Cross reference to a related application: this application claims priority to European application No. 17306904.8 -filed on December 22, 2017-, the whole content of this application being incorporated herein by reference for all purposes.

The present invention concerns a haptic system including an electroactive polymer composite layer comprising core-shell particles and at least one polymer and electrodes for the activation of said layer. It pertains to polymer composite layer capable of providing a haptic response to an electrical stimulus. It also pertains to devices comprising said system.

Electroactive polymers, or EAPs, are polymers that exhibit a change in size or shape when stimulated by an electric field, sometimes these polymers are also qualified as electromechanically active polymers. The most common

applications of this type of material are in actuators and sensors. In some applications the actuator comprising EAP thin layer provides a haptic response to a user. It is the case for a haptic device such as a touch screen that may generate immediately a feedback vibration when the touch screen is activated by the user input. In haptic applications the actuator is generally capable of generating haptic feedback in accordance with the stimulus detected for example with pressure detected.

In most of the applications, the actuator need to be sensitive to low voltage i.e. give the highest response to the lowest electrical stimulus. Moreover, the use of EAP that can be processed in the form of a film allows preparing very thin actuators.

EAP may be a dielectric elastomer used as a film deposited between two electrodes. Applying voltage between electrodes leads to an electrostatic attraction between the electrodes and to compression of the elastomer. The shape of the elastomer is recovered when the voltage is removed.

EAP may be a piezoelectric polymer such as polyvinylidene fluoride (PVDF) or polyvinylidene fluoride-co-trifluoroethylene (P(VDF-TrFE)). When a voltage is applied across a piezoelectric layer comprised sandwiched between 2 electrodes, a reverse piezoelectric effect will lead to a bending deflection of the structure. The strain of a piezoelectric polymer layer is linearly proportional to the applied electric field. Strain, here and hereafter in the document, is to be understood as a change of shape or size of an object/layer due to externally applied field.

EAP may be an electro strictive polymer which exhibits a strain behavior under an electric field which is quadratically proportional to the electric field.

Electroactive polymers having high dielectric permittivity and low dielectric loss are highly desirable for the manufacture of devices having low electrical consumption and/or having high mechanical response to electrical stimuli which are properties useful for electromechanical applications such as haptic actuators. On one hand, low electrical consumption is economically valuable because it spares resources and also because it increases the shelf life of the device by avoiding over-heating during operating. On the other hand, enhanced response to stimuli provides better sensitivity of the device.

Polymer composites having high dielectric permittivity can be a material of choice for manufacturing actuators useful in haptic devices. Generally in composites, polymers are used as matrix where they bring excellent thermal and mechanical properties while fillers, which are dispersed in the polymer matrix, bring functionalities. Polymer matrix in composite is also responsible for high flexibility and high processability.

Thus, dispersing in a polymer matrix conductive nano-objects having low percolation threshold because of their high aspect ratio, is a strategy to achieve high dielectric permittivity materials. However the addition of fillers that may be conductive such as metal nanowires can also increase dielectric loss and leakage current because of forming a conduction network.

The dielectric permittivity and dielectric loss of a material are related to each other by the dissipation factor tan d also named dielectric loss tangent:

tan /> = e"/e'

wherein e” is the dielectric loss and s’ the dielectric permittivity. The lower the dielectric loss tangent the better is the performance of a dielectric material because it represents a good balance between low dielectric loss e” and high dielectric permittivity e’ .

There are numerous publications concerning systems using electroactive polymer layers as actuator for haptic applications.

US9164586 describes a haptic system based on electromechanical polymer (EMP) layers. The authors compare the behaviors of actuators comprising respectively dielectric elastomer layers, piezoelectric polymer layers and electro strictive polymer layers and give the main characteristics of such actuators. The electromechanical polymers are free of any core-shell particles.

US2016185915 describes the use of a system comprising an electroactive polymer layer consisting of polysilo xanes substituted by chloro or fluoro groups optionally crosslinked in haptic application. As a consequence of the presence of halogen groups, the intensity of polarization of the siloxane polymer is improved and the dielectric constant thereof is also improved. Vibratory acceleration of the layer at actuation is also improved. Nothing is said about further enhancement of the properties of such a system by improving the dielectric constant and reducing dielectric losses.

