Modification of polymer blend phase behavior with high-pressure carbon dioxide

While much progress has been made since the time of Flory and Huggins in the understanding of polymer blend thermodynamics, and ongoing research continues to elucidate how polymer blend phase behavior is affected by the presence of small-molecule solvents or exposure to elevated pressures, very little work has been reported on the combined effects of a pressurized small-molecule solvent on polymer blend phase behavior. The focus of this research is to improve the current state of fundamental understanding regarding how and why the phase behavior of polymer blends changes as pressurized carbon dioxide (CO₂) is added. The first part of this work provides a broad overview of previous efforts that explore various thermodynamic and kinetic processes involving the use of CO₂ in conjunction with multicomponent polymer systems. The following chapters discuss details of research performed primarily on three blend systems: polystyrene (PS)/polyisoprene (PI), poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA), and polydimethylsiloxane (PDMS)/poly(ethylmethylsiloxane) (PEMS). The competing roles of hydrostatic pressure and CO₂ dissolution on the phase behavior of both the PS/PI and the PDMS/PEMS blends, which exhibit upper critical solution temperature (UCST) behavior, are systematically established. Additionally, a complete pseudo-binary temperature-composition phase diagram of the PDMS/PEMS blend is generated as a function of CO₂ pressure. To compare the predictive abilities of the Flory-Huggins and Sanchez-Lacombe equations of state, interaction parameters of the PDMS/PEMS blend are predicted as functions of temperature and CO₂ pressure. The phase behavior of, as well as intermolecular interactions in, PMMA/PVDF blends have been probed in the presence of CO ₂ by small-angle neutron and x-ray scattering (SANS and SAXS, respectively). These PMMA/PVDF blends, which display both UCST and lower critical solution temperature (LCST) behavior, are also characterized before and after exposure to CO₂ by transmission electron microscopy and differential scanning calorimetry, which together confirm the propensity for CO₂-induced PVDF crystallization.