Copolymers, piezoelectric materials, piezoelectric films, and piezoelectric elements

Abstract

To provide a copolymer that serves as a piezoelectric material, enabling the production of piezoelectric films with high heat resistance and piezoelectric characteristics.SOLUTION: The copolymer includes: a structural unit represented by formula (1) (R1 is any one selected from a hydrogen atom, a methyl group, a dimethyl group, an ethyl group, an isopropyl group, an isobutyl group, a phenyl group, and a benzyl group, R2 is a hydrogen atom or a methyl group, or R1 and R2 form a benzooxazolidinone skeleton together with an oxazolidinone ring); and a structural unit represented by formula (2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to copolymers, piezoelectric materials, piezoelectric films, and piezoelectric elements. [Background technology]

[0002] Conventionally, PZT (PbZrO3-PbTiO3 solid solution), a ceramic material, has been widely used as the piezoelectric material for forming the piezoelectric element in piezoelectric devices. However, PZT has the disadvantage of being brittle because it is a ceramic containing lead. Therefore, there is a need for a piezoelectric material that has a low environmental impact and is highly flexible.

[0003] To address these requirements, polymeric piezoelectric materials can be considered. Examples of 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. Therefore, conventional piezoelectric materials made of ferroelectric polymers lose their piezoelectric properties at high temperatures, and their physical properties such as elastic modulus also deteriorate. Consequently, piezoelectric elements with piezoelectric materials made of conventional ferroelectric polymers have had a narrow temperature range in which they can be used.

[0004] Furthermore, amorphous polymer piezoelectric materials acquire piezoelectric properties by cooling while polarization at temperatures near the glass transition temperature. Amorphous polymers lose their piezoelectric properties at temperatures near the glass transition temperature. Therefore, amorphous polymer piezoelectric materials with high glass transition temperatures and good heat resistance are required.

[0005] Examples of amorphous polymer piezoelectric materials with high glass transition temperatures include vinylidene cyanide-vinyl acetate copolymers (see, for example, Patent Document 1). However, vinylidene cyanide-vinyl acetate copolymers require the use of vinylidene cyanide, which is difficult to handle, as a raw material monomer. Specifically, vinylidene cyanide readily undergoes homopolymerization with even trace amounts of moisture in the atmosphere. For this reason, polymers using vinylidene cyanide as a raw material monomer exhibit large variations during manufacturing, making them difficult to use as amorphous polymer piezoelectric materials.

[0006] Furthermore, polymers using 2-fluoroacrylonitrile as a starting monomer, which, like vinylidene cyanide, has a nitrile group, are known (see, for example, Non-Patent Document 1). 2-fluoroacrylonitrile has good stability. However, conventional polymers using 2-fluoroacrylonitrile as a starting monomer have a low glass transition temperature and insufficient heat resistance. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 1991 / 013922 [Non-patent literature]

[0008] [Non-Patent Document 1] Journal of the Chemical Society of Japan, 1985, (10), P.1862~1868 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Conventionally, there has been a demand for polymer piezoelectric materials that can produce piezoelectric films with high heat resistance and piezoelectric properties. The present invention has been made in view of the above circumstances, and aims to provide a copolymer that can be used as a piezoelectric material to obtain a piezoelectric film with high heat resistance and piezoelectric properties.

[0010] Furthermore, the present invention aims to provide a piezoelectric material that contains the copolymer of the present invention and yields a piezoelectric film with high heat resistance and piezoelectric properties. Furthermore, the present invention aims to provide a piezoelectric film containing the copolymer of the present invention that has high heat resistance and piezoelectric properties, and a piezoelectric element having the piezoelectric film of the present invention that also has high heat resistance and piezoelectric properties. [Means for solving the problem]

[0011] To solve the above problems, the following means are provided. A copolymer according to one aspect of the present invention is a copolymer having a structural unit represented by the following general formula (1) and a structural unit represented by the following formula (2).

[0012] [ka] (In general formula (1), R 1 R is one of the following: hydrogen atom, methyl group, dimethyl group, ethyl group, isopropyl group, isobutyl group, phenyl group, benzyl group, 2 is a hydrogen atom or a methyl group, or R 1 and R 2 It forms a benzoxazolidinone skeleton together with the oxazolidinone ring. [Effects of the Invention]

[0013] The copolymer of the present invention has a structural unit represented by general formula (1) and a structural unit represented by formula (2). For this reason, the copolymer of the present invention can be used as a piezoelectric material to obtain piezoelectric films with high heat resistance and piezoelectric properties. Furthermore, since the piezoelectric material of the present invention contains the copolymer of the present invention, a piezoelectric film with high heat resistance and piezoelectric properties can be obtained. Furthermore, the piezoelectric film of the present invention includes the copolymer of the present invention. For this reason, the piezoelectric film of the present invention and the piezoelectric element of the present invention having the piezoelectric film of the present invention have excellent heat resistance and piezoelectric properties. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is the 1H-NMR measurement chart for Example 3. [Figure 2] Figure 2 is the 19F-NMR measurement chart for Example 3. [Figure 3] Figure 3 is the 1H-NMR measurement chart for Example 8. [Figure 4] Figure 4 is the 19F-NMR measurement chart for Example 8. [Modes for carrying out the invention]

[0015] To solve the above problems, the inventors focused on polymers that use stable monomers having a nitrile group (-C≡N) as raw material monomers and conducted extensive research. As a result, it was found that copolymers having structural units containing an oxazolidinone skeleton and structural units derived from acrylonitrile become piezoelectric materials that produce piezoelectric films with good heat resistance and piezoelectric properties. The reason for this is presumed to be that, in the above copolymer, the structural units containing the highly polar oxazolidinone skeleton disrupt the ordered structure that can be formed by the nitrile groups, which are polar groups derived from acrylonitrile, making it difficult for the nitrile groups to orient themselves in a way that cancels out each other's polarity.

