WO2016084031A1

WO2016084031A1 - Carbon nanotube-based material and polymerized ionic liquid for production of an actuator

Info

Publication number: WO2016084031A1
Application number: PCT/IB2015/059152
Authority: WO
Inventors: David GENDRON; Alberto Ansaldo; Davide Francesco RICCI; Grzegorz BUBAK; Giulio Sandini; Luca CESERACCIU
Original assignee: Fondazione Istituto Italiano Di Tecnologia
Priority date: 2014-11-27
Filing date: 2015-11-26
Publication date: 2016-06-02
Also published as: WO2016084031A9

Abstract

Process for the preparation of an electrode material for an actuator comprising mixing carbon nanotubes with a gelling ionic liquid in order to obtain a gelled mixture. The ionic liquid is a polymerised ionic liquid.

Description

Carbon nanotube-based material and polymerized ionic liquid for production of an actuator

This invention relates in general to the actuators known as carbon nanotube and solid electrolyte actuators based on ionic liquids (salts in the liquid phase capable of producing a gel when mixed with carbon nanotubes).

Document US 7315106 B2 describes an actuator formed from two layers of electrodes, each insulated from the other, comprising a gel formed of CNT, an ionic liquid and a supporting polymer. It is known that the supporting polymer provided to impart the required mechanical characteristics to the composite nevertheless has an adverse effect on the electrical conductivity which can be achieved with the material, in that in general it is an electrically insulating component.

In order to produce an electrode material without supporting polymer, an actuator comprising a thin electrically conducting film formed of an ionic liquid (IL) and carbon nanotubes (CNT) having a particular aspect ratio has been proposed in US 8004146 B2. The above document thus describes an actuator having mechanical and electrical properties suitable for the production of an actuator without having a supporting polymer within the CNT/IL composition. From a manufacturing point of view the device described in US 8004146 B2 is limited by the need to have a relatively high aspect ratio for the CNT (not less than 10⁴), or CNT having a length of not less than 50 μηι.

The document "Dramatic Effect of Dispersed Carbon Nanotubes on the Mechanical and Electroconductive Properties of Polymers Derived from Ionic Liquids" [1] describes the method for synthesising a conductive plastics material, known as "bucky plastics material", having a reduced CNT content (3-10% w/w) obtained by the polymerisation of a mixture of IL + CNT in which the CNT act as "organisers" of the spatial organisation of the ionic liquid (gelling agents) prior to polymerisation. It has however been found that the films produced with the abovementioned material are generally brittle or in any event not sufficiently strong for application as actuators. One object of the invention is to provide an electrode material for actuators based on carbon nanotubes and ionic liquid which is capable of at least partly overcoming the disadvantages of known materials.

With this in view an object of the invention comprises a process for the preparation of an electrode material for an actuator comprising

mixing carbon nanotubes with a gelling ionic liquid in order to obtain a gelled mixture,

in which the ionic liquid is a polymerised ionic liquid.

Another object of the invention is an electrode material which can be obtained using a process according to the invention comprising an electrically conductive plastics material capable of changing volume after charging with ions, the said plastics material comprising a mixture of carbon nanotubes and polymerised ionic liquid.

For the purposes of this invention, by polymerised ionic liquid (PIL), otherwise known as polymer ionic liquid or poly(ionic liquid), is meant a special type of polyelectrolyte which has one IL species in each of its repetitive units. As a consequence the cationic or anionic centres are linked to the repetitive units in the polymer chain. It is worth noting that although the IL are in the liquid state at temperatures close to ambient temperature, PIL are generally solid at such temperatures [2, 3]).

Thanks to the mixture of CNT with a polymerised ionic liquid as a starting material, and the surprising discovery that the abovementioned PIL mixed with CNT forms a gel, the invention has electrical conductivity, electrical capacitance and mechanical properties that are considered to be better than in the known art, both in general and in the context of actuation systems.

Also, with regard to the CNT/IL/polymer mixture described in US 7315106 B2 the material according to the invention comprises a CNT/PIL mixture without supporting polymer, in that the polymerised ionic liquid performs the mechanical functions which were previously performed by the supporting polymer. This simplifies the process of producing the material in question, and therefore actuators produced from such material.

Furthermore, in comparison with the material described in US 8004146 B2, the material according to the invention does not have any limitations as regards the dimensional characteristics of the nanotubes for the purposes of obtaining a material having optimum mechanical and electrical characteristics for the production of an actuator.

