ORGANIC PIEZOELECTRIC FILM

An object is to provide an organic piezoelectric film with a high level of piezoelectricity and a high level of transparency. The organic piezoelectric film of the present disclosure has a total light transmittance of 90% or more, an internal haze value of 0.2%/μm or less per unit film thickness or an internal haze value of 4% or less, and piezoelectric constant d33 of 10 pC/N or more after heating at 110° C. for 10 minutes.

TECHNICAL FIELD

The present disclosure relates to an organic piezoelectric film.

BACKGROUND ART

Organic piezoelectric films are formed from a polymer, which is an organic substance, and have piezoelectricity, i.e., the property of converting applied force into voltage, or the property of converting applied voltage into force. Organic piezoelectric films have various applications in areas in which piezoelectricity is used (e.g., sensors, actuators, touch panels, haptic devices, which have the functionality of providing feedback of tactile sensations to users, vibration-powered generators, speakers, and microphones). In particular, for organic piezoelectric films placed in a location where visibility is important (e.g., windows and displays), there is demand for a low level of retardation (phase difference) in addition to a high level of transparency in order to maintain visibility. For example, sufficient light permeability is required if an organic piezoelectric film is placed on a display. Additionally, retardation must be low to avoid interference with components of the display (e.g., a polarizer and a retarder).

Polyvinylidene fluoride (PVDF) films are typically used as organic piezoelectric films. To impart good piezoelectricity to a PVDF film, the PVDF film must be uniaxially stretched and subjected to polarization treatment (e.g., PTL 1).

CITATION LIST

Patent Literature

SUMMARY

The present disclosure includes the following subject matter.

(1) An organic piezoelectric film, havinga total light transmittance of 90% or more,an internal haze value of 0.2%/μm or less per unit film thickness, anda piezoelectric constant d33of 10 pC/N or more after heating at 110° C. for 10 minutes.
(2) An organic piezoelectric film, comprising a vinylidene fluoride-based copolymer film containing an acicular crystal,the organic piezoelectric film havinga maximum crystal length of 800 nm or less, anda piezoelectric constant d33of 10 pC/N or more after heating at 110° C. for 10 minutes.
(3) A method for producing an organic piezoelectric film, comprisingstep A of preparing an unstretched and non-polarized vinylidene fluoride-based polymer film by casting;step B of polarizing the unstretched and non-polarized vinylidene fluoride-based polymer film; andstep C of heating the unstretched vinylidene fluoride-based polymer film at any time relative to step B, wherein step A includes step A3 of performing heating treatment at a temperature within the range of (T−10) ° C. to (T+5) ° C. when the melting point of the vinylidene fluoride-based polymer is T° C.

The present disclosure provides an organic piezoelectric film with a high level of piezoelectricity and a high level of transparency. Additionally, the present disclosure provides an organic piezoelectric film with a high level of piezoelectricity and a high level of transparency even after being heated (e.g., heated at 110° C. for 10 minutes).

DESCRIPTION OF EMBODIMENTS

All publications, patents, and patent applications cited in the present specification are incorporated herein by reference in their entirety.

Terms

Unless otherwise mentioned, the symbols and abbreviations in the present specification can be understood in the context of the present specification in the meaning commonly used in the technical field to which the present disclosure belongs.

In the present specification, the terms “comprise” and “contain” are used with the intention of including the terms “consisting essentially of” and “consisting of.”

Unless otherwise mentioned, the steps, treatments, or operations described in the present specification may be performed at room temperature.

In the present specification, room temperature can refer to a temperature within the range of 10 to 40° C.

In the present specification, the expression “Cn-m” (n and m are each an integer, and n<m) indicates that the number of carbon atoms is n or more and m or less, as could be generally understood by a person skilled in the art.

Organic Piezoelectric Film

The organic piezoelectric film according to the present disclosure contains a piezoelectric polymer, which is an organic substance. Examples of piezoelectric polymers include, but are not limited to, vinylidene fluoride-based polymers, vinylidene cyanide-based polymers, odd-chain nylon, and polylactic acid. The piezoelectric polymer may be a single piezoelectric polymer or a combination of two or more piezoelectric polymers. The piezoelectric polymer is preferably a vinylidene fluoride-based polymer. The vinylidene fluoride-based polymer is preferably a polarized vinylidene fluoride-based polymer. In the present specification, the term “polarized” means that an electric charge is imparted to a surface. Specifically, the polarized vinylidene fluoride-based polymer may be an electret or piezoelectric substance, or a ferroelectric substance.

The vinylidene fluoride-based polymer contains at least vinylidene fluoride (VDF) as a polymerization component (monomer). The vinylidene fluoride-based polymer may be a homopolymer of vinylidene fluoride (polyvinylidene fluoride) or a copolymer of vinylidene fluoride with one or more monomers copolymerizable with vinylidene fluoride (a vinylidene fluoride-based copolymer). One or more monomers copolymerizable with vinylidene fluoride may be one or more halogen-containing monomers, one or more halogen-free monomers, or a combination of these monomers.

Examples of halogen-free monomers include α-olefins (e.g., ethylene and propylene), unsaturated dicarboxylic acids or derivatives thereof (e.g., maleic acid and maleic anhydride), vinyl ethers (e.g., ethyl vinyl ether), allyl ethers (e.g., allyl glycidyl ether), vinyl esters (e.g., vinyl acetate), acrylic acid or esters thereof, and methacrylic acid or esters thereof.

The vinylidene fluoride-based polymer is preferably a copolymer of vinylidene fluoride with one or more monomers copolymerizable with vinylidene fluoride (a vinylidene fluoride-based copolymer). The lower limit of the proportion of vinylidene fluoride in the copolymer (the molar ratio of the repeating unit derived from vinylidene fluoride (—CH2—CF2—) to the total repeating units) is preferably 65 mol %, 66 mol %, 67 mol %, 68 mol %, 69 mol %, 70 mol %, 71 mol %, 72 mol %, or 73 mol %. The upper limit of the proportion of vinylidene fluoride in the copolymer is preferably 85 mol %, 84 mol %, 83 mol %, or 82 mol %.

The copolymer is preferably at least one member selected from the group consisting of vinylidene fluoride-trifluoroethylene copolymers and vinylidene fluoride-tetrafluoroethylene copolymers.

The lower limit of the compositional ratio of vinylidene fluoride to trifluoroethylene (the molar ratio of the repeating unit derived from vinylidene fluoride (—CH2—CF2—) to the repeating unit derived from trifluoroethylene (—CF2—CHF—)) in the vinylidene fluoride-trifluoroethylene copolymer is preferably 65/35, more preferably 70/30, and still more preferably 73/27. The upper limit of the compositional ratio is preferably 85/15, and more preferably 82/18. The compositional ratio is preferably within the range of 65/35 to 85/15, more preferably 70/30 to 85/15, and still more preferably 73/27 to 82/18.

The vinylidene fluoride-trifluoroethylene copolymer may contain only vinylidene fluoride and trifluoroethylene as polymerization components, or may contain vinylidene fluoride, trifluoroethylene, and one or more other monomers as polymerization components (e.g., a ternary copolymer and a quaternary copolymer). The one or more other monomers can be selected, for example, from the group consisting of monomers listed as examples of “one or more monomers copolymerizable with vinylidene fluoride” (excluding trifluoroethylene, however). The lower limit of the proportion of one or more other monomers is, for example, 0.01 mol %, 0.05 mol %, or 0.1 mol %. The upper limit of the proportion of one or more other monomers is, for example, 10 mol %, 5 mol %, or 1 mol %.

The lower limit of the compositional ratio of vinylidene fluoride to tetrafluoroethylene in the vinylidene fluoride-tetrafluoroethylene copolymer (the molar ratio of the repeating unit derived from vinylidene fluoride (—CH2—CF2—) to the repeating unit derived from tetrafluoroethylene (—CF2—CF2—)) is preferably 65/35, more preferably 66/34, and still more preferably 67/33. The upper limit of the compositional ratio is preferably 85/15, more preferably 82/18, and still more preferably 80/20. The compositional ratio is preferably within the range of 65/35 to 85/15, more preferably 66/34 to 82/18, and still more preferably 67/33 to 80/20.

The vinylidene fluoride-tetrafluoroethylene copolymer may contain only vinylidene fluoride and tetrafluoroethylene as polymerization components, or may contain vinylidene fluoride, tetrafluoroethylene, and one or more other monomers as polymerization components (e.g., a ternary copolymer and a quaternary copolymer). The one or more other monomers can be selected, for example, from the group consisting of monomers listed as examples of “one or more monomers copolymerizable with vinylidene fluoride” (excluding tetrafluoroethylene, however). The lower limit of the proportion of one or more other monomers is, for example, 0.01 mol %, 0.05 mol %, or 0.1 mol %. The upper limit of the proportion of one or more other monomers is, for example, 10 mol %, 5 mol %, or 1 mol %.

The piezoelectric polymer preferably has a weight average molecular weight (Mw) of 50000 or more and 2000000 or less. A piezoelectric polymer with a weight average molecular weight within this range is preferred due to excellent processability and excellent piezoelectricity.

In particular, the weight average molecular weight is preferably 100000 or more and 2000000 or less. The lower limit of the weight average molecular weight is more preferably 200000 or more, still more preferably 300000 or more, and most preferably 600000 or more. The upper limit of the weight average molecular weight is more preferably 1900000 or less, still more preferably 1800000 or less, and most preferably 1100000 or less.

The weight average molecular weight (Mw) can be measured, for example, by gel permeation chromatography (GPC).

