Revealing spatially heterogeneous relaxation in a model nanocomposite

The detailed nature of spatially heterogeneous dynamics of glycerol-silica nanocomposites is unraveled by combining dielectric spectroscopy with atomistic simulation and statistical mechanical theory. Analysis of the spatial mobility gradient shows no glassy layer, but the alpha-relaxation time near the nanoparticle grows with cooling faster than the alpha-relaxation time in the bulk and is similar to 20 times longer at low temperatures. The interfacial layer thickness increases from similar to 1.8 nm at higher temperatures to similar to 3.5 nm upon cooling to near bulk T-g. A real space microscopic description of the mobility gradient is constructed by synergistically combining high temperature atomistic simulation with theory. Our analysis suggests that the interfacial slowing down arises mainly due to an increase of the local cage scale barrier for activated hopping induced by enhanced packing and densification near the nanoparticle surface. The theory is employed to predict how local surface densification can be manipulated to control layer dynamics and shear rigidity over a wide temperature range. (C) 2015 AIP Publishing LLC.