Visualizing the Toughening Mechanism of Nanofiller with 3D X‑ray Nano-CT: Stress-Induced Phase Separation of Silica Nanofiller and Silicone Polymer Double Networks

Adding silica nanofiller in silicone rubber can toughen the matrix 3 orders in terms of fracture energy, which is far larger than most other nanofiller–rubber systems. To unveil the astonishing toughening mechanism, we employ in situ synchrotron radiation X-ray nanocomputed tomography (Nano-CT) technique with high spatial resolution (64 nm) to study the structural evolution of silica nanofiller in silicone rubber matrix at different strains. The imaging results show that silica nanofiller forms three-dimensional connected network, which couples with silicone chain network to construct a double-network structure. Stress-induced phase separation between silica nanofiller and silicone polymer chain networks is observed during tensile deformation. Unexpectedly, though the spatial position and morphology of nanofiller network changes greatly at large strains, the connectivity of nanofiller network shows negligible reduction. This indicates that nanofiller network undergoes destruction and reconstruction simultaneously, during which silica nanofiller serves as reversible high functionality cross-linker. The reversible bonding between silica nanofiller and silicone rubber or between nanofiller particles can dissipate mechanical energy effectively, which may account for the 3 orders enhancement of toughness.在硅橡胶中添加二氧化硅纳米填料，可使基体的断裂能提升三个数量级，这一增幅远超绝大多数其他纳米填料-橡胶体系。为揭示这一惊人的增韧机制，我们采用具备高空间分辨率（64 nm）的原位同步辐射X射线纳米计算机断层扫描（synchrotron radiation X-ray nanocomputed tomography, Nano-CT）技术，探究不同应变条件下硅橡胶基体中二氧化硅纳米填料的结构演化过程。成像结果显示，二氧化硅纳米填料可形成三维连通网络，并与硅链网络耦合构建双网络结构。拉伸变形过程中，可观测到二氧化硅纳米填料与硅聚合物链网络之间发生应力诱导相分离。出乎意料的是，尽管大应变下纳米填料网络的空间位置与形貌发生显著变化，但其连通性几乎无衰减。这表明纳米填料网络同时经历破坏与重构过程，在此过程中二氧化硅纳米填料充当可逆的高功能交联剂。二氧化硅纳米填料与硅橡胶之间，抑或纳米填料颗粒之间的可逆键合可有效耗散机械能，这或许正是韧性提升三个数量级的核心缘由。