Methods for making two-phase light-transmissive electrode layer with controlled conductivity

Abstract

A method of making a two-phase light-transmissive electrode layer comprising a first phase made of a highly electronically-conductive matrix and a second phase made of a polymeric material composition having a controlled volume resistivity.

Claims

1. A method of making a two-phase light-transmissive electrode layer for incorporation into an electro-optic assembly, the method comprising: coating a composition comprising a light-transmissive polymeric material and a conductive additive onto a substrate, the substrate contacting a highly electronically-conductive matrix; and drying the composition to create a two-phase light-transmissive electrode layer.

2. The method of claim 1, further comprising roll milling the composition comprising the light-transmissive polymeric material and the conductive additive prior to coating.

3. The method of claim 1, further comprising removing the substrate after drying to produce a light-transmissive two-phase electrode layer without a substrate.

4. The method of claim 1, wherein the conductive additive is a salt, a polyelectrolyte, a polymer electrolyte, or a solid electrolyte.

5. The method of claim 1, wherein the conductive additive is polyethylene glycol.

6. The method of claim 1, wherein the volume resistivity of the composition comprising a light-transmissive polymeric material and a conductive additive is 110.sup.7 to 110.sup.12 Ohm-cm.

7. The method of claim 1, wherein the highly electronically-conductive matrix comprises carbon nanotubes, silver nanowires, a metal coated open foam structure, or a printed mesh of wires.

8. The method of claim 1, wherein the light-transmissive polymeric material comprises a polyurethane, a vinyl acetate, a vinyl acetate ethylene, an epoxy, or a polyacrylic.

9. The method of claim 1, wherein the light-transmissive polymeric material comprises a conductive polymer selected from PEDOT-PSS, polyacetylene, polyphenylene sulfide, and polyphenylene vinylene.

10. The method of claim 1, wherein the substrate is a polymeric film having a thickness in the range of about 1 to about 25 mil (25 to 634 m).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic cross-section through a basic front plane laminate (10) of an electro-optic display having a conductive layer of the present invention.

(2) FIG. 2 is a schematic cross-section, similar to that of FIG. 1.

DETAILED DESCRIPTION

(3) Accordingly, this invention provides an electro-optic display comprising a two-phase, light-transmissive electrically-conductive layer containing a first phase made from a highly conductive matrix and a second phase made from a polymeric material composition having a controlled volume resistivity. An electro-optic display normally comprises a layer of electro-optic material and at least two other layers disposed on opposite sides of the electro-optic material, at least one of these being a light-transmissive electrically-conductive layer. The term light-transmissive is used herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will be normally be viewed through the electrically-conductive layer and adjacent substrate (if present).

(4) The first phase may be a conductive matrix. The term conductive matrix is used herein to describe the first phase array of highly conductive regions of the electrode layer. The matrix may be constructed from carbon nanotubes, silver nanowires, a metal coated open foam structure, a printed mesh of wires, or any such material that does not fully cover the viewing surface. The first phase may be light-transmissive or light-absorbing with a diameter size ranging from nanometers to millimeters. The matrix may be arranged regularly (i.e., grid-like, hexagonally, webbed, etc.) or irregularly (i.e., randomly). The matrix may be arranged to be very thin or sparsely populated such that the electrode layer is light-transmissive.

(5) The second phase may be made from an intrinsically conductive polymer or a polymer mixed with a conductive additive. The second phase, while significantly less conductive compared to the first phase, has conductive properties (i.e., a resistivity of not more than 110.sup.12 Ohm-cm). Preferably, the second phase has a resistivity of approximately 110.sup.7 to 110.sup.12 Ohm-cm. The second phase may be ionically or electronically conductive. Typically, the second phase surrounds or coats the first phase to create less conductive regions around the conductive matrix. This provides mechanical integrity and protection to the first phase, which may be fragile due to its structure. If the first phase is light-absorbing and the second phase is light-transmissive, a high ratio of less conductive areas to highly conductive areas is desirable. More preferably, the first phase makes up less than 10% of the viewing area while the second phase makes up the remaining viewing area.

