Polymer degradation kinetics are usually studied by means of kinetic analysis methods originally devised for solid-state processes. These approaches include isoconversional methods, which yield the activation energy values as a function of the reacted fraction and model-fitting methods, that attempt to fit the experimental data to a set of predefined mathematical functions proposed assuming a given reaction mechanism [1]. Determining in a precise manner the kinetic parameters require not only reliable experimental data but also awareness of the strength and limitations of the methods employed. For instance, a widespread error is the use of empirical kinetic models without physical meaning, such as first order or nth order kinetic functions to fit the experimental data. Those methods are often used as only options due to their simplicity and the good results it often provides in terms of fit quality [2]. However, despite the ability of such functions to closely accommodate most experimental data arrays, the goodness of fit alone does not guarantee of correctness of the model. This is important for the inappropriate assumption of the kinetic model strongly compromises any predictions or lifetime estimations. For instance, many recent works prove that autoaccelerative models can better describe the degradation of many polymers such as cellulose, polystyrene and others [3, 4]. This type of functions, in which the reaction rate first increases up to a certain value of conversion before starting the usual decay, are especially appropriate to model the progressive breakage of the polymeric chains and the ulterior volatilization of the produced fragments. While conventional model-fitting methods of kinetic analysis would yield relatively similar kinetic parameters for both nth order and autoaccelerative models, the implications of an incorrect assignation are significant in terms for predictive capability [5]. In this talk, we would provide an overview of the different methods of kinetic analysis available for polymer degradation studies and we will highlight the errors committed in any half-life predictions when the kinetic parameters are determined by fitting the experimental data to an erroneous kinetic model, with emphasis in chain scission driven polymer degradation reactions. 

References 

1. Vyazovkin, S., et al., ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data. Thermochimica Acta, 2011. 520(1): p. 1-19. 

2. Sanchez-Jimenez, P.E., et al., Limitations of model-fitting methods for kinetic analysis: Polystyrene thermal degradation. Resources Conservation and Recycling, 2013. 74: p. 75-81. 

3. Sanchez-Jimenez, P.E., et al., An improved model for the kinetic description of the thermal degradation of cellulose. Cellulose, 2011. 18(6): p. 1487-1498. 

4. Sánchez-Jiménez, P.E., et al., Nanoclay nucleation effect in the thermal stabilization of a polymer nanocomposite: A kinetic mechanism change. Journal of Physical Chemistry C, 2012. 116(21): p. 11797-11807. 

5. Sanchez-Jimenez, P.E., et al., A new model for the kinetic analysis of thermal degradation of polymers driven by random scission. Polymer Degradation and Stability, 2010. 95(5): p. 733- 739.