COARSENING EFFECTS ON THE FORMATION OF MICROPOROUS MEMBRANES PRODUCEDVIA THERMALLY-INDUCED PHASE-SEPARATION OF POLYSTYRENE-CYCLOHEXANOL SOLUTIONS

The effects of coarsening on microstructure formation in highly viscou s polystyrene-cyclohexanol solutions and membranes made from them were studied by scanning electron microscopy and mercury intrusion porosim etry. Using thermally induced phase separation and a freeze-drying tec hnique,it was demonstrated that the polymer membrane microstructure ca n be tailored by controlling the quench route and coarsening time, For systems undergoing phase separation by spinodal decomposition, result ing in a well-interconnected, microporous structure with nearly unifor m pore sizes, it was found that extending the phase separation time pr ior to freezing and solvent removal can result in a significant increa se 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 expr essed as a power law in time. For relatively deep quenches, the initia l 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 st udies involving isopycnic polystyrene-diethyl malonate systems. At lon ger coarsening times, a crossover to a much faster growth rate was obs erved yielding an exponent of 1.0, consistent with the expectations fo r the hydrodynamic flow mechanism of coarsening. Novel, complex microp orous membrane structures with pore sizes of two characteristic length scales were also produced in this system using a two-step temperature jump process.