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EP1161482A1 - Composite capable of rapid volume change - Google Patents

Composite capable of rapid volume change

Info

Publication number
EP1161482A1
EP1161482A1 EP00901491A EP00901491A EP1161482A1 EP 1161482 A1 EP1161482 A1 EP 1161482A1 EP 00901491 A EP00901491 A EP 00901491A EP 00901491 A EP00901491 A EP 00901491A EP 1161482 A1 EP1161482 A1 EP 1161482A1
Authority
EP
European Patent Office
Prior art keywords
composite
gel
conjugated polymer
polymer
change
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP00901491A
Other languages
German (de)
French (fr)
Inventor
Ib Johannsen
Peter Sommer-Larsen
Keld West
Jacob Hooker
Elisabeth Smela
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Riso National Laboratory
Original Assignee
Riso National Laboratory
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Riso National Laboratory filed Critical Riso National Laboratory
Priority to EP00901491A priority Critical patent/EP1161482A1/en
Publication of EP1161482A1 publication Critical patent/EP1161482A1/en
Withdrawn legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L101/00—Compositions of unspecified macromolecular compounds
    • C08L101/12—Compositions of unspecified macromolecular compounds characterised by physical features, e.g. anisotropy, viscosity or electrical conductivity
    • C08L101/14—Compositions of unspecified macromolecular compounds characterised by physical features, e.g. anisotropy, viscosity or electrical conductivity the macromolecular compounds being water soluble or water swellable, e.g. aqueous gels
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03G—SPRING, WEIGHT, INERTIA OR LIKE MOTORS; MECHANICAL-POWER PRODUCING DEVICES OR MECHANISMS, NOT OTHERWISE PROVIDED FOR OR USING ENERGY SOURCES NOT OTHERWISE PROVIDED FOR
    • F03G7/00—Mechanical-power-producing mechanisms, not otherwise provided for or using energy sources not otherwise provided for
    • F03G7/008—Mechanical-power-producing mechanisms, not otherwise provided for or using energy sources not otherwise provided for characterised by the actuating element
    • F03G7/012—Electro-chemical actuators
    • F03G7/0121—Electroactive polymers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03G—SPRING, WEIGHT, INERTIA OR LIKE MOTORS; MECHANICAL-POWER PRODUCING DEVICES OR MECHANISMS, NOT OTHERWISE PROVIDED FOR OR USING ENERGY SOURCES NOT OTHERWISE PROVIDED FOR
    • F03G7/00—Mechanical-power-producing mechanisms, not otherwise provided for or using energy sources not otherwise provided for
    • F03G7/029—Mechanical-power-producing mechanisms, not otherwise provided for or using energy sources not otherwise provided for characterised by the material or the manufacturing process, e.g. the assembly

Definitions

  • the present invention relates to a novel composite capable of rapid volume change, the use of said composite as well as an actuator comprising the composite.
  • An interpenetrating network of a polyelectrolyte gel and a ⁇ - conjugated electron conducting polymer was constructed by thermal cross-linking of a polyacrylic acid-polyvinyl alcohol mixture followed by gas state polymerisation of polypyrrole using FeCI to polymerise the polypyrrole.
  • the length variations of the composite was measured for various applied potential differences. In general the length variations are of 4 to 7 %, and are very slow, in that the changes appear within a period of 30 minutes up to several hours.
  • the object of the present invention is to provide a composite capable of rapid volume change, for example for use in an actuator in which the composite is controlla- ble by electrical and/or chemical stimuli. Furthermore, the strength of the composite is important.
  • the present invention relates to a composite comprising one conjugated polymer capable of changing oxidation-state upon the onset of an electrical or chemical stimulus, and an ion-sensitive gel capable of changing volume, the polymer and the gel being morphologically interrelated, and wherein the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer, and 50 % of the total volume change of the composite is achieved within a period of at most 5 minutes from the onset of the stimulus to the conjugated polymer.
  • the composite of the present invention has several advantages over composites previously described in the prior art. Firstly it changes volume with greater speed.
  • the composite described in the present invention is highly flexible and may there- fore accommodate large volume changes without mechanical failure. This is due to the polymeric nature of the electrode used to drive the ion-sensitive change, such as a pH change.
  • Another aspect of the present invention concerns the use of a composite for the production of an actuator.
  • the present invention relates to an actuator comprising a composite as defined by the invention.
  • the present invention relates to a sensor comprising a composite as defined below.
  • the actuator is an artificial muscle or an hydraulic actuator or a bilayer actuator or a micro actuator.
  • the present invention also relates to a process of causing a volume change in a composite comprising a conjugated polymer capable of changing oxidation-state and an ion-sensitive gel capable of changing volume comprising the steps of:
  • the present invention concerns a method of producing a composite as defined below, comprising the steps of:
  • the present invention relates to a method of producing an actuator comprising the steps of:
  • the conjugated polymer cause a 50 % of the total volume change of the composite in at most 5 minutes from the onset of a stimulus to the conjugated polymer.
  • Figure 1 A conjugated polymer characterised by alternating single and double bonds along its backbone.
  • Fig. 2a Illustrates different emulsions.
  • the morphology of these composites may be varied and include micelles (Fig. 2a), tubes (Fig. 2b), layers (Fig. 2c) and a less ordered structure (Fig. 2d).
  • This figure shows a multi-layered composite structure with alternating polymer and gel layers.
  • This figure illustrates the distance A, defined as the distance between two opposing surfaces of either the gel or the polymer.
  • This figure is an illustration of the force density.
  • the composite expands in the z- direction, and the force is delivered in the z-direction.
  • the force is divided by the area perpendicular to the z-direction (shown in grey).
  • Figure 7a and 7b show the expanded state and contracted state of two different composites, Fig. 7a with a volume changeable polymer and Fig. 7b, wherein the polymer does not contract. In both, the gel is depicted as white parts in the black matrix of polymer.
  • Figure 8a, b, c and d show different morphologies obtained for the PAni/Polyvinyl alcohol/Polyacrylic acid gels.
  • the gel is depicted as white parts in the black matrix, a) PAni/Gel (3/1 , w/w) dry film, low HCI during processing; b) PAni/Gel wet film 0.01 M HCI; c) PAni/Gel wet film 0.01 M NaOH; d) PAni/Gel (4/1, w/w) dry film, high
  • the object of the present invention is to provide a novel composite comprising a conjugated polymer and an ion-sensitive gel capable of rapidly changing its volume.
  • the conjugated polymers may undergo oxidation- reduction, or redox, reactions in which electrons are donated (reduction) or removed (oxidation) from the polymer. Ion transport may accompany this electron transport in order to maintain charge neutrality if the polymer is in contact with an electrolyte.
  • the gel of the composite according to the invention is a three-dimensional network of cross-linked macromolecular polyelectrolytes, in which the gel can be swollen by solvents.
  • the network may be created with various types of cross-linking points in the gel, such as chemical-covalent bonds, ionic bindings, hydrogen bonds and physically bindings, such as crystals and phase-separated areas.