Finally, unlike the method described in [1], an already polymerised ionic liquid is used as the starting material and this is mixed with the CNT in order to gel the latter. This particular method ensures that a composite having optimum mechanical and electrical characteristics for the production of actuators without supporting polymer is obtained.

The inventors have produced CNT/PIL composites having a conductivity of approximately 100 S/cm in the case of mixtures containing 28% w/w of CNT. This characteristic derives from the possibility of mixing the CNT with the PIL after the latter has polymerised.

To sum up, the action of mixing CNT with an already polymerised ionic liquid makes it possible to:

purify the PIL after synthesis, removing any monomers and short chains which may be present, if considered necessary (for example by means of dialysis)

produce CNT/PIL composites having mechanical characteristics which are suitable for the production of solid devices without other supporting polymers

- achieve better electrical conductivity than that reported in the literature

obtain a composite CNT/PIL film having physical characteristics suitable for the production of actuators, that is:

- electrical capacitance

- electrical conductivity

- mechanical properties

- a change in volume following electrical charging.

Further characteristics and advantages of the invention will be apparent from the following detailed description with reference to the appended drawings provided purely by way of a non-limiting example in which:

Figure 1 is a diagrammatical representation of a conventional actuator,

Figures 2 and 3 are block diagrams illustrating a process for the fabrication of an electrode film and a process for the production of an actuator according to this invention respectively;

Figure 4 is a graph showing thermogravimetric diagrams for some samples of PIL/CNT composite according to the invention which have the same percentage composition (quantities of CNT with respect to the PIL) but different polyionic liquids; from the shape of the graph it will be seen how it is possible to control the thermal stability of the composite through choosing the appropriate ionic liquid

Figure 5 is a graph showing stress/strain curves for some samples of CNT/PIL composite according to the invention, which have different Young's moduli; the Young's modulus of the composite may be adjusted as desired through a wide range of values by suitably selecting the molecular weight and composition of the PIL

Figures 6 to 8 show SEM images of some samples of CNT/PIL composite according to the invention obtained with different PIL. In all cases it will be noted how there is complete interpenetration between the various PIL and the CNT, a fact which makes it virtually impossible to distinguish the polymer matrix (PIL) from the CNT themselves. The scale is 10 μηι.

With reference to Figure 1, this diagrammatically illustrates an example of a polymer actuator, indicated as a whole by 1.

Actuator 1 comprises a first and second electrode layer 2, 3, each containing electrically conducting material and both capable of changing dimensions along at least one direction of deformation through the effect of introducing charge. In particular the direction of deformation of electrode layers 2, 3 is substantially perpendicular to the direction of layering, that is the direction in which the various layers of actuator 1 are arranged. Actuator 1 can therefore deform through the effect of the combination of changes in the dimensions of first and second electrode layers 2, 3.

In general, in order to avoid asymmetric behaviour by the actuator for the same electrical input, and therefore the charge delivered to the electrodes, first and second electrode layers 2, 3 are made of the same material (same shape and same density). As an alternative they may be made of different materials, but are capable of producing identical deformations for the same delivered charge.

Each of electrode layers 2, 3 comprises a mixture of carbon nanotubes (hereinafter CNT) and polymerised ionic liquid (hereinafter PIL).

The carbon nanotubes which can be used in this invention are a carbon-based material having a structure in which a sheet of graphene is wound into a cylindrical shape. The carbon nanotubes are approximately subdivided into three groups according to the construction of the perimeter wall, that is single wall nanotubes (SWNT), double wall nanotubes (DWNT) and multiple wall nanotubes (MWNT). There are many types of carbon nanotubes depending upon the structure of the graphene sheet, for example chiral (helical), zigzag and armchair types. Any type of carbon nanotube may be used in this invention as long as it belongs to one of the mentioned categories of nanotubes.

The concentration of carbon nanotubes in the electrode layers is preferably greater than 10% by weight with respect to the polymerised ionic liquid.

Various known types of polymerised ionic liquids may be used in this invention, but it is preferable that the polymerised ionic liquid be stable at ambient temperature and be in liquid or solid form at ambient temperature or close to ambient temperature. Polymerised ionic liquids may be obtained by various methods (for example free radical polymerisation, organometallic bonding), which may result in the preparation of the following types of polymers, including but not limited to copolymers, statistical copolymers or cross-linked polymers. Examples of polymerised ionic liquids which are preferable in this invention include a polymerised cationic backbone (imidazole, pyridinium, quaternary ammonium) and an anion (X ) represented by the following general formulae:

(!) (II) (III)

In the general formulae (I, II, III) R¹, R² and R_x represent C_1-12 alkyl groups having a straight or branched chain, or an alkyl group having a straight or branched chain with ether, ester, amide, thioether, carbonyl or thiocarbonyl bonds. Both R¹ and R² may have an alkyl chain bonded to an aromatic ring (which may or may not be heterocyclic) such as imidazole, pyridinium, phosphonium.

