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Description  |
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FIELD OF THE INVENTION
This invention relates generally to optically transparent conductive
polymer electrodes and films and their fabrication as fluid phase-cast
coatings, free-standing and supported films and the like. The transparent
conductive coatings and films are fabricated using functionalized protonic
acids to induce processibility of electrically conductive substituted or
unsubstituted polyaniline and blends of electrically conductive
substituted or unsubstituted polyaniline with amorphous bulk polymers
(polyblends), and to induce solubility of the electrically conductive
substituted or unsubstituted potyaniline and the polyblends in organic
liquids or fluid (melt) phases of solid polymers so as to permit the
fluid-phase processing.
BACKGROUND OF THE INVENTION
There is an established need for transparent conducting materials for use
as transparent electrodes, coatings and films. Such transparent materials
are required in a variety of device applications, including for example
the electrodes for liquid crystal displays and transparent conductive
coatings for antistatic applications. Other applications include, for
example, use as electrodes on polymer (or composite) objects to enable
electroplating and as conductive transparent films for packaging and
electrically shielding electronic goods.
Only a few materials exhibit the combination of relatively high electrical
conductivity and optical transparency. The most widely used examples are
mixed inorganic oxide materials, for example indium/tin oxide (ITO) and
other related oxides.
Although ITO has adequate properties for many uses in technology, ITO also
has many disadvantages, including the following:
(i) High vacuum technology (for example sputtering) is required for
application of transparent conducting ITO or related mixed oxides onto a
substrate. This vacuum technology is expensive, since it requires major
capital investment for the equipment needed for application. Furthermore,
application, for example by sputtering, onto curved or complex surfaces is
difficult.
(ii) The precise chemical stoichiometry and morphology required for
transparent conducting ITO or related mixed oxides is difficult to achieve
and difficult to control. As a result, the preparation of transparent
conducting ITO or related mixed oxide films is often treated as a trade
secret. Thus, the routine manufacture of transparent conducting ITO or
related mixed oxide films for transparent electrode applications requires
precise and detailed process control.
(iii) Transparent conducting ITO or related mixed oxide films are brittle.
Thus, when applied onto flexible substrates (for example onto free
standing polymer films as substrates), the coated substrate is delicate
and the transparent electrode is easily shattered.
(iv) Patterning of the transparent conducting ITO or related mixed oxides
requires etching the insoluble ITO or related mixed oxide material.
Although possible, relatively highly corrosive etching solutions are
required.
Thus, there is a need for materials which are both electrically conducting
and optically transparent, and which can be applied or fabricated directly
from fluid phases at easy-to-obtain conditions. There is also a need for
conductive devices and films which are mechanically robust and have
specific and easily controlled compositions.
There exists prior art in the area of transparent conducting coatings made
from conductive polymers:
(1) Shacklette et al (U.S. Pat. No. 4,963,206, Oct. 16, 1990) applied a
conductive polyaniline film onto Aclar by exposing the Aclar film to a
mixture of aniline tosylate and ammonium persulfate in an aqueous solution
of tosic acid. Thus the conductive polyaniline film was polymerized in
situ onto the substrate.
(2) Fukunishi et al (JP application no. 63145326, Jun. 17, 1988) used
similar techniques to prepare polymer composites by in situ polymerization
of pyrrole and aniline.
(3) Takahashi et al (JP application no. 63268733) prepared thin
semitransparent films by electrolytic polymerization.
(4) Sakai et al (JP application no. 63215772, Sep. 8, 1988) manufactured
conductive polymer compositions by polymerizing monomers capable of
forming anionic polymer electrolytes in the presence of polymers of
.pi.-conjugated structure. Transparent thin films were deposited
electrolytically.
There is no known prior art in which optical quality transparent conducting
polymer films have been Acast directly from a fluid phase (melt or
solution) in the conductive form (without need for subsequent doping) as
the pure conductive polymer or as polyblends containing the conductive
polymer.
The present invention employs polyanilines as conductive polymers. The
following is a general summary of art concerning these materials.
Kobayashi Tetsuhiko et al., J. Electroanal Chem., "Electrochemical
Reactions Concerned With electrochromism of Polyaniline Film-Coated
Electrodes," 177 (1984) 281-291, describes various experiments in which
spectroelectro-chemical measurement of a polyaniline film coated electrode
were made. French Patent No. 1,519,729, French Patent of Addition 94,536;
U.K. Patent No. 1,216,549; "Direct Current Conductivity of Polyaniline
Sulfate," M. Donomedoff, F. Kautier-Cristojini, R. ReSur-vail, M.