  • the gel can hold a large fraction of solvent, typically water.
  • an ion-sensitive gel is meant a volume responsive gel sensitive to ionic strength, which in turn means that the gel changes volume in response to a change, in the gel phase, of pH or ionic strength.
  • ionic strength is meant changes in salt or ion concentrations, and in the present context pH changes are included.
  • a pH-responsive gel according to the invention changes volume due to a change in the charging of the polymer chain, which is a chemical reaction where acidic or basic groups on the polymer are ionised or neutralised.
  • An ionic strength responsive gel changes volume due to changes in the ion concentration of a solution, which is in equilibrium with the gel.
  • the concentration of ions inside the gel-phase is determined by the equilibrium.
  • the gel-phase comprises the whole system and it is the concentration of ions in the gel-phase, which are changed.
  • an ionic strength responsive gel will contract, when the ion-concentration is increased further. This is mainly due to a lowering of osmotic forces and due to screening of Coulomb forces between charged groups on the polymer chain. This is a general effect independent of the nature of the ions. The effect depends on the nature of the solvent.
  • the gel is a pH-sensitive gel.
  • An example of a gel sensitive to pH is a polyacid that reversibly contracts and expands upon exposure to acid and base (alkali).
  • a pH-sensitive gel is an extreme example of an ion- sensitive gel.
  • the raising or lowering of hydrogen-ion concentration in the solution primarily charge or uncharge the polymer chain.
  • a general effect of changing the concentration of other ions totally depends on the pKa of the acid in the example above.
  • PAN polyacrylonitrile
  • gel sensitive to salt concentration is cross-linked collagen, which undergoes chemical melting and crystallisation. This system is thus based on a phase transition or order-disorder transition.
  • the gel comprises a monomer or repeating monomer units independently selected from the group of acrylic acid, vinyl phosphate, acrylamide, styrene, urethane, ethylene oxide, propyl- ene oxide, vinyl alcohol, acrylonitrile, and collagen or a derivative thereof.
  • the oxidation-state of the conjugated polymer can be electrochemically controlled.
  • the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer. Thereby, the volume change is electrochemically controllable.
  • the gel or the conjugated polymer of the invention is "..in response to the change..” it is meant to indicate that the volume change of the gel or the conjugated polymer is caused as a direct or indirect consequence of the change of oxidation-state of the conjugated polymer.
  • the volume change of the composite may be due to the volume change of the gel.
  • the volume change of the gel may preferably be an expansion of at least 5 % of the starting volume or the most contracted volume.
  • the volume change of the gel may be an expansion of at least 7.5 % of the starting volume or the most contracted volume, and in an even more preferred embodiment the volume change of the gel may be an expansion of at least 10 % of the starting volume or the most contracted volume.
  • volume change is defined as an expansion.
  • a change of volume may also be a contraction.
  • the volume change may be at least a 20 % expansion, more preferred at least a 50 % expansion. Since the gel itself may be capable of volume changes of up to at least 800 %, even greater volume changes of the com- posite are envisaged.
  • the expansion of the composite may physically speaking be a substantially three dimensional expansion.
  • the expansion of the composite according to the invention may be substantially linear.
  • a substantially linear expansion is in the present context defined as a one-dimensional expansion.
  • the linear expansion of the composite may be up to 1.5 %, such as up to 3 %, or 5 %, or 7.5 %, or 10 %, or 20 %, or even up to 30 %.
  • the conjugated polymer may initiate a volume change in the gel, and the gel may act as a source/sink of ions for the conjugated polymer.
  • the ion concentration at the surface of the polymer changes upon electrochemical cycling as illustrated by equation 1-3 below.
  • the pH at the surface of the conducting polymers e.g. polypyrrole and polyaniline changes upon electrochemical cycling, falling during oxidation. The number of ions taken up and released by these polymers is very high.
  • doping refers to the oxidised state where electrons are re- moved from the polymer. This may be exemplified by the equation:
  • P + represents the doped (oxidised) state of the polymer and P° the undoped (reduced, neutral) state.
  • P°(A " ) indicates that the anion A ' is incorporated in the polymer as a dopant, (aq) represents an anion in the aqueous or solvent phase, and e " represents an electron. If the anion is immobile, then the following situation can occur:
  • the anion may be immobile i.e. it is not able to move in and out of the polymer.
  • the conjugated polymer of the composite of the present invention preferably comprises an immobile anion such as alkyl sulphates, alkyl sulphonates, dodecylben- zenesulphonate, polystyrenesulphonate, polyacrylic acid, National, polyacrylomido- propylsulfonic acid, ethylenglycol, ethanol, organic carbonates, and esters.
  • an immobile anion such as alkyl sulphates, alkyl sulphonates, dodecylben- zenesulphonate, polystyrenesulphonate, polyacrylic acid, National, polyacrylomido- propylsulfonic acid, ethylenglycol, ethanol, organic carbonates, and esters.
  • Some polymers are self-doped, i.e. the anion is a substituent on the polymer, and is thus immobile.
  • An example of a self-doped polymer is a
  • the conjugated polymer of the composite comprises any mobile anion such as tosylate, triflate, CIO 4 “ , BF “ , PF 6 “ , Cl “ , NO 3 ' , hydro- gensulphates and sulphates.
  • the conjugated polymer of the composite comprises an immobile cation.
  • the cation is preferably a mono- charged cation selected from tetra-alkyl ammonium salt, tri-alkyl ammonium salt, alkyl amines, polyimines and phosphonium salts.
  • volume change of the composite is dependent on the chemical components of the composite, and of the physical interrelations of the chemical components, i.e. the morphology of the gel and the conjugated polymer.
  • the composites may vary in chemical and physical nature.
  • the volume change of the composite is predominantly due to the volume change of the gel, and to a lesser extent due to the volume change of the conjugated polymer.
  • it is the equal volume change of both the gel and the conjugated polymer respectively which is responsible for the volume change of the composite.
  • it is primarily the volume change of the conjugated polymer, which causes a volume change of the composite, and secondarily the gel causing the volume change of the composite.
  • one embodiment of the present invention relates to a composite comprising one conjugated polymer capable of changing oxidation-state upon the onset of an electrical or chemical stimulus, and an ion-sensitive gel capable of changing volume, the polymer and the gel being morphologically interrelated and wherein the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer.
  • the volume change of the composite relies almost entirely on the volume change of the gel.
  • the speed of the volume change of the composite is particularly fast due to the nature of the composite, which enables a greater volume change because it is the gel rather than the conjugated polymer determining the volume change.
  • the volume change of the composite is caused by the equal volume change of the gel and the conjugated polymer respectively, in response to a change in oxidation level of the polymer.
  • the volume change of the composite is predominantly due to the volume change of the conjugated polymer in response to a change in oxidation level of the polymer.
  • the volume change of the polymer is estimated to be at the most 10 %.
  • the total volume change of the composite according to the invention must be achieved within a time frame of at the most 1 hour.
  • the total volume change is defined as being the volume change obtained after 1 hour after the onset of a stimulus described by the invention.
