Photooxidation of poly(para-xylylene)

The photochemical behaviour of poly(para-xylylene) (PPX or parylene N) under polychromatic irradiation has been investigated by FTIR, UV–Visible and emission analysis. The various photoproducts formed have been identified by chemical titration, derivatisation reactions, physical treatments associated with the analysis by mass spectrometry of the gas phase and HPLC analysis of the low molecular weight fragments after methanolic extraction. The experimental results indicate that direct excitation of PPX units can occur at λ>300 nm. In the presence of oxygen, radicalar species formed in direct photolysis are able to induce the oxidation of the methylene groups of PPX through hydrogen abstraction. The photo-unstable secondary hydroperoxides that are then formed lead subsequently to the formation of various oxidised species that have been identified. In parallel, the detection of aliphatic carboxylic acids indicates the oxidation and the cleavage of the aromatic rings. As a consequence of the chain scissions provoked by oxidation of the methylene groups located on both sides of the aromatic rings, the photoproducts are predominantly low molecular weight oxidation species.