SOLID-STATE C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY OF ETHYLENE-VINYL ALCOHOL COPOLYMERS - MORPHOLOGICAL PARTITIONING OF HYDROXYLS

Cross-polarization/magic angle spinning (CP/MAS) C-13 nuclear magnetic resonance (n.m.r.) spectra of several random ethylene (E)/vinyl alcoh ol (VOH) copolymers, including both homopolymers, have been obtained. Contributions to these spectra arising from the crystalline and non-cr ystalline regions of these materials have been isolated and from these spectra the average concentrations of the comonomers in the crystalli ne phase is determined. For melt-crystallized samples, the composition of the crystals is very close to stoichiometric, thereby supporting t he notion, for example, that VOH and E residues may act as interstitia l-or vacancy-type defects in the polyethylene or poly(vinyl alcohol) l attices, respectively. Assuming the validity of a previously proposed assignment scheme for the methine multiplets visible in the E-rich com position range, we conclude that there is no significant discriminatio n against VOK-VOH sequences in the crystalline regions. In one sample containing 18 mol% VOH, it was shown that simultaneous with VOH incorp oration, other 'defects' were, within the signal-to-noise, rejected fr om 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 reacti ons) as well as acetate branches (from incomplete hydrolysis of the pr ecursor copolymer). In contrast to the melt-crystallized samples, copo lymers precipitated from isopropanol show a slight bias in their cryst alline regions towards a more E-rich composition, presumably because E -rich stems are more available at the time of crystallization via thei r decreased solubility in this solvent. Finally, a few observations ar e made regarding the PVOH homopolymer in the dry and slightly hydrated state. Spectral changes are not entirely consistent with trends repor ted in the literature. Thus, a question is raised whether possible var iations in molecular mobility within the PVOH crystalline regions can give rise to systematic differences in crystalline-phase spectra isola ted by different C-13 n.m.r. spectroscopic methods.