GB2587611A - A method for producing polymeric surface modification layers - Google Patents

Abstract

A method of coating a substrate comprises (i) mixing polystyrene, polymethyl methacrylate, and solvent to form a polymer solution, (ii) applying the polymer solution to the substrate to form a coating, (iii) drying the coating to produce a polymeric layer, (iv) exposing the polymeric layer to reactive polar species to form a modified polymeric layer. The solvent may be anisole and the polymer solution may be applied by spin-coating. Step (iv) may comprise UV/ozone or O2 plasma treatment. The method may comprise (v) heating the modified polymeric layer at 100-200°C for 5-30 seconds. A method of forming a printed substrate comprises printing onto the modified polymeric layer. Typically, the printing step uses an ink containing a metal oxide and a polar solvent, e.g. mixtures of 2-ethoxyethanol, ethylene glycol, and ethanolamine. The printed substrate may be heated to ≥350°C to remove the polymer layer. Typically, the substrate is a silicon wafer.

Description

A METHOD FOR PRODUCING POLYMERIC SURFACE MODIFICATION LAYERS

Field of the Invention

The present invention relates to polymeric layers and particularly, although not exclusively, to polymeric surface adjustment layers suitable for various printing applications.

Background

The regulation of wetting has an important technological role in many applications, including printing of functional inks on solid, non-permeable substrates, which is an essential part of printed electronics. This technology enables cost-effective fabrication of electronic components on various substrates, such as glass, plastics, ceramics and metal foils. In printed electronics, for example, layer-on-layer printing is commonplace, such that each new pattern is printed on an inhomogeneous surface (i.e. a surface with pre-deposited layer(s)). This can result in complex and unpredictable wetting behaviour, characterized by differing contact angles on areas with pre-deposited structures and areas which are the bare surface of the substrate. Thus resolution or stability of the printed pattern can be compromised. There is a significant risk of short circuit if lines or other printed elements connects where they are not designed to do so.

The wetting of a particular surface by a liquid is described by advancing and receding contact angles of the liquid. The advancing contact angle is the maximum possible contact angle; the contact line advances when the momentary contact angle exceeds the advancing contact angle. The receding contact angle is the smallest possible contact angle; the contact line recedes when the momentary contact angle is smaller than the receding contact angle. The difference between both angles is called the contact angle hysteresis. The contact angle hysteresis is omnipresent. It prevents a drop from sliding on inclined surfaces, such as rain drops resting on windows, and has a crucial role in the stability of printed patterns and drying.

In inkjet printing, the ink-substrate interaction would ideally result in a large advancing contact angle and a receding contact angle of zero. A large advancing contact angle is ideal for high resolution, while a receding contact angle of zero is ideal for the stability of the pattern. An ideal surface (atomically smooth and chemically homogeneous), in principle, would exhibit a contact angle hysteresis of zero, i.e., equal advancing and receding contact angles. However, it is known that surface imperfections, such as spatial topographical or chemical variations, result in a contact angle hysteresis.

Generally, smoother and chemically homogeneous surfaces exhibit smaller contact angle hysteresis in comparison to rough, chemically inhomogeneous surfaces.

Wetting and spreading of the ink on a given substrate can be manipulated either by (i) modifying the surface properties of a substrate, or (ii) locally restricting the spreading of the ink.

(i) Modifying the surface properties of a substrate This approach relies on producing a partial wetting of the substrate. Here, the ink-substrate interaction needs to be well balanced to retain the stability and definition of the printed features. A common approach to adjusting the wetting of the substrate is by surface treatment, such as wet cleaning, plasma/corona treatment, and UV/ozone treatments. These methods are usually used to reduce the contact angle and prepare the surface for further use in coating applications where good wetting (very small contact angle) is desired. On the other hand, printing applications require precise adjustment of the contact angle to yield partial wetting. This can be achieved by employing a wetting-control layer prior to the printing step.

Jang et al. (Adv. Electron. Mater., 2015, 1500086) used a poly(methyl methacrylate) (PMMA) layer as the wetting-control layer. A thin layer was deposited onto the glass substrate via spin-coating. This procedure was followed by a drying at 200 °C for 10 min. A UV/ozone treatment increased the surface energy of the PMMA layer and reduced the contact angle of the functional ink. The PMMA surface modification layer was smooth, and contact angle hysteresis of the ink was small as indicated by the contact line de-pinning at low substrate temperatures during printing.

