Polymer Actuators and Sensors

Abstract

To provide a polymer actuator including a dielectric elastomer layer which has an increased dielectric constant and has dielectric breakdown (decrease in dielectric breakdown field strength) suppressed while maintaining flexibility (elastic modulus).SOLUTION: A polymer actuator includes a dielectric elastomer layer and two electrodes arranged on both surfaces of the dielectric elastomer layer. The dielectric elastomer layer is formed of a cured product of a composition including: a resin component containing a polyrotaxane and a diene-based polymer having hydroxyl groups; and grafted carbon nanotubes obtained by bonding a (meth)acrylic polymer having at least one of a polyethylene glycol chain and a linear or branched hydrocarbon chain in a side chain to carbon nanotubes by graft polymerization.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to polymer actuators and sensors, and more particularly to polymer actuators and sensors that include a dielectric elastomer layer. [Background technology]

[0002] Polymer actuators having a dielectric elastomer layer containing polyrotaxane have been known as polymer actuators with large displacements (for example, JP 2017-66318 A (Patent Document 1)). However, conventional polymer actuators having a dielectric elastomer layer containing polyrotaxane do not necessarily have sufficiently high output, and there has been a demand for higher output.

[0003] In general, improving the dielectric constant of a polymer actuator having a dielectric elastomer layer is considered effective for increasing the output. Furthermore, incorporating a high-dielectric-constant filler, such as barium titanate or titania, is considered effective for improving the dielectric constant of the dielectric elastomer layer. However, since many high-dielectric-constant fillers are made of ceramics with high elastic moduli, incorporating such high-dielectric-constant fillers into a dielectric elastomer layer improves the dielectric constant of the dielectric elastomer layer. However, incorporating such high-dielectric-constant fillers into a dielectric elastomer layer significantly increases the elastic modulus, resulting in a problem of reduced displacement of the polymer actuator during operation. For this reason, there has been a demand for a technology that improves the dielectric constant of a polymer actuator having a dielectric elastomer layer while maintaining the flexibility of the dielectric elastomer layer.

[0004] On the other hand, when a dielectric elastomer layer is blended with a conductive filler, carbon nanotubes, the carbon nanotubes are polarized within the dielectric elastomer layer when an electric field is applied, which is expected to produce a significant dielectric effect. However, because carbon nanotubes are conductive, they tend to form conductive paths within the dielectric elastomer layer, making them susceptible to dielectric breakdown. For this reason, the amount of carbon nanotubes blended must be kept low enough to prevent dielectric breakdown in the dielectric elastomer layer, making it difficult to sufficiently improve the dielectric constant of the dielectric elastomer layer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-66318 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the problems associated with the above-mentioned conventional technology, and aims to provide a polymer actuator and sensor having a dielectric elastomer layer that improves the dielectric constant while maintaining flexibility (elastic modulus) and suppresses dielectric breakdown (decrease in dielectric breakdown field strength). [Means for solving the problem]

[0007] As a result of extensive research aimed at achieving the above object, the present inventors have found that by blending grafted carbon nanotubes, in which a (meth)acrylic polymer having a polyethylene glycol chain in its side chain is bonded to a carbon nanotube by graft polymerization, into a dielectric elastomer layer containing polyrotaxane, it is possible to improve the dielectric constant and suppress dielectric breakdown (decrease in dielectric breakdown field strength) while maintaining the flexibility (elastic modulus) of the dielectric elastomer layer. However, even in such a dielectric elastomer layer, the dielectric constant is not necessarily sufficiently high, and further improvement is desired.

[0008] As a result of further intensive research to achieve the above-mentioned object, the inventors have discovered that by blending grafted carbon nanotubes, in which a (meth)acrylic polymer having at least one of a polyethylene glycol chain and a linear or branched hydrocarbon chain on its side chain is bonded to the carbon nanotubes by graft polymerization, into a dielectric elastomer layer made of a resin component containing polyrotaxane and a diene-based polymer having a hydroxyl group, the grafted carbon nanotubes can be well dispersed in the resin component, further improving the dielectric constant while maintaining the flexibility (elastic modulus) of the dielectric elastomer layer, and making it possible to suppress dielectric breakdown (reduction in dielectric breakdown field strength), thereby completing the present invention.

[0009] That is, the polymer actuator of the present invention comprises a dielectric elastomer layer and two electrodes arranged on both sides of the dielectric elastomer layer, and is characterized in that the dielectric elastomer layer is made of a cured product of a composition containing a resin component including polyrotaxane and a diene-based polymer having a hydroxyl group, and grafted carbon nanotubes in which a (meth)acrylic polymer having at least one of a polyethylene glycol chain and a linear or branched hydrocarbon chain in its side chain is bonded to a carbon nanotube by graft polymerization.

[0010] In the polymer actuator of the present invention, the content of the grafted carbon nanotubes in the composition is preferably in the range of 0.1 to 5 parts by mass per 100 parts by mass of the resin component, and the content of the carbon nanotubes in the grafted carbon nanotubes is preferably in the range of 1 to 50% by volume.

[0011] The sensor of the present invention is characterized by comprising the polymer actuator of the present invention.

