JP7600801B2 - Polymeric piezoelectric material and piezoelectric element using same - Google Patents

Description

The present invention relates to a polymeric piezoelectric material and a piezoelectric element using the polymeric piezoelectric material.

Piezoelectric elements are used in applications such as pressure sensors that utilize the piezoelectric effect, which generates an electric charge when stress is applied, and actuators that utilize the inverse piezoelectric effect, which generates deformation such as expansion and contraction when an electric field is applied.
As piezoelectric materials used in such piezoelectric elements, polymeric piezoelectric materials that are lightweight and have excellent flexibility, moldability, processability, etc. are known, and polyvinylidene fluoride (PVDF)-based ferroelectric polymers are representative. However, with PVDF-based polymers, it is difficult to obtain stable piezoelectricity in air because the movement of water and ions contributes to the driving of the piezoelectric element.

In response to this, for example, Patent Documents 1 and 2 propose polyurethane elastomers as polymeric piezoelectric materials that provide good piezoelectric properties in air.
Also, Non-Patent Document 1 discloses a methacrylate dielectric elastomer having a graft chain of high molecular weight polydimethylsiloxane on the side chain.

JP 2014-100046 A JP 2014-100047 A

Advanced Materials, 2017, Vol. 29, 1604209

However, the polyurethane elastomers described in Patent Documents 1 and 2 have molecular chains in a random coil shape between crosslinks, so when they are used in actuators, they need to go through a stretching process to suppress hysteresis loss, and it is difficult to obtain a high elongation rate.

On the other hand, the methacrylate dielectric elastomer described in Non-Patent Document 1 has a structure in which the molecular chains are extended due to the dense brush-like presence of high molecular weight side chains, so no stretching process is required. However, because it has side chains made of high molecular weight polydimethylsiloxane, it cannot be said that its relative dielectric constant is sufficient for use as a piezoelectric material.

Therefore, from the perspective of further improving piezoelectricity, it is desirable to develop polymeric piezoelectric materials with high elongation and high dielectric constant.

The present invention aims to solve these problems by providing a polymeric piezoelectric material that has a low elastic modulus, a high elongation rate, a high dielectric constant, and good piezoelectric properties, and a piezoelectric element using the same.

The present invention is based on the discovery that a polymeric piezoelectric material with a low elastic modulus, high elongation rate, and high dielectric constant can be obtained by using a specific urethane acrylate having a polyether chain and a urethane bond.

The present invention provides the following means.
[1] A polymer having a structural unit based on a monofunctional urethane (meth)acrylate,
The polymeric piezoelectric material, wherein the monofunctional urethane (meth)acrylate is one or more monomers selected from the reaction products of the following (i) to (iii):
(i) An equimolar reaction product of a polyether monol and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product having one isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule.
(ii) an equimolar reaction product of a polyether monol, a diisocyanate, and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product having one group reactive with an isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule.
(iii) an equimolar reaction product of a polyether polyol and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product having one isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule.
[2] The polymeric piezoelectric material according to [1], wherein the monofunctional urethane (meth)acrylate has a number average molecular weight of 3,000 to 25,000.
[3] The polymeric piezoelectric material according to [1] or [2], wherein the monofunctional urethane (meth)acrylate has a glass transition temperature of −55° C. or lower.
[4] The polymeric piezoelectric material according to any one of [1] to [3], wherein a ratio of oxyethylene groups in all oxyalkylene groups of the monofunctional urethane (meth)acrylate is 0 to 90 mass%.
[5] The polymeric piezoelectric material according to any one of [1] to [4], wherein the polymer has a storage modulus of 5 to 150 kPa at −20 to 80° C.
[6] A piezoelectric element comprising the polymeric piezoelectric material according to any one of [1] to [5] sandwiched between at least one pair of electrodes.
[7] An actuator comprising the piezoelectric element according to [6].
[8] A pressure sensor comprising the piezoelectric element according to [6].

The present invention provides a polymeric piezoelectric material that has a low elastic modulus, a high elongation rate, a high dielectric constant, and good piezoelectric properties, and a piezoelectric element using the same.

The definitions and meanings of the terms and expressions used in this specification are given below.
The term "(meth)acryloyloxy group" is a general term for an acryloyloxy group and a methacryloyloxy group.
"(Meth)acrylate" is a general term for acrylate and methacrylate.
Similarly, "(meth)acrylic acid" is a general term for acrylic acid and methacrylic acid.
Unless otherwise specified, the "number of functional groups" refers to the number of (meth)acryloyloxy groups in one molecule.
The term "average number of functional groups" refers to the average number of (meth)acryloyloxy groups in one molecule, unless otherwise specified, with the formula weight based on a chemical formula or the number average molecular weight being one unit.
The term "monofunctional urethane (meth)acrylate" refers to a urethane (meth)acrylate having an average number of functional groups in one molecule that is substantially 1. A urethane (meth)acrylate having an average number of functional groups in one molecule that is 0.7 to 1.4, preferably 0.8 to 1.3, is considered to be a urethane (meth)acrylate having substantially one (meth)acryloyloxy group in one molecule, that is, a monofunctional urethane (meth)acrylate.
By "equimolar reaction product" is meant that the molar ratio of the reacting compounds is substantially 1, and a reaction product in which the molar ratio is 0.7 to 1.4, preferably 0.8 to 1.3, is considered to be an equimolar reaction product.
Similarly, the "equal number of moles" of reactive groups (or compounds) means that the molar ratio of the reactive groups (or compounds) is substantially 1, and when the molar ratio is 0.7 to 1.4, preferably 0.8 to 1.3, the molar numbers of the reactive groups (or compounds) are considered to be equal.
The "hydroxyl value" is determined by measurement in accordance with JIS K 1557: 2007. The "hydroxyl-equivalent molecular weight" is a value calculated from the formula 56100/(hydroxyl value)×(number of active hydrogens in the initiator).
The "NCO index" in the reaction between an isocyanate group-containing compound and a hydroxyl group-containing compound is a value expressed as a percentage of the equivalent ratio of the isocyanate groups of the isocyanate group-containing compound to the hydroxyl groups of the hydroxyl group-containing compound.
Unless otherwise specified, the term "molecular weight" refers to the formula weight based on a chemical formula, or, in the case of a compound having a molecular weight distribution, the number average molecular weight. The "number average molecular weight" is a polystyrene-equivalent molecular weight determined by gel permeation chromatography (GPC) based on a calibration curve prepared using standard polystyrene samples.

[Polymer piezoelectric materials]
The polymeric piezoelectric material of the present invention (hereinafter, also simply referred to as "piezoelectric material") contains a polymer having a constitutional unit based on a monofunctional urethane (meth)acrylate, and the monofunctional urethane (meth)acrylate is one or more monomers selected from the reaction products of the following (i) to (iii):
(i) An equimolar reaction product of a polyether monol and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product having one isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule.
(ii) an equimolar reaction product of a polyether monol, a diisocyanate, and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product having one group reactive with an isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule.
(iii) an equimolar reaction product of a polyether polyol and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product having one isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule.

The above polymeric piezoelectric materials have a low elastic modulus, high elongation rate, and high dielectric constant, and have good piezoelectric properties.

