1H and 13C NMR studies of water and heavy water absorption in poly(vinyl alcohol) hydrogels

Abstract

Solid state nuclear magnetic resonance measurements of slightly syndiotactic poly(vinyl alcohol) (PVOH) films have been used to explore the effect of H 2O and 2H 2O absorption on hydrogen bonding, and hence molecular mobility and conformations, in these hydrogels. Gels of water concentration from 5 to 60 wt% were studied. 1H NMR spectra show switching of methine proton conformation at similar levels (ca. 30 wt%) of water and heavy water absorption, indicative of a threshold reduction of hydrogen bonding and increased molecular mobility. The methine 13C NMR spectra display triad structure line narrowing at a similar level of absorption which is commensurate with a 1:1 matching of PVOH monomer units with water or heavy water molecules. 1H NMR chemical shifts are used to study the mobility environments (or ‘states’) of water and line widths are used to follow solvent mobility. Discrete environments of H 2O and 2H 2O are observed as multiple peaks in the 1H line at 4.8 ppm (5.0 ppm for 2H 2O), and these results are supported by thermogravimetric measurements. The main features of the 13C NMR spectra suggest that (polymer mobility) line narrowing occurs due to the break up of inter- and intra-molecular hydrogen bonding through interaction of hydroxyl groups with H 2O or 2H 2O molecules and not due to the concomitant reduction of crystallinity.