Investigation on the mechanism of the volume phase transition of gels and the effect of electrostatic interaction

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We have investigated the effect of the introduction of ionized groups or hydrophilic groups into N-isopropylacrylamide (NIPA) gel on the volume phase transition. Copolymer gels NIPA with acrylic acid, stirene sulfonate, and acrylamide were prepared and their swelling behaviors were examined. We have found that the transition like swelling behavior of NIPA gels completely disappeared by the introduction of very small amounts of electric charges. This effect of electric charge is, however, suppressed by the addition of salt. Differential scanning calorimetry was also carried out on the NIPA gels and the copolymer gels.I.Thermal behavior of NIPA gels. For a gel of large size, a broad but clear endothermic peak was observed near the transition temperature even if the volume change of gel is continuous with temperature. This indicates that the dehydration of gel chains, which drives, the volume phase transition, is an endothermic reaction. For a gel of the smallest size, the two successive … More endothermic peaks were observed for the volume phase transition near the transition temperature.II.Thermal behavior of copolymer gels. We have found for the gels containing the ionized groups that the volume change is continuous with temperature and the corresponding enthalpy change is small. Two explanation might be possuble, the dehydration is suppresed in spite of the reduced volum, or the endothermic energy due to the dehydration is compensated with an exothermic energy of some unknown mechanism. We have found that the reduced reaction enthalpy increases up to the original level by adding salt. We have found a new type of volume phase transition for copolymer gel of NIPA and acrylic acid at pH=2, even when the monomer content of the latter was as high as 10%. The thermal behavior including the endothermic energy was found to be the same as that of NIPA gel except for that the transion temperature was low. This indicates that the hydrophobicity of unionized acrylic acid is the same as that of NIPA. Less