A CRITICAL COMPARISON OF PROCEDURES FOR EVALUATING RATE CONSTANTS IN THERMAL-DEGRADATION, ILLUSTRATED BY PYROLYSIS-GC RESULTS FROM 4 POLYMERS

When studying the thermal degradation of macromolecular compounds that give volatiles, several methods are available for verifying the exten t to which the rate of evolution of volatiles is first order in the ma ss of remaining sample. Five such methods are discussed, and each has its own demands for experimental data, and its own way of plotting the data. Each of the methods also provides a value for the first-order r ate constant. For a given sample, it is shown that the value of the ra te constant obtained by each of the methods is not necessarily the sam e. A principal reason for this is the different way in which the asymp totic limit of product yield is explicitly or implicitly determined in each of the methods. This problem is exacerbated when the evolving vo latiles leave a residue that becomes progressively different from the original sample. A further problem arises when the method requires tha t the same sample is repeatedly heated and cooled in order to obtain t he kinetic data. The evidence then suggests that the data become more unreliable as the heating cycles progress. Pyrolysis-g.c. results for PMMA, PIE, PSt and PBuAc are used to illustrate the utilisation and li mitations of the methods. The general conclusion is that the 'normal l og plot method' is best for obtaining absolute values of rate constant s, and that sequential methods may be convenient for comparing relativ e rates. (C) 1998 Elsevier Science Limited. All rights reserved.