A study of thermal degradation of poly(aryl-ether-ether-ketone) using stepwise pyrolysis/gas chromatography/mass spectrometry

Abstract

Thermal degradation of poly(aryl-ether-ether-ketone) (PEEK) was investigated using stepwise pyrolysis/gas chromatography/mass spectrometry (Py/GC/MS) by consecutively heating the sample at fixed temperature intervals to achieve narrow temperature pyrolysis conditions. The individual mass chromatograms of the various pyrolysis products were correlated with the pyrolysis temperature in order to elucidate the pyrolysis mechanism. 4-Phenoxyphenol and 1,4-diphenoxybenzene were detected at 450°C, indicating that the degradation of PEEK was initiated by selective cleavage at chain ends and chain branches. Phenol was detected as the major pyrolysis product by further pyrolysis to 650 °C, which was attributed to the random main chain cleavage of ether groups. At temperatures above 650 °C, chain cleavage at carbonyl ends became the dominant pyrolysis pathway. The high char yield suggested that random main chain scission was accompanied by carbonization. Carbonization was presumably the dominant pyrolysis mechanism at temperatures above 750°C. Results obtained with the new method indicate the interesting potentials of the technique for the investigation of polymeric materials. © 1997 John Wiley & Sons, Ltd. Received 15 August 1997; Revised 17 October 1997; Accepted 20 October 1997 Rapid Commun. Mass Spectrom. 11, 1987-1995(1997 Pyrolysis uses heat alone to break down large molecules into molecules of lower mass due to thermal fission and, occasionally, of higher mass due to intermolecular reaction. The nature of the fragmentation and recombination products is determined largely by the heating parameters and ambient conditions. The analysis of pyrolysis products (pyrolysates) thus provides an alternative means to study polymeric and complex materials that are not suitable for direct analysis by popular techniques such as chromatography and mass spectrometry. From an analytical viewpoint, the applications of analytical pyrolysis are limited by the complexity of the pyrolysates. As a result, the integration of pyrolysis with Fourier transform infrared spectroscopy (Py/FTIR), mass spectrometry (Py/MS), and gas chromatography (Py/GC) is necessary for the routine analysis of polymeric and complex materials. Py/MS offers the advantages of the speed of analysis, the ease of automated data processing, and the richness in chemical information. Py/GC is capable of separating isomers and is a useful tool for quantitative analysis. The integration of pyrolysis with GC/MS (Py/GC/MS) could therefore provide additional sensitivity and selectivity, and extend the application fields of pyrolysis.

Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies

“…18 Rapid and significant mass loss occurs just below 600°C resulting in the volatilisation of around 45% of the polymer mass, the remaining polymer mass appears to be carbonaceous char. This has also been observed by other authors and has been attributed to the loss of, mainly, phenols as decomposition products, 21 although carbon monoxide (CO) and carbon dioxide (CO 2 ) have also been identified as evolving rapidly over this temperature range, 22 possibly as a by-product of the decomposition of PEEK to phenols. This is followed by a slower process of volatilisation of the residue, with over 35% still present even at 1000°C.…”

“…All have a peak pyrolysis temperature of 650°C, reach maximum amounts at 750°C and decrease rapidly to trace amounts above 900°C. 22 This is similar to the pattern shown by dibenzofuran and so indicates that all are formed by the recombination of free radicals produced by ether or carbonyl scission of the main polymer chain. 22 This process produces biphenyl as shown ( Figure 10): Figure 10.…”

“…It is believed that as most of the products of PEEK decomposition contain terminal hydroxyl groups and there are few with aldehyde units that the ether links are less thermally stable. 21 Furthermore, work has been carried out on the initial decomposition of PEEK and its products using coupled techniques such as Thermogravimetry/Mass Spectometery (TG/MS) 19 and Pyrolysis Gas Chromatography/Mass Spectrometry (pyGC/MS) 22 as well as a combination of the two. 27 Table 3 shows the volatile decomposition products of PEEK determined by stepwise pyGC/MS arranged by peak temperature of production.…”

“…As the randomness of the main chain scission increases, other volatile products such as diphenyl ether and CO and CO 2 are formed at 650°C and dibenzofuran, biphenyl and naphthalene at and above 750°C. 22 At temperatures above 650°C, chain cleavage at terminal carbonyls is believed to be the primary pyrolysis pathway. Phenol has been identified as a major decomposition product, its yield being greater than benzene at temperatures above 650°C.…”

“…The following mechanism has been proposed: Recombination of Adjacent Radicals to form Dibenzofuran. 22 Dibenzofuran has also been identified in the decomposition products of poly(1,4-phenylene oxide) and is commonly observed as a dimer, 21 but the corresponding carbonyl derivative was not observed indicating that the ether group is more stable than the ketone group. However, work completed by Tsai et al found benzophenone to be a minor product of PEEK decomposition at a peak pyrolysis temperature of 750°C.…”