Thermodynamics of Interaction between Water and Lightly Crosslinked Hydrogels of Acrylic and Metacrylic Acids

Abstract

Equilibrium isothermal sorption and microcalorimetry are used to study the effect the ratio of units of different nature in the macromolecular network of hydrogels of copolymers of acrylic (AA) and methacrylic (MAA) acids have on the thermodynamic parameters of interaction with water. Isotherms of the sorption of water vapor on dried hydrogels and the concentration dependences of the enthalpy of swelling of partially swollen hydrogels in excess of water are measured experimentally. The values of effective Flory–Huggins integral parameter χ is determined, along with its components χH of enthalpy and χS of entropy. The values of the Flory–Huggins parameter grow naturally upon an increase in the content of MAA units in the copolymers, which indicates weaker interaction with water and is consistent with a reduction in their degree of swelling. In contrast, the values of χH fall as the content of MAA units in the copolymers grows. This suggests that water interacts more energetically with the PMAA network than with PAA, despite the more hydrophilic nature of the latter. The values of χS are positive for all systems, which corresponds to the negative noncombinatorial entropy of gel swelling. Results are discussed in terms of the balance of hydrophilic and hydrophobic hydration in AA and MAA gels.