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FIELD OF THE INVENTION
The present invention relates to conductive polymers and their use in
electronic applications. More particularly, the present invention relates
to the preparation of polypyrrole and its use in preparing printed circuit
boards and through hole plating. Even more particularly, the present
invention relates to photosensitive solutions of pyrrole and direct
metallization processes for preparing electronic circuits on
non-conducting substrates.
BACKGROUND OF THE DISCLOSURE
The trend toward miniaturization, integration and automated assembly in the
electronics industry is forcing designers to continually increase the
component density in integrated circuit manufacturing, interconnection and
packaging. Current demand for increasingly complex PWBs has resulted in
increasingly stringent requirements for all production steps. To produce
high-quality boards at competitive prices means keeping production costs
down. This in turn means less consumption of environmentally toxic
chemicals, reduced number of manufacturing steps, shorter process times,
and a greater need for automation.
The introduction of double-sided, followed by multilayer boards, was
achieved by metallization of plated through-holes with electroless copper.
For the last 25 years, 98 percent of the PWBs manufactured used this
technology. However, electroless deposition of copper requires a potent
reducing agent, such as formaldehyde--a reported carcinogen. Most
electroless copper solutions contain cyanide and chelating agents, which
are difficult to remove from waste streams. Besides the normal drag-out
associated with wet processing, "bail-out" (required to maintain solution
balance and periodic bath changes) renders waste treatment of electroless
copper far more expensive than electroplated copper. Stripping copper from
racks and tanks with nitric acid is another environmental and waste
treatment concern associated with electroless copper.
In the conventional subtractive plated-through-hole (PTH) process, copper
foil is laminated onto an insulating substrate (typically polyimide,
epoxy-fiberglass, etc.). Holes are drilled through the copper-clad
laminate to allow insertion of components. Then the typical smear and
etch-back process uses an alkaline permanganate solution followed by a
hydrofluoric acid solution to remove resin smear and glass fibers from the
walls of the holes in preparation for the plating process.
In the conventional process a seed or catalyst, usually a noble metal salt,
is then applied to the circuit board. Next, by means of electroless copper
deposition about 10-20 microns of copper is deposited on the surfaces of
through-hole walls, providing electrical continuity from one side of the
panel to the other. Electroless copper deposition is a seven-step process
with interval rinses with water that become contaminated with copper
sulfate/EDTA/formaldehyde bath components. Following electroless copper
deposition, copper is electrodeposited over the entire board surface and
sensitized walls of through-holes, usually to a thickness of 0.001 in.
A negative-, or plating-resist, pattern is then applied and registered to
both sides of the material. Resist covers all areas of the foil where base
copper conductor is not required, and the surplus conductor will
subsequently be etched off. The panels are imaged in preparation for the
actual circuitry pattern by a conventional photolithographic process. In
this process photoresist is applied as a thin film to the substrate and is
subsequently exposed in an image-wise fashion through a photomask. The
mask (Mylar) is then removed. The areas in the photoresist that are
exposed to light are made either soluble or insoluble in a specific
solvent termed a developer. In the case of a negative resist, the
non-irradiated regions are dissolved leaving a negative image. This is
achieved in the development process.
The next plating step is to electrodeposit copper and a thin layer of a
suitable etch-resist plating, usually solder or gold. The original plating
resist, screen or photoresist, is removed, and the circuit pattern is
defined by etching away exposed copper in a suitable etchant (e.g.
ammonium persulfate). During this process, 90% of the copper plating is
removed by etching, thus producing large volumes of sludge and rinse
water.
Recently, the U.S. Environmental Protection Agency's Waste Reduction
Innovative Technology Evaluation (WRITE) Program has been established in
the printed wiring board manufacturing industry in order to perform
technical and economic evaluations of the volumes and/or toxicity of
wastes produced from the manufacture, processing and use of materials.
Environmental concerns associated with electroless copper metallization,
have fostered interest in direct metallization processes. Despite numerous
attempts over the last 10 years, conversion to a direct metallization
process has not gained widespread acceptance, and only about five percent
of PWB manufacturers worldwide have eliminated metallization by
electroless copper.
