U.S. patent application number 09/935493 was filed with the patent office on 2002-04-25 for polymer-wrapped single wall carbon nanotubes.
Invention is credited to Colbert, Daniel T., O'Connell, Michael, Smalley, Richard E., Smith, Ken A..
Application Number | 20020048632 09/935493 |
Document ID | / |
Family ID | 26921584 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048632 |
Kind Code |
A1 |
Smalley, Richard E. ; et
al. |
April 25, 2002 |
Polymer-wrapped single wall carbon nanotubes
Abstract
The present invention relates to new compositions of matter and
articles of manufacture comprising SWNTs as nanometer scale
conducting rods dispersed in an electrically-insulating matrix.
These compositions of matter have novel and useful electrical,
mechanical, and chemical properties including applications in
antennas, electromagnetic and electro-optic devices, and
high-toughness materials. Other compositions of matter and articles
of manufacture are disclosed. including polymer-coated and polymer
wrapped single-wall nanotubes (SWNTs), small ropes of
polymer-coated and polymer-wrapped SWNTs and materials comprising
same. This composition provides one embodiment of the SWNT
conducting-rod composite mentioned above, and also enables creation
of high-concentration suspensions of SWNTs and compatibilization of
SWNTs with polymeric matrices in composite materials. This
solubilization and compatibilization, in turn, enables chemical
manipulation of SWNT and production of composite fibers, films, and
solids comprising SWNTs.
Inventors: |
Smalley, Richard E.;
(Houston, TX) ; Colbert, Daniel T.; (Houston,
TX) ; Smith, Ken A.; (Katy, TX) ; O'Connell,
Michael; (Pearland, TX) |
Correspondence
Address: |
Winstead Sechrest & Minick P.C.
5400 Renaissance Tower
Dallas
TX
75270-2199
US
|
Family ID: |
26921584 |
Appl. No.: |
09/935493 |
Filed: |
August 23, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60227604 |
Aug 24, 2000 |
|
|
|
60268269 |
Feb 13, 2001 |
|
|
|
Current U.S.
Class: |
427/230 ; 222/1;
423/460 |
Current CPC
Class: |
B82Y 10/00 20130101;
H01C 17/0652 20130101; Y02E 60/364 20130101; Y10T 428/2935
20150115; Y10S 977/746 20130101; Y02E 60/36 20130101; C08K 9/08
20130101; H05K 1/0373 20130101; H01L 51/0052 20130101; Y10S 977/745
20130101; Y10S 977/783 20130101; Y10S 977/75 20130101; Y10S 977/748
20130101; Y10T 428/2991 20150115; H01L 51/0048 20130101; H01Q
17/002 20130101; Y10T 428/2998 20150115; D01F 11/14 20130101; B82Y
30/00 20130101; H05K 1/162 20130101 |
Class at
Publication: |
427/230 ;
423/460; 222/1 |
International
Class: |
D01F 009/12; B67B
007/00; G01F 011/00; B05D 007/22; C09C 001/56 |
Goverment Interests
[0006] This invention was made with U.S. Government support under
Grant No. NCC9-77 awarded by the National Aeronautical and Space
Administration. The U.S. Government may have certain rights in the
invention.
Claims
What is claimed is:
1. A method of associating a polymer with the sidewalls of a
plurality of individual single-wall carbon nanotubes, comprising:
(a) providing a purified single-wall carbon nanotube material
substantially free of amorphous carbon; (b) dispersing said
single-wall carbon nanotube material in the polymer by a
combination of high-shear mixing and ultrasonication; (c) adding
salt to bring the solution to a desired concentration of salt by
weight; (d) centrifuging the solution; (e) decanting the solution;
(f) redispersing the material in water by mechanical agitation; and
(g) passing the material through at least one filter.
2. The method in accordance with claim 1, wherein the salt is
selected from the group consisting of an alkali metal salt and an
alkaline earth metal salt.
3. The method in accordance with claim 2, wherein the salt is
sodium chloride.
4. The method in accordance with claim 1, further comprising
applying a high-gradient magnetic field to the material to remove
ferromagnetic particles.
5. The method in accordance with claim 1, wherein said dispersing
step (b) comprises dispersing said single-wall carbon nanotube
material in at least about 1% polystyrene sulfonate in water.
6. The method in accordance with claim 3, wherein the concentration
of sodium chloride is at least about 10%.
7. The method in accordance with claim 1, wherein said centrifuging
step (d) comprises centrifuging at least about 60,000 g for at
least about 20 minutes.
8. The method in accordance with claim 1, wherein the polymer is an
amphiphilic polymer
9. A method in accordance with claim 1, wherein the polymer is
selected from the group consisting of: polyvinyl pyrrolidone (PVP),
polystyrene sulfonate (PSS), poly(1-vinyl pyrrolidone-co-vinyl
acetate) (PVP/VA), poly(1-vinyl pyrrolidone-coacrylic acid),
poly(1-vinyl pyrrolidone-co-dimethylaminoethyl methacrylate),
polyvinyl sulfate, poly(sodium styrene sulfonic acid-co-maleic
acid), dextran, dextran sulfate, bovine serum albumin (BSA),
poly(methyl methacrylate-co-ethyl acrylate), polyvinyl alcohol,
polyethylene glycol, and polyallyl amine.
10. The method in accordance with claim 1, wherein the single-wall
carbon nanotubes are coated with at least two different
polymers.
11. A method for making polymer-coated single-wall carbon nanotubes
comprising dispersing single-wall carbon nanotubes and a polymer in
a solvent by a method selected from the group consisting of mixing,
sonication, heating and combinations thereof.
12. A method in accordance with claim 11, wherein the single-wall
carbon nanotubes are substantially free of amorphous carbon.
13. A method in accordance with claim 11, wherein the single-wall
carbon nanotubes are coated with at least two polymers.
14. A method accordance with claim 11, wherein the polymer and the
plurality of individual single wall carbon nanotubes are added to
the solvent sequentially.
15. A method in accordance with claim 11, wherein the polymer and
the plurality of individual single-wall carbon nanotubes are added
to the solvent simultaneously.
16. A method in accordance with claim 11, wherein the solvent
comprises water and the polymer is water-soluble.
17. A method in accordance with claim 11, wherein the solvent
further comprises a surfactant.
18. A method in accordance with claim 11, wherein the concentration
of single-wall carbon nanotubes in the solvent is between about 0.1
grams/liter and about 5 grams/liter.
19. A method in accordance with claim 11, wherein the concentration
of polymer in the solvent is between about 1.0 percent and about
5.0 percent by weight.
20. A method in accordance with claim 11, wherein the solvent is
heated to a temperature at least about 40.degree. C.
21. A method in accordance with claim 11, wherein the solvent is
heated to a temperature of between about 50.degree. C. and about
60.degree. C.
22. A method in accordance with claim 11, wherein the solvent is
heated between about 0.1 hours and about 100 hours.
23. A method in accordance with claim 11, wherein the solvent is
heated between about 1 hour and about 50 hours.
24. A method in accordance with claim 11, further comprising the
step of extruding the polymer-wrapped nanotubes with a second
polymer to form an encapsulated nanotube-polymer composite.
25. A method in accordance with claim 11, further comprising the
step of removing the polymer coat from the nanotubes by contacting
the coated nanotubes with a solvent having a low surface
tension.
26. A method in accordance with claim 25, wherein the solvent
comprises a chlorinated hydrocarbon.
27. A method in accordance with claim 11, further comprising the
step of aligning the nanotubes by application of an external field
selected from the group consisting of an electrical field, a
magnetic field and a shear flow field.
28. A method for making polymer-coated aggregates of single-wall
carbon nanotubes comprising dispersing aggregates of single-wall
carbon nanotubes and a polymer in a solvent by a method selected
from the group consisting of mixing, sonication, heating and
combinations thereof.
29. A method in accordance with claim 28, wherein the aggregates of
single-wall carbon nanotubes comprises ropes of single-wall carbon
nanotubes which are substantially aligned along their longitudinal
axes.
30. A method in accordance with claim 28, wherein the aggregates of
single-wall carbon nanotubes comprises bundles of single-wall
carbon nanotubes which are substantially aligned along their
longitudinal axes.
31. A method in accordance with claim 28, wherein the aggregates of
single-wall carbon nanotubes are coated with at least two different
polymers.
32. A method in accordance with claim 28, wherein the single-wall
carbon nanotubes in the aggregates are substantially free of
amorphous carbon.
33. A method accordance with claim 28, wherein the polymer and the
aggregates of single-wall carbon nanotubes are added to the solvent
sequentially.
34. A method in accordance with claim 28, wherein the polymer and
the aggregates of single-wall carbon nanotubes are added to the
solvent simultaneously.