US6423412 shows that electro strictive copolymers obtained by high energy irradiation of fluorinated copolymers such as polyvinylidenefluoride-co- trifluoroethylene (P(VDF-TrFE) copolymer) exhibit a high dielectric constant and low dielectric loss and can be used advantageously as actuators. Nothing is said about the improvement of the dielectric properties of these electro strictive copolymers and about the improvement of some actuators made from them that could be obtained by addition of any core-shell particles.

O.Pabst et al. in Organic Electronics 2013, 14, 3423-3429 describes ink-jet printed P(VDF-TrFE) layer between 2 silver electrodes onto a

polyethyleneterephtalate (PET) substrate for preparing thin film actuator at a process temperature not exceeding l30°C. They observed significant actuator deflection at voltage not exceeding 400V. The authors are silent about upgrading of the actuator by adding core shell particles to the piezoelectric polymer layer.

Le, M.Q. et al. in Scientific Reports (2015) 5, 11814 describe the use of electro strictive polymer poly(vinylidenefluoride-trifluoroethylene- chlorofluoroethylene) (P(VDF-TrFE-CFE) copolymer) filled with a bis(2- ethylhexyl)phthalate (DEHP). According to the author, the addition of the phthalate plasticizer gives rise to an enhanced dielectric permittivity and to a decreased young modulus, both being responsible for an increased

electro strictive strain under relatively low electric field. However, an increase of the dielectric losses is concurrently observed as well as a decrease of dielectric breakdown strength.

According to all the above, the inventors have found some needs in the domain of electroactive polymers for haptic applications.

There is a need for improved actuator comprising electroactive polymer for haptic applications. Whatever the electroactive category of polymer which is chosen is, i.e. elastomeric electroactive polymer, piezoelectric polymer or electro strictive polymer, there is a need for an actuator including electroactive polymer layer exhibiting high strain in response to a relatively low electric field.

There is a need for said electroactive polymer layer to be activated by excitation having a large range of frequencies and to vibrate also at a large range of frequencies.

There is also a need for said electroactive polymer layer to exhibit high vibratory acceleration at actuation.

There is a need for thin actuators that can be used in portable devices and that can be processed easily starting from cheap materials.

There is also a need for actuators exhibiting dielectric breakdown at relatively high driving fields.

All these needs and others are advantageously fulfilled by a haptic system including

- at least one electroactive polymer composite layer comprising core-shell particles and at least one polymer and,

- electrodes for the activation of said electroactive polymer composite layer.

By core shell particle is meant particle composed of biphasic materials which have an inner core structure and an outer shell made of different components.

The core- shell particle suitable for the invention is generally any particle having dimensions below 50 pm. It is preferably a submicronic particle or nanoparticle. Nanoparticle is meant particle of any shape with dimensions ranging from 1 nm to 100 nm, including tube and fibers having only 2

dimensions below 100 nm i.e nano tubes, nano fibers and nano wires and including sheets having only one dimension (thickness) below 100 nm i.e. nanosheet such as graphene.

Generally, the particles suitable for the invention have an aspect ratio of at least 2. Preferably, the aspect ratio is of at least 5, more preferably of at least 10, even more preferably of at least 15 and the most preferably of at least 20. The particles have usually an aspect ratio of at most 5000, preferably of at most 1000, more preferably of at most 500 and even more preferably of at most 200.

The aspect ratio is the ratio of length to width of a particle (ISO 13794 :

1999). An average aspect ratio may be determined by the skilled person by image processing of transmission electron microscopy (TEM) or scanning electron microscopy (SEM) pictures.

When the aspect ratio is close to 1 the particle tends to be spherical. Then when the aspect ratio of the particle increases, the shape moves from rod to nanowires.

In some preferred embodiments the core shell particles suitable for the invention comprise a conductive core. A conductive core means a core which is essentially composed or even composed of a conductive material. The conductive core comprises generally at least 95 wt. % of a conductive material, preferably at least 97 wt. % and more preferably at least 99 wt. %.

Generally, the conductive core comprises conductive material selected from the list consisting of copper, silver, gold, zinc and carbon.

Preferably, the conducting material is silver or copper and more preferably is silver.

The conductive core is generally selected from the list consisting of silver or copper nanowires, silver, copper, gold or zinc nanoparticles, graphene and carbon nanotubes.

Preferably, the conducting core is silver or copper nanowires and more preferably is silver nanowires.

