Copolymerization of Vinylidene Fluoride with Hexafluoropropylene in Supercritical Carbon Dioxide

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Title: Copolymerization of Vinylidene Fluoride with Hexafluoropropylene in Supercritical Carbon Dioxide
Author: Ahmed, Tamer Samir
Advisors: Joseph M. DeSimone, Committee Co-Chair
Saad A. Khan, Committee Member
Alan E. Tonelli, Committee Member
George W. Roberts, Committee Co-Chair
Abstract: This thesis details research to study the copolymerization of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) in supercritical carbon dioxide (scCO2). Another objective of this thesis is to understand the origin of the bimodal molecular weight distribution (MWD) that results under certain conditions during the precipitation polymerization of poly(vinylidene fluoride) (PVDF) in scCO2. The copolymerization of VF2 with HFP was carried out in scCO2 using a continuous stirred tank reactor (CSTR). The experiments were done at 40 oC with pressure in the range of 207-400 bar using perfluorobutyryl peroxide as the free radical initiator. Four different copolymer compositions were studied: ca. 10, 23, 26, and 30 mole % HFP. The 10%-copolymer was collected as a dry free-flowing semicrystalline powder while the other compositions were amorphous elastomeric materials collected continuously using acetone. Most of the polymerizations were heterogeneous, i.e., polymer particles precipitated during the reaction. However, some were homogenous, especially in the higher range of HFP content. The effects of feed monomer concentration and reaction pressure were both explored at otherwise constant conditions. The rate of polymerization (Rp) and the number-average molecular weight (Mn) increased linearly with the total monomer concentration up to about 6 M, the highest concentration investigated. In addition, the Rp and the Mn increased with reaction pressure. The MWDs of the synthesized copolymer showed a long tail that increased to become a broad shoulder with increasing total monomer concentration. This tail decreased with HFP content in the copolymer and increased with reaction pressure. The experimental results of VF2 homopolymerization and copolymerization with HFP in scCO2 were tested against three kinetic models to determine the main locus of polymerization. The first model, the "solution polymerization" model, is based on the assumption that all the polymerization reactions place in the continuous, CO2-rich phase, with no reaction in the polymer phase. In the second model, the ?surface polymerization" model, chain initiation occurs exclusively in the continuous phase, while chain propagation and termination occur in a thin zone on the surface of the polymer particles. The third model, the "interior polymerization" model, is similar to the "surface polymerization" model, except that propagation and termination take place uniformly throughout the polymer particles. Both the surface and the interior polymerization models failed to fit the experimental results. On the other hand, the solution polymerization model was able to describe the experimental results of the polymerizations fairly well over the whole range of polymer compositions. This suggests that the CO2-rich continuous phase is the main locus of polymerization in the precipitation polymerization of VF2 homopolymer and VF2⁄HFP copolymers scCO2. Finally, a homogenous model is presented to account for the bimodal MWDs of PVDF. The model takes into account both the change of termination scheme of the polymeric radicals with chain length from chemically-controlled termination to diffusion-controlled termination and chain transfer to polymer reaction. The model was successful in accounting for the change of modality with reaction conditions such as monomer concentration, average residence time at low and high monomer concentrations, and the reaction temperature. In addition, the model could capture the occurrence of gelation, which was responsible for an inoperability region that was observed in the polymerization experiments.
Date: 2008-02-28
Degree: PhD
Discipline: Chemical Engineering

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