WO2019073013A1

WO2019073013A1 - Stabilization of silver nanowires coated with oxide in various solvents

Info

Publication number: WO2019073013A1
Application number: PCT/EP2018/077809
Authority: WO
Inventors: Colbeau-Justin FRÉDÉRIC; Sanseau OLIVIER; Mignard MATHILDE
Original assignee: Rhodia Operations
Priority date: 2017-10-12
Filing date: 2018-10-11
Publication date: 2019-04-18

Abstract

The present invention concerns a suspension of silver nanowires coated with at least one oxide in a polar aprotic solvent comprising a surfactant, optionally comprising a polymer, and concerns a process for preparing such a suspension.

Description

Stabilization of silver nanowires coated with oxide in various solvents

This application claims priority to European application No.17306384.3 filed on October 12, 2017 the whole content of this application being

incorporated herein by reference for all purposes.

The present invention concerns a suspension of silver nanowires coated with at least one oxide in a polar aprotic solvent in the presence of a surfactant and the process for preparing said suspension. It also concerns a suspension of silver nanowires coated with at least one oxide in a solution comprising a polymer dissolved in the polar aprotic solvent in the presence of a surfactant and process for preparing film from this suspension. Finally, the invention concerns the resulting films.

US 2011/0045272 discloses the synthesis of silver nanowires conducted by a polyol process involving the reduction of an Ag metal precursor. Silver nanowires are synthesized by mixing reagents i.e. silver salt,

polyvinylpyrolidone (PVP) and glycerol. The polyol serves both as the solvent and as the reducing agent. The shape and the size of the nanostructures are influenced by the relative amount of PVP and of the metal salt. A post-synthesis treatment of the silver nanowires is conducted by wash cycles by a polar aprotic solvent, namely a ketone which selectively and progressively removes the metal structures having aspect ratio of less than 10. Accordingly silver nanowires are purified by precipitation/sedimentation after addition to a silver nanowires suspension in water of the ketone. In US 2011/0045272, ketone is clearly used for the purpose of destabilizing the nanowires suspension.

Y.Yin et al. in NanoLetters, 2002, vol.2, n°.4, p. 427-430 describes the direct coating of silver nanowires with amorphous silica. Silver nanowires are prepared by reduction of AgN0₃ with ethylene glycol using PVP as structure- directing agent. After synthesis, the silver nanowires are rinsed with acetone. The coating of the silver nanowires is conducted via a sol-gel process involving tetraethyl orthosilicate (TEOS) and ammonia. No primer is required before the condensation of silica onto the surface of silver. Once again, the ketone, namely acetone, is used for the purpose of purifying the uncoated silver nanowires before the sol-gel process. C.Chen et al. in Composites Science and Technology, 2014, vol. 105, p. 80-85 proposes the preparation of silver nanowires coated with silica synthesized via a sol-gel approach. After synthesis the coated silver nanowires are dispersed in ethanol and mixed with epoxy resin and a curing agent to give a crosslinkable composite precursor mixture.

X.Huang et al. in Polymer International, 2014, vol. 63, p. 1324-1331 describes the preparation of Ti0₂ coated Silver micro/nanowires composites with poly(arylene ether nitrile) which requires the preparation of a suspension of the coated silver nanowires in N-methyl pyrrolidone (NMP). Nothing is said about the stability of the suspension. Moreover, the solvent removal from the composite is performed by evaporation at temperature as high as 200°C, which is not cost-effective and which may be detrimental in some temperature sensitive applications.

The prior art such as above described consider polar aprotic solvents such as ketones and more particularly acetone as a solvent convenient for washing the silver nanowires after synthesis.

The prior art generally considers the dispersion of silver nanowires in polar pro tic solvents such as water, alcohol and mixture thereof. This represents a limitation for the possible applications of silver nanowires. Indeed, while it is commonly admitted that suspension is a convenient way to handle nano-objects safely, the versatility of use of the nano-objects is strongly related with the nature of the solvent.

Just as matter of example the suspension of silver nanowires in solvents that are good solvents for organic polymers may be helpful to prepare compositions, composites comprising these polymers via a solvent process. Accordingly, the suspension of silver nanowires in solvents that are good solvents for fluorinated polymers may be helpful to prepare composites of these fluorinated polymers via solvent processes.

Suspensions in solvents are highly desirable for safe handling of silver nanowires.

Stable suspensions are also wished to allow controlling of the amount of silver nanowires collected when samples are taken for an application.

Suspensions of silver nanowires in solvents are stable to a certain extent. After a while, sedimentation of silver nanowires is observed. Generally, this sedimentation is accompanied by some aggregation of the nanowires and further dispersion is impossible even when submitting the media to high energy through vigorous stirring or through ultrasonic bath treatment. There is also a need for redispersible silver nano wires suspensions in solvents after sedimentation has occurred.

Suspensions of silver nanowires in polar aprotic solvents are needed to ensure the versatility of the applications.

Suspensions of silver nanowires in solvents that are good solvents for organic polymers, especially fluorinated polymers, are sought after for preparing composites via a solvent process and for making films of these composites.

Suspensions in solvents having relatively low boiling points are also highly desirable to ensure an easy removal of these solvents required in most applications.

Finally, there is a need for stable suspensions of silver nanowires in solvents having low toxicity.

