Preparation temperature dependence of the dissociation enthalpy of hydrophobic binding in N-isopropylacrylamide gels

The thermal properties of N-isopropylacrylamide (NIPA) gels in water have been investigated with differential scanning calorimetry (DSC). The dissociation enthalpy of hydrophobic bonding per NIPA monomer, Delta H-N and the number of water molecules associated with the dissociation, (n - n(0)), were evaluated as a function of the gel preparation temperature, T-prep, where n and n(0) are the number of water molecules per NIPA mole before and after dissociation, respectively. It was found that Delta H-N is independent of T-prep. On the other hand, both n and no increase with increasing T-prep, while the difference, i.e., (n - n(0)) is constant irrespective of T-prep. These experimental findings suggest that the enthalpy and the number of water molecules associated with the dissociation are independent of T-prep although the number of trapped water molecules increases with T-prep in both the swollen and shrunken states.