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

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)-C-circle-NH- and (circle)(C=O)NH-, under air in addition to the two radical intermediates, (CH2)-C-circle- and CH2-(CH)-C-circle-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 beta-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)-C-circle-NH- radical, indicating that the initial radical of thermo-oxidative degradation of PA66 is -(CH)-C-circle-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.

Automated summary

By using spin-trapping electron spin resonance (ST-ESR) technique, we successfully detected the radical intermediates produced during the thermo-oxidative degradation of polyamide 66 (PA66) in air. The presence of oxygen, which interferes with ESR measurements, did not hinder the ST-ESR analysis. We found the presence of two new radical intermediates, -(CH)-C-circle-NH- and (circle)(C=O)NH-, in addition to the previously reported ones, under air.