Salt effect on elastic properties of shrunken N-isopropylacrylamide gel

The tensile moduli of poly(N-isopropylacrylamide) gels in salt-free and NaCl solutions were measured by changing the temperature. A partition of NaCl between the gel and the solution outside the gel was examined. It was found that the tensile force relaxed after elongation and that the relaxation moduli depended on the temperature T and NaCl concentration C-S. The relaxation of the tensile modulus, Y(t), for the shrunken gel was fitted to Y(t) = (Y-i - Y-e)(1 + t/tau)(-beta)+Ye, where t is the time after elongation. The beta and tau values were found to be independent of C-S. The moduli at initiation, Y-i, and at equilibrium, Y-e, increased with T in the salt-free and NaCl solutions. The transitional changes of these moduli were observed at the temperatures at which the volumes of the gels changed transitionally. Y-i and Y-e of the shrunken gel increased with C-S, whereas those of the swollen gel were insensitive to C-s. The NaCl concentration was approximately a 300th part of the concentration of monomeric units of the NIPA chain in the shrunken gel. This suggests that a small amount of NaCl molecules changes the structure of the clustered water molecules surrounding the chains to restrict chain freedom and increases the moduli. The effect of NaCl molecules on the state of the clustered water molecules was detected as a change in the transition enthalpy, which was obtained by differential scanning calorimetry measurements.