Ultra fast oleophobic-hydrophilic switching surfaces

Abstract

The invention provides a coating which exhibits fast oleophobic-hydrophilic switching behaviour with, for example, equilibration of high oil contact angle (hexadecane=80) and low water contact angle (<10) values which occur within 10 s of droplet impact. These optically transparent surfaces display excellent anti-fogging and self-cleaning properties. The magnitude of oleophobic-hydrophilic switching can be further enhanced by the incorporation of surface roughness and in one embodiment the coating is applied to a surface in the form of a mesh in order to form an effective filter.

Claims

1. A coating which is oleophobic and hydrophilic with a switching parameter magnitude of at least 60 and a switching speed of less than 10 seconds.

2. A coating according to claim 1 wherein the coating has a switching parameter magnitude of at least 90.

3. A coating according to claim 1 wherein the coating has a switching speed of less than 1 second.

4. A coating according to claim 1 wherein the coating comprises a co-polymer-fluorosurfactant complex.

5. A coating according to claim 4 wherein the coating forms a fast switching oleophobic-hydrophile polyelectrolyte fluorosurfactant surface.

6. A coating according to claim 4 wherein the co-polymer-fluorosurfactant complex is dipped or spin coated.

7. A coating according to claim 6 wherein the co-polymer-fluorosurfactant complex is by dipped or spin coated from a dimethylformamide (DMF) solvent.

8. A coating according to claim 7 wherein the coating has a surface with an AFM RMS roughness in the range of 1-5 nm.

9. A coating according to claim 8 wherein the surface is prepared for use as a coating with at least one anti-fogging application and self-cleaning applications.

10. A coating according to claim 1 wherein the coating includes a mixed solvent which is used to create a roughened surface effect on the coating.

11. A coating according to claim 10 wherein the said co-polymer-fluorosurfactant complex is applied by spin coating a dimethylformamide methanol solvent mixture.

12. A coating according to claim 11 wherein the roughness of the surface of the coating is increased by allowing the evaporation of at least one components of a mixed solvent mixture that is used to form the coating, as the coating dries.

13. A coating according to claim 1, wherein the coating further comprises an article which acts as a base.

14. A coating according to claim 13 wherein the base is a mesh, the mesh is configured to treat a material including a first component and a second different component, and at least one component of the material remains on the coating surface to be subsequently removed therefrom and the second different component of the material passes over the coating and hence through the mesh.

15. A coating according to claim 14 further comprising another layer of material, wherein the material which passes through the mesh is collected as a fluid or absorbed into the another layer of material.

16. A coating according to claim 13, wherein the base is a filter further comprising the mesh being part of a filtration system.

17. A coating according to claim 16 wherein the filtration system has a number of filters, at least one of which includes a mesh to which the coating is applied.

18. A coating according to claim 5, wherein the oleophobic-hydrophilic polyelectrolyte-fluorosurfactant surfaces is created by utilising a maleic anhydride co-polymer.

19. A coating according to claim 1, wherein the coating is a polymer-surfactant complex.

Description

(1) Specific embodiments of the invention are now described with reference to the accompanying figures wherein:

(2) FIG. 1 illustrates microlitre water and hexadecane droplets dispensed onto copolymer spin coated from acetone solvent and copolymer-fluorosurfactant complex surfaces spin coated from dimethylformamide solvent.

(3) FIG. 2 illustrates microlitre droplet oil static contact angles and contact angle hysteresis on poly(ethylene-alt-maleic anhydride)-fluoro surfactant complex surfaces spin coated from dimethylformamide solvent as a function of liquid alkane chain length.

(4) FIG. 3 illustrates hexadecane, octane, olive oil, and motor oil droplets (left to right) on: (a) uncoated glass slide; and (b) poly(ethylene-alt-maleic anhydride)-fluorosurfactant complex surface solvent cast from dimethylformamide.

(5) FIG. 4 illustrates a demonstration of anti-fogging following exposure to water vapour (fogging): on uncoated glass slide and poly(ethylene-alt-maleic anhydride)-fluorosurfactant complex solvent cast from dimethylformamide.

