Physical and Life Sciences Directorate , Lawrence Livermore National Laboratory , Livermore , California 94550 , United States.
Department of Chemical Engineering , University of California , Davis , California 95616 , United States.
J Phys Chem B. 2019 Sep 19;123(37):7926-7935. doi: 10.1021/acs.jpcb.9b07159. Epub 2019 Sep 9.
Chemical reactions involving the polydimethylsiloxane (PDMS) backbone can induce significant network rearrangements and ultimately degrade macro-scale mechanical properties of silicone components. Using two levels of quantum chemical theory, we identify a possible electronic driver for chemical susceptibility in strained PDMS chains and explore the complicated interplay between hydrolytic chain scissioning reactions, mechanical deformations of the backbone, water attack vector, and chain mobility. Density functional theory (DFT) calculations reveal that susceptibility to hydrolysis varies significantly with the vector for water attacks on silicon backbone atoms, which matches strain-induced anisotropic changes in the backbone electronic structure. Efficient semiempirical density functional tight binding (DFTB) calculations are shown to reproduce DFT predictions for select reaction pathways and facilitate more exhaustive explorations of configuration space. We show that concerted strains of the backbone must occur over at least a few monomer units to significantly increase hydrolysis susceptibility. In addition, we observe that sustaining tension across multiple monomer lengths by constraining molecular degrees of freedom further enhances hydrolysis susceptibility, leading to barrierless scission reactions for less substantial backbone deformations than otherwise. We then compute chain scission probabilities as functions of the backbone degrees of freedom, revealing complicated configurational interdependencies that impact the likelihood for hydrolytic degradation. The trends identified in our study suggest simple physical descriptions for the synergistic coupling between local mechanical deformation and environmental moisture in hydrolytic degradation of silicones.
涉及聚二甲基硅氧烷 (PDMS) 主链的化学反应会引起显著的网络重排，最终导致硅酮部件的宏观机械性能下降。我们使用两级量子化学理论，确定了应变 PDMS 链中化学敏感性的可能电子驱动力，并探索了水解链断裂反应、主链机械变形、水攻击矢量和链迁移率之间的复杂相互作用。密度泛函理论 (DFT) 计算表明，水解敏感性随水对硅主链原子攻击矢量的变化而显著变化，这与主链电子结构的应变诱导各向异性变化相匹配。高效半经验密度泛函紧束缚 (DFTB) 计算表明，它可以再现 DFT 对选定反应途径的预测，并促进更全面地探索构象空间。我们表明，必须至少在几个单体单元上发生协同应变，才能显著增加水解敏感性。此外，我们观察到通过约束分子自由度在多个单体长度上维持张力进一步提高了水解敏感性，导致对于较小的主链变形，没有势垒的断裂反应。然后，我们将链断裂概率作为主链自由度的函数进行计算，揭示了影响水解降解可能性的复杂构象相互依存关系。我们研究中确定的趋势表明，在硅酮的水解降解中，局部机械变形和环境湿度之间的协同耦合可以用简单的物理描述来表示。
