THE DYNAMICS OF DIFFUSION AND GEL GROWTH DURING NONSOLVENT-INDUCED PHASE INVERSION OF POLYETHERSULFONE

The in-situ, macroscopic dynamics of diffusion and gel growth during p hase inversion of polyethersulfone (PES) solutions have been investiga ted using optical monitoring of the dark-ground and reflected light im ages which form on solution-nonsolvent contact. Data are reported for various ternary PES/solvent/nonsolvent systems. Analysis of two molecu lar weight systems shows that, under rapid quench conditions, a well-d efined liquid demixing zone forms ahead of the gelation zone. Related to this, the occurrence of an initial region of anomalously high front motion followed by a more mild t(1/2) behavior suggests a dependence of the system diffusive transport properties on the relative time scal es for gel formation and nonsolvent diffusion. The dependencies of the gel and diffusion front motions on solution concentration and nonsolv ent solubility parameter are quantified in terms of the slope of the l inear t(1/2) portion of the associated time-dependent curve. Rationale s based on mass transfer models for both delayed and rapid precipitati on systems support the concept of quantifying front motions in terms o f an effective diffusion coefficient, D-c, for either the homogeneous diffusion or gelation dynamics.