Composites made from Pickering emulsions

Abstract

The present invention relates to a method of making a composite material, the method comprising the steps of: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, wherein the discontinuous liquid phase comprises a polysiloxane and a curing agent; (2) placing the Pickering emulsion formed in step (1) in a sealed system for a time sufficient to at least partially cure the polysiloxane; and (3) allowing any remaining liquid to evaporate.

Description

[Technical field]

[0001] The present invention relates to a composite material made from a Pickering emulsion comprising a two-dimensional (2D) material, polysiloxane (silicone), and a curing agent, and a method for making such a material. [Background technology]

[0002] A composite (or hybrid) material is a solid material made from two or more constituent materials that have significantly different physical or chemical properties that, when combined, produce a material with properties different from those of the individual components. The individual components remain separate and unique within the final structure, differentiating composite materials from mixtures and solid solutions.

[0003] Composite materials can therefore combine desirable properties of their constituent materials to create new materials that may be preferable to traditional non-composite materials for a variety of reasons: for example, composite materials may be stronger, lighter, and / or cheaper than traditional materials.

[0004] Composite materials may be considered to include a matrix or continuous phase and a filler or discontinuous phase. For example, concrete is a composite material that typically includes loose stones or aggregates (i.e., fillers) held in a matrix of cement. Other composite materials include ceramic matrix composites, which have ceramic fibers (e.g., silicon carbide fibers) embedded in a ceramic matrix (e.g., silicon carbide).

[0005] The nature and structure of the filler or discontinuous phase strongly influences the final properties of the composite. Recently, research has focused on using layered two-dimensional (2D) materials, such as graphene, as fillers in composites due to their interesting and potentially useful properties. For example, graphene has high thermal and electrical conductivity.

[0006] Polysiloxane elastomers, also known as silicone elastomers, are a type of polymer that is flexible, lightweight, heat stable and chemically resistant. Polysiloxane elastomers have a wide range of applications due to their favorable properties. For example, polysiloxane elastomers can be used to form molds for casting other materials, seals, cookware and medical implants. Therefore, it is desirable to create composite materials in which the properties of silicone elastomers are modified or enhanced by the addition of filler materials such as graphene.

[0007] The easiest and most common way to form a composite material involves simply mixing the filler with a matrix, which results in a material with a uniform distribution of filler particles. When this approach is used for 2D filler materials such as graphene, the 2D material is randomly distributed in the matrix.

[0008] For example, Non-Patent Document 1 discloses a process that includes forming graphene nanosheets and mixing the nanosheets with homemade "Silly Putty" (a lightly crosslinked, highly viscoelastic polysiloxane).

[0009] Similarly, US Patent No. 5,399,633 discloses a process comprising dispersing functional graphene sheets in a polar solvent, adding a vinyl-terminated polysiloxane, removing the solvent, adding a crosslinker and a hydrosilylation catalyst, and curing the resulting mixture to provide a nanocomposite composition comprising a silicone elastomer matrix and functional graphene sheets as a filler.

[0010] However, the chemical resistance of silicone makes it difficult to form composites due to the difficulty in blending graphene with some control over the structure. The processes disclosed in [1] and [1] do not allow any control over the integration of 2D materials into the elastomer, and therefore the 2D materials are randomly distributed and arranged in the matrix.

[0011] US Patent No. 5,999,336 discloses a process for forming a conductive composite material that includes immersing an elastomer in a solution of a conductive 2D material such as graphene and applying energy to incorporate the 2D material into the elastomer. However, this process also lacks control over the integration of the 2D material into the elastomer, and as a result, the 2D material is also randomly distributed and arranged in the matrix.

[0012] It is therefore desirable to provide a method of forming a composite containing a polysiloxane elastomer and a 2D material, where the 2D material is integrated into the matrix in a controlled manner, resulting in enhanced properties of the composite material. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] US Patent Application Publication No. 2011 / 0178224 [Patent Document 2] US Patent Application Publication No. 2016 / 0287175 [Non-patent literature]

[0014] [Non-Patent Document 1] Boland et al., Science, 2016, vol. 354, no. 6317, pp. 1257-1260 Summary of the Invention [Means for solving the problem]

[0015] In one aspect, the invention provides a method of making a composite material, comprising the steps of: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a curing agent; (2) placing the Pickering emulsion formed in step (1) in a sealed system for a time sufficient to at least partially cure the polysiloxane; (3) allowing any remaining liquid to evaporate; The present invention provides a method comprising:

[0016] In another aspect, the present invention is directed to a composite material obtainable by or formed by the above method.

[0017] In another aspect, the present invention is directed to a strain sensor comprising the composite material described above or formed by the method described above.

[0018] In another aspect, the present invention is directed to a pressure sensor comprising the composite material described above or formed by the method described above.

[0019] In another aspect, the invention provides a method of making a composite material, comprising the steps of: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the continuous liquid phase comprising a polysiloxane and a curing agent; (2) allowing the polysiloxane to at least partially cure; The present invention provides a method comprising:

[0020] In another aspect, the invention is directed to a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase and a 2D material, wherein the continuous liquid phase comprises a polysiloxane and a curing agent.

[0021] In the above aspect, the preferred polysiloxane is PDMS.

[0022] In the above aspects, the preferred 2D material is graphene. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram of droplets in a Pickering emulsion. [Diagram 2] 1 is a table showing whether a series of liquid phases are immiscible or miscible with each other. [Diagram 3] The predicted emulsion geometry is shown as a function of the surface tension of the "water" and "oil" phases. [Figure 4] FIG. 1 is a schematic diagram of a process for forming a Pickering emulsion. [Diagram 5] 1 is an SEM image of a composite material of the present invention. [Figure 6] 1 is an SEM image of an elastomeric sphere described herein. [Figure 7] 1 is a plot of electrical conductivity versus graphene loading level for materials of the present invention and materials known in the art. [Figure 8] 1 is a plot of R / R0 as a function of strain time for a strain sensor of the present invention. [Figure 9] 1 is a series of SEM images showing the formation of a composite material of the present invention. [Figure 10] 1 illustrates toughness versus interdiffusion time for materials formed by the method of the present invention. [Figure 11] 1 shows electrical conductivity versus interdiffusion time for materials formed by the method of the present invention. [Figure 12] 1 shows the Young's modulus as a function of graphene loading for various composites of the present invention. [Figure 13] 1 shows the yield strain as a function of graphene loading for various composites of the present invention. [Figure 14] 1 shows a strain sensor of the present invention in a relaxed state. [Figure 15] 1 shows a strain sensor of the present invention in extension. [Figure 16] 1 shows the electrical response of a strain sensor of the present invention upon extension. [Figure 17] 1 shows the electrical response when the strain sensor of the present invention is pressed against the carotid artery. [Figure 18] The Fourier transform from FIG. [Figure 19] 1 shows the electrical response when a strain sensor of the present invention is pressed against the chest. [Figure 20] The Fourier transform from FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Pickering Emulsion As is well known in the art, an emulsion is a mixture of two or more immiscible liquids in which droplets of one liquid (the dispersed or discontinuous phase) are dispersed in the other liquid (the continuous phase). Examples of emulsions include vinaigrette, homogenized milk, and mayonnaise. Emulsions often contain surfactants, which act to stabilize the emulsion by increasing the kinetic stability. Surfactants generally have a polar or hydrophilic portion and a non-polar or hydrophobic portion. Surfactant molecules orient themselves with the polar portion toward the more polar liquid phase and the non-polar portion toward the less polar liquid phase. Thus, the surfactant molecules form a layer between the dispersed and continuous phases, which helps to stabilize the emulsion.

[0025] In contrast, a Pickering emulsion is an emulsion stabilized by solid particles, which adsorb at the interface between the two liquid phases in the emulsion. Figure 1 shows the solid-stabilized droplets in a Pickering emulsion, which includes a continuous phase (1) and a dispersed (discontinuous) phase (2), with solid particles (3) forming a layer or interface between the two phases.

[0026] 2D materials The 2D materials used in the present invention act as solid stabilizers in Pickering emulsions.

[0027] The 2D material used in the present invention can be any 2D material that is not a silicate and is not functionalized. Thus, graphene oxide, as well as silicates including silica, clays, functionalized clays, and surface-modified clays, are not included in the definition of suitable 2D materials for use in the present invention.

[0028] Suitable non-functionalized 2D materials for use in the present invention include graphene, borophene, germanene, silicene, stanene, phosphorene, bismuthene, hexagonal boron nitride (h-BN), MXenes, 2D perovskites, and transition metal dichalcogenides (TMDs). TMDs are of the formula MX 2 where M is a transition metal and X is a chalcogen atom (S, Se or Te). An example of a TMD is molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), molybdenum ditelluride (MoTe 2 ), niobium diselenide (NbSe 2 ), tungsten disulfide (WS 2 ), tungsten diselenide (WSe 2 ) and hafnium disulfide (HfS 2 ) are 2D materials consisting of layers of transition metal carbides, nitrides or carbonitrides that are a few atoms thick. Examples include Ti 2 C, V 2 C, Nb 2 C, Mo 2 C, Ti 3 C 2 , Ti 3 CN, Zr 3 C 2 , Hf 3 C 2 , Ti 4 N 3 , Nb 4 C 3 , Ta 4 C 3 , Mo 2 TiC 2 , Cr 2 TiC 2 and Mo2 Ti 2 C 3 Includes.

[0029] Those skilled in the art will recognize that 2D materials can be selected to provide the desired properties of the material formed from the Pickering emulsion. For example, graphene may be selected to provide electrical conductivity. Alternatively, hexagonal boron nitride may be selected to provide thermal conductivity. Other 2D materials, such as molybdenum disulfide and tungsten disulfide, are semiconductors.

[0030] The 2D material is a non-functionalized or unfunctionalized 2D material. Non-functionalized or unfunctionalized means that the 2D material has not undergone any surface modification to change its surface tension. Thus, the term "non-functionalized 2D material" excludes, for example, graphene oxide.

[0031] Preferably, the 2D material is graphene, hexagonal boron nitride (h-BN), phosphorene or a transition metal dichalcogenide (TMD), more preferably graphene, hexagonal boron nitride or molybdenum disulfide. Most preferably, the 2D material is graphene.

[0032] As will be appreciated by those skilled in the art, in order to form a Pickering emulsion, the 2D material should have a surface tension between that of the two liquid phases.

