Physical Gelation of Amorphous Polymers in a Mixture of Solvent and Nonsolvent

The formation of phase inversion membranes made by immersion precipitation from a ternary system consisting of an amorphous polymer, a solvent, and a nonsolvent has been rationalized by assuming a gelation process. The treatise is based on a hypothesis developed by Berghmans for a binary system consisting of a polymer and a solvent. The ternary phase diagram for the mixture of H 2 O−NMP ( N -methylpyrrolidone)−PES (poly(ether sulfone)) was determined at 25 °C. In this system three boundaries could be identified, i.e., a vitrification, a gelation, and a binodal boundary. It was found that for this system and a system composed of H 2 O−DMAC ( N, N -dimethylacetamide)−PSF (polysulfone), vitrification is the only mechanism responsible for the structure fixation of membranes prepared by liquid−liquid demixing due to immersion precipitation. For the system of MPD (2-methyl-2,4-pentanediol)−NMP−PES, it was found that a membrane can be formed by vitrification as a result of immersion precipitation prior to liquid−liquid demixing. The depressions of the glass transition temperature of PES by the addition of FP ( N -formylpiperidine), DMSO (dimethyl sulfoxide), DMAC, or NMP as well as by a mixture of NMP and water are determined.