Our group at the Naval Research Laboratory is studying hierarchical arrangements of materials at multiple length scales and how such arrangements can be used to yield novel properties. We are investigating nanocomposites comprising a thermotropic liquid crystalline polymer (LCP) matrix reinforced with carbon nanofibers for potential structure + conduction multifunctional applications. The LCP matrix is known for its inherent hierarchical microstructure, and its fracture surface is typically characterized by fibrils ranging in size from nanometer to micrometer. The carbon nanofibers being compounded with the LCP matrix are vapor-grown carbon nanofibers (VGCF) and pre-processing techniques are being developed to eventually replace VGCF with single-wall carbon nanotubes (SWNT). Composites with VGCF content of 0, 1, 2, 5 and 10 wt.% were extruded using a twin-screw extruder to yield monofilaments in the range of 0.5 to 2 mm in diameter. The mechanical properties of extruded filaments were determined via quasi-static tensile tests and fracture surfaces examined under a scanning electron microscope. Porosity and hierarchical fibrillar structures were commonly observed in the fracture surfaces of tensile tested LCP and LCP-VGCF filaments. The LCP-VGCF filaments showed a maximum increase in strength and modulus of 20% and 35%, respectively, at 1-2 wt.% VGCF content. The dependence of mechanical properties on VGCF content was attributed to the interplay between the extrusion process parameters, VGCF dispersion and molecular alignment of LCP. In another set of experiments, LCP was thermo-mechanically pre-processed using a laboratory scale double-roll mixer and extruded using a Maxwell mixing extruder to yield monofilaments in the range of 0.2 to 0.7 mm. At 0.2 mm diameter, filaments of un-pre-processed and pre-processed neat LCP showed almost identical mechanical properties. At 0.7 mm diameter, however, pre-processed LCP filaments showed 10% and 30% degradation in strength and modulus, respectively, relative to un-pre-processed LCP. The lowered mechanical properties of pre-processed LCP were attributed to its chemical degradation during thermo-mechanical processing. Over the diameter range from 0.2 to 2 mm and irrespective of prior processing or extrusion method, the modulus and strength of neat LCP filaments increased with decreasing diameter. The strength and modulus dependence on filament diameter could be explained by the "skin-core" effect typically seen in liquid crystalline polymers. Future work will involve optimizing processing parameters for simultaneous enhancements in mechanical properties and electrical/thermal conductivity in LCP-VGCF/LCP-SWNT filaments.

1.
Rohatgi, A., Kosmatka, J. B., Vecchio, K. S., Harvey, K. P., Nguyen, P. and Harach, D. J., 2001, “Development of Multifunctional Metallic-Intermetallic Laminate Composites with Particulate Based Damping”, Session MA-2, 203, Proc. 16th ASC Technical Conference, Virginia, USA.
2.
Baucom, J. N., Thomas, J. P., Pogue III, W. R. and Qidwai, M. A., 2004, “Authophagous Structure-Power Systems”, Proc. 11th Annual Symposium on Smart Structures and Materials, California, USA, 5387, pp. 96–105.
3.
Leung
A. C.
,
Matic
P.
,
Delsanto
P. P.
and
Hirsekorn
M.
,
2004
, “
A Parametric Sonic Crystal Modal Analysis Using Finite Modeling
”, 59816,
Proc. International Mechanical Engineering Congress of the American Society of Mechanical Engineers, California, USA
,
31
, pp.
229
236
.
4.
Tirrell, D. A., Aksay, I., et al., 1994, “Hierarchical Structures in Biology as a Guide for New Materials Technology”, 0309046386, National Academy Press, Washington, D.C., USA.
5.
Shofner
M. L.
,
Rodriguez-Macias
F. J.
,
Vaidyanathan
R.
and
Barrera
E. V.
,
2003
, “
Single Wall Nanotube and Vapor Grown Carbon Fiber Reinforced Polymers Processed by Extrusion Freeform Fabrication
”,
Composites Part A: Applied Science and Manufacturing
,
34A
(
12)
, pp.
1207
1217
.
6.
Sawyer
L. C.
and
Jaffe
M.
,
1986
, “
The Structure of Thermotropic Copolyesters
”,
Journal of Materials Science
,
21
(
6)
, pp.
1897
1913
.
7.
Zulle
B.
,
Demarmels
A.
