Water sorption of flexible networks based on 2-hydroxyethyl methacrylate triethylenglycol dimethacrylate copolymers

Cross-linked films of copolymers of 2-hydroxyethyl methacrylate (HEMA) and triethylenglycol dimethacrylate (TEGDMA) were prepared by free radical polymerization initiated at 60 degrees C. The swelling behaviour of films prepared with 2, 5 and 10 mol% of TEGDMA was followed gravimetrically and the diffusion coefficients were determined according to the Fickian law at several temperatures in the interval 25-50 degrees C. The apparent activation energy was independent of the composition of the networks, giving a value of 15.5 kJ mol(-1) which corresponds to a typical diffusion process. The freezing point, the enthalpy of fusion and the amount of non-freezable water were determined by differential scanning calorimetry (DSC) on hydrated samples with different hydration degree. There is a critical hydration interval (W = 20-30 wt%) in which it is possible to distinguish the different states of water in the cross-linked hydrogel. The variation of the enthalpy Delta H-f with the hydration degree gives a maximum amount of non-freezable water of about 23 wt%, independent of the TEGDMA content of copolymer systems. (C) 1997 Elsevier Science Ltd.