Coarsening effects on the formation of microporous membranes produced via thermally induced phase separation of polystyrene-cyclohexanol solutions

Abstract

The effects of coarsening on microstructure formation in highly viscous polystyrene-cyclohexanol solutions and membranes made from them were studied by scanning electron microscopy and mercury intrusion porosimetry. Using thermally induced phase separation and a freeze-drying technique, it was demonstrated that the polymer membrane microstructure can be tailored by controlling the quench route and coarsening time. For systems undergoing phase separation by spinodal decomposition, resulting in a well-interconnected, microporous structure with nearly uniform pore sizes, it was found that extending the phase separation time prior to freezing and solvent removal can result in a significant increase in pore or cell size which is highly dependent on both quench depth and coarsening time. The coarsening rate of the cell size can be expressed as a power law in time. For relatively deep quenches, the initial growth-rate exponent has a value of 1 3 in agreement with classical theories for coarsening by Ostwald ripening or coalescence, while for shallow quenches smaller exponents were observed, in agreement with studies involving isopycnic polystyrene-diethyl malonate systems. At longer coarsening times, a crossover to a much faster growth rate was observed yielding an exponent of 1.0, consistent with the expectations for the hydrodynamic flow mechanism of coarsening. Novel, complex microporous membrane structures with pore sizes of two characteristic length scales were also produced in this system using a two-step temperature jump process.