Studies on different types of hydrogen bonds in poly(vinyl alcohol) films by 1H CRAMPS and solid-state two-dimensional 1H-13C heteronuclear correlation analyses

Abstract

Different hydrogen-bonded states of OH groups in poly(vinyl alcohol) (PVA) films with different tacticities have been characterized by 1H combined rotation and multiple pulse spectroscopy (CRAMPS), solid-state 13C NMR and two-dimensional (2D) 1H-13C heteronuclear correlation (HETCOR) analyses. The newly developed line shape analysis for the 1H CRAMPS spectra reveals the existence of three types of OH groups in different hydrogen-bonded states below Tg; intramolecular and intermolecular hydrogen bonded OH (OHintra, OHinter) groups and OH (OHf) groups free from hydrogen bonding. Above Tg, another resonance line which can be assigned to highly mobile OH (OHr) groups in the non-crystalline region appears as a result of the reduction in mole fractions of the OHintra and OHinter groups, in good accord with the confirmation of the rubbery component by the solid-state 13C NMR line shape analysis. In contrast, the mole fraction of the OHf groups stays constant at -60-160°C, indicating that these OH groups are preferentially distributed in the crystalline region. In order to examine the correlations of the results obtained by 1H CRAMPS and solid-state 13C NMR analyses, 2D 1H-13C HETCOR spectra have been also measured at room temperature for atactic and main-chain deuterated atactic PVA films. As a result, evident correlations between 1H OH and 13C CH constituent lines are observed at a cross polarization (CP) contact time of 120 μm, indicating the validity of the assignments for the 1H OH and 13C CH resonance lines made previously as well as in this work.