Piezoelectric film comprising ionic liquid and electrophoretic display film comprising piezoelectric film

Abstract

To provide a piezoelectric film containing an ionic liquid and an electrophoretic display film containing the piezoelectric film.SOLUTION: A piezoelectric film (410) comprising polyvinylidene fluoride (PVDF) and 10% or less (by weight) of an ionic liquid is disclosed. The film has a greater amount of beta phase and can be poled using an external field without additional processing such as stretching. The film does not require an external power source and can be used to create a piezophoretic display film (405) that can be patterned for use as a security marker, authentication film, or sensor.SELECTED DRAWING: Figure 4A

Description

[Background technology]

[0001] (Reference to Related Application) This application claims priority to U.S. Provisional Application No. 63 / 314,702, filed February 28, 2022. All patents and publications disclosed herein are incorporated by reference in their entirety.

[0002] Electrophoretic displays (EPDs) are non-emissive devices based on the electrophoresis of charged pigment particles dispersed in a solvent or solvent mixture. The displays typically include two electrodes placed opposite each other, which provide an electric field to drive the movement of the charged pigment particles. One of the electrodes is usually transparent. When a voltage difference is applied between the two electrodes, the pigment particles migrate to one side or the other, causing either the color of the pigment particles or the color of the solvent (if colored) to be seen from the viewing side. Electrophoretic fluids typically contain a non-polar solvent and one or more sets of charged particles. The particles can have different optical properties (colors), different charges (positive or negative), different charge magnitudes (zeta potentials), and / or different absorption properties (broadly, light absorption, broadly, light reflection, or selective absorption or selective reflection). In cases where there are multiple sets of particles with opposite charge polarities, application of an electric field can cause one set of particles to appear at the viewing surface while other particles are driven away from the viewing surface.

[0003] Many electrophoretic displays are bistable, meaning their optical state persists after the activating electric field is removed. Bistability is primarily due to an induced dipole charge layer around the charged pigment, which results from complex interactions between the pigment, charge control agent, and free polymer dispersed in the solvent. Bistable displays can persist for several years in their last addressed optical state before being switched again with the application of a new driving field.

[0004] Driving an electrophoretic display requires a power source to provide an electric field between the electrodes. The power source is typically a battery, which provides power to the electrodes via drive circuitry. One or more electrodes may be incorporated into an active matrix backplane. The power source may be, for example, a photovoltaic cell, a fuel cell, or a power supply operating from wall current.

[0005] The power source can also be a piezoelectric element, which generates an electric charge through physical movement or thermal expansion, as described in U.S. Pat. No. 5,930,026 (incorporated by reference in its entirety). Because electrophoretic displays are typically constructed as films, it is preferable to bond the electrophoretic film to a piezoelectric film. The resulting display is thinner and more flexible. However, only piezoelectric polymers can be made into films that can be bonded to electrophoretic films. In general, crystalline piezoelectric materials are very fragile and not suitable for incorporation into electrophoretic displays.

[0006] Piezoelectric polymer films, including polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyamide, and parylene-C, require a high relative concentration of polarizable regions to be useful. For example, in PVDF, the fluorine atoms in the polymer chains must be locally aligned to achieve a suitable piezoelectric effect. However, as a polymer, when melted, PVDF chains typically adopt their lowest energy state, known as the alpha phase, in which the fluorine atoms are misaligned and as far apart as possible from each other. The alpha phase of PVDF has very little piezoelectric response. Other phases, such as beta and gamma, have fluorine atoms primarily aligned in the same direction and have much greater piezoelectric response. To achieve sufficient beta phase concentration, it is often necessary to stretch piezoelectric polymer films such as PVDF after formation from the melt to induce stress domains that increase polarizability. The stretched polymer film can then be poled to align the beta phase to increase the piezoelectric response, i.e., the strength of the local electric field.

[0007] However, stretching a polymer film after formation introduces inhomogeneities into the film thickness. Stretching can also cause the film to "crack," distorting the light passing through it. It is possible to avoid stretching a piezoelectric polymer film by adding charged inorganic particles, such as clay and carbon black, which increase the local polarizability. However, at the concentration scale where charged additive particles are effective, composite piezoelectric polymer films typically lose their transparency due to a combination of scattering and absorption by the additive particles. Therefore, it is very difficult to produce thin, transparent piezoelectric films with uniform thickness and sufficient beta (or gamma) phase to be useful for piezoelectric applications. Summary of the Invention [Means for solving the problem]

[0008] Disclosed herein are piezoelectric films formed from a mixture of polyvinylidene fluoride (PVDF) and an ionic liquid, and methods for fabricating such piezoelectric films. Due to the presence of the ionic liquid, the resulting piezoelectric film has a higher concentration of beta phase than PVDF alone, and the film does not need to be stretched to increase the proportion of beta phase. Therefore, as discussed below, piezoelectric films can be made thin and transparent and may be suitable for poling, for example, using an electric field. Because piezoelectric films are not stretched, they do not suffer from optical stretching strain, tearing, and thickness variations across the film. Such thickness variations would result in different optical responses in low-voltage electrophoretic films when such films are bonded to piezoelectric films. Therefore, the piezoelectric films described herein enable the creation of security markers, authentication tags, indicators, and strain sensors that function well with electrophoretic films. Such films do not require any external power. They can be switched between optical states using simple mechanical motion (see, for example, Figures 1A and 1B). Nevertheless, it is feasible to use the piezoelectric films described herein with powered electrophoretic displays.

[0009] In a first aspect, the present invention includes a piezoelectric film comprising polyvinylidene fluoride (PVDF) and less than 10% (wt / wt) of an ionic liquid. In some embodiments, the piezoelectric film is less than 10 μm thick. In some embodiments, the ionic liquid comprises an alkyl-substituted imidazolium cation, an alkyl-substituted pyridinium cation, a pyridine-derived N-heterocyclic cation, a fluorinated counteranion, a sulfated counteranion, dicyanamide (N(CN)), a quaternary ammonium cation, or a combination thereof. In some embodiments, the alkyl-substituted imidazolium cation is 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1-butyl-2,3-dimethylimidazolium (DBMIM), 1-octyl-3-methylimidazolium (OMIM), 1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium (DAMI), 1-decyl-3-methylimidazolium (DMIM), 1-dodecyl-3-methyl-dodecylimidazolium, and 1-butyl-2,3-dimethylimidazolium (BMMIM). In some embodiments, the pyridine-derived N-heterocyclic cation is 4-methyl-N-butyl-pyridinium (MBPy) or N-octylpyridinium (CPy). In some embodiments, the fluorinated counteranion is tetrafluoroborate (BF), hexafluorophosphate (PF), bis-trifluoromethanesulfonimide (NTf), or trifluoromethanesulfonic acid (OTf). In some embodiments, the quaternary ammonium cation is tetraethylammonium (TEA) or tetrabutylammonium (TBA). In some embodiments, the sulfated counteranion is hydrogen sulfate (HSO), methyl sulfate (MeOSO), trifluoromethyl sulfate (CFOSO), ethyl sulfate (EtOSO), or perfluoroethyl sulfate (CFCFOSO). In some embodiments, the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM) (BF). In some embodiments, the film is optically transparent.In some embodiments, the piezoelectric film further comprises a copolymer selected from trifluoroethylene (TrFE), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE).

[0010] In some embodiments, the electrophoretic display film can be produced with a thickness (top to bottom) of less than 100 μm and includes a first adhesive layer, an electrophoretic medium layer, a patterned PVDF film with zones of differential polarization, and a flexible, light-transmitting electrode layer. The PVDF film can be any of the types described above. In some embodiments, the electrophoretic medium layer includes a plurality of microcapsules containing a non-polar fluid and charged pigment particles, which migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the microcapsules are bonded to each other using a polymer binder. In some embodiments, the electrophoretic medium layer includes a plurality of microcells containing a non-polar fluid and charged pigment particles, which migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells using a sealing layer. In some embodiments, the film is less than 50 μm thick. In some embodiments, the patterned piezoelectric layer comprises polyvinylidene fluoride (PVDF). In some embodiments, the PVDF is poled to produce zones of differential polarization. In some embodiments, the flexible light-transmitting electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible light-transmitting electrode layer comprises poly(3,4-ethyleneoxythiophene) (PEDOT). In some embodiments, the invention includes an electrophoretic display film assembly comprising a release sheet bonded to an electrophoretic display film as described above, the release sheet being bonded to a first adhesive layer. In some embodiments, a second adhesive layer is bonded to the flexible light-transmitting electrode layer, and the second release sheet is bonded to a second adhesive layer. In some embodiments, the electrophoretic display film can be produced with a thickness (top to bottom) of less than 100 μm and comprises a first adhesive layer, a patterned PVDF film comprising zones of differential polarization, an electrophoretic medium layer, and a flexible light-transmitting electrode layer.The patterned PVDF film can be made according to any of the PVDF films described above. In some embodiments, the electrophoretic display film can be produced with a thickness (top to bottom) of less than 100 μm and comprises an adhesive layer, an electrophoretic medium layer, a patterned PVDF film with zones of differential polarization, and a conductive adhesive layer. The patterned PVDF film can be made according to any of the PVDF films described above.

