Kinetics and mechanisms of flash pyrolysis of poly(methyl methacrylate) (PMMA)

Flash pyrolysis (600-1000 degrees C/s) of a film of anionically polymerized poly(methyl methacrylate) (PMMA) was conducted at controlled temperatures in the 380-600 degrees C range by T-jump/FTIR spectroscopy. In the endothermic decomposition process, the methyl methacrylate monomer (MMA greater than or equal to 90%) and CO2 (less than or equal to 10%) are the only pyrolysis products detected. Arrhenius parameters for evolution of MMA and CO2 were determined under a pressure of 2 and 11 atm Ar for PMMA with MW = 2000 and 131,000. A zeroth-order rate model was used. A slope break occurs at 460-500 degrees C in the Arrhenius plots. Below this range for MMA, E-a = 43-66 kcal/mol and ln(A/s)= 26-44, which are comparable to previous reports for random C-C scission kinetics. For CO2, E-a = 39-46 kcal/mol and ln(A/s) = 23-27, which are comparable to the decarboxylation rates of other organic compounds. Above 460-500 degrees C, increased control of the rates by desorption/diffusion of MMA and CO2 is suggested by the lower values of E-a in the 12-30 kcal/mol range and ln(A/s) in the 5.7-17 range. Thus, the limiting temperature of pyrolysis of PMMA is about 500 degrees C before the deconsolidation rate of the pyrolysis products begins to take control from the bulk decomposition rate in determining the rate of product evolution. (C) 1997 by The Combustion Institute.