WO2009125202A1 - Polymer films - Google Patents

Abstract

The present invention provides a process for producing a superhydrophobic, microscopically textured polymer surface on a substrate, comprising the steps of: (i) providing a solution comprising a hydrophobic polymer or precursor(s) of a hydrophobic polymer; (ii) forming an aerosol of the solution; (iii) passing the aerosol through a space in order to solidify the polymer or react the precursors to form a solid polymer; and (iv) depositing polymer particulates onto a substrate.

Description

POLYMER FILMS

The present invention relates to a process for producing a microscopically textured polymer surface on a substrate. The invention also relates to the use of aerosol assisted deposition (AAD) for producing microscopically textured polymer surfaces on a substrate. Surfaces thus produced may be superhydrophobic or superhydrophilic.

Background

Hydrophobic surfaces, in particular superhydrophobic surfaces, can be self-cleaning and antimicrobial. In particular, superhydrophobic surfaces may exhibit the "lotus effect". This effect is named after the lotus plant and refers to the self-cleaning action which results from the surface chemistry and microscopic structure of the plant's leaves. Droplets of water roll off the surfaces of the leaves, taking foreign bodies such as mud, tiny insects and bacteria with them.

Superhydrophilic surfaces can also be self-cleaning, as a result of their good wettability.

Surfaces coated with such films therefore have good drainage properties, and the enhanced sheeting of water acts to sweep away contaminants.

One way of making superhydrophobic surfaces from hydrophobic materials, or superhydrophilic surfaces from hydrophilic materials, is to introduce surface roughness.

Typically, such surface roughness may be provided by chemical etching (e.g. etching of aluminium by immersing in sodium hydroxide), physical roughening (e.g. by sand-blasting), or by providing micro- or nano-scale particulates or structures (e.g. carbon nanotubes). Such methods have the drawback of being expensive to implement.

Aerosol assisted deposition (AAD) uses a liquid-gas aerosol to transport soluble precursors to a substrate. For example, aerosol assisted chemical vapour deposition (AACVD) has traditionally been used when a conventional atmospheric pressure CVD precursor proves involatile or thermally unstable. EP 1301341 describes a combustion chemical vapour deposition (CCVD) process for applying a polymer to a substrate. The application of a heated gas to the substrate is said to result in smooth, uniform films. Summary of the invention

In one aspect of the invention there is provided a process for producing a superhydrophobic, microscopically textured polymer surface on a substrate, comprising the steps of:

(i) providing a solution comprising a hydrophobic polymer or precursor(s) of a hydrophobic polymer; (ii) forming an aerosol of the solution;

(iii) passing the aerosol through a space in order to solidify the polymer or react the precursors to form a solid polymer; and

(iv) depositing polymer particulates onto a substrate.

In another aspect of the present invention, there is provided a process for producing a superhydrophilic, microscopically textured polymer surface on a substrate, comprising the steps of:

(i) providing a solution comprising a hydrophilic polymer or precursor(s) of a hydrophilic polymer; (ii) forming an aerosol of the solution; (iii) passing the aerosol through a space in order to solidify the polymer or react the precursors to form a solid polymer; and (iv) depositing polymer particulates onto a substrate.

In one aspect of the invention there is provided a process for producing a microscopically textured polymer surface on a substrate, comprising the steps of:

(i) providing a solution comprising the polymer or precursor(s) of the polymer;

(ii) forming an aerosol of the solution; (iii) passing the aerosol through a heated space; and

(iv) depositing polymer particulates onto a substrate. In one embodiment, the polymer is a hydrophobic material, such that a superhydrophobic polymer surface is produced. In another embodiment, the polymer is a hydrophilic material, such that a superhydrophilic polymer surface is produced.

AAD is a particularly suitable technique for use in the present invention. Thus, the present invention also provides the use of AAD, preferably use of AACVD apparatus, for producing a microscopically textured polymer surface on a substrate.

In one preferred embodiment, the present invention provides a process for producing a superhydrophobic (or superhydrophilic), microscopically textured polymer surface on a substrate, comprising the steps of:

(i) providing a solution comprising precursor(s) of a hydrophobic (or hydrophilic) polymer; (ii) forming an aerosol of the solution;

(iii) passing the aerosol through a space in order to react the precursors to form a solid polymer; and (iv) depositing polymer particulates onto a substrate,

wherein steps (ii) to (iv) are effected using AACVD apparatus. Preferably, the space in step (iii) is a heated space, preferably at a temperature of at least 8O⁰C.

The present invention also provides use of a hydrophobic or hydrophilic polymer to render a substrate superhydrophobic or superhydrophilic, by forming on said substrate a microscopically textured polymer surface, by a process as described above.

In a further embodiment, the present invention provides use of a hydrophobic or hydrophilic polymer to render a substrate self cleaning and/or antimicrobial, by forming on said substrate a superhydrophobic or superhydrophilic microscopically textured polymer surface, by a process as described above.

In a further embodiment, the present invention provides the use of a polymer, such as a hydrophobic polymer or hydrophilic polymer, for producing a microscopically textured surface on a substrate, comprising depositing the polymer onto the substrate by AAD, preferably using AACVD apparatus.

In further embodiments, the present invention provides a substrate having a microscopically textured polymer surface coated thereon obtainable by the process or use described above, and a microscopically textured polymer surface obtainable by the process or use described above.

Drawings

Figure 1 Schematic of the Aerosol Assisted CVD experimental apparatus (1 - glass substrates; 2 - graphite heating block; 3 - flask containing precursor solution; 4 - ultrasonic humidifier; 5 - flask inlet valve; 6 - nitrogen gas in; 7 - bypass valve; 8 - to exhaust).

Figure 2 Measurement of water droplet contact angles. Contact angle θ of water interaction with surface film. The photo is taken so that the contact angle is obvious and can be accurately established.

Figure 3 Measurement of water droplet slipping angles. Slipping angle θ' is the angle to which the surface is raised to when the water droplet begins to move from the spot it was placed.

Figure 4 Shows the effect of temperature change on the elastomer (Sylgard® 184) curing times. Data is provided by the Dow Corning materials description sheet.

Figure 5 Copper substrate with elastomer deposition carried out using AACVD at substrate temperatures of 21O⁰C. The left part shows the coated area; the right of the picture shows the area uncovered by polymer.

Figure 6 XRD spectra obtained from: (a) Plain copper plate, (b) Dip coated polymer on copper plate, (c) Thinner part of film on the dip coated polymer on copper plate, (d) Polymer deposited on copper plate via AACVD with a substrate temperature of 24O⁰C and (e) Polymer deposited via AACVD onto SiO2 barrier glass at a substrate temperature of 240⁰C. Figure 7 IR spectrum of dip coated elastomer (with glass vibrations removed as background) shows clear presence of CH bond stretches at 2960 and 2905cm^"1.

Figure 8 SEM images of S YLGARD® 184 Silicone Elastomer applied to SiO₂ barrier glasses using AACVD. Using a substrate temperature of (a/b) 270, (c) 300 and (d) 36O⁰C. (a), (c) and (d) show top- views of the coating, whereas (b) is a profile view. Magnification and scale bar are also shown.

Figure 9 Close-up SEM image of SYLGARD® 184 Silicone Elastomer applied to SiO₂ barrier glass using AACVD, with a substrate temperature of 27O⁰C. The black scale bar relates to 1 μm. Magnification is x20,000.

Figure 10 Photograph used in the determination of the contact angle made of a water droplet with an elastomer film. The contact angle (shown in black), reached a maximum of 167.5° on a film deposited using a substrate temperature of 360⁰C.

Figure 11 SEM image of SYLGARD® 184 Silicone Elastomer (left) and Dyneon™ FC-2120 Fluoroelastomer (right) AACVD depositions onto glass substrate, carried out at 27O⁰C. Magnification is x5000 with scale bar inset.

Figure 12 SEM image (left) of AACVD deposition of NuSiI Med-4850 where substrate temperature was 42O⁰C, scale bar shown (magnification x5000). Photo (right) shows the maximum contact angle achieved on this surface (170°).