[0012] While the reason why the present invention enables the dielectric constant to be further improved and the dielectric breakdown (reduction in the breakdown field strength) to be suppressed while maintaining the flexibility (elastic modulus) of the dielectric elastomer layer is not entirely clear, the inventors speculate as follows. Specifically, when the grafted carbon nanotubes of the present invention are blended into a dielectric elastomer layer, the grafted carbon nanotubes are polarized within the dielectric elastomer layer when an electric field is applied, which is thought to improve the dielectric constant. Furthermore, because the carbon nanotubes are in the form of ultrafine fibers, they are relatively easy to follow the deformation of the dielectric elastomer layer, and it is thought that the flexibility (elastic modulus) of the dielectric elastomer layer is maintained. Furthermore, in a dielectric elastomer layer blended with the grafted carbon nanotubes of the present invention, a graft layer is formed by a (meth)acrylic polymer having at least one of a polyethylene glycol chain and an acrylic chain in its side chain, and this graft layer is thought to serve as an insulating layer. Furthermore, because the polyethylene glycol chains and acrylic chains of the grafted carbon nanotubes of the present invention have a high affinity with diene-based polymers having hydroxyl groups, they are presumably well dispersed in a dielectric elastomer layer made of a resin component containing polyrotaxane and diene-based polymers having hydroxyl groups, and are therefore less likely to form aggregates. Therefore, the insulation provided by the graft layer and the good dispersion of the grafted carbon nanotubes are presumably responsible for preventing contact between the carbon nanotubes, making it less likely that conductive paths will be formed and reducing dielectric breakdown (reduction in breakdown field strength). Furthermore, the good dispersion of the grafted carbon nanotubes is presumably responsible for applying a voltage uniformly to all the carbon nanotubes in the dielectric elastomer layer, further improving the dielectric constant. [Effects of the Invention]

[0013] According to the present invention, it is possible to obtain a polymer actuator and a sensor having a dielectric elastomer layer in which the grafted carbon nanotubes are well dispersed without forming aggregates of 1 μm or more, and which further improves the dielectric constant while maintaining flexibility (elastic modulus), thereby suppressing dielectric breakdown (decrease in dielectric breakdown field strength). [Brief explanation of the drawings]

[0014] [Figure 1] 1 is an optical microscope photograph of the cured film obtained in Example 1. [Figure 2] 1 is an optical microscope photograph of the cured film obtained in Example 2. [Figure 3] 1 is an optical microscope photograph of the cured film obtained in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below based on preferred embodiments thereof.

[0016] The polymer actuator of the present invention comprises a dielectric elastomer layer and two electrodes arranged on both sides of the dielectric elastomer layer, and the dielectric elastomer layer is made of a cured product of a composition containing a resin component including polyrotaxane and a diene-based polymer having a hydroxyl group, and grafted carbon nanotubes in which a (meth)acrylic polymer having at least one of a polyethylene glycol chain and a linear or branched hydrocarbon chain in its side chain is bonded to the carbon nanotube by graft polymerization.

[0017] (Polyrotaxane) Polyrotaxane is a molecular assembly having a structure in which a rod-shaped molecule (axis molecule) passes through the hole of a cyclic molecule (ring-shaped molecule), and bulky moieties (end groups) are bonded to both ends of the rod-shaped molecule. In the present invention, such polyrotaxane is not particularly limited, and conventionally known polyrotaxanes can be used.

[0018] The ring-shaped molecule is not particularly limited, and examples thereof include cyclodextrins (e.g., α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin), crown ethers, cyclophanes, calixarenes, cucurbiturils, and cyclic amides. These ring-shaped molecules may be used alone or in combination. Furthermore, such ring-shaped molecules may have a substituent or a side chain. Among these ring-shaped molecules, cyclodextrins having caprolactone chains as graft chains are preferred from the viewpoint of obtaining appropriate flexibility and dielectric constant. Furthermore, cyclodextrins modified with hydrocarbon groups (e.g., alkyl groups or aryl groups) are preferred from the viewpoints of improving the moisture resistance of the dielectric elastomer layer, making it difficult for water molecules to be mixed into the dielectric elastomer layer, thereby suppressing the occurrence of hydrolysis reactions and deterioration of insulating properties, and ensuring the dispersibility and solubility of the polyrotaxane in the resin component.

[0019] The axial molecule is not particularly limited, and examples thereof include polyethylene glycol, polypropylene glycol, polyethylene, polypropylene, polyisoprene, polyisobutylene, polybutadiene, polylactic acid, polytetrahydrofuran, polydimethylsiloxane, polyvinyl alcohol, and polyvinyl methyl ether. These axial molecules may be used alone or in combination of two or more. Among these axial molecules, polyethylene glycol is preferred because it has a low glass transition temperature and allows the ring-shaped molecule to move smoothly.

[0020] Furthermore, the bulky terminal group is not particularly limited, and examples thereof include a dinitrophenyl group, a cyclodextrin group, an adamantane group, a trityl group, a fluorescein group, a pyrene group, a substituted benzene ring, a polycyclic aromatic ring, and a steroid group. These terminal groups may be used alone or in combination of two or more. Among these terminal groups, the dinitrophenyl group, the cyclodextrin group, the adamantane group, the trityl group, the fluorescein group, and the pyrene group are preferred, with the adamantane group being more preferred, from the viewpoints of being easily introduced into the terminal of the axial molecule and having sufficient bulk to prevent the ring-shaped molecule from coming off.

[0021] (Diene polymer) The diene polymer (diene rubber) used in the present invention is a diene polymer having a hydroxyl group, and the hydroxyl group may be present at the end of the diene polymer or may be present on a side chain by a graft reaction or the like.

[0022] Examples of the diene polymers (diene rubbers) include homopolymers of dienes such as butadiene, isoprene, and chloroprene (for example, butadiene rubber, isoprene rubber, and chloroprene rubber) and copolymers (butadiene copolymers such as styrene-butadiene rubber and acrylonitrile-butadiene rubber, and isoprene copolymers such as styrene-isoprene rubber and acrylonitrile-isoprene rubber). Examples of copolymerizable monomers in the diene copolymers include (meth)acrylate, styrene, and acrylonitrile.

