Equivalence between polymer nanocomposites and thin polymer films: Effect of processing conditions and molecular origins of observed behavior |
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Authors: | S Sen Y Xie A Bansal H Yang K Cho L S Schadler S K Kumar |
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Institution: | (1) Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA;(2) Polymer Applications Laboratory, GE Global Research, Niskayuna, NY 12309, USA;(3) Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, NY 12180, USA;(4) Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk, 790-784, Korea;(5) Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA |
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Abstract: | Recently we established a quantitative equivalence in
thermomechanical properties between polystyrene-silica nanocomposites and
planar freestanding polystyrene thin films. This equivalence was quantified
by drawing a direct analogy between film thickness and an appropriate
experimental particle spacing. Using these findings, here we unequivocally
show that the glass transition process in confined geometries is controlled
by the mean volume fraction of polymer that is affected by the presence of
surfaces. Since separate signatures of the bulk and the surface layers are
never found, we can clearly rule out any simple “two layer” model which
postulates the existence of surfaces which are dynamically decoupled from
the bulk. Rather, we argue that the modification of properties at the
surfaces propagates into the bulk through a spatial gradient: macroscopic
experimental techniques average over these gradients and yield a broadened
signature relative to the bulk polymer. In a second aspect of this paper we
focus on the role of processing conditions on the results obtained. We have
developed a new method of processing the nanocomposites which results in a
better dispersion of the nanoparticles in the matrix. However, these samples
did not show the unique glass transition behavior seen in the first set of
nanocomposites discussed above. This indicates that processing conditions
can profoundly affect the nature of the particle-polymer interface which
controls the macroscopic behavior of these important systems. |
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