A NEW APPROACH FOR MEASURING THE RATE OF PYROLYSIS OF CROSS-LINKED POLYMERS - EVALUATION OF DEGRADATION RATE CONSTANTS FOR CROSS-LINKED PMMA

Low molecular weight samples of PMMA have previously been observed to degrade at 400''C via scission-initiated chain unzip in which the majo rity of the molecules depropagate to the end and are effectively termi nated in this way. It was considered by the authors that if a cross-li nk were present within the PMMA molecule the unzip may be halted and t he overall rate of degradation of the PMMA sample thereby reduced. In the present work, samples of cross-linked low molecular weight PMMA ha ve been prepared by free radical polymerisation using azo-bis-isobutyr onitrile as an initiator, carbon tetrachloride as a transfer agent and ethylene glycol dimethacrylate as a cross-linking agent. The rate of evolution of MMA monomer from these polymers has been measured and com pared with equivalent samples prepared without a cross-linking agent. The samples have all been prepared in situ on the filament subsequentl y used for pyrolysis. This novel approach has not yet been previously reported, and is the ideal approach for studying the kinetics of intra ctable cross-linked material. It has been found that cross-linking low molecular weight PMMA does inhibit the rate of evolution of MMA monom er at 400 degrees C. An explanation of this may be attributed to cross -link units in the chain impeding the unzipping reaction. (C) 1998 Els evier Science Limited. All rights reserved.