Jozefowicz, L. T. Yu, and R. Buyer, J. Chim. Phys. Physicohim. Brol., 68,
1055 (1971); "Continuous Current Conductivity of Macromolecular
Materials," L. T. Yu, M. Jozefowicz, and R. Buyer, Chim. Macromol., 1,469
(1970); "Polyaniline Based Filmogenic Organic Conductive Polymers,"-D.
LaBarre and M. Jozefowicz, C.R. Read Sci., Ser. C, 269, 964 (1969);
"Recently Discovered Properties of Semiconducting Polymers," M.
Jozefowicz, L. T. Yu, J. Perichon, and R. Buyer, J. Polym. Sci., Part C,
22, 1187 (1967); "Electrochemical Properties of Polyaniline Sulfates," F.
Cristojini, R.-De Surville, and M. Jozefowicz, Cr. Read. Sci., Ser. C,
268, 1346 (1979); "Electrochemical Cells Using Protolytic Organic
Semiconductors," R. De Surville, M. Jozefowicz, L. T. Yu, J. Perichon, R.
Buvet, Electrochem. Ditn. 13, 1451 (1968); "Oligomers and Polymers
Produced by Oxidation of Aromatic Amines," R. De Surville, M. Jozefowicz,
and R. Buvet, Ann. Chem. (Paris), 2, 5 (1967) "Experimental Study of the
Direct Current Conductivity of Macromolecular Compound," L. T. Yu, M.
Borredon, M. Jozefowicz, G. Belorgey, and R. Buyer, J. Polym. Sci. Polym.
Symp., 16, 2931 (1967); "Conductivity and Chemical Properties of
Oligomeric Polyaniline," M. Jozefowicz, L. T. Yu, G. Belorgey, and R.
Buyer, J. Polym. Sci., Polym. Symp., 16, 2934 (1967); "Products of the
Catalytic Oxidation of Aromatic Amines," R. De Surville, M. Jozefowicz,
and R. Buyer, Ann. Chem. (Paris), 2, 149 (1967); "Conductivity and
Chemical Composition of Macromolecular Semiconductors," Rev. Gen. Electr.,
75 1014 (1966); "Relation Between the Chemical and Electrochemical
Properties of Macromolecular Semiconductors," M. Jozefowicz and L. T. Yu,
Rev. Gen. Electr., 75, 1008 (1966); "Preparation, Chemical Properties, and
Electrical Conductivity of Poly-N-Alkyl Anilines in the Solid State," D.
Muller and M. Jozefowicz, Bull. Soc. Chem. Fr., 4087 (1972).
U.S. Pat. Nos. 3,963,498 and 4,025,463 describe oligomeric polyanilines and
substituted polyanilines having not more than 8 aniline repeat units which
are described as being soluble in certain organic solvents and which are
described as being useful in the formation of semiconductors compositions.
European Patent No. 0017717 is an apparent improvement in the compositions
of U.S. Pat. Nos. 3,963,498 and 4,025,463 and states that the polyaniline
can be formed into a latex composite through use of the oligomers of
polyaniline and a suitable binder polymer.
High molecular weight polyaniline has emerged as one of the more promising
conducting polymers, because of its excellent chemical stability combined
with respectable levels of electrical conductivity of the doped or
protonated material. Processing of polyaniline high polymers into useful
objects and devices, however, has been problematic. Melt processing is not
possible, since the polymer decomposes at temperatures below a softening
or melting point. In addition, major difficulties have been encountered in
attempts to dissolve the high molecular weight polymer.
Recently, it was demonstrated that polyaniline, in either the conducting
emeraldine salt form or the insulating emeraldine base form, can be
processed from solution in certain strong acids to form useful articles
(such as oriented fibers, tapes and the like). By solution processing from
these strong acids, it is possible to form composites, or polyblends of
polyaniline with other polymers (for example polyamides, aromatic
polyamides (aramids), etc.) which are soluble in certain strong acids and
thereby to make useful articles. "Electrically Conductive Fibers of
Polyaniline Spun from Solutions in Concentrated Sulfuric Acid," A.
Andreatta, Y. Cao, J. C. Chiang, A. J. Heeger and P. Smith, Synth. Met.,
26, 383 (1988); "X-Ray Diffraction of Polyaniline," Y. Moon, Y. Cao, P.