All these needs and others are met by a suspension of silver nanowires coated with at least one oxide selected from the list consisting of titanium, zirconium, hafnium, vanadium, aluminum, gallium, indium, silicon, germanium and tin oxides and mixtures thereof in a polar aprotic solvent, wherein said suspension comprises at least one surfactant.

The oxide is often 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 and titanium dioxides and mixtures thereof. It is more preferably silicon dioxide.

In some embodiments the oxide is selected from the list consisting of hafnium, vanadium, gallium, indium, germanium and tin oxides and mixtures thereof.

Suitable silver nanowires may be any commercially available silver nanowires coated with at least one oxide and the suspension according to the present invention is not linked to a particular route for synthesizing silver nanowires coated with at least one oxide.

However, the oxide coating is generally obtained by a sol-gel process conducted in the presence of silver nanowires.

The sol-gel process can be seen as the hydrolysis and the condensation of metal alkoxides to give a three dimensional network of 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 oxides are generally titanium, zirconium, aluminum, or silicon alkoxides.

Precursors of silicon oxide can be but are not limited to

tetramethylorthosilicate (TMOS), Tetraethylorthosilicate (TEOS) or

tetraisopropylorthosilicate (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).

Some other precursors for the oxides are generally hafnium, vanadium, gallium, indium, germanium or tin alkoxides such as hafnium isopropoxide, vanadium (V) oxytriisopropoxide, tetraethyl orthogermanate, gallium (III) isopropoxide, indium ethoxide or tin tert-butoxide.

The synthesis of the 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 nano wires in a mixture comprising water and at least one alcohol, 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 100°C.

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

Some other inorganic precursors for the oxides are generally vanadates, germanates or stannates such as potassium or sodium and preferably sodium vanadates, germanates or stannates.

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

After the sol-gel process the silver nanowires coated with oxide are generally recovered from the reaction mixture. For example, the silver nanowires coated with 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 oxide with regard to the total weight of silver nanowires coated with at least one 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 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 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. 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 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 used for removing impurities can be any solvent. However, in certain embodiments several dispersion-centrifugation cycles are run with the polar aprotic solvent according to the invention.

The dispersion of silver nanowires coated with at least one oxide can be obtained by adding energy to their mixture with the solvent. A sonication treatment can be used but any other mean of providing energy to the mixture can be considered.

The uncoated silver nanowires which may be used for the coating by sol- gel process have generally an aspect ratio of at least 10, preferably of at least 15 and even preferably of at least 20. The raw silver nanowires 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 suspension according to the present invention is not linked to a particular route for synthesizing uncoated silver nanowires. Suitable but non limiting routes can be found as a matter of example in X.-Z. Xiang et al, Rare Metals (2016), 35(4), 289-298.

However, silver nanowires are generally prepared by reduction of a silver salt in the presence of polyols which act as solvent and as reducing agent with the help of a structure-directing agent. This method, namely polyols method, combines several advantages allowing large-scale preparation of silver nanowires. For example, AgN0₃ can be the silver salt, ethylene glycol or propylene glycol the polyol and PVP the directing agent. The reaction is generally stopped by quenching using cold water.

The silver nanowires are generally under the form of a slurry comprising alcohol, water or mixtures thereof. Alcohol can be methanol, ethanol, propanol, butanol, ethylene glycol, propylene glycol or mixtures thereof. However, they may be used under the form of a solid material.

The content of silver in the silver nanowires slurry can be determined by inductively coupled plasma optical emission spectrometry (ICP-OES). For this purpose, the intensity of the emission measured for a sample diluted in nitric acid 5% aqueous solution at the silver specific wavelength (eg. 328.068 and 338.289 nm) is compared to a calibration curve in a similar range of concentration of silver standards obtained in similar analytical conditions.

The silver nanowires are generally recovered from the slurry by sedimentation resulting from centrifugation. Thus, the supernatant comprising alcohol or/and water can be removed and the nanowires isolated. The nanowires can also be isolated by vacuum- filtering the sedimented suspension.

The nanowires can be further submitted to several dispersion- centrifugation cycles in the slurry comprising alcohol, water or mixtures thereof to remove impurities. Among impurities one can consider silver nanoparticles having an aspect ratio below 10. Often, silver nanowires are submitted to at least 2 dispersion-centrifugation cycles, sometimes to at least 3 dispersion- centrifugation cycles, rarely to at least 4 dispersion-centrifugation cycles in order to remove impurities.

The solvent used in the suspension according to the present invention is a polar aprotic solvent. According to IUPAC a polar aprotic solvent is a solvent with high dielectric constant and a sizable permanent dipole moment that cannot donate suitably labile hydrogen atoms to form strong hydrogen bonds.

Polar aprotic solvents suitable for the suspension according to the invention are solvents having a dielectric constant measured at 20 °C generally over 5. The dielectric constant of the polar aprotic solvent according to the invention is preferably more than 10, more preferably more than 12 and even more preferably more than 15.

The dielectric constant of solvents can be determined using for example BI-870 Dielectric Constant Meter available from Brookhaven Instruments Corporation, following the recommendations of the provider.

Suitable polar aprotic solvents are solvents having a dipole moment generally over 0.5 Debye. The dipole moment of the solvent is preferably more than 1.0 Debye, more preferably more than 1.5 Debye and even more preferably more than 2.0 Debye.

