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

研究了N-异丙基丙烯酰胺(NIPA)凝胶中离子基团或亲水性基团的引入对其体积相变的影响。制备了NIPA与丙烯酸、苯乙烯磺酸盐、丙烯酰胺的共聚凝胶，并对其溶胀行为进行了研究。我们发现，通过引入极少量的电荷，NIPA凝胶的类似溶胀行为的转变完全消失。然而，这种电荷效应被盐的加入所抑制。还对NIPA凝胶和共聚凝胶进行了差示扫描量热分析。I.NIPA凝胶的热行为。对于较大尺寸的凝胶，即使凝胶的体积随温度的变化是连续的，在转变温度附近也会观察到一个宽而清晰的吸热峰。这表明驱动体积相变的凝胶链的脱水是一个吸热反应。对于最小尺寸的凝胶，连续的两个…在相变温度附近观察到较多的吸热峰。II.共聚凝胶的热行为。我们发现，对于含有电离基团的凝胶，其体积随温度的变化是连续的，相应的热变很小。两种解释可能是可能的，尽管体积减小，但脱水被抑制，或者由于脱水而产生的吸热能被某种未知机制的放热能补偿。我们发现，加入盐后，还原反应热增加到原来的水平。发现NIPA与丙烯酸共聚凝胶在pH=2时，即使在单体含量高达10%时，也会发生一种新的体积相变。除了转变温度较低外，该凝胶的热行为包括吸热能与NIPA凝胶相同。这表明，非离子化的丙烯酸的疏水性与NIPA相同。较少

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