title: Revealing spatially heterogeneous relaxation in a model nanocomposite.
abstract: 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 α-relaxation time near the nanoparticle grows with cooling faster than the α-relaxation time in the bulk and is ∼20 times longer at low temperatures. The interfacial layer thickness increases from ∼1.8 nm at higher temperatures to ∼3.5 nm upon cooling to near bulk Tg. 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.
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Shiwang
authors.initials: S
authors.lastname: Cheng
authors.affiliation: Department of Materials Science and Chemistry, Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Stephen
authors.initials: S
authors.lastname: Mirigian
authors.affiliation: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Jan-Michael Y
authors.initials: JM
authors.lastname: Carrillo
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Vera
authors.initials: V
authors.lastname: Bocharova
authors.affiliation: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Bobby G
authors.initials: BG
authors.lastname: Sumpter
authors.affiliation: Department of Materials Science and Chemistry, Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Schweizer
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Alexei P
authors.initials: AP
authors.lastname: Sokolov