Matav2 et al. (Langmuir, 2017, 33 (43), 11893-11900) used layers of poly(styrene), poly(methyl methacrylate), poly(vinyl alcohol), or poly(vinyl pyrrolidone) to adjust the surface properties of glass substrates. The wetting of the investigated inks depended on the chemical nature of the polymeric layer. For a more precise adjustment of wetting, a method involving a thermal decomposition of poly(methyl methacrylate) layer on glass substrate at 350 °C was proposed. Similarly as in the approach by Jang et al., the polymeric layers were smooth and exhibited a relatively small contact angle hysteresis.

(ii) Locally restricting the spreading of the ink An alternative approach of wetting regulation is by restraining the spreading of the functional ink by producing low surface energy patterns on well wetting surfaces.

Nguyen et al. (Appl. Mater. Interfaces, 2014, 6, 4011-4016) used patterned surfaces with a combination of hydrophilic and hydrophobic properties to restrain the spreading of the liquid.

Selective wetting was achieved via a two-step hydrophilic-hydrophobic coating of 3-aminopropyl trimethoxysilane (APTMS) and 3M Novec 1700 and Novec 7100DL Engineered Fluid respectively on PET surfaces; this was followed by a selective hydrophilic treatment (either atmospheric 02/Ar plasma or UV/ozone surface treatment) with the aid of a nickel stencil. Regions with large water contact angle of 105° and regions with small contact angle of water of <5° were achieved on a single surface. This process employs mechanical nickel stencil and thus limits the otherwise easy pattern modification at the printing stage.

Li et al. (Appl. Mater Interfaces, 2017, 9, 8194-8200) used surface-energy patterns to restrain the spreading of the functional ink on glass substrates. The surface energy template was prepared by spin coating a thin layer of CYTOPS polymer and afterwards printing pure solvent to etch the hydrophobic CYTOP® layer via coffee stain effect. The process retains the digital definition of the printing pattern; however, the resolution of the template depends on printer characteristics and the contact angle of etching solvent on the underlying surface.

Godard et al. (Adv. Mater Technol., 2018, 1800168) used low surface energy alkanethiolate-based templates, which bind to metal surface by chemisorption to form a self-assembled monolayer. The templates were inkjet-printed onto platinized silicon wafers to form surface energy template, which restrained the spreading of the functional ink. The process retains digital definition of the printing pattern; however, the resolution of the template depends on the printer characteristics and wetting of alkanethiolate ink.

US9248686B2 relates to a printing element having at least one polymer layer which has photoimageable constituents and a chemically functionalized polymer to make the polymer layer either more hydrophobic or hydrophilic, which provide differential wetting with hydrophilic inks. A fluorinated polymer layers is selectively modified under light using a photomask. Unexposed polymer can be dissolved away to leave a patterned surface.

There remains a need to develop fast, reliable and cost-effective techniques which can adjust the wetting of arbitrary solid optical grade substrates, resulting in a large contact angle hysteresis.

The present invention has been devised in light of the above considerations.

Summary of the Invention

The present inventors have devised a method to improve the wetting properties of a surface.

In general, the present surface modification layers provide more universal and efficient regulation of wetting in the deposition of several overlapping layers, which is required for printed electronics. The regulation of wetting typically proceeds by a two-step process: (i) covering the whole surface by a thin polymeric layer and (ii) treating the polymeric layer by a surface modification method (UV/03 or plasma treatment) to adjust the wetting. The problem occurs as the existing/known polymeric layers are intrinsically smooth (Ra<1 nm) and chemically homogeneous and thus exhibit relatively small contact angle hysteresis.

The hysteresis is generally sufficient for printing of stable patterns and provides moderate resolution; however, the printed patterns may de-wet upon drying. A lower receding contact angle and a larger contact angle hysteresis benefits higher printing resolution as well as improved stability upon drying by limiting de-pinning. The methods of the present invention can lead to a reduction of receding contact angle and/or an increase of contact angle hysteresis. Even if the contact angle hysteresis is not increased, the methods of the present invention allow it to be held reasonably steady while significantly reducing the receding contact angle, thereby improving the wetting properties of the surface.

Discussed herein is the formation of a nano-textured polymeric layers. A spontaneous phase separation between poly(methyl methacrylate) (PMMA) and poly(styrene) (PS) leads to a thin layer with a highly topographically and chemically inhomogeneous surface (Fig.1 and Fig. 2).

