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BACKGROUND OF THE INVENTION
The invention relates to the use of polyaniline or derivatives thereof for
absorbing electromagnetic radiation, including microwaves, radar waves,
infrared waves, visible light waves, and ultraviolet waves. The invention
further relates to the use of the radiation absorbing polyaniline
compositions to modulate another electromagnetic beam. The invention also
relates to the modification of the electromagnetic response of polyaniline
compositions by chemical or electrochemical means. The invention further
relates to electronic and microelectronic devices based on the chemical
and physical properties of polyaniline and its derivatives, and the
control of the properties.
When a spectrum of radiant energy is directed into a sample of some
substances, several things may happen to the energy: (1) it may pass
through the sample with little absorption taking place and therefore,
little energy loss. (2) The direction of propagation of the beam may be
altered by reflection, refraction, or diffraction. Scattering of the beam
by particulate suspended matter may also be involved. (3) The radiant
energy may be absorbed entirely or in part. The absorption involves a
transfer of energy to the medium, and the absorption process is a specific
phenomenon related to characteristic molecular and electronic structures;
the wavelengths of certain components of the radiation may be absorbed
while others pass through essentially undisturbed, depending on the
characteristics of the substance. Components of the radiation are absorbed
if its energy matches that energy which is required to raise molecular or
ionic components of the sample from one energy level to another. Those
energy transitions may involve vibrational, rotational, or electronic
states. After it has been absorbed, that energy may be emitted as
fluorescence, utilized to initiate chemical reactions, or actually
dissipated as heat energy.
When molecules interact with radiant energy in the visible and ultraviolet
region, the absorption consists in displacing an outer electron in the
molecule, although sometimes the energy of the far ultraviolet is
sufficient to exceed the energy of dissociation of certain bonds.
The absorption of radiant energy is a highly specific property of the
molecular structure, and the frequency range within which energy can be
absorbed is specifically dependent upon the molecular structure of the
absorbing material. The smaller the energy difference between the ground
state and the excited electronic state, the lower will be the frequency of
absorption (i.e., the longer the wavelength). Chemical compounds with only
single bonds involving sigma-valency electrons exhibit absorption spectra
only below approximately 150 millimicrons. In covalently saturated
compounds containing heteroatoms, such as nitrogen, oxygen, sulfur, and
halogen, unshared p-electrons are present in addition to sigma electrons.
Excitation promotes a p-orbital electron into an antibonding sigma
orbital, such as occurs in ethers, amines, sulfides, and alkyl halides. In
unsaturated compounds absorption results in the displacement of
pi-electrons. Molecules containing single absorbing groups, called
chromophores, undergo electronic absorption transitions at characteristic
wavelengths, and the intensity of the absorption will be proportional to
the number of that type of chromophore present in the molecule. Marked
bathochromic shifts (absorption at longer wavelengths) occur when --OH,
--NH.sub.2, and --SH, for example, replace hydrogen in unsaturated groups.
It is desirable for certain applications to have a material whose radiation
absorption characteristics and index of refraction can be easily and
reversibly modulated. Various polymeric materials have been investigated
including polyacetylene, polymethylacrylonitrile, pyrazoline,
tetracyanoethylene, tetracyanonaphthoquinodimethane,
tetracyanoquinodimethane, polydiacetylene, polypyrrole,
poly(N-methyl-pyrrole), polyphenylene vinylene, and polythiophene. Some of
these polymeric materials are known to exhibit photoresponsive effects,
but the materials have deficiencies when considered for certain
electromagnetic applications. For example, polyacetylene and
polydiacetylene are nonaromatic, possess unacceptable absorption band
gaps, have limited photoresponse, are air sensitive, generally cannot be
derivatized, and are not readily soluble and therefore cannot be easily
deposited as a thin film from solution. In addition, most materials
previously investigated for electromagnetic radiation absorption are not
readily tunable, i.e., the photoresponses of the materials cannot be
reversibly modulated by an external source of energy.
