INK COMPOSITION COMPRISING THERMO-RESPONSIVE

POLYMER

TECHNICAL FIELD

The present invention relates to an ink composition comprising a carbon material, or layered oxide, nitride or transition metal dichalcogenide, and a thermo-responsive polymer, methods of preparing it, methods of printing an article with the ink composition.

BACKGROUND

The field of 3D printing and additive manufacturing for the printing of carbon material-based products is underdeveloped when compared to the printing of polymer or metal products. Printing inks for 3D objects often require complicated formulations with multiple additives, for example dispersants, binders, that are specifically formulated for each material to be printed. The inks also require the fine control of pH, and/or mixtures of volatile solvents. There is therefore a need for simplified inks which have the required resolution and structural stability for printing 3D articles from carbon materials or layered oxides, nitrides or transition metal dichalcogenides.

SUMMARY OF THE INVENTION

It has been found that an ink composition comprising carbon material or layered oxide, nitride or transition metal dichalcogenide and a thermo-responsive polymer provides an ink which has the required resolution and structural stability for the printing of 3D articles. Accordingly, in a first aspect, the present invention provides an ink composition comprising a component A, an aqueous solvent, and a thermo-responsive polymer, wherein component A is selected from carbon material or layered oxide, nitride or transition metal dichalcogenide, and the ratio (by weight) of component A to the thermo-responsive polymer is 0.01 : 1 to 20: 1.

In a second aspect, the present invention relates to a method of producing a printed article comprising the step of printing an ink composition according to the first aspect to form an article.

In a third aspect, the present invention relates to a method of preparing an ink composition according to the first aspect wherein the method: (a) is a method comprising the step of adding component A to an aqueous solution of thermo-responsive polymer at a temperature below the lower critical solution temperature (LCST) of the polymer; or

(b) is a method comprising adding component A and a thermo-responsive polymer to a slurry comprising component A; wherein component A is selected from carbon material, layered oxide, nitride or transition metal dichalcogenide.

In a fourth aspect, the present invention provides a use of a composition according to the first aspect, as an ink for 3D printing. In a fifth aspect, the present invention provides an article obtained by printing a composition according to the first aspect.

BRIEF DESCRIPTION OF THE FIGURES

Specific embodiments of the invention are described below by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows the chemical structure of Pluronic F127 (F127).

Figure 2 shows the changes in Theological properties of Pluronic F127 with temperature.

Figure 3 shows the Theological properties of graphene oxide (GO) and F127 inks.

Figure 4 shows a comparison of the viscosity and rheology of an ink comprising different concentrations of GO and F127. Figure 5 shows 3D printed structures with GO-F127 inks.

Figure 6 shows the microstructure of the reduced 3D printed woodpile-type structure.

Figure 7 shows the internal microstructure of ink filaments.

Figure 8 shows the Raman analysis of the 3D printed structure comparing the top and bottom of the printed article. Figure 9 shows TGA analysis of the freeze dried Graphene Oxide.

Figure 10 shows the rheological properties of an ink composition with 0.78 wt% GO.

Figure 11 shows 3D printed structure with 0.78 wt% GO of GO/F127 ink. Figure 12 shows rheological properties of 2 wt% GO content GO/F127 ink. DETAILED DESCRIPTION

The meanings of the terms used herein are explained below, and the present invention will be described in detail. The molecular weight of a polymer species is given as the number average molecular weight M _n , except where otherwise specified. M _n may be determined by gel permeation

chromatography, mass spectrometry or ¹ H NMR.

An ink composition according to the present invention comprises a component A, a thermo- responsive polymer and an aqueous solvent, wherein component A comprises a carbon material or a layered oxide, nitride or transition metal dichalcogenide.

Carbon materials according to the present invention include, but are not limited to, graphite, graphene, graphene oxide (which may also be referred to as chemically modified graphene (CMG)), reduced graphene oxide (rGO), active carbon, carbon black, carbon nanotubes or carbon fullerenes, or mixtures thereof. The carbon material may also include multi-layered material comprising two or more layers of graphene, including graphite nanoplatelets (GNP), chemically modified graphene materials and materials made using graphene or another graphene material as a precursor. Carbon materials may be nanomaterials. Nanomaterials are materials with at least one external dimension in the size range from about 1 to 100 nm. Carbon nanomaterials are carbon materials with at least one external dimension in the size range from about 1 to 100 nm.

Graphene is a single-atom thick sheet of hexagonally arranged, sp ² -bonded carbon atoms. Graphene may have a specific surface area of up to 2600 m ² /g. Graphite nanoplatelets (GNP) are stacks of graphene sheets. They may have a thickness and/or lateral dimension less than 100 nm. They may be stacks with a total thickness of each graphite platelet of around about 2 nm and an average diameter of 1 to 2 μιη. GNPs have a high aspect ratio. Graphene nanoribbons may be single-atom-thick strips of hexagonally arranged, sp2- bonded carbon atoms and which have a longer lateral dimension which exceeds the shorter lateral dimension by at least an order of magnitude.

