DYNAMICAL MECHANICAL MEASUREMENTS IN DRY PHEMA AND ITS HYDROGELS

The elastic modulus and tg delta of poly(2-hydroxyethyl methacrylate) (PHEMA) and its hydrogels have been measured over the frequency range 0.330 Hz and the temperature range 120-450 K. The mechanical spectra reveal the presence of a low-temperature gamma-relaxation plus a shoulder (beta-relaxation) that merges into the alpha-relaxation. The absorbed water lowers the temperature and rises the height of the lowest temperature peak (the gamma-peak for dry PHEMA) while lowering both temperature and height of the alpha-peak. This behaviour is interpreted in terms of hydrogen-bonding effects between water and polymer which weaken the intra- and/or interchain interactions. Furthermore, the calorimetric analysis in dry polymer is consistent with the mechanical behaviour in the higher-temperature region.