WO2017123881A1 - Permanently grafted glaciophobic nanomaterials and methods of making - Google Patents

Abstract

The present disclosure provides for substrates for glaciophobic polymer-nanoparticle composites, glaciophobic polymer-nanoparticle composites, methods of making glaciophobic polymer-nanoparticle composites, methods of making films on substrates, and the like.

Description

PERMANENTLY GRAFTED GLACIOPHOBIC NANOMATERIALS

AND METHODS OF MAKING

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application Serial No. 62/278,067, having the title "PERMANENTLY GRAFTED GLACIOPHOBIC NANOMATERIALS AND METHODS OF MAKING," filed on January 13, 2016, the disclosure of which is incorporated herein in by reference in its entirety.

BACKGROUND

Ice accumulation on surfaces is problematic in daily life as it can cause breakage of power transmission lines, freezing on roads, and, as any homeowner can attest, buildup of ice inside of domestic freezers as well as issues with aircraft wings and wind turbines.

However, current technologies to address ice accumulation are not satisfactory due to deficiencies their inability to function effectively in harsh conditions and environments.

SUMMARY

Embodiments of the present disclosure provide substrates for glaciophobic polymer- nanoparticle composites, glaciophobic polymer-nanoparticle composites, methods of making glaciophobic polymer-nanoparticle composites, methods of making films on substrates, and the like.

An embodiment of the present disclosure includes a substrate with a film of a glaciophobic polymer-nanoparticle composite bound to the surface of the substrate. The glaciophobic polymer-nanoparticle composite has a first state which includes a polymer and nanoparticles, wherein the polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety. Prior to exposure to UV radiation, the substrate contains C-H bonds on its surface and the nanoparticle contains C-H bonds on its surface; after exposure to UV radiation, the glaciophobic polymer-nanoparticle composite has a second state including a first portion of photo cross-linkable moieties that are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross-linkable moieties that are covalently bonded to the nanoparticle through C-C bonds upon H abstraction.

An embodiment of the present disclosure also includes a composition comprising a glaciophobic polymer-nanoparticle composite in a first state, wherein the glaciophobic polymer-nanoparticle composite includes a polymer and nanoparticles dispersed in the polymer, wherein the polymer includes a hydrocarbon backbone that contains a photo cross- linkable moiety, a fluorinated moiety, and an alkyl moiety. An embodiment of the present disclosure also includes methods of making a film on a substrate, by exposing the substrate containing C-H bonds on its surface to a glaciophobic polymer-nanoparticle composite in a first state. Said glaciophobic polymer-nanoparticle composite includes a polymer and nanoparticles dispersed in the polymer. The polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety, wherein the nanoparticles contain C-H bonds on their surface. A film of the glaciophobic polymer-nanoparticle composite is formed in a second state that is bound to the surface of the substrate. After exposure to UV radiation, a first portion of the photo cross-linkable moieties are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross-linkable moieties are covalently bonded to the nanoparticle through C-C bonds upon H abstraction.

An embodiment of the present disclosure also includes a method of making a film on a substrate, by exposing the substrate containing C-H bonds on its surface to a glaciophobic polymer-nanoparticle composite in a first state. The glaciophobic polymer-nanoparticle composite includes a polymer and nanoparticles dispersed in the polymer. Said polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety, and the nanoparticles contain C-H bonds on their surface. A film of the glaciophobic polymer-nanoparticle composite is formed in a second state that is bound to the surface of the substrate. After exposure to UV radiation, a first portion of the photo cross-linkable moieties are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross-linkable moieties are covalently bonded to the nanoparticle through C-C bonds upon H abstraction.

Other compositions, structures, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional compositions, structures, methods, features and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings.

Figure 1 shows BP conversion of p(BP/F/iBA) and p(BP/F/nBA) at experimental temperature of 20 °C and 50 °C as monitored via UV/Vis Spectrometry of the decreasing intensity of the η-ττ* absorbance at 260 nm (Inset: Spectra for p(BP/F/iBA) at 20 °C).

Figure 2 is FTIR spectra of p(BP/F/iBA) film on Si0₂ (bottom) before and (top) after UV exposure. Figures 3A-E is a morphology study of p(iBP/F/iBA)/nanoparticle composite on alkylated Si02 substrates. Fig. 3A is a representative top view SEM images. Fig. 3B illustrates a zoom-in SEM image of an asperity. Fig. 3C is a side view SEM image of coated substrate cross section. Fig. 3D illustrates a surface profile thickness of the coated surface. Fig. 3E illustrates a schematic cross-sectional profile of water in contact with a composite coated surface that illustrates the Cassie-Wenzel transition.

Figure 4 shows optical images taken after the icing experiment that demonstrates the anti-icing property of p(BP/F/iBA)/nanoparticle composite coated silicon wafer before (left) and after (right) abrasion.

Figure 5 shows the relationship of the factor of free energy barrier between heterogeneous nucleation on spherical surface and homogeneous nucleation (f) versus nanoparticle radius (R).

Figures 6A-B are SEM images of p(iBP/F/iBA) / nanoparticle composites coated on Si0₂ substrates with different polymer / nanoparticle ratios. The mass ratio of polymer / nanoparticle is (Fig. 6A) 2 and (Fig. 6B) 2.8 (w/w).

Figures 7A-B Abrasion study of polymer/nanoparticle composites coated Si0₂ substrates. Optical images of substrates coated with (Fig. 7B) p(hexafluorobutyl

methacrylate) and (Fig. 7B) p(BP/F/iBA)/ nanoparticle composite after one cycle of abrasion. Inset: image of water droplet on the substrate.

