Nitrogen-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 6"' smectic form. The y 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 a crystal form transforms above 95 "C to the pseudohexagonal 6 crystal form. 15N spin-lattice (T,) relaxation experiments confirmed decreasing relative mobility in the order 6' > 6 > a consistent with increasing molecular rigidity and density. Previous studies suggested that the a-6 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. Combining 15N T, 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.