Detailed description of the invention

The process of the present invention provides a microscopically textured polymer surface deposited on a substrate. The microscopically textured surface has a rough texture. Roughness can generally be defined by a roughness factor r as defined below. A "rough" surface has a value of r of greater than 1 :

r = actual surface area (taking into account protrusions)/planar (or geometric) surface area. Preferably, the roughness r of the microscopically textured polymer surface of the present invention is greater than 1.5, for example greater than 2, preferably greater than 5, more preferably greater than 10. In one embodiment, the roughness factor is greater than 20, preferably greater than 50, more preferably greater than 100. In one embodiment, the roughness factor is, for example, from 1.5 to 100, such as from 2 to 50 or 2 to 20.

In the present case, roughness can usefully be defined using a combination of the mean distance between adjacent peaks on the surface and the mean height of the peaks. Preferably, the mean distance between adjacent peaks of the rough surface should be from 0.1 to 100 μm, preferably 0.1 to 30 μm, more preferably 0.1 to 10 μm. Preferably, the mean height of the peaks should be at least 0.1 μm. Although the maximum height of the peaks is of less importance, in practice the mean peak height will be from 0.1 to 100 μm, preferably 1 to 30 μm, more preferably 1 to 10 μm. In one embodiment, the height to width ratio of the peaks is greater than 1. Such surface properties can be measured using any suitable technique known to those skilled in the art.

In one preferred embodiment, the microscopically textured polymer surface is a superhydrophobic surface. A superhydrophobic surface may be defined as one on which the contact angle of water is at least 140°, preferably at least 150°.

In another embodiment, the microscopically textured polymer surface is a superhydrophilic surface. A superhydrophilic surface may be defined as one on which the contact angle of water is 0 to 30°, preferably less than 10°. In some cases the contact angle of a superhydrophilic surface can be or can approach zero.

The contact angle is the angle that the tangent makes with the surface at the point of contact of a water droplet, the maximum being 180°. The higher the contact angle, the more hydrophobic the surface. Conversely, the lower the contact angle, the more hydrophilic the surface. The contact angle may be measured using any suitable technique, for example using a contact angle goniometer or by measurement from a photograph.

According to one preferred embodiment of the present invention, a superhydrophobic surface is formed from a hydrophobic polymer: it is the combination of the hydrophobic, low surface energy, polymer with the microscopically rough texture which leads to the superhydrophobicity of the surface. Similarly, introducing microscopically rough texture to a hydrophilic polymer may form a superhydrophilic surface. As would be understood by those skilled in the art, the Cassie-Baxter model can be used to rationalise contact angles on surfaces, and hence superhydrophobic or superhydrophilic properties, from interfacial energies.

A hydrophobic polymer may be defined as a polymer for which the contact angle of water on a flat, unstructured, surface of the polymer is greater than 90°. A hydrophilic polymer may be defined as a polymer for which the contact angle of water on a flat, unstructured, surface of the polymer is less than 90°.

The process of the present invention will now be described in more detail.

Solution

Substantially any polymer that can be dissolved in a suitable solvent, or whose precursors can be dissolved in a suitable solvent, can be used in the present invention, including inorganic, organic and organometallic polymers (including copolymers). Some examples of suitable polymers include polyesters, polyamides, polyurethanes, fluoropolymers, polyolefϊns, vinyl polymers, silicones, silicone rubbers and polysiloxanes. In one embodiment, the polymer is a polysiloxane or polyurethane. Where soluble, the polymer itself may be used directly. Alternatively, soluble polymer precursor(s) may be used, as described further below.

One or more polymers (or, where appropriate, precursor(s)) may be used. Thus, in one embodiment a mixture of polymers (or polymer precursor(s)) may be used. These may be mixed in a single solution, or provided as separate solutions and deposited via separate aerosol streams.

In another embodiment, a solution of polymer precursor(s) is used. For example, it is possible to use one or more monomers, with or without a polymerisation catalyst or agent (such as a (reactive) curing agent). The monomers may be chosen such that any of the above-mentioned polymers will be formed on polymerisation. As used herein, reference to monomers should be understood as embracing oligomers or polymers which can react further to form the polymer deposited on the substrate. It is also possible to use a solution of polymer precursors that will undergo crosslinking reactions, and the solution may thus include a suitable crosslinking agent or curing catalyst or agent. For example, a solution comprising linear polymers and a crosslinking agent may be used in order to form a crosslinked polymer upon reaction.

The precursor(s) and any curing agent may be mixed in a single solution, or used as separate solutions to produce separate aerosol streams. In mixed solutions any polymerisation, crosslinking or curing catalyst/agent should be substantially inactive at room temperature, such that reaction does not occur before aerosolisation of the solution. If necessary, an inhibitor may be added to the solution to inactivate the polymerisation, crosslinking or curing catalyst/agent.

In one preferred embodiment, the polymer is a hydrophobic polymer, as described above, or precursor(s) of said hydrophobic polymer. Examples of suitable hydrophobic polymers include hydrophobic polyesters, hydrophobic polyamides, hydrophobic polyurethanes, fluoropolymers, polyolefins, hydrophobic vinyl polymers and hydrophobic silicones, silicone rubbers and polysiloxanes. Examples of such hydrophobic polymers include fluoroalkylsilanes, poly(tetrafluoroethylene) (PTFE) and silicone elastomers. In one embodiment, the hydrophobic polymer is a polysiloxane or polyurethane. Where appropriate, precursor(s) of these polymers may be used, thus forming the finished polymer on the substrate. Thus, the hydrophobic polymer may be provided as a solution of the polymer itself, or as precursor(s) as described above.

It maybe advantageous to select a more hydrophobic polymer, i.e. having a greater contact angle of water on a flat, unstructured, surface, e.g. from 100 to 140°, preferably 110 to 140°. As would be evident to one skilled in the art, use of a more hydrophobic polymer would increase hydrophobicity when deposited as a rough surface.

In a preferred embodiment, the hydrophobic polymer is a silicone elastomer, and the solution in step (i) comprises a silicone base and a curing agent. Typically, the silicone base comprises a siloxane comprising [R₂SiO] units, for example HOSi(R₂)OSi(R₂)OH, wherein R is a C₁-C₄ alkyl or C₆-C₁₀ aryl group, e.g. methyl. Typically, the curing agent comprises a siloxane comprising [RR' SiO] units, wherein R and R' may be the same or different, and are H, Ci-C₄ alkyl or C₆-C]₀ aryl, preferably H and/or methyl. Alternatively the curing agent may comprise a compound of formula R-SiX₃, wherein R is H, C₁-C₄ alkyl (e.g. methyl) or C₆-C₁₀ aryl, and X is amine (-NHR, wherein R is C₁-C₄ alkyl or C₆-C₁₀ aryl), acetate (-OC(O)CH₃), alcohol Or-OCH₂CH₃. Suitable examples of such silicone elastomers include Sylgard® 184 and NuSiI Med-4850.

In one embodiment, the polymer is a hydrophilic polymer, or precursor(s) of said hydrophilic polymer. Examples of suitable hydrophilic polymers include hydrophilic polyesters, hydrophilic polyamides, hydrophilic polyurethanes, hydrophilic vinyl polymers and hydrophilic silicones, silicone rubbers and polysiloxanes. Where appropriate, precursor(s) of these polymers may be used, thus forming the finished polymer on the substrate. Thus, the hydrophilic polymer may be provided as a solution of the polymer itself, or as precursor(s) as described above.

The polymer or precursor(s) should be dissolved in a suitable solvent, such that an aerosol can be formed from the solution. The polymer or precursor(s) should be highly soluble in the chosen solvent, most preferably completely soluble. The solvent must be capable of forming an aerosol.

Examples of suitable solvents include toluene, benzene, hexane, cyclohexane, methyl chloride, acetonitrile, chloroform, methylene chloride, acetone, methanol, ethanol, water, and other non-polar or polar solvents. A mixture of two or more different solvents may be used, provided the solvents are miscible.

Typical concentrations of polymer or precursor(s) in the solvent range from about 1 to about 100 g/L, preferably about 5 to about 20 g/L. As mentioned above, the solution may further comprise additives such as polymerisation/crosslinking inhibitors.

