Patent Grant
7242134
The present invention relates to an electromechanical actuator of the type utilizing a conducting polymer for effecting a desired action with change(s) in the volume of the polymer in response to an applied potential. The electromechanical actuator finds application in numerous fields of technology. There is also provided a method of providing the electromechanical actuator.
Electromechanical actuators based on conducting polymers are known in the art and find use as artificial muscles as well as in a myriad of other applications. Operation of such actuators is achieved by redox processes. The actuators may be arranged for providing a bending motion or a uniaxial force.
The bending or axial force is achieved as a result of volume changes in the conducting polymer upon application of a potential to the polymer. More particularly, the volume changes occur as a result of the injection or expulsion of counter ions into or from the polymer during these processes. For instance, upon electrochemically oxidizing a neutral conducting polymer film by the application of an anodic potential to the polymer in an electrolyte, positive charges are generated along the polymer resulting in counter ions being forced to enter the polymer from the electrolyte. As a result, there may be a significant increase in the volume of the conducting polymer. During reduction with a change in the potential, electrons are injected into the solid eliminating the positive charges and forcing the counter ions and solvated molecules to be expelled into the electrolyte. The result of this is that the volume of the conducting polymer decreases and the polymer returns to its neutral state. By harnessing the changes in volume, the electromechanical actuator can be utilized to achieve the desired action such as in the case of an artificial muscle. The expansion and contraction movement of the conducting polymer may also be utilized such as for example, for operating a piston in a miniature pump and other mechanical type applications.
However, attempts to improve the efficiency of performance of electromechanical actuators can impact on the mechanical and/or electromechanical properties of the actuator. It is, therefore, desirable to enhance efficiency of performance while minimizing any impact on the mechanical and/or electromechanical properties of such actuators.
It is an aim of the present invention to ameliorate one or more of the problems of the prior art or to at least provide a useful alternative.
In a first aspect of the present invention there is provided an electromechanical actuator for effecting a desired action, comprising:
a conducting polymer and a conductor for conducting voltage along the polymer from one end region of the polymer to an opposite end region of the polymer,
wherein the conductor is adapted for axially extending and retracting with axial expansion and contraction of the polymer, and extends along the polymer from the first end region to the opposite end region.
Typically, the conductor will be arranged for extending and retracting with axial expansion and contraction of the polymer. Hence in another aspect of the present invention there is provided an electromechanical actuator for effecting a desired action, comprising:
a conducting polymer and a conductor for conducting voltage along the polymer from one end region of the polymer to an opposite end region of the polymer;
wherein the conductor is arranged for axially extending and retracting with expansion and contraction of the polymer, and extends along the polymer from the first end region to the opposite end region.
Preferably, the conductor will be wound in a helix along the polymer. However, it will be understood that the conductor may be arranged in other configurations that allow the conductor to axially extend and contract in concert with the polymer. For example, the conductor may be arranged in a concertina pattern along the polymer.
Accordingly, in another aspect of the present invention there is provided an electromechanical actuator for effecting a desired action, comprising:
a conducting polymer and a conductor wound in a helix along the polymer for conducting voltage from one end region of the polymer to an opposite end region of the polymer.
Typically, the conductor of an actuator of the present invention will extend along a majority of the length of the conducting polymer and most preferably, along substantially the entire length of the conducting polymer.
Typically, the conductor will be in intimate contact with the conducting polymer along substantially the entire length of the polymer. Preferably, the conductor will be embedded in the polymer.
Preferably, the conducting polymer will be in the form of a tube. The tube may have a cross-section lying in a plane extending substantially perpendicularly to a longitudinal axis of the tube of any desired shape. Generally, the shape of the cross-section of the tube will be substantially circular.
Hence, in another aspect of the present invention there is provided an electromechanical actuator for effecting a desired action, comprising
a tube of conducting polymer having an internal passageway for receiving an electrolyte.
Preferably, the electromechanical actuator will further comprise an electrical connector for facilitating electrical connection to the conductor. Typically, the connector will also be in direct electrical contact with the polymer. Most preferably, an electrical connector will be connected to each end region of the conductor, respectively.
