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The segmental relaxation dynamics of poly(methyl methacrylate)/silica (PMMA/SiO 2 ) nanocomposites with different compositions ( ϕ SiO 2 ) near and above the glass transition temperature were investigated by mechanical spectroscopy. At ϕ SiO 2 ≤ 0.5%, the α peak temperature hardly changes with ϕ SiO 2 , but that of α ' relaxation composed of Rouse and sub-Rouse modes decreases by 15 °C due to the increase of free volume. At ϕ SiO 2 ≥ 0.7%, both α and α ' relaxations shift to high temperatures because of the steric hindrance introduced by nanoparticle agglomeration. On the other hand, with increasing ϕ SiO 2 , the peak height for α relaxation increases at ϕ SiO 2 ≤ 0.5% and then decreases at ϕ SiO 2 ≥ 0.7%, but that for α ' relaxation shows an opposite behavior. This is because at low ϕ SiO 2 , the short-chain segments related to α relaxation can easily bypass the particles, but the longer-chain segments related to α ' relaxation cannot. At high ϕ SiO 2 , the polymer chains were bound to the nanoparticles due to the physical adsorption effect, leading to the decrease of relaxation unit concentration involved in α relaxation. However, the dissociation of those bonds with heating and the concentration heterogeneity of polymer chains result in the increase of peak height for α ' relaxation.

Opposite Effects of SiO2 Nanoparticles on the Local α and Larger-Scale α’ Segmental Relaxation Dynamics of PMMA Nanocomposites