NYLON 13,13 ANALYSIS BY X-RAY AND SOLID-STATE NMR - TREATMENT-DEPENDENT CRYSTAL FORMS

Nylon 13,13 samples were prepared by various solvent-casting and thermal-treatment methods. Melt-quenched material showed broad, amorphous-like peaks in the X-ray powder pattern (at 2theta=21-degrees, 4.2 angstrom) and both the C-13 and N-15 solid-state n.m.r. spectra. Two different crystal forms of this polymer could be generated using different methods: a type A form was obtained by extended annealing just below the melting temperature and by film casting from 1,1,1,3,3,3-hexafluoroisopropanol, while a type B form developed on precipitating from m-cresol into methanol or film casting from m-cresol. The X-ray pattern for type A was similar to those of other nylons possessing an alpha-crystal form, although the n.m.r. data, especially the strong, sharp N-15 peak at 88.7 ppm, was more consistent with a gamma-phase. The type B crystals gave an X-ray pattern similar to those of other gamma-form nylons and to the pseudohexagonal phase observed in some homo- and copolyamides. The n.m.r. data for the type B form (especially the sharp N-15 peak at 87.3 ppm) seemed to indicate an amide conformation of c. 30-degrees with respect to the planes of the attached all-trans methylene chains; this value is half-way between those of the alpha and gamma-crystal forms of most nylons (N-15 chemical shifts of c. 84 and 89 ppm, respectively). We believe these crystal forms are unique to nylon 13,13 but have insufficient data to further describe the molecular conformations and crystalline packings that correspond to each.