CN109787502B - Electroactive polymers based on negative poisson's ratio dielectric elastomers - Google Patents
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Abstract
The invention discloses a novel electroactive polymer based on a negative Poisson ratio dielectric elastomer, which comprises an elastic membrane and flexible electrodes on two sides of the elastic membrane; the flexible electrodes on 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 anode and the cathode of an external voltage. When voltage is applied to the flexible electrodes on the two sides, the elastic film 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, thereby having more excellent electromechanical properties than the traditional dielectricity electroactive polymer.
Description
Technical Field
The invention relates to an electroactive polymer, in particular to an electroactive polymer based on a negative poisson ratio dielectric elastomer.
Background
The negative Poisson ratio material is also called as an Auxetic material (Auxetic), is a functional material with a negative Poisson ratio, and can expand laterally in a direction perpendicular to a load when the material is subjected to tensile deformation; and when the material is compressively deformed, lateral contraction occurs perpendicular to the direction of the load. Therefore, the material can be automatically concentrated at a loading position so as to be capable of bearing the load more effectively, and the rigidity of the material is nonlinearly increased along with the increase of the load, so that the negative poisson ratio material has higher shear modulus and rebound toughness and excellent mechanical properties.
The electroactive polymer is a flexible functional material which can generate displacement and load change under the excitation of an electric field and voltage, and in addition, the displacement and the change of the load condition of the electroactive polymer can also cause the remarkable change of the electric field and the voltage, so the load, the displacement, the electric field and the voltage condition of the electroactive polymer are mutually coupled, and the change of any one condition can cause the change of one parameter condition or several parameter conditions. Electroactive polymers can be mainly classified into two major types, ionic and electric field types: the ionic electroactive polymer realizes conversion between electric energy and mechanical energy by taking chemical energy as transition, and has the advantages of low driving voltage, large deformation, slow response and low energy density, so the ionic electroactive polymer is difficult to be suitable for energy absorption components under dynamic working conditions. Electric field type electroactive polymers can be further classified into piezoelectric type and dielectric type: the piezoelectric electroactive polymer can generate the electrical stress under the excitation of an electric field, directly realizes the conversion between the electrical energy and the mechanical energy, but has smaller deformation and lower efficiency; the dielectric electroactive polymer realizes energy conversion through electrostatic coulomb force generated by electrodes on two sides under the excitation of an electric field, and has the characteristics of quick response, large deformation (the maximum area strain can reach 380 percent), large energy density and high energy conversion efficiency (the highest energy conversion efficiency can reach 90 percent). Based on the above characteristics, dielectric electroactive polymers are also commonly referred to as artificial muscles. Another advantage of dielectric electroactive polymers is that they are inexpensive and therefore are expected to find wide application.
Under the excitation of an electric field and voltage, positive charges and negative charges can be respectively accumulated on flexible electrodes on two sides of a traditional dielectric electroactive polymer, so that an electrostatic effect is generated and coulomb force is formed, the coulomb force acts on the thickness direction of the electroactive polymer, the electroactive polymer is compressed along the thickness direction to generate lateral stretching, the thickness size is reduced, the area is increased, along with the continuous reduction of the thickness of the electroactive polymer, the problems of mechanical damage, electric breakdown, electromechanical coupling instability and the like are easy to occur, and the large-scale application of the electroactive polymer is not facilitated.
Disclosure of Invention
The invention aims to solve the technical problem that the rigidity and the bearing capacity of the traditional dielectric electroactive polymer can be reduced when the deformation caused by electrostatic coulomb force is large in the background technology, and provides the electroactive polymer based on the negative poisson ratio dielectric elastomer.
The invention adopts the following technical scheme for solving the technical problems:
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.
As a further optimization of the electroactive polymer based on the negative poisson's ratio dielectric elastomer of the present invention, the porous dielectric elastomer material is extruded into a mold having three orthogonal dimensions that are smaller than its own dimension when three orthogonal compressive forces are applied to the porous dielectric elastomer material.
As a further optimization scheme of the electroactive polymer based on the negative Poisson ratio dielectric elastomer, the negative Poisson ratio dielectric elastomer material is made of negative Poisson ratio polyurethane or negative Poisson ratio polyolefin blend, and the flexible electrodes on two sides are made of any one of electrode carbon powder, silver paste, metal films, carbon resin, carbon nano tubes, hydrogel electrolyte and graphene.