In addition to the environmental concerns about electroless copper
metallization, circuit board manufacture using this process can require as
many as 15 to 20 steps (including rinses), involving 70 minutes of
processing time. This obviously creates a roadblock for achieving a
flee-flowing process. Electronics manufacturers have not realized or
appreciated the benefits that direct metallization can provide. These
include reduced waste treatment/processing costs, lower chemical costs,
improved efficiency/reliability, and the elimination of a time-consuming
procedure.
Electronically conducting polymers have often been categorized as
non-processable and intractable, because of their insolubility in the
conducting form. Only recently has it been shown that polymers such as
polyaniline can be dissolved using functionalized sulfonic acids. For
polypyrrole, this can be achieved by using its derivatives [e.g., poly
(3-octylpyrrole)] which are known to be soluble in different solvents, or
by treatment in dilute aqueous sodium hypochlorite solutions, ammonia or
mono-, di- or tri-substituted amine (co)solvents. Another method of
solubilizing polypyrrole is the process of polypyrrole chain deprotonation
in basic solutions, which causes a transformation of conducting
polypyrrole into a non-conducting polymer of quinoid structure.
The lack of processability of conducting polymer materials, e.g., solution
or melt processing, infusability and poor mechanical properties, e.g.,
ductility, have slowed down their emerging commercial applications. While
electrochemical preparation of conducting polymers has been shown to be
the most satisfactory process from the viewpoint of fundamental
investigations, it is likely to be inappropriate for the large-scale
industrial production of bulk quantities of these materials. This is
particularly true where large molecular entities, e.g., copolymers or
different additives, need to be incorporated into conducting polymer
matrices in order to obtain tailored performance characteristics.
In order to compete with more-advanced interconnect systems, such as hybrid
circuits and multichip modules (MCMs), future PWBs will have to be
designed so that their size and cost advantages can be used to find a
wider range of applications. This will require PWBs with increased
conductor density. To accomplish this, finer lines and spaces (<5 mils),
smaller vias (<12 mils), thinner multilayer boards (<0.032 in), and
improved insulation resistance will be necessary. Finer lines and pitch
will require high-resolution imaging and precision etching. The presence
of plated-through-holes (PTHs 0.062-0.04 in) and vias (<0.10 in) in
ever-increasing numbers, will present a challenge in laminating, drilling
and metallization.
Consequently, there remains a need for improved direct metallization
processes for preparing electronic circuits on non-conducting substrates.
It would be desirable to have a direct metalization process that avoids
polymer solubility problems, can easily incorporate additives, does not
depend upon electroless-copper plating, minimizes hazardous chemicals and
copper plating solutions, requires fewer process steps, provides
simplified through-hole metallization, and facilitates increased conductor
densities.
SUMMARY OF THE INVENTION
The present invention provides a method for forming an electronically
conducting polymer on a substrate comprising the steps of forming a
solution comprising a pyrrole monomer and an electron acceptor, wherein
the molar ratio of pyrrole:electron acceptor is between about 0.5 and
about 20; applying a film of the solution onto a substrate;
photopolymerizing portions of the film to form electronically conducting
polypyrrole; and optionally washing the unpolymerized portion of the film
from the substrate and activating the polypyrrole with palladium bromide.
The substrate may be either conducting or nonconducting and either rigid
or flexible.
The electron acceptor is selected from the group consisting of silver
salts, e.g., AgNO.sub.3, AgClO.sub.4 and AgNO.sub.2, with the most
preferred being AgNO.sub.3. The solution may further comprise a
photoinitiator, another monomer such as aniline, a flexibilizer selected
from the group consisting of polyethylene glycol diglycidyl ether, dodecyl
sulfate and dodecylbenzene sulfonate.
The invention further includes a method of forming an electronic circuit on
a substrate comprising the steps of forming a solution comprising a
pyrrole monomer and an electron acceptor; applying a film of the solution
onto the substrate; photopolymerizing portions of the film to form
electronically conducting polypyrrole; washing the unpolymerized portion
of the film from the substrate; activating the polypyrrole with aqueous
palladium bromide; and electrodepositing copper onto the activated
polypyrrole. This method is particularly well suited to direct
metallization of printed wiring boards having through-holes in very few
steps.
BRIEF DESCRIPTION OF THE DRAWINGS
The file of this patent contains at least one drawing executed in color.
Copies of this patent with color drawing(s) will be provided by the Patent
and Trademark Office upon request and payment of the necessary fee.
