Processing physically crosslinked polyvinyl alcohol hydrogel into solid foams via freeze-drying: The role of hydrogel state

Processing hydrogels into solid foams or aerogels via freeze-drying broaden their applications. However, one challenge is its inherent processing instability, often leading to dimensional shrinkage and structural collapse of the resultant solid foams. To address this, our study investigated the role of hydrogel state in the solid foam formation. Specifically, polyvinyl alcohol (PVA) hydrogel, synthesized through repeated freeze-thaw (FT) cycles, was investigated. It was found that PVA concentration and FT cycles played a vital role; specifically, higher PVA concentration and more FT cycles negatively affected solid foam formation. These hydrogels exhibited an increase in hydrogel viscoelasticity and crystallizability of the most flexible chains, indicative of strong and denser network structure. We proposed that the final solid foam structure was determined by a balance between the volumetric expansion due to water-to-ice transition during freezing and the intense inter-polymeric chain attractions within the denser network during sublimation.