Kinetics and thermodynamics of the water sorption of 2-hydroxyethyl methacrylate/styrene copolymer hydrogels

A series of polymer hydrogels based on 2-hydroxylethyl methacrylate and styrene were synthesized by bulk polymerization. The kinetic and thermodynamic swelling properties of these hydrogels were studied. The swelling mechanism of the hydrogels followed Fickian behavior. The diffusion coefficients, initial swelling rate, and maximum water uptake all decreased with the styrene content increasing because of the hydrophobicity of styrene. The swelling process was exothermic from 278 to 315 K, and the greater the styrene content was, the lower the enthalpy of mixing was in magnitude. The polymer-water interaction parameter, reflecting thermodynamic interactions, increased with increasing styrene content in the polymers. The negative values and trend of the actual partial molar enthalpy and entropy of dilution could be explained by the structuring of water through enhanced hydrogen bonding and hydrophobic interactions.