  • at least 50 % of the total volume change of the composite is achieved within a period of 5 minutes from the onset of the electrical stimulus to the conjugated polymer, preferably at least 50% of the volume change is achieved within a period of less than 10 seconds, more preferably less than 1 second, even more preferably less than 0.1 second.
  • the speed of the response is defined by the time it takes the composite to change volume by an amount (V ma ⁇ - V m ⁇ n )/2, where V m i n is the fully contracted volume and V m ax is the fully expanded volume.
  • the total volume change has taken place within the periods defined above. Accordingly, preferably the total volume change has occurred within a period of 1 second.
  • substantially the same direction is meant that the polymer and the gel, respectively are acting in the same reaction and that their respective volume changes are not counteracting one another.
  • both components either expand or contract upon an electrical or chemical stimulus.
  • equation (2) or (3) mentioned above applies.
  • the conjugated polymer contracts, expelling cations into and lowering the pH of the gel.
  • a pH-sensitive gel with ionizable acid groups responds by also contracting, and/or the gel responds to the increased cation concentration by contracting.
  • the gel "gets out of the way" of the conjugated polymer, as illustrated in Fig. 7a, allowing the composite to change volume by ⁇ V e p * Fep+ ⁇ Vgei * Fg e ⁇ , where ⁇ V ep is the volume change of the conjugated polymer, ⁇ V ge ⁇ the volume change of the gel, and F the volume fraction of each component.
  • the process is reversed
  • equation (1) above applies.
  • the conjugated polymer expands, taking up anions and lowering the pH of the gel.
  • the pH-sensitive gel for example with ionizable base groups, responds by also expanding, and/or the gel responds to the lowered anion concentration by expanding.
  • the volume change in the conjugated polymer magnifies the volume change in the gel. In the figure above, the arrows are reversed during oxidation.
  • Another preferred combination is a conjugated polymer that does not change volume and a gel that changes volume in response to the external signal.
  • a conjugated polymer that does not change volume and a gel that changes volume in response to the external signal.
  • An example of this type a PPy layer that does not change volume significantly, but a polyacryl acid gel that responds to the ion fluxes induced by the polypyrrole, contracting dur- ing oxidation and expanding during reduction, vide Fig. 7b.
  • the oxidation state of the conjugated polymer can be electrochemically controlled, whereby the response of the whole composite depends on the voltage applied.
  • the predetermined volume change is obtained by applying from -3V to
  • +3V to the polymer such as from -1.5V to +1.5V, or even -1.0V to 0V.
  • the pH at the surface changes from 10.5 at -0.6V (vs. Ag/AgCI) to 3.5 at +0.6V.
  • the voltage may be applied by any suitable means such as an elec- trode, which is an integral part of the composite.
  • the stimulus to the composite is chemical.
  • the chemicals are preferably iodine or ammonia.
  • the term morphological interrelated defines the physical relationship between the two components in the composite, namely the conjugated polymer and the gel.
  • the conjugated polymer is in intimate contact throughout its volume with the gel. This intimate contact is a feature providing for the rapid volume change of the composite.
  • the structure of the composite according to the invention may have any suitable structure, for example an interpenetrating network, or an emulsion, or the structure may be layered.
  • An interpenetrating network is a blend of two materials on the molecular level. Each material forms a connected structure throughout the composite.
  • the composite components do not blend on a molecular level but blend in small regions of each type of component (see Figure 2a-2d).
  • the composite may be a layered structure, such as a multi-layered structure, in which the conjugated polymer and gel are placed in separate phases.
  • the layers may be relatively thick, for example several micrometers.
  • the layers may be joined in a variety of ways, such as gluing or melting them together, polymerising one ma- terial over the other, dipping one into the other and co-extrusion.
  • the multi-layers may be made from a large sheet consisting of one . layer of each material that is folded or rolled.
  • the composite may have the geometry of fibres or fibre bundles of one component in a matrix of the other component (see Figure 4).
  • the conducting polymer regions must preferably be connected throughout the composite, which means that a current is able to pass through the whole conducting material.
  • the dimensions of the polymer and the gel are of great importance to the speed with which the composite of the present invention changes its volume. If each layer or region of either polymer or gel, in particular gel is too small, the composite will be unable to change volume, possibly due to an insignificant capability of swelling by solvent uptake
  • each layer or region of either polymer or gel is preferably more than 50 Angstrom, more preferred more than 250 A, and even more preferred more than 500 A.
  • each layer or region is preferably not more than 1 mm in at least one plane. This may be defined in the following statement: in any point throughout the gel or the polymer, distance A (defined below) is less than 1 mm in at least one plane through said point. In the present context, the distance A is defined as the distance between two opposing surfaces of either the gel or the polymer ( Figure 5).
  • Flexibility of the composite is another important feature. In order to accommodate the large volume changes without mechanical failure, the composite must be flexible. This is ensured by the component of the composite being flexible at all stages of contraction and expansion.
  • the flexibility of the composite is due to the polymer material.
  • the electrode used to apply stimulus is polymeric rather than metallic, the flexibility is increased.
  • the strength of the composite i.e. the strength to push against a load
  • the term strength may be defined as the measurement of force density.
  • the force density is defined as the force divided by the area of the composite orthogonal to the measured force. The force density is measured in the direction in which the expansion or contraction of the composite is utilised (see Figure 6).
  • the force generated by the composite may be measured using a balance, load cell another force transducer, or weights.
  • Another way to calculate the force is by multiplying the expansion of the composite by the Young's modulus.
  • the expansion in each direction may be measured using a ruler, profilometer, or other means.
  • the Young's modulus may be measured using a rheometer or a similar device. It is important to stress that the measurement of the Young's modulus must be carried out in the same physical environment as the one in which the composite operates. This is due to the fact that the Young's modulus is affected by whether a polymer or a gel is either wet or dry. According to the present invention the Young's modulus is measured in a liquid environment.
  • the Young's modulus is at least 10 MPa (mega Pascal), preferably at least 100 MPa, more preferably at least 500 MPa, yet more preferably at least 1000 MPa, even more preferably at least 2000 MPa, and most preferably at least 3000 MPa.
  • the mechanical failure by overloading, ripping or cracking may be defined by the ultimate elongation and failure stress.
  • the ultimate elongation is a measure of how much the composite may be passively elongated without breaking.
  • the ultimate elongation is preferably up to 35 % of the original length, such as up to 25 %.
  • Failure stress is a measure of the force necessary to pull the composite apart.
  • the failure strength is preferably as high as possible, such as at least 2 MPa, preferably above 3 MPa, such as 4-12 MPa.
  • the composite may be used for any purpose where its capability of controllable volume change and other characteristics are useful, such as an actuator. Accordingly, in another aspect of the present invention the composite is used for the production of an actuator.
  • an actuator or a sensor may comprise a composite as defined above.
  • the actuator may be an artificial muscle or an hydraulic actuator or a bilayer actuator or a micro actuator.
