morphology. A.2. Experimental Materials Reagent grade styrene (S), methyl acrylate (MA) and butyl acrylate (BA) (Merck) were distilled under reduced nitrogen atmospbere in order to remave inhibitor. The middle fraction was cut and stared at 4"C. Befare use tbe cross linking agents divinyl benzene (DVB) (to be used in case of styrene) and ethylene diacrylate (EDA) (to be used in case of acrylic monomer) were wasbed witb a NaOH salution in order to remave inhibitor. The water was distilled twice. Potassium persulphate (Merck p.a.), n-dodecyl mercaptan (Merck p.a.) and Aerosol MA 80 (sodium dibexyl sulfosucclnate) (Cyanamid) were used witbout furtber purification. Appendix A Seed latex preparadon The seed latices were prepared in a glass reactor under 1.7 atm. nitrogen pressure at 50"C for MAand BA, and at 70"C forS. The recipe is given in Table (A.1). After approximately 24 hours the temperature was increased to 90"C for 3 hours in order to dissociate the remaioder of the initiator. After polymerization initiator fragments, unreacted monomer and the major part of the emulsifier were removed by dialysis. Seeded polymerizations The seeded polymerizations (see Table (A.2)) were carried out under 1.7 atm. nitrogen pressure in a 600 mi glass reactor normally thermostated at 50"C. In the reactor monomer and cross-linking agent or mercaptan were allowed to swell the seed latex for one hour before the initiator ~S20g. dissolved in 5 mi water, was added. In an attempt to minimize secondary nucleation and to promote the growth of the seed particles, no additional water was added in order to keep a high partiele number in the reactor. In order to prevent coagulation, however, it appeared to be necessary to add additional Aerosol MA 80 (dissolved in water) after 4 hours (during interval lli of the emulsion polymerization). The amount of added cross-linker was sufficient to highly cross link the polymer. The polymers formed appeared to be insoluble in THF. The monomer fpolymer ratio (M/P) in the seeded polymerizations was chosen to be 4. This would result in a 1.7 times growth in diameter provided the particles remain spherical and no secondary nucleation or coagulation occurs during the seeded polymerization. Partiele Morphology 193. Emu/sion copolymerization In order to investigate the copolymer partiele morphology, one emulsion copolymerization was carried out at so·c using the procedures described in chapter 3. The initial monoroer ratio (S/MA)0 was 0.33 (moljmol) and tbe initial monoroer to water ratio (M/W>o was 0.2 (g/g). Sodium dodecyl sulfate (SDS) 1 was used as surfactant (0.0116 mol· L" ) and potassium persulfate as initiator 1 (1.233 mmol·L- ). TEM Partiele morphology and size were examined by means of transmission electron microscopy (TEM). The use of several different staining techniques (witb or witbout UV hardening) proved to be useful. Ru04 was used as a preferendal staining agent to distinguish between the polystyrene (dark) and the acrylate ester domains (light). Altematively, phosphotungstic acid (PTA) was used as negative staining agent in order to better delineate the partiele edges, especially for the acrylate polymer domains, which remaio almost unstained by Ru04• UV hardeDing (i.e., photo-cross-linking) of the latex particles prevented the soft PMA and PBA particles to be flattened on the grid27). Table A.l. Seed latex recipe (MA and BA: 50"C. S: 70"C) . . cross-linked (g) non cross-linked (g) MAor BA or S 30.0 MA or BA or S 30.0 water 750.0 water 750.0 ~S20s 0.2 ~S208 0.2 DVBorEDA 1.5 n-dodecyl mercaptan 0.3 Aerosol MA 80 2.0 Aerosol MA 80 2.0 Appendix A Table A.2 Second stage laiex recipe (50•C). time cross-linked (g) non cross-linked (g) MAorBAorS 30.0 MAorBAorS 30.0 t=O seed latex 150.0 seed latex 150.0 EDAorDVB 1.5 n-dodecyl mercaptan 0.3 t=1 hr ~208 in 5 mi H:P 0.2 KzS20 8 in 5 mi H20 0.2 t=4 hr Aerosol MA 80 2.0 Aerosol MA 80 2.0 water 195.0 water 195.0 A.3 Results and dlscussion StnJctured laiex particles Sixteen different structured (styrene-acrylate) latex particles were prepared differing in polymerization sequence (styrene first, acrylate second or vice versa), in condition of cross-linking (only seed, only second stage polymer, both stages or non-cross-linked), and in type of acrylate (methyl acrylate or butyl acrylate). In this appendix the structured latex particles are coded in the following manner: first stage monomer(cr)-second stage monomer(cr), where "er" means cross-linked. For instance, Ser-MA means MA bas been polymerized on a cross linked polystyrene seed. It appeared that there is a difference in the ocrurenee of secondary nucleation of latex particles. Second stage S polymerization on a polyacrylic seed did oot result in secondary nucleation. Under the experimental conditions employed, however, it bas been impossible to avoid secundary nucleation in case of polymerization of MA or BA in the presence of PS seed latex particles. These PBA and PMA latex particles caooot be observed by means of TEM using conventional staining techniques, because they are too soft and flow during Partiele Morphology 195 drying on the grid. However, UV hardeDing of the latex particles befare drying on the grid prevented or at least minimized this flow. Secondary nucleation was also noticed in the case of polymerization of MA or BA in the presence of a cross-linkerand PS latex particles (S-MAcr and S-BAcr). UV hardening was not necessary in this case. As a consequence of the large .amount of secondary nucleation, in both cases the original PS seed latex particles hardly grew. Most