Structure and Strength of Silica-PDMS Nanocomposites
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Structure and Strength of Silica-PDMS Nanocomposites Adrian Camenzind1, Thomas Schweizer2, Michael Sztucki3 and Sotiris E. Pratsinis1 1
Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland 2 Institute of Polymers, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH8093 Zurich, Switzerland 3 European Synchrotron Radiation Facility (ESRF), BP 220, F-38043 Grenoble Cedex, France ABSTRACT Commercially available SiO2 nanoparticles (Aerosil, Degussa) with varying primary particle diameter, specific surface area (SSA), degree of aggregation and structure (fractal dimension) were compounded into PDMS-based nanocomposites. Thin sections of cured nanocomposites were analyzed with TEM and small and ultra-small angle X-ray scattering (U/SAXS) with respect to nanocomposite structure such as: filler primary particle, aggregate (chemically or sinter-bonded particles) and agglomerate (physically-bonded particles). Tensile tests (Young’s modulus) were used to determine the nanocomposite strength which increased with increasing filler volume fraction (up to 12 vol%) consistent with “bound rubber” theory. INTRODUCTION The processing of flame-made oxide filler nanoparticles (e.g. SiO2, Al2O3 or TiO2) into polymers was lately summarized [1] focusing on particle surface modification and compounding and the resulting mechanical and optical properties of nanocomposites as a function of particle characteristics. The current understanding of nanoparticle formation in flames and the systematic control of particle size and morphology (aggregate or agglomerate formation) is highlighted in Fig. 1: The evolution of primary particle, dp (solid line), and collision, dc (dashed-doted line), diameters in a flame reactor as a function of residence time is shown. The characteristic temperature profile (T) in such reactors can be described with a steep increase (by fuel/precursor combustion) and fast cooling (convection by gas entrainment and radiation). Full coalescence of particles takes place at high flame temperatures. Particle sintering (coalescence) slows down as the flame temperature drops downstream of the combustion zone resulting in aggregated particles (partially coalesced) as indicated by the splitting of particle diameter into dp and dc in Fig. 1. Further downstream at even lower temperatures, particle sintering stops and the primary particle size, dp, reaches an asymptotic value so the aggregate diameter (dcH) no longer increases also. Subsequent coagulation of aggregated particles, however, results in larger agglomerate particles (and dc) that can be broken (e.g. post-processing [2]) into its building blocks, aggregate and/or primary particles. Nitrogen adsorption, small angle X-ray scattering (SAXS) or microscopic counting allows to determine the primary particle size, dp. Experimental determination of aggregate size dimensions, dcH, however, is much harder by microscopic counting [3] and light scatteri
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