Organic polymers have long been studied for electronic transport and, more
recently, for optical properties. The first organic polymers prepared were
electrically insulating with conductivities as low as 10.sup.-14 (ohms
cm).sup.-1. The insulating properties are the result of all the electrons
in the polymer being localized in the hybrid-atom molecular orbital bonds,
i.e. the saturated carbon framework of the polymer. These insulators,
which include polymers such as poly(n-vinylcarbazole), or polyethylene,
have extremely large band gaps with energy as high as 10 eV required to
excite electrons from the valence to the conduction band. Electrical
applications of insulating organic polymers are limited to insulating or
supporting materials where low weight and excellent processing and
mechanical properties are desirable.
High electrical conductivity has been observed in several conjugated
polymer or polyene systems. The first and simplest organic polymer to show
high conductivity was "doped" polyacetylene In the "doped" form its
conductivity is in excess of 200 (ohm cm).sup.-1. Although polyacetylene
was first prepared in the late 1950's, it was not until 1977 that this
polyene was modified by combining the carbon chain with iodine and other
molecular acceptors to produce a material with metallic conductivity.
Polyaniline is a family of polymers that has been under intensive study
recently because the electronic and optical properties of the polymers can
be modified through variations of either the number of protons, the number
of electrons, or both. The polyaniline polymer can occur in several
general forms including the so-called reduced form (leucoemeraldine base),
possessing the general formula
##STR2##
the partially oxidized so-called emeraldine base form, of the general
formula
##STR3##
and the fully oxidized so-called pernigraniline form, of the general
formula
##STR4##
In practice, polyaniline generally exists as a mixture of the several forms
with a general formula (I) of
##STR5##
When 0<y<1 the polyaniline polymers are referred to as
poly(paraphenyleneamineimines) in which the oxidation state of the polymer
continuously increases with decreasing value of y. The fully reduced
poly(paraphenyleneamine) is referred to as leucoemeraldine, having the
repeating units indicated above corresponding to a value of y=1. The fully
oxidized poly(paraphenyleneimine) is referred to as pernigraniline, of
repeat unit shown above corresponds to a value of y=0. The partly oxidized
poly(paraphenyleneimine) with y in the range of greater than or equal to
0.35 and less than or equal to 0.65 is termed emeraldine, though the name
emeraldine is often focused on y equal to or approximately 0.5
composition. Thus, the terms "leucoemeraldine", "emeraldine" and
"pernigraniline" refer to different oxidation states of polyaniline. Each
oxidation state can exist in the form of its base or in its protonated
form (salt) by treatment of the base with an acid.
The use of the terms "protonated" and "partially protonated" herein
includes, but is not limited to, the addition of hydrogen ions to the
polymer by, for example, a protonic acid, such as mineral and/or organic
acids. The use of the terms "protonated" and "partially protonated" herein
also includes pseudoprotonation, wherein there is introduced into the
polymer a cation such as, but not limited to, a metal ion, M.sup.+. For
example, "50%" protonation of emeraldine leads formally to a composition
of the formula
##STR6##
which may be rewritten as
##STR7##
Formally, the degree of protonation may vary from a ratio of [H.sup.+
]/[--N.dbd.]=0 to a ratio of [H.sup.+ ]/[--N.dbd.]=1. Protonation or
partial protonation at the amine (--NH--) sites may also occur.
The electrical and optical properties of the polyaniline polymers vary with
the different oxidation states and the different forms. For example, the
leucoemeraldine base, emeraldine base and pernigraniline base forms of the
polymer are electrically insulating while the emeraldine salt (protonated)
form of the polymer is conductive. Protonation of emeraldine base by
aqueous HCl (1M HCl) to produce the corresponding salt brings about an
increase in electrical conductivity of approximately 10.sup.10 ;
deprotonation occurs reversibly in aqueous base or upon exposure to vapor
of, for example, ammonia. The emeraldine salt form can also be achieved by
electrochemical oxidation if the leucoemeraldine base polymer or
electrochemical reduction of the pernigraniline base polymer in the
presence of an electrolyte of the appropriate pH. The rate of the
electrochemical reversibility is very rapid; solid polyaniline can be
switched between conducting, protonated and nonconducting states at a rate
of approximately 10.sup.5 Hz for electrolytes in solution and even faster
with solid electrolytes (E. Paul, et al., J. Phys. Chem. 1985, 89,
1441-1447) The rate of electrochemical reversibility is also controlled by
the thickness of the film, thin films exhibiting a faster rate than thick
films. Polyaniline can then be switched from insulating to conducting form
as a function of protonation level (controlled by ion insertion) and
oxidation state (controlled by electrochemical potential). Thus, in
contrast to, for example, the polypyrrole mentioned above, polyaniline can
be turned "on" by either a negative or a positive shift of the
electrochemical potential, because polyaniline films are essentially
insulating at sufficiently negative (approximately 0.00 V vs. SCE) or
positive (+0.7 V vs. SCE) electrochemical potentials. Polyaniline can also
then be turned "off" by an opposite shift of the electrochemical
potential.