seeded systems resulted in sphericallatex particles, however the ones coded S-MA. S-BA, BA-S, MAcr-S and MAcr-Scr contained different structures. Both S-MA and S-BA latices had structured latex particles exhibiting the so called partially localized structure (Figure (A.l)) in the presence of a buge amount of secondary nucleation polyacrylate latex particles (Figure Alb). The 'void' structure (Figure (Ala)) can be made visible by TEM with PTA or Ru04 staining tecbniques without the use of UV hardening. An explanation for these structures bas already been given by Okubo10>. The original shape of the latex particles is spherical with a partially localized structure. However, durlog drying of the latex on the grid in behalf of TEM, the small soft PBA or PMA parts flow from the seed particles on the grid. The result is known as the 'void' structure. The original sbape is spherical and can be made visible after UV hardening of the latex (Figure Alb). The structure of BA-S latex particles appeared to be dependent on the reaction temperature applied. The latex particles exhibited a 'polymerie oil in oil' (POO) or 'raspberry' structure when prepared at so•c (Figure A.2a). However, when preparedat a higher temperature (SO•c or 9o•q they sbowed the partially localized ('void') strucure, in agreement with literature10> (Figure A2b). This behaviour can be explained by the general rule tbat the most bydrophylic polymer is most likely to be found at tbe water-partiele interface. But low viscosity and thus high temperature is required to achieve the equilibrium morphology'>. However, under the applied experimental conditions MA-S latex particles did not follow this general rule and were spherical. The fact that monomer hydrophilicity is apparently not the only parameter cantrolling the partiele morphology was already demonstrated by Dimonie12> who reported 196 Appendix A anomalous inverted core-shell morphology in case of S-MA latex particles. Although BAcr-S(cr) latices exhibited sphericallatex particles, MAcr-S(cr) latices exln'bited 'snow-man' type partiele morphology (Figure A3). This anomalous behaviour is not yet understood.. Figure A.I. TEM micrograph of composile (S-MA) latex particles with partially localized morphology. (a) exhibiting the 'void' sttucture; PTA staining (b) exhibiting the 'real' shape and secondaly nucleation; UV/RuO 4 staining. Figure A.2 TEM micrographof composite (BA-S) latex particles. RuO4 stained latices. (a) preparedat 50*C exhibiting the 'raspberry' or POO sttucture (b) preparedat 90•c exhibiting the 'void' structure. Partiele Morphology 197 Figure A.3. TEM micrographof PTA/Ru04 stained ~now man' like MAcr S (seed cross-linked PMA/second stage PS) composite latex particles (a) ovetView, (b) detaiL Copolymer latex particles A S-MA emulsion copolymerization was carried out exbibiting a strong composition drift towards more MA-rich monomer feed: (S/MA)0 = 0.33 (mol/mol) and (M/W)0 = 0.2 (g/g) (see chapters 6 and 7). The high conversion copolymer latex was analysed with TEM using several different staining techniques. The micrographs given in Figure (A.4) show a core-shell type of latex partiele morphology and do not indicate the presence of particles formed by secondary nucleation. Applying a straightforward staining with Ru04 the micrograph (Figure (A.4a)) reveals the existence of a PMA shell. The white circles of flown soft PMA can be seen around the dark core consisting of the more rigid copolymer. This can be explained by the fact that the PMA shell flows from the core during drying on the grid, thereby pushing away the surfactant (SDS). The dark circles around the white circles represent the surfactant. 198 Appendix A This phenomenon cannot be observed when UV hardeDing of the particles is applied (Figure (A4b), because UV hardeDing prevents the flow of the PMA shell. Furthermore, the dark surfactant circles disappear when the latex is dialysed (removal of surfactant) prior to Ru04 staining (Figure (A4c)). In this case PMA cannot be observed, since the contrast between the PMA and tbe background is negligible. Figure A.4. TEM micrograph of S-MA heterogenenous copolymer latex particles after Ru04 staining. (a) only Ru04 staining (b) UV hardenins prior to Ru04 staining, (c) dialysed prior to Ru04 staining. Partiele Morphology A.4 Conclusions Under the described experimental conditions, polymerizations of MA or BA on PS seed latices result in secondary nucleation as proved by using TEM after UV hardeDing of the latices. The final composite latex partiele morpbologies strongly diverge and are affected by many process parameters. Various aspects remain unclear and further research is required. During the S-MA emulsion copolymerization with (S/MA)0 = 0.33 (mol/mol), where secondary nucleation could be expected on the grounds of the occurrence of a strong composition drift, this phenomenon was not observed. However, it is very likely that the particles have a core-shell type of morphology. So the assumption of a constant number of particles used in the model calculations in this thesis seems to be valid. However, the assumption of uniform/homogeneous particles appears not to be correct. As outlined in chapter 6 a core-shell morphology of the latex particles may affect polymerization rate, because monomer (MA) concentradon may vary within the heterogeneaus latex particle. On the other hand it bas been demonstrated in chapter 7 that a possible core-shell morphology does not affect composition drift and as a consequence, will not significantly affect copolymer microstructure. 