Direct observation of the thermo-oxidative degradation of PA66 by spin-trapping ESR analysis

Abstract

We succeeded in detecting the radical intermediates produced during the thermo-oxidative degradation of polyamide 66 (PA66) in air between 80°C to 240°C, the temperature range for practical applications, using spin-trapping electron spin resonance (ST-ESR) even in the presence of oxygen, which interferes with ESR measurements. As a result, ST-ESR analysis has newly revealed the presence of two more radical intermediates, −•CH−NH− and •(C=O)NH−, under air in addition to the two radical intermediates, •CH2− and −CH2−•CH−CH2−, already found in the previous report, which was examined under oxygen-free conditions. The formation of those four radicals can be explained by hydrogen abstraction and β-scission in the autoxidation mechanism. The onset temperature of the rapid oxidation reaction observed in chemiluminescence (CL) measurement coincides with the one for generating the −•CH−NH− radical, indicating that the initial radical of thermo-oxidative degradation of PA66 is −•CH−NH− radical generated by selective oxygen attack on the NH-adjacent methylene group. This finding strongly supports previous reports that concluded hydrogen abstraction from the NH-adjacent methylene group is the main reaction under air.