The conductivity of polyaniline is known to span 10 orders of magnitude and
to be sensitive to pH and other chemical parameters. It is well known that
the resistance of films of both the emeraldine base and 50% protonated
emeraldine hydrochloride polymer decrease by a factor of approximately 3
to 4 when exposed to water vapor. The resistance increases only very
slowly on removing the water vapor under dynamic vacuum. The polyaniline
polymer exhibits conductivities of approximately 1 to 5 Siemens per
centimeter (S/cm) when approximately half of its nitrogen atoms are
protonated. Electrically conductive polyaniline salts, such as fully
protonated emeraldine salt [(--C.sub.6 H.sub.4 --NH-- C.sub.6 H.sub.4
--NH.sup.+)--Cl.sup.- ].sub.x, have high conductivity (10.sup.-4 to
10.sup.+2 S/cm) and high dielectric constants (20 to 200) and have a
dielectric loss tangent of from below 10.sup.-3 to approximately 10.sup.1.
Dielectric loss values are obtained in the prior art by, for example,
carbon filled polymers, but these losses are not as large as those
observed for polyaniline.
Polyaniline has been used to coat semiconductor photoelectrodes, to serve
as an electrochromatic display material, and to suppress corrosion of
iron.
While the preparation of polyaniline polymers and the protonated
derivatives thereof is known in the art, it is novel herein to use these
compositions for the attenuation of electromagnetic radiation,
particularly microwaves, radar waves, infrared waves, visible waves, and
ultraviolet waves. A need exists for a polymeric material which can be
designed to absorb microwaves, radar waves, infrared waves, visible waves,
and ultraviolet waves. In addition, a need exists for a method of
absorbing the electromagnetic radiation to modulate another
electromagnetic beam. A need also exists for a method for the modification
of the electromagnetic properties of polyaniline compositions by chemical
or electrochemical means.
SUMMARY OF THE INVENTION
The present invention relates to the use of polyaniline or derivatives
thereof for absorbing electromagnetic radiation, including microwaves,
radar waves, infrared waves, visible waves, and ultraviolet waves as
needed. The invention further relates to the use of the
radiation-absorbing polyaniline compositions to modulate another
electromagnetic beam. The invention also relates to the modification of
the electrical and optical properties of polyaniline compositions by
chemical or electrochemical means. The invention further relates to
electronic and microelectronic devices based on the chemical and physical
properties of polyaniline and its derivatives.
While the invention relates to both microwave responses and nonlinear
optical responses of polyaniline and its derivatives, the inventors
believe that these phenomena are of different physical origins. The
photoresponse is believed to be the result of the reorganization of
chemical bonds and to be microscopic. The time frame is believed to be
approximately 10.sup.-13 to 10.sup.-12 seconds (a rate of 10.sup.12 to
10.sup.13 Hz). The use of polyaniline compositions to achieve the
microwave attenuation of the present invention, however, is believed to be
due to a local reorganization of the electronic density on the order of
10.sup.1 to 10.sup.2 Angstroms and on a time frame of approximately
10.sup.-10 seconds. Both the photoresponse and the microwave attenuation
phenomenae are believed to be due to the absorption of electromagnetic
radiation by the pi electron systems of the polyaniline polymer and its
derivatives.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-7 illustrate alternative embodiments of the invention utilizing the
optical properties.