(6) The polymeric material may be any polymeric material that fulfills the particular needs of the end-use application. Examples of suitable polymeric materials include polyurethane, vinyl acetate, vinyl acetate ethylene, epoxy, a polyacrylic-based adhesive, or combinations thereof. These adhesive materials may be solvent based or aqueous based. An example of a particular polyurethane that may be used is described in U.S. Pat. No. 7,342,068, issued Mar. 11, 2008, which is incorporated herein by reference in its entirety and assigned to Air Products and Chemicals, Inc.

(7) FIG. 1 is a schematic cross-section through a basic front plane laminate (10) of an electro-optic display having a conductive layer of the present invention. Typically, the light-transmissive electrically-conductive layer (14) will be carried on a light-transmissive substrate (12), which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The substrate will be typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 m), preferably about 2 to about 10 mil (51 to 254 m). The substrate (12) forms the viewing surface of the final display and may have one or more additional layers, for example, a protective layer to absorb ultra-violet radiation, barrier layers to prevent ingress of moisture, or anti-reflection coatings.

(8) The conductive layer (14) comprises the two-phase electrode layer. A layer of electro-optic medium (16) is in electrical contact with the conductive layer (14). The electro-optic medium shown in FIG. 1 is an opposite charge dual particle encapsulated electrophoretic medium having a plurality of microcapsules, each of which comprises a capsule wall (18) containing a hydrocarbon-based liquid (20) in which are suspended negatively charged white particles (22) and positively charged black particles (24). The microcapsules are retained within a binder (25). Upon application of an electrical filed across the electro-optic layer (16), the white particles (22) move to the positive electrode and the black particles (24) move to the negative electrode, so that the layer (16) appears, to an observer viewing the display through the substrate (12), white or black depending upon whether the layer (14) is positive or negative relative to the backplane at any point within the final display.

(9) The front plane laminate (10) as shown in FIG. 1 further comprises a layer of lamination adhesive (26) adjacent the electro-optic medium layer (16) and a release sheet (28) covering the adhesive layer (26). The release layer (28) is peeled from the adhesive layer (26) and the adhesive layer is laminated to a backplane to form the final electro-optic display.

(10) FIG. 2 shows a modified front plane laminate (generally designated 10) which is identical to the laminate 10 shown in FIG. 1, except that a thin second conductive layer 30 is provided on the surface of the release layer 28 facing the electrophoretic medium. The release layer 28 and conductive layer 30 can be formed from an aluminized PET film, and such films are readily available commercially. Although not shown in FIG. 2, in some cases, depending upon the properties of the electro-optic medium, it may be necessary or desirable to provide a coating of a low surface energy material, such as a silicone, over the second conductive layer 30 to prevent this layer sticking to the electro-optic medium. Although this low energy material introduces an additional electrical resistance into the system, the resistance of most electro-optic media is sufficiently high that the presence of the additional resistance is not a problem. If necessary, the driving voltage used during testing (see below) may be adjusted to allow for the additional resistance.

(11) FIG. 2 shows the conductive layer 14 grounded and a drive voltage V, sufficient to cause a change in the optical state of the electro-optic medium and preferably equal to the drive voltage used in the final display, applied to the second conductive layer 30. Contact with the second, conductive layer 30 may be achieved by peeling a small area of the release layer 28 away from the adhesive layer 26; contact with the conductive layer 14 may be achieved by similar peeling and either removing the electro-optic medium from a small area of the laminate or by piercing this medium with, for example, crocodile clips. Advantageously, an alternating, preferably square wave, voltage is applied to the second conductive layer 30, so that the entire sheet of laminate 10 flashes black and white.

(12) The two-phase conductive layer of the present invention may be the front electrode of an electro-optic display, which is the electrode located on the side closest to the viewing surface. In an electro-optic display that is fully light-transmissive or has two viewing surfaces, the two-phase conductive layer of the present invention may be both the front and back electrodes.

