Solution C-13 NMR characterization of nylon 66: Quantitation of cis amide conformers, acid and amine end groups, and cyclic unimers

Using a 2,2,2-trifluoroethanol/deuteriochloroform solvent mixture (TFE/CDCl 3), well-resolved solution C-13 NMR spectra of nylon 66 samples were obtain ed that showed several low-intensity peaks not seen in other, more acidic s olvents. These low-intensity peaks were correlated to the methylene carbons near cis amide conformers, acid and amine end groups, and within cyclic ol igomers. On the basis of relative peak intensities, the quantitative conten ts of these components for a typical sample of nylon 66 were determined to be 1.1 mol % cis amide conformers, 1.0 mol % acid end groups, and 0.5 mol % amine end groups. A number-average molecular weight of 30 177 g/mol was ca lculated from end group concentrations, which is a typical value for commer cial nylon 66 samples. The chemical shift values of methylene carbons conta ined in end groups and in cyclic oligomers were found to be pH sensitive an d therefore dependent on the relative concentrations of acid and amine end groups present in each sample. Understanding this behavior, in conjunction with the peak assignments described here, allows identification and quantit ation of structural units and contaminants important in controlling chemica l and physical properties.