FIGS. 8-11 illustrate waveguides utilizing the microwave absorption
properties of the invention for absorbing microwaves propagated through
the waveguide. FIG. 12 illustrates an alternative embodiment in which a
surface is coated with a material embodying the present invention for
preventing microwave reflections from the coated material.
FIGS. 13 and 14 illustrate alternative embodiments in which a microwave
strip conductor is coated with material embodying the present invention.
FIGS. 15 and 1 illustrate microwave strip embodiments including an
electrolyte for the controlled variation of the microwave absorption
properties along the propagation axis of the microwave strip conductors.
FIGS. 17 and 18 illustrate embodiments utilizing thermally responsive films
which have materials embodying the present invention distributed within
the film.
FIG. 19 is a graphical plot illustrating the variation of the loss tangent
as a function of protonation.
DETAILED DESCRIPTION
The dielectric loss of the polyaniline polymeric compositions can,
according to the present invention, be controlled by the design of the
chemical composition of the polyaniline polymer, the oxidative state of
the polymer, and the degree of doping, including but not limited to,
protonation and pseudoprotonation of the polymer. Thus, by the addition of
electron-withdrawing or electron-donating groups to the nitrogen atoms
and/or to the C.sub.6 rings of the leucoemeraldine, emeraldine, or
pernigraniline polyaniline compositions, the dielectric loss tangent can
be varied. For example, addition of a methyl group to each C.sub.6 ring to
form poly(ortho-toluidine) leads to a dielectric loss tangent that can be
varied from 10.sup.-2 to 10.sup.0. By the present invention dielectric
loss tangents can be varied from 10.sup.-2 to approximately 20 by varying
the form of the polyaniline, the degree, site and type of substituents. In
the prior art, carbon filled silicone rubber or carbon filled epoxy paints
or carbon bonded to fabric produce non-magnetic dielectric losses at
microwave frequencies. A preferred embodiment of the present invention for
attaining maximum dielectric loss is the emeraldine salt, wherein y is in
the range of from approximately 0.4 to 0.6 and the protonation is
approximately one proton per imine nitrogen, i.e., [H.sup.+ ]/[--N.dbd.]
is equal to approximately one.
The addition of electron-withdrawing or electron-donating groups to the
polyaniline composition can facilitate the design of a polymeric material
with desired absorption and transmission bands. Known electron-donating
groups to be substituted onto the C.sub.6 ring and operative in the
present invention can include, but are not limited to, --OCH.sub.3,
--CH.sub.3, halogens (electron-donating by way of a resonance effect),
--NR.sub.2, --NHCOR, --OH, --O--, --SR, --OR, and --OCOR. These groups or
atoms possess one or more unshared electron pairs on the atom adjacent to
the ring. Known electron-withdrawing groups can include halogens
(electron-withdrawing by way of an inductive effect), --NO.sub.2, --COOH,
--COOR, --COR, --CHO, and --CN. Thus, the addition of electron-donating
groups to the rings of polyaniline augments the charge delocalization. The
added opportunities for resonance stabilization of the pi to pi* excited
stat provided by electron-donating groups causes a marked lowering in the
requirement for excitation energy, and thus a decreased frequency (longer
wavelength) of absorption. Conversely, the addition of
electron-withdrawing groups diminishes the opportunities for resonance
stabilization, causing an increase in the requirement for excitation
energy, and thus an increased frequency (shorter wavelength) of
absorption. Thus, for example, protonation of --NH.sub.2 changes it to
--NH.sub.3 +; this group no longer has an unshared pair of electrons to
participate in charge delocalization. Alteration of --OH to the ion,
--O.sup.-, provides further opportunity for participation of unshared
electrons on oxygen in charge delocalization. Thus, the change of H to
NH.sub.2 is bathochromic; NH.sub.2 to NH.sub.3.sup.+ is hypschromic; OH to
O.sup.- is bathochromic; and both of the changes, OH to OCOCH.sub.3 and NH
to NHCOCH.sub.3 (acetylation), are hypsochromic.
The electron-withdrawing or electron-donating group can be present on the
C.sub.6 rings or the nitrogen atoms of the polyaniline composition at any
desired percentage of the available sites. The electron-withdrawing or
electron-donating groups are added to the C.sub.6 ring sites or the
nitrogen atom sites by chemical techniques known to those skilled in
synthetic organic chemistry.