200 Relerences 1. D.R. Paul, S. Newman, "Polymer Blends", Academie Press., New York, 1978 2. M. Okubo, Y. Katsuta, K. Inoua, K. Nakamae, T. Matsumoto, J. Adhes. Soc. Jpn., 1" 278 (1980) 3. M. Okubo, M. Seike, T. Matsumoto, Prepr. Polym. Microsphere Symp. Jpn., 1, 2S (1980) 4. J.A. Manson, C.H. Sperlin, in "Polymer Blends and Composites", Plenum Press., New York, 1976 5. N. Sütterlin, MacromoL Chem., SuppL, 10/11, 403 (1985) 6. H. Kast, MakromoL Chem., SuppL, 10/11, 447 (1985) 7. D.A. Greenhill, DJ. Hourston, Polym. Mat. ScL Eng., 60, 644 (1989) 8. B. Scblund, J. Guillot, C. Pichot, A Cruz, Polymer, 60, 1883 (1989) 9. M. Okubo, MakromoL Chem., MakromoL Symp., 35/36, 307 (1990) 10. M. Okubo, Y. Katsuta, T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 20, 45 (1982) 11. A Cruz-Rivera, L. Rios-Guerrero, C. Monnet, B. Schlund, J. Guillot, C. Pichot, Polymer, 30, 1872 (1984) 12. V.L. Dimonie, M.S. El-Aasser, J.W. Vanderhoff, Polym. Mat. Sci Eng., 58, 821 (1988) 13. E.S. Daniels, M.S. El-Aasser, A Klein, J.W. Vanderhoff, Polym. Mat. ScL Eng., 58, 1104 (1988) 14. J. C. Daniel, MakromoL Chem., SuppL, 10/11, 359 (1985) 15. I. Cho, K. Lee, J. AppL Polym. ScL, 30, 1903 (1985) 16. M.P. Merkel, V.L. Dimonie, M.S. EI-Aasser, J.W. Vanderhoff, Polym. Mat. ScL Eng., Sl, 320 (1985) . 17. V.L. Dimonie, M.S. El-Aasser, A Klein, J.W. Vanderhoff, J. Polym. Sci, Polym. Chem. Ed., 22, 2197 (1984) 18. DJ. Hourston, R. Satgurunathan, H. Varma, J. AppL Polym. Sd., 31, 1955 (1986) 19. M. Okubo, Y. Katsuta, T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 18, 481 (1980) 20. V.L. Dimonie, Y. Chen, M.S. El-Aasser, J. W. Vanderhoff, "lnterfacial Phenomena Cantrolling Partiele Morphology of Composite Latexes", presented at the 2nd International Symposium on Copolymerization and Copolymers in Dispersed Media, Lyon, France, 1989 21. M.S. El-Aasser, Y.C. Chen, V. Dimonie, 'Vevelopment ?[ Partiele Morphology in Composite Latexes", presented at the 33 IUPAC International Symposium on Macromolecules, Montréal, Canada, 1990 22. M. Okubo, M. Ando, A. Yamada, Y. Katsuta, T. Matsumoto, J. Polym. Sci, Polym. Lett. Ed., 19, 143 (1981) 23. S.H. Maron, M.E. Elder, I.N. Ulevitch, J. Colloid Sci, 9, 89 (1954) 24. W. Ramirez-Marquez, J. Guillot, MakromoL Chem., 189, 361 (1988) 25. J. Guillot, MakromoL Chem., MacromoL Symp., 35/36, 269 (1990) 26. H.R. Sheu, M.S. El-Aasser, J.W. Vanderhoff, Polym. Mat. Sci Eng., 59, 1185 (1988) 27. I.A. Maxwell, D.H. Napper, R.G. Gilbert, J. Chem. Soc., Faraday Trans. I, 83, 1449 (1987) J 2 D COLOC and NOESY NMR 201 Appendix B Investigation of the Methoxy Proton Region in 1H NMR Spectra of Styrene-Methyl Acrylate Copolymers by means of COLOC and NOESY NMR SUMMARY: Merely on a statistical basis two different assignments at a triad level for the methoxy proton region in the 1H NMR spectra of styrene (S)-methyl acrylate (MA) copolymers are correct. They result, however, in different co-isotacticity parameters. In order to discriminate between these two assignments some 20 COLOC NMR and 20 NOESY NMR experiments were carried out. From the results obtained it can be concluded that neither of the two peak assignments on triad level is correct. Pentad effects have to be taken into account. B.l Introduetion High resolution Nuclear Magnetic Resonance (NMR) spectroscopy bas been particularly effective in the determination of.the intramolecular chain structure 1 of (co)polymers ). NMR methods may provide information on the average composition, tacticity, and sequence distribution of copolymers. Furthermore, two dimensional NMR methods have recently become available giving additional configurational and conformational information2.3>. During the last decade 20 NMR techniques have been widely applied to structure determinations of smalt organic molecules as well as of biological macromolecules such as proteins, 4 nucleic acids and polypeptides ). Recently, the application of 20 NMR techniques on structure elucidation of synthetic polymers appeared to be powerfut as welll.S,tO,n). 202 Appendix B The chemical shift of the methoxy protons in the conventional 10 1H NMR spectra of styrene-methyl acrylate copolymers is known to be affected by both sequentia! and configurational effects6l. Three different regions within the methoxy proton signa! region can be observed. On a statistica} basis two different assignments are in agreement with the observed data at a td.w;llevet6l: (1) The lto-Yamashita (1-Y) assignment7) is well known and leads to an unexpected and unlikely high degree of co-isotacticity (uSM = 0.9) for the statistical (i.e., random) copolymers. (2) Furthermore, Nikman and Harwood15>introduced an alternative assignment that results into a more realistic aSM of 0.3 or o:F>. In order to discriminate between the two assignments or at least reduce the uncertainty of the peak assignment, three long-range 20 heteronuclear chemical shift correlation (20 COWC NMR) experiments8>were carried out on well-defined statistical S-MA copolymers in an attempt to establish the connectivity over three honds of the carbonyl C=O resonances and the protons of the OCH3 resonances. A detailed description of the tecbnique (pulse sequence) bas been given by KesslerB>. Recently, this COWC tecbnique bas also been applied by Fang to characterize carboxyl-terminated butadiene-acrylonitrile copolymerS>. Additionally, attempts were performed to use 2D NOESY (Nuclear Overhauser Effect Spectroscopy) NMR in order to find the correct peak assignment. Two dimensional NOESY is a NMR technique, specially tailored to detect spatial relations over short distances (generally < 5Á). Exchange of magnetization (nuclear Overhauser effects) by through-space dipole-dipole interaction during the mixing time produces an off-diagonal resonance or cross peak. By this technique, the close proximity ( thus (co)-isotacticity configuration) of styrene units to the methoxy group can directly be shown as cross-peaks. The conventional 10 spectrum is on the diagonal. This technique has already been applied to the related styrene (S) - methyl methacrylate (MMA) copolymer 9 10 system. Heffner et aL • > publisbed 20 NOESY studies on a1ternatin& S·MMA copolymer. The cross-peaks observed in the 20 NOESY spectrum between the aromatic and the aliphatic parts of the spectrum were consistent with the literature assignments of the 10 methoxy 1H NMR signals. Very recently, l D COLOC and NOESY NMR l03 1 bowever, Aerdts et aL u) reassigned tbe metboxy H NMR signals (based on pentad level interactions) for statistica! S-MMA copolymers. They found evidence supporting a new assignment at pentad level in 20 NOESY NMR experiments on statistica! and alternatinK S-MMA copolymers. B.l Experimental The 20 NMR experiments were performed on well purified low conversion (bomogeneous) solution copolymers of styrene and methyl acrylate. The copolymerization and purification procedures have been described in chapter 3. The COLOC experiments11>have been used for the observation of long range couplings between C=O 13C carbon atoms and methoxy protons. The Kessler Griesinger pulse sequence was used8>. On concentrated samples (25% w/v in CD03) 250 scans were accumulated per t1 value with a recycle delay of 1 sec on a 400-MHz (Broker AM 400) spectrometer">. The initial data matrix was 2000 Hz (1K) and 800Hz (128 t1 values). Before Fourier transformation, double zero filling was used in the F1 dimension. Moreover, shifted sine-bell window function 'IC/4 and 'IC/8 were applied for the F1 and F2 domains, respectively. Oelay times (01 and 0 2) were both set to 25 msec. 12 The phase sensitive 20 NOESY > experiments employed a 'IC/2- t1 -'IC/2- 1 14 1 m - 'lf/2 - tz pulse sequence 3, >. NOESY bas been used for the observation of dipole-dipole interactions between neighbouring S and MA units. The 20 NOESY experiments were recorded on a Broker AM 600 spectrometerb>. The copolymer sample was dissolved (2% (w/v)) in hexachlorobutadiene with 10 vol% benzene-d6 as locking agent. The spectra were recorded at a temperature of 353 K. The process data matrix consisted of 512*512 points coverlog5882Hz in botb dimensions. The repetition time was set at 2 sec with 16 scans collected • courtesy of OSM Research BV, Geleen, The Netherlands b courtesy of SON/NWO, Central NMR facilities, Nijmegen, The Netherlands 204 Appendix B for each of the 256 spectra. The optima! mixing time ( T .) appeared to be 2 s. The spin-lattice relaxation times (T1) of the aromatic, metho:xy and aliphatic protons were 650, 500 and 350 ms, respectively. Before Foutier transformation, the phase sensitive spectra were processed with sine-bell squared window multiplication with a shift of 4 in the F1 direction and a shift of 2 in the F2 direction. B.3 Results and discussion At a triad level two different peak assignments of the metho:xy proton region fit the observed data in an acceptable manner: (1) Ito's assignment7) and (2) an alternative assignment15>. Both assignments are based on the observations (Fig. (3.12) that MMM triads resonate at 3.65 ppm and exhibit no tacticlty splittings, and that the phenyl rings in the next neighbour position give shielding effects, being more pronounced in an isotactic configuration. The 10 theoretically possible MA-centred triads are not resolved in 10 distinct resonances in the 1H NMR spectra. Up to now the methoxy peak region bas been analysed in termsof the lto-Yarnashita (1-Y) assignment7), originally proposed for styrene-methyl methacrylate copolymers. According to Ito and Yarnashita7) the three different groups of peaks (X, Y, Z) (see Fig. (3.12)) can be assigned to the following combination of MA-centred triads: (B.la) (B.1b) F1 = u FMMs + 2u(1-u) FsMs (B.1c) where Fxt F1 and Fz are normalized peak areasof peak X ranging from 3.68- 35, Y ranging from 3.5 - 3.35 and Z ranging from 3.35 - 3.0 ppm. 2 D COLOC and NOESY NMR 205. Tbe followiog alternative tentative assignment, that also fits the lD 1U NMR data, bas been reported by Nikman and UarwoodlS). (B.2a) (B.2b) Fz = 2a(l-o) FSMs + (l·ol FSMS (B.2c) Tbe resonance assignments for (1-o)2SMS and alSMS can be interchanged, giving a:=l-o. B.3. COLOC experiments Tbe three 2D COLOC spectra are given in Figure (B.la-c). From tbe contour plot of the COLOC experiment on the copolymer witb F, = 0.12 (Figure (B.la)), the alternative assignment bas to be rejected in favour of the earlier 1- y assignment, because of the presence of the peak that correlates the X peak in the 1U NMR spectrum with the MMS peak in the 13C NMR spectrum.. Moreover, as depicted in the COLOC contour plots of the copolymers with Fs = 0.46 (Fig. (B.lb)) and F, = 057 (Fig. (B.lc)) the X peak correlates with part of the SMS and the MMS peak in the 13C NMR spectrum. Tbis also favours Ito's assignment and would refute the alternative assignment. Uowever, in the case of F1 = 0.46 the MMM peak clearly correlates with the Y peak instead of correlating with the X peak. Tbe latter would be expected based on the 1U NMR spectrum of methyl acrylate homopolymer (see Fig. (3.12)). Tbis observation in Figure (B.3b) refutes the 1-Y assignment (eq. (B.1)). Uowever, it cannot be unambiguously excluded that some kind of artifacts are (partly) responsible for the apparently COntradietory COLOC results, since COLOC is a not extensively used technique for copolymers4>. 