The structure of physical poly(vinyl alcohol) (PVA) hydrogels prepared by subjecting a PVA/ D 2O 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.

The crystallinity of freeze/thaw poly(vinyl alcohol) (PVA) hydrogels, either fresh or aged or obtained by dipping dried freeze/thaw gel samples in water immediately after their preparation, was investigated by using different techniques. Free induction decays obtained from 1H NMR experiments provide the most accurate measurement of the degree of crystallinity of these systems. Values thus obtained are in a good agreement with data obtained by X-ray diffraction for all the samples under study. The degrees of crystallinity, determined by using differential scanning calorimetry (DSC), instead, are lower than those obtained by the other two methods, for all the gel samples, but the aged gels. This result is due to the occurrence of the gel−sol transition during the heating scan which is characterized by the endothermic melting of the crystallites and the exothermic solubilization and solvation of PVA chains in water. In as-prepared and rehydrated gels, the endothermic and exothermic effects overlap, which leads to an underestimated value of the degree of crystallinity. For aged samples, the crystallites are larger and more perfect; the corresponding melting endotherms are narrower and shifted toward higher temperatures, which permits the separation of the endothermic and exothermic effects and leads to a more accurate measurement of the degree of crystallinity. Thus, the comparative analysis of the degree of crystallinity in PVA hydrogels measured by different techniques provides indirect information concerning their complex structure.

The structure of a bis-urea based reversible polymer is investigated using capillary viscosimetry, infrared spectroscopy, small-angle neutron scattering, and rheology. The highly viscoelastic solutions obtained in toluene are due to the formation of long and rigid fibrillar species. The cross section of these wires is measured and is shown to likely contain two or three molecules per axial repetition unit.