New vistas in the photo-oxidation of nylon 6

Matrix-assisted laser desorption ionization mass spectrometry (MALDI) is an excellent method to use in determining the structure of the molecules produced in the photo-oxidative degradation of nylon 6 at 60 degreesC in air. The MALDI spectra of photo-oxidized nylon 6 (Ny6) show the presence of over 40 compounds, as compared to only 3 in the blank Ny6 sample. The extremely favorable event here is that the MALDI spectra present so many new well-resolved peaks, which provide information on the structure and end groups of the oxidation products. The structural analysis of the photo-oxidation products provided by the MALDI spectra allowed us to draw a detailed map of the photo-oxidation mechanisms of Ny6 (Schemes 1-4). The presence of the majority of the oligomers listed in Table 1 had not been revealed before. Our results extend and modify the currently accepted picture for the photo-oxidation mechanisms of Ny6. In fact, our results confirm previous insights that the hydrogen abstraction and subsequent formation of a hydroperoxide intermediate actually occur but also reveal that Norrish I and Norrish II chain-cleavage reactions play an important role in the photo-oxidation process of Ny6. This essential feature of the process had been missed in previous studies focused on UV and IR techniques. The peculiarity of our approach consists of using a high-sensitivity and nonaveraging technique such as mass spectrometry, which allows the detection and monitoring of each new oligomers formed during the oxidation process. This is a remarkable result, and it should be expected that future MALDI studies may have an impact on the current views of photo-oxidation processes of other polymer systems.