[0033] The surface tension of 2D materials can be estimated using known techniques such as those known from Hernandez et al., Langmuir, 2010, vol. 26, no. 5, pp. 3208-3213 and Hernandez et al., Nature Nanotechnology, 2008, vol. 3, no. 9, pp. 563-568. This technique is based on the maximum feasible concentration of the material in the dispersion. The surface tensions of various 2D materials are known in the art. For example, the surface tension of graphene is estimated to be in the range of about 41 to about 43 mN / m. The surface tensions of most 2D materials are similar. Preferably, the surface tension of the 2D material is in the range of about 40 to about 45 mN / m.

[0034] The amount of 2D material that should be present in a Pickering emulsion may depend on the size of the dispersed phase droplets in the emulsion, the total volume of the dispersed phase, the properties of the 2D material and / or the properties of the liquid phase.

[0035] In general, the particles of the 2D material are present in a Pickering emulsion in an amount sufficient to form a monolayer of particles around each of the dispersed phase droplets. Thus, in general, for a given dispersed phase volume, the smaller the dispersed phase droplet size, the greater the amount of 2D material required.

[0036] The Pickering emulsion formed in the present invention may be formed by first exfoliating a layered 3D material to form a 2D material, and secondly forming a Pickering emulsion stabilized by said 2D material. By exfoliating the layered 3D material before forming the emulsion, it is possible to ensure that the 3D material is well exfoliated (i.e., the formed 2D material consists of particles of 2D material that are not more than a few layers thick) before the emulsion is formed. Because the particles of the 2D material are not more than a few layers thick, it is possible to form a monolayer of particles of 2D material around each droplet of the dispersed phase in the emulsion using less 2D material than would be necessary if each particle of the 2D material contained more layers of 2D material.

[0037] In this context, few layers means 1 to about 10, preferably 1 to about 5, more preferably 1 layer of atoms or formula units. Thus, particles of 2D material are preferably 1 to about 10, preferably 1 to about 5, more preferably 1 layer of atoms or formula units thick.

[0038] Alternatively, the 2D materials may be commercially available, for example, graphene and 2D boron nitride can be obtained commercially from Thomas Swan & Co. Ltd.

[0039] The 2D material is generally present in the Pickering emulsion in an amount of less than about 15% by volume, more preferably less than about 10% by volume, and most preferably less than about 5% by volume, based on the volume of the dispersed phase. Generally, the 2D material is present in the Pickering emulsion in an amount of at least about 0.001% by volume, such as from about 0.01 to about 15% by volume, preferably from about 0.05 to about 10% by volume, and most preferably from about 0.1 to about 5% by volume, based on the volume of the dispersed phase.

[0040] The volume percent of a 2D material may be calculated by measuring the mass of the 2D material present prior to the formation of the emulsion (measured by weighing or by absorbance spectroscopy). This is divided by the density of the bulk 3D material to obtain the volume of the 2D material. This is then divided by the volume of the dispersed phase and multiplied by 100 to express as a percentage. For example, 0.225 mg of graphene may be formed in 1 mL of cyclohexanone, after which 1 mL of water is added to form a Pickering emulsion in which the water forms the dispersed phase. In this example, the volume of graphene is 0.000225 g / 2.25 g / mL = 0.0001 mL (graphite has a density of 2.25 g / mL). The volume percent of graphene based on the volume of the dispersed phase (water in this case) is (0.0001 / 1)*100 = 0.01 volume percent.

[0041] Alternatively, the 2D material is generally present in the Pickering emulsion in an amount of less than about 30% by volume, more preferably less than about 20% by volume, and most preferably less than about 15% by volume, based on the volume of the polysiloxane and the hardener. Generally, the 2D material is present in the Pickering emulsion in an amount of at least about 0.1% by volume, such as from about 0.1 to about 30% by volume, preferably from about 0.25 to about 20% by volume, and most preferably from about 0.5 to about 15% by volume, based on the volume of the polysiloxane and the hardener.

[0042] Alternatively, the amount of 2D material may be expressed as a weight percent, calculated as the weight of the 2D material divided by the weight of the dispersed phase (which can be calculated using the volume and density of the dispersed phase). In this case, the 2D material is generally present in the Pickering emulsion in an amount of less than about 30% by weight, more preferably less than about 20% by weight, more preferably less than about 15% by weight, and most preferably less than about 10% by weight, based on the weight of the dispersed phase. Generally, the 2D material is present in the Pickering emulsion in an amount of at least about 0.001% by weight, such as from about 0.01 to about 20% by weight, preferably from about 0.1 to about 15% by weight, and most preferably from about 0.2 to about 10% by weight, based on the weight of the dispersed phase.

[0043] Alternatively, the 2D material is generally present in the Pickering emulsion in an amount of less than about 50% by weight, more preferably less than about 40% by weight, and most preferably less than about 35% by weight, based on the weight of the polysiloxane and the curing agent. Generally, the 2D material is present in the Pickering emulsion in an amount of at least about 0.2% by weight, such as from about 0.2 to about 50% by weight, preferably from about 0.5 to about 40% by weight, and most preferably from about 1.0 to about 35% by weight, based on the weight of the polysiloxane and the curing agent.

[0044] Typically, the 2D materials used in the present invention form a layer having a thickness of one atom or a formula unit. These layers typically have a thickness of about 1 to about 5 nm. Thus, the 2D materials used in the present invention are generally in the form of particles or flakes having a thickness of about 1 to about 50 nm, more preferably about 1 to about 10 nm, and most preferably about 1 to about 5 nm. As used herein, the term "particle" includes flakes. Particles generally have an aspect ratio (length to thickness) greater than about 50. Thus, the particles of the 2D material may have an (average) length of about 5 nm to about 5000 nm, preferably about 50 nm to about 2000 nm, more preferably about 100 nm to about 1000 nm, and more preferably about 200 to about 500 nm, where the length corresponds to the longest dimension in the direction of the layer of the flake or particle.

[0045] The particles may have an approximately round or square shape when viewed perpendicular to the 2D plane. Thus, the width of the particles may be approximately the same as the length. Alternatively, the particles of the 2D material may have an approximately rectangular shape when viewed perpendicular to the 2D plane. Thus, the particles may have an (average) width of about 2.5 nm to about 2500 nm, preferably about 20 nm to about 1000 nm, more preferably about 50 nm to about 700 nm, and more preferably about 100 to about 300 nm, where the width is perpendicular to the length and corresponds to the longest dimension of the particle in the direction of the layer. The aspect ratio (length to width) of the particles is preferably less than about 3.

[0046] Thus, the 2D material may be considered a "nanomaterials". The size (e.g., length and width) and thickness of the particles of the 2D material can be measured using atomic force microscopy, transmission electron microscopy, or dynamic light scattering techniques.

[0047] The particles must be small enough to effectively coat the droplets of the dispersed phase in the emulsion. The smaller the particles of the 2D material, the smaller the droplets can remain coated by the particles.

[0048] liquid phase One skilled in the art will understand that to form a Pickering emulsion, one of the liquid phases must have a surface tension higher than that of the 2D material and the other liquid phase must have a surface tension lower than that of the 2D material. The two liquid phases must also be immiscible.

[0049] Figure 2 is a table showing whether a series of liquid phases are immiscible or miscible with each other. In Figure 2, "0" indicates combinations that are miscible and "1" indicates combinations that are immiscible. Whether other combinations of liquids are miscible or immiscible can be easily determined by simple mixing experiments.

[0050] Advantageously, graphene and similar 2D materials are known to have a surface tension between that of water and many liquids that are incompatible with water. This avoids the need to surface modify or functionalize the 2D material to adjust its surface tension before a Pickering emulsion can be formed. Pickering emulsions can be formed without it using a non-functionalized 2D material and two immiscible liquids, such as water and a liquid that is immiscible with water. In contrast, clays and other silicates require surface modification to adjust the surface tension before a Pickering emulsion can be formed. Such modifications are well known to impair the properties of 2D materials. It is therefore highly advantageous to be able to avoid surface modification of the 2D materials used in the present invention.

[0051] The surface tension of most liquids is well known in the art (see, for example, "Themophysical Properties of Chemicals and Hydrocarbons," Carl L. Yaw, William Andrew, Norwich, NY, 2008). Alternatively, the surface tension of a liquid can be easily characterized experimentally using the Wilhelmy plate technique (as described, for example, in "Understanding Solvent Spreading for Langmuir Deposition of Nanomaterial Films: A Hansen Solubility Parameter Approach," Large et al., Langmuir, ACS: American Chemical Society, 2017, DOI: 10,1021 / acs.langmuir.7b03867). Such a method can be carried out at 25° C. using a Nima PS4 surface pressure sensor. The surface tensions of some common liquids are given in Table 1 below. All surface tensions referred to herein are surface tensions measured at 25° C.

[0052] [Table 1]

[0053] Each liquid phase may comprise a single liquid with the required surface tension, or may comprise a mixture of liquids, provided that each liquid phase has the required surface tension relative to the other liquid phase and the 2D material. For example, a mixture of liquids may be used to adjust the surface tension or tune the emulsion properties (e.g., viscosity). For example, a mixture of water and ethylene glycol has a surface tension between that of pure water and pure ethylene glycol.

[0054] Optionally, one or both of the liquid phases may contain other components, but the 2D material acts as a stabilizer for the Pickering emulsion. As a result, unlike conventional emulsions, a surfactant stabilizer is not required. Thus, preferably, no surfactant is present in the Pickering emulsion used in the present invention.

[0055] One of the liquid phases may be considered to represent the "water" phase and should have a surface tension higher than that of the 2D material, for example, about 1 to about 35 mN / m higher, preferably about 2 to about 10 mN / m higher, more preferably about 3 to about 8 mN / m higher than the surface tension of the 2D material.

[0056] When absolute values are considered, this liquid phase preferably has a surface tension of at least about 43 mN / m, more preferably at least about 45 mN / m, and most preferably at least about 48 mN / m.

[0057] This liquid phase may form a continuous liquid phase (i.e., water-in-oil emulsion) or a discontinuous liquid phase (i.e., oil-in-water emulsion). As further discussed below, this liquid phase generally forms a continuous liquid phase when the surface tension of this phase is less than about 55 mN / m, preferably less than about 50 mN / m. Thus, in this case, this liquid phase preferably has a surface tension of about 43 mN / m to about 55 mN / m, more preferably about 45 mN / m to about 50 mN / m.

