Physical and Life Sciences Directorate , Lawrence Livermore National Laboratory , Livermore , California 94550 , United States.
Department of Chemical Engineering , University of California, Davis , Davis , California 95616 , United States.
J Phys Chem B. 2018 Dec 20;122(50):12201-12210. doi: 10.1021/acs.jpcb.8b09636. Epub 2018 Dec 7.
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 likelihood 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.
我们使用基于量子的分子动力学模拟集合来预测聚二甲基硅氧烷和聚二苯基硅氧烷模型共聚物中辐射诱导的苯基基团的化学变化。我们的模拟涵盖了广泛的高度多孔和凝聚相密度范围，包括湿和干条件。我们观察到，在没有水的情况下，激发的苯基基团往往会从其他甲基或苯基侧基中提取氢以产生苯，其中欠氢化的基团引发随后的链内环化反应。这些系统还产生了由单个聚二苯基硅氧烷亚基中两个苯基基团完全提取形成的二苯基部分的少量产物。相比之下，我们发现水的存在促进了游离苯和硅醇侧基的形成，降低了链内环化反应的可能性，并完全抑制了二苯基物质的形成。此外，我们预测水在链断裂反应中起着关键作用，这表明环境湿度和辐射之间可能存在协同作用，这可能会促进更大聚合物网络的改变。这些结果可能会对解释加速老化实验产生影响，在这些实验中，聚合物分解反应和网络重排被认为对随后的机械性能有重大影响。