FIG. 1 is a graph illustrating the dependence of conductivity on the type
and concentration of dopant anions present in photopolymerizable
pyrrole-based formulations;
FIG. 2 is a graph illustrating the dependence of electrical resistance of
photopolymerized polypyrrole films on the electron acceptor:monomer mole
ratio in the starting formulation;
FIG. 3 is a graph illustrating the dependence of curing time on the
concentration of photoinitiator;
FIG. 4 is a graph illustrating the dependence of conductivity on
photoinitiator concentration;
FIG. 5 is a graph illustrating the dependence of curing time on the amount
of dodecyl sulfate used as flexibilizer;
FIG. 6 is a graph illustrating the dependence of curing time and
conductivity on the amount of dodecyl sulfate used as flexibilizer;
FIG. 7 is a graph illustrating the dependence of resistance of
photopolymerized polypyrrole films on the ratios of pyrrole:aniline
monomers;
FIG. 8 is a photograph of alumina substrates with laser patterned lines of
electronically conducting polypyrrole;
FIG. 9 contains scanning electron micrographs of the cross-sections of
polypyrrole films formed electrochemically and photochemically;
FIG. 10 contains scanning electron micrographs of the surfaces of
polypyrrole films formed electrochemically and photochemically;
FIG. 11 is a flow chart comparing the steps of conventional PWB fabrication
with those of the present invention;
FIG. 12 is a photograph of black conducting polypyrrole lines on a
fiberglass/epoxy PWB substrate patterned using UV illumination through a
shadow mask, wherein one line has been electrodeposited with copper; and
FIG. 13 is a photograph of a copper-on-polypyrrole plated 0.025 inch
diameter through-hole.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides a novel process for using conducting
polymers for direct metallization of nonconducting surfaces which is
capable of metallizing both PWB conductor lines and through-holes. The
process is highly compatible with lithographic processes used in
manufacturing PWBs. The proposed process satisfies the criteria required
for designing future PWBs: (a) environmentally conscious manufacturing,
and (b) high-resolution conductor line imaging.
The formulations of the present invention include a salt that serves both
as an electron acceptor for oxidation of the monomer(s) and as a dopant to
preserve electroneutrality in the oxidized polymer. Preferred electron
acceptors undergo very slow oxidation of the monomer in the dark (1-2
days) and have the highest conductivities.
Polypyrrole (PPY) can be chemically prepared using inorganic (Fe.sup.3+ and
Cu.sup.2+ ions) or organic (chloranil) electron acceptors. When these
inorganic acceptors are added to pyrrole-containing solutions a powdery
polymer material results almost immediately after the addition. Therefore,
cations having too high an oxidation potential are not suitable for
photopolymerization of polypyrrole. Several attempts were made to use
organic electron acceptors, but photopolymerization of black conductive
PPY films was unsuccessful. It has been shown that electron acceptors with
proper oxidation potential (e.g. Ag.sup.+, Fe.sup.3+ or Cu.sup.2+ ions)
and dopant (e.g. NO.sub.3.sup.-, BF.sub.4.sup.-, tosylate, etc.) play a
decisive role in determining the conductivity of the conducting polymer
film. The preferred electron acceptors are the silver salts (such as
AgNO.sub.3, AgClO.sub.4 and AgNO.sub.2), with the most preferred being
silver nitrate, AgNO.sub.3.
Molar ratios of monomer to electron acceptor ranging between about 2 and
about 100 are effective for producing electronically conductive polymer
films. However, the electrical conductivity of the polymers decreases with
decreasing concentration of electron acceptor (increasing monomer to
electron acceptor ratio).
A polymer network can be formed by promoting the polymerization of a
monomer, oligomer, or mixtures of monomers and/or oligomers.
Polymerization is a chain reaction that can develop very rapidly,
especially when intense UV radiation is used to produce the initiating
species. This UV-curing reaction leads ultimately to a three-dimensional
polymer network. Since most of the monomers or oligomers commonly employed
do not produce initiating species with a sufficiently high yield upon UV
exposure, it is preferred to introduce a photoinitiator that will allow
the polymerization to start. A typical UV-curable formulation, therefore,
will contain two basic components: (i) a photoinitiator, and (ii) a
monomer, oligomer, or a mixture of monomers and/or oligomers.
