STUDIES OF THE BREAKDOWN MECHANISM OF POLYMERS: I. THE THERMAL DECOMPOSITION OF SOME PHENYLENE-R-POLYMERS

Abstract : Samples of eight different phenylene-R polymers were subjected to isothermal heating in three progressive steps. The gaseous decomposition products were collected and analyzed by mass spectroscopy. The corresponding residues were obtained in separate, identical runs and identified by weight, elemental analysis, and IR spectra. The data collected were used to postulate an overall breakdown mechanism. The first reaction occurring during the decomposition of these polymers at lower temperatures (below 450 C) is the removal of pendant groups, and the cleavage of the linkage between the rings. Polyphenylene sulfide shows the least amount of ring breakdown, as indicated by very little methane formation. Sulfur is removed as hydrogen sulfide, and the ruptured bond may partially heal, forming C-C-bonds. Sulfones appear to behave in a similar manner, the SO2 linkage being removed as sulfur dioxide. The breakdown of ethers and phenols is always accompanied by a considerable amount of ring cleavage and destruction, the decomposition products being CO, CO2, and H2O. Crosslinking with evolution of hydrogen is the predominant reaction of all polymers at the higher temperatures (up to 620 C). The end products, which may contain some O and especially S in the form of stable, fused ring systems, are crosslinked polymer carbons with unsaturated, aliphatic chains rather than completely aromatic, fused ring systems. Possible exceptions are polyphenylene sulfide and polyphenylene.