(13) For an electro-optic display, it is critical to control the electrical properties of the polymeric material; otherwise, the display may experience diminished optical performance. Optical performance may be enhanced when the bulk resistance of the polymeric material is not more than 110.sup.12 Ohm-cm.

(14) Conductive Polymers

(15) In another form, the present invention provides a two-phase electrode layer wherein the second phase is an ionic conductive polymer in which one ion can migrate through the polymeric material while the other cannot. This type of ionic material prevents ions diffusing out of the polymeric material and potentially damaging other layers (for example, organic semiconductor layers) into which the ions diffuse.

(16) Ionic conduction has been shown to occur by a hopping mechanism, in which dissociated free ions translate among ionic aggregates (ion pairs and higher aggregates), most of these aggregates being essentially neutral. In accordance with the present invention, only one of the anion and cation of the ionic material is capable of motion. The fixed ion is constrained to a single location, while the mobile ion is still free to migrate. An example of an appropriate ionic material is a polymeric salt, for example, an ionic salt of a polymeric carboxylate. In this case, the carboxylate ion is effectively immobile because it is attached to the polymer chain, and can only move with the polymer as a whole. The cationic counter ion, on the other hand, can freely participate in hopping motions, and the rate at which it can move depends on the strength of the electrostatic interaction with the anionic carboxylate, the concentration of carboxylate-counter ion aggregates in the adhesive medium, the viscosity of the medium, and the free energy of solvation of the counter ion by the medium.

(17) As described in U.S. Patent Publication No. 2009/0122389, filed Nov. 4, 2008, assigned to E Ink Corporation, large cations are advantageous, in that they have relatively low electrostatic energies of attraction to the ionic aggregate states, and therefore dissociate readily from them. As an example, a quaternary ammonium hydroxide could be used to neutralize the carboxylic functions on the polyurethane, leading to a quaternary ammonium carboxylate polymer capable of supporting ionic conduction as described above.

(18) It is desirable that the ionic material be chosen such that the conductivity of the final polymeric material after drying can be modified and adjusted by varying the carboxylic acid content of the polyurethane, and also by the cation used. For example, in the aforementioned system where a carboxylic group on the polyurethane is neutralized with a quaternary ammonium hydroxide, at a given carboxylic acid content, the conductivity would be expected to increase in the order:

(19) tetramethylammonium<tetraethylammonium<tetrabutylammonium, etc.

(20) Phosphonium salts could also be used, and should be somewhat more conductive than the nitrogen containing analogs because of the larger size of the central atom. Other cationic species (e.g., complex ions of metals) may also be useful for this purpose. Solubility of the ionic material in the polymer material is not an issue in this approach, since the ions are an intrinsic part of the medium and cannot therefore phase separate as a separate crystalline phase.

(21) The acidic component of the polymeric material may also be made more acidic by replacing a carboxylic acid component by a group with a higher dissociation constant, for example, a sulfate monoester, sulfonic acid, sulfinic acid, a phosphonic acid, phosphinic acid group or phosphate ester, as long as there is at least one dissociable proton present. Quaternary salts and other large cations would still be expected to be most useful as counter ions because of their large size, and relatively high degree of ionic dissociation in dried adhesive media of low polarity. Nitrogen-based acids could also be used if attached to sufficiently electron-withdrawing functions (e.g., RSO.sub.2NHSO.sub.2R)). In this case almost any mobile ion could be used, including tertiary ammonium, because the mobile ion will exist in the protonated form even in the dried adhesive. However, mobile ions based on larger amines (i.e., ones with longer alkyl tails) might still be preferable, because they are effectively larger in size and therefore the ion pairs comprising them would be more dissociable. Alternatively, a carboxylate group on the polymer could be used with a mobile ion that is not a strong Bronsted acid, i.e., which does not have an acidic proton, such as the quaternary cations discussed above.

