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

摘要

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 ~(13)C NMR spectroscopy.The fully relaxed dipolar decoupling(DD)/MAS ~(13)C 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,~(13)C spin-lattice relaxation time(T_(1C))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 T_(1C)-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 f_a for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probabilityf_t 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 rodlike 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 2 theta 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.