Thermal decomposition processes in aromatic polycarbonates investigated by mass spectrometry

The thermal decomposition pathways leading to the formation of volatile compounds and to char residue in poly(bisphenol A carbonate) (PC), poly(resorcinol carbonate) (PRC), and poly-(hydroquinone carbonate) (PHC) have been investigated by mass spectrometry. The structure of the volatile compounds obtained in the temperature range 300-700 degrees C, by direct pyrolysis mass spectrometry (DPMS), suggests that these polycarbonates undergo thermal decomposition by a number of different pyrolysis processes. In the initial stage of the thermal degradation are generated cyclic oligomers by an intramolecular exchange reaction, whereas the evolution of CO2 and H2O is spread over all the pyrolysis temperature range, being responsible for the formation of ether bridges (decarboxylation) and phenolic end groups (hydrolysis). A disproportionation reaction of the BPA isopropylidene bridges of PC itself takes place at higher temperature yielding phenyl and isopropylidene end groups, whereas pyrolysis products containing dibenzofuran units are formed by dehydrogenation of ether units. The formation of compounds containing xanthone and fluorenone units, most likely generated by isomerization of the aromatic carbonate functional groups and successive condensation reaction, has also been observed. In an additional set of experiments, the isothermal pyrolysis of PC was achieved, isothermally, at 350 and 400 degrees C, and then the exhaustive and selective aminolysis of the carbonate functional groups, still present in the pyrolysis residue, has been performed. The aminolyzed residue was then analyzed by fast atom bombardment (FAB) mass spectrometry to detect the compounds eventually formed by molecular rearrangements of PC chains. The FAB-MS spectra showed the presence of compounds containing several consecutive xanthone and ether units, indicating that at this temperature the isomerization and the condensation processes leading to these structures are quite extensive. These units undergo aromatization and cross-linking processes, leading to a graphite-like charred residue as the temperature increases.