Smith and A. J. Heeger, Polymer Communications, 30, 196 (1989); "Influence
of the Chemical Polymerization Conditions on the Properties of
Polyaniline," Y. Cao, A. Andreatta, A. J. Heeger and P. Smith, Polymer,
30, 2305 (1990); "Magnetic Susceptibility of Crystalline Polyaniline," C.
Fite, Y. Cao and A. J. Heeger, Sol. State Commun., 70, 245 (1989);
"Spectroscopy and Transient Photoconductivity of Partially Crystalline
Polyaniline," S. D. Phillips, G. Yu, Y. Cao, and A. J. Heeger, Phys. Rev.
B 39, 10702 (1989); "Spectroscopic Studies of Polyaniline in Solution and
in the Solid State," Y. Cao and A. J. Heeger, Synth. Met. 32, 263, (1989);
"Magnetic Susceptibility of One-Dimensional Chains in Solution," C. Fite,
Y. Cao and A. J. Heeger, Solid State Commun., 73, 607 1990); "Electrically
Conductive Polyblend Fibers of Polyaniline and Poly(p-phenylene
terephthalamide)," A. Andreatta, A. J. Heeger and P. Smith, Polymer
Communications, 31, 275 (1990); "Polyaniline Processed From Sulfuric Acid
and in Solution in Sulfuric Acid: Electrical, Optical and Magnetic
Properties," Y. Cao, P. Smith and A. J. Heeger in Conjugated Polymeric
Materials: Opportunities in Electronics, Opto-electronics, and Molecular
Electronics, ed. J. L. Bredas and R. R. Chance (Kluwer Academic
Publishers, The Netherlands, 1990).
U.S. Pat. No. 4,983,322 describes solutions and plasticized compositions of
electrically conductive substituted and unsubstituted polyanilines and
methods of forming such solutions or compositions and use of same to form
conductive articles. The polyaniline materials were made soluble by the
addition of an oxidizing agent such as FeCl.sub.3. Since the resulting
compounds are charge transfer salts, highly polar solvents were required;
specifically solvents were needed with dielectric constants equal to or
greater than 25 and with dipole moments equal to or greater than 3.25.
Starting with the insulating emeraldine base form, polyaniline can be
rendered conducting through two independent doping routes:
(i) Oxidation either electrochemically (by means of an electrochemical
charge transfer reaction) or chemically (by means of chemical reaction
with an appropriate oxidizing agent such as FeCl.sub.3);
(ii) Protonation through acid-base chemistry by exposure to protonic acids
(for example, in aqueous environment with pH less than 2-3). (1)
`Polyaniline`: Protonic Acid Doping of the Emeraldine Form to the Metallic
Regime by J.-C. Chiang and Alan G. MacDiarmid, Synthetic Metals 13 193
(1986). (2) A Two-Dimensional-Surface `State` Diagram for Polyaniline by
W. R. Salaneck, I. Lundstrom, W.-S Huang and A. G. MacDiarmid, Synthetic
Metals 13, 297 (1986).
These two different routes lead to distinctly different final states. In
(i), the oxidation causes a change in the total number of .pi.-electrons
on the conjugated chain and thereby renders it conductive. In (ii), there
is no change in the number of electrons; the material is rendered
electrically conductive by protonation of the imine nitrogen sites.
A need exists for techniques and materials to facilitate the fabrication of
shaped transparent conductive polyaniline articles, especially articles
made from bulk material (conductive polyanilines and/or composites, or
polyblends of conductive polyaniline with other polymers) and films,
fibers and coatings.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to overcome the
aforementioned disadvantages of the prior art and, primarily, to provide
electrically conducting and optically transparent bodies, coatings and
films for uses such as electrodes, these materials being fabricated using
functionalized protonic acids to induce processibility of electrically
conductive polyaniline and blends of electrically conductive polyaniline
with amorphous bulk polymers. The functionalized protonic acids induce
solubility of electrically conductive polyaniline and blends of
electrically conductive polyaniline in amorphous bulk polymers, in organic
liquids and in fluid (melt) phases of solid bulk polymers and prepolymers.
It is additionally an object of the present invention to utilize the
processing advantages associated with the soluble conducting polyblends
made from polyaniline with amorphous bulk polymers to make possible
routine fabrication transparent electrodes on flat substrates and/or
substrates with complex curved surfaces.
