Mechanical properties of poly(vinyl methyl ether) hydrogels below and above their volume phase transition

The mechanical properties of radiation cross-linked poly(vinyl methyl ether) hydrogels below and above the volume phase transition (VPT) under isobar conditions were studied. The viscoelastic properties as a function of radiation dose, radiation source and polymer concentration at the state of irradiation were examined. Increased radiation doses led to higher cross-linking densities and higher moduli. Hydrogels irradiated with -rays were much harder than those obtained with electron beam irradiation at the same radiation dose. It was found that the modulus strongly increased by up to 1 order of magnitude at a temperature of the VPT of about 37 °C. In the collapsed state at temperatures well above the VPT a frequency dependence of the E() moduli in the range 0.1–22 Hz was detected, indicating viscoelastic behavior. To study the influence of solvent quality on the modulus of the hydrogels, rheological measurements were performed in water, 2-propanol and cyclohexane. A scaling exponent for the modulus according to de Gennes (G^2.25) was not found. Possible reasons for deviations (G^3.54) on poly(vinyl methyl ether) hydrogels were discussed in the context of deviations from ideal networks.