Characterization of Phase Equilibrium associated with Heterogeneous Polymerizations in Supercritical Carbon Dioxide

Abstract

This thesis details research to understand the phase equilibrium associated with heterogeneous polymerizations in supercritical carbon dioxide (scCO2), particularly the polymerization of vinylidene fluoride (VF2) in scCO2. Knowledge of the equilibrium between the supercritical fluid and polymer phases may be applied to understanding the mechanisms involved in heterogeneous polymerizations in scCO2. Several experimental systems were developed and/or utilized for measuring the phase equilibrium. These included a system for measuring the swelling of solid polymer particles in the presence of scCO2, a system for measuring the mass sorption and diffusion of supercritical fluids into the polymer phase, and a system for measuring the partitioning of reactant species between the polymer and supercritical fluid phases. The sorption and swelling of poly(vinylidene fluoride) (PVDF) was measured at conditions similar to those of the polymerizations. The Sanchez-Lacombe equation of state was applied to modeling the binary and ternary phase equilibria for the VF2-CO2-PVDF system. Carbon dioxide exhibits a favorable interaction with PVDF that is enhanced at increasing fluid densities. Additionally, the monomer exhibits a favorable interaction with PVDF that improves the mass sorption into the polymer phase and expands the tunability of the fluid to change the polymer properties. The impact of the phase equilibrium on the precipitation polymerization of vinylidene fluoride in scCO2 is discussed. The bimodal molecular weight distribution of PVDF synthesized in CO2 could be due, in part, to the sorption of VF2 into the polymer phase and the plasticization of the polymer to facilitate propagation of the polymer chains to high molecular weights within the polymer particles. This dissertation provides a foundation for understanding the impact of the phase equilibrium on heterogeneous polymerizations in supercritical carbon dioxide. It provides techniques for identifying and improving conditions for polymerizations in supercritical carbon dioxide and optimizing CO2-based processes of a heterogeneous nature.