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
when an electroactive polymer based on the negative poisson ratio dielectric elastomer is excited by an electric field and voltage, the electroactive polymer can be deformed to show a negative poisson ratio characteristic, namely, after the electroactive polymer is electrified, the electroactive polymer can contract along the thickness direction, the length direction and the width direction of the electroactive polymer also contract simultaneously, the area is reduced, and therefore the response of the electroactive polymer is completely opposite to that of the traditional dielectric electroactive polymer;
the negative poisson's ratio material has higher shear modulus and rebound toughness, and has excellent properties in the aspect of mechanical properties, and compared with the traditional dielectric electroactive polymer, the electroactive polymer based on the negative poisson's ratio dielectric elastomer has the outstanding characteristics that the rigidity can be increased nonlinearly along with the increase of voltage, the bearing capacity of the material can be greatly improved, and the application range of the electroactive polymer can be greatly expanded.
Drawings
FIGS. 1(A) and 1(B) are a schematic view of a conventional dielectric electroactive polymer and a schematic view of an electromechanical deformation, respectively;
FIGS. 2(A), 2(B) are schematic diagrams of electroactive polymers and electromechanical deformations based on negative Poisson's ratio dielectric elastomers, respectively;
FIGS. 3(A) and 3(B) are schematic diagrams of electromechanical deformations when a conventional dielectric electroactive polymer and an electroactive polymer based on a negative Poisson's ratio dielectric elastomer are used as actuators, respectively;
fig. 4 is a graph of mechanical force versus electrical force in an electroactive polymer based on a negative poisson's ratio dielectric elastomer.
Detailed Description
Compared with the traditional electroactive polymer, the electroactive polymer has higher shear modulus and rebound toughness and excellent properties in the aspect of mechanical properties. Under the excitation of different voltages or electric fields, the electroactive polymer can achieve different mechanical properties, the size and rigidity of the material can be changed in real time, the bearing capacity of the material is improved, the application range of the material is further expanded, and the integration, electronization, informatization and intellectualization of the elastic element, the vibration reduction element, the sensor element, the actuator element and the energy recovery element can be simultaneously realized.
The following further describes embodiments of the present invention with reference to the drawings.
The invention discloses an electroactive polymer based on a negative Poisson ratio dielectric elastomer, which comprises an elastic membrane and flexible electrodes on two sides 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.
As a further optimization of the electroactive polymer based on the negative poisson's ratio dielectric elastomer of the present invention, the porous dielectric elastomer material is extruded into a mold having three orthogonal dimensions that are smaller than its own dimension when three orthogonal compressive forces are applied to the porous dielectric elastomer material.
As a further optimization scheme of the electroactive polymer based on the negative Poisson ratio dielectric elastomer, the negative Poisson ratio dielectric elastomer material is made of negative Poisson ratio polyurethane or negative Poisson ratio polyolefin blend, and the flexible electrodes on two sides are made of any one of electrode carbon powder, silver paste, metal films, carbon resin, carbon nano tubes, hydrogel electrolyte and graphene.
FIG. 1(A) shows a schematic diagram of a conventional dielectric electroactive polymer, which is a sandwich-like sandwich structure, wherein the sandwich material is a conventional elastic film, and may be silica gel, acrylic, polyurethane or other dielectric elastomer material. The upper and lower sides are flexible electrodes, and materials such as electrode carbon powder, silver paste, metal films, carbon grease, carbon nanotubes, hydrogel electrolyte, graphene and the like can be adopted. The dielectricThe initial length, width and thickness of the electroactive polymer are respectively L1、L2、L3Wherein L is3Is 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 effect on the mechanical properties of the electroactive polymer.
FIG. 1B shows a schematic diagram of electromechanical deformation of a conventional dielectric electroactive polymer, wherein upper and lower flexible electrodes are respectively connected to positive and negative electrodes of a high voltage DC power supply, wherein the voltage of the high voltage DC power supply is phi, the dielectric electroactive polymer is equivalent to a capacitor, and current cannot pass through an elastic membrane, so that + -Q charges are respectively accumulated on the upper and lower flexible electrodes to generate electrostatic effect and coulomb force acting on the thickness direction of the dielectric electroactive polymer, thereby increasing the thickness of the electroactive polymer from L3Is reduced to l3The length and width dimensions are respectively from L1、L2Increase to l1、l2In this case, the stress states of the dielectric electroactive polymer in three directions are respectively P1、P2、P3. Phi, Q, P and l in this system3Are coupled state parameters, where a change in any one state affects the other three state parameters.
FIG. 2(A) shows a schematic diagram of an electroactive polymer based on a negative Poisson's ratio dielectric elastomer, also in a sandwich-like sandwich structure, where the sandwich material is an elastic membrane and a negative Poisson's ratio dielectric elastomer material is used. The upper and lower sides are flexible electrodes. The electroactive polymer has initial length, width and thickness dimensions L1、L2、L3Wherein L is3Is the sum of the thicknesses of the elastic membrane and the flexible electrodes on the two sides. The young's modulus of the flexible electrode material on both sides should be much smaller than that of the elastic membrane to reduce its effect on the mechanical properties of the electroactive polymer.