(6) FIG. 5 illustrates a demonstration of self-cleaning: (a) uncoated glass slide and poly(ethylene-alt-maleic anhydride)-fluorosurfactant complex coating solvent cast from dimethylformamide fouled with hexadecane; and (b) after quick rinse with water.

(7) FIG. 6 illustrates (a) AFM height images and RMS roughness values for poly(styrene-co-maleic anhydride)-fluorosurfactant complex surfaces spin coated from different vol % dimethylformamide-methanol solutions; (b) AFM RMS roughness and hexadecane-water static contact angle switching parameter of poly(styrene-co-maleic anhydride)-fluorosurfactant complex surfaces as a function of dimethylformamide-methanol solvent mixture composition; and (c) correlation between hexadecane-water static contact angle switching parameter of poly(styrene-co-maleic anhydride)-fluorosurfactant complex surfaces and AFM RMS roughness.

(8) FIG. 7 illustrates a demonstration of oil-water separation: hexadecane-water mixture dispensed onto (a) uncoated stainless steel mesh; (b) stainless steel mesh dip coated with poly(styrene-co-maleic anhydride)-fluorosurfactant complex in 33 vol % dimethylformamide-66 vol % methanol solvent mixture; and (c) inclined coated stainless steel mesh dip coated with poly(styrene-co-maleic anhydride)-fluorosurfactant complex in 33 vol % dimethylformamide-66 vol % methanol solvent mixture acting as oil-water separator (oil and water are shown to be collected into separate beakers).

(9) FIG. 8 illustrates oleophobic-hydrophilic switching parameters for nominally flat surfaces reported in the literature: (a) Zhang, (b) Antonietti, (c) Turri, (d) Youngblood, (e) Sawada, (f) Badyal, (g) Sawada, (h) Badyal, (i) poly(ethylene-alt-maleic anhydride)-fluorosurfactant (RMS=1.10.3 nm), (j) poly(styrene-alt-maleic anhydride)-fluorosurfactant (RMS=2.70.3 nm), (k) poly(styrene-co-maleic anhydride)-fluorosurfactant (smooth RMS=5.31 nm), and (l) poly(styrene-co-maleic anhydride)-fluorosurfactant (rough RMS=2463 nm).

(10) In accordance with the invention, polished silicon (100) wafers (Silicon Valley Microelectronics, Inc.) and glass slides (Academy Science Ltd.) were used as flat substrates. Poly(ethylene-alt-maleic anhydride) (Vertellus Specialties Inc.), poly(styrene-alt-maleic anhydride) (Apollo Scientific Ltd.), or poly(styrene-co-maleic anhydride) (Polyscope Polymers BV) were dissolved in acetone (+99.8%, Sigma Aldrich Ltd.) at a concentration of 2% (w/v). The cationic fluorosurfactant (Zonyl FSD, DuPont Ltd.) employed for complexation was dissolved in high purity water at a concentration of 5% (v/v) and then added to the copolymer solution. The precipitated solid was collected from the liquid phase and dissolved at a concentration of 2% (w/v) in dimethylformamide (99%, Fisher Scientific UK Ltd.) for preparation of smooth surfaces and, in the case of the poly(styrene-co-maleic anhydride)-fluorosurfactant complex, varying composition dimethylformamide-methanol (99%, Sigma Aldrich Ltd.) solvent mixtures were utilised to produce rough surfaces. Spin coating was carried out using a photoresist spinner (Cammax Precima) operating at 2000 rpm. For the oil-water separation experiments, stainless steel mesh (0.16 mm wire diameter, 0.20 mm square holes, The Mesh Company Ltd.) was dip coated in the copolymer-fluorosurfactant complex solution and the solvent was allowed to evaporate.

(11) Glass transition temperatures of the copolymer and copolymer-fluorosurfactant complexes were measured by differential scanning calorimetry (DSC, Pyris 1, Perkin Elmer Inc.).

