13C and 15N Solid-State NMR of Copolymers of Nylons 6 and 7: Observation of a Stable Pseudohexagonal Phase

A series of nylon copolymers were synthesized and characterized based on the even-odd A-B monomer combination caprolactam and enantholactam. The nylon 6-co-nylon 7 samples displayed physical properties which directly correlate with copolymer composition. The degrees and types of crystallinity for these materials were examined using a combination of thermal, NMR, and X-ray measurements. The homopolymers each displayed highly ordered α-crystallites while the copolymers possessed less-ordered domains that decreased in α-like characteristics down to the 1 :1 copolymer, which possessed a thermodynamically stable pseudohexgonal ordered phase. The 13 C T 1 values determined for the annealed 1 :1 copolymer were shorter than those of crystalline regions of annealed homopolymers. In fact, they were comparable to values observed for amorphous domains in melt-quenched nylon 6 and nylon 7 homopolymers but significantly longer than the values for a precipitated and annealed 1 :1 nylon 11-co-nylon 12 sample. Annealing at temperatures ca. 10 °C below the DSC T m increased the perfection of the copolymer ordered domains based on both NMR and X-ray measurements but did not convert the pseudohexagonal phase to α-crystallites.