Enthalpy of Swelling of Potassium Polyacrylate and Polymethacrylate Hydrogels. Evaluation of Excluded-Volume Interaction

The enthalpy of swelling over the entire concentration range in potassium polyacrylate and polymethacrylate hydrogels with varying charge density was measured by means of isothermal Calvet microcalorimetry at 298 K. Concentration plots of the enthalpy of swelling at polymer volume fractions up to 50% were used for evaluation of Flory‐Huggins binary parameter of interaction. Apparent values of χ determined directly from experimental plots revealed extremely strong concentration dependence at high degrees of swelling. This was taken as evidence for the overlap of electrostatic and excluded‐volume interactions. The separate evaluation of these contributions to the enthalpy of swelling was performed using the recent theory developed for the enthalpy of dilution in semi‐dilute polyelectrolyte solutions. The mean value of the parameter χ over a wide range of swelling for the uncharged poly(acrylic acid) and poly(methacrylic acid) gel was found equal to 0.21 and −0.50 respectively. The ionization of the gel network due to the introduction of potassium salt residues resulted in the progressive decrease in the quality of the solvent. At 40–50% of ionization, the binary interaction parameter was about 2.8 both for potassium polyacrylate and polymethacrylate hydrogels. That value is much more than the critical value for liquid‐liquid phase separation in non‐electrolyte polymer solutions.