Comparison of the photochemical and thermal degradation of bisphenol-A polycarbonate and trimethylcyclohexane-polycarbonate

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 (lambda = 254 nm) and long (lambda > 300 nm) wavelengths in the absence and in the presence of oxygen or thermo-oxidised at 170 degreesC, 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 trimethyleyclohexylidene 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. (C) 2001 Elsevier Science Ltd. All rights reserved.