  • the industrial applications of the actuators comprising a composite according to the present invention are vast. Some of the applications may be as dextrous robotic grippers, smart valves, ad- vanced consumer products and toys. Other potential applications are within the medico technical field, such as stearable catheters and other minimal invasion surgical instruments.
  • Yet another aspect of the present invention relates to the process of causing a vol- ume change in a composite as described above comprising the steps of applying an electrical or chemical stimulus to the conjugated polymer, causing a change of the oxidation-state of the polymer, and arranging the ion-sensitive gel in contact with the conjugated polymer in a predetermined pattern of morphological interrelation in order to allow the gel to respond to the change in oxidation-state of the polymer by a change in volume.
  • a further aspect of the present invention concerns a method of producing a composite as defined above comprising the steps of selecting a conjugated polymer and an ion-sensitive gel, then combining the conjugated polymer and the ion-sensitive gel in a predetermined pattern of morphological interrelation in order to allow the composite to react followed by applying an electrical or chemical stimulus to the conjugated polymer and having the change in oxidation-state of the conjugated polymer cause a 50 % of the total volume change of the composite in at most 5 minutes from the onset of the stimulus to the conjugated polymer.
  • the following is an example of a composite of a conducting polymer and an ion-sensitive gel having electro-chemical properties and being capable of swelling in basic solution.
  • a morphologically interrelated composite film of polyaniline with a polyvinylalcohol (PVA)/polyacrylic acid (PAA) gel (PAni) was developed.
  • a solution of PVA, PAA and PAni in N-methyl-2-pyrroiidinone (NMP) was prepared.
  • NMP N-methyl-2-pyrroiidinone
  • G glutaraldehyde
  • HCI glutaraldehyde
  • a film of this solution was heated which resulted in a phase separated structure and a concomitant cross-linking of the PVA and PAA to form a PAni/gel composite.
  • Various composite structures were obtained. Examples of these structures are included below in Figure 8 (e.g. compare Figure 8a and 8d).
  • the gel composite shrank and expanded upon addition to acidic and basic solutions, respectively.
  • the film bent towards the Kapton® upon reduction and vice versa upon oxidation in acidic solution (0.01 M HCI, -0.2V/0.85V, Ag/AgCI ref.).
  • the PAni/gel composite showed good electro-chemical activity. It is expected that upon oxidation in strong acidic solution (e.g. 1 M HCI), that PAni in emeraldine salt (ES) state will contract. This will produce a bending away from the Kapton® upon oxidation. This would suggest that for the composite the bending may be due to the pH change induced by the PAni.

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Abstract

The present invention relates to a composite capable of rapid volume change, the use of said composite as well as an actuator comprising the composite. Thus, the invention relates to a composite comprising one conjugated polymer capable of changing oxidation-state upon the onset of an electrical or chemical stimulus, and an ion-sensitive gel capable of changing volume, the polymer and the gel being morphologically interrelated, and wherein the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer, and 50 % of the total volume change of the composite is achieved within a period of at most 5 minutes from the onset of the stimulus to the conjugated polymer.

Description

Composite capable of rapid volume change
The present invention relates to a novel composite capable of rapid volume change, the use of said composite as well as an actuator comprising the composite.
Background of the invention
The development of polymer actuators for orthotics, prosthetics and advanced robotics has been a subject of much research.
For example, Okuzaki H. and Kunugi T. ("Rapid Communication - Electrically induced Contraction of Polypyrrole Film in Ambient Air" Journal of Polymer Science: Part B: Polymer Physics, Vol. 36, pp. 1591-1594 (1998)) found that a polypyrrole solid film contracts by applying a DC voltage in ambient air. The degree of contrac- tion attained 1.5 % by applying 3 V.
A higher degree of contraction has been shown in for example Sawai T. ("Electrical control of the volume of pH-sensitive micro-gels", J. Electroanal. Chem., Vol. 297 (1991), pp. 399-407), where it was demonstrated that the volume of a pH-sensitive gel was electrically controlled in a pH modulator cell composed of a polypyrrole- coated platinum mesh electrode. The pH-sensitive gel was dispersed in an aqueous solution. The gel volume was reversibly modulated by approx. 30 % in response to the electrically controlled pH of the solution. A time lag was observed supposedly due to the slow diffusion of the ions in the solution. However, this laboratory set-up was not useful as an actuator because of the gel being dispersed in the aqueous solution.
Others have conducted tests wherein an interpenetrating network of a polymer and a gel has been made. An example is Mansouri J. et al ("Assembly of conducting polymer networks inside hydrogel structures", SPIE Vol. 2716 (1996), pp. 8-19) describing the assembly of a hydrogel structure wherein a conducting polymer has been grown (polymerised) in the pores of the hydrogel creating a network of the two components. Another example is Chiarelli P. et al ("A polymer composite showing electrocontrac- tile response", Polymer Gels (1991 ), pp. 195-205, ed. by D. De Rossi et al, Plenum Press, New York). An interpenetrating network of a polyelectrolyte gel and a π- conjugated electron conducting polymer was constructed by thermal cross-linking of a polyacrylic acid-polyvinyl alcohol mixture followed by gas state polymerisation of polypyrrole using FeCI to polymerise the polypyrrole. The length variations of the composite was measured for various applied potential differences. In general the length variations are of 4 to 7 %, and are very slow, in that the changes appear within a period of 30 minutes up to several hours.
Summary of the invention
The object of the present invention is to provide a composite capable of rapid volume change, for example for use in an actuator in which the composite is controlla- ble by electrical and/or chemical stimuli. Furthermore, the strength of the composite is important.
Accordingly, the present invention relates to a composite comprising one conjugated polymer capable of changing oxidation-state upon the onset of an electrical or chemical stimulus, and an ion-sensitive gel capable of changing volume, the polymer and the gel being morphologically interrelated, and wherein the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer, and 50 % of the total volume change of the composite is achieved within a period of at most 5 minutes from the onset of the stimulus to the conjugated polymer.
The composite of the present invention has several advantages over composites previously described in the prior art. Firstly it changes volume with greater speed.
Other advantages of the present composite is an improved strength determined by the conjugated polymer. Further the oxidation state of the conjugated polymer can be electrochemically controlled and thus the response of the composite depends on the applied voltage.
The composite described in the present invention is highly flexible and may there- fore accommodate large volume changes without mechanical failure. This is due to the polymeric nature of the electrode used to drive the ion-sensitive change, such as a pH change.
Another aspect of the present invention concerns the use of a composite for the production of an actuator.
Further the present invention relates to an actuator comprising a composite as defined by the invention.
In another aspect the present invention relates to a sensor comprising a composite as defined below.
In yet another aspect of the present invention the actuator is an artificial muscle or an hydraulic actuator or a bilayer actuator or a micro actuator.
The present invention also relates to a process of causing a volume change in a composite comprising a conjugated polymer capable of changing oxidation-state and an ion-sensitive gel capable of changing volume comprising the steps of:
- applying an electrical or chemical stimulus to the conjugated polymer, causing a change of the oxidation-state of the polymer,
arranging the ion-sensitive gel in contact with the conjugated polymer in a predetermined pattern of morphological interrelationship in order to allow the gel to respond to the change in oxidation-state of the polymer by a change in volume.
