Monte Carlo Simulation of Linear Polymer Thermal Depolymerization under Isothermal and Dynamic Modes

Kinetics of linear polymer thermal depolymerization under isothermal and dynamic TGA modes was simulated by the Monte Carlo method. The simulation was carried out on model arrays having the same initial degree of polymerization and different width (polydispersity index, ) at three constant temperatures and five heating rates. Kinetics of the process in both modes is described by the Avrami equation, the exponent in which decreasing as the distribution width increases. Treatment of the model kinetic curves of degradation using the nonlinear regression method by the Avrami equation, under both isothermal and dynamic modes, gives correct activation energy and pre-exponential factor values independently of the initial PDI. Data obtained in the dynamic mode were also treated by two isoconversion methods, widely applied to kinetic analysis of TGA curves (Flynn-Wall-Ozawa method and Kissinger-Akahira-Sunose (KAS) method). 1. Introduction Thermogravimetric analysis (TGA) method is widely applied to the investigation of various polymers and polymeric composites. This is a simple and reliable method that allows estimation of thermal resistance of various materials and obtaining of information on kinetics and the mechanisms of thermal degradation processes. At present, different investigators are using about 20 various methods for quantitative treatment of the TGA thermograms for determination of thermal degradation kinetic parameters, the activation energy, first of all. The most general description of a single-stage thermal degradation of a polymer under the dynamic TGA mode looks as follows: where is the conversion determined from the expression )/( ); , , and are current, initial, and final masses of the sample, respectively; is the heating rate; is the absolute temperature; is the gas constant; and are the pre-exponential factor and the activation energy of degradation, respectively. The kinetic function depends on a particular degradation mechanism. The majority of the methods applied to kinetic analysis of TGA thermograms use different approximate solutions of (1) in the form of linear approximations. TGA data may also be treated by the method of nonlinear regression [1, 2], (1) being solved numerically. The main kinetic functions , used for description of thermal processes in the solid phase, are classified in [3]. Unfortunately, all these kinetic descriptions are rather formal and can hardly be bound to the real physicochemical processes proceeding in the polymers. One of the frequently observed kinetic mechanisms of thermal degradation of polymers is