Optional subsequent surface treatment(s) with UV/ozone and/or post-baking as described herein can further reduce the receding contact and and/or increase the contact angle hysteresis of the layer.

In a first aspect, the invention provides a method for coating a substrate, the method comprising the steps of (a) mixing poly(styrene), poly(methyl methacrylate) and a solvent to form a polymer solution, (b) applying the polymer solution to the substrate to form a polymer solution coating, (c) drying the polymer solution coating to form a polymeric layer on the substrate, and (d) exposing the polymeric layer to a reactive polar species, to form a modified polymeric layer on the substrate.

In some embodiments the polymeric layer is exposed to UV/ozone in step (d). This decreases the receding contact angle and increases the contact angle hysteresis of polar solvents.

In some embodiments the polymeric layer is exposed to 02 plasma.in step (d). This decreases the receding contact angle and increases the contact angle hysteresis of polar solvents.

In some embodiments the surface is further modified by heating the modified polymeric layer at 100 to 200 °C for 5 to 30 seconds. This may further increase the advancing contact angle and the contact angle hysteresis.

In a second aspect, the invention provides a method for forming a printed substrate, the method comprising the steps of (i) forming a coated substrate by the method of the first aspect, and (ii) printing onto the coated substrate.

In some embodiments step (ii) is carried out using an ink comprising a polar solvent. Polar solvents have a low receding contact angle and large contact angle hysteresis on substrates prepared as described and are particularly useful for printing functional layers In some embodiments the polar solvent is a mixture of 2-ethoxyethanol (2EE), ethylene glycol (EG) and ethanolamine (EA). This solvent system for printing functional inks results in uniform, flat layers.

In some embodiments the ink comprises a metal oxide. Metal oxide inks decompose at high temperatures to form a functional layer.

In some embodiments the printed substrate is heated to a temperature of 350°C or greater, to remove the polymeric layer. This enables the polymeric layer to act as a temporary surface modification which will not be involved in the function of the printed layer.

In some embodiments the substrate is a silicon wafer.

In some embodiments the substrate used in step (b) of the method of the first or second aspect has a pre-printed layer on the surface onto which the polymer solution is applied in step (b). In this way, multiple functional layers can be accurately printed on top of each other through repetition of the above method.

In a third aspect, the invention provides a substrate obtainable by the method of the first or second aspect.

In some embodiments the polymeric layer on the substrate has a thickness of 10 nm or less. This thickness prevents cracking of the printed functional layer during decomposition and reduces the amount of carbon at the interface.

In some embodiments the distance between phase separated islands is <1 pm. This ensures the phase separated islands are relatively small compared to the feature or features being printed.

The present invention also relates to coated materials (that is, a substrate with a polymeric coating or layer thereon) obtained or obtainable by the methods described herein. Described herein are substrates having a polymeric layer thereon; printed substrates having a polymeric layer thereon and a printed pattern on that layer; and printed substrates having a printed pattern and the substrate with the polymeric layer removed.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows a schematic drawing of the topography of a textured PMMA/PS polymeric layer illustrating a heterogeneous surface with separated phases of PMMA and PS.

Figure 2 shows a schematic drawing of a cross section of the textured PMMA/PS polymeric layer illustrating the separated phases of PMMA and PS.

Figure 3 shows an AFM topography scan over the surface of an 8-nm-thick PMMA/PS polymeric layer.

Figure 4 shows the contact angle of ethylene glycol on PMMA-coated glass and PMMA/PS-coated glass upon increasing the drop volume (corresponding to the advancing contact angle) and subsequently decreasing the drop volume (corresponding to the receding contact angle). Both polymeric films were treated by UV/ozone for 120 s and post-baked at 150 °C for 15 s. The dJ.4 is the instantaneous diameter of a sessile drop normalized to its maximum diameter reached in the measurement.

Figure 5 shows a square pattern with dimensions of 500x500 pm2 printed on PMMA-coated glass and PMMA/PS-coated glass upon drying. A white dotted line outlines the initial shape of a printed square. (a) and (c) show the printed square immediately after printing, while (b) and (d) show the same feature after 30s of drying.

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

In the present invention, a mixture of PMMA and PS in a solvent is deposited onto a surface. The layer undergoes phase separation to produce uniformly distributed topographical features when dried. The resultant layer has a reduced receding contact angle and/or an increased contact angle hysteresis.

