Copolymer, piezoelectric material, piezoelectric film, and piezoelectric element

Abstract

This copolymer comprises: a structural unit derived from vinylidene cyanide and represented by formula (1); a structural unit derived from a vinyl ester and represented by formula (2); and a structural unit derived from a (meth)acrylic acid ester and represented by formula (3).

Description

Copolymer, piezoelectric material, piezoelectric film and piezoelectric element

[0001] The present invention relates to a copolymer, a piezoelectric material, a piezoelectric film, and a piezoelectric element. This application claims priority to Japanese Patent Application No. 2024-025313, filed on February 22, 2024, the contents of which are incorporated herein by reference.

[0002] Conventionally, the piezoelectric material forming the piezoelectric body of a piezoelectric element has been a ceramic material, PZT (PbZrO ３ -PbTiO ３ PZT (a type solid solution) is widely used. However, PZT contains lead and is a ceramic, which makes it brittle. For this reason, there is a demand for piezoelectric materials that are environmentally friendly and highly flexible.

[0003] One piezoelectric material that can meet these requirements is a polymeric piezoelectric material. Polymeric piezoelectric materials include ferroelectric polymers such as polyvinylidene fluoride (PVDF) and vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)). However, these ferroelectric polymers have insufficient heat resistance. As a result, piezoelectric elements made of conventional ferroelectric polymers lose their piezoelectric properties at high temperatures, and their physical properties, such as elastic modulus, also deteriorate. Therefore, piezoelectric elements having piezoelectric elements made of conventional ferroelectric polymers have a narrow usable temperature range.

[0004] Another piezoelectric material is amorphous polymer piezoelectric material, which acquires piezoelectricity by cooling while polarizing it at a temperature near its glass transition temperature. Amorphous polymers lose their piezoelectric properties when heated to temperatures near their glass transition temperature. Therefore, amorphous polymer piezoelectric materials with high glass transition temperatures and good heat resistance are in demand.

[0005] An example of an amorphous polymeric piezoelectric material with a high glass transition temperature is vinylidene cyanide-vinyl acetate copolymer (P(VDCN-VAc)) (see, for example, Patent Document 1).

[0006] International Publication No. 1991 / 013922

[0007] Piezoelectric films for piezoelectric elements include those obtained by poling piezoelectric material sheets containing copolymers. To obtain piezoelectric films with high piezoelectric properties, it is preferable to perform poling on the piezoelectric material sheets at high temperatures and high electric field strengths. However, poling at high temperatures and high electric field strengths can easily cause dielectric breakdown in the piezoelectric material sheets. Therefore, it has been difficult to increase the temperature and / or electric field strength during poling to obtain piezoelectric films with high piezoelectric properties.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a copolymer that can be suitably used as a piezoelectric material, from which a piezoelectric film with excellent piezoelectric properties can be obtained even when polarization treatment is performed under conditions that make it difficult for dielectric breakdown to occur.

[0009] Another object of the present invention is to provide a piezoelectric material that contains the copolymer of the present invention and that can provide a piezoelectric film with high piezoelectric properties even when subjected to polarization treatment under conditions that make dielectric breakdown unlikely. Another object of the present invention is to provide a piezoelectric film with high piezoelectric properties that contains the piezoelectric material (copolymer) of the present invention and that can be produced with a high yield by performing polarization treatment under conditions that make dielectric breakdown unlikely, and a piezoelectric element with high piezoelectric properties that includes the piezoelectric film of the present invention.

[0010] In order to solve the above problems, the following means are provided. Note that hereinafter, the structural formulas and general formulas of compounds may also be simply referred to as formulas. A copolymer according to one aspect of the present invention is a copolymer containing a structural unit derived from vinylidene cyanide represented by the following formula (1), a structural unit derived from a vinyl ester represented by the following formula (2), and a structural unit derived from a (meth)acrylic acid ester represented by the following formula (3):

[0011] (In formula (2), R １ is an organic group having 1 to 15 carbon atoms or a hydrogen atom. ２ is an organic group or a hydrogen atom. ３ is a hydrogen atom or a methyl group.

[0012] A copolymer according to one aspect of the present invention includes a structural unit derived from vinylidene cyanide represented by formula (1), a structural unit derived from a vinyl ester represented by formula (2), and a structural unit derived from a (meth)acrylic acid ester represented by formula (3). Therefore, the copolymer according to one aspect of the present invention can be suitably used as a piezoelectric material from which a piezoelectric film with excellent piezoelectric properties can be obtained even when subjected to a polarization treatment under conditions that make it difficult for dielectric breakdown to occur.

[0013] FIG. 1A shows the polymer of Example 1. １ H-NMR measurement chart ( １ H-NMR chart, １ 1H-NMR spectrum), showing chemical shifts (δ) in the range of 1.4 to 4.4 ppm. １ 2 is a H-NMR measurement chart showing the chemical shift (δ) in the range of 4.6 to 7.6 ppm. １ 3 is a H-NMR measurement chart of the polymer of Example 5. １ 4 is a H-NMR measurement chart of the polymer of Example 7. １ 1 is a H-NMR measurement chart.