Figures 8A-C demonstrate the robustness of p(BP/F/iBA) and p(BP/F/nBA) nanocomposites against abrasion: SEM images of (Fig. 8A) p(BP/F/iBA) and (Fig. 8B) p(BP/F/nBA) nanocomposites coated Si0₂ surfaces after 28 abrasion cycles (dashed box: abraded area); (Fig. 8C) Plot of mechanical abrasion cycles and static water CA after every other abrasion cycle for p(BP/F/iBA) and p(BP/F/nBA) coated composites.

Figures 9A-B are optical images taken after the icing experiment that demonstrates the anti-icing property of p(BP/F/iBA)/nanoparticle composite coated polypropylene substrate before (Fig. 9A) and after (Fig. 9B) abrasion.

Figure 10 is the size distribution (measured by dynamic light scattering) of the particles (-60 nm) that are used in the composite.

Figures 11 A and 11 B illustrate embodiments of the polymer of the glaciophobic polymer-nanoparticle composite in the first state prior to exposure to UV irradiation.

Fig. 12 illustrates a schematic of the glaciophobic polymer-nanoparticle composite in the second state after to exposure to UV irradiation where C-C bonds are formed with the polymer, the nanoparticles and the surface of the substrate. DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, and as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, material science, and the like, which are within the skill of the art.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the probes disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 °C and 1 atmosphere.

Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence where this is logically possible.

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.

Discussion:

Embodiments of the present disclosure provide for compositions including

glaciophobic polymer-nanoparticle composite, methods of forming a film including the glaciophobic polymer-nanoparticle composite, substrates including the glaciophobic polymer-nanoparticle composite, and the like. Embodiments of the present disclosure can be used as anti-icing agents and may be advantageous over other technologies due to the ability of the glaciophobic polymer-nanoparticle composite to form covalent C-C bonds with substrates having surfaces including C-H groups prior to reaction with the glaciophobic polymer-nanoparticle composite. The covalent bonding allows the glaciophobic polymer- nanoparticle composite to have high durability to better handle the harsh environmental conditions that often accompany icing conditions.

In general, embodiments of the present disclosure include glaciophobic polymer- nanoparticle composites including a polymer and nanoparticles, where the nanoparticles include C-H groups on its surface prior to UV irradiation. In an embodiment, the copolymers can include a photo cross-linkable moiety (e.g., benzophenone), a fluorinated moiety (e.g., hexafluorobutyl), and an alkyl moiety, where under mild UV irradiation, generate a densely cross-linked network of polymer and well-dispensed nanoparticles, where the nanoparticles and surface of the substrate are covalently bound to one another to form a film composed of glaciophobic polymer-nanoparticle composite material. Investigations into the embodiments revealed icephobicity after Taber testing with multiple abrasion cycles using a 300 g load, demonstrating excellent mechanical resistance.

Now having described the composition in general, the composition includes the glaciophobic polymer-nanoparticle composite. The glaciophobic polymer-nanoparticle composite can be in a first state or a second state. In the first state, the glaciophobic polymer-nanoparticle composite includes a polymer and nanoparticles. In the second state, the glaciophobic polymer-nanoparticle composite, after UV irradiation, forms C-C bonds with the nanoparticles. In addition, upon UV irradiation C-C bonds can be formed with the substrate having a C-H surface. The composite also forms covalent bonds with itself via inter or intramolecular crosslinking with other polymers. In an embodiment of the glaciophobic polymer-nanoparticle composite in the first state, the polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety as branches off of the hydrocarbon backbone. In the second state, the photo cross- linkable moiety forms C-C bonds with the nanoparticle and/or the surface of the substrate via H abstraction. In an embodiment, the polymer can have a molecular weight of about 2000 kDa to 1 ,000,000 kDa.

In an embodiment, the hydrocarbon backbone can include multiple different repeat units. In an embodiment, the hydrocarbon backbone includes three different repeat units each different repeat unit including one of the photo cross-linkable moiety, the fluorinated moiety, and the alkyl moiety. The hydrocarbon backbone can be linear or branched and can be cross-linked. The hydrocarbon backbone can be a random copolymer, alternating copolymer, a block copolymer or the like. In an embodiment, one or more of the units can be a methacrylate unit, where the methacrylate unit can include one of the photo cross- linkable moiety, the fluorinated moiety, or the alkyl moiety or where different methacrylate unit includes the photo cross-linkable moiety, the fluorinated moiety, or the alkyl moiety. In an embodiment, one or more of the units can be an acrylate unit, where the acrylate unit can include one of the photo cross-linkable moiety, the fluorinated moiety, or the alkyl moiety or where different acrylate unit includes the photo cross-linkable moiety, the fluorinated moiety, or the alkyl moiety. In still other embodiments, the repeat unit can be a styrene derivative, an acrylamide derivative, a methacrylamide derivative, or a combination thereof.