In this manner, a polyaniline composition is prepared which when produced
in a flexible sheet form or which is coated onto a flexible substrate can
be used to absorb electromagnetic radiation. Thus, a means of rendering an
object undetectable to electromagnetic radiation such as radar is produced
by the present invention by draping over the object the flexible
polyaniline film or the coated flexible substrate, such as a cloth fabric
or fishnet. Furthermore, by coating electromagnetic radiation-absorbing
polyaniline compositions onto fibers, and then producing woven or
non-woven fabrics from the coated fibers, cloth or clothing which is
radiation absorbing could be produced, according to the present invention.
In another embodiment, fibers of polyaniline itself or a derivative
thereof, or fibers of polyaniline copolymerized with another polymer can
be drawn or extruded and subsequently woven into electromagnetic radiation
absorbing fabric, garments, coverings, and the like. In this manner radar
absorbing clothing can be produced.
A further advantage of the present invention is that the polyaniline
compositions and derivatives thereof have, or can be designed to have,
desired processability in terms of, for example, viscosity, flexural
strengths, solubility, adhesion to substrates, crosslinking, melting
point, weight, adaptability to filler loading and the like. This is
achieved by varying as desired the degree of protonation, the state of
oxidation, and the type and degree of substituents on the polymer. Certain
substituents may be preferred for the facilitation of desired processing
parameters, such as increasing or decreasing solubility, altering
extrusion parameters (rheology), achieving a specific viscosity, and the
like. Derivatization is also useful for achieving compatibility with a
copolymer, facilitating the tunability of the polyaniline composition for
non-linear optics applications, and for specific wavelength absorption,
such as microwave attenuation or a particular photoresponse.
The polyaniline compositions useful in the present invention can be coated
by a variety of techniques onto substrates of choice. The polyaniline
polymers can be applied to substrates according to the present invention
by spray coating, dip coating, spin casting, transfer roll coating,
brush-on coating, and the like. The polyaniline polymers can also be
electrochemically deposited onto conductive substrates by known
electrochemical deposition techniques.
According to the present invention, polyaniline can also be entrained
within a matrix of, or copolymerized with, another polymer material to
thereby produce a blend or a composite. Thus, polyaniline could be
dispersed in, for example, polyethylene, polyimide, cellulose nitrate, and
the like, and also can be coated onto fibrous materials. In addition,
derivatization of the polyaniline compositions can enhance compatibility
and processability of the polymer with other polymers.
In addition, the polyaniline compositions can be cast as thin films from a
solvent solution, and the solvent evaporated to produce free standing
films. The polyaniline films can be stacked as a composite with other
polyaniline films, with films of polyaniline copolymerized with another
polymer, or with non-polyaniline polymers and/or copolymers. Depending on
the desired type and degree of substitution of the polyaniline with
various crosslinkable functional moieties, the films produced can be cured
in deeper sections, that is, thicker films or articles can also be
produced by known polymer preparation techniques. Such thicker polyaniline
materials will have some utility in certain non-linear optics
applications, but will be even more preferred in certain radiation
absorption applications, such as microwave attenuation.
Polyaniline will absorb electromagnetic radiation in the visible spectrum,
in the infrared range, and in the ultraviolet range. Thus, the present
invention further relates to a method of absorbing infrared, visible, or
ultraviolet waves comprising exposing the polyaniline to infrared,
visible, or ultraviolet waves, whereby the infrared, visible, or
ultraviolet waves are absorbed by the polyaniline. The present invention
also relates to a method for absorbing microwave radiation comprising
exposing polyaniline to microwave radiation, whereby the microwave
radiation is absorbed by the polyaniline.
Because polyaniline compositions are shown by the present invention to
absorb electromagnetic radiation, another object of the invention is
electromagnetic shielding. A thin film of polyaniline within, for example,
the walls of television sets, computers, electronic machinery, and places
for the storage of electronic data, such as computer semiconductor
memories, will effectively absorb continuous and intermittent
electromagnetic radiation from wires, coils, cathode tubes, etc.
