Photo- and thermo-oxidation of poly(p-phenylene-vinylene) and phenylene-vinylene oligomer

This paper reports a study of the photo- and thermo-degradation of poly(p-phenylene-vinylene)-type (PPVs) materials, including a five-ring n-octyloxy substituted phenylene-vinylene oligomer (Ooct-OPV5) and poly(p-phenylene-vinylene) (PPV). The modifications in the chemical structure of the thin films submitted to UV-visible light irradiation and thermal oxidation were analysed with infrared spectroscopy, and the oxidation products were identified by derivatisation reactions. Results showed that the photochemical and thermal behaviour of the Ooct-OPV5 oligomer was similar to that of MDMO-PPV (poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene), which is a polymer used in organic solar cells. Additionally, the identification of the products resulting from the oxidation of the vinylene bonds was simpler in Ooct-OPV5 and PPV compared to MDMO-PPV In contrast, the oxidation mechanisms of PPV, which has no ether substituent, and MDMO-PPV are not identical; the disappearance of the double bonds in PPV does not involve the formation of aromatic ketones. It was also shown that the absence of ether substituents does not decrease the rate of photo-oxidation of PPV compared to MDMO-PPV. Finally, as the same mechanisms proposed for Ooct-OPV5 and PPV occur under both photo- and thermo-oxidative conditions of ageing, this confirms that singlet oxygen does not play a decisive role in the photo-oxidation of PPVs. (C) 2011 Elsevier Ltd. All rights reserved.