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Description  |
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BACKGROUND OF THE INVENTION
Conductive polymers have been widely investigated due to growing interest
in their use in, e.g., anti-static coatings, conductive paints,
electromagnetic shielding, electrode coatings and the like. Polyaniline is
a relatively air-stable conductive polymer which has previously been
prepared by oxidative polymerization of aniline monomer in aqueous media
by ammonium persulfate or potassium dichromate. The use of doped, i.e.,
protonated, polyaniline has been limited by its intractability, i.e., it
decomposes without melting and is essentially insoluble. Undoped
polyaniline has low solubility in polar solvents such as
dimethylformamide, dimethyl sulfoxide, n-methylpyrrolidinone, and also in
80 percent by weight formic acid or acetic acid (see Angelopoulos et al.,
Mol. Cryst. Liq. Cryst., 1988, 160. 151-163). However, the undoped
polyaniline is not electrically conductive.
Recently, another conductive polymer, polypyrrole, has been prepared in a
colloidal or latex form by the use of water-soluble polymers, such as
poly(vinyl alcohol-co-acetate), poly(vinyl pyrrolidone), and methyl
cellulose, as polymeric surfactants or steric stabilizers.
Accordingly, it is an object of this invention to provide a processable
electrically conductive polymer composition containing a polymerized
amino-substituted aromatic monomer such as aniline.
It is a further object of this invention to provide a processable
electrically conductive latex polymer composition.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the
purposes of the present invention, as embodied and broadly described
herein, the present invention provides processable electrically conductive
polymer compositions including: colloidal particles of an oxidized,
polymerized amino-substituted aromatic monomer; a stabilizing effective
amount of a random copolymer containing amino-benzene type moieties as
side chain constituents; and dopant anions. -n one embodiment of the
invention, the processable electrically conductive polymer composition
includes: colloidal particles of oxidized, polymerized aniline monomer; a
stabilizing effective amount of a random copolymer selected from the group
consisting of a copolymer of a 2-, 3- or 4-vinyl pyridine monomer and
para-aminostyrene monomer, or a partially esterified copolymer of vinyl
alcohol and vinyl acetate, said vinyl copolymer partially esterified with
para-aminobenzoate; and dopant anions. The dopant anions comprise chloride
ions.
The present invention further provides dispersions of electrically
conductive colloidal polymer particles including: a suitable dispersion
medium; and, electrically conductive colloidal polymer particles
comprising an oxidatively, polymerized amino-substituted aromatic monomer,
a stabilizing effective amount of a random copolymer containing
amino-benzene type moieties as side chain constituents, and dopant anions.
The present invention still further provides a process of preparing
processable electrically conductive polymer compositions by oxidatively
polymerizing under mild conditions a polymerizable amino-substituted
aromatic monomer in an acidic aqueous reaction medium, the acidic aqueous
reaction medium including a suitable oxidant for the aromatic monomer and
a stabilizing effective amount of a random copolymer containing
amino-benzene type moieties as side chain constituents.
DETAILED DESCRIPTION
The present invention concerns processable electrically conductive polymer
compositions and processes of preparing such compositions. An oxidative
polymerization reaction is employed in the preparation of the conductive
polymers of this invention. Such a reaction can be performed on a variety
of oxidatively polymerizable monomers. Suitable oxidatively polymerizable
monomers include amino-substituted aromatic monomer compounds, such as
aniline and substituted anilines. The substituent groups (other than with
amino groups) may include alkyl, aryl, aralkyl, alkaryl, hydroxy, methoxy,
chloro, bromo, and nitro groups. The substituent groups can be selected
consistent with the desired polymerization conditions and the desired
properties of the resultant polymer composition. Aniline is particularly
preferred as the amino-substituted aromatic monomer.
By "processable" is meant that the electrically conductive polymer
compositions have good film-forming characteristics and can be processed
by conventional coating techniques.
The processable electrically conductive polymer compositions include a
stabilizing effective amount of a random copolymer containing
amino-benzene type moieties as side chain constituents. While not wishing
to be bound by any particular theory, it is believed that the random
copolymer functions as a steric stabilizer for the conductive polymer,
e.g., the polyaniline, formed in the oxidative polymerization process,
thereby preventing precipitation and resulting in a colloidal polyaniline
composition. It is further believed that the copolymer functions by
chemically grafting onto the polyaniline particles. The random copolymers
generally contain only a minor fraction or mole percentage of the
amino-benzene type moieties, i.e., from about to about 10 mole percent,
although inclusion of greater mole percentages may be possible. The random
copolymer can be, e.g., a copolymer of a vinyl pyridine and an
amino-substituted styrene, preferably para-aminostyrene, or can be a
partially esterified vinyl alcohol polymer or a partially esterified
copolymer of vinyl alcohol and vinyl acetate, the partial esterification
being with para-aminobenzoate.