[0023] Among these diene polymers, hydrophobic diene polymers such as butadiene rubber, isoprene rubber, butadiene copolymers, and isoprene copolymers are preferred, from the viewpoint of avoiding the introduction of moisture due to moisture absorption, which promotes a decrease in the breakdown voltage, and butadiene rubber and isoprene rubber are more preferred.

[0024] (grafted carbon nanotubes) The grafted carbon nanotubes (grafted CNTs) used in the present invention are carbon nanotubes (CNTs) to which a (meth)acrylic polymer having at least one of a polyethylene glycol chain and a linear or branched hydrocarbon chain on its side chain has been bonded by graft polymerization.

[0025] The degree of polymerization of ethylene glycol in the polyethylene glycol chain is preferably 2 to 100, more preferably 2 to 80, even more preferably 3 to 50, even more preferably 3 to 20, and particularly preferably 4 to 10. If the degree of polymerization of ethylene glycol is below the lower limit, the resulting grafted CNTs tend to aggregate and not be uniformly dispersed in the dielectric elastomer layer, resulting in an insufficient improvement in dielectric constant and insufficient suppression of dielectric breakdown (reduction in dielectric breakdown field strength).On the other hand, if the degree of polymerization exceeds the upper limit, the CNT content in the grafted CNTs tends to decrease, and the amount of grafted CNTs required to obtain the effect of improving the dielectric constant tends to increase.

[0026] Furthermore, the degree of polymerization of the (meth)acrylic monomer in the (meth)acrylic polymer having a polyethylene glycol chain in its side chain is preferably 50,000 to 100,000, more preferably 50,000 to 10,000, even more preferably 5 to 1,000, and particularly preferably 5 to 100. If the degree of polymerization of the (meth)acrylic monomer is below the lower limit, the grafted CNTs will not be uniformly dispersed in the dielectric elastomer layer, and the dielectric constant will not be sufficiently improved, and dielectric breakdown (reduction in dielectric breakdown field strength) will tend not to be sufficiently suppressed. On the other hand, if the degree of polymerization exceeds the upper limit, the CNT content in the grafted CNTs will decrease, and the amount of grafted CNTs required to obtain the effect of improving the dielectric constant will tend to increase.

[0027] The hydrocarbon chain may be linear or branched, saturated or unsaturated. Examples of such hydrocarbon chains include alkyl and alkenyl groups having 2 to 100 carbon atoms (preferably 3 to 50, more preferably 4 to 20). Examples of the alkyl group include 2-ethylhexyl, butyl, hexyl, isobutyl, and stearyl. Examples of the alkenyl group include oleyl.

[0028] Of such polyethylene glycol chains and linear or branched hydrocarbon chains, polyethylene glycol chains are preferred, and among the polyethylene glycol chains, preferred are those having a degree of ethylene glycol polymerization of 2 to 100 (more preferably 2 to 80, even more preferably 3 to 50, still more preferably 3 to 20, and particularly preferably 4 to 10) and having an alkyl group having 1 to 12 carbon atoms at the terminal (more preferably a methyl group, an ethyl group, or a butyl group).

[0029] The CNT content in the grafted CNT is preferably 1 to 50% by volume, more preferably 1 to 30% by volume. If the CNT content in the grafted CNT is less than the lower limit, the effect of the CNT cannot be obtained sufficiently, and for example, the dielectric constant tends not to be sufficiently improved. On the other hand, if the CNT content exceeds the upper limit, the graft layer (insulating layer) becomes relatively small, and the effect of the graft layer cannot be obtained sufficiently, and for example, dielectric breakdown (reduction in dielectric breakdown field strength) tends not to be sufficiently suppressed.

[0030] The average diameter of the CNTs in the grafted CNTs is preferably 0.0004 to 0.05 μm, more preferably 0.0004 to 0.03 μm, even more preferably 0.0004 to 0.002 μm, and particularly preferably 0.0004 to 0.001 μm. If the average diameter of the CNTs is less than the lower limit, defects in the CNTs tend to increase, making them less polarizable. On the other hand, if the average diameter of the CNTs exceeds the upper limit, the proportion of carbon not involved in polarization within the CNTs increases, making the CNTs as a whole less polarizable. The average length of the CNTs is preferably 0.5 to 100 μm, more preferably 0.5 to 50 μm, even more preferably 0.5 to 10 μm, and particularly preferably 0.5 to 5 μm. If the average length of the CNTs is less than the lower limit, the polarization effect of the grafted CNTs will not be sufficient, and the dielectric constant will tend not to improve sufficiently. On the other hand, if the average length of the CNTs exceeds the upper limit, the grafted CNTs will not be uniformly dispersed within the dielectric elastomer layer, and the dielectric constant will not improve sufficiently, and dielectric breakdown (decrease in dielectric breakdown field strength) will tend not to be sufficiently suppressed.

[0031] Furthermore, the content of the grafted CNTs in the composition for forming the dielectric elastomer layer is preferably 0.01 to 5 parts by mass, and more preferably 0.03 to 2 parts by mass, per 100 parts by mass of the resin component for forming the dielectric elastomer layer. If the content of the grafted CNTs is less than the lower limit, the effect of the CNTs is not fully obtained, and for example, the dielectric constant tends to not be sufficiently improved. On the other hand, if the content of the grafted CNTs is more than the upper limit, the grafted CNTs tend to aggregate and not be uniformly dispersed in the dielectric elastomer layer, so the dielectric constant does not improve sufficiently and dielectric breakdown (a decrease in the breakdown field strength) tends to not be sufficiently suppressed.