[Polymer]
The polymeric piezoelectric material of the present invention contains a polymer having a structural unit based on a monofunctional urethane (meth)acrylate, and the polymer is obtained by a photopolymerization reaction or a thermal polymerization reaction of a (meth)acryloyloxy group of the monofunctional urethane (meth)acrylate.
Monofunctional urethane (meth)acrylate has flexible graft chains that do not contribute to crosslinking, and thus has the advantages of a low glass transition temperature, small temperature dependence of storage modulus over a wide temperature range of -20 to 80°C, and small residual strain. Therefore, unlike conventional polyurethane elastomers, the polymeric piezoelectric material of the present invention does not require a film stretching process to develop piezoelectricity, and has good piezoelectricity with suppressed hysteresis loss.

The content of the structural unit based on the monofunctional urethane (meth)acrylate in the structural units of the polymer is preferably 50 parts by mass or more, more preferably 70 to 100 parts by mass, and even more preferably 80 to 100 parts by mass, per 100 parts by mass of all structural units, from the viewpoint of achieving good piezoelectricity due to the monofunctional urethane (meth)acrylate in the piezoelectric material.

The monofunctional urethane (meth)acrylate in the present invention is one or more monomers selected from the reaction products of the above (i) to (iii) (hereinafter also referred to as "monomer 1-1", "monomer 1-2" and "monomer 1-3"). The monofunctional urethane (meth)acrylate monomer constituting the polymer may be used alone or in combination of two or more kinds.
The (meth)acryloyloxy group of the monofunctional urethane (meth)acrylate is preferably an acryloyloxy group from the viewpoint of the polymerization reaction rate for obtaining the polymer.

<Monomer 1-1>
Monomer 1-1 is a reaction product of (i), and is an equimolar reaction product of polyether monool (i-1) and a compound (i-2) having a (meth)acryloyloxy group, and the compound (i-2) having a (meth)acryloyloxy group is a compound having one isocyanate group per molecule and one or two (meth)acryloyloxy groups per molecule.

As monomer 1-1, a compound represented by formula (1) is preferred.

In formula (1), R ¹ is a monovalent organic group having one or two (meth)acryloyloxy groups.
R ¹² is preferably an alkylene group having 2 to 8 carbon atoms, more preferably an alkylene group having 2 to 4 carbon atoms. A plurality of R ¹² present in one molecule may be the same or different. When two or more types of R ¹² are present in one molecule, the chain of -OR ¹² - may be block or random. R ¹² is preferably one or more types selected from an ethylene group, a propylene group, a 1,2-dimethylethylene group, and a 1-ethylethylene group, more preferably one or more types selected from an ethylene group and a propylene group.

It is also preferred that (OR ¹² ) is a unit based on a monomer a having one epoxy group and an ether bond other than the ether bond of the epoxy group in one molecule. The unit based on the monomer a is preferably a unit represented by formula (11). The monomer a may be used alone or in combination of two or more kinds.

In formula (11), R ¹⁰¹ is a monovalent group represented by -R ¹⁰³ -O-R ¹⁰⁴ , and R ¹⁰² is a hydrogen atom or a monovalent group represented by -R ¹⁰⁵ -O-R ^106. R ¹⁰³ and R ¹⁰⁵ are each independently a linear or branched alkylene group having 1 to 3 carbon atoms, and R ¹⁰⁴ and R ¹⁰⁶ are each independently a linear or branched alkyl group having 1 to 18 carbon atoms.
The alkylene group of R ¹⁰³ and R ¹⁰⁵ is each independently preferably a methylene group, an ethylene group, an n-propylene group or an isopropylene group, more preferably a methylene group or an ethylene group, and further preferably a methylene group.
R ¹⁰⁴ and R ¹⁰⁶ each independently have 1 to 14 carbon atoms, more preferably 1 to 12 carbon atoms, and even more preferably 2 to 10 carbon atoms.
Examples of the linear alkyl group for R ¹⁰⁴ and R ¹⁰⁶ include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, an n-octyl group, an n-decyl group, a lauryl group, a cetyl group, and a stearyl group, and preferably each independently is a methyl group, an ethyl group, or an n-butyl group. A branched alkyl group has a structure in which a hydrogen atom in the linear alkyl group (excluding a hydrogen atom bonded to a terminal carbon) is substituted with an alkyl group. Examples of the substituting alkyl group include a methyl group and an ethyl group. A preferred branched alkyl group is a 2-ethylhexyl group.

The monomer a is preferably a monomer represented by formula (12).

R ¹⁰¹ and R ¹⁰² in formula (12) are the same as R ¹⁰¹ and R ¹⁰² in formula (11).

Examples of the monomer represented by formula (12) include methyl glycidyl ether, butyl glycidyl ether, 2-ethylhexyl glycidyl ether, lauryl glycidyl ether, and hexyl glycidyl ether, and preferably butyl glycidyl ether or 2-ethylhexyl glycidyl ether.

In formula (1), R ¹³ is an alkyl group having 1 to 20 carbon atoms. R ¹³ is preferably an alkyl group having 1 to 8 carbon atoms, more preferably a methyl group, an ethyl group, or a butyl group, and even more preferably a butyl group.
a is an integer from 20 to 600, preferably an integer from 35 to 500, and more preferably an integer from 65 to 250.

(Polyether monol (i-1))
The polyether monool (i-1) in the monomer 1-1 is a compound having an initiator residue, a polyether chain, and a hydroxyl group corresponding to the number of active hydrogens of the initiator, the compound being obtained by ring-opening polymerization of an alkylene oxide and/or the monomer a with an initiator having an active hydrogen-containing group and having one or more active hydrogens.

Examples of the active hydrogen-containing group of the initiator include a hydroxyl group, a carboxyl group, and an amino group having one hydrogen atom bonded to a nitrogen atom. The active hydrogen-containing group of the initiator is preferably a hydroxyl group or a carboxyl group, more preferably a hydroxyl group, and even more preferably an alcoholic hydroxyl group.

Examples of initiators having one active hydrogen include monohydric alcohols, monohydric phenols, monovalent carboxylic acids, and amine compounds having one hydrogen atom bonded to a nitrogen atom. The initiator is preferably a monohydric aliphatic alcohol or a monohydric aliphatic carboxylic acid, and more preferably a monohydric aliphatic alcohol. A polyoxyalkylene monool having a lower molecular weight than the target polyether monool may also be used as the initiator.

The number of carbon atoms of the monohydric aliphatic alcohol as the initiator is preferably 1 to 20, more preferably 2 to 8. Specific examples of the monohydric aliphatic alcohol as the initiator include ethanol, propanol, 2-propanol, and n-butanol.
The number of carbon atoms of the monovalent aliphatic carboxylic acid used as the initiator is preferably 2 to 20, more preferably 2 to 8, including the carbon atoms of the carboxy group.

When the alkylene oxide and the monomer a undergo ring-opening polymerization, the ratio of the mass of the monomer a to the total mass of the two is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 10 to 80% by mass, from the viewpoint of good piezoelectricity of the piezoelectric material.

The number of carbon atoms in the alkylene oxide is preferably 2 to 8, and more preferably 2 to 4. Specific examples of the alkylene oxide include propylene oxide, ethylene oxide, 1,2-butylene oxide, and 2,3-butylene oxide.

