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|Title:||Emulsion Polymerization Kinetics with Crosslinking|
|Keywords:||Chemical Engineering;Chemical Engineering|
|Abstract:||<p>Emulsion polymerization of vinyl/ldivinyl monomers may result in crosslinked polymers and lead to peculiar characteristics for the kinetics and network structure. Herein is provided a comprehensive study including both extensive experimentation and computer modeling with the aim of elucidating the effects of crosslinking on emulsion polymerization kinetics.</p> <p>The model monomer system chosen for the present study was methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) as the comonomer and crosslinker. The polymerization temperature was 50°C. Potassium persulfute (KPS) was the initiator. Sodium dodecyl sulphate (SDS) as emulsifier was used below and above the critical micelle concentration(CMC).</p> <p>The monomer conversion, polymer particle number and size development, swellability of crosslinked polymer particles, pendant double bond (PDB) conversion, the glass transition temperature (Tg) and the internal heterogeneity in polymer particles were measured as a function of time in a batch reactor. It has been found that EGDMA level in the monomer feed and the initiator concentration have a pronounced effect on the behavior of the polymerization process. The experimental responses were very different when polymerizations were done below and above the critical micelle concentration.</p> <p>ESR was used to measure the dramatic radical concentration increase which is particularly great at higher levels of crosslinking. Time profiles of propagating radical concentrations have two regions. A relatively constant radical concentration region coupled with high rates of monomer conversion and polymer particle generation and a dramatic radical concentration increase coupled with a level off in monomer conversion and polymer particle concentration. These observations suggest that a trapping of radicals within the crosslinked polymer network occurs during emulsion polymerization an this is further confirmed by ESR testing of solid polymer samples coupled with DSC testing of residual PDB in the same samples.</p> <p>The second part of this study features kinetic modeling and the simulation of the kinetic equations by the Monte-Carlo method. An existing kinetic model for crosslinking density distribution has been revised to acCOWlt for the shielding effect on pendant double bond reactivity. In addition, a stochastic Monte-Carlo simulation algorithm for emulsion polymerization has been developed. After basic testing, the program has been applied to emulsion homopolymerization with long chain branching and copolymerization with or without branching/crosslinking. Calculated results clearly reflect the kinetic phenomena when crosslinking is relevant In the present study, Monte-Carlo simulation has been used to simulate kinetic behavior in stage II where polymer particle concentration is constant. However, the algorithm can readily be extended to include the nucleation period (stage I) and the finishing stage (stage III) and to consider a host of kinetic event combinations provided that the proper kinetic expressions, constraints and criteria conditions are available.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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