The present invention also describes techniques for further adjusting the surface energy of the layer to further reduce the receding contact angle and/or increase the contact angle hysteresis by reaction with polar species.

Furthermore, the present invention describes techniques to increase the advancing contact angle and contact angle hysteresis by annealing.

Furthermore, the invention also describes decomposition of the polymeric layer after printing.

Selection of Substrate The PMMA/PS layers of this invention can be successfully applied as a surface modification layer on many different types of surface. For inkjet printing applications, numerous optical-grade substrates used in electronics can be selected. Suitable substrates include silicon wafers, glasses of various types, polished ceramic substrates and metal foils and polymers such as polyimide. A particularly useful substrate is a silicon wafer. In the case where a silicon wafer is used, the substrate can be suitably passivated by deposition of an inert layer, for example a layer of A1203. The substrate can also be a material mentioned above with a pre-printed functional layer on the surface.

The substrates suitable for wetting modification are not limited by size, composition, or geometry. The present invention is suitable for wetting modification of planar, nonplanar and curved substrates, as the PMMA/PS layer of this invention can be uniformly applied to these substrates.

Polymer Deposition As a first step, the PMMA and PS raw materials are dissolved in a solvent. Suitable solvents include, for example, anisole. Suitable molecular weights for the PMMA and PS may independently be about 1 kDa to 1500 kDa, for example 100 to 1000 kDa, for example, 400 to 600 kDa, for example about 500 kDa A suitable mass ratio of PMMA to PS (PMMA:PS) is about 0.5:1 to 1:0.5. Suitably the mass ratio is about 1:1. The topographical properties of the layer can be adjusted by modifying mass ratio of the two polymers, as is known in the art.

As a second step, the surface of the substrate is coated by applying the polymer solution to the substrate by any suitable technique, for example by spin coating, spray coating or any other coating technique.

As a third step, the polymer solution coating is dried. This may be done by any suitable method. For example, it may be done by leaving the coated substrate to stand in standard conditions, or optionally by heating. A suitable heating temperature is 100 to 200°C. A suitable heating time is 0.5 to 10 minutes, suitably 1 to 5 minutes. It is appreciated that drying conditions may influence the morphology of the polymeric layer as is known in the art.

Of course, a suitable drying regime may be chosen depending on the solvent of the polymeric solution.

This forms the polymeric layer on the substrate.

During this time, the PMMA and PS present in the polymer solution coating undergo a phase separation. It is believed that the resulting polymeric layer contains islands of PMMA and PS phases (Fig. 1 and Fig. 2), which makes it chemically heterogeneous. This creates a surface with an increased contact angle hysteresis compared to a chemically homogeneous surface of similar surface roughness. The present inventors have found that the as-prepared polymeric films have low surface energy and low surface polarity, resulting in a larger contact angle of many polar solvents, for example diols (such as ethylene glycol), polyols (such as glycerol), water, formamide, dimethyl sulfoxide, and ethanolamine. Non-polar solvents, such as toluene, benzene, octane, have a decreased contact angle on the low surface energy polymer films described herein compared to on uncoated polar substrates such as glass or ceramics.

The as-prepared polymeric films described herein result in a larger contact angle hysteresis of many polar and non-polar solvents, for example ethylene glycol, ethanolamine, glycerol, toluene, benzene and octane, due to the surface defects present on the film.

The PMMA/PS layers formed by the methods described herein exhibit periodic topographical features < 1 pm wide, for example about 400 nm wide with a peak-to-valley height <10 nm, for example about 4 nm (Fig. 3). It is thought that each feature acts as a topographical defect and effectively increases the pinning energy of the wetting liquid and, consequently, its contact angle hysteresis. A root mean square roughness, measured using atomic force microscopy, of the present PMMA/PS layer on a glass substrate is about 1.5 nm, which is sufficiently small to prevent deteriorative effects on printed functional layers.

The polymeric layer formed by the methods described herein has a thickness which is suitably 10 nm or less, for example 8 nm or less to prevent cracking and minimize its effects on the adhesion of the functional oxide layer, and reduce the amount of residual carbon at the interface after decomposition.

The surface morphology of the PMMA/PS polymeric layer of this invention can be controlled by the PMMA to PS ratio, the polymer molecular weight of each polymer, the concentration of the polymers in the coating solution, deposition parameters, and thermal treatment of the layer as can be understood by those skilled in this technical field.