Examples of anions (X^") comprise tetrafluoro boric acid ions (BF₄ ^~) and other perfluoro derivatives such as BF₃CF₃ ^" or BF₃C₂F₅ ^", tris(trifluoromethanesulfonyl)imidic acid ions ((CF₃S0₂)₂N^"), hexafluorophosphoric acid ions (PF₆ ^"), perchloric acid ions (C10₄ ^~), trifluoromethanesulfonic acid ions (CF₃S0₃ ^"), dicyanamide ions ((CN)₂N^"), trifluoroacetic acid ions (CF₃C0₂ ^"), carboxylic acid ions and organic halogen ions.

Specific examples of polymerised ionic liquids are those in which the cation is a poly(l- vinyl-3-butylimidazole) ion and the anion is a halogen bromide ion (Br ), a tetrafluoroboric acid ion (BF₄ ^") or a bis(trifluoromethanesulfonyl)imidic acid ion ((CF₃S0₂)₂N^"). There is however no limitation to possible combinations.

The material described above is therefore based on carbon nanotubes and polymer ionic liquid.

For it to be effectively usable for the production of an actuator the material must have at the same time:

electrical conductivity (which, if low, restricts charging times)

electrical capacitance (which determines the quantity of ions which can be accumulated on the surface)

quantities of nanotubes (the greater the quantity, the greater the electrical conductivity, electrical capacitance and elastic modulus)

ability of the polymer ionic liquid to gel the nanotubes (if not gelled the surface area of the nanotubes is not available for electrical capacitance)

mechanical properties of the polymerised ionic liquid (necessary to ensure a high elastic modulus, or conversion of deformation on the mesoscopic scale to macroscopic work)

ion conductivity in the electrolyte (which if low restricts charging times)

packing of the CNT, which must be "dense" in order to ensure actuation (percentage in the composite) but "separate" (thanks to the gelling) to ensure the intercalation of ions which causes the material to change in volume.

In particular the carbon nanotubes have a crucial role in the material in that they provide a series of properties all of which are necessary for the purposes of actuation:

1) electrical conductivity: necessary for charging and discharging the device

2) surface area: necessary in order to have a surface exposed to the ionic liquid and therefore capable of accumulating charge

3) mechanical strength: necessary in order to transform the forces generated within the device into external forces/displacements of the device.

The process of gelling between PIL and CNT makes the surface area of the nanotubes accessible to the ions. The inaccessible surface area does not contribute to the accumulation of charge and as a consequence to actuation.

As the exact principle of actuation has not been clearly demonstrated, what is known empirically is that the insertion of ions between the nanotubes (intercalation of ions) is the cause of the change in volume. As a consequence the concentration of nanotubes must be sufficient to ensure that the nanotubes are isolated and can therefore interact with each other giving rise to deformation of the material.

The material according to the invention in fact has organised structures (bundles) of nanotubes which can be accessed by the ions (see Figure 7) which are ideal for the purpose of ensuring charging (provided by the accessible surface area), rate of charging (provided by the high electrical conductivity), and actuation on a macroscopic scale (provided by the strong nanotube-nanotube interactions mediated by the ions, which is impossible in the case of composites having a low nanotube content).

As may be seen in Figure 5 the material according to the invention has a high elastic modulus, such as to ensure efficient transfer of the force generated on the mesoscopic scale into useful work at the macroscopic scale, and therefore conversion of the electrical energy accumulated in the device in the form of ionic charge into mechanical work.

Actuator 1 also comprises a layer of solid polymer electrolyte 4 placed between first and second electrodes 2, 3. This layer of solid polymer electrolyte 4 is electrically insulating and conducts ions. The solid electrolyte may be any electrically insulating material which conducts ions produced by using plastics materials mixed with an ionic liquid in different compositions or gels, such as agarose, and containing salts. The salts may also be dissolved in any high boiling point solvent and may be trapped in the polymer matrix in the form of solution. These materials are in fact commonly identified by the term solid polymer electrolyte (SPE).

