NATURAL ABUNDANCE N-15 CP MAS NMR OF SOLID POLYAMIDES - A TECHNIQUE SENSITIVE TO COMPOSITION AND CONFORMATION IN THE SOLID-STATE

High resolution solid state N-15 nuclear magnetic resonance (n.m.r.) of several AB and AA-BB polyamides were obtained at the nitrogen natural abundance level. Resonances at 84 and 89 ppm (relative to solid glycine) clearly correspond to alpha and gamma-crystal forms, respectively. In addition, a broad intermediate peak (84-89 ppm) is assigned to rigid non-crystalline amorphous and interphase regions in these semicrystalline polymer samples. Confirmation of these assigments involved analysis of N-15-enriched samples of nylon 6 and nylon 11. In addition, a high-temperature delta-form for nylon 11 (labelled sample) was found to give a peak at 86.6 ppm while a metastable delta'-form obtained by quenching from the melt gave an identical chemical shift value. Cross-polarization/magic angle spinning (CP/MAS) spectra coupled with spin-lattice relaxation measurements of the labelled samples further confirmed the identity of peaks for amorphous regions and the various individual crystal forms. Commercial amorphous nylons were also examined by N-15 CP/MAS and found to give a broad envelope of resonances (80-90 ppm) indicative of random conformations around the amide groups. The origin of the chemical shift differences is rationalized in terms of polymethylene chain conformation relative to the plane of the amide group. Semi-empirical molecular orbital calculations of model amides show electron density variations which correlate with the shielding and deshielding of the nitrogen atom consistent with this interpretation. Based on these results, N-15 solid state n.m.r. is found to be a sensitive technique for examining local conformations in solid polyamides even with natural abundance N-15 samples.