CN109787502B - Electroactive polymers based on negative Poisson's ratio dielectric elastomers - Google Patents

Abstract

The invention discloses a novel electroactive polymer based on a negative Poisson's ratio dielectric elastomer, comprising an elastic film and flexible electrodes on both sides of the elastic film; the flexible electrodes on both sides of the elastic film are uniformly coated on the upper and lower sides of the elastic film On the surface, the thickness is smaller than the thickness of the elastic film, and the Young's modulus is smaller than that of the elastic film, which are respectively used to connect with the positive and negative electrodes of the external voltage. When the voltage is applied to the flexible electrodes on both sides, the elastic film shrinks along the thickness, length, and width directions at the same time, the volume decreases, the material density, stiffness and bearing capacity increase, and the failure of mechanical damage, electrical breakdown, and electromechanical coupling instability The limit is increased, resulting in better electromechanical properties than traditional dielectric electroactive polymers.

Description

Electroactive polymers based on negative Poisson's ratio dielectric elastomers

technical field

The present invention relates to an electroactive polymer, in particular to an electroactive polymer based on a negative Poisson's ratio dielectric elastomer.

Background technique

Negative Poisson's ratio materials, also known as auxetic materials, are a class of functional materials with a negative Poisson's ratio. When the material undergoes tensile deformation, lateral expansion will occur in the direction perpendicular to the load; When compressive deformation occurs, lateral shrinkage occurs in the direction perpendicular to the load. Therefore, the material will automatically focus on the loading place to be able to bear the load more effectively, and the stiffness of the material will increase nonlinearly with the increase of the load, so the material with negative Poisson's ratio has higher shear modulus and resilience. , with excellent mechanical properties.

Electroactive polymers are a class of flexible functional materials that can generate displacement and load changes under electric field and voltage excitation. In addition, changes in their displacement and load conditions can also cause significant changes in electric field and voltage. Therefore, the load of electroactive polymers , displacement, electric field and voltage states are coupled with each other, and the change of any one state will cause the change of one or several other parameter states. Electroactive polymers can be mainly divided into ionic and electric field types: ionic electroactive polymers use chemical energy as a transition to realize the conversion between electrical energy and mechanical energy. Its advantages are low driving voltage and large deformation, but the response It is slow and has low energy density, so it is difficult to apply to energy-absorbing parts under dynamic conditions. Electric field type electroactive polymers can be further divided into piezoelectric type and dielectric type: piezoelectric type electroactive polymers will generate electrical stress in the material itself under the excitation of electric field, and directly realize the conversion between electrical energy and mechanical energy, but the deformation is relatively small. It is small and has low efficiency; the dielectric electroactive polymer realizes energy conversion through the electrostatic Coulomb force generated by the electrodes on both sides under the excitation of the electric field, which is characterized by fast response, large deformation (maximum area strain up to 380%), and energy density. Larger and with high energy conversion efficiency (up to 90%). Based on the above characteristics, dielectric electroactive polymers are often called artificial muscles. Another advantage of dielectric electroactive polymers is that they are inexpensive and therefore expected to be widely used.

Under the excitation of electric field and voltage, the traditional dielectric electroactive polymer will accumulate positive and negative charges on the flexible electrodes on both sides respectively, resulting in electrostatic effect and the formation of Coulomb force. The Coulomb force acts on the thickness direction of the electroactive polymer, The electroactive polymer is compressed along the thickness direction and stretched laterally, the thickness size decreases, and the area increases. , which is not conducive to the large-scale application of electroactive polymers.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to solve the defect of reducing the stiffness and bearing capacity of the traditional dielectric electroactive polymer in the background technology when the deformation caused by the electrostatic Coulomb force is large. The electroactive polymer of the Songpi dielectric elastomer, through the application of the negative Poisson's ratio of the electroactive polymer, the shape, density and stiffness of the electroactive polymer can be changed in real time, and the traditional dielectric electroactive polymerization can be improved. As the deformation increases, its stiffness and bearing capacity decrease, and it can realize the integration, electronization, informationization and intelligence of elastic elements, damping elements, sensor elements, actuator elements and energy recovery elements at the same time. .