It is advantageous to use a solvent having a relatively low boiling point when the suspension is aimed to be used in an application requiring removal of this solvent. Removal at lower temperature of low boiling point solvent is cost effective and may be useful when conducted in the presence of temperature sensitive compounds or materials. Suitable polar aprotic solvents are solvents having a normal boiling point generally below 250°C, preferably below 200°C, more preferably below 150°C and even more preferably below 100°C.

Polar aprotic solvents useful for the invention are solvents having a normal boiling point generally above 40 °C and preferably above 50°C.

Normal boiling point is measured at atmospheric pressure by any method well known by the skilled person.

Dielectric constant, dipole moment and boiling point of pure solvents can be found in the "Handbook of Organic Solvent Properties" edited by

I.M.Smallwood and published by Elsevier in 1996 (ISBN : 978-0-340-64578-9).

The solvents convenient for the suspension according to the invention are generally chosen in the list consisting of ketones, ethers, esters, amides, nitriles, sulfoxides and mixtures thereof.

For sake of example, among amides, dimethylformamide,

dimethylacetamide and mixtures thereof may be used; among esters, ethylacetate may be used; among ethers, 2-methyltetrahydrofuran, tetrahydrofuran and mixture thereof may be used; among nitriles, acetonitrile may be used and among sulfoxides, dimethylsulfoxide may be used.

Still for sake of example mixture of ethers with amides such as a mixture of 2-methyltetrahydrofurane with dimethylformamide may be used.

In a preferred embodiment, the solvent is selected from ketones and mixtures thereof. Acetone, methylethyl ketone, diethyl ketone, cyclopentanone, cyclohexanone and mixtures thereof are advantageously used and methylethyl ketone may be preferred.

The suspension according to the invention comprises at least one surfactant. 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/ vapour and/or at other interfaces. In the present invention, the surfactant has the effect of stabilizing the suspension of the coated silver nano wires in the polar aprotic solvent.

The surfactant can be selected from the list consisting of anionic, cationic, amphoteric, non- ionic surfactants and mixtures thereof.

Suitable anionic surfactants are generally chosen from the list consisting of phosphates, sulfonates, sulfates, carboxylates and mixtures thereof. Suitable cationic surfactants are generally chosen from the list consisting of phosphonium, ammonium and pyridinium salts. Ammonium salts

corresponding to the formula (1) are preferred :

R₂

+

R!— N-R₄ X

3 Formula (1)

wherein R_{l s} R₂, R₃ and R4, which may be the same or different represent H or a Ci-C₃o 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.

The inventors have found advantageous to use cetyl trimethyl ammonium bromide.

Suitable amphoteric surfactants are generally chosen from the list consisting of betaines, sulfobetaines 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.

Suitable non- ionic surfactants 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, and R₇, 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_nH2_n-0- wherein -C_nH2_n- 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 poly(propylene glycol) or poly(propylene 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 poly(ethylene 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 poly(propylene 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.

In the suspension according to the invention, the weight ratio of surfactant with regard to Ag metal is generally of at least 0.1 wt. %. It is often of at least 0.5 wt. % and possibly 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 possibly of at most 150 wt. %.

The Ag content in the suspension according to the invention is generally of at least 50 ppm. It is preferably of at least 100 ppm. The Ag content is generally determined by ICP-OES. The mass fraction of Ag is expressed in parts per million (ppm).

The Ag content in the suspension is generally of at most 200000 ppm. It is preferably of at most 50000 ppm, more preferably of at most 20000 ppm and even more preferably of at most 10000 ppm.

Another object of the invention is a suspension of silver nano wires coated with at least one oxide in a polar aprotic solvent comprising a surfactant, wherein at least one polymer is dissolved in the solvent.

The polymer is generally selected from the list consisting of polyamides, polysulfones, polyesters, polyethers, polyphenylene sulfides, polyvinylidene chlorides, polyketones, polyimides, polyetherimides, polyamideimides and fluorinated polymers.

Suitable poly condensation engineering polymers used in certain embodiments include at least partially aromatic polyamides, polyarylethersulfones, polyphenylenesulfides, polyamideimides, polyimides, polyetherimides and polyaryletherketones polymers.

Suitable at least partially aromatic polyamides are polyphthalamides formed by polycondensation reaction between at least one aromatic dicarboxylic acide and a diamine. In some embodiments the aromatic dicarboxylic acid is terephtalic acid. In other embodiments the polyphthalamide further comprises isophtalic acid residues. Suitable polyphthalamides for use in the present invention are available as AMODEL® polyphthalamides from Solvay Specialty Polymers.

Other at least partially aromatic polyamides suitable for use in the present invention are formed from the reaction between an aliphatic dicarboxylic acid e.g. adipic acid and an aromatic diamine e.g. m- xylene diamine. IXEF® polyarylamide from Solvay Specialty Polymer is an example of such suitable at least partially aromatic polyamide.

Polyarylethersulfones are other aromatic polycondensation polymers suitable for use in the present invention. They comprise arylene units linked with each other either by ether linkages or by sulfone linkages, both types of linkage being present in the polymer. UDEL^® polysulfone and polyphenylsulfone commercially available as RADEL^® R from Solvay Specialty Polymers are examples of suitable polyarylethersulfones.

