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

摘要

High resolution solid state 15N NMR of several AB and AA-BB polymides were obtained at the nitrogen natural abundance level. Resonances at 84 and 89 ppm (relative to solid glycine) clearly delineate alpha and gamma crystal forms, respectively. In addition, intermediate resonances are seen which have not been identified by x ray or IR. These resonances are thought to belong to rigid noncrystalline or amorphous regions in these semicrystalline polymer samples. Amorphous nylons have also been examined using this technique and found to have a broad envelope of resonances indicative of random conformations of amide groups. The origin of the chemical shift difference is rationalized in terms of chain conformation about the plane of the amide group. Semi-empirical MO 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, 15N solid state NMR is found to be a sensitive technique for examining local conformations in solid polyamides. Polyamides. (mjm)