Solid-state 13C nuclear magnetic resonance spectroscopy of ethylene/vinyl alcohol copolymers: morphological partitioning of hydroxyls

Abstract

Cross-polarization/magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (n.m.r.) spectra of several random ethylene (E)/vinyl alcohol (VOH) copolymers, including both homopolymers, have been obtained. Contributions to these spectra arising from the crystalline and non-crystalline regions of these materials have been isolated and from these spectra the average concentrations of the comonomers in the crystalline phase is determined. For melt-crystallized samples, the composition of the crystals is very close to stoichiometric, thereby supporting the notion, for example, that VOH and E residues may act as interstitial- or vacancy-type defects in the polyethylene or poly(vinyl alcohol) lattices, respectively. Assuming the validity of a previously proposed assignment scheme for the methine multiples visible in the E-rich composition range, we conclude that there is no significant discrimination against VOH—VOH sequences in the crystalline regions. In one sample containing 18 mol% VOH, it was shown that simultaneous with VOH incorporation, other ‘defects’ were, within the signal-to-noise, rejected from the crystalline regions. These defects, mainly short-chain branches totalling about 8 branch points per 1000 main-chain carbons, included ethyl and butyl-plus-longer branches (from polymerization side reactions) as well as acetate branches (from incomplete hydrolysis of the precursor copolymer). In contrast to the melt-crystallized samples, copolymers precipitated from isopropanol show a slight bias in their crystalline regions towards a more E-rich composition, presumably because E-rich stems are more available at the time of crystallization via their decreased solubility in this solvent. Finally, a few observations are made regarding the PVOH homopolymer in the dry and slightly hydrated state. Spectral changes are not entirely consistent with trends reported in the literature. Thus, a question is raised whether possible variations in molecular mobility within the PVOH crystalline regions can give rise to systematic differences in crystalline-phase spectra isolated by different 13C n.m.r. spectroscopic methods.