X-ray photon correlation spectroscopy (XPCS) is a powerful tool for investigating the dynamics of polymer nanocomposites. In this study, XPCS was utilized to analyze the dynamics of a matrix-free nanocomposite consisting of poly(methyl methacrylate)-b-poly(butyl acrylate)-g-silica nanoparticles (PMMA-b-PBA-g-SiNPs) during uniaxial elongation and stress relaxation. The effect of the microphase-separated state of PMMA-b-PBA on the dynamics of the SiNP core in the PMMA-b-PBA-g-SiNPs was confirmed at the initial state with temperature-dependence. The SiNPs exhibited a hyperdiffusive and ballistic-like motion in the temperature range of 298–423 K, including at temperatures above and below the glass transition temperature (Tg) of the outer PMMA layer. The SiNPs velocity increased rapidly when the temperature approached the Tg of PMMA. During uniaxial elongation, the PMMA-b-PBA-g-SiNPs exhibited anisotropic dynamics between two directions, namely parallel and perpendicular to elongation. The SiNPs velocity in the direction perpendicular to elongation was faster than those parallel to elongation, despite the small strain, high weight ratio of SiNP, and the large size of SiNP in the PMMA-b-PBA-g-SiNPs. A decrease in the relaxation rate of the SiNPs was also observed by XPCS during stress relaxation at strain = 0.2, which is in the plastic deformation region.
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