Effect of the introduced charge on the thermal behavior of N-isopropylacrylamide gels in water and NaCl solutions

Effects of the introduced charge on the thermal behavior of N-isopropylacrylamide (NIPA) gel were investigated by differential scanning calorimetry (DSC) for copolymer gels of NIPA and sodium styrenesulfonate (NIPA-SSNa) with various charge densities of the chains and ionic strengthes. As the SSNa contents of the NIPA-SSNa gel increased, the sharp endothermic peak of the NIPA gel in water changed into a broad one and the excess enthalpy change per mole of NIPA monomer (Delta H) decreased. The Delta H value is most likely related to the amount of the dehydration of NIPA chains. In NaCl solutions, however, the sharp endothermic peak was observed again for the NIPA-SSNa gel and the Delta H value was similar to that of the NIPA gel. We found that the DSC curve was very similar to the derivative profile of the swelling curve with respect to temperature both in water and in NaCl solutions. The good correspondence between the derivative curve and the DSC curve suggests that the volume shrinkage of the gel and the specific heat change of the gel are closely related. This similarity most probably originates from the fact that the temperature-induced volume shrinkage of the gel is caused by the dehydration of NIPA chains. On the basis of this correlation, the effect of the introduced charge on the DSC curve (i.e., the shape of the endothermic peak and the Delta H value) is explained in terms of the Donnan osmotic pressure of counterions.