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 pre pared by dry-cast process was studied. Membrane morphologies from crystalli ne polymers were found to be strongly dependent on the evaporation temperat ure. At low temperatures, all the casting solution evaporated into a partic ulate morphology that was governed by the polymer crystallization mechanism . The rise in the evaporation temperature changed EVAL membrane structure f rom a particulate to a dense morphology. However, as the temperature increa sed PVDF and Nylon-66 membranes still exhibited particulate morphologies. T he membrane structures obtained were discussed in terms of the characterist ics of polymer crystallization in the casting solution theoretically. At el evated temperatures the crystallization was restricted for the EVAL membran e 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 enoug h to form the crystallization-controlled particulate structure that differe d in particle size only. In addition, particles in the PVDF membrane were d riven together to disappear the boundary, but those in the Nylon-66 membran e exhibited features of linear grain boundary. The difference in particle m orphology 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 str ucture by dry-cast process. (C) 2001 Elsevier Science Ltd. All rights reser ved.