The choice of the photoinitiator is of prime importance in light-induced
polymerizations, since it directly governs the cure rate. A suitable
photoinitiator system must possess high absorption in the emission range
of the light source. The photoinitiator must also form an excited state
having a short lifetime to avoid quenching by oxygen or the monomer and
split into reactive radicals or ionic species with the highest possible
quantum yield. Other factors to be considered in selecting the proper
photoinitiator include solubility in the monomer, storage stability and
the nature of the photo-products, which should not be colored, toxic or
induce some degradation of the polymer upon aging. Photoinitiators can be
classified into three major categories, depending on the kind of mechanism
involved in their photolysis: (i) radical formation by photo-cleavage;
(ii) radical generation by hydrogen abstraction, and (iii) cationic
photoinitiators.
Cationic photoinitiators have proven to be particularly useful in the
photopolymerization of polypyrrole from pyrrole monomers in solution.
Besides their specificity, cationic-initiated photopolymerizations have
the advantage of being insensitive to atmospheric oxygen. In the absence
of nucleophilic reagents, the chain reaction will thus continue to develop
after the illumination has ceased and provide a beneficial post-cure
effect that can be enhanced by thermal treatment. The preferred
post-photopolymerization thermal treatment involves heating the polymer at
temperatures between about 80 and about 120 degrees Celsius for about
three hours, with the most preferred temperature being about 100 degrees
Celsius.
Thermally stable photoinitiators for cationic polymerizations of commercial
significance include the onium salts, such as triarylsulfonium and
diaryliodonium, with complex metal halide anions. A key feature of these
photoinitiators is the low nucleophilicity of the anions which reduces
termination processes and allows ambient temperature cationic
polymerization to proceed. The absence of air inhibition represents a
distinguishing feature of cationic, as compared to radical,
polymerization.
The photoinitiators investigated included a titanocene radical
photoinitiator (such as IRGACURE.TM. 784 available from Ciba Geigy,
located in Ardsley, N.Y.), a cationic ferrocinium photoinitiator
(IRGACURE.TM. 261 available from Ciba Geigy, located in Ardsley, N.Y.),
triaryl sulphonium PF.sub.6.sup.- salts (such as CYRACURE.TM. 6990,
available from Union Carbide, located in Danbury, Conn.), triaryl
sulphonium SbF.sub.6.sup.- salts (such as CYRACURE.TM. 6974, available
from Union Carbide, located in Danbury, Conn.). The photoinitiators are
preferrably added to the monomer in amounts less than about 8 weight
percent, with the most preferred amounts being between about 0.2 to about
0.8 weight percent.
Photopolymerization of pyrrole alone, or pyrrole mixed with a
photoinitiator such as titanocene, yields a transparent yellow film
exhibiting insulating properties. Resistances of over 20 M.OMEGA. are
measured by an ohmeter. When AgNO.sub.3, an electron acceptor, is
dissolved into the pyrrole prior to curing, a black polymer film
characteristic of conducting polypyrrole is formed.
In general, both electropolymerized and photopolymerized polypyrrole films
suffer from poor mechanical properties. They lack flexibility, either as
stand alone films or as coatings. Three approaches have been found to
improve the mechanical properties of photopolymerized polypyrrole: (i)
incorporating large amphiphilic (surfactant) organic,anions into the
polypyrrole structure, (ii) photo-copolymerizing a suitable comonomer
material with pyrrole, and (iii) including commercial flexibilizers. The
preferred surfactants are large anionic surfactants, such as the sodium
salts of dodecyl sulfate (DDS) and dodecylbenzene sulfonate (DDBS). The
preferred comonomer is aniline. The preferred flexibilizer is polyethylene
glycol diglycidyl ether.
In accordance with the present invention, formulations can include a
mixture of monomers which can be photopolymerized to form copolymers.
While photo-copolymerizations can be achieved with many monomer pairings,
the preferred monomer pairs for the lithographic production of an
electronically conducting copolymer on a non-conducting substrate are
comprised of: (i) pyrrole in combination with: (ii) a sub-stoichiometric
amount of silver nitrate (molar ratio of pyrrole to silver nitrate is 8:1)
and with (iii) fifteen mole percent aniline relative to pyrrole. The mixed
monomer formulation is then diluted with an equivalent volume of
acetonitrile to provide good contact with the substrate.