Specifically, the molecular weight can be calculated based on the results of measurement by GPC under the following conditions with polystyrene standards as criteria.

Conditions

The melting point of the piezoelectric polymer can be any temperature that does not allow the substrate to degrade, and may be, for example, 100° C. or higher, 105° C. or higher, 110° C. or higher, or 115° C. or higher; and may be 200° C. or lower, 190° C. or lower, 180° C. or lower, 170° C. or lower, or 160° C. or lower. The melting point can be measured, for example, as the maximum value in a heat-of-fusion curve obtained by increasing the temperature at a rate of 10° C./minute by differential scanning calorimetry (DSC) in accordance with the measurement method for the transition temperature of plastics (JIS K 7121).

The lower limit of the content of the piezoelectric polymer in the organic piezoelectric film according to the present disclosure is, for example, 10 mass %, 20 mass %, 30 mass %, 40 mass %, 50 mass %, 60 mass %, or 70 mass %, and preferably 80 mass %, more preferably 85 mass %, and still more preferably 90 mass %. The upper limit of the content is not limited, and may be, for example, 100 mass % or 99 mass %.

The organic piezoelectric film according to the present disclosure may further contain a polymer other than the piezoelectric polymer. Examples of polymers other than the piezoelectric polymer include polycarbonate, polyester (e.g., polyethylene terephthalate and polyethylene naphthalate), polyamide, silicone resin, polyether, polyvinyl acetate, acrylic resin, methacrylic resin, and polyolefin (e.g., polyethylene and polypropylene). The content of the polymer other than the piezoelectric polymer is not limited, and is, for example, 100 parts by mass or less, 80 parts by mass or less, 60 parts by mass or less, 40 parts by mass or less, 20 parts by mass or less, 10 parts by mass or less, 5 parts by mass or less, or 1 part by mass or less, per 100 parts by mass of the piezoelectric polymer.

The organic piezoelectric film according to the present disclosure may consist of a piezoelectric polymer (or a piezoelectric polymer and a polymer other than a piezoelectric polymer), or may contain a piezoelectric polymer and additives. Specific examples of the latter case include films containing an inorganic substance dispersed in the piezoelectric polymer.

Specific examples of additives include fillers (e.g., inorganic oxide particles), affinity improvers, heat stabilizers, UV absorbers, pigments, and combinations of one or two or more of these. Preferable examples include inorganic oxide particles, and combinations of inorganic oxide particles and affinity improvers.

Preferable examples of inorganic oxide particles include at least one member selected from the group consisting of inorganic oxide particles (B1) to (B3) described below.

Inorganic oxide particles (B1) are particles of an oxide of a metal element of Group 2, 3, 4, 12, or 13 of the periodic table, or inorganic oxide composite particles thereof.

Preferable examples of (B1) include particles of an oxide of Be, Al, Mg, Y, or Zr. These particles are preferable because they are versatile and inexpensive, and have high volume resistivity.

More preferable examples of (B1) include particles of at least one inorganic oxide selected from the group consisting of Al2O3, MgO, ZrO2, Y2O3, BeO, and MgO·Al2O3. These particles are preferable due to their high volume resistivity.

Even more preferable examples of (B1) include Al2O3having a γ-type crystalline structure. These particles are preferable due to their large specific surface area and good dispersibility in piezoelectric polymers.

Inorganic oxide particles (B2) are particles of an inorganic composite oxide represented by formula: M1a1M2b1Oc1, wherein M1is a metal element of Group 2, M2is a metal element of Group 4, a1 is within the range of 0.9 to 1.1, b1 is within the range of 0.9 to 1.1, c1 is within the range of 2.8 to 3.2, and M1and M2each can be one or two or more metal elements.

Preferable examples of the metal element of Group 2 include Mg, Ca, Sr, and Ba.

Preferable examples of the metal element of Group 4 include Ti and Zr.

Preferable examples of (B2) include particles of at least one inorganic oxide selected from the group consisting of BaTiO3, SrTiO3, CaTiO3, MgTiO3, BaZrO3, SrZrO3, CaZrO3, and MgZrO3. These particles are preferable due to their high volume resistivity.

Inorganic oxide particles (B3) are an oxide of a metal element of Group 2, 3, 4, 12, or 13 of the periodic table, or inorganic oxide composite particles of silicon oxide.

Specific examples of (B3) include particles of at least one inorganic oxide selected from the group consisting of 3Al2O3·2SiO2, 2MgO·SiO2, ZrO2·SiO2, and MgO·SiO2.

The inorganic oxide particles are not necessarily required to be highly dielectric, and can be appropriately selected according to the intended use of the organic piezoelectric film. For example, versatile and inexpensive inorganic oxide particles (e.g., (B1), in particular, Al2O3particles and MgO particles) can be used to improve volume resistivity. The relative permittivity (1 kHz, 25° C.) of a single type of these inorganic oxide particles (B1) is generally less than 100, and preferably within the range of 10 or less.

For inorganic oxide particles, ferroelectric inorganic oxide particles (e.g., a relative permittivity (1 kHz, 25° C.) of 100 or more; e.g., (B2) and (B3)) may be used for the purpose of improving permittivity. Inorganic materials that form ferroelectric inorganic oxide particles include, but are not limited to, composite metal oxides, complexes of such oxides, solid solutions of such oxides, and sol-gel forms of such oxides.

The relative permittivity (25° C., 1 kHz) of inorganic oxide particles is preferably within the range of 10 or more. From the viewpoint of increasing the permittivity of the organic piezoelectric film, the relative permittivity is preferably within the range of 100 or more, and more preferably 300 or more. The upper limit of the relative permittivity is not limited, and is generally about 3000.

The relative permittivity (ε) (25° C., 1 kHz) of inorganic oxide particles is a value calculated from the capacitance (C) measured with an LCR meter, electrode area (S), and sintered body thickness (d) according to the formula C=εε0×S/d (ε0: permittivity in vacuum).

The inorganic oxide particles preferably have a smaller average primary particle size; in particular, nanoparticles having an average primary particle size of 1 μm or less are preferable. A small amount of such inorganic oxide nanoparticles uniformly dispersed can significantly improve the electrical insulation properties of the organic piezoelectric film. The average primary particle size is preferably within the range of 800 nm or less, more preferably 500 nm or less, and still more preferably 300 nm or less. The average primary particle size is preferably within the range of 10 nm or more, more preferably 20 nm or more, and still more preferably 50 nm or more, from the viewpoint of difficulty in production, difficulty in uniform dispersion, and costs.

The average primary particle size of inorganic oxide particles is determined using a laser diffraction-scattering particle size distribution analyzer (trade name: LA-920, Horiba, Ltd.) or an equivalent instrument.

From the viewpoint of transparency, the organic piezoelectric film preferably contains no inorganic oxide particles; however, to the extent that transparency is not impaired, the organic piezoelectric film may contain inorganic oxide particles in an amount within the range of 0.01 to 300 parts by mass, and more preferably 0.1 to 100 parts by mass, per 100 parts by mass of the piezoelectric polymer.

The lower limit of the content of inorganic oxide particles is preferably 0.1 parts by mass, more preferably 0.5 parts by mass, and still more preferably 1 part by mass, from the viewpoint of improved electrical insulation properties.

The upper limit of the content of inorganic oxide particles is preferably 200 parts by mass, more preferably 150 parts by mass, and still more preferably 100 parts by mass, from the viewpoint of uniformly dispersing inorganic oxide particles in the piezoelectric polymer and preventing a decrease in electrical insulation properties (withstand voltage) and a decrease in tensile strength of the film.

If the organic piezoelectric film contains inorganic oxide particles, the organic piezoelectric film may further contain an affinity improver.

The affinity improver can increase the affinity between the inorganic oxide particles and the piezoelectric polymer, uniformly disperse the inorganic oxide particles in the piezoelectric polymer, firmly bond the inorganic oxide particles and the piezoelectric polymer to suppress the formation of voids, and increase relative permittivity.

Specific examples of affinity improvers include coupling agents, surfactants, and epoxy group-containing compounds.

Examples of coupling agents include organic titanium compounds, organic silane compounds, organic zirconium compounds, organic aluminum compounds, and organic phosphorus compounds.

Preferable examples of organic titanium compounds include alkoxytitanium and titanium chelate because of their good affinity with inorganic oxide particles.

The organic silane compound may be of a high molecular weight or a low molecular weight. Examples include alkoxysilanes (e.g., monoalkoxysilane, dialkoxysilane, trialkoxysilane, and tetraalkoxysilane), vinylsilanes, epoxysilanes, aminosilanes, methacryloxy silanes, and mercaptosilanes. If an alkoxysilane is used, hydrolysis can further improve volume resistivity (i.e., improve electrical insulation properties), which is the effect of surface treatment.

Examples of organic zirconium compounds include alkoxyzirconium and zirconium chelates.

Examples of organic aluminum compounds include alkoxyaluminum and aluminum chelates.

Examples of organic phosphorus compounds include phosphite esters, phosphate esters, and phosphate chelates.

The surfactant as an affinity improver may be of a high molecular weight or a low molecular weight, but is preferably of a high molecular weight, from the viewpoint of thermal stability.

Examples of surfactants include nonionic surfactants, anionic surfactants, and cationic surfactants.

Examples of nonionic surfactants include polyether derivatives, polyvinylpyrrolidone derivatives, and alcohol derivatives. Preferable examples includes polyether derivatives because of their good affinity with inorganic oxide particles.