In other embodiments the polymers suitable for use in the present invention are fluorinated polymers. 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 for the invention are chosen among vinylidene fluoride homopolymers or copolymers in order to provide advantageously high chemical resistance.

Vinylidene fluoride copolymers comprise generally at least 50 % by moles, very often 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 from 0 to 15 % by moles of repeat units derived from monomers selected from the list consisting of vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, 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 , R₂ and R₃ are equal to or different from each other, are independently selected from hydrogen atom and a C1-C4 group, and

R4 is selected from hydrogen atom and C1-C12 group optionally comprising at least one heteroatom.

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

hydroxypropyl(meth)acrylate; hydroxyethylhexyl(meth)acrylates.

The (meth)acrylic monomer is preferably selected among:

- hydroxyethylacrylate (HEA) of formula:

OH

- 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.

In some other embodiments, the suitable fluorinated polymer is a fluoropolymer [polymer (F)] comprising: - recurring units derived from vinylidene fluoride (VDF);

- from 10% to 50% by moles of recurring units derived from

trifluoroethylene (TrFE); and

- from 0.01 % to 10% by moles of recurring units derived from at least one (meth)acrylic monomer [monomer (MA)] having formula (II) here below:

wherein:

- R'i, R'₂ and R'₃, equal to or different from each other, are independently selected from a hydrogen atom and a Ci-C₃ hydrocarbon group, and - R'_OH represents a hydrogen atom or a Ci -C₅ hydrocarbon moiety comprising at least one hydroxy 1 group.

The polymer (F) of the invention comprises preferably from 15% to 48% by moles, more preferably from 16% to 45% by moles, even more preferably from 17% to 40% by moles of recurring units derived from trifluoroethylene (TrFE).

The (meth)acrylic monomer [monomer (MA)] preferably complies with formula (III) here below:

wherein:

- R"i and R"₂, equal to or different from each other, are independently selected from a hydrogen atom and a Ci-C₃ hydrocarbon group, preferably R"i and R"₂ being hydrogen atoms,

- R"₃ is a hydrogen atom, and

- R"_OH represents a hydrogen atom or a C₁-C5 hydrocarbon moiety comprising at least one hydroxy 1 group. Non- limitative examples of (meth)acrylic monomers (MA) notably include acrylic acid, methacrylic acid, hydroxyethyl(meth)acrylate,

hydroxypropyl(meth)acrylate, hydroxyethylhexyl(meth)acrylate.

The monomer (MA) is more preferably selected from the folio wings: - hydroxyethylacrylate (HEA) of formula: H

- 2-hydroxypropyl acrylate (HP A) of either of formulae:

- acrylic acid (AA) of formula:

- and mixtures thereof.

The monomer (MA) is even more preferably acrylic acid (AA) or hydroxyethylacrylate (HEA).

The polymer (F) of the invention may further comprise recurring units derived from one or more other fluorinated comonomers [comonomer (F)].

The term "fluorinated comonomer [comonomer (F)]" is hereby intended to denote an ethylenically unsaturated comonomer comprising at least one fluorine atom.

The comonomer (F) may further comprise one or more other halogen atoms such as chlorine, bromine and iodine atoms. Non- limitative examples of suitable comonomers (F) notably include the folio wings:

(i) C₂-Cs perfluoroolefins such as tetrafluoroethylene (TFE) and hexafluoropropylene (HFP);

(ii) perfluoroalkylethylenes of formula CH₂=CH-R_fo, wherein Ra is a C₂-

C₆ perfluoroalkyl group;

(iii) chloro- and/or bromo- and/or iodo-C₂-C₆ fluoroolefins such as chlorotrifluoroethylene (CTFE);

(iv) perfluoroalkylvinylethers of formula CF₂=CFOR_fi, wherein RA is a Ci-C₆ perfluoroalkyl group, such as perfluoromethylvinylether (PMVE) and perfluoropropylvinylether (PPVE);

(v) (per)fluorooxyalkylvinylethers of formula CF₂=CFOXo, wherein X₀ is a Ci-Ci₂ oxyalkyl group or a Ci-Ci₂ (per)fluorooxyalkyl group having one or more ether groups, e.g. perfluoro-2-propoxy-propyl group;

(vi) (per)fluoroalkylvinylethers of formula CF₂=CFOCF₂OR¾ wherein R^ is a Ci-C₆ (per)fluoroalkyl group, e.g. -CF₃, -C₂F₅, -C₃F₇, or a Ci-C₆

(per)fluorooxyalkyl group having one or more ether groups, e.g. -C₂F₅-0-CF₃;

(vii) functional (per)fluorooxyalkylvinylethers of formula CF₂=CFOYo, wherein Y₀ is selected from a Ci-Ci₂ alkyl group or (per)fluoroalkyl group, a Ci- C₁₂ oxyalkyl group and a Ci-Ci₂ (per)fluorooxyalkyl group having one or more ether groups, Yo comprising a carboxylic or sulfonic acid group, in its acid, acid halide or salt form;

(viii) fluorodioxoles, especially perfluorodioxoles.

The comonomer (F) is preferably free of hydrogen atoms.

Most preferred fluorinated comonomers (F) are chlorotrifluoroethylene

(CTFE), perfluoromethylvinylether (PMVE), tetrafluoroethylene (TFE), hexafluoropropylene (H FP).

