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submitted on 01.10.2018 and posted on 02.10.2018by 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.