Graphene oxide (GO) is an oxidized form of graphene. Graphene oxide may be prepared by oxidation and exfoliation of graphite. Graphene oxide may comprise, for example, at least 20 atomic % oxygen. Graphene oxide may be provided as a multi-layered material comprising two or more layers of oxidized graphene. The graphene oxide may also be exfoliated into individual layers. Graphene oxide may be provided as flakes. Flakes of graphene oxide may comprise single monolayers of graphene oxide or several layers. The flakes may have an average thickness of several nanometres (for example up to about 10 nm). The flakes may have an average lateral dimension (maximum diameter) of up to about 300 pm, up to about 200 pm, up to about 150 pm, up to about 100 pm, up to about 80 pm, up to about 60 pm, up to about 50 pm, preferably up to about 30 pm, preferably up to about 25 pm, for example about 20 pm. The flakes may have an average lateral dimension of at least about 5 pm, preferably at least about 10 pm. The average lateral dimension and average thickness is the arithmetic mean of the lateral dimension and thickness, respectively. The lateral dimensions of the GO flakes may be measured using optical and/or scanning electron microscopy. The thickness may be determined using atomic force microscopy or transmission electron microscopy.

Reduced graphene oxide (rGO) is graphene oxide which has been reductively processed, for example, by thermal or chemical treatment. rGO may be provided as flakes, with corresponding dimensions as discussed above for GO.

Carbon nanotubes (CNTs), according to the present invention, include, but are not limited to single-wall carbon nanotubes (SWNTs or SWCNTs), double-wall carbon nanotubes (DWNTs or DWCNTs), multi-wall carbon nanotubes (MWNTs or MWCNTs), small diameter carbon nanotubes, large wall carbon nanotubes, and combinations thereof. Carbon nanotubes may be any type of nanotube, that is, it may be any hollow tubular structure having at least one dimension measuring on the nanometer scale. For example, the nanotube may have a smallest inner diameter measuring between about 0.5 nm to about 50 nm, such as about 0.5 nm to about 20 nm, for example between about 0.7 nm to about 10 nm, e.g. between about 0.8 nm to about 2 nm. Small diameter multi-wall carbon nanotubes may be medium-sized carbon nanotubes with diameters of around 5 to 10 nm and lengths of several micrometers. Large diameter multi-wall carbon nanotubes may be large nanotubes with diameters of around about 100 nm and lengths of several tens of micrometers. The nanotube may be of any length. For example, the nanotube may have a length between about 5 nm to about 2 mm, for example, about 5 to about 500 pm. The nanotube may have a length of 2 mm. The specific surface area of these small diameter multi-wall carbon nanotubes may be around 250 m ² /g for 10 nm tubes, and 500 m ² /g for 5 nm tubes.

Component A may alternatively comprise a layered material selected from layered oxides, layered nitrides and layered transition metal dichalcogenides. A layered oxide as described herein is a metal (generally a transition metal, such as one or more of Ti, Nb, Mn, Ta, W) oxide material comprising a layered structure. Layered oxides include multi-layered material comprising two or more layers of oxide or individual oxide layers. The oxide layers may comprise a geometrical, such as octahedral, arrangement of atoms and may comprise alkali metal cations (K ⁺ , Rb ⁺ , Cs ⁺ ) occupying the interlayer space. The layered oxides according to the present invention include, but are not limited to, calcium niobium oxide, manganese oxide, niobium oxide, caesium tungsten oxide or molybdenum trioxide. The precursors of the layered oxides may be described as M0 ₆ octahedral units where M may be Ti, Nb, Mn, Ta, V,W) and alkali metal cations (K ⁺ , Rb ⁺ , Cs ⁺ ) occupying the interlayer space. The layered oxides may be exfoliated into individual layers and thus may be provided as a multi-layered material or as an individual layer.

A layered nitride as described herein is a metal (generally a transition metal such as Ti, Zr, Sr, or metalloid, such as boron) nitride material comprising a layered structure. Layered nitrides include multi-layered material comprising two or more layers of nitrides or individual nitride layers. Layered nitrides may be functionalised with oxygen prior to preparation of the ink composition. An example of a layered nitride is boron nitride.

A layered transition metal dichalcogenide (i.e. comprising ME ₂ , wherein M is a transition metal e.g. Ti, Nb, Mn, Ta, V, W, Zr, Sr) and E is a chalcogenide(e.g. S, Se, Te)) may be provided as a multi-layered material or as an individual layer.

For the purpose of this invention, the term "polymer" is given the meaning of a molecular structure formed by a plurality (e.g. at least 5) monomers connected by covalent bonds. Preferably, the polymer may comprise at least about 10, about 30, about 50, or about 100 monomers. The polymer may comprise up to about 130, about 200, about 250, or about 350 monomers. The polymer may comprise from about 10 to about 350 monomers, about 30 to about 250 monomers, about 50 to about 200 monomers, or about 100 to about 130 monomers. The monomers may all be identical. In this case, the polymer is said to be a homopolymer. There may be more than one type of monomer present in the polymer. In this case, the polymer is said to be a copolymer, which in the context of this invention, may have 2 types of monomer or more than two types of monomer. The monomers forming the copolymer may be arranged in any way. The monomers in a copolymer may be arranged randomly (random copolymer), alternating (alternating copolymer), or in blocks (block copolymer), or any combination thereof. Block polymers or block copolymers may have two or more repeating distinct blocks. Block copolymers with three distinct blocks are triblock copolymers. The polymer may be a linear polymer comprising a single main polymer chain. The polymer may be a branched polymer comprising a main polymer chain with at least one polymeric side chain.