The oxyalkylene group of the polyether monool (i-1) preferably consists of only an oxypropylene group or a combination of an oxypropylene group and a group other than the oxypropylene group. The oxyalkylene group other than the oxypropylene group is preferably an oxyethylene group.
The proportion of oxypropylene groups in all oxyalkylene groups in the polyether monool (i-1) is preferably 10 to 100% by mass, more preferably 20 to 100% by mass. When the initiator is a polyoxyalkylene monool having a lower molecular weight than the target polyether monool, the oxyalkylene groups in the initiator are regarded as oxyalkylene groups in the obtained polyether monool (i-1).

From the viewpoint of ease of production of the monofunctional urethane (meth)acrylate, the oxyalkylene groups of the monofunctional urethane (meth)acrylate are preferably all oxypropylene groups or oxypropylene groups and oxyethylene groups.
When the monofunctional urethane (meth)acrylate has many oxyethylene groups, the dielectric constant of the polymer tends to be improved, but if the number is too large, the polymer is likely to crystallize. From this viewpoint, when the monofunctional urethane (meth)acrylate has oxyethylene groups, the ratio of oxyethylene groups in all oxyalkylene groups is preferably 90% by mass or less, more preferably 10 to 85% by mass, and even more preferably 20 to 80% by mass. The same applies to the case of the monomer 1-2 or 1-3 described later.

In the polyether monool (i-1), a polyoxyalkylene monool having a low hydroxyl value, i.e., a high molecular weight, can be produced by ring-opening polymerization of an alkylene oxide having 3 or more carbon atoms, preferably propylene oxide, as an initiator in the presence of a composite metal cyanide complex catalyst.
The polyoxyalkylene monool having a low hydroxyl value includes polyoxyalkylene monools having a hydroxyl value of 40 mgKOH/g or less.
A polyoxyalkylene monool having a low hydroxyl value and an oxyethylene group can be produced by using a polyoxyalkylene monool having a high hydroxyl value and an oxyethylene group, for example, a hydroxyl value of 50 mgKOH/g or more, as an initiator and ring-opening polymerizing an alkylene oxide having 3 or more carbon atoms, preferably propylene oxide, in the presence of a composite metal cyanide complex catalyst.
In the polyether monool, the polyoxyalkylene monool having a high hydroxyl value and the polyoxyalkylene monool having a high hydroxyl value as the initiator can also be produced by using an alkali catalyst such as potassium hydroxide.

In the production of polyoxyalkylene monool, the initiator and alkylene oxide to be added to the reaction system are usually ones with low moisture content, with moisture removed by degassing under reduced pressure or the like. Usually, the lower the moisture content of the initiator in the production of polyoxyalkylene monool, the better, preferably 500 ppm by mass or less, more preferably 300 ppm by mass or less. If the moisture content is within the above range, the amount of polyoxyalkylene diol produced from water is suppressed, and as a result, the amount of by-products caused by polyoxyalkylene diol is suppressed, and it is easy to adjust the upper limit of the average number of hydroxyl groups in the obtained polyoxyalkylene monool to 1.2 or less.

The lower the moisture content of polyether monol (i-1) used as the raw material for monomer 1-1, the better. It is preferably 300 mass ppm or less, more preferably 250 mass ppm or less, and even more preferably 50 to 200 mass ppm relative to polyether monol (i-1). When the moisture content is within the above range, the generation of by-products of moisture and isocyanate group-containing compounds is low, and the stability of monomer 1-1, which is the reaction product, is improved. Furthermore, it is easy to suppress changes in the appearance of the polymer over time, and it is easy to obtain a piezoelectric material with good piezoelectric properties.

The average number of hydroxyl groups in one molecule of the polyether monol (i-1) is preferably 0.80 to 1.20, more preferably 0.90 to 1.10.
The hydroxyl value of the polyether monool (i-1) is preferably from 1.6 to 18.1 mgKOH/g, more preferably from 2.8 to 14.0 mgKOH/g, and further preferably from 3.1 to 11.2 mgKOH/g.

The polyether monool (i-1) in monomer 1-1 may be a mixture of two or more polyether monools. In this case, each polyether monool is preferably a polyoxyalkylene monool that falls within the above category.

An example of polyether monool (i-1) is one represented by formula (1a).

In formula (1a), R ¹² , R ¹³ and a have the same symbols as those in formula (1).

(Compound (i-2) Having a (Meth)acryloyloxy Group)
The compound (i-2) having a (meth)acryloyloxy group is a compound having one isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule. The compound (i-2) having a (meth)acryloyloxy group is preferably a (meth)acrylate having an isocyanate group bonded to an aliphatic hydrocarbon group or an alicyclic hydrocarbon group, more preferably an isocyanate alkyl (meth)acrylate.
The number of carbon atoms in the alkylene group excluding the isocyanate group of the isocyanatoalkyl group is preferably 8 or less, and more preferably 4 or less.

An example of the compound (i-2) having a (meth)acryloyloxy group is a compound represented by formula (1b).

In formula (1b), R ¹¹ is a hydrogen atom or a methyl group, and is preferably a hydrogen atom.
s is an integer of 1 to 4, preferably 1 or 2.

Specific examples of the compound (i-2) having a (meth)acryloyloxy group include 2-isocyanate ethyl (meth)acrylate, isocyanate methyl methacrylate, etc. Commercially available products include, for example, Karenz AOI (registered trademark; hereafter abbreviated) and Karenz MOI (registered trademark) (both manufactured by Showa Denko K.K.).

An example of the compound (i-2) having a (meth)acryloyloxy group is a compound represented by formula (1c).

In formula (1c), R ¹¹ is a hydrogen atom or a methyl group, and is preferably a hydrogen atom.
R ¹⁴ is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and is preferably a methyl group.
t is an integer of 1 to 8, preferably an integer of 1 to 4, and more preferably an integer of 1 to 2.
u is an integer of 0 to 4, preferably an integer of 0 to 2.

Specific examples of the compound (i-2) having a (meth)acryloyloxy group include 2,2-(bisacryloyloxymethyl)propyl isocyanate and 1,1-(bisacryloyloxymethyl)ethyl isocyanate (product name "Kalends BEI (registered trademark)", manufactured by Showa Denko K.K.), and preferably 1,1-(bisacryloyloxymethyl)ethyl isocyanate.

For example, when polyether monool (i-1) is polyoxypropylene monool, monomer 1-1 is preferably one or more selected from the compounds represented by formula (1-1-1), formula (1-1-2) and formula (1-1-3).

In formula (1-1-1), formula (1-1-2), and formula (1-1-3), m, n1, and n2 each independently represent an integer of preferably 20 to 600, more preferably an integer of 35 to 500, and even more preferably an integer of 65 to 250.
Bu is a butyl group.

<Monomer 1-2>
Monomer 1-2 is a reaction product of (ii), which is an equimolar reaction product of polyether monool (ii-1), diisocyanate (ii-2) and a compound (ii-3) having a (meth)acryloyloxy group, and the compound (ii-3) having a (meth)acryloyloxy group is a compound having one group reactive with an isocyanate group per molecule and having one or two (meth)acryloyloxy groups per molecule.

As monomer 1-2, a compound represented by formula (2) is preferred.

