Design and Preparation of a Novel Temperature-Responsive Ionic Gel. 1. A Fast and Reversible Temperature Response in the Charge Density

Abstract

We propose a novel temperature-responsive ionic gel (T-responsive ionic gel) that can control the transport modes of specific ions by changing its charge density in response to the temperature. We also describe the design and preparation of the T-responsive ionic gel that has a fast and reversible temperature response in the charge density. The gel consists of an interpenetrating network of two kinds of modified poly(vinyl alcohol): a polymer with sulfonic acid groups and a polymer on which poly(N-isopropylacrylamide) [poly(NIPAAm)] chains were grafted. Phase separation in the gel at temperatures above the lower critical solution temperature (LSCT) of poly(NIPAAm) gives steep decrease in the water content of the gel at temperatures around the LCST, while the ion-exchange capacity does not change with temperature. The charge density, which is defined as the division of the ion-exchange capacity by the water content, increases with increasing temperature. The measurements of the charge density change over stepwise changes of temperature between 10 and 50 °C indicate that the value of the charge density changes from 0.07 to 0.24 mol dm-3 within a period of 1 min and the temperature response of the charge density is reversible in all the temperature changing steps. Permeation experiments in a dialysis system consisting of the gel membrane and mixed KCl and CaCl2 solutions show that the membrane can control the transport modes of Ca2+ ions by changing the temperature in two ways: downhill transport (transport along their own concentration gradient in a system) at temperatures below the LCST of poly(NIPAAm) and uphill transport (transport against their concentration gradient) at temperatures above the LCST.