[0011] In a second aspect, the present invention includes a method for fabricating a piezoelectric film comprising polyvinylidene fluoride (PVDF). The method includes providing a powdered or pelleted polymer comprising PVDF; combining an ionic liquid with PVDF at less than 10% (ionic liquid weight / PVDF weight) to form a mixture; diluting the mixture with an aprotic solvent to form a slurry; casting the slurry onto a release substrate to form a slurry film; heating the slurry film to form a piezoelectric film comprising PVDF; and removing the piezoelectric film comprising PVDF from the release substrate. In some embodiments, the aprotic solvent comprises dimethylformamide (DMF), dimethylacetamide, or 1-methyl-2-pyrrolidone. In some embodiments, the powdered or pelleted polymer comprising PVDF further comprises a copolymer selected from trifluoroethylene (TrFE), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE). In some embodiments, the piezoelectric film is less than 10 μm thick. In some embodiments, the ionic liquid comprises an alkyl-substituted imidazolium cation, an alkyl-substituted pyridinium cation, a pyridine-derived N-heterocyclic cation, a fluorinated counteranion, a sulfated counteranion, dicyanamide (N(CN)2), a quaternary ammonium cation, and combinations thereof. In some embodiments, the alkyl-substituted imidazolium cation is 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1-butyl-2,3-dimethylimidazolium (DBMIM), 1-octyl-3-methylimidazolium (OMIM), 1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium (DAMI), 1-decyl-3-methylimidazolium (DMIM), 1-dodecyl-3-methyl-dodecylimidazolium, and 1-butyl-2,3-dimethylimidazolium (BMMIM). In some embodiments, the pyridine-derived N-heterocyclic cation is 4-methyl-N-butyl-pyridinium (MBPy) or N-octylpyridinium (C8Py).In some embodiments, the fluorinated counteranion is tetrafluoroborate (BF), hexafluorophosphate (PF), bis-trifluoromethanesulfonimide (NTf), or trifluoromethanesulfonic acid (OTf). In some embodiments, the quaternary ammonium cation is tetraethylammonium (TEA) or tetrabutylammonium (TBA). In some embodiments, the sulfated counteranion is hydrogen sulfate (HSO), methyl sulfate (MeOSO), trifluoromethyl sulfate (CFOSO), ethyl sulfate (EtOSO), or perfluoroethyl sulfate (CFCFOSO). In some embodiments, the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM) (BF). In some embodiments, the film is optically transparent. In some embodiments, less than 1% (ionic liquid weight / PVDF weight) ionic liquid is combined with PVDF to form a mixture.

[0012] In some embodiments, an electrophoretic display film can be constructed with a PVDF film prepared using the method described above. The electrophoretic display film can be prepared by bonding a film of polyvinylidene fluoride (PVDF) to a polymer film comprising an acrylate, vinyl ether, or epoxide to produce a piezoelectric microcell precursor film; bonding the piezoelectric microcell precursor film to a flexible light-transmitting electrode layer; bonding the light-transmitting electrode layer to a first release film using a first adhesive layer; embossing the piezoelectric microcell precursor film to produce an array of microcells, each having a bottom, walls, and a top opening; filling the microcells with an electrophoretic medium through the top opening; and sealing the top openings of the filled microcells with a water-soluble polymer. In some embodiments, the method further includes bonding the water-soluble polymer to a second release film using a second adhesive layer. In some embodiments, the method further includes removing the first release film to produce an electrophoretic display film that is less than 100 μm thick. In some embodiments, the electrophoretic medium layer comprises a plurality of microcells containing a non-polar fluid and charged pigment particles, which migrate toward or away from the piezoelectric layer when the piezoelectric layer is bent, and the non-polar fluid and charged pigment particles are sealed within the microcells using a sealing layer. In some embodiments, the PVDF is polarized to create zones exhibiting differential polarization. In some embodiments, the flexible light-transmitting electrode layer comprises a metal oxide comprising tin or zinc. In some embodiments, the flexible light-transmitting electrode layer comprises poly(3,4-ethyleneoxythiophene) (PEDOT). In some embodiments, the polyvinylidene fluoride film is patterned using an electric field to create areas of different polarization. In some embodiments, the method further comprises patterning the completed electrophoretic display film using an electric field to create areas of different polarization in the polyvinylidene fluoride film. The present invention provides, for example, the following items. (Item 1) A piezoelectric film comprising polyvinylidene fluoride (PVDF) and less than 10% (wt / wt) ionic liquid. (Item 2) Item 10. The piezoelectric film of item 1, wherein the piezoelectric film has a thickness of less than 10 μm. (Item 3) The ionic liquids may contain alkyl-substituted imidazolium cations, alkyl-substituted pyridinium cations, pyridine-derived N-heterocyclic cations, fluorinated counter anions, sulfated counter anions, dicyanamide (N(CN) 2 Item 1, wherein the piezoelectric film comprises a quaternary ammonium cation, a quaternary ammonium cation, or a combination thereof. (Item 4) the alkyl-substituted imidazolium cation is 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1-butyl-2,3-dimethylimidazolium (DBMIM), 1-octyl-3-methylimidazolium (OMIM), 1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium (DAMI), 1-decyl-3-methylimidazolium (DMIM), 1-dodecyl-3-methyl-dodecylimidazolium, and 1-butyl-2,3-dimethylimidazolium (BMMIM); or The pyridine-derived N-heterocyclic cation is 4-methyl-N-butyl-pyridinium (MBPy) or N-octylpyridinium (C 8 Py), or The fluorinated counter anion is tetrafluoroborate (BF 4 ), hexafluorophosphate (PF 6 ), bis-trifluoromethanesulfonimide (NTf 2 ), or trifluoromethanesulfonic acid (OTf), or the quaternary ammonium cation is tetraethylammonium (TEA) or tetrabutylammonium (TBA), or The sulfated counter anion is hydrogen sulfate (HSO 4 ), methyl sulfate (MeOSO 3 ), trifluoromethyl sulfate (CF 3 OSO 3 ), ethyl sulfate (EtOSO 3 ), or perfluoroethyl sulfate (CF 3 CF 2 OSO 3 ) or The ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM) (BF 4 4. The piezoelectric film according to item 3, wherein (Item 5) Item 10. The piezoelectric film according to item 1, wherein the piezoelectric film is optically transparent. (Item 6) Item 10. The piezoelectric film of item 1, further comprising a copolymer selected from trifluoroethylene (TrFE), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE). (Item 7) 1. An electrophoretic display film having a thickness (top to bottom) of less than 100 μm, said electrophoretic display film comprising, in order: a first adhesive layer (520); an electrophoretic medium layer (530); Item 1, a patterned piezoelectric film (410, 560) comprising zones of differential polarization (460, 470); a flexible, light-transmitting electrode layer (580); An electrophoretic display film comprising: (Item 8) 1. An electrophoretic display film having a thickness (top to bottom) of less than 100 μm, said electrophoretic display film comprising, in order: a first adhesive layer (520); Item 1, a patterned piezoelectric film (410, 560) comprising zones of differential polarization (460, 470); an electrophoretic medium layer (530); a flexible, light-transmitting electrode layer (580); An electrophoretic display film comprising: (Item 9) 1. An electrophoretic display film having a thickness (top to bottom) of less than 100 μm, said electrophoretic display film comprising, in order: an adhesive layer (520); an electrophoretic medium layer (530); Item 1, a patterned piezoelectric film (410, 560) comprising zones of differential polarization (460, 470); A conductive adhesive layer (570) and An electrophoretic display film comprising: (Item 10) the electrophoretic medium layer (530) comprises a plurality of microcapsules (990) containing a non-polar fluid (425) and charged pigment particles (423, 427), which migrate toward or away from the patterned piezoelectric film (410, 560) when the patterned piezoelectric film (410, 560) is bent, and the microcapsules (990) are bonded to one another using a polymeric binder (995); or 10. An electrophoretic display film according to any one of items 7-9, wherein the electrophoretic medium layer (530) comprises a plurality of microcells (420) containing a non-polar fluid (425) and charged pigment particles (423, 427), the charged pigment particles (423, 427) moving towards or away from the patterned piezoelectric film (410, 560) when the patterned piezoelectric film (410, 560) is bent, and the non-polar fluid (425) and charged pigment particles (423, 427) are sealed within the microcells using a sealing layer (430, 540). (Item 11) 10. The electrophoretic display film according to any of items 7-9, wherein the electrophoretic display film is less than 50 μm thick. (Item 12) 10. The electrophoretic display film according to any one of items 7-9, wherein the flexible light-transmitting electrode layer (580) comprises a metal oxide comprising tin or zinc, or poly(3,4-ethyleneoxythiophene) (PEDOT). (Item 13) Item 7. An electrophoretic display film assembly comprising a release sheet (510) bonded to the electrophoretic display film described in item 7, wherein the release sheet (510) is bonded to the first adhesive layer (520). (Item 14) Item 14. The electrophoretic display film assembly of item 13, further comprising a second adhesive layer bonded to the flexible light-transmitting electrode layer and a second release sheet bonded to the second adhesive layer. [Brief explanation of the drawings]