In formula (2), ^R2 is a monovalent organic group having one or two (meth)acryloyloxy groups.
R ²² is preferably an alkylene group having 2 to 8 carbon atoms, more preferably an alkylene group having 2 to 4 carbon atoms. A plurality of R ²² present in one molecule may be the same or different. When two or more types of R ²² are present in one molecule, the chain of -OR ²² - may be block or random. R ²² is preferably selected from an ethylene group, a propylene group, a 1,2-dimethylethylene group, and a 1-ethylethylene group, more preferably one or more types selected from an ethylene group and a propylene group.
Similarly to (OR ¹² ) in formula (1), (OR ²² ) is also preferably a unit based on a monomer a having one epoxy group and an ether bond other than the ether bond of the epoxy group in one molecule. The preferred embodiment of the monomer a is the same as that of the monomer 1-1.
R ²³ is an alkyl group having 1 to 20 carbon atoms, preferably an alkyl group having 2 to 8 carbon atoms, and more preferably a butyl group.
^R24 is a divalent group obtained by removing two isocyanate groups from a diisocyanate. Examples of diisocyanates will be described later.
b is an integer of 20 to 600, preferably an integer of 35 to 500, and more preferably an integer of 65 to 250.

(Polyether monol (ii-1))
The polyether monol (ii-1) is the same as the polyether monol (i-1) in the monomer 1-1, and the preferred embodiments are also the same.

An example of the polyether monool (ii-1) is one represented by formula (2a).

In formula (2a), R ²² , R ²³ and b have the same meanings as the same symbols in formula (2).

(Diisocyanate (ii-2))
The diisocyanate (ii-2) is a compound having two isocyanate groups in one molecule.
Examples of the diisocyanate (ii-2) include non-yellowing aromatic diisocyanates, aliphatic diisocyanates, alicyclic diisocyanates, and various modified products of these diisocyanates (modified products having two isocyanate groups). The diisocyanates may be used alone or in combination of two or more.
From the viewpoint of good piezoelectric properties of the piezoelectric material, the diisocyanate (ii-2) is preferably at least one selected from the group consisting of aliphatic diisocyanates and alicyclic diisocyanates.

Specific examples of non-yellowing aromatic diisocyanates include xylylene diisocyanate and tetramethylxylylene diisocyanate.
Specific examples of the aliphatic diisocyanate include 1,6-hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and lysine diisocyanate.
Alicyclic diisocyanates include isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 2,5-norbornane diisocyanate, and 2,6-norbornane diisocyanate.

An example of diisocyanate (ii-2) is a compound represented by formula (2b).

In formula (2b), R ²⁴ has the same meaning as the same symbol in formula (2).
As the diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate are preferred from the viewpoint of good piezoelectric properties of the piezoelectric material.

(Compound (ii-3) Having a (Meth)acryloyloxy Group)
The compound (ii-3) having a (meth)acryloyloxy group is a compound having one group reactive with an isocyanate group in one molecule and having one or two (meth)acryloyloxy groups in one molecule.
Examples of the group that reacts with an isocyanate group include a hydroxyl group and an amino group having a nitrogen atom bonded to a hydrogen atom. The number of hydroxyl groups and the number of hydrogen atoms bonded to the nitrogen atom in the group that reacts with an isocyanate group are preferably each 1. In addition, the group that reacts with an isocyanate group is preferably a hydroxyl group bonded to an aliphatic hydrocarbon group or an alicyclic hydrocarbon group.

As the compound (ii-3) having a (meth)acryloyloxy group, hydroxyalkyl (meth)acrylates and hydroxycycloalkyl (meth)acrylatenates are preferred, and hydroxyalkyl (meth)acrylates in which the hydroxyalkyl group has 8 or less carbon atoms are particularly preferred.

An example of the compound (ii-3) having a (meth)acryloyloxy group is a compound represented by formula (2c).

In formula (2c), R ²¹ is a hydrogen atom or a methyl group, and is preferably a hydrogen atom.
p is an integer of 1 to 4, and preferably an integer of 1 or 2.

Specific examples of the compound (ii-3) having a (meth)acryloyloxy group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 6-hydroxyhexyl (meth)acrylate. Commercially available products include Light Ester HO-250(N), Light Ester HOP(N), Light Ester HOA(N), Light Ester HOP-A(N), and Light Ester HOB(N) (all manufactured by Kyoeisha Chemical Co., Ltd.), and 4-HBA (manufactured by Osaka Organic Chemical Industry Ltd.).

An example of the compound (ii-3) having a (meth)acryloyloxy group is a compound represented by formula (2d).

In formula (2d), R ²¹ is a hydrogen atom or a methyl group, and is preferably a hydrogen atom.
R ²⁵ is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and is preferably a methyl group.
q is an integer of 1 to 8, preferably an integer of 1 to 4, and more preferably an integer of 1 to 2.
r is an integer of 0 to 4, preferably an integer of 0 to 2.

Specific examples of the compound (ii-3) having a (meth)acryloyloxy group include 2,2-(bisacryloyloxymethyl)propan-1-ol and 1,1-(bisacryloyloxymethyl)ethane-1-ol, with 1,1-(bisacryloyloxymethyl)ethane-1-ol being preferred.

<Monomer 1-3>
Monomer 1-3 is a reaction product of (iii), and is an equimolar reaction product of polyether polyol (iii-1) and a compound (iii-2) having a (meth)acryloyloxy group, and the compound (iii-2) having a (meth)acryloyloxy group is a compound having one isocyanate group per molecule and one or two (meth)acryloyloxy groups per molecule.

As the monomer 1-3, a compound represented by the formula (III) is preferable.
R ³ -NH-C(=O)-Z ¹ ...(III)
In formula (III), ^R3 is a monovalent organic group having one or two (meth)acryloyloxy groups.
^Z1 is a residue of a polyether polyol obtained by removing one hydrogen atom from one of the hydroxyl groups in the polyether polyol.

More preferably, monomer 1-3 is a compound represented by formula (3).

In formula (3), ^R3 is the same as ^R3 in formula (III).
R ³² is preferably an alkylene group having 2 to 8 carbon atoms, more preferably an alkylene group having 2 to 4 carbon atoms. A plurality of R ³² present in one molecule may be the same as or different from each other. When two or more types of R ³² are present in one molecule, the chain of -OR ³² - may be block or random. R ³² is preferably one or more types selected from an ethylene group, a propylene group, a 1,2-dimethylethylene group, and a 1-ethylethylene group, more preferably one or more types selected from an ethylene group and a propylene group.
Similarly to (OR ¹² ) in formula (1), (OR ³² ) is also preferably a unit based on a monomer a having one epoxy group and an ether bond other than the ether bond of the epoxy group in one molecule. The preferred embodiment of the monomer a is the same as that of the monomer 1-1.
c is an integer of 20 to 600, preferably an integer of 35 to 500, and more preferably an integer of 65 to 250.

(Polyether polyol (iii-1))
The polyether polyol (iii-1) is a compound obtained by ring-opening polymerization of an alkylene oxide and/or the monomer a with an initiator having an active hydrogen-containing group and having two or more active hydrogens, and having an initiator residue, a polyether chain, and hydroxyl groups corresponding to the number of active hydrogens of the initiator.

