Studies of the thermal volume transition of poly(N-isopropylacrylamide) hydrogels by high-sensitivity differential scanning microcalorimetry. 2. Thermodynamic functions

We report the first accurate measurements of the partial heat capacity of poly(N-isopropylacrylamide) hydrogels with varying cross-link density. When the cross-link density is increased, the transition broadens and the transition temperature decreases, while the enthalpy, entropy, and heat capacity increment of the transition do not practically change. The transition heat capacity increment is negative, Δ_tc_p = -0.63±0.04 J/g/K. This indicates the formation of a hydrophobic core of the gel upon the transition. The partial heat capacity of polymer network in the gel approaches the partial heat capacity of the unfolded linear poly(N-isopropylacrylamide) at low temperatures, indicating a complete disordering of the gel under these conditions. On the basis of the calorimetric data, thermodynamic functions of the transition were calculated from 0 to 150 °C. They allow one to compare enthalpic and entropic contributions to the stabilization of the collapsed gel. This state is found to be most stable at about 100 °C, and a reswelling transition could be expected only above 150 °C. Contributions of the dehydration of apolar and polar groups as well as residual factors to the transition enthalpy, entropy, and free energy were calculated. The role of apolar dehydration, i.e., of the hydrophobic effect, was not found to be predominant. Apparently, interactions of residues (van der Waals interactions and/or hydrogen bonding) contribute mainly to the stabilization of the collapsed state.