[0058] Conversely, this liquid phase generally forms a discontinuous liquid phase when the surface tension of this phase is greater than about 55 mN / m, preferably greater than about 65 mN / m, and most preferably greater than about 70 mN / m.

[0059] Preferably, this liquid phase contains glycerol, water, formamide, diethylene glycol, ethylene glycol, propylene glycol, or a combination thereof. More preferably, this liquid phase contains water, propylene glycol, ethylene glycol, or a combination thereof.

[0060] When it is desired that the liquid phase form the continuous phase, the liquid phase preferably comprises, alternatively consists essentially of, or consists of ethylene glycol. Due to its immiscibility with many organic solvents, ethylene glycol is a preferred liquid phase.

[0061] Alternatively, when it is desired that the liquid phase form a discontinuous phase, the liquid phase preferably comprises water. Alternatively, the continuous liquid phase consists essentially of water, or consists of water.

[0062] The other liquid phase, which may be considered to represent the "oil" phase, should have a surface tension lower than that of the 2D material, e.g., about 5 to about 35 mN / m lower, preferably about 10 to about 35 mN / m lower, more preferably about 15 to about 25 mN / m lower than the surface tension of the 2D material.

[0063] When absolute values ​​are considered, it is preferred that the liquid phase have a surface tension of less than about 40 mN / m, more preferably less than about 35 mN / m, and most preferably less than about 30 mN / m.

[0064] The liquid phase includes the polysiloxane and the curing agent, and may also include one or more organic solvents. Any organic solvent or solvents present in this phase should be miscible with the polysiloxane and the curing agent. Suitable organic solvents include, but are not limited to, hexane, acetone, tetrahydrofuran, chlorobenzene, diethyl ether, ethyl acetate, toluene, xylene, pentanol, butanol, propanol, ethanol, methanol, chloroform, acrylonitrile, dichloromethane, and combinations thereof. Preferably, the organic solvent is selected from ethyl acetate, dichloromethane, and combinations thereof, more preferably a combination of ethyl acetate and dichloromethane.

[0065] An optional solvent may be used to adjust the viscosity of the liquid phase. For example, the liquid phase may have a viscosity of about 0.001 to about 10 Pa·s, preferably about 0.01 to about 1 Pa·s, more preferably about 0.1 to about 0.5 Pa·s, and most preferably about 0.2 to about 0.3 Pa·s. The presence of a solvent may also help prevent premature curing of the polysiloxane.

[0066] The liquid phase may contain about 80 to about 95% by weight of polysiloxane and about 20 to about 5% by weight of curing agent, based on the total weight of polysiloxane and curing agent. Preferably, the liquid phase contains about 87 to about 93% by weight of polysiloxane and about 13 to about 7% by weight of curing agent, based on the total weight of polysiloxane and curing agent. The weight ratio of polysiloxane to curing agent may be about 15:1 to about 5:1, preferably about 12:1 to about 8:1, and most preferably about 10:1. The amount of curing agent should be sufficient to at least partially cure the polysiloxane. Preferably, the amount of curing agent is sufficient to completely cure the polysiloxane.

[0067] When a solvent is present, the solvent may be present in any amount suitable to dissolve the polysiloxane and curing agent and / or reduce the viscosity and / or density of the liquid phase to a desired level. Matching the viscosities of the two liquid phases helps maximize the local shear rate during mixing, thereby facilitating a smaller average droplet size for the same energy input. The amount of solvent present depends at least in part on the viscosity of the solvent and the nature and viscosity of the polysiloxane. Preferably, the liquid phase comprises from about 10 to about 90% by weight of the solvent, more preferably from about 40 to about 85% by weight of the solvent, and most preferably from about 60 to about 80% by weight of the solvent.

[0068] Phase arrangement Depending on the surface tension of the "water" phase, the "oil" phase either forms a discontinuous phase (the "water" phase forms a continuous phase) or forms a continuous phase (the "water" phase forms a discontinuous phase). Thus, the arrangement of the liquid phases depends primarily on the properties (and thus the surface tension) of the "water" phase.

[0069] As discussed above, the placement of the liquid phase depends primarily on the nature of the "water" phase. Thus, when it is desired that the polysiloxane be in the discontinuous phase, the liquid phase not containing the polysiloxane (i.e., the "water" phase) should have a surface tension of less than about 55 mN / m, preferably less than about 50 mN / m. Suitable liquid phases include, but are not limited to, ethylene glycol, propylene glycol, and combinations thereof, preferably ethylene glycol.

[0070] Conversely, when it is desired that the polysiloxane be in the continuous phase, the liquid phase not containing the polysiloxane (i.e., the "water" phase) should have a surface tension of greater than about 55 mN / m, preferably greater than about 65 mN / m, and most preferably greater than about 70 mN / m. Suitable liquid phases include, but are not limited to, glycerol, water, formamide, diethylene glycol, and combinations thereof, preferably water.

[0071] To form some of the polysiloxane elastomeric spheres and composites described herein, the polysiloxane needs to be the discontinuous phase.

[0072] If necessary, the configuration of any two phases can be determined by mixing the phases together and observing the type of emulsion that forms. Alternatively, the configuration of phases in a Pickering emulsion can be predicted based on the surface energies of the phases. In particular, the following equation (1) indicates the point at which phase inversion occurs:

[0073]

number

[0074] Here, γ o , γ w and γ s are the surface energies of the oil phase, the water phase, and the solid, respectively.

[0075] For liquids, the surface energy (γ) is equal to the sum of the surface tension (Γ) and the surface entropy, where the surface entropy is 29 mJ / m for liquids at room temperature. 2 The surface tension can be determined experimentally, for example, using the Wilhelmy plate method as discussed above.

[0076] For 2D materials, γ s The value of γ can be determined experimentally. This is the time when an emulsion changes from an oil-in-water emulsion to a water-in-oil emulsion or vice versa. o and γ w This can be done by experimentally determining the value of and substituting said value into equation (1).

[0077] For example, ethyl acetate is sometimes used as the "oil" phase (surface tension (Γ) = 23.3 mN / m), which results in a γ o becomes constant (i.e. 23.3+29=52.3mN / m). Next, γ w Varying volume ratios of ethylene glycol and deionized water can be used to change the value of . Ethyl acetate is less dense than water and ethylene glycol, so oil-in-water droplets float while water-in-oil droplets sink. Thus, the transition between oil-in-water and water-in-oil emulsions can be seen by the naked eye. For graphene being a 2D material, this inversion occurs when the surface tension (Γ) of the "water" phase is 52 mN / m. Thus, γ w is 52+29=81mN / m.

[0078] In equation (1), γ o = 52.3mN / m and γ w = 81mN / m, for graphene, γ s= 66 mN / m. For other 2D materials, γ s The value can be determined in a similar manner.

[0079] Figure 3 shows the s = 66 mN / m (i.e., for a system containing graphene as the 2D material). Also shown in Figure 3 are the surface tensions of PDMS, ethyl acetate (EA), dichloromethane (DCM), ethylene glycol (EG), and deionized water.

[0080] Figure 3 may be used to determine the predicted configuration of any given Pickering emulsion where the 2D material is graphene by plotting the surface tension of the "oil" and "water" phases. For example, when the two liquid phases are PDMS and ethylene glycol and the 2D material is graphene, the intersection between the "PDMS" and "EG" lines is below the solid line and in the "o / w" region, so an oil-in-water emulsion forms. Conversely, when the two liquid phases are PDMS and water and the 2D material is graphene, the intersection between the "PDMS" and "water" lines is above the solid line and in the "w / o" region, so a water-in-oil emulsion forms.

[0081] Polysiloxane Polysiloxanes are also called silicones.

[0082] Polysiloxanes, or silicones, are polymers made up of repeating units of siloxane, a chain of alternating silicon and oxygen atoms combined with carbon, hydrogen, and sometimes other elements. Polysiloxanes thus have an inorganic silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with organic side chains attached to the silicon atoms such that each silicon atom is tetravalent. Silicones thus have the general chemical formula [R 2 SiO] n where R is an organic group and n is an integer greater than 1.

[0083] By varying the -Si-O- chain length and the nature of the organic side chains, silicones can be synthesized with a wide variety of properties and compositions.

[0084] Polysiloxane or silicone elastomers can be formed by crosslinking individual polymer chains to form a 3D network. The process of crosslinking polysiloxanes to form polysiloxane elastomers is also known as curing.

[0085] The nature of the polysiloxane used in the present invention is not critical, and therefore any polysiloxane that can be cured to form a polysiloxane elastomer may be used in the present invention. Any curing mechanism known in the art may be used to form the polysiloxane elastomer.

[0086] Suitable polysiloxanes for use in the present invention and their curing mechanisms for forming polysiloxane elastomers are well known in the art, for example from "Chemistry and Technology of Silicones", W. Noll, Academic Press, New York (1968); "Synthesis and Properties of Silicone and Silicone-Modified Materials", Clarsson et al., 2003; and "Inorganic Polymers", 2nd Edition, Mark et al., 2005. For example, suitable polysiloxanes for use in the present invention are commercially available from Dow Inc. or Wacker, Inc.

[0087] Polysiloxane, as used herein, may include a single polysiloxane or a blend of two or more polysiloxanes.

[0088] Suitable polysiloxanes for use in the present invention include, but are not limited to, those having the structure of formula (I). Here,

[0089]

Chemical formula

[0090] each of R R 1 R 2 R 3 R 4 R 5 and R 6 is independently H or an organic group; X is an organic group; Y is H or an organic group; n is any integer greater than 1.

[0091] Other suitable polysiloxanes include, but are not limited to, those having the structure of formula (II). Here,

[0092]

Chemical formula

[0093] each of R R 1 R 2 R 3 R 4 R 5 and R 6 is independently H or an organic group; X is an organic group; Y is H or an organic group; W is an organic group; Z is H or an organic group; n and m are each any integer greater than 1.

[0094] Preferably, n is from 100 to 100,000.

[0095] Preferably, m is 100 to 100,000.

[0096] The organic group may optionally contain one or more of the following functional groups: alkene, alcohol, aldehyde, ketone, carboxylic acid, aryl, ether, ester, amine, imine, or amide.