[0013] [Figure 1A] 1A shows a side view of a piezoelectric display film of the present invention, which includes star-shaped areas of differential polarization. Three exemplary positions, namely, convex, neutral, and concave, are shown from the side. The total thickness of the piezoelectric display film can be less than 100 μm, for example, less than 50 μm, for example, less than 25 μm.

[0014] [Figure 1B] 1B shows a top view of a piezoelectric display film of the present invention, which includes star-shaped areas of differential polarization. Three exemplary positions, convex, neutral, and concave, are shown from above. When the piezoelectric display film is bent, the areas of differential polarization result in oppositely charged particles appearing at the viewing surface.

[0015] [Figure 2A] FIG. 2A shows an exemplary thin layer of piezoelectric material on a substrate.

[0016] [Figure 2B] 2B illustrates how areas of differential polarization can be created in a thin layer of piezoelectric material by using the strong electric field of a corona discharge. By moving the piezoelectric material closer and farther from the discharge, the amount of polarization can be spatially controlled.

[0017] [Figure 2C] 2C illustrates how areas of differential polarization can be created in a thin layer of piezoelectric material by using the strong electric field of a corona discharge. A conductive mask is used to pattern the piezoelectric material and create areas of differential polarization.

[0018] [Figure 2D] FIG. 2D illustrates a polarization pattern that can be achieved using the methods of FIGS. 2B and 2C.

[0019] [Figure 3A] FIG. 3A illustrates a side view of a piezoelectric film poled in the A direction.

[0020] [Figure 3B] FIG. 3B illustrates a top view of a piezoelectric film poled in the A direction.

[0021] [Figure 3C] FIG. 3C illustrates a side view of a piezoelectric film poled in the G direction using a conductive mask.

[0022] [Figure 3D] FIG. 3D illustrates a top view of a piezoelectric film poled in the G direction using a conductive mask.

[0023] [Figure 4A]FIG. 4A shows an exemplary thin layer of piezoelectric microcellular precursor film on a substrate.

[0024] [Figure 4B] 4B illustrates how the strong electric field of a corona discharge can be used to create areas of differential polarization within a thin layer of piezoelectric material in a piezoelectric microcellular precursor film. By moving the piezoelectric microcellular precursor film closer and farther from the discharge, the amount of polarization can be spatially controlled.

[0025] [Figure 4C] 4C illustrates a method for creating areas of differential polarization within a thin layer of piezoelectric material of a piezoelectric microcellular precursor film by using the strong electric field of a corona discharge. A conductive mask is used to pattern the piezoelectric material of the piezoelectric microcellular precursor film to create areas of differential polarization.

[0026] [Figure 4D] FIG. 4D illustrates a poling pattern within a piezoelectric microcellular precursor film that can be achieved using the methods of FIGS. 3B and 3C.

[0027] [Figure 5A] FIG. 5A is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0028] [Figure 5B] FIG. 5B is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0029] [Figure 5C] FIG. 5C is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0030] [Figure 5D] FIG. 5D is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0031] [Figure 6A] FIG. 6A is a schematic cross-sectional view of an embodiment of a piezoelectric display.

[0032] [Figure 6B] FIG. 6B is a schematic cross-sectional view of an embodiment of a piezoelectric display.

[0033] [Figure 7] FIG. 7 details a method for producing a piezophoretic film or (optionally) a display.

[0034] [Figure 8A] FIG. 8A is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0035] [Figure 8B] FIG. 8B is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0036] [Figure 9A] FIG. 9A is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0037] [Figure 9B] FIG. 9B is a schematic cross-sectional view of an embodiment of a piezoelectric film.

[0038] [Figure 10A] FIG. 10A is a schematic cross-sectional view of an embodiment of a piezoelectric display.

[0039] [Figure 10B] FIG. 10B is a schematic cross-sectional view of an embodiment of a piezoelectric display.

[0040] [Figure 10C] FIG. 10C is a schematic cross-sectional view of an embodiment of a piezoelectric display.

[0041] [Figure 11] FIG. 11 details a method for producing piezoelectric films containing ionic liquids.

[0042] [Figure 12] FIG. 12 shows the infrared spectrum of the piezoelectric film containing the ionic liquid and a comparison with the native polymer and other methods of increasing the beta phase in the polymer. DETAILED DESCRIPTION OF THE INVENTION

[0043] Disclosed herein are piezoelectric films containing ionic liquids and methods for making piezoelectric films containing ionic liquids. The resulting films have a higher level of beta phase and can be poled using an external field without additional processing, such as stretching. The films are optically transparent. The films are generally flexible. Some films are less than 100 μm thick. In some embodiments, the piezoelectric film can be combined with an electrophoretic medium to produce a piezoelectric film, and the resulting piezoelectric film can be patterned after fabrication using a high-voltage electric field. This feature allows end users of the piezoelectric film to address the piezoelectric material during production, for example, using corona discharge, which can include, for example, a bar code or serial number that is only visible when the piezoelectric film is manipulated. Such films are useful as security markers, authentication films, indicators, or sensors. The films are generally flexible. Some films are less than 100 μm thick. In some embodiments, the piezoelectric film is less than 50 μm and can be folded without breaking. Displays formed using the film do not require an external power source.

[0044] The term "electro-optic," as applied to a material or display, is used herein to refer to a material having first and second display states that differ in at least one optical property in its conventional sense in the imaging arts, and that can be changed from its first display state to its second display state by the application of an electric field to the material. The optical property is typically color perceptible to the human eye, but can also be another optical property such as optical transmittance, reflectance, luminescence, or, in the case of displays intended for machine reading, pseudocolor in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

[0045] The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states that differ in at least one optical property, such that after any given element is driven with a finite-duration address pulse to exhibit either the first or second display state, that state will persist after the address pulse is terminated for at least several times, e.g., at least four times, the minimum duration of the address pulse required to change the state of the display element. U.S. Pat. No. 7,170,670 indicates that some grayscale-capable particle-based electrophoretic displays are stable not only in their extreme black-and-white states but also in their intermediate gray states, and the same is true for several other types of electro-optic displays. Displays of this type are properly referred to as "multistable" rather than bistable, although for convenience the term "bistable" may be used herein to encompass both bistable and multistable displays.

[0046] The term "gray state" is used herein in its conventional sense in the imaging arts to refer to a state intermediate between two extreme pixel optical states, and does not necessarily imply a black-to-white transition between these two extreme states. For example, several of the E INK patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, such that the intermediate "gray state" is actually light blue. In fact, as already noted, a change in optical state may not be a change in color at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of a display and should generally be understood to include extreme optical states that are not strictly black and white, such as the aforementioned white and dark blue states. The term "monochrome" may be used hereinafter to refer to a display or drive scheme that drives pixels exclusively to its two extreme optical states, with no intervening gray states.

[0047] The term "pixel" is used herein in its conventional sense in the display art to mean the smallest unit of a display capable of producing all the colors the display itself can show. In full-color displays, each pixel typically consists of multiple sub-pixels, each of which is capable of displaying less than all the colors the display itself can show. For example, in most conventional full-color displays, each pixel consists of a red sub-pixel, a green sub-pixel, a blue sub-pixel, and optionally a white sub-pixel, each of which is capable of displaying a range of colors from black to the brightest version of its specified color.

[0048] Several types of electro-optic displays are known. One type of electro-optic display uses an electrochromic medium, for example, in the form of a nanochromic film consisting of electrodes formed at least in part from a semiconducting metal oxide and a plurality of dye molecules attached to the electrodes that can reverse their color change. See, for example, O'Regan, B., et al., Nature 1991, 353, 737 and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Patent Nos. 6,301,038, 6,870,657, and 6,950,220. This type of medium is also typically bistable.