Should the fluorinated comonomer (F) be present, the polymer (F) of the invention comprises typically from 2% to 20% by moles, preferably from 3% to 18% by moles, more preferably from 4% to 15% by moles of recurring units derived from said fluorinated comonomer (F).

Good results were obtained using P(VDF-TrFE-CTFE) terpolymer Solvene® T provided by Solvay Specialty Polymers as the fluorinated polymer [polymer (F)].

The suspension wherein at least one polymer is dissolved in the solvent according to the invention contains generally at least 50 wt. %, preferably at least 60 wt. %, more preferably at least 70 wt. % of the solvent, based on the total weight of the suspension. Besides, the suspension contains generally at most 95 wt. %, preferably at most 90 wt. % of the solvent, based on the total weight of the suspension.

The suspension contains at least 5 wt. % of the polymer, often at least 10 wt. %. Besides, the suspension contains generally at most 50 wt. % of the polymer, often at most 40 wt. %, possibly at most 30 wt. % of the polymer, based on the total weight of the suspension.

Good results can be obtained with suspension comprising from 10 to 30 wt. % of the polymer and from 90 to 70 wt. % of the solvent, wherein the aforementioned wt. % are based on the total weight of the suspension.

The Ag content in the suspension comprising polymer is generally of at least 50 ppm. It is often of at least 100 ppm.

The Ag content in the suspension is generally of at most 200000 ppm. It is often of at most 50000 ppm, sometimes of at most 20000 ppm and rarely of at most 10000 ppm.

The molar ratio between Ag and the surfactant is generally of at least 0.05. It is often of at least 0.1

Besides, the molar ratio between Ag and the surfactant is generally of at most 1000. It is often of at most 500 and possibly of at most 250.

The weight ratio of surfactant with regard to Ag metal is generally of at least 0.1 wt. %. It is often of at least 0.5 wt.% and possibly 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 possibly of at most 150 wt.%.

In some embodiments, the suspension according to the invention, having or not at least one polymer dissolved in the solvent, may additionally comprise at least one other ingredient; for example, it may comprise a stabilizer, a plasticizer or a processing aid. Good results can be obtained with suspension composed essentially of, or even composed of the silver nanowires coated with at least one oxide, the surfactant, optionally the polymer and the polar aprotic solvent.

The suspension according to the invention is often free or substantially free of any additive such as plasticizers, processing aids or stabilizing agents.

The suspension according to the invention is stable in the meaning that the time which is needed before visually observing the nanowires sedimentation is long enough for example to take a sample containing a predictable amount of silver nano wires.

Another object of the invention is to propose a process for preparing a suspension of silver nanowires coated with at least one oxide in a polar aprotic solvent comprising the step of dispersing the silver nanowires coated with at least one oxide in a polar aprotic solvent in the presence at least one surfactant.

Dispersing the silver nanowires coated with at least one oxide in a polar aprotic solvent in the presence of a surfactant so as to obtain a stable suspension is generally conducted by sonication. However, any other mean of providing energy to the mixture comprising the silver nanowires coated with at least one oxide, the surfactant and the polar aprotic solvent can be considered.

Dispersing can be conducted, for example and not in a limitative way, by using vessels equipped with agitator and optionally baffles, static mixers or high shear dispersers.

Still another object of the invention is to propose a process for preparing a suspension of silver nanowires coated with at least one oxide in a polar aprotic solvent comprising a surfactant, wherein at least one polymer is dissolved.

In a first embodiment, the process comprises the step of dispersing the silver nanowires coated with at least one oxide in a polar aprotic solvent, wherein at least one polymer is dissolved, in the presence of at least one surfactant.

Dispersing can be conducted as previously described.

In a second embodiment, the process comprises the steps of

- dispersing silver nanowires coated with at least one oxide in a polar aprotic solvent in the presence of at least one surfactant so as to obtain a suspension (a),

- preparing a solution (b) comprising at least one polymer and a polar aprotic solvent,

- mixing the suspension (a) with the solution (b) so as to obtain said suspension.

Dispersing the silver nanowires coated with at least one oxide, in the presence of a surfactant, in a polar aprotic solvent so as to obtain the suspension (a) is generally conducted by sonication. However, any other mean of providing energy to the mixture as previously described may be used.

The solution (b) may be obtained by dissolving the fluorinated polymer in the polar aprotic solvent under stirring possibly accompanied by heating. Just for sake of example, mixing the suspension (a) with the solution (b) can be performed by mechanical stirring of the mixture in a vessel. In some embodiment this mechanical stirring is accompanied by further sonication.

The sonication may advantageously allow the homogeneous dispersion of the silver nanowires in the composition.

In some embodiments, the process according to the invention may comprise a prior step of oxide coating of the silver nanowires by a sol-gel process as previously described.

In some other embodiments it may also comprise a step of synthesis of uncoated silver nanowires as previously described prior to the step of metal coating.

The time which is needed before the nanowires sedimentation is visually observed can be a way to assess the stability of the suspension according to the invention. Accordingly, the longer the time for sedimentation is, the more stable the suspension.

Sedimentation of the suspension according to the invention is a reversible process and the coated silver nanowires can be dispersed again for example by a further sonication treatment.