In some other preferred embodiments the core shell particles suitable for the invention comprise a semi-conductive core. A semi-conductive core means a core which is essentially composed or even composed of a semi-conductive material. The semi-conductive core comprises generally at least 95 wt. % of a semi-conductive material, preferably at least 97 wt. % and more preferably at least 99 wt. %.

Generally, the semi-conductive material is selected from the list consisting of Si, Si-Ge, GaAs, InP, GaN, SiC, ZnS, ZnSe, CdSe, and CdS. Preferably, the semi-conducting material is selected from the list consisting of GaAs, SiC, ZnS and CdS. More preferably, the semi-conducting material is SiC.

Generally the semi-conducting core is selected from the list consisting of

GaAs, SiC, ZnS and CdS nanoparticles.

Generally the core shell particles suitable for the invention comprise an insulating shell. An insulating shell means that the shell is essentially composed or even composed of an insulating material. The insulating shell comprises generally at least 95 wt. % of an insulating material, preferably at least 97 wt. % and more preferably at least 99 wt. %. This insulating material may be selected from the list consisting of insulating metal oxides, metal halogenides, metal sulfides or organic materials.

Just for the sake of example, MgO, AI ₂ O ₃ , Si0 ₂ , Ti0 ₂ , Zr0 ₂ and SrTiCL can be cited as suitable metal oxides, AgCl and CuCl as suitable metal halogenides, Ag ₂ S and CuS as metal sulfides.

Suitable organic materials are generally selected from polymers such as polyvinylchloride (PVC), hydrogenated nitrile rubber (HNBR), stabilizing agents such as polyvinylpyrrolidone (PVP) and alcoxysilanes or surfactants such as poly(alkylene oxide)s. Poly(alkylene oxide)s suitable for use in the present invention are polymers essentially all or all the repeating units of which comply with general formula -C _n H _2n- 0- wherein -C _n H _2n - represents a divalent alkylene group with n ranging from 2 to 10. Such poly(alkylene oxide) s may be terminated by a hydroxyl group. Particularly suitable poly(alkylene oxide) s are those wherein n ranges from 2 to 4, preferably from 2 to 3, more preferably wherein n = 2. The poly( alkylene oxide)s may be either linear or branched.

Linear poly( alkylene oxides) are generally preferred.

Specific examples of suitable poly( alkylene oxide) s include

polyoxyalkylene polyols, such as polyoxyethylene glycol (also known as poly(ethylene glycol) or poly(ethylene oxide)), polyoxyethylene triol, polyoxyethylene tetraol, polyoxypropylene glycol (also commonly referred to as polypropylene glycol) or polypropylene oxide), polyoxypropylene triol, polyoxypropylene tetraol, polyoxybutylene glycol, polyoxypentane glycol, polyoxyhexane glycol, polyoxyheptane glycol, and polyoxyoctane glycol. These polymers may be used either individually or in combinations of two or more; for example, it can be cited random copolymers of ethylene oxide and propylene oxide, and polypthylene oxide)-poly(propylene oxide) block copolymers.

The hydroxyl end groups of the poly(alkylene oxide)s may according to a preferred embodiment be partly or fully substituted by alkoxide groups, preferably methoxy or alkoxy. Methods for converting hydroxyl groups of poly(alkylene oxide) s into alkoxy groups are known to the skilled man and described in the literature.

Just for sake of example the following core- shell combinations may be advantageously used in the present invention:

-Ag or Cu nanowire and Ag or Cu nanoparticle core coated with Ag ₂ 0, Si0 ₂ , Al ₂ 0 ₃ , Ti0 ₂ or Zr0 ₂ shell; especially Ag nanowires coated with

Si0 ₂ and Ag nanowires coated with Ti0 ₂ , -Zn nanoparticle core coated with ZnO shell,

-SiC nanoparticles coated with Si02,

-graphene or carbon nanotube core coated with HNBR or PVC.

The electroactive polymer suitable for the invention may be any dielectric polymer. By dielectric polymer is meant any polymer that has the ability to polarize in response to an applied field. The dielectric polymer is characterized by a dielectric permittivity.

