Guest Lecture: Kostas Steiakakis – Walking on the Potential Energy Landscape: Mapping Degradation Pathways in Polymeric Solids

The kinetics of chemical ageing in most commercial plastics remains 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 timescales associated with slow-progressing degradative reactions. We introduce a new mechanistic framework in which the infrequent reaction events that govern the long-time-scale evolution of the chemistry of any macromolecular solid are described as successive elementary transitions of its 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 the chemical paths, thus describing its chemical space. 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 forcefield, tailored for the accurate description of the reactions propagating polymers 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, the energetics and rates of the elementary reactions in the glassy state can be extracted, motivating a discussion over the impact of stereochemical properties of the matrix on the oxidation thermochemistry and kinetics.