GENERALIZED COMPOSITE DEGRADATION KINETICS FOR POLYMERIC SYSTEMS UNDER ISOTHERMAL AND NONISOTHERMAL CONDITIONS

A composite degradation methodology is extended to the conversion-dependence function in order to explain the importance of multiple reaction mechanisms which might be considered to be involved in degradation processes. Based on two elementary reaction mechanisms, a specific form of the model equation is derived, which is capable of describing various types of degradation behavior showing sigmoidal rate as well as deceleratory rate. The conversion-dependence function is derived to be independent of the Arrhenius-type reaction constant or temperature, and thus the kinetic parameters are determined by analytic methods that have been developed for isothermal and dynamic-heating experiments without any modification or additional assumptions. The developed model equation is tested by predicting the isothermal master curve of polyether-ether-ketone (PEEK), which is used as a model system in this study. The activation energies of the model system are analyzed using comparable methods for isothermal and dynamic experiments, which compare favorably in terms of the activation energy as a function of conversion. The resulting model equation, based on the kinetic parameters determined by isothermal experiments, can accurately predict both isothermal and dynamic-heating thermogravimetry utilizing the same constants and identical reaction mechanisms without additional assumption.