Additionally the present invention concerns a method of producing a composite as defined below, comprising the steps of:
-selecting a conjugated polymer and an ion-sensitive gel
-combining the conjugated polymer and the ion-sensitive gel in a predetermined pattern of morphological interrelation in order to allow the composite to react to an electrical or chemical stimulus, and thereby having the change in oxidation- state of the conjugated polymer cause 50 % of the total volume change of the composite in at most 5 minutes from the onset of the stimulus to the conjugated polymer.
Furthermore the present invention relates to a method of producing an actuator comprising the steps of:
-selecting a composite as defined above
-applying an electrode to the composite in such a manner that the conjugated polymer is able to change oxidation-state, thereby obtaining an actuator, wherein the conjugated polymer cause a 50 % of the total volume change of the composite in at most 5 minutes from the onset of a stimulus to the conjugated polymer.
Drawings
The following is a description of the drawings mentioned in the text.
Figure 1 A conjugated polymer characterised by alternating single and double bonds along its backbone.
Figure 2
Illustrates different emulsions. The morphology of these composites may be varied and include micelles (Fig. 2a), tubes (Fig. 2b), layers (Fig. 2c) and a less ordered structure (Fig. 2d).
Figure 3
This figure shows a multi-layered composite structure with alternating polymer and gel layers.
Figure 4
Shows a cross-section of a composite structure in the form of fibres or fibre bundles
(black dots) of the gel component in a matrix of the polymer. Figure 5
This figure illustrates the distance A, defined as the distance between two opposing surfaces of either the gel or the polymer.
Figure 6
This figure is an illustration of the force density. The composite expands in the z- direction, and the force is delivered in the z-direction. The force is divided by the area perpendicular to the z-direction (shown in grey).
Figure 7
Figure 7a and 7b show the expanded state and contracted state of two different composites, Fig. 7a with a volume changeable polymer and Fig. 7b, wherein the polymer does not contract. In both, the gel is depicted as white parts in the black matrix of polymer.
Figure 8
Figure 8a, b, c and d show different morphologies obtained for the PAni/Polyvinyl alcohol/Polyacrylic acid gels. The gel is depicted as white parts in the black matrix, a) PAni/Gel (3/1 , w/w) dry film, low HCI during processing; b) PAni/Gel wet film 0.01 M HCI; c) PAni/Gel wet film 0.01 M NaOH; d) PAni/Gel (4/1, w/w) dry film, high
HCI during processing.
Detailed description of the invention
The object of the present invention is to provide a novel composite comprising a conjugated polymer and an ion-sensitive gel capable of rapidly changing its volume.
The conjugated polymer (= conducting polymer) of the composite according to the invention is an organic macromolecuie characterised by alternating single and dou- ble bonds along its backbone. The conjugated polymers may undergo oxidation- reduction, or redox, reactions in which electrons are donated (reduction) or removed (oxidation) from the polymer. Ion transport may accompany this electron transport in order to maintain charge neutrality if the polymer is in contact with an electrolyte. In a preferred embodiment the conjugated polymer_comprises monomer units selected independently from the group of pyrrole, aniline, thiophene, acetylene, phen- ylene, p-phenylene vinylene, p-phenylene sulphide, thienylenevinylene, furane, azulene, and isothianaphthene or a derivative thereof.
The gel of the composite according to the invention is a three-dimensional network of cross-linked macromolecular polyelectrolytes, in which the gel can be swollen by solvents. The network may be created with various types of cross-linking points in the gel, such as chemical-covalent bonds, ionic bindings, hydrogen bonds and physically bindings, such as crystals and phase-separated areas. Thus, the gel can hold a large fraction of solvent, typically water. By an ion-sensitive gel is meant a volume responsive gel sensitive to ionic strength, which in turn means that the gel changes volume in response to a change, in the gel phase, of pH or ionic strength. By ionic strength is meant changes in salt or ion concentrations, and in the present context pH changes are included. A pH-responsive gel according to the invention changes volume due to a change in the charging of the polymer chain, which is a chemical reaction where acidic or basic groups on the polymer are ionised or neutralised.
An ionic strength responsive gel changes volume due to changes in the ion concentration of a solution, which is in equilibrium with the gel. The concentration of ions inside the gel-phase is determined by the equilibrium. In one extreme, the gel-phase comprises the whole system and it is the concentration of ions in the gel-phase, which are changed.
In equilibrium, a balance between elastic forces of the cross-linked polymer network, osmotic forces due to free ions in the gel-phase, interactions between the polymer and the solution, and interactions in-between the polymer chains, determine the volume.
At a high ion concentration of the solution in equilibrium with the gel, typically for concentrations (more specific - an ionic strength) above 0.1 M in water, an ionic strength responsive gel will contract, when the ion-concentration is increased further. This is mainly due to a lowering of osmotic forces and due to screening of Coulomb forces between charged groups on the polymer chain. This is a general effect independent of the nature of the ions. The effect depends on the nature of the solvent.
In one aspect of the present invention, the gel is a pH-sensitive gel. An example of a gel sensitive to pH is a polyacid that reversibly contracts and expands upon exposure to acid and base (alkali). A pH-sensitive gel is an extreme example of an ion- sensitive gel. Here the raising or lowering of hydrogen-ion concentration in the solution primarily charge or uncharge the polymer chain. Hence, a general effect of changing the concentration of other ions totally depends on the pKa of the acid in the example above.
Exposure to base produces charges on the polymer chain, which are balanced by free counter-ions. The gel expands due to an uptake of water driven by osmotic forces. An electrostatic repulsion of ions on the polymer chain also causes an ex- pansion of the coiled chain structure that exists in an acid environment.
Furthermore, polyacrylonitrile (PAN) can be treated to produce pH-sensitivity with induced expansions that can be up to 800 %. The expansion and contraction is explained as two states corresponding to two different dielectric constants of the solu- tion in the gel. This gel is also sensitive to osmotic pressure or ion concentration.
An example of a gel sensitive to salt concentration is cross-linked collagen, which undergoes chemical melting and crystallisation. This system is thus based on a phase transition or order-disorder transition.
In another preferred embodiment of the present invention the gel comprises a monomer or repeating monomer units independently selected from the group of acrylic acid, vinyl phosphate, acrylamide, styrene, urethane, ethylene oxide, propyl- ene oxide, vinyl alcohol, acrylonitrile, and collagen or a derivative thereof.
It is the purpose of the present invention to provide a composite having superior physical and chemical properties in particular with respect to control of volume change and amount of volume change. This is achieved by taking advantage of the conjugated polymers capability of changing oxidation-state upon the onset of an electrical or chemical stimulus. Thus, the oxidation-state of the conjugated polymer can be electrochemically controlled. The volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer. Thereby, the volume change is electrochemically controllable.
