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.