206 Appendix B ___t__ (a)JL... (b) . (c} v--vv·~~. v ~ Figure B.I. Contour plots of three 2D COLOC NMR experiments on low conversion salution copolymers (Fs = 0.12 (a/i 0.4ó (b) and 0.57 (c)), showing long-range correlation between carbonyl C atoms and protons ofthe methoxy group. 2 D COLOC and NOESY NMR 8.3.2 1H NOESY experiments Corresponding with the results of Heffner et al. tO) and Aerdts et al.ll), obtained on S-MMA copolymers, ,any cross-peaks between the aromatic and the methoxy protons are expected to be caused by SMS triads in co-isotacticlty configuration (small distance, strong NOB effect) or co-heterotacticlty configuration (moderate NOB effect), whereas co-syndiotactic configuration does oot result in a cross-peak (large distance between tbe aromatic and the methoxy protons) (see Fig. {B.2)). In Figure (B.3) a NOESY contour plot is given for a statistica! S-MA copolymer with F, = 0.53. As expected on the grounds of the close proximity, strong cross-peaks are found between the mutual aliphatic protons and the mutual aromatic protons. Furthermore, strong cross-peaks are observed between the aromatic protons and tbe aliphatic protons (already observed applying a Tm = 300 ms). Surprisingly, no cross-peaks were found between the aliphatic and methoxy protons. A small cross-peak can be observed between the aromatic and the Y peak of the methoxy proton area. The occurrence of interaction between the Y part of the methoxy proton region and the aromatic protons rejects lto's peak assignment in favour of the alternative assignment. From the COLOC and NOBSY experiments it can be concluded that no peak assignment on triad level can be correct for the methoxy proton region. This is supported by the 1H NMR spectrum of the alternating S-MA copolymer (see Figure (3.14A)) of which the methoxy proton region (determined by SMS triads) is completely different from the methoxy region of statistica! copolymers with high S content thus having a high SMS trlad fraction. The influence of pentads probably explains the unexpected difference. For instance, the MSMSM pentad in the alternating copolymer obviously resonates at different ppm than the SSMSS pentad present in the statistica! copolymer with a high S content (Figure (3.14)). Moreover, the MMMMM pentad resonates at 3.64 ppm, whereas the SMMMM and SMMMS pentads resonate at higher fields (Figures (3.12) and (B.l.b)). 208 Appendix B 0 ' H H .· ·. cohetero cosyndio coiso Figure B.2 Various configurations of SMS triads. • 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 , 4.5 5.0 .. 5.5 0 " .0 .5 7.0 7.5 ppm llPII 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5. Figure B.3. Contow plot of a 6()().MHz 2D NOESY NMR experiment with 1 m = 2 s of a low conversion solution (statistica/) S-MA copolymer. 2 D COWC and NOESY NMR B.4 Conclusion No suitable peak assignment (at a triad level) can be found for the methoxy proton region of 1H NMR spectra of statistical S-MA copolymer. This is indicated by COLOC atid NOESY experiments, and the observed difference between the methoxy proton region of alternating copolymers and statistical copolymers with a high S content. Pentad effects have to be taken into account. The methoxy proton region of S·MA copolymers, however, is resolved in merely three distinct groups (X. Y, Z). Therefore, no attempts were undertaken to find an assignment on pentad level. Nevertheless, in chapter 3 it bas been demonstrated by 13C NMR, unambiguously providing the MA-centred triad fractions, that both the Ito-Y amashita and the alternative assignment for the 1H NMR spectra can be used to determine the MA-centred triad fractions without co-isotacticity information. 210 Keterences 1. HJ. Harwood, "Problems in Aromatic Copolymer Structure~ in "Natura/ and Synthetic High Polymers", VoL 4, NMR (Ed. P. Diehl et al.), Springer Verslag, Berlin, 1970 2. FA Bovey, in "Comprehensive Polymer Sci. ", Ed. G. Allen and J.C. Bevington, Part 1, 17, "Stnu:ure of Chains by Solution NMR Spectroscopy", Pergamon Press, 339, 1989 3. F.A Bovey, PA Mirau, MakromoL Chem., MacromoL Symp., 34, 1 (1990) 4. W.R. Croasmun, R.M.K. Carlson, 'Two-Dimensional NMR Spectroscopy", VCH Publishers, lnc., New York, 1987 5. T. Fang, Macromolecules, 23, 2145 (1990) 6. This thesis: Chapter 3 7. K. lto, Y. Yamashita,J. Polym. Sci.: Part B: Polym. Lett. Ed., 3, 637 (1965) 8. H. Kessler, C. Griesinger, J. Zarbock, H.R. Loosli, J. Magn. Reson., 57, 331 (1984) 9. P.A Mirau, F.A Bovey, AB. Tonelli, S.A Heffner, Macromolecules, 20, 1701 (1987) 10. S.A Heffner, F.A Bovey, LA Verge, PA Mirau, AB. Tonelli, Macromolecules, 19, 1628 (1986) 11. AM. Aerdts, J.W. de Haan, AL. German, G.P.M. van der Velden, submitted to Macromolecules 12. DJ. States, R.A Haberkorn, DJ. Ruben, J. Magn. Reson., 48, 286 (1982) 13. J. Jeener, B.H. Meier, P. Bachmann, R.R. Ernst, J. Chem. Phys., 71, 4546 (1979) 14. S. Macura, R.R. Ernst, MoL Phys., 41, 95 (1980) 15. M.K. Niknam, Ph.D. Dissertation, Univ. of Akron, Akron, OH, USA (1985), Diss. Abstr. Int. B., 1985, 46 (4), 1196. Avail. through Univ. Microfilms, Int. Order No. DA 85 13339 Summacy 211 Summacy Emulsion copolymerization is a widely used process, and the synthetic latices produced find a hroad range of applications in the coating, ink, and plastic industry. Emulsion (co)polymerization is a very complex process due to its heterogeneons and colloidal nature. In industry many emulsion polymerization processes