In some preferred embodiments, the dielectric polymer is an elastomer. Generally, this elastomer is selected from the list consisting of acrylic

elastomers, butadiene acrylonitrile rubber (NBR), polyisoprene (IR), butyl rubber (HR), chlorobutyl rubber (CIIR), bromobutyl rubber (BUR),

polybutadiene (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), ethylene propylene rubber (EPM), ethylene propylene diene rubber (EPDM), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubbers, fluoro silicone rubbers, fluoroelastomers, perfluoroelastomers, ethylene- vinyl acetate copolymer(EVA), thermoplastic olefins (TPO) and thermoplastic polyurethanes (TPU). It is preferably selected from thermoplastic polyurethanes, fluoro silicone rubbers and silicone rubbers and more preferably from silicone rubbers.

In some other preferred embodiments, the polymer is a fluorinated polymer. By the term fluorinated polymer is meant: polymer comprising repeat units derived from at least one fluorinated monomer. By the term fluorinated monomer is meant ethylenically unsaturated monomer comprising at least one fluorine atom.

In certain embodiments, the fluorinated polymers suitable are chosen among vinylidene fluoride (VF ₂ ) homopolymers or copolymers which provide advantageously high chemical resistance.

Vinylidene fluoride copolymers comprise generally at least 60 % by moles, preferably at least 75 % by moles, more preferably at least 85 % by moles and possibly at least 95 % by moles of repeat units derived from vinylidene fluoride.

Vinylidene fluoride copolymers comprise generally up to 40% by moles, in particular from 0 to 15% by moles, of repeat units derived from monomers selected from the list consisting of vinyl fluoride (VF), trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chloro trifluoroethylene (CTFE),

tetrafluoroethylene, hexafluoropropylene (HFP), hexafluoroisobutylene, pentafluoropropene, 3,3,3-trifluoropropene, perfluoromethylvinylether and mixtures thereof.

Vinylidene fluoride copolymers may also comprise repeat units derived from at least one (meth)acrylic monomer. (Meth)acrylic monomers include monomers having the formula (I) thereafter :

wherein:

Ri, R ₂ and R ₃ are equal to or different from each other, are independently selected from hydrogen atom and a Ci-C ₄ group, and

R ₄ is selected from hydrogen atom and C i -C 1 ₂ group optionally comprising at least one heteroatom.

Non limitative examples of (meth)acrylic monomers are notably acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate,

hydro xypropyl(meth)acrylate; hydro xyethylhexyl(meth)acrylates.

The (meth)acrylic monomer is preferably selected among:

- hydroxyethylacrylate (HEA) of formula:

- 2-hydroxy propyl acrylate (HP A) of either of formulae:

- acrylic acid (AA) of formula:

- and mixtures thereof.

The repeat units derived from the (meth)acrylic monomer are comprised in the copolymer in an amount of preferably from 0% to 15 % by moles and more preferably from 0% to 10 % by moles.

Good results can be obtained with terpolymers VF ₂ -TrFE-CTFE or VF ₂ - TrFE-CFE, copolymers VF ₂ -HFP, copolymers VF ₂ -TrFE and copolymers VF ₂ - CTFE, wherein VF ₂ is vinylidenefluoride, TrFE is trifluoroethylene, CTFE is chlorotrifluoroethylene, CFE is chlorofluoroethylene and HFP is

hexafluoropropylene .

The glass transition temperature of the fluorinated polymer is generally of at most 50 °C, preferably of at most 20 °C, more preferably of at most 0 °C and even preferably of at most -5 °C. Besides, the glass transition temperature of the fluorinated polymer is generally of at least -60 °C, preferably of at least -50 °C and more preferably of at least -40 °C.

Glass transition can be measured by differential scanning calorimetry (DSC) well known by the skilled person.

In some other embodiments, the dielectric polymer may be a polyolefin such as polypropylene or polyethylene.

In some preferred embodiments the polymer suitable for the invention is a piezoelectric polymer such as PVDF, P(VDF-TrFE) copolymer or P(VDF-TrFE- CFE) terpolymer. Accordingly, the strain of a piezoelectric polymer layer is linearly proportional to the applied electric field.