(12) Microlitre sessile drop contact angle analysis was carried out with a video capture system (VCA2500XE, AST Products Inc.) using 1.0 L dispensation of de-ionised water (BS 3978 grade 1), hexadecane (99%, Sigma Aldrich Ltd.), tetradecane (+99%, Sigma Aldrich Ltd.), dodecane (99%, Sigma Aldrich Ltd.), decane (+99%, Sigma Aldrich Ltd.), octane (+99%, Sigma Aldrich Ltd.), heptane (99%, Sigma Aldrich Ltd.), hexane (+99%, Sigma Aldrich Ltd.), and pentane (+99%, Sigma Aldrich Ltd.). Advancing and receding contact angles were measured by respectively increasing and decreasing the droplet size until the contact line was observed to move. Oil repellency was further tested using motor engine oil (GTX 15W-40, Castrol Ltd.) and olive cooking oil (Tesco PLC). Switching parameters were determined by calculating the difference between equilibrium hexadecane and water contact angles.

(13) Atomic force microscopy (AFM) images were collected in tapping mode at 20 C. in ambient air (Nanoscope III, Digital Instruments, Santa Barbara, Calif.) using a tapping mode tip with a spring constant of 42-83 N m.sup.1 (Nanoprobe). Root-mean-square (RMS) roughness values were calculated over 100100 m scan areas.

(14) Anti-fogging was tested by exposing the coated surfaces to a high purity water spray from a pressurised nozzle (RG-3L, Anest Iwata Inc.). Self-cleaning was tested by dispensing oil droplets onto a surface followed by rinsing with high purity water. Oil-water separation was tested by pouring an agitated mixture of oil and water over stainless steel mesh which has been dip coated with copolymer-fluorosurfactant complex. Oil Red O (75% dye content, Sigma Aldrich Ltd) and Procion Blue MX-R (35% dye content, Sigma Aldrich Ltd.) were employed as oil and water dispersible dyes respectively in order to enhance visual contrast (similar results were obtained in absence of dye).

(15) In the present invention, fast-switching oleophobic-hydrophilic polyelectrolyte-fluorosurfactant surfaces were created by utilising three different maleic anhydride copolymers, shown in Scheme 1, below. In order to systematically investigate the role of polymer backbone structure, these comprised poly(ethylene-alt-maleic anhydride) alternating copolymer as a reference standard (based on previously reported polyelectrolyte-fluorosurfactant switching studies); poly(styrene-alt-maleic anhydride) where the aforementioned alternating copolymer ethylene segments are replaced with styrene segments; and finally poly(styrene-co-maleic anhydride), which is a copolymer comprising singe maleic anhydride units alternating with styrene block segments (because maleic anhydride does not homopolymerise).

(16) ##STR00001##

(17) With regard to surface switching of the costing the results show, via Differential scanning calorimetry (DSC), that the poly(ethylene-alt-maleic anhydride) copolymer has a higher glass transition temperature compared to the poly(styrene-alt-maleic anhydride), which can be attributed to the larger molecular weight of the former and less ordering due to the stiff and bulky styrene groups for the latter, see Table 1 below. In the case of the poly(styrene-co-maleic anhydride) copolymer, the presence of a single glass transition temperature is consistent with block styrene segments alternating with single maleic anhydride units (since a plausible alternative diblock copolymer structure should display two respective glass transition temperatures), Scheme 1. Also, its higher glass transition temperature compared to the poly(styrene-alt-maleic anhydride) alternating copolymer stems from a combination of higher molecular weight and favourable intermolecular interactions between adjacent styrene units contained within the block styrene segments.

(18) TABLE-US-00001 TABLE 1 Glass transition temperatures of copolymers and copolymer-fluorosurfactant complexes Glass Transition Maleic Temperature/ C. Anhydride Molecular Copolymer- Content/ Weight/g Co- Fluorosurfactant Copolymer wt. % mol.sup.1 polymer Complex Poly(ethylene-alt- 50 60,000 155 157 maleic anhydride) Poly(styrene-alt- 50 50,000 120 131 maleic anhydride) Poly(styrene-co- 26 80,000 160 138 maleic anhydride)

(19) Following fluoro surfactant complexation, both the poly(ethylene-alt-maleic anhydride) and poly(styrene-alt-maleic anhydride) copolymer-fluoro surfactant complexes display raised glass transition temperatures, which suggests a greater degree of ordering upon surfactant complexation, and is consistent with previous studies relating to copolymer-surfactant complex systems, Table 1.

