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siloxane_dephenylation_kroonblawd_goldman_lewicki.pdf (7.19 MB)

Chemical Degradation Pathways in Siloxane Polymers Following Phenyl Excitations

preprint
submitted on 01.10.2018, 22:52 and posted on 02.10.2018, 15:25 by Matthew Kroonblawd, Nir Goldman, James Lewicki
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.

History

Email Address of Submitting Author

kroonblawd1@llnl.gov

Institution

Lawrence Livermore National Laboratory

Country

United States

ORCID For Submitting Author

0000-0002-5009-5998

Declaration of Conflict of Interest

No conflict of interest

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in The Journal of Physical Chemistry B

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