[0016] Therefore, the inventors focused on the dipole moment of a copolymer having structural units containing an oxazolidinone skeleton and structural units derived from acrylonitrile, and conducted extensive research to further improve the piezoelectric properties of a piezoelectric film containing the above copolymer. However, it was difficult to improve the piezoelectric properties of the piezoelectric film containing this copolymer for the following reasons.

[0017] In other words, in order to improve the piezoelectric properties of a piezoelectric film containing the above copolymer, it is desirable that the nitrile groups contained in the structural units derived from acrylonitrile in the copolymer are not oriented in such a way that their dipole moments cancel each other out. However, polymers using acrylonitrile as a raw material monomer have low symmetry in their dipole moments with respect to the polymer main chain direction. Furthermore, even when acrylonitrile is copolymerized with a compound in which a vinyl group is bonded to the nitrogen atom of an oxazolidinone skeleton, it was not possible to sufficiently suppress the cancellation of the dipole moments of the nitrile groups derived from acrylonitrile. For this reason, it was difficult to further improve the piezoelectric properties of a piezoelectric film containing the above copolymer.

[0018] Therefore, the inventors have conducted extensive research on polymers using a stable monomer having a nitrile group as a raw material, instead of acrylonitrile, which yields a polymer with high symmetry in the dipole moment with respect to the polymer main chain direction. As a result, we found that a copolymer having a specific structural unit containing an oxazolidinone skeleton and a structural unit derived from 2-fluoroacrylonitrile is sufficient.

[0019] In other words, in copolymers having a specific structural unit containing an oxazolidinone skeleton and a structural unit derived from 2-fluoroacrylonitrile, the fluoro groups contained in the structural unit derived from 2-fluoroacrylonitrile form intermolecular hydrogen bonds, resulting in strong intermolecular interactions. As a result, it is estimated that piezoelectric films using this copolymer will have better heat resistance compared to copolymers using structural units derived from acrylonitrile instead of structural units derived from 2-fluoroacrylonitrile.

[0020] Furthermore, because the copolymer contains nitrile and fluoro groups in the structural units derived from 2-fluoroacrylonitrile, it exhibits high symmetry in its dipole moment with respect to the polymer main chain direction. Therefore, the copolymer suppresses the cancellation of dipole moments and can exhibit high polarity. From this, it is presumed that a piezoelectric film using the copolymer will yield better piezoelectric properties compared to a copolymer using structural units derived from acrylonitrile instead of structural units derived from 2-fluoroacrylonitrile.

[0021] Furthermore, the inventors produced a copolymer (polymer) having a specific structural unit containing an oxazolidinone skeleton and a structural unit derived from 2-fluoroacrylonitrile, and confirmed that it has good heat resistance and that a piezoelectric film using it as a piezoelectric material has good piezoelectric properties, which led to the invention of the present invention.

[0022] Furthermore, the inventors considered using compounds in which the fluoro group (-F) of 2-fluoroacrylonitrile is replaced with other halogen groups (chloro group (-Cl), bromo group (-Br), iodo group (-I)) as a raw material monomer. However, compounds in which the fluoro group of 2-fluoroacrylonitrile is replaced with other halogen groups proved difficult to copolymerize with compounds in which a vinyl group is bonded to the nitrogen atom of an oxazolidinone skeleton. For this reason, it was not possible to produce copolymers of compounds in which a vinyl group is bonded to the nitrogen atom of an oxazolidinone skeleton with compounds in which the fluoro group of 2-fluoroacrylonitrile is replaced with other halogen groups as a raw material monomer.

[0023] The present invention includes the following embodiments. [1] A copolymer having structural units represented by the following general formula (1) and structural units represented by the following formula (2).

[0024] [ka] (In general formula (1), R 1 is any one selected from a hydrogen atom, a methyl group, a dimethyl group, an ethyl group, an isopropyl group, an isobutyl group, a phenyl group, and a benzyl group, and R 2 is a hydrogen atom or a methyl group, or R 1 and R 2 form a benzoxazolidinone skeleton together with the oxazolidinone ring.)

[0025] [2] In the general formula (1), the copolymer according to [1], wherein R 1 is a hydrogen atom and R 2 is a hydrogen atom or a methyl group. [3] The copolymer according to [1] or [2], wherein the content of the structural unit represented by the formula (2) is 10 to 80 mol%.

[0026] [4] A piezoelectric material containing the copolymer according to any one of [1] to [3]. [5] A piezoelectric film containing the copolymer according to any one of [1] to [3]. [6] A piezoelectric element having the piezoelectric film according to [5] and electrodes disposed on one surface and the other surface of the piezoelectric film, respectively.

[0027] Hereinafter, the copolymer, piezoelectric material, piezoelectric film, and piezoelectric element of the present invention will be described in detail. [Copolymer] The copolymer (polymer) of the present embodiment has a structural unit represented by general formula (1) and a structural unit represented by formula (2).