It is possible to add other optional components to the solution, for example colouring agents, antimicrobial or antibacterial agents, or agents to promote adhesion to the substrate.

In one embodiment, one or more photosensitisers may be added to the solution, in order to co- deposit said photosensitiser(s) with the polymer. Preferred photosensitisers have absorption maxima in the visible wavelength range. The photosensitiser is suitably chosen from porphyrins (e.g. haematoporphyrin derivatives, deuteroporphyrin), phthalocyanines (e.g. zinc, silicon and aluminium phthalocyanines), chlorins (e.g. tin chlorin e6, poly-lysine derivatives of tin chlorin e6, m-tetrahydroxyphenyl chlorin, benzoporphyrin derivatives, tin etiopurpurin), bacteriochlorins, phenothiaziniums (e.g. toluidine blue O, methylene blue, dimethylmethylene blue), phenazines (e.g. neutral red), acridines (e.g. acriflavine, proflavin, acridine orange, aminacrine), texaphyrins, cyanines (e.g. merocyanine 540), anthracyclins (e.g. adriamycin and epirubicin), pheophorbides, sapphyrins, fullerene, halogenated xanthenes (e.g. rose bengal), perylenequinonoid pigments (e.g. hypericin, hypocrellin), gilvocarcins, terthiophenes, benzophenanthridines, psoralens and riboflavin. Other possibilities are arianor steel blue, tryptan blue, crystal violet, azure blue cert, azure B chloride, azure 2, azure A chloride, azure B tetrafluoroborate, thionin, azure A eosinate, azure B eosinate, azure mix sice, and azure II eosinate.

Another way to incorporate photosensitisers into the superhydrophobic or superhydrophilic surfaces produced according to the present invention is by swell encapsulation into the deposited polymer surfaces. For example, the deposited films may be contacted with a solution of a solvent, for example acetone, comprising a suitable photosensitiser. The solvent can then be evaporated. Such a method typically does not substantially affect the rough microstructure of the surfaces, but has the advantage that decomposition (e.g. thermal decomposition) of the photosensitiser may be avoided during the deposition procedure.

In one embodiment, preferred photosensitisers inlcude toluidine blue O, methylene blue, dihaematoporphyrin ester, tin chlorin e6, indocyanine green, rose bengal, methylene violet, erythrosine B and nile blue sulphate.

In one embodiment, a mixture of two or more photosensitisers is used. Suitable photosensitisers include those mentioned above. In a preferred embodiment, the mixture of photosensitisers comprises toluidine blue O and rose bengal, preferably in a 1 : 1 ratio.

Typically, the photosensitiser is added in an amount of at most 1 part photosensitiser (or total photosensitisers where a mixture is used) to 100 parts polymer (weight or molar basis). Preferably, the ratio of photosensitiser(s) to polymer is from 1 : 1000 to 1 : 10000, although the amount of photosensitiser(s) could be as low as ppm.

In another embodiment, the polymer/precursor solution may comprise nanoparticles, in order to co-deposit said nanoparticles with the polymer, for example nanoparticles comprising a metal, preferably nanoparticles comprising a main group metal or a transition metal, such as _^ .

11 ^*

gold, silver or copper or an alloy thereof, particularly silver, silver alloy and/or silver oxide. The nanoparticles may be charge-stabilized or ligand-stabilized.

In one embodiment, the solution may comprise both nanoparticles, in particular metallic nanoparticles such as those comprising gold, silver copper or an alloy thereof, and photosensitiser(s), such as those mentioned above. The nanoparticles and photosensitiser may be separate entities, in which case one preferred combination is gold nanoparticles and methylene blue or toluidine blue O as photosensitiser. Alternatively, the nanoparticles and photosensitiser may be associated, e.g. as a metallic nanoparticle-ligand-photosensitiser conjugate, in which in a preferred embodiment the metallic nanoparticle comprises gold, the ligand comprises tiopronin and the photosensitiser comprises toluidine blue O. Suitable examples of the aforementioned combinations of nanoparticles and photosensitiser, both separate and conjugated, may be found in WO 2008/015453, the disclosure of which is incorporated herein by reference.

In another embodiment, the solution may comprise a metal, for example chosen from transition metals or lanthanides. Preferably, the metal is copper, silver, gold, zinc, cadmium, mercury, tin, lead or gallium, or mixtures thereof. Alloys of such metals may also be used, preferably alloys of silver and gold. The metal may be nanoparticulate, as described above, or for example in a colloid composition. The metal may be in its elemental form or present as metal ions, e.g. in a metal halide. This may require the use of separate solutions for aerosolisation, e.g. one comprising the polymer (or precursors thereof), and the other comprising the metal or metal ions, to avoid problems with solvent incompatibility.

Another example of a further component in the polymer/precursor solution is titanium dioxide, alone or in combination with silver or silver oxide. Again, it may be necessary to use separate solutions for aerosolisation to avoid problems with solvent incompatibility.

Deposition of polymer

The process of the present invention comprises steps of (ii) forming an aerosol of the solution comprising polymer or polymer precursor(s) as described above, (iii) passing the aerosol through a space in order to solidify the polymer or react the precursors to form a solid polymer, and (iv) depositing polymer particulates onto a substrate. The steps leading to deposition of the polymer are described in more detail below.

Formation of an aerosol

An aerosol may be generated from the solution using any suitable technique. For example, an aerosol may be generated ultrasonically or by nebulisation.

For ultrasonic generation, the solution is typically placed in an ultrasonic humidifier at high frequency. The frequency required for aerosol generation depends on the solvent used. Typically, a frequency of 10 to 100 kHz is used, preferably 20 to 70 kHz, more preferably 30 to 50 kHz. The solution may be placed directly in contact with the piezoelectric crystal of the ultrasonic humidifier, or alternatively placed in a container, such as a thin plastic container, or a glass flask in which the base has been thinned to around a quarter of the usual thickness.

Nebulisation to form an aerosol may be achieved, for example, using the top of a conventional spray can (thus, an aerosol spray can may be used) or a nebuliser of the type typically used for asthma medication.

Alternatively, an aerosol may be formed using an electrospray method.

Typically, the mean particle size of the aerosol droplets is from 0.1 to 100 μm, such as 0.1 to 30 μm, for example 0.1 to 10 μm. Control of the aerosol droplet size could be used to control the microscopic texture, and thus the degree of hydrophobicity/hydrophilicity, of the deposited polymer surface. Where the solution in step (i) comprises polymer and not precursors, the aerosol droplet size is typically chosen such that the solvent has substantially evaporated at the time the polymer reaches the substrate. Where the solution in step (i) comprises precursors rather than polymer, the aerosol droplet size is typically chosen such that reaction to form a polymer is substantially completed at the time it reaches the substrate. Thus where polymer precursor(s) are used, control of aerosol droplet size can help control the extent of reaction that may take place during step (iii) of the process.

Deposition onto a substrate

The aerosol of the solution is directed through a space towards the substrate using a flow of a transport gas in order to solidify the polymer or react the precursors to form a solid polymer. The gas may be an inert gas, preferably nitrogen. Mixtures of gases may be used, such as nitrogen and hydrogen or nitrogen and oxygen. The space may be heated in order to promote solvent evaporation and, where necessary, polymerisation or crosslinking.

The polymer should be solidified before hitting the target substrate, to the extent that it will substantially retain its shape when deposited. For example, the polymer may form "beads", or particulates as it travels through the space towards the substrate, which then build up on the substrate to form a microscopically textured surface. Thus, the polymer need not be completely solidified, so long as it solidifies enough to form a microscopically textured surface on deposition.

If no heating is used, it may be necessary to adjust the flow of transport gas, and/or adjust the distance travelled by the aerosol particles before reaching the substrate, in order to ensure that solvent is removed and the polymer substantially solidifies before reaching the substrate. Where polymer precursors are used, they should have sufficient time to react and form a substantially solid polymer before reaching the substrate. In many cases, heating the space will promote such solidification and reaction.

