The phase transition of gels containing N-isopropylacrylamide (NIPA) was investigated by differential scanning calorimetry (DSC) and swelling measurements. The enthalpy of dissociation of the hydrophobic interaction per molar unit of NIPA chains (ΔH N), the transition temperature (T c), and the number of water molecules associated with an NIPA monomer, (n) were evaluated as a function of polymer concentration. Significant differences in ΔH N and T c were found between two systems:  poly(NIPA-ran-acrylic acid) (NIPA/AAc; a weakly charged gel) and poly(NIPA-ran-dimethylacrylamide) (NIPA/DMAA; a neutral gel). ΔH N decreases with increasing comonomer concentration. However, a larger decrease in ΔH N was observed for NIPA/AAc than for NIPA/DMAA, which is accounted for by the strong hydrophilic effect of the charged AAc comonomers. No noticeable copolymer concentration dependence in n was observed in both systems. It is suggested that there are two types of water molecules, i.e., one associated with the phase transition, (n − n 0), and the other the lower limit for the hydrophobic hydration, n 0. The roles of these water molecules are discussed in relation to the volume phase transition.

No abstract

No abstract