Protection against unwanted or unknown electronic surveillance of rooms
can be achieved by the application of polyaniline to the walls, floor, and
ceiling. Similarly, electrical wires can be shielded by the incorporation
of a layer of polyaniline material into the plastic insulator coating on
the wires with the advantage of grounding and static free property.
In addition, polyaniline can be used to make a remote thermal switch by
exposing the polyaniline composition to microwave radiation. The
polyaniline composition absorbs the radiation, which heats up the
polyaniline, which in turn, can trigger a thermocouple placed in contact
with the polyaniline composition. Upon removal of the source of
microwaves, the polyaniline composition will cool and cause the
thermocouple to switch back. By this manner a thermal switch is produced.
Thus, the present invention relates to a composition for absorbing
electromagnetic radiation, wherein said electromagnetic radiation
possesses a wavelength generally in the range of from about 1000 Angstroms
to about 50 meters, wherein said composition comprises a polyaniline
composition of the formula I, above, or a protonated salt thereof, where y
is in the range of approximately 0.2 to 0.8, and the degree of
protonation, i.e., x=[H.sup.+ ]/[--N.dbd.], varies from x=0 through x=1.
The instant invention further relates to a method of applying heat to a
substrate, said method comprising the steps of:
(a) applying to a substrate a microwave radiation-absorbing polyaniline
composition, or a partially protonated salt thereof;
(b) exposing the microwave radiation-absorbing polyaniline composition, for
example, a partially protonated salt thereof, to microwave radiation,
whereby the microwave radiation-absorbing polyaniline composition, or the
partially protonated salt thereof, absorbs the microwave radiation,
resulting in the generation of thermal energy within the polyaniline
composition. This heat can be localized, transferred from the polyaniline
composition or the salt to a substrate and utilized to accomplish desired
results, such as, but not limited to, joining of materials. Thus, two
materials which have been placed in contact or close proximity with each
other and in contact with a polyaniline composition can be adhered to each
other upon the exposure of the polyaniline composition to sufficient
microwave radiation to heat and thus melt or at least soften at least one
of the materials to enable fusing. The frequency, duration and/or
intensity of the microwave radiation necessary to achieve the desired
adhesion of the two materials will vary depending on the nature of the
materials to be adhered and on the degree and type of protonation and/or
substitution, if any, on the polyaniline. The preferred frequency of the
microwave radiation to be absorbed by the polyaniline compositions to
thereby induce localized heating is from about 10.sup.9 Hz to about
10.sup.11 Hz. The polyaniline composition can be applied to one or both of
the materials in any pattern, such as a grid pattern, stripes, spots, or
the like as desired. The polyaniline can be applied via solution coating,
adhesion of films, vapor deposition, extrusion of gels containing
polyaniline, and other known application techniques.
In a preferred embodiment of the present invention directed toward the
adhering of two or more materials by the absorption of microwave radiation
by polyaniline, at least one of the materials to be adhered is a plastic.
In another embodiment of the present invention one of the materials to be
adhered is a silicate-containing material, such as, for example quartz or
glass. In this manner, a plastic can be adhered to a glass fiber, such as
an optical fiber, by means of exposure of the polyaniline to microwave
radiation.
According to the present invention, polyaniline compositions can also be
utilized to absorb radar waves possessing a wavelength in the general
range of from about 0.01 cm to about 100 cm. The absorption of radar waves
by the polyaniline composition would assist in rendering objects coated
with the polyaniline composition relatively invisible to radar detection.
Therefore, the instant invention further relates to a method for absorbing
radar waves comprising exposing a polyaniline composition or a partially
protonated salt thereof to radar waves whereby the polyaniline composition
or the salt thereof absorbs at least some of the radar waves. The
invention further relates to a method for reducing the detectability by
radar of an object comprising applying to the object a polyaniline
composition or a partially protonated salt thereof in an amount sufficient
to absorb at least some, and preferably all, radar radiation to which the
object may be exposed.