[0049] Another type of electro-optic display is the electrowetting display developed by Philips and described in Hayes, R.A., et al., "Video-Speed ​​Electronic Paper Based on Electrowetting," Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 shows that such electrowetting displays can be bistable.

[0050] Another type of electro-optic display that has been the subject of intensive research and development for many years is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared to liquid crystal displays.

[0051] Electro-optical devices typically comprise a layer of electrophoretic material and at least two other layers disposed on opposite sides of the electrophoretic material, one of these two layers being an electrode layer. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongated row electrodes and the other into elongated column electrodes extending perpendicular to the row electrodes, with pixels defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode, and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electrophoretic display intended for use with a stylus, print head, or similar movable electrode separate from the display, only one of the layers adjacent to the electrophoretic layer comprises an electrode, and the layer opposite the electrophoretic layer is typically a protective layer intended to prevent the movable electrode from damaging the electrophoretic layer.

[0052] Numerous patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise a multiplicity of small capsules, each of which itself comprises an internal phase containing electrophoretically movable particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymer binder to form a coherent layer positioned between two electrodes. Techniques described in these patents and applications include the following: (a) electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814); (b) capsules, binders, and encapsulation processes (see, e.g., U.S. Patent Nos. 6,922,276 and 7,411,719); (c) films and subassemblies containing electro-optical materials (see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564); (d) backplanes, adhesive layers, and other auxiliary layers and methods used in displays (see, e.g., U.S. Patent Nos. 7,116,318 and 7,535,624); (e) color formation and color control (see, e.g., U.S. Patent Nos. 7,075,502 and 7,839,564); (f) methods of driving displays (see, e.g., U.S. Patent Nos. 7,012,600 and 7,453,445); (g) display applications (see, e.g., U.S. Patent Nos. 7,312,784 and 8,009,348); (h) non-electrophoretic displays (see U.S. Patent Nos. 6,241,921, 6,950,220, 7,420,549, and 8,319,759, and U.S. Patent Application Publication No. 2012 / 0293858); (i) Microcell structures, wall materials, and methods of forming the microcells (see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906); (j) Methods for filling and sealing microcells (see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088).

[0053] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus giving rise to so-called "polymer-dispersed electrophoretic displays," in which the electrophoretic medium consists of multiple discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and that the discrete droplets of electrophoretic fluid in such polymer-dispersed electrophoretic displays may be considered capsules or microcapsules, even though no discrete capsule membrane is associated with each individual droplet. See, for example, the aforementioned U.S. Patent No. 6,866,760. Therefore, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0054] A related type of electrophoretic display is the microcell electrophoretic display, also known as a MICROCUP®. In a microcell electrophoretic display, the charged particles and fluid are not encapsulated in microcapsules, but instead are held within a plurality of cavities formed in a carrier medium, typically a polymeric film. See, e.g., U.S. Patent Nos. 6,672,921 and 6,788,449 (both of which are incorporated by reference in their entirety).

[0055] Although electrophoretic media are often opaque (e.g., because in many electrophoretic media the particles substantially block the transmission of visible light through the display) and can operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "obscured mode," in which one display state is substantially opaque and one is light transmissive. See, e.g., U.S. Patent Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode. See U.S. Patent No. 4,418,346. Other types of electro-optic displays may also be capable of operating in a obscured mode. Electro-optic media operating in the shielding mode can be useful in multilayer structures for full-color displays, in which at least one layer adjacent to the viewing surface of the display operates in the shielding mode to expose or obscure a second layer more remote from the viewing surface.

[0056] Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices and offer additional advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (The use of the word "printing" is intended to include all forms of printing and coating, including, but not limited to, pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coatings such as knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, silk screen printing processes, electrostatic printing processes, thermal printing processes, inkjet printing processes, electrophoretic deposition (see U.S. Pat. No. 7,339,715), and other similar techniques.) Thus, the resulting display can be flexible. Furthermore, because the display medium can be printed using a variety of methods, the display itself can be made inexpensively.

[0057] The aforementioned U.S. Patent No. 6,982,178 describes a method for assembling solid electro-optic displays (including particle-based electrophoretic displays) that is well adapted for mass production. Essentially, this patent describes a so-called "front plane laminate" ("FPL"), which comprises, in order, a light-transmitting conductive layer, a layer of a solid electro-optic medium in electrical contact with the conductive layer, an adhesive layer, and a release sheet. Typically, the light-transmitting conductive layer may be supported on a light-transmitting substrate, which is preferably flexible in the sense that the substrate may be manually wound around a 10-inch (254 mm) diameter drum (for example) without permanent deformation. The term "light-transmitting" is used in this patent and herein to mean that the layer so designated transmits sufficient light to enable an observer looking through the layer to observe a change in the display state of the electro-optic medium, typically visible through the conductive layer and adjacent substrate (if present); if the electro-optic medium exhibits a change in reflectance at non-visible wavelengths, the term "light-transmitting" should, of course, be interpreted to refer to the transmission of the relevant non-visible wavelengths. The substrate is typically a polymer film and will usually have a thickness within the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). The conductive layer may conveniently be a thin metal or metal oxide layer, e.g., aluminum or ITO, or may be a conductive polymer. Aluminum or ITO coated poly(ethylene terephthalate) (PET) films are commercially available, for example, as "Aluminized Mylar" ("Mylar" is a registered trademark) from EI du Pont de Nemours & Company (Wilmington, Del.), and such commercial materials can be used with good results in front plane lamination.

[0058] Assembly of an electro-optic display using such a front plane laminate can be accomplished by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium, and conductive layer to the backplane. This process is well suited to mass production, as the front plane laminate can typically be mass-produced using roll-to-roll coating techniques and then cut into pieces of any size required for use with a particular backplane.

[0059] U.S. Patent No. 7,561,324 describes a so-called "dual release sheet," which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Patent No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling electro-optic displays from the front plane laminates already described, but involving two separate laminations; typically, in a first lamination, the double release sheet is laminated to the front plane electrodes to form the front plane assembly, and then, in a second lamination, the front plane assembly is laminated to the backplane to form the final display, although the order of these two laminations can be reversed, if desired.

[0060] The subject matter presented herein particularly relates to piezoelectric display structural designs that do not require a power source (e.g., a battery or wired power supply, a photovoltaic source, etc.) for the electrophoretic display to operate. Assembly of such electrophoretic displays is therefore simplified. In some embodiments, the piezoelectric material and the electrophoretic medium are directly laminated together. The electrophoretic medium may be contained within microcells, microcapsules, as described above, or the electrophoretic medium may be dispersed in a polymer matrix. In some embodiments, the piezoelectric material is poled (i.e., written) using a high-voltage electric field after the piezoelectric film or piezoelectric display is produced, as discussed below.

[0061] Piezoelectricity is an electric charge that accumulates in a solid material in response to applied mechanical stress. Suitable polymer materials for the subject matter disclosed herein include polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyamide, and parylene-C. The piezoelectric films and displays described herein use piezoelectricity to drive charged pigments in an electrophoretic medium. Thus, when a piezoelectric material bonded to an electrophoretic medium layer is manipulated, the color of the electrophoretic material at the viewing surface changes. For example, by bending or stressing a piece of piezoelectric material, a voltage can be generated, which can be used to cause the color pigments in the electrophoretic material to move. When segments of piezoelectric material with different polarizations are used, or when areas of differential polarization are created within the piezoelectric film, an electrophoretic medium with two types of oppositely charged pigments can be used to create patterns with high contrast ratios, as shown in FIGS. 1A and 1B. As used herein, the term "contrast ratio" (CR) in reference to electro-optic displays (e.g., electrophoretic displays) is defined as the ratio of the brightness of the brightest color (white) to the darkest color (black) that the display is capable of producing. Typically, a high contrast ratio, or CR, is a desirable aspect of a display.