[0028] In the structural unit represented by formula (1) of the copolymer of the present embodiment, R 1 is any one selected from a hydrogen atom, a methyl group, a dimethyl group, an ethyl group, an isopropyl group, an isobutyl group, a phenyl group, and a benzyl group, and R 2 is a hydrogen atom or a methyl group. In the copolymer of the present embodiment, R 1 and R 2As described above, it can be easily manufactured. Furthermore, the copolymer of this embodiment has R of the structural unit shown in formula (1). 1 and R 2 As described above, it can be used as a material for piezoelectric films with good heat resistance and piezoelectric properties. The R of the structural unit shown in formula (1) 1 and R 2 Since it is nonpolar, it is preferable that it has a small volume. This is because the proportion of the volume of the polar portion in the entire copolymer increases relatively, contributing to the improvement of the piezoelectric properties of the piezoelectric film using it. Specifically, R 1 However, it is a hydrogen atom, R 2 However, it is preferable that it be a hydrogen atom or a methyl group. Furthermore, as is evident from the examples, it can be used as a piezoelectric film material with good heat resistance and piezoelectric properties, in particular, R 1 However, it is a hydrogen atom, R 2 However, a methyl group is preferred.

[0029] The structural unit shown in equation (1) is R 1 and R 2 However, it may also form a benzoxazolidinone skeleton together with the oxazolidinone ring. The R of the structural unit represented by formula (1) in the copolymer of this embodiment 1 and R 2 However, even when it forms a benzoxazolidinone skeleton together with the oxazolidinone ring, it can be easily manufactured and used as a piezoelectric film material with good heat resistance and piezoelectric properties.

[0030] In the copolymer of this embodiment, there are no particular restrictions on the arrangement order of the repeating units, the structural units represented by formula (1) and the structural units represented by formula (2). Furthermore, in the copolymer of this embodiment, the number of structural units represented by formula (1) and the number of structural units represented by formula (2) may be the same or different. Therefore, the copolymer of this embodiment may have alternating arrangement portions in which the structural units represented by formula (1) and the structural units represented by formula (2) are arranged alternately, random arrangement portions in which the structural units represented by formula (1) and the structural units represented by formula (2) are arranged without order, and block arrangement portions having portions in which the structural units represented by formula (1) are arranged continuously and portions in which the structural units represented by formula (2) are arranged continuously, distributed in any proportion. In the copolymer of this embodiment, it is preferable to include alternating arrangement portions because the fluorogroups and nitrile groups contained in the structural units represented by formula (2) are less likely to be oriented in a way that cancels out each other's polarity, making it usable as a piezoelectric material with good heat resistance and piezoelectric properties.

[0031] The copolymer of this embodiment preferably contains 10 to 80 mol% of the structural unit represented by formula (1), more preferably 20 to 70 mol%, and even more preferably 30 to 60 mol%. When the content of the structural unit represented by formula (1) is 10 mol% or more, the copolymer becomes even more heat resistant. Furthermore, when the content of the structural unit represented by formula (1) is 80 mol% or less, it is possible to prevent the piezoelectric film containing the copolymer from becoming hard and brittle due to an excessive content of the structural unit represented by formula (1). In addition, when the content of the structural unit represented by formula (1) is 80 mol% or less, it is possible to suppress the decrease in the insulation resistance of the copolymer due to moisture absorption by the structural unit represented by formula (1).

[0032] The copolymer of this embodiment preferably contains 10 to 80 mol% of the structural unit represented by formula (2), more preferably 20 to 70 mol%, and even more preferably 30 to 60 mol%. When the content of the structural unit represented by formula (2) is 10 mol% or more, the copolymer has high insulation resistance and can form a flexible piezoelectric film. Furthermore, when the content of the structural unit represented by formula (2) is 80 mol% or less, it becomes easier to ensure the content of the structural unit represented by formula (1). As a result, the fluorogroups and nitrile groups contained in the structural unit represented by formula (2) are less likely to be oriented in a way that cancels out each other's polarity, resulting in a copolymer that can form a piezoelectric film with better heat resistance and piezoelectric properties.

[0033] The copolymer of this embodiment may optionally contain one or more structural units other than the structural unit represented by formula (1) and the structural unit represented by formula (2). Examples of other structural units include structural units derived from known monomers or oligomers having polymerizable unsaturated bonds, such as acrylonitrile. In the copolymer of this embodiment, the total content of structural units represented by formula (1) and structural units represented by formula (2) is preferably 50% by mass or more, more preferably 80% by mass or more, and may be 90% by mass or more, or may consist only of structural units represented by formula (1) and structural units represented by formula (2).

[0034] The weight-average molecular weight (Mw) of the copolymer in this embodiment is preferably 10,000 to 1,000,000. If 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 manufactured. If the weight-average molecular weight (Mw) of the copolymer is 1,000,000 or less, it can be easily dissolved in a solvent, and a piezoelectric film can be easily manufactured using a coating solution dissolved in a solvent.

[0035] "Method for producing copolymers" The copolymer of this embodiment can be produced, for example, by radical copolymerization using a known method with a starting monomer containing a compound from which the structural unit represented by formula (1) is derived, 2-fluoroacrylonitrile, and a polymerization initiator such as azobisbutyronitrile. The polymerization conditions, such as reaction temperature and reaction time, when producing the copolymer of this embodiment can be appropriately determined according to the composition of the raw material monomers.