As stated above, upon UV irradiation, H abstraction occurs and a C-C bond between the photo cross-linkable moiety and the nanoparticle (e.g., having a C-H on the surface) and/or the surface of the substrate (e.g., having a C-H on the surface) to form the glaciophobic polymer-nanoparticle composite in the second state. In an embodiment of the glaciophobic polymer-nanoparticle composite in the first state, the photo cross-linkable moiety can include an aryl ketone (about 340 to 400 nm), an aryl azide group (about 250 to 450 nm or about 350 to 375 nm), a diazirine group (about 340 to 375 nm), and the compound can include a combination of these groups. In an embodiment, the aryl ketone group can include benzophenone (about 340 to 380 nm), acetophenone (about 340 to 400 nm), a naphthylmethylketone (about 320 to 380 nm), a dinaphthylketone (about 310 to 380 nm), a dinaphtylketone derivative (about 320 to 420 nm), or derivatives of each of these. In an embodiment, the photo cross-linkable moiety is a benzophenone group. In an embodiment, the aryl azide group can include phenyl azide, alkyl substituted phenyl azide, halogen substituted phenyl azide, or derivatives of each of these. In an embodiment, the diazirine group can include 3,3 dialkyl diazirine (e.g., 3,3 dimethyl diazirine, 3, 3 diethyl diazirine), 3,3 diaryl diazirine (e.g., 3,3 diphenyl diazirine), 3-alkyl 3-aryl diazirine, (e.g., 3- methyl-3-phenyl diazirine), or derivatives of each of these. One skilled in the art will understand the resulting structure of the glaciophobic polymer-nanoparticle composite in the second state after H abstraction of each of the various photo cross-linkable moieties. An example of H abstraction is illustrated in Fig. 1 1 when the photo cross-linkable moiety is benzophenone.

In an embodiment, the fluorinated moiety can include a linear or branched fluorinated hydrocarbon moiety (e.g., C1 to C10) including one or more F moieties, a cyclo hydrocarbon moiety (e.g., C4 to C12) including one or more F moieties, and the like. For example the alkyl moiety can be a fluorinated alkyl such as hexafluorobutyl; 2,2,3,3,4,4,5,5,6,6,7,7- Dodecafluoroheptyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9, 10, 10, 1 1 , 1 1 , 12, 12, 12- Heneicosafluorododecyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9, 10, 10, 10-Heptadecafluorodecyl methacrylate; 2,2,3,3,4,4,4-Heptafluorobutyl acrylate; 2,2,3,3,4,4,4-Heptafluorobutyl methacrylate; 2,2,3,4,4,4-Hexafluorobutyl acrylate; 2,2,3,4,4,4-Hexafluorobutyl methacrylate; 1 , 1 , 1 ,3,3,3-Hexafluoroisopropyl acrylate; 1 , 1 , 1 ,3,3,3-Hexafluoroisopropyl methacrylate; 2,2,3,3,4,4,5,5-Octafluoropentyl acrylate; 2,2,3,3,4,4,5,5-Octafluoropentyl methacrylate; 2,2,3,3,3-Pentafluoropropyl acrylate; 2,2,3,3,3-Pentafluoropropyl methacrylate;

1 H , 1 H,2H,2H-Perfluorodecyl acrylate; 2,2,3,3-Tetrafluoropropyl methacrylate;

3,3,4, 4,5,5,6,6, 7,7,8, 8, 8-Tridecafluorooctyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,8- Tridecafluorooctyl methacrylate; 2,2,2-Trifluoroethyl methacrylate; 1 , 1 , 1-Trifluoro-2- (trifluoromethyl)-2-hydroxy-4-methyl-5-pentyl methacrylate; or 2-[(1 ', 1 ', 1 '-Trifluoro-2'- (trifluoromethyl)-2'-hydroxy)propyl]-3-norbornyl methacrylate.

In an embodiment, the alkyl moiety can be a C1 to C10 moiety, in particular a C2-C4 moiety. In an embodiment, the alkyl moiety can be a C4 moiety such as n- or i-butyl moiety.

In an embodiment, the nanoparticle can be a silica nanoparticle, a metal

nanoparticle, a metal oxide nanoparticle, a metal composite nanoparticle (e.g., core/shell structure), and the like. In an embodiment, the metal nanoparticle or oxide thereof can include a transition metal such as copper, gold, silver, platinum, palladium, and oxides thereof. In an embodiment, the metal composite can include a core/shell structure where the core and shell are made of different materials such as silica, metal, metal oxides, and the like. In an embodiment, the nanoparticle can have a spherical shape, a substantially spherical shape, a polygonal shape, an irregular shape, and the like. In an embodiment, the nanoparticle has a longest dimension (e.g., diameter) of about 10 to 1000 nm, 10 to 500 nm, about 40 to 100 nm, or about 50 to 80 nm. In an embodiment, the polymer: nanoparticle ratio can be about 1 to 5, about 1 to 3, or about 1.5 to 2.5.

In an embodiment, the polymer, in the first state, as shown in Fig. 1 1 A, can have the following structure:

, where R1 is the photo cross-linkable moiety, R2 is the fluorinated moiety, R3 is a H or first alkyl group, R4 is the alkyl moiety, and R5 is a H or second alkyl group, and each of m, n, and o are independently 1 to 10,000, 1 to 1000, or 1 to 100. In an embodiment, R3 and R5 are independently selected from H, an aliphatic group (substituted or unsubstituted and/or linear, branched, or cyclic) (e.g., alkyl, alkenyl, alkynyl), an aryl group (substituted or unsubstituted), or a heteroaryl group (substituted or unsubstituted). In an embodiment, R3 and R5 are independently H or a C1 to C3 alkyl group (e.g., methyl group).