A thermo-responsive polymer is a polymer that undergoes a reversible temperature dependent phase transition (e.g. from a liquid to gel transition or from a gel to liquid transition). Accordingly, a solution or suspension of a thermo-responsive polymer will have different rheological properties at different temperatures. The temperature at which the phase transition occurs (e.g. the change in rheology occurs) is the lower critical solution temperature (LCST). For example, below the LCST, a thermo-responsive polymer is a low viscosity solution (i.e. the polymer is a fluid). Above the LCST, the thermo-responsive polymer is a solid gel (i.e. the polymer is able to retain its shape). At the LCST, a thermo- responsive polymer transitions from a low viscosity solution to a solid gel. This may happen, for example, by forming micelle aggregates.

LCST can be measured by measuring the rheology of the polymer. For example, the storage modulus G' and the loss modulus G" can be measured as a function of temperature and the LCST is taken to be when G'=G". For example, G' and G" as a function of temperature can be measured using a temperature ramp at a fixed strain of 0.15% and a fixed frequency of 396 rad.s ^"1 in a Discovery Hybrid Rheometer RH1 (TA Instruments). Alternatively, the LCST of a polymer may be determined from the absorbance of the polymer at 450 nm measured against temperature using a UV vis spectrophotometer. In a first aspect, the invention relates to an ink composition comprising a component A, an aqueous solvent, and a thermo-responsive polymer, wherein component A is selected from carbon material or layered oxide, nitride or transition metal dichalcogenide, and the ratio (by weight) of component A to the thermo-responsive polymer is 0.01 :1 to 20: 1.

The ink composition comprises a ratio (by weight) of component A to the thermo-responsive polymer of about 0.01 :1 to about 20:1. The ratio of component A to thermo-responsive polymer may be up to about 10:1 , up to about 9:1 , up to about 5: 1 , up to about 3:1 , up to about 2: 1 , or up to about 1.5: 1 , or up to about 1 : 1. The ratio of component A to thermo- responsive polymer may be at least about 0.02: 1 , about 0.05: 1 or about 0.5:1. For example, the ratio may be about 0.02: 1 to about 10: 1 , about 0.02: 1 to about 9: 1 , about 0.02: 1 to about 5:1 , about 0.05:1 to about 2: 1 or about 0.05: 1 to about 1 : 1. In any of these ranges, the ratio of component A to thermo-responsive polymer may be at least about 0.5:1. Preferably, the ratio of component A to thermo-responsive polymer may be about 1 :1. In some embodiments, the ink composition comprises at least 0.1 wt% of component A. The ink composition may comprise at least about 0.5 wt%, at least about 0.7 wt%, at least about 0.9 wt%, at least about 3 wt%, at least about 4 wt%, at least about 4.5 wt%, at least about 5 wt% of component A. The ink composition may comprise up to about 20 wt%, up to about 10 wt%, up to about 8 wt%, up to about 7 wt%, up to about 6.5 wt%, up to about 6 wt% of component A. The ink composition may comprise about 0.1 to about 20 wt % of component A. For example, the ink composition may comprise about 0.7 wt% to about 10 wt%, about 0.8 wt% to about 9 wt%, about 1 wt% to about 8 wt%, about 2 wt% to about 8 wt%, about 2 wt% to about 7 wt%, or about 2 wt% to about 6 wt% of component A. More preferably, the composition comprises about 4-8 wt%, about 4-7 wt% or about 5-6 wt% of component A. The composition may comprise 6 wt% component A.

The weight percent of component A in an ink composition is based on the total weight of the thermo-responsive polymer, the weight of component A and the weight of the aqueous solvent. Component A may be a carbon material. Preferably, component A may be graphene oxide.

Component A may be a layered oxide selected from the group of calcium niobium oxide, manganese oxide, niobium oxide, caesium tungsten oxide and molybdenum trioxide.

The following definitions of a thermo-responsive polymer apply to all aspects and embodiments of the invention described herein. The thermo-responsive polymer may be a hydrogel.

The thermo-responsive polymer may be a copolymer.

Preferably, the thermo-responsive polymer may be a triblock copolymer.

The thermo-responsive polymer may be selected from: poly(N-isopropyl acrylamide), poly(N,N-diethylacrylamide), poly(N-vinylalkylamide), poly(N-vinylcaprolactam),

polyphosphazenes, Pluronic polymers, Tetronic polymers, polysaccharide polymers, chitosan, and PLGA-PEG-PLGA triblock copolymers.

