title: Fragility and cooperative motion in a glass-forming polymer-nanoparticle composite.
abstract: Polymer-nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperatureTgdue to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperatureT- termed the fragilitymof glass-formation - is comparatively poorly understood. In the present work, we calculate bothTgandmof a model polymer nanocomposite by molecular dynamics simulations. We systematically consider howTgandmvary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and two polymer-NP interactions. We find large positive and negative changes inTgandmthat can be interpreted in terms of the Adam-Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. This provides a molecular perpective of fragility changes due to the addition of NPs and for glass formation more generally. We also contrast the behavior along isobaric and isochoric approaches toTg, since these differing paths can be important to compare experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and fundamental significance for understanding the properties glass-forming materials more broadly.
authors.affiliation: Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA.
authors.forename: Beatriz A Pazmiño
authors.initials: BA
authors.lastname: Betancourt
authors.affiliation: Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
authors.forename: Jack F
authors.initials: JF
authors.lastname: Douglas
authors.affiliation: Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA.
authors.forename: Francis W
authors.initials: FW
authors.lastname: Starr