Short Time Dynamics of Solvent Molecules and Supramolecular Organization of Poly (vinyl alcohol) Hydrogels Obtained by Freeze/Thaw Techniques

The tensile mechanical properties of poly(vinyl alcohol) (PVA) hydrogels obtained by freeze/thaw techniques have been investigated by measuring the stress−strain curves. This analysis has been extended to gels obtained by drying the freeze/thaw PVA hydrogels immediately after their preparation and then rehydrating the so-obtained dried samples. The effect of aging and the drying/rehydration procedure on the viscoelastic behavior of PVA hydrogels in the freshly prepared state has been also analyzed. The stress−strain curves of PVA hydrogels indicate that the stress at any strain and the stress at break of gels increase with increasing the number of freeze/thaw cycles. Rehydrated gels are tougher than as-prepared gels due to the increase of polymer concentration. The shear storage modulus, G‘, at low frequency and low strain amplitude increases as the number of freeze/thaw cycles (n) increases. It reaches a plateau for n = 3−5 cycles; moreover, for a given number of freeze/thaw cycles, aged and rehydrated gels always exhibit lower G‘ values than as-prepared gels. This indicates that the stiffness of our gels decreases upon aging and rehydration. The gels prepared by imposing a single cycle make an exception. In the latter case, the drying/rehydration procedure produces physical properties that are higher in the resulting gels than in the as-prepared state. A structural parameter related to network mesh size is derived from measurements of the dynamic storage moduli at low values of strain frequency and amplitude. Finally, the intradiffusion coefficient of water in the gels has been measured through 1H PGSE-NMR experiments. The latter experiment indicates that the polymer-poor regions embedded in the network scaffolding (due to interconnected polymer-rich regions) are in turn interconnected. Therefore, the network structure of freeze/thaw PVA gels may be described in terms of two bicontinuous phases.