[0014] In order to solve the above problems, the present inventors have focused on the energy required for polarization treatment in piezoelectric materials containing copolymers of vinylidene cyanide and vinyl ester, which are piezoelectric materials with a high glass transition temperature (Tg), and have conducted extensive research as described below.

[0015] Copolymers of vinylidene cyanide and vinyl esters tend to form alternating copolymers. Alternating copolymers have a high degree of uniformity in the arrangement of the structural units that make up the copolymer. Therefore, when a piezoelectric material containing an alternating copolymer is subjected to a polarization process, the main chain of the alternating copolymer is less likely to rotate, and the polar groups contained in the alternating copolymer are less likely to be oriented in a specific direction. Therefore, piezoelectric materials containing alternating copolymers require a high amount of energy to align the polarization direction. Therefore, in order to obtain a piezoelectric film with excellent piezoelectric properties, the polarization process must be performed at high temperatures and high electric field strengths, even though this increases the risk of dielectric breakdown.

[0016] The inventors therefore considered that by using other monomers in addition to vinylidene cyanide and vinyl ester as raw material monomers, the uniformity of the arrangement of the structural units forming the copolymer could be reduced, thereby reducing the energy required for polarization treatment. They then conducted extensive research into various compounds copolymerizable with vinylidene cyanide for use as other monomers.

[0017] Compounds copolymerizable with vinylidene cyanide include (meth)acrylic acid esters such as methyl (meth)acrylate. However, copolymers of vinylidene cyanide and (meth)acrylic acid esters have low glass transition temperatures. For example, a copolymer of vinylidene cyanide and methyl (meth)acrylate, a (meth)acrylic acid ester, has a glass transition temperature (Tg) of 146°C. Therefore, piezoelectric materials containing this copolymer are prone to dielectric breakdown upon polarization and cannot be polarized at high temperatures or high electric field strengths. Therefore, even if a piezoelectric material containing a copolymer of vinylidene cyanide and methyl (meth)acrylate is polarized, a piezoelectric film with high piezoelectric properties cannot be obtained. For these reasons, those skilled in the art would not consider using (meth)acrylic acid esters such as methyl (meth)acrylate as the other monomer.

[0018] However, the present inventors have surprisingly found that the energy required for polarization can be effectively reduced by using a (meth)acrylic acid ester such as methyl (meth)acrylate as another monomer. That is, the inventors have found that a piezoelectric material containing a copolymer obtained by copolymerizing a specific (meth)acrylic acid ester with vinylidene cyanide and a specific vinyl ester requires a low energy for polarization, and can be suitably used as a piezoelectric material from which a piezoelectric film with excellent piezoelectric properties can be obtained even when polarization is performed under conditions that are unlikely to cause dielectric breakdown.

[0019] The reason for this is presumably that when a specific (meth)acrylic acid ester is used together with vinylidene cyanide and a specific vinyl ester as raw material monomers, the inclusion of structural units derived from the specific (meth)acrylic acid ester has an unexpectedly significant effect of lowering the energy barrier for aligning polar groups in the copolymer in the same direction, while the effect of lowering the glass transition temperature (Tg) of the copolymer is slight.

[0020] Furthermore, the present inventors produced a copolymer obtained by polymerizing vinylidene cyanide, a specific vinyl ester, and a specific (meth)acrylic acid ester, and confirmed that a piezoelectric film with excellent piezoelectric properties could be obtained when subjected to polarization treatment under the same conditions as a copolymer not containing a structural unit derived from the specific (meth)acrylic acid ester, and thus arrived at the present invention.

[0021] The present invention includes the following aspects.

[0022] [1] A copolymer comprising a structural unit derived from vinylidene cyanide represented by the following formula (1), a structural unit derived from a vinyl ester represented by the following formula (2), and a structural unit derived from a (meth)acrylic acid ester represented by the following formula (3):

[0023] (In formula (2), R １ is an organic group having 1 to 15 carbon atoms or a hydrogen atom. ２ is an organic group or a hydrogen atom. ３ is a hydrogen atom or a methyl group.

[0024] [2] The copolymer according to [1], wherein the content of the structural unit derived from the vinyl ester represented by the formula (2) is 80 to 99 parts by mole, and the content of the structural unit derived from the (meth)acrylic acid ester represented by the formula (3) is 1 to 20 parts by mole, relative to 100 parts by mole of the structural unit derived from vinylidene cyanide represented by the formula (1).

[0025] [3] In the formula (2), R １[4] The copolymer according to [1] or [2], wherein in the formula (3), R is a linear alkyl group having 1 to 3 carbon atoms. ２ [5] The copolymer according to any one of [1] to [3], wherein R is an organic group having 1 to 15 carbon atoms. ２ [6] The copolymer according to [4], wherein in the formula (3), R is an alkyl group having 1 to 8 carbon atoms which may be substituted with a cyano group. ２ is an organic group represented by the following formula (4):

[0026] (n in formula (4) is a value that makes the number average molecular weight of the organic group represented by formula (4) 415 to 19,915.)