[0032] Furthermore, in the composition for forming the dielectric elastomer layer, the content of the grafted CNT is preferably 0.1 to 20 parts by mass, more preferably 0.2 to 10 parts by mass, relative to 100 parts by mass of the polyrotaxane. If the content of the grafted CNT is less than the lower limit, the effect of the CNTs is not fully obtained, and for example, the dielectric constant tends to not be sufficiently improved. On the other hand, if the content exceeds the upper limit, the grafted CNTs tend to aggregate and not be uniformly dispersed in the dielectric elastomer layer, so the dielectric constant does not improve sufficiently and dielectric breakdown (a decrease in the breakdown field strength) tends to not be sufficiently suppressed.

[0033] There are no particular limitations on the method for synthesizing such grafted CNTs. For example, they can be synthesized by graft polymerizing polyethylene glycol (meth)acrylate monomers or alkyl (meth)acrylate monomers onto initiator-modified carbon nanotubes (initiator-modified CNTs), in which a polymerization initiator is bonded to a carbon nanotube, as described below.

[0034] That is, first, the following formula (1):

[0035] [ka]

[0036] As shown in the figure, a diamine (H2N-X-NH2) is reacted with hydrochloric acid (HCl) to synthesize a diazonium salt (ClN2-X-NH3Cl) as a polymerization initiator, and the resulting diazonium salt is reacted with a diamine (H2N-X-NH2) as shown in the figure to synthesize a diazonium salt (ClN2-X-NH3Cl) as a polymerization initiator.

[0037] [ka]

[0038] As shown in the figure, this diazonium salt is reacted with carbon nanotubes (CNTs) to synthesize aminated CNTs (CNT-X-NH2), and then the aminated CNTs are reacted with the diazonium salts of the following formula (3):

[0039] [ka]

[0040] As shown in Figure 1, the aminated CNTs (CNT-X-NH2) are reacted with a polymerization initiator (Br-Y-Br) to synthesize initiator-modified CNTs (CNT-X-NH-Y-Br).

[0041] Next, the following formula (4):

[0042] [ka]

[0043] [wherein R 1 represents a hydrogen atom or a methyl group, and R 2 represents an alkyl group, preferably an alkyl group having 1 to 12 carbon atoms, more preferably a methyl group, an ethyl group, or a butyl group; n represents the degree of polymerization of ethylene glycol, preferably n=2 to 100, more preferably n=2 to 80, even more preferably n=3 to 50, still more preferably n=3 to 20, and particularly preferably n=4 to 10; R 3 represents a linear or branched, saturated or unsaturated hydrocarbon group (preferably having 2 to 100 carbon atoms, more preferably 3 to 50 carbon atoms, and even more preferably 4 to 20 carbon atoms), and k represents the degree of polymerization of the (meth)acrylic monomer, preferably k=50,000 to 100,000, more preferably k=50,000 to 10,000, even more preferably k=5 to 1,000, and particularly preferably k=5 to 100. As shown in Figure 1, the grafted CNTs according to the present invention can be obtained by graft polymerizing a polyethylene glycol (meth)acrylate monomer or an alkyl (meth)acrylate monomer onto the initiator-modified CNTs.

[0044] The diamine is not particularly limited as long as it can introduce an amino group into the carbon nanotube. The polymerization initiator is not particularly limited as long as it can react with the amino group introduced into the carbon nanotube to introduce the polymerization initiator into the carbon nanotube and can serve as an initiator for graft polymerization of polyethylene glycol (meth)acrylate monomers or alkyl (meth)acrylate monomers.

[0045] Furthermore, in the method for synthesizing grafted CNTs, other functional groups may be introduced into carbon nanotubes instead of amino groups to synthesize functional group-containing CNTs, these functional group-containing CNTs may be reacted with a polymerization initiator to synthesize initiator-modified CNTs, and polyethylene glycol (meth)acrylate monomers or alkyl (meth)acrylate monomers may be graft polymerized onto these initiator-modified CNTs.

[0046] The reaction conditions for these synthesis methods are not particularly limited, and conventionally known reaction conditions can be appropriately adopted, and it is preferable to adopt optimized reaction conditions.

[0047] (Other resin components) The resin components for forming the dielectric elastomer layer may optionally contain other resin components, such as a polysiloxane-containing block copolymer or a polysiloxane-free polymer (excluding the diene-based polymer having a hydroxyl group). The polysiloxane is not particularly limited, but examples thereof include polydimethylsiloxane, polymethylphenylsiloxane, and modified polydimethylsiloxane. The polysiloxane-containing block copolymer is not particularly limited, but examples thereof include polycaprolactone-polysiloxane block copolymer, polyadipate-polysiloxane block copolymer, and polyethylene glycol-polysiloxane block copolymer. The polysiloxane-free polymer is not particularly limited, but examples thereof include polypropylene glycol, polytetramethylene glycol, polycarbonate, polycaprolactone, polyethylene adipate, polybutylene adipate, and polypropylene glycol monobutyl ether.

[0048] The dielectric elastomer layer of the present invention can be formed by curing a composition containing the grafted CNTs and a resin component containing at least the polyrotaxane and the diene polymer having hydroxyl groups. Therefore, the composition preferably contains a crosslinking agent as needed.

[0049] Such a crosslinking agent can be appropriately selected depending on the resin components contained in the composition, such as the polyrotaxane, the diene polymer having a hydroxyl group, the block copolymer containing the polysiloxane, and the polymer not containing the polysiloxane, and is not particularly limited, and examples thereof include aliphatic polyols, aliphatic polyethers, aliphatic polycarbonates, and block copolymers thereof, each having a functional group. Examples of the functional group include an isocyanate group, a blocked isocyanate group, and an epoxy group.

