15N NMR Characterization of Multiple Crystal Forms and Phase Transitions in Polyundecanamide (Nylon 11)
UNIVERSITY OF SOUTHERN MISSISSIPPI HATTIESBURG DEPT OF POLYMER SCIENCE
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Polyundecanamide nylon 11 was synthesized with 99 Nitrogen 15 labeling of the amide nitrogen. Polymer samples were thermally treated to give the stable triclinic alpha-crystal form and the metastable delta-smectic form. The gamma-form was obtained by precipitation from neat trifluoroacetic acid upon evaporation. 15N CPMAS NMR clearly differentiated the samples by chemical shift. Confirming previously reported x-ray and thermal analysis, the alpha- crystal form transforms above 95 C to the pseudohexagonal delta-crystal form. 15N spin lattice T1 relaxation experiments confirmed decreasing relative mobility in the order delta delta alpha consistent with increasing molecular rigidity and density. Previous studies suggested that the alpha-gamma transition involves the onset of rapid hydrogen bond disruption and reformation 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 T1 information with wide-angle x-ray data, a model is presented which accounts for the crystal-crystal transition as well as the relative stability of the three crystal forms.
- Physical Chemistry
- Polymer Chemistry