New method for evaluation of kinetic parameters and mechanism of degradation from pyrolysis-GC studies: Thermal degradation of polydimethylsiloxanes

Thermal degradation of polydimethylsiloxane (PDMS) polymers having hydroxyl (PS) and vinyl (PS-V) terminals was studied by pyrolysis-gas chromatograph y (PGC) in the temperature range from 550 to 950 degrees C. The degradation products were primarily cyclic oligomers ranging from trimer (D-3) to cycl omer D-11 and minor amounts of linear products and methane. The product com position varied significantly with pyrolysis temperature and extent of degr adation. A new method was developed to derive a mass loss-temperature curve (pyrothermogram, PTG) and to determine the kinetic parameters of decomposi tion (k, n, and E-a) from sequential pyrolysis studies. It was shown that i sothermal rate constants can be derived from repeated pyrolysis data. Good agreement between the rate constants derived from the two methods validates the methodology adopted. This was further confirmed from thermogravimetric studies. The E-a values for the decomposition of PS and PS-V derived from sequential pyrolysis were 40 +/- 2 and 46 +/- 2 kcal mol(-1), respectively. Various mechanisms for the degradation of PDMS were reviewed and discussed in relation to the PGC results. (C) 1999 John Wiley & Sons, Inc.