(22) Polymeric materials in which a cation is the fixed ion can be constructed by using quaternary ammonium groups in the polymer backbone or as side chains, and preferably using large anions (e.g., hexafluorophosphate, tetrabutylborate, tetraphenylborate, etc.) as the mobile ions. The quaternary ammonium groups could be replaced by phosphonium, sulfonium or other cationic groups without dissociable hydrogen, including those formed by complexation with metallic cations. Examples of the latter include polyether/lithium ion inclusion complexes, especially cyclic polyethers (e.g. 18-crown-6) or polyamine complexes with transition metal ions. In this case the anionic mobile ion could include those types of ions listed above, plus more strongly basic materials such as carboxylates or even phenolates.

(23) Alternative fixed cation polymeric materials include polymers containing repeating units derived from basic monomers, for example poly(vinylpyridine), poly(-dimethylaminoethyl acrylate), etc. and copolymers containing such groups, in conjunction with mobile anions that are not good Bronsted acceptors (e.g., sulfonates, sulfates, hexafluorophosphate, tetrafluoroborate, bis(methanesulfonyl)imidate, phosphates, phosphonates, etc.). Quaternary salts derived from such amino monomers may also be used, for example, poly(N-methyl or benzyl(vinylpyridinium)), poly(N-alkyl (or alkaryl)-N-vinyl-imidazolium), and poly(-trimethylammonioethyl)acrylate or methacrylate) salts, as well as vinyl copolymers comprising these ionic groups. As before, larger mobile ions are preferred.

(24) These chemical modification techniques are not restricted to polyurethanes but can be applied to any polymer of suitable structure. For example, vinyl-based polymers can contain either anions or cation fixed ions.

(25) In another form of the present invention, the polymeric material may contain one or more conducting polymers selected from PEDOT-PSS, polyacetylene, polyphenylene sulfide, polyphenylene vinylene and combinations thereof. The thickness of the polymeric material may range from approximately 0.1 m to 20 m. The polymeric material may be (a) water-soluble or water dispersible and coated from water, (b) soluble or dispersible in organic solvent and coated from organic solvent, or (c) coated in monomeric form or oligomeric form and then polymerized by UV, irradiation, heating, or other methods known in the art. The additive incorporated into a polymeric material may be formed in situ; in other words, one or more precursor materials may be incorporated into the polymeric material where the precursor material(s) can react with one another, or with the polymeric material, or be changed by exposure of the polymeric material to conditions (for example, exposure to heat, light, or magnetic or electric fields) which cause a change in the precursor materials to form the final additive.

(26) Diffusible Ionic Additives

(27) The second phase may contain one or more ionic additives, for example, (a) a salt, a polyelectrolyte, a polymer electrolyte, a solid electrolyte, and combinations thereof; or (b) a non-reactive solvent, a conductive organic compound, and combinations thereof.

(28) In one form, the additive may be a salt such as an inorganic salt, organic salt, or combination thereof, as described in U.S. Pat. No. 7,012,735, filed Mar. 26, 2004, assigned to E Ink Corporation. Exemplary salts include potassium acetate, and tetraalkylammonium salts, especially tetrabutylammonium salts such as the chloride. Further examples of salts include salts such as RCF.sub.3SOF.sub.3, RClO.sub.4, LiPF.sub.6, RBF.sub.4, RAsF.sub.6, RB(Ar).sub.4 and RN(CF.sub.3SO.sub.2).sub.3 where R may be any cation, such as Li.sup.+, Na.sup.+, H.sup.+, or K.sup.+. Alternatively, R may include ammonium groups of the form N.sup.+R1R2R3R4. A preferred salt is tetrabutylammonium hexafluorophosphate.

(29) In another form, the additive may be a salt having anions containing at least three fluorine atoms as described in U.S. Pat. No. 8,446,664, filed Apr. 4, 2011, assigned to E Ink Corporation. The salt may, for example, have a hexafluorophosphate anion. The salt may also have an imidazolium cation. Exemplary salts include 1-butyl-3-methylimidazolium hexafluorophosphate (hereinafter BMIHFP), 1-butyl-3-methylpiperidinium hexafluorophosphate, 1-butyl-3-methylpyridinium hexafluorophosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, sodium hexafluorophosphate, 1-butyl-3-methylimidazolium trifluoromethanesulfonate and 1-butyl-3-methylimidazolium boron tetrafluoride. A preferred salt is BMIHFP. This preferred salt is liquid at 25 C. and can be dispersed directly in an aqueous polymer dispersion or latex without the use of any solvent. Alternatively, since the preferred salt is soluble in water in an amount of about 1 percent at 25 C., this salt can be added in the form of a dilute aqueous solution. Addition of the salt as an aqueous solution avoids the introduction of any undesirable organic solvent into the binder.

