Effects of continuous water flow on the swelling properties of polyelectrolyte hydrogels

We report the swelling behavior of a polyelectrolyte copolymer gel in water, which consists of poly(N-isopropylacrylamide) and sodium acrylate. The diameter of the cylindrical gels was measured at room temperature under a continuous flow of solvent water (equivalent to an infinite amount of water; open system). After a sufficient water flow, the diameter of the gel in the limited amount of water (closed system) was measured as a function of the temperature. The gel in the open system was found to shrink as a result of the continuous flow of water, and the gel in the closed system began to swell again at the phase transition point by increasing the temperature. The effects of the degree of initial ion exchange by the water flow on the macroscopic swelling behavior were discussed in terms of the exchange of counterions (the ion dissociation of carboxyl groups) and of the creation and destruction of intermolecular forces (hydrogen bonding). It was concluded that the amount of solvent water determines the environmental variables, such as the pH and ion concentration, which affects the swelling properties of polyelectrolyte hydrogels; intermediate re-swollen states can be observed in a closed system.