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 ra dical polymerization initiated at 60 degrees C. The swelling behaviour of films prepared with 2, 5 and 10 mol% of TEGDMA was followed gravim etrically 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 o f the networks, giving a value of 15.5 kJ mol(-1) which corresponds to a typical diffusion process. The freezing point, the enthalpy of fusi on and the amount of non-freezable water were determined by differenti al scanning calorimetry (DSC) on hydrated samples with different hydra tion 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 wit h the hydration degree gives a maximum amount of non-freezable water o f about 23 wt%, independent of the TEGDMA content of copolymer systems . (C) 1997 Elsevier Science Ltd.