When stating that the gel or the conjugated polymer of the invention is "..in response to the change.." it is meant to indicate that the volume change of the gel or the conjugated polymer is caused as a direct or indirect consequence of the change of oxidation-state of the conjugated polymer.
In one aspect of the invention the volume change of the composite may be due to the volume change of the gel. In one embodiment the volume change of the gel may preferably be an expansion of at least 5 % of the starting volume or the most contracted volume. In a more preferred aspect of the invention the volume change of the gel may be an expansion of at least 7.5 % of the starting volume or the most contracted volume, and in an even more preferred embodiment the volume change of the gel may be an expansion of at least 10 % of the starting volume or the most contracted volume.
In the present context the volume change is defined as an expansion. However, since the composite reacts by expanding and contracting depending on the oxidation-state of the polymer, a change of volume may also be a contraction. In a preferred embodiment the volume change may be at least a 20 % expansion, more preferred at least a 50 % expansion. Since the gel itself may be capable of volume changes of up to at least 800 %, even greater volume changes of the com- posite are envisaged.
According to the invention the expansion of the composite may physically speaking be a substantially three dimensional expansion. However, in another aspect the expansion of the composite according to the invention may be substantially linear. A substantially linear expansion is in the present context defined as a one-dimensional expansion. The linear expansion of the composite may be up to 1.5 %, such as up to 3 %, or 5 %, or 7.5 %, or 10 %, or 20 %, or even up to 30 %.
When a conjugated polymer and an ion-sensitive gel are combined, the conjugated polymer may initiate a volume change in the gel, and the gel may act as a source/sink of ions for the conjugated polymer. When current is applied to a conjugated polymer the ion concentration at the surface of the polymer changes upon electrochemical cycling as illustrated by equation 1-3 below. Likewise the pH at the surface of the conducting polymers, e.g. polypyrrole and polyaniline changes upon electrochemical cycling, falling during oxidation. The number of ions taken up and released by these polymers is very high.
The redox reactions referred to above may also be referred to as doping and de- doping. Accordingly, doping refers to the oxidised state where electrons are re- moved from the polymer. This may be exemplified by the equation:
(1) P+(A") + C+(aq) + e <-→ P° + A"(aq) + C+(aq)
where P+ represents the doped (oxidised) state of the polymer and P° the undoped (reduced, neutral) state. P°(A") indicates that the anion A' is incorporated in the polymer as a dopant, (aq) represents an anion in the aqueous or solvent phase, and e" represents an electron. If the anion is immobile, then the following situation can occur:
(2) P+(A") + C+(aq) + e <-→ P°(AO+)
In this case, cations enter the polymer in the neutral state for charge compensation. In both cases, there may be solvent transport in or out of the polymer associated with the ion movements. A mixed situation in which both equations 1 and 2 are ap- plicable may also occur.
In the conjugated polymer, the anion may be immobile i.e. it is not able to move in and out of the polymer.
The conjugated polymer of the composite of the present invention preferably comprises an immobile anion such as alkyl sulphates, alkyl sulphonates, dodecylben- zenesulphonate, polystyrenesulphonate, polyacrylic acid, Nation, polyacrylomido- propylsulfonic acid, ethylenglycol, ethanol, organic carbonates, and esters. Some polymers are self-doped, i.e. the anion is a substituent on the polymer, and is thus immobile. An example of a self-doped polymer is a polyaniline, exemplified by equation (3):
(3) P+ + C+(aq) + e" <— > P° (C+)
In another aspect of the invention, the conjugated polymer of the composite comprises any mobile anion such as tosylate, triflate, CIO4 ", BF ", PF6 ", Cl", NO3 ', hydro- gensulphates and sulphates.
In yet another aspect of the invention, the conjugated polymer of the composite comprises an immobile cation. In this respect, the cation is preferably a mono- charged cation selected from tetra-alkyl ammonium salt, tri-alkyl ammonium salt, alkyl amines, polyimines and phosphonium salts.
An aspect of the present invention is that the volume change of the composite is dependent on the chemical components of the composite, and of the physical interrelations of the chemical components, i.e. the morphology of the gel and the conjugated polymer. According to the invention the composites may vary in chemical and physical nature.
Therefore, in one embodiment of the invention the volume change of the composite is predominantly due to the volume change of the gel, and to a lesser extent due to the volume change of the conjugated polymer. In another embodiment it is the equal volume change of both the gel and the conjugated polymer respectively which is responsible for the volume change of the composite. In yet a further embodiment it is primarily the volume change of the conjugated polymer, which causes a volume change of the composite, and secondarily the gel causing the volume change of the composite.
It follows that one embodiment of the present invention relates to a composite comprising one conjugated polymer capable of changing oxidation-state upon the onset of an electrical or chemical stimulus, and an ion-sensitive gel capable of changing volume, the polymer and the gel being morphologically interrelated and wherein the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer. In this particular embodiment the volume change of the composite relies almost entirely on the volume change of the gel. Here, the speed of the volume change of the composite is particularly fast due to the nature of the composite, which enables a greater volume change because it is the gel rather than the conjugated polymer determining the volume change.
In another embodiment of the present invention the volume change of the composite is caused by the equal volume change of the gel and the conjugated polymer respectively, in response to a change in oxidation level of the polymer.
Furthermore, in yet another embodiment of the present invention the volume change of the composite is predominantly due to the volume change of the conjugated polymer in response to a change in oxidation level of the polymer. According to the invention in composites wherein the volume change is mostly due to a change in the volume of the conjugated polymer, the volume change of the polymer is estimated to be at the most 10 %.
For all practical purposes the total volume change of the composite according to the invention must be achieved within a time frame of at the most 1 hour. Hence, in the present context the total volume change is defined as being the volume change obtained after 1 hour after the onset of a stimulus described by the invention. According to the invention, at least 50 % of the total volume change of the composite is achieved within a period of 5 minutes from the onset of the electrical stimulus to the conjugated polymer, preferably at least 50% of the volume change is achieved within a period of less than 10 seconds, more preferably less than 1 second, even more preferably less than 0.1 second. The speed of the response is defined by the time it takes the composite to change volume by an amount (Vmaχ - Vmιn )/2, where Vmin is the fully contracted volume and Vmax is the fully expanded volume.
In a more preferred embodiment, 75%, or even 100% of the total volume change has taken place within the periods defined above. Accordingly, preferably the total volume change has occurred within a period of 1 second.
It is an object of the present invention to provide for a composite wherein the poly- mer and the gel are preferably selected such that any volume change of the polymer and of the gel are in substantially the same directionjn response to a change in oxidation level. By "substantially the same direction" is meant that the polymer and the gel, respectively are acting in the same reaction and that their respective volume changes are not counteracting one another. Thus, preferably both components either expand or contract upon an electrical or chemical stimulus.
This can be accomplished in a number of ways, two of which are described below.