The components of photopolymerizable solutions are mixed in a glass vial
that excludes the penetration of light. The solutions are then sonicated
to help dissolution and homogenization of the formulation. Since a slow
chemical polymerization of pyrrole takes place over a period of one to two
days in the presence of Ag.sup.+ ions, it is preferred that fresh
photopolymerizable formulations be prepared immediately prior to
polymerization.
A thin layer of the formulation is then cast and evenly spread on the
surface of a selected substrate typically having a surface area of between
about 1 and about 4 square centimeters (cm.sup.2). The preferred methods
of spreading the formulation over the substrate to achieve a thin layer
having uniform thickness include brush coating, spraying, dipping and spin
coating, with the most preferred method being spin coating.
After casting of the photopolymerizable solution onto a substrate and
formation of an air-dried nonconducting film, the oxidation process is
initiated by irradiation. The preferred irradiation methods are those
which selectively expose only discrete regions or lines on the coated
substrate, such as exposure by ultraviolet light through a contact mask,
direct laser imaging, or electron beam imaging. Using these methods, thin
polymer patterns (lines and through-holes) are readily polymerized on
various conducting and nonconducting substrates. Multiple coating-curing
cycles (up to 10 layers) can be carried out in order to produce thick
uniform films.
Photopolymerizations according to the present invention can be accomplished
with a 200-watt mercury-xenon lamp focused through a lens vertically
downward onto a circular area of less than one centimeter diameter. All
the optical accessories should be made of fused silica in order to pass
high energy UV as well as visible light.
The present invention uses irradiation as the driving force to induce
electron transfer from the monomer species in a cast solution film to the
electron acceptor, also present in the formulation. As the concentration
of oxidized polymer increases, coupling between the oxidized monomer units
begins. This process continues, resulting in growth of the conducting
polymer chains. Since the polymer is oxidized, the anion present in the
formulation intercalates into the polymer, maintaining electroneutrality.
The photopolymerization process does not require a conducting substrate for
deposition to take place, and conducting polymer films and/or lines of
various thickness, typically between about 5 and about 300 microns can be
readily photopolymerized on typical PWB substrates (fiberglass/epoxy,
polyimide) and MCM (alumina) as well as on metals, ceramic, silicon, GaAs,
glass, paper, Teflon, Mylar and polystyrene substrates. The process of the
present invention is much simpler than techniques known in the art and
offers a high potential and flexibility for adaptation to a variety of PWB
technologies.
The photopolymerization process of the invention includes the following
steps:
(i) a photopolymerizable formulation is applied on a substrate;
(ii) after air-drying, a dry negative prepolymer film is exposed to laser
light, an electron beam or to a UV lamp through a shadow mask;
(iii) the illumination induces photopolymerization of the prepolymer film
at exposed areas rendering the exposed areas insoluble; and
(iv) the non-polymerized (non-illuminated) areas are washed off with an
environmentally benign solvent (acetone) or water, leaving a pattern of
conducting polymer lines.
The main advantage of the photopolymerization process, compared to
electrochemical and/or chemical polymerizations, is that it allows
properties of conducting polymer films to be easily designed and optimized
by incorporating molecular species into the polymer structure. For
example, it is possible to change the conductivity of the polymer by
controlling the amount of the electron acceptor and dopant anions present
in the formulations. The same oxidatively coupled cationic polymer is
formed through photopolymerization as through electrochemical
polymerization, except that the anion/monomer ratio is much higher (1:1.3)
compared to that found in electrochemically formed films (1:4). This is a
desirable feature because with more anions in the polymer matrix, more
charge can be introduced onto the polymer chains and, consequently, higher
conductivities may be achieved.
EXAMPLE 1
A separate investigation involving both photopolymerization and thermal
polymerization processes was performed on samples having two different
electron acceptor salts, AgNO.sub.3 and AgTs, at rather low concentrations
(pyrrole:electron acceptor=50:1). Four samples were cured at the same time
either thermally or by photopolymerization. Curing times were determined
by observing the solidification of the surface and by applying a simple
pencil hardness test, often used in the polymer coating industry for
semiquantitative determination of curing quality. The results are
summarized in Table 1.