35. A method in accordance with claim 28, wherein the solvent
comprises water and the polymer is water-soluble.
36. A method in accordance with claim 28, wherein the solvent
further comprises a surfactant.
37. A method in accordance with claim 28, wherein the concentration
of the aggregates of single-wall carbon nanotubes in the solvent is
between about 0.1 gram/liter and about 5 gram/liter.
38. A method in accordance with claim 28, wherein the concentration
of the polymer in the solvent is between about 1.0 percent and
about 5.0 percent by weight.
39. A method in accordance with claim 28, wherein the solvent is
heated to a temperature at least about 40.degree. C.
40. A method in accordance with claim 28, wherein the solvent is
heated to a temperature of between about 50.degree. C. and about
60.degree. C.
41. A method in accordance with claim 28, wherein the solvent is
heated between about 0.1 hours and about 100 hours.
42. A method in accordance with claim 28, wherein the solvent is
heated between about 1 hour and about 50 hours.
43. A method in accordance with claim 28, further comprising the
step of extruding the polymer-wrapped aggregates of single-wall
carbon nanotubes with a second polymer to form a composite of
single-wall carbon nanotube aggregates.
44. A method in accordance with claim 28, further comprising the
step of removing the polymer coat from the polymer-coated
aggregates of the single-wall carbon nanotubes by contacting the
polymer-coated aggregates of single-wall carbon nanotubes with a
solvent having a low surface tension.
45. A method in accordance with claim 44, wherein the solvent
comprises a chlorinated hydrocarbon.
46. A method in accordance with claim 28, further comprising the
step of aligning the polymer-wrapped aggregates of single-wall
carbon nanotubes by application of an external field selected from
the group consisting of an electrical field, magnetic field, and
shear flow field.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This patent application claims priority from U.S.
Provisional Application No. 60/227,604, filed on Aug. 24, 2000, and
from U.S. Provisional Application No. 60/268,269, filed on Feb. 13,
2001, both of which are incorporated herein by reference.
[0002] This patent invention is related to the following
corresponding PCT/U.S. Patent Applications, all of which also claim
priority from the above referenced provisional patent applications
and all of which PCT/U.S. Patent Applications are also incorporated
herein by reference:
[0003] International Application No. PCT/US ______,
"POLYMER-WRAPPED SINGLE WALL CARBON NANOTUBES" to Smalley et al.,
(Attorney Docket No. 11321-P014WO), filed concurrent to the date of
this Application;
[0004] U.S. patent application Ser. No. ______, "POLYMER-WRAPPED
SINGLE WALL CARBON NANOTUBES" to Smalley et al., (Attorney Docket
No. 11321-P014US), filed concurrent to the date of this
Application; and
[0005] U.S. patent application Ser. No. ______, "POLYMER-WRAPPED
SINGLE WALL CARBON NANOTUBES" to Smalley et al., (Attorney Docket
No. 11321-P035US), filed concurrent to the date of this
Application.
FIELD OF THE INVENTION
[0007] The present invention relates generally to carbon nanotubes,
and more particularly relates to materials containing single-wall
carbon nanotubes and non-covalently derivatized single-wall carbon
nanotubes.
BACKGROUND OF THE INVENTION
[0008] Fullerenes are closed-cage molecules composed entirely of
sp.sup.2-hybridized carbons, arranged in hexagons and pentagons.
Fullerenes (e.g., C.sub.60) were first identified as closed
spheroidal cages produced by condensation from vaporized
carbon.
[0009] Fullerene tubes are produced in carbon deposits on the
cathode in carbon arc methods of producing spheroidal fullerenes
from vaporized carbon. Ebbesen et al. (Ebbesen I), "Large-Scale
Synthesis Of Carbon Nanotubes," Nature, Vol. 358, p. 220 (Jul. 16,
1992) and Ebbesen et al., (Ebbesen II), "Carbon Nanotubes," Annual
Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are
referred to herein as carbon nanotubes. Many of the carbon
nanotubes made by these processes were multi-wall nanotubes, i.e.,
the carbon nanotubes resembled concentric cylinders. Carbon
nanotubes having up to seven walls have been described in the prior
art. Ebbesen II; Iijima et al., "Helical Microtubules Of Graphitic
Carbon," Nature, Vol. 354, p. 56 (Nov. 7, 1991).
[0010] In defining single-wall carbon nanotubes, it is helpful to
use a recognized system of nomenclature. In this application, the
carbon nanotube nomenclature described by M. S. Dresselhaus, G.
Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon
Nanotubes, Chap. 19, especially pp. 756-760, (1996), published by
Academic Press, 525 B Street, Suite 1900, San Diego, Calif.
92101-4495 or 6277 Sea Harbor Drive, Orlando, Fla. 32877 (ISBN
0-12-221820-5), which is hereby incorporated by reference, will be
used. The single wall tubular fullerenes are distinguished from
each other by double index (n,m) where n and m are integers that
describe how to cut a single strip of hexagonal "chicken-wire"
graphite so that its edges join seamlessly when it is wrapped to
form a cylinder. When the two indices are the same, m=n, the
resultant tube is said to be of the "arm-chair" (or n,n) type,
since when the tube is cut perpendicular to the tube axis, only the
sides of the hexagons are exposed and their pattern around the
periphery of the tube edge resembles the arm and seat of an arm
chair repeated n times. Arm-chair tubes are one form of single-wall
carbon nanotubes; they are truly metallic, and have extremely high
electrical conductivity. Other nanotube geometries, where (n-m)/3
is an integer are semi-metallic, i.e. they have a small band-gap
and are good electrical conductors at temperatures relevant to the
operation of almost all electronic materials and devices. The
remaining nanotube geometries where (n-m)/3 is not an integer are
semiconductors, having a band-gap in the neighborhood of 1 eV,
which varies with inversely with their individual diameters. See
Odom et al, J. Phys. Chem. B, vol. 104 p.2794 (2000). In addition,
all single-wall nanotubes are the stiffest molecules known, and
have extremely high thermal conductivity and tensile strength. See
Yakobson and Smalley, Am. Sci. vol. 85, p. 324 (1997).
[0011] Single-wall carbon nanotubes have been made in a DC arc
discharge apparatus of the type used in fullerene production by
simultaneously evaporating carbon and a small percentage of Group
VIII transition metal from the anode of the arc discharge
apparatus. See Iijima et al., "Single-Shell Carbon Nanotubes of 1
nm Diameter," Nature, Vol. 363, p.603 (1993); Bethune et al.,
"Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic
Layer Walls," Nature, Vol. 63, p. 605 (1993); Ajayan et al.,
"Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon
Nanotube Synthesis," Chem. Phys. Lett., Vol. 215, p. 509 (1993);
Zhou et al., "Single-Walled Carbon Nanotubes Growing Radially From
YC.sub.2 Particles," Appl. Phys. Lett., Vol. 65, p.1593 (1994);
Seraphin et al., "Single-Walled Tubes and Encapsulation of
Nanocrystals Into Carbon Clusters," Electrochem. Soc., Vol. 142, p.
290 (1995); Saito et al., "Carbon Nanocapsules Encaging Metals and
Carbides," J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et
al., "Extrusion of Single-Wall Carbon Nanotubes Via Formation of
Small Particles Condensed Near an Evaporation Source," Chem. Phys.
Lett., Vol. 236, p. 419 (1995). It is also known that the use of
mixtures of such transition metals can significantly enhance the
yield of single-wall carbon nanotubes in the arc discharge
apparatus. See Lambert et al., "Improving Conditions Toward
Isolating Single-Shell Carbon Nanotubes," Chem. Phys. Lett., Vol.
226, p. 364 (1994).
[0012] While this arc discharge process can produce single-wall
nanotubes, the yield of nanotubes is low and the tubes exhibit
significant variations in structure and size between individual
tubes in the mixture. Individual carbon nanotubes are difficult to
separate from the other reaction products and purify.
[0013] An improved method of producing single-wall nanotubes is
described in U.S. Pat. No. 6,183,714, entitled "Method of Making
Ropes of Single-Wall Carbon Nanotubes," incorporated herein by
reference in its entirety. This method uses, inter alia, laser
vaporization of a graphite substrate doped with transition metal
atoms, preferably nickel, cobalt, or a mixture thereof, to produce
single-wall carbon nanotubes in yields of at least 50% of the
condensed carbon. The single-wall nanotubes produced by this method
are much more pure than those produced by the arc-discharge method.
Because of the absence of impurities in the product, the
aggregation of the nanotubes is not inhibited by the presence of
impurities and the nanotubes produced tend to be found in clusters,
termed "ropes," of 10 to 5000 individual single-wall carbon
nanotubes in parallel alignment, held together by van der Waals
forces in a closely packed triangular lattice.