The alkylene oxide is preferably an alkylene oxide having a carbon number of 2 to 4. Specific examples of the alkylene oxide having a carbon number of 2 to 4 include propylene oxide, ethylene oxide, 1,2-butylene oxide, and 2,3-butylene oxide.
Moreover, the monomer a is preferably a monomer represented by the above formula (12). Examples of the monomer represented by formula (12) include methyl glycidyl ether, butyl glycidyl ether, 2-ethylhexyl glycidyl ether, lauryl glycidyl ether, and hexyl glycidyl ether. From the viewpoint of good piezoelectric properties of the piezoelectric material, butyl glycidyl ether or 2-ethylhexyl glycidyl ether is preferable.

When the alkylene oxide and the monomer a undergo ring-opening polymerization, the ratio of the mass of the monomer a to the total mass of the two is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 10 to 80% by mass, from the viewpoint of good piezoelectricity of the piezoelectric material.

The active hydrogen-containing group of the initiator includes a hydroxyl group, a carboxyl group, and an amino group having a hydrogen atom bonded to a nitrogen atom. The active hydrogen-containing group of the initiator is preferably a hydroxyl group, and more preferably an alcoholic hydroxyl group.

Examples of initiators having two or more active hydrogen atoms include water, polyhydric alcohols, polyhydric phenols, polycarboxylic acids, and amine compounds having two or more hydrogen atoms bonded to nitrogen atoms. The initiator is preferably water or a dihydric aliphatic alcohol, more preferably a dihydric aliphatic alcohol. A polyoxyalkylene polyol having a lower molecular weight than the target polyether polyol may also be used as the initiator.

The carbon number of the dihydric aliphatic alcohol used as the initiator is preferably 2 to 8. Specific examples of the dihydric aliphatic alcohol used as the initiator include ethylene glycol, propylene glycol, polypropylene glycol such as dipropylene glycol, and 1,4-butanediol.

The oxyalkylene groups of the polyether polyol (iii-1) are preferably composed of only oxypropylene groups or a combination of oxypropylene groups and other groups. The oxyalkylene groups other than oxypropylene groups are preferably one or more selected from oxyethylene groups and oxytetramethylene groups. The proportion of oxypropylene groups in all oxyalkylene groups of the polyether polyol (iii-1) is preferably 10 to 100 mass%, more preferably 20 to 100 mass%.
When the initiator is a polyoxyalkylene polyol having a lower molecular weight than the target polyether polyol, the oxyalkylene group in the initiator is regarded as the oxyalkylene group in the obtained polyether polyol (iii-1).

Of the polyether polyols (iii-1), a polyoxyalkylene polyol having a low hydroxyl value, i.e., a high molecular weight, can be produced by ring-opening polymerization of an alkylene oxide having 3 or more carbon atoms, preferably propylene oxide, as an initiator in the presence of a composite metal cyanide complex catalyst.
Examples of polyoxyalkylene polyols having a low hydroxyl value include polyoxyalkylene polyols having a hydroxyl value of 40 mgKOH/g or less.
Of the polyether polyols (iii-1), a polyoxyalkylene polyol having a low hydroxyl value and an oxyethylene group can be produced by ring-opening polymerization of an alkylene oxide having 3 or more carbon atoms, preferably propylene oxide, using a polyoxyalkylene polyol having a high hydroxyl value and an oxyethylene group, for example a hydroxyl value of 50 mgKOH/g or more, as an initiator in the presence of a composite metal cyanide complex catalyst.
Among the polyether polyols (iii-1), the polyoxyalkylene polyol having a high hydroxyl value and the polyoxyalkylene polyol having a high hydroxyl value serving as the initiator can also be produced using an alkali catalyst such as potassium hydroxide.

The average number of hydroxyl groups in one molecule of the polyether polyol (iii-1) is preferably 1.60 to 2.00, more preferably 1.70 to 2.00, and even more preferably 1.80 to 1.96. A polyether polyol having an average number of hydroxyl groups in one molecule of 1.60 to 2.00 is sometimes called a polyether diol.
The hydroxyl value of the polyether polyol (iii-1) is preferably from 1.6 to 18.1 mgKOH/g, more preferably from 2.8 to 14 mgKOH/g.

Polyether polyol (iii-1) may be a mixture of two or more polyether polyols. In this case, each polyether polyol is preferably a polyether polyol that falls within the above category.

An example of polyether polyol (iii-1) is one represented by formula (3a).

In formula (3a), R ³² and c have the same meanings as the same symbols in formula (3).

(Compound (iii-2) Having a (Meth)acryloyloxy Group)
The compound (iii-2) having a (meth)acryloyloxy group is a compound having one isocyanate group in one molecule and one or two (meth)acryloyloxy groups in one molecule. The compound (iii-2) having a (meth)acryloyloxy group is the same as the compound (i-2) having a (meth)acryloyloxy group in the monomer 1-1, and the preferred embodiments are also the same.

From the viewpoint of good piezoelectricity of the piezoelectric material, the molecular weight of the monofunctional urethane (meth)acrylate is preferably 3,000 to 25,000, more preferably 4,000 to 23,000, and even more preferably 5,000 to 20,000. When two or more types of monomers (monofunctional urethane (meth)acrylates) are used in combination, it is preferable that the molecular weight of each is within the above range.

From the same viewpoint, the monofunctional urethane (meth)acrylate preferably has a glass transition temperature of -55°C or lower, more preferably from -85 to -58°C, and even more preferably from -80 to -60°C.
The glass transition temperature of the monofunctional urethane (meth)acrylate in the present invention is a value determined by dynamic viscoelasticity measurement of a homopolymer of the monofunctional urethane (meth)acrylate. Specifically, it is determined by the method described in the examples.

From the viewpoint of good piezoelectric properties of the piezoelectric material, the polymer having a structural unit based on a monofunctional urethane (meth)acrylate has a storage modulus at −20 to 80° C. of preferably 5 to 150 kPa, more preferably 5 to 130 kPa, and even more preferably 5 to 100 kPa.
The storage modulus of the polymer in the present invention is a value determined by dynamic viscoelasticity measurement, specifically, by the method described in the Examples.

[Method for producing polymer]
The polymer of the present invention can be obtained by reacting a polymerizable composition containing a monofunctional urethane (meth)acrylate.
The polymerizable composition preferably contains a polymerization initiator, and, if necessary, other components.
From the viewpoint of good piezoelectric properties of the piezoelectric material, the content of the monofunctional urethane (meth)acrylate in the polymerizable composition is preferably 50% by mass or more, more preferably 70 to 99% by mass, and even more preferably 80 to 99% by mass.
The components in the polymerizable composition are preferably mixed uniformly, and can be mixed using a known mixing device such as a rotary-revolution type stirring and defoaming device, a homogenizer, a planetary mixer, a three-roll mill, a bead mill, etc. The components may be mixed simultaneously or by successive addition.

<Polymerization initiator>
The polymerization initiator for initiating the addition polymerization of the monofunctional urethane (meth)acrylate may be a photopolymerization initiator or a thermal polymerization initiator, and any known initiator may be used. Preferably, a photoradical polymerization initiator or a thermal radical polymerization initiator is used.
When a photopolymerization initiator is used, the polymerizable composition can be reacted quickly without heating to a high temperature, and the polymer and its surroundings are not thermally damaged. From the viewpoint of controlling the polymerization reaction, the photopolymerization initiator is preferably one that can be used with ultraviolet irradiation having a wavelength of 380 nm or less.
On the other hand, when a thermal polymerization initiator is used, a polymer can be obtained by heating etc. even in a place where it is difficult to irradiate with ultraviolet light, visible light, etc. As the thermal polymerization initiator, one that can be used by heating within a range of 50 to 120°C is preferable from the viewpoint of controlling the polymerization reaction.