Pluronic polymers are triblock copolymers based on polypropylene oxide (PPO) and polyoxyethylene oxide (PEO). Preferably, the polymer is a polyoxyethlyene- polyoxypropylene-polyoxyethylene triblock copolymer. Preferably, the thermo-responsive polymer is a Pluronic polymer. In a preferred embodiment, the polymer may be Pluronic F127 (poloxamer 407). Pluronic F127 is a triblock copolymer comprising blocks of PEO and PPO, structure units of (PEO- PPO-PEO). Pluronic F127 has an average structure of (PE0 ₉₉ -PP0 ₆ 7-PE0 ₉ 9). The number average molecular weight (M _n ) of Pluronic F127 is 12600 Dalton.

Tetronic polymers are tetrafunctional block copolymers based on ethylene oxide and propylene oxide monomers.

Polysaccharide polymers are derived from saccharide monomers. The monosaccharide monomers may be bound together by glycosidic bonds. Polysaccharides may include, for example, cellulose, cellulose derivatives, e.g. hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose.

Polyphosphazenes are inorganic/organic hybrid polymers which contain alternating phosphorus and nitrogen atoms.

The polymer may comprise poly(lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG). The polymer may be a triblock copolymer comprising PLGA-PEG-PLGA.

In any of the embodiments herein, the thermo-responsive polymer may have an M _n of at least 5000 Da, for example 5000 Da to 20 kDa, or 10 kDa to 15 kDa.

Preferably, the thermo-responsive polymer may have an LCST in the range of between about 0 °C to about 50 °C. Preferably, the LCST may be in the range of between about 5 °C to about 30 °C or about 15 °C to 30 °C , more preferably between about 10 °C to about 25 °C, more preferably between about 15 °C to about 20 °C. The LCST of the polymer may be at least about 0 °C, at least about 12 °C , at least about 15 °C, at least about 16 °C, at least about 23 °C. The LCST may be when G"=G\

The aqueous solvent may be water or a mixed solvent system comprising water and one or more other solvents (which are preferably miscible with water). Preferably, the solvent may be water. An aqueous solvent enables the full formation of hydrogen bonds to obtain a change in the rheological properties of the composition. Organic solvent, such as an alcohol, for example methanol or ethanol, may be used in the aqueous solvent. Up to about 20 wt%, up to about 15 wt%, up to about 10 wt%, up to about 5 wt% of an alcohol with respect to the total weight of water may be used.

More preferably, the ink composition may comprise a ratio of component A to thermo- responsive polymer of about 1 : 1 , and the composition may comprise about 5 to 6 wt% of component A. In a third aspect, the ink composition may be prepared by:

(a) a method of adding component A to an aqueous solution of thermo-responsive polymer at a temperature below the lower critical solution temperature (LCST) of the polymer, wherein component A is selected from a carbon material, layered oxide, nitride or transition metal dichalcogenide; or

(b) a method comprising adding component A and a thermo-responsive polymer to a slurry comprising component A, wherein component A is selected from a carbon material, layered oxide, nitride or transition metal dichalcogenide. In option (b) preferably the slurry comprises an initial concentration of about 10 g/L to about 15 g/L of component A.

Preferably, the slurry may be an aqueous slurry. The slurry may be concentrated by evaporating water. Preferably, component A and the thermo-responsive polymer are added as powders to the slurry of component A. Preferably, the addition of thermo-responsive polymer is carried out at a temperature below LCST.

The following method features may apply to options (a) or (b) above. The addition of component A with the thermo-responsive polymer may take place below about 20 °C, below about 15°C, below about 10 °C, below about 7 °C, below about 5 °C, below about 0 °C.

The method of preparing an ink composition may further comprise a step of cooling the composition comprising component A and the thermo-responsive polymer following the addition of component A.

The method of preparing an ink composition may comprise a step of mixing or dispersing component A and the thermo-responsive polymer. Dispersion may mean the preparation of a homogenous composition which does not contain aggregates of component A. The dispersion may take place, for example, using a high centrifugal planetary mixer. Component A may be added in portions to the ink composition. The method may comprise repeated addition of component A to the composition.

The steps of addition of component A, mixing of component A and cooling of the composition following the addition of component A may be repeated.

The method of preparing an ink composition may comprise a further step of defoaming the mixed composition following dispersion. Defoaming may be done using a planetary mixer. Defoaming eliminates bubbles and provides a very homogenous composition, which provides a continuous flow during the printing process.

The method may further comprise a step of allowing the prepared composition to settle before printing. Settling may take up to about 1 hour, up to about 2 hour, up to about 6 hours, up to about 12 hours, up to about 24 hours, up to about 36 hours, up to about 48 hours. Settling may take place at room temperature.

In order to print self-supporting 3D structures, an ink must have well controlled viscoelastic properties that enable a stable flow through the deposition nozzle and then "set" immediately retaining the shape. The inks must have a shear thinning behaviour (viscosity decreases with shear stress) so they can be easily injected through a small nozzle. At the same time, they should exhibit a solid-like (G'>G") behaviour, and high values of viscoelastic properties (storage modulus, G').

Preferably, the storage moduli G' of the ink composition may be at least about 1 kPa at a strain below 1 %. The storage moduli G' may be at least about 5kPa, at least about 10kPa, at least about 20 kPa, at least about 30 kPa at a strain below 1 %, at least about 40 kPa, at least about 100 kPa at a strain below 1 %.