In an embod shown in Fig. 1 1 B, has the

following structure:

Now having described the glaciophobic polymer-nanoparticle composite, methods of forming a film on a substrate are now described. In general, the method of making the film on a substrate includes exposing the substrate containing C-H bonds on its surface to a glaciophobic polymer-nanoparticle composite in the first state. The film can be formed including the glaciophobic polymer-nanoparticle composite that is bound to the surface of the substrate. A first portion of the photo cross-linkable moieties are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross- linkable moieties are covalently bonded to the nanoparticle through C-C bonds upon H abstraction when the glaciophobic polymer-nanoparticle composite is in the second state.

Although not intending to be bound by theory, the interplay between the components can provide advantages. One advantage is that the appropriate polymer: nanoparticle ratio results in a better composite. Figure 6A illustrates that a nanoparticle:polymer composition can have the appropriate nanoscale and microscale roughness to provide an icephobic surface. Figure 6B illustrates that the nanoparticles are too embedded in the polymer matrix. The surface is still quite hydrophobic but ice can nucleate on this substrate. The ratio of nanoparticles to polymer is too low. Figure 6A has a polymer: nanoparticle ratio of 2. Figure 6B has a polymernanoparticle ratio of about 3 (2.8). Also, the size of the nanoparticles can be advantageously selected. Particles that are too small, or too large, can lead to surfaces where ice can nucleate. Particles of about 50-80 nm in size are advantageous, specifically for silica nanoparticles.

In an embodiment, the glaciophobic polymer-nanoparticle composite has a polymer: nanoparticle ratio of about 2 (e.g., about 1 .5 to 2.5, about 1 .75 to 2.25. about 1.9 to 2.1 , or about 2) with nanoparticles of about 50-80 nm, resulting in an appropriate nanoscale and microscale roughness to provide an icephobic surface without areas where ice can nucleate.

In an embodiment, the polymer of the glaciophobic polymer-nanoparticle composite in the first state functions to at least undergo a photochemical change to covalently bond with a surface or a layer on the surface of the article having a C-H group (e.g., the nanoparticle and/or the surface of the substrate). In an embodiment, the compound is covalently bonded via the interaction of the compound with a UV light (e.g., about 250 nm to 500 nm or about 340 to 370 nm) that causes a C-C bond to form between the compound and the surface having a C-H group or a layer on the surface having the C-H group. The UV light can be generated from a UV light source such as those known in the art.

In other words, the glaciophobic polymer-nanoparticle composite can be attached to the surface or the layer on the surface through a photochemical process (C-C bond formation through H abstraction) and as such is easy and inexpensive to achieve. Once the covalent bonds are formed, the compound layer is strongly bound to the surface and can withstand very harsh conditions such as extended washing steps, exposure to harsh environmental conditions (e.g., heat, cold, icing, humidity, lake, river, and ocean conditions (e.g., above and/or under water)), and the like.

In an embodiment, the surface of the substrate has C-H groups that can interact (e.g., form C-C bonds) with the glaciophobic polymer-nanoparticle composite upon exposure to UV light. In an embodiment, the substrate has a layer (also referred to as a

"functionalized layer") (e.g., a thin film or self-assembling layer) disposed on the surface of the structure. The functionalized layer includes C-H bonds that can interact (form C-C bonds) with the glaciophobic polymer-nanoparticle composite upon exposure to UV light. The substrate can be exposed to UV light in many different ways such as direct exposure to a UV light source, exposure to UV light during the spray coating process, exposure to UV light during the dip coating process, exposure to UV light during the spincoating process, exposure to UV light during dip padding, exposure to UV light during nip padding, exposure to UV light during kiss rolling, and exposure to UV light during the drop-casting process.

Either during application of the glaciophobic polymer-nanoparticle composite or once the glaciophobic polymer-nanoparticle composite is disposed on the surface of the substrate, UV light is directed onto the glaciophobic polymer-nanoparticle composite on the surface. As described above, the UV light causes a photochemical reaction to occur between the glaciophobic polymer-nanoparticle composite and the C-H groups on the surface and/or the nanoparticles to form one or more covalent bonds (C-C bonds) between the glaciophobic polymer-nanoparticle composite and the surface of the substrate and/or nanoparticle.

The wavelength of the UV light can be selected based on the photo cross-linkable moiety. In general, the UV light can be active to form the C-C bonds at about 250 to 500 nm, about 340 to 400 nm, or about 360 to 370 nm. The specific wavelength(s) that can be used for a particular photo cross-linkable moiety are described herein. In an embodiment, the UV light can be active to form the C-C bonds at a wavelength of about 340 to 370 nm. In an embodiment, the UV light can be active to form the C-C bonds at a wavelength of about 365 nm.

Once the process is complete, a substrate including a film of a glaciophobic polymer- nanoparticle composite bound in the second state is the result. Fig. 12 illustrates a schematic of the glaciophobic polymer-nanoparticle composite in the second state after to exposure to UV irradiation where C-C bonds are formed with the nanoparticles and the surface of the substrate. The film of a glaciophobic polymer-nanoparticle composite in the second state is bound to the surface of the substrate. A first portion of the photo cross- linkable moieties are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross-linkable moieties are covalently bonded to the nanoparticle through C-C bonds upon H abstraction. In an embodiment, the film can have a thickness of about 200 nm to 2 millimeters. In an embodiment, the

polymer: nanoparticle ratio is about 2. Embodiments of glaciophobic polymer-nanoparticle composites that can produce a combination of nanoscale and microscale roughness that is an advantageous result.

