Predicted pathways for chemical degradation in siloxane polymers

Chemical degradation can result in undesirable changes in the performance of functional materials over the service lifetime, but the underlying atomic-scale processes are often subtle to probe with experiments. We use ensembles of quantum molecular dynamics (QMD) simulations to predict the chemical reactions that follow radiation-induced excitations of phenyl groups in a model copolymer of polydimethylsiloxane and polydiphenylsiloxane. Initial benzene-forming proton transfer reactions are predicted to induce subsequent reactions including intrachain cyclization and chain scission reactions. Water is found to play a crucial role in chain scission reactions, which indicates a possible synergistic effect between environmental humidity and radiation that could promote alterations of a larger polymer network. An approach for obtaining probability distributions of reaction intermediates and products by coupling high throughput semiempirical QMD simulations with automated graph-based structure recognition tools is discussed.