[0014] PCT/US/98/04513 entitled "Carbon Fibers Formed From
Single-Wall Carbon Nanotubes" and which is incorporated by
reference, in its entirety, discloses, inter alia, methods for
cutting and separating nanotube ropes and manipulating them
chemically by derivatization to form devices and articles of
manufacture comprising nanotubes. Other methods of chemical
derivatization of the side-walls of the carbon nanotubes are
disclosed in PCT/US99/21366 entitled "Chemical Derivatization of
Single Wall Carbon Nanotubes to Facilitate Solvation Thereof, and
Use of Derivatized Nanotubes," which is incorporated by reference,
in its entirety.
[0015] Another method for producing single-wall carbon nanotubes is
described in PCT/US99/25702, (entitled "Gas-Phase Nucleation and
Growth of Single-Wall Carbon Nanotubes from High Pressure CO")
incorporated herein in its entirety by reference, which describes a
process involving supplying high pressure (e.g., 30 atmospheres) CO
that has been preheated (e.g., to about 1000.degree. C.) and a
catalyst precursor gas (e.g., Fe(CO).sub.5) in CO that is kept
below the catalyst precursor decomposition temperature to a mixing
zone. In this mixing zone, the catalyst precursor is rapidly heated
to a temperature that results in (1) precursor decomposition, (2)
formation of active catalyst metal atom clusters of the appropriate
size, and (3) favorable growth of single-wall carbon nanotubes
("SWNTs") on the catalyst clusters. Preferably a catalyst cluster
nucleation agency is employed to enable rapid reaction of the
catalyst precursor gas to form many small, active catalyst
particles instead of a few large, inactive ones. Such nucleation
agencies can include auxiliary metal precursors that cluster more
rapidly than the primary catalyst, or through provision of
additional energy inputs (e.g., from a pulsed or CW laser) directed
precisely at the region where cluster formation is desired. Under
these conditions SWNTs nucleate and grow according to the Boudouard
reaction. The SWNTs thus formed may be recovered directly or passed
through a growth and annealing zone maintained at an elevated
temperature (e.g., 1000.degree. C.) in which tubes may continue to
grow and coalesce into ropes.
[0016] In yet another method for production, single-walled carbon
nanotubes have been synthesized by the catalytic decomposition of
both carbon monoxide and hydrocarbons over a supported metal
catalyst. Under certain conditions, there is no termination of
nanotube growth, and production appears to be limited only by the
diffusion of reactant gas through the product nanotube mat that
covers the catalyst. "Catalytic Growth of Single-Wall Carbon
Nanotubes from Metal Particles" (PCT/US99/21367) incorporated in
its entirety by reference, details a catalyst-substrate system that
promotes the growth of nanotubes that are predominantly
single-walled tubes in a specific size range, rather than the large
irregular-sized multi-walled carbon fibrils that are known to grow
from supported catalysts. This method allows bulk catalytic
production of predominantly single-wall carbon nanotubes from metal
catalysts located on a catalyst-supporting surface.
[0017] Carbon nanotubes, and in particular, single-wall carbon
nanotubes, are useful for making electrical connectors in micro
devices such as integrated circuits or in semiconductor chips used
in computers because of the electrical conductivity and small size
of the carbon nanotube. The carbon nanotubes are useful as antennas
at optical frequencies, as constituents of non-linear optical
devices, and as probes for scanning probe microscopy such as are
used in scanning tunneling microscopes (STM) and atomic force
microscopes (AFM). The carbon nanotubes may be used in place of or
in conjunction with carbon black in tires for motor vehicles, as
elements of a composite materials to elicit specific physical,
chemical or mechanical properties in those materials (e.g.
electrical and/or thermal conductivity, chemical inertness,
mechanical toughness, etc). The carbon nanotubes themselves and
materials and structures comprising carbon nanotubes are also
useful as supports for catalysts used in industrial and chemical
devices and processes such as fuel cells, hydrogenation, reforming
and cracking.
[0018] Individual SWNT and ropes of single-wall carbon nanotubes
exhibit metallic conductivity, i.e., they will conduct electrical
charges with a relatively low resistance. Nanotubes and ropes of
nanotubes are useful in any application where an electrical
conductor is needed, for example as an additive in electrically
conductive polymeric materials, paints or in coatings. Nanotubes
and ropes of nanotubes have a propensity to aggregate into large
networks, which are themselves electrically conductive, and this
property enables them to form such networks when they are suspended
in a matrix of a different material. The presence of this network
alters the electrical properties of a composite that includes
nanotubes.
[0019] Single-wall carbon nanotubes have outstanding properties as
field-emitters for electrons, and serve well as the active element
in cold-cathodes in any applications that involve emission of
electrons, such as microwave power tubes and video displays.
SUMMARY OF THE INVENTION
[0020] Prior art reveals materials comprising SWNTs which have
substantial bulk electrical conductivity derived from the
conductive network of SWNTs. This invention provides a new class of
materials that have low electrical conductivity, but contain
individual nanotubes and small ropes of individual nanotubes which
are themselves good electrical conductors, and serve as small
conducting rods immersed in an electrically-insulating matrix.
These materials represent a new composition of matter that has
novel electrical and electronic properties that are distinct from
those of previously known nanotube-containing materials.
[0021] This invention also provides another new composition of
matter which is a single-wall carbon nanotube that is coated or
wrapped with one or more polymer molecules. This coating or
wrapping is one form of a non-covalent derivatization. "Coating"
and "wrapping" are used interchangeably herein, and refer to the
presence of a polymer molecule in contact with the exterior of a
SWNT, whether the polymer molecule covers all or only part of the
SWNT's exterior. The use of either "coating" or "wrapping" does not
imply that the polymer is necessarily wrapped around a SWNT in a
regular or symmetrical configuration. Also provided is a
composition of matter comprising individual nanotubes and small
ropes of nanotubes that are coated or wrapped with polymer
molecules. The polymer that covers these small ropes and tubes
attaches to the SWNTs through weak chemical forces that are
primarily non-covalent in nature. These forces are due primarily to
polarization, instead of sharing of valence electrons. Thus the
properties of the SWNTs involved are substantially the same as that
of free, individual SWNTs and small ropes of SWNTs. (A measured
property is "substantially the same" if the measurements for this
property are plus or minus twenty-five percent (.+-.25%)). Thus
SWNTs and small ropes of SWNTs that are electrically-conductive
retain their individual electrical conductivity and other
properties when they are coated with the polymer. Their high
electrical conductivity makes the individual nanotubes highly
electrically polarizable, and materials containing these highly
polarizable molecules exhibit novel electrical properties,
including a high dielectric constant when the SWNTs are
electrically isolated from one another.
[0022] A polymer wrapping or coating greatly inhibits the Van der
Waals attraction normally observed between separate SWNT and small
ropes of SWNT. The polymer also interacts with solvents. The
combination of the Van der Waals inhibition and polymer-solvent
interaction causes the wrapped nanotubes to be much more readily
suspended at high concentrations in solvents. This enables creation
of high-concentration SWNT solutions and suspensions, which in turn
substantially enables manipulation of SWNT into bulk materials of
many kinds, including films, fibers, solids, and composites of all
kinds. This invention also includes treatment of the polymer-coated
SWNT to remove the polymer coating and restore the pristine
SWNT.
[0023] Aggregations of polymer-coated SWNTs produce a new material
that has novel electrical properties such as a high dielectric
constant. The novel electrical properties are isotropic in
compositions where the SWNT are essentially randomly oriented with
one another, such as in an electrically-insulating matrix.
Anisotropic behavior of such materials will be obtained when the
SWNTs are aligned by the application of an external aligning field,
such as a magnetic field, electrical field, or shear flow field.
Aggregations of polymer coated SWNTs are disclosed in which the
SWNTs are substantially aligned in an insulating matrix and provide
a new form of electrically-conducting rod composite, where the
conducting rods have cross sectional dimensions on the nanometer
scale and lengths of hundreds of nanometers or more, and the
electrical properties of the composite are highly anisotropic.
[0024] The polymer wrappings or coatings can be chosen to make the
polymer-associated SWNTs and ropes of SWNTs compatible with
matrices of other materials to facilitate fabrication of
composites. Composite materials of polymer-associated SWNTs
suspended in a polymer matrix, provide a new form of polymeric
composite. This composite material is novel in that the structure
of the nanotube suspended is (in its cross sectional dimensions)
substantially smaller than the typical scale length of the
individual polymer molecules in the matrix. This microscopic
dimensional compatibility minimizes the propensity of the composite
to fail mechanically at the interface between the matrix and the
SWNTs, producing a composite material with enhanced mechanical
properties such as strain-to-failure, toughness, and resistance to
mechanical fatigue. This composite also has the novel electrical
properties discussed above, when the properties of SWNT in the
matrix allow them to be electrically isolated from one another. In
embodiments where there is incomplete electrical isolation between
the SWNT, the mechanical enhancement of the composite is due to the
similarity of scale in both the polymer and single wall carbon
nanotube. Because of this similarity the SWNT and polymer establish
a substantially more intimate contact than if their dimensional
scales would not have been the same.
