WO2003081761A2

WO2003081761A2 - Conducting polymer activators based on microporous asymmetric membranes

Info

Publication number: WO2003081761A2
Application number: PCT/US2003/008405
Authority: WO
Inventors: Hsing-Lin Wang; Benjamin R. Mattes
Original assignee: The Regents Of The University Of California
Priority date: 2002-03-21
Filing date: 2003-03-18
Publication date: 2003-10-02
Also published as: AU2003224714A8; WO2003081761A3; AU2003224714A1

Abstract

A method and apparatus for producing monolithic actuators (figure 6). Conducting polymer monolithic membranes, using polyaniline, polypyrrole, or polythiophene, are produced and used as monolithic actuators (figure 6). The actuators (figure 6) can be activated chemically, electrochemically, and by acidic vapors.

Description

CONDUCTING POLYMER ACTIVATORS BASED ON MICROPOROUS

ASYMMETRIC MEMBRANES

The present invention generally relates to conducting polymer actuators, and, more specifically to conducting polymer actuators based on polyaniline microporous asymmetric membranes. This invention was made with

Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION Conducting polymer actuators have been in use for some time to provide controllable bending when stimulated by electricity or chemicals. These actuators potentially can find application as artificial muscles or actuators with properties approaching those of natural muscles, as micro-machine components, as robotic components, and as medical instrumentation. This is possible because of the small size in which the polymer actuators can be made, and the long life cycles of which they are capable.

Most prior art conducting polymer actuators have been constructed by laminating a conducting polymer thin film onto a piece of nonconducting, flexible substrate, such as adhesive tape, polymer film, or a thin sputtered metal layer, for example, creating a bimorph structure whose volume is not affected by an electric field or other external stimuli. When such a structure is in the presence of a particular external stimulus, the conducting polymer may expand and the bimorph structure bends toward the substrate side. With different stimuli, the conducting polymer may contract, causing the structure to bend in the opposite direction, toward the conducting polymer side. As a conducting polymer expands due to electrochemical oxidation, counterions are incorporated as dopants in the conducting polymer matrix leading to volume expansion of the conducting polymer, while the volume of the associated flexible substrate remains constant. This results in a bending of the bimorph structure toward the substrate. Conversely, electrochemical reduction releasing counterions from the conducting polymer matrix causes contraction of the conducting polymer, and the bimorph structure bends in the opposite direction, toward the conducting polymer layer.

Many forms of these bimorph structures have been demonstrated. A bimorph structure has been made using two different conducting polymers that exhibit bending upon electrochemical doping. Others have constructed a bimorph polypyrrole/polyethylene chemical actuator that generates deflection upon exposure to ammonia gas

However, after many such cycles, these bimorph structures tend to delaminate and fail. This is due to the physical adhesion between layers cannot sustain repeated volume alteration at the interface. In addition, the efficiency of bimorph devices is reduced due to the weight of the bimorph device and to the amount of input energy that is consumed by the stress generated at the interface. Linear actuators also have been constructed by encapsulating conducting polymer films in one case and fibers in another in a polyelectrolyte matrix. With this structure, one or both ends is firmly fixed during a redox cycle so that the volumetric deformation generates linear strain. Recently, a novel polythiophene- based conducting polymer gel has been demonstrated from which linear actuators having a porous structure are produced. This porous structure allows facile diffusion of electrolyte and solvent in and out of the actuators.

A particular need exists for polymer actuators that will bend in the presence of many different stimuli, as well as operate for an extended period of time. With this sort of actuator available, the applications for such devices could quickly expand. The applications for such devices are myriad and include Micro Adaptive Flow Control (MAFC) technologies. MAFC enables, among other things, control of large-scale aerodynamic flows using small-scale actuators that are important in the design and performance of many critical airplane and submarine components. Other applications lie in micomachined structures, advanced robotics, amphibious machines, and medical devices.

It is therefore an object of the present invention to provide electrochemical actuators that cannot delaminate.

It is another object of the present invention to provide electrochemical actuators that have structural stability superior to that of conventional bimorph actuators.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a method of making microporous asymmetric membranes comprising the steps of pouring a concentrated conducting polymer solution onto a glass substrate; using a Gardner blade to create a uniform wet film of the concentrated polyaniline solution having a predetermined thickness; immersing the uniform wet film in a water bath; and drying the uniform wet film to obtain the microporous asymmetric membranes.

