Simulation Framework for the Chemical Degradation in Polymeric Solids

The kinetics of chemical aging in most commercial plastics have remained long debated, since the nature of dense polymeric solids inhibits the in situ experimental investigation of complex degradation paths, while traditional computational techniques fail to reach the time scales associated with slow-progressing degradative reactions. In this work, a novel mechanistic framework is introduced in which the infrequent reaction events that govern the long-time-scale evolution of the chemistry of any amorphous solid are described as successive elementary transitions of the atomistic configuration between local minima on its energy landscape. For each elementary reaction event, the corresponding transition state is identified, allowing the estimation of the free-energy barrier and, thereby, of the transition rate constant by means of transition state theory. The result is a network of states populated by the stationary states that are visited by the system along chemical paths. We demonstrate the applicability of the presented approach for the study of complex reaction schemes by applying it to the study of the autoxidation of glassy polystyrene. The introduction of an appropriately trained reactive force field, ReaxFF-lg/CHOpox, tailored for the accurate description of the reactions propagating polymer oxidation, i.e., peroxy radical and hydroperoxide formation, in the glassy state, allows the large-scale sampling of potential reaction paths in situ. From the created network of states, we extracted the energetics and rates of the elementary reactions in the glassy state. For both reactions, the broad distribution of free-energy barriers, spanning over many orders of magnitude, is indicative of the significant impact that the local dense environment has on reaction kinetics and highlights the importance of studying solid-state reactions in situ.