[0025] The above described materials are useful in mechanical or
structural applications where enhanced material or structural
mechanical properties are desired. The materials are also useful in
electronic applications requiring a high-dielectric constant
material, including but not limited to, capacitor dielectrics,
circuit board materials, waveguide materials, optical
index-matching materials, materials that absorb electromagnetic
radiation, materials that re-direct electromagnetic radiation,
optoelectronic materials, antenna arrays, and materials for
suspending antennas, electrically-loading antennas to change their
required length, and support of antenna arrays that provides
advantages in the spacing of the individual antennas. These
materials also serve as the active element for a range of
transducers in that they change their physical dimensions in
response to applied electric and magnetic fields. Applied in a way
that they can come into contact with other chemicals, they also
change dimensionally and electronically in response to adsorption
of chemicals on the nanotube surface, and therefore serve as
chemical sensors and transducers that respond with a change of
electrical and mechanical properties to the presence of specific
chemicals.
[0026] For those materials in which the SWNT are mobile (such as in
a suspension or solution of SWNT, produced by known means for
suspending or solubilizing SWNT known in the art), the materials'
mechanical, electrical and electromagnetic properties are modified
by application of an electric field that changes the orientation or
location of the SWNT segments contained therein. These materials
enable new electromechanical elements, fluids with
electrically-controllable viscosity, opto-electronic elements where
the character (e.g. intensity, polarization) of a electromagnetic
wave transmitted through the SWNT-containing material is modified
as a consequence of application of an electric field to the
material.
[0027] Nanometer-scale Conducting-rod Materials
[0028] One aspect of the present invention is directed to the
creation of novel SWNT-containing materials that have novel
electronic and/or electromagnetic properties derived from the
electrical conductivity and high electrical polarizability of the
nanotubes. The invention includes means for creation of such
materials, articles of manufacture incorporating such materials,
and the materials themselves.
[0029] Electrically-conductive single-wall carbon nanotubes are the
first known molecules that are excellent electrical conductors. An
example of this carbon polymer is shown in FIG. 1. This large
molecule can easily be manipulated through chemical means well
known to those skilled in the art of polymer chemistry and by new
means that are specific to the nanotube structure. Means of such
chemical manipulation are disclosed in PCT/US/98/04513, cited
above, as well as in Chemical Derivatization of Single Wall Carbon
Nanotubes to Facilitate Solvation Thereof, and Use of Derivatized
Nanotubes (PCT/US99/21366), incorporated herein in its entirety by
reference. This invention applies the chemical facility of this
molecule to enable new materials that have novel electrical and
electromagnetic properties. This invention is novel in that it
creates materials from individual SWNT, on a truly molecular basis.
Prior art (see Grimes et al. Chem. Phys. Lett., Vol. 319, p. 460,
2000) has shown that bundles of SWNT in a polymer matrix provide a
composite material with a high dielectric constant. In that work,
loading a polyethyl methacrylate matrix with mechanically-ground
SWNT in bundles (nanotube ropes) of .about.10 nm diameter provided
a material with a bulk dielectric constant ranging between 10 and
140 for SWNT loadings ranging between 4 wt % and 23 wt %,
respectively. The bundle lengths in that work were .about.1 to 5
microns. The theoretical work of Lagarkov and Sarychev (Phys. Rev.
B, vol. 53., no. 10, p. 6318, 1996), which is incorporated by
reference in its entirety, illustrates that the ultimate dielectric
constant of a composite material comprising electrically conducting
rods in an electrically-insulating matrix depends strongly on the
aspect ratio (length divided by diameter) of the conducting rods,
and that for small concentrations of rods that the dielectric
constant scales as the square of the aspect ratio. The dielectric
constant is dependent on the concentration of the rods, peaking at
the percolation threshold for the material. The peak value attained
again depends on the aspect ratio of the conducting rods.
Individual SWNT and small bundles of individual SWNT are molecular
"conducting rods", and composite materials comprising individual
SWNT and small bundles of SWNT in an electrically-insulating matrix
are a direct embodiment of the predictions of Lagarkov and Sarychev
and attain dielectric constants in excess of 1000.
[0030] Such materials with tailored electronic properties such as a
large dielectric constant, .epsilon., or a controllable electric
susceptibility tensor, .epsilon., are of significant importance in
applications that include integrated logic circuits, radio
frequency circuits, antennas, and energy storage. Such materials
are also useful to manipulate electromagnetic radiation within the
material and at its surfaces and interfaces. The response of
conducting rod composite materials also demonstrates dispersive
behavior, which is dependent on the frequency of the
electromagnetic field to which the material is subjected. This
dispersive behavior becomes resonant when the rod length is
comparable to the wavelength of the incident radiation. (Lagarkov
and Sarychev Phys. Rev. B, vol. 53. p. 6318). PCT/US/98/04513
entitled "Carbon Fibers Formed From Single-Wall Carbon Nanotubes"
describes means for making individual SWNT and small SWNT bundles
(<10 nm in diameter) available in lengths comparable to the
wavelength of visible light, and therefore enables creation of
"conducting rod composites" that have novel behavior in the visible
light wavelength region, which include a high absorptivity for
electromagnetic waves. Such materials have an extinction
coefficient from about 0.001 to about 10 and a large real
dielectric constant, which may be positive or negative, from about
-100 to about 1000, and a substantial paramagnetic response for
time-dependent excitation, even though the material's constituents
are essentially-non magnetic. These novel characteristics, in turn
give rise to implementations of the conducting-rod-composite
material in optical applications including new configurations for
lenses, reflectors, polarizers, and wavelength selectors. Clearly,
in compositions of matter and devices where the SWNT are mobile,
the electromagnetic properties of the composition or device are
easily modified by the change in orientation or location of the
SWNT within the material in response to an applied field or its
gradient. Even in materials where the SWNT are mechanically
constrained, however, the known response of nanotubes to electric
fields, wherein their electronic property and, therefore, their
conducting behavior is modified by strong electric (see Slepyan et
al. Phys. Rev. B, vol. 57, p. 9485 (1998)) and magnetic (see Lee et
al, Solid State Communications vol. 115, p. 467 (2000)) fields
permits the conducting-rod-composite material to function in
electro-optic devices such as switches, modulators, and amplifiers.
Likewise, the known response of nanotube electronic property and
conductivity to electric and magnetic fields, to mechanical strains
(Hertel et al., J. Phys. Chem. B, vol. 102, p. 910 (1998)) and to
chemical adsorption on their surfaces (see Jing-Kong et al.,
Science, vol. 287 p. 622 (2000)) combined with the strong optical
response of the composite material permits fabrication of
optically-interrogated electrical, mechanical, and chemical
sensors.
[0031] The nanotube conducting-rod composite materials comprising
SWNT in a solid matrix are also electro-strictive,
magneto-strictive and chemi-strictive which is to say that they
change their physical dimensions in response to electric fields,
magnetic fields, and chemisorption of atoms and molecules on their
surfaces. This property permits their use as sensors and
transducers in all the same ways as are known in the arts of other
electro- and magneto-strictive materials as sensors and
electromechanical transducers, and admits chemically-sensitive
transduction as well. In some embodiments the dimensional change is
more effective when an electromagnetic field is applied to the
material in conjunction with the electric, magnetic, or chemical
influence that initiates the dimensional change.
[0032] In all the above applications, the response of the material
is made anisotropic when the nanotube elements of the material
possess a collective net orientation in one direction. An
appropriate choice of this orientation is used to enhance or tailor
the response of a device to a particular purpose. For instance, the
dielectric constant of a material of oriented nanotube segments
will be enhanced for electric fields parallel to the mean direction
of the constituent nanotube orientation. The mechanical
electrostrictive response, depends on the elastic properties of the
insulating matrix and the relative orientation of the applied
electric field and net orientation of the constituent SWNT.
[0033] Polymer Coating of SWNTs and Small Ropes of SWNTs
[0034] According to one embodiment of the present invention,
nanotubes are solubilized or suspended, primarily as individual
tubes, in a liquid by associating them robustly with linear
polymers compatible with the liquid used. (If, for example, the
liquid is water, suitable polymers include, for example, polyvinyl
pyrrolidone and polystyrene sulfonate.) This association is
characterized by tight, relatively uniform association of the
polymers with the sides of the nanotubes. This association of
polymers with the sides of the nanotubes wraps the tubes in a way
that inhibits electrical and physical contact between the
individual tubes. The suspension or solution thus created provides
an example of a novel dielectric material wherein the SWNT are
mobile and do not agglomerate. When the wrapped nanotubes are
removed from solution, the polymer wrapping remains, and the tubes
form an aggregate in which the individual tubes are substantially
electrically-isolated from one another. The material thus formed
represents a "conducting rod composite" as described above.
