Decoupled Polymer Dynamics in Weakly Attractive Poly(methyl methacrylate)/Silica Nanocomposites

In this work, polymer dynamics of the weakly attractive poly(methyl methacrylate) (PMMA)/silica nanocomposites with different molecular weights of the matrix, interaction strength, nanoparticle sizes, and particle fractions were investigated. We demonstrated by temperature-dependent Fourier transform infrared spectroscopy that the conformation of adsorbed chains in the interfacial layer would undergo a transition process on increasing the temperature above T-g, which we speculated might be ascribed to the partial desorption of adsorbed segments. This conformation transition led to an increment in the fractional free volume, which may account for the variation of the glass-transition temperature T-g and the Vogel temperature T-infinity upon changing the matrix molecular weights and surface properties of particles. Using the isomonomeric friction adjustment, we found that the increased monomeric friction mainly originated from the conformation loss due to adsorption and spatial constraint. The entanglement relaxation time under the isofriction condition still followed the prediction of the tube model and was well related to the enhanced entanglement due to the topological constraint on the trapped chains from the loops of adsorbed chains. However, the isofriction terminal relaxation time was dependent on the interaction strength. In weakly interacting systems, the isofriction terminal relaxation remained the same as that for bulk PMMA, which might be induced by the simultaneous desorption of adequate segments, resulting in the release of topological constraints from the adsorbed chains. In contrast, strong interaction strength might make the desorption too weak to release the topological entanglement constraint, and the enhanced entanglement could lead to an increment in the isofriction terminal relaxation. We conjectured that the decoupling between the relaxation of the entangled strands and polymer chains might result from the simultaneous desorption of multiple adsorbed segments.