Examples of photopolymerization initiators include 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methylpropiophenone, diethoxyacetophenone, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropanone, 1-(4-dodecylphenyl)-2-hydroxy-2-methylpropanone, 4-(2-hydroxyethoxy)-phenyl(2-hydroxy-2-propyl)ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, and benzoin-n-bu These include ethyl ether, benzoin phenyl ether, benzyl dimethyl ketal, benzophenone, benzoylbenzoic acid, methyl benzoylbenzoate, 4-phenylbenzophenone-4-methoxybenzophenone, thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-dichlorothioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, methylphenyl glyoxylate, benzyl, camphorquinone, etc.

Examples of thermal polymerization initiators include azo compounds; organic peroxides such as hydroperoxides, dialkyl peroxides, peroxy esters, diacyl peroxides, peroxydicarbonates, peroxyketals, and ketone peroxides. Specific examples of thermal radical polymerization initiators include azobisisobutyronitrile, benzoyl peroxide, t-butylperoxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoyl)peroxyhexane, t-butylperoxybenzoate, t-butyl peroxide, cumene hydroperoxide, dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-dibutylperoxyhexane, 2,4-dichlorobenzoyl peroxide, 1,4-di(2-t-butylperoxyisopropyl)benzene, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, methyl ethyl ketone peroxide, and 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate.

From the viewpoint of an appropriate polymerization rate, the content of the polymerization initiator in the polymerizable composition is preferably 0.001 to 20 parts by mass, more preferably 0.01 to 10 parts by mass, and even more preferably 0.1 to 7 parts by mass, per 100 parts by mass of the monofunctional urethane (meth)acrylate.

When a polymerizable composition is irradiated with light to obtain a polymer, the light source can be appropriately set according to the light absorption ability of the photopolymerization initiator to be blended, and for example, an ultraviolet light emitting diode (LED), a low pressure mercury lamp, a high pressure mercury lamp, a mercury xenon lamp, a metal halide lamp, a tungsten lamp, an arc lamp, an excimer lamp, an excimer laser, a semiconductor laser, a YAG laser, a laser system combining a laser and a nonlinear optical crystal, a high frequency induced ultraviolet generator, etc. can be used as the light source. The integrated light amount is, for example, about 0.01 to 5 J/ ^cm2 .
From the viewpoint of stabilizing the physical properties of the polymer, a heat treatment may be further performed after the light irradiation. Usually, the heating temperature is about 40 to 200° C., and the heating time is about 1 minute to 15 hours. The physical properties of the polymer can also be stabilized by leaving the polymer at rest at room temperature (about 15 to 25° C.) for about 1 to 48 hours.

When a polymerizable composition is subjected to a heat treatment to obtain a polymer, the heating temperature is usually about 40 to 250°C, and the heating time is about 5 minutes to 24 hours. Preferably, the heating time is shortened when the heating temperature is high, and the heating time is lengthened when the heating temperature is low.

<Other Ingredients>
The polymerizable composition may contain other components in addition to the monofunctional urethane (meth)acrylate and the polymerization initiator from the viewpoint of good handling and good piezoelectricity of the piezoelectric material. Examples of the other components include other monomer components other than the monofunctional urethane (meth)acrylate, catalysts (tertiary amine compounds, quaternary ammonium compounds, bismuth 2-ethylhexanoate, etc.), colorants such as pigments and dyes, silane coupling agents, tackifier resins, antioxidants, light stabilizers, metal deactivators, rust inhibitors, antiaging agents, moisture absorbents, hydrolysis inhibitors, defoamers, fillers, etc. Also, a solvent may be included. These other components in the polymerizable composition may be blended in a content within a range that does not impair the effects of the present invention.

The other monomer components are compounds that copolymerize with the epoxy compound and the monofunctional urethane (meth)acrylate, and may be used alone or in combination of two or more. Examples of the other monomer components include (meth)acrylates such as polyfunctional urethane (meth)acrylates having two or more (meth)acryloyloxy groups, alkyl (meth)acrylates, hydroxyl group-containing (meth)acrylates, and amino group-containing (meth)acrylates.

The polymeric piezoelectric material of the present invention may contain any additives, if necessary, in addition to the above-mentioned polymer, from the viewpoint of ease of molding, good piezoelectricity, etc. Also, by-products during the production of the polymer may be contained. However, the content of other components, such as additives, other than the polymer in the piezoelectric material is within a range that does not impair the effects of the present invention. The content of the polymer in the piezoelectric material is preferably 50% by mass or more, more preferably 60 to 100% by mass, and even more preferably 70 to 100% by mass.

The dielectric constant of the polymeric piezoelectric material at 1 kHz (temperature 25°C, humidity 40% RH) is preferably 4 to 20, more preferably 5 to 20, from the viewpoint of good piezoelectricity. The dielectric constant is specifically determined by the method described in the examples.

The polymeric piezoelectric material can be molded into a desired shape for use, and from the perspective of use in piezoelectric elements, it is preferably molded into a film, sheet, or cable. The thickness is preferably 10 to 1200 μm, more preferably 20 to 1000 μm, and even more preferably 50 to 900 μm.

Examples of methods for molding a polymeric piezoelectric material include a method using a molding die, and a method in which a polymerizable composition is sandwiched between release films with spacers arranged therebetween and polymerized to obtain a film-like molded product.
The polymeric piezoelectric material of the present invention can provide good piezoelectric properties without requiring a stretching process as in the case of conventional polyurethane elastomers.

[Piezoelectric element]
The piezoelectric element of the present invention comprises the polymeric piezoelectric material of the present invention sandwiched between at least one pair of electrodes. From the viewpoint of good piezoelectricity, the polymeric piezoelectric material is preferably in the form of a film as described above.
The piezoelectric element can be manufactured, for example, by forming electrodes on both sides of a film-like polymeric piezoelectric material.
Examples of the material of the electrode include metals such as gold, platinum, aluminum, silver, and copper, carbon, carbon nanotubes, and conductive resins and conductive elastomers in which these are dispersed in resin. The electrodes can be formed by, for example, plasma CVD, ion sputter coating, vacuum deposition, screen printing, or other known coating methods.

The piezoelectric element of the present invention is made of a polymeric piezoelectric material with a low elastic modulus, high elongation rate, and high dielectric constant, and can achieve high elasticity with a relatively low voltage drive, making it possible to configure an actuator with excellent piezoelectric properties. In addition, hysteresis loss in charge generation due to stress application is suppressed, making it suitable for configuring a high-precision pressure sensor.
Therefore, the piezoelectric element of the present invention can be used as an actuator in, for example, medical robots, artificial muscles, wearable assist robots, touch screens, touch display devices, Braille block devices, etc., and is also useful as a pressure sensor for converting vibrations, ultrasound, tactile pressure, etc. into electrical signals in the medical and industrial fields.

The present invention will be described in detail below based on examples, but the present invention is not limited to the following examples.