[0097] For example, suitable organic groups include alkyl groups (e.g., methyl, ethyl, propyl, or butyl), -OH, alkoxy groups, esters (e.g., acetoxy groups), alkenyl groups (e.g., -(CH 2 )n-CH=CH 2 , where n is any integer, for example, 0, 1, 2, or 3), and aryl groups (for example, phenyl).

[0098] Preferred organic groups include methyl, OH, and —(CH 2 ) n -CH=CH 2 More preferred organic groups include methyl and -OH.

[0099] Therefore, preferably, R 1 , R 2 , R 3 , R 4 , R 5 and R 6 each independently represents H, methyl, -CH=CH 2 , acetoxy or -OH. More preferably, R 1 , R 3 , R 4 and R 6 is methyl, and R 2 and R 5 are each independently H, methyl, -CH=CH 2 Or -OH.

[0100] Preferably, X is methyl, -OH, acetoxy, -(CH 2 ) n -CH=CH 2or phenyl. More preferably, X is methyl or -CH=CH 2 , most preferably methyl.

[0101] Preferably, Y is H, methyl, -OH, acetoxy, -(CH 2 ) n CH=CH 2 or phenyl. More preferably, Y is H, methyl or -CH=CH 2 , most preferably methyl.

[0102] Preferably, W is methyl, -OH, acetoxy, -(CH 2 ) n -CH=CH 2 or phenyl. More preferably, W is methyl or -CH=CH 2 , most preferably methyl.

[0103] Preferably, Z is H, methyl, -OH, acetoxy, -(CH 2 ) n -CH=CH 2 or phenyl. More preferably, Z is H, methyl or -CH=CH 2 , most preferably methyl.

[0104] Thus, preferably, the polysiloxane is a compound of formula (I) in which R 1 , R 3 , R 4 and R 6 is methyl; R 2 and R 5 are each independently H, methyl, -CH=CH 2 , acetoxy or -OH; X is methyl; Y is H or methyl.

[0105] Alternatively, and preferably, the polysiloxane is a compound of formula (II) R 1 , R 3 , R 4 and R 6 is methyl; R 2 and R 5 are each independently H, methyl, -CH=CH 2 , acetoxy or -OH; X, W and Z are methyl; Y is H or methyl.

[0106] The polysiloxane can be polydimethylsiloxane (PDMS), which can be, for example, non-functionalized PDMS, hydroxy-terminated PDMS, or vinyl-terminated PDMS.

[0107] As used herein, polysiloxanes may contain alkenyl groups (e.g., vinyl groups). Thus, polysiloxanes may be polymers of formula (I) or (II), where R 1 ~R 6 At least one of the X, Y, W and Z groups comprises an alkenyl group, preferably a vinyl group.

[0108] Such polysiloxanes are alkenyl-terminated polysiloxanes, preferably vinyl-terminated polysiloxanes (i.e., R 1 ~R 6 At least one of the groups has an alkenyl group, preferably a vinyl group), such as alkenyl-terminated polydimethylsiloxane (alkenyl-terminated PDMS), preferably vinyl-terminated polydimethylsiloxane (vinyl-terminated PDMS).

[0109] Alternatively, polysiloxane as used herein may include polysiloxanes having hydrolyzable groups (e.g., ester or amide groups, preferably esters). Thus, polysiloxanes may be polymers of formula (I) or (II), where R1 ~R 6 At least one of the X, Y, W and Z groups comprises an ester or amide group, preferably an ester group. Preferred ester groups include acetoxy (-OAc).

[0110] Alternatively, polysiloxane as used herein may include polysiloxanes having alcohol groups. Thus, polysiloxanes may be polymers of formula (I) or (II), R 1 ~R 6 At least one of the X, Y, W and Z groups has or is preferably -OH.

[0111] The polysiloxanes used in the present invention may have any viscosity, but should preferably be liquid at room temperature and pressure (25°C and 1 atm). Alternatively, the polysiloxanes may be solid at room temperature but soluble in a solvent. If the polysiloxane is liquid but has too high a viscosity, it will be necessary to include a solvent in the liquid phase of the Pickering emulsion containing the polysiloxane to reduce the viscosity of the liquid phase. Thus, the polysiloxanes used herein preferably have a viscosity of about 0.01 to about 10 Pa·s, more preferably about 0.1 to about 1 Pa·s, and even more preferably about 0.5 to about 1 Pa·s. A range of polysiloxanes with different viscosities are commercially available.

[0112] Hardener A curing agent or crosslinking agent is required to cure or crosslink polysiloxane to form polysiloxane elastomer. Any suitable curing agent or crosslinking agent may be used, and those skilled in the art will know the curing agents useful in forming polysiloxane elastomer. Suitable curing systems and curing agents are known, for example, from "Silicone resins and their combinations", European Coatings Literature, Heilen, 2005; and "Biomaterials Science-An Introduction to Materials in Medicine", 2nd ed., Elsevier Academic Press, Ratner et al., 2004.

[0113] Two component products comprising a polysiloxane and a suitable hardener are commercially available, for example QSIL 216 from Farnell UK.

[0114] There are a variety of different types of curing systems, including: (1) hydrosilylation-based systems; (2) condensation cure systems; and (3) radical cure systems.

[0115] (1) Hydrosilylation System In hydrosilylation silicone curing systems, two different chemical groups (compounds containing Si-H groups and polysiloxanes containing alkenyl groups, preferably vinyl groups) react in the presence of a catalyst, often a platinum catalyst. This type of reaction system is well known in the art, for example in U.S. Pat. No. 3,989,668 and U.S. Pat. No. 5,399,668.

[0116] The reaction is outlined below:

[0117] [ka]

[0118] Thus, in this system, the polysiloxane is a polysiloxane having one or more alkenyl groups (e.g., vinyl groups). Suitable polysiloxanes having one or more alkenyl groups are discussed above.

[0119] In this system, the curing agent comprises: (i) a compound having one or more Si-H groups; and (ii) a hydrosilylation catalyst.

[0120] The compound containing Si-H groups can be any such compound. For example, suitable compounds include tetrakis(dimethylsiloxy)silane or poly(methylhydrosiloxane).

[0121] The nature of the hydrosilylation catalyst is not particularly limited and may be any conventional hydrosilylation catalyst, such as a platinum-based hydrosilylation catalyst. Suitable catalysts are known in the art, for example in US Pat. No. 5,399,668 and US Pat. No. 3,989,668.

[0122] Specific examples of suitable catalysts include elementary platinum, platinum-vinylsiloxane complexes (e.g., Karstedt's catalyst); platinum phosphine complexes (e.g., Pt(PPh 3 ) 4 ); platinum phosphite complexes (e.g. Pt[P(OPh) 3 ] 4 );RhCl 3 ;PdCl 2 2H 2 O;TiCl 4 ; and Pt(acac) 2 Catalysts may be used alone or in combination, including but not limited to:

[0123] (2) Condensation cure system Condensation cure systems use water (e.g., atmospheric moisture) to drive the curing process. Such processes may be carried out at room temperature and pressure (i.e., 25° C. and 1 atm). Systems of this type are also known in the art, for example from U.S. Pat. No. 4,562,238.

[0124] In this process, silane crosslinkers exposed to water (e.g., ambient moisture) undergo a hydrolysis step that generates silanol (Si-OH) groups, which condense with hydrolyzable groups on the polysiloxane until the system is fully cured.

[0125] Thus, in this system the polysiloxane generally has hydrolyzable groups, such as ester or amide groups (preferably esters, e.g. -OAc). Alternatively or in addition, the polysiloxane may already have Si-OH groups and thus be a hydroxy-terminated polysiloxane.

[0126] Thus, in this system, the curing agent comprises a silane crosslinker. Any suitable silane crosslinker may be used. For example, the curing agent may be a silane having an alkoxy, acetoxy, ester, epoxy or oxime group, preferably an alkoxy group. Acetoxysilanes are a preferred class of curing agents. Examples of suitable curing agents include methyltrimethoxysilane or methyltriacetoxysilane.

[0127] Additionally, a condensation catalyst may also be included in the curing agent, such as a tin-based catalyst. Suitable catalysts are well known in the art, for example, see U.S. Patent No. 3,989,668, and include dibutyltin dilaurate.

[0128] (3) Radical curing system Free radicals, i.e., atoms, molecules or ions that have an unpaired valence electron, may also be used to cure polysiloxanes to form polysiloxane elastomers.

[0129] Generally, radicals are formed when a radical initiator decomposes to form radicals that react with side chains on the polysiloxane chains to generate radicals. These, in turn, react with radicals formed from other polysiloxane chains, thereby chemically crosslinking the polysiloxane chains. For an organic peroxide initiator (R denotes an organic group), this mechanism is shown below:

[0130] [ka]

[0131] The radicals can be formed using any suitable radical initiator, such as peroxides, azo compounds, and halogens. The conditions necessary to form the radicals from the radical initiator are known in the art and often include heat or ultraviolet light.

[0132] In this system, the polysiloxane can be any suitable polysiloxane having C—H bonds. For example, PDMS can be used.

[0133] In this system, the curing agent is a radical initiator, such as a peroxide, an azo compound, or a halogen. Any suitable radical initiator can be used, and examples of suitable radical initiators are known in the art.

[0134] Peroxides are preferred radical initiators, with organic peroxides being particularly preferred. For example, the organic peroxide may be selected from diacyl peroxide, dicumyl peroxide, di-tert-butyl peroxide or dichlorobenzoyl peroxide.

[0135] Emulsions The droplets in the Pickering emulsion formed preferably have a (number) representative (average) (mean) diameter of about 10,000 μm or less, more preferably about 1,000 μm or less, and most preferably about 100 μm or less. In general, the droplets have a (number) representative diameter of at least about 100 nm, more preferably at least about 200 nm, and most preferably at least about 1 μm. For droplets larger than about 10 μm, optical microscopy can be used to measure the representative diameter. Below this size, dynamic light scattering (DLS) can be used to measure the representative diameter.

[0136] Generally, the Pickering emulsions described herein comprise at least about 85% by volume liquid, preferably at least about 90% by volume liquid, and more preferably at least about 95% by volume liquid, which liquid comprises both a continuous liquid phase and a dispersed liquid phase.

[0137] Preferably, the Pickering emulsion comprises about 50 to about 75% by volume of a continuous liquid phase, based on the total volume of the liquid phase, and about 25 to about 50% by volume of a dispersed liquid phase, based on the total volume of the liquid phase.