In a preferred embodiment of the method for reducing the detectability by
radar of an object it is desirable to coat the object in such a way as to
produce a gradient of absorption to minimize reflectance. Such a gradient
of polyaniline material can be achieved by varying the degree of
protonation of the polymer or the degree of substitution on either the
C.sub.6 ring or the nitrogen atoms or both with a chemical substituent
such that an incoming radar beam first encounters a polyaniline
composition with little or no protonation, i.e., a material with limited
absorption of radiation. As the beam further advances along the gradient
of polyaniline material covering the object, the beam encounters
polyaniline polymer with continually increasing degrees of protonation,
and hence increasing degrees of electromagnetic absorption. In this
manner, little or no reflection of the beam is produced and the object is
not detectable by a radar wave reflection.
The present invention further relates to a method of electrochemical
switching of the polymeric state of a polyaniline composition. By
contacting the polyaniline composition with an electrolyte,
electrochemical switching of the polymeric state can be significantly
accelerated, being accomplished on a time scale of approximately 10.sup.-5
seconds. By contacting the polyaniline composition with a solid
electrolyte, electrochemical switching of the polymeric state can be even
further accelerated, being accomplished on a time scale of less than
approximately 10.sup.-7 seconds. For electromagnetic radiation absorption,
such as the absorption of microwave radiation, electrochemical switching
of the polymeric state can turn the polymeric material from radiation
transparent to radiation absorbing, or vice versa, depending on the nature
and direction of the electrochemical switching. For non-linear optics,
electrochemical switching can change the important absorption and/or
transmission bands for the probe and modulator beams, such as, for
example, in switching from the emeraldine base form to the emeraldine salt
form of polyaniline. The range of the absorption bands for the base and
the salt can be shifted bathochromically (i.e., shifted to longer
wavelengths) or hypsochromically (i.e., shifted to shorter wavelengths) as
may be desired according to the characteristics of the available probe
beam, the available modulator beam, or the available detector or sensor,
or any combination thereof.
Polyaniline compositions can also be used according to the present
invention as a photoactive switch by manipulation of the index of
refraction of the polyaniline compositions. Because of the extremely rapid
photoresponse of the polyaniline polymer, it is therefore useful according
to the present invention in nonlinear optical devices. The time dependence
of the photo bleaching of the polymer is on the order of picoseconds. For
example, the application of a laser beam of wavelength 6250 Angstroms (2.0
eV) to polyaniline polymer produces significant photoinduced bleaching
(i.e., increased transmission) in broad energy bands of 8,265 Angstroms to
4,590 Angstroms (approximately 1.5 eV to 2.7 eV) and again at 3,760
Angstroms to 2,880 Angstroms (approximately 3.3 eV to 4.3 eV).
Simultaneously laser beam photoinduced absorption (i.e., decreased
transmission) for polyaniline occurs at 24,800 Angstroms to 8,265
Angstroms (approximately 0.5 eV to 1.5 eV) and from 4,590 Angstroms to
3,760 Angstroms (2.7 eV to 3.3eV). Photoinduced absorption and bleaching
occur in polyaniline compositions in less than 10.sup.-12 seconds. These
photoinduced changes in absorption correspond to changes in the index of
refraction at these wavelengths. These changes in optical constants have
broad application in nonlinear optical signal processing and optical
communications, which according to the present invention, are useful as
means to switch, modulate, multiplex, focus, and provide optical
bistability for commercial systems.
Polyaniline is therefore useful in nonlinear optical signal processing
according to the present invention. For example, a thin film coating of
polyaniline can be applied to a phototransmissive substrate. In one
embodiment of the present invention, a probe beam of light of a given
wavelength is then propagated through the noncoated side of the substrate
onto the coating at the critical angle to the polyaniline such that the
probe beam is wave-guided in the phototransmissive substrate. To activate
the desired switching property of the polyaniline coating, a pump beam of
light, also called a modulator beam, of a different wavelength or some
wavelength is applied to the coating through the coated or noncoated side
of the substrate at a second angle such that the index of refraction of
the polyaniline composition is changed by the absorption by the
polyaniline of the electromagnetic radiation of the modulator beam. The
wavelength of the modulator beam can vary widely, but is preferably within
the range of from about 12,100 Angstroms (1.5 eV) to about 21,775
Angstroms (2.7 eV). The change in the refractive index of the polyaniline
composition coating alters the transmissive property of the polyaniline
and allows the probe beam to be refracted or otherwise modified by the
polyaniline coating. This refraction or other modification of the probe
beam can, for example, be used to trigger a photocell, initiate or
terminate an optical signal, encode information on the probe beam, or the
like. By these means is produced a low cost, stable means of optical
signal processing.