[Preparation of polymerizable composition]
Various polymerizable compositions were prepared for the production of polymeric piezoelectric materials.

[Raw material compounds]
Details of the raw material compounds for the polymerizable composition are as follows.
DMC-TBA: zinc hexacyanocobaltate-tert-butyl alcohol complex AOI: 2-acryloyloxyethyl isocyanate; Karenz AOI, manufactured by Showa Denko K.K. Photopolymerization initiator: 2-hydroxy-2-methylpropiophenone; Irgacure 1173, manufactured by BASF PPG monomethacrylate: polypropylene glycol monomethacrylate; Blenmer (registered trademark) PP-1000, manufactured by NOF Corporation; molecular weight approximately 376
Trifunctional PPG: Polypropylene glycol number average molecular weight 6,000
Bifunctional PPG (1): Polypropylene glycol; number average molecular weight 3,000
Bifunctional PPG (2): Polypropylene glycol; number average molecular weight 1,000
TDI: Tolylene diisocyanate; "Coronate (registered trademark) T-80" manufactured by Tosoh Corporation; a mixture of isomers of 2,4-TDI and 2,6-TDI (mixture ratio 80/20)
・Lead octylate: "Lead hexoate 24%", Toei Kako Co., Ltd.

The number average molecular weight of the monofunctional urethane acrylate in Preparation Examples 1 to 3 was measured by gel permeation chromatography (GPC) under the following measurement conditions.
<Measurement conditions>
Equipment used: "HLC-8120GPC", manufactured by Tosoh Corporation Columns used: The following two types of columns were connected in series in this order: "TSKgel (registered trademark) G7000H _XL ", manufactured by Tosoh Corporation, 1 column "TSKgel (registered trademark) GMH _XL ", manufactured by Tosoh Corporation, 2 columns Column temperature: 40°C
Detector: Refractive index (RI) detector Eluent: Tetrahydrofuran Flow rate: 0.8 mL/min Sample concentration: 0.5% by mass
Sample injection volume: 100 μL
・Standard sample: polystyrene

The glass transition temperature of the monofunctional urethane acrylate was determined by dynamic viscoelasticity measurement as follows.
0.3 parts by mass of a photoradical polymerization initiator ("Irgacure 819", manufactured by BASF) was added to 100 parts by mass of a monofunctional urethane acrylate, and mixed with a planetary stirrer. The resulting mixture was irradiated under a nitrogen gas atmosphere with a conveyor-type UV irradiator (manufactured by Oak Manufacturing Co., Ltd.; mercury xenon lamp, illuminance 100 mW/ ^cm2 , accumulated light amount 3 J/ ^cm2 ) to obtain a cured sample of the monofunctional urethane acrylate (homopolymer; length 15 mm, width 5 mm, thickness 2 mm).
The loss modulus of the cured sample was measured using a dynamic viscoelasticity measuring device ("EXSTAR TMA/SS6100", manufactured by Seiko Instruments Inc.; tensile mode, temperature range: -80 to 130°C, heating rate: 3°C/min, measurement frequency: 1 Hz, strain: 1%), and the temperature at the peak was taken as the glass transition temperature.

Preparation Example 1
In a pressure-resistant reactor equipped with a stirrer and a nitrogen inlet tube, 0.5 g of DMC-TBA and 112 g of n-butanol as an initiator were placed, and 4,887 g of propylene oxide (hereinafter abbreviated as "PO") was added at a constant rate under a nitrogen atmosphere at 130°C to obtain intermediate A. In a pressure-resistant vessel, 1,148 g of intermediate A and 0.2 g of DMC-TBA were placed, and 2,325 g of PO was added at a constant rate over 5.5 hours under a nitrogen atmosphere at 130°C.
It was confirmed that the internal pressure of the pressure-resistant reactor had stopped decreasing, and polyether monol (hydroxyl value: 5.6 mg KOH/g, hydroxyl value-based molecular weight: 10,020, average number of hydroxyl groups: 1.08) was obtained.
Into a reaction vessel equipped with a stirrer and a nitrogen inlet tube, 500.0 g of the polyether monool, 7.04 g of AOI (NCO index 100), and 0.04 g of a 25% by mass toluene solution of bismuth 2-ethylhexanoate were placed and stirred at 70° C. for 3 hours to obtain a monofunctional urethane acrylate (number average molecular weight 16,200, glass transition temperature −69° C.).
Polymerizable composition 1 was prepared by mixing 100 parts by mass of the monofunctional urethane acrylate (a) and 2 parts by mass of a photopolymerization initiator.

Preparation Example 2
Into a pressure vessel were placed 1,148 g of Intermediate A in Preparation Example 1 and 0.2 g of DMC-TBA, and then under a nitrogen atmosphere at 130° C., 1,997 g of PO and 856 g of ethylene oxide (hereinafter abbreviated as “EO”) were added at a constant rate over 5.5 hours.
It was confirmed that the internal pressure of the pressure-resistant reactor had stopped decreasing, and polyether monol (hydroxyl value: 5.23 mg KOH/g, hydroxyl value-based molecular weight: 10,730, average number of hydroxyl groups: 1.08, EO constitutional units: 21% by mass) was obtained.
Into a reaction vessel equipped with a stirrer and a nitrogen inlet tube, 296.1 g of the polyether monool, 3.90 g of AOI (NCO index 100), and 0.024 g of a 25% by mass toluene solution of bismuth 2-ethylhexanoate were placed and stirred at 70° C. for 3 hours to obtain a monofunctional urethane acrylate (b) (number average molecular weight 16,230, glass transition temperature −74° C.).
Polymerizable composition 2 was prepared by mixing 100 parts by mass of the monofunctional urethane acrylate (b) and 2 parts by mass of a photopolymerization initiator.

Preparation Example 3
In a pressure-resistant reactor equipped with a stirrer and a nitrogen inlet tube, 0.5 g of DMC-TBA and 740 g of n-butanol as an initiator were placed, and 3260 g of PO was added at a constant rate under a nitrogen atmosphere at 130° C. to obtain intermediate B. In a pressure-resistant vessel, 538 g of intermediate B and 0.4 g of DMC-TBA were placed, and 1120 g of PO and 6347 g of EO were added at constant rates over 7.2 hours under a nitrogen atmosphere at 130° C.
It was confirmed that the internal pressure of the pressure-resistant reactor had stopped decreasing, and polyether monol (hydroxyl value 9.61 mg KOH/g, hydroxyl value-based molecular weight 5838, average number of hydroxyl groups 1.03, EO constitutional units: 79% by mass) was obtained.
Into a reaction vessel equipped with a stirrer and a nitrogen inlet tube, 292.9 g of the polyether monol, 7.08 g of AOI (NCO index 100), and 0.024 g of a 25% by mass toluene solution of bismuth 2-ethylhexanoate were placed and stirred at 70° C. for 3 hours to obtain a monofunctional urethane acrylate (c) (number average molecular weight 8,230, glass transition temperature −74° C.).
Polymerizable composition 3 was prepared by mixing 100 parts by mass of the monofunctional urethane acrylate (c) and 2 parts by mass of a photopolymerization initiator.

Preparation Example 4
Polymerizable composition 4 was prepared by mixing 100 parts by mass of PPG monomethacrylate and 2 parts by mass of a photopolymerization initiator.

