Comparison of the photochemical and thermal degradation of bisphenol-A polycarbonate and trimethylcyclohexane–polycarbonate
Résumé
The photochemical and thermal behaviour of bisphenol-A polycarbonate (PC) and trimethylcyclohexane–polycarbonate (TMC–PC) have been compared. The ageing of films, irradiated at short (λ=254 nm) and long (λ>300 nm) wavelengths in the absence and in the presence of oxygen or thermo-oxidised at 170 °C, has been analysed by different spectroscopic and chromatographic methods. A dual photochemistry is shown to account for the photodegradation of TMC–PC, as previously reported for PC. Under excitation at the shortest wavelengths, the mechanism involves photo-Fries rearrangements of the aromatic carbonate units and a photo-induced oxidation of the aliphatic moieties. Under excitation at 254 nm, the second photo-Fries rearrangement is lowered in TMC–PC at the expense of the formation of yellowing structures. Moreover, under short and long wavelengths exposures, the rate of photo-oxidation of TMC–PC was observed to be higher than that of PC. Such effects have been attributed to a reduced mobility of the macromolecules and to a steric effect due to the trimethylcyclohexylidene structure that contains tertiary carbon atoms. Experimental results confirmed that, regarding oxidation which initially involves hydrogen abstraction, tertiary and secondary sites of TMC–PC are more oxidisable than the primary aliphatic ones contained in PC.
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