Examples of polymers for solid polymer electrolyte comprise polyvinylidene fluoride (PVdF), poly(vinylidene fluoride-hexafluoro propylene) (P(VdF-HFP), poly(vinylidene fluoro-tetrafluoroethylene) (P(VdF-TFE)), poly(vinylidene fluoro-trifluoroethylene) (P(VdF-TrFE)), polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), polyacrylonitrile (PAN), PVC/PAN mixture, polyhydroxyethylmethacrylate (PHEMA), poly(ethylene oxide) (PEO) and polymers based on styrene/divinyl benzene.

Examples of ionic liquids for the solid polymer electrolyte comprise any salts in such a concentration as to achieve sufficient ion conductivity (10^~6 S/cm or higher) in the composite. One particular example comprises ionic liquids based on imidazole, which are liquids at ambient temperature. Other examples of ionic liquids which might be used comprise piperidinium, pyrrolidinium and quaternary ammonium salts.

Although Figure 1 shows a bimorphic actuator with two active layers, the invention is not limited to this configuration and can be applied to the production of other architectures, for example multidirectional architectures such as that described in publication WO 201 1/161651 A2 by the same Applicant, which are capable of expanding and contracting and simultaneously folding following the application of different electrical potentials to the ends of the electrodes.

Electrode layers 2, 3 may be made using different techniques such as casting, lamination, moulding, spraying and filtering.

Solid polymer electrolyte layer 4 may be produced by the deposition and evaporation of a solution containing a supporting polymer and an ionic liquid. As an alternative, polymer electrolyte layer 4 may be produced by immersing a porous polymer membrane in a bath containing ionic liquid, in such a way as to soak the polymer membrane with ionic liquid. Advantageously this membrane may be a preformed PVDF membrane which is commercially available from various companies (GE, Millipore, Pall) and is normally used for western blot. This membrane (typically 140 μιη thick with pores of 0.2 μιη) can in fact be effectively converted into a solid electrolyte by prolonged immersion (several hours) in an ionic liquid or a mixture of ionic liquid and suitable solvents such as, by way of example, propylene carbonate (PC). The chemical affinity between the PVDF and the ionic liquid has the result that through this procedure the membrane is stably soaked with the ionic liquid itself, taking on the properties of a solid electrolyte.

The main advantages of this approach are:

- ion conductivity which is improved by 1000-2000% (one order of magnitude) in comparison with a membrane obtained by conventional methods based on deposition of a solution by evaporation, for the same type of ionic liquid;

- the process is carried out at ambient temperature, while the process based on deposition by the evaporation of a solution requires a temperature of approximately 70- 100°C to dissolve the polymer;

- equipment for shaping and forming a film of the membrane is not required, as only immersion in ionic liquid is required;

- process times are decidedly shorter (the solid electrolyte film is ready for subsequent stages in the process after approximately 2 hours, while the liquid procedure generally requires 12 hours of mixing at approximately 80°C followed by 2 days of drying the membrane to remove the solvents);

- special equipment such as a vacuum drying stove is not required.

The material obtained by soaking the membrane with ionic liquid has mechanical properties which differ from those of the starting material (pristine membrane) in that it deforms plastically via elongation of up to approximately 120%. This makes it possible to change the thickness of the membrane by applying a uni- or multi-axial stress to it, bringing about plastic deformation which causes thinning of the membrane at the same time as increasing its geometrical surface area.

With reference to Figure 2, an example of a process for preparing an electrode material for an actuator will now be described.

Initially (step 101), the polymerised ionic liquid is prepared, or an already available polymerised ionic liquid is used. Techniques for the preparation of PIL are known in the art, and do not constitute an object of this invention.

The PIL is then mixed with the carbon nanotubes and an organic solvent (steps 102, 103). As solvents, use may for example be made of dimethylacetamide (DMAc), dimethylformamide (DMF), methylpentanone, tetrahydrofuran (THF), l-methyl-2- pyrrolidone (NMP), etc.

In particular, in one experiment conducted by the inventors, carbon nanotubes (30 mg, 28% w/w) and poly(ionic liquid) (78 mg) were placed in a reaction flask and 5 mL of DMAc were added as solvent. The flask was then stoppered and the mixture was agitated at 80°C for 16 hours. After cooling to ambient temperature the resulting gel mixture had solidified to the extent that it would not pour out of the flask when the latter was turned upside-down. After adding 2 mL of DM Ac to the flask the gel was subjected to further sonication for 45 minutes.

