Thermodynamic properties of copolymeric hydrogels based on 2-hydroxyethyl methacrylate and a zwitterionic methacrylate

The copolymerisation of 2-hydroxyethyl methacrylate and a zwitterionic methacrylate, namelyN,N-dimethyl-N-methacryloxyethyl-N-(3-sulphopropyl)-ammonium betaine (SPE), in the presence of a tetrafunctional crosslinker has been effected to 100% conversion by γ-irradiation. The resultant xerogels of different compositions were swollen to equilibrium in water to yield hydrogels. Volumetric swelling and compression-strain measurements were made over the temperature range 278–343 K. All these copolymers showed an increasing volumetric swelling with temperature, but the derived values of the partial molar enthalpy, entropy and Gibbs free energy of dilution showed certain differences which were interpreted on the basis of copolymer dyad distribution.