Thermal degradation mechanisms of polyetherimide investigated by direct pyrolysis mass spectrometry

The thermal degradation mechanisms of poly[2,2′-bis(3,4-dicarboxyphenoxy)phenylpropane-2-phenylenediimide] ( PEI ) have been investigated by thermogravimetry (TG) and by direct pyrolysis mass spectrometry (DPMS). TG data show that PEI has a main decomposition step centred at about 510°C followed by a less marked step in the 600–650°C temperature range and leaving about 60% of charred residue at 800°C. The total ion curve (TIC) of a purified PEI sample, obtained by DPMS, closely reproduces the two maxima appearing in the derivative TG (DTG) curve, whereas the TIC curve of a crude PEI sample shows two less pronounced maxima in the temperature range of 250–450°C due to low molar mass compounds, which volatilize undecomposed in the high vacuum of the MS. The structure of the pyrolysis compounds obtained in the first thermal degradation step of a purified PEI sample suggest that they are mainly formed by the scission of: i) the isopropylidene bridge of bisphenol A; ii) the oxygen-phthalimide bond; iii) the phenyl-phthalimide bond, which are apparently the weakest bonds of PEI . Extensive hydrogen transfer reactions and subsequent condensation reactions may account for the high amount of char residue. The pyrolysis compounds obtained in the second degradation step (620°C) are mainly constituted of CO₂, benzene, aniline, benzonintrile, phenylenediamine, and dibenzonitrile, which may be generated by further thermal degradation reactions of pyrolysis compounds containing N—H phthalimide as end groups. Another degradation processes which may account for CO₂ formation is the hydrolysis of the imide moiety to form poly(amic acid) units which produce an aromatic amide structure by decarboxylation. The pyrolysis of an aromatic polyamide ( NOMEX ) was then studied for comparison. The structure of the pyrolysis products detected by the DPMS analysis of both polymers allowed a detailed schematization of the thermal degradation pathways involved in the degradation of PEI and on the reactions leading to the formation of the charred residue.