It is another objective of the invention to disclose a film comprising - silver nanowires coated with at least one oxide selected from the list consisting of titanium, zirconium, hafnium, vanadium, aluminum, gallium, indium, silicon, germanium and tin oxides and mixtures thereof,

- a polar aprotic solvent in an amount not exceeding 0.5 wt. %, preferably not exceeding 0.1 wt. % based on the total weight of the film,

- a surfactant, and

- at least one polymer.

In some embodiments the film according to the invention comprises at least one oxide selected from the list consisting of titanium, zirconium, aluminum and silicon oxides and mixtures thereof.

In some other embodiments the film according to the invention comprises at least one oxide selected from the list consisting of hafnium, vanadium, gallium, indium, germanium and tin oxides and mixtures thereof.

Suitable polar aprotic solvents are solvents having a normal boiling point generally below 200°C, preferably below 180°C, more preferably below 150°C and even more preferably below 100°C. Suitable polymers comprised in the film according to the invention are those present in the suspension according to the invention as previously described.

It is therefore desirable and is an object of the invention to propose a process for preparing a film comprising the steps of

- casting the suspension according to the invention wherein at least one polymer is dissolved on a substrate, so as to form a swollen film,

- removing from the swollen film the polar aprotic solvent so as to obtain the film according to the invention.

The substrate can notably be a plate of a chemically inert material and casting can be performed using for example a doctor blade device. In a certain embodiment, the chemically inert material is glass.

Removal of the solvent can be obtained for example by heating the swollen film in an oven but any other process well known by the skilled person can be used.

Optionally, after a first solvent removal, the film may be dried under vacuum to remove residues of the solvent that may be included therein.

The film 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 polymer with regard to the total weight of the film. Besides, the film comprises generally at most 99.5 wt. %, often at most 99.0 wt. % and sometimes at most 98wt. % of polymer.

The film according to the invention comprises generally at least 0.5 wt. %, often at least 1 wt. % and sometimes at least 2 wt. % of silver nanowires coated by oxide with regard to the total weight of the film. Besides, the film comprises generally at most 50 wt. %, often at most 40wt. %, sometimes at most 30 wt. % and rarely at most 20 wt. % of silver nanowires.

The weight ratio of surfactant with regard to Ag metal in the film according to the invention is the same as the ratio described for the suspension according to the invention.

The film obtained by the process according to the invention has generally an average thickness not exceeding 200 μιη, preferably not exceeding 100 μιη, more preferably not exceeding 50 μιη. The average thickness of the film according to the invention is at least 5 μιη, preferably at least 10 μιη, more preferably at least 15 μιη. In a most preferred embodiment, the average thickness of the film ranges froml5 μιη to 50 μιη. The thickness may be measured using a digital micrometer onto the film recovered from the process described above. The value can be the average value of at least ten different measurements made along the film and distanced of at least 1 cm.

The film may additionally comprise at least one other ingredient; for example, it may comprise stabilizing agents, plasticizers or a processing aids.

Good results can be obtained with films composed essentially of, or even composed of the silver nanowires coated with at least one oxide, the surfactant, and the polymer.

Finally, the present invention relates to a device comprising such a film.

For example, said device may be a sensor (in particular a haptic sensor), an actuator (in particular a haptic actuator), an energy harvesting or an energy storage device. Generally said device can be used in haptic applications.

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.

Examples:

Silver nanowires were synthesized using the classical polyol route in propylene glycol using PVP as directing agent. At the end of the synthesis the reaction was quenched with cold water. The resulting slurry was composed of silver nanowires and silver nanoparticles in a propylene glycol / water mixture. In the following examples the slurry was submitted to a pretreatment consisting of centrifugation followed by elimination of the the supernatant in order to remove potential silver nanoparticles from nanowires.

Content of silver element into the slurry was 1360 ppm as determined by ICP-OES performed on PlasmaQuant® PQ 9000 from Analytik Jena. The measurement was carried out after digestion of a sample in nitric acid (eg. 0.2- 0.3 g of slurry with 4 mL of nitric acid 65%). The limpid solution was diluted in a nitric acid 5% aqueous solution taking into account the expected silver concentration. The intensity measured on the Silver specific wavelength (eg. 328.068 and 338.289 nm) was compared to a calibration curve in the range of 0.05 to 2 mg/L of silver standards obtained in similar analytical conditions in order to determine the amount in the diluted solution. The amount in the solution was obtained by calculation using the dilution factor. In the examples below deagglomeration of the silver nanowires and dispersion in the solvents were performed through sonication in a water bath using a Bransonic^® 221 sonifier during 5 min. at 48 kHz and 50W.

The stability of the suspension was assessed by noting the needed time for observing the sedimentation of the silver nanowires after sonication.

Attempts to redisperse silver nanowires in the solvent were performed through sonication in a water bath during 5 min. at 48 kHz and 50W one day after sedimentation was observed.

