Solid-state C-13 NMR and H-1 CRAMPS investigations of the hydration process and hydrogen bonding for poly(vinyl alcohol) films

The hydration process and resulting hydrogen bonding for atactic poly(vinyl alcohol) (A-PVA) films have been investigated by high-resolution solid-sta te NMR spectroscopy. In H-1 CRAMPS spectra for the A-PVA films which are dr ied after soaked in deuterated water for different periods, the prominent d ecrease in intensity is observed within 5 min for the resonance line assign ed to the intermolecular hydrogen-bonded OH (OHinter) groups while such an intensity reduction occurs over about 30 min for the line ascribed to the i ntramolecular hydrogen bonded-OH (OBintra) groups. This fact indicates that the exchange between the OH protons and deuteriums of deuterated water is more preferably induced for the OHinter groups compared to the case of the OHintra groups in the noncrystalline region. In contrast, all OH protons in the crystalline region, including OH protons free from hydrogen bonding th at are selectively allowed to exist in this region, are found to undergo th e exchange with deuteriums at a much slower rate. It is also found by the C -13 spin-lattice relaxation analysis that there exist three components with different T-IC values assigned to the crystalline, less mobile, and mobile components in the A-PVA films with the water content of 0.82 g-water/g-PVA . The line shape analysis of the CH resonance line for the crystalline comp onent reveals that the probability of the formation of intramolecular hydro gen bond in the meso sequence significantly increases by the addition of wa ter. In the less mobile component the intra- and intermolecular hydrogen bo nds are still kept irrespective of the enhanced molecular motion induced by the hydration but the gauche conformation markedly increases in mole fract ion probably as a result of partial breaking of hydrogen bonding. In contra st, the mobile component is subjected to rapid exchanges among different co nformations and hydrogen bonds, resulting in the appearance of the sharp tr iplet assignable to the CH carbons in the mm, mr, and rr sequences like the spectrum in solution. Finally the size of the crystallites is estimated to be about 26 nm by the analysis of the H-1 spin diffusion process from the water-swollen noncrystalline region to the crystalline region.