[0036] The compounds from which the structural unit shown in formula (1) originates are compounds in which the structural unit shown in formula (1) and the oxazolidinone skeleton and the atoms bonded to the carbon atoms of the oxazolidinone skeleton are the same, and a vinyl group is bonded to the nitrogen atom of the oxazolidinone skeleton. Examples of compounds from which the structural unit represented by formula (1) originates include N-vinyl-oxazolidinone, N-vinyl-5-methyloxazolidinone, N-vinyl-4-methyloxazolidinone, N-vinyl-4,4-dimethyloxazolidinone, N-vinyl-4-ethyloxazolidinone, N-vinyl-4-propyloxazolidinone, N-vinyl-4-isopropyloxazolidinone, N-vinyl-4-isobutyloxazolidinone, N-vinyl-4-phenyloxazolidinone, N-vinyl-4-benzyloxazolidinone, and N-vinyl-2-benzoxazolidinone, which are appropriately determined depending on the structure of the copolymer of this embodiment, which is the target product.

[0037] "Piezoelectric materials" The piezoelectric material of this embodiment includes the copolymer of this embodiment. The copolymer of this embodiment included in the piezoelectric material of this embodiment may be one type or two or more types. Furthermore, the piezoelectric material of this embodiment may, if necessary, include one or two or more known polymers other than the copolymer of this embodiment together with the copolymer of this embodiment.

[0038] "Piezoelectric film" The piezoelectric film of this embodiment includes the copolymer of this embodiment. The piezoelectric film of this embodiment can be manufactured, for example, by the method shown below. The piezoelectric material of this embodiment, including the copolymer of this embodiment, is dissolved in a known solvent such as N,N-dimethylformamide to prepare a coating solution. Next, the coating solution is applied to a peelable substrate to a predetermined thickness to form a coating film. As the substrate, known materials such as those made of resin films such as polyethylene terephthalate (PET) can be used. The method of applying the coating solution can be a known method depending on the coating thickness, viscosity of the coating solution, etc. After that, the coating film is dried to remove the solvent in the coating film and obtain a piezoelectric material sheet.

[0039] Subsequently, the piezoelectric material sheet is peeled from the substrate, electrodes made of a known conductive material such as aluminum are placed on one side and the other side of the piezoelectric material sheet, and a voltage is applied at a temperature near the glass transition temperature of the piezoelectric material forming the piezoelectric material sheet, and then the sheet is cooled while the voltage is applied. This acquires piezoelectric properties. Through the above steps, a sheet-like piezoelectric film is obtained. The electrodes used to achieve piezoelectricity may be used as components to form the piezoelectric element, or they may be removed.

[0040] "Piezoelectric element" The piezoelectric element of this embodiment comprises a piezoelectric film and electrodes disposed on the surface of the piezoelectric film. Specifically, it includes a sheet-like piezoelectric film and electrodes disposed on one surface and the other surface of the piezoelectric film, respectively. As the electrode material, known conductive materials such as aluminum can be used. The piezoelectric element of this embodiment can be manufactured, for example, by providing electrodes on one surface and the other surface of a piezoelectric film, respectively, using a known method such as vapor deposition.

[0041] The copolymer of this embodiment has structural units represented by general formula (1) and structural units represented by formula (2). Therefore, the copolymer of this embodiment can be used as a piezoelectric material to obtain piezoelectric films with high heat resistance and piezoelectric properties. Furthermore, since the piezoelectric material of this embodiment includes the copolymer of this embodiment, a piezoelectric film with high heat resistance and piezoelectric properties can be obtained. Furthermore, the piezoelectric film of this embodiment includes the copolymer of this embodiment. For this reason, the piezoelectric film of this embodiment and the piezoelectric element of this embodiment having the piezoelectric film of this embodiment have excellent heat resistance and piezoelectric properties.

[0042] Although embodiments of the present invention have been described in detail above, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention. [Examples]

[0043] "Example 1" In a 100 ml Schlenk tube, 0.6 ml (4 mmol) of N-vinyl oxazolidinone represented by the following general formula (11) and 0.9 ml (12 mmol) of 2-fluoroacrylonitrile were mixed, and 12.4 mg (0.08 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.6 g of the polymer of Example 1. The yield was 40%.

[0044] [ka] (In general formula (11), R 2 (This is a hydrogen atom.)

[0045] For the polymer of Example 1, an NMR (nuclear magnetic resonance) spectrometer (product name JNM-ECA500, manufactured by JEOL Ltd.) was used, and dimethyl sulfoxide d6 (DMSO-d6) was used as the solvent. 1 H-NMR measurement and 19 The molecular structure was identified by performing 1F-NMR measurements. As a result, the polymer of Example 1 is a structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2It was confirmed that the copolymer has a hydrogen atom and a structural unit shown in formula (2). Also, Example 1 1 H-NMR spectrum and 19 The composition ratio was calculated from the integral values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 1 was 76%.

[0046] Example 2 In a 100 ml Schlenk tube, 0.9 ml (4 mmol) of N-vinyl-oxazolidinone and 0.4 ml (6 mmol) of 2-fluoroacrylonitrile were mixed, and 7.8 mg (0.05 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.7 g of the polymer of Example 2. The yield was 62%.