[0035] In another embodiment of this invention, the polymeric
wrappings around the tubes are cross-linked by introduction of a
linking agent, forming a different material in which individual,
electrically-isolated SWNT are permanently suspended in a solid
cross-linked polymeric matrix.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] Various features and objects of the invention will be best
appreciated with reference to a detailed description of specific
embodiments of the invention, which follows, when read in
conjunction with the accompanying figures, wherein:
[0037] FIG. 1 is a schematic view of a single-wall carbon
nanotube--one carbon atom resides at each line intersection in the
figure. Depending on the production parameters, the nanotube
diameter ranges from about 0.6 to about 1.6 nanometers, and the
normal tube segment lengths range between 30 and 3000
nanometers.
[0038] FIGS. 2A-2C show schematically how polyvinyl pyrrolidone
(PVP) conforms to the SWNT, forming a wrapping that electrically
isolates one tube from another. In particular, these figures show
possible wrapping arrangements of polyvinyl pyrrolidone (PVP) on an
(8,8) SWNT. A double helix (FIG. 2A), a triple helix (FIG. 2B), and
an internal-rotation induced switch-back (FIG. 2C) are shown.
[0039] FIGS. 3A-3D show tapping-mode atomic force micrograph (AFM)
images of PVP-SWNTs on a functionalized substrate. A 5 .mu.m-wide
height image (FIG. 3A) and amplitude image (FIG. 3B), and a 1
.mu.m-wide expanded height image (FIG. 3C) and amplitude image
(FIG. 3D) are shown. The measured feature heights are consistent
with individual tubes wrapped with one layer of PVP.
[0040] FIGS. 4A-4B show typical filter cake formed by filtering
un-wrapped SWNTs from organic or surfactant suspension, consisting
of large ropes (FIG. 4A), and filter cake formed by filtering
PVP-wrapped SWNTs (FIG. 4B). Scale bars are 500 nm. The
spaghetti-like structures shown in FIG. 4A form because the strong
Van der Waals attraction for the bare tubes cause their aggregation
into "ropes". The "wrapping" distances the tubes from one another,
essentially eliminating the short-range Van der Waals attraction,
and the tubes do not form ropes. The clear difference in the
morphology of these filter cakes demonstrates the effectiveness of
isolation of one tube from its neighbors.
[0041] FIG. 5 is an optical-micrograph showing birefringent domains
of PVP-coated SWNT material as observed between crossed polarizers.
Scale bar is 0.04 mm.
[0042] FIG. 6 is a Scanning-electron-micrograph of a material
comprising >10.sup.4 individually-wrapped SWNT where the
wrapping polymer has been cross-linked according to one embodiment
of the present invention.
[0043] FIGS. 7A-7C show PVP-SWNT lengths as separated by gel
electrophoresis.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
[0044] In the disclosure that follows, in the interest of clarity,
not all features of actual implementations are described. It will
of course be appreciated that in the development of any such actual
implementation, as in any such project, numerous engineering
decisions must be made to achieve the developers' specific
objectives, which will vary from one implementation to another.
Moreover, attention will necessarily be paid to proper engineering
practices for the environment(s) in question. It will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the relevant fields.
[0045] One aspect of the present invention is a composition that
comprises one or more single-wall carbon nanotubes, each of which
is coated at least in part with a polymer molecule. FIG. 1
illustrates a single wall carbon nanotube (SWNT), and FIGS. 2A-2C
shows three possible arrangements of polymer molecules wrapped
around a SWNT. FIG. 2 is illustrative of one possible means in
which the similar sizes of the SWNT and polymer enable an
individual polymer molecule to contact an individual SWNT at many
points, establishing an effective and intimate contact between the
polymer and SWNT. Although the arrangements shown in FIGS. 2A-2C
are regular and symmetrical, it should be understood that the
invention also encompasses arrangements in which the polymer is
wrapped in a non-regular and/or nonsymmetrical manner around a
SWNT. In some embodiments of the invention, the polymer is present
as a mono-molecular layer on the SWNT. However, it is also possible
to have a thicker layer of polymer present. Likewise, while in some
embodiments of the invention the polymer wraps or coats only part
of the exterior surface of the SWNT, as shown in FIGS. 2A-2C, in
other embodiments of the invention the SWNT may be completely
coated (i.e., encapsulated) by polymer.
[0046] A variety of polymers can be used in the present invention.
In some embodiments, amphiphilic polymers, such as polymer
surfactants, are especially useful. An amphiphilic polymer is one
that contains both hydrophobic and hydrophilic segments. Water
soluble polymers are preferred in embodiments in which the solvent
used to form the composition comprises water. It is also possible
to use two or more different polymers to wrap the SWNT. Among the
many polymers that can be used in the present invention are
polyvinyl pyrrolidone (PVP), polystyrene sulfonate (PSS),
poly(1-vinyl pyrrolidone-co-vinyl acetate) (PVP/VA), poly(1-vinyl
pyrrolidone-coacrylic acid), poly(1-vinyl
pyrrolidone-co-dimethylaminoethyl methacrylate), polyvinyl sulfate,
poly(sodium styrene sulfonic acid-co-maleic acid), dextran, dextran
sulfate, bovine serum albumin (BSA), poly(methyl
methacrylate-co-ethyl acrylate), polyvinyl alcohol, polyethylene
glycol, polyallyl amine, and mixtures thereof.
[0047] The polymer is not covalently bonded to the SWNT, therefore,
the electronic, electrical, thermal, and mechanical properties of
the coated single-wall carbon nanotube are substantially the same
as those of an uncoated single-wall carbon nanotube. In certain
embodiments of the invention, the coated carbon nanotube is
prepared by a process that includes dispersing SWNT and polymer in
a solvent by a method selected from the group consisting of mixing,
sonication, and a combination thereof. In other embodiments of the
invention, the coated carbon nanotube is prepared by a process that
includes: (a) dispersing SWNT and polymer in a solvent (either
simultaneously or sequentially) by a method selected from the group
consisting of mixing, sonication, and a combination thereof; and
(b) adding salt in a quantity effective to promote wrapping of
polymer on the carbon nanotube, whereby polymer becomes wrapped on
the exterior of the carbon nanotube. The SWNT can be produced by
methods including the laser-oven method (described in U.S. Pat. No.
6,183,714, which is incorporated herein by reference) or the
"HiPco" process (described in PCT/US99/25702, which is incorporated
herein by reference). Preferably the SWNTs used in the process are
substantially free (e.g., contain less than about 5% by weight)
amorphous carbon.
[0048] The polymer in some embodiments of the invention is
suspended in the solvent in step (a), while in other embodiments
the polymer is dissolved in the solvent. As mentioned above, when
an aqueous solvent is used, preferably the polymer is
water-soluble. Optionally, the solvent can also comprise a
surfactant that promotes wrapping of polymer on the carbon
nanotube, such as sodium dodecyl sulfate (SDS). In some embodiments
of the process, the concentration of carbon nanotubes in the
solvent in step (a) is between about 0.1 gram/liter to about 5.0
gram/liter, and the concentration of polymer in the solvent in step
(a) is between about 1.0 percent and about 5.0 percent by weight.
It is often useful to heat the solvent in step (a) to a temperature
at least about 40.degree. C., such as between about 50.degree. C.
and about 60.degree. C., for about 0.1 to about 100 hours, more
preferably for about 1 to about 50 hours.
[0049] Without being bound by theory, it is believed that the
polymer has a greater affinity for the SWNT than for the solvent in
which it is dissolved and/or suspended, even in a dilute solution.
This affinity promotes wrapping of polymer molecules around SWNTs,
and effectively solubilizes the SWNTs.
[0050] When the composition comprises a plurality of single-wall
carbon nanotubes, each of which is coated at least in part with a
polymer molecule, it is possible to align the SWNT by application
of an external electrical field, magnetic field, or shear flow
field. When this is done, preferably the plurality of carbon
nanotubes are substantially aligned along their longitudinal axis
(e.g., within 250 of having parallel longitudinal axes). Thus, one
particular embodiment of the invention is a composition that
comprises a plurality of single-wall nanotubes that are
substantially aligned with one another to define a composition that
is substantially anisotropic in its electrical, mechanical and
thermal characteristics.