[0027] [7] In the formula (3), R ３ [6] The copolymer according to any one of [1] to [6], wherein is a hydrogen atom.

[0028] [8] A piezoelectric material comprising the copolymer according to any one of [1] to [7]. [9] A piezoelectric film comprising the copolymer according to any one of [1] to [7].

[10] A piezoelectric element comprising the piezoelectric film according to [9] and electrodes disposed on one surface and the other surface of the piezoelectric film.

[0029] The copolymer, piezoelectric material, piezoelectric film, and piezoelectric element of this embodiment will be described in detail below. [Copolymer] The copolymer (polymer) of this embodiment contains a structural unit derived from vinylidene cyanide represented by formula (1) (hereinafter sometimes referred to as a "first structural unit"), a structural unit derived from a vinyl ester represented by formula (2) (hereinafter sometimes referred to as a "second structural unit"), and a structural unit derived from a (meth)acrylic acid ester represented by formula (3) (hereinafter sometimes referred to as a "third structural unit"). In this specification, "(meth)acrylic acid" is a general term for acrylic acid and methacrylic acid.

[0030] In the copolymer of this embodiment, the arrangement order of the repeating units, that is, the first structural unit, the second structural unit, and the third structural unit, is not particularly limited. Therefore, the copolymer of this embodiment may have, for example, an alternating arrangement portion in which the first structural unit and the second structural unit are alternately arranged, and a random arrangement portion in which the first structural unit, the second structural unit, and the third structural unit are arranged in no order. Furthermore, the copolymer of this embodiment may have one or more of the block arrangement portions in which the first structural units are successively arranged, the block arrangement portion in which the second structural units are successively arranged, and the block arrangement portion in which the third structural unit is successively arranged, in any proportion. Since the copolymer of this embodiment can be used as a piezoelectric material with excellent piezoelectric properties, it is preferable that it has an alternating arrangement portion consisting of the first structural unit and the second structural unit, and the random arrangement portion.

[0031] The copolymer of this embodiment is an amorphous (non-crystalline) polymer. A piezoelectric film obtained by poling a piezoelectric material containing the copolymer of this embodiment has good piezoelectric properties due to the orientation of C-CN dipoles possessed by the structural unit derived from vinylidene cyanide represented by formula (1). The structural unit derived from vinylidene cyanide represented by formula (1) forms a molecular chain with a large dipole moment together with the structural unit derived from vinyl ester represented by formula (2). The structural unit derived from (meth)acrylic acid ester represented by formula (3) reduces the uniformity of the arrangement of the structural units in the molecular chains forming the copolymer, thereby reducing the energy required for polarization.

[0032] In the structural unit derived from the vinyl ester represented by formula (2), R １ is an organic group having 1 to 15 carbon atoms or a hydrogen atom. １ is an organic group having 15 or less carbon atoms or a hydrogen atom, so R １The volume of the copolymer of this embodiment is small, and the volume ratio of molecular chains containing C—CN dipoles that contribute to high polarity can be secured. Therefore, a piezoelectric material containing the copolymer of this embodiment can form a piezoelectric film with high piezoelectric properties by performing a polarization treatment. The copolymer of this embodiment contains R １ The polysiloxane may contain a plurality of different structural units.

[0033] R １ is an organic group having 1 to 15 carbon atoms, １ Examples of R include an alkyl group having 1 to 15 carbon atoms which may have a substituent, an aromatic ring group which may have a substituent, a phosphonic acid group which may have a substituent, a heterocyclic group which may have a substituent, and an aliphatic ring group which may have a substituent. １ When is one of these groups, the substituents it may have include halogeno groups such as a fluoro group and a chloro group, a nitrile group, and an alkyl group.

[0034] R １ It is preferable that the piezoelectric material containing the copolymer of the present embodiment does not contain a halogen element, because by performing a polarization treatment on the piezoelectric material containing the copolymer of the present embodiment, the halogen element is released from the copolymer, which can prevent the piezoelectric material from deteriorating and causing dielectric breakdown.

[0035] R １ is an alkyl group having 1 to 15 carbon atoms which may have a substituent, １ Examples of the alkyl group include a linear alkyl group having 1 to 15 carbon atoms, a dichloromethyl group, a trichloromethyl group, a difluoromethyl group, a trifluoromethyl group, a 2-cyanoethyl group, an isobutyl group, a t-butyl group, a 2-ethylhexyl group, a 2-(acetoacetyloxy)ethyl group, etc. Examples of the linear alkyl group having 1 to 15 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, an n-pentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, an n-nonyl group, an n-decyl group, an n-undecyl group, an n-dodecyl group, an n-tridecyl group, an n-tetrasil group, and an n-pentadecyl group.

[0036] R １is an aromatic ring group which may have a substituent, R １ Examples of the alkyl group include a phenyl group, a 2-methylphenyl group, a 3-methylphenyl group, a 4-methylphenyl group, a 4-t-butylphenyl group, a 4-nitrophenyl group, a 4-cyanophenyl group, and a naphthyl group.