In other preferred embodiments the polymer suitable for the invention is an electro strictive polymer which exhibits a strain behavior under electric field which is quadratically proportional to said electric field. These materials are materials of choice for preparing actuator for haptic application since the highest mechanical response is obtained for a given applied electric field. Just for the sake of example electro strictive polymers can be obtained by high energy irradiation of the fluorinated polymers as previously described or by addition of plasticizer to the same polymers. Electro strictive polymers can also be obtained by improving the dielectric permittivity of neat polymer such as the dielectric elastomers and the fluorinated polymers as previously described. For the purpose of increasing dielectric permittivity dispersion of conductive particles into said neat polymer is a known strategy.

In some preferred embodiments, the electroactive polymer composite layer comprises a composition comprising at least one fluorinated polymer and silver nanowires coated with at least one metal oxide wherein the fluorinated polymer is as previously described.

The metal oxide is generally selected from the list consisting of titanium, zirconium, aluminum and silicon oxides and mixtures thereof. It is preferably selected from the list consisting of silicon oxides and titanium oxides. It is more preferably silicon dioxide. Generally, the metal oxide coating is obtained by a sol-gel process conducted in the presence of silver nanowires. However, any commercially available silver nanowires coated with at least one metal oxide can be used in the process.

The sol-gel process can be seen as the hydrolysis and the condensation of metal alkoxides giving a three dimensional network of metal oxides. Just for sake of example, sol-gel process steps are described by L.Hench et al. in

Chemical Review, 1990, vol.90, n°.l, 33-70.

The sol-gel process is generally conducted in a reaction medium comprising at least one alcohol and water. It is conducted in a reaction medium comprising a volume ratio of alcohol and water generally of at most 10/1, preferably of at most 8/1, more preferably of at most 6/1 and even more preferably at most 5/1. Besides, the volume ratio of alcohol and water is generally of at least 1/5, preferably of at least 2/5, more preferably of a least 3/5 and even more preferably of at least 4/5.

Precursors for the metal oxides are generally titanium, zirconium, aluminum, or silicon alkoxides.

Precursors of silicon oxide can be but are not limited to

tetramethylortho silicate (TMOS), Tetraethylortho silicate (TEOS) or

tetraisopropylortho silicate (TPOS).

Precursors of aluminum oxide can be but are not limited to aluminum- (isopropoxide) or aluminum- (2-butoxide).

Precursor of zirconium oxide can be but is not limited to zirconium- (isopropoxide) and precursors of titanium oxide can be but are not limited to titanium- (2-ethoxide) or titanium- (isopropoxide).

The synthesis of the metal oxide via the sol-gel process can be catalyzed by the use of an acid or a basic catalyst. For example, in the former case HC1 may be involved while in the latter case ammonia may be used. In a preferred embodiment, ammonia is used as basic catalyst.

The sol-gel process can be performed by adding under stirring to a suspension of silver nanowires in a mixture comprising water and at least one alcohol, metal oxide precursors and catalyst, all these components being as previously described.

The sol-gel process can be conducted at room temperature. It is often conducted at a temperature of at least 40°C, possibly at a temperature of at least 60°C, sometimes at a temperature of at least 80°C and rarely at a temperature of at least l00°C.

In some other embodiments the sol-gel process is performed using inorganic precursors.

Inorganic precursors for the metal oxides are generally titanates, zirconates, aluminates or silicates.

Inorganic precursors are generally alkali metal or earth alkaline metal titanates, zirconates, aluminates or silicates. They are preferably alkali metal, more preferably potassium or sodium and even more preferably sodium titanates, zirconates, aluminates or silicates.

The synthesis of the metal oxide via the sol-gel process involving inorganic precursors can be catalyzed by the use of an acid catalyst. In a preferred embodiment, HC1 is used as acid catalyst.

The sol-gel process can be performed by adding under stirring to a suspension of silver nano wires in a mixture comprising water and optionally an alcohol, inorganic metal oxide precursors and catalyst, all these components being as previously described.

After the sol-gel process the silver nanowires coated with metal oxide are generally recovered from the reaction mixture. For example, the silver nanowires coated with metal oxide can be recovered by sedimentation resulting from centrifugation. Thus, the alcohol/water supernatant can be removed and the coated nanowires isolated. The coated nanowires can also be isolated by vacuum- filtering the sedimented suspension.

The weight ratio of metal oxide with regard to the total weight of silver nano wires coated with at least one metal oxide is generally of at least 1 wt. %, often of at least 5 wt. % and possibly of at least 10 wt. %. The ratio is generally of at most 25 wt. %, often of at most 20 wt. % and possibly of at most 15 wt. %.

