X-ray scattering studies of flow-induced alignment in<br/><br/>model polymer nanocomposites

Wednesday, September 16, 2009 - 9:50am - 10:35am
EE/CS 3-180
Wesley Burghardt (Northwestern University)
The property enhancements associated with dispersion of nanoparticles
in polymers will depend not only on the state of dispersion achieved
during the synthesis or formulation of the nanocomposite, but also on
the degree and direction of particle alignment induced during
subsequent processing. Here we present data on flow-induced
orientation in two classes of nanoparticle dispersions, based on
multi-walled carbon nanotubes (MWNTs) and organically modified clay.
Particles are dispersed in viscous but Newtonian matrices (uncured
epoxy resin and oligomeric polybutene, respectively) to allow focus
on the fundamentals of flow-induced particle orientation free from
complications associated with polymer melt viscoelasticity. Small-
and wide-angle x-ray scattering under shear are used to probe
flow-induced anisotropy in the particle orientation distribution.
Both samples show particle alignment increasing with shear rate. In
dilute MWNT dispersions, flow-induced alignment is correlated with
break-down of large aggregates. In more concentrated dispersion,
unexpected rapid relaxation of flow-induced alignment suggests that
some of the observed alignment stems from elastic distortion of
entangled nanotubes within clusters. In the clay dispersions,
attempts are made to explore the relationship between particle
orientation and bulk rheological behavior. In both systems, we have
explored the relation between anisotropy measures extracted from
small- and wide-angle x-ray scattering, which probe fluid structure
at different length scales.
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