Douglas G. Powell

Natural abundance 15N CP/MAS n.m.r. of solid polyamides : a technique sensitive to composition and conformation in the solid state

Abstract High resolution solid state 15 N nuclear magnetic resonance (n.m.r.) of several AB and AA-BB polyamides were obtained at the nitrogen natural abundance level. Resonances at 84 and 89 ppm (relative to solid glycine) clearly correspond to α and γ crystal forms, respectively. In addition, a broad intermediate peak (84–89 ppm) is assigned to rigid non-crystalline amorphous and interphase regions in these semicrystalline polymer samples. Confirmation of these assignments involved analysis of 15 N-enriched samples of nylon 6 and nylon 11. In addition, a high-temperature δ form for nylon 11 (labelled sample) was found to give a peak at 86.6 ppm while a metastable δ′ form obtained by quenching from the melt gave an identical chemical shift value. Cross-polarization/magic angle spinning (CP/MAS) spectra coupled with spin-lattice relaxation measurements of the labelled samples further confirmed the identity of peaks for amorphous regions and the various individual crystal forms. Commercial amorphous nylons were also examined by 15 N CP/MAS and found to give a broad envelope of resonances (80–90 ppm) indicative of random conformations around the amide groups. The origin of the chemical shift differences is rationalized in terms of polymethylene chain conformation relative to the plane of the amide group. Semi-empirical molecular orbital 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, 15 N solid state n.m.r. is found to be a sensitive technique for examining local conformations in solid polyamides even with natural abundance 15 N samples.

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 T1N, T1H and T1ρ for each sample at 300 K. The crystalline resonance was found to have T1N values of 125–416 s, while the down-field peak had two measurable T1N values, one component with a T1 of 1–3 s and a second with the longer T1 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. T1ρ measurements were consistent with two-phase morphology although two-component decay for the amorphous region was not observed. 1 H 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 (σ33) 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 + % 15 N 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. 15 N 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. 15 N spin-lattice (T1) 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. 15 N CSA spectra showed that the hydrogen-bonded amide units remain conformationally rigid in the crystal lattice despite increasing librational motion. Combining 15 N T 1 information with wide-angle X-ray data, a model is presented which accounts for the crystal-to-crystal transition.

A low-volume insert for solid-state magic-angle spinning NMR of small samples

Abstract : Easily machined ceramic inserts with low volume capacities were compared to standard rotors packed completely with samples or with silica gel-diluted samples of adamantane and glycine. A disc shaped sample cavity gave better signal-to-noise than a cylindrically-shaped one for comparable sample size and number of cylindrically-shaped one for comparable sample size and number of scans, and gave the narrower peak widths than even the full rotor. The inserts are inexpensive and easy to use while facilitating high-speed rotor spin-up and recovery of the noncontaminated samples.