The present invention adopts the following technical solutions for solving the above-mentioned technical problems:

An electroactive polymer based on a negative Poisson's ratio dielectric elastomer, comprising an elastic film and flexible electrodes on both sides of the elastic film;

The flexible electrodes on both sides of the elastic film are evenly coated on the upper and lower surfaces of the elastic film, the thickness is smaller than the thickness of the elastic film, and the Young's modulus is smaller than the Young's modulus of the elastic film. catch;

The elastic film adopts a negative Poisson's ratio dielectric elastomer material, which is prepared by applying compressive forces in three orthogonal directions at the same time when the porous dielectric elastomer material is heated to a temperature slightly higher than its thermal softening temperature range;

When a voltage is applied to the flexible electrodes on both sides of the elastic film, the elastic film shrinks along the thickness direction, the length direction, and the width direction at the same time, the volume decreases, the material density, stiffness and bearing capacity increase, mechanical damage, electrical breakdown, electromechanical coupling The failure limit for instability is increased.

As a further optimized solution of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer of the present invention, when the porous dielectric elastomer material exerts compressive forces in three orthogonal directions, the porous dielectric elastomer material is extruded by extruding the porous dielectric elastomer material. A method of pressing into a mold whose dimensions in three orthogonal directions are all smaller than its own dimensions.

As a further optimized solution of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer of the present invention, the negative Poisson's ratio dielectric elastomer material adopts a negative Poisson's ratio polyurethane or a negative Poisson's ratio polyolefin blend, The flexible electrodes on both sides are made of any one of electrode carbon powder, silver paste, metal film, carbon grease, carbon nanotubes, hydrogel electrolyte, and graphene.

Compared with the prior art, the present invention adopts the above technical scheme, and has the following technical effects:

When the electroactive polymer based on negative Poisson's ratio dielectric elastomer is excited by electric field and voltage, its deformation will exhibit negative Poisson's ratio characteristics, that is, the electroactive polymer will shrink along the thickness direction after electrification, and its length and width will be reduced. The direction also shrinks at the same time, and the area is reduced, so its response is completely opposite to that of traditional dielectric electroactive polymers;

Negative Poisson's ratio materials have high shear modulus and resilience toughness, and have excellent properties in terms of mechanical properties, while electroactive polymers based on negative Poisson's ratio dielectric elastomers are comparable to traditional dielectric-type electroactive polymers. Compared with polymers, its outstanding feature is that the stiffness will increase nonlinearly with the increase of voltage, and the load-bearing capacity of the material will also be greatly improved, which can greatly expand the application range of electroactive polymers.

Description of drawings

1(A) and 1(B) are a schematic diagram of a traditional dielectric electroactive polymer and a schematic diagram of electromechanical deformation, respectively;

Fig. 2(A) and Fig. 2(B) are a schematic diagram and a schematic diagram of electromechanical deformation of an electroactive polymer based on a negative Poisson's ratio dielectric elastomer, respectively;

3(A) and FIG. 3(B) are schematic diagrams of electromechanical deformation when conventional dielectric electroactive polymers and electroactive polymers based on negative Poisson’s ratio dielectric elastomers are used as actuators, respectively;

Figure 4 is a graph of mechanical force versus electric field force in an electroactive polymer based on a negative Poisson's ratio dielectric elastomer.

Detailed ways

The electroactive polymer in the present invention belongs to a kind of negative Poisson's ratio material. Compared with the traditional electroactive polymer, it has higher shear modulus and resilience toughness, and has excellent properties in terms of mechanical properties. . Under the excitation of different voltages or electric fields, electroactive polymers can achieve different mechanical properties, realize real-time change of material size and stiffness, improve the bearing capacity of materials, and then expand the application range of materials, and can realize elastic elements at the same time. , Vibration reduction components, sensor components, actuator components and energy recovery components are integrated, electronic, informatized and intelligent.