Example 1

Ice accumulation on surfaces is problematic in daily life as it can cause breakage of power transmission lines, freezing on roads, and, as any homeowner can attest, buildup of ice inside of domestic freezers.^1"3 It also raises potential issues in industrial applications such as disturbing the aerodynamics of aircraft wings and wind turbines, resulting in increased maintenance and repair costs every year.^4"5 The mechanism of ice formation under subzero conditions and the design of ice-repellent coatings has been an area of intense study for decades.^6"10 Such coatings are expected to minimize ice adhesion^11"14 or delay ice formation on surfaces.^{2, 8}' ^5"16 Superhydrophobic surfaces, which are characterized by a high water contact angle (CA) and low water contact hysteresis (CAH), have been observed to exhibit anti-icing characteristics.^{11"13, 7"18} Due to the small contact area between the freezing droplet and the rough surface, an air layer is formed at the interface, reducing the ice-solid interaction under superhydrophobic conditions. Two approaches have been utilized to fabricate anti-icing surfaces with micro/nanoscale roughness: (1) patterning substrates such as silicon¹⁵ or metal^{13, 6, 9-20} with ordered nano-scale features by etching or

photolithography,⁷ followed by a top-coat of a fluorinated self-assembled monolayer (SAM) or polymer; or (2) depositing inorganic nanoparticles (Ag, ZnO, Ti0₂ , etc.) directly onto surfaces,^{2, 0- 2} which is likely a more feasible and scalable process. The drawback however is that superhydrophobic surfaces are very fragile due to the susceptibility of the micro- or nanoscale features to mechanical abrasion or environmental conditions. Most

superhydrophobic coatings are attached to the surface via non-covalent interactions such as van der Waals forces, electrostatic interactions, or hydrophobic interactions.^21-22 These shortcomings limit the coating's widespread application in harsh environments and lead to delamination, desorption, and dewetting.

Benzophenone (BP) photochemistry has been thoroughly studied and widely used as a photoactive tethering reagent to functionalize a wide variety of materials.^23-27 The extensive use of BP can be attributed to the following advantages: (1) The BP moiety attaches to C-H bonds in a wide range of chemical environments, including commercial plastics and fabrics,^28-32 (2) BP can be manipulated in ambient atmosphere and activated with mild UV light (345-365 nm), which avoids oxidative damage to many materials,²⁴ (3) BP is both thermally and chemically more stable and synthetically more versatile than most other tethering functionality, such as sulfonyl azides,³³ diazo esters,³⁴ aryl azides,³⁵ and diazirines.^36-37 Under irradiation, BP absorbs photons, resulting in the promotion of an election from n-orbital of the carbonyl oxygen to the TT*-orbital and the formation of biradicaloid triplet state. The electron-deficient oxygen abstracts a hydrogen atom from neighboring C-H group, followed by the combination of two carbon radicals and generation of a new C-C bond. This photochemistry has been used to permanently attach polymer films to a broad selection of C-H surfaces in many different applications, such as microfluidic devices,^38-39 biosensing, ^40-4 antimicrobial coatings,³⁰ patterned sheets,^42-43 and organic semiconductors.⁴⁴

In this disclosure, we describe a robust anti-icing composite that includes of well- dispersed silica nanoparticles within cross-linked, fluorinated polymer network. The surface attachment of the composite was readily achieved by UV-initiated cross-linking between BP moieties and surface-bound C-H groups. The composite demonstrated impressive anti-icing capabilities due to its fluorinated hierarchical micro/nano-scale surface structure. The composite also resisted relatively harsh mechanical stress, which is attributed to the dense covalent network between the copolymer and nanoparticles, as well as the substrate surface. To the best of our knowledge, this is the first demonstration of the covalent immobilization of hierarchical anti-icing coatings onto surface that can withstand high abrasive forces.

Experimental Section

Materials. Triphenylphosphine, iodomethane, potassium te/f-butoxide, 2,2'-azobis(2- methylpropionitrile) (AIBN), 2,2,3,4,4,4-Hexafluorobutyl methacrylate, isobutylmethacrylate (iBMA), and n-butyl acrylate (nBA) were obtained from Alfa Aesar. 4-Bromobenzaldehyde, benzonitrile, and isobutyltrichlorosilane were purchased from TCI. Tetraethoxysilane was obtained from UTC and magnesium turnings were obtained from Eastman Organic.

Cetyltrimethylammonium bromide was purchased form Acros Organic and isobutyl acrylate from Sigma-Aldrich. Inhibitor was removed from monomers by passing through a neutral aluminum column. Silicon wafers with native oxide were used as substrates. Unless otherwise noted all compounds were used as received.

Instrumental Methods. UV-vis spectroscopy was performed on a Cary Bio spectrophotometer (Varian). The UV light sources were a Compact UV lamp (UVP) and FB- UVXL-1000 UV Crosslinker (Fisher Scientific) with bulbs of wavelength at 254 nm for small (1 x 2 cm) and large (3.6 x 4.2 cm) substrates, respectively. The substrates were held a certain distance from the light source during irradiation to obtain power of 7.5 mW/cm².