Surface Modification The surface energy of the polymeric layer may optionally be further increased by exposure to reactive polar species (for example, oxygen or nitrogen). Suitable treatments include exposure to UV/ozone (03), 02-plasma, nitrogen plasma or other polymer surface modification techniques known in the art. This further treatment can further decrease the contact angles of polar solvents, in particular the receding contact angle, and thus may further increase the contact angle hysteresis. Such surface modifications increase the contact angle of non-polar solvents.

The contact angle and contact angle hysteresis of the surface can be adjusted by varying the exposure time to the polar species. Exposure to polar species decreases the contact angles of polar solvents. By this method a suitable exposure time can be chosen to achieve the desired properties based on the polymeric film and the solvent used, and the receding contact angle may be reduced to, or close to, zero.

Suitable exposure times for UV/03 modification using a commercial UV/03 cleaner depend on polymer film and preferred contact angle to be achieved. For example suitable exposure times may be about 5 to 50 seconds or about 15 to 300 seconds. Exposure times may be for example about 30 to 300 seconds, for example about 30 to 120 seconds, for example about 30, about 60, about 90 or about 120 seconds. Suitable exposure times for 02-plasma modification using RF oxygen plasma operating at 50 Pa working pressure and power of 50W are, for example, 0.1 to 60 S. Suitable wavelengths for the UV radiation are, for example, about 100 to 350 nm, for example about 150 to 300 nm, suitably about 175 to 275 nm. Particular examples include 185 nm and 254 nm. Under these UV conditions molecular 02 dissociates to form oxygen radicals which react with molecular 02 to produce ozone.

A suitable exposure distance is about 10 to 50 mm, as determined by the specifications of the ozone cleaner.

A suitable exposure power is about 5 to 50 pW/cm2, for example about 10 to 30 pW/cm2, for example about 20 pW/cm2.

The contact angle of the polymeric layer may also optionally be increased by ageing at room temperature in the ambient environment. However this is slow for printing applications and the effect is minimal. It is desirable for printing to proceed soon after exposure to polar species.

Therefore, in some embodiments, the contact angle may be further increased after exposure to the polar species by baking at about 100 to 200 °C, preferably about 150 °C. A suitable baking time is about 5 to 30 seconds, suitably about 10 to 20 seconds, for example about 15 seconds.

This baking is thought to further increase the advancing contact angle and the contact angle hysteresis.

By the techniques described herein, for example, a contact angle hysteresis for ethylene glycol greater than 43° can be achieved for PMMA/PS layers as described herein after exposure to UV/ozone for 120s and then baking at 150 °C for 15 s. This is about a 200 % larger contact angle hysteresis of that of conventional PMMA layers (see Fig. 4).

Printing The present invention provides a surface with a high contact angle hysteresis which has a large impact on drying of printed structures. The coated surfaces of the present invention can be used for the printing of electronic components. The PMMA/PS layer can be used for wetting adjustments of many inks, for example commercial silver inks, inks containing 40 vol% or more of one of ethylene glycol, ethanolamine, 1,3-propanediol, diethylene glycol or any other solvent with similar surface tension, or inks containing 10 vol% or more of glycerol.

Printing of lanthanum nickelate precursor ink with ternary solvent composition of 2-ethoxyethanol (2EE), ethylene glycol (EG) and ethanolamine (EA) on PMMA layers treated by UV/ozone for 150 s (Matav2 et al., App!. Phys. Lett., 2018, 113, 012904) results in an advancing contact angle of the ink on such a film of 10°. However, despite the low advancing contact angle of the treated PMMA film, the drying results in an evaporation-induced contact line de-pinning (see Fig. 5, PMMA), which deteriorates the pattern definition.

In contrast to these known, smooth PMMA layers treated with UV/ozone, which are characterized by this dynamic contact-line behaviour of the drying drop, the nano-textured PMMA/PS layers described herein do not exhibit any wetting instability (see Fig. 5, PMMA/PS) and can be used to deposit well-defined structures. The larger contact angle hysteresis of PMMA/PS polymeric layer of this invention enables higher printing resolution, better quality of the pattern definition, and the better morphology of dried deposits.

Decomposition Once an ink or other functional layer has been printed onto the polymeric layer described herein, the polymeric layer can be removed by thermal decomposition. The polymeric layer readily decomposes at temperatures below that at which the printed functional layer decomposes. Suitable temperatures are over 350 °C.

The preferred thickness of the present polymeric layers (suitably 10 nm or less, for example 8 nm or less) prevents cracking or peeling of the printed functional layer during this process and reduces the amount of carbon at the interface.