The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

The invention discloses an electroactive polymer based on a negative Poisson's ratio dielectric elastomer, comprising an elastic film and flexible electrodes on both sides of the elastic film;

The flexible electrodes on both sides of the elastic film are evenly coated on the upper and lower surfaces of the elastic film, the thickness is smaller than the thickness of the elastic film, and the Young's modulus is smaller than the Young's modulus of the elastic film. catch;

The elastic film adopts a negative Poisson's ratio dielectric elastomer material, which is prepared by applying compressive forces in three orthogonal directions at the same time when the porous dielectric elastomer material is heated to a temperature slightly higher than its thermal softening temperature range;

When a voltage is applied to the flexible electrodes on both sides of the elastic film, the elastic film shrinks along the thickness direction, the length direction, and the width direction at the same time, the volume decreases, the material density, stiffness and bearing capacity increase, mechanical damage, electrical breakdown, electromechanical coupling The failure limit for instability is increased.

As a further optimized solution of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer of the present invention, when the porous dielectric elastomer material exerts compressive forces in three orthogonal directions, the porous dielectric elastomer material is extruded by extruding the porous dielectric elastomer material. A method of pressing into a mold whose dimensions in three orthogonal directions are all smaller than its own dimensions.

As a further optimized solution of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer of the present invention, the negative Poisson's ratio dielectric elastomer material adopts a negative Poisson's ratio polyurethane or a negative Poisson's ratio polyolefin blend, The flexible electrodes on both sides are made of any one of electrode carbon powder, silver paste, metal film, carbon grease, carbon nanotubes, hydrogel electrolyte, and graphene.

Figure 1(A) shows a schematic diagram of a traditional dielectric electroactive polymer, which is a sandwich-like structure, wherein the core material is a traditional elastic film, which can be made of silica gel, acrylic, polyurethane or other dielectric elastomer materials. The upper and lower sides are flexible electrodes, which can be made of electrode carbon powder, silver paste, metal film, carbon grease, carbon nanotubes, hydrogel electrolyte, graphene and other materials. The initial length, width and thickness of the dielectric electroactive polymer are respectively L ₁ , L ₂ and L ₃ , where L ₃ is the sum of the thicknesses of the dielectric elastomer film and the flexible electrodes on both sides. The Young's modulus of the flexible electrode material on both sides should be much smaller than that of the dielectric elastomer film to reduce its influence on the mechanical properties of the electroactive polymer.

Figure 1(B) shows a schematic diagram of the electromechanical deformation of a traditional dielectric electroactive polymer. The upper and lower flexible electrodes are connected to the positive and negative poles of the high-voltage DC power supply, where the voltage of the high-voltage DC power supply is Φ, and the dielectric The electroactive polymer is equivalent to a capacitor, and the current cannot pass through the elastic membrane, so ±Q charges are accumulated at the upper and lower flexible electrodes respectively, resulting in an electrostatic effect and the formation of Coulomb force, which acts on the thickness of the dielectric electroactive polymer. direction, so that the thickness of the electroactive polymer decreases from L ₃ to l ₃ , and the length and width dimensions increase from L ₁ and L ₂ to l ₁ and l ₂ , respectively. At this time, the dielectric electroactive polymer The stress states in the three directions are P ₁ , P ₂ and P ₃ respectively. In this system Φ, Q, P and l ₃ are mutually coupled state parameters, and the change of any state will affect the other three state parameters.

Figure 2(A) shows a schematic diagram of an electroactive polymer based on a negative Poisson's ratio dielectric elastomer, which is also a sandwich-like sandwich structure, wherein the sandwich material is an elastic film, and a negative Poisson's ratio dielectric elastomer material is used. The upper and lower sides are flexible electrodes. The initial length, width and thickness of the electroactive polymer are respectively L ₁ , L ₂ and L ₃ , where L ₃ is the sum of the thicknesses of the elastic membrane and the flexible electrodes on both sides. The Young's modulus of the flexible electrode material on both sides should be much smaller than that of the elastic film to reduce its influence on the mechanical properties of the electroactive polymer.