[0037] R １ is an optionally substituted phosphonic acid group, R １ Examples of R include a dimethylphosphonylmethyl group, a diethylphosphonylmethyl group, and a diphenylphosphonylmethyl group. １ is an optionally substituted heterocyclic group, R １ Examples of the alkyl group include a furfuryl group, a tetrahydrofurfuryl group, a 2-oxotetrahydrofuran-3-yl group, and a 4-methyl-2-oxotetrahydro-2H-pyran-4-yl group.

[0038] R １ is an optionally substituted aliphatic cyclic group, R １ Examples of the alkyl group include a 1-adamantyl group, a 2-methyl-2-adamantyl group, a 2-ethyl-2-adamantyl group, a 2-isopropyl-2-adamantyl group, an isobornyl group, and a dicyclopentanyl group.

[0039] R １ Among these, R is preferably a linear alkyl group having 1 to 3 carbon atoms, and particularly preferably a methyl group. １ is a linear alkyl group having 1 to 3 carbon atoms, R １ This results in a smaller volume, ensuring a larger volume fraction of molecular chains containing C-CN dipoles, which contribute to high polarity. As a result, a piezoelectric material containing this copolymer can be polarized to form a piezoelectric film with even better piezoelectric properties.

[0040] In the structural unit derived from the (meth)acrylic acid ester represented by formula (3), R ２ is an organic group or a hydrogen atom, and R ３ is a hydrogen atom or a methyl group. ２is an organic group or a hydrogen atom, and R ３ is a hydrogen atom or a methyl group, so R ２ and R ３ Therefore, the volume of the piezoelectric material containing the copolymer of this embodiment can be reduced by performing a polarization process to form a piezoelectric film with excellent piezoelectric properties.

[0041] The copolymer of the present embodiment contains, as a structural unit derived from a (meth)acrylic acid ester represented by formula (3), R ２ and / or R ３ The polysiloxane may contain a plurality of different structural units.

[0042] R ２ is an organic group or a hydrogen atom. ２ As the organic group having 1 to 15 carbon atoms, an organic group having 1 to 15 carbon atoms or an organic group represented by the above formula (4) is preferred. As the organic group having 1 to 15 carbon atoms, R in the structural unit derived from vinyl ester represented by formula (2) is preferred. １ The same groups as those which may be present as R ２ is an organic group having 1 to 15 carbon atoms, R ２ Examples of R include the above-mentioned alkyl groups having 1 to 15 carbon atoms which may have a substituent, aromatic ring groups which may have a substituent, phosphonic acid groups which may have a substituent, heterocyclic groups which may have a substituent, and aliphatic ring groups which may have a substituent. ２ When R is one of these groups, the substituents that R may have include １ Similarly, examples of the halogen atom include a halogeno group such as a fluoro group or a chloro group, a nitrile group, and an alkyl group.

[0043] R ２ is R １ may be the same as or different from R ２ is R １ Similarly, it is preferable that the piezoelectric material does not contain a halogen element, because by performing a polarization treatment on a piezoelectric material containing the copolymer of this embodiment, the halogen element is released from the copolymer, which can prevent the piezoelectric material from deteriorating and causing dielectric breakdown.

[0044] R ２ When R is an organic group having 1 to 15 carbon atoms, it is preferably an alkyl group having 1 to 8 carbon atoms which may be substituted with a cyano group, more preferably any one selected from a 2-ethylhexyl group, a methyl group, and a 2-cyanoethyl group, and particularly preferably a 2-ethylhexyl group. ２ When R is an alkyl group having 1 to 8 carbon atoms which may be substituted with a cyano group, the inclusion of a structural unit derived from the vinyl ester represented by formula (2) reduces the uniformity of the arrangement of the structural units forming the copolymer, thereby achieving a significant effect of reducing the energy required for polarization treatment. ２ The same effects can be obtained when is an organic group represented by the above formula (4).

[0045] R ３ is a hydrogen atom or a methyl group, and is preferably a hydrogen atom. ３ The volume of R becomes smaller, and the volume ratio of molecular chains containing C—CN dipoles that contribute to high polarity is secured to a greater extent. ３ A piezoelectric material containing a copolymer in which is a hydrogen atom can be polarized to form a piezoelectric film with improved piezoelectric properties.

[0046] In the copolymer of the present embodiment, the content of the structural unit (second structural unit) derived from the vinyl ester represented by formula (2) is preferably 80 parts by mole to 99 parts by mole, and the content of the structural unit (third structural unit) derived from the (meth)acrylic acid ester represented by formula (3) is preferably 1 part by mole to 20 parts by mole, relative to 100 parts by mole of the structural unit (first structural unit) derived from vinylidene cyanide represented by formula (1).

[0047] When the content of the second structural unit relative to 100 molar parts of the first structural unit is 80 to 99 molar parts, the first structural unit and the second structural unit can sufficiently form a molecular chain with a large dipole moment, including a C—CN dipole that contributes to piezoelectric properties. As a result, a piezoelectric material containing the copolymer can be subjected to a polarization treatment to form a piezoelectric film with superior piezoelectric properties. The content of the second structural unit is more preferably 85 to 99 molar parts.