Examples of anionic surfactants include polymers containing sulfonic acid, carboxylic acid, or a salt thereof. Preferable examples include acrylic acid derivative polymers and methacrylic acid derivative polymers because of their good affinity with piezoelectric polymers.

Examples of cationic surfactants include amine compounds, compounds having a nitrogen-containing heterocyclic ring (e.g., imidazoline), and halogenated salts thereof.

The epoxy group-containing compound as an affinity improver may be a low-molecular-weight compound or a high-molecular-weight compound. Specific examples include epoxy compounds and glycidyl compounds, and preferable examples include low-molecular-weight compounds having one epoxy group, from the viewpoint of affinity with piezoelectric polymers.

More preferable examples of epoxy group-containing compounds include compounds represented by the following formula:

wherein R is a hydrogen atom, a methyl group, a C2-10hydrocarbon group optionally having an intervening oxygen atom or nitrogen atom, or an optionally substituted aromatic ring group; l is 0 or 1; m is 0 or 1; and n is an integer of 0 to 10.

Examples of the compound represented by the above formula include compounds having a ketone group or an ester group, and more specifically compounds represented by the following formulas:

The content of the affinity improver can be preferably within the range of 0.01 to 30 parts by mass, more preferably 0.1 to 25 parts by mass, and still more preferably 1 to 20 parts by mass, per 100 parts by mass of the inorganic oxide particles, from the viewpoint of uniform dispersion and a high degree of relative permittivity of the resulting organic piezoelectric film.

The organic piezoelectric film is preferably a non-stretched film. The organic piezoelectric film is preferably a cast film. Preparing the organic piezoelectric film in the form of a cast film results in a highly uniform thickness, for example, achieving a variation coefficient of film thickness of 10% or less, as described below.

Physical Properties

The organic piezoelectric film preferably has the following physical properties. In the present specification, unless otherwise indicated, each type of physical properties refers to physical properties measured without performing heating as a pretreatment before measurement. When the phrase “after heating at 110° C. for 10 minutes” is included, “physical properties” means physical properties measured after heating is performed at 110° C. for 10 minutes as a pretreatment. The target of heating at 110° C. for 10 minutes can be an organic piezoelectric film after production. For example, if the organic piezoelectric film is produced according to the method including step A to step C described below, the target is the organic piezoelectric film obtained after step C. In an embodiment, the organic piezoelectric film preferably has at least the total light transmittance described below, the internal haze value per unit film thickness described below, and piezoelectric constant d33after heating at 110° C. for 10 minutes described below. In another embodiment, the organic piezoelectric film preferably has at least the crystal size (in particular, average crystal length) described below and piezoelectric constant d33after heating at 110° C. for 10 minutes described below. In still another embodiment, the organic piezoelectric film preferably has at least the rate of change in internal haze value per unit film thickness described below, and the rate of change in piezoelectric constant d33described below.

In these embodiments, the organic piezoelectric film preferably further has at least one type of physical properties selected from the group consisting of the film thickness described below, the retardation described below, the YI value described below, the residual polarization amount described below, and the crystallinity described below. The organic piezoelectric film preferably further has at least one type of physical properties selected from the group consisting of the variation coefficient of piezoelectric constant d33described below, the variation coefficient of film thickness described below, and the area described below.

Total Light Transmittance

Method for Determining Total Light Transmittance

In the present specification, the total light transmittance can be measured according to JIS K-7361 with a haze meter (trade name: NDH-7000SP, Nippon Denshoku Industries Co., Ltd.) or an equivalent instrument.

From the viewpoint of transparency, the lower limit of total light transmittance is preferably 90%, more preferably 918, still more preferably 92%, and particularly preferably 93%. The lower limit of total light transmittance after heating at 110° C. for 10 minutes can also be set to the same values. The upper limit of total light transmittance is not limited, and can be, for example, 99.99%, 99.9%, or 99%. The upper limit of total light transmittance after heating at 110° C. for 10 minutes can also be set to the same values. The total light transmittance is preferably 90% or more (e.g., within the range of 90 to 99.99%), more preferably 91% or more (e.g., within the range of 91 to 99.99%), and still more preferably 92% or more (e.g., within the range of 92 to 99.99%). The range of total light transmittance after heating at 110° C. for 10 minutes can be set to the same ranges.

Internal Haze Value

Method for Determining Internal Haze Value

In the present specification, the internal haze value (inner haze) is determined according to JIS K 7361 with a haze meter (trade name: NDH-7000SP, Nippon Denshoku Industries Co., Ltd.) or an equivalent instrument in a haze (turbidity) test in which the film is inserted into a glass cell filled with water, and the haze value is measured.

From the viewpoint of transparency, the upper limit of the internal haze value is preferably 6%, more preferably 5.5%, and still more preferably 5%. The upper limit of the internal haze value can be set even lower, such as 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5%, or 1%. From the viewpoint of transparency, the upper limit of the internal haze value after heating at 110° C. for 10 minutes is preferably 7%, more preferably 6.5%, and still more preferably 68. The upper limit of the internal haze value after heating at 110° C. for 10 minutes can also be set even lower, such as 5.5%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5%, or 1%. The lower limit of the internal haze value is not limited, and can be, for example, 0.01%, 0.05%, or 0.1%. The lower limit of the internal haze value after heating at 110° C. for 10 minutes can also be set to the same values. The internal haze value is preferably 6% or less (e.g., within the range of 0.01 to 6%), more preferably 5.5% or less (e.g., within the range of 0.01 to 5.5%), and still more preferably 5% or less (e.g., within the range of 0.01 to 5%). The internal haze value after heating at 110° C. for 10 minutes is preferably 7% or less (e.g., within the range of 0.01 to 7%), more preferably 6.5% or less (e.g., within the range of 0.01 to 6.5%), and still more preferably 6% or less (e.g., within the range of 0.01 to 6%). Even if the internal haze value is within these ranges, the organic piezoelectric film can exhibit a high level of piezoelectricity.

Rate of Change in Internal Haze Value (%)

In the present specification, “the rate of change in internal haze value” refers to the absolute value of the value determined according to the following formula: ((internal haze value after heating at 110° C. for 10 minutes)−(internal haze value))/(internal haze value)×100.

From the viewpoint of thermal stability, the upper limit of the rate of change in internal haze value is preferably 140%, more preferably 120%, still more preferably 100%, and yet more preferably 908. The upper limit of the rate of change in internal haze value can be set even lower, such as 80%, 70%, 60%, or 50%.

The lower limit of the rate of change in internal haze value is not limited, and can be, for example, 1%, 2%, 3%, 4%, or 5%.

The rate of change in internal haze value is preferably 140% or less (e.g., within the range of 1 to 140%), more preferably 120% or less (e.g., within the range of 1 to 120%), and still more preferably 100% or less (e.g., within the range of 1 to 100%).

Ratio of Internal Haze Value (%)/Film Thickness (μm)

From the viewpoint of transparency, the upper limit of the ratio of internal haze value (%)/film thickness (μm) is preferably 0.2, more preferably 0.15, still more preferably 0.1, and particularly preferably 0.05. The upper limit of the ratio of internal haze value (%) after heating at 110° C. for 10 minutes/film thickness (μm) can also be set to the same values. The lower limit of the ratio of internal haze value (%)/film thickness (μm) is not limited, and can be, for example, 0.0001, 0.0005, 0.001, or 0.005. The lower limit of the ratio of internal haze value (%) after heating at 110° C. for 10 minutes/film thickness (μm) can also be set to the same values. The ratio of internal haze value (%)/film thickness (μm) is preferably 0.2 or less (e.g., within the range of 0.0001 to 0.2), more preferably 0.15 or less (e.g., within the range of 0.0001 to 0.15), and still more preferably 0.1 or less (e.g., within the range of 0.0001 to 0.1). The range of the ratio of internal haze value (%) after heating at 110° C. for 10 minutes/film thickness (μm) can also be set to the same ranges. Even if the ratio is within these ranges, the organic piezoelectric film can exhibit a high level of piezoelectricity.

Rate of Change in Ratio of Internal Haze Value (%)/Film Thickness (μm)

In the present specification, “the rate of change in the ratio of internal haze value (%)/film thickness (μm)” refers to the absolute value of the value determined according to the following formula: ((r after heating at 110° C. for 10 minutes)−(r))/(r)×100, wherein r represents the ratio.

From the viewpoint of thermal stability, the upper limit of the rate of change in r is preferably 140%, more preferably 120%, still more preferably 100%, and yet more preferably 90%. The upper limit of the rate of change in r can be set even lower, such as 80%, 70%, 60%, or 50%.

The lower limit of the rate of change in r is not limited, and can be, for example, 1%, 2%, 3%, 4%, or 5%.

The rate of change in r is preferably 140% or less (e.g., within the range of 1 to 140%), more preferably 120% or less (e.g., within the range of 1 to 120%), and still more preferably 100% or less (e.g., within the range of 1 to 100%).

Method for Determining Piezoelectric Constant d33

In the present specification, piezoelectric constant d33is measured by applying a force of 1.0 N at 110 Hz with a piezometer system (PM300, Piezotest) or an equivalent instrument. Piezoelectric constant d33is measured at 10 non-arbitrarily selected points on a film, and the arithmetic mean value is taken as piezoelectric constant d33. Ten non-arbitrary points on a film can be selected, for example, by selecting 10 points at 50-mm intervals on a straight line. The term “arbitrary” means that the variation coefficient, described later, is intended to be small.

The actual measured value of piezoelectric constant d33is positive or negative depending on the front or back of the film to be measured. In the present specification, the absolute value is described as the value of piezoelectric constant d33.