By "latex" or "colloidal" is meant that the polymer composition forms
particles having dimensions of from about 30 to about 500 nanometers which
remain relatively uniformly dispersed or suspended throughout the
particular dispersion medium.
The ratio of the stabilizing copolymer to the amino-substituted aromatic
monomer in the resultant product can vary from about 10/90 to about 25/75
by weight.
The stabilizing effective amount of the random copolymer containing
amino-benzene type moieties as side chain constituents in the
polymerization reaction medium will vary with the choice of particular
stabilizing polymer. Generally, from about 0.4 to about 1.2 g of the
copolymer per 100 ml of reaction medium will prevent precipitation of the
conductive polymer and yield the colloidal composition. Lesser amounts may
not prevent agglomeration, while greater amounts may result in excess
copolymer being included in the colloidal particles and adversely
affecting conductivity.
The oxidant used in the oxidative polymerization process must be compatible
with the random copolymer containing the amino-benzene type moieties as
side chain constituents and capable of oxidatively polymerizing the
amino-substituted aromatic monomer. By "compatible" is meant that the
combination of the oxidant and the stabilizing copolymer does not result
in the formation of significant insoluble precipitate within the reaction
medium in which the polymerization reaction is conducted. By "significant"
is meant that whatever insoluble precipitate forms may be easily filtered
off without adversely reducing the yield of conductive polymer
composition. Further, the oxidant must be a relatively mild oxidant, i.e.,
the oxidant must not polymerize the aromatic monomer at a rate whereat
precipitation may result even in the presence of the stabilizing
copolymer. It may be possible to increase the level of the copolymer to
allow stronger oxidants, but this may result in decreased conductivity in
the resultant polymer composition due to the additional amount of
copolymer. Potassium iodate is a preferred oxidant for the combination of
aniline monomer and the like with the amino-benzene type moiety containing
copolymers. Other useful oxidants for the present invention may include
well-known oxidants for aniline such as ammonium persulfate and ferric
chloride depending upon suitability with the particular stabilizing
polymer.
The concentration of oxidant in the reaction medium can vary. Generally,
the oxidant concentration will be from about 0.01 to 0.1 molar.
Preferably, the oxidant concentration is about 0.04 molar.
The reaction medium for the oxidative polymerization reaction is an acidic,
aqueous medium. Generally, any strong mineral acid such as hydrochloric
acid, sulfuric acid, and nitric acid can be used to provide the acidic
medium. However, nitric acid and sulfuric acid are less preferred than
hydrochloric acid due to their non-volatility. Hydrochloric acid is the
preferred acid and provides a sufficiently acidic medium during the
preparation stage while its volatility provides advantages during
subsequent processing stages. The concentration of acid in the reaction
medium is generally from about 0.5 to about 2.0 molar, more preferably
from about 1.0 to about 1.25 molar. At lower concentrations of acid, the
reaction rate is generally too low resulting in lower yield, while higher
concentrations of acid can result in degradation of the conductive polymer
and reduce conductivity.
The reaction medium may also contain organic solvent materials as long as
such organic solvent materials are water-miscible and compatible with the
oxidant, the stabilizing copolymer and the amino-substituted aromatic
monomer. Suitable organic solvent materials may include dimethylformamide,
dimethyl sulfoxide, and alcohols such as methanol or ethanol.
The temperature of the reaction medium is typically room temperature, i.e.,
about 20.degree. C., although the reaction may be conducted at higher or
lower temperatures if desired. The choice of temperature can depend upon
the solubility of reactants and products at the particular temperature.
As a result of oxidizing the amino-substituted aromatic monomer, e.g.,
aniline, in a hydrochloric acid medium, chloride ions from the HCl medium
are incorporated as dopant anions in the resultant electrically conductive
colloidal polymer composition.
