Stretch-induced crystallization (SIC) and phase transitions of poly­(dimethylsiloxane) (PDMS) have been studied with the in situ synchrotron radiation wide-angle X-ray scattering technique (WAXS) during tensile deformation at temperatures ranging from −45 to −65 °C. The phase transitions during tensile deformation go through different processes at different temperature regions, where four phases are involved in namely oriented amorphous (OA), mesophase, α form, and β form crystals. We found that SIC of the α form can proceed via two different multistage ordering processes with either the mesophase or β form as the structural intermediate. Further cyclic tensile experiments demonstrate that the transition from the β to α form is a reversible process controlled by stress, which is attributed to the different helical pitches in β and α forms. A nonequilibrium phase diagram of SIC and phase transitions are constructed in strain–temperature space, which is of great significance for practical applications of PDMS at low temperature.

Stretch-induced crystallization (SIC) and structural transitions of poly(dimethylsiloxane) (PDMS) filled with different contents of nanosilica were studied with an in situ synchrotron radiation wide-angle X-ray scattering technique during uniaxial tensile deformation at temperatures from −40 to −65 °C. With low filler contents (10 and 25 phr), two different multistage kinetic pathways, namely, (i) amorphous−mesophase-α′−α forms and (ii) amorphous−mesophase-β′−β−α forms, are first observed during SIC of PDMS at −45 and −50 °C, respectively. However, with higher filler contents (40 and 55 phr), only α and β forms are observed during SIC of PDMS. The nonequilibrium SIC structural evolution diagrams of PDMS with different filler contents are constructed in strain−temperature space. Further cyclic tensile experiments demonstrate that the transitions of α′−α and β′−β are reversible and controlled by stress. Under preset strains, cooling results in the emergence of β, β′, α′, and α forms in sequence with the increase of the preset strains, which precisely follows the structural evolution diagram of SIC for PDMS with the low filler content. β′ and α′ are the independent crystal structures which occur before the SIC of β and α forms, respectively, rather than intermediate transient states for β−α transition. These results demonstrate the complexity of SIC with different structural intermediates in PDMS which is influenced not only by temperature and strain but also by the filler content and the kinetic process.

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.