Solid-state NMR characterization of copolymers of nylon 11 and nylon 12

Abstract

Solid-state 13C and 15N NMR spectroscopy, in conjunction with differential scanning calorimetry, wide-angle X-ray diffraction and infrared spectroscopy, were used to characterize a series of nylon 11 and 12 copolymers with mole percentages of nylon 12 monomer of 0, 15, 35, 50, 65, 85, and 100%. Monotonic melting point ( T m) and heat of fusion depressions were observed for the copolymer series with the 65 mol% nylon 12 copolymer having the lowest apparent crystallinity and T m at 148°C. Solid-state 15N NMR spectra showed a smooth shift of the main peak position for the as-prepared copolymers from 84 ppm for the α-form of pure nylon 11 to 89 ppm for the γ-form of pure nylon 12. Similar behavior was seen for FTIR amide V and VI modes which are also sensitive to the α- and γ-crystal forms. 13C NMR T 1 measurements showed that the overall most mobile sample was the 65:35 copolymer. The amide group of the 1:1 copolymer was labelled using 15N-labelled amino acids available through the Gabriel synthesis; an annealed, solution-cast film of this sample showed a T 1N value of 349 s, similar to values seen for annealed nylon 11 and nylon 12 homopolymers. The WAXS pattern for the 65 mol% nylon 12 sample showed a sharp peak at 2 θ=21.3, overlapping a broad peak centered at 2 θ=21.0. These are consistent with the values seen for γ-form nylon 12. The 1:1 copolymer ( 15N labelled) was shown to be polymorphic, like the homopolymers after specific treatments, with a γ-like phase formed upon solvent casting, and an α-like phase dominating for as-polymerized material and precipitated flakes.