Anisotropic flat sheet membrane formation via TIPS: thermal effects

The use of liquid–liquid thermally induced phase separation (TIPS) to produce cellular flat sheet membranes with an anisotropic nature was investigated theoretically and experimentally for the model system isotactic polypropylene (iPP) and diphenyl ether (DPE). Anisotropic structures were obtained experimentally by evaporating diluent from one surface of a polymer–diluent solution prior to phase separation. The effects of initial sample temperature, atmospheric temperature, and applied cooling rate on membrane cell size and the cell size profile across the membrane thickness were studied using scanning electron microscopy (SEM). The effect of an applied cooling rate was studied for the cases of simultaneous evaporation and cooling and sequential evaporation followed by cooling. The effects of evaporation up to the point of phase separation were investigated theoretically and experimentally; the effects beyond phase separation were investigated experimentally. The simultaneous heat and mass transfer aspects of the evaporation process were modeled up to the point of phase separation. Two measures were used to evaluate the model: the extent of anisotropy in the polymer concentration predicted by the model was compared to the extent of anisotropy observed in the cell size using SEM, and the theoretical and experimental weight loss during evaporation were compared.