ABSTRACT:To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, X-ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0°C and well below 0°C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3-8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ϳ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as-formed liquid-like microstructure, characterized in the electron microscope by rounded ϳ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores.