Opposite Effects of SiO(2) Nanoparticles on the Local α and Larger-Scale α’ Segmental Relaxation Dynamics of PMMA Nanocomposites

The segmental relaxation dynamics of poly(methyl methacrylate)/silica (PMMA/SiO(2)) nanocomposites with different compositions ([Formula: see text]) near and above the glass transition temperature were investigated by mechanical spectroscopy. At [Formula: see text] ≤ 0.5%, the α peak temperature hardly changes with [Formula: see text] , but that of α’ relaxation composed of Rouse and sub-Rouse modes decreases by 15 °C due to the increase of free volume. At [Formula: see text] ≥ 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 [Formula: see text] , the peak height for α relaxation increases at [Formula: see text] ≤ 0.5% and then decreases at [Formula: see text] ≥ 0.7%, but that for α’ relaxation shows an opposite behavior. This is because at low [Formula: see text] , 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 [Formula: see text] , 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.