Thermodynamics of water sorption in acrylic homonetworks and IPNs

The equilibrium thermodynamic properties of poly(hydroxyethyl acrylate) and poly(ethyl acrylate)-i-poly(hydroxyethyl acrylate) hydrogels are investigated starting from the water sorption isotherms of the systems. Partial enthalpy and entropy of the sorbed water in the gel differ markedly from the values of pure water at the lowest water contents, and tend to those of liquid water as saturation is approached. The residual mixing free energy is calculated, as a means of assessing the intensity of the water-polymer interaction. Its small positive magnitude shows that water-polymer hydrogen bonds are labile compared to water-water and polymer-polymer hydrogen bonds, and thus the stability of the gel state is still mainly due to the combinatorial entropic contribution to the mixing free energy. An equation correctly describing the sorption isotherms, when combined with the thermodynamic equations, can deliver the true water-polymer interaction parameter and its dependence on the polymer volume fraction in the gel.