title: Bound Layers "Cloak" Nanoparticles in Strongly Interacting Polymer Nanocomposites.
abstract: Polymer-nanoparticle (NP) interfacial interactions are expected to strongly influence the properties of nanocomposites, but surprisingly, experiments often report small or no changes in the glass transition temperature, Tg. To understand this paradoxical situation, we simulate nanocomposites over a broad range of polymer-NP interaction strengths, ε. When ε is stronger than the polymer-polymer interaction, a distinct relaxation that is slower than the main α-relaxation emerges, arising from an adsorbed "bound" polymer layer near the NP surface. This bound layer "cloaks" the NPs, so that the dynamics of the matrix polymer are largely unaffected. Consequently, Tgdefined from the temperature dependence of the routinely measured thermodynamics or the polymer matrix relaxation is nearly independent of ε, in accord with many experiments. Apparently, quasi-thermodynamic measurements do not reliably reflect dynamical changes in the bound layer, which alter the overall composite dynamics. These findings clarify the relation between quasi-thermodynamic Tgmeasurements and nanocomposite dynamics, and should also apply to thin polymer films.
authors.affiliation: Department of Physics, Wesleyan University , Middletown, Connecticut 06459, United States.|Department of Molecular Biology & Biochemistry, Wesleyan University , Middletown, Connecticut 06459, United States.
authors.forename: Francis W
authors.identifier: 0000-0002-2895-6595
authors.initials: FW
authors.lastname: Starr
authors.affiliation: Materials Science and Engineering Division, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
authors.forename: Jack F
authors.initials: JF
authors.lastname: Douglas
authors.affiliation: Department of Chemical Engineering, Columbia University , New York, New York 10027, United States.
authors.forename: Dong
authors.initials: D
authors.lastname: Meng
authors.affiliation: Department of Chemical Engineering, Columbia University , New York, New York 10027, United States.
authors.forename: Sanat K
authors.initials: SK
authors.lastname: Kumar
keywords: bound polymer; glass transition; interfacial dynamics; polymer nanocomposite