Combinations
Immobile anion or self-doped polymer
In this case, equation (2) or (3) mentioned above applies. During oxidation, the conjugated polymer contracts, expelling cations into and lowering the pH of the gel. A pH-sensitive gel with ionizable acid groups responds by also contracting, and/or the gel responds to the increased cation concentration by contracting. The gel "gets out of the way" of the conjugated polymer, as illustrated in Fig. 7a, allowing the composite to change volume by ΔVep*Fep+ΔVgei*Fgeι, where ΔVep is the volume change of the conjugated polymer, ΔVgeι the volume change of the gel, and F the volume fraction of each component. During reduction, the process is reversed
• Mobile anion
In this case, equation (1) above applies. During oxidation, the conjugated polymer expands, taking up anions and lowering the pH of the gel. The pH-sensitive gel, for example with ionizable base groups, responds by also expanding, and/or the gel responds to the lowered anion concentration by expanding. The volume change in the conjugated polymer magnifies the volume change in the gel. In the figure above, the arrows are reversed during oxidation.
Another preferred combination is a conjugated polymer that does not change volume and a gel that changes volume in response to the external signal. An example of this type: a PPy layer that does not change volume significantly, but a polyacryl acid gel that responds to the ion fluxes induced by the polypyrrole, contracting dur- ing oxidation and expanding during reduction, vide Fig. 7b. As stated above, the oxidation state of the conjugated polymer can be electrochemically controlled, whereby the response of the whole composite depends on the voltage applied.
Generally, the predetermined volume change is obtained by applying from -3V to
+3V to the polymer, such as from -1.5V to +1.5V, or even -1.0V to 0V.
On polypyrrole, the pH at the surface changes from 10.5 at -0.6V (vs. Ag/AgCI) to 3.5 at +0.6V. The voltage may be applied by any suitable means such as an elec- trode, which is an integral part of the composite.
In another aspect of the present invention the stimulus to the composite is chemical. The chemicals are preferably iodine or ammonia.
The term morphological interrelated defines the physical relationship between the two components in the composite, namely the conjugated polymer and the gel. In the present invention the conjugated polymer is in intimate contact throughout its volume with the gel. This intimate contact is a feature providing for the rapid volume change of the composite.
The structure of the composite according to the invention may have any suitable structure, for example an interpenetrating network, or an emulsion, or the structure may be layered.
An interpenetrating network is a blend of two materials on the molecular level. Each material forms a connected structure throughout the composite.
In emulsions the composite components do not blend on a molecular level but blend in small regions of each type of component (see Figure 2a-2d).
The composite may be a layered structure, such as a multi-layered structure, in which the conjugated polymer and gel are placed in separate phases. The layers may be relatively thick, for example several micrometers. The layers may be joined in a variety of ways, such as gluing or melting them together, polymerising one ma- terial over the other, dipping one into the other and co-extrusion. The multi-layers may be made from a large sheet consisting of one. layer of each material that is folded or rolled. Furthermore, the composite may have the geometry of fibres or fibre bundles of one component in a matrix of the other component (see Figure 4).
The conducting polymer regions must preferably be connected throughout the composite, which means that a current is able to pass through the whole conducting material.
The dimensions of the polymer and the gel are of great importance to the speed with which the composite of the present invention changes its volume. If each layer or region of either polymer or gel, in particular gel is too small, the composite will be unable to change volume, possibly due to an insignificant capability of swelling by solvent uptake
Accordingly, the dimensions of each layer or region of either polymer or gel is preferably more than 50 Angstrom, more preferred more than 250 A, and even more preferred more than 500 A.
Furthermore, in order to reduce the distance of ion transport it has been found that each layer or region is preferably not more than 1 mm in at least one plane. This may be defined in the following statement: in any point throughout the gel or the polymer, distance A (defined below) is less than 1 mm in at least one plane through said point. In the present context, the distance A is defined as the distance between two opposing surfaces of either the gel or the polymer (Figure 5).
Flexibility of the composite is another important feature. In order to accommodate the large volume changes without mechanical failure, the composite must be flexible. This is ensured by the component of the composite being flexible at all stages of contraction and expansion.
Furthermore, the flexibility of the composite is due to the polymer material. In particular where the electrode used to apply stimulus is polymeric rather than metallic, the flexibility is increased. The strength of the composite (i.e. the strength to push against a load) is predominantly determined by the swelling properties of the gel. The term strength may be defined as the measurement of force density. The force density is defined as the force divided by the area of the composite orthogonal to the measured force. The force density is measured in the direction in which the expansion or contraction of the composite is utilised (see Figure 6).
The force generated by the composite may be measured using a balance, load cell another force transducer, or weights. In case of the latter, weights may be attached to or placed onto the composite material and the maximum force may be calculated from the equation F (force) = ma, where m is the mass and a = g = 9.8 m/s2.
Another way to calculate the force is by multiplying the expansion of the composite by the Young's modulus. The expansion in each direction may be measured using a ruler, profilometer, or other means. The Young's modulus may be measured using a rheometer or a similar device. It is important to stress that the measurement of the Young's modulus must be carried out in the same physical environment as the one in which the composite operates. This is due to the fact that the Young's modulus is affected by whether a polymer or a gel is either wet or dry. According to the present invention the Young's modulus is measured in a liquid environment.
In a preferred embodiment of the present invention the Young's modulus is at least 10 MPa (mega Pascal), preferably at least 100 MPa, more preferably at least 500 MPa, yet more preferably at least 1000 MPa, even more preferably at least 2000 MPa, and most preferably at least 3000 MPa.
The mechanical failure by overloading, ripping or cracking may be defined by the ultimate elongation and failure stress.
The ultimate elongation is a measure of how much the composite may be passively elongated without breaking. The ultimate elongation is preferably up to 35 % of the original length, such as up to 25 %. Failure stress is a measure of the force necessary to pull the composite apart. The failure strength is preferably as high as possible, such as at least 2 MPa, preferably above 3 MPa, such as 4-12 MPa.
In the composite both of these properties are predominantly determined by the properties of the conjugated polymer.
The composite may be used for any purpose where its capability of controllable volume change and other characteristics are useful, such as an actuator. Accordingly, in another aspect of the present invention the composite is used for the production of an actuator.
In yet another aspect of the present invention an actuator or a sensor may comprise a composite as defined above.
Further in the present invention the actuator may be an artificial muscle or an hydraulic actuator or a bilayer actuator or a micro actuator. The industrial applications of the actuators comprising a composite according to the present invention are vast. Some of the applications may be as dextrous robotic grippers, smart valves, ad- vanced consumer products and toys. Other potential applications are within the medico technical field, such as stearable catheters and other minimal invasion surgical instruments.
Yet another aspect of the present invention relates to the process of causing a vol- ume change in a composite as described above comprising the steps of applying an electrical or chemical stimulus to the conjugated polymer, causing a change of the oxidation-state of the polymer, and arranging the ion-sensitive gel in contact with the conjugated polymer in a predetermined pattern of morphological interrelation in order to allow the gel to respond to the change in oxidation-state of the polymer by a change in volume.