In a further aspect of the present invention, and in accordance with its principles and objectives a monolithic actuator comprises a chemical solution, with a conducting polymer monolithic membrane immersed in the chemical solution, the conducting polymer monolithic membrane bending in one direction when the chemical solution is acidic and bending in another direction when the chemical solution is basic. In a still further aspect of the present invention, and in accordance with its principles and objectives a monolithic actuator comprises a chemical solution, with a conducting polymer monolithic membrane immersed in the chemical solution. A source of electrical power is connected to the conducting polymer monolithic membrane such that the conducting polymer monolithic membrane bends in one direction for electrical power of one polarity and in another direction for electrical power of the other polarity.

In a still further aspect of the present invention, and in accordance with its principles and objectives a monolithic actuator comprising an acidic vapor, and a conducting polymer monolithic membrane surrounded by the acidic Vapor, where the conducting polymer monolithic membrane bends.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGURE 1 is a digital rendering of the membrane of the present invention bending when exposed to chemicals.

FIGURE 2 is scanning electron microscope view of the cross section of a membrane according to the present invention.

FIGURE 3 is a graph of bending angle versus time for a membrane according to the present invention immersed in chemicals of varying concentration.

FIGURE 4 is a graph of bending angle versus time for a membrane according to the present invention of varying thicknesses.

FIGURE 5 is a graph of bending angle versus time for a membrane according to the present invention in the presence of an acidic vapor. FIGURE 6 is a schematical illustration of a membrane according to the present invention connected to an electrode for an electrochemical application.

FIGURES 7A AND 7B are digital renderings of an electrochemical membrane according to the present invention bending to the left and right, respectively. DETAILED DESCRIPTION

The present invention provides monolithic electrochemical actuators that are capable of repetitive operations with no danger of delamination. The invention can most easily be understood through reference to the following examples and figures. For the production of membranes according to the present invention, polyaniline emeraldine base (EB) powder can be obtained from Neste Oy. N- methyl-2-pyrrolidone (99% Aldrich) and heptamethylenimine (98% Acros) are used with further purification.

A typical example of preparing highly concentrated polyaniline ("PANI") solution involved the following: 4.14 g of the N-methyl-2-pyrrolidone (NMP) was mixed with 0.747 g of the heptamethylenimine This mixture was placed inside a 10 ml Teflon® vial. 1.15 g of EB powder was added to this solution while stirring over a period of 5 second period. The resulting solution was then placed inside a 60 °C over for 20 minutes until the solution became homogeneous and flowed freely. The mass content of EB powder in this solution was 20% w/w. Based on this described procedure, concentrated EB solutions up to 35 wt% can be prepared.

An example of the preparation of polyaniline integrally skinned asymmetric membranes (hereinafter "PANI ISAMs") follows. Preparation of the PANI ISAMs is carried out using the phase inversion technique heretofore invented by others. The previously prepared concentrated PANI solution was poured on top of a glass substrate and a homogenous thin solution layer was made using the Gardner blade, followed by immersion in a nonsolvent bath, such as a water bath. Rapid solvent-nonsolvent exchange led to polymer precipitation and to the formation of a thin skin layer supported by a porous substructure.

Once the membrane had formed, it was left in the water bath for more than 3 days, with the water being replaced every 24 hours to ensure completed removal of the NMP and the heptamethylenimine. After 72 hours, the membrane was removed from the water bath, tap dried with a Kimwipe®, then air-dried for 24 hours. Finally, the membrane was placed inside a vacuum oven under dynamic pump for 24 hours.

The electrochemical actuators of the present invention are based on strips cut from these membranes. Typically, these actuators are approximately 50 mm long and 5 mm wide. In one test of the actuator, a strip of membrane was fixed at one end, with the other end free. The strip was immersed in an aqueous HCI solution that was at room temperature. The strip, almost immediately, began to bend. An angular plot was placed behind the actuator strip to monitor the bending angle using a Panasonic® Digital Video Camcorder model PV-DV800 to record the bending angle. This camera was connected to a Pentium III Dell® computer through an ATI DV-Wonder PCI card, and controlled using ULEAD® VideoStudio-5® software, Version 5.00.0006. Two still shots of the strip bending are illustrated in Figure 1. This combination of components allowed calculation of bending rates for the actuator strips. As will be discussed in more detail below, an acidic solution bend the actuator strips in one direction, and a basic solution bend them in the opposite direction.