[0138] Preferably, the liquid phase forming the continuous phase and the liquid phase forming the dispersed phase are present in a volume ratio of about 3:1 to about 1:1 (continuous liquid phase to dispersed liquid phase), most preferably about 2:1 to about 1:1, most preferably about 3:2.

[0139] The process of making Pickering emulsion In order to form the material of the present invention, it is necessary to form a Pickering emulsion. Processes for making Pickering emulsions (i.e., step (1) of the method of the present invention) are known in the art. One suitable method is to (1a) exfoliating a layered 3D material in a solvent to produce particles of a 2D material; (1b) forming a dispersion of particles of the 2D material in a first liquid phase; (1c) adding a second liquid phase and homogenizing the dispersion of the 2D material in the first liquid phase and the second liquid phase, thereby forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase and the 2D material; It may include:

[0140] As discussed above, to form a Pickering emulsion, one of the liquid phases should have a surface tension higher than that of the 2D material and the other liquid phase should have a surface tension lower than that of the 2D material. Furthermore, the first and second liquid phases should be immiscible.

[0141] The first and second liquid phases and 2D materials used in the above method may include the liquids and materials discussed above. The polysiloxane and hardener may be present in the first or second liquid phase. Preferably, the polysiloxane and hardener are present in the first liquid phase, which preferably forms a discontinuous phase.

[0142] Step (1a) Suitable methods are known in the art for exfoliating layered 3D materials in a solvent to form particles of 2D materials.For example, the method for exfoliating layered 3D materials to produce particles of 2D materials may include applying energy, such as ultrasound, to the layered 3D materials in the solvent.Alternatively, shear forces may be applied to the layered 3D materials in the solvent.Suitable methods are disclosed in WO2012 / 028724, WO2014 / 001519 and US2016 / 0009561.

[0143] Step (1b) In some embodiments, the solvent used in step (1a) corresponds to the first liquid phase, and therefore step (1b) is simply a consequence of carrying out step (1a). In this case, the solvent must be immiscible with the second liquid phase. However, it is preferred that the solvent used in step (1a) is different from the first liquid phase to allow better exfoliation of the 2D material.

[0144] When the solvent used in step (1a) is different from the first liquid phase, step (1b) may further comprise removing at least a portion of the solvent before adding the first liquid phase to form a dispersion of the 2D material in the first liquid phase. Thus, in this case, step (1b) comprises: (1b1) removing at least a portion of the solvent and then adding a first liquid phase, thereby forming a dispersion of particles of the 2D material in the first liquid phase; Includes.

[0145] Preferably, a majority (at least about 50% by weight) of the solvent is removed in step (1b1). More preferably, at least about 80% by weight, and most preferably at least about 95% by weight (e.g., about 100% by weight) of the solvent is removed. Alternatively, all of the solvent may be removed.

[0146] Alternatively, the first liquid phase may comprise a solvent and one or more additional components, such as a polysiloxane and a curing agent. In this case, step (1b) comprises: (1b2) adding one or more components, thereby forming a dispersion of particles of the 2D material in a first liquid phase; Includes.

[0147] In this case, the first liquid phase comprises the solvent used in step (1a) and one or more components added in step (1b2). Thus, the first liquid phase preferably comprises the solvent, polysiloxane and curing agent used in step (1a).

[0148] If desired, the solvent may be removed by any suitable process. For example, the dispersion may be centrifuged (e.g., at 5000 g for 24 hours) to settle the 2D material, after which the supernatant (i.e., the solvent) may be discarded and the 2D material transferred into the first liquid phase. Alternatively, vacuum filtration may be used to prepare a "wet cake" of the exfoliated material, which may be redispersed in the first liquid phase.

[0149] If the solvent used in step (1a) is different from the first liquid phase, it is preferred that the solvent used in step (1a) is miscible with the first liquid phase to prevent any unwanted emulsion formation upon transfer of the particles into the first liquid phase. More preferably, the solvent is miscible with both the first and second liquid phases to minimize any deviation in the specific surface tension of the two phases due to the presence of any remaining solvent.

[0150] The solvent used in step (1a) depends in part on the material to be exfoliated. As discussed above, methods for exfoliating 3D materials to form 2D materials are known in the art, for example from WO 2012 / 028724, WO 2014 / 001519, US 2016 / 0009561 and Hernandez et al., Langmuir, 2010, vol. 26, no. 5, pp. 3208-3213. Thus, those skilled in the art can select the appropriate solvent for the 2D material to be exfoliated.

[0151] For example, the solvent may be selected from N-methyl-2-pyrrolidone (NMP), N-cyclohexyl-2-pyrrolidone (CHP), 1,3-dimethyl-2-imidazolidinone (DMEU), N-ethyl-2-pyrrolidone (NEP), isopropanol, acetone, cyclopentanone (CPO) and cyclohexanone (CHO).

[0152] Preferably, the solvent used in step (1a) has a surface tension of about 30 to about 50 mN / m. More preferably, the solvent used in step (1a) has a surface tension that is approximately the same as that of the 2D material. Thus, the solvent used in step (1a) preferably has a surface tension of about 40 to about 50 mN / m, more preferably about 40 to about 45 mN / m.

[0153] Thus, preferably, the solvent is selected from N-methyl-2-pyrrolidone (NMP), N-cyclohexyl-2-pyrrolidone (CHP), cyclopentanone (CPO) and cyclohexanone (CHO), more preferably cyclopentanone (CPO) and cyclohexanone (CHO), with cyclopentanone being particularly preferred, especially for the exfoliation of graphite to form graphene.

[0154] When the solvent used in step (1a) corresponds to or is contained within the first liquid phase (i.e. step (1b) comprises step (1b2)), the solvent is preferably selected from cyclopentanone, cyclohexanone and combinations thereof. In this case, the second liquid phase preferably comprises ethylene glycol.

[0155] Alternatively, the solvent used in step (1a) may comprise a mixture of water and a surfactant. Any suitable surfactant may be used, such as an ionic or non-ionic surfactant. The surfactant is ideally water-soluble. Triton™ X-100 (polyethylene glycol tert-octylphenyl ether) is one example of a suitable non-ionic surfactant, and sodium cholate is one example of a suitable ionic surfactant.

[0156] The surfactant may be present in the solvent used in step (1a) in an amount of about 0.01 to about 0.05% by weight, preferably about 0.02 to about 0.03% by weight, based on the weight of water.

[0157] If the solvent in step (1a) comprises water and a surfactant, it is necessary to remove at least a portion of the surfactant before forming the Pickering emulsion. Thus, step (1b) comprises step (1b1) as described above. In this case, step (1b1) comprises removing at least a portion of the mixture of water and surfactant, and then adding a first liquid phase, for example water (without any surfactant).

[0158] Step (1c) Step (1c) involves adding a second liquid to the dispersion of the 2D material in the first liquid phase and then homogenizing the two liquid phases and the 2D material to form a Pickering emulsion.

[0159] The homogenizing step may simply involve subjecting the mixture to mechanical agitation, for example by mixing or shaking the two liquid phases and the 2D material. Preferably, the mixture is homogenized by subjecting it to high shear forces, ultrasonic mixing or by using a microfluidizer. A microfluidizer is preferred as it allows for control of the droplet size and droplet size distribution in the resulting Pickering emulsion.

[0160] The viscosity of the resulting Pickering emulsion depends on the droplet size, the volume percentage of the dispersed phase and / or the viscosity of the continuous phase.

[0161] The process discussed above is illustrated in schematic form in Figure 4. Figure 4 shows a process in which a 2D material (10) is formed by exfoliating a layered 3D material in a solvent (11), after which the 2D material (10) is transferred (e.g. by a centrifugation process) to a first liquid phase (12) and a second liquid phase (13) is added. The liquid phases (12, 13) are then mixed and the emulsion is homogenized (e.g. by a high shear mixing process such as sonication or microfluidization) to form a Pickering emulsion (14).

[0162] Alternatively, suitable 2D materials, such as graphene, are commercially available. When a commercially available 2D material is used, step (1a) is not necessary and the method of forming a Pickering emulsion may simply include dispersing particles of the 2D material in a first liquid phase, adding a second liquid phase, and homogenizing the dispersion. Alternatively, the first liquid phase, the second liquid phase and the particles of the 2D material could simply be mixed and homogenized.

[0163] material As used herein, the term "composite material" means a solid material made from two or more component materials that remain separate and distinct within the finished structure.

[0164] Once the Pickering emulsion has been made (i.e., step (1) of the method of the present invention), the materials of the present invention can be formed. Different materials may be formed depending on the nature of the Pickering emulsion, in particular which liquid phase forms the continuous phase within the Pickering emulsion. Furthermore, different materials may be formed from the same Pickering emulsion depending on the conditions under which the polysiloxane is allowed to cure.

[0165] In particular, as described further below, when a Pickering emulsion in which polysiloxane is part of the discontinuous phase is placed in a sealed environment such that the continuous liquid phase cannot easily evaporate, a composite material forms that includes a matrix phase of polysiloxane elastomer and a network of dispersed 2D material.

[0166] Without wishing to be bound by theory, it is believed that in this method, polysiloxane polymer chains in the discontinuous phase of the Pickering emulsion are able to diffuse across the 2D material coating and through the continuous phase of the Pickering emulsion. A curing (or cross-linking) process then occurs. As a result, polysiloxane polymers that have diffused from one droplet form "bridges" with polysiloxane polymers from neighboring droplets. Over time, these bridges increase in number and size until the previously separated polymers form a new matrix phase of polysiloxane elastomer. This matrix phase has a well-defined and highly ordered network of 2D material that is derived from the 2D material that coated the droplets in the Pickering emulsion.

[0167] The physical properties of the resulting composite material depend at least in part on the length of time the emulsion is left in a sealed environment: for example, a longer time allows for more diffusion of the polysiloxane across the 2D material into the continuous phase and cross-linking with polysiloxane chains from other droplets.

[0168] In contrast, if the same Pickering emulsion is placed in an unsealed environment, the continuous liquid phase is allowed to at least partially evaporate while the polysiloxane hardens. This results in the formation of "spheres" of polysiloxane elastomer coated with the 2D material. The polysiloxane "spheres" can then be separated, for example by using a sieve, from any liquid phase remaining after the polysiloxane has hardened. Without wishing to be bound by theory, it is believed that in this process, the continuous liquid phase is at least partially evaporated and the polysiloxane hardens within the droplets formed in the Pickering emulsion before the polysiloxane can diffuse through the 2D material.