The lower limit of piezoelectric constant d33is preferably 13 pC/N, more preferably 14 pC/N, still more preferably 15 pC/N, and even more preferably 16 pC/N. The lower limit of piezoelectric constant d33can be set even higher, such as 17 pC/N. The lower limit of piezoelectric constant d33after heating at 110° C. for 10 minutes is preferably 10 pC/N, more preferably 11 pC/N, still more preferably 12 pC/N, and even more preferably 13 pC/N. The upper limit of piezoelectric constant d33is not limited, and can be, for example, 100 pC/N, 50 pC/N, 35 pC/N, or 30 pC/N. The upper limit of piezoelectric constant d33after heating at 110° C. for 10 minutes can also be set to the same values. Piezoelectric constant d33is preferably 13 pC/N or more (e.g., within the range of 13 to 100 pC/N), more preferably 14 pC/N or more (e.g., within the range of 14 to 100 pC/N), and still more preferably 15 pC/N or more (e.g., within the range of 15 to 100 pC/N). Piezoelectric constant d33after heating at 110° C. for 10 minutes is preferably 10 pC/N or more (e.g., within the range of 10 to 100 pC/N), more preferably 11 pC/N or more (e.g., within the range of 11 to 100 pC/N), and still more preferably 12 pC/N or more (e.g., within the range of 12 to 100 pC/N). Even if piezoelectric constant d33is within these ranges, the organic piezoelectric film can exhibit a high level of piezoelectricity.

Rate of Change in Piezoelectric Constant d33(%)

In the present specification, “the rate of change in piezoelectric constant d33” refers to the absolute value of the value determined according to the following formula: ((d33after heating at 110° C. for 10 minutes)−(d33))/(d33)×100.

From the viewpoint of thermal stability, the upper limit of the rate of change in piezoelectric constant d33is preferably 50%, more preferably 45%, and still more preferably 40%. The upper limit of the rate of change in piezoelectric constant d33can be set even lower, such as 35%, 30%, 25%, 20%, or 15%.

The lower limit of the rate of change in piezoelectric constant d33is not limited, and can be, for example, 0.5% or 1%.

The rate of change in piezoelectric constant d33is preferably 50% or less (e.g., within the range of 0.5 to 50%), more preferably 45% or less (e.g., within the range of 0.5 to 45%), and still more preferably 40% or less (e.g., within the range of 0.5 to 40%).

Variation Coefficient of Piezoelectric Constant d33

The variation coefficient of piezoelectric constant d33is the ratio of the standard deviation of piezoelectric constant d33to the arithmetic mean.

From the viewpoint of in-plane uniformity, the upper limit of the variation coefficient of piezoelectric constant d33is preferably 2, more preferably 1.5, and still more preferably 1. From the viewpoint of production costs, the lower limit of the variation coefficient of piezoelectric constant d33is preferably 0.01. The variation coefficient of piezoelectric constant d33is preferably 2 or less (e.g., within the range of 0.01 to 2), more preferably 1.5 or less (e.g., within the range of 0.01 to 1.5), and still more preferably 1 or less (e.g., within the range of 0.01 to 1).

Retardation

Method for Determining Retardation

In the present specification, the retardation is determined by measurement with a RETS-100 testing instrument for phase difference films or optical materials (trade name, Otsuka Electronics Co., Ltd.) or an equivalent instrument by using a film sample cut to a size equal to or greater than 2 cm×2 cm. In the present specification, a value of 550 nm is used as the value of retardation.

The upper limit of the retardation is preferably 9000 nm, more preferably 8500 nm, and even more preferably 8000 nm, from the standpoint of optical properties. The upper limit of the retardation can be set to an even lower value, such as 7000 nm, 6000 nm, 5000 nm, 4000 nm, 3000 nm, 2000 nm, 1000 nm, 500 nm, 100 nm, 50 nm, or 30 nm.

The lower limit of the retardation is not limited, and can be, for example, 0.1 nm, 0.5 nm, or 1 nm.

The retardation can be preferably within the range of 0.1 to 100 nm, more preferably within the range of 0.5 to 50 nm, and even more preferably within the range of 1 to 30 nm.

Ratio of Retardation [nm]/Film Thickness [μm] The upper limit of the ratio of retardation [nm]/film thickness [μm] is preferably 10, more preferably 5, even more preferably 1, and still more preferably 0.5, from the standpoint of optical properties.

The lower limit of the ratio of retardation [nm]/film thickness [μm] is not limited, and can be, for example, 0.01, 0.02, 0.03, 0.04, 0.05, or 0.1.

The ratio of retardation [nm]/film thickness [μm] is preferably within the range of 0.01 to 10, more preferably within the range of 0.01 to 1, and even more preferably within the range of 0.01 to 0.5.

Degree of Crystallinity

Method for Determining Degree of Crystallinity

In an X-ray diffraction pattern obtained by placing the film sample directly on a sample holder with an aperture, and performing X-ray diffraction measurement at a diffraction angle 2θ of 10 to 40°, a baseline is set as a straight line connecting a diffraction intensity at a diffraction angle 2θ of 10° and a diffraction intensity at a diffraction angle 2θ of 25°, and the area surrounded by the baseline and a diffraction intensity curve is separated into two symmetric peaks by profile fitting. Of these peaks, a peak with a larger diffraction angle 2θ is taken as the crystalline peak, and a peak with a smaller diffraction angle 2θ is taken as the amorphous halo peak. Under these conditions, the value expressed as 100×(area of crystalline peak)/(sum of area of crystalline peak and area of amorphous halo peak) is defined as the degree of crystallinity.

The lower limit of the degree of crystallinity can be preferably 40%, more preferably 45%, even more preferably 50%, still even more preferably 55%, particularly preferably 60%, and particularly more preferably 65%, from the viewpoint of piezoelectricity. The lower limit of the degree of crystallinity after heating at 110° C. for 10 minutes can be set to the same values.

The upper limit of the degree of crystallinity can be, for example, 99%, 95%, or 90%. The upper limit of the degree of crystallinity after heating at 110° C. for 10 minutes can also be set to the same values.

The degree of crystallinity is preferably 40% or more (e.g., within the range of 40% to 99%), more preferably 50% or more (e.g., within the range of 50% to 99%), and even more preferably 60% or more (e.g., within the range of 60% to 99%). The range of the degree of crystallinity after heating at 110° C. for 10 minutes can also be set to the same ranges.

Even if the degree of crystallinity is within these ranges, a high level of transparency can be achieved.

YI Value

In the present specification, the YI value refers to a yellowness index determined according to JIS K 7105. In the present specification, the YI value is determined by transmission measurement with a spectrophotometer (CM-5, produced by Konica Minolta, Inc.) or an equivalent instrument using a film sample cut to a size equal to or greater than 2 cm×2 cm.

The upper limit of the YI value is preferably 4, more preferably 3, and even more preferably 2. The upper limit of the YI value after heating at 110° C. for 10 minutes can also be set to the same values.

The lower limit of the YI value is not limited, and can be, for example, 0.1 or 0.2. The lower limit of the YI value after heating at 110° C. for 10 minutes can also be set to the same values.

The YI value is preferably 4 or less (e.g., within the range of 0.1 to 4), more preferably 3 or less (e.g., within the range of 0.1 to 3), and even more preferably 2 or less (e.g., within the range of 0.1 to 2). The range of the YI value after heating at 110° C. for 10 minutes can also be set to the same ranges.

Rate of Change in YI Value (%) In the present specification, the “rate of change in YI value” refers to the absolute value of the value determined by the following formula: ((YI value after heating at 110° C. for 10 minutes)−(YI value))/(YI value)×100.

From the viewpoint of thermal stability, the upper limit of the rate of change in YI value is preferably 150%, more preferably 100%, even more preferably 90%, still more preferably 80%, and particularly preferably 70%.

The lower limit of the rate of change in YI value is not limited, and is, for example, 1% or 5%.

The rate of change in YI value is preferably 150% or less (e.g., within the range of 1% to 150%), more preferably 100% or less (e.g., within the range of 1% to 100%), and even more preferably 80% or less (e.g., within the range of 1 to 80%).

Residual Polarization Amount (mC/m2)

Method for Determining Residual Polarization Amount

A sample film is obtained by patterning an aluminum electrode (flat electrode) by vacuum deposition on the center (5 mm×5 mm) of a film cut to 20 mm×20 mm, and then attaching two lead electrodes (3 mm×80 mm) made of aluminum foil reinforced with insulating tape to the flat electrode with conductive double-sided tape. The sample film, a function generator, a high-voltage amplifier, and an oscilloscope are incorporated into a Sawyer-Tower circuit, and a triangular wave is applied to the sample film (maximum: ±10 kV). The response of the sample film is measured using the oscilloscope to obtain the residual polarization amount at an applied electric field of 80 MV/m.

The residual polarization amount is preferably 30 mC/m2or more (e.g., within the range of 30 to 200 mC/m2or 30 to 100 mC/m2), more preferably 40 mC/m2or more (e.g., within the range of 40 to 90 mC/m2), and even more preferably 50 mC/m- or more (e.g., within the range of 50 to 80 mC/m2).

Crystal Size

The crystal size is not limited. The organic piezoelectric film preferably comprises an acicular crystal or fibrous crystal, and preferably has the following average crystal length, average crystal width, and/or maximum crystal length.