Thermally cured polymer films, either with AgNO.sub.3 or AgTs as electron
acceptor, were of very poor quality, rough and lacked a uniform color,
indicating nonhomogeneous polypyrrole films. Thermal curing of the first
layer proceeded with incomplete coverage of the exposed substrate surface
and curing resembled that of simple drying of the solution. On the other
hand photopolymerization of the first layer resulted in a completely
covered substrate surface. When more layers were added, curing times
became longer, because of the penetration of freshly added formulation
into the existing layers. Curing times for films where silver tosylate was
added as the electron acceptor salt were longer than for AgNO.sub.3
-containing samples. This was expected, because diffusion of larger
(organic) anions into polymer films being formed (in order to satisfy the
neutrality of an oxidized polymer) is much slower than for smaller anions,
like nitrates. Thermal curing, required 2-3 times longer curing times than
the process of photopolymerization. This is evident especially where
tosylates are used as electron acceptors.
TABLE 1
__________________________________________________________________________
COMPARISON OF PHOTOPOLYMERIZED AND THERMALLY POLYMERIZED POLYPYRROLE
FILMS
(PYRROLE/ELECTRON ACCEPTOR MOLAR RATIO WAS 50:1; PHOTOINITIATOR: 3 wt %
IRGACURE 261)
AgNO.sub.3 AgTs
LAYER ELECTRON PHOTO- THERMALLY PHOTO- THERMALLY
NUMBER
ACCEPTOR POLYMERIZED
POLYMERIZED
POLYMERIZED
POLYMERIZED
__________________________________________________________________________
FIRST CURE 68 69 68 69
LAYER TEMPERATURE
.degree.C.
CURING TIME
2 3.5 3 18
min
POLYMER FILM
Smooth, incomplete
smooth, green-
incomplete
APPEARANCE
black, brittle
coverage, black, coverage,
gray-black,
brittle gray-green black,
rough, brittle rough, brittle
FIFTH CONDUCTIVITY
9.7 .times. 10.sup.-3
3.8 .times. 10.sup.-2
4.1 .times. 10.sup.-4
6.0 .times. 10.sup.-5
LAYER S cm.sup.--1
CURE 68 68 67 68
TEMPERATURE
.degree.C.
CURING TIME
7 8 9 20
min
POLYMER FILM
smooth, gray-white-black,
smooth, green-
gray-white-black,
APPEARANCE
black, brittle
rough, brittle
black, rough, brittle
brittle
__________________________________________________________________________
From the results of this experiment it is evident that the photochemical
polymerization process proceeds faster than thermal polymerization and
produces more smooth and uniform polypyrrole films. The thermal
polymerization process is obviously different in nature, possibly based on
a chemical polymerization mechanism at elevated temperatures, leading to
the formation of a partially silver-filled non-conducting polypyrrole
matrix.
EXAMPLE 2
In order to improve the mechanical properties of PPY films three different
electron acceptor salts were investigated: AgNO.sub.3, AgTs and
AgBF.sub.4. It has been reported that incorporation of tosylate anions
improves the mechanical properties of electrochemically formed PPY films.
Thus, these three electron acceptor salts were added to photopolymerizable
formulations using pyrrole: acceptor molar ratios ranging from 100:1 to
4:1, the latter being closest to the ratio of pyrrole monomer to positive
charge found in electrochemically polymerized films. FIG. 1 shows the
dependence of electrical conductivity on the concentration of electron
acceptors (AgNO.sub.3 and AgTs) added to the formulations. Both curves
exhibit a maximum conductivity value of approximately 0.1-0.3 S cm.sup.-1
at pyrrole: salt molar ratios between about 3:1 and about 8:1. A steep
decrease in conductivity occurred at molar ratios higher than 15:1. In the
case of AgTs, at low added salt concentrations, the conductivities were
several orders of magnitude lower than those for polymer films
photopolymerized with AgNO.sub.3. The data shown in FIG. 1 includes films
of different thicknesses, where all of them were photopolymerized and then
peeled off from Al substrates. Although the thinner films were less
brittle and less fragile, no improvement in mechanical properties was
observed for films photopolymerized with tosylates.
In experiments performed involving different substrates it was found that
comparisons between photopolymerized polypyrrole films were best achieved
if polystyrene was used as the substrate, and if the films under
investigation were cured at the same time, which assured the same curing
conditions. Polystyrene showed satisfactory wettability for a whole range
of film compositions used.