[0169] Alternatively, if a Pickering emulsion is formed with polysiloxane in the continuous phase and the polysiloxane is allowed to harden, a composite material is formed that contains a continuous matrix phase of polysiloxane elastomer and a network of cavities where the 2D material coats the surface of the polysiloxane around the cavities, similar in structure to a sponge or other porous solid material.

[0170] Voids in the material can be formed by evaporation of the discontinuous liquid phase, or, if any liquid phase is trapped in the polysiloxane elastomer, it can be removed by disturbing the elastomeric material (e.g., by compressing it), thereby creating pathways for the discontinuous liquid phase to escape the matrix.

[0171] Thus, one aspect of the method of the present invention comprises: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a curing agent; (2) placing the Pickering emulsion formed in step (1) in a sealed system for a time sufficient to at least partially cure the polysiloxane; (3) allowing any remaining liquid to evaporate; Includes.

[0172] This method is illustrated in Examples 3 and 6.

[0173] The present invention also provides a composite material formed by the above method.

[0174] As discussed above, in this process, it is believed that during step (2) of the method above, the polysiloxane polymer chains in the discontinuous phase of the Pickering emulsion are able to diffuse across the 2D material coating and through the continuous phase of the Pickering emulsion, whereupon the curing (or crosslinking) process occurs.

[0175] Step (2) may include placing the Pickering emulsion in a sealed system for a time sufficient to fully cure the polysiloxane. Alternatively, step (2) may include placing the Pickering emulsion in a sealed system for a time sufficient to partially cure the polysiloxane, after which the material is removed from the sealed environment. The curing can then be completed by heating the composite material, for example in an oven.

[0176] Step (2) may include placing the Pickering emulsion in a sealed system or environment for at least 24 hours, preferably at least about 48 hours, and more preferably at least 7 days. The exact time depends on the amount of 2D material in the Pickering emulsion, and generally longer times may be required when more 2D material is present. The Pickering emulsion may be placed at room temperature and pressure (25° C. and 1 atm). Alternatively, the Pickering emulsion may be placed at elevated temperature, for example about 40° C. to about 50° C., and optionally at elevated pressure. In this case, the curing time may be shorter.

[0177] Step (3) of the above method includes allowing any remaining liquid to evaporate. The liquid includes liquid from the first continuous phase and any solvent from the first discontinuous phase. This step may include allowing the product of step (2) to remain in the unsealed system for a time sufficient to allow the liquid to evaporate. For example, this step may include allowing the product of step (2) to remain in the unsealed system for at least about 1 hour, preferably at least about 6 hours.

[0178] The evaporation may be carried out at room temperature and pressure (25° C. and 1 atm.) Alternatively, step (3) may include placing the product of step (2) in an unsealed system at an elevated temperature, for example, from about 30° C. to about 70° C.

[0179] After evaporation of any remaining liquid (i.e., step (3)), a composite material, e.g., a composite film, is formed having a matrix phase of polysiloxane elastomer and a network of highly ordered 2D material.

[0180] The shape of the composite depends on the system in which curing is performed. For example, a composite film may be formed when the Pickering emulsion is allowed to cure in a thin layer. Alternatively, the Pickering emulsion may be placed in a mold to form a molded composite product. The total volume of the composite formed is smaller than the volume of the Pickering emulsion due to the evaporation of any remaining liquid in step (3).

[0181] For example, when the Pickering emulsion contains graphene-coated PDMS, the method can produce a composite film of cured PDMS elastomer with discrete graphene networks. An SEM image of such a film is shown in Figure 5, where the lighter lines are the graphene networks in the PDMS matrix.

[0182] Also disclosed herein is a process that includes allowing the polysiloxane to cure while in an unsealed system where the continuous liquid phase and any solvent from the discontinuous phase can evaporate. In this method, the continuous liquid phase evaporates and the polysiloxane cures before the polysiloxane chains diffuse through the 2D material. Thus, as discussed above, this method results in the formation of "spheres" of polysiloxane elastomer coated with the 2D material.

[0183] As used herein, the term "balls" refers to spherical particles or nearly spherical particles. The spheres are preferably spherical in shape.

[0184] Therefore, (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a curing agent; (2) allowing the continuous liquid phase to at least partially evaporate and the polysiloxane to at least partially cure; Disclosed herein is a method for making polysiloxane elastomer spheres coated with a 2D material, comprising:

[0185] This method is illustrated by reference to Example 2.

[0186] Step (1) may include the process disclosed above for forming a Pickering emulsion.

[0187] In the case of Pickering emulsions (i.e. water-in-oil emulsions) where the "water" phase forms the discontinuous phase, a hydrophobic container (e.g. silanized glass) is preferably used to prevent the droplets from popping, as the high surface tension water phase would tend to stabilize the surface energy mismatch between the larger glass and the 2D material.

[0188] Preferably, step (2) includes allowing the continuous liquid phase to evaporate and the polysiloxane to at least partially cure.

[0189] Preferably, step (2) includes allowing the continuous liquid phase to evaporate and the polysiloxane to fully cure.

[0190] Alternatively, step (2) may include allowing the continuous liquid phase to partially evaporate and the polysiloxane to harden, after which the elastomeric spheres are separated (e.g., with a sieve) from the remaining liquid phase.

[0191] Step (2) may be carried out by placing the Pickering emulsion formed in step (1) in an unsealed environment, optionally at an elevated temperature, for a period of time.

[0192] For example, step (2) may include placing the Pickering emulsion formed in step (1) in an unsealed environment at room temperature and pressure (25° C., 1 atm) for about 2 hours to about 24 hours. Increasing the temperature increases the rate of curing of the polysiloxane and / or increases the rate of evaporation of the continuous liquid phase, thus decreasing the time required to perform step (2). Thus, step (2) may include exposing the Pickering emulsion formed in step (1) to an unsealed environment at about 60° C. to about 120° C. for about 10 minutes to about 2 hours.

[0193] During this evaporation process, the polysiloxane polymers within the droplets undergo a curing or cross-linking process to form a polysiloxane elastomer. Thus, after the continuous liquid phase is evaporated and curing takes place, multiple discrete polysiloxane elastomer spheres coated with the 2D material are formed.

[0194] Thus, when the Pickering emulsion contains graphene-coated PDMS, graphene-coated cured PDMS elastomer spheres are formed by this method. An SEM image of such graphene-coated silicone particles is shown in Figure 6.

[0195] The discrete spheres have a texture similar to dry sand.

[0196] Generally, the coated spheres formed by the methods discussed above are smaller in size than the droplets trapped within the Pickering emulsion, since any solvent present in the discontinuous phase of the Pickering emulsion also evaporates, thereby reducing the size of the spheres formed. Thus, the (number) representative (average) diameter of the spheres is preferably about 5000 μm or less, more preferably about 500 μm or less, and most preferably about 100 μm or less. Generally, the spheres have a (number) representative diameter of at least about 50 nm, more preferably at least about 100 nm, and most preferably at least about 1 μm.

[0197] Thus, the spheres may have a size ranging from about 50 nm to about 5000 μm, preferably from about 100 nm to about 500 μm, and more preferably from about 1 to about 100 μm.

[0198] Also disclosed herein are 2D material coated polysiloxane elastomer spheres that may be formed by the above methods.

[0199] Preferably, the sphere comprises about 0.1 to about 50% by weight of the 2D material and about 50 to about 99.9% by weight of the polysiloxane elastomer. More preferably, the sphere comprises about 0.5 to about 40% by weight of the 2D material and about 60 to about 99.5% by weight of the polysiloxane elastomer. Most preferably, the sphere comprises about 1 to about 35% by weight of the 2D material and about 65 to about 99% by weight of the polysiloxane elastomer.

[0200] The discrete spheres discussed above may be used as filler particles in a composite material and may be combined with any suitable matrix material (e.g., a matrix material that is compatible with the 2D material) to form a composite material.

[0201] Therefore, (1) incorporating polysiloxane elastomer spheres coated with a 2D material into a matrix material; Also disclosed herein is a method of making a composite material, comprising:

[0202] (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a curing agent; (2) allowing the continuous liquid phase to at least partially evaporate and the polysiloxane to at least partially cure, thereby forming polysiloxane elastomer spheres coated with a 2D material; (3) incorporating the polysiloxane elastomer spheres formed in step (2) into a matrix material; Also disclosed herein is a method of making a composite material, comprising:

[0203] Preferably, step (2) includes allowing the continuous liquid phase to evaporate and the polysiloxane to at least partially cure, thereby forming polysiloxane elastomer spheres coated with the 2D material.

[0204] Preferably, step (2) involves allowing the continuous liquid phase to evaporate and the polysiloxane to fully cure, thereby forming polysiloxane elastomer spheres coated with the 2D material.

[0205] Step (1) may include the process disclosed above for forming a Pickering emulsion.

[0206] In step (3), the spheres formed in step (2) are added to a matrix material as fillers, thereby forming the composite material of the present invention.

[0207] Composites formed using the polysiloxane elastomer spheres described above as a filler material have superior properties to composites formed by simply adding 2D materials to a matrix material. For example, the pre-assembly of the 2D materials on the surface of the spheres prevents the 2D materials from agglomerating, and therefore all of the 2D materials act to modify the properties of the composite.

[0208] Furthermore, when "loose" 2D material is added, it can be randomly distributed throughout the matrix phase. In contrast, in the system described above, where the 2D material is added in the form of "spheres" of polysiloxane elastomer coated with the 2D material, the 2D material cannot occupy the volume of material occupied by the polysiloxane elastomer. Thus, a continuous network of 2D material can be formed at a lower filling rate than in a random system. This effect has been described for carbon nanotubes in Jurewicz et al. (Journal of Physical Chemistry B, vol. 115, no. 20, 2011, pp. 6395-6400).

[0209] Also disclosed herein are composite materials formed by the methods described above.

[0210] For example, disclosed herein is a composite material comprising a matrix phase and a plurality of filler particles, the filler particles comprising discrete polysiloxane elastomer spheres coated with a 2D material.

