Structure and rheological properties of poly(vinyl alcohol) hydrogels obtained by freezing/thawing cycles

The relationships between the structure and the rheological behavior of poly (vinyl alcohol) (PVA) hydrogels prepared by freeze/thaw cycles were investigated as a function of polymer concentration (5 wt%-25 wt% ). The degree of crystallinity and apparent dimensions of crystallite were determined by XRD. Rheological properties of PVA solutions including modulus and creep properties were characterized by controlled stress rheometer. Measurements of the shear storage and loss modulus were performed at a fixed frequency of 1 Hz and a low stress, under conditions where the deformation imposed on the gel structure is entirely reversible. The storage modulus at low frequency and low strain amplitude increases as the concentration increases,but the increase rate becomes clearly slow when the concentration is higher than 20 wt% . The storage modulus at 1 Hz was determined and related to the degree of crystallinity and concentration. Based on these measurements it was concluded that a minimum crystallinity and concentration were required for these PVA samples to exhibit gel behaviour. A more detailed comparison of XRD and rheological data led to the conclusion that the storage modulus was mainly controlled by the PVA crystallinity while the hydrogen bond interactions had a much smaller contribution, when the concentration was lower than 15 wt% . A structural parameter related to the network mesh size is derived from measurements of the dynamic storage moduli at low values of strain frequency and amplitude. The storage modulus of PVA hydrogels at different temperatures was analyzed on the basis of a theoretical model based on the scaling approach. The creep response of PVA hydrogels was investigated. With increase in concentration, these hydrogels exhibited an evolution from a linear viscoelastic response to a nonlinear viscoelastic response.