[0047] Regarding the polymer of Example 2, the same procedure was followed as with the polymer of Example 1. 1 H-NMR measurement and 19 The molecular structure was identified by performing F-NMR measurements. As a result, the polymer of Example 2, like the polymer of Example 1, has a structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that the copolymer has a hydrogen atom and a structural unit shown in formula (2). Also, Example 2 1 H-NMR spectrum and 19 The composition ratio was calculated from the integrated values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 2 was 59%.

[0048] "Example 3" In a 100 ml Schlenk tube, 0.6 ml (6 mmol) of N-vinyl-oxazolidinone and 0.4 ml (6 mmol) of 2-fluoroacrylonitrile were mixed, and 9.1 mg (0.06 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.5 g of the polymer of Example 3. The yield was 48%.

[0049] Regarding the polymer of Example 3, the same procedure was followed as with the polymer of Example 1. 1 H-NMR measurement and 19 The molecular structure was identified by 1F-NMR measurement. Figure 1 shows the molecular structure of Example 1. 1 This is the H-NMR measurement chart for Example 3. Figure 2 shows the results for Example 3. 19 This is a chart showing F-NMR measurements. As a result, the polymer of Example 3, like the polymer of Example 1, has structural units represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that the copolymer has a hydrogen atom and a structural unit shown in formula (2). Also, Example 3 1 H-NMR spectrum and 19 The composition ratio was calculated from the integral values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 3 was 50%.

[0050] "Example 4" In a 100 ml Schlenk tube, 0.6 ml (6 mmol) of N-vinyl-oxazolidinone and 0.3 ml (4 mmol) of 2-fluoroacrylonitrile were mixed, and 7.9 mg (0.05 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.6 g of the polymer of Example 4. The yield was 58%.

[0051] Regarding the polymer of Example 4, the same procedure was followed as with the polymer of Example 1. 1H-NMR measurement and 19 The molecular structure was identified by 1F-NMR measurement. As a result, the polymer of Example 4, like the polymer of Example 1, has a structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that the copolymer has a hydrogen atom and a structural unit shown in formula (2). Also, Example 4 1 H-NMR spectrum and 19 The composition ratio was calculated from the integrated values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 4 was 41%.

[0052] Example 5 In a 100 ml Schlenk tube, 0.6 ml (12 mmol) of N-vinyl-oxazolidinone and 0.4 ml (6 mmol) of 2-fluoroacrylonitrile were mixed, and 9 mg (0.05 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.7 g of the polymer of Example 5. The yield was 59%.

[0053] Regarding the polymer of Example 5, the same procedure was followed as with the polymer of Example 1. 1 H-NMR measurement and 19 The molecular structure was identified by 1F-NMR measurement. As a result, the polymer of Example 5, like the polymer of Example 1, has a structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that the copolymer has a hydrogen atom and a structural unit shown in formula (2). Also, Example 5 1 H-NMR spectrum and 19 The composition ratio was calculated from the integral values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 5 was 25%.

[0054] "Example 6" 0.9 ml (8 mmol) of N-vinyl-5-methyloxazolidinone (R in general formula (11)) in a 100 ml Schlenk tube. 2 However, a compound containing a methyl group was mixed with 0.3 ml (4 mmol) of 2-fluoroacrylonitrile, and 10.3 mg (0.06 mmol) of azobisisobutyronitrile was added, and the mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.7 g of the polymer of Example 6. The yield was 56%.

[0055] Regarding the polymer of Example 6, the same procedure was followed as with the polymer of Example 1. 1 H-NMR measurement and 19 The molecular structure was identified by 1F-NMR measurement. As a result, the polymer of Example 6 was found to be a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that the copolymer has a methyl group and a structural unit represented by formula (2). Also, Example 6 1 H-NMR spectrum and 19 The composition ratio was calculated from the integral values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 6 was 74%.

[0056] Example 7 In a 100 ml Schlenk tube, 0.7 ml (6 mmol) of N-vinyl-5-methyloxazolidinone and 0.3 ml (4 mmol) of 2-fluoroacrylonitrile were mixed, and 8.3 mg (0.05 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.8 g of the polymer of Example 7. The yield was 56%.

[0057] Regarding the polymer of Example 7, the same procedure was followed as with the polymer of Example 1. 1 H-NMR measurement and 19The molecular structure was identified by 1F-NMR measurement. As a result, the polymer of Example 7, like the polymer of Example 6, has a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that the copolymer has a methyl group and a structural unit represented by formula (2). Also, Example 7 1 H-NMR spectrum and 19 The composition ratio was calculated from the integrated values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 7 was 65%.

[0058] "Example 8" In a 100 ml Schlenk tube, 0.5 ml (4 mmol) of N-vinyl-5-methyloxazolidinone and 0.3 ml (4 mmol) of 2-fluoroacrylonitrile were mixed, and 6.2 mg (0.04 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.6 g of the polymer of Example 8. The yield was 76%.

[0059] Regarding the polymer of Example 8, the same procedure was followed as for the polymer of Example 1. 1 H-NMR measurement and 19 The molecular structure was identified by 1F-NMR measurement. Figure 3 shows the molecular structure of Example 8. 1 This is the H-NMR measurement chart for Example 8. Figure 4 shows the results for Example 8. 19 This is a chart showing F-NMR measurements. As a result, the polymer of Example 8, like the polymer of Example 6, has structural units represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that the copolymer has a methyl group and a structural unit represented by formula (2). Also, Example 8 1 H-NMR spectrum and 19The composition ratio was calculated from the integrated values ​​of each signal in the 1F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 8 was 53%.