The reason for this action of the actuator strips lies in the fact that the density of the PANI ISAMs varies from the dense skin side to the porous opposite side as shown in Figure 2 This Scanning Electron Microscope (SEM) view of a cross-section of a PANI ISAM shows this porosity density gradient that is important in determining the performance of chemical actuators. Immersing the actuator strips in an acidic solution causes doping of the polyaniline chains resulting in dopants (counterions) being drawn into the polymer network. Insertion of the counterions into the actuator membrane strips causes density- dependent volume expansions that vary within the membrane that causes bending toward the porous side. Placing such a membrane in a basic solution removes these counterions and causes bending toward the dense skin side. Testing has shown that immersing a 200 μm thick undoped PANI ISAM into a 1 M HCI aqueous solution and thereafter into a basic solution can go through an angle change of 180° in fewer than 5 seconds. This relatively rapid response is likely due to the porous membrane structure allowing rapid diffusion of dopants into and out of the membrane. Additionally, the thickness of the skin layer of the membrane is less than 0.3 μm, so little time is necessary for the doping to reach the interior of the membrane.

Detail tests were conducted to study the effect of the HCI concentration on actuator membranes prepared from 20 wt% EB solution. The results of this test are illustrated in Figure 3, where it is shown that the bending angle (angle per time period) increases as the HCI concentration increases. Higher concentration causes faster diffusion of dopants into the actuator membrane and is consistent with the hypothesis that the bending angle of the chemical actuator is governed by protonic doping. Similarly, adding LiCI salt to the HCI solution also results in a faster bending angle. This can be explained by the Donan phenomenon, where during the doping process the NaCI is added to the HCI solution, with chloride as the common ion. The common ion then amplifies the ion concentration difference at the polymer-water interface, thereby shifting the equilibrium and allowing more efficient protonic doping.

Additionally, the response time and bending motion of the chemically driven actuator membranes can be fined-tuned by other parameters, such as membrane thickness. As shown in Figure 4, the bending rate increases with decreasing membrane thickness. For example, more than 600 seconds was required for the 206 μm membrane in a 0.1 M HCI aqueous solution to bend 90°, but only 20 seconds for a 62 μm membrane to do the same. It should be noted that all the membranes used in producing Figure 4 were prepared from the same solution.

Actuators according to the present invention also have exhibited bending in the presence of various acid vapors. A PANI membrane was prepared according to the teachings of the present invention having a thickness of 155 μm. This PANI membrane was cut into strips of 50.0 mm in length and 5.0 mm in width. One such strip was fixed at one end by a clamp, leaving the opposite end free to move. An aqueous solution of 50 ml concentrated HCI was placed into a glass jar that thereafter was sealed for 10 minutes to allow the HCI vapor to fill the glass jar. After this period, the actuator strip was put into the glass jar above the liquid surface. After a short time, the actuator strip became doped by the HCI vapor and began to bend. The bending angle as a function of time was recorded and the movement of the free end of the actuator strip was calculated and is shown in Figure 5.

The present invention can also form electrochemical actuators. This is accomplished through application of a voltage to PANI ISAMs immersed in an electrolyte as shown in schematic form in Figure 6. In this case, platinum electrode 5_1 grasps an end of membrane 52, which has a skin side 52a and a porous side 52b. The membrane 52, as a monolithic actuator, can be based on a 60 μm thick PANI ISAM suspended in a 1.0M HCI aqueous solution. Referring now to Figures 6A and 6B, which are digital images, membrane 52 again is shown attached to platinum electrode 51 as well as reference electrode 61, which provides the other pole for connection of an electrical potential. As shown, membrane 52 bends to the left in Figure 7A with one polarity, and bends to the right in Figure 7B with the opposite polarity applied.

In testing, a square wave voltage was applied that ranged from -1 V to 1.5 V at a frequency of 0.25 Hz was applied between platinum electrode 5_1 and reference electrode 6 _. . Although the extent of the bending angle of membrane 52 decreases gradually with the number of cycles (from ± 30° to ±15°), the efficacious electrochemical behavior of membrane 52 not only remains intact, but also shows a slight increase. This result indicates a fairly stable structural and electrochemical ability of membrane 52 to operate over long periods of time. However, as is evident, the extent of the bending angle of membrane 52 does not reach the same degree as the actuators in a purely chemical environment. As indicated, electrochemical actuators bend a maximum of 45° to each side, while chemical actuators can reach up to 90° in each direction.

