Folding and birefringence behavior of poly(vinyl alcohol) hydrogel film induced by freezing and thawing

Abstract

Band-like folds with high aspect ratio and birefringence behavior were observed on an in situ formed thin poly(vinyl alcohol) (PVA) hydrogel film via freezing–thawing treatment of PVA aqueous solution coated on glass. The crystallites generated during the freezing of the PVA solution cross-linked the PVA to form the hydrogel film. The volume expansion of the hydrogel film due to the absorption of condensed water in thawing induced the formation of folds. These folds show interesting birefringence behavior. The morphology, crystallization and birefringence behavior of the folds were characterized by polarized optical microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. A plausible principle for the fold formation is also discussed. It has been found that the moderate interaction between the hydrogel film and the substrate and the existence of condensed water on the frozen hydrogel film play important roles in the appearance of the folds.