[0211] A composite material comprising a matrix phase and filler particles, the filler particles comprising: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a crosslinker; (2) allowing the continuous liquid phase to at least partially evaporate and the polysiloxane to harden; Also disclosed herein is a composite material formed by a method comprising:

[0212] For example, the filler particles may be incorporated into the matrix material by a composite formation process, such as in an extrusion process where the filler particles are extruded along with the desired matrix phase. Suitable processes and conditions are well known in the art.

[0213] The matrix phase into which the filler particles discussed above are incorporated may be selected from any matrix phase that is compatible with the selected 2D material.Preferably, the matrix phase is a polymer, such as natural rubber latex, polyolefin, polyester, polyacrylate, or polysiloxane elastomer.For example, the matrix phase may be a polysiloxane elastomer, where the polysiloxane is the same or different from the polysiloxane in the filler particles.

[0214] For example, graphene-coated cured PDMS spheres can be added to a PDMS matrix, thereby forming a graphene-containing PDMS composite.

[0215] In each of the methods and materials discussed above, the polysiloxanes and 2D materials may be selected from any of those discussed herein.

[0216] In composite materials including discrete polysiloxane elastomer spheres as discussed above, the discrete polysiloxane elastomer spheres (i.e., filler particles) may comprise up to about 90% by weight of the composite material. For example, the filler particles may comprise from about 10 to about 90% by weight of the composite material, such as from about 20 to about 80% by weight of the composite material. Alternatively, the filler particles may comprise from about 30 to about 70% by weight of the composite material.

[0217] The properties of the composite material are influenced by the properties of the matrix phase and the properties of the filler particles, for example the properties of the 2D material affect the properties of the filler particles and therefore the properties of the composite material.

[0218] For example, electrically conductive 2D materials such as graphene may be used to impart electrically conductive properties to composite materials.

[0219] In another aspect, the invention provides a method for making a composite material that includes a continuous matrix phase of a polysiloxane elastomer and a network of cavities, where a 2D material coats the surface of the polysiloxane around the cavities. This type of material resembles in structure a sponge, or other porous solid material.

[0220] This material may be formed by forming a Pickering emulsion containing a polysiloxane and a curing agent in a continuous phase, and then allowing the polysiloxane to cure. The discontinuous liquid phase is allowed to evaporate.

[0221] Therefore, this composite material has (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the continuous liquid phase comprising a polysiloxane and a curing agent; (2) allowing the polysiloxane to at least partially cure; The composite may be formed by a method including:

[0222] Step (2) may include allowing the polysiloxane to at least partially cure and the discontinuous liquid phase to evaporate, which may be performed in an unsealed environment.

[0223] Alternatively, the discontinuous liquid phase may become trapped in the composite as the polysiloxane cures in step (2). In this case, it may be necessary to deform (e.g., by compressing) the resulting elastomer in order to create pathways for the trapped discontinuous liquid phase to escape, e.g., by evaporation. This method therefore involves: (3) agitating the material formed in step (2) to remove any remaining discontinuous liquid phase; It may further include:

[0224] The present invention also includes a composite material formed by the above method.

[0225] Purpose The materials of the present invention have a wide variety of uses, for example, as discussed above, the discrete polysiloxane elastomer spheres coated with the 2D material are used as filler particles in composites to impart functionality to the composite.

[0226] For example, if the 2D material is electrically conductive (such as graphene), the spheres may be added to a matrix material to impart electrical conductivity. Such materials may be used as electrical sensors or as antistatic materials.

[0227] Alternatively or additionally, polysiloxane elastomer spheres may be used to provide mechanical reinforcement to the matrix material.

[0228] The discrete polysiloxane elastomer spheres described herein may also be used in water filtration devices where a 2D material (e.g., graphene) acts as a filter.

[0229] When a composite is formed by placing the Pickering emulsion in a sealed system, the resulting composite may be used in a strain gauge.

[0230] The electrical properties and strain range of the composite materials of the present invention invite their use as strain sensors. Nanocomposites are attractive candidates for next generation strain sensors due to their elasticity, but widespread adoption by industry has been hindered by nonlinear effects such as hysteresis and creep, which make accurate, repeatable strain readings a current challenge.

[0231] The sensitivity of a strain gauge is usually quantified by the relative change in resistance for a given strain. This is known as the gauge factor G f Commercial strain sensors are typically based on metal foil gauges, in which a significant portion of the strip resistivity arises from the geometry, which varies according to Poisson's ratio. Accuracy and reliability are preferred, rather than sensitivity and strain range, and such devices generally have a gauge factor (G) of about 2 to 6. f ) and typically fail at strains of 5% or less. In contrast, the composite materials formed herein may have gauge factors of greater than 20.

[0232] Gauge factor G f is expressed by the following equation (2):

[0233]

number

[0234] where ν is the Poisson's ratio, ρ is the resistivity, and ρ 0 is the initial resistivity, R is the resistance, R 0 is the initial resistance and ε is the applied strain.

[0235] Thus, in one aspect, the present invention comprises: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a curing agent; (2) maintaining the Pickering emulsion formed in step (1) in a sealed system for a time sufficient to cure the polysiloxane; (3) allowing any remaining liquid to evaporate; The present invention provides a strain gage comprising a composite material formed by a method comprising:

[0236] A strain gauge may be formed by simply attaching electrodes to the composite material and measuring the current flow during stretching.

[0237] The sensitivity of the strain gauge described herein is high enough to be used to track the respiration rate and pulse of people wearing the device. For example, the strain gauge could be incorporated into a band similar to a "fitness tracker" or even embedded into the fabric of an article of clothing such as a baby's sleepwear. Such a device would provide a comfortable, non-invasive way to monitor a subject's respiration and heart rate. This could be useful in any field where it is desirable to monitor respiration rate and heart rate, such as sleep apnea, heart rate and respiration rate during exercise, or monitoring a baby's respiration rate and heart rate.

[0238] The composite materials described herein may also be used as pressure sensors. The materials are suitable for use as pressure sensors for reasons similar to those that make them useful as strain sensors, e.g., due to their electrical properties.

[0239] The composite materials described herein may also be used as electrodes for energy storage devices such as supercapacitors and batteries.

[0240] Alternatively, composite materials that include a continuous matrix phase with a network of cavities may be used as a sponge-like material to absorb oil: the oil will bind to the composite material while the water will not, and thus these materials could be used to help clean up oil spills in the ocean. EXAMPLES

[0241] material A two-component product containing PDMS and a platinum hardener (QSIL 216) was purchased from Farnell UK.

[0242] Graphene powder (CAS: 1034343-98-0) was purchased from Thomas Swan & Co. Ltd (Elicarb Premium Grade Graphene Powder).

[0243] Dichloromethane, ethyl acetate and ethylene glycol were purchased from Sigma Aldrich.

[0244] Reference Example 1 - Formation of Pickering Emulsion 22 mg of graphene powder was ultrasonically irradiated in 4.98 g (3.76 mL) of dichloromethane (DCM) at less than 10° C. for 1 h to disperse the graphene in DCM.

[0245] Separately, 2 mL of QSIL 216 (containing PDMS (1.75 g) and curing agent (0.175 g) in a weight ratio of 10:1) was mixed with 2.02 g (2.24 mL) of ethyl acetate (EA).

[0246] The DCM containing the graphene particles was then added to the mixture of PDMS, curing agent, and ethyl acetate and homogenized by vigorous shaking for 30 seconds. The total volume of the mixture formed was 8 mL.

[0247] Next, 13.36 g (12 mL) of ethylene glycol was added, resulting in a ratio of ethylene glycol to DCM / EA / PDMS / curing agent of 60:40 by volume. Shear mixing was then carried out at 10,000 rpm for 2 minutes to form Sample 1.

[0248] The above procedure was repeated using the amounts of graphene shown in Table 2 below to form Samples 2-12.

[0249] [Table 2]

[0250] Reference Example 2 - Formation of filler particles The Pickering emulsion formed in Example 1 was rehomogenized by shaking for 30 seconds and transferred by pipette into a glass Petri dish.

[0251] The Petri dish with the emulsion was placed in a 30° C. oven for 1 hour, after which the temperature was increased by 10° C. per hour until it reached 70° C., then maintained overnight.

[0252] An SEM image of the filler particles formed by this method is shown in FIG.

[0253] Example 3 - Formation of a Composite Film The Pickering emulsion formed in Example 1 was left in a sealed container for 95 days before being rehomogenized by shaking for 30 seconds and transferred by pipette into a glass Petri dish.

[0254] The Petri dish with the emulsion was placed in a 30° C. oven for 1 hour, after which the temperature was increased by 10° C. per hour until it reached 70° C., then maintained overnight.

[0255] An SEM image of the filler particles formed by this method is shown in FIG.

[0256] Example 4 - Conductivity The conductivity of the composite film formed in Example 3 was examined. The results of the conductivity versus graphene loading are plotted in FIG.

[0257] FIG. 7 also plots the electrical conductivity versus graphene loading for the composites formed according to the present invention.

[0258] As can be seen from FIG. 7, the composite material of the present invention exhibits significantly higher conductivity than the material formed by Boland et al. at the same graphene loading level.

[0259] Without wishing to be bound by theory, it is believed that this is at least in part due to the process of the present invention providing control over the arrangement of the 2D material, since the 2D material is ordered in the Pickering emulsion before the composite is made. Thus, the composite formed by the method of the present invention has a 2D material with a high degree of orientation. In contrast, the process disclosed in Boland et al. results in graphene with a random arrangement within the composite, resulting in a lower final electrical conductivity.

[0260] Furthermore, the isolated nature of the 2D material network in the composite material of the present invention means that all graphene contributes to the electrical conductivity of the composite material. In contrast, in composite materials where "loose" graphene is added (e.g., as in Boland et al.), there are some aggregated graphenes that form disconnected clusters that do not contribute to the overall electrical conductivity. This results in lower electrical conductivity in the material formed in Boland et al. at any given graphene loading level. Thus, the material of the present invention achieves high electrical conductivity at low loadings of 2D materials.

[0261] Example 5 - Strain Sensor A composite film formed according to Example 3 was strained to failure and its electrical properties were measured using a mechanical testing stage (Texture Analyzer, Stable Microsystems) and a Keithley 2126B probe station.

[0262] Silver electrodes, roughly 5-7 mm wide and 25-30 mm long, were applied to both ends of the composite film samples. Sandpaper was used to insulate the metal clamps used to secure each sample in the mechanical testing stage, and each sample was connected to a probe station via alligator clips attached to tin foil strips secured against both silver-coated ends.