[0048] When the content of the second structural unit relative to 100 molar parts of the first structural unit is 80 to 99 molar parts, and the content of the third structural unit relative to 100 molar parts of the first structural unit is 1 molar part or more, the uniformity of the arrangement of the structural units in the molecular chains forming the copolymer is reduced, and the effect of reducing the energy required for polarization treatment is more significantly obtained. ３ When is a methyl group, it is more preferably 3 molar parts or more.

[0049] Furthermore, when the content of the second structural unit relative to 100 molar parts of the first structural unit is 80 to 99 molar parts, and the content of the third structural unit relative to 100 molar parts of the first structural unit is 20 molar parts or less, the first structural unit and the second structural unit can sufficiently form a molecular chain with a large dipole moment. As a result, the glass transition temperature (Tg) of the copolymer is less likely to be low, and by subjecting a piezoelectric material containing this to a polarization treatment, a piezoelectric film with superior piezoelectric properties can be formed. More preferably, the content of the third structural unit is 15 molar parts or less.

[0050] The copolymer of the present embodiment may, as necessary, contain one or more structural units other than the structural unit derived from vinylidene cyanide represented by formula (1), the structural unit derived from a vinyl ester represented by formula (2), and the structural unit derived from a (meth)acrylic acid ester represented by formula (3). Examples of the other structural units include structural units derived from known monomers or oligomers having a polymerizable unsaturated bond.

[0051] Of the structural units contained in the copolymer of the present embodiment, the total content of the structural unit derived from vinylidene cyanide represented by formula (1), the structural unit derived from a vinyl ester represented by formula (2), and the structural unit derived from a (meth)acrylic acid ester represented by formula (3) is preferably 80% by mass or more, more preferably 90% by mass or more, and may be only the structural unit represented by formula (1), the structural unit represented by formula (2), and the structural unit represented by formula (3).

[0052] The weight-average molecular weight (Mw) of the copolymer of this embodiment is preferably 10,000 to 1,000,000 (10,000 to 1,000,000). When the weight-average molecular weight (Mw) of the copolymer is 10,000 or more, the film-forming properties are good, and a piezoelectric film containing the copolymer of this embodiment can be easily produced. When the weight-average molecular weight (Mw) of the copolymer is 1,000,000 or less, the copolymer can be easily dissolved in a solvent, and a piezoelectric film can be easily produced using a coating liquid dissolved in a solvent.

[0053] "Method for Producing Copolymer" The copolymer of the present embodiment can be produced, for example, by mixing raw material monomers including vinylidene cyanide represented by formula (2-1), vinyl ester represented by formula (2-2), and (meth)acrylic acid ester represented by formula (2-3) with a solvent such as normal heptane, and radically copolymerizing the mixture using a polymerization initiator such as azobisbutyronitrile by a known method.

[0054] (In formula (2-2), R １ is an organic group having 1 to 15 carbon atoms or a hydrogen atom. ２ is an organic group or a hydrogen atom. ３ is a hydrogen atom or a methyl group.

[0055] R in the vinyl ester represented by formula (2-2) １ is an organic group having 1 to 15 carbon atoms or a hydrogen atom, and R in the structural unit derived from the vinyl ester represented by the above formula (2) １ is the same as

[0056] R in the (meth)acrylic acid ester represented by formula (2-3) ２ is an organic group or a hydrogen atom, and R in the structural unit derived from the (meth)acrylic acid ester represented by the above formula (3) ２ The R in the (meth)acrylic acid ester represented by formula (2-3) is the same as ３ is a hydrogen atom or a methyl group, and R in the structural unit derived from the (meth)acrylic acid ester represented by the above formula (3) ３Similarly, R in the structural unit derived from the (meth)acrylic acid ester represented by the above formula (3) is preferably a hydrogen atom. ２ is an organic group represented by formula (4), it is preferable to use a (meth)acrylic acid ester represented by the following formula (2-4) as a raw material monomer.

[0057] (n in formula (2-4) is a value that makes the number average molecular weight of the (meth)acrylic acid ester represented by formula (2-4) 500 to 20,000.)

[0058] n in formula (2-4) is the same as n in formula (4). Formula (4) is obtained by subtracting C from formula (2-4). ４ H ５ O ２ Since this is a structural formula excluding the , the molecular weight of formula (4) is the value obtained by subtracting 85 from the molecular weight of formula (2-4). Therefore, when the number average molecular weight of the (meth)acrylic acid ester represented by formula (2-4) is 500 to 20,000, the number average molecular weight of the organic group represented by formula (4) is 415 to 19,915. The number average molecular weight of the (meth)acrylic acid ester represented by formula (2-4) is measured, and the number average molecular weight of the organic group represented by formula (4) is calculated from the number average molecular weight. The number average molecular weight of the (meth)acrylic acid ester represented by formula (2-4) is a polystyrene-equivalent molecular weight determined by gel permeation chromatography (GPC) based on a calibration curve prepared using a standard polystyrene sample, and is measured, for example, under the following conditions. <Measurement Conditions> Column: Shodex LF404 (manufactured by Resonaq Co., Ltd.) Mobile phase solvent: THF Flow rate: 1.0 ml / min Column temperature: 40°C Standard material: TSK Standard Polystyrene (manufactured by Tosoh Corporation)

