Measurement and Correlation of the Swelling Pressure of N-Isopropylacrylamide Gel

Swelling pressures of N-isopropylacrylamide gels of various cross-linking densities were measured from their swollen state to shrunken state by swelling the gels in polymer aqueous solutions whose concentrations were adjusted to show the desired osmotic pressures. The swelling pressures of the gels were determined to be equal to the osmotic pressure of the aqueous solution in which the gel was swollen. The measured swelling pressures were in the range 0–6000 Pa. It was found that the binodal curve of the gel–gel transition of N-isopropylacrylamide gels, which was estimated from the measured swelling pressure data, was similar to that in liquid–liquid equilibria of poly(N-isopropylacrylamide) aqueous solutions, especially in the vicinity of a critical region (namely near its lower critical solution temperature, LCST). The parameters of the prediction model proposed by Hooper et al. were fitted to the measured swelling pressure data of the gels of various cross-linking densities and applied to the calculation of the swelling equilibria of the gels. The calculation results were in good agreement with the experimental data, suggesting that the addition of the mixing and elastic contributions in osmotic pressure of the gel could be a useful assumption for the prediction of swelling equilibria of the gel, i.e., that the nature of the gel could be approximately expressed by means of its elasticity.