A Quantum-Based Approach to Predict Primary Radiation Damage in Polymeric Networks

Abstract

Initial atomistic-level radiation damage in chemically reactive materials is thought to induce reaction cascades that can result in undesirable degradation of macroscale properties. Ensembles of quantum-based molecular dynamics (QMD) simulations can accurately predict these cascades, but extracting chemical insights from the many underlying trajectories is a labor-intensive process that can require substantial a priori intuition. We develop here a general and automated graph-based approach to extract all chemically distinct structures sampled in QMD simulations and apply our approach to predict primary radiation damage of polydimethylsiloxane (PDMS), the main constituent of silicones. A postprocessing protocol is developed to identify underlying polymer backbone structures as connected components in QMD trajectories. These backbones form a repository of radiation-damaged structures. A scheme for extracting and updating a library of isomorphically distinct structures is proposed to identify the spanning set and aid chemical interpretation of the repository. The analyses are applied to ensembles of cascade QMD simulations in which the four element types in PDMS are selectively excited in primary knock-on atom events. Our approach reveals a much higher degree of combinatorial complexity in this system than was inferred through radiolysis experiments. Probabilities are extracted for radiation-induced network changes including formation of branch points, carbon linkages, cycles, bond scissions, and carbon uptake into the Si−O siloxane backbone network. The general analysis framework presented here is readily extendable to modeling chemical degradation of other polymers and molecular materials and provides a basis for future quantum-informed multiscale modeling of radiation damage.

Polymer Degradation through Chemical Change: A Quantum-based Test of Inferred Reactions in Irradiated Polydimethylsiloxane

“…Hundreds of QMD simulations are used to predict the dynamic reaction cascades that follow from initial ionization events, which are modeled with a primary knock-on atom (PKA) approach 36 using an established simulation protocol. 11 The simulations probe reaction cascades under ideal vacuum conditions and in the presence of environmental water, which is a common and potentially reactive contaminant 7,10,17,31,37,38 that is difficult to eliminate in experiments.…”

“…We used the DFTB parameter set pbc-0-3 (available at http://www.dftb.org), a typical off-the-shelf parameter set for silicon-containing systems that we previously applied to study siloxane chemistry. 10,11,17 Universal force field dispersion terms 45 were used for E Disp . The electronic structure was evaluated without spin polarization at the Γ-point only using Fermi-Dirac thermal smearing 46 and an SCC accuracy threshold of 2.7 × 10 −5 eV (10 −6 H).…”

“…Common vectors for degradation arise from the ready uptake of environmental water [4][5][6][7] or absorption of radiation. [7][8][9][10][11][12] Understanding the degradation responses of PDMS has enormous practical applications, ranging from assessing environmental ima Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States; E-mail: kroonblawd1@llnl.gov pacts [13][14][15] to establishing models for the aging and lifetime prediction of silicone components. [4][5][6][7][8][9][10][11][12]16,17 Ionizing radiation is a common cause for polymer degradation that can arise during component sterilization, 1,18,19 as a consequence of planned operation conditions, and as a driver in accelerated aging studies.…”

“…[7][8][9][10][11][12] Understanding the degradation responses of PDMS has enormous practical applications, ranging from assessing environmental ima Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, United States; E-mail: kroonblawd1@llnl.gov pacts [13][14][15] to establishing models for the aging and lifetime prediction of silicone components. [4][5][6][7][8][9][10][11][12]16,17 Ionizing radiation is a common cause for polymer degradation that can arise during component sterilization, 1,18,19 as a consequence of planned operation conditions, and as a driver in accelerated aging studies. [7][8][9][10][11][12] In 1955, Charlesby proposed 20 a set of reaction schemes for crosslinking in irradiated PDMS that were quickly refined by others 21,22 and were based on IR vibrational spectroscopy, titration, mass spectra of off-gassing species, and chromatography (see Figure 1).…”

“…[4][5][6][7][8][9][10][11][12]16,17 Ionizing radiation is a common cause for polymer degradation that can arise during component sterilization, 1,18,19 as a consequence of planned operation conditions, and as a driver in accelerated aging studies. [7][8][9][10][11][12] In 1955, Charlesby proposed 20 a set of reaction schemes for crosslinking in irradiated PDMS that were quickly refined by others 21,22 and were based on IR vibrational spectroscopy, titration, mass spectra of off-gassing species, and chromatography (see Figure 1). These reactions respectively connect the evolution of hydrogen (H 2 ), methane (CH 4 ), and ethane (C 2 H 6 ) with the formation of silethylene (Si-C-C-Si), silmethylene (Si-C-Si), and silicon-silicon (Si-Si) crosslinks.…”