If desired, the further electrical connector may also be in direct electrical contact with the polymer at a spaced distance from the first mentioned electrical connector.
An actuator as described above may be arranged within an outer actuator comprising a tubular conducting layer allowing bundling of the actuators together.
Accordingly, in another aspect of the present invention there is provided an electrochemical actuator for effecting a desired action, comprising:
an outer actuator comprising a tubular conducting polymer having a hollow interior;
an inner actuator comprising a further conducting polymer; and
an electrolyte in a space defined between the inner actuator and the outer actuator;
wherein the actuator is arranged within the outer actuator and lies along the hollow interior of the outer actuator for axial extension or retraction within the outer actuator.
In yet another aspect of the present invention there is provided an electromechanical actuator for effecting a desired action, comprising;
an outer actuator comprising a tubular conducting polymer having a hollow interior;
an inner actuator comprising a further conducting polymer and a conductor for conducting voltage along the further polymer from one end region of the further polymer to an opposite end region of the further polymer; and
an electrolyte in a space defined between the inner actuator and the outer actuator;
wherein the conductor is adapted for axial extension or retraction with expansion or contraction of the further polymer and the inner actuator is arranged within the outer actuator and lies along the hollow interior of the outer actuator for axial extension or retraction within the outer actuator.
Preferably, the inner actuator will be arranged within the outer actuator for extension or retraction of the further conducting polymer in opposition to axial expansion or contraction of the tubular conducting polymer.
In another aspect of the present invention there is a method of providing an electromechanical actuator for effecting a desired action, comprising:
arranging a conductor for conducting voltage along a conducting polymer from one end region of the polymer to an opposite end region of the polymer and so that the conductor extends along the polymer from the one end region to the opposite end region for axially extending and contracting with expansion and contraction of the polymer.
Typically the method will further comprise embedding the conductor in the polymer. This may be achieved by coating the conductor with polymer using any suitable method known in the art.
An electrochemical actuator of the invention may for instance be manufactured by winding the conductor around a temple to form a helix along the template and subsequently coating the template and/or the helix with polymer. Preferably, the polymer will be coated on the conductor by electrodeposition.
Most preferably, the conducting polymer will be electrodeposited onto the template and the conductor.
Hence, in another aspect the present invention extends to a method of providing an electromechanical actuator for effecting a desired action, comprising:
electrodepositing a conducting polymer onto a conductor wound in a helix to form a tube of the polymer in which the helix is embedded.
In yet another aspect of the present invention there is provided a method of providing an electromechanical actuator for effecting a desired action, comprising:
(a) winding a conductor around a template to form a helix along the template; and
(b) electrodepositing a conducting polymer onto the helix to form a tube of the polymer in which the helix is embedded;
wherein the helix is in electrical contact with the tube of polymer for conducting a voltage along the tube from one end region of the tube to an opposite end region of the tube.
Typically, the conducting polymer will be electrodeposited onto the helix while the helix is wound around the template. Preferably, the method will further comprise the steps of:
(c) removing the template from the helix;
(d) connecting an electrical connector to one or each end region of the conductor respectively, for facilitating electrical connection with the conductor; and
(e) securing the conductor to the or each electrical connector.
Preferably, an electrical connector will be inserted into the one end region of the polymer and another said electrical connector into the opposite end region of the polymer.
In yet another aspect there is provided a method of providing an electromechanical actuator for effecting a desired action, comprising:
forming a polymer body on a conductor for conducting voltage along the polymer body from one end region of the polymer body to an opposite end region of the polymer body and axially extending and retracting wit expansion and contraction of the polymer body.
In another aspect of the present invention there is provided an electrochemical actuator for effecting a desired action, comprising;
a tube of conducting polymer having a longitudinal passageway for receiving an electrolyte.
In still another aspect of the present invention there is a method for providing an electrochemical actuator for effecting a desired action, comprising:
coating a template with a conducting polymer; and
removing the template to provide a tube of the polymer having a longitudinal passageway for receiving an electrolyte.