Preparation Example 5
A reaction vessel was charged with 36.4 parts by mass of trifunctional PPG and 54.5 parts by mass of bifunctional PPG (1), and the mixture was dehydrated under reduced pressure for 1 hour while stirring, and the atmosphere in the reaction vessel was replaced with nitrogen. 9.1 parts by mass of TDI was added to the reaction vessel, and the mixture was reacted at 90° C. for 6 hours to synthesize an isocyanate-terminated prepolymer.
Next, 74.6 parts by mass of trifunctional PPG, 18.7 parts by mass of bifunctional PPG (2), and 0.8 parts by mass of lead octylate were mixed and degassed under reduced pressure to prepare a mixed solution. 100 parts by weight of the isocyanate-terminated prepolymer was added to this mixed solution, and the mixture was mixed and degassed using a planetary mixer (rotating and revolving mixer, manufactured by Thinky Corporation) to obtain a polymerizable composition 5.

[Production of film and piezoelectric element]
A film, which is a molded article of a polymeric piezoelectric material, was prepared using each of the polymerizable compositions 1 to 5, and a piezoelectric element was manufactured using the film. Examples 1 to 3 are working examples, and Examples 4 and 5 are comparative examples.

Example 1
The polymerizable composition 1 was poured into a mold with a polyethylene film attached, and the polyethylene film side of the glass plate with the polyethylene film attached was placed on top of the polymerizable composition. Under a nitrogen gas atmosphere, the glass plate side was irradiated with ultraviolet light using a conveyor-type UV irradiator (manufactured by Oak Manufacturing Co., Ltd.; mercury xenon lamp, illuminance 100 mW/cm ² , cumulative light amount 3 J/cm ² ) to polymerize the composition. After leaving the composition for 12 hours, the polyethylene films on both sides were peeled off to obtain a film with a thickness of 150 μm.
The obtained film was fixed in a circular polypropylene frame having a diameter of 60 mm, and carbon grease was applied to both sides of the film in a circular shape having a diameter of 15 mm and a thickness of about 50 μm, thereby producing the piezoelectric element of Example 1.

[Examples 2 to 4]
Films were prepared in the same manner as in Example 1, except that polymerizable composition 2 to 4 were used instead of polymerizable composition 1 in Example 1, and piezoelectric elements of Examples 2 to 4 were manufactured.

Example 5
The polymerizable composition 5 was poured into a mold to which a polyethylene film had been attached, and the glass plate to which the polyethylene film had been attached was placed so that the polyethylene film side was in contact with the polymerizable composition 5. After polymerization was carried out by heating at 80° C. for 1 hour, the polyethylene films on both sides were peeled off, and the film was biaxially stretched (2 times the magnification) to obtain a polyurethane elastomer film having a thickness of 150 μm.
The obtained film was fixed in a circular polypropylene frame having a diameter of 60 mm, and carbon grease was applied to both sides of the film in a circular shape having a diameter of 15 mm and a thickness of about 50 μm, thereby producing the piezoelectric element of Example 4.

[evaluation]
The films and piezoelectric elements produced in Examples 1 to 5 were evaluated for the following items. The evaluation results are shown in Table 1.

[Storage modulus]
The storage modulus E' of the film was measured using a dynamic viscoelasticity measuring device ("EXSTAR6000 DMS6100", Hitachi High-Tech Science Corporation; tensile mode, temperature range -80 to 130°C, heating rate 3°C/min, measurement frequency 1 Hz, strain 1%).
The storage modulus was evaluated as follows: when the storage modulus E' in the temperature range of -20 to 80°C was 100 kPa or less, it was rated as A; when it was more than 100 kPa and 150 kPa or less, it was rated as B; and when it was more than 150 kPa, it was rated as C.
It can be said that the lower the storage modulus E' of the film, the easier it is to obtain a high elongation rate when a voltage is applied, and the better the piezoelectricity can be expressed.

[Elongation rate]
The piezoelectric element was connected to a DC high voltage power supply ("HJPM-5R0.6-SP", manufactured by Matsusada Precision Co., Ltd.) via a copper foil, and a voltage of 10 MV/m was applied. The film area _S1 before the voltage application and the film area _S2 after the voltage application were measured, and the elongation rate ΔS [%] was calculated by the following formula.
ΔS=(S ₂ /S ₁ -1) ^1/2 ×100
The elongation rate was evaluated as A when the elongation rate ΔS was 30% and as B when it was less than 30%.
It can be said that the higher the elongation rate ΔS of a film, the better the piezoelectric properties it has.

[Dielectric constant]
A test piece measuring 5 cm x 5 cm was cut out from the film, and aluminum foil having a thickness of 11 μm was laminated on the top and bottom surfaces to form a laminate. The dielectric constant was measured at room temperature (25°C), humidity (40% RH), and 1 kHz using a potentio/galvanostat ("SP-150", manufactured by Biologic Corporation).
It can be said that the higher the relative dielectric constant, the better the piezoelectric properties can be exhibited.

From the evaluation results shown in Table 1, it can be said that the films (Examples 1 to 3) made using monofunctional urethane acrylate have a low storage modulus, a large elongation rate, and good piezoelectric properties. It was also confirmed that the monofunctional urethane acrylate with a higher proportion of oxyethylene groups (EO structural units) (Examples 2 and 3) exhibited a higher relative dielectric constant.

Claims (8)

The polymer includes a structural unit based on a monofunctional urethane (meth)acrylate,
The polymeric piezoelectric material, wherein the monofunctional urethane (meth)acrylate is one or more monomers selected from the reaction products of the following (i) to (iii):
(i) An equimolar reaction product of a polyether monol and a compound having a (meth)acryloyloxy group,
The reaction product, wherein the compound having a (meth)acryloyloxy group is a compound having one isocyanate group in one molecule and one (meth)acryloyloxy group in one molecule.
(ii) an equimolar reaction product of a polyether monol, a diisocyanate, and a compound having a (meth)acryloyloxy group,
The compound having a (meth)acryloyloxy group is a reaction product which has one group reactive with an isocyanate group in one molecule and one (meth)acryloyloxy group in one molecule.
(iii) an equimolar reaction product of a polyether polyol and a compound having a (meth)acryloyloxy group,
The reaction product, wherein the compound having a (meth)acryloyloxy group is a compound having one isocyanate group in one molecule and one (meth)acryloyloxy group in one molecule. The polymeric piezoelectric material according to claim 1, wherein the monofunctional urethane (meth)acrylate has a number average molecular weight of 3,000 to 25,000. The polymeric piezoelectric material according to claim 1 or 2, wherein the monofunctional urethane (meth)acrylate has a glass transition temperature of -55°C or lower. The polymeric piezoelectric material according to any one of claims 1 to 3, wherein the ratio of oxyethylene groups to all oxyalkylene groups of the monofunctional urethane (meth)acrylate is 0 to 90 mass%. The polymer piezoelectric material according to any one of claims 1 to 4, wherein the polymer has a storage modulus of 5 to 150 kPa at -20 to 80°C. A piezoelectric element in which the polymeric piezoelectric material according to any one of claims 1 to 5 is sandwiched between at least one pair of electrodes. An actuator comprising the piezoelectric element according to claim 6. A pressure sensor including the piezoelectric element according to claim 6.