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.