The silver nanowires coated with at least one metal oxide suitable for the invention have generally an aspect ratio of at least 10, preferably of at least 15 and even preferably of at least 20. The silver nanowires coated with metal oxide have usually an aspect ratio of at most 5000, preferably of at most 1000, even more preferably of at most 500 and the most preferably at most 200.

The aspect ratio is the ratio of length to width of a particle (ISO, 1999). An average aspect ratio may be determined by image processing of TEM or SEM.

The coated nanowires can be further submitted to several dispersion- centrifugation cycles in a solvent to remove impurities. Among impurities one can consider species involved in the sol-gel process such as alcohol, acid or base catalyst or small metal oxide particles or any chemical which is not coated silver nanowires. Often, the coated silver nanowires are submitted to at least 2 dispersion-centrifugation cycles in the solvent, sometimes to at least 3 dispersion-centrifugation cycles, rarely to at least 4 dispersion-centrifugation cycles. The solvent may be the polar aprotic solvent suitable for the invention as will be described below.

The composition according to the invention comprises generally at least 50 wt. %, often at least 60 wt. %, sometimes at least 70 wt. % and rarely at least 80 wt. % of fluorinated polymer with regard to the total weight of the composition. Besides, the composition comprises generally at most 99.5 wt. %, often at most 99.0 wt. % and sometimes at most 98 wt. % of polymer.

The composition according to the invention comprises generally at least 0.5 wt. %, often at least 1 wt. % and sometimes at least 2 wt. % of silver nano wires coated by metal oxide with regard to the total weight of the composition. Besides, the composition comprises generally at most 50 wt. %, often at most 40 wt. % of silver nanowires.

The composition according to the invention is generally free of any surfactant. This is a desirable situation when the composition is aimed to be used in very demanding applications. For example, it may be the case when the composition is intended to be used for preparing materials comprising silver nanowires coated with at least one metal oxide and fluorinated polymers for some electronic applications.

In certain embodiments, it is substantially free of any surfactant. In this case the weight ratio of surfactant with regard to Ag metal is generally of at most 0.01 wt. %, often of at most 0.005 wt. %.

In other embodiments the composition comprises at least one surfactant. When the surfactant is present, the weight ratio of surfactant with regard to Ag metal is generally of at least 0.1 wt. %. It is preferably of at least 0.5 wt.% and more preferably of at least 1 wt.%.

Besides, the weight ratio of surfactant with regard to Ag metal is generally of at most 250 wt.%. It is often of at most 200 wt.% and more possibly of at most 150 wt.%.

According to IUPAC a surfactant is a substance which lowers the surface tension of the medium in which it is dissolved, and/or the interfacial tension with other phases, and, accordingly, is positively adsorbed at the liquid/ vapor and/or at other interfaces.

The surfactant that may be present in the composition according to the invention can be selected from the list consisting of anionic, cationic, amphoteric, non-ionic surfactants and mixtures thereof.

Anionic surfactants suitable for the invention are generally chosen from the list consisting of phosphates, sulfonates, sulfates, carboxylates and mixtures thereof.

Cationic surfactants suitable for the invention are generally chosen from the list consisting of phosphonium, ammonium and pyridinium salts. Ammonium salts corresponding to the formula (1) are preferred :

Formula (1)

wherein Ri, R ₂ , R ₃ and R ₄ , which may be the same or different represent H or a C1-C30 hydrocarbyl or heterohydrocarbyl group and,

wherein X is an halogen atom or an alkyl sulfate group.

The term“hydrocarbyl” as used herein refers to a group only containing carbon and hydrogen atoms. The hydrocarbyl group may be saturated or unsaturated, linear, branched or cyclic. If the hydrocarbyl is cyclic, the cyclic group may be an aromatic or non-aromatic group.

The term“heterohydrocarbyl” as used herein refers to a hydrocarbyl group wherein one or more of the carbon atom(s) is/are replaced by a heteroatom, such as Si, S, N or O. Included within this definition are heteroaromatic rings, i.e. wherein one or more carbon atom within the ring structure of an aromatic ring is replaced by a heteroatom.