[0059] The raw material monomers are prepared so that the proportions of vinylidene cyanide represented by formula (2-1), vinyl ester represented by formula (2-2), and (meth)acrylic acid ester represented by formula (2-3) correspond to the proportions of structural units derived from vinylidene cyanide represented by formula (1), structural units derived from vinyl ester represented by formula (2), and structural units derived from (meth)acrylic acid ester represented by formula (3) contained in the target copolymer, respectively. Polymerization conditions such as reaction temperature and reaction time when producing the copolymer of this embodiment can be appropriately determined depending on the composition of the raw material monomers, etc.

[0060] "Piezoelectric Material" The piezoelectric material of this embodiment includes the copolymer of this embodiment. The piezoelectric material of this embodiment may include only one type of copolymer of this embodiment, or two or more types of copolymers of this embodiment. Furthermore, the piezoelectric material of this embodiment may, if necessary, include one or more types of known polymers other than the copolymer of this embodiment, together with the copolymer of this embodiment.

[0061] "Piezoelectric Film" The piezoelectric film of this embodiment includes the copolymer of this embodiment. The piezoelectric film of this embodiment can be produced, for example, by the method described below. The piezoelectric material of this embodiment, including the copolymer of this embodiment, is dissolved in a solvent to form a coating liquid. As the solvent, a known solvent such as N,N-dimethylformamide can be used. Next, the coating liquid is applied to a peelable substrate to a predetermined thickness to form a coating film. As the substrate, a known material such as a resin film can be used. As a method for applying the coating liquid, a known method can be used depending on the coating thickness, viscosity of the coating liquid, and the like. Thereafter, the coating film is dried to remove the solvent in the coating film, resulting in a piezoelectric material sheet. The piezoelectric material sheet may be subjected to a stretching treatment as necessary.

[0062] The piezoelectric material sheet is then peeled off from the substrate, and electrodes made of a known conductive material such as aluminum are placed on one side and the other side of the piezoelectric material sheet. A voltage is then applied to the piezoelectric material sheet at a predetermined electric field strength via the electrodes placed on both sides at a predetermined polarization temperature near the glass transition temperature of the piezoelectric material forming the piezoelectric material sheet. The piezoelectric material sheet is then cooled with the voltage still applied. This results in piezoelectricity. A sheet-like piezoelectric film is obtained through the above process. The electrodes used to achieve piezoelectricity may be used as components for forming a piezoelectric element, or may be removed.

[0063] "Piezoelectric Element" The piezoelectric element of this embodiment has the piezoelectric film of this embodiment and electrodes arranged on one side and the other side of the piezoelectric film. Specifically, it can have a sheet-like piezoelectric film and electrodes arranged on one side and the other side of the piezoelectric film. Known conductive materials such as aluminum can be used as the material for the electrodes. The piezoelectric element of this embodiment can be manufactured, for example, by providing electrodes on one side and the other side of the piezoelectric film by a known method such as vapor deposition.

[0064] The copolymer of this embodiment contains a structural unit (first structural unit) derived from vinylidene cyanide represented by formula (1), a structural unit (second structural unit) derived from a vinyl ester represented by formula (2), and a structural unit (third structural unit) derived from a (meth)acrylic acid ester represented by formula (3). Therefore, the copolymer of this embodiment has a lower energy barrier for aligning polar groups in the copolymer in the same direction, compared to, for example, a copolymer containing only the first structural unit and the second structural unit. Therefore, the copolymer of this embodiment can produce a piezoelectric film with excellent piezoelectric properties even when subjected to polarization treatment under conditions that are unlikely to cause dielectric breakdown.

[0065] That is, when a piezoelectric film manufactured using the copolymer of this embodiment and a piezoelectric film manufactured using a copolymer containing only the first structural unit and the second structural unit are subjected to polarization treatment under the same conditions, the piezoelectric film manufactured using the copolymer of this embodiment has higher piezoelectric properties than the piezoelectric film manufactured using the copolymer containing only the first structural unit and the second structural unit. Furthermore, the copolymer of this embodiment has superior heat resistance compared to the copolymer containing only the first structural unit and the third structural unit. Therefore, the copolymer of this embodiment can be polarized at a higher temperature and / or a higher electric field strength than the copolymer containing only the first structural unit and the third structural unit, and a piezoelectric film with higher piezoelectric properties can be formed. Therefore, the copolymer of this embodiment is suitable as a piezoelectric material.

[0066] Furthermore, since the piezoelectric material of this embodiment contains the copolymer of this embodiment, a piezoelectric film with excellent piezoelectric properties can be obtained even when polarization treatment is performed under conditions where dielectric breakdown is unlikely to occur. Furthermore, the piezoelectric film of this embodiment contains the copolymer of this embodiment. Therefore, the piezoelectric film of this embodiment and the piezoelectric element of this embodiment having the piezoelectric film of this embodiment have excellent piezoelectric properties.