A further aspect of the present invention concerns a method of producing a composite as defined above comprising the steps of selecting a conjugated polymer and an ion-sensitive gel, then combining the conjugated polymer and the ion-sensitive gel in a predetermined pattern of morphological interrelation in order to allow the composite to react followed by applying an electrical or chemical stimulus to the conjugated polymer and having the change in oxidation-state of the conjugated polymer cause a 50 % of the total volume change of the composite in at most 5 minutes from the onset of the stimulus to the conjugated polymer.
Experimentais
The following is an example of a composite of a conducting polymer and an ion- sensitive gel having electro-chemical properties and being capable of swelling in basic solution.
A morphologically interrelated composite film of polyaniline with a polyvinylalcohol (PVA)/polyacrylic acid (PAA) gel (PAni) was developed. A solution of PVA, PAA and PAni in N-methyl-2-pyrroiidinone (NMP) was prepared. To this was added solutions of glutaraldehyde (GA) and HCI. A film of this solution was heated which resulted in a phase separated structure and a concomitant cross-linking of the PVA and PAA to form a PAni/gel composite. Various composite structures were obtained. Examples of these structures are included below in Figure 8 (e.g. compare Figure 8a and 8d).
For the films in Figures 8a-8c, the following procedure was used:
1.5 g of a 4 wt % solution of PVA in NMP and 1.5 g of a 4 wt % solution of PAA in NMP were mixed well. 40 mg of finely powdered PAni (emeraldine base) was added very slowly by vigorous stirring followed by 200 μl of 1 % aqueous GA. After being well mixed the solution was sonicated for another 60 minutes. Then by vigorous stirring, 200 μl of H2O and 200 μl of 0.01 M HCI were added. Films were then prepared by either solution casting or spin coating onto various substrates. The films were then immediately placed in a co-invection oven at 75 °C for 60 minutes followed by further drying in a vacuum oven at 40 °C for 60 minutes. Before cycling, the films were soaked in the analyte solution for a minimum of 1 hour.
The gel composite shrank and expanded upon addition to acidic and basic solutions, respectively. For the composite in Figure 8, the film bent towards the Kapton® upon reduction and vice versa upon oxidation in acidic solution (0.01 M HCI, -0.2V/0.85V, Ag/AgCI ref.). The PAni/gel composite showed good electro-chemical activity. It is expected that upon oxidation in strong acidic solution (e.g. 1 M HCI), that PAni in emeraldine salt (ES) state will contract. This will produce a bending away from the Kapton® upon oxidation. This would suggest that for the composite the bending may be due to the pH change induced by the PAni.

Claims

Claims
1. A composite comprising,
one conjugated polymer capable of changing oxidation-state upon the onset of an electrical or chemical stimulus, and
an ion-sensitive gel capable of changing volume,
the polymer and the gel being morphologically interrelated, and
wherein the volume change of the gel is in response to the change of the oxidation-state of the conjugated polymer, and
50 % of the total volume change of the composite is achieved within a period of at most 5 minutes from the onset of the stimulus to the conjugated polymer.
2. The composite according to claim 1 , wherein the conjugated polymer comprises monomer units selected independently from the group of pyrrole, aniline, thio- phene, acethylene, p-phenylene, p-phenylene vinylene, p-phenylene sul- fide, thienylenevinylene, furane, azulene, and isothianaphthene or a derivative thereof.
3. The composite according to claim 1 or 2, wherein the composite comprises an immobile anion.
4. The composite according to claim 3, wherein the immobile anion may be alkyl sulphates, alkyl sulphonates, dodecylbenzenesulphonate, polystyrenesulpho- nate, polyacrylic acid, Nafion, and polyacrylomidopropylsulphonic acid, eth- ylenglycol, ethanol, organic carbonates, and esters.
5. The composite according to claim 1 or 2, wherein the composite comprises a mobile anion.
6. The composite according to claim 5, wherein the_ mobile anion may be tosylate, triflate, CIO4 ", BF4 ', PF6 ', Cl', NO3 ", hydrogensulphates and sulphates.
7. The composite according to claim 1 ,2, 5 or 6, wherein the composite comprises an immobile cation.
8. The composite according to claim 7, wherein the immobile cation is selected from tetra-alkyl ammonium salt, tri-alkyl ammonium salt, alkyl amines, polyimi- nes and phosphonium salts.
9. The composite according to any of the preceding claims, wherein the gel comprises a monomer or repeating monomer units independently selected from the group of acrylic acid, vinyl phosphate, acrylamide, styrene, urethane, ethylene oxide, propylene oxide, vinyl alcohol, acrylonitrile, and collagen or a derivative thereof.
10. The composite according to any of the preceding claims, wherein the gel is a pH-sensitive gel.
11. The composite according to any of the preceding claims, wherein the volume change of the gel is an expansion of at least 10 % of the starting volume.
12. The composite according to any of the preceding claims, wherein the stimulus is an electrical stimulus.
13. The composite according to claim 12, wherein the stimulus is applied by an electrode being an integral part of the composite.
14. The composite according to any of the preceding claims, having the structure of an interpenetrating network.
15. The composite according to any of the claims 1-14, having the structure of an emulsion.
16. The composite according to any of the claims 1-1_4, wherein the structure is layered.
17. The composite according to any of the preceding claims, wherein an internal distance A is less than or equal to 1 mm.
18. The composite according to any of the preceding claims, wherein 50 % of the total volume change of the composite is achieved within a period of less than 10 sec.
19. The composite according to any of the preceding claims, wherein the Young's modulus of the composite is at least 10 MPa.
20. The use of a composite according to claims 1-19, for the production of an ac- tuator.
21. An actuator comprising a composite as defined in any of the claims 1-19.
22. A sensor comprising a composite as defined in any of the claims 1-19.
23. The actuator according to claim 21 , which is an artificial muscle or an hydraulic actuator or a bilayer actuator or a micro actuator.
24. A process of causing a volume change in a composite comprising a conjugated polymer capable of changing oxidation-state and an ion-sensitive gel capable of changing volume comprising the steps of:
applying an electrical or chemical stimulus to the conjugated polymer, causing a change of the oxidation-state of the polymer,
arranging the ion-sensitive gel in contact with the conjugated polymer in a predetermined pattern of morphological interrelationship in order to allow the gel to respond to the change in oxidation-state of the polymer by a change in volume.
25. A method of producing a composite comprising the steps of:
-selecting a conjugated polymer and an ion-sensitive gel
-combining the conjugated polymer and the ion-sensitive gel in a predetermined pattern of morphological interrelation in order to allow the composite to react to an electrical or chemical stimulus, and thereby having the change oxidation- state of the conjugated polymer cause 50 % of the total volume change of the composite in at most 5 minutes from the onset of the stimulus to the conjugated polymer.
26. A method of producing an actuator comprising the steps of:
-selecting a composite as defined above
-applying an electrode to the composite in such a manner that the conjugated polymer is able to change oxidation-state, thereby obtaining an actuator, wherein the conjugated polymer cause a 50 % of the total volume change of the composite in at most 5 minutes from the onset of a stimulus to the conjugated polymer.
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