Distearyl dimethyl ammonium chloride, distearyl dimethyl ammonium bromide, lauryl trimethyl ammonium chloride, lauryl trimethyl ammonium bromide, cetyl trimethyl ammonium chloride, cetyl trimethyl ammonium bromide, alkyl dimethyl benzyl ammonium chloride, alkyl dimethyl benzyl ammonium bromide, cetyl pyridinium chloride, cetyl pyridinium bromide, didecyl dimethyl ammonium chloride and didecyl dimethyl ammonium bromide are examples of adequate quaternary ammonium surfactants.

In a preferred embodiment cetyl trimethyl ammonium bromide is advantageously used. Amphoteric surfactants suitable for the invention are generally chosen from the list consisting of betaines, sulfo betaines and amine oxides.

Cocamidopropyl dimethyl betaine and lauramidopropyl betaine, coco hydroxypropyl sulphobetaine and dodecyl hydroxypropyl sulphobetaine, coco N, N - dimethylamine-N-oxide and N, N - dimethyldodecylamine-N-oxide are examples of respectively adequate betaines, sulfobetaines and amine oxides.

Non-ionic surfactants suitable for the invention are generally chosen from the list consisting of alkoxylates, pyrrolidinones, glycerides, glycosides and amines.

Lauryl alcohol ethoxylate, nonyl phenol ethoxylate, stearyl alcohol ethoxylate and cetostearyl alcohol ethoxylate are examples of appropriate alkoxylates .

Suitable amines correspond to the formula NR5R6R7 wherein R5, R ₆ and R ₇ , which may be the same or different represent H or a C1-C30 hydrocarbyl or heterohydrocarbyl group; n-dodecylamine is an example of appropriate amine. The inventors have found advantageous to use n-dodecylamine. Good results were obtained using Fentamine® A12 available from Solvay Novecare.

In some embodiments, suitable non-ionic surfactants are chosen from poly(alkylene oxide)s. Poly(alkylene oxide)s suitable for use in the present invention are polymers essentially all or all the repeating units of which comply with general formula -C _n H _2n- 0- wherein -C _n Fl _2n - represents a divalent alkylene group with n ranging from 2 to 10. Such poly(alkylene oxide) s may be terminated by a hydroxyl group. Particularly suitable poly(alkylene oxide) s are those wherein n ranges from 2 to 4, preferably from 2 to 3, more preferably wherein n = 2. The poly( alkylene oxide)s may be either linear or branched.

Linear poly( alkylene oxides) are generally preferred.

Specific examples of suitable poly( alkylene oxide) s include

polyoxyalkylene polyols, such as polyoxyethylene glycol (also known as poly(ethylene glycol) or poly(ethylene oxide)), polyoxyethylene triol, polyoxyethylene tetraol, polyoxypropylene glycol (also commonly referred to as polypropylene glycol) or polypropylene oxide), polyoxypropylene triol, polyoxypropylene tetraol, polyoxybutylene glycol, , polyoxypentane glycol, polyoxyhexane glycol, polyoxyheptane glycol, and polyoxyoctane glycol. These polymers may be used either individually or in combinations of two or more; for example, it can be cited random copolymers of ethylene oxide and propylene oxide, and polypthylene oxide)-poly(propylene oxide) block copolymers. The hydroxyl end groups of the poly(alkylene oxide)s may according to a preferred embodiment be partly or fully substituted by alkoxide groups, preferably methoxy or alkoxy. Methods for converting hydroxyl groups of poly(alkylene oxide) s into alkoxy groups are known to the skilled man and described in the literature.

Certain suitable oxyalkylene-containing compounds suitable in the compositions in accordance with the instant invention are amine-terminated poly(alkylene oxide) s, in particular amine-terminated poly(ethylene oxide) s or amine-terminated polypropylene oxide) s, including copolymers comprising both mentioned types of oxyalkylene units which are commercially available under the tradename Jeffamine® from Huntsman Chemical Corporation.

In certain cases poly(alkylene oxide) s having a number average or weight average molecular weight of at least 20,000, preferably at least 200,000 and even more preferably at least 1,000,000 are advantageous. In other cases average molecular weights of at most 20,000, preferably at most 10,000 and even more preferably at most 1000 are useful. The molecular weight of the poly(alkylene oxides) suitable may also be optimized. For example, a methoxy-terminated poly(ethylene oxide) having a number average or a weight average molecular weight of at most 2,000 may be used.