Figure 2(B) shows a schematic diagram of the electromechanical deformation of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer. The upper and lower flexible electrodes are respectively connected to the positive and negative poles of the high-voltage DC power supply, where the voltage of the high-voltage DC power supply is Φ, at this time, the electroactive polymer is equivalent to a capacitor, and the current cannot pass through the elastic membrane, so ±Q charges are accumulated at the upper and lower flexible electrodes respectively, resulting in electrostatic effect and the formation of Coulomb force, acting on the negative Poisson’s ratio dielectric. The thickness direction of the electro-elastomer electro-active polymer reduces the thickness of the electro-active polymer from L ₃ to l ₃ , and because the elastic film will shrink laterally when subjected to a vertical load, therefore, the electro-active polymer The dimensions of the length and width of the polymer are reduced from L ₁ and L ₂ to l ₁ and l ₂ , respectively, which is completely opposite to the response of conventional dielectric electroactive polymers. At this time, the stress states of the electroactive polymer in three directions are respectively P ₁ , P ₂ and P ₃ . In this system Φ, Q, P and l ₃ are mutually coupled state parameters, and the change of any state will affect the other three state parameters.

The invention discloses a method for estimating the electromechanical response of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer, comprising the following steps:

Let the initial length, width and thickness of the dielectric elastomer based on negative Poisson's ratio be L ₁ , L ₂ and L ₃ respectively, the voltage on both sides is Φ, and ±Q charges are accumulated at the upper and lower flexible electrodes respectively, so that the electric The thickness of the active polymer is reduced from L ₃ to l ₃ , the length and width dimensions are reduced from L ₁ and L ₂ to l ₁ and l ₂ , respectively, and the stretching ratios in the three directions are λ ₁ =l ₁ /L ₁ , λ ₂ =l ₂ /L ₂ and λ ₃ =l ₃ /L ₃ ;

At this time, the stress states of the electroactive polymer in three directions are P ₁ , P ₂ and P ₃ respectively, and the true stresses in the three directions are σ ₁ =P ₁ /l ₂ l ₃ , σ ₂ =P ₂ /l ₁ l ₃ and σ ₃ =P ₃ /l ₁ l ₂ ; the real electric field strength of the elastic membrane is E=Φ/l ₃ =Φ/ ₃ L ₃ , and the real electric displacement is D=Q/l ₁ l ₂ ;

Let the Helmholtz free energy of the electroactive polymer be F, and its density be W=F/(L ₁ L ₂ L ₃ );

The change of Helmholtz free energy in the case of small perturbation is:

δF=P ₁ δl ₁ +P ₂ δl ₂ +P ₃ δl ₃ +ΦδQ (1)

Where δQ=Dl ₂ δl ₁ +Dl ₁ δl ₂ +l ₁ l ₂ δD, δl ₁ , δl ₂ , δD are the changes of l ₁ , l ₂ , and D in the case of minor disturbances, respectively;

Divide both sides of equation (1) by L ₁ L ₂ L ₃ , then we have:

δW=(σ ₁ +ED)λ ₂ λ ₃ δλ ₁ +(σ ₂ +ED)λ ₁ λ ₃ δλ ₂ +σ ₃ λ ₁ λ ₂ δλ ₃ +Eλ ₁ λ ₂ λ ₃ δD (2)

Set the Helmholtz free energy density as a function of four independent variables W=W(λ ₁ ,λ ₂ ,λ ₃ ,D), after substituting into equation (2), we have:

Since λ ₁ , λ ₂ , λ ₃ and D are four independent variables, at the equilibrium position we have:

Since there is a linear relationship between the electric field strength and electrical displacement experienced by the elastic film, that is, E=D/ε, where ε is the dielectric constant of the dielectric elastomer. Integrating equation (2) with D and keeping λ ₁ , λ ₂ and λ ₃ unchanged, we get:

where W _s is the strain energy function of the elastic membrane, and formula (5) and E=D/ε are substituted into formula (4):

The strain energy function adopts the Ogden model, then there are:

where α _i , _ui and β _i are the material parameters of the elastic film, which can be obtained by fitting the material experimental data of the elastic film, N is the order of the Ogden model, and i is a natural number greater than or equal to 1 and less than or equal to N;

Substituting equation (7) into equation (6), we get:

The electromechanical state of the electroactive polymer based on the negative Poisson's ratio dielectric elastomer is estimated by equation (8), and the load and voltage can be obtained from P ₁ =σ ₁ l ₂ l ₃ , P ₂ =σ ₂ l ₁ l ₃ , P ₃ =σ ₃ l ₁ l ₂ and Φ=Eλ ₃ L ₃ are estimated.