Generally, one end of the electromechanical actuator of the invention will be adapted for being secured to a support and an opposite end for being secured to a load in order to effect the desired action. The support and/or the load may be movable by the actuator relative to a fixed reference point upon actuation of the electromechanical actuator in use.
The conducting polymer may be any polymer capable of undergoing a volume change and/or change in one or more mechanical properties (eg. stiffness) in response to redox processes and which is deemed suitable for use in the provision of an electromechanical actuator of the type to which the present invention relates. The polymer may comprise a polymeric material consisting of a single polymer or a mixture of a number of different polymers. Accordingly, the term “conducting polymer” is to be taken to include within its scope a mixture of polymers one or more of which are capable of undergoing redox processes.
The conductor used for conducting voltage along the polymer will typically have greater conductivity (κ) compared to the conducting polymer utilized. The conductor may be formed from any material deemed suitable. For example, the conductor may be another conducting polymer such as a polyaniline fiber or thread. Preferably, however, the conductor will be a metal such as platinum, gold, silver or other metal with sufficient flexibility to extend and contract in concert with expansion and contraction of the conducting polymer. Most preferably, the conductor will be a wire.
Preferably, the template will also be conductive and most preferably, will be a length of metal such as a metal strip, wire or the like. Generally, the template will consist of the same material as used for the conductor.
The electrical connector(s) may be any short length of conducting metal. Preferably, the or each electrical connector will consist of the same material as used for the conductor.
The conductor will generally be secured to the or each electrical connector by wrapping the connector tightly around the connector(s) or by gluing, heat welding or other suitable means. Preferably, the conductor will be glued to the connector(s) by a suitable epoxy resin.
By forming the conducting polymer in the shape of a tube, it has been found that improved characteristics of the actuator may be obtained compared to the conducting polymer when provided in strip form. In particular, one or more of the electronic, mechanical and/or electrochemical properties of the actuator may be enhanced. While not being bound by theory, it is believed that a tube configuration has enhanced electrolytic efficiency compared to an actuator in the form of a strip as more of the conducting polymer comprising the tube is electrochemically accessible than a corresponding strip of the polymer.
It is further believed the provision of the conductor further enhances electrolytic efficiency by reducing voltage (iR) drops along the conducting polymer, enabling longer fibers to be used while retaining efficient activation capability.
Electromechanical actuators of the present invention find use in a broad range of applications. For example, an electromechanical actuator may be utilized as an artificial muscle in prosthetic or robotic applications, as an actuator in pumps for microfluidic applications, as an actuator in anti-vibration systems, as an actuator for electronic Braille screens, as an actuator in medical devices such as steerable catheters, and fibers in wearable prosthetics such as gloves that assist in gripping. Accordingly, an electromechanical actuator may be woven into a fiber or mesh, or otherwise lie along and be secured to fabric or mesh for causing movement of same upon actuation of the actuator.
Accordingly, in another aspect, there is provided apparatus incorporating an electromechanical actuator of the present invention. The electromechanical actuator will typically be provided in a suitable electrolyte. The electrolyte may be a liquid or solid electrolyte, and the actuator may be immersed in the electrolyte or otherwise coated with the electrolyte.
In yet another aspect of the present invention there is provided a device incorporating an electromechanical actuator for effecting a desired action together with an electrolyte, wherein the electromechanical actuator comprises:
A conducting polymer and a conductor for conducting voltage along the polymer from one end region of the polymer to an opposite end region of the polymer,
wherein the conductor is adapted for axially extending and retracting with expansion and contraction of the polymer, and extends along the polymer from the one end region to the opposite end region.
Preferably, the electrolyte will be an ionic liquid electrolyte. Most preferably, the ionic liquid electrolyte will comprise an ionic liquid selected from the group consisting of 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl) amide (EMI.TFSA) and 1-butyl-3-methylimidazolium hexafluorophosphate (MBI.PF6).