[0050] The method for forming the dielectric elastomer layer is not particularly limited, and examples thereof include a method in which the composition is coated by a known film-forming method such as spin coating, slit die coating, screen printing, or inkjet printing, and then cured.

[0051] The polymer actuator of the present invention comprises a dielectric with an electrode layer, which consists of such a dielectric elastomer layer and two electrodes arranged on both sides of the dielectric elastomer layer. There are no particular restrictions on the shape of the dielectric with an electrode layer, and examples include a cylindrical shape formed by multiple spiral wounds, or a curtain-like shape formed by bending and folding the dielectric in a wavy pattern.

[0052] The electrodes are not particularly limited, and known electrodes used in conventional actuators can be used. Examples include conductive polymer films made of silicone, natural rubber, resin, etc., in which conductive particles such as noble metals (e.g., silver nanowires, etc.) and carbon (e.g., carbon black, carbon nanotubes, graphene, etc.) are dispersed.

[0053] The sensor of the present invention is equipped with the polymer actuator of the present invention. [Example]

[0054] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. The proportion of carbon nanotubes in the grafted carbon nanotubes was determined by the following method.

[0055] [Proportion of carbon nanotubes in grafted carbon nanotubes] The grafted carbon nanotubes were placed in a platinum pan and subjected to thermogravimetric analysis using a thermal analyzer (Rigaku Corporation, "THERMO PLUS II"), heated from room temperature to 1000°C at a rate of 10°C / min under a nitrogen flow. The mass loss from room temperature to 600°C was taken as the mass of the grafted component to determine the weight fraction of the carbon nanotubes in the grafted carbon nanotubes. Furthermore, the density of the grafted component was calculated as 1 g / cm. 3 , the density of carbon nanotubes is 2g / cm 3 The volume fraction of carbon nanotubes in the grafted carbon nanotubes was calculated as follows:

[0056] (Synthesis Example 1) <Synthesis of initiator> First, the following formula (5):

[0057] [ka]

[0058] p-Chloromethylbenzyl alcohol was synthesized by the reaction shown in the following formula. Specifically, 41.5 g of p-xylylene-α,α'-diol, 150 ml of concentrated hydrochloric acid, and 600 ml of toluene were mixed in a flask and stirred at room temperature for 8 hours. The toluene layer was then washed four times with 100 ml of water and dried over anhydrous sodium sulfate. The solvent was then removed by evaporation and vacuum drying, and the resulting solid was recrystallized using ethyl acetate to obtain p-chloromethylbenzyl alcohol (yield: 29.0 g, 62%).

[0059] Next, the following formula (6):

[0060] [ka]

[0061] p-Chloromethylbenzyl 2-bromo-2-methylpropanoate was synthesized by carrying out the reaction shown in the following formula. Specifically, 20 g of the p-chloromethylbenzyl alcohol, 10.8 ml of pyridine, and 100 ml of dehydrated ether were placed in a flask under a nitrogen atmosphere, mixed, and cooled on ice. Further, a solution of 15.8 ml of 2-bromo-2-methylpropanoic acid bromide in 30 ml of dehydrated ether was added dropwise while stirring under ice cooling. The resulting solution was allowed to return to room temperature and stirred for 2 hours. The resulting reaction solution was washed three times with 20 ml of water and then dried by adding anhydrous sodium sulfate. The solvent was then removed by evaporation and vacuum drying to obtain p-chloromethylbenzyl 2-bromo-2-methylpropanoate as an oil (yield: 39.0 g, 100%). Regarding this oil, 1 H-NMR measurement confirmed no impurities, and the product was used as is in the following reaction.

[0062] Next, the following formula (7):

[0063] [ka]

[0064] p-Bromomethylbenzyl 2-bromo-2-methylpropanoate was synthesized by the reaction shown in the following formula: 38.8 g of p-chloromethylbenzyl 2-bromo-2-methylpropanoate, 26.1 g of sodium bromide, and 100 ml of N,N-dimethylformamide (DMF) were placed in a flask and stirred at 60°C for 1 hour under a nitrogen atmosphere. The precipitated salt was removed by filtration. 3 g of sodium bromide was then added, and the mixture was stirred at 60°C for 1 hour under a nitrogen atmosphere. The precipitated salt was removed by filtration. 1 g of sodium bromide was then added, and the mixture was stirred at 60°C for 1 hour under a nitrogen atmosphere. The precipitated salt was removed by filtration. The resulting filtrate was evaporated at 70°C to remove the solvent and then dried under vacuum. 30 ml of water was added to the resulting mixture, and the aqueous layer was extracted twice with 50 ml of chloroform. The chloroform layer was then washed with 50 ml of water and dried by adding anhydrous sodium sulfate. Thereafter, the solvent was removed by evaporation and vacuum drying to obtain p-bromomethylbenzyl 2-bromo-2-methylpropanoate (yield: 41.7 g, 94%).

[0065] (Synthesis Example 2) <Synthesis of initiator-modified carbon nanotubes> First, the following equation (8):

[0066] [ka]

[0067] A diazonium salt was synthesized by carrying out the reaction shown in the following formula. Specifically, 12.5 g of 4,4'-oxydianiline was dissolved in a solution prepared by diluting 18 ml of concentrated hydrochloric acid with 180 ml of water, and the solution was stirred and cooled on ice. An aqueous solution of 4.40 g of sodium nitrite in 50 ml of water was added dropwise to this solution, adjusting the dropping rate so that the solution temperature remained below 5°C. At this time, the solution changed color from colorless to yellow. Stirring was then continued for an additional 30 minutes while cooling on ice, yielding an aqueous diazonium salt solution.