Figure 3(A) shows a schematic diagram of the electromechanical deformation of a conventional dielectric electroactive polymer as an actuator, where the traditional dielectric electroactive polymer film is fully constrained at both ends and subjected to a concentrated load F at the midpoint and maintained No change, the dotted line is the equilibrium position before power-on. When a voltage is applied on both sides of the conventional dielectric electroactive polymer, its equilibrium position becomes the position shown by the solid line, and its actuation displacement can be seen downward. This phenomenon shows that the stiffness of the traditional dielectric electroactive polymer film decreases after electrification, and the higher the voltage, the smaller the stiffness.

Figure 3(B) shows a schematic diagram of the electromechanical deformation of an electroactive polymer based on a negative Poisson's ratio dielectric elastomer as an actuator, where the electroactive polymer film is fully constrained at both ends and subjected to a concentrated load at the midpoint. F and remain unchanged, the dotted line is the equilibrium position before power-on. When a voltage is applied on both sides of the electroactive polymer film, its equilibrium position becomes the position shown by the solid line, and it can be seen that its actuation displacement is upward, contrary to the traditional dielectric electroactive polymer. This phenomenon indicates that the stiffness of the electroactive polymer film increases after electrification, and the higher the voltage, the greater the stiffness.

Figure 4 shows the relationship between mechanical force and electric field force in electroactive polymers of negative Poisson's ratio dielectric elastomers. In equilibrium, the electric force is equal to the mechanical force. When the voltage, charge and capacitance of the electroactive polymer change so that the electric field force exceeds the mechanical force, as shown in point 1, in order to reach the equilibrium position, the mechanical force continues to increase, and the area and thickness of the electroactive polymer decrease , the density increases, and finally the balance between the electric field force and the mechanical force is reached, that is, the point 2 is reached. During this process, part of the electrical energy is converted into mechanical energy. On the other hand, when the load and deformation of the electroactive polymer change so that the mechanical force exceeds the electric field force, as shown in point 3, in order to reach the equilibrium position, the electric field force continues to increase, and the two sides of the electroactive polymer are flexible The voltage of the electrodes rises, eventually reaching a balance between the electric field force and the mechanical force, that is, reaching point 4, during which part of the mechanical energy is converted into electrical energy. In the upper left area of the equilibrium state curve in the figure, the electroactive polymer can act as an actuator device, and in the lower right area of the equilibrium state curve, it works as an energy recovery (or generator) or sensor device.

When the electroactive polymer can be used as an actuator device, it can convert electrical energy into mechanical energy. Compression in the thickness direction reduces the area and thickness of the electroactive polymer, so that it undergoes a certain amount of displacement to achieve the actuation function. Different actuation requirements can be achieved by changing the connected power supply voltage Φ and the load P to be subjected to. Compared with the actuators made by traditional dielectric electroactive polymers, electroactive polymers deform in the opposite direction in length and width under the action of electric field force, and the density and stiffness of the material will change. As the electric field force increases and the nonlinearity increases, the actuator can be subjected to higher supply voltage Φ and load P.

Electroactive polymers convert mechanical energy into electrical energy when used as energy recovery, or generators. The basic principle is: when the electroactive polymer is subjected to a vertical load, the elastic film is forced to shrink and the thickness is reduced; the flexible electrodes on both sides of the material are connected to a relatively low-voltage loop, and a certain amount of energy will accumulate on the flexible electrodes at both ends. Electric charge; disconnect the relatively low voltage loop, reduce the size of the vertical load, make the elastic film gradually stretch, the thickness increases, the charges on the flexible electrodes on both sides are gradually pushed away, and the voltage increases; a flexible electrode is connected at both ends of the material. The relatively high-voltage circuit outputs electric energy under high voltage, thereby realizing energy recovery.