[0062] 1A and 1B illustrate side and top views of an exemplary piezoelectric phoretic display 100 according to the presently disclosed subject matter. In this embodiment, a piezoelectric material of the present invention is laminated to an electrophoretic medium layer (discussed below), and one or more electrodes are included to provide a suitable electric field to propel the electrophoretic particles toward (or away from) a viewing surface. In the embodiment shown in FIGS. 1A and 1B, a second area 120 of the piezoelectric film of the piezoelectric phoretic display 100 is polarized in the opposite direction to the first area 110. Thus, when the piezoelectric phoretic display 100 is manipulated from a neutral state (Position 2) to either a first (Position 1) or second (Position 3) optical state, the first and second areas (110, 120) will achieve different colors within the two areas. In the case of an electrophoretic medium having a set of black and white oppositely charged particles, a high-contrast image will be formed, for example, as shown in FIG. 1B. Because the first and second areas (110, 120) of piezoelectric material can be polarized with good resolution (as discussed below), various images / information can be encoded to "appear" when the piezoelectric phoretic display 100 is manipulated. For example, a security ribbon can be generated to exist in a neutral state as a gray strip, but when the security ribbon is bent, the ribbon will display a security seal, such as the star shape shown in FIG. 1B. Of course, the security seal could alternatively include a barcode, numbers, words, phone numbers, internet addresses, QR codes, photographs, half-tone images, or logos.

[0063] The piezoelectric film of the present invention comprises a piezoelectric polymer and less than 10% ionic liquid. The piezoelectric film has a sufficient amount of polarizable polymer domains to be useful as a piezoelectric material without the need for stretching / pulling or heat treatment such as annealing. The piezoelectric polymer can be polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyamide, parylene-C, or a combination thereof; however, PVDF is preferred because it is commercially available in powder and pellet form, for example, from Sigma-Aldrich or Arkema. A preferred formulation is available under the trade name KYNAR®. PVDF can include copolymers such as trifluoroethylene (TrFE), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE). PVDF copolymers are also available as raw materials; however, PVDF copolymers are typically 100 to 10,000 times more expensive than simple PVDF. The ionic liquid may be present in the final film at a concentration of 10% by weight or less of the ionic liquid relative to the polymer, but the active piezoelectric film may contain 5% by weight or less of the ionic liquid, for example, 2% by weight or less of the ionic liquid, for example, 1% by weight or less of the ionic liquid, for example, 0.5% by weight or less of the ionic liquid, for example, 0.1% by weight or less of the ionic liquid, for example, 500 ppm by weight or less of the ionic liquid, for example, 250 ppm by weight or less of the ionic liquid, for example, 100 ppm by weight or less of the ionic liquid. The piezoelectric film can be made very thin, for example, 10 μm or less, for example, 7 μm or less, for example, 5 μm or less, for example, 3 μm or less, for example, about 1 μm thick. Using such a film, for example, as discussed below, it is possible to produce a piezoelectric film of 100 μm or less.

[0064] Suitable ionic liquids for incorporation into the piezoelectric films of the present invention include alkyl-substituted imidazolium cations, alkyl-substituted pyridinium cations, pyridine-derived N-heterocyclic cations, fluorinated counter anions, sulfated counter anions, dicyanamide (N(CN)2), quaternary ammonium cations, or combinations thereof. For example, alkyl-substituted imidazolium cations can include 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-3-methylimidazolium (EMIM), 1-butyl-2,3-dimethylimidazolium (DBMIM), 1-octyl-3-methylimidazolium (OMIM), 1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium (DAMI), 1-decyl-3-methylimidazolium (DMIM), 1-dodecyl-3-methyl-dodecylimidazolium, and 1-butyl-2,3-dimethylimidazolium (BMMIM). Pyridine-derived N-heterocyclic cations can include 4-methyl-N-butyl-pyridinium (MBPy) or N-octylpyridinium (CPy). The fluorinated counteranion may include tetrafluoroborate (BF), hexafluorophosphate (PF), bis-trifluoromethanesulfonimide (NTf), or trifluoromethanesulfonic acid (OTf). The quaternary ammonium cation may include tetraethylammonium (TEA) or tetrabutylammonium (TBA). The sulfated counteranion may include hydrogen sulfate (HSO), methyl sulfate (MeOSO), trifluoromethyl sulfate (CFOSO), ethyl sulfate (EtOSO), or perfluoroethyl sulfate (CFCFOSO). In a preferred embodiment, the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM) (BF). EMIM-BF is commercially available from Sigma-Aldrich.

[0065] An exemplary method for producing a piezoelectric film with an ionic liquid is shown in FIG. 11. The method begins with combining a raw piezoelectric polymer (or raw piezoelectric copolymer) with an aprotic solvent (such as dimethylformamide (DMF), dimethylacetamide, dimethylsulfoxide (DMSO), or 1-methyl-2-pyrrolidone (NMP)) with a suitable amount of ionic liquid, as in step 10. In a preferred embodiment, the piezoelectric polymer is PVDF, the aprotic solvent is DMF, and the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM) (BF4). The ingredients are mixed together with heating to produce a slurry, as shown in step 20. Typically, only 2-10 wt. % aprotic solvent is required to produce a usable melt. The ingredients only need to be heated to approximately 70°C with regular stirring, such as with a Teflon paddle. It is also possible to produce a melt of only PVDF and (EMIM)(BF4) by starting with a dry mixture (i.e., no solvent) and raising the temperature to about 180°C; however, casting this hot mixture requires a specially fabricated die. Once the slurry is prepared, it can be cast onto a web / release agent using a roll-to-roll process of the type described in U.S. Pat. No. 6,930,818 (incorporated by reference in its entirety). In a preferred embodiment, the web / release agent is PET, and the slot die is only about 5 μm above the surface of the web (see steps 30 and 40). The web with the cast piezoelectric polymer / ionic liquid slurry then moves to an oven area, where the DMF is removed using heat (about 160°C), as shown in step 50. The coated web is allowed to cool, at which point the piezoelectric film bearing the ionic liquid can be peeled from the web, or the web can be sliced ​​into sections for later use, i.e., as shown in step 60.

[0066] The resulting piezoelectric material has a high proportion of beta phase (see examples), as shown in Figures 2A-3D, and can be polarized using a strong localized electric field. As discussed above, it is known that piezoelectric films can be stimulated to move between polarization states using various external stresses, such as mechanical stretching, heat, electromagnetic fields, and applied forces. The piezoelectric effect is closely related to the generation of electric dipole moments in solids. The dipole density or polarization (P) corresponds to the dipole moment / volume of the crystallographic unit cell and is typically expressed as C / m 2 The resulting dipole density P is measured as ρ = 1 / 2 . The resulting dipole density P is a vector field (i.e., differential polarization) that is specific for a particular region of the material. Similar to magnets, dipoles near each other tend to align in regions (Weiss domains). The Weiss domains in the piezoelectric films of the present invention are primarily clusters of beta phase that are aligned together. When initially produced, the domains within a larger film are typically randomly oriented (into or out of the film surface, laterally, etc.). However, using various multi-step processes, the domains can be aligned to produce localized areas of differential polarization. The process of aligning these regions is known as poling.

[0067] An exemplary method for poling a thin film of piezoelectric polymer with an ionic liquid is illustrated in Figures 2A-2D. As shown in Figures 2B and 2C, a thin film of piezoelectric material 210 can be polarized using a high-voltage corona discharge 230 with spatial focusing. Suitable corona discharge equipment is available, for example, from Simco-Ion (Alameda, CA). Such devices can generate localized 10-50 kV fields, e.g., 30 kV fields, e.g., 20 kV fields, that can be brought within a few microns of the piezoelectric material to be polarized. Spatial focusing can be achieved using a steering electric field and / or gas flow, which focuses / steers the flow of ions emanating from the corona discharge. As shown in Figure 2B, the high-voltage corona discharge 230 can be moved in three dimensions to create areas of differential polarization, i.e., pattern the piezoelectric material 210. Alternatively, the piezoelectric material 210 can be mounted on an XYZ stage, which allows the film workpiece to approach the high-voltage corona discharge 230 in a controlled manner. In an alternative embodiment, a conductive mask 240 can be used to protect areas of the piezoelectric material 210 from the high-voltage corona discharge 230, as shown in FIG. 2C . The conductive mask can be made, for example, from conductive stainless steel or another conductive material that can withstand proximity to the corona discharge. Alternative masks made from charge-absorbing or charge-blocking materials, such as glass, plastic, or rubber, would also work. When the high-voltage corona discharge 230 is moved over the thin film of piezoelectric material 210, the thin film of piezoelectric material 210 is polarized only in areas where the conductive mask 240 does not cover the thin film of piezoelectric material 210. Additionally, the polarity of the high-voltage corona discharge 230 can be reversed so that some areas are polarized in a first direction, some areas are polarized in a second direction, and some areas can be randomly polarized or unpolarized. See also Figures 3A-3D.

