Solid state 15N nuclear magnetic resonance of 15N-labeled nylon 6 and nylon 11: Observation of multiple crystalline forms and amorphous regions

Abstract

The solid state 15N nuclear magnetic resonance (NMR) characterization of nylon 6 and nylon 11 is reported. Nylon 6 (20% 15N enriched) was prepared by anionic polymerization of isotopically enriched caprolactam, and NMR samples were prepared by quenching from the melt, and by slow cooling and annealing. Cross-polarization (CP)-magic-angle-spinning (MAS) spectra of the 15N-enriched samples showed a single sharp peak (α crystal form) at 84.2 ppm (relative to glycine) and a broader resonance at 87.2 ppm. Relaxation experiments were conducted to determine T 1N, T 1H and T 1 ρ for each sample at 300 K. The crystalline resonance was found to have T 1N values of 125–416 s, while the down-field peak had two measurable T 1N values, one component with a T 1 of 1–3 s and a second with the longer T 1 of 19–29 s. The two components of the non-crystalline peak are thought to belong to a liquid-like amorphous region and a more rigid “interphase” region lying between the crystalline and amorphous regions. T 1 ρ measurements were consistent with two-phase morphology although two-component decay for the amorphous region was not observed. 1H T 1 measurements were apparently dominated by spin diffusion that masked any differences between the regions. The chemical shift anisotropy (CSA) of static samples was also obtained. Motion in the amorphous region was monitored by observing an isotropic peak at elevated temperatures. At temperatures above 100 °C, the most deshielded ( σ 33) component was lost from the CSA spectrum, suggesting a previously unreported motion occuring in the rigid crystalline region. This motion is speculated to be associated with the intermolecular hydrogen bond between adjacent chains. Polyundecanamide (nylon 11) was synthesized with 99 + % 15N labeling of the amide nitrogen. Polymer samples were thermally treated to give the stable triclinic α crystal 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. The α crystal form transformed above 95 °C to the pseudohexagonal δ crystal form, confirming previously reported X-ray and thermal analysis. 15N spin-lattice ( T 1) relaxation experiments confirmed decreased relative mobility in going from the δ to the α form consistent with increased molecular rigidity and density. Previous studies suggested that the α-to-δ transition involves the onset of rapid hydrogen bond disruption and re-formation within the crystal lattice above the 95 °C transition temperature. 15N CSA spectra showed that the hydrogen-bonded amide units remain conformationally rigid in the crystal lattice despite increasing librational motion. Combining 15N T 1 information with wide-angle X-ray data, a model is presented which accounts for the crystal-to-crystal transition.