Role of boric acid in the formation of poly(vinyl alcohol)–iodine complexes in undrawn films

Abstract

The role of boric acid in the formation of poly(vinyl alcohol) (PVA)–iodine complexes in undrawn films has been investigated by using wide-angle X-ray diffraction (WAXD) and high-resolution solid-state 13C NMR spectroscopy. From UV–vis absorption spectroscopy, it is confirmed that boric acid is necessary for the formation of the complexes in films that are treated with I 2/KI aqueous solutions at relatively low I 2 concentrations. The WAXD profiles indicate that, irrespective of the presence of iodine, crystallite sizes perpendicular to the chain axis become smaller by the addition of boric acid in the swelling media. Moreover, small crystallites and surficial parts of larger crystallites may be partially dissolved in the swelling process with water and boric acid suppresses the re-crystallization in the drying process with or without iodine. The 13C spin–lattice relaxation time analysis reveals that there exist two components called the mobile and the less mobile components in the films and the latter component, which contains the complexes and the crystalline component, is increased in the fraction by the presence of boric acid. The evaluation of the CH resonance line shows that some of the intermolecular hydrogen bonds are broken by boric acid, which increases the intramolecular hydrogen bonds. The CH 2 lineshape analysis also reveals that the gauche fraction is appreciably increased in the less mobile component by the addition of boric acid. These facts suggest that boric acid may promote the formation of PVA–iodine complexes particularly in the surficial areas of the crystallites probably by reducing the molecular mobility of the PVA chains by causing cross-linking among them.