[0068] Using the technique shown in FIGS. 2B and 2C, it is straightforward to produce a thin film of piezoelectric material 210 with areas of differential polarization P1 and P2, shown as 260 and 270 in FIG. 2D. The areas of differential polarization 260 and 270 do not necessarily have equal and opposite polarities; however, such an arrangement generally provides a better contrast ratio when a two-particle electrophoretic medium is used with the thin film of piezoelectric material 210. For example, as shown in FIG. 2D, the first area 260 can be polarized toward the viewer, while the second area 270 can be polarized away from the viewer. This technique is further illustrated in FIGS. 3A-3D, which show how a single area 360 of a thin film of piezoelectric material deposited on a substrate 320 can be polarized to have a polarization vector pointing out of the page, as shown in FIG. 3B. Thus, when the thin film of piezoelectric material is manipulated (bent), it will preferentially drive one polarity of the electrophoretic particles toward the viewing surface. As shown in FIG. 3C, a second area 370 of the thin film of piezoelectric material can be polarized in a different direction, with or without the addition of a conductive mask 340, depending on the needs of the application, resulting in several patterned combinations of polarity and magnitude. As shown in FIG. 3D, some portions of 370 are polarized toward the viewing surface, but shading is created by the conductive mask 340. Thus, when the piezoelectric material is manipulated (bent), it will preferentially drive one polarity of the electrophoretic particles toward the viewing surface, except in the areas where the polarization is masked, which will remain a neutral color phase, thereby creating a pattern, such as a security seal.

[0069] 2A-3D illustrate various techniques that can be used to create areas of differential polarization in a thin film of piezoelectric material 210. As illustrated in FIGS. 4A-4D, these same techniques can also be used to create areas of differential polarization in a thin piezoelectric medium film 405. As shown in FIG. 4A, a thin film of piezoelectric material 410 can be bonded to a layer of electrophoretic microcell 420 to create piezoelectric medium film 405. The thin film of piezoelectric material 410 can be bonded to the layer of electrophoretic microcell 420 using an adhesive layer (not shown), or the thin film of piezoelectric material 410 can be directly, i.e., slot coated onto the layer of electrophoretic microcell 420, as discussed above with respect to FIG. 2A. Electrophoretic microcell 420 is typically formed from a polymer such as an acrylate, vinyl ether, or epoxide, as described in detail in, for example, U.S. Patent Nos. 6,930,818, 7,052,571, 7,616,374, 8,361,356, and 8,830,561, all of which are incorporated by reference in their entireties. In some embodiments, a layer of electrophoretic microcell 420 may be filled with an electrophoretic medium 425, typically including two or more electrophoretic particles 423 and 427 having different electrophoretic mobilities and optical properties. The electrophoretic medium 425 may be sealed with a sealing layer 430, preferably a water-soluble sealing layer, as described in U.S. Patent Nos. 7,560,004, 7,572,491, 9,759,978, or 10,087,344 (all of which are incorporated by reference in their entireties). In some embodiments, a layer of electrophoretic microcells 420 is produced on a release material, filled with electrophoretic medium 425, and sealed with sealing layer 430, and the filled and sealed electrophoretic microcells 420 are then used as a substrate for the production of a thin film of piezoelectric material 410. The resulting structure is a thin piezoelectric medium film 405. In other embodiments, the thin film of piezoelectric material 410 is laminated to an acrylate, vinyl ether, or epoxide film that is a precursor to the layer of electrophoretic microcells 420.The combined thin film of piezoelectric material 410 and precursor material is then embossed on the precursor side (discussed below) and subsequently filled with electrophoretic medium 425 and sealed with sealing layer 430 to produce thin piezoelectric medium film 405. In yet another embodiment (not shown in FIGS. 4A-4D ), a complete microcellular front plane laminate of the type described in U.S. Pat. No. 7,158,282 and commercially available from E Ink Corporation can be used as a substrate for the thin film of piezoelectric material 410, which can be poled as described below. Notably, when a front plane laminate material is used, the final structure additionally includes a conductive layer, which is typically optically transparent. The front plane laminate can be oriented so that the optically transparent electrode layer contacts the thin film of piezoelectric material 410, or the front plane laminate can be flipped over so that the sealing layer contacts the thin film of piezoelectric material 410.

[0070] Once the thin piezoelectric medium film 405 is produced, the thin film of piezoelectric material 410 can be addressed as described with respect to FIGS. 2A-3D above. That is, the thin film of piezoelectric material 410 can be polarized using a high-voltage corona discharge 230 with spatial focus, as shown in FIG. 4B , for example, by mounting the thin piezoelectric medium film 405 on an XYZ stage that allows the film workpiece to approach the high-voltage corona discharge 230 in a controlled manner. In an alternative embodiment, a conductive mask 240 can be used to protect areas of the thin piezoelectric medium film 405 from the high-voltage corona discharge 230, as shown in FIG. 4C . As discussed with respect to FIGS. 2A-3D , the polarity of the high-voltage corona discharge 230 can be reversed, so that some areas are polarized in a first direction, some areas are polarized in a second direction, and some areas are randomly polarized or unpolarized. As shown in FIG. 2D above, poling the thin film of piezoelectric material 410 in thin piezoelectric medium film 405 results in areas of polarization P1 and P2 exhibiting differences shown as 460 and 470 in FIG. 4D . Importantly, because thin piezoelectric medium film 405 can be fabricated prior to poling, it is feasible for the end customer to control the final step of creating the desired polarization design in thin piezoelectric medium film 405. Thus, if the final product will include a security seal or serial number, the security seal or serial number can be installed after the final product is completed and verified, etc. For example, a U.S. $100 bill is printed at the U.S. Treasury with a metallic ink, along with a serial number, and at the same time, the security ribbon comprising thin piezoelectric medium film 405 can be polarized to create a verification code corresponding to the serial number. This feature eliminates many logistical issues and associated costs, for example, by not having to match pre-fabricated security markers to specific products further downstream in the supply chain.

[0071] The techniques described above can be used to achieve a wide variety of thin piezophoretic films, as illustrated in the following figures.

[0072] As shown in Figures 5A-6B and 8A-10C, a piezoelectric film or display comprises a layered stack of several components, including a thin piezoelectric film and a layer of electrophoretic medium. The piezoelectric material can be PVDF-EMIM-BF4; however, any of the materials listed above can be used, as they can be fabricated into very thin films. The electrophoretic medium typically contains one or more sets of charged particles that migrate through a nonpolar solvent in the presence of an electric field. The electrophoretic medium is typically contained within microcapsules, microcells, or dispersed droplets. The electrophoretic medium can also be contained within an open valley or well sealed within a larger flexible container. The piezoelectric films and displays exemplified herein can be made very thin, e.g., 100 μm or less in thickness, e.g., 70 μm or less in thickness, e.g., 50 μm or less in thickness, e.g., 35 μm or less in thickness, e.g., 20 μm or less in thickness, e.g., 10 μm or less in thickness. Such thin materials can bend without breaking or leaking and are unobtrusive when incorporated into final products such as paper or banknotes. In addition, many piezoelectric films or displays include layers, all of which are optically transparent and / or thin enough to be optically transparent, thus allowing the piezoelectric response to be viewed from above and below. In such piezoelectric films or displays, when a first image is viewable from the top surface, e.g., position 1 in FIG. 1B, the bottom surface will typically show an inverted image, e.g., position 3 in FIG. 1B. However, if the electrophoretic medium contains three layers, e.g., When incorporating these types of particles, the top and bottom may not show inverse images due to mixed particle conditions on one of the two surfaces.

[0073] Piezoelectric films or displays often include at least one electrode layer, which may be optically transparent and flexible. Suitable materials include commercially available ITO-coated PET, which may be used as a substrate for fabrication. In some other embodiments, flexible and transparent conductive coatings including other transparent conductive oxides (TCOs) may be used, such as zinc oxide, zinc tin oxide, indium zinc oxide, aluminum zinc oxide, indium tin zirconium oxide, indium gallium oxide, indium gallium zinc oxide, or fluorinated versions of these oxides, such as fluorine-doped tin oxide. In many of the embodiments described herein, poly(3,4-ethyleneoxythiophene) polystyrene sulfonate (PEDOT:PSS) is used because it has excellent bending properties and is optically transparent. While its overall conductivity is not as high as, for example, PET / ITO, PEDOT:PSS is sufficient to provide the electric field necessary to drive electrophoretic particles within the electrophoretic medium. Other materials include polymers, typically optically transparent polymers doped with conductive materials such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. In some cases, the electrode layer is a metal film such as a copper, silver, gold, or aluminum film or foil. Metal-coated polymer films may also be suitable for use as electrode layers. The resistance of the electrode layer may be 500 ohm-meters or less, e.g., 100 ohm-meters or less, e.g., 1 ohm-meter or less, e.g., 0.1 ohm-meters or less, e.g., 0.01 ohm-meters or less. (For comparison, electrophoretic medium layers typically have a resistance of about 10 7 ~10 8 ohm-meter resistance, and the piezoelectric material 11 ~10 14 It has a resistance in ohms.)

