Natural Abundance 15N CP/MAS of Nylons and Aramids: A Sensitive Technique for Examining Crystalline Composition and Conformation in Solid Polyamides

摘要

We have examined a variety of polyamides and aramids using natural abundance 15N CP/MAS NMR. These have included commercially available A-B and AA-BB nylons, Kevlar and poly(p-benzamide) as well as a number of block and graft copolymers synthesized in our laboratory. Excellent correlation was observed between the type of crystalline form adopted by polyamides (alpha and gamma modifications) and the chemical shift of the rigid amide nitrogens. Values for the two forms center around 84 ppm (relative to glycine at 0 ppm) and 88 ppm, respectively. For some samples, unexplained peaks between the two main crystalline peaks are tentatively assigned to rigid amide conformations that are not x ray active but are seen by NMR. Aramids show similar complexity. Block and graft copolymers display characteristic peaks for composition (aryl vs alkyl nitrogen substituents) and for conformation and crystallinity. (MJM)