are empirically developed, often resulting in process conditions far from optimal. Modelling of emulsion (co )polymerization is a necessity for a hetter onderstanding of the process enabling a hetter control of the polymer synthesis, in turn resulting in optimal product quality. This thesis descrihes an investigation of the typical aspects of emulsion g;tpolymerization aiming at a better and more quantitative onderstanding of the effects of the process parameters on the chemical microstructure of the copolymer. The quintessence of modelling emulsion copolymerization is the proper description of the heterogeneity of the emulsion system in combination with the different properties of the monomers, viz., reactivity, water solubility, and swellability of the latex particles. It has been demonstrated by using the model developed and described in chapter 4 that "apparent" reactivity ratios, which are still frequently used, are generally incapable of descrihing the course of emulsion copolymerizations in an adequate manner. During this investigation the main model system was the monomerpair styrene - methyl acrylate. But also romparisous have been made with other monomer pairs exhibiting different monomer partitioning and/or reactivity properties, viz., styrene- n-butyl acrylate, styrene - acrylic acid and methyl acrylate - n-butyl acrylate. Detailed elucidation of copolymer microstructure is a prerequisite to thorough and fundamental research in this field. On the one hand, the copolymer microstructure directly reflects the chemical processes occurring in the reaction loci. On the other hand, copolymer microstructure controls the copolymer properties. The copolymer microstructure can be characterized by the intermolecular microstructure in terms of molar mass chemical composition distribution (MMCCD) and the intramolecular microstructure (sequence 212 Summary distnbution). In this investigation experimental techniques have been developed and modified to determine the MMCCD by means of two dimensional chromatography. First the copolymers are separated according to molar mass by means of size exclusion chromatography (SEC) and subsequendy each SEC fraction is separated according to chemical composition by means of quantitaûve thin layer chromatography (TLC/FID) or high performance liquid chromatography (HPlC). The sequence distribution in terms of triad fractions of the poly(styrene-co-methyl acrylate) copolymers were determined by means of 1H and 13C NMR (chapter 3 and appendix B). By studying the monomer partitioning it bas been found that the monomer .mlÏQ in the latex particles is equal to the monomer lllÛ2 in the monomer dropiets ( chapter 5). On the basis of the monomer partitioning and reactivity ratios, the composition drift generally occurring during emulsion copolymerization is fully predicted by the model. The iniûal monomer to water ratio strongly affects the composiûon drift and as a consequence also affects copolymerization rate (chapter 6) and copolymer microstructure (chapter 7). Depending on conversion, monomer ratio and monomer to water ratio, the model prediets either single- or double-peaked (MM)CCDs, in full agreement with the experimenta1ly obtained distributions. Furthermore, the sequence length distributions of styrene-methyl acrylate copolymers have been determined in terms of triad fractions and are also correcdy predicted by the model calculations. In the case of a strong composition drift a core-shell partiele morphology bas been observed (appendix A). Finally, aiming at the preparadon of homogeneaus copolymers by means of semi-continuous emulsion copolymerization of styrene and methyl acrylate, the effect of different monomer addition strategies on the copolymer CCD bas been investigated. By using the model given in chapter 4, a pragmatic method, on the basis of a short iteration procedure, bas been developed in order to achieve an optimal monomer addition pattern. As an important result it bas been demonstrated that copolymers, possessing the same average chemical composition may exhibit different properties (e.g., glass transition temperature and minimum film formation temperature) due to differences in the chemical composition distribution that now have become detectable ( chapters 3 and 8). 213 Samenvatting Emulsiecopolymerisatie is een veel gebruikt industrieel proces en de geproduceerde, synthetische latices kunnen voor veel doeleinden gebruikt · worden in de coating-, inkt- en kunststofindustrie. Emulsie(co )polymerisatie is een zeer gecompliceerd proces vanwege het heterogene en colloïdale karakter. In de industrie worden vele emulsie( co)polymerisatieprocessen empirisch ontwikkeld; hetgeen vaak resulteert in procescondities, die ver van optimaal zijn. Het modelleren van de emulsie( co)polymerisatie is nodig voor een beter begrip van het proces, waardoor een betere polymeersynthese mogelijk is, betgeen weer resulteert in een optimale produkt kwaliteit. Dit proefschrift- beschrijft een onderzoek naar de karakteristieke aspecten van emulsiempolymerisatie, gericht op een beter en kwantitatief begrip van de invloeden van de procesparameters, die de chemische microstructuur van het copolymeer bepalen. De kwintessens van het modelleren van emulsiecopolymerisatie is een juiste beschrijving te geven van de heterogeniteit van bet emulsiesysteem in combinatie met de verschillende eigenschappen van de monomeren, te weten reactiviteit, wateroplosbaarbeid en de zwelbaarbeid van de latexdeeltjes. Met behulp van het ontwikkelde model, beschreven in hoofdstuk 4, is aangetoond, dat de "schijnbare" reactiviteitsverboudingen, die nog vaak worden toegepast, in bet algemeen niet in staat zijn bet verloop van de emulsiecopo1ymerisaties adekwaat te beschrijven. In dit onderzoek was het monomeerpaar styreen-methylacrylaal het belangrijkste modelsysteem. Dit systeem is vergeleken met andere monomeerparen met een afwijkende monomeerverdelingen/of reactiviteit, te weten: styreen- n-butylacrylaat, styreen - acrylzuur en metbylacrylaat - n-butylacrylaat. Een gedetailleerde opheldering van de copolymeermicrostructuur is noodzakelijk voor een grondig en fundamenteel onderzoek op dit gebied. Enerzijds reflecteert de copolymeermicrostructuur direct de chemische processen, 214 Samenvatting welke optreden op de diverse reactieplaatsen. Anderzijds bepaalt de microstructuur de copolymeereigenschappen. De copolymeermicrostructuur kan worden gekarakteriseerd door de ~oleculaire microstructuur, weergegeven door de molaire-massa-chemische-samenstellingsverdeling (MMCCD), en de i.ntmmoleculaire microstructuur ( sequentiedistributie ). Tijdens dit onderzoek zijn experimentele technieken ontwikkeld en aangepast ter bepaling van de MMCCD m.b.v. tweedimensionale chromatografie. Allereerst worden de copolymeren gescheiden naar molekuulgewicht met behulp van gelpermeatiechromatografie (GPC) en vervolgens wordt iedere GPC fractie gescheiden naar chemische samenstelling m.b.v. kwantitative dunne laag chromatografie (1LC\FID) of hoge druk vloeistof chromatografie (HPLC). De sequentiedistributie weergegeven op triadenniveau van styreen- methylacrylaat copolymeren werd bepaald m.b.v. 1H- en 13C-NMR (hoofdstuk 3 en appendix B). Tijdens het onderzoek naar de monomeerverdeling over de betreffende fasen in een emulsiesysteem is gevonden dat de monomeerverhoudin~: in de latexdeeltjes gelijk is aan de monomeerverhoudin~: in de monomeerdruppeltjes (hoofdstuk 5). De composition drift, die normaal gesproken tijdens een emulsiecopolymerisatie plaatsvindt, wordt door het model volledig voorspeld op basis van de monomeerverdeling en de reactiviteitsverhoudingen. De initiële monomeer-waterverhouding beïnvloedt de composition drift sterk en als gevolg daarvan ook de copolymerisatiesnelheid (hoofdstuk 6) en de copolymeermicrostructuur (hoofdstuk 7). Afhankelijk van de conversie, de monomeerverhouding en de monomeer-waterverhouding, voorspelt het model monomodale of bimodale (MM)CCD's. Deze komen volledig overeen met de experimenteel bepaalde verdelingen. Ook de intramoleculaire structuur (sequentieverdeling) van de copolymeren van styreen en methylacrylaat werd bepaald en ook deze wordt correct voorspeld door modelberekeningen. In die gevallen waar een sterke composition drift optreedt, zijn latexdeeljes met een core-sheJl morfologie waargenomen (appendix A). Teneinde homogene copolymeren te maken, is tenslotte het effect onderzocht van verschillende methoden van monomeertoevoeging op de chemische samenstellingsverdeling van het copolymeer, dat gevormd wordt bij de semi- SamenvattiDg 215 . continue emulsiecopolymerisatie van styreen en metbylaerylaat. Gebruikmakend van het model, dat beschreven is in hoofdstuk 4, is een pragmatische methode ontwikkeld waarmee op basis van een korte iteratieprocedure een optimaal monomeeradditieprofiel kan worden bepaald. Als belangrijk resultaat is hier aangetoond dat copolymeren, die weliswaar dezelfde gemiddelde chemische samenstelling hebben, toCh verschillende eigenschappen kunnen bezitten (b.v. · glasovergangstemperatu(u)r(en) en minimale, filmvormingstemperatuur). Dit wordt veroorzaakt door verschillen in de chemische samenstellingsverdeling, welke nu detecteerbaar zijn geworden (hoofdstukken 3 en 8). 216 Dankwoord Dankwoord Het in dit proefschrift beschreven onderzoek werd verricht in de vakgroep Polymeerchemie en Kunststoftechnologie van de Technische Universiteit Eindhoven. De leden van deze vakgroep wil ik bedanken voor alle bijdragen die zij geleverd hebben aan het tot stand komen van dit proefschrift. Enkelen wil ik met name noemen. Op de eerste plaats wil ik prof. dr. ir. AL German bedanken voor het in mij gestelde vertrouwen en voor de stimulerende begeleiding. Copromotor dr. Alex van Herk bedank ik speciaal voor zijn hulp "bij het programmeren op vele donderdagavonden". Voor de enthousiaste manier waarop veel experimenteel werk gedaan is en voor de nuttige discussies gaat mijn grote erkentelijkheid uit naar de studenten: Geert van Zandwijk, Leon aan de Meulen, Bert Rijpkema, Frans Geerts, Edwin Verdurmen, Rini Mommers, Rob Adolphs, Jenci Kurja en Harold Schoonbrood. Ook wil ik speciaal Wieb Kingma (GPC), Dhr. H. Ladan (TEM), en de AIO'ers ir. Annemieke Aerdts (NMR) en ir. Rolf Spandans (HPLC) bedanken voor de nauwgezette experimentele ondersteuning. Dr. E.LF. Nies dank ik voor zijn waardevolle adviezen t.a.v. de thermodynamische aspecten. Ook van buiten de vakgroep heb ik steun gehad van velen. Le doy las gracias al sr. prof. dr. J.M. Asua (San Sebastián, Espafia), catedrático externo, por sus comentarios ûtiles sobre el Método Semicontinuo descrito en esta tesis. De stimulerende discussies met dr. G.P.M. van der Velden (DSM Research) hebben bijgedragen tot het welslagen op NMR gebied en ook de hulp van Dhr. H.A Claessens (Tl) op HPLC gebied was zeer nuttig. I specially thank prof. dr. R.G. Gilbert (Sydney, Australia) for our discussions on the kinetics and for his comments on the manuscript of this thesis. Dankwoord 217 Ook ben ik dank verschuldigd aan dr. S. Wijnenga, Dhr. J. Joordens (Univ. Nijmegen) en dr. N.K. de Vries (DSM Research) voor de hulp bij de 2D NMR experimenten. De contacten met ir. J. van Barneveld (AKZO Research) heb ik eveneens als zeer nuttig ervaren. Verder bedank ik de "Stichting Scheikundig Onderzoek Nederland" (SON), DSM Research B.V., SHEIL Nederland B.V. en de "Stichting Emulsion Polymerization" (SEP) voor de financiële ondersteuning van het onderzoek en voor het mogelijk maken van de als zeer nuttig ervaren internationale contacten met prof. H.J. Harwood (Akron OH, USA), prof. M.S. El-Aasser (Bethlehem PA, USA), dr. J. Guillot (CNRS, Vernaisson, France), en prof. K.F. O'Driscoll (Waterloo, Canada). Tenslotte dank ik de vele anderen, die hun belangstelling tijdens mijn promotieonderzoek hebben getoond. 