[0067] The above describes the embodiments of the present invention in detail, but each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the requirements of the present invention.

[0068] (Synthesis of copolymer) "Examples 1 to 14, Comparative Examples 1 to 4" In a 100 ml Schlenk tube, vinylidene cyanide represented by the following formula (2-1) and R １ and a vinyl ester having the following formula (2-2) and a vinyl ester having the R ２ and R ３A (meth)acrylic acid ester having the formula (2-3) and normal heptane as a solvent was mixed in the ratios shown in Tables 1 to 4. Azobisisobutyronitrile (AIBN) was then added as a polymerization initiator in the amounts shown in Tables 3 and 4, and the mixture was allowed to react at 70°C for 1 hour. The reaction product was poured into 100 ml of ethanol and washed with stirring. The product was then filtered and dried to obtain a polymer (copolymer) having the mass shown in Tables 3 and 4. The yields are shown in Tables 3 and 4. In Example 14, a (meth)acrylic acid ester manufactured by JNC Corporation, product name: FM-0711, was used. This (meth)acrylic acid ester was represented by the formula (2-4) below, and had a number average molecular weight (Mn) of 940 and a viscosity of 9 cSt at 25°C, as measured by the method described in the embodiment.

[0069]

[0070] (n in formula (2-4) is a value that gives a number average molecular weight of 500 to 20,000 of the (meth)acrylic acid ester represented by formula (2-4). In Example 14, n in formula (2-4) was a value that gave a number average molecular weight of 940 of the (meth)acrylic acid ester represented by formula (2-4).)

[0071]

[0072]

[0073]

[0074]

[0075] The polymers of Examples 1 to 14 and Comparative Examples 1 to 4 thus obtained were each subjected to NMR (nuclear magnetic resonance) analysis using a nuclear magnetic resonance (NMR) apparatus (trade name JNM-ECA500, manufactured by JEOL Ltd.) and dimethyl sulfoxide d6 (DMSO-d6) as a solvent. １ The molecular structure was identified by measuring the H-NMR chart.

[0076] １ ​From the results of the H-NMR chart measurement, it was confirmed that the polymers of Examples 1 to 14 were copolymers having a structural unit represented by formula (1), a structural unit represented by formula (2), and a structural unit represented by formula (3). １ H-NMR measurement chart ( １ H-NMR chart, １ 2 shows the H-NMR spectrum of the polymer of Example 3. １ 3 is a H-NMR measurement chart of the polymer of Example 5. １ 4 is a H-NMR measurement chart of the polymer of Example 7. １ 1 is a H-NMR measurement chart.

[0077] Also, １ From the results of the H-NMR chart measurement, it was confirmed that the polymers of Comparative Examples 1, 3, and 4 were copolymers having a structural unit represented by formula (1) and a structural unit represented by formula (2). It was also confirmed that the polymer of Comparative Example 2 was a copolymer having a structural unit represented by formula (1) and a structural unit represented by formula (3).

[0078] In addition, the polymers of Examples 1 to 14 and Comparative Examples 1 to 4 １ The composition ratios of the polymers of Examples 1 to 14 and Comparative Examples 1 to 4 were determined from the integrals of each signal in the H-NMR spectra. The content (parts by mol) of the structural unit represented by formula (2) and the content (parts by mol) of the structural unit represented by formula (3) relative to 100 parts by mol of the structural unit represented by formula (1) contained in each polymer, calculated from the results, are shown in Tables 5 and 6, respectively.

[0079]

[0080]

[0081] (Manufacturing of Piezoelectric Films) "Examples 101 to 114, Examples 201 to 214, Comparative Examples 101 to 104, and Comparative Examples 201 to 209" Using the respective polymers of Examples 1 to 14 and Comparative Examples 1 to 4 as piezoelectric materials, piezoelectric films were manufactured by the methods described below.

[0082] A piezoelectric material was dissolved in N,N-dimethylformamide as a solvent to prepare a 20% by mass polymer solution (coating solution). The obtained polymer solution was applied to a PET film (trade name: Lumirror (registered trademark), manufactured by Toray Industries, Inc.) as a substrate so that the thickness after drying would be 50 μm, forming a coating film. The coating film formed on the PET film was then dried on a hot plate at 120°C for 6 hours to remove the solvent in the coating film, and a piezoelectric material sheet was obtained.

[0083] The obtained piezoelectric material sheet was peeled from the PET film, and aluminum electrodes were provided on one side and the other side of the piezoelectric material sheet by vapor deposition. The electrodes of the piezoelectric material sheet were then electrically connected to a high-voltage power supply HARB-20R60 (manufactured by Matsusada Precision Co., Ltd.), and the sheet was maintained for 30 minutes at the polarization temperatures shown in Tables 7 to 10 while an electric field with the field strength shown in Tables 7 to 10 was applied. The sheet was then slowly cooled to room temperature with the voltage still applied, and subjected to a poling treatment to obtain a sheet-like piezoelectric film.

