Allison M. Sikes

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.

Synthesis and characterization of in situ anionically polymerized p-aminobenzoylcaprolactam using di-and tri-functional initiator systems

Summaryp-Aminobenzoylcaprolactam has been polymerized anionically in a two-step process to produce aramide-amide copolymers. Both drawn fibers and unoriented films may be produced using this method. The samples were characterized using FTIR, solution NMR, thermal analysis, viscosity, and mechanical testing. FTIR and NMR revealed the incorporation of both the aramid and amide linkages into the polymer backbone. TGA results gave a lower decomposition temperature than that of both aramide and nylon 6 materials. Intrinsic viscosities averaged 0.5 dL/g for the drawn copolymer samples polymerized by the di-functional initiator. Light microscopy displayed crystalline domains that did not appear to melt even up to 300°C. Mechanical testing showed that the initial moduli of drawn fiber samples to be in the range of 30–80 MPa, while homopolymers of nylon 6 and p-benzamide possessed values of 25 MPa and 2.70×103 MPa, respectively.