Effect of freeze-thawing on polyvinyl alcohol hydrogel: Films before and after solvent evaporation casting

Freeze-thawing (F/T), a process widely reported in literature for crosslinking Polyvinyl alcohol (PVA), is widely known in the science community to develop toxic-free hydrogels. Despite extensive characterization, optimizing both the viscous and elastic properties of hydrogels remains challenging. To address this issue, we studied the F/T mechanisms. The common understanding is due to the hydrogel transition that occurs within the solid-liquid interface upon freezing and subsequent thawing. However, in this study, we demonstrate that it is possible to crosslink PVA films prior F/T, i.e. after solvent-evaporation casting. This observation translates to crystallinity, swelling, and mechanical properties. When F/T was applied to cast films, the PVA exhibited an increase in melting enthalpy from 47.3 to 53.7 J/g and a rise in melting temperature from 207.5 to 223.0 °C. Moreover, post-casting F/T increased the storage modulus.