Investigation of the structure of poly(vinyl alcohol)–iodine complex hydrogels prepared from the concentrated polymer solutions

Abstract

The structure and dynamics of poly(vinyl alcohol) (PVA)–iodine complex hydrogels that were prepared from concentrated PVA solutions have been characterized by high-resolution solid-state 13C NMR spectroscopy. The fully relaxed dipolar decoupling (DD)/MAS 13C NMR spectrum indicates that the hydrogels contain at least two components, a highly mobile and broader components. The former is assigned to the soluble or well water-swollen PVA chains that are not closely associated with the PVA–iodine complexes, whereas the latter may be mainly ascribed to the aggregated PVA chains that are produced by the formation of the PVA–iodine complexes because no diffraction peaks due to the conventional PVA crystallites are observed by wide-angle X-ray diffractometry. Furthermore, 13C spin-lattice relaxation time (T1C) analyses reveal that the broader component is composed of the highly restricted component probably assignable to PVA molecular chain aggregates containing the PVA–iodine complexes and the less mobile component. As for the former component, their CH resonance line measured by the T1C-filtering method is successfully resolved into 7 constituent lines by the least-squares curve fitting. The statistical analysis of the integrated intensities of the constituent lines thus obtained also reveals that the probability fa for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability ft of the trans conformation are, respectively, as high as 0.86 and 0.88. This fact indicates that the PVA molecular chain aggregates containing the PVA–iodine complexes should be composed of PVA segments with the trans-rich conformation and the PVA–iodine complexes therein may also be formed with these several trans-rich segments surrounding the rod-like polyiodine cores in agreement with the so-called aggregation model. Moreover, several new diffraction peaks that should be interpreted in terms of the hexagonal structure are observed for the PVA–iodine complex hydrogels in the low 2θ region in the wide-angle X-ray diffraction (WAXD) profile measured by a strong X-ray source at SPring-8. This suggests the necessity of more detailed WAXD characterization to propose a new structure model, which should be referred to as the hexagonal aggregation model, for the PVA–iodine complexes.