Method for Determining Average Crystal Length, Average Crystal Width, and Maximum Crystal Length

A scanning electron microscope (SEM) image of the surface of a film sample is observed. An electron microscope image at a magnification selected from the range of 10,000× to 100,000× according to the size of the constituent acicular crystals is observed, with the proviso that the sample, observation conditions, and magnification are adjusted to satisfy the following conditions (1) and (2).

(1) A single straight line X is drawn at any point in an observed image, with 20 or more acicular crystals intersecting the straight line X.
(2) In the same observed image, a straight line Y perpendicularly intersecting the straight line X is drawn, with 20 or more acicular crystals intersecting the straight line Y.

In the thus-obtained electron microscope image, the crystal lengths (long diameters) and crystal widths (short diameters) of at least 20 acicular crystals intersecting the straight line X and at least 20 acicular crystals intersecting the straight line Y (i.e., at least 40 crystals in total) are read. At least 3 or more sets of such electron microscope images are observed, and the crystal lengths and widths of at least 40 crystals×3 sets (i.e., at least 120 crystals) are read. The crystal lengths and crystal widths read in this manner are averaged to obtain the average crystal length and average crystal width, and the longest crystal length from among the crystal lengths read is defined as the maximum crystal length.

The upper limit of the average crystal length is preferably 450 nm, more preferably 400 nm, even more preferably 350 nm, still more preferably 300 nm, particularly preferably 250 nm, and particularly more preferably 200 nm.

The average crystal length is preferably 450 nm or less (e.g., within the range of 1 to 450 nm), more preferably 300 nm or less (e.g., within the range of 1 to 300 nm), and even more preferably 200 nm or less (e.g., within the range of 1 to 200 nm).

The upper limit of the maximum crystal length is preferably 800 nm, more preferably 700 nm, even more preferably 600 nm, still more preferably 500 nm, and particularly preferably 400 nm. From the viewpoint of transparency, the upper limit of the maximum crystal length is preferably the minimum wavelength of visible light (380 nm), more preferably 350 nm, and even more preferably 300 nm.

The lower limit of the maximum crystal length is not limited, and is, for example, 10 nm.

The maximum crystal length is preferably 800 nm or less (e.g., within the range of 10 to 800 nm), more preferably 600 nm or less (e.g., within the range of 10 to 600 nm), and more preferably 380 nm or less (e.g., within the range of 10 to 380 nm).

The upper limit of the average crystal width is preferably 100 nm, more preferably 80 nm, even more preferably 60 nm, and still more preferably 50 nm. The upper limit of the average crystal width can be set to an even lower value, such as 40 nm, 35 nm, or 30 nm.

The lower limit of the average crystal width is not limited, and is, for example, 1 nm.

The average crystal width is preferably 100 nm or less (e.g., within the range of 1 to 80 nm), more preferably 80 nm or less (e.g., within the range of 1 to 100 nm), and even more preferably 60 nm or less (e.g., within the range of 1 to 60 nm).

When the average crystal length and average crystal width are within the above ranges, piezoelectricity and transparency are both excellent.

Film Thickness

Method for Determining Film Thickness

In the present specification, the thickness of the film is measured with a photoelectric digital length measurement system (Digimicro MH-15M, Nikon) or an equivalent instrument at 10 points every square centimeter over the entire film in the plane direction, and the average value is determined to be the thickness of the film.

The lower limit of the film thickness is preferably 1 μm, more preferably 5 μm, even more preferably 10 μm, still more preferably 15 μm, and particularly preferably 20 μm so that the film can be used as a self-supported film. Although it is not so limited, when the film is formed on a support, such as glass or a PET film, the lower limit of the film thickness is preferably 10 nm, more preferably 30 nm, and even more preferably 50 nm, from the viewpoint of obtaining a smooth surface without defects.

The upper limit of the film thickness is not limited, and can be preferably, for example, 1000 μm, 900 μm, or 800 μm, from the viewpoint of obtaining flexibility.

The film thickness is preferably 1 μm or more (e.g., within the range of 1 to 1000 μm), more preferably 5 μm or more (e.g., within the range of 5 to 1000 μm), and even more preferably 10 μm or more (e.g., within the range of 10 to 1000 μm) so that the film can be used as a self-supported film. When the film is formed on a support, such as glass or a PET film, the film thickness is preferably 10 nm or more (e.g., within the range of 10 nm to 1000 μm), more preferably 30 nm or more (e.g., within the range of 30 nm to 1000 μm), and even more preferably 50 nm or more (e.g., within the range of 50 nm to 1000 μm).

Variation Coefficient of Film Thickness

Method for Determining Variation Coefficient of Film Thickness

In the present specification, the variation coefficient of film thickness is defined as the variation coefficient of values measured at 10 points every square centimeter over the entire film in the plane direction.

From the viewpoint of in-plane uniformity, the upper limit of the variation coefficient of the film thickness is preferably 10%, and more preferably 5%.

The lower limit of the variation coefficient of the film thickness is not limited, and can be, for example, 0.01%, 0.05%, or 0.1%.

The variation coefficient of the film thickness is preferably 10% or less (e.g., within the range of 0.01 to 10%), and more preferably 5% (e.g., within the range of 0.01 to 5%).

The lower limit of the area is not limited, and is preferably 9 cm2, and more preferably 10 cm2, from the viewpoint of industrial productivity. The lower limit of the area can be set to an even higher value, such as 50 cm2, 100 cm2, 200 cm2, 300 cm2, 400 cm2, or 500 cm2. The lower limit is not limited to the above values when printing or application is performed with respect to a minute area, and can be, for example, 0.1 μm2, 1 μm2, 10 μm2, 50 μm2, or 100 μm2.

The upper limit of the area is not limited, and can be, for example, 4000 m2, 3000 m2, 2000 m2, 1000 m2, or 500 m2. The lower limit is not limited to the above values when printing or application is performed with respect to a minute area, and can be, for example, 10 mm2, 5 mm2, or 1 mm2.

The area is preferably 9 cm2or more (e.g., within the range of 9 cm2to 4000 m2), and more preferably 10 cm2or more (e.g., within the range of 10 cm2to 4000 m2). This range corresponds to the range of the area produced by a roll-to-roll process.

The organic piezoelectric film of the present disclosure can be applied to various uses. Specific examples of uses include sensors (e.g., touch sensors, vibration sensors, biosensors, and tire sensors (sensors installed on the inner surface of tires)), actuators, touch panels, haptic devices (devices that have the ability to provide feedback of tactile sensations to users), vibration-powered generators (e.g., vibration-powered floors and vibration-powered tires), speakers, and microphones.

Production Method

The organic piezoelectric film of the present disclosure can be produced, for example, by a production method comprising at least one of the following steps A, B, and C, preferably a method comprising steps A to C:step A of preparing a non-polarized film containing a piezoelectric polymer (e.g., an unstretched and non-polarized film) by a casting method;step B of polarizing a non-polarized film (e.g., an unstretched and non-polarized film); andstep C of heating a non-polarized film (e.g., an unstretched and non-polarized film), or heating the film at any time relative to step B.

Step A: Film Preparation Step

The method for producing a non-polarized film by a casting method comprises, for example,(A1) dissolving or dispersing a piezoelectric polymer (e.g., a vinylidene fluoride-based polymer) and desired components mentioned above (e.g., inorganic oxide particles and an affinity improver) in a solvent to prepare a liquid composition;(A2) applying (casting or coating) the liquid composition to a substrate;(A3) heating and drying the substrate to which the liquid composition has been applied at a first temperature; and(A4) heating the substrate heated at the first temperature at a second temperature that is higher than the first temperature. These steps are preferably performed by a roll-to-roll process, from the viewpoint of industrial productivity. Steps (A3) and (A4) may also be performed by sequential transfer from a zone of exposure to a first temperature to a zone of exposure to a second temperature that is higher than the first temperature.

In step (A1), in the preparation of the liquid composition, the dissolution temperature is not limited and can be suitably selected according to the type of solvent for use. From the viewpoint of promoting dissolution and preventing film coloring, the dissolution temperature is preferably equal to or higher than room temperature, and equal to or lower than the solvent vaporization temperature or 80° C.

Preferable examples of the solvent, from the standpoint of coloring prevention, include ketone solvents (e.g., methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetone, diethyl ketone, dipropyl ketone, and cyclohexanone), ester solvents (e.g., ethyl acetate, methyl acetate, propyl acetate, butyl acetate, and ethyl lactate), ether solvents (e.g., tetrahydrofuran, methyltetrahydrofuran, and dioxane), and amide solvents (e.g., dimethylformamide (DMF) and dimethylacetamide). These solvents may be used singly or in a combination of two or more. The amount of the amide solvent contained in the solvent is preferably 50 mass % or less.

In step (A2), the liquid composition may be cast (or applied) onto a substrate according to a commonly used method, such as knife coating, cast coating, roll coating, gravure coating, blade coating, rod coating, air doctor coating, or slot die. Of these, gravure coating or slot die is preferable from the standpoint of easy handling, less irregular thickness of the resulting film, and excellent productivity.

The substrate for use may be, for example, a plastic film, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resin, cycloolefin-based polymer, or polyimide (PI), or a metal film, such as aluminum (Al), stainless steel (SUS), or copper (Cu). Of these, polyethylene terephthalate (PET) is preferable from the viewpoint of its versatility, cost, and productivity. Further, the substrate preferably has a smooth surface so as not only to improve the peelability of the cast-coated film, but also to obtain a smooth surface of the resulting film.

