New Method for Evaluation of Kinetic Parameters and Mechanism of Degradation from Pyrolysis-GC Studies: Thermal Degradation of Polydimethylsiloxanes

摘要

Thermal degradation of polydimethylsiloxane (PDMS) polymers having hydroxyl (PS) and vinyl (PS-V) terminals was studied by pyrolysis-gas chromatography (PGC) in the temperature range from 550 to 950 deg C. The degradation products were primarily cyclic oligomers ranging from trimer (D_3) to cyclomer D_11 and minor amounts of linker products and methane. The product composition varied signficantly with pyrolysis temperature and extent of degradation. A new method was developed to derive a mass loss-temperature curve (pyrothermogram, PTG) and to determine the kinetic parameters of decomposition (k, n, and E_a) from sequential pyrolysis studies. It was shown that isothermal 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.