[0060] "Example 9" In a 100 ml Schlenk tube, 0.4 ml (3 mmol) of N-vinyl-5-methyloxazolidinone and 0.3 ml (4 mmol) of 2-fluoroacrylonitrile were mixed, and 5.5 mg (0.03 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.4 g of the polymer of Example 9. The yield was 57%.

[0061] Regarding the polymer of Example 9, the same procedure was followed as with the polymer of Example 1. 1 H-NMR measurement and 19 The molecular structure was identified by 1F-NMR measurement. As a result, the polymer of Example 9, like the polymer of Example 6, has a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that the copolymer has a methyl group and a structural unit represented by formula (2). Also, Example 9 1 H-NMR spectrum and 19 The composition ratio was calculated from the integral values ​​of each signal in the F-NMR spectrum. As a result, the content of the structural unit represented by formula (2) in the polymer of Example 9 was 38%.

[0062] "Example 10" In a 100 ml Schlenk tube, 0.5 ml (4 mmol) of N-vinyl-5-methyloxazolidinone and 0.6 ml (8 mmol) of 2-fluoroacrylonitrile were mixed, and 4.4 mg (0.03 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.6 g of the polymer of Example 10. The yield was 51%.

[0063] For the polymer of Example 10, in the same manner as the polymer of Example 1, 1 1H-NMR measurement and 19 19F-NMR measurement were performed to identify the molecular structure. As a result, the polymer of Example 10, similar to the polymer of Example 6, has a structural unit represented by the general formula (1) (in the general formula (1), R 1 is a hydrogen atom, and R 2 is a methyl group.) and a structural unit represented by the formula (2). It was confirmed that it is a copolymer having these. Also, for the 1 1H-NMR spectrum and 19 19F-NMR spectrum of Example 10, the composition ratio was calculated from the integral values of each signal. As a result, the content of the structural unit represented by the formula (2) contained in the polymer of Example 10 was 24%.

[0064] "Comparative Example 1" Polyacrylonitrile (trade name 181315, manufactured by Sigma-Aldrich) was used as the polymer of Comparative Example 1. "Comparative Example 2" Poly(acrylonitrile-CO-methyl acrylate) (trade name 517941, manufactured by Sigma-Aldrich) was used as the polymer of Comparative Example 2.

[0065] "Comparative Example 3" In a 100 ml Schlenk tube, 0.4 ml (4 mmol) of N-vinyl-oxazolidinone and 1.2 ml (16 mmol) of acrylonitrile were mixed, 11.5 mg (0.07 mmol) of azobisisobutyronitrile was added, and the mixture was reacted at 60 °C for 2 hours. The reaction product was poured into 200 ml of methanol for reprecipitation, and after filtration and drying, 1.1 g of the polymer of Comparative Example 3 was obtained. The yield was 78%.

[0066] For the polymer of Comparative Example 3, in the same manner as the polymer of Example 1, 1 1H-NMR measurement was performed to identify the molecular structure. As a result, the polymer of Comparative Example 3 has a structural unit represented by the general formula (1) (in the general formula (1), R 1and R 2 It was confirmed that it is a copolymer having a structural unit derived from acrylonitrile and (wherein R is a hydrogen atom). Also, from the integral values of the respective signals in the 1 1H-NMR spectrum of Comparative Example 3, the composition ratio was calculated. As a result, the content of the structural unit derived from acrylonitrile contained in the polymer of Comparative Example 3 was 70%.

[0067] "Comparative Example 4" 0.4 ml (4 mmol) of N-vinyl-oxazolidinone and 0.6 ml (9 mmol) of acrylonitrile were mixed in a 100 ml Schlenk tube, 7.8 mg (0.05 mmol) of azobisisobutyronitrile was added, and the mixture was reacted at 60 °C for 2 hours. The reaction product was poured into 200 ml of methanol for reprecipitation, and the precipitate was separated by filtration and dried to obtain 0.7 g of the polymer of Comparative Example 4. The yield was 70%.

[0068] For the polymer of Comparative Example 4, in the same manner as the polymer of Example 1, 1 1H-NMR measurement was carried out to specify the molecular structure. As a result, the polymer of Comparative Example 4 was a copolymer having a structural unit represented by the general formula (1) (wherein R and R in the general formula (1) are hydrogen atoms) and a structural unit derived from acrylonitrile. It was confirmed that 1 and R 2 are hydrogen atoms). Also, from the integral values of the respective signals in the 1 1H-NMR spectrum of Comparative Example 4, the composition ratio was calculated. As a result, the content of the structural unit derived from acrylonitrile contained in the polymer of Comparative Example 4 was 59%.

[0069] "Comparative Example 5" In a 100 ml Schlenk tube, 0.4 ml (4 mmol) of N-vinyl oxazolidinone and 0.4 ml (7 mmol) of acrylonitrile were mixed, and 6.8 mg (0.04 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.5 g of the polymer of Comparative Example 5. The yield was 68%.

[0070] For the polymer of Comparative Example 5, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 5 was found to have a structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that it is a copolymer having a hydrogen atom and structural units derived from acrylonitrile. Also, Comparative Example 5 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 5 was 49%.