Hence, in still another aspect of the present invention there is provided a device incorporating an electromechanical actuator for effecting a desired action together with an ionic liquid electrolyte, wherein the electromechanical actuator comprises:
a conducting polymer and a conductor for conducting voltage along the polymer from one end region of the polymer to an opposite end region of the polymer;
wherein the conductor is adapted for axially extending and retracting with expansion and contraction of the polymer, and extends along the polymer from the one end region to the opposite end region.
Unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’) ‘comprising’, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
The features and advantages of the present invention will become further apparent from the following description of preferred embodiments of the invention.
a is a graph comparing stress generated by unplatinised and platinised polypyrrole films;
b is a schematic diagram illustrating the effect of iR (voltage) drop on the actuation of polypyrrole film;
Inherently conducting polymers (ICPs) such as polypyrroles, polythiophenes and polyanilines (I-III shown below) have attracted considerable attention over the past two decades.
The electronic and ion exchange redox properties of these materials has led to their use in application areas as diverse as chemical sensors, polymer batteries, polymer containing supercapacitors, membrane separations and artificial muscles. All of these applications utilise the chemical or physical changes that accompany the facile redox processes occurring at these polymers.
Electromechanical actuators based on inherently conducting polymers can be viewed as simple electrochemical cells in which the application of a potential creates dimensional changes in one or more of the electrode materials. The ability to efficiently inject or extract charge from the polymer(s) utilised without mechanical degradation of the system determines the overall actuator performance possible. Hence, the electrochemical properties of polymer(s) utilised dictate the level of performance obtainable.
Conducting polymers are oxidised/reduced according to Equations (1) and (2) set out below using polypyrrole as an example:
A− is a dopant anion, X+ is a cation from the supporting electrolyte, n is an integer of from 1 to X and is most usually 3 or 4. The symbol m represents the number of repeat units of the polymer thereby determining the molecular weight of the polymer.
For small mobile anions (A−) the process described by Eq. 1 predominates whereas for larger immobile anions (such as polyelectrolytes), processes described by Eq. 2 will predominate. In practice, for most anions, a mixture of both processes occurs. Accompanying anion expulsion (Eq. 1) is a decrease in volume of the conducting polymer. Alternatively, if cations are incorporated into the polymer (Eq. 2) during the redox reaction, the volume of the polymer increases(1). These dimensional changes may be translated into a bending motion using a bimorph(2) as illustrated in
To maximise energy efficiency, the conducting polymer should be oxidised/reduced at minimal potentials and the process not be limited by kinetic effects. However, with all conducting polymers the latter is an inherent limitation since movement of ions through the electrolyte and polymer is diffusion controlled.
Transitions induced by polymer oxidation/reduction may have an effect on the ability of a polymer to actuate(4). For instance, a polymer becomes more resistive (that is, resistance R increases) with electrochemical reduction making subsequent reduction or oxidation more difficult since:
E=Eapp−iR (3)
where E is the potential at the polymer, Eapp is the potential applied by an external power source and i is the current. Change in the electronic properties of the polymer makes efficient charge injection throughout the polymer, especially to the reduced state, desirable.
Chemical properties of a polymer can also change dramatically with properties such as hydrophobicity being dependent on the oxidation state(4). This, in turn, influences which electrochemical mechanism (Eq. 1 or 2) predominates. For example, if hydrophobicity of a polymer dramatically increases upon reduction it is easier to extract anions from the polymer than inject highly hydrated cations into the polymer.
In addition, mechanical properties of a polymer can be greatly influenced by lie potential applied(5) and hence, the redox state of the polymer. In this regard, a polymer can become significantly more ductile in the reduced state, and such changes in mechanical properties may well influence the efficiency of an electromechanical actuator.
The above illustrates that actuator performance and efficiency are dependent on the ability to inject or extract charge from the polymer with low energy consumption. The ease of charge injection/extraction is reflected in a parameter denoted as electrolytic efficiency (EE). The electrolytic efficiency is a measure of the ability to access all the available electrochemical sites of a polymer that can contribute to actuation. Specifically, the electrolytic efficiency of a system can be defined as:
The effect of polymer thickness on electrolytic efficiency of a polymer film deposited on a glassy carbon disc electrode (ie. with substantially ideal electrical connection) is shown as a function of polymer thickness in
While the polymer(s) and electrolyte used determine the maximum performance of an electromechanical actuator that can be expected, practical issues such as the efficiency of the electrical connection to the actuator may also be a limiting factor.