The desired temperature of the heated space will depend on the particular polymer or precursor(s) being used. In particular, if a polymerisation or crosslinking reaction is desired to take place between precursor(s) in the solution, the temperature in the heated space should be high enough for said reaction to occur. In practice, the temperature of the heated space may be at least 8O⁰C. In some cases, a temperature of 200 to 300⁰C may be preferable. As would be evident to one skilled in the art, where precursors are used, slower curing rate polymers typically require a higher temperature than fast-curing polymers.

The maximum temperature will depend on both the polymer and the substrate, but in general should be such that the polymer does not decompose. Usually for organic polymers, this maximum temperature may be in the range of about 300 to 35O⁰C, or up to about 400⁰C. However for some polymers, such as those used in flame retardancy such as polyphosphazenes or polyboryl-based polymers, higher temperatures may be possible, for example up to about 400-700°C. When the solution comprises a photosensitiser (or other additional temperature-sensitive component(s)) the maximum temperature that may be used without degradation may be about 300⁰C. In one embodiment, the temperature should preferably be the highest temperature that can be used without degrading the polymer (or other temperature-sensitive component(s), if present). One skilled in the art would readily be able to determine the maximum temperature for a given polymer through routine experimentation.

The heated space may be heated, for example, using a heat source such as a heated block placed at a suitable distance from the substrate on which deposition occurs.

After deposition, if necessary the apparatus is allowed to cool, preferably to room temperature.

The process may be carried out using any suitable AAD technique. In one preferred embodiment, the process of the present invention is carried out using AACVD apparatus. In particular, steps (ii) to (iv) of the process as set out above may be effected using AACVD apparatus. Using this technique, a polymer or precursor is transformed into an aerosol before deposition using a conventional CVD reactor. Examples of suitable CVD reactors are cold- wall horizontal-bed CVD reactors, cold-wall shower-head reactors and hot-wall reactors. AACVD is particularly suitable for use when starting from an initial solution comprising polymer precursor(s).

Preferably, a cold-wall horizontal-bed reactor is used. A CVD apparatus suitable for use in the present invention is described in Chem. Vap. Dep. 1998, 4, 222-225, the disclosure of which is incorporated herein by reference in its entirety. In such an apparatus wherein two substrates are used - a "bottom plate" in contact with a heating block and a "top plate" separated from the bottom plate by a heated space - the microscopically textured film may deposit on the top plate (i.e. the plate not having direct contact with the heating block) and/or on the heated bottom plate. The bottom plate may be at room temperature, or heated to a temperature of about 300°C. The top plate is not heated directly, but rather through convection of the heated gas between the plates, and can be up to about 100°C, e.g. about 50 to 80°C, cooler than the heated plate.

Provided the substrate is capable of having a polymer film deposited on its surface, the substrate is not critical to the invention. Example of suitable substrates include glass substrates, for example glass slides, films, panes or windows. Preferred glass substrates have a barrier layer of silica to stop diffusion of ions from the glass into the deposited film. Typically, the silica barrier layer is 50nm thick.

Other examples of substrates are temperature-insensitive materials such as metals, metal oxides, nitrides, carbides, suicides and ceramics. Such substrates may be, for example, in the form of windows, tiles, wash basins or taps.

Further examples of possible substrates include textiles, fabrics, polymers and wood.

The substrate may be pre-coated with polymer in order to increase adherence of the deposited polymer surface. Thus, the substrate may be dip coated or spray coated with a polymer that is the same as, or different to, the polymer to be deposited by the process of the present invention. Preferably, the substrate is pre-coated with the same polymer as that to be used for deposition.

The substrate may also comprise an antimicrobial material, or the substrate surface on which the polymer is to be deposited may be have an antimicrobial coating, for example with materials such as those described in WO 2007/051994 or WO 2008/015453 (the disclosures of which are incorporated herein by reference), or may incorporate other light activated antimicrobial agents.

The substrate may be heated, unheated or even cooled. Use of a heated substrate e.g. at a temperature of 100 to 300⁰C, preferably 200 to 25O⁰C, may be beneficial in improving adherence of the deposited polymer surfaces to the substrate.

The process of the present invention allows microscopically textured polymer surfaces to be deposited on a substrate. Without wishing to be bound by theory, it is believed that the process of passing an aerosolised solution of polymer or precursor(s) through a (heated) space allows evaporation of the solvent and any reaction to occur in the gas phase, resulting in the formation of small "beads" of solid polymer which then substantially retain this form when they deposit on the substrate.

One advantage of this process is its simplicity. A further advantage is its flexibility in allowing a range of polymers to be deposited as microscopically textured surfaces. Furthermore, CVD in particular is a widely used industrial technique in fields such as microelectronics and window glass coating, and has a number of well known advantages, not least the deposition of adherent, conformal films. The process can be easily incorporated into float glass production lines, and has fast growth times, deposition taking from 1 second to 1 minute. The process of the present invention can use a single-step deposition.

The process of the present invention, in particular the process comprising using AACVD apparatus, is particularly advantageous, because films may be deposited onto any suitable substrate. Since the coating layer conforms to the surface of the substrate, the substrate is not limited as to its shape, size or conformation. Another advantage is that the deposited surfaces can have good durability, forming tenacious coatings that are not easily wiped or otherwise removed from the surface of the substrate.

Films according to the invention The microscopically textured polymer surfaces obtainable by the process of the present invention preferably have a mean thickness of from about 0.5 to about 500μm, preferably from about 1 to about 150 μm, preferably from about 1 to about 50 μm, more preferably from about 1 to about 10 μm.

The microscopically textured polymer surfaces have a rough texture, as described above, and comprises deposited polymer particulates. Typically, the mean particule size of the particulates is from 0.1 to 100 μm, such as 0.1 to 30 μm, for example 0.1 to 10 μm.

In one embodiment, the microscopically textured polymer surfaces obtainable by the process of the present invention are substantially not light transmissive. Thus, the visible transmittance, defined by the equation T = I/IQ (where / is the intensity of transmitted light and Io is the intensity of incident light) is less than 0.3, preferably less than 0.2, more preferably less than 0.1. Thus in one embodiment, the deposited surfaces are substantially opaque.

In one embodiment, the microscopically textured polymer surfaces obtainable by the process of the present invention are not antireflective, such that the reflectance R when light meets the surface-air interface at normal incidence, calculated using the following equation:

where «₀ and «s are the refractive indices of the surface and air respectively, is greater than 0.5 (50%), for example greater than 0.7 (70%), preferably greater than 0.8 (80%).

When a hydrophobic polymer is used, the resulting superhydrophobic polymer surfaces may be used in self-cleaning applications or as anti-microbial coatings. Biofilms may not substantially attach to such surfaces. These surfaces also exhibit the "lotus effect", whereby any water applied to the surface forms balls that spin on the surface and do not wet the surface effectively. The spinning action mimics what happens in nature and removes matter including dust, bacteria and viruses from the surface, as well as preventing biofilm formation. Thus in one embodiment the present invention also provides a process for removing microbes from a superhydrophobic polymer surface according to the present invention (or a substrate having such a superhydrophobic polymer surface coated thereon), comprising applying water to the surface (for example washing the surface with water).

The superhydrophobic polymer surfaces are also difficult to stain due to their water-repellent nature. This property may find application in particular in textiles, such as for clothing, furniture and carpets.

When a hydrophilic polymer is used, the resulting superhydrophilic polymer surfaces may be used, for example, as anti-fog coatings for glass, mirrors, etc. Since water applied to such superhydrophilic surfaces "sheets" off the surface, sweeping away contaminants, these surfaces can also be used in self-cleaning applications. Thus, in a further embodiment the present invention also provides a process for removing microbes from a superhydrophilic polymer surface according to the present invention (or a substrate having such a superhydrophilic polymer surface coated thereon), comprising applying water to the surface (for example washing the surface with water).