Copolymers comprising oxyethylene and oxypropylene units in random or block distribution may also be suitable and respective products are commercially available under the tradename Pluronics® from BASF and Synperonics® from CRODA.

The composition according to the invention is generally free of any solvent.

Often it is substantially free of any solvent. In this case a polar aprotic solvent can be present generally in an amount not exceeding 0.5 wt. %, often not exceeding 0.2 wt. % sometimes not exceeding 0.1 wt. % based on the total weight of the composition.

The electroactive polymer composite layer suitable for the invention may be obtained by any method well known by the skilled person.

Just for the sake of example it may be prepared by casting of a composition comprising core- shell particles, at least one polymer and a solvent of the polymer on a substrate so as to form a swollen film followed by solvent removal so as to obtain a film corresponding to the layer. It may be prepared and then assembled with the other elements of the haptic system or it may be prepared directly onto an inert substrate or onto an electrode beforehand deposited onto an inert substrate.

The electroactive polymer composite layer suitable for the invention generally comprises at least 50 wt. %, often at least 60 wt. %, sometimes at least 70 wt. % and rarely at least 80 wt. % of polymer with regard to the total weight of the composite layer. Besides, the layer comprises generally at most 99.5 wt.

%, often at most 99.0 wt. % and sometimes at most 98 wt. % of polymer.

The electroactive layer according to the invention comprises generally at least 0.5 wt. %, often at least 1 wt. % and sometimes at least 2 wt. % of core shell particles with regard to the total weight of said electroactive layer. Besides, the electroactive layer comprises generally at most 50 wt. %, often at most 40 wt. % of core-shell particles.

In some embodiments, the electroactive polymer composite layer also comprises at least one other ingredient; for example, it may comprise a stabilizing agent, a plasticizer or a processing aid.

The polymer composite layer has generally an average thickness not exceeding 500 pm, preferably not exceeding 200 pm, more preferably not exceeding 100 pm. The average thickness of the layer is at least 5 pm, preferably at least 10 pm, more preferably at least 15 pm. In a most preferred embodiment, the average thickness of the layer ranges froml5 pm to 100 pm.

The thickness may be measured using a digital micrometer onto the layer recovered from the process described above. The value can be the average value of at least ten different measurements made along the layer and distanced of at least 1 cm.

Electrodes suitable for activating the electroactive polymer composite layer according to the invention can be obtained by any method well known by the skilled person.

Suitable electrodes may comprise materials selected from the list consisting of Au, Ag, Pt, Al, Ni, Pd, Cu, Mo, Ti, Cr, Al-Cu alloy, ITO (Indium- Tin Oxide), conducting polymers (like PEDOT, in particular PEDOT-PSS or PANI) and C.

Preferred electrodes comprise Ag, Au or Pt and more preferred electrodes comprise Ag.

In some embodiments, the electroactive layer may be located between the pair of driving electrodes. In some other embodiments, when patterned the driving electrodes may be located on the same side of the electroactive layer and preferably interdigited.

Still in other embodiments, the stack comprising electroactive layer and electrodes may be attached to a substrate. Suitable substrates may be, without being exhaustive, PET (polyethylene terephalate), PEN (polyethylene naphtalate), polyimide, PEEK (polyether ether ketone) or glass.

Generally, the electroactive polymer composite layer according to the invention is capable of being activated by an excitation having a frequency ranging from 1 to 10000 Hz. Preferably, it is capable of being activated by an excitation having a frequency of at least 50 Hz, more preferably of at least 100 Hz. Besides, it is capable of being activated by an excitation having a frequency of at most 400 Hz, more preferably of at most 300 Hz.

Generally, the electroactive polymer composite layer according to the invention is capable of vibrating at a frequency ranging from 1 to 10000 Hz. Preferably, it is capable of vibrating at a frequency of at least 50 Hz, more preferably of at least 100 Hz. Besides, it is capable of vibrating at a frequency of at most 400 Hz, more preferably of at most 300 Hz.

Generally, the electroactive polymer composite layer according to the invention is capable of deforming with an acceleration ranging from 5 to 150 m/s . Preferably, it is capable of deforming with an acceleration of at least 10 m/s 2 and more preferably of at least 20 m/s 2. Besides, it is preferably capable of deforming with an acceleration of at most 120 m/s and more preferably of at most 100 m/s ² .

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.