(376bi) Quantum-Based Predictions for Network-Altering Chemical Reactions in Siloxane Polymer Systems

Chemical reactions involving the siloxane backbone can induce significant network rearrangements and ultimately degrade macro-scale mechanical properties of silicone components. High throughput atomistic modeling tools, such as semi-empirical quantum molecular dynamics, can yield detailed chemical understanding of these processes that is difficult to elucidate from experiments alone. Using large ensembles of simulations, we investigate the interconnections between mechanical loading, irradiation, and environmental moisture on chemical reactions in systems containing polydimethylsiloxane (PDMS) and polydiphenylsiloxane (PDPS). An approach to probe the chemistry underlying network rearrangements is proposed that couples graph-based structure recognition tools with efficient quantum simulations. Radiation-induced excitations of siloxane side groups are found to induce subsequent reactions resulting in intrachain cyclization and chain scissioning. A possible synergistic effect between water and radiation is identified that could promote alterations of a larger polymer network. Physical origins for increased chemical susceptibility in strained PDMS chains are proposed and explored in the context of hydrolysis reactions. Chain scission probabilities for PDMS under attack by water are extracted as functions of the backbone degrees of freedom, revealing complicated configurational inter-dependencies that increases the likelihood for hydrolytic degradation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Approved for unlimited release, LLNL-ABS-771404.