Furthermore, where a surface obtainable by the process of the present invention comprises an antimicrobial agent, such as an antimicrobial compound or (nanoparticulate) metal, such as silver, the surface can exhibit an inherent antimicrobial effect, separate from the "self- cleaning" effects described above. Where the surface comprises one or more photosensitisers (with or without separate or conjugated nanoparticles), an antimicrobial effect can be achieved by irradiating the surface with light of a suitable wavelength. Thus, the surface may be exposed to a light source comprising radiation having a wavelength, or a range of wavelengths, within the range of wavelengths absorbed by the photosensitiser, preferably near or corresponding to the wavelength of maximum absorption of the photosensitiser (λmax). In one embodiment, it is preferred that the photosensitiser demonstrates antimicrobial activity when exposed to visible light, i.e. λmax is between 380 and 780nm. For example, toluidine blue O demonstrates antimicrobial activity when irradiated with light having a wavelength of 633nm. Where two or more photosensitisers are used, preferably these have absorption maxima at different wavelengths within the visible spectrum (e.g. toluidine blue O and rose bengal). In general, any light source that emits light of an appropriate wavelength may be used. The source of light may be any device or biological system able to generate monochromatic or polychromatic light, coherent or incoherent light, especially visible white light. Examples include a fluorescent light source, laser, light emitting diode, arc lamp, halogen lamp, incandescent lamp or an emitter of bioluminescence or chemiluminescence. In certain circumstances, sunlight maybe suitable. For further details, see WO 2008/015453, the disclosure of which is incorporated herein by reference.

The self-cleaning/antimicrobial properties of superhydrophobic and superhydrophilic polymer surfaces prepared according to the present invention may find application in hospitals and other places where microbiological cleanliness is necessary, for example food processing facilities, dining areas or play areas. The films may be applied to any suitable surface in order to provide antimicrobial properties, for example metal surfaces such as taps and metal work surfaces, ceramic surfaces, such as wash basins and toilets or glass surfaces, such as doors and windows. It is also envisaged that the films could be applied to furniture, such as beds and tables, or to medical equipment and instruments. Further, the films could be applied to the screens of electronic equipment, such as televisions, computers and hand held devices, in particular touch screens. In one aspect, the present invention does not extend to the use of the films in methods of treatment of the human or animal body by surgery or therapy, or in methods of diagnosis conducted on the human or animal body. Another example of an application of the microscopically textured polymer surfaces of the present invention, in particular superhydrophobic polymer surfaces or superhydrophilic polymer surfaces, is in water treatment systems and pipelines, e.g. to prevent biofouling and other blockages. An advantage of providing superhydrophobic polymer surfaces or superhydrophilic surfaces in such applications is that the self-cleaning effect can operate without the need for any light source.

Another example of an application of the microscopically textured polymer surfaces of the present invention, for example the superhydrophobic polymer surfaces or superhydrophilic polymer surfaces, is in paints.

EXAMPLES

With the exception of S YLGARD® 184 Silicone Elastomer that was purchased from R. W. Greeff, NuSiI Med-4850 obtained from UCL Eastman Dental Institute and Dyneon™ FC- 2120 Fluoroelastomer provided by Dyneon™ UK Office, all chemicals used in this investigation were purchased from Sigma-Aldrich Chemical Co; chloroform, toluene, ethanol, methylene blue solution (1%), HAuCL₄.3H₂O, tetraoctylammonium bromide (TOAB), NaBH₄, dilute H₂SO₄, Na₂SO₄, tetraethylortho silicate (TEOS), acetone, 4-methyl-2- pentanone (MIBK), butane-2-one.

Example 1 - Aerosol Assisted CVD (AACVD) of SYLGARD® 184 Silicone Elastomer

Elastomer Deposition:

Precursor synthesis. SYLGARD® 184 Silicone Elastomer (0.5Og) was dissolved in chloroform (5OmL) with rapid stirring for 5 min. To prevent premature curing, upon mixing of the two-part elastomer, chloroform was added immediately and used directly after stirring was complete.

AACVD. The depositions were carried out in a cold-walled horizontal-bed CVD reactor (Figure 1). The reactor contained a top and bottom plate for deposition to occur, both composed of SiO₂ barrier glass (1; dimensions: 145 x 45 x 5mm) supplied by Pilkington PIc. Deposition was carried out on the barrier layer to prevent ion transfer from the bulk glass to the film generated. The CVD reactor was heated by a carbon block 2, which the bottom plate was placed on. The top plate was positioned 8mm above and parallel to the bottom plate, the complete assembly enclosed within a quartz tube. The aerosol of the precursor solution was generated using an ultrasonic humidifier 4. The aerosol generated was moved to the reactor using a nitrogen gas flow via PTFE and glass tubing, where it entered between the top and bottom plate. The reactor waste gas left via an exhaust (Schematic shown in Figure 1).

The nitrogen flow carried the vapour from the flask until all liquid was gone, which took typically 30-35 min. The nitrogen flow was left on for a further 10 min (at heated temperature) and then switched off, the heater was also switched off and the reactor was allowed to cool. The cooled plates were removed and handled in air, with the top plate having the deposited film. The reactor temperature was varied (90 to 36O⁰C) to find a suitable deposition temperature. Also the nitrogen flow rate was adjusted (0.8 to 1.2L/min) to discover the favoured velocity of the carrier gas for deposition. The amount of elastomer used was also varied from the 0.5g, this to note the effect of the effect on the deposited films' hydrophobicity.

Use of different substrate materials. Instead of using SiO₂ barrier glass, copper plates with dimensions of 145 x 45 x 3mm were used. The copper substrate was roughened to observe the change in hydrophobicity with a physical change in substrate microstructure. The method used was sand-blasting using high velocity sand particles. The same AACVD method was used, with depositions taking place at reactor temperatures of 180 and 210⁰C. Nitrogen flow rate was maintained at l.OL/min. Deposition was also carried out on steel sheet of dimensions 145 x 45 x 0.5mm.

Film Characterisation: The films were analysed while adhered to the SiO₂ barrier glass substrate unless otherwise stated. The samples were gold-sputtered and analysed using field emission scanning electron microscopy (SEM) using a Jeol JSM- 6301 F operating at 5kV. Powder X-ray diffraction (XRD) patterns were measured on a Siemens D5000 diffractometer using monochromated Cu Ka radiation (λ = 1.5406 A°) in the reflection mode. UV-vis spectra were also taken using a Thermo Spectronic Helios Alpha single beam instrument over a range of 300-1000nm. IR spectroscopy was employed over the range of 2200-4000Cm^"1 using a Perkin Elmer FT-IR Spectrum RXl instrument. Adherence/Scratch tests were also carried out using Scotch® tape and a metal scalpel. Atomic force microscopy measurements were performed on a Veeco Dimension 3100 in air. Water Droplet Contact Angles:

The water contact angles are a measurement of the hydrophobicity of the surface, and as discussed previously are a measurement of both the surface material's ability to repel water combined with the microstructure of the surface. Photos were taken of water droplets sitting on the surface and contact angle of the water droplet and surface were observed (Figure 2). Methylene blue was added to the water droplet so that a clear image of its shape could be observed. This method gives the actual contact angle to a certain degree of accuracy, without making assumptions that are often made in computational methods. Spots were tested from a range of areas over the plate.

Water Droplet Slipping Angles:

A water droplet's ability to wet a surface can be demonstrated by how it moves on that surface. The slipping angle is the angle to which the surface can be tipped until the water droplet begins to slide. A water droplet of known volume (60μL) was placed onto the surface and thus tipped until the water droplet moved from the spot where it was placed. The angle at which the droplet began to move was noted (Figure 3). Many different spots were measured from each surface.

Comparative Example 1 - Dip-Coating of SYLGARD® 184 Silicone Elastomer The elastomer was applied to SiO₂ barrier glass and also to copper substrate, both used in the AACVD coating method. The substrate was dipped into a 2:1 (chloroform: elastomer) mixture, and withdrawn at an arbitrary rate. The coated substrate was then placed in a vacuum desiccator at room temperature and left to cure for 48 hours. The substrate was withdrawn from the desiccator and thereafter stored and handled in air. These dip-coated samples were analysed using powder XRD, UV- vis and IR spectroscopy as described above for Example 1.

Results for Example 1 and Comparative Example 1 are discussed below.