Surface topography was analyzed with a FEI Inspec F FEG scanning electron microscopy at 20 keV. Infrared spectroscopy studies of polymer coated films were carried out using a Thermo-Nicolet model 6700 spectrometer equipped with a variable angle grazing angle attenuated total reflection (GATR-ATR) accessory (Harrick Scientific). Water contact angles were measured by using a DSA 100 drop shape analysis system (KRUSS) with a computer- controlled liquid dispensing system. Water droplets with volume of 1 μΙ_ were used to measure the static contact angle. The advancing and receding contact angles were measured by expanding and retracting the water droplet on the substrate. The glass transition temperature (T_g) of copolymer was measured by using a differential scanning calorimeter DSC 823^e (Mettler Toledo). Data was stored and manipulated using the software STARe DB V9.20 (Mettler Toledo). Samples were scanned from -60 °C to 150 °C at a rate of 10 °C/min. Four heating and cooling cycles were conducted. Nanoparticle sizing was determined using a Zetasizer Nano Series (Malvern) with dynamic light scattering (DLS).

Methyltriphenylphosphonium Iodide: Triphenyl phosphine (12 g, 45.75 mmol), iodomethane (8.44 g, 59.8 mmol) and toluene (100 ml_) were stirred at room temperature for 16 h under a nitrogen atmosphere. The reaction mixture was filtered and a pure white solid was collected. The solid was washed with diethyl ether and air-dried. Yield: 17.17 g (93%). H NMR (500 MHz, CDCI₃, δ): 7.84-7.67 (m, 15H, Ar H), 3.25 (d, J = 13.3 Hz, 3H; CH₃). 4-Bromostyrene: Methyltriphenylphosphonium iodide (10.66 g, 26.4 mmol) was suspended in tetrahydrofuran (15 mL) and cooled to 0 °C in a three-neck round-bottom flask. Potassium te/f-butoxide (3.23 g, 28.8 mmol) was dissolved in THF (10 mL) and added drop- wise to the round-bottom flask. The reaction mixture was stirred at 0 °C for 20 min and at room temperature for 10 min. 4-Bromobenzyldeheyde (4.44 g, 24.0 mmol) was dissolved in THF (10 mL) and added to the flask. The reaction was stirred at room temperature overnight. The solvent was removed under reduced pressure. The solid mixture was taken up in diethyl ether, and washed with water and brine, then dried over magnesium sulfate. The solvent was removed with a rotary evaporator. The crude product was purified on a silica gel column using a hexane:ethyl acetate (4:1) eluent. Yield: 3.97 g (90%). H NMR (500 MHz, CDCIs, δ): 7.44 (d, J = 8.8 Hz, 2H, Ar H), 7.27 (d, J = 8.2 Hz, 2H, Ar H), 6.65 (dd, J = 1 1.1 Hz, 1 H; CH), 5.74 (d, J = 17.6 Hz, 1 H; CH₂) 5.28 (d, J = 11.1 Hz, 1 H; CH₂). C NMR (300 MHz, CDCI₃, δ): 136.61 , 135.94, 132.27, 128.21 , 125.98, 116.54.

4-Vinylbenzophenone (4VBP): Magnesium turnings (0.65 g, 26.6 mmol) and a flake of iodine were dissolved in dry THF (5 mL) in a three-neck round-bottom flask under nitrogen atmosphere at 0 °C. 4-Bromostyrene (4.48 g, 24.5 mmol) was dissolved in anhydrous THF (5 mL) and added to the flask drop-wise from an additional funnel. The reaction mixture was stirred overnight at room temperature. A catalytic amount of copper (I) bromide and benzonitrile (2.27 g, 22.0 mmol) were added by syringe, and the reaction was stirred for 16 h. Sulfuric acid (15 mL, 15% V/V) was added to quench the reaction, followed by extraction with diethyl ether (50 mL). The organic layer was washed with water and brine, dried over magnesium sulfate and concentrated with a rotary evaporator. The crude product was purified by column chromatography (hexane:ethyl acetate, 4:1). Yield: 2.23 g (49%). 1 H NMR (500 MHz, CDCI3, δ): 7.81 (d, J = 7.6 Hz, 2H, Ar H), 7.80 (d, J = 8.4 Hz, 2H, Ar H),

7.59 (t, J = 7.6 Hz, 1 H, Ar H), 7.51 (d, J = 8.3 Hz, 2H, Ar H), 7.49 (t, J = 7.8 Hz, 2H, Ar H), 6.79 (dd, J = 10.8 Hz, 1 H; CH), 5.90 (d, J = 17.6 Hz, 1 H; CH2), 5.41 (d, J = 1 1.2 Hz, 1 H; CH2). 1 C NMR (300 MHz, CDCI3, δ): 116.59, 128.27, 129.09, 129.92, 130.53, 132.31 , 135.98, 136.65, 137.72, 141.53, 196.1 1.

Terpolymer: The polymer was synthesized by free radical polymerization of vinyl benzophenone (0.16 g, 0.79 mmol), hexafluorobutyl methacrylate (1.18 g, 4.72 mmol), and n-butyl or isobutyl acrylate (0.10 g, 0.79 mmol) in toluene (3 mL) with AIBN (0.0038 g, 0.0024 mmol) as the initiator in a Schlenk flask under nitrogen atmosphere. The degassed mixture was stirred at 65 °C for 16 h. The resulting polymer was precipitated in cold ethanol, filtered, and dried under vacuum. Yield: 0.31 g (67%). H NMR (CDCI₃): δ, 7.73 (bs, 4H);

7.60 (bs, 1 H); 7.48 (bs, 2H); 7.14 (bs, 2H); 4.91 (bs, 6H); 4.34 (bs, 12H); 2.62 (bs, 1 H), 1.89 (bs, 19H); 1.48 (bs, 2H); 1.26 (bs, 2H); 1.01 (bs, 6H) 0.937(bs, 10H); 0.72 (bs, 4H). The number average molecular weight (M_n) of two copolymers was -137 kDa. PDI of the copolymers that contains n-butyl and isobutyl acrylate were 1.9 and 2.0, respectively.

