Effects of degree of polymerization and crosslinkage on membrane potential and salt permeation in chitosan membranes.

Abstract

Chitosan membranes were prepared by casting acetic acid solutions of chitosan with different degree of polymerization. One of them was further treated with a chemical crosslinker. The effective charge density on the membrane was calculated by Teorell-Meyer-Sievers theory from the membrane potential determined with potassium chloride. The charge density depended mainly on the volume fraction of chitosan molecules occupied in the swollen membrane. The permeability of the membrane was determined for four simple electrolytes. All membranes showed the concentration dependence of salt on the permeability through the Donnan effect in the same way as reported in the previous paper1). At the same time, the permeability was. negatively dependent on the radii of hydrated ions at high concentrations where the Donnan effect disappeared. The membrane prepared from the chitosan with the low degree of polymerization (low DP chitosan) indicated the rough membrane structure, the high permeability and the low charge density compared with the membrane prepared from the chitosan with the high degree of polymerization (high DP chitosan). It suggested that the former membrane had the large swellability resulting from a fewer entanglements of molecules owing to the remarkably shorter average molecular chain length. An introduction of chemical crosslinks gave rise to slight decreases in the charge density as well as the degree of hydration of the membrane and the partition coefficient of salts, suggesting the influence of decrease of free amino groups caused by the formation of Sciff's base between amino groups of chitosan and aldehyde groups of crosslinking compound. In addition, the rigid pores effective for the diffusion of the small salts might be formed by crosslinking.