[0068] Next, the following formula (9):

[0069] [ka]

[0070] Aminated carbon nanotubes (aminated CNTs) were synthesized by the reaction shown in the following formula. Specifically, 4.0 g of carbon nanotubes (CNTs, Nanosil 7000 manufactured by Nanocyl Corporation), 500 ml of DMF, and 300 ml of water were mixed, and the diazonium salt aqueous solution was added to the resulting mixture. The resulting dispersion was heated to 50°C under a nitrogen atmosphere. The resulting reaction solution was filtered, and the filter cake was washed with 500 ml of DMF. Further, DMF was added to disperse the solid components, followed by the addition of 1 g of triethylamine. The resulting dispersion was filtered, and DMF was added to disperse the solid components. This series of operations (filtration → DMF addition → dispersion) was repeated three times to wash the solid components. The solid components obtained by filtration were then heated at 80°C for 10 hours and then vacuum-dried at room temperature for 12 hours to obtain aminated CNTs (yield: 5.91 g).

[0071] Next, the following formula (10):

[0072] [ka]

[0073] Initiator-modified carbon nanotubes (initiator-modified CNTs) were synthesized by carrying out the reaction shown in the following formula. Specifically, 5.85 g of the aminated CNTs, 8 g of p-bromomethylbenzyl 2-bromo-2-methylpropanoate (initiator) obtained in Synthesis Example 1, 0.98 g of 1,8-bis(dimethylamino)naphthalene, and 350 ml of dehydrated DMF were mixed, and the resulting mixture was subjected to ultrasonic treatment for 18.5 hours under a nitrogen atmosphere. The resulting reaction solution was poured into 1.5 L of methanol and then filtered. The resulting solid component was poured into 350 ml of DMF and dispersed by ultrasonic treatment. The resulting dispersion was poured into 1.5 L of methanol and then filtered. This series of operations (methanol pouring → filtration) was repeated twice, and the resulting solid component was then vacuum-dried at 60°C to obtain initiator-modified CNTs (yield: 5.95 g).

[0074] (Synthesis Example 3) <Synthesis of crosslinking agent> 100 g of polycaprolactone-grafted polypropylene glycol was placed in a three-necked recovery flask and stirred in a nitrogen stream in an oil bath at 90°C. 7.45 g of 1,3-bis(isocyanatomethyl)cyclohexane (Takenate 600, manufactured by Mitsui Chemicals, Inc.) was slowly added dropwise to the resulting solution over 1 hour, and the mixture was stirred for an additional 2 hours to obtain an oligomer.

[0075] A three-necked recovery flask was charged with 16.66 g of 1,3-bis(isocyanatomethyl)cyclohexane ("Takenate 600" manufactured by Mitsui Chemicals, Inc.) and stirred in a 90°C oil bath under a nitrogen stream. A solution of 80 g of the oligomer in 80 g of toluene was slowly added dropwise to the resulting solution over 2 hours, followed by stirring for an additional 2 hours. The liquid temperature was then lowered to 40°C, and 10.94 g of 2-butanone oxime (manufactured by Tokyo Chemical Industry Co., Ltd.) was slowly added dropwise, ensuring that the liquid temperature did not exceed 60°C. After the addition was complete, the mixture was stirred at 40°C for 5 hours to obtain a crosslinking agent consisting of polypropylene glycol (Mn: 5422) with end-blocked isocyanate groups. This crosslinking agent was added to butyl acetate to prepare a 50% by mass crosslinking agent solution.

[0076] Example 1 <Synthesis of grafted carbon nanotubes> The following formula (11):

[0077] [ka]

[0078] Grafted carbon nanotubes (grafted CNTs) were synthesized by carrying out the reaction shown in the following formula. Specifically, 0.38 g of the initiator-modified CNTs obtained in Synthesis Example 2, 28.6 mg of copper (I) bromide, 10 g of polyethylene glycol monomethyl ether methacrylate (average molecular weight: 950, n = approximately 19 in the formula (11) above) (manufactured by Aldrich), and 30 ml of dehydrated N,N-dimethylacetamide (dehydrated DMAC) were mixed together, and the resulting mixture was subjected to a dispersion treatment for 2 hours by irradiating with ultrasound while stirring at 400 rpm using a mechanical stirrer under a nitrogen atmosphere at 60°C. 60 μL of N,N,N',N",N"-pentamethyldiethylenetriamine (PMDATA) was added to the resulting dispersion under a nitrogen atmosphere, and then degassing and nitrogen introduction were repeated five times. The mixture was then stirred for 6 hours using a mechanical stirrer under a nitrogen atmosphere at 60°C and 400 rpm. The resulting reaction solution was centrifuged (28,000 rpm, 60 minutes), and the solid component was dispersed in 50 ml of acetonitrile and then purified by centrifugation (28,000 rpm, 60 minutes). The resulting solid component was vacuum dried at 40°C for 2 days to obtain grafted CNT (1) in which polyethylene glycol monomethyl ether methacrylate was grafted onto carbon nanotubes (CNT) (yield: 2.2 g). The proportion of carbon nanotubes in this grafted CNT (1) was determined according to the method described above and was found to be 13.8 mass% (7 volume%).

