Large-Scale Morphology of Dispersed Layered Silicates
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Large-Scale Morphology of Dispersed Layered Silicates Dale W. Schaefer,1,4 Ryan S. Justice,1 Hilmar Koerner,2 Richard Vaia,3 Chungui Zhao,4 Mingshu Yang,4 and Jim Vale5 1 Chemical and Materials Engineering, University of Cincinnati, Cincinnati , OH, 45221-0012 USA. 2 University of Dayton Research Institute, Dayton, Ohio, USA 3 Air Force Research Laboratory, WPAFB, OH, USA. 4 Institute of Chemistry, Chinese Academy of Sciences, Beijing, China. 5 Givaudan Flavors Corporation, Cincinnati, OH 45216, USA
ABSTRACT Ultra small angle x-ray scattering is used to probe the morphology of highly dispersed montmorillonite (MMT) in water and polyamide-66. In water the scattered intensity, I(q) shows a q-2 dependence for q > 0.01 Å-1, where q is the magnitude of the scattering vector. This is as expected for a two dimensional sheet-like object. On larger scales (smaller q) mass-fractal character is evident up to the radius-of-gyration of the individual scattering entities. The scattering profile is interpreted using a semi-flexible sheet model in which flat, disk-like entities of radius = 80 Å (an areal persistence length) are fractally distributed on large scales with a mass fractal dimension of 2.65. These size scales correspond to a scattering entity comprised of one or a few crumpled sheets. No evidence of inter-particle correlations is found at concentrations below the gel point. In polyamide-66 loaded with organically modified MMT long-range fractal behavior is also observed but with larger fractal dimension. INTRODUCTION The extent of dispersion of layered silicates (clays) in polymer nanocomposites is reflected in both small-angle and ultra small-angle x-ray scattering (SAXS and USAXS). In the SAXS regime diffraction peaks reveal the layer spacing and layer perfection. As multilayer stacks (tactoids) exfoliate and disperse, the layer peaks shift to smaller q and broaden. [1] USAXS probes larger length scales and reveals the size and perfection of the individual sheet-like platelets as well as the morphology of tactoids. In addition, since these systems percolate at very low volume fractions, interparticle correlations are expected to influence the USAXS profile by supressing the scatteirng at small q. Here we examine the dispersion of montmorillonite (MMT) in water and polyamide-66 by USAXS. Although the data extend into the SAXS regime, the focus of the work is on the large-scale morphology. The goal is to characterize the morphology of fully dispersed systems where there is no layer diffraction and to ascertain the effect of inter-particle correlations at concentrations approaching the percolation threshold. Several USAXS and USANS studies of MMT dispersions exist, most of which address the issue of phase separation and gelation. [2-8] These studies show evidence of power-law behavior in the scattering profile at small q, but there is little agreement on the power-law exponent (-P) or the origin of the apparent fractal correlations. Exponents ranging from 1.8 < P < 3.5 are seen in systems similar to those we stu
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