Electroactive polymers convert mechanical energy into electrical energy when used as sensor devices. The basic principle is: when the electroactive polymer is subjected to a vertical load, the elastic film is forced to shrink and the thickness is reduced; when the flexible electrodes on both sides of the material are connected to a circuit with a certain voltage, a certain amount of charge will be accumulated on the flexible electrodes at both ends; When the vertical load decreases, the elastic film gradually stretches, the thickness increases, the capacitance decreases, and the charge of the flexible electrodes on both sides gradually decreases; when the vertical load increases, the elastic film gradually shrinks, the thickness decreases, the capacitance increases, and the two The charge of the side flexible electrodes gradually increases. Therefore, by measuring the capacitance value or the amount of charge on the flexible electrodes on both sides, the change in load can be calculated.

Negative Poisson’s ratio dielectric elastomers belong to a class of negative Poisson’s ratio materials. When an external mechanical force is applied, a negative Poisson’s ratio is produced. Therefore, electroactive polymers are compared with traditional dielectric electroactive polymers. , under the action of external excitation, it will produce the opposite deformation effect, so that some more excellent performance can be obtained. For example, under the action of load, the density and stiffness of electroactive polymers will increase nonlinearly with the increase of load due to the negative Poisson's ratio, so they can withstand greater than traditional electroactive polymers. load, recover more energy, and can withstand a greater breakdown voltage.

By designing a certain control strategy and control system, the multifunctional coupling of variable stiffness, actuation, energy recovery, and sensing of electroactive polymers of negative Poisson's ratio dielectric elastomers can be realized.

Electroactive polymers based on negative Poisson's ratio dielectric elastomers can be fabricated to include, but are not limited to, real-time variable buffer elements, energy absorbing elements, vibration damping elements, spring-damping structures, sensors, actuators, and energy recovery elements .

It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It should also be understood that terms such as those defined in the general dictionary should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as herein, are not to be taken in an idealized or overly formal sense. explain.

The specific embodiments described above further describe the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (3)

1. An electroactive polymer based on a negative poisson's ratio dielectric elastomer comprising an elastic membrane, and flexible electrodes on either side of the elastic membrane;

the flexible electrodes on the two sides of the elastic membrane are uniformly coated on the upper surface and the lower surface of the elastic membrane, the thickness of the flexible electrodes is smaller than that of the elastic membrane, and the Young modulus of the flexible electrodes is smaller than that of the elastic membrane and is respectively used for being connected with the positive electrode and the negative electrode of an external voltage;

the elastic membrane is made of a negative Poisson ratio dielectric elastomer material, and the elastic membrane is prepared by heating a porous dielectric elastomer material to a temperature slightly higher than the thermal softening temperature range of the porous dielectric elastomer material and simultaneously applying compression forces in three orthogonal directions;

when voltage is applied to the flexible electrodes on the two sides of the elastic membrane, the elastic membrane contracts along the thickness direction, the length direction and the width direction at the same time, so that the size is reduced, the density, the rigidity and the bearing capacity of the material are increased, and the failure limit of mechanical damage, electric breakdown and electromechanical coupling instability is improved.

2. The negative poisson's ratio dielectric elastomer-based electroactive polymer of claim 1, wherein the porous dielectric elastomer material is compressed into a mold having three orthogonal dimensions that are less than its own dimension when three orthogonal compressive forces are applied to the porous dielectric elastomer material.

3. The electroactive polymer based on a negative poisson's ratio dielectric elastomer as claimed in claim 1, wherein the negative poisson's ratio dielectric elastomer material is negative poisson's ratio polyurethane or negative poisson's ratio polyolefin blend, and the flexible electrodes on both sides are made of any one of electrode carbon powder, silver paste, metal film, carbon grease, carbon nano-tubes, hydrogel electrolyte and graphene.

Images