N-15 NMR CHARACTERIZATION OF MULTIPLE CRYSTAL FORMS AND PHASE-TRANSITIONS IN POLYUNDECANAMIDE (NYLON 11)

Polyundecanamide (nylon 11) was synthesized with 99+%15N labeling of the amide nitrogen. Polymer samples were thermally treated to give the stable triclinic a crystal form and the metastable δ’” smectic form. The γ form was obtained by precipitation from neat trifluoroacetic acid upon evaporation.15N CP/MAS NMR clearly differentiated the samples by chemical shift. Confirming previously reported X-ray and thermal analysis, the α crystal form transforms above 95 °C to the pseudohexagonal δ crystal form. 15N spin-lattice (T1) relaxation experiments confirmed decreasing relative mobility in the order δ’ > δ > α consistent with increasing molecular rigidity and density. Previous studies suggested that the α-δ transition involves the onset of rapid hydrogen bond disruption and re-formation within the crystal lattice above the 95 °C transition temperature.15N chemical shift anisotropy (CSA) spectra showed that the hydrogen-bonded amide units remain conformationally rigid in the crystal lattice despite increasing librational motion. Combining15N T1 information with wide-angle X-ray data, a model is presented that accounts for the crystal-crystal transition as well as the relative stability of the three crystal forms. © 1990, American Chemical Society. All rights reserved.