X-ray Diffraction Analysis of Poly(vinyl alcohol) Hydrogels, Obtained by Freezing and Thawing Techniques

The structure of physical poly(vinyl alcohol) (PVA) hydrogels prepared by subjecting a PVA/D2O solution (11% w/w PVA) to freeze (−22 °C)/thaw (+25 °C) cycles has been systematically investigated by X-ray powder diffraction technique as a function of the number of cycles and aging time. The structural analysis has been extended to PVA "dried gels" and PVA hydrogels obtained by rehydrating the dried samples. The results of the present analysis confirm that highly stable PVA hydrogels, with a water uptake higher than 80%, may be obtained upon freeze/thaw cycles. The X-ray diffraction profiles of PVA hydrogels have been interpreted in terms of three components: "free water", crystalline PVA aggregates, and swollen amorphous PVA. The degree of crystallinity and the size of the crystals increase with increasing the number of freeze/thaw cycles and the aging time. Our results support the hypothesis that PVA hydrogels have a porous structure, with pores mainly occupied by water. The porous walls consist of swollen amorphous PVA while the crystalline domains act as knots of the gel network. The presence of crystalline knots ensures a high dimensional stability of the gel and induces elastic properties. Long time aging in sealed vials at room temperature induces large variations in the structure of freeze/thaw PVA hydrogels. The porous structure formed during freeze/thaw cycles in PVA hydrogels, instead, is not greatly altered upon drying and during the successive rehydration step; rehydrated gels, indeed, recover almost completely volume, shape, and physical properties of the as-formed freeze/thaw PVA hydrogels. Thus, the outstanding physical and mechanical properties of freeze/thaw PVA/hydrogels in the as-prepared state, may be preserved even for a long time, drying the samples immediately after the preparation and then restored when needed, upon rehydration of the dried samples.