A structural study of water in a poly(vinyl alcohol) gel by 17O NMR spectroscopy

Abstract

17O NMR spectra of poly(vinyl alcohol) (PVA) gel as prepared by repeating the freezing/thawing cycle were measured, in order to elucidate the structure and dynamics of water in PVA gel. From these experimental results, it was found that the 17O chemical shift position of water in PVA gel moves downfield with an increase in the number of freezing/thawing cycles. Ab initio MO GIAO-CHF and FPT INDO calculations on the hydrogen-bonded water model suggest that such a downfield shift comes from a reduction of hydrogen-bond length between water molecules or between water molecule and hydroxyl group of PVA. 17O spin–lattice relaxation times ( T 1) of water in the gel were measured, in order to elucidate dynamics of water in the gel. The T 1 value is decreased with an increase in the number of freezing/thawing cycles. This means that the mobility of water in the gel is reduced with an increase in the degree of gelation. Further, 17O NMR spectra of PVA gel, which was kept at room temperature after the temperature elevation until 80°C, were measured. Some peaks of water that come from different structures in the PVA gel appeared. Further, detailed discussion on the structure of water in PVA gel was discussed.