Results and Discussion SYLGARD® 184 Silicone Elastomer

The formation of a hydrophobic surface requires a hydrophobic material to construct the surface from. This material was chosen because of the potentially high hydrophobicity of silicone polymers. The SYLGARD® 184 silicone elastomer is comprised of two parts, a silicone base and a curing agent in a 10:1 (base:curing agent) ratio. Upon mixing of the two components the medium viscosity liquid will cure into a flexible transparent elastomer. The hardening of the elastomer is due to an increase in cross-linking within the silicone network, thus increasing its rigidity. The curing is greatly dependant on the temperature in which it is carried out in (Figure 4). The curing can take place in air or if completely sealed. The cured elastomer is stable up to temperature of 250⁰C and becomes brittle when exposed to greater temperatures.

It is the composition of such a polymer and the temperature-dependant curing that makes it ideal for applications in AACVD. If both components can be transported in an aerosol, to a reactor which is at elevated temperature, curing will be able to take place; thus depositing hydrophobic material onto the substrate. The side chains from the silicone network contain hydrophobic alkyl groups Formula (I) shows a general chemical structure of silicones. The R/R' alkyl groups contain (CH₂) and other hydrophobic groups act to repel water. The water repelling properties of the elastomer will thus provide a hydrophobic surface.

The scheme below shows a typical silicone curing reaction, wherein the formation of Si-O bonds would result in a hardening of the elastomer. This demonstrates possible variations of curing agents; by a change in X.

HO + 2HX

— N R -O C CH₃ H₂ -O C -CH₃

X= H S

Alcohol Amine Acetate Synthesis:

The first factor that needed to be considered in order to carry out a deposition was the solvent the polymer could be dissolved in, as an aerosol would have to be generated. Because the silicone is hydrophobic a polar solvent such as water could not be used, therefore a nonpolar solvent had to be used. The best solvent found to dissolve both components of the elastomer was chloroform. Dip-coated samples were prepared in order to judge the hydrophobic potential of the elastomer, and this also allows a comparison to the film made by AACVD.

The films prepared using the AACVD method were all deposited on the top plate of substrate. The deposition temperature was raised from 90⁰C (in increments of 30⁰C). At low temperatures (90 and 120⁰C) the deposited material was left uncured. This is probably due to the slower curing time, and also the slower evaporation of the chloroform solvent. Between 150 and 27O⁰C solid deposition occurred, however the elastomer showed signs of decomposition at 300⁰C and above.

Physical Characterisation:

Dip-coated SYLGARD® 184 silicone elastomer. After curing had taken place the elastomer had hardened to a rubber-like consistency and was transparent in appearance. There were no air bubbles present in the elastomer due to the curing having been carried out under vacuum. The elastomer could not be removed with Scotch ® tape or be peeled off either the SiO₂ or the copper surface; however it could be scratched off using the metal scalpel using moderate effort.

AACVD of SYLGARD® 184 silicone elastomer. The films formed were white and appeared cloudy on the glass substrate. Light was let through the coatings however the amount of light let through (transmitted) in the SiO₂ barrier glass samples decreased as deposition temperature was increased. The amount of material deposited rose with temperature (up to 300⁰C) due to a faster curing rate and increased evaporation of solvent during the CVD process. This increased amount of material would explain the decrease in transmitted light. At temperatures of 300⁰C and above there were signs of decomposition of the polymer, denoted by yellow/brown patches that appeared firstly on the bottom substrate at 300⁰C, and thus extended to the top substrate at higher temperatures. Figure 5 shows a copper substrate with elastomer deposition carried out using AACVD at substrate temperatures of 21O⁰C. The left part shows the coated area; the right of the picture shows the area uncovered by polymer.

Powder x-ray Diffraction: Because the SiO2 barrier glass substrate is an amorphous material and has a very broad peak in the spectra (figure 8, e), it would only aid in detecting any crystalline material giving sharp peaks that would stick up though this broad peak. In none of the analyses of the elastomer deposited by AACVD onto SiO₂ barrier glass was there any evidence found for the deposited material being crystalline (no sharp peaks to suggest crystalline order). To confirm this, the coatings on the copper substrates were analysed as copper gave very sharp peaks (Figure 6) that could be distinguished from any amorphous material. Figure 6 shows the key XRD data.

The XRD data for the dip coated polymer (b/c), confirms a highly amorphous chemical structure, with very broad peaks. The copper's highly ordered structure can be seen in its spectrum (a). For the AACVD deposition the same amorphous peaks can be observed (at 2- Theta = 12, d), however there is a difference in the spectra (2-Theta = 29). The polymer deposited by AACVD shows a variation in chemical structure. Although there is this difference between dip coating and CVD films; the miscellaneous peak is still broad which suggests the structure being amorphous. The difference in structure could be rationalised by visualising the method of film formation; the AACVD method could proceed by gas/aerosol phase curing and then deposition onto the substrate, whereas the dip-coated polymer cures from a liquid phase. This difference in curing mechanisms would explain the difference in chemical structure.

Thus from the XRD data the dip-coated polymer had a highly amorphous structure confirmed. The coatings formed by AACVD show a differing structure when compared to dip-coating, however the structure still shows an amorphous structure (with a lack of long range order in the elastomers structure). The spectra of the SiO₂ glass (Figure 6, e) shows the amorphous glass with no clear peaks that can be definitively identified.

Infra-red Analysis:

The SiO₂ barrier glass absorbed some of the IR radiation (< 2200cm^"1), so the only region visible was the greater than 2200cm^"1. The only distinct peaks available from the spectra were signals at 2960cm^"1 and 2905cm^"1 which correspond to C-H stretches. The shape of these signals were most clearly seen in the dip-coated samples (Figure 7), as they were transparent and did not absorb the IR radiation. If the glass signals were removed (by using glass in the background scan) the peaks can be clearly visible. As AACVD deposition temperatures increased the transmittance to IR radiation became less; however the C-H peaks were still visible. The presence of C-H bonds is essential to the material's action as a hydrophobic surface.

SEM Imaging:

The images gained from this analysis provide information on surface roughness. The SEM images give an aid for the visualising the surface roughness, however the images only show one point of view, and in order to get a true sense of the surfaces roughness more than one view must be taken (figure 10 a/b).

The images taken show that the elastomer deposited via AACVD has a very rough microstructure with areas to trap air under a water droplet (which can be important in the creation of hydrophobic surfaces). Figure 10 (b) shows tower-like structures protruding from the glass substrate. These tower-like structures were not as prevalent in low temperature depositions, and became more prominent up to a deposition temperature of 270⁰C. Above this temperature the bulk of the surface microstructure did not greatly change; however its shape did. At raised temperatures (>270°C) the elastomer coating contained greater areas of material that were clumped together, and the rounded shape of lower temperature depositions (figure 8, a) were lost. This is most likely due to decomposition of the polymer causing a breakdown of the polymer's organosilicate network.

The coating at 270⁰C formed at a thickness of approximately 2-3 μm although the glass substrate is not completely covered, the glass can be clearly seen beneath the elastomer deposition. The protrusions form in a seemingly random pattern of agglomerating particles (Figure 9), the likes of which extended throughout the sample. Thus with the IR analysis in combination with these SEM images; not only is the polymer shown to be hydrophobic, but the AACVD allows the formation of an extremely rough surface structure. It is both these factors that suggest the deposited film to be potentially very hydrophobic. The extent and nature of its hydrophobicity was determined using water droplet contact angle and slipping angle testing. Water Droplet Contact Angles:

The surfaces were tested using distilled water with some methylene blue dye to make contact angles easier to see. The presence of methylene blue proved to have no notable effect on the contact angle. The contact angle made with a flat dip-coated elastomer film was approximately 95°, therefore S YLGARD® 184 Silicone Elastomer is hydrophobic (contact angle greater than 90°). Any increase on this contact angle will be due to the surface microstructure increasing the surface's hydrophobicity. The method of contact angle measurement (shown Figure 10), used images to determine the actual measurement. Upon inspection of the images it was found that there was a range of contact angles that could be found on one surface; a film made by AACVD at deposition temperature of 240⁰C gave a maximum contact angle of 165.0°, however another water droplet on the same surface gave a 145.0° contact angle. This discrepancy in contact angle measurements could be down to two factors:

• The film generated by AAVCD could be unevenly spread over the substrate, with a slightly heavier deposition of the elastomer toward the middle of the plate. This could result in differing physical properties (i.e. hydrophobicity) across the substrate.

