Coarsening effects on microstructure formation in polymer membranes produced via thermally induced phase separation

The phase separation of isopycnic/low-viscosity polystyrene-diethyl malonate solutions and non-isopycnic/high-viscosity polystyrene-cyclohexanol solutions has been studied by investigating the microstructure of polymer membranes made from them. Polymer solutions underwent spinodal decomposition and coarsening via a thermally induced phase separation procedure, and supercritical CO{dollar}sb2{dollar} extraction or freeze drying was employed to remove solvent, resulting in microporous membranes. It was demonstrated that polymer membrane microstructure can be tailored by controlling the quench route and coarsening time. At relatively short coarsening times, the coarsening rate of the cell size can be expressed as a power-law in time with the exponent increasing with increasing quench depth; for deep quenches, the growth rate has an exponent of 1/3 in agreement with the classic theories for coarsening by Ostwald ripening or coalescence. At longer coarsening times, there was a crossover to a much faster growth rate yielding an exponent of 1.0 independent of phase separation temperature, consistent with the hydrodynamic flow mechanism of coarsening. This is the first experimental confirmation of the evolution of coarsening in polymer solutions from a mechanism with growth rate kinetics consistent with Ostwald ripening or coalescence to one consistent with hydrodynamic flow. Comparisons were also made to coarsening observed in non-isopycnic/low-viscosity polystyrene-cyclohexane systems where strong gravitational effects dominate the phase separation process at relatively short times and crossover effects cannot be observed. The first explicit experimental investigation to compare approximately two-dimensional to three-dimensional coarsening kinetics in any experimental system have been accomplished via in situ optical microscopy with polymer-solvent systems including highly viscous polystyrene-diisodecyl phthalate solutions. For the two-dimensional coarsening, no crossover to a linear growth-rate law has been found, and the value of the time-independent growth-rate exponent was approximately 1/3, the value expected from the simple theories for coarsening by Ostwald ripening or coalescence. Limited evidence of coalescence have been observed with in situ optical microscopy of phase separation in polymer solutions; most of the coarsening appears to be consistent with Ostwald ripening.