Extraordinary Flow Characteristics of Nanotube-Filled Polymer
Materials
Introduction
There
has been intense interest in composites of polymers and carbon
nanotubes (CNT) because of the large transport property (conductivity,
elasticity, viscosity, thermal conductivity) changes exhibited
by these additives for relative low CNT concentrations (=
1 % by vol.). Here we show that the unusual rheology leads to
a striking result: a complete suppression of die swell during
extrusion. Thus CNTs can be used as processing aids while at
the same time also improving the final properties.
Experimental Approach
The
percolation threshold for conductivity (yellow circles)
and stiffness occurs at ~1% volume fraction.
We explore the implications of a striking
effect observed by Lin-Gibson et al.; that nanotubes dispersed
in a polymeric fluid display “negative normal forces”
when they are sheared. We constructed a device to simultaneously
measure conductivity and rheology to study how the network
structure is affected by shear. We examine the processing
behavior by extruding the nanocomposites in a capillary
rheometer.
What kind of transport properties can be expected from
this peculiar state of matter, beyond an anticipated
increase in stiffness, electrical and thermal conductivity?
Results
In contrast to unfilled PP, above the
percolation threshold the normal stresses of CNT/PP nanocomposites
become negative. Apparently, both the large-scale
deformation of the whole nanotube network as well as the
nanotube deformation on a local scale contribute to this
unusual phenomena
The decrease of both transport properties
(viscosity and conductivity) with increasing rate of shear
reflects changes in the network structure, which becomes
less effective at transmitting shear stress and electrical
current. Both s and h are sensitive to subtle changes
in the local contact geometry of the nanotube network
and have a similar qualitative origin
Dispersion of CNT in polymer matrices
leads to the elimination of die-swell. This enhanced
control of the processing, creates the potential for extrusion
of small-scale structures, where die-swell otherwise distorts
the geometry of the product.
Further Implications
Our observations of strongly non-linear rheology under flow
(shear thinning and large negative normal stresses) imply that
these fluids should exhibit other ‘anomalous’ flow
characteristics (e.g., droplet distortion and thread break-up,
etc.) that are quite unlike Newtonian fluids and which are crucial
for their processing.
Publications
S. B. Kharchenko, J. F. Douglas, J. Obrzut, E. Grulke and
K. B. Migler
“Flow Properties of Nanotube-Filled Polymer Materials”
(Nature Materials, August 2004).
Contributors:
Semen B. Kharchenko, Jack F. Douglas, Jan Obrzut, and Kalman
B. Migler
Collaborator:
Erik Grulke (University of Kentucky)
Processing Characterization Group
Polymers Division
Materials Science and Engineering Laboratory