It is to be noted that printing, and optionally subsequent removal of the polymeric layer by decomposition, can be carried out after drying the polymeric coating solution; or, if present, the surface modification by exposure to a reactive polar species; or, if present, the surface modification by baking.

It is also to be noted that the processes described herein can be repeated, in order to print multiple patterns onto a substrate. For example, a substrate onto which a pattern has already been printed using the methods described herein (optionally with surface modification and/or decomposition as described herein) can be used as the beginning substrate for a further polymeric layer deposition and printing (optionally with surface modification and/or decomposition as described herein). * ;The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. ;For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. ;Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. ;Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and "include", and variations such as "comprises", "comprising", and "including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. ;It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about" in relation to a numerical value is optional and means for example +/-10%. ;Examples ;EXAMPLE -I ;PMMA (polymethylmethacrylate) and PS (polystyrene) with molecular weight of 500 kDa are used as starting materials. ;0.1 g of PMMA and 0.1 g of PS were combined with 40 mL of anisole and agitated at room temperature until complete dissolution of the polymers. ;A polymeric layer was prepared by completely covering a glass substrate with the polymer solution and spin-coating at 3000 rpm for 30 s. The polymeric layer on the substrate was dried on a hotplate at 150°C for 2 min, resulting in an 8 nm thick polymeric layer. ;Contact angles were measured using Kruss DSA 20E tenziometer by observing the sessile drops in side-view at 22°C +/-2°C and relative humidity of 30% +/-10%. Advancing contact angles were measured by increasing the drop volume at the rate of 5 pL/min using a thin syringe, while receding contact angles were measured by decreasing the drop volume using a thin syringe at the rate of -5 pUmin. The advancing and receding contact angles for various solvents are shown in Table 1. ;Table 1 ;Solvent/ink Advancing Contact Angle (°) Receding Contact Angle (°) Contact Angle Hysteresis (°) ethylene glycol 64 45 19 water 90 72 18 ethanolamine 62 41 21 glycerol 80 65 15 1,2-propanediol 58 15 43 lanthanum nickelate precursor ink in ethylene glycol (0.1M) 66 48 18 lanthanum nickelate precursor ink in 2-ethoxy ethanol, ethylene glycol, ethanolamine at 20:16:64 ratio (0.1M) 47 23 24 ;COMPARATIVE EXAMPLE I ;Contact angles of various solvents were measured for a smooth PMMA polymeric surface prepared in the same way as in Example 1, using 0.2 g of PMMA alone instead of 0.1 g PMMA + 0.1 g of PS. The results are shown in Table 2 ;Table 2 ;Solvent/ink Advancing Contact Angle (°) Receding Contact Angle (°) Contact Angle Hysteresis (°) ethylene glycol 55 43 12 water 76 64 12 ethanolamine 56 37 19 glycerol 68 60 8 1,2-propanediol 46 22 24 lanthanum nickelate precursor ink in 2-ethoxy ethanol, ethylene glycol, ethanolamine at 20:16:64 ratio (0.1M) 42 25 17 ;EXAMPLE 2 ;A PMMA/PS polymeric layer was deposited on a glass substrate as specified in Example 1. The polymeric layer was then treated by UV/ozone (simultaneous 185 nm and 254 nm illumination, the dominant wavelengths of the ozone cleaner, 20 pVV/cm2 for 254 nm wavelength at distance of 10 to 50 mm, as determined by the specifications of the ozone cleaner) at room temperature for different times. Ozone is formed via UV mediated dissociation of molecular oxygen. ;The advancing and receding contact angles for various solvents are shown in Table 3. ;It was observed that the exposure of the PMMA/PS layer to UV/ozone did not result in significant change in surface roughness and morphology. ;Table 3 ;Solvent/ink UV/Ozone Exposure Time (s) Advancing Contact Angle (°) Receding Contact Angle (°) Contact Angle Hysteresis (°) ethylene glycol 30 54 31 23 ethylene glycol 60 45 19 26 ethylene glycol 120 27 0 27 water 30 62 12 40 water 60 47 0 47 water 120 44 0 44 lanthanum nickelate precursor ink in 2-ethoxy ethanol, ethylene glycol, ethanolamine at 20:16:64 ratio (0.1M) 30 14 0 14 COMPARATIVE EXAMPLE 2 Contact angles of various solvents were measured for a smooth PMMA polymeric surface prepared and modified with UV/ozone in the same way as in Example 2. The results are shown in Table 4. ;Table 4 ;Solvent/ink UV/Ozone Advancing Receding Contact Angle Hysteresis (°) Exposure Contact Contact Time (s) Angle (°) Angle (°) ethylene glycol 30 45 25 20 ethylene glycol 60 42 17 25 ethylene glycol 120 0 0 0 water 30 62 36 26 water 60 61 31 30 water 120 47 23 24 lanthanum nickelate precursor ink in 2-ethoxy ethanol, ethylene glycol, ethanolamine at 20:16:64 ratio (0.1M) 30 0 0 0 ;EXAMPLE 3 ;A PMMA/PS polymeric layer was deposited on a glass substrate as specified in Example 1. ;The polymeric layer was then treated by UV/ozone for 120 s, under the conditions set out in Example 2. After the UV/ozone treatment, the polymeric layer was heated at 150°C for 15s. ;The advancing and receding contact angles for various solvents are shown in Table 5. ;It was observed that the exposure of the PMMA/PS layer to UV/ozone and post-baking at 150°C did not result in significant change in surface roughness and morphology. ;Table 5 ;Solvent/ink UV/Ozone Exposure Time (s) Advancing Contact Angle (°) Receding Contact Angle (°) Contact Angle Hysteresis (°) ethylene glycol 120 53 <10 >43 water 120 73 10 63 lanthanum nickelate precursor ink in 2-ethoxy ethanol, ethylene glycol, ethanolamine at 20:16:64 ratio (0.1M) 120 20 0 20 COMPARATIVE EXAMPLE 3 Contact angles of various solvents were measured for a smooth PMMA polymeric surface prepared and modified with UV/ozone and heating at 150°C for 15s in the same way as in Example 3. The results are shown in Table 6. ;Table 6 ;Solvent/ink UV/Ozone Exposure Advancing Contact Receding Contact Angle (0) Contact Angle Hysteresis (°) Time (s) Angle (°) ethylene glycol 120 51 36 15 water 120 72 54 18 lanthanum nickelate precursor ink in 2-ethoxy ethanol, ethylene glycol, ethanolamine at 20:16:64 ratio (0.1M) 120 26 10 16 Figure 4 shows the results for ethylene glycol in comparison to those obtained using a similar method wherein PMMA alone is used as a polymeric layer instead of PMMA/PS as described herein. ;References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. The entirety of each of these references is incorporated herein. *