[0263] The composite was then strained and its electrical resistance was measured, and the relationship between the change in resistance and the applied strain was determined.

[0264] FIG. 8 shows the R / R ratio as a function of strain (X) for the composite made from sample 12 when strained to failure. 0 R = resistance, R 0 = initial resistance.

[0265] G=18.3 and X=R / R as the applied strain 0 =e Gx Plotting gives an excellent agreement that holds up to >80% strain.

[0266] We found that this exponential function was well-shaped (different from most linear gauge factors) at larger strains. Furthermore, characterization of the electromechanical properties over a wide range of samples and loading levels revealed a response that was consistent enough to be calibrated. Thus, the composites of the present invention are capable of measuring strain rather than simply sensing it.

[0267] The composite films tested were able to withstand >1000 cycles at 36% strain and >100 cycles at 74% strain before failure.

[0268] Compared to both linear and nonlinear strain sensors in the literature, the sensors tested here exhibited the largest absolute change in resistance reported, which is likely due to efficient nanosheet packing and distribution, allowing for superior conductivity at lower loading levels and mitigating the impact on the operating strain range.

[0269] Example 6 - Formation and testing of composite films Five identical oil-in-water emulsions were made using the method set out in Example 1, with 7.3% by volume of graphene relative to the volume of QSIL 216.

[0270] These emulsions were left in a sealed environment for 21 to 501 hours, respectively, before being poured into glass Petri dishes and cured via stepwise temperature increases as described in Example 3.

[0271] SEM (Zeiss Sigma field emission gun SEM) microscopy and Raman spectroscopy (Renishaw inVia microscopy) were performed on the stress fracture cross sections. The results are shown in Figure 9, which shows the transition from elastomeric spheres to a composite film of the invention as rest (or interdiffusion) time increases.

[0272] As discussed above, the change in the composition of the composite with increasing standing or interdiffusion time is likely due to the fact that, given sufficient time, polymer chains can diffuse through the graphene shell into neighboring droplets, eventually resulting in a macroscopically continuous film.

[0273] While the ratio of PDMS to curing agent (10:1) greatly influences the degree of crosslinking, emulsion formation and polymer diffusivity are expected to be governed by the viscosity of the oil phase at room temperature. One strategy to shorten the interdiffusion time (i.e., the time the emulsion is allowed to sit undisturbed in a sealed system) is to use a lower molecular weight PDMS, which has a reduced viscosity and a higher diffusion coefficient.

[0274] The toughness of the various composite films versus interdiffusion time is shown in Figure 10. As can be seen, the mechanical toughness increases dramatically only when a significant fraction of the chains diffuse beyond the graphene shell, allowing interdroplet bridging and changing the dominant failure mechanism from overcoming van der Waals adhesion forces to chain pull-out or scission.

[0275] A doubling of conductivity was also observed upon increasing density from elastomer spheres to a continuous film, as shown in Figure 11 (which shows conductivity versus interdiffusion time), which is believed to be due to the disappearance of the void space between the elastomer spheres, which increases the electrical contact area between the nanosheets and reduces the porosity of the composite film.

[0276] Example 7 - Comparison with randomly distributed composite material The morphology of the conductive network in the composite of the present invention, visible in the SEM of the fracture surface shown in Figure 9, is significantly different from the graphene network found in conventional randomly distributed composites. Due to the fabrication method, the graphene is strongly confined to the droplet interface in the liquid system, which means that the network structure is preserved after curing. It is therefore intuitive that when the graphene sheets are all confined in close proximity, the quality of the electrical bond will be high due to the reduction in the tunneling distance between the nearest neighbors. In contrast, in a random percolated network, a significant proportion of the conductive fillers do not contribute to the conductive pathways near the percolation threshold, resulting in a negligible contribution to the macroscopic conductivity and a less efficient use of materials compared to the present system.

[0277] Furthermore, assembling the graphene network in a reduced volume (i.e., at the oil-water interface) requires less total filler to realize macroscopic conductive pathways, while interfacial tension orients the graphene sheets tangentially to the droplet surface, both of which act to increase the number of conductive junctions and improve their quality by providing superior inter-sheet contact compared to randomly distributed networks.

[0278] This is why the composite material of the present invention exhibits significantly higher electrical conductivity than the material formed by Borland et al. (wherein the 2D material is randomly distributed and oriented in the matrix) at the same graphene loading level, as shown in FIG. 7 and discussed in Example 4.

[0279] A further comparison between the composite material of the present invention and the material formed by Boland et al. is shown in FIGS.

[0280] FIG. 12 plots the Young's modulus of various composite films formed by the method set out in Example 3 as a function of graphene loading, showing a linear trend over the range of data. Data for the random composites disclosed in Boland et al. are plotted for comparison. A much larger increase in Young's modulus with orientation level is observed in the composites of Boland et al. It is important to note that the nanocomposites used for strain sensing need to remain soft enough to be compatible with the surface being measured, e.g., human skin. Thus, the much weaker dependence of Young's modulus on loading level observed in the composites of the present invention may prove advantageous in the design of skin sensors.

[0281] In contrast to the Young's modulus observed in the composites claimed in this invention, which is lower compared to that disclosed in Boland, FIG. 13 shows that the composites of this invention have a significantly higher yield strain than those of Boland et al. The yield strain is nearly constant, but both systems show a decrease with increasing graphene content. Viscoelastic sensors with a significant viscous component do not recover when strained beyond the yield point. However, the high elasticity of the sensors of this invention allows the material to operate over a much wider strain range.

[0282] The structure of the composite material claimed in this invention, highlighted in Figure 9, consists of graphene shells around a bare PDMS "core." These shells, interdiffusing with the PDMS chains, are responsible for increasing the Young's modulus (due to interfacial stress transfer between the matrix and graphene).

[0283] Example 8 - Body movement detection The electrical response of the strain sensor formed in Example 5 was examined by subjecting it to strain scenarios typical of the human body, including finger bending, pulses, and breathing. Figures 14 and 15 show the sensor in a relaxed state when taped to an index finger (Figure 14) and in a maximally bent and strained state (Figure 15).

[0284] In Figure 16, the electrical response to multiple finger bends over small (<10°), medium (~45°) and large (~90°) bend radii is shown (top, middle and bottom, respectively). When fully relaxed, the sensor is ~4cm in length and rises to ~5cm, or ~25% strain, upon large bends.

[0285] We then placed sensors on both the neck and chest of a human subject and recorded the electrical response. When the sensor was gently pressed against the carotid artery, a pulse was clearly detectable and a narrow peak at 59 bpm was extracted from the Fourier transform (Figure 18) (Figure 17).

[0286] When placed on the chest, the sensor was able to detect both the high-strain, low-frequency modes associated with respiration and the high-frequency, low-strain modes associated with pulse (see Figure 19). The fact that the pulse signal is easily discernible above the respiration modes speaks to the versatility of the device and its potential as a biomedical sensor. The inset in Figure 19 is the pulse waveform with the baseline drift induced by respiration removed. This was also subjected to a Fourier transform, revealing a maximum at 65 bpm, typical of a resting heart rate (Figure 20).

Claims

1. 1. A method of making a composite material, comprising: (1) forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and a 2D material, the discontinuous liquid phase comprising a polysiloxane and a curing agent; (2) placing the Pickering emulsion formed in step (1) in a sealed system for a time sufficient to at least partially cure the polysiloxane; (3) allowing any remaining liquid to evaporate; The method includes:

2. 10. The method of claim 1, wherein the 2D material is graphene, hexagonal boron nitride, phosphorene, or a transition metal dichalcogenide.

3. The method of claim 2 , wherein the 2D material is graphene.

4. The polysiloxane is represented by formula (I) or formula (II): 【Chemistry A】 【Chemistry B】 A compound of the formula: R １ , R ２ , R ３ , R ４ , R ５ and R ６ each is independently H or an organic group; X is an organic group; Y is H or an organic group; W is an organic group; Z is H or an organic group; n and m are each any integer greater than 1; The method according to any one of claims 1 to 3.

5. The method of claim 4 , wherein the polysiloxane is PDMS.

6. The method of any one of claims 1 to 5, wherein the continuous liquid phase comprises glycerol, water, formamide, diethylene glycol, ethylene glycol, propylene glycol, or combinations thereof.

7. 7. The method of claim 6, wherein the continuous liquid phase comprises water, propylene glycol, ethylene glycol, or a combination thereof.

8. 8. The method of any one of claims 1 to 7, wherein the discontinuous liquid phase further comprises a solvent selected from the group consisting of hexane, acetone, tetrahydrofuran, chlorobenzene, diethyl ether, ethyl acetate, toluene, xylene, pentanol, butanol, propanol, ethanol, methanol, chloroform, acrylonitrile, dichloromethane, and combinations thereof.

9. 9. The method of claim 8, wherein the solvent is selected from the group consisting of ethyl acetate, dichloromethane, and combinations thereof.

10. 10. The method of any one of claims 1 to 9, wherein step (2) comprises placing the Pickering emulsion in a sealed system for at least 24 hours.

11. 11. The method of claim 10, wherein step (2) comprises placing the Pickering emulsion in a sealed system for at least 7 days.

12. 12. The method of any one of claims 1 to 11, wherein step (3) comprises placing the product of step (2) in an unsealed system for at least 1 hour.

13. 13. The method of any one of claims 1 to 12, wherein step (3) comprises placing the product of step (2) in an unsealed system at a temperature between 30°C and 70°C.

14. The Pickering emulsion is (1a) exfoliating a layered 3D material in a solvent to produce particles of a 2D material; (1b) forming a dispersion of particles of the 2D material in a first liquid phase; (1c) adding a second liquid phase and homogenizing the dispersion of the 2D material in the first liquid phase and the second liquid phase, thereby forming a Pickering emulsion comprising a continuous liquid phase, a discontinuous liquid phase, and the 2D material; The method of any one of claims 1 to 13, wherein the polymer is formed by a process comprising:

15. The method of claim 14 , wherein the polysiloxane and curing agent are present in the first liquid phase.

16. A composite material formed by the method of any one of claims 1 to 15.

17. A strain sensor comprising the composite material of claim 16.

18. A pressure sensor comprising the composite material of claim 16.

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