• The depositions could also be sensitive to physical wear, and because some of the hydrophobic material could have been removed during this wear, the contact angle would have been affected by a lack of elastomer local to the droplet being measured.

The water droplets were dropped onto the surface at random and had an arbitrary volume.

Contact angles were taken from each side of a droplet as they did differ substantially in some cases. Table 1 lists the contact angle measurements obtained, and shows data gathered from contact angle measurements made on SiO₂ barrier glass substrate under differing deposition conditions. The contact angle of dip-coated elastomer was improved upon in every case, thus implying a massive increase in surface roughness. Table 1

The results show an increase of average, and maximum contact angle with increasing deposition temperature. This would suggest that at higher temperatures there is an increase in the elastomer's coverage of the surface as well as an increased surface roughness. The maximum contact angle was 167.5° (Figure 10, super hydrophobic) at a deposition temperature of 360⁰C, this surface also yielded the greatest average value and the largest minimum contact angle. The minimum contact angle obtained by each surface shows a roughly similar increase with deposition temperature. A pattern is not followed exactly because the minimum contact angle is most likely due to an area of the hydrophobic coating that has been worn away, thus reducing the contact angle greatly. This effect is local to each water droplet analysed, and specific to the particular treatment (and thus wear) of each substrate plate, therefore this value can be seen as slightly less significant. The copper substrates also used provided similar patterns in contact angle measurements.

Water Droplet Slipping Angles:

As well as contact angles to analyse the hydrophobicity of the surface, the slipping angle of water on the surface was also used. If the slipping angle is low, so the droplet easily falls off the surface, there is less attraction of the water droplet to the surface. If there is air trapped underneath a water droplet resting on a surface, because there is less water-surface contact the droplet is more likely to slip off at lower tipping angles.

Low temperature sample would often hold the 60μL water droplet when held at 90°. Even in higher temperature samples there were parts of the surface that gave outlying results in relation to the most common. There was a decrease in slipping angle as the deposition temperature was raised, up to 360⁰C. This surface provided typical slipping angles of 5-15° and the lowest slipping angle recorded was 4.2°. The droplets tended to roll and spin down the surface, in a mechanism similar to the lotus effect, observed in the natural world. This wetting mechanism (termed the Cassie-Baxter model) is used in the self-cleaning of plants in order to remove bacteria and dirt from the plant's surfaces, thus because this deposited surface demonstrates a similar mechanism it has potential self-cleaning applications.

Summary:

The material used for surface construction has been shown to be hydrophobic, by a flat dip-coated surface giving a 95° contact angle. As well as the presence of C-H bonds on the surface, confirmed by IR analysis, which are proven to have hydrophobic properties, the surface structure has been proven not to be flat but have a highly rough texture. This roughness of the surface can be improved by increasing deposition temperature, however signs of elastomer decomposition were found at 300⁰C and greater. Surface hydrophobicity was found to yield a contact angle of 167.5° at its maximum (super hydrophobic), using deposition temperatures of 36O⁰C. The surface had typical water droplet slipping angles of 5- 15°, and the rolling mechanism of water droplets could useful in self-cleaning surfaces.

Comparative Example 2 - AACVD of Dyneon™ FC-2120 Dyneon™ FC-2120 is a carbon-based fluoroelastomer, a pre-formed polymer with no curing agent incorporated. Dyneon™ FC-2120 (0.125g) was placed into a MIBK: ethanol mixture (95:5 ratio; 50 mL) and stirred rapidly until the polymer dissolved (approximately 45 mins).

AACVD deposition was performed as in Example 1 , except that deposition typically took 90- 100 mins. The resulting films were characterized as described in Example 1.

Results and Discussion

Comparison ofSylgard®lS4 (SEl) with Dyneon™ FC-2120 fluoroelastomer (FE) The cured dip-coated films were featureless and approximately flat (shown by AFM/SEM measurements), and so any change in hydrophobicity from these surfaces will be due to alterations of surface topology (assuming complete coverage of the substrate). The water contact angles on these smooth surfaces show that the dip-coated FE film had the greater hydrophobicity (Table 2), thus if surface roughness is induced to the same extent on both surfaces the FE surface should have a greater hydrophobicity. The SEl and FE films started to decompose at substrate temperatures at and above 330°C. AU hydrophobic depositions occurred on the top substrate plate, this suggests the temperature of the elastomer must be less than deposition temperatures as the top plate was not heated directly. The films appeared as white-opaque films with the FE film being slightly more transparent to light. The SEl films were resistant to Scotch® tape, but could be scraped off with a metal scalpel. FE films were more resistant, surviving Scotch® tape and needing more effort to be scraped off the substrate. Elastomer coverage of the substrate increased as the temperature was increased, which improved contact angle results, with the highest contact angle measurements being recorded between 240-330°C (Table 2). Both elastomer films were shown to be amorphous (XRD) with no sharp signals. The presence of C-F (FE) and C-H (SEl) bonds were verified with peaks in the respective infra-red regions.

Table 2 Deposition type and conditions with average (ten separate measurements) water droplet contact angles for Sylgard® 184 (SEl) and Dyneon™ FC-2120 (FE). Depositions occurred onto glass substrates.

For both elastomers, the AACVD depositions resulted in a change in surface topology with respect to the dip-coated films (Figure 11). As described in Example 1, the SEl resulted in a highly rough surface, with average contact angle of 160°, and maximum contact angle of 167° at a deposition temperature of 33O°C. The FE however showed little improvement on the dip- coated surface contact angle (99°), with an average contact angle of 104°, and a maximum contact angle of 118° at a deposition temperature of 330⁰C. In the FE depositions, 0.125g of elastomer was used in the precursor solution; however an increase in delivery amount of precursor did not result in any increase in contact angles at any temperature, and there was also no substantial change in surface morphology (confirmed by SEM/AFM). _{. .}

30

Without wishing to be bound by theory, it can be postulated that in the FE experiments, when the solvent is evaporated by the elevated temperature of the CVD reactor, instead of curing to a solid, the elastomer is softened and behaves more like a liquid. In this case, when the elastomer makes contact with the substrate the result is a flatter film where further deposition of FE adds to the already molten film. Thus in this case the FE did not form a highly rough development of tower-like structures, as was seen in the AACVD of the curable SEl (Figure 11). The features in the FE film were 1 μm in height (imaged using AFM), and were as large as 10 μm across. In contrast, the nodule like structures in the SEl film were 3 μm in height (measured using side on SEM), and could not be fully imaged using AFM due to their magnitude.

The FE was confirmed to be soluble in three ketones (acetone, butan-2-one and MIBK). Using acetone resulted in a very volatile aerosol, where the acetone evaporated before reaching the CVD reactor, thus no deposition occurring. The use of butan-2-one did not result in any detectable change in the surface structure (image using SEM) or contact angle. This suggests that the higher temperatures required to evaporate the solvent result in the melting of the elastomer and destruction of any potential surface roughness.

Thus in order to achieve a superhydrophobic surface using the FE polymer, it would be necessary to modify parameters such as the aerosol droplet size, the distance through which the aerosol travels (e.g. the size of the space in step (Ui)), and/or the temperature.

Example 3 - AACVD of Nusil Med-4850

Nusil Med-4850 is a two part thermally cured silicone elastomer (1 :1 silicone base:curing agent), similar to the Sylgard® 184 polymer.

Nusil Med-4850 (0.5Og) was dissolved in chloroform (8OmL) with rapid stirring for 10 min. AACVD deposition was performed as in Example 1 , except that deposition typically took 1 hour, and the reactor temperature was varied from 90 to 42O⁰C. The resulting films were characterised as described in Example 1.

