Studies on swelling behaviors, mechanical properties, network parameters and thermodynamic interaction of water sorption of 2-hydroxyethyl methacrylate/novolac epoxy vinyl ester resin copolymeric hydrogels

A series of the highly cross-linked hydrogels has been developed by the syntheses of bulk copolymers based on 2-hydroxyethyl methacrylate (HEMA) and novolac epoxy vinyl ester resin (NEVER). The resulting hydrogels were investigated on their equilibrium water content (EWC), swelling behaviors and tensile properties. Dynamic swelling behaviors of copolymeric hydrogels indicate that the swelling process of these polymers follows Fickian behavior. The EWC decreased and volume fraction of polymer in hydrogel (ϕ2) increased with NEVER content increasing due to its hydrophobicity. The increase of ionic strength of swelling medium or temperature results in a decrease in EWC and an increase in values of ϕ2. Young's modulus and tensile strength of hydrogels, as well as effective cross-link density (ve), increased as NEVER content increased or ionic strength of swelling medium increased, attributing to increasing interaction between hydrophobic groups and polymer-polymer interaction with an increase in NEVER content or in ionic strength. The polymer-solvent interaction parameter χ reflecting thermodynamic interaction was also studied. As NEVER content, ionic strength of swelling medium or temperature increased, the values of χ increased. The values of χ and its two components χH and χS varied with increasing T. The negative values and trend of the enthalpy and entropy of dilution derived from values of χH and χS, could be explained on the basis of structuring of water through improved hydrogen bonding and hydrophobic interaction.