(20) In contrast, for the poly(styrene-co-maleic anhydride)-fluorosurfactant complex, the glass transition temperature is lower compared to that of the parent copolymer; this may be due to disruption of the favourable intermolecular interactions between adjacent styrene units contained within the block segments (something which is absent for the parent alternating copolymers).

(21) Spin coating of all three copolymer-fluorosurfactant complexes dissolved in dimethylformamide (DMF) onto silicon wafers and glass slides produced smooth films (AFM RMS roughness=1-5 nm), see Table 2 below. In all cases, a time period of 10 s was sufficient for the water contact angles to reach their final static values (in fact, the poly(styrene-alt-maleic anhydride)-fluorosurfactant system underwent instantaneous water wetting); whereas hexadecane droplets remained stationary, FIG. 1 and Table 2. Copolymer-fluorosurfactant complex surfaces that were prepared using an alternative quaternary ammonium cationic fluorosurfactant (S-106A, Chemguard) displayed similar oleophobic-hydrophilic switching behaviour. This was also found to be the case for copolymer-fluorosurfactant complex surfaces created using a cationic copolymer (poly(styrene-alt-maleimide), SMA 10001, Cray Valley HSC) and an anionic fluorosurfactant (Capstone FS-63, Dupont Ltd.). Control experiments utilising any of the parent copolymers (in the absence of fluorosurfactant complexation) showed the converse trend, with an absence of superhydrophilicity and instantaneous spreading of hexadecane droplets, shown in Table 2, below.

(22) TABLE-US-00002 TABLE 2 Microlitre water and hexadecane static contact angles for copolymer spin coated from acetone solvent; copolymer-fluorosurfactant complex surfaces (smooth) spin coated from dimethylformamide (DMF) solvent; and poly(styrene-co- maleic anhydride)-fluorosurfactant complex surfaces (rough) spin coated from 33 vol % DMF-66 vol % methanol. Water droplets were allowed to relax for 10 seconds to reach equilibrium prior to final static contact angle measurement. No relaxation in contact angle was observed for hexadecane droplets. AFM surface roughness values are included for comparison. Static Water AFM RMS Contact Roughness/ Angle/ Hexadecane Contact Angle/ nm 0 s 10 s Static Advancing Receding Hysteresis Poly(ethylene- 4.4 1 38 2 22 2 Wets alt-maleic anhydride) Poly(ethylene- 1.1 0.3 88 2 <10 74 1 76 2 72 2 4 2 alt-maleic anhydride)- fluorosurfactant Poly(styrene-alt- 6.7 1 68 2 66 2 Wets maleic anhydride) Poly(styrene-alt- 2.7 0.3 <10 <10 80 2 85 2 66 2 19 2 maleic anhydride)- fluorosurfactant Poly(styrene-co- 10.3 1 90 2 90 2 Wets maleic anhydride) Poly(styrene-co- 5.3 1 36 2 23 2 80 2 88 2 66 2 22 2 maleic anhydride)- fluorosurfactant Poly(styrene-co- 246 3 <10 <10 112 5 125 5 <10 >115 maleic anhydride)- fluorosurfactant 33 vol % DMF- 66 vol % methanol

(23) Oil repellence of the poly(ethylene-alt-maleic anhydride)-fluorosurfactant complex surfaces was found to improve (higher contact angle and lower hysteresis) with increasing hydrocarbon length of straight chain alkane droplets, FIG. 2. A similar trend was observed for both of the poly(styrene-maleic anhydride)-fluorosurfactant complex surfaces. Furthermore, olive oil and motor engine oil spreading were shown to be inhibited on all three types of copolymer-fluorosurfactant complex surfaces, FIG. 3. Note, hexadecane and octant droplets are dyed with Oil Red O (Sigma Aldrich Ltd) to show contrast (similar results were obtained in the absence of dye).

(24) Extremely low water contact angles are highly desirable for anti-fogging applications. Copolymer-fluorosurfactant complex dip coated glass slides using dimethylformamide solvent were found to retain their transparency (anti-fogging) during water vapour exposure, FIG. 4.

(25) Self-cleaning properties were demonstrated by rinsing off fouling oils with just water, FIG. 5. This is consistent with the high receding contact angle measured for hexadecane, Table 2.