(30) Alternatively, the fluorine-containing salt may have a tetrafluoroborate anion, a tetraphenylborate anion, a bis(trifluoromethane)sulfonamide anion (triflimide), a tetra(pentafluorophenyl)borate anion, a tetrakis(3,5-bis(trifluoromethyl)phenyl)borate anion or a trifluoromethanesulfonate anion (triflate), for example, 1-butyl-3-methylimidazolium boron tetrafluoride or 1-butyl-3-methylimidazolium trifluoromethanesulfonate. The fluorine-containing salt may be present in an amount of from about 50 to about 10,000 ppm based upon the solids content of the polymeric material, and generally from about 100 to about 1000 ppm.

(31) In other embodiments, the polymer electrolyte is a polyelectrolyte. Polyelectrolytes are typically polymers in which about 10% or more of the molecule is composed of a functional group capable of ionizing to form a charged species. Examples of certain functional groups within a polyelectrolyte include, but are not limited to, carboxylic acids, sulfonic acids, phosphoric acids, and quaternary ammonium compounds. These polymers can be combined with organic or inorganic salts or alternatively used alone. Examples of polyelectrolytes, include but are not limited to, polyacrylic acid, polystyrene sulfonate, poly(2-vinylpyridine), poly(4-vinylpyridine), poly(dimethylammonium chloride, poly(dimethylaminoethyl methacrylate), poly(diethylaminoethyl methacrylate) and may comprise a salt of a polyacid such as, but not limited to, an alkali metal salt of polyacrylic acid. A preferred polyelectrolyte is the sodium salt of polyacrylic acid.

(32) In a further form of the present invention, the additive is a polymer electrolyte. The term polymer electrolyte as used herein describes a polymer that is capable of solubilizing a salt. The solubility of the salt in these polymers can be enhanced by the presence of oxygen and/or nitrogen atoms in the polymer which form ether, carbonyl, carboxylic acids, primary secondary, tertiary, and quaternary amino groups, sulphonic, acids etc. Examples of polymer electrolytes include polyether compounds such as polyethylene oxide, polypropylene oxide, polytetramethyleneoxide, polyamines such as polyethyleinimine, polyvinyl pyrrolidinone, polymers which contain quaternary ammonium groups such as N.sup.+R1R2R3R4 wherein R1, R2, R3, and R4 are each independently a H or a straight, branched, or cyclic alkyl group having from 1 to 25 carbon atoms and where the counter ion can be either selected from any organic or inorganic anion.

(33) Still other additives may include a non-reactive solvent that may improve or, alternatively, hinder the mobility of the ions in the solution. Examples of suitable non-reactive solvents include water, diethyl ether, dipropyl ether, diethyleneglycol, glyme, diglyme, N-methylpyrrolidone, etc. In still another embodiment, a conductive organic compound can be used as the additive. Some non-limiting examples of these compounds include polyaniline, polythiophene, polypyrrole, poly-3,4-dioxyethylene thiophene, and derivatives of these materials in their n- or p-doped states.

(34) It is also known, as disclosed in U.S. Pat. No. 7,256,766, filed May 10, 2002, assigned to E Ink Corporation, that an optical biasing element may be provided in one of the layers of an encapsulated electrophoretic display to adjust the appearance of the display. Such an optical biasing element may be added to the polymeric material. The electrical properties of the polymeric material containing such an optical biasing element may be optimized by use of the additives described herein.

