Thermodynamical analysis of the hydrogel state in poly(2-hydroxyethyl acrylate)

The thermodynamical properties of water and polymer in a hydrogel are analyzed from experimental equilibrium sorption isotherms at different temperatures in the case of poly(2-hydroxyethyl acrylate). The equations of the thermodynamics of mixtures allow to calculate the specific partial thermodynamical properties of water and polymer in the hydrogel as a function of its water content. Partial volume of water in the gel at low activities is slightly lower than the specific volume of water in the liquid state. The partial enthalpy and entropy differ significantly from those of pure water at the lowest water contents, and tend to those of pure water for activities close to one. The mutual interaction between the gel's components is discussed through the residual part of the mixing free energy. This function is positive for all gel compositions, suggesting that, on the average, the water-polymer interactions are not stronger than the water-water or polymer-polymer interactions, and that the affinity of water to mix homogeneously with the polymer has a combinatorial origin. The composition dependence of the Flory-Huggins interaction parameter is calculated and explained in terms on the residual free energy. This parameter is compared with the apparent interaction parameter, obtained fitting point by point the experimental sorption isotherm to the Flory-Huggins isotherm. The difference often found between the sorption capacity of an hydrogel when equilibrated in saturated water vapour and in liquid water is discussed.