(26) Further enhancement of the oleophobic-hydrophilic surface switching behaviour was investigated for the poly(styrene-co-maleic anhydride)-fluorosurfactant system by varying the casting solvent mixture composition, FIG. 6. Diluting dimethylformamide with methanol gives rise to an increase in surface roughness, which is attributable to the poor solubility of the styrene block segments in methanol. This solvent-induced roughness lowers the static water contact angle (<10) whilst concurrently raising the static hexadecane contact angle (>110), to yield a hexadecane-water switching parameter exceeding 100, FIG. 6. Control experiments showed a lack of surface roughness enhancement by varying the dimethylformamide-methanol solvent composition for poly(ethylene-alt-maleic anhydride)-fluorosurfactant and the poly(styrene-alt-maleic anhydride)-fluorosurfactant complex solutions, which is consistent with the absence of low methanol solubility styrene block segments being present in the alternating copolymer structures, Scheme 1.

(27) Oil-water separation efficacy was tested using copolymer-fluorosurfactant complex coatings dip coated onto stainless steel mesh. These were then suspended over a sample vial followed by dispensing an agitated oil-water mixture. The water component was observed to pass through the mesh whilst the oil (hexadecane) remained suspended on the mesh surface, FIG. 7. These meshes were then inclined at an angle, and pouring the oil-water mixture over them yielded separation efficiencies as high as 98% in the case of the poly(styrene-co-maleic anhydride)-fluorosurfactant complex surface (attributable to the dimethylformamide-methanol solvent mixture induced roughness enhancement of the oil-water switching parameter), FIG. 7 and Table 3. Similar behaviour was observed for octane- and motor oil-water mixtures. Hexadecane was dyed with Oil Red O (Sigma Aldrich Ltd.) to show contrast (similar results were obtained in the absence of dye). The absence of solvent induced roughness resulted in lower oil-water separation efficiencies for the two alternating copolymer-fluorosurfactant complex systems.

(28) TABLE-US-00003 TABLE 3 Oil-water separation efficiencies for copolymer- fluorosurfactant complex dip coated stainless steel mesh from 33 vol % dimethylformamide-66 vol % methanol solvent mixtures. AFM RMS Oil-Water Separation Switching Surface Roughness/nm Efficiency.sup.a/% Poly(ethylene-alt-maleic 1.1 0.3 0 anhydride) + fluorosurfactant Poly(styrene-alt-maleic 2.7 0.3 48 4 anhydride) + fluorosurfactant Poly(styrene-co-maleic 246 3 98 2 anhydride) + fluorosurfactant .sup.a100% efficiency corresponds to complete separation of water from hexadecane.

(29) Previously reported polymer-fluoro surfactant complex surfaces which display oleophobic-hydrophilic switching behaviour rely on the inherent hydrophilicity of the base polymer. For instance, in the case of solvent cast ionic polymer-fluorosurfactant complex surfaces, the fluorinated surfactant tails segregate at the air-solid interface, thereby aligning the hydrolysed counterionic groups towards the near-surface region as a consequence of their strong electrostatic attraction towards the ionic surfactant head. This interfacial interaction leads to an enhanced concentration of hydrophilic groups in the near-surface region compared to the parent polymer. It has been proposed that such polymer-fluorosurfactant surfaces are able to exhibit oleophobic-hydrophilic switching behaviour due to the existence of defect sites or holes at the fluorinated surfactant tail air-solid interface through which water molecules can penetrate down towards the complexing counterion hydrophilic sub-surface. This description helps to explain why all three copolymer-fluorosurfactant complex systems in the present study display lower final static water contact angles compared to their parent base copolymers, FIG. 1 and Table 2.

