Chemical Degradation Pathways in Siloxane Polymers Following Phenyl Excitations

Abstract

We use ensembles of quantum-based molecular dynamics simulations to predict the chemical reactions that follow radiation-induced excitations of phenyl groups in a model copolymer of polydimethylsiloxane and polydiphenylsiloxane. Our simulations span a wide range of highly porous and condensed phase densities, and include both wet and dry conditions. We observe that in the absence of water, excited phenyl groups tend to abstract hydrogen from other methyl or phenyl side groups to produce benzene, with the under-hydrogenated group initiating subsequent intrachain cyclization reactions. These systems also yield minor products of diphenyl moieties formed by the complete abstraction of both phenyl groups from a single polydiphenylsiloxane subunit. In contrast, we find that the presence of water promotes the formation of free benzene and silanol side groups, reduces the likelyhood for intrachain cyclization reactions, and completely suppresses the formation of diphenyl species. In addition, we predict that water plays a critical role in chain scission reactions, which indicates a possible synergistic effect between environmental moisture and radiation that could promote alterations of a larger polymer network. These results could have impact in interpreting accelerated aging experiments, where polymer decomposition reactions and network rearrangements are thought to have a significant effect on the ensuing mechanical properties. 1 Introduction Polysiloxane-based materials, or silicones, are widely used in a range of technological applications where favorable long term mechanical response, chemical inertness, and shape filling factor over a broad temperature range are required. Applications include space-filling rubber or foam gaskets and shock absorbers to biomedical implants, lubricants, and adhesives. 1,2 Many applications of silicone rubber and foam components have specific requirements for mechanical properties such as elastic moduli (bulk, Young's, and shear), viscoelastic response, and hardness. The mechanical properties of silicones are often tuned by controlling

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

“…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.…”

Mechanochemical synthesis of glycine oligomers in a virtual rotational diamond anvil cell

“…53 Atomistic modeling approaches such as semiempirical quantum-based molecular dynamics (QMD) using density functional tight binding [54][55][56] (DFTB) can accurately predict chemistry under extreme conditions. 24,36,44,[57][58][59][60] Using QMD offers a powerful route to perform computer "experiments" that can help selectively target and interpret more expensive and labor-intensive laboratory experiments. The DFTB method is derived from density functional theory 61,62 (DFT) and offers a competitive balance between computational accuracy and expense that allows for the practical generation of nanoseconds worth of QMD trajectory.…”

Synthesis of functionalized nitrogen-containing polycyclic aromatic hydrocarbons and other prebiotic compounds in impacting glycine solutions

“…The computational efficiency of DFTB allows for initiating many independent simulations concurrently to generate trajectories that can approach chemical equilibrium timescales. [48][49][50][51] This relatively high throughput allows for running multiple independent MD simulations and gathering of ensemble statistics, which can inform the interpretation of noisy experimental results and validate coarse-grained models (e.g., equations of state) for large-scale simulations.…”