Claims (3)

1. Claims: 1. A method for coating a substrate, the method comprising the steps of: (a) mixing poly(styrene), poly(methyl methacrylate) and a solvent to form a polymer solution; (b) applying the polymer solution to the substrate to form a polymer solution coating; (c) drying the polymer solution coating to form a polymeric layer on the substrate; (d) exposing the polymeric layer to a reactive polar species, to form a modified polymeric layer on the substrate.
2. 2. A method according to claim 1, wherein in step (d) the polymeric layer is exposed to UV/ozone.
3. 3. A method according to claim 1, wherein in step (d) the polymeric layer is exposed to 02 plasma 4. A method according to any of claims 1 to 3, comprising the further step of: (e) heating the modified polymeric layer at 100 to 200°C for 5 to 30 seconds.5. A method for forming a printed substrate, the method comprising the steps of: (i) forming a coated substrate by a method according to any one of claims 1 to 4; and (ii) printing onto the coated substrate.6. A method according to claim 5 wherein step (ii) is carried out using an ink comprising a polar solvent.7. A method according to claim 6 wherein the polar solvent is a mixture of 2-ethoxyethanol (2EE), ethylene glycol (EG) and ethanolamine (EA) A method according to any of claims 5 to 7 wherein the ink comprises a metal oxide.9. A method according to any of claim 5 to 8, comprising the further step of: (iii) heating the printed substrate to a temperature of 350°C or greater, to remove the polymeric layer.10. A method according to any of claims Ito 9 wherein the substrate is a silicon wafer.11. A method according to any of claims 1 to 10 wherein the substrate used in step (b) has a pre-printed layer on the surface onto which the polymer solution is applied in step (b).12. A substrate obtainable by a method according to any one of claims 1 to 11.13. A substrate according to claim 12 wherein the polymeric layer has a thickness of 10 nm or less 14. A substrate according to any of claims 12 to 13 wherein the distance between phase separated islands is <1 pm.