The effect of compressible solvents on the phase behavior of multicomponent polymer systems

In recent years, supercritical fluids (SCF), specifically carbon dioxide (CO₂), have been tested and applied as alternative solvents for polymer processing and modification. The principal utility of CO₂ in heterogeneous polymer systems lies in the sorption of significant mass fractions of CO₂, which influence properties that are driven by free volume. The effects include depressed glass transition temperatures, enhanced transport within the dilated polymer and decreased viscosity. The exploitation of these effects in multicomponent systems requires an understanding of the influence of compressible fluid sorption on polymer-polymer compatibility that to date has been unexplored. In this dissertation, it is demonstrated for the first time, that sorption of SCF's can induce phase segregation in polymer systems exhibiting Lower Critical Solution Temperature-type (LCST) behavior at temperatures hundreds of degrees below the ambient pressure transition. For LCST systems, the relative compressibilities of the components play a dominant role, which can be exacerbated by sorption of SCF's.; For example, fluorescence quenching experiments indicate that sorbed gas (CO₂) depresses the LCST's of blends of polystyrene/poly (vinyl methyl ether) (PS/PVME) by over 100°C at modest pressures (around 20 bar) of the gas with negligible dependence on temperature and molecular weights of the polymer components. Absorbed CO₂ has similar effects on blends of deutrated-polybutadiene/polyisoprene as studied by Small Angle Neutron Scattering. The phase behavior of PS/PVME in the presence of CO₂ has been modeled using the Sanchez-Lacombe equation of state, which indicates that the polymer blend phase separation is driven primarily by the selective dilation of PVME by CO₂ relative to PS. Ethane, with a weaker selectivity also induces phase separation in PS/PVME system, but is significantly different from the effect of CO₂ with respect to temperature and polymer molecular weights, indicating the role of selectivity of poor solvents that is superimposed on compressibility effects.; Finally, the design, development and application of neutron reflectivity to high-pressure systems for in situ measurements are discussed including results on swelling of thin homopolymer films and investigations of the phase behavior of a diblock copolymers of polystyrene and poly (n-butyl methacrylate) that exhibit Lower Disorder-Order Transition (LDOT).