After the oxidative polymerization reaction is completed the resultant
conductive colloidal polymer particles can be separated from the reaction
medium, e.g., by centrifugation and decantation of the liquid. The
colloidal conductive particles, e.g., colloidal polyaniline particles, can
be redispersed in a dispersion medium such as de-ionized water, or dilute
HCl solutions. The redispersion can be accomplished by the use of, e.g.,
ultrasonics. Generally, the dispersion medium may be any solvent in which
the stabilizing copolymer is soluble.
Pressed pellets prepared from the conductive polymer particles have
solid-state conductivities as high as from about 0.5 to about 2.0 siemens
per centimeter (Scm.sup.-1), these values despite the presence of the
stabilizing copolymer in the conductive composition. Conductivities may be
reduced at higher values for the stabilizing copolymer:aromatic monomer
ratio.
A surprising feature of those conductive latex polymer compositions
including the vinyl pyridine-containing stabilizing random copolymers is
that they exhibit a reversible base/acid-induced
flocculation-restabilization behavior. That is, the dispersed colloidal
conductive polymer particles can be flocculated by addition of sufficient
base to result in a pH of greater than about 4 or 5 and then redispersed
or restabilized by addition of an acid to shift the pH back to less than
about 3 or 4. While not wishing to bound by the present explanation, it is
believed that this reversible behavior is related to the pyridine in the
stabilizing copolymer. Upon the addition of base, aggregation is believed
to result from the deprotonation of the pyridine and a resultant decrease
in the solubility of the stabilizing copolymer. Addition of acid, then
results in the reprotonation of the pyridine and a resultant increase in
the solubility.
Another surprising feature of the conductive latex polymer compositions of
the present invention is the morphology of the colloidal particles. The
microscopic polyaniline-based particles have a nonspherical "rice-grain"
morphology when viewed under a transmission electron microscope. That is,
the particles have a length of about 120 nanometers (nm) with a standard
deviation of 20% and a width of about 60 nm, or an aspect ratio of about
2:1.
In the preparation of the processable electrically conductive polymer
compositions of the present invention, the polymerizable amino-substituted
aromatic monomer is added to an acidic aqueous reaction medium containing
a suitable oxidant for the amino-substituted aromatic monomer, e.g.,
potassium iodate, and a stabilizing effective amount of a random copolymer
containing amino-benzene type moieties as side chain constituents. The
admixture is stirred at room temperature for a sufficient time to complete
the reaction and the resultant conductive colloidal polymer particles
separated and purified by means well known to those skilled in the art.
The present invention is more particularly described in the following
examples which are intended as illustrative only, since numerous
modifications and variations will be apparent to those skilled in the art.
EXAMPLE A
A copolymer of 4-vinyl pyridine and para-aminostyrene was prepared under an
inert atmosphere as follows: A reaction vessel equipped with heating means
and stirring means was charged with 150 milliliters (ml) of ethanol and
heated to reflux (about 60.degree. C.) Para-aminostyrene (3.0 ml) and
4-vinyl pyridine (30.0 ml) were quickly added to the refluxing solvent,
followed by addition of 0.15 gram (g) of 2,2'-azobisisobutyrolnitrile
(AIBN) to the admixture. The admixture was then refluxed at about
70.degree. C. with stirring for 49 hours. The resultant copolymer was
precipitated by addition of excess n-hexane. The solvents were poured off,
the precipitate dissolved in methanol, and reprecipitated by addition of
water. Purification was completed by repeated dissolving in 1,4-dioxan,
followed by addition of water to reprecipitate. The resultant polymer had
a viscosity average molecular weight of about 105,000 and contained from
about 5 to about 6 mole percent amino-benzene type moieties as determined
by NMR.
EXAMPLE B
A copolymer of 2-vinyl pyridine and para-aminostyrene was prepared in a
similar manner to example A. The resultant polymer had a weight average
molecular weight of about 77,000 as determined by gel permeation
chromatography (GPC).
EXAMPLE C
A copolymer of 2-vinyl pyridine and para-aminostyrene was prepared in a
similar manner to example A with the exception that toluene was used
instead of ethanol and only 2.80 ml of para-aminostyrene was used. The
resultant product was purified by allowing the toluene to evaporate,
redissolving the precipitate in dioxin and reprecipitating with water. The
resultant polymer had a number average molecular weight of about 21,500
and a weight average molecular weight of about 54,000 as determined by
GPC.