[0079] <Preparation of grafted CNT-containing polybutadiene film> 3.94 g of polyrotaxane having caprolactone side chains ("SH3400P" manufactured by ASM Co., Ltd., ring-shaped molecule: cyclodextrin having caprolactone side chains, axial molecule: polyethylene glycol (molecular weight: 35,000, terminal group: adamantane group),

[0080] [ka]

[0081] 8.74 g of hydroxyl-terminated liquid polybutadiene ("Poly bd R-15HT" manufactured by Idemitsu Kosan Co., Ltd.) represented by the formula (Idemitsu Kosan Co., Ltd.), 7.7 g of toluene, 0.2 g of a surface conditioner ("DBL-C31" silicone additive manufactured by Gelest, a toluene solution with a concentration of 30% by mass), 0.41 g of a hydrolysis inhibitor ("Carbodilite V-09GB" manufactured by Nisshinbo Chemical Inc., a toluene solution with a concentration of 30% by mass), and 0.24 g of an antioxidant ("Irganox 1726" manufactured by BASF, 2,4-bis(dodecylthiomethyl)-6-methylphenol) were mixed. To the resulting solution, 0.5 parts by mass (0.07 parts by mass in terms of CNTs) of the grafted CNTs (1) dispersed in toluene at a concentration of 3% by mass was added per 100 parts by mass of the resin components (the polyrotaxane + the hydroxyl-terminated liquid polybutadiene). Further, 1.94 g of the crosslinker solution (crosslinker concentration: 50% by mass) obtained in Synthesis Example 3 was added in terms of crosslinker to prepare a resin solution in which the grafted CNTs (1) were dispersed. After degassing the resin solution, the resin solution was applied to a substrate film using a slit die coater. The resulting coating was vacuum dried at 130°C for 2 hours and cured. The resulting cured film (grafted CNTs (1)-containing polybutadiene film) was then peeled off from the substrate film. The cured film had a thickness of 0.05 mm.

[0082] Example 2 <Preparation of grafted CNT-containing polybutadiene film> A cured film (grafted CNT (1)-containing polybutadiene film) was prepared in the same manner as in Example 1, except that the amount of grafted CNT (1) added was changed to 1.4 parts by mass (0.2 parts by mass in terms of CNT) per 100 parts by mass of the resin component. The thickness of this cured film was 0.05 mm.

[0083] Example 3 <Synthesis of grafted carbon nanotubes> The following formula (12):

[0084] [ka]

[0085] Grafted carbon nanotubes (grafted CNTs) were synthesized by carrying out the reaction shown in the following formula. Specifically, the procedure was the same as in Example 1 except that 10 g of 2-ethylhexyl methacrylate was used instead of polyethylene glycol monomethyl ether methacrylate, and grafted CNTs (2) in which 2-ethylhexyl methacrylate was graft-polymerized onto carbon nanotubes (CNTs) were obtained (yield: 1.8 g). The proportion of carbon nanotubes in this grafted CNTs (2) was determined according to the method described above and was found to be 13.5 mass % (7 volume %).

[0086] <Preparation of grafted CNT-containing polybutadiene film> A cured film (grafted CNT (2)-containing polybutadiene film) was prepared in the same manner as in Example 1, except that 1.9 parts by mass (0.26 parts by mass in terms of CNT) of the grafted CNT (2) was added to 100 parts by mass of the resin components (the polyrotaxane + the hydroxyl-terminated liquid polybutadiene) instead of the grafted CNT (1). The thickness of this cured film was 0.05 mm.

[0087] (Comparative Example 1) <Preparation of polybutadiene film> A cured film (polybutadiene film) was produced in the same manner as in Example 1 except that the grafted CNT (1) was not added. The thickness of this cured film was 0.05 mm.

[0088] (Comparative Example 2) <Production of CNT-containing polybutadiene film> A cured film (CNT-containing polybutadiene film) was produced in the same manner as in Example 1 except that 0.25 parts by mass of non-grafted carbon nanotubes (CNT, "Nanocyl 7000" manufactured by Nanocyl) was added to 100 parts by mass of the resin component instead of the grafted CNT (1). The thickness of this cured film was 0.05 mm.

[0089] 〔Optical microscope observation〕 The surface of the obtained cured film was observed with an optical microscope. Figures 1 to 3 are optical microscope photographs of the cured films obtained in Examples 1 to 3, respectively.

[0090] 〔Initial elastic modulus〕 From the obtained cured film, dumbbell-shaped No. 7 test pieces were produced according to JIS K6251. For these test pieces, using a tensile testing machine ("Autograph AGS-X 10N" manufactured by Shimadzu Corporation), with a grip distance of 20 mm and a tensile speed of 100 mm / min, a tensile test was carried out until the elongation reached 100% of the effective length. Then, it was contracted at the same speed as during elongation until it reached 0%, and a stress-strain curve was obtained. The range of 1% to 5% elongation of this stress-strain curve was linearly approximated, and the slope was taken as the initial elastic modulus. The results are shown in Table 1.

[0091] 〔Breaking strength〕 [5]] From the obtained cured film, dumbbell-shaped No. 7 test pieces were produced according to JIS K6251. For these test pieces, using a tensile testing machine ("Autograph AGS-X 10N" manufactured by Shimadzu Corporation), with a grip distance of 20 mm and a tensile speed of 100 mm / min, a tensile test was carried out until it broke, and the breaking strength was determined from the stress value at the time of breakage. The results are shown in Table 1. 〔Hysteresis loss〕 "Hysteresis loss" refers to the mechanical energy loss (hysteresis loss) in one cycle of deformation and recovery according to JIS K6400, where strain in a tensile test of the material is used instead of deformation. It was measured as follows, in accordance with the method described in JP 2011-241401 A. Specifically, dumbbell-shaped No. 7 test specimens were prepared from the resulting cured films according to JIS K6251. Tensile tests were conducted on these test specimens using a tensile tester (Shimadzu Corporation, Autograph AGS-X 10N) at a gripper distance of 20 mm and a tensile speed of 0.2 mm / sec to obtain stress-strain curves. This measurement was performed 10 times (10 cycles), and the reduction ratio of the area during contraction to the area during extension in the stress-strain curve for each cycle was calculated. The average value for cycles 2 to 10 was calculated and used as the hysteresis loss (%). The results are shown in Table 1.

