Thermal degradation mechanisms of polyetherimide investigated by direct pyrolysis mass spectrometry

The thermal degradation mechanisms of poly[2,2'-bis(3,4-dicarboxyphenoxy)ph enylpropane-2-phenylenediimide] (PEI) have been investigated by thermogravi metry (TG) and by direct pyrolysis mass spectrometry (DPMS). TG data show t hat PEI has a main decomposition step centred at about 510 degrees C follow ed by a less marked step in the 600-650 degrees C temperature range and lea ving about 60% of chanted residue at 800 degrees C. The total ion curve (TI C) of a purified PEI sample, obtained by DPMS, closely reproduces the two m axima appearing in the derivative TG (DTG) curve, whereas the TIC curve of a crude PEI sample shows two less pronounced maxima in the temperature rang e of 250-450 degrees C due to low molar mass compounds, which volatilize un decomposed in the high vacuum of the MS. The structure of the pyrolysis com pounds obtained in the first thermal degradation step of a purified PEI sam ple suggest that they are mainly formed by the scission of: i) the isopropy lidene bridge of bisphenol A; ii) the oxygen-phthalimide bond; iii) the phe phenyl-phthalimide bond, which are apparently the weakest bonds of PEI. Ex tensive hydrogen transfer reactions and subsequent condensation reactions m ay account for the high amount of char residue. The pyrolysis compounds obt ained in the second degradation step (620 degrees C) are mainly constituted of CO2 benzene, aniline, benzonintrile, phenylenediamine, and dibenzonitri le, which may he generated by further thermal degradation reactions of pyro lysis compounds containing N-H phthalimide as end groups. Another degradati on processes which may account for CO2 formation is the hydrolysis of the i mide moiety to form poly(amic acid) units which produce an aromatic amide s tructure by decarboxylation. The pyrolysis of an aromatic polyamide (NOMEX) was then studied for comparison. The structure of the pyrolysis products d etected by the DPMS analysis of both polymers allowed a detailed schematiza tion of the thermal degradation pathways involved in the degradation of PEI and on the reactions leading to the formation of the charred residue.