Results and discussion

Comparison of Sylgard® 184 (SEl) and NuSiI Med-4580 (SE2) Both polymers comprise a two part kit (silicone base and curing agent), whose curing depends on temperature. When mixed SEl cures after 48 hours at room temperature, whereas the SE2 elastomer was not fully cured after 48 hours of curing, and was left a total of 72 hours. The SE2 was also more viscous prior to curing, with a substantial aerosol only able to be formed upon dilution of the elastomer (80 mL of CHCL₃, cf. 50 mL required for SEl). The dip coated surfaces showed very similar properties, both having an average contact angle of 95°, and were shown to be flat using AFM measurements. Thus the cured polymers have very similar energies of interaction with water, and any difference in contact angles in the AACVD- deposited surfaces must be due to the differences in surface topology. Results are show in Table 3.

Table 3 Deposition temperature with average (ten separate measurements) water droplet contact angles for Sylgard® 184 (SEl) and NuSiI Med-4850 (SE2) on glass substrates. (*) Indicates visible signs of decomposition on films, (-) indicates a substantial decomposition of the polymer during the AACVD deposition.

The SE2 films were similar in appearance to the SEl films, i.e., white-opaque films. Both films showed the presence of C-H bonds in the IR spectra. Both films were resistant to the scotch tape test, with SE2 higher temperature depositions being noticeably more resistant to the scalpel scratching (however the films were still able to be removed). The contact angles for the films (Table 3) demonstrate the differences in curing behaviour. Whilst SEl decomposed at a 360⁰C substrate temperature, SE2 was resistant up to 42O⁰C, at which temperature decomposition was seen, as shown by a white deposition occurring on the bottom plate. SE2 did not reach superhydrophobic contact angles until 330-360⁰C. The maximum contact angle observed for SE2 was 170° (Figure 12) at a deposition temperature of 420⁰C, even more hydrophobic than the maximum SEl contact angle achieved (167°). The higher required deposition temperature for SE2 is most likely due to the slower curing rate of the polymer along with the greater proportion of solvent used, requiring more energy to be removed. This higher temperature also aids the adherence of the elastomer to the substrate.

The water droplet slip angles of the SE2 42O⁰C deposition were recorded, the average being 6°, with some droplets only requiring a tip angle of 2° to slip off. Both SEl and SE2 higher temperature depositions demonstrate such low slipping angles, due to the Cassie Baxter model nature of the wetting.

Thus, the present examples demonstrate that it is possible to produce a superhydrophobic, microscopically textured surface from a hydrophobic polymer using the process of the present invention. One skilled in the art would predict that similarly, a superhydrophilic surface would be produced using a hydrophilic polymer, due to the formation of a microscopically rough texture.

In one embodiment, the present invention provides a process for producing a microscopically textured polymer surface on a substrate, comprising the steps of: (i) providing a solution comprising the polymer or precursor(s) of the polymer; (ii) forming an aerosol of the solution; (iii) passing the aerosol through a heated space; and (iv) depositing polymer particulates onto a substrate.

Thus, in one embodiment, the present invention provides a process as described above, comprising using aerosol assisted deposition (AAD); a process as described above, comprising using aerosol assisted chemical vapour deposition (AACVD) apparatus; a process as described above, wherein the heated space is at a temperature of at least 8O⁰C; a process as described above, wherein the substrate comprises metal, ceramic, glass, polymer, wood, fabric or textile; a process as described above, wherein the polymer is polysiloxane or polyurethane; a process as described above, wherein the solution comprises precursor(s) of the polymer; a process as described above, wherein the precursor(s) is capable of forming a crosslinked polymer; a process as described above, wherein the solution further comprises a crosslinking agent or curing catalyst; a process as described above, wherein the solution further comprises one or more photosensitisers; a process as described above, wherein the solution comprises toluidine blue O and rose bengal; a process as described above, wherein the solution further comprises nanoparticles; a process as described above, wherein the solution comprises metallic nanoparticles and one or more photosensitisers; a process as described above for producing a superhydrophobic polymer surface on a substrate, wherein the polymer is a hydrophobic material.

A process as described above for producing a superhydrophilic polymer surface on a substrate, wherein the polymer is a hydrophilic material; use of a polymer for producing a microscopically textured surface on a substrate, comprising depositing the polymer onto the substrate by AAD; use of a hydrophobic polymer for producing a superhydrophobic surface on a substrate, comprising depositing the polymer onto the substrate by AAD; use of a hydrophilic polymer for producing a superhydrophilic surface on a substrate, comprising depositing the polymer onto the substrate by AAD; use of AAD for producing a microscopically textured polymer surface on a substrate; use of AAD for producing a superhydrophobic or superhydrophilic polymer surface on a substrate; a substrate having a microscopically textured polymer surface coated thereon, which substrate is obtainable by the process as described above or the use as described above; a microscopically textured polymer surface obtainable by the process as described above or the use as described above; or use of a substrate or a surface as described above, wherein the polymer is a hydrophobic polymer and the surface is superhydrophobic, or wherein the polymer is a hydrophilic polymer and the surface is superhydrophilic, in self-cleaning applications.

Claims

1. A process for producing a superhydrophobic, microscopically textured polymer surface on a substrate, comprising the steps of:

(i) providing a solution comprising a hydrophobic polymer or precursor(s) of a hydrophobic polymer; (ii) forming an aerosol of the solution;

(iii) passing the aerosol through a space in order to solidify the polymer or react the precursors to form a solid polymer; and

(iv) depositing polymer particulates onto a substrate.

2. A process according to claim 1, wherein steps (ii) to (iv) are effected using aerosol assisted chemical vapour deposition (AACVD) apparatus.

3. A process according to claim 1 or claim 2, wherein the space in step (iii) is a heated space.

4. A process according to claim 3, wherein the heated space is at a temperature of at least 80⁰C.

5. A process according to any one of claims 1 to 4, wherein the polymer is a polysiloxane or polyurethane.

6. A process according to any one of the preceding claims, wherein the solution comprises precursor(s) of the polymer.

7. A process according to claim 6, wherein the precursor(s) is capable of forming a crosslinked polymer.

8. A process according to claim 6 or claim 7, wherein the solution further comprises a crosslinking agent or curing catalyst or curing agent.

9. A process according to any one of the preceding claims, wherein the solution further comprises one or more photosensitisers.

10. A process according to claim 9, wherein the solution comprises toluidine blue O and rose bengal.

11. A process according to any one of the preceding claims, wherein the solution further comprises nanoparticles.

12. A process according to any one of the preceding claims, wherein the solution comprises metallic nanoparticles and one or more photosensitisers.

13. A process for producing a superhydrophilic, microscopically textured polymer surface on a substrate, comprising the steps of (i) providing a solution comprising a hydrophilic polymer or precursor(s) of a hydrophilic polymer; (ii) forming an aerosol of the solution; (iii) passing the aerosol through a space in order to solidify the polymer or react the precursors to form a solid polymer; and (iv) depositing polymer particulates onto a substrate.

14. A process according to claim 13, wherein steps (ii) to (iv) are effected according to any one of claims 2 to 4

15. Use of aerosol assisted deposition (AAD) for producing a superhydrophobic or superhydrophilic polymer surface on a substrate.

16. Use according to claim 15, wherein the AAD is effected using AACVD apparatus.

17. A substrate having a superhydrophobic, microscopically textured polymer surface coated thereon, which substrate is obtainable by the process of any one of claims 1 to 12 or the use of claim 15 or claim 16.

18. A substrate having a superhydrophilic, microscopically textured polymer surface coated thereon, which substrate is obtainable by the process of claim 13 or claim 14, or the use of claim 15 or claim 16.

19. Use of a hydrophobic or hydrophilic polymer to render a substrate superhydrophobic or superhydrophilic, by forming on said substrate a microscopically textured polymer surface, by a process according to any one of claims 1 to 14.

20. Use of a hydrophobic or hydrophilic polymer to render a substrate self cleaning and/or antimicrobial, by forming on said substrate a superhydrophobic or superhydrophilic microscopically textured polymer surface, by a process according to any one of claims 1 to 14.