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| 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 % volume
fraction). NIST's experience in the area of dielectric and rheological
measurement, in conjunction with expertise in modeling, puts it in
a unique position to lead the development of new processing concepts
required by industry to utilize this important new class of materials.
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| Kalman B. Migler and Jack
F. Douglas |
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The combination of extended shape, rigidity and deformability allows
carbon nanotubes (CNT) to be mechanically dispersed in polymer matrices
in the form of disordered network structures exhibiting a gel-like
rheology. Our measurements on representative network-forming multi-wall
carbon nanotube (MWNT) dispersions in polypropylene (PP) indicate
that these materials exhibit extraordinary flow-induced property changes.
Specifically, electrical conductivity  and
steady shear viscosity 
both decrease strongly with increasing shear rate
and these nanocomposites exhibit impressively large and negative normal
stress differences, a rarely reported phenomenon in soft condensed
matter. We illustrate the practical implications of these non-linear
transport properties by showing that MWNT eliminate die swell in our
nanocomposites, an effect crucial for their processing. |
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| The strong interest in CNT 'nanocomposites' stems from their ability
to affect thermal, electrical and rheological properties for relatively
small concentrations of this type of additive. These additives have
found manufacturing applications in electrostatic painting, protective
coatings for electronic components, and flammability reduction. Utilization
of CNT for more complex applications however, requires an understanding
of how processing conditions (mixing, molding, extrusion) influence
nanocomposite properties |
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| Despite the high elastic modulus of CNT, their small cross-sectional
dimensions and large aspect ratio allows them to bend substantially
in response to inter-tube interactions under processing conditions.
This bending leads to the formation of a disordered 'web-like' (see
Fig. 1) structure of substantial mechanical integrity. The presence
of a nanotube network interpenetrating the polymer matrix creates
additional contributions to nanocomposite viscoelasticity that can
have a radical effect on the processing characteristics of these materials.
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| Figure 1: Optical microscopy image of 1 % by volume MWNT/PP
nanocomposite (obtained using a 100x objective) demonstrates good
dispersion of the MWNT and reveals a polydispersity in nanotube length
and shape. The MWNT volume fraction in this figure equals Ø=
0.01, which is close to the geometrical percolation concentration
where the CNT network first forms, and where the conductivity and
stiffness of the nanocomposite increase by orders of magnitude (see
Fig. 2). |
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| In Fig. 2, we characterize the large changes in viscoelasticity
and conductivity for which polymer composites containing CNT are well
known. Simultaneous measurements of s and the shear moduli (G', G")
characterize the elastic and viscous properties of our composites.
G' can be thought of as a measure of 'stiffness' and G" provides
a measurement of viscous resistance to deformation. The ratio (G'/G")
or 'loss tangent' (tan Ø), is a measure of the composite 'firmness'
and we compare this basic quantity to s. We observe that both (G'/G")
and s increase with ø and that this variation becomes
rapid for MWNT volume fraction Ø in the range from 0.0025 to
0.01. We see that adding MWNT to the PP matrix increases the conductivity
by an impressive seven orders of magnitude as a percolating network
structure forms. G' and G" become frequency independent as f
is varied through the 'gelation concentration',Øc=0.01. |
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In order to manufacture MWNT nanocomposites into usable shapes,
we must understand how the network structure acts to influence their
processing behavior. The linear rheological and electrical transport
properties (Fig. 2) are strongly altered by flow, as Fig. 3 indicates.
Notably, both the conductivity and the viscosity(
) exhibit a strong thinning. The viscosity decreases over the full
range of shear explored here, whereas the conductivity shows a plateau
region at low shear. Moreover, a positive normal force  N
is observed in our nanocomposite for Ø=< Øc, where
the matrix dominates the rheological response (Fig.3), but N
becomes large and negative for Ø>=Øc, compensating
the large N exhibited
by the matrix polymer. (A negative N
in nanotube dispersions was reported by Lin-Gibson, et al.) This has
significant processing consequences. |
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| Figure 2: Characterization of conductivity and viscolelasticity
of MWNT/ PP nanocomposites (Ø = 0.025; T = 200 °C). Inset:
Shear modulus as a function of frequency for a range of nanotube concentrations. |
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| Figure 3: Normal stress measurements showing slightly
positive normal stress for pure PP and increasingly negative normal
stress as the MWNT fraction increases. Inset: Conductivity and viscosity
as a function of shear rate for (Ø = 0.025; T = 200 °C). |
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| Figure 4: Comparison of PP extrudate with (A) and without
(B) added nanotubes. The red dashed lines correspond to the die size. |
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| Since the extrusion of the nanocomposite is a basic processing operation
for which normal forces are known to be important, we extruded a nano-composite
sample (Ø = 0.025) and found that the cross-section actually
shrinks upon extrusion (Fig. 4). This striking effect is contrasted
with the extrusion of pure PP where a nearly 6-fold increase in cross-sectional
area is observed. Evidently, the CNT change the qualitative nature
of the polymer flow. |
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| The suppression of die swell of extruded polymers by adding a relatively
small amount of MWNT (Ø=0.01) offers a powerful tool for controlling
dimensional characteristics and surface distortion in manufacturing
composites. 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) that are quite unlike
Newtonian fluids. Understanding these flow characteristics is crucial
for their processing. |
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| For More Information on this Topic |
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| Semen Kharchenko, Jan Obrzut and Eric Hobbie |
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| S. Lin-Gibson, J.A. Pathak, E.A. Grulke, H. Wang, and E.K. Hobbie,
"Elastic Flow Instability in Nanotube Suspensions," Physical
Review Letters 92, 048302-(1-4) (2004). |
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| S.B. Kharchenko, J.F. Douglas, and J. Obrzut, E.A. Grulke, K.B.
Migler, "Extraordinary Flow Characteristics of Nanotube-Filled
Polymer Materials," Nature Materials, in press. |
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