The basis of this difference lies in the spatially dependent internal stress- gradient of membrane 52. It is to noted that when the same PANI ISAM is exposed over long periods to acid or base solutions, it may attain total bending angles exceeding 180°, and ultimately to the formation of loose spirals, while a electrochemically driven PANI ISAM actuator exhibits considerably smaller bending angles through a complete redox cycle. This difference results from membrane 52 operating in a 1.0M HCI aqueous solution, where during the redox cycle, polyaniline undergoes conformational changes as its oxidation state changes from pernigraniline base to emeraldine base to leucoemeraldine base. Regardless of the oxidation states, PANI contains either secondary amine or a combination of secondary amines and tertiary amines. These amines are subject to protonation in acidic media. This means that the volumetric deformation of membrane 52 is not caused by dopants moving into and out of membrane 52, but by electrochemically induced conformational changes at different oxidation states. This all indicates that, while a particular PANI ISAM may exhibit good chemical actuator behavior, it may not also exhibit good electrochemical actuator behavior. The deformation mechanism of electrochemical actuators is likely affected by a combination of factors that contribute to changes in polymer conformation in the three oxidation states. Transforming this change in polymer conformation into bending movement requires intimate contact between polymer chains. Therefore, the present invention presents improved electrochemical actuators that have membranes with a denser structure of greater than 0.53 g/cm³, and a density gradient across the membrane.

Although the present invention has been described using polyaniline, it is most probable that the actuator operation described could be achieved with other materials. It is likely that other conducting polymers, such as polypyrrole and polythiophene and their derivatives could be used. The invention illustrates the potential advantages of constructing monolithic membrane actuators having fast response times, long working cycles, and high working efficiencies. The present invention is directed toward the development of high strength, fast response time, low operating potential conducting polymer actuators. The actuators of the present invention demonstrate controlled movement at a fine scale. Therefore, they can find application in the Micro Adaptive Flow Control (MAFC) technologies. MAFC enables control of large-scale aerodynamic flows using small-scale actuators that are critical in the design and operation of many critical aircraft and submarine components. Other potential applications of the present invention include use in micromachined structures, advanced robotics, amphibious machines, and medical devices.

The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

What is claimed is:

1. A method of making microporous asymmetric membranes comprising the steps of: pouring a concentrated conducting polymer solution onto a glass substrate; using a Gardner blade to create a uniform wet film of said concentrated polyaniline solution having a predetermined thickness; immersing said uniform wet film in a water bath; and drying said uniform wet film to obtain said microporous asymmetric membranes.

2. The method as described in Claim 1 wherein said concentrated conducting polymer solution is a concentrated polyaniline solution.

3. The method as described in Claim 1 , wherein said concentrated polyaniline solution is concentrated to greater than 10 wt%.

4. The method as described in Claim 1 , wherein said concentrated polymer solution is a concentrated polypyrrole solution.

5. The method as described in Claim 1 , wherein said concentrated polymer solution is a concentrated polythiophene solution.

6. A monolithic actuator comprising: a chemical solution; a conducting polymer monolithic membrane immersed in said chemical solution, said conducting polymer monolithic membrane bending in one direction when said chemical solution is acidic and bending in another direction when said chemical solution is basic.

7. The monolithic actuator as described in Claim 6, wherein said chemical solution is a HCI aqueous solution.

8. The monolithic actuator as described in Claim 6, wherein said conducting polymer monolithic membrane is a polyaniline monolithic membrane.

9. The monolithic actuator as described in Claim 6, wherein said conducting polymer monolithic membrane is a polypyrrole monolithic membrane.

10. The monolithic actuator as described in Claim 6, wherein said conducting polymer monolithic membrane is a polythiophene monolithic membrane.

1 1. A monolithic actuator comprising: a chemical solution; a conducting polymer monolithic membrane immersed in said chemical solution; a source of electrical power connected to said conducting polymer monolithic membrane such that said conducting polymer monolithic membrane bends in one direction for electrical power of one polarity and in another direction for electrical power of the other polarity.

12. The monolithic actuator as described in Claim 12, wherein said conducting polymer monolithic membrane is a polyaniline monolithic membrane.

13. The monolithic actuator as described in Claim 12, wherein said conducting polymer monolithic membrane is a polyaniline monolithic membrane.

14. The monolithic actuator as described in Claim 12, wherein said conducting polymer monolithic membrane is a polypyrrole monolithic membrane.

15. The monolithic actuator as described in Claim 12, wherein said conducting polymer monolithic membrane is a polythiophene monolithic membrane.

16. A monolithic actuator comprising: an acidic vapor; a conducting polymer monolithic membrane surrounded by said acidic vapor, wherein said conducting polymer monolithic membrane bends.

17. The monolithic actuator as described in Claim 16, wherein said conducting polymer monolithic membrane is a polyaniline monolithic membrane.

18. The monolithic actuator as described in Claim 16, wherein said conducting polymer monolithic membrane is a polypyrrole and its derivatives monolithic membrane.

19. The monolithic actuator as described in Claim 16, wherein said conducting polymer monolithic membrane is a polythiophene and its derivatives monolithic membrane.

20. The monolithic actuator as described in Claim 16, wherein said acidic vapor is a HCI acidic vapor.