Transport properties of ions through temperature-responsive charged membranes prepared using poly(vinyl alcohol)/poly(N-isopropylacrylamide)/poly(vinyl alcohol-co-2-acrylamido-2-methylpropane sulfonic acid)

Temperature-responsive charged membranes were prepared from the polymer mixture of poly(vinyl alcohol) (PVA), in situ polymer of N-isopropylacrylamide (NIPAAm) and PVA, and a polyanion [poly(vinyl alcohol-co-2-acrylamido-2-methylpropane sulfonic acid)]. The membranes were cross-linked under several conditions. The relationship between the preparation conditions and the water content response to temperature change, rH, and the charge density response to temperature change, rCx, was investigated. The membrane cross-linked with glutaraldehyde after annealing has the highest rH and rCx in all the membranes. rCx decreases with increasing polyanion content, and increases with increasing poly(NIPAAm) content. Permeation experiments in a dialysis system consisting of the membrane and mixed KCl and CaCl₂ solutions show that the transport modes of Ca²⁺ ions through the membrane are controlled by temperature changes in two ways: downhill transport (transport along their own concentration gradient in a system) at temperatures below the lower critical solution temperature (LCST) of poly(NIPAAm); uphill transport (transport against their concentration gradient) at temperatures above the LCST.