The storage modulus of the ink composition is measured by oscillation measurement, using a Discovery Hybrid Rheometer RH1 (TA Instruments) with a parallel plate with 40 mm diameter. The oscillation measurement is performed with a solvent trap cover to prevent solvent evaporation. The oscillation settings are: amplitude sweep at a fixed frequency of 63 Hz or 396 rad/s, under strains from 0.0001 up to 1000%, at 25°C.

There are hydrophobic interactions with the unoxidized graphitic islands on the basal plane of GO and hydrogen bond interactions between the protonated functional groups in the GO and the functional groups on the hydrophilic blocs of the polymer. Moreover, there is an adsorptive interaction between the hydrophobic groups and the hydrophobic carbon material surface. This allows the formation of a carbon material/polymer 3D network linked by non- covalent interactions and an increase in the loss and storage modulus. The non-covalent nature of the interactions allows the network links to break down under shear enabling the flow of the resulting ink composition, for example, through nozzles. The links heal once the stress is released and the carbon material/thermo-responsive polymer networks recover their gel-like structure and set after printing. If the ink is passed through a nozzle, the ink sets immediately after passing through the nozzle and retains the shape of the filament, holding the printed structure. In a second aspect, the invention provides a method of producing a printed article comprising the step of printing an ink composition according to the first aspect to form an article.

The method may comprise printing an ink in combination with one or more additional ink compositions. The additional ink composition may be an ink composition according to the first aspect of the invention or may be a different ink composition.

The printing method may further comprise the step of drying and optionally curing the printed article.

The method may further comprise the step of preparing an ink composition according to the first aspect.

The rheological properties of an ink composition according to the invention enable its use for production of three dimensional articles. The ink composition is preferably for use in 3D printing and is of particular use in extrusion filament printing. The ink composition may also be used for coating, injection moulding and tape casting. Preferably, the printing may be extrusion filament printing. In extrusion filament printing, an ink composition is printed by extrusion of a filament of the ink composition through a printing nozzle. Printing of thin filaments is possible and the printing nozzle may have a diameter of, for example, 100-1000 μηη, preferably 100-500 pm (giving a corresponding printed filament diameter). Filaments can be built up in layers to provide a 3D printed article. The ink may be used for continuous extrusion.

The drying may be according to methods known in the art. Drying may be in a convective oven with or without humidity control. Drying may also take place in a microwave oven. Drying may comprise lyophilisation. Drying may comprise supercritical drying. The drying step may comprise lyophilising the printed article. Freeze-drying, or lyophilising helps avoid shrinkage and allows the formation of 3D objects with smooth surfaces while preserving fine printing features down to the low micrometre range.

The curing may comprise the step of heating the article. The heating temperature may be between 200-2400 °C. Preferably, the heating may be at a temperature of between 900- 1000°C. The heating may be under a reducing environment. Preferably, the curing comprises the step of heating the article at a temperature of between 200-2400 °C under a reducing atmosphere.

Preferably, when the article is a graphene oxide article, the article is chemically reduced to a graphene article. Preferably, the reduction of the graphene oxide article may take place under a reducing atmosphere with an argon/H ₂ atmosphere comprising 10% H ₂ . These reductive conditions allow Pluronic F127 to be removed and reduce GO to restore the properties of graphene.

The article may be chemically reduced to a graphene article. Chemical reduction may involve reduction in the presence of hydrazine or ascorbic acid. The reducing atmosphere may comprise argon/H ₂ atmosphere comprising 10% H ₂ .

The inks of the present invention may be used to prepare electronic components such as supercapacitors.

In the context used herein, "about" may refer to a variation of ± 10%.

All features of each of the aspects of the invention as described above can be applied to all other aspects of the invention mutatis mutandis.

The present invention will now be explained in more detail by reference to the following non- limiting examples:

Examples

Ink compositions were prepared as follows: A temperature responsive polymer (Pluronic F127) stock solution with a concentration of 25 wt% in Pluronic was prepared in distilled water and stored in a refrigerator. This stock solution is prepared as follows. By using a centrifugal planetary mixer, Pluronic F127 powder is dissolved in water by successive mixing cycles of 5 min at 2000 rpm and then cooling down in an ice bath for 10 min, up to total dissolution. The starting graphene oxide (GO) water based slurry comprises a 10 g/L GO slurry prepared using the modified method of Tour et al. (Marcano, D.C., Kosynkin, D.V., Berlin, J.M., Sinitskii, A., Sun, Z., Slesarev, A., Alemany, L.B., Lu, W., Tour, J.M. Improved synthesis of graphene oxide (2010) ACS Nano, 4 (8), pp. 4806-4814) and subsequently freeze dried. The freeze dried GO was used to prepare the inks. Example 1 2.7 g of the Pluronic F127 (25 wt% stock solution) and 7.5 g of distilled water were added in a HPDE beaker. Then small amounts of freeze dried GO (i.e 30mg) are added and mixed with the help of a centrifugal planetary mixer for 3-5 min at 2000 rpm. The beaker is afterwards immersed in an ice/water bath for 10 min in order to decrease temperature below 15°C before adding more freeze dried GO (30mg). Mixing/Ice Cooling/Freeze dried GO addition steps are subsequently repeated to the desired concentration of GO in the ink and until good dispersion of the GO is achieved and no agglomerates are present and ink/paste looks shiny. In this example up to 0.675 g of dried GO were added. The final concentration of GO in the ink is 6 wt% and the ratio g GO/g dried F127 is 1/1. The resulting composition is finally defoamed using a planetary mixer at 2200 rpm for 1 min, and allowed to settle for at least 2 hours, and up to more than 24 hours at room temperature.

