Accuracy of the isoconversional principle in thermal analysis of complex condensed phase reactions

This study explores the isoconversional principle by showing, through both inference and calculation on simulated reactions, that the conversion function of complex reactions may depend not only on the degree of conversion but also on temperature. We demonstrate from mathematical perspective that the isoconversional principle is exact only when the isoconversional temperature interval (ΔTα) approaches zero, a condition not attainable in practice. Instead, the kinetic triplet—activation energy, pre-exponential factor, and conversion function—derived by isoconversional methods are only approximations of their corresponding values at the mean isoconversional temperatures. Using both simulated reactions and the thermal decomposition of the polymer coating from a commercial optical fiber, we show that kinetic triplets are influenced by conversion degree, reaction temperature, and the applied temperature program (thermal history). This study additionally introduces a quantitative criterion for assessing the adequacy of kinetic datasets for use in isoconversional analysis. The proposed criterion is applied to examine the kinetic datasets corresponding to the thermal decomposition of poly(methyl methacrylate) as well as the glass and melt crystallization kinetics of poly(butylene 2,5-furandicarboxylate).