Improvement in actuation performance may be obtained by platinizing a conducting polymer film in order to minimize iR (ie voltage) drop effects along the length of the actuator as indicated in
For most efficient performance an actuator should not only allow efficient injection and extraction of charge, but should also desirably enhance or at least not interfere with the mechanical and electromechanical properties of the device. In the example illustrated in
The electrolyte used to facilitate the redox processes plays an important role. If the original dopant anion (A−) shown in the above examples is released during the reduction process, subsequent ICP oxidation will involve uptake of electrolyte anions. If on the other hand the reduction process leads to cation (X+) incorporation that is, the anion is not expelled during reduction, then X+ will be taken up from the electrolyte source.
The electrolyte also plays an important role in determining the performance of ICPs at extreme potentials. In particular, at extreme anodic potentials polypyrroles undergo irreversible degradation which is attributed to nucleophilic attack on the polymer from solvent or electrolyte ions. Breakdown of the solvent or electrolyte can also prevent some of the electronic properties of ICPs from being accessed. For example, at extreme negative potentials some polythiophenes can be n-doped18-20 provided the solvent or electrolyte does not break down before this occur.
Interestingly, the electrolyte can also have a direct impact on the mechanical properties of ICPs even after simple immersion before any electrical stimuli are applied. Variation in mechanical properties as the oxidation state of the polymer is changed has also been found to be electrolyte dependent.
In some electrochemical systems containing ICPs the use of conventional liquid electrolytes is not practical, with leakage or loss due to evaporation resulting in limited working life. Thus, there is a need for alternative systems. For example, both polyvinylalcohol (containing H3PO4) and polyethylene oxide (containing LiCF3SO3) platicised with poly(ethylene) glycol have been evaluated for use in polypyrrole based redox supercapacitors with specific capacitances in the range 40-84 Fg−1 being obtained. These electrolytes have ionic conductivities in the range 10−4-10−3 Scm−1. Alternatively, polyethylene oxide-based solid polymer electrolytes have been used in polypyrrole based batteries(7).
Actuators of the present invention are illustrated in
The manufacture the actuator will now be described with reference to
If desired, the template wire 4 may be left in position within the longitudinally extending interior passageway of the polymer tube rather than removing it as indicated above.
A further embodiment of an electromechanical actuator of the present invention is illustrated in
In use, the device is connected to a power supply such that the inner actuator functions as the cathode and the outer actuator the anode. Accordingly, when a voltage is applied, the inner actuator 4 axially expands and the outer actuator axially retracts. The inner and outer actuators therefore move axially in opposition to one another. The arrangement provides enhanced efficiency as the same electrolyte is used for obtaining axial movement of both actuators. The conducting polymers of the inner and outer actuators may be the same or different.
As will be appreciated, rather than an outer actuator as illustrated in
Suitable polymers that may be used in an actuator of the present invention include, but are not limited to polyaniline, polypyrrole and polythiophene, derivatives thereof, and mixtures of the foregoing.
Suitable derivatives include alkyl, alkoxy, amine and alcohol derivatives of polyaniline, polypyrrole, polythiophene. Such derivatives are well known in the art. Examples include poly(3-alkylthiophene)s, poly(3-alkylpyrrole)s, poly(methoxyaniline)s and poly(alkylaniline)s. Specific polymers that may find use in electromechanical actuators of the present invention include poly(3,4-ethylenedioxypyrrole), poly(3-heptoxythiophene), poly(3-dodecylthiophene), poly(2,5-dimethoxyaniline), poly(2,2-bithiophene) and poly(3,4-ethylenedioxythiophene) Of particular interest are inherently conducting polymers containing redox groups such as ferrocene that introduce additional charge (and thereby expansion) capacity such as for example poly[(3-ferrocenylpropyl) pyrrole]. Copolymers containing any of the monomers identified above are may also find application in an electromechanical actuator of the present invention.