In Table 2 results are given for PPY films photopolymerized using different
silver salts, and their mixtures, added at pyrrole:salt molar ratios of
8:1. All the films yielded conductivity values within an order of
magnitude of each other (approximately 0.1 to 0.4 S cm.sup.-1), except in
the case of AgBF.sub.4 which displayed a conductivity value two orders of
magnitude lower. AgBF.sub.4 -containing films possessed the poorest
mechanical properties, and required the longest curing times for complete
curing. When mixed with AgNO.sub.3 in equimolar concentrations, but
keeping the total pyrrole/salt ratio constant (8:1), the conductivity of
polypyrrole films improved and approached the values measured for
AgNO.sub.3 alone.
From the data presented in FIG. 1 and Table 2, it was concluded that
AgNO.sub.3 added to photopolymerizable formulations in amounts
corresponding to 10-15 mol %, provide the necessary electron acceptor
properties for photopolymerization to take place, and gives the amount of
NO.sub.3.sup.- anions required for charge balance inside the polymer.
Thus, AgNO.sub.3 is the optimal choice of electron acceptor for the
photopolymerization of pyrrole.
TABLE 2
__________________________________________________________________________
CONDUCTIVITY OF PHOTOPOLYMERIZED PPY FILMS CONTAINING
DIFFERENT ANIONS (ELECTRON ACCEPTOR: Ag.sup.+ ; PHOTOINITIATOR: 3
wt % IRGACURE 261; PYRROLE/SALT RATIO = 8:1).
ELECTRON
STAND ALONE FILMS FILMS ON POLYSTYRENE
ACCEPTOR
CONDUCTIVITY
THICKNESS
CONDUCTIVITY
THICKNESS
SALT S cm.sup.-1
mm S cm.sup.-1
mm
__________________________________________________________________________
AgNO.sub.3
0.425 62 0.158 34
AgTs 0.197 88 0.179 53
AgBF.sub.4 0.0018 57
AgNO.sub.3 .backslash.AgTs
0.212 168
AgNO.sub.3 .backslash.AgBF.sub.4
0.375 51
__________________________________________________________________________
EXAMPLE 3
A series of experiments were performed to examine the electrical resistance
of photopolymerized polypyrrole films as a function of monomer/electron
acceptor mole ratio in the starting formulation. A mole ratio range of
20:1 to 0.5:1 (pyrrole:silver nitrate) was investigated. The solutions
were prepared in one ml of pyrrole monomer and varying amounts of silver
nitrate. Pyrrole films of constant thickness (ca. 60 microns) were
produced. A minimum in resistance (Van der Pauw method) of ca. 80 .OMEGA.
was observed at a 1:1 mole ratio of monomer to silver nitrate. Results
shown in FIG. 2 demonstrate that by simple adjustment of the concentration
of starting formulation components (monomer and electron acceptor) an
order of magnitude change in resistance could be obtained.
EXAMPLE 4
Simple tests of thick film curing were performed by simultaneous
illumination of formulations containing photoinitiators added at 3 wt % to
an 8:1, pyrrole:AgNO.sub.3 solution. Exposure to UV light was brought
about from the top of miniature glass vials (0.7 cm dia. and 1.1 cm
height) containing different photoinitiators. The process of
photopolymerization was closely followed under low illumination conditions
(corresponding to a temperature of 30.degree.-32.degree. C.), in order to
determine the changes taking place during photopolymerization. In all four
vials the polymerization process went through different stages which
affected the color of the bulk and/or surface layers of the formulations
and the speed of solidification. From this simple experiment it was
observed that cationic photoinitiators exhibited faster curing rates than
radical photoinitiators. Especially, Irgacure 261 demonstrated better
curing (in line with weak absorption of 366 nm light), as evidenced by a
deeper and more homogeneous blackening and solidification of the entire
formulation volume in the glass vial.
Although the choice of photoinitiator between triaryl-sulfonium salts and
the ferrocinium photoinitiator, all three being cationic photoinitiators,
was not conclusive, the ferrocinium photoinitiator is more suitable for
photopolymerization of pyrrole because it allows deeper light penetration
through the black solidified surface layer. Ferrocinium photoinitiators
have been found to be successful for the photopolymerization of epoxides,
which have been used in this work as potential copolymers with polypyrrole
(see later).
The effect of ferrocinium photoinitiator concentration on the curing time
of PPY films is shown in FIG. 3. Formulations containing increasing
amounts of photoinitiator were applied at different thicknesses on Al and
glass substrates, and were cured simultaneously. Curing time was
determined by observing solidification and by the pencil hardness test.
