Phase structure of poly(vinyl alcohol) single crystals as revealed by high-resolution solid-state 13C n.m.r. spectroscopy

Abstract

The phase structure composed of the crystalline and non-crystalline phases, in particular the structure of the non-crystalline phase and hydrogen bonding, has been examined for poly(vinyl alcohol) (PVA) single crystals with different molecular weights by cross-polarization/magic angle spinning 13C n.m.r., differential scanning calorimetry and X-ray measurements. Although the molecular weight is vigorously decreased for the single crystals conventionally prepared from triethylene glycol solution, no reduction in molecular weight occurs when they are crystallized in a triethylene glycol/glycerol mixed solvent after dissolution at relatively low temperatures. According to the method previously reported, the triplets of the CH resonance lines of the crystalline and non-crystalline components have been analysed to obtain information about the intramolecular and intermolecular hydrogen bonds. It is found that the probability of formation of intramolecular hydrogen bonds in the meso sequence greatly differs between the crystalline and non-crystalline components, slightly or insignificantly depending on the molecular weight and the crystallization condition. It is finally concluded that PVA single crystals are composed of a crystalline phase with a thickness of about 8.0 nm and a non-crystalline overlayer with a thickness of about 2.0 nm. Moreover, the non-crystalline overlayer should consist of short loops with rather restricted molecular mobility compared to the mobility of the amorphous component.