It is possible to print less GO concentrated suspensions by decreasing the g dried GO/ g F127 ratio. For example, inks have been successfully printed with the following compositions prepared following the same protocol described above. Example 2

28 g of the Pluronic F127 stock solution and 3 g of distilled water were added in a HPDE beaker. Then small amounts of freeze dried GO (i.e 30mg) are added and mixed with the help of a centrifugal planetary mixer for 3-5 min at 2000 rpm. The beaker is afterwards immersed in an ice/water bath for 10 min in order to decrease temperature below 15°C before adding more freeze dried GO (30mg). Mixing/Ice Cooling/Freeze dried GO addition steps are subsequently repeated to the desired concentration of GO in the ink and until good dispersion of the GO is achieved and no agglomerates are present and ink/paste looks shiny. In this example up to 0.250 g of dried GO were added. The final concentration of GO in the ink is 0.78 wt% and the ratio g GO/g dried F127 is 0.035/1. The resulting composition is finally defoamed using a planetary mixer at 2200 rpm for 1 min.

Example 3

4.44 g of the Pluronic F127 stock solution and 0.56 g of distilled water were added in a HPDE beaker. Then small amounts of freeze dried GO (i.e. 30mg) are added and mixed with the help of a centrifugal planetary mixer for 3-5 min at 2000 rpm. The beaker is afterwards immersed in an ice/water bath for 10 min in order to decrease temperature below 15°C (LCST of the stock solution where it behaves as a liquid) before adding more freeze dried GO (30mg). Mixing/Ice Cooling/Freeze dried GO addition steps are subsequently repeated until the desired concentration of GO in the ink is obtained and good dispersion of the GO is achieved with no agglomerates present and ink/paste looks shiny. In this example up to 0.100 g of dried GO were added. The final concentration of GO in the ink is 2 wt% and the ratio g GO/g dried F127 is 0.09/1. The resulting composition is finally defoamed using a planetary mixer at 2200 rpm for 1 min. Example 4

10 g of GO slurry (11.78 mg/ml) was added to an HDPE beaker. Then small amounts of freeze dried GO (i.e. 30mg) and grinded F127 powder are added and mixed with the help of a centrifugal planetary mixer for 3-5 min at 2000 rpm. Mixing/Freeze dried GO addition steps are subsequently repeated until the desired concentration of GO in the ink is obtained and good dispersion of the GO is achieved with no agglomerates present and ink/paste looks shiny. In this example up to 0.49 g of dried GO were added. The final concentration of GO in the ink is 7.31 wt% and the ratio g GO/g dried F127 is 1.25/1. The resulting composition is finally defoamed using a planetary mixer at 2200 rpm for 1 min.

3D printing was performed at 27°C in a controlled temperature chamber. The ink is placed in a 3cc HDPE syringe and a nozzle from 100-500 microns can be used for printing by adjusting the wt% GO content of the ink and also the Pluronic:GO ratio.

GO:F127 inks were used to print 3D objects using a robotic deposition device (Robocad 3.0, 3-D Inks, Stillwater, OK). The diameter of the printing nozzles ranged from 100 to 500 pm (EFD precision nozzles, EFD, East Providence, Rl, USA). The inks were prepared at least 24 h before printing to ensure a stable rheological response. Typically for 5-6wt% GO and 1 :1 ratio of GO:F127 is printed by a 500 microns nozzle. Inks from 5-7.5 wt% GO and 1 :1 ratio of GO:F127 are printed by a 500 microns nozzle. The printing speed is from 8mm/s to 12 mm/s. 3D printed GO structures were frozen in liquid nitrogen and subsequently freeze-dried (Freezone 4.5, Labconco Corporation). Once dried, samples were reduced in a tubular furnace (Carbolite Furnaces) at 900 °C for 1 h in a 10%H ₂ /90%Ar atmosphere.