[0051] In some embodiments of the invention, the plurality of
carbon nanotubes form an aggregate (such as a rope or bundle)
preferably having an overall diameter less than about 10 nm. It is
also possible to cross-link the polymer molecules on two or more
adjacent SWNTs, thereby producing a cross-linked composite
material. Another option is to place the coated SWNTs in a mass of
a second polymer, whereby the coated carbon nanotubes are
encapsulated in the second polymer (such as suspended in a polymer
matrix). For example, the coated SWNT can be combined with a second
polymer and extruded through a die to form a composite
material.
[0052] Another embodiment of the invention is a method of producing
a conducting rod composition of matter. This method comprises:
associating a linear polymer with the sidewalls of a plurality of
individual single-wall carbon nanotubes; solubilizing said
plurality of single-wall carbon nanotubes in a solvent such as
water; and removing said single-wall carbon nanotubes along with
their associated polymers from said solvent to form an aggregate in
which said individual single-wall carbon nanotubes are
substantially electrically isolated from one another.
[0053] Yet another embodiment of the invention is a method of
associating a polymer with the sidewalls of a plurality of
individual single-wall carbon nanotubes, comprising: providing a
purified single-wall carbon nanotube material substantially free of
amorphous carbon; dispersing said single-wall carbon nanotube
material in a polymer by a combination of high-shear mixing and
ultrasonication; adding salt to bring the solution to a desired
concentration of salt by weight; centrifuging the solution;
decanting the solution; redispersing the material in water by
mechanical agitation; and passing the dispersion through at least
one filter. Preferably the salt is an alkali metal salt or an
alkaline earth metal salt, such as sodium chloride. Optionally, a
high gradient magnetic field (with gradients in excess of 1
Tesla/meter) can be applied to the filtered material to remove
ferromagnetic particles. In certain specific embodiments of this
method, the method further comprises dispersing said single-wall
carbon nanotube material in at least about 1% polystyrene
sulfonate, the desired concentration of sodium chloride is at least
about 10%, and further comprises centrifuging at least about 60,000
g for at least about 20 minutes.
[0054] After the SWNTs have been coated with polymer, it is
possible to remove the polymer coating from the nanotubes by
contacting them with a solvent having a low surface tension.
Suitable polymer removal solvents include chlorinated hydrocarbons
such as methylene chloride. The choice of solvent will depend in
part on the chemical nature of the polymer present on the
SWNTs.
[0055] Another embodiment of the invention is a dielectric material
comprising a plurality of single-wall carbon nanotubes, wherein
each nanotube is coated at least in part with a polymer molecule,
and wherein the coated nanotubes are supported in an
electrically-insulating material. Suitable electrically insulating
materials in which the coated nanotubes can be suspended include
poly(methyl methacrylate), polystyrene, polypropylene, nylon,
polycarbonate, polyolefin, polyethylene, polyester, polyimide,
polyamide, epoxy, and phenolic resin. In at least some embodiments,
this composite dielectric material has anisotropic electrical
properties. Preferably the circumferences of the nanotubes in this
dielectric material are on average less than one-tenth the average
length of the individual polymer molecules.
[0056] Another embodiment of the invention is a composition that
comprises a bundle of single-wall nanotubes, each nanotube being
coated at least in part with a polymer, the nanotubes being
substantially aligned along their longitudinal axis. The coated
nanotubes form part of a chemical sensor or transducer that
responds with a change of electrical and mechanical properties to
the presence of specific chemicals.
[0057] Yet another embodiment of the invention is a composite
dielectric material that comprises a continuous phase of dielectric
material and a plurality of SWNT dispersed therein. Each SWNT is
electrically isolated from adjacent SWNTs by at least a partial
coat of polymer on the exterior surface of the SWNT.
[0058] Certain detailed embodiments of the invention are described
below.
EXAMPLE 1
Suspension
[0059] Material is first purified to remove catalyst and amorphous
carbon. See "Large-scale purification of single-wall carbon
nanotubes: process, product and characterization," A. G. Rinzler,
J. Liu, H. Dai, P. Nikolaev, C. B. Huffman, F. J. Rodriguez-Macias,
P. J. Boul, A. H. Lu, D. Heymann, D. T. Colbert, R. S. Lee, J. E.
Fischer, A. M. Rao, P. C. Eklund, R. E. Smalley, Applied Physics A,
67, 29 (1998)) and provisional U.S. patent application No.
60/268,228, entitled "Purified Single-Wall Carbon Nanotubes" filed
Feb. 12, 2001, and provisional U.S. patent No. 60/284,419, entitled
"Method for Purifying Single-Wall Carbon Nanotubes and Composition
Thereof," filed Apr. 17, 2001, both of which are incorporated
herein by reference in their entirety. The SWNT material is then
dispersed in 1% polystyrene sulfonate (PSS) at a concentration of
50 mg/L by a combination of high-shear mixing and sufficient
ultrasonication to ensure that primarily individual SWNTs were
present. Enough sodium chloride is added to bring the solution to
10% NaCl by weight, and the dispersion is centrifuged (60,000 g, 20
min.), decanted, and re-dispersed in water by mechanical agitation.
This series is repeated twice with enough sodium chloride to make
5% NaCl and 3% NaCl solutions, respectively. The resulting
dispersion is passed successively through a Whatman filter, then 8
.mu.m, 3 .mu.m, and 1 .mu.m polycarbonate track-etched filters, and
finally a high-gradient magnetic field that removes the
ferromagnetic catalyst particles remaining in the suspension.
EXAMPLE 2
Removal of Wrapping (Optional)
[0060] The PSS-SWNT complexes were dissociated by ultrasonication
in concentrated phosphoric acid for 1 hour followed by washing with
water. Microprobe analysis of the resulting SWNTs showed a decrease
of sulfur content by over two orders of magnitude, to within
baseline noise. Similarly, a suspension of NMR-silent PVP-SWNTs,
upon addition of tetrahydrofuran, recovered the PVP NMR spectrum,
indicating that the polymer and SWNTs were dissociated.
EXAMPLE 3
Formation of a Simple Solid Nanotube-conducting-rod Composite
Material
[0061] PVP-wrapped SWNTs, upon centrifuging in water at 200,000 g
for 2 hours, formed a gelatinous pellet that is ca. 2% SWNTs and
2-4% PVP by weight. Examination of 10-50 .mu.m thick films between
crossed polarizers revealed large (ca. 100 .mu.m), well-defined
birefringent domains, suggesting that the material behaved
nematicly (FIG. 6). This further shows that the nanotube surface
area is uniformly covered by the associated polymer, allowing
formation of relatively large aggregates wherein the
polymer-wrapped SWNT are substantially aligned with one another.
The highly-polarized optical transmission observed is
characteristic of a conducting-rod composite.
EXAMPLE 4
Formation of a Cross-linked Nanotube Conducting-rod Composite
Material
[0062] To produce another embodiment of this invention, the
following were mixed: 0.1% by total wt. SWNTs, 1% by total wt. PVP
(50 kD), an equal amount by wt. of polyallylamine (70 kD), and 1.1
molar equivalents (with respect to PVP) of ethylene dichloride
(EDC) (coupling agent). The SWNT were already wrapped by the PVP/VA
(10:1 by wt.) when the polyallylamine was added. Shortly after, the
EDC was added. The mixture was vortexed and filtered using a 200 nm
PTFE membrane. The resulting "bucky paper" was dried at room
temperature for 2 days and examined with a scanning electron
microscope, producing the image shown in FIG. 6.
EXAMPLE 5
Preparation of Polymer-wrapped Single-wall Nanotubes
[0063] SWNTs were produced by both the laser oven methods (see A.
G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C. B. Huffman, F. J.
Rodriguezmacias, P. J. Boul, A. H. Lu, D. Heymann, D. T. Colbert,
R. S. Lee, J. E. Fischer, A. M. Rao, P. C. Eklund, R. E. Smalley,
Appl. Phys. A 67 (1998) ("Rinzler 1998") 29) and HiPco (see P.
Nikolaev, M. J. Bronikowski, R. K. Bradley, F. Rohmund, D. T.
Colbert, K. A. Smith, R. E. Smalley, Chem. Phys. Lett. 313 (1999)
("Nikolaev 1999") 91). As received, laser-oven SWNT material
(dispersed in toluene) was filtered (Whatman #41) and washed with
methanol and then water. The SWNTs were then homogenized with a
high-shear mixer (Polyscience X-520) and refiltered repeatedly
until the filtrate was clear and colorless. HiPco SWNT material was
purified by gas-phase oxidation, hydrochloric acid extraction, and
high-temperature annealing.
