Kinetic Modeling of the Thermal Degradation of Methacrylate Copolymers by Thermogravimetric Methods

Abstract

The thermal degradation kinetics of a random copolymer of isobutyl methacrylate / lauryl methacrylate, produced by free-radical solution polymerization is investigated over a temperature range of 350 to 750 K, using dynamic thermogravimetric experiments. Heat treatment of the copolymer affects the main polymer backbone and side chains. The thermal degradation of the copolymer proceeds in three distinct steps of weight loss: the first and easiest step is initiated by scissions of head-to-head linkages representing one type of defect in the polymer backbone; the second and more difficult step is initiated by scissions at the vinylidene chain ends; the third and most energetic step is initiated by random scissions within the polymer chain.The time evolution of molecular weight distribution (MWD) is measured by gel permeation chromatography (GPC). The most pronounced changes in the trend of the average molecular weight are observed during the transition from each degradation step to the subsequent one. A continuous distribution kinetic model based on a population balance is developed to describe the observed degradation behaviour of the copolymer. This comprehensive model conforms to the special mechanisms for random chain-scission and chain-end depolymerization. The pseudo-kinetic rate parameters for each degradation step are estimated to be respectively equal to 1.1 10-8, 5.6 10-8 and 1.08 10-7 mol g-1 min-1. The average calculated activation energies are respectively 89.2, 116.4 and 134.8 kJ/mol.Global kinetic parameters of degradation are also determined using dynamic thermogravimetric (TGA/DTGA) data. The model-fitting and model-free isoconversional methods are used to retrieve the kinetic parameters of the degradation process. The model-free isoconversional method can satisfactorily describe the dependence of the activation energy on the conversion and is recommended over the model-fitting methods for obtaining the reliable and consistent kinetic parameters of polymer degradation.