(30) The oleophobic-hydrophilic behaviour of such polymer-fluorosurfactant complex surfaces can be quantified in terms of a switching parameter (for instance, the difference in measured static contact angle between hexadecane and water droplets), FIG. 8. Most previous studies have tended to quote water contact angles only after allowing the droplet to stabilise over several minutes on the surface because of the slow rate at which water molecules penetrate through towards the hydrophilic sub-surface to manifest surface switching (although the surface initially is hydrophobic). In the present invention, the time taken to reach a final static water contact angle is much shorter (<10 s) for all copolymer-fluorosurfactant systems. Furthermore, both styrene-containing copolymer-fluorosurfactant complex surfaces reach a final static water contact angle value much quicker than their ethylene-containing copolymer counterpart due to the bulky styrene side group providing a lower packing efficiency for the former, and thereby facilitating a faster penetration of water into the hydrophilic sub-surface, FIG. 1. This explanation is consistent with the styrene-based copolymer-fluoro surfactant complexes having lower glass transition temperatures, Table 1. In addition, for the case of the poly(styrene-alt-maleic anhydride) copolymer, the more disordered nature of the alternating styrene side groups provides a greater level of polymer chain mobility, which allows the fluorinated alkyl chains to reorient themselves more readily at the solid-air interface (culminating in instantaneous water wetting and high hexadecane contact angle values, FIG. 1 and Table 2).

(31) The high receding hexadecane contact angle and low surface roughness of copolymer-fluoro surfactant complex surfaces spin coated from dimethylformamide solvent make them ideal for self-cleaning and anti-fog applications, Table 2 and FIGS. 3-5. Such surfaces are easily cleaned by rinsing in water (which replaces the oil-solid interaction with a much more favourable water-solid interaction, i.e. switching).

(32) Dissolving the poly(styrene-co-maleic anhydride)-fluorosurfactant complex in a dimethylformamide-methanol solvent mixture prior to film formation enhances surface roughness due to the poor solubility of the styrene block segments in methanol. This surface roughness is capable of improving hydrophilicity due to increased surface area (Wenzel wetting) and oleophobicity due to the ability to trap air (Cassie-Baxter wetting), Table 2. A key advantage of this approach is that it circumvents the need for introducing roughness as a separate step through the incorporation of additional materials or by mixing roughening particles into the copolymer-fluorosurfactant complex solution. It is envisaged that a range of different solvents or coating methods (e.g. spray coating) may be used to introduce surface roughness for the enhancement of the switching parameter for other types of polymer-surfactant complex systems.

(33) Coating of steel mesh with such roughened poly(styrene-co-maleic anhydride)-fluorosurfactant complex surfaces (prepared from dimethylformamide-methanol solvent mixtures) provides two length scales of roughness (steel mesh pores plus solvent-induced film roughness) both of which help to lower oil contact angle hysteresis (improve oil repellency). When combined with the inherent high switching parameter, oil-water separation with >98% efficiency is attained, Table 3. This performance matches existing oleophobic-hydrophilic systems for oil-water separation (which however tend to be far more complex in nature and fabrication methods). Although there are more efficient separation processes (99.999% efficiency) based on membrane filtration where small pores allow the passage of water whilst blocking oils, such filters have low volume throughput and can be easily clogged with excess oil (requiring cleaning or replacement).

(34) One embodiment of the current methodology would be to deploy it for pre-treatment filters installed upstream of conventional membrane filters, thereby ensuring removal of the majority of oil-based contaminants so as to minimise the amount of oil reaching the membrane filters (and therefore avoid blockage as well as maximise efficiency). Such oil-water separators could potentially help to tackle the environmental impact of the gas, oil, metal, textile, and food processing industries.

(35) Solvent cast copolymer-fluorosurfactant complexes have been found to display large magnitude oleophobic-hydrophilic switching behaviour as well as rapid switching speeds. Further enhancement in switching performance is achieved by combining surface chemical functionality and roughness. These ultra-fast switching oleophobic-hydrophilic surfaces have been shown to display excellent anti-fog, self-cleaning, and oil-water separation properties.

(36) Thus smooth copolymer-fluorosurfactant complex film surfaces are found to exhibit fast oleophobic-hydrophilic switching behaviour. Equilibration of high oil contact angle (hexadecane=80) and low water contact angle (<10) values occurs within 10 s of droplet impact. These optically transparent surfaces display excellent anti-fogging and self-cleaning properties. The magnitude of oleophobic-hydrophilic switching can be further enhanced by the incorporation of surface roughness to an extent that it reaches a sufficiently high level (water contact angle<10 and hexadecane contact angle>110) which, when combined with the ultra-fast switching speed, yields oil-water mixture separation efficiencies exceeding 98%.