Example 1:

3 g of the slurry was centrifugated at 4500 rpm during 15min. to remove the silver nanoparticles and solvent. Silver nanowires were then dispersed under stirring in 40.3 g of an ethanol/water mixture (5 / 1 vol. / vol.) and the suspension was heated at 40°C in an oil bath before 1 ml of a solution of ammonia 28 wt. % in water was added. Then 2.6 mg (12.4.10^"6 mol.) of tetraethylortho silicate (TEOS) was added dropwise to the solution under stirring and the reaction conducted during 90 min. The suspension was then centrifugated at 4500 rpm during 15 min. to remove the solvent and unreacted TEOS. Silver nanowires coated with silica (AgNW@Si0₂) were recovered and then dispersed in 25 g of methylethyl ketone (MEK). The cycle centrifugation-dispersion in MEK was repeated 3 times. During the last dispersion in MEK, 4.8 mg (13.2.10^"6 mol.) of cetyl trimethyl ammonium bromide (CTAB) were added. Finally, the dispersion was treated using ultrasonic bath to deagglomerate nanowires. The resulting suspension was stable for at least 2 hours. After sedimentation nanowires were successfully redispersed through sonication in a water bath.

Example 2:

3 g of the slurry was centrifugated at 4500 rpm during 15min. to remove the silver nanoparticles and solvent. Silver nanowires were then dispersed under stirring in 40.3 g of an ethanol/water mixture (5 / 1 vol. / vol.) and the suspension was heated at 40°C in an oil bath before 1 ml of a solution of ammonia 28 wt. % in water was added. Then 2.6 mg (12.4.10^"6 mol.) of tetraethylortho silicate (TEOS) was added dropwise to the solution under stirring and the reaction conducted during 90 min. The suspension was then centrifugated at 4500 rpm during 15 min. to remove the solvent and unreacted TEOS. Silver nanowires coated with silica (AgNW@Si0₂) were recovered and then dispersed in 25 g of methylethyl ketone (MEK). The cycle centrifugation-dispersion in MEK was repeated 3 times. During the last dispersion in MEK, 2.3 mg (12.4.10^"6 mol.) of n-dodecylamine were added. Finally, the dispersion was treated using ultrasonic bath to deagglomerate nanowires. The resulting suspension was stable for at least 1 hour. After sedimentation nanowires were successfully redispersed through sonication in a water bath.

Example 3:

3 g of the slurry was centrifugated at 4500 rpm during 15min. to remove the silver nanoparticles and solvent. Silver nanowires were then dispersed under stirring in 40.3 g of an ethanol/water mixture (5 / 1 vol. / vol.) and the suspension was heated at 40°C in an oil bath before 1 ml of a solution of ammonia 28 wt. % in water was added. Then 2.6 mg (12.4.10^"6 mol.) of tetraethylortho silicate (TEOS) was added dropwise to the solution under stirring and the reaction conducted during 90 min. The suspension was then centrifugated at 4500 rpm during 15 min. to remove the solvent and unreacted TEOS. Silver nanowires coated with silica (AgNW@Si0₂) were recovered and then dispersed in 25 g of 2-methyltetrahydrofuran (2-Me THF). The cycle centrifugation-dispersion in 2- Me THF was repeated 3 times. During the last dispersion in 2-Me THF, 5.4 mg (14.8.10^"6 mol.) of cetyl trimethyl ammonium bromide (CTAB) were added. Finally, the dispersion was treated using ultrasonic bath to deagglomerate nanowires. The resulting suspension was stable for at least 20 minutes. After sedimentation nanowires were successfully redispersed through sonication in a water bath.

Example 4:

2.9 g of the slurry was centrifugated at 4500 rpm during 15min. to remove the silver nanoparticles and solvent. Silver nanowires were then dispersed under stirring in 39.5 g of ethanol. Then 6.5μ1 (19.1.10 ⁶ mol.) of titanium butoxide were added. The suspension was transferred into a Teflon liner which was introduced in a stainless steel autoclave at 120°C, for 12h. The suspension was then centrifugated at 4500 rpm during 15 min to remove the solvent and unreacted titanium butoxide. Silver nanowires coated with titanium oxide (AgNW@Ti0₂) were recovered and then dispersed in 28 g of MEK. The cycle centrifugation-dispersion in MEK was repeated 3 times. During the last dispersion in MEK, 4.2 mg (11.5.10^~6 mol.) of cetyl trimethyl ammonium bromide (CTAB) were added. Finally, the dispersion was treated using ultrasonic bath to deagglomerate nanowires. The resulting suspension was stable for at least 2 hours. After sedimentation nanowires were successfully redispersed through sonication in a water bath. Comparative example 1:

3 g of the slurry was centrifugated at 4500 rpm during 15min. to remove the silver nanoparticles and solvent. Silver nano wires were then dispersed under stirring in 25 g of methylethyl ketone (MEK). During the dispersion in MEK, 8.0 mg (21.9.10^"6 mol.) of cetyl trimethyl ammonium bromide (CTAB) were added. Finally, the dispersion was treated using ultrasonic bath to deagglomerate nanowires. The resulting suspension was not stable.

Comparative example 2:

2.97 g of the slurry was centrifugated at 4500 rpm during 15min. to remove the silver nanoparticles and solvent. Silver nanowires were then dispersed under stirring in 40.3 g of an ethanol/water mixture (5 / 1 vol. / vol.) and the suspension was heated at 40°C in an oil bath before 1 ml of a solution of ammonia 28 wt. % in water was added. Then 2.6 mg (12.4.10^"6 mol.) of tetraethylortho silicate (TEOS) was added dropwise to the solution under stirring and the reaction conducted during 90 min. The suspension was then

centrifugated at 4500 rpm during 15 min. to remove the solvent and unreacted TEOS. Silver nanowires coated with silica (AgNW@Si0₂) were recovered and then dispersed in 25 g of cyclohexane. The cycle centrifugation-dispersion in cyclohexane was repeated 3 times. During the last dispersion in cyclohexane, 5.6 mg (15.9.10^"6 mol.) of cetyl trimethyl ammonium bromide (CTAB) were added. Finally, the dispersion was treated using ultrasonic bath to deagglomerate nanowires. The resulting suspension was not stable.