Rheology measurements

The flow behaviour and viscoelastic properties of the Pluronic stock solution and graphene inks were measured in a Discovery Hybrid Rheometer HR1 (TA Instruments). The flow and viscoelastic fingerprints experiments were carried out with a parallel plate (ø = 40 mm) and a solvent trap cover. The viscoelastic properties (G ', G ") were assessed with strain and frequency sweeps, and the effect of the temperature was monitored with a temperature ramp. In detail, viscoelastic fingerprints and linear viscosity region (LVR) were evaluated with stress-controlled amplitude sweeps at a fixed frequency of 396 rads ^"1 (63Hz), and stress- controlled frequency sweeps at a fixed strain of 0.15%. Rheological studies were performed on the Pluronic® F-127 solution (25 wt% in water) and the graphene inks for 3D-printing (Figures 3, 10 and 12). The purpose of the rheology tests was to assess the flow behaviour of the paste (viscosity against shear rate by applying a flow sweep) and the magnitude of the viscoelastic properties (storage modulus, G', and loss modulus, G", by applying an oscillation measurement), to determine whether the ink will be printable or not.

The viscoelastic fingerprints of the stock Pluronic® F-127 solution and the ink for 3D-printing were measured using the following conditions:

Amplitude sweep: the strain is varied from 0.0001 % to 1000% at a fixed frequency of 396 rad s ^"1 (angular frequency), at 25°C. Frequency sweep: the frequency is varied from 0.1 to 100 Hz at fixed 0.15% (strain), at 25°C.

The flow behaviour of the Pluronic® F-127 solution and the graphene inks for 3D-printing were measured applying a flow ramp (0-100 s-1).

Additionally, a temperature ramp was applied at fixed strain and frequency to follow the change of the viscoelastic properties with the temperature and identify the LSCT transition temperature of the pluronic stock solution and the graphene ink.

Figure 1 shows the chemical structure of Pluronic™ F127. Pluronic™ is a commercial thermo-responsive hydrogel (BASF Pluronic F-127) and is a triblock copolymer comprising (poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) (PEO-PPO-PEO) triblocks. F127 displays a low critical solution temperature (LCST of about 16°C), resulting in a transition from a low viscosity solution to a solid gel when it is heated up.

Figure 2 shows the rheological study of a 25wt% stock solution of Pluronic F127. Pluronic F127 shows a LCST at 16°C. When the stock solution is below 16°C, G">G' and it behaves as a liquid. Above the LSCT, G'>G" and the solution becomes a gel with a solid behaviour. Figure 3 shows the rheological study of an ink of 6 wt% GO and 1 : 1 ratio of GO: F 127. Figure 3(a) shows a flow ramp test in which the ink shows a shear thinning behaviour (viscosity decreases with shear stress) so they can be easily injected through a small nozzle. Viscoelastic fingerprints (b) and (d). Figure 3(b) shows a frequency sweep of the ink: by changing the frequency at a fixed amplitude (strain) the ink exhibits a solid-like (G'>G") behaviour, and high values of viscoelastic properties (storage modulus, G'). Figure 3(c) shows a temperature ramp test for the ink: a transition temperature cannot be seen as in the stock Pluronic (Fig 2) solution as the modulus G' is too high in a broad range of temperature. Figure 3(d) shows an amplitude sweep: by changing the amplitude at a fixed frequency up to 10% strain the ink shows predominantly solid like behaviour (G'>G").This region is called linear viscoelasticity region (LVR) when G' and G" are stable and independent of frequency, strain or time. When increasing the amplitude of the sinusoidal then the structure breaks down and G -G".

Figure 4 shows a comparison between the inks of examples 1 to 3 in terms of viscosity and storage modulus.

Figure 5 shows images of the lattices printed with 6wt% GO:F127 ink through a 500 microns nozzle on graphite substrate. Self-standing structures have been achieved either after drying or after thermal reduction.

Figure 6 shows FE SEM images showing the filaments of the printed objects.

Figure 7 shows FE SEM images showing the internal structures of the printed objects. Filaments show internal 2-8 microns porous features. Figure 8 shows the Raman spectra of the annealed printed lattices at 900°C. The thermal reduction is a complex process that involves the removal of intercalated H ₂ 0 molecules and oxide groups, defect formation, lattice contraction, folding and unfolding of the layers and layer stacking. Furthermore, the honeycomb hexagonal lattice is recovered to some extent leading to a more ordered material. The main features in the Raman spectra of carbons are the so-called G and D peaks, which lie at around 1560 and 1360 cm ^"1 respectively for visible excitation. Figure 8 shows that the reduced lattice D and G peaks are sharpened and moreover the 2D peak appears at 2700 cm ^"1 due to the thermal annealing effect. Bottom part of the printed lattice shows higher degree of order (lower D band) as it has been preferentially oriented during printing. Figure 9 shows the thermogravimetric analysis of the dried GO in air. GO starts to loose functional groups at temperatures over 50°C. It is completely burnt at 440°C in the presence of air. Figure 10 shows Theological study of an ink of 0.78 wt% GO and 0.035/1 GO/F127. (a) Flow ramp test, the ink shows a shear thinning behaviour (viscosity decreases with shear stress) so they can be easily injected through a small nozzle. Viscoelastic fingerprints (b) and (d), (b) Frequency sweep: by changing the frequency at a fixed amplitude (strain) the ink exhibits a solid-like (G'>G") behaviour, and high values of viscoelastic properties (storage modulus, G'). (d) Amplitude sweep: by changing the amplitude at a fixed frequency up to 6 %strain the ink shows predominantly solid like behaviour (G'>G") This region is called linear viscoelasticity region (LVR) when G' and G" are stable and independent of frequency, strain or time. But when increasing the amplitude of the sinusoidal then the structure breaks down and G -G". G" is higher than G' over 6% strain and the inks behaves more like a liquid, (c) Temperature ramp test: due to the low wt% solids content in this ink the transition temperature can be seen as in the stock Pluronic (Fig 2) solution as the modulus G" surpasses G' at ~15°C.