FIG. 2(B) shows the electromechanical deformation of electroactive polymer based on negative Poisson's ratio dielectric elastomer, wherein the upper and lower flexible electrodes are respectively connected with the positive and negative electrodes of high-voltage DC power supplyWherein, the voltage of the high voltage direct current power supply is phi, and the electroactive polymer is equivalent to a capacitor at the moment, and the current can not pass through the elastic membrane, so that +/-Q charges are respectively accumulated at the upper and lower flexible electrodes to generate electrostatic effect and form coulomb force to act on the thickness direction of the electroactive polymer of the dielectric elastomer with the negative Poisson ratio, so that the thickness of the electroactive polymer is L-L3Is reduced to l3And because the elastic membrane undergoes lateral contraction when subjected to a vertical load, the length and width dimensions of the electroactive polymer vary from L1、L2Is reduced to l1、l2In contrast to the response of conventional dielectric electroactive polymers. At this time, the stress states of the electroactive polymer in three directions are respectively P1、P2、P3. Phi, Q, P and l in this system3Are coupled state parameters, where a change in any one state affects the other three state parameters.
The invention discloses an electromechanical response estimation method of an electroactive polymer based on a negative Poisson's ratio dielectric elastomer, which comprises the following steps:
let the initial length, width and thickness of the dielectric elastomer based on the negative Poisson ratio be L1、L2、L3The voltage on two sides is phi, and the flexible electrodes on the upper and lower sides accumulate plus or minus Q charges respectively to make the thickness of the electroactive polymer from L3Is reduced to l3The length and width dimensions are respectively from L1、L2Is reduced to l1、l2The elongation in three directions is respectively lambda1＝l1/L1、λ2＝l2/L2And λ3＝l3/L3；
At the moment, the stress states of the electroactive polymer in three directions are respectively P1、P2、P3The true stress in three directions is σ1＝P1/l2l3、σ2＝P2/l1l3And σ3＝P3/l1l2(ii) a Reality of the elastic filmElectric field intensity E ═ phi/l3＝Φ/3L3The real potential shift is D ═ Q/l1l2；
The Helmholtz free energy of the electroactive polymer is F, and the density of the electroactive polymer is W ═ F/(L)1L2L3)；
The change in helmholtz free energy with minor perturbations is:
F＝P1l1+P2l2+P3l3+ΦQ (1)
wherein Q ═ Dl2l1+Dl1l2+l1l2D，l1、l2D is respectively under the condition of small disturbance1、l2A change in D;
dividing both sides of formula (1) by L1L2L3Then, there are:
W＝(σ1+ED)λ2λ3λ1+(σ2+ED)λ1λ3λ2+σ3λ1λ2λ3+Eλ1λ2λ3D (2)
the helmholtz free energy density is set as a function W (λ) of four independent variables1,λ2,λ3D), after the formula (2) is substituted:
due to lambda1、λ2、λ3And D are four independent variables, so in the equilibrium position:
the elastic film is subjected to a linear relationship between the electric field strength and the electric displacement, i.e., E ═ D/, where the dielectric constant of the dielectric elastomer is. Integrating the formula (2) with D and keeping lambda1、λ2And λ3Without change, we obtained:
wherein WsSubstituting the formula (5) and E ═ D/formula (4) as the strain energy function of the elastic film:
the strain energy function adopts an Ogden model, and then:
wherein alpha isi、uiAnd betaiThe material parameters of the elastic film can be obtained by fitting the material experimental data of the elastic film, wherein N is the order of the Ogden model, and i is a natural number which is more than or equal to 1 and less than or equal to N;
substituting formula (7) for formula (6) yields:
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 estimated from P1＝σ1l2l3、P2＝σ2l1l3、P3＝σ3l1l2And Φ ═ E λ3L3And (6) estimating.
Fig. 3(a) shows a schematic diagram of electromechanical deformation when a conventional dielectric type electroactive polymer film is used as an actuator, wherein both ends of the conventional dielectric type electroactive polymer film are fully constrained and bear a concentrated load F at the midpoint and are kept constant, and the dotted line is an equilibrium position before power-on. When a voltage is applied across a conventional dielectric electroactive polymer, its equilibrium position changes to that shown by the solid line, and its actuation displacement is seen to be downward. This phenomenon indicates that the conventional dielectric electroactive polymer film decreases in rigidity after being energized, and the higher the voltage, the smaller the rigidity.