[0064] SWNT material from both sources was dispersed in 1% sodium
dodecyl sulfate (SDS) in water at a concentration of 50 mg/L by a
combination of high-shear mixing and sufficient ultrasonication to
ensure that primarily individual SWNTs were present, as evaluated
by AFM. See J. Liu, M. J. Casavant, M. Cox, D. A. Walters, P. Boul,
W. Lu, A. J. Rimberg, K. A. Smith, D. T. Colbert, R. E. Smalley,
Chem. Phys. Lett. 303 (1999)125. Enough polyvinyl pyrrolidone (PVP)
was added to the mixture to result in a 1% solution by weight,
which was then incubated at 50.degree. C. for 12 hours. The mixture
was passed through a 1 .mu.m track-etched polycarbonate filter.
Catalyst particles remaining from the SWNT synthesis were then
removed by passing the dispersion through a High Gradient Magnetic
Separator (HGMS). Residual SDS and polymer were removed by at least
three cycles of high-speed centrifugation (200,000 g, 2 hours),
decanting, and redispersion in pure water by mechanical agitation
including mild (ten minutes or less) ultrasonication to produce a
stable solution of PVP wrapped SWNTs (PVP-SWNTs) in water,
uniformly dispersible up to 1.4 g/L.
EXAMPLE 6
Solubilization by Polymer Wrapping
[0065] AFM images of PVP-SWNT supramolecular aggregates adsorbed
onto amine-functionalized substrates show SWNT height and length
distributions consistent with the notion that most of the complexes
consist of a single SWNT associated with at most a monolayer of
polymer, and a smaller percentage of ropes consisting of more than
one SWNT (FIGS. 3A-3D). The solutions formed are stable for months,
and easily pass through a 1 .mu.m track-etched polycarbonate filter
membrane. When the material is dried, it is easily redissolved into
pure water with minimal ultrasonication, in dramatic contrast to
dried non-polymer-wrapped SWNT material in any other solvent
system.
[0066] Solubility is a measure of an equilibrium between the
dissolved phase and the aggregated phase, so one component to
increasing solubility could be the destabilization of the solid.
The smooth, uniform interaction surface along pristine SWNTs allows
a remarkably robust van der Waals interaction between them. The
pair-wise interaction potential between parallel SWNTs has recently
been determined by a continuum model. See L. A. Girifalco, M.
Hodak, R. S. Lee, Phys. Rev. B 62 (2000) 13104. For every nanometer
of overlap between two (10,10) tubes, the binding energy is 950
meV. Thus, summed for a typical SWNT length of 100 nm embedded in a
rope, the cohesive energy is a staggering 2.9 keV. It might be
expected, therefore, that modifications to the SWNTs that disrupt
the uniform interactions along their lengths in a hexagonally
packed crystal would shift their equilibrium in solvents toward the
dissolved phase. Such modifications could be side-wall
functionalization, (see P. J. Boul, J. Liu, E. T. Mickelson, C. B.
Huffman, L. M. Ericson, I. W. Chiang, K. A. Smith, D. T. Colbert,
R. H. Hauge, J. L. Margrave, R. E. Smalley, Chem. Phys. Lett. 310
(1999) ("Boul 1999") 367) end-cap functionalization, (see J. Chen,
M. A. Hamon, H. Hu, Y. S. Chen, A. M. Rao, P. C. Eklund, R. C.
Haddon, Science 282 (1998) 95) or wrapping the SWNTs with a
polymer. In filtered papers made from non-wrapped SWNT dispersions
(FIG. 4A), the tubes self-assemble into mats of tangled, seemingly
endless ropes, wherein a majority of the tubes are in direct van
der Waals contact with other SWNTs along their entire length. See
Rinzler 1998, 29. Filtered papers of PVP-SWNTs, on the other hand,
exhibit no such large-scale structure (FIG. 4B).
EXAMPLE 7
Robust Association between Polymer and SWNTs
[0067] A modified flow field-flow fractionation (FFF) technique was
used to test the stability of polymer wrapping. 20 .mu.L of a
dissolved 0.46 g/L PVP-SWNTs were injected into the Flow FFF
instrument (Universal Fractionator Model F-1000, FFFractionation,
LLC) with zero crossflow and a channel flow of water with 0.02%
sodium azide (a bactericide). When the sample entered the
cross-flow region, the channel flow was halted and a cross-flow of
0.5 mL/min. was initiated, pinning the sample against the
accumulation membrane. The sample was washed by the crossflow
against the membrane for 40 min., after which the cross-flow was
halted and the channel flow reinitiated, allowing the sample to be
collected when it exited the Flow FFF. The final samples were
compared to the starting material by tapping-mode AFM, and found to
be unchanged within experimental error. The observed length
distribution was 137.+-.130 nm before and 167.+-.138 nm after. In
all cases, a large percentage of the tubes had heights consistent
with known individual nanotube heights plus a monolayer coating of
the polymer. No nanotubes were observed surviving this treatment in
a control experiment where the SWNTs were suspended by Triton
X-100.
[0068] During the sample preparation, the Nuclear Magnetic
Resonance (NMR) signal for the polymer disappeared after several
centrifugation/decanting- /resuspension cycles, although by
absorption spectroscopy there is still a significant amount of
polymer left in solution. We conclude that the polymer that is
tightly associated with the SWNTs is dramatically broadened by some
combination of factors, including inhomogeneities in the local
magnetic field induced by the diameter and helicity dependent
diamagnetism of the SWNTs themselves (i.e., an `antenna` effect),
the slow motion of such large objects in solution preventing
rotational averaging, and the at least partial alignment of the
SWNTs in the magnetic field. See J. Hone, M. C. Llaguno, N. M.
Nemes, A. T. Johnson, J. E. Fischer, D. A. Walters, M. J. Casavant,
J. Schmidt, R. E. Smalley, Appl. Phys. Lett. 77 (2000) 666 and B.
W. Smith, Z. Benes, D. E. Luzzi, J. E. Fischer, D. A. Walters, M.
J. Casavant, J. Schmidt, R. E. Smalley, Appl. Phys. Lett. 77 (2000)
663. Therefore, NMR spectroscopy quantifies the amount of free
polymer in solution, and by subtraction from the total amount of
polymer as determined by absorption spectroscopy, the amount of
polymer that is associated with the SWNTs is quantified. These NMR
measurements are possible due to recent developments in the
purification techniques for SWNTs, particularly with respect to
removing residual metal catalyst from the samples.
[0069] Taken together, these results are strong evidence that the
polymer and SWNT comprise a single entity that can be manipulated
as a whole, rather than solubilization by a dynamic equilibrium of
the supramolecular association.
EXAMPLE 8
Tight, Uniform Wrapping
[0070] The individual PVP-SWNTs appear by AFM to be of uniform
diameter along their lengths with heights consistent with monolayer
coverage of the SWNTs, supporting the interpretation that the
polymer is uniformly wrapped around the tubes rather than
associated with the side-walls at various points as random
coils.
[0071] PVP-SWNTs, after centrifuging in water at 200,000 g for 2
hours, formed a gelatinous pellet that was found to be ca. 2% SWNTs
and 2% PVP by weight. Examination of 10-50 .mu.m thick films
between crossed polarizers revealed large (ca. 100 gm),
well-defined birefringent domains, suggesting that the material
behaves nematicly (FIG. 5). This further supports the
interpretation that the nanotube surface area is uniformly covered
by the associated polymer. The system is almost certainly a nematic
solid, and may require the traditional addition of highly entropic
side chains to transform the material into a true liquid
crystal.
[0072] At maximum coverage, the 40 kD PVP:SWNT ratio by weight was
found to be 5:8 for HiPco material and 1:1 for laser oven material.
360 kD PVP was found to associate with the SWNTs at higher ratios,
1.7:1 and 2:1 for laser-oven material and HiPco material,
respectively. It is likely that these longer polymers are more
likely to entangle during the wrapping process, thus associating
more unwrapped polymer for a given surface-area coverage.
Furthermore, assuming the same polymer/SWNT surface area ratio as
observed in the 40 kD case, which corresponds to the simple case in
which the polymer executes a helical wrapping of the tube, covering
the tube with a monolayer of polymer (such as the illustrated forms
in FIGS. 2A and 2B), a single strand of 360 kD PVP would more than
cover the entire surface area of a single SWNT of the typical
observed length.
EXAMPLE 9
Wrapping: A General Phenomenon
[0073] SWNTs can also be successfully solubilized by wrapping with
other polymers, including polystyrene sulfonate (PSS), poly(1-vinyl
pyrrolidone-co-vinyl acetate) (PVP/VA), poly(1-vinyl
pyrrolidone-coacrylic acid), poly(1-vinyl
pyrrolidone-co-dimethylaminoeth- yl methacrylate), polyvinyl
sulfate, poly(sodium styrene sulfonic acid-co-maleic acid),
dextran, dextran sulfate, and bovine serum albumin (BSA). Although
polymers including poly(methyl methacrylate-co-ethyl acrylate),
polyvinyl alcohol, polyethylene glycol, and polyallyl amine were
initially unsuccessful in solubilizing SWNT by wrapping, subsequent
tests indicated that they can successfully solubilize SWNT by
wrapping.
