Comparison of the photodegradation of parylene C and parylene N

In the present paper the photochemical behaviours of poly(para-xylylene) (PPX-N or parylene N) and poly(2-chloro-para-xylylene) (PPX-C or parylene C) are compared. The photodegradation of films irradiated at λ>300 nm was studied by FTIR, UV-visible and emission analysis, chemical titration and derivatisation reactions, physical treatments and HPLC identification after methanolic extraction. The experimental results show that two successive steps are involved in the photooxidation of PPX-C. Direct photolytical processes, related to the 2-chloro-substitution and involving the formation of specific UV and IR absorbing structures, are initially implied in the photooxidation of PPX-C. Then the same reactions as those previously reported for PPX-N account for the photodegradation of PPX-C, involving a photo-induced oxidation of the methylene groups as well as the oxidation of the aromatic rings. As a consequence of the numerous chain scissions, the photoproducts are predominantly low molecular weight oxidation species. Contrary to the rate of photodiscoloration, the rate of photooxidation of PPX-C was observed to be lower than that of PPX-N. Such an effect was attributed to a five time lower permeability to oxygen of PPX-C film combined with the light attenuation resulting from the formation in the first steps of oxygenated exposure of UV-absorbing photolysis products.