In addition, blends of polymer systems may also be utilized. Blends of polymers may be obtained by mixing inherently conducting polymers with polyvinyl alcohol, polymethacrylate, poly(butylacrylate-vinyl acetate), polyurethane or polyester. Such blends may be used to improve the mechanical properties of the overall material and provide more processing options. Inherently conducting polymers containing selected additive components such as metallic fibers, particles or carbon nanotubules to improve strength and/or conductivity may also be utilized.
A range of dopants (A−) may also be incorporated into the conducting polymers such dopants are well known in the art. Of particular interest are those dopants that increase mechanical strength of the polymer such as sulphated poly(β-hydroxy ether) when used in polythiophenes and dopants incorporating redox groups such as poly(1-vinyl ferrocene-2-sulfonate) which may improve performance of the actuator.
While the conducting polymer may be electrodeposited onto the conductor of an actuator of the invention, the coating of the polymer may be achieved by any other suitable method. For instance, as indicated above, the conductor may be coated with the polymer by dipping the conductor into molten polymer. Alternatively, soluble polymers may be cast around the conductor and polymers that melt may be extruded around the template and conductor.
Desired characteristics of an electrolyte for use in electromechanical actuators include adequate conductivity, mechanical properties, adhesion during flexing and mechanical/electrical stability, easily processible and that the electrolyte be able to function in air. An electrolyte may for instance be contained in a porous membrane or a gel such as a polyacrylamide gel. Suitable electrolyte systems are also conventionally known in the art and any appropriate electrolyte may be utilized. For example, polyethylene glycol containing LiClO4 has been used in the past as a solid polymer electrolyte for polypyrrole based microactuators(8). The use of polyacrylonitrile or Kynar based non-aqueous electrolytes and water based polyacrylamide hydrogel ion source/sinks containing various perchlorate salts have also been investigated for their applicability in polypyrrole based actuators(9). Those studies indicated that the optimum electrolyte for use with polypyrrole based actuators was a polyacrylonitrile plasticized with propylene carbonate and ethylene carbonate containing 1.0 M NaClO4.
Further examples of electrolytes that may be used include simple salt electrolytes such as sodium chloride solutions and salt containing buffers. The salt concentration is typically in a range of from 0.1 to 1.0 M. Buffers that may be utilised include 0.1 to 1.0 M phosphate and acetate buffers.
Organic electrolytes such as acetonitrile or polypropylene carbonate containing salts such as tetraethyl ammonium hexafluorophosphate in a concentration typically in a range of from 0.1 to 0.25 M may also be used.
The pH of an electrolyte will depend on the inherently conducting polymer used. For polypyrrole and derivatives of this polymer, the pH will typically be in a range of from pH 2 to 7.
Preferred electrolytes also include ionic liquids (organic salts that are molten at or about room temperature) and particularly ionic liquids containing polymers. Ionic liquids function well as electrolytes in electrochemical actuator systems and have excellent environmental and electrochemical stability. Indeed, it has been shown that dramatically improved electromechanical efficiency and lifetime can be achieved when ionic liquids are used as the electrolyte in artificial muscle systems based on ICPs and that electrochromic windows based on ICPs using ionic liquids as electrolyte can be cycled in excess of 1,000,000 times(10). Examples of ionic liquid electrolytes include 1-butyl-3-methyl imidazolium hexafluorophosphate (BMI.PF6) and 1-ethyl-3-methyl imadazolium bis(trifluoromethanesulfonyl)amide (EMI.TFSA) and combinations of the cations and anions indicated in Scheme 1.
The invention will now be further described with reference to a number of examples.
For the purpose of demonstrating characteristics of electromechanical actuators of the invention, a number of actuators were prepared and tested using polypyrrole polymer with hexaflourophosphate (PF6−) as a dopant as indicated below.
wherein n=2-4, and m=the number of repeat units of the polymer.