It is additionally an object of the present invention to provide
transparent conducting bodies, coatings and films, such as electrodes
formed of conducting polyblend films made from polyaniline with amorphous
bulk polymers, said materials being mechanically robust and flexible.
It is additionally an object of the present invention to provide
transparent conducting materials formed from polyblends of polyaniline
with amorphous bulk polymers. Said polyblends are re-soluble in common
organic solvents, thus enabling the use of photolithographic techniques
for patterning the transparent material; said techniques being, for
example, routinely used in the semiconductor industry.
Additional objects, advantages and novel features of the invention will be
set forth in part in the description which follows, and in part will
become apparent to those skilled in the art on examination of the
following, or may be learned by practice of the invention. The objects and
advantages of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims. These compositions include a conductive polyaniline in
intimate admixture (solution or the like) with a substrate material. The
term "substrate" is used to indicate a range of nonconductive and
semiconducting (i.e., conductivity of 10.sup.-8 S-cm.sup.-1 or less)
organic solvents and polymers of dielectric constant below about 22.
"Polymers" include solids, melts and prepolymers (oligomers).
In one general aspect, this invention provides transparent electrodes
formed of electrically conducting substrated polymeric compositions
comprising polyaniline of film and fiber-forming molecular weight, a
substrate and functionalized protonic acid in which the counterion has
been functionalized to be compatible with the substrate. The substrate can
be nonconductive or semiconductive solid polymer. It can be this polymer
in melted (molten) form. It can separately or in addition be organic
solvent which can be partially or completely removed during processing. As
used herein, a "functionalized protonic acid" is a protonic acid,
generally denoted as H.sup.+ (M.sup.- -R.sub.p), in which the counter-ion
anionic species, (M.sup.- -R.sub.p), contains R.sub.p which is a
functional group or a connection to a polymer backbone which is chosen to
be compatible with the substrate. Typically the substrate is nonpolar or
weakly polar.
In a more specific aspect of the invention, the transparent conductor is
fabricated with polyaniline (PANi) which has been protonated to the
conducting emeraldine salt form using camphor-sulfonic acid (CSA) as the
functionalized protonic acid; said conducting polyaniline complex
(protonated polyaniline/camphor-sulfonic acid) being soluble in
meta-cresol. The conducting polyaniline complex is co-dissolved in
meta-cresol at a desired ratio, for example 5% w/w polyaniline complex to
meta-cresol, with a nonconductive bulk polymer, poly(methyl-methacrylate)
(PMMA), at a desired ratio, for example 50% w/w PMMA to meta-cresol, to
form a solution of the polyblend, polyaniline complex with PMMA. The
solution is then spin-cast onto a suitable substrate to yield an optical
quality transparent film thin conducting polyblend film electrode with
final composition 10% w/w polyaniline complex to PMMA. The concentrations
of either of the final components can be varied and controlled by changing
the concentrations in the pre-prepared solution prior to spin-casting onto
the substrate.
Specific advantages of the transparent conductor of this invention over the
prior art include the following:
(i) Because the transparent conducting body, coating or film is a stable
soluble polymer blend, the conducting transparent film can be applied by
casting from solution (for example, spin-casting, drop-casting, etc). This
can be carried out in ambient atmosphere with no need for vacuum
technology.
(ii) Since the transparent conductor is cast onto the substrate directly
from solution, said material can be cast onto complex, curved surfaces.
(iii) The precise chemical stoichiometry is pre-determined by the
concentrations of the conducting polyaniline complex and the PMMA in the
solution used for casting the electrode film. Thus, the manufacture of
transparent conducting films for electrode applications and the like is
routine.
(iii) Transparent conducting materials fabricated from the conducting
polyaniline complex/PMMA polyblends are flexible and mechanically robust.
Thus, when applied onto flexible support surfaces (for example onto free
standing polymer films) the coated surfaces are robust.
(iv) Since transparent conducting films fabricated from the conducting
polyaniline complex/polymer polyblends are re-soluble in common organic
solvents, transparent electrodes and other conductor forms can be
patterned using photo-lithographic techniques; said techniques being, for
example, routinely used in the semiconductor industry.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention will be further described with reference being made to the
accompanying drawings in which
FIG. 1 is a series of visible transmittance spectrographs for three
PANi/CSA films;
FIG. 2 is a graph of conductivity of PANi/CSA/PMMA films at various PANi
concentrations;
FIG. 3 is a series of visible transmittance spectrographs for PANi/CSA/PMMA
films;
FIG. 4 is a se | | |
