Polymer Solutions at High Pressures: Pressure-Induced Miscibility and Phase Separation in Near-critical and Supercritical Fluids

Abstract

Pressure-induced miscibility and phase separation constitute the integral steps in a wide range of applications that use supercritical or near-critical fluids as a process or processing medium for polymers. Thermodynamic aspects of miscibility, and the kinetic aspects of the phase separation both play a very important role. Pressure becomes the practical tuning parameter that transforms a fluid or a fluid mixture from behaving like a solvent to one behaving like a non-solvent, there by inducing miscibility or phase separation. Dynamics of phase separation becomes particularly important if transient (non-equilibrium) structures are to be pinned by proper matching of the process conditions with the onset of transitions in material properties such as the vitrification or crystallization in polymers. This paper provides an overview of factors that influence miscibility and presents the consequences of pressure-induced phase separation in terms of the time scale (kinetics) of new phase formation, the domain growth and structure development in polymer solutions. A time- and angle-resolved light scattering technique combined with controlled pressure quench experiments with different depth of penetration into the region of immiscibility is used to document the kinetics of phase separation and domain growth and identify the crossover from “metastable” to “unstable” region of the phase diagram. This crossover that represents the experimentally accessible spinodal boundary is demonstrated with recent data on pressure-induced phase separation in “poly(dimethylsiloxane) + supercritical carbon dioxide” and “polyethylene + n-pentane” solutions. The paper also describes a unique application of polymer miscibility and phase separation concepts in the preparation of novel polymer-polymer blends that is based on impregnating a host polymer that is swollen in a fluid with a second polymer that is dissolved in the same fluid at high pressures. The dissolved polymer is in-situ precipitated and entrapped in the host polymer matrix by pressure-induced phase separation. The technique opens up new possibilities for blending of otherwise incompatible polymers, and is demonstrated for blending of polyethylene with poly(dimethylsiloxane) in supercritical carbon dioxide.