In an alternative embodiment, the beam to be modulated is refracted by the
phototransmissive substrate and reflected off the polyaniline coating on
the backside of the substrate such that the beam is then reflected
repeatedly between the front side of the substrate and the polyaniline
coated back side of the substrate. This reflection continues within the
phototransmissive substrate until the modulating beam is caused to impinge
on the polyaniline coating, whereby the index of refraction of the
polyaniline coating is altered by the absorption of the electromagnetic
radiation of the modulator beam, altering the propagation of the probe
beam. In this manner the polyaniline coating has acted as a switch which
is reversibly controlled by the presence of the pump or modulating beam to
increase or decrease the modulation (both intensity and direction) of the
probe beam. Because of the very rapid photoresponse rate of the
polyaniline polymer, the refractive index can be altered at gigahertz to
terrahertz rates, thereby providing a method for the rapid modulation of
optical data signals.
In yet another preferred embodiment, the beam to be modulated is caused to
impinge upon a thin coating of polyaniline which is on a phototransmissive
substrate. A portion of the beam is reflected, the remainder refracted,
transmitted, and partly absorbed Application of a modulator beam at a
second angle changes the index of refraction of the polyaniline thereby
altering the direction and the percentage of the probe beam transmitted
and reflected. The preferred embodiment has the probe beam incident on the
polyaniline at the critical angle and the modulator beam preferably of
wavelength between 12,100 Angstroms (1.5 eV) and 21,800 Angstroms (2.7
eV).
Thus, the present invention further relates to a method of changing the
refractive index of polyaniline comprising the steps:
(a) applying polyaniline to a phototransmissive substrate;
(b) applying a first beam of light of wavelength x at the critical angle y
to the polyaniline surface; and
(c) applying a second beam of light of wavelength z to the polyaniline
surface, whereby the second beam is absorbed by the polyaniline changing
the index of refraction of the polyaniline, whereby the transmission of
the first beam through the phototransmissive substrate is altered. The
preferred wavelength x of the first or probe beam of light is dependent on
the form of polyaniline utilized. For emeraldine base polymer, the
preferred wavelength x of the first or probe beam of light is in one or
more of the ranges of approximately 0.6 eV to 4.2 eV; 0.8 to 1.1 eV; 1.6
to 2.4 eV; 2.8 to 3.2 eV; and 3.4 to 4.3 eV. The preferred wavelengths
will vary depending on the degree of protonation of the polyaniline
polymer and the nature of the substituents, if any, on the polymer. For
the emeraldine base polymer, the preferred wavelength z of the second or
modulating beam of light is in the range of approximately 1.7 eV to 2.7
eV. The preferred wavelength of the second or modulating beam is
determined by the oxidation state, protonation level, and substituents of
the polymer. For the leucoemeraldine polymer the preferred wavelengths of
the probe beam are in the range of 24,800 Angstroms to 8,265 Angstroms
(0.5 to 1.5 eV) and 4,590 Angstroms and 3,760 Angstroms, with greater
preferred modulator beam wavelength of 3,760 Angstroms to 2,880 Angstroms.
For pernigraniline, the preferred probe and modulator wavelength are
similar to emeraldine.
The photoswitching phenomenon can, according to the present invention, also
be used to couple a light signal from one optical fiber to another optical
fiber. The two optical fibers are positioned in close contact with each
other and with a thin film of polyaniline composition between them. The
polyaniline composition is then exposed to a modulating beam. The
modulating beam changes the index of refraction of the polyaniline such
that "crosstalk" between the two optical fibers is obtained. This allows
the optical signal within either of the optical fibers to be coupled to
the other fiber as desired, but without permanent physical alteration of
either fiber. In addition, the coupling can be turned on and off as
desired by the manipulation of the index of refraction and, because of the
very rapid photoresponse rate of the polyaniline polymer, the refractive
index can be altered and coupling achieved at gigahertz to terrahertz
rates.
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