In step (A3), the substrate to which the liquid composition has been applied may be heated and dried according to typical heating and drying methods for film formation. The heating and drying may be preferably performed, for example, by passing the substrate to which the liquid composition has been applied through a high-temperature furnace (or drying furnace) by a roll-to-roll process. Alternatively, the heating and drying may be performed in batch mode to form a film.

The lower limit of the first temperature (heating and drying temperature) can be suitably selected according to the type of solvent for use (or vaporization temperature), and is, for example, 20° C., preferably 30° C., more preferably 40° C., even more preferably 50° C., still more preferably 60° C., particularly preferably 70° C., and more particularly preferably 80° C. The upper limit of the heating and drying temperature is, for example, 200° C., 190° C., 180° C., 170° C., 160° C., 150° C., 140° C., 130° C., or 120° C. The heating and drying temperature is, for example, within the range of 50 to 140° C., within the range of 60 to 130° C., within the range of 70 to 120° C., or within the range of 80 to 130° C. The heating and drying temperature may be constant or varied. The heating and drying temperature may be varied, for example, from low temperature (e.g., 40 to 100° C.) to high temperature (e.g., 120 to 200° C.). The method of varying the heating and drying temperature is not limited. For example, the heating and drying temperature may be varied by transfer between multiple drying zones set to different temperatures.

The lower limit of the time for heating at the first temperature (heating and drying time) is, for example, 1 second, preferably 10 seconds, and even more preferably 30 seconds. The upper limit of the heating and drying time is, for example, 60 minutes, preferably 30 minutes, and even more preferably 10 minutes. The heating and drying time is, for example, within the range of 10 seconds to 60 minutes, preferably within the range of 30 seconds to 10 minutes, and more preferably within the range of 30 seconds to 10 minutes.

In step (A4), the second temperature is preferably at or near the melting point. In the present specification, the temperature at or near the melting point can be (the melting point−12° C.) or more and (the melting point+12° C.) or less. The second temperature is more preferably (the melting point−11° C.) or more, (the melting point−10° C.) or more, (the melting point−9° C.) or more, (the melting point−8° C.) or more, (the melting point−7° C.) or more, (the melting point−6° C.) or more, or (the melting point−5° C.) or more. Further, the second temperature is more preferably (the melting point+11° C.) or less, (the melting point+10° C.) or less, (the melting point+9° C.) or less, (the melting point+8° C.) or less, (the melting point+7° C.) or less, (the melting point+6° C.) or less, or (the melting point+5° C.) or less. The second temperature is preferably within the range of the melting point−12° C. to the melting point+12° C., more preferably within the range of the melting point−10° C. to the melting point+10° C., and more preferably within the range of the melting point−10° C. to the melting point+5° C. or within the range of the melting point−5° C. to the melting point+5° C.

The second temperature varies depending on the type of piezoelectric polymer, and is preferably 110° C. or more, more preferably 115° C. or more, even more preferably 120° C. or more, and still more preferably 125° C. or more. The second temperature is preferably 150° C. or less, more preferably 145° C. or less, even more preferably 140° C. or less, and still more preferably 135° C. or less. The second temperature is preferably within the range of 110 to 150° C., more preferably within the range of 120 to 140° C., and even more preferably within the range of 125 to 135° C.

The time for heating at the second temperature is, for example, 1 minute or more, preferably 5 minutes or more, and more preferably 10 minutes or more. Further, the time for heating at the second temperature is, for example, 60 minutes or less, and preferably 30 minutes or less. The time for heating at the second temperature is, for example, within the range of 1 to 60 minutes, preferably within the range of 5 to 60 minutes, and more preferably within the range of 10 to 60 minutes.

By heating at the second temperature, a high level of crystallinity can be maintained even if the crystals of the piezoelectric polymer become finer. This allows the organic piezoelectric film to achieve both a high level of transparency and a high level of piezoelectricity.

The production method for the non-polarized film may further comprise (A5) heating the substrate heated at the second temperature at a third temperature.

The third temperature is preferably at or near the crystallization temperature. In the present specification, the temperature at or near the crystallization temperature can be (the crystallization temperature−12° C.) or more and (the crystallization temperature+12° C.) or less. The third temperature is more preferably (the crystallization temperature−11° C.) or more, (the crystallization temperature−10° C.) or more, (the crystallization temperature−9° C.) or more, (the crystallization temperature−8° C.) or more, (the crystallization temperature−7° C.) or more, (the crystallization temperature−6° C.) or more, (the crystallization temperature−5° C.) or more, (the crystallization temperature−4° C.) or more, or (the crystallization temperature−3° C.) or more. Further, the third temperature is more preferably (the crystallization temperature+) 11° C. or less, (the crystallization temperature+10° C.) or less, (the crystallization temperature+9° C.) or less, (the crystallization temperature+8° C.) or less, (the crystallization temperature+7° C.) or less, (the crystallization temperature+6° C.) or less, (the crystallization temperature+5° C.) or less, (the crystallization temperature+4° C.) or less, or (the crystallization temperature+3° C.) or less. The third temperature is preferably within the range of the crystallization temperature−10° C. to the crystallization temperature+10° C., more preferably within the range of the crystallization temperature−5° C. to the crystallization temperature+5° C., and even more preferably within the range of the crystallization temperature−3° C. to the crystallization temperature+3° C. The third temperature varies depending on the type of piezoelectric polymer, and is preferably 108° C. or more, more preferably 110° C. or more, even more preferably 113° C. or more, and particularly preferably 115° C. or more. Further, the third temperature is preferably 128° C. or less, more preferably 125° C. or less, even more preferably 123° C. or less, and particularly preferably 121° C. or less. The third temperature is preferably within the range of 108 to 128° C., more preferably within the range of 113 to 123° C., and even more preferably within the range of 115 to 121° C.

The time for heating at the third temperature is, for example, 1 minute or more, preferably 3 minutes or more, and even more preferably 5 minutes or more. The time for heating at the third temperature is, for example, 10 minutes or less. The time for heating at the third temperature is, for example, within the range of 1 to 10 minutes, and preferably 3 to 10 minutes.

Heating at the third temperature is preferred to promote crystal growth, suppress changes in properties due to heating, and obtain a film with a high piezoelectric constant.

On the other hand, depending on the film to be obtained, heating at the third temperature may be omitted to prioritize the aspect of industrial productivity.

The thickness of the non-polarized film prepared in step A may be set according to the film to be obtained.

Step B: Polarization Treatment Step

The polarization treatment can be performed by a commonly used method, preferably corona discharge treatment.

The conditions for the corona discharge treatment may be suitably set based on common knowledge in the related technical field. Although either negative corona or positive corona may be used for corona discharge, negative corona is preferably used because of the ease of polarization of non-polarized films.

The corona discharge treatment is not limited. The treatment can be performed, for example, by applying voltage to a non-polarized film using linear electrodes as described in JP2011-181748A and JP2016-219804A, by applying voltage to a non-polarized film using needle-shaped electrodes, or by applying voltage to a non-polarized film using grid electrodes.

In order to suppress the in-plane variation of piezoelectric constant d33of the obtained polarized film, it is desirable that the distance between each needle-shaped electrode and/or linear electrode and the film is constant; that is, it is desirable that there is no (or extremely small) in-plane variation in the distance between each electrode and the film (specifically, the difference between the longest distance and the shortest distance is preferably within 15 mm, and more preferably within 10 mm).

In addition, for example, when voltage is continuously applied by a roll-to-roll process, it is desirable that the film is brought into close contact with the roll appropriately and uniformly by applying a constant tension to the film.

For example, when voltage is continuously applied using linear electrodes by a roll-to-roll process, the direct current electric field is within the range of, for example, −10 to −25 kV; however, this varies depending on the distance between the linear electrodes and the non-polarized film, the film thickness, and the like. The rate of treatment is within the range of, for example, 10 to 1200 cm/min.

Alternatively, in addition to corona discharge, the polarization treatment may be performed, for example, by applying voltage to the non-polarized film with the film sandwiched between plate electrodes. More specifically, when voltage is applied to the non-polarized film with the film sandwiched between plate electrodes, the following conditions, for example, may be applied: a direct current electric field within the range of 0 to 400 MV/m (preferably 50 to 400 MV/m) and a voltage application time within the range of 0.1 seconds to 60 minutes.

Step C: Heat Treatment Step

Preferably, step C is optionally performed at any time relative to step B. More specifically, step C may be performed before step B, simultaneously with step B, or after step B. When step C is performed after step B, the heat treatment in step C can be performed on the polarized film obtained in step B or a part in which polarization has been completed in step B. More specifically, while the polarization treatment in step B is performed, the heat treatment in step C may also be performed on the part in which the polarization treatment has been completed.

The heat treatment is not limited, and preferably includes, for example, sandwiching the film between two metal plates and heating the metal plates, heating a roll of the film in a constant-temperature chamber, heating a metal roll and bringing the film into contact with the heated metal roll in the production of the film by a roll-to-roll process, or passing the film through a heated oven by a roll-to-roll process. When step C is performed after step B, the polarized film may be heat-treated singly, or the film may be stacked on another type of film or a metal foil to form a laminated film and the laminated film may be heat-treated. In particular, when the heat treatment is performed at high temperatures, the latter method is preferable because wrinkles are less likely to be formed in the polarized film.

The temperature for the heat treatment may vary depending on the type of polarized film to be heat-treated, and is preferably within the range of the melting point of the polarized film to be heat-treated−100° C. to the melting point of the polarized film to be heat-treated+40° C.

More specifically, the temperature for the heat treatment is preferably 80° C. or more, more preferably 85° C. or more, and even more preferably 90° C. or more.