Increasing the amount of photoinitiator from 1 to 8 wt % decreased the
curing time by approximately a factor of two. FIG. 4 shows that increasing
amounts of photoinitiator present in the films causes a slight decrease in
conductivity.
EXAMPLE 5
Organic anions chosen were DDS (dodecyl sulfate, sodium salt) and DDBS
(dodecylbenzene sulfonate, sodium salt). They were added to the already
optimized formulation to yield the highest conductivity, i.e.,
pyrrole:AgNO.sub.3 ratio of 8:1 and 3 wt % of Irgacure 261 photoinitiator.
Amounts added to the formulation are expressed as pyrrole/surfactant molar
ratios. Polypyrrole films where photopolymerized from these formulations
under different illumination conditions and on various substrates. A
postcure thermal treatment at the highest lamp irradiance was applied
after photocuring. This is recommended by Ciba-Geigy for completion of
curing processes when Irgacure 261 photoinitiator is used.
Photopolymerization along the area of the substrate covered by the
formulation was followed by observing black solidifying zones smoothly
spreading on the substrate. It was evident that these additives helped
diffusion of polymerizing components in the thin formulation layer. Curing
was generally slower than for the films without surfactant additives.
Films obtained showed a significant improvement in mechanical properties.
They were very flexible compared to the films that did not contain
surfactant additives. It was possible to bend these films, whether coated
on an aluminum sheet or on polystyrene, through angles greater than
90.degree. without breaking them. Additives acting as surfactants greatly
improved the adherence to the substrate. More importantly, films thus
formulated retained good conductivity. DDBS was less soluble in pyrrole
and gave rise to films of lower flexibility when compared to films with
DDS as additive. Table 3 compares conductivities for DDS- and
DDBS-containing films, added to pyrrole:surfactant molar ratios of 15:1.
TABLE 3
__________________________________________________________________________
CONDUCTIVITY OF PHOTOPOLYMERIZED PPY FILMS WITH LARGE
ORGANIC ANIONS AS FLEXIBILIZERS (PYRROLE/AgNO.sub.3 = 8:1;
PYRROLE/SURFACTANT = 15:1; PHOTOINITIATOR: 3 wt % IRGACURE 261;
CURING TIME: FAST, 1.9 W cm.sup.-2 WITH THERMAL POSTCURE: 2.3 W
cm.sup.-2).
ADDITIVE DDS DDBS
SUBSTRATE CONDUCTIVITY
THICKNESS
CONDUCTIVITY
THICKNESS
MATERIAL S cm.sup.-1
mm S cm.sup.-1
mm
__________________________________________________________________________
STAND ALONE
0.21 163 0.20 215
STAND ALONE
0.51 61 0.59 75
STAND ALONE
0.134 224
PY/AgNO.sub.3 = 5/1
POLYSTYRENE
0.48 39 0.32 62
POLYMER FILM
smooth, black, smooth, black,
APPEARANCE
curing time: 1.3 min/layer,
curing time: 2 min/layer,
very flexible flexible
__________________________________________________________________________
FIGS. 5 and 6 show variations in curing time and conductivity of films
photopolymerized with different concentrations of DDS additive. It was
possible to follow the curing progress at two stages: corresponding to
surface solidification and when curing was completed. Both plots exhibit
the same slope, showing that the curing time is longer with increasing
amounts of DDS in the films. Films with higher concentrations of
surfactant additive became soft. The electrical conductivity of the films
was within the range 0.1-0.5 S cm.sup.-1. A minimum in electrical
conductivity, evident at ratios between 30:1 to 50:1, is probably due to
an artifact in that the resistivity probe tips penetrated into the soft
films at ratios greater than 30:1 and hence, displayed conductivity values
higher than those for the films of measured thickness. It was found that
films containing between 10:1 and 20:1 of pyrrole:DDS additive, possess
the greatest flexibility and conductivity.
EXAMPLE 5
A series of experiments were performed to examine the electrical resistance
of polymer films photopolymerized from mixtures of pyrrole and aniline
monomers. Solutions of silver nitrate (AgNO.sub.3), pyrrole, and aniline
were prepared in one ml of acetonitrile. Equivalent molar amounts of
AgNO.sub.3 and various proportions of pyrrole and aniline were prepared in
a large volu | | |