[0071] "Comparative Example 6" In a 100 ml Schlenk tube, 0.4 ml (4 mmol) of N-vinyl-oxazolidinone and 0.3 ml (4 mmol) of acrylonitrile were mixed, and 5.9 mg (0.04 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 0.6 g of the polymer of Comparative Example 6. The yield was 87%.

[0072] For the polymer of Comparative Example 6, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 6 was found to be the structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that it is a copolymer having a hydrogen atom and structural units derived from acrylonitrile. Also, Comparative Example 6 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 6 was 24%.

[0073] "Comparative Example 7" In a 100 ml Schlenk tube, 1.2 ml (12 mmol) of N-vinyl-oxazolidinone and 0.1 ml (2 mmol) of acrylonitrile were mixed, and 0.9 mg (0.03 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 4.5 g of the polymer of Comparative Example 7. The yield was 73%.

[0074] Regarding the polymer of Comparative Example 7, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 7 was found to have a structural unit represented by general formula (1) (R in general formula (1)). 1 and R 2 It was confirmed that it is a copolymer having a hydrogen atom and structural units derived from acrylonitrile. Also, Comparative Example 7 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 7 was 14%.

[0075] "Comparative Example 8" 0.5 ml (4 mmol) of N-vinyl-5-methyloxazolidinone (R in general formula (11)) in a 100 ml Schlenk tube. 2 However, a compound containing a methyl group was mixed with 1.0 ml (16 mmol) of acrylonitrile, and 10.7 mg (0.07 mmol) of azobisisobutyronitrile was added, and the mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, and 0.9 g of the polymer of Comparative Example 8 was obtained by filtration and drying. The yield was 68%.

[0076] Regarding the polymer of Comparative Example 8, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 8 was found to have a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that it is a copolymer having a methyl group and structural units derived from acrylonitrile. Also, Comparative Example 8 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 8 was 76%.

[0077] "Comparative Example 9" In a 100 ml Schlenk tube, 0.9 ml (8 mmol) of N-vinyl-5-methyloxazolidinone and 1.0 ml (16 mmol) of acrylonitrile were mixed, and 9.8 mg (0.05 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 1.2 g of the polymer of Comparative Example 9. The yield was 69%.

[0078] For the polymer of Comparative Example 9, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 9 was found to have a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that it is a copolymer having a methyl group and structural units derived from acrylonitrile. Also, Comparative Example 9 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 9 was 57%.

[0079] "Comparative Example 10" In a 100 ml Schlenk tube, 1.4 ml (12 mmol) of N-vinyl-5-methyloxazolidinone and 1.0 ml (16 mmol) of acrylonitrile were mixed, and 10.8 mg (0.07 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 1.5 g of the polymer of Comparative Example 10. The yield was 67%.

[0080] For the polymer of Comparative Example 10, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 10 was found to have a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that it is a copolymer having a methyl group and structural units derived from acrylonitrile. Also, Comparative Example 10 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 10 was 44%.

[0081] "Comparative Example 11" In a 100 ml Schlenk tube, 1.4 ml (12 mmol) of N-vinyl-5-methyloxazolidinone and 0.8 ml (12 mmol) of acrylonitrile were mixed, and 9.1 mg (0.06 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 1.3 g of the polymer of Comparative Example 11. The yield was 60%.

[0082] For the polymer of Comparative Example 11, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 11 was found to be the structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2It was confirmed that it is a copolymer having a methyl group and structural units derived from acrylonitrile. Also, Comparative Example 11 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 11 was 28%.

[0083] "Comparative Example 12" In a 100 ml Schlenk tube, 1.4 ml (12 mmol) of N-vinyl-5-methyloxazolidinone and 0.4 ml (6 mmol) of acrylonitrile were mixed, and 14.8 mg (0.09 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. The reaction product was added to 200 ml of methanol for reprecipitation, filtered, and dried to obtain 1.1 g of the polymer of Comparative Example 12. The yield was 60%.

[0084] For the polymer of Comparative Example 12, the same procedure was followed as for the polymer of Example 1. 1 The molecular structure was identified by 1H-NMR measurement. As a result, the polymer of Comparative Example 12 was found to be a structural unit represented by general formula (1) (R in general formula (1)). 1 R is a hydrogen atom, 2 It was confirmed that it is a copolymer having a methyl group and structural units derived from acrylonitrile. Also, Comparative Example 12 1 The composition ratio was calculated from the integrated values ​​of each signal in the 1H-NMR spectrum. As a result, the content of structural units derived from acrylonitrile in the polymer of Comparative Example 12 was 13%.

[0085] "Comparative Example 13" In a 100 ml Schlenk tube, 0.7 ml (8 mmol) of N-vinyl oxazolidinone and 0.6 ml (8 mmol) of 2-chloroacrylonitrile were mixed, and 12.2 mg (0.07 mmol) of azobisisobutyronitrile was added. The mixture was reacted at 60°C for 2 hours. However, 2-chloroacrylonitrile decomposed during the reaction, and no polymer was obtained.

[0086] "Comparative Example 14" 0.9 ml (8 mmol) of N-vinyl-5-methyloxazolidinone (R in general formula (11)) in a 100 ml Schlenk tube. 2 However, a compound containing a methyl group was mixed with 0.6 ml (8 mmol) of 2-chloroacrylonitrile, and 13.5 mg (0.08 mmol) of azobisisobutyronitrile was added, and the mixture was reacted at 60°C for 2 hours. However, 2-chloroacrylonitrile decomposed during polymerization, and no polymer was obtained.

