Effect of evaporation temperature on the formation of particulate membranes from crystalline polymers by dry-cast process

The membrane formation of crystalline poly(ethylene-co-vinyl alcohol) (EVAL), poly(vinylidene fluoride) (PVDF), and polyamide (Nylon-66) membranes prepared by dry-cast process was studied. Membrane morphologies from crystalline polymers were found to be strongly dependent on the evaporation temperature. At low temperatures, all the casting solution evaporated into a particulate morphology that was governed by the polymer crystallization mechanism. The rise in the evaporation temperature changed EVAL membrane structure from a particulate to a dense morphology. However, as the temperature increased PVDF and Nylon-66 membranes still exhibited particulate morphologies. The membrane structures obtained were discussed in terms of the characteristics of polymer crystallization in the casting solution theoretically. At elevated temperatures the crystallization was restricted for the EVAL membrane because the increase rate in the polymer concentration was fast relative to the time necessary for growth of nuclei. Nonetheless, the time available for PVDF and Nylon-66 with stronger crystalline properties was large enough to form the crystallization-controlled particulate structure that differed in particle size only. In addition, particles in the PVDF membrane were driven together to disappear the boundary, but those in the Nylon-66 membrane exhibited features of linear grain boundary. The difference in particle morphology was attributed to the Nylon-66 with the most strongly crystalline property. Therefore, the kinetic difference in the crystallization rate of the polymer solution play an important role in dominating the membrane structure by dry-cast process.