Monitoring nylon-6,6 thermochemistry by solid-state NMR

Solid-state ¹³C and ¹⁵N NMR spectroscopy was used to study the structure and thermo-chemistry of nylon-6,6. Experiments were performed on samples of nylon-6,6 heated in the presence of labeled adipic acid and hexamethylenediamine (HMD) monomers as well as labeled bis(hexamethylene-triamine) (BHMT), a molecule previously implicated as a potential nylon cross-linker. The uptake and subsequent reaction of labeled materials were characterized by a variety of solid-state NMR techniques. Reaction of the central (α) nitrogen of BHMT to form an amide occurs very rapidly. With time, transamidation causes this position to become indistinguishable from the bulk amide nitrogen in nylon. Rotational-echo double resonance (REDOR), a solid-state NMR experiment that permits the quantitative measurement of ¹³C-¹⁵N labeled chemical bonds, was used to detect the formation and breaking of carbon-nitrogen bonds due to polymerization and transamidation. The reaction of adipic acid and hexamethylenediamine (HMD) and the transamidation reaction between the resulting oligomers and bulk nylon were studied by heating nylon-6,6 in the presence of [¹³C]adipic acid and [¹⁵N]HMD and characterizing the resulting polymers with REDOR. The combination of stable isotope labeling and NMR spectroscopy illustrated in this work should be applicable to the study of a wide variety of polymers.