[0074] Although not to be bound by theory, a rational
interpretation of the examples indicates that the wrapping of the
SWNTs by water-soluble polymers is a general phenomenon, driven
largely by a thermodynamic drive to eliminate the hydrophobic
interface between the tubes and their aqueous medium. Within this
interpretation, changing the solvent system to remove the strong
hydrophobic thermodynamic penalty should induce the PVP-SWNT
complexes to dissociate, and in fact NMR-silent PVP-SWNTs, upon
addition of tetrahydrofuran, recovers the PVP NMR spectrum.
[0075] This can be understood by estimating the dominant
thermodynamic factors in wrapping an idealized water-soluble
polymer around a SWNT. Due to the free-rotation about the backbone
bonds in the polymers studied here, they generally form random
coils in solution. The entropic cost of forcing a linear polymer
into a wrapping conformation around a nanotube can be estimated as
being at most that of restricting each polymer backbone bond to one
of its three rotational minima, which is simply
.DELTA.S=-k.multidot.ln(W)=-k.multidot.ln(3.sup.n-2)=-k(n-2)ln(3),
[0076] where n is the number of backbone carbon atoms. In an effort
to evaluate the generality of a proposed thermodynamic driving
force, we will consider the isoenergetic case, recognizing that
polymers with favorable enthalpic interactions can likely be chosen
(e.g., PVP is a close polymer analog to n-methyl pyrrolidone (NMP),
an excellent solvent for laser-oven SWNTs. See K. D. Ausman, R.
Piner, O. Lourie, R. S. Ruoff, M. Korobov, J. Phys. Chem. B 104
(2000) 8911. From the observed 40 kD PVP:SWNT mass ratio and
assuming a 1.0 nm diameter tube. See Nikolaev (1999), 91. We find
an average of 8.1 monomer units per nanometer of HiPco SWNT,
resulting in an entropic penalty of ca. 56 J/mol K. Assuming a
negligible enthalpic contribution to the free energy as discussed
above, this gives a maximum free energy penalty for polymer
conformational restriction at 25.degree. C. of 17 kJ/mol per nm of
SWNT wrapped.
[0077] Offsetting this effect is the loss of hydrophobic surface
achieved by shielding the nanotube from the water in which it is
immersed, which for SWNTs can be estimated from the surface tension
of the corresponding hydrophobic cavity. For a cavity the size of a
1.0 nm diameter nanotube, this is ca. 136 kJ/mol for each nm of
SWNT length at room temperature. Clearly, the free energy cost of
forcing the polymeric wrapping into a regular wrapping arrangement
is significantly smaller than the gain achieved by overcoming the
hydrophobic penalty between the SWNTs and their surrounding
water.
[0078] Of the successful wrapping polymers, the ionic polymers
provide an instructive contrast to the nonionic case described.
PSS, for example, solubilized SWNT material up to 4.1 g/L. However,
during the wrapping procedure, the higher ionic strength of the
solutions induced aggregation during the wrapping step as a
consequence of electric double-layer solubilization, requiring that
the dispersion and association steps take place simultaneously.
This double-layer effect could be used to good advantage, however,
in the purification step through intentional salting-out of the
ionic-polymer wrapped SWNTs, significantly reducing the required
centrifugation forces. The absorption spectroscopy/NMR technique
for quantifying the amount of wrapping polymer present was verified
in this case by electron microprobe analysis for sulfur content.
PSS-SWNTs had less-reproducible polymer to SWNT mass ratios than
did PVP-SWNTs, ranging up to 1:2 and 2:1 for laser oven and HiPco
materials, respectively. This irreducibility is a result of a lower
binding of polymer to the tubes, as evinced by the continued
removal of polymer by repeated
centrifugation/decanting/resuspension steps. Removing the PSS
wrapping by ultrasonication in concentrated phosphoric acid for 1
hour followed by washing with water reduced the sulfur microprobe
signal by over two orders of magnitude, to within baseline
noise.
EXAMPLE 10
Molecular Picture
[0079] From the key observations described, solubilization with
near-monolayer coverage of tightly-associated polymer around
individual SWNTs, a molecular-level picture of the association
geometry is suggested: helical wrapping. A single tight coil,
however, would necessarily introduce significant bond-angle strain
in the polymer backbone, enough to offset the thermodynamic drive
for wrapping described above. Multi-helical wrapping (FIGS. 2A-2C),
on the other hand, allows high surface-area coverage with low
backbone strain, where, at least locally, multiple strands of
polymer coil around the SWNT at close to their nascent backbone
curvature. Given such a picture, it is natural to expect that
successive strands of polymer wrapping would have different binding
constants, particularly when the polymer strand is charged as in
the case of PSS, explaining the lower binding of PSS relative to
PVP.
EXAMPLE 11
Biologically Relevant Conditions
[0080] PSS-SWNTs are largely solubilized by the electric
double-layer effect, making solution stability sensitive to its
ionic strength. HiPco PSS-SWNT solutions at a concentration of 11
mg/L of tubes salt out at NaCl concentrations of 20.+-.5 mM, and
MgCl.sub.2 concentrations of 2.+-.1 mM. PVP-SWNTs also salt out,
albeit at higher ionic strengths (135.+-.20 mM NaCl and 10.+-.2
MgCl.sub.2 for 11 mg/L of tubes), suggesting that these tubes also
carry a charge, similar to the dimethylformamide and
dimethylsulfoxide dispersions of SWNTs previously reported. See
Boul (1999), 367. The solution stability of PVP-SWNTs at these high
ionic strengths raises the possibility that a similar system will
be solubilized in biologically-relevant conditions. BSA and
dextrans also stably solubilized SWNTs at high ionic strengths,
more directly making the connection to the biological world.
EXAMPLE 12
Applications
[0081] The solubilization of SWNTs by the procedures described here
opens up the possibility of functionalization chemistry, both on
the tubes themselves and on the wrapping polymer, and
solution-phase separations. For example, purification of the SWNTs
from any residual catalytic material can be efficiently performed
by high gradient magnetic separation. Also, PVP-SWNTs can be
length-fractionated by gel electrophoresis. Fractions, as measured
from a maximum migration of 18 cm along a 0.5% agarose gel in a
glycine/SDS buffer and subjected to 100 V for five hours, were
investigated by AFM to reveal significant separation by length
(FIG. 7).
[0082] Single-walled carbon nanotubes have been reversibly
solubilized in water by wrapping them with a variety of linear
polymers. It was demonstrated that the association between the
polymer and the SWNT is robust, not dependent upon the presence of
excess polymer in solution, and is uniform along the sides of the
tubes. A general thermodynamic driving force for such wrapping in
an aqueous environment has been identified. This solubilization
provides a route to more precise manipulation, purification,
fractionation, and functionalization than was before possible, as
well as allowing SWNTs to be introduced to biologically-relevant
systems.
EXAMPLE 13
High Dielectric Constant material
[0083] A suspension of PVP-wrapped nanotubes was prepared according
to the procudure of example 5. After the sample was completely
reacted, it is dried for further analysis. This was done by using
the aspirator filtration apparatus with a 100 nm or less pore size
track-etched polycarbonate filtration membrane. The SWNTs were
captured on the filter membrane and form a bucky paper. This bucky
paper was allowed to air-dry to remove as much of the liquid as
possible.
[0084] The bucky paper prepared in this way with dimensions
approximately 0.8 cm by 1.2 cm and a thickness of 0.012 cm was
clamped securely between two lead/indium foil electrodes. The
electrodes were connected to a Hewlett Packard 4284A (20 Hz-1 MHz)
Precision LCR Meter. This measurement indicated a dielectric
constant of ranging between 600 and 800 over the frequency range of
1 megahertz to 500 hertz with an applied RMS ac voltage of 200 mV
applied between the foil electrodes.
[0085] From the foregoing detailed description of specific
embodiments of the invention, it should be apparent that methods
and apparatuses involving materials containing single-wall carbon
nanotubes and non-covalently derivatized single-wall carbon
nanotubes have been disclosed. Although specific embodiments and
implementation alternatives have been disclosed herein, this has
been done solely for the purposes of describing the invention in
its various aspects, and is not intended to be limiting with the
scope of the invention as defined in the claims accompanying this
application and/or a non-provisional counterpart to this
application. It is contemplated that various substitutions,
alterations, and or modifications, including but not limited to
those which may have been discussed or suggested herein, may be
made to the disclosed embodiment(s) without departing from the
spirit and scope of the invention.
* * * * *