Figure 11 shows an example of self-standing printed article with GO/ Pluronic F127 (0.035/1) 0.78wt% GO solids ink trough a 500 microns nozzle, printed on a graphite substrate.

Figure 12 shows the rheological study of an ink of 2 wt% GO and 0.09/1 GO/F127. (a) Flow ramp test, the ink shows a shear thinning behaviour (viscosity decreases with shear stress) so they can be easily injected through a small nozzle. Viscoelastic fingerprints (b) and (d), (b) Frequency sweep: by changing the frequency at a fixed amplitude(strain) the ink exhibits a solid-like (G'>G") behaviour, and high values of viscoelastic properties (storage modulus, G')- (d) Amplitude sweep: by changing the amplitude at a fixed frequency up to 10 % strain the ink shows predominantly solid like behaviour (G'>G").This region is called linear viscoelasticity region (LVR) when G' and G" are stable and independent of frequency, strain or time. But when increasing the amplitude of the sinusoidal then the structure breaks down and G'=G". G" is higher than G' over 10% strain and the inks behaves more like a liquid, (c) Temperature ramp test: increasing the wt %GO solids content allows for the transition temperature to be shifted at lower temperatures if we compare with the stock Pluronic (Fig 2) solution as the modulus G" surpasses G' at ~5°C.

High purity CMGs have been successfully synthesized (average flake size > 10 pm) and functionalised with small amounts of Pluronic F127 (CMG:F127, 1 : 1) at temperatures below LCST. At room temperature (above LCST) the CMG flakes and F127 micelles assemble into a network, forming a soft solid with high viscosity and strong elastic behaviour. Inks with concentrations between 2 and 7.5 wt% - prepared from freeze-dried CMG (CMG:F127, 1 :1 (dry weight)) in a 25 wt% F127 stock solution are examples of inks according to the invention which display the appropriate flow behaviour and viscoelasticity for printing.

Further examples of ink formulations are described below.

Small amounts of freeze dried GO powder and F127 powder may also be added to a GO slurry (initial concentration of 14 mg/L) to increase GO concentration, also increasing the GO/F127 ratio and to prepare an ink formulation. The addition takes place following a sequence of manual mixing and centrifugal planetary mixing at 2000 rpm.

Pastes of reduce graphene oxide and other carbon materials can be prepared by manually grinding rGO, CNT or their mixtures along with F127 powder. Once the mixtures are ground up, small amounts of water can be added to prepare an ink formulation following a sequence of manual mixing and centrifugal planetary mixing at 2000 rpm. For example: a paste was prepared with 500 mg of rGO/CNT mixture and 55 mg F127, and 1.8mL of distilled water. Ink formulations comprising rGO only and CNT only were prepared according to this method.

CMG synthesis was performed in a custom-built reactor designed to manipulate up to 10 L of concentrated acids. In a typical synthesis, a 9:1 mixture of concentrated H ₂ SO ₄ /H ₃ PO ₄ (3:0.3 L) was mixed with 24 g of natural graphite flakes (150-500 pm sieved, Aldrich), followed by the addition of 144 g of KMn0 ₄ (6 wt%). This slightly exothermic reaction rose the temperature up to 35-40 °C during the mixing process. Afterwards, the temperature controller was set at 50 °C while stirring vigorously at 400 rpm for 18 h. Once completed, the reactor was cooled down to room temperature and the dropwise addition of 1.72 L of aqueous H ₂ 0 ₂ (2 wt %) stopped the oxidation. The graphene oxide suspension was washed using repeated centrifugation at 9000 rpm (Thermo Scientific Sorvall LYNX 6000 Superspeed Centrifuge) and re-dispersion in double-distilled water. The work-up was carried out until the supernatant water of the centrifuged CMG was close to pH 6, typically occurring after 16 washing cycles. Low speed (<1000 rpm) centrifugation cycles were performed to remove any un-exfoliated graphite particles.

The average lateral dimensions of the graphene oxide flakes are measured as follows: a drop of very diluted GO solution is placed on a Si wafer and allowed to dry. Using an optical microscope, photos at different magnitudes (from 5x to 100x) are taken to measure the lateral size of over 100 flakes using the software ImageJ. Alternatively, transmission electron microscopy (TEM) and/or scanning TEM microscopy (STEM) are used to measure the lateral dimensions using the software ImageJ. TEM and STEM were carried out using an FEI Titan 80-300 S/TEM operated at 80 kV. Raman spectra were recorded with a Renishaw Raman inVia microscope using a 432 nm excitation laser source at a laser power of 1.5 mW. The spectra were collected over an area of 25x25 pm ² and an average spectrum was calculated.

While preferred embodiments of the present invention have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.