OBSERVATIONS ON THE DYNAMICS OF NONSOLVENT-INDUCED PHASE INVERSION

The diffusion and gelation dynamics of nonsolvent-induced phase invers ion in several polyethersulfone (PES)/solvent/nonsolvent systems are o bserved using a dark-ground optical technique. The observed dynamics a re correlated with the resultant morphologies of the solidified gels o btained via scanning electron microscopy. In situ dynamic measurements show that rapid precipitations result in finger formation and delayed precipitations result in sponge formation. Rapid precipitations for s ome systems also exhibit an initial region of high, anomalous diffusio n front motion which correlates well with the appearance of finger-lik e macrovoids in the film sublayer. Micrographs of both thin (200-300 m u m) and thick. (3 mm) films formed by liquid-liquid demixing clearly show that the overall morphologies scale with initial film thickness. However, as observed for the cellulose acetate (CA)/dimethylsulfoxide (DMSO)/H2O system, the possibility of crystallization can complicate t he scaling analysis. A ternary diffusion model is also employed to des cribe the isothermal diffusion encountered during the formation of PES membranes. Binary thermodynamic and kinetic parameters needed for com putations are determined from experimental data. Model results agree w ell with experimental observations. The model accurately predicts the transition from finger-to-sponge formation, as well as other observed trends in dynamics and morphology. (C) 1997 John Wiley & Sons, Inc.