[0084] The piezoelectric constant d of each of the piezoelectric films of Examples 101 to 114, Examples 201 to 214, Comparative Examples 101 to 104, and Comparative Examples 201 to 209 obtained in this manner was measured by the following method. ３３ The results are shown in Tables 7 to 10.

[0085] (piezoelectric constant d ３３ Measurement method for the piezoelectric constant d) The piezoelectric film was attached to the measurement device using a pin with a tip diameter of 1.5 mm as a sample fixing jig. ３３ The measuring device used was a piezometer system PM200 manufactured by PIEZOTEST. ３３ The measured value of the piezoelectric constant d is a positive or negative value depending on the front and back of the piezoelectric film being measured. ３３ The absolute value of the actual measurement is recorded as the value.

[0086]

[0087]

[0088]

[0089]

[0090] Tables 7 to 10 show the contents (parts by mol) of the structural unit represented by formula (2) and the contents (parts by mol) of the structural unit represented by formula (3) relative to 100 parts by mol of the structural unit represented by formula (1) contained in the polymers of Examples 1 to 14 and Comparative Examples 1 to 4 used as piezoelectric materials.

[0091] As shown in Tables 7 and 9, the piezoelectric films of Examples 101 to 108 and Examples 201 to 208, which were manufactured using the polymers (copolymers) of Examples 1 to 8, had a piezoelectric constant d ３３ The piezoelectric properties were good.

[0092] Furthermore, in Comparative Example 202, in which the copolymer of Comparative Example 1, which does not have the structural unit represented by formula (3), was used and the polarization temperature was set to 170° C., dielectric breakdown occurred. Furthermore, in Comparative Example 203, in which the copolymer of Comparative Example 1 was used and the electric field strength was set to 120 MV / m, dielectric breakdown also occurred. Therefore, it was found that when the copolymer of Comparative Example 1 was used, it was difficult to increase the polarization temperature and electric field strength in order to obtain a piezoelectric film with excellent piezoelectric properties.

[0093] Furthermore, as shown in Table 7, in Comparative Example 102 produced using the copolymer of Comparative Example 2 which does not have the structural unit represented by formula (2), dielectric breakdown occurred when polarization treatment was performed at a polarization temperature of 165°C and an electric field strength of 80 MV / m.

[0094] As shown in Tables 8 and 10, the piezoelectric films of Examples 109 to 114 and Examples 209 to 214, which were manufactured using the polymers (copolymers) of Examples 9 to 14, had a piezoelectric constant d ３３ The piezoelectric properties were good.

[0095] Furthermore, breakdown occurred in Comparative Examples 206 and 208, which used the copolymers of Comparative Examples 3 and 4, which did not have the structural unit represented by formula (3), and which were set at a polarization temperature of 170° C. Furthermore, breakdown also occurred in Comparative Examples 207 and 209, which used the copolymers of Comparative Examples 3 and 4, and which were set at an electric field strength of 120 MV / m. Therefore, it was found that when the copolymers of Comparative Examples 3 and 4 were used, it was difficult to increase the polarization temperature and electric field strength in order to obtain a piezoelectric film with excellent piezoelectric properties.

[0096] The copolymer of this embodiment is suitably applied to piezoelectric materials and piezoelectric films.

Claims

1. A copolymer comprising a structural unit derived from vinylidene cyanide represented by the following formula (1), a structural unit derived from a vinyl ester represented by the following formula (2), and a structural unit derived from a (meth)acrylic acid ester represented by the following formula (3): (In formula (2), R １ is an organic group having 1 to 15 carbon atoms or a hydrogen atom. ２ is an organic group or a hydrogen atom. ３ is a hydrogen atom or a methyl group.

2. The copolymer according to claim 1, wherein the content of the structural units derived from the vinyl ester represented by formula (2) is 80 to 99 parts by mole, and the content of the structural units derived from the (meth)acrylic acid ester represented by formula (3) is 1 to 20 parts by mole, relative to 100 parts by mole of the structural units derived from vinylidene cyanide represented by formula (1).

3. In the formula (2), R １ is a linear alkyl group having 1 to 3 carbon atoms.

4. In the formula (3), R ２ is an organic group having 1 to 15 carbon atoms.

5. In the formula (3), R ２ The copolymer according to claim 4, wherein is an alkyl group having 1 to 8 carbon atoms which may be substituted with a cyano group.

6. In the formula (3), R ２ The copolymer according to claim 1 , wherein is an organic group represented by the following formula (4): (n in formula (4) is a value that makes the number average molecular weight of the organic group represented by formula (4) 415 to 19,915.) 7. In the formula (3), R ３ The copolymer according to claim 1 , wherein is a hydrogen atom.

8. A piezoelectric material comprising the copolymer according to any one of claims 1 to 7.

9. A piezoelectric film comprising the copolymer according to any one of claims 1 to 7.

10. A piezoelectric element comprising the piezoelectric film according to claim 9 and electrodes disposed on one surface and the other surface of said piezoelectric film.

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