(433c) Quantum-Based Modeling of Radiation Damage in Siloxane Polymers

Exposing materials to ionizing radiation is a reliable means of sterilization and is a common approach taken in accelerated aging experiments. Initial atomistic-level radiation damage in chemically reactive materials such as silicones is thought to induce a sequence of network-altering events that lead to undesirable macroscale degradation, including permanent set and mechanical failure. We develop a multiscale approach based on semiempirical quantum molecular dynamics (QMD) to predict and analyze primary radiation damage in polydimethylsiloxane (PDMS) and gauge the macroscale consequences of that damage by connecting with a constitutive model. Large ensembles of QMD simulations are used to predict the initial reaction cascades that follow from primary knock-on atom radiation events. A graph-based analysis is developed to automatically identify changes to the backbone structure and quantify mechanically relevant network alterations including formation of cross-links and chain scissions. Distinguishing characteristics of radiation coupling to different parts of the PDMS backbone are explored. Results from QMD modeling and the graph-based analysis are used to inform a simplified constitutive model for the mechanical response of irradiated elastomers. 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-809007.