(35) The optimum amount of additive will of course vary widely with the exact polymeric material and the exact additive used, and the desired volume resistivity of the final mixture. However, by way of general guidance it may be indicated that a concentration of from about 10.sup.5 to about 10.sup.4 moles of additive per gram of polymeric material has been found to give useful results. When the additive is a salt, this range is for 1:1 salts such as tetrabutylammonium chloride, tetrabutylammonium hexafluorophosphate and potassium acetate; if 1:2 salts such as sodium carbonate or calcium chloride are used, lower concentrations, of the order of 10.sup.6 moles of salt per gram of polymeric material may suffice. The volume resistivity of polymeric materials typically varies in a predictable manner with the concentration of the additive, and hence the final choice of how much additive should be added to achieve a desired volume resistivity may readily be determined empirically.

(36) Although small amounts of salts have been added to polymers used as binders and lamination adhesives in prior art electro-optic displays, for example as biocides to protect the polymers from biological degradation during extended storage, such salts are typically used up during storage as they perform their biocidal or similar function. In contrast, the additives used in the present invention are intended to be permanent constituents of the polymeric material since they are intended to effect a permanent adjustment in the conductivity thereof. Also, the optimum amounts of additives used are typically substantially greater than the amounts of salts used as biocides etc.

(37) Hydroxyl Containing Low Molecular Weight Polymer/Glycol Additive

(38) Another form of the present invention relates to an electro-optic display having an electrode layer comprising a first phase made from a conductive matrix and a second phase made from a polymer mixed with an additive wherein the additive is a low number average molecular weight (Mn not greater than about 5000) polymer containing hydroxyl. As described in U.S. Pat. No. 7,349,148, filed Dec. 20, 2006, assigned to E Ink Corporation, a preferred polymer for this purpose is poly(ethylene glycol) (PEG), desirably having Mn not greater than about 2000. In this additive, the protons move rather than the electrons. The optimum concentration of hydroxyl containing polymer additive for any particular system is best determined empirically, but by way of general guidance it may be said that the optimum concentration of typically around 10.sup.6 to 10.sup.5 moles per gram of polymeric material.

(39) If, as is typically the case, the electrode layer is formed by coating a film of a solution of the second phase on to a substrate containing the first phase and drying to form the electrode layer, the additive will normally simply be dissolved or dispersed in the solution of the polymeric material before coating. The additive may be added to the solution neat or may be dissolved in an aqueous solution, non-aqueous solution, or combination thereof. It is of course necessary to ensure that the additive is uniformly dispersed throughout the polymeric material in order to prevent variations in conductivity within the final conductive layer, but those skilled in coating technology will be familiar with routine techniques, such as lengthy agitation on a roll mill, for ensuring such uniform dispersion. The two-phase electrode layer may be used with the substrate in place. Alternatively, once set, the second phase may add sufficient structure to the first phase such that the substrate may be removed.

(40) The choice of the specific additive to be used is governed largely by considerations of compatibility with the electrodes and solubility in the polymeric material to which the additive is to be added. If, as is typically the case, the additive is to be added to an aqueous polymeric material, the additive should be chosen to have good water solubility, so that among salts alkali metal and substituted ammonium salts are generally preferred. Care should be taken to ensure that the additive does not cause aggregation of the polymer particles. Also, the additive should desirably not cause major changes in the pH of the polymeric material, and should not chemically react with the electrodes or polymeric material or other parts of the final display with which it eventually comes into contact, for example the backplane.

(41) The addition of one or more additives greatly expands the range of polymeric materials, which can be used in electro-optic displays. In particular, the addition of one or more additives enables the use of polymeric materials which have mechanical properties highly desirable in electro-optic displays but which have volume resistivities in their pure states too high to be useful. Also, since some electro-optic displays, especially encapsulated electrophoretic displays and electrochromic displays, are sensitive to moisture, the addition of the one or more additive may be used to replace water-based polyurethane dispersions hitherto used in such displays with non-hygroscopic and/or hydrophobic polymeric materials.

(42) The polymeric material may contain components other than the additive used to adjust its volume resistivity; for example, the polymeric material may also contain a dye or other colorant.

(43) It will be appreciated that the modified polymeric materials disclosed herein may be useful in applications other than electro-optic displays.

(44) It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.