EXAMPLE D
A modified polyvinyl alcohol was prepared as follows: A copolymer (20.00 g)
of vinyl alcohol (88%) and vinyl acetate (12%) having an average molecular
weight of about 125,000 (available from Aldrich) was dissolved in 300 ml
of dimethyl formamide and heated to about 130.degree. C. To this admixture
was added first 2.5 ml of triethylamine, followed by 3.01 g of
4-nitrobenzoyl chloride. This admixture was heated for about 19 hours.
About 16 ml of phenylhydrazine was then added dropwise to the reaction
solution and the solution maintained at 130.degree. C. for 21 hours. The
resultant product was twice precipitated in a dimethyl formamide/acetone
mixture, redissolved in water and finally reprecipitated in
tetrahydrofuran. The purified product had a weight average molecular
weight of about 40,000 as determined by GPC and contained from about 1 to
about 2 mole percent amino-benzene type moieties as determined by NMR.
EXAMPLE E
Another modified polyvinyl alcohol was prepared similarly to that of
example D with the following changes in amounts: 10 g of the vinyl alcohol
copolymer; 1.5 g of 4-nitrobenzoyl chloride; 8 ml of phenylhydrazine; and
no triethylamine.
EXAMPLE 1
A reaction vessel was charged with 100 ml of 1.25 molar hydrochloric acid,
0.50 g of the copolymer from example A, and 0.90 g of potassium iodate.
The admixture was stirred at room temperature as 1.00 ml of aniline was
added. Stirring was continued for about 44 hours. The resultant admixture
was centrifuged at 10,000 rpm to separate the colloidal polymer particles.
A pressed pellet of the dried particles had a conductivity of about 0.5
siemens per centimeter (Scm.sup.-1). Analysis of the colloidal polymer
particles gave the following composition: %C, 46.80; %H, 4.82; %N, 8.31;
%Cl, 13.06; %0, 10.30; and %1, 14.60. While not completely understood, the
presence of the iodine may be explained in a variety of ways. The iodine
may be present as iodide ions and be a codopant anion with the chloride
anions. Alternatively, the iodine may be substituted on the aniline ring,
complexed with the aniline ring, complexed with the pyridine group, or
encapsulated within the stabilizing polymer.
EXAMPLE 2
A reaction vessel was charged with 100 ml of 1.25 molar hydrochloric acid,
0.50 g of the copolymer of Example B, and 0.90 g of potassium iodate. The
admixture was stirred at room temperature as 1.00 ml of aniline was added.
Stirring was continued for about 72 hours. The resultant dark-green
colloid was centrifuged to separate the colloidal particles. Analysis of
the colloidal polymer particles gave the following composition: %C, 45.55;
%H, 4.50; %N, 7.88; %Cl, 10.10, %O, 9.58; and %I, 20.60. As in Example 1,
the presence of the iodine may be explained in a number of ways.
EXAMPLE 3
Two additional runs in the manner of example 1 were completed using (a)
0.51 g of the copolymer from example E and (b) 1.00 g of the copolymer of
example C respectively. The resultant colloidal product of run (a) was
centrifuged at 4,500 rpm for 60 minutes and then redispersed by
ultrasonics in water. The dispersion was then spread onto a substrate and
allowed to dry 25.degree. C. overnight under ambient conditions. The
resultant film exhibited a conductivity of about 2.0 Scm.sup.-1. The
product of run (b) was also a colloidal dispersion.
EXAMPLE 4
A reaction vessel was charged with 100 ml of 1.25 molar hydrochloric acid,
0.75 g of the copolymer of Example D, and 0.90 g of potassium iodate. The
admixture was stirred at room temperature as 1.00 ml of aniline was added.
Stirring was continued for about 91 hours, followed by centrifuging to
remove the bulk of the liquids. The resultant polymer composition was
redispersed in water and filtered to remove minor insoluble material. This
dispersion was used to successfully cast conductive polymeric films onto
plastic substates.
The results of the present examples demonstrate that a processable
conductive colloidal polymer composition can be prepared by utilizing
appropriate stabilizing polymers such as the random copolymer of vinyl
pyridine and para-animostyrene and poly(vinyl alcohol-co-vinyl
acetate-co-vinyl para aminobenzoate).
Although the present invention has been described with reference to
specific details, it is not intended that such details should be regarded
as limitations upon the scope of the invention, except as and to the
extent that they are included in the accompanying claims.
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