Specifically, the actuators were prepared:
(a) as a strip by electrodeposition onto a platinum (Pt) plate from a solution containing 0.06 M pyrrole and 0.05 M PPy/PF6 in propylene carbonate at a current density of 0.15 mA/cm2;
(b) as a tube but with no helical wire conductor 2 using the method described above the reference to
(c) as a tube with a helical wire conductor 2 using the method described above with reference to the
The tube configuration results in improved electronic, mechanical and electrochemical properties as summarised below in Table 1. Mean values are shown.
The electrochemical efficiency of the tube configuration compared to the flat film indicates that more of the tube is electrochemically accessible than the corresponding strip. However, even with the tube configuration, enhanced electrolytic efficiency and actuation was obtained with just one and then both ends of the tube connected to the short wire inserts 8 and 10 suggesting improved electrical connection with the polymer was obtained utilising the wire inserts as indicated in Table 2.
A number of tube actuators of the invention incorporating helical conductors were prepared and the performance of three samples is shown in Table 3.
Resistance of the actuators were measured after locating wire inserts 8 and 10 in each end of the polymer tube, respectively.
All the polymer helices were between 45 and 55 mm long.
All polymer helices tested were from the same batch and prepared under a current density of 0.15 mA/cm2 for 24 hours.
A platinum (Pt) wire helix was used with a pitch of 25 turns/cm.
In all cases, the inclusion of the helical wire in the actuator resulted in improved electrochemical and actuator performance. By forming the conductor wire 2 into a helix, the wire is able to readily extend and contract in length with expansion and reduction of the volume of the polymer tube.
The effect of the pitch of the helical wire 2 on actuator performance was investigated. The results set out in Table 4 suggests use of low pitch provides better performance as indicated by the increase in strain obtained. The increase in strain at lower pitch is in agreement with the increase in electrochemical efficiency at lower pitch as indicated in
An actuator was prepared as described in Example 1 and illustrated in
All solutions were thoroughly deoxygenated with nitrogen prior to use. A current density of 0.15 mA/cm2 was applied. The polymerization solution consisted of propylene carbonate containing 0.06 M pyrrole momomer and 0.05 M tetrabutylammonium hexafluorophosphate (TBA.PF6). 1% (v/v) H2O was added to ensure that the water content was constant. The pyrrole, TBA.PF6 and propylene carbonate were obtained from Sigma Chemical Co. The pyrrole was distilled before use. The cell temperature was controlled at −28° C. The polymerisation process was maintained for 6 hours and a polymer tube formed around the platinum wire template. Following electrodeposition the platinum template was removed. The resulting hollow tube with the helical platinum wire conductor embedded in the inner wall was then washed with propylene carbonate and stored wet.
Electrical connection to the hollow tube was provided using two shorter platinum wires (1-1.5 cm) inserted into each end of the tube and subsequently glued in position with polystyrene hot melt. The electrical contact was checked by measuring the electrical resistance between the two ends. Values of 4 to 8 ohms were typically obtained.
The use of the ionic liquids BMI.PF6 and EMI.TFSA as electrolytes for the electromechanical actuator prepared in Example 4 was compared with propylene carbonate containing 0.1M tertiary butylammonium hexafluorophosphate (TBA.PF6).
1-butyl-3-methylimidazolium hexaflourophosphate (BMI.PF6) [IL1]
1-Ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl) amide (EMI.TFSA) [IL2]
Cyclic voltammetry studies were conducted in a three electrode electrochemical cell using a platinum working electrode onto which the PPy/PF6 composite had been deposited. A Ag/Ag+ reference electrode and platinum mesh auxiliary electrode were used and measurements conducted in the propylene carbonate (TBA.PF6) or ionic liquid electrolytes. Absorption spectra (250-1100 nm) were obtained using a Shimadzu Model UV-1601 spectrophotometer and polymer films grown galvanostatically (1 mA/cm2 for 3 min) on indium-tin-oxide (ITO) coated glass.