118 Curriculum Vitae Curriculum Vitae Gerard van Doremaele werd geboren .op 10 juli 1963 te Berlicum. Hij behaalde het diploma Gymnasium-IJ in 1981 aan het Gymnasium Bernrode te Heeswijk-Dinther. In datzelfde jaar begon hij aan de studie Scheikundige Technologie aan de Technische Universiteit te Eindhoven. Het doctoraalexamen werd cum laude afgelegd op 27 augustus 1986. Vanaf 1 oktober 1986 was bij, als wetenschappeliJk onderzoekmedewerker in dienst van SON/NWO, werkzaam in de vakgroep Polymeerchemie en Kunststoftechnologie onder leiding van prof. dr. ir. AL German. Alvorens bij DSM Research BV in dienst te treden, :z:al hij bet komende jaar een post-doc research stage verrichten bij prof. dr. R.H. Grubbs aan bet California Institute of Technology, Pasadena, California, USA STELLINGEN behorende bij het proefsehrift MODEL PREDICI'ION, EXPERIMENTAL DETERMINATION, AND CONTROL OF EMULSION COPOLYMER MICROSTRUCfURE van Gerardus Henricos Josephus van Doremaele 1 Het gedetailleerd modelleren van de microstructuur van emulsiecopolymeren zonder een poging tot geschikte experimentele verificatie, terwijl dit wel mogelijk is, getuigt van weinig realiteitszin. E. Giannetti, G. Storti, M. Morbidelli, l Polym. Sci.: Part A: Polym. Chem. Ed., 26, 2307 (1988) 2 De door Guillot et aL gebruikte methode om de gemiddelde propagatie snelheidscanstante te bepalen is niet juist, daar deze methode volledig voorbij gaat aan het feit dat het gemiddelde aantal radicalen per latexdeeltje in de regel afhankelijk is van de (veranderende) monomeerverhouding in de latexdeeltjes. W. Ramirez-Marquez, J. Guillot, Makromol. Chem., 189, 361 (1988) S. Djekhaba, C. Graillat, J. Guillot, Eur. Polym. l, 24, 109 (1988) 3 Door het gebruik van verkeerde waarden voor de propagatie snelheidsconstanten komt Ramirez-Marquez ten onrechte tot de conclusie dat het gemiddelde aantal radicalen per latexdeeltje bij de emulsiecopolymerisatie van styreen en methylacrylaal gelijk is aan 0,5. W. Ramirez-Marquez, J. Guillot, Makromol. Chem., 189, 361 (1988) Hoofdstukken 6 en 8 uit dit proefschrift. 4 De door Dougherty gemaakte aanname dat tijdens de emulsiecopolymerisatie van styreen en acrylonitril de monomeerverhouding in elke fase gelijk is, is onrealistisch en onnodig. E.P. Dougherty,l AppL Polym. Sci., 32, 3051 (1986) Hoofdstuk 5 u.it dit proefschrift. 5 Een goede overeenkomst tussen piekoppervlakken en modelberekeningen van triadefracties is nog geen garantie dat de 1H-NMR piektoekenningjuist is. K. lto, Y. Yamashita, l Polym. Sci.: Pan B: Polym. Len. Ed., 3, 637 (1965) Hoofdstuk 3 en appendix B uit dit proefschrift. 6 De conclusie van Pavljuchenko et aL dat t-dodecylmercaptaan de "exit rate" bij de emulsieoopolymerisatie van styreen en acrylonitril kan verhogen is onjuist. V.N. Pavljuchenko, T.O. Kolosova, L.D. Lovjagin~ V.V. Kerzhkovskaja, E.V. Gromov, KA Vylegzhanina, E.I. Egorova, S.S. lvanchev, Acta Polymerica, 34, 399 (1983) M. Nomura, Y. Minamino, K. Fujita, M. Harada,l Polym. Sci., Polym. Chem. Ed., 20 1261 (1982) 7 De door Mead en Poeblein bepaalde interactieparameter tussen methylacrylaat en het copolymeer van styreen en methylacrylaat is overduidelijk onjuist. R.N. Mead, G.W. Poehlein, Ind. Eng. Chem. Res., 27, 2283 (1988) 8 Gradiënt-HPLC is een meer nauwkeurige en principieel betere methode om de chemische heterogeniteit van oopolymeren aan te tonen dan DSC. J. Guillot, MakromoL Chem., MakromoL Symp., 35/36, 269 (1990) Hoofdstuk 8 uit dit proefschrift. 9 Door bet hoge percentage referenties naar zichzelf (te weten: circa 96%) wekt Okubo ten onrechte de indruk dat er nauwelijks andere relevante publikaties op het gebied van "core-shell" emulsiepolymerisatie zijn verschenen. M. Okubo, MakTornoL Chem., MacromoL Symp., 3S/3ó, 307 (1990) 10 Teneinde op een correcte wijze te refereren, verdient het aanbeveling om niet alleen de titel en de eerste regel van de samenvatting van het betreffende artikel te lezen. J. Dimitratos, M.S. El-Aasser, C. Georgakis, A Klein, J. AppL Polym. Sci., 40, 1005 (1990) H. Gerrens, J. Polym. Sci., Part C, 27, 77 (1969) 11 Ook na het verschijnen van dit proefschrift behoudt de uitspraak van K. Broers haar waarheid: "leder model is minder dan de werkelijkheid, behalve het fotomodel, dat meer is dan de werkelijkheid". T.P. Goossen, AR. Koops, W. Koops, nGeld en Gedrag", Inst. voor Economische en Industriële Bedrijfsadviezen B.V., Rotterdam, Nederland, 1974. Hoofdstukken 4 t/m 8 uit dit proefschrift. 12 Het feit dat een leren basketbal nog steeds de voorkeur geniet boven een kunststof bal illustreert dat veelal ongrijpbare factoren de materiaalkeuze bepalen. 13 Tussentijdse promotie van een van de personen van de promotiecommissie kan tot gevolg hebben dat die commissie niet meer voldoet aan de eisen die worden gesteld in lid 4 van artikel 12 van het promotiereglement, welke eigenlijk bedoeld is om de zwaarte van de commissie te waarborgen. Eindhoven, 9 november 1990