The temperature for the heat treatment is also preferably 170° C. or less, more preferably 160° C. or less, and even more preferably 140° C. or less.

The time for the heat treatment is typically 10 seconds or more, preferably 0.5 minutes or more, more preferably 1 minute or more, and even more preferably 2 minutes or more.

The upper limit of the time for the heat treatment is not limited, and the time for the heat treatment is typically 60 minutes or less.

The conditions for the heat treatment are preferably such that the temperature for the heat treatment is 90° C. or more, and the time for the heat treatment is 1 minute or more.

Roll of Organic Piezoelectric Film

The organic piezoelectric film can be preferably stored and shipped as a roll.

The roll according to one embodiment of the present disclosure may consist of the film, may have a form in which the film laminated with a protective film or the like is wound, or may comprise a core of a paper tube or the like and the film wrapped around the core.

The roll of the film preferably has a width within the range of 50 mm or more and a length within the range of 20 m or more.

The roll of the film can be prepared, for example, by winding the film using an unwinding roller and a winding roller.

From the standpoint of suppressing the deflection of the film, it is preferable to set the unwinding roller and the winding roller parallel to each other, as is usually performed. In order to improve the slipperiness of the film, the roller preferably used is a roller with good slipperiness, specifically a roller coated with a fluororesin, a plated roller, or a roller coated with a release agent.

The uneven thickness of the film causes unevenness in the thickness of the roll, such as “high edge” (the end of the roll is thicker than the axial center of the roll; when the film thickness at both ends is lower than that in the center, the both ends are recessed compared to the center; or when the thickness changes in an inclined manner from one end to the other, the end with a lower film thickness is recessed), which may cause the formation of wrinkles. This may also cause deflection of the film (curve in a state in which no tension other than the tension due to gravity is applied) when the film is unwound.

In general, for the purpose of preventing the high edge of the roll, the edge of the film, which is the edge of the roll, is slit using a slitter. When the thickness of the film is uneven over a wide range from the edge of the film, it is difficult to prevent the high edge and recesses of the roll only by slitting the edge.

Further, in general, the high edge, recesses, and deflection are more likely to occur in films that are wider (e.g., 100 mm or more) and longer (e.g., 50 m or more).

However, since the piezoelectric film has a highly uniform thickness, even when the film is wide (e.g., 100 mm or more) and long (e.g., 50 m or more), the high edge, recesses, and deflection can be suppressed in the resulting roll without any treatment on the film or simply by slitting the edge of the film, which is the edge of the roll, using a slitter.

The edges (film edges) removed by slitting can be collected and recycled as a raw material for the film.

The roll of the film has a highly uniform thickness, and the ratio of the thickness of the roll at the thicker end to the thickness of the roll in the axial center is preferably within the range of 70% to 130% so as to suppress the deflection of the film unwound from the roll of the film.

Further, it is preferable that at least the surface material of the rollers used for producing the film and the roll thereof is polytetrafluoroethylene (PTFE), chrome plating, or stainless steel (SUS).

As a result, wrinkles on the film can be suppressed.

Piezoelectric Material

The piezoelectric material according to one embodiment of the present disclosure may be a laminate and comprises the organic piezoelectric film and an electrode provided on at least one surface of the organic piezoelectric film.

Specific examples of the electrode include indium tin oxide (ITO) electrodes, tin oxide electrodes, aluminum electrodes, metal nanowires, metal nanoparticles (e.g., silver nanoparticles), and organic conducting resins.

The piezoelectric material may be a laminate comprising the organic piezoelectric film, a positive electrode layer (or an upper electrode layer) provided on one surface of the organic piezoelectric film, and a negative electrode layer (or a lower electrode layer) provided on the other surface of the organic piezoelectric film.

The piezoelectric material may have an insulating layer on the surface of the electrode layer on which the organic piezoelectric film is not laminated. The piezoelectric material may also have a cover (e.g., an electromagnetic shielding layer) on the surface (or top surface) of the electrode layer on which the organic piezoelectric film is not laminated.

The method for producing the piezoelectric material comprises, for example:preparing the organic piezoelectric film; andproviding an electrode on at least one surface of the organic piezoelectric film.

In the step of providing an electrode, the method of forming the electrode generally includes heat treatment. Specific examples include a method of forming a film from an electrode material by a physical vapor deposition method (e.g., vacuum evaporation, ion plating, or sputtering) or by a chemical vapor deposition method (e.g., plasma CVD), and a method of applying an electrode material to a substrate.

The lower limit of the temperature for the heat treatment is, for example, 25° C., preferably 40° C., and more preferably 50° C.

The upper limit of the temperature for the heat treatment is the melting point of the polarized film to be heat-treated−3° C., such as 220° C., preferably 180° C., more preferably 150° C., and even more preferably 130° C.

The temperature for the heat treatment can be within the range of, for example, 25 to 220° C., and preferably within the range of 40 to 130° C.

The time for the heat treatment is generally within the range of 10 seconds or more, preferably 1 minute or more, more preferably 10 minutes or more, and even more preferably 15 minutes or more.

The present disclosure includes the following subject matter.

Item 1. An organic piezoelectric film, havinga total light transmittance of 90% or more,an internal haze value of 0.2%/μm or less per unit film thickness, anda piezoelectric constant d33of 10 pC/N or more after heating at 110° C. for 10 minutes.
Item 2. The organic piezoelectric film according to Item 1, comprising a vinylidene fluoride-based copolymer film.
Item 3. The organic piezoelectric film according to Item 2, wherein the vinylidene fluoride-based copolymer is at least one member selected from the group consisting of vinylidene fluoride-tetrafluoroethylene copolymers and vinylidene fluoride-trifluoroethylene copolymers.
Item 4. The organic piezoelectric film according to Item 2 or 3, wherein the proportion of vinylidene fluoride in the vinylidene fluoride-based copolymer is within the range of 60 to 85 mol %.
Item 5. The organic piezoelectric film according to any one of Items 1 to 4, having a film thickness of 10 nm to 1000 μm.
Item 6. The organic piezoelectric film according to any one of Items 1 to 5, having a retardation of 500 nm or less.
Item 7. The organic piezoelectric film according to any one of Items 1 to 6, having a YI value of 4 or less.
Item 8. The organic piezoelectric film according to any one of Items 1 to 7, having a residual polarization amount of 40 mC/m2or more.
Item 9. The organic piezoelectric film according to any one of Items 1 to 8, which has a degree of crystallinity of 40% or more, the degree of crystallinity being expressed as 100×(area of crystalline peak)/(sum of area of crystalline peak and area of amorphous halo peak), the degree of crystallinity being determined from an X-ray diffraction pattern obtained by placing a film sample directly on a sample holder with an aperture, and performing X-ray diffraction measurement at a diffraction angle 2θ of 10 to 40° in the following manner:a baseline is set as a straight line connecting a diffraction intensity at a diffraction angle 20 of 10° and a diffraction intensity at a diffraction angle 20 of 25°,an area surrounded by the baseline and a diffraction intensity curve is separated into two symmetric peaks by profile fitting, andof these peaks, a peak with a larger diffraction angle 2θ is taken as the crystalline peak, and a peak with a smaller diffraction angle 2θ is taken as the amorphous halo peak.
Item 10. An organic piezoelectric film, comprising a vinylidene fluoride-based copolymer film containing an acicular crystal, the organic piezoelectric film having a maximum crystal length of 800 nm or less, and a piezoelectric constant d3of 10 pC/N or more after heating at 110° C. for 10 minutes.
Item 11. The organic piezoelectric film according to Item 10, having an average crystal length of 450 nm or less.
Item 12. The organic piezoelectric film according to Item 10 or 11, which has a degree of crystallinity of 40% or more, the degree of crystallinity being expressed as 100×(area of crystalline peak)/(sum of area of crystalline peak and area of amorphous halo peak), the degree of crystallinity being determined from an X-ray diffraction pattern obtained by placing a film sample directly on a sample holder with an aperture, and performing X-ray diffraction measurement at a diffraction angle 2θ of 10 to 40° in the following manner:a baseline is set as a straight line connecting a diffraction intensity at a diffraction angle 20 of 10° and a diffraction intensity at a diffraction angle 20 of 25°,an area surrounded by the baseline and a diffraction intensity curve is separated into two symmetric peaks by profile fitting, andof these peaks, a peak with a larger diffraction angle 2θ is taken as the crystalline peak, and a peak with a smaller diffraction angle 2θ is taken as the amorphous halo peak.
Item 13. The organic piezoelectric film according to any one of
Items 1 to 12, for use in one or more members selected from the group consisting of sensors, actuators, touch panels, haptic devices, vibration-powered generators, speakers, and microphones.
Item 14. A piezoelectric material that is a laminate, the piezoelectric material comprising the organic piezoelectric film of any one of Items 1 to 12, and an electrode provided on at least one surface of the organic piezoelectric film.
Item 15. A method for producing an organic piezoelectric film, comprisingstep A of preparing an unstretched and non-polarized vinylidene fluoride-based polymer film by casting;step B of polarizing the unstretched and non-polarized vinylidene fluoride-based polymer film; andstep C of heating the unstretched vinylidene fluoride-based polymer film at any time relative to step B, wherein step A includes step A3 of performing heating treatment at a temperature within the range of (T−10) ° C. to (T+5) ° C. when the melting point of the vinylidene fluoride-based polymer is T° C.

EXAMPLES

One embodiment of the present disclosure is described in more detail below with reference to Examples; however, the present disclosure is not limited thereto.