Fig. 3(B) shows a schematic diagram of electromechanical deformation when an electroactive polymer based on a negative poisson's ratio dielectric elastomer, in which the two ends of the electroactive polymer membrane are fully constrained and subjected to a concentrated load F at the midpoint and held constant, is used as an actuator, and the dashed line is the equilibrium position before power-on. When a voltage is applied across the electroactive polymer film, its equilibrium position changes to that shown by the solid line, which is seen to displace upward in actuation, as opposed to a conventional dielectric electroactive polymer. This phenomenon indicates that the stiffness of the electroactive polymer film increases after energization, and the higher the voltage, the greater the stiffness.
Figure 4 shows the mechanical force versus electrical force in an electroactive polymer of a negative poisson's ratio dielectric elastomer. In the equilibrium state, the electric and mechanical forces are equal. When the voltage, charge and capacitance of the electroactive polymer change such 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, 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, i.e., point 2 is reached, during which part of the electric 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 continuously increases, the voltage of the flexible electrodes on both sides of the electroactive polymer increases, and finally the balance between the electric field force and the mechanical force is reached, that is, point 4 is reached, in the process, part of the mechanical energy is converted into electric energy. In the upper left region of the equilibrium curve, the electroactive polymer may be used as an actuator device, and in the lower right region of the equilibrium curve, the electroactive polymer may be used as an energy recovery (or generator) or sensor device.
Electroactive polymers, when used as actuator devices, convert electrical energy into mechanical energy based on the following basic principles: when the material is connected with a power supply, the electrodes on two sides of the electroactive polymer accumulate charges under the action of voltage, and the generated electric field force is compressed along the thickness direction, so that the area and the thickness of the electroactive polymer are reduced, and the electroactive polymer is displaced by a certain amount to achieve an actuating function. Different actuation requirements can be achieved by varying the applied supply voltage Φ and the load P experienced. Compared with the actuator made of the traditional dielectric electroactive polymer, under the action of an electric field force, the deformation directions of the electroactive polymer in the length direction and the width direction are opposite, the density and the rigidity of the material are increased nonlinearly along with the increase of the electric field force, and the actuator can bear higher power supply voltage phi and load P.
Electroactive polymers, when used as energy recovery (or generator), convert mechanical energy into electrical energy. The basic principle is as follows: when the electroactive polymer is subjected to a vertical load, the elastic membrane is forced to contract, and the thickness is reduced; a loop with relatively low voltage is connected to the flexible electrodes on the two sides of the material, and certain charges are gathered on the flexible electrodes on the two ends; a loop with relatively low voltage is disconnected, the size of the vertical load is reduced, the elastic membrane is gradually stretched, the thickness is increased, the charges of the flexible electrodes on the two sides are gradually pushed away, and the voltage is increased; and flexible electrodes at two ends of the material are connected with a loop with a relatively high voltage to output electric energy under the high voltage, so that energy recovery is realized.
Electroactive polymers, when used as sensor devices, convert mechanical energy into electrical energy. The basic principle is as follows: when the electroactive polymer is subjected to a vertical load, the elastic membrane is forced to contract, and the thickness is reduced; the flexible electrodes at two sides of the material are connected with a loop of a certain voltage, and certain charges are gathered on the flexible electrodes at the two ends; when the vertical load is reduced, the elastic membrane is gradually stretched, the thickness is increased, the capacitance is reduced, and the electric charge quantity of the flexible electrodes on the two sides is gradually reduced; when the vertical load is increased, the elastic film is gradually contracted, the thickness is reduced, the capacitance is increased, and the electric charge quantity of the flexible electrodes on the two sides is gradually increased. Therefore, by measuring the capacitance or the amount of charge on the flexible electrodes on both sides, the amount of change in the load can be calculated.
The negative poisson ratio dielectric elastomer belongs to a class of negative poisson ratio materials, and can generate negative poisson ratio characteristics when external mechanical force is applied, so that compared with the traditional dielectric electroactive polymer, the electroactive polymer can generate opposite deformation effect under the action of external excitation, and thus, more excellent performances can be obtained. For example, under the action of a load, the density, the rigidity and other characteristics of the electroactive polymer can be increased nonlinearly with the increase of the load due to the negative poisson ratio characteristic, so that the electroactive polymer can bear larger load, recover more energy and bear larger breakdown voltage compared with the traditional electroactive polymer.
By designing a certain control strategy and a control system, the multifunctional coupling of variable stiffness, actuation, energy recovery, sensing and the like of the electroactive polymer of the negative poisson ratio dielectric elastomer 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 cushioning elements, energy absorbing elements, vibration damping elements, spring-damper 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 will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The above-mentioned embodiments, objects, technical solutions and advantages of the present invention are further described in detail, it should be understood that the above-mentioned embodiments are only illustrative of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in 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.
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