,
Plummer
C. J. G.
,
Schneider
T.
and
Kausch
H. H.
,
1992
, “
The Morphology and Tensile Strength in Filled and Unfilled Thermotropic Liquid-Crystalline Polymer Injection Mouldings
”,
Journal of Materials Science Letters
,
11
(
21)
, pp.
1411
1413
.
8.
Blundell
D. J.
,
Chivers
R. A.
,
Curson
A. D.
,
Love
J. C.
and
MacDonald
W. A.
,
1988
, “
The Relationship of Chain Linearity of Aromatic Liquid-Crystal Polyesters to Molecular Orientation and Stiffness of Mouldings
”,
Polymer
,
29
(
8)
, pp.
1459
1467
.
9.
Chung
T. S.
,
1988
, “
Production of Ultrahigh-Modulus Liquid-Crystal Polymer Rods
”,
Journal of Polymer Science Part B-Polymer Physics
,
26
(
7)
, pp.
1549
1552
.
10.
Tibbetts
G. G.
and
McHugh
J. J.
,
1999
, “
Mechanical Properties of Vapor-Grown Carbon Fiber Composites with Thermoplastic Matrices
”,
Journal of Materials Research
,
14
(
7)
, pp.
2871
2880
.
11.
Finegan
I. C.
,
Tibbetts
G. G.
,
Glasgow
D. G.
,
Ting
J. M.
and
Lake
M. L.
,
2003
, “
Surface Treatments for Improving the Mechanical Properties of Carbon Nanofiber/Thermoplastic Composites
”,
Journal of Materials Science
,
38
(
16)
, pp.
3485
3490
.
12.
Lozano
K.
,
Bonilla-Rios
J.
and
Barrera
E. V.
,
2001
, “
A Study on Nanofiber-Reinforced Thermoplastic Composites (II): Investigation of the Mixing Rheology and Conduction Properties
”,
Journal of Applied Polymer Science
,
80
(
8)
, pp.
1162
1172
.
13.
Yang
S.
,
Lozano
K.
,
Lomeli
A.
,
Foltz
H. D.
and
Jones
R.
,
2005
, “
Electromagnetic Interference Shielding Effectiveness of Carbon Nanofiber/Lcp Composites
”,
Composites Part A: Applied Science and Manufacturing
,
36
(
5)
, pp.
691
697
.
14.
Kuriger
R. J.
,
Alam
M. K.
,
Anderson
D. P.
and
Jacobsen
R. L.
,
2002
, “
Processing and Characterization of Aligned Vapor Grown Carbon Fiber Reinforced Polypropylene
”,
Composites Part A: Applied Science and Manufacturing
,
33
(
1)
, pp.
53
62
.
15.
Breuer
O.
and
Sundararaj
U.
,
2004
, “
Big Returns from Small Fibers: A Review of Polymer/Carbon Nanotube Composites
”,
Polymer Composites
,
25
(
6)
, pp.
630
645
.
16.
Jacobsen
R. L.
,
Tritt
T. M.
,
Guth
J. R.
,
Ehrlich
A. C.
and
Gillespie
D. J.
,
1995
, “
Mechanical-Properties of Vapor-Grown Carbon-Fiber
”,
Carbon
,
33
(
9)
, pp.
1217
1221
.
17.
Tibbetts
G. G.
and
Beetz
C. P.
,
1987
, “
Mechanical-Properties of Vapor-Grown Carbon-Fibers
”,
Journal of Physics D: Applied Physics
,
20
(
3)
, pp.
292
297
.
18.
Chen
Y. M.
and
Ting
J. M.
,
2002
, “
Ultra High Thermal Conductivity Polymer Composites
”,
Carbon
,
40
(
3)
, pp.
359
362
.
19.
Rohatgi, A., Baucom, J. N., Pogue III, W. R. and Thomas, J. P., 2005, “Mechanical Characterization of a Liquid-Crystalline Polymer Nanocomposite”, Proc. 63rd Annual Technical Conference (ANTEC), Massachusetts, USA, pp. 1883–1887.
20.
Endo
M.
,
Kim
Y. A.
, et al.,
2002
, “
Structural Characterization of Cup-Stacked-Type Nanofibers with an Entirely Hollow Core
”,
Applied Physics Letters
,
80
(
7)
, pp.
1267
1269
.
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