In the Examples below, the weight average molecular weight (Mw) of the vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer was calculated by gel permeation chromatography (GPC) under the following conditions with polystyrene standards as criteria.

In the Examples below, the melting point of the vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer was measured as the maximum value in a heat of fusion curve obtained by increasing the temperature at a rate of 10° C./min by differential scanning calorimetry (DSC) in accordance with the testing methods for transition temperatures of plastics (JIS K 7121).

The following electrodes were used in the Examples described later.

Electrodes Used

(1) A needle-shaped electrode rod with electrode needles (needle-shaped electrodes) (R=0.06 mm) arranged in a row at 10-mm intervals on the center line of a 20-mm-wide (10-mm-thick, 500-mm-long) brass rod.
(2) A needle-shaped electrode rod with electrode needles (R=0.06 mm) arranged in a row at 15-mm intervals as in (1).
(3) A tungsten linear electrode with a diameter of 0.1 mm (length: 500 mm).

For the organic piezoelectric film described later, the total light transmittance, internal haze value, piezoelectric constant d33, retardation, and film thickness were measured according to the following methods.

Total Light Transmittance

The total light transmittance was measured with a haze meter (trade name: NDH-7000SP, Nippon Denshoku Industries Co., Ltd.) according to JIS K-7361.

Internal Haze Value

The film was inserted into a quartz cell filled with water, and the internal haze value was measured according to JIS K-7136 with an NDH-7000SP (trade name, Nippon Denshoku Industries Co., Ltd.).

Piezoelectric constant d33was measured with a piezometer system (PM300, Piezotest). In this measurement, a sample was clipped at 1 N, and the charge generated by applying a force of 1.0 N at 110 Hz was read.

Degree of Crystallinity

An X-ray diffraction pattern measurement was performed at a diffraction angle 2θ of 10 to 40° by placing the film sample directly on a sample holder with an aperture. In the obtained X-ray diffraction pattern, a baseline was set as a straight line connecting a diffraction intensity at a diffraction angle 2θ of 10° and a diffraction intensity at a diffraction angle 2θ of 25°, and the area surrounded by the baseline and a diffraction intensity curve was separated into two symmetric peaks by profile fitting. Of these peaks, a peak with a larger diffraction angle 2θ was taken as the crystalline peak, and a peak with a smaller diffraction angle 2θ was taken as the amorphous halo peak. Under these conditions, the value expressed as 100×(area of crystalline peak)/(sum of area of crystalline peak and area of amorphous halo peak) was defined as the degree of crystallinity.

YI Value

The YI value was measured according to JIS K 7105.

Residual Polarization Amount

Patterning of an aluminum electrode (flat electrode) was performed by vacuum deposition on the center (5 mm×5 mm) of a sample film cut to 20 mm×20 mm. Two lead electrodes (3 mm×80 mm) made of aluminum foil reinforced with insulating tape were then attached to the flat electrode with conductive double-sided tape. The sample film, a function generator, a high-voltage amplifier, and an oscilloscope were incorporated into a Sawyer-Tower circuit, and a triangular wave was applied to the sample film (maximum: ±10 kV). The response of the sample film was measured using the oscilloscope to obtain the residual polarization amount at an applied electric field of 80 MV/m.

Retardation

The retardation was determined by measurement with a RETS-100 testing instrument for phase difference films or optical materials (trade name, Otsuka Electronics Co., Ltd.) using a film sample cut to a size equal to or greater than 2 cm×2 cm. A value of 550 nm was used as the value of retardation.

Film Thickness

The thickness of the film was measured with a photoelectric digital length measurement system (Digimicro MH-15M, Nikon) at 10 points every square centimeter over the entire film in the plane direction, and the thickness of the film was determined from the average value.

Average Crystal Length and Maximum Crystal Length

A scanning electron microscope (SEM) image of the surface of a film sample was observed. An electron microscope image at a magnification selected from the range of 1,000× to 100,000× according to the size of the constituent acicular crystals was observed, with the proviso that the sample, observation conditions, and magnification were adjusted to satisfy the following conditions (1) and (2).

(1) A single straight line X was drawn at any point in the observed image, with 20 or more acicular crystals intersecting the straight line X.
(2) In the same observed image, a straight line Y perpendicularly intersecting the straight line X was drawn, with 20 or more acicular crystals intersecting the straight line Y.

In the thus-obtained electron microscope image, the crystal lengths (long diameters of acicular crystals) of at least 20 acicular crystals intersecting the straight line X and at least 20 acicular crystals intersecting the straight line Y (i.e., at least 40 crystals in total) were read. At least 3 or more sets of such electron microscope images were observed, and the crystal lengths of at least 40 crystals×3 sets (i.e., at least 120 crystals) were read. The crystal lengths read in this manner were averaged to obtain the average crystal length, and the longest crystal length was defined as the maximum crystal length.

Average Crystal Width

The average crystal width was determined by analyzing the electron microscope images used to determine the average crystal length. Specifically, in the obtained electron microscope images, the crystal widths (short diameters of acicular crystals) of at least 20 acicular crystals intersecting the straight line X and at least 20 crystals intersecting the straight line Y (i.e., at least 40 crystals in total) were read. At least 3 or more sets of such electron microscope images were observed, and the crystal widths of at least 40 crystals x 3 sets (i.e., at least 120 crystals) were read. The crystal widths read in this manner were averaged to obtain the average crystal width.

Examples 1 to 6 and Comparative Examples 1 to 3

A vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer (VDF/TFE=75/25 (molar ratio), weight average molecular weight (Mw)=1050000, melting point=131.6° C.) was dissolved in methyl ethyl ketone (MEK) to prepare a paint with a solids content of 20 wt %. The paint was then cast onto a PET film with a bar coater (casting), and heated and dried at 80° C. for 10 minutes to form a copolymer film, which was then treated at the second temperature shown in Table 1 for 0.5 hours.

The copolymer film peeled off from the PET film had a film thickness shown in Table 1. The copolymer film was subjected to the following polarization treatment.

Polarization Treatment

In an ISO class 7 clean room (humidity: 20% to 30%), as outlined inFIG.1, a SUS ground electrode, which was a grounded stage1(length: 320 mm, width: 220 mm), was maintained at 25° C., and a non-polarized copolymer film2cut to a size of 50 mm×50 mm was mounted thereon. A PET film3(340×240 mm) was mounted on the top thereof so that its central portion covered the central portion of the non-polarized copolymer film2. As the upper electrode, needle-shaped electrodes (first electrodes E1) arranged at 10-mm intervals, and a wire electrode formed of a single tungsten wire (r=0.1 mm) (second electrode E2), were positioned at a distance of 10 mm above the copolymer film and parallel to the top surface of the ground electrode. Then, DC voltages applied to the needle-shaped electrodes and the wire electrode were set from 0 kV to the applied voltage of 11 kV and 15 kV, respectively (Trek 610D high-voltage supplier).

The non-polarized copolymer film2placed on stage1was subjected to polarization treatment by allowing it to pass through the needle-shaped electrodes E1and wire electrode E2at a rate of the movement of stage1of 3000 mm/min. The resulting polarized copolymer film was left to stand in a drying furnace maintained at 80° C. for heat treatment to thus obtain an organic piezoelectric film.

FIG.2shows scanning electron microscope images of Example 1 and Comparative Example 3.

Examples 7 to 9

Organic piezoelectric films were obtained in the same manner as in Example 1, except that heating at the third temperature shown in Table 1 was further performed for 10 minutes after heating at the second temperature.

Example 10 and Comparative Example 4

A vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer (VDF/TFE=80/20 (molar ratio), weight average molecular weight (Mw)=230000, melting point=124.9° C.) was dissolved in methyl ethyl ketone (MEK) to prepare a paint with a solids content of 20 wt %. Thereafter, the same procedure as in Example 1 was performed to thus obtain an organic piezoelectric film.

Examples 11 and 15

A vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer (VDF/TFE=67/33 (molar ratio), weight average molecular weight (Mw)=670000, melting point=146.8° C.) was dissolved in methyl ethyl ketone (MEK) to prepare a paint with a solids content of 20 wt %. Thereafter, the same procedure as in Examples 1 to 6 was performed to thus obtain an organic piezoelectric film.

Examples 12 to 14

A vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer (VDF/TFE=74/26 (molar ratio), weight average molecular weight (Mw)=730000, melting point=133.4° C.) was dissolved in methyl ethyl ketone (MEK) to prepare a paint with a solids content of 14 wt %. The paint was then filtered with a filter, and the filtrate was applied to a PET film with a die coater. Drying was further performed to thus form a copolymer film on the PET film. The drying was performed in a drying furnace having four zones (2 m per zone) in which the drying temperatures were set to 80° C., 100° C., 129° C., and 129° C., respectively.

The copolymer film peeled off from the PET film had a film thickness shown in Table 1. A voltage was continuously applied to the copolymer film using linear electrodes by a roll-to-roll process, and the polarization treatment was conducted. Thereafter, the same procedure as in Example 1 was performed to thus obtain an organic piezoelectric film.

Comparative Example 5

A vinylidene fluoride (VDF)-tetrafluoroethylene (TFE) copolymer (VDF/TFE=80/20 (molar ratio), weight average molecular weight (Mw)=230000, melting point=124.9° C.) was dissolved in methyl ethyl ketone (MEK) to prepare a paint with a solids content of 20 wt %. Thereafter, the same procedure as in Example 7 was performed to thus obtain an organic piezoelectric film.

Table 1 below shows the evaluation results of the organic piezoelectric films produced in the Examples and Comparative Examples.