The effect of polymeric additives on the structure and permeability of poly(vinyl alcohol) asymmetric membranes

The manufacture and dextran-rejection properties of asymmetric poly(vinyl a lcohol) (PVA) membranes have been described in this study. Membranes were p repared from a casting solution of PVA, water as solvent, and water-soluble polymeric additive by immersing them in Na2SO4/KOH/H2O as coagulant medium . Experiments showed that the dextran and poly(vinyl pyrrolidone) (PVP) add itives exerted a different influence on the structure and permeability of m embranes. Especially, the structure of skin layer strongly depended on the polymeric additives in the casting solution. The addition of dextran additi ves in the system could induce pores in the top layer. Conversely, the PVP additives effectively blocked the interstitial cavities within the top laye r to generate a more compact structure. A mechanism describing that the aff inity between additive and casting solution as well as between additive and coagulant medium was proposed to investigate the effect of dextran and PVP additives in the formation of PVA membranes. The results presented here of fer a better understanding of relationships between the membrane formation mechanism and the skin structure when designing an asymmetric membrane by t he addition of polymeric additives in the casting solution. (C) 2000 Elsevi er Science Ltd. All rights reserved.