[0074] Piezoelectric films or displays often include at least one adhesive layer formed from a polymer such as acrylic or polyurethane, polyurethane, polyurea, polycarbonate, polyamide, polyester, polycaprolactone, polyvinyl alcohol, polyether, polyvinyl acetate derivatives such as poly(ethylene-co-vinyl acetate), polyvinyl fluoride, polyvinylidene fluoride, polyvinyl butyral, polyvinyl pyrrolidone, poly(2-ethyl-2-oxazoline), acrylic or methacrylic copolymers, maleic anhydride copolymers, vinyl ether copolymers, styrene copolymers, diene copolymers, siloxane copolymers, cellulose derivatives, gum arabic, alginic acid, lecithin, or polymers derived from amino acids. The adhesive may additionally include one or more low-dielectric polymers or oligomers, ionic liquids, or conductive fillers such as carbon black, metal flakes, metal whiskers, carbon nanotubes, silicon nitride nanotubes, or graphene. Adhesives containing such charged and / or conductive materials are conductive adhesives. The polymers and oligomers used in the adhesive layer may have functional groups for chain extension or cross-linking during or after lamination. The adhesive layer is generally 10 6 ohm*cm~10 8 ohm*cm, preferably 10 12 It may have a resistivity value of less than ohm*cm.

[0075] Among the polymers and oligomers mentioned above, polyurethanes, polyureas, polycarbonates, polyesters, and polyamides, especially those equipped with functional groups, are preferred due to their superior adhesive and optical properties and high environmental resistance. Examples of functional groups include, but are not limited to, -OH, -SH, -NCO, -NCS, -NHR, -NRCONHR, -NRCSNHR, vinyl, or epoxide, and derivatives thereof, including cyclic derivatives. The "R" in the above-mentioned functional groups can be hydrogen or alkyl, aryl, alkylaryl, or arylalkyl of up to 20 carbon atoms, which alkyl, aryl, alkylaryl, or arylalkyl can optionally be substituted or interrupted by N, S, O, or halogen. "R" is preferably hydrogen, methyl, ethyl, phenyl, hydroxymethyl, hydroxyethyl, hydroxybutyl, etc. Functionalized polyurethanes, such as hydroxyl-terminated polyester polyurethanes or polyether polyurethanes, isocyanate-terminated polyester polyurethanes or polyether polyurethanes, or acrylate-terminated polyester polyurethanes or polyether polyurethanes, are particularly preferred.

[0076] In many embodiments, a piezoelectric film or display will often include a release sheet. The release material may be used temporarily to facilitate processing of the piezoelectric film or display, for example, during embossing, filling, cutting, etc. In other embodiments, the release material may be used to deliver the final piezoelectric film or display that will be adhered to a final product. In some cases, the release material will protect a functional adhesive layer that will be used to operate the piezoelectric film or display prior to its placement in a final product. The release material may be formed from a material selected from the group consisting of polyethylene terephthalate (PET), polycarbonate, polyethylene (PE), polypropylene (PP), paper, and laminating or cladding films thereof. The release material may be metallized to facilitate quality control measurements and / or to control static electricity during handling, shipping, and downstream incorporation into products. In some embodiments, a silicone release coating may be applied to the release material to improve release properties.

[0077] Although not shown in Figures 5A-6B and 8A-10C, the piezoelectric film or display may include an additional edge seal and / or barrier material that allows the piezoelectric film or display to maintain a desired humidity level, for example, to prevent leakage of non-polar solvents or adhesives, and to prevent the intrusion of water, dust, or gas. The barrier material can be any flexible material, typically a polymer with a low to negligible WVTR (water vapor transmission rate). Suitable materials include polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, cyclic olefin, and combinations thereof. If the piezoelectric film or display will be exposed to particularly harsh conditions, a flexible glass such as WILLOW® glass (Corning, Inc.) can be used for the barrier layer. The edge seal can be a metallized foil or other barrier foil that is covered and adhered over the edge of the piezoelectric film or display. The edge seal may also be formed from a dispensed sealant (thermally, chemically, and / or radiation cured), polyisobutylene, or an acrylate-based sealant, which may be crosslinked. In some embodiments, the edge seal may be a sputtered ceramic such as alumina or indium tin oxide, or an advanced ceramic available from companies such as Vitex Systems, Inc. (San Jose, CA).

[0078] In general, the layers of the piezoelectric film 501-504 can be arranged / laminated in an order that yields the best performance for the end application. For example, as shown in FIG. 5A, the piezoelectric film 501 can be prepared by disposing microcell precursor material on a release material 510 that includes a release material adhesive 520. The microcell precursor can then be embossed or photolithographically processed to produce an array of microcells 530. The microcells 530 can be cured thermally or with electromagnetic radiation, such as UV light. The microcells 530 can then be filled with an electrophoretic medium and sealed with a sealing layer 540, as discussed above with respect to FIG. 4A. (It should be understood that the microcells 530 adjacent to the sealing layer 540 are filled with an electrophoretic medium comprising charged particles in a non-polar solvent, although the electrophoretic medium is not shown in subsequent figures.) The piezoelectric layer 560 can be laminated to the sealing layer 540 using an adhesive 550, which will typically be an optically transparent adhesive formed from one of the materials listed above. Finally, a flexible electrode 580 would be bonded to the piezoelectric film using a conductive adhesive 570. Such a piezoelectric film 501 can then be manipulated by handling the release material 510 until such time as the stack without the release material 510 is attached to the final product. In the piezoelectric film 501, the piezoelectric layer 560 is typically poled to create areas of differential polarization before the flexible electrode 580 is bonded to the piezoelectric film. In some embodiments, the flexible electrode 580 and the conductive adhesive 570 can be replaced with a thin layer of transparent conductive oxide, such as ITO. The ITO can be sputtered directly onto the piezoelectric layer 560 .

[0079] A closely related, but alternative, stack is shown in Figures 5B-5D. In Figure 5B, a piezoelectric film 502 is produced in which a piezoelectric layer 560 is prepared prior to fabrication on a separate release material 510. For example, the piezoelectric layer 560 can be a PVDF-ionic liquid film that has been poled to create a security pattern. The piezoelectric layer 560 is then bonded to a sealed microcell layer 530, which is already bonded to a flexible electrode 580. Notably, in the piezoelectric film 502, the openings in the microcell layer 530 face away from the piezoelectric layer 560, which can promote good bonding between the microcell layer 530 and the piezoelectric layer 560. This bonding can be improved with the introduction of a primer 535 to improve adhesion of the piezoelectric layer 560 to the microcell material, typically a polymer comprising an acrylate, vinyl ether, or epoxide. Primer 535 can be a polar oligomeric or polymeric material, such as a polyhydroxy-functionalized polyester acrylate (e.g., BOMAR® BDE1025 from Dymax) or an alkoxylated acrylate, such as ethoxylated nonylphenol acrylate (e.g., SR504 from Sartomer), ethoxylated trimethylolpropane triacrylate (e.g., SR9035 from Sartomer), or ethoxylated pentaerythritol tetraacrylate (e.g., SR494 from Sartomer). Examples of polar polymers suitable for use in primer 535 include solvent urethane polymers, such as Irostic® polymers.

[0080] Of course, it is also possible to construct the stack such that the openings of the microcell layer 530 face towards the piezoelectric layer 560, as in the piezoelectric film 504 illustrated in Figure 5D. As a further alternative, shown in Figure 5C, the piezoelectric film 503 is positioned such that the openings of the microcell layer 530 face away from the piezoelectric layer 560, however the piezoelectric layer 560 is directly bonded to the flexible electrode 580.

[0081] The piezoelectric films (501, 502, 503, 504) shown in Figures 5A-5D can be converted into piezoelectric displays (601, 602) with the addition of a second flexible electrode 680 in place of the release layer in Figures 5A-5D. The piezoelectric displays (601, 602) will typically also include a second conductive adhesive 670; however, note that in some cases, the conductive adhesive 670 alone may be sufficient to provide the electric field necessary to switch the electrophoretic material. In addition, it is also possible to directly coat the bottom of the microcell layer 530 (Figure 6A) or the sealing layer 540 (Figure 6B) with a thin layer of transparent conductive oxide to create the second electrode. Furthermore, a conductive metal foil can be used as the second flexible electrode 680 if it is not necessary to be visible through both the top and bottom of the piezoelectric displays (601, 602). As shown in Figures 6A and 6B, it is typical to add a release material 510 to the completed piezoelectric display (601, 602) to improve handling and to provide an easy-to-use adhesive for attaching the piezoelectric displays (601, 602). In some embodiments, the piezoelectric display 601 can be formed by simply bonding the piezoelectric layer 560 to a commercial front plane laminate, which includes a second flexible electrode 680 and a sealed microcell layer 530 containing the electrophoretic medium. In such cases, the piezoelectric layer 560 is typically polarized to create areas of differential polarization before the front plane laminate is bonded to the piezoelectric layer 560. While the piezoelectric display (601, 602) of Figures 6A and 6B is shown with the piezoelectric layer 560 above the sealed microcell layer 530, it should be understood that the piezoelectric layer 560 can also be placed below the sealed microcell layer 530 to produce a piezoelectric display similar to Figures 5B and 5D.