The gelled mixture was then shaped in order to obtain a gel film. In particular, in the experiment mentioned, the gelled mixture was poured into a PTFE mould (step 104).

The gel was then dried (step 105) to obtain the finished composite film. In particular, in the experiment mentioned, the mould was heated to 40°C and held at that temperature for 16 hours, or until the film was completely dry. An example of a process for the production of an actuator will now be described with reference to Figure 3.

In the context of this process a first and second electrode layer are fabricated from the composite CNT/PIL material obtained as described above. In particular, in one experiment performed by the inventors, each electrode layer was prepared by subjecting the composite film to a pressure of 1 metric tonne for 1 minute at 150°C (step 201). The composite films were first protected by placing them between polyimide films 0.008 mm thick.

The layer of solid polymer electrolyte was also fabricated. This layer may be produced in the manners described above.

The layer of solid polymer electrolyte and the electrode layers are then assembled by placing the layer of solid polymer electrolyte between the electrode layers (step 202); the assembly so obtained is then pressed (step 203). In particular, in the experiment mentioned, the assembly was subjected to pressing for 1 hour at 130°C after first being protected between two films of polyimide.

In order to render the device safer and more long-lasting it is possible to encapsulate it within a thin protective film (indicative thicknesses between 0.1 and 2 μπι) of a poly(p- xylylene) derivative - commercially known as Parylene - or poly(2-chloro-p-xylylene) - commercially known as Parylene-C. The main advantages provided by this technique are:

increased intrinsic safety for the device, which is electrically and physically insulated from the surrounding environment;

reduced possibility of the accidental release of substances such as CNT and IL;

longer service life for the device, thanks to lesser exposure to substances present in the surrounding environment and in particular exposure to water and oxygen;

the possibility of using encapsulated devices in hostile environments, or in liquid, including physiological solutions, thus making it possible for them to be used in medical devices such as active catheters and in general medical devices which require a shaped actuator supplied with a low voltage (less than 4 V);

the possibility of using antibacterial Parylene coatings.

Bibliographic references

[1] Fukushima T, et al, Small 2 (2006), 4, 554-560.

[2] Marcilla R, et al, J. Polym. Sci. Part A Polym. Chem. (2004), 42, 208-212.

[3] Yuan J, et al, Polymer 52 (201 1), 1469-1482.

Claims

1. A method for preparing an electrode material for an actuator, comprising

mixing carbon nanotubes with a gelling ionic liquid to obtain a gelled mixture, characterised in that the ionic liquid is a polymerised ionic liquid.

2. A method according to claim 1, in which the carbon nanotubes and ionic liquid are mixed with an organic solvent.

3. A method according to claim 1 or 2, in which said mixing comprises

applying temperature and agitating the carbon nanotubes with the ionic liquid.

4. A method according to any of the preceding claims, further comprising

forming the gelled mixture to obtain a gel film.

5. A method for producing an actuator comprising

fabricating first and second electrode layers, said fabricating the electrode layers comprising a method according to any of the preceding claims,

fabricating a solid polymer electrolyte layer,

assembling the solid polymer electrolyte layer with the electrode layers by placing the solid polymer electrolyte layer between the electrode layers, and

pressing the so obtained assembly.

6. A method according to claim 5, in which fabricating the solid polymer electrolyte layer comprises depositing and evaporating a solution containing a support polymer and an ionic liquid.

7. A method according to claim 5, in which fabricating the solid polymer electrolyte layer comprises

soaking a porous polymer membrane with ionic liquid by immersing the membrane in a bath containing an ionic liquid.

8. A method according to claim 7, in which fabricating the solid polymer electrolyte layer further comprises

plastically thinning the solid polymer electrolyte layer by applying a mono- or multi- axial stress.

9. A method according to any of claims 5 to 8, further comprising

encapsulating the layer assembly within a protective film.

10. A method according to claim 9, in which the protective film comprises material selected from the group consisting of poly-p-xylylene and poly(2-chloro-p-xylylene).

11. An electrode material obtainable by a method according to any of claims 1 to 4, and comprising an electrically conductive plastics material capable of changing volume in response to ion charging, said plastics material being composed of a mixture of carbon nanotubes and polymerised ionic liquid.

12. A material according to claim 1 1, having a carbon nanotube content greater than 10% by weight.

13. A material according to claim 11 or 12, having a Young's modulus greater than 50 MPa.

14. A material according to any of claims 11 to 13, in which the carbon nanotubes have a length shorter than 50 μηι or an aspect ratio lower than 10⁴.