[0087] For each of Examples 1 to 10 and Comparative Examples 3 to 14, the R in the structural unit represented by general formula (1) contained in the polymer obtained after synthesis was determined. 2 Table 1 shows the types of nitrile groups, the names of the monomers containing nitrile groups used in the synthesis, and the amount of nitrile groups (content of structural units shown in formula (2)) contained in the polymer obtained after synthesis. Furthermore, the names of the polymer compounds used in Comparative Example 1 and Comparative Example 2 are shown in Table 1, respectively.

[0088] [Table 1]

[0089] The glass transition temperature (Tg) of the polymers in Examples 1 to 10 and Comparative Examples 1 to 12 was measured using the method described below. The results are shown in Table 1. (Method for measuring glass transition temperature (Tg)) Using a high-sensitivity differential scanning calorimeter (product name, DSC6200, manufactured by Seiko Instruments Inc.), the following heating and cooling operations were performed under a nitrogen atmosphere: heating from 30°C to 200°C at a heating rate of 20°C / min, cooling from 200°C to 30°C at a cooling rate of 40°C / min, and heating from 30°C to 200°C at a heating rate of 20°C / min. The inflection point during the second heating cycle was determined and defined as the glass transition temperature (Tg).

[0090] Furthermore, piezoelectric films were manufactured using the polymers of Examples 1 to 10 and Comparative Examples 1 to 12 as piezoelectric materials, respectively, by the methods described below, and the piezoelectric constant d33 was measured. The results are shown in Table 1.

[0091] (Manufacturing of piezoelectric films) A piezoelectric material was dissolved in N,N-dimethylformamide, 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 was 50 μm, forming a coating film. Subsequently, the coating film formed on the PET film was dried on a hot plate at 120°C for 6 hours to remove the solvent from the coating film, thereby obtaining a piezoelectric material sheet.

[0092] The obtained piezoelectric material sheet was peeled from the PET film, and electrodes made of aluminum were attached to one side and the other side of the piezoelectric material sheet by vapor deposition. Then, the electrodes of the piezoelectric material sheet were electrically connected to a high-voltage power supply HARB-20R60 (manufactured by Matsusada Precision Co., Ltd.), and an electric field of 100 MV / m was applied and maintained at 140°C for 15 minutes. After that, the piezoelectric material sheet was slowly cooled to room temperature while the voltage was still applied, and a polling process was performed to obtain a sheet-like piezoelectric film.

[0093] (Method for measuring piezoelectric constant d33) A pin with a tip diameter of 1.5 mm was used as a sample fixing jig to attach the piezoelectric film to the measuring device. For measuring the piezoelectric constant d33, a PM200 piezometer system from PIEZOTEST was used. The measured value of the piezoelectric constant d33 will be either positive or negative depending on whether the piezoelectric film being measured is facing in the front or back direction. In this specification, the absolute value of the measured piezoelectric constant d33 will be given.

[0094] As shown in Table 1, the polymers of Examples 1 to 10 were found to have a higher glass transition temperature (Tg) and better heat resistance compared to the polymers of Comparative Examples 1 to 2. Furthermore, the polymers in Examples 1 to 10 all had sufficiently high glass transition temperatures (Tg) and good heat resistance.

[0095] As shown in Table 1, the piezoelectric films of Examples 1 to 10, which contain the polymers of Examples 1 to 10, had a higher piezoelectric constant d33 and better piezoelectric properties compared to the piezoelectric film of Comparative Example 1, which contains the polymer of Comparative Example 1, and the piezoelectric film of Comparative Example 2, which contains the polymer of Comparative Example 2. Furthermore, the piezoelectric films containing polymers in Examples 1 to 10 all had higher piezoelectric constants d33 and better piezoelectric properties compared to the piezoelectric films containing polymers in Comparative Examples 3 to 12.

[0096] In particular, the piezoelectric films of Examples 3-5, 8, and 9, which contain polymers with a content of 30-60 mol% of the structural unit shown in formula (2), have the R of the structural unit shown in formula (1). 2 Compared to piezoelectric films containing polymers with the same properties but containing less than 30 mol% or more than 60 mol% of the structural units shown in formula (2), the piezoelectric constant d33 was higher and the piezoelectric properties were better.

Claims

1. A copolymer having structural units represented by the following general formula (1) and structural units represented by the following formula (2). 【Chemistry 1】 (In general formula (1), R １ R is one of the following: hydrogen atom, methyl group, dimethyl group, ethyl group, isopropyl group, isobutyl group, phenyl group, benzyl group, ２ is a hydrogen atom or a methyl group, or R １ and R ２ It forms a benzoxazolidinone skeleton together with the oxazolidinone ring.

2. In the above general formula (1), R １ However, it is a hydrogen atom, R ２ The copolymer according to claim 1, wherein the atom is a hydrogen atom or a methyl group.

3. The copolymer according to claim 1 or claim 2, wherein the content of the structural unit represented by formula (2) is 10 to 80 mol%.

4. A piezoelectric material comprising the copolymer according to claim 1 or claim 2.

5. A piezoelectric film comprising the copolymer according to claim 1 or claim 2.

6. A piezoelectric element having a piezoelectric film according to claim 5, and electrodes disposed on one surface and the other surface of the piezoelectric film, respectively.

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