[0082] A method for incorporating a piezoelectric polymer film containing an ionic liquid into a thin piezoelectric film 801 (see FIG. 8A) is described with respect to FIG. 7. First, a thin film 960 of piezoelectric polymer and ionic liquid is produced as described above. In step 720, the piezoelectric film 960 is removed from the substrate. The piezoelectric film 960 can be 10 μm or less in thickness, for example, 5 μm or less in thickness, for example, 3 μm or less in thickness. Stretching the piezoelectric film to increase the number of beta phase domains is typically not necessary, but is not excluded from the process. The piezoelectric film 960 is then poled using a suitable electric field, as discussed above. In step 730, a release material 910 is provided along with an adhesive 920, and the release material 910 and adhesive 920 are subsequently laminated to the piezoelectric film 960 in step 740. The piezoelectric film 960 is then coated with / bonded to an electrophoretic layer in step 750. The electrophoretic layer can be a sealed microcell layer including filled microcells 930 and a sealing layer 940, as shown in Figures 9A and 9B, or alternatively, the electrophoretic layer can include an encapsulated electrophoretic medium 990 within a polymer binder 995. Bonding of the piezoelectric film 960 to the electrophoretic layer can be facilitated with an intervening primer layer 935, for example, using one of the primer materials discussed above. If the electrophoretic layer is a sealed microcell layer, the microcells 930 can be positioned so that the sealing layer 940 is adjacent to the piezoelectric film 960 (as in Figure 8A), or the microcells 930 can be positioned so that the sealing layer 940 is positioned on the opposite side from the piezoelectric film 960 (i.e., as in Figure 8B). As a final step 760, an electrode layer 980 is produced, either bonded to / deposited on the microcell 930, as in Figure 8A, or bonded to / deposited on the sealing layer 940, as in Figure 8B. As described above, the electrode layer 980 may comprise a flexible conductive material such as PEDOT:PSS, or may comprise a directly deposited (e.g., sputtered or evaporated) transparent conductive oxide (TCO).In some embodiments, the electrode 980 may comprise a pre-fabricated film of ITO on a polymer substrate such as PET. The piezoelectric film 801, comprising the directly deposited TCO electrode layer 980, the thin piezoelectric layer 960, and the thin layer of microcells 930 (approximately 10 μm thick), is remarkably thin (i.e., less than 25 μm thick, excluding the release material 910), which allows the piezoelectric film 801 to bend without breaking and is unobtrusive when attached to an object such as a banknote. A corresponding piezoelectric film 901 including microcapsules can also be fabricated with a total thickness of less than 25 μm. Of course, alternative structures using a thin piezoelectric film 960 are also possible, such as positioning the piezoelectric film 960 between the electrode 980 and the electrophoretic layer, i.e., the layer of microcapsules 990, as shown in FIG. 9B . Alternatively, the electrode 980 in FIGS. 8A-9B may be replaced with a conductive adhesive (not shown) or a conductive adhesive along with an additional release layer (not shown).

[0083] 6A and 6B, the piezoelectric films of FIGS. 8A-9B can include a second electrode layer to form corresponding displays (1001, 1002, 1003), as shown in FIGS. 10A-10C. Both the electrode layer 980 and the second electrode layer 1080 can comprise a flexible conductive material such as PEDOT:PSS, or both the electrode layer 980 and the second electrode layer 1080 can comprise a directly deposited (e.g., sputtered or evaporated) transparent conductive oxide (TCO), or some combination thereof. Again, in cases where both the electrode layer 980 and the second electrode layer 1080 use thin TCO films, the resulting piezoelectric displays (1001, 1002, 1003) can be made very thin, i.e., less than 25 μm thick, excluding the release material 910. In some embodiments, electrode layer 980 is bonded to / deposited on microcell 930, as in Figure 10A. In other embodiments, electrode layer 980 is bonded to / deposited on sealing layer 940, as in Figure 10B. The assembly of piezoelectric displays 1001 and 1002 can also be used with microcapsules 990 containing an electrophoretic medium held together with binder 995, thus producing piezoelectric display 1003, as shown in Figure 10C. Alternatively, electrodes 980 / 1080 in Figures 10A-10C can be replaced with a conductive adhesive (not shown) or a conductive adhesive along with an additional release layer (not shown).

[0084] It should be understood that the electrodes do not need to be bonded to the piezoelectric film prior to embossing the stack comprising the piezoelectric film and microcell precursor material. Rather, a stack comprising a release material, adhesive, piezoelectric film, and microcell precursor can be prepared, and the microcell precursor can then be embossed, filled, and sealed as described above. Alternatively, a stack comprising a release material, adhesive, electrodes, piezoelectric film, and microcell precursor can also be prepared, and the microcell precursor can then be embossed, filled, and sealed as described above.

[0085] It should be understood that the piezoelectric films and displays described herein can be combined with other known techniques for generating security markers or authenticity indicators. For example, the piezoelectric film or display may additionally include a translucent upper layer that does not change optical properties when the piezoelectric film is manipulated. For example, a smiley face upper layer may include eyes constructed from a piezoelectric display so that the eyes appear to blink when the layered material is bent. In some embodiments, an image or shape may be printed or laminated onto a single-color (e.g., white) background, and one must view through the piezoelectric film to see the pre-arranged pattern. Thus, when not in use, only the single color is visible to the viewer, i.e., the printed image or shape will be hidden. However, the printed image or shape will be displayed when the device is manipulated. It is also feasible to adhere the piezoelectric film or piezoelectric display to a separate light-transmitting polymer film contained within the target product (e.g., banknote), so that the pattern in the piezoelectric layer is only visible when the target product is held up and manipulated relative to a light source.

[0086] (Example) A slurry of PVDF and 10% 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM) (BF4) in DMF was prepared using mixing and slight application of heat above room temperature. The slurry was cast onto a PET web using a slot die coater approximately 5 μm above the web at an operating speed of approximately 10 feet per minute. The web was placed directly into a 160°C oven, where it appeared as a thin piezoelectric film after approximately 5 minutes. The resulting film was removed from the PET backing and mounted in a thin film mount for FTIR spectroscopy. The IR absorption spectrum of the resulting PVDF-EMIM-BF4 film (star line) is shown in Figure 12, compared with a commercial PVDF-TrVDF copolymer film from TE Connectivity (Norwood, MA) (triangle line), PVDF solution-melted in DMF and cast (no ionic liquid) (open circle line), and PVDF solution-melted in DMF and cast, then strained once the film was cooled (no ionic liquid) (open square line). [The cast and strained PVDF spectrum (open square line) is offset to make it easier to distinguish from the other spectra; however, the baselines are indeed very similar for the PVDF-TrVDF copolymer film and the PVDF-EMIM-BF4 film.]

[0087] As can be seen from the spectra in Figure 12, the PVDF-EMIM-BF4 film of the present invention has excellent properties evidenced by a high beta phase concentration. In particular, the 974 cm -1 The absorption at 1,280 cm is mainly absent for the PVDF-EMIM-BF4 film. -1 and 840 cm -1The beta peak at is prominent in the PVDF-EMIM-BF4 film, as well as in the copolymer and tensioned samples, which are known to have a high proportion of beta domains. Rough estimates of the percentage beta phase, based on the area under the indicated peak with a normalized baseline, are: cast PVDF = 50% beta, PVDF-TrVDF copolymer film = 85% beta, cast and tensioned PVDF film = 80% beta, and PVDF-EMIM-BF4 film = nearly 90% beta. It is noteworthy that the cast and tensioned PVDF film was not completely transparent in the optical spectrum and exhibited some waviness when a light source was viewed through the film. The other three films were transparent to visible light and had good optical properties.

[0088] It will be apparent to those skilled in the art that numerous changes and modifications can be made to 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 sense, and not in a restrictive sense.

Claims

[Claim 1] The invention described in this specification.

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