A detailed re-examination of the photo-oxidation mechanism of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) at both long and short irradiation wavelengths under accelerated conditions is reported. The formation rate of polydimethylphenoxy (PDMP) radicals is observed both in the presence and in the absence of ultra-violet (u.v.) light by using electron spin resonance spectroscopy. In order to explain the great stability of PDMP radicals, the bimolecular decay observed in the dark is discussed. In addition, spin-trapping experiments using nitrosodurene reveal the existence of benzylic-type radicals under u.v. irradiation conditions. Under mild oxidative conditions, i.e. thermo-oxidation at 80 degrees C, the changes in the Fourier-transform infra-red spectra after 1215 h show the formation of two main absorption bands at 1694 and 1658 cm(-1) assigned to aromatic aldehydes and quinonic groups, respectively. The poor photo-stability of these two primary oxidation products generates, by fast photolytic conversion to benzoic acids and esters, a broad band centred at 1734 cm(-1) under accelerated photo-oxidative conditions. Furthermore, the cyclic voltammetry of PPO solution in the dark indicates both the existence of superoxide radical anions in the absence of u.v. light and the electro-catalytic effect possibly induced by quinonic groups. In this work direct formation of radical cations and superoxide anions by a photo-induced electron-transfer reaction is suggested under photo-oxidative conditions. The catalytic role of quinonic groups on the further generation of superoxide anions and PDMP radicals at long irradiation wavelengths and in the dark is also considered on the overall PPO photodegradation.
A reappraisal of the photo-oxidation mechanism at short and long wavelengths for poly(2.6-dimethyl-1.4-phenylene oxide)
Scoponi M;Amadelli R;
1996
Abstract
A detailed re-examination of the photo-oxidation mechanism of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) at both long and short irradiation wavelengths under accelerated conditions is reported. The formation rate of polydimethylphenoxy (PDMP) radicals is observed both in the presence and in the absence of ultra-violet (u.v.) light by using electron spin resonance spectroscopy. In order to explain the great stability of PDMP radicals, the bimolecular decay observed in the dark is discussed. In addition, spin-trapping experiments using nitrosodurene reveal the existence of benzylic-type radicals under u.v. irradiation conditions. Under mild oxidative conditions, i.e. thermo-oxidation at 80 degrees C, the changes in the Fourier-transform infra-red spectra after 1215 h show the formation of two main absorption bands at 1694 and 1658 cm(-1) assigned to aromatic aldehydes and quinonic groups, respectively. The poor photo-stability of these two primary oxidation products generates, by fast photolytic conversion to benzoic acids and esters, a broad band centred at 1734 cm(-1) under accelerated photo-oxidative conditions. Furthermore, the cyclic voltammetry of PPO solution in the dark indicates both the existence of superoxide radical anions in the absence of u.v. light and the electro-catalytic effect possibly induced by quinonic groups. In this work direct formation of radical cations and superoxide anions by a photo-induced electron-transfer reaction is suggested under photo-oxidative conditions. The catalytic role of quinonic groups on the further generation of superoxide anions and PDMP radicals at long irradiation wavelengths and in the dark is also considered on the overall PPO photodegradation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