[0092] [Breakdown field strength] The resulting cured film was attached to a disk electrode, taking care to minimize air bubbles between the film and the electrode. A cylindrical electrode was then placed on top of the cured film, and the film was degassed using a vacuum device. This was then placed in a breakdown tester, and a voltage was applied between the electrodes at room temperature (20±15°C) and room humidity (65±20%) at a rate of 10 V / 0.1 s using a power supply. After this, a state of essentially no current flow was reached, and the voltage was measured when the current reached 1.2 μA or greater, and the breakdown field strength was calculated. The results are shown in Table 1.

[0093] [Volume resistivity] Gold was vapor-deposited on both sides of the obtained cured film using an autofine coater (JEC-3000FC manufactured by JEOL Ltd.) to prepare electrode films (8 mm diameter), and the volume resistivity was measured using a microcurrent meter (Super Insulation Meter SM7120 manufactured by Hioki E.E. Corporation. The results are shown in Table 1.

[0094] [Dielectric constant] Gold was evaporated onto both sides of the resulting cured film using an auto-fine coater (JEOL Ltd., "JEC-3000FC") to form electrode films (15 mm diameter). The capacitance was measured using a precision impedance analyzer (Agilent Corp., "4294A") with a dielectric constant measurement probe, and the relative dielectric constant was calculated. The results are shown in Table 1.

[0095] [Table 1]

[0096] As shown in FIGS. 1 to 3, it was confirmed that the grafted CNTs were well dispersed in the cured films obtained in Examples 1 to 3, without forming aggregates of 1 μm or more.

[0097] As shown in Table 1, when grafted carbon nanotubes, in which a (meth)acrylic polymer having a polyethylene glycol chain on its side chain is bonded to a carbon nanotube by graft polymerization, were added to a dielectric elastomer layer made of a cured product of a resin component containing a polyrotaxane and a diene polymer having a hydroxyl group (Examples 1 and 2), the dielectric constant was confirmed to be improved compared to when no grafted carbon nanotubes were added (Comparative Example 1) (Example 1: 10% improvement, Example 2: 30% improvement, relative to Comparative Example 1). Furthermore, it was confirmed that the breaking strength was improved, and mechanical properties suitable for polymer actuators were exhibited. Furthermore, no significant change was observed in the initial modulus of elasticity, confirming that the properties as an elastomer (particularly flexibility) were not impaired. Furthermore, no significant change was observed in the hysteresis loss, confirming that the properties as a dielectric elastomer were not impaired.

[0098] Furthermore, when grafted carbon nanotubes, in which a (meth)acrylic polymer having an alkyl chain on the side chain is bonded to carbon nanotubes by graft polymerization, were added to a dielectric elastomer layer made of a cured product of a resin component containing polyrotaxane and a diene polymer having a hydroxyl group (Example 3), it was confirmed that the relative dielectric constant was improved compared to when no grafted carbon nanotubes were added (Comparative Example 1) (Example 3: 45% improvement over Comparative Example 1). Furthermore, although the breaking strength decreased slightly, no significant changes were observed in the initial modulus of elasticity and hysteresis loss, confirming that the properties of the dielectric elastomer were not impaired.

[0099] Furthermore, it was found that when ungrafted carbon nanotubes were added (Comparative Example 2), the breakdown field strength was significantly reduced compared to when they were not added (Comparative Example 1).On the other hand, it was found that when the grafted carbon nanotubes according to the present invention were added (Examples 1 to 3), the reduction in breakdown field strength was suppressed compared to when ungrafted carbon nanotubes were added (Comparative Example 2).

[0100] From the above results, it was found that by adding grafted carbon nanotubes in which a (meth)acrylic polymer having at least one of a polyethylene glycol chain and an alkyl chain on its side chain is bonded to the carbon nanotubes by graft polymerization to a dielectric elastomer layer consisting of a cured resin component including polyrotaxane and a diene-based polymer having a hydroxyl group, it is possible to increase the relative dielectric constant while maintaining the initial elastic modulus and suppress a decrease in the breakdown field strength. [Industrial Applicability]

[0101] As described above, according to the present invention, the grafted carbon nanotubes are well dispersed without forming aggregates of 1 μm or more, and it is possible to form a dielectric elastomer layer that improves the dielectric constant while maintaining flexibility (elastic modulus) and suppresses dielectric breakdown (decrease in dielectric breakdown field strength). Therefore, the polymer actuator of the present invention, which includes such a dielectric elastomer layer, can be used in a variety of fields, such as industrial and nursing robots, artificial muscles, sensors, and haptics.

Claims

1. a dielectric elastomer layer and two electrodes disposed on opposite sides of the dielectric elastomer layer; A polymer actuator characterized in that the dielectric elastomer layer is made of a cured product of a composition containing a resin component including polyrotaxane and a diene-based polymer having a hydroxyl group, and grafted carbon nanotubes in which a (meth)acrylic polymer having at least one of a polyethylene glycol chain and a linear or branched hydrocarbon chain in its side chain is bonded to the carbon nanotubes by graft polymerization.

2. 2. The polymer actuator according to claim 1, wherein the content of the grafted carbon nanotubes in the composition is in the range of 0.1 to 5 parts by mass with respect to 100 parts by mass of the resin component.

3. 3. The polymer actuator according to claim 1, wherein the content of the carbon nanotubes in the grafted carbon nanotubes is in the range of 1 to 50% by volume.

4. A sensor comprising the polymer actuator according to any one of claims 1 to 3.

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