Using the propylene carbonate containing TBA.PF6 as electrolyte, degradation in the polymer electroactivity was observed if the anodic potential range was extended beyond +1.30 V. This degradation is attributed to the irreversible “over oxidation” previously observed for polypyrroles at anodic potentials where the polymer backbone becomes susceptible to attack from nucleophiles in the electrolyte. When the limit was extended to +1.55 V (
When the polypyrrole was deposited on ITO glass and cycled over the same potential range the changes in the UV-visible spectra observed at long wavelengths
Similarly, in EMI.TFSA (
Actuation and displacement tests on an electromechanical actuator prepared as in Example 4 were carried out in each of the electrolytes utilised in Example 5 using two different test systems. The displacement test involved the use of a two electrode cantilever system as illustrated in
The amount of displacement was measured from a video recording of the cantilever movement and the strain calculated according to the original length of the actuator. A repetitive pulse (500 mS duration) was applied over a number of potential ranges and the maximum strain as well as the average s rate over the 500 mS period determined. Specifically, 100 cycles were applied at each potential range selected starting with ±1 V and increasing to ±5 V. The strain data recorded were average values over the 500 mS period. The electrolytic efficiency was determined during these experiments from current flow. As expected, in all electrolytes the electrolytic efficiency, strain and strain rate increased with increasing magnitude of the applied potential (see for example Table 5).
At most potentials (all except ±5V) there is a higher electrolytic efficiency in propylene carbonate TBA.PF6 electrolyte than either ionic liquid electrolyte, reflecting the higher ionic conductivity of the propylene carbonate electrolyte compared with the ionic liquids. Interestingly, at ±5V the electrolytic efficiencies observed are approximately the same (50-56%) in all electrolytes, yet there is significant differences in the % strain generated. In the ionic liquid electrolytes a higher strain is generated for a given charge passed when a large potential is applied. These different characteristics reflect fundamental differences in the actuation mechanism of PPy.PF6 in ionic liquids compared with the propylene carbonate based electrolyte. The results may also be affected by Faradaic reactions occurring in the propylene carbonate electrolyte that do not contribute to actuation but still consume charge.
The actuator was then cycled over several thousand cycles using propylene carbonate (TBA.PF6) electrolyte. Each cycle involved application of ±5V (1 Hz). After 3,600 cycles the strain recorded decreased to 0.3% (
Actuator strain data was obtained using a conventionally known beam balance arrangement(10) and stain versus applied potential was plotted. The expansion contraction of the polymer was recorded using a linear variable distance transducer when a ±2.5V triangle waveform with a potential scan rate of 50 mV/s was applied. For EMI.TFSA this experiment was repeated under different loads. The results are also illustrated in
The direction of actuation was clearly shown to be reversed in the ionic liquids compared with the propylene carbonate based electrolyte (see
Further evidence that the cation motion dominates the actuation mechanism can be observed from the contrast in the maximum strain obtained form the two ionic liquids under conditions of rapid testing (such as cycling shown in
Another, unique, feature of actuation in ionic liquids compared to other electrolytes is the absence of osmotic effects. In ionic liquids there is no solvent present so osmosis does not occur. Interestingly, the strain amplitude, under equivalent conditions, is slightly higher for ionic liquid electrolytes (where osmosis is absent) compared with TBA.PF6 in propylene carbonate (where osmosis is expected).
From the above results, ionic liquids therefore provide an alternative electrolyte for electromechanical actuators. The wide electrochemical potential window provides an added degree of stability enabling many thousands of actuator cycles. In addition, an unexpected result has been obtained in terms of actuator mechanism where cation inclusion/expulsion predominates in determining the mode of actuation. The actuator performance in terms of strain under load and cycle life time, was significantly improved in the ionic liquid electrolytes compared with the conventional propylene carbonate based organic electrolyte used.
Although the present invention has been described hereinbefore with reference to a number of preferred embodiments, the skilled addressee will understand that numerous modifications and improvements are possible without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| PR9071 | Nov 2001 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU02/16080 | 11/25/2002 | WO | 00 | 9/7/2004 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
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| 20050006989 A1 | Jan 2005 | US |