From the examples according to the invention it is clear that dispersion of silver nanowires coated with oxide in the presence of a surfactant resulted in stable suspension in polar aprotic solvents. Moreover, silver nanowires coated with oxide were successfully redispersed, in the presence of a surfactant, in polar aprotic solvent after sedimentation while uncoated silver nanowires were not.

Example 1 shows that the dispersion of silver nanowires coated with oxide, namely Si0₂, in a polar aprotic solvent, namely MEK, comprising CTAB as surfactant, gave stable suspension while comparative example 1 reveals that the dispersion, in the same polar aprotic solvent comprising CTAB as surfactant, of uncoated silver nanowires gave a suspension which was not stable.

Examples 2 shows that the dispersion of silver nanowires coated with Si0₂ in MEK gave stable suspension in the presence of n-dodecylamine.

Example 3 reveals that MEK could be replaced by another polar aprotic solvent i.e. 2-Me THF. Example 4 gives evidence that Ti0₂ could replace Si0₂.

Comparative example 1 shows that, in the presence of a surfactant, the dispersion in a polar aprotic solvent of uncoated silver nanowires gave a suspension which was not stable.

Finally, comparative example 2 highlights that the dispersion of silver nanowires coated with oxide into non-polar aprotic solvent, namely cyclohexane, resulted in suspension which was not stable even in the presence of a surfactant.

Through these examples the applicant has shown that silver nanowires coated with oxide give stable suspensions and can be redispersed after sedimentation in polar aprotic solvents in the presence of a surfactant.

Claims

C L A I M S

1. A suspension of silver nanowires coated with at least one oxide selected from the list consisting of titanium, zirconium, hafnium, vanadium, aluminum, gallium, indium, silicon, germanium and tin oxides and mixtures thereof in a polar aprotic solvent, wherein said suspension comprises at least one surfactant.

2. The suspension according to claim 1, wherein the surfactant is an amine corresponding to the formula NR₅R₆R₇ wherein R5, R^ and R₇, which may be the same or different represent H or a C1-C30 hydrocarbyl group.

3. The suspension according to claim 2, wherein the amine is n- dodecylamine.

4. The suspension according to claim 1, wherein the surfactant is a cationic surfactant.

5. The suspension according to claim 4, wherein the cationic surfactant is chosen from the list consisting of phosphonium, ammonium and pyridinium salts.

6. The suspension according to claim 5, wherein the ammonium salts correspond to formula (1)

Formula (1)

wherein R_{l s} R₂, R3 and R4, 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.

7. The suspension according to claim 6, wherein the ammonium salt is cetyl trimethyl ammonium bromide.

8. The suspension according to any one of the preceding claims, wherein the solvent is selected from the list consisting of ethers, esters, ketones, amides, nitriles, sulfoxides and mixtures thereof.

9. The suspension according to any one of the preceding claims, wherein the solvent has a normal boiling point below 200°C.

10. The suspension according to any one of the preceding claims, wherein the oxide is selected from the list consisting of titanium, zirconium, aluminum, and silicon oxides and mixtures thereof.

11. The suspension according to any one of the preceding claims but claim 10, wherein the oxide is selected from the list consisting of hafnium, vanadium, gallium, indium, germanium and tin oxides and mixtures thereof.

12. The suspension according to any one of the preceding claims, wherein at least one polymer is dissolved in the solvent.

13. A process for preparing the suspension according to any one of claims 1 to 12 comprising the step of dispersing silver nanowires coated with at least one oxide in a polar aprotic solvent in the presence of at least one surfactant.

14. A process for preparing the suspension according to claim 12 comprising the step of dispersing silver nanowires coated with at least one oxide in a solution comprising at least one polymer dissolved in a polar aprotic solvent, in the presence of at least one surfactant.

15. A process for preparing the suspension according to claim 12 comprising the steps of

16. The process according to any one of claims 13 to 15, wherein dispersing silver nanowires coated with at least one oxide is conducted by sonication.

17. A process for preparing a film comprising the steps of

- casting the suspension according to claim 12 on a substrate, so as to form a swollen film,

- removing from the swollen film the polar aprotic solvent so as to obtain the film.

18. A film comprising

- silver nanowires coated with at least one oxide selected from the list consisting of titanium, zirconium, hafnium, vanadium, aluminum, gallium, indium, silicon, germanium and tin oxides and mixtures thereof,

- a polar aprotic solvent in an amount not exceeding 0.5 wt. % based on the total weight of the film,

- a surfactant, and -at least one polymer.

19. The film according to claim 18, wherein the oxide is selected from the list consisting of titanium, zirconium, aluminum and silicon oxides and mixtures thereof.

20. The film according to claim 18, wherein the oxide is selected from the list consisting of hafnium, vanadium, gallium, indium, germanium and tin oxides and mixtures thereof.

21. A device comprising the film of any one of claims 18 to 20.

22. The device according to claim 21, which is a sensor, an actuator, an energy harvesting or an energy storage device.