Surface Modified Silica Nanoparticle: An ammonia solution (10.8 g, 29% w/w) was added to deionized water (442 ml_) and heated to 50 °C, followed by addition of

cetyltri methyl ammonium bromide (0.279 g) with rapid stirring. The solution was cooled to room temperature and tetraethoxysilane (1.394 ml_) was added. The solution was stirred vigorously for 2 h. The water was removed by centrifugation and the white solid was dried in a vacuum oven at 60 °C for 16 h. Yield: 0.40 g. The silica nanoparticles were suspended in toluene (40 ml_). Isobutyltrichlorosilane (7.662 g) was then added to the suspension and stirred rapidly for 16 h. The resulting suspension was centrifuged and supernatant was discarded. The solid was washed by resuspending in THF (30 ml_), vortexing and centrifuging. This process was repeated three times. The white solid was dried in a vacuum oven overnight.

Preparation of Surface-Bound Terpolymer-Nanoparticle Composite: A silicon wafer (3.6 x 4.2 cm) was pre-treated with a self-assembled monolayer (SAM) of

isobutyltrichlorosilane (iBTS) using solution deposition of iBTS in toluene (0.1 mM) solution overnight. The terpolymer-nanoparticle composite was prepared by mixing 12 mg of polymer and 6 mg of nanoparticle in toluene/acetone (500/100 μΙ_) and sonicating for 5 min. The suspension was then applied on the alkylated silicon wafer by drop casting. After air-drying, the coated substrate was irradiated with UV light (254 nm, 7.5 mW / cm²) for 30 min to covalently graft the polymer-nanoparticle composite to the substrate surface using the pendant benzophenone moiety on the polymer.

Abrasion Test. The robustness of the surface-bound copolymer-nanoparticle coating on the substrate was evaluated by a modified Taber test using eraser rubbing. A pencil (# 2, HB, Ticonderoga) latex free eraser was pressed against the coated substrate mounted to a scale with 300 g force and rubbed back and forth (considered as one rubbing cycle) in a straight line over the surface. After a given amount of rubbing cycles, the damage of the coating was examined by measuring the static contact angle and supercooled water impaction.

Icing Experiments. Freezing rain was simulated under a laboratory condition by impinging supercooled water on the samples at subzero temperatures. The preparation of supercooled water, the cold substrate, and decanting water onto the copolymer-nanoparticle coating were conducted in an explosion-proof lab freezer (temperature: -18 °C, relative humidity: 40%). Supercooled water was prepared by carefully cooling still, pure water (18 ΜΩ) in the freezer for 2 h. Bare and coated substrates were placed in freezer in advance, followed by pouring supercooled water (100 ml_) from a 20 cm height. The anti-icing effectiveness of the coating was evaluated by visualizing ice-formation upon impact of supercooled water and recorded by photography and video. The anti-icing capability of the coating was examined before and after abrasion. The icing experiment was repeated 3 times for each sample.

Scheme 1. Outline of the S nthesis and Chemical Structure of the Terpolymers

Results and Discussion

The terpolymers, p(BP/F/nBA) and p(BP/F/iBA), which contain benzophenone (BP), hexafluorobutyl (F), and butyl side chains (BA), were prepared by free-radical polymerization of the three monomers (Scheme 1). The terpolymer compositions were checked by NMR spectroscopy, which revealed that the polymer composition matched the pendant group feed ratio (BP: F : BA = 1 : 6 : 1). The hexafluorobutyl side-chain constitutes the majority of the total polymer pendant groups due to hydrophobicity requirement of the polymer. Fluorinated materials possess high hydrophobicity because fluorine has a low polarizability due to its dense electron cloud, which results in weak van der Waals interaction between

fluorocarbons and water.⁴⁵ The benzophenone moiety acts as a cross-linker between the polymer and any organic interface through hydrogen abstraction followed by C-C

recombination under UV irradiation. Carbon radical stability affects the hydrogen abstraction rate, which further governs the crosslinking kinetics. Previous studies have demonstrated that β-fluorine substitution gives rise to radical destabilization relative to the ethyl radical,⁴⁶ so a more reactive hydrogen is necessary to enhance the cross-linking efficiency. The butyl side-chain was chosen to provide this reactive hydrogen. Two copolymers containing n-butyl and isobutyl acrylate (nBA, iBA) were prepared in order to compare the cross-linking kinetics, mechanical strength, and hydrophobicity between the copolymers with a varying reactivity of abstractable hydrogens.

The cross-linking kinetics of copolymers p(BP/F/nBA) and p(BP/F/iBA) were investigated by UV-vis spectroscopy on an iBTS functionalized quartz substrate. The polymer film was deposited on the alkylated quartz by drop-casting with polymer solution (10 μΙ_, 10 mg/mL in CHCI₃). Upon absorption of 254 nm light, the promotion of one electron from a nonbonding orbital to the antibonding ττ* orbital of the carbonyl group on the BP yields a biradicaloid triplet state where the electron-deficient oxygen n orbital interacts with surrounding weak C-H δ bonds, resulting in H abstraction to complete the half-filled n orbital. The two resulting carbon radicals then combine to form a new C-C bond. This process is monitored by the absorbance decrease in n - ττ* transition of BP at 260 nm, as shown in the inset of Fig. 1. The conversion of BP in p(BP/F/iBA) and p(BP/F/nBA) as function of irradiation energy (E, J/cnT ) was determined by plotting the decay of this peak (Fig. 1) at room temperature and elevated temperature. Initial absorbance was normalized to 1 for both polymers. The data can be described by a single-exponential decay:

Anormal = e-^E/Ec + K 0)

where E_c is the characteristic energy for BP conversion and K is the constant absorbance at infinite energy. The decay of the benzophenone triplet is first order,^47"48 accordingly the rate constant is determined as f^e = £ <²>

where t is the irradiation time. The E_c and k of the two polymers at two different irradiation temperatures are reported in Table 1. Photoinitiation studies have indicated that radical stability of the alkyl group governs the rate of reactivity of BP.⁴⁹ As ketyl radical stability increases with carbon substitution, rate constants for hydrogen abstraction increase in the order of primary, secondary and tertiary. Consequently, iBA is expected to have higher reactivity than nBA due to the tertiary proton. However, when irradiated at room temperature, the E_c of iBA is higher than nBA by 1.8 J/cm². This is likely explained by the segmental motion difference between the two polymer backbones at room temperature. The DSC traces and glass transition temperature (T_g) of two copolymers are given in Table 1. Under irradiation, the actual surface temperature of the quartz substrate measured by an infrared thermometer is 31.1 °C. Thus segmental chain movement of p(BP/F/nBA) may occur since the local heating caused by UV irradiation is sufficient to allow the polymer to be raised above its T_g (29.2 °C) during crosslinking. On the other hand, p(BP/F/iBA), with a T_g considerably higher than room temperature (35.4 °C), does not have sufficient free-volume to encounter surrounding C-H groups in the solid state, i.e. hydrogen abstraction is limited. This assumption is further confirmed by plotting the BP decay and calculating E_c and k at 50 °C, which is well above T_g of both polymers. Under this condition, both polymers have sufficient segmental motion and the reaction rate is governed by the stability of the radical. It should be noted that the difference of E_c and k between two types of butyl side chain is not remarkable (only by 0.5 J cm^"2 and 0.21 s^"1 , respectively). A plausible explanation is likely that the nBA pendant group contains more 2° hydrogens than iBA. Additionally, although the iBA contains a 3° hydrogen, the reactivity of the 3° C-H is only one order of magnitude higher than 2° C-H. For this reason, the BP cross-linking reaction rate of iBA and nBA are comparable.

Table 1. Irradiation characteristics and glass transition temperatures of the

terpolymers used in this study.

Copolymer Irradiation Temp E_c (J/cm²) k (s ¹) T_g (°C)

(°C)

BP/F/iBA 25 °C 6.4 1.17 35.4

50 °C 4.0 1.88

BP/F/nBA 25 °C 4.6 1.63 29.2

50 °C 4.5 1.67

The completion of the photochemical cross-linking of the terpolymer was also indicated by GATR-FTI R. Figure 2 shows the IR spectra of a p(BP/F/iBA) film coated on silicon wafer before (bottom) and after (top) UV irradiation. In Figure 2 (bottom) and (top), the ester C-0 and C=0 stretches are assigned at 1 187 and 1746 cm^"1. The peak at 1284 cm^"1 is due to C-F stretching of hexafluorobuyl pendant group. The C-C ring vibrations is assigned to 1607 cm^"1. The peak at 1658 cm^"1 in Figure 2 (bottom) represents the C=0 stretch of BP chromophore. A significant reduction of this peak, as seen in Figure 2 (top), is support for the photodegradation of the BP chromophore and formation of the covalent grafting after irradiation.

Terpolymer/Nanoparticle Composites. Scheme 2 illustrates the covalent attachment strategy for copolymer/nanoparticle composites to C-H containing surfaces upon UV irradiation. The surface-bound copolymer/surface-alkylated nanoparticle composite was prepared by drop-casting the polymer/NP solvent suspension on alkyl-functionalized silicon wafers and performing photochemical irradiation. In order to obtain a uniform dispersion of the polymer and nanoparticles, the mixture was sonicated vigorously in toluene/acetone co- solvent and then applied to the substrate surface by drop casting. Upon solvent evaporation, the resulting polymer/nanoparticle mixture remained on the substrate surface with a macroscopically smooth topography. The composite was then irradiated for 45 min to ensure complete BP photo-cross-linking. When the nanoparticles are in the composite, the BP moiety of the copolymer reacts non-selectively with the C-H groups on the silane-treated silicon substrate surface, the nanoparticle surface and the pendent butyl side-chain of polymer itself, resulting in formation of a fluorinated polymeric network embedded with NPs, covalently attached to the substrate surface.

Surface Morphology. Figures 3A-E demonstrate the surface morphology of the UV- cured composite on a silicon substrate. In the SEM images (Fig. 3A), many asperities are seen across the surface, which indicates microscale surface roughness. When magnified (Fig. 3B), the protrusions can be observed to include aggregates of nanoparticles, which imply nano-scale roughness. The diameter of the nanospherical feature is ~ 70 nm based on SEM, which is -10 nm higher than the average size of the unfunctionalized silica

nanoparticle measured by DLS (60 nm, Fig. 10). The micro/nano-scale roughness is also observed by the SEM side view image of the cross section of the composite coated on substrate in Fig 3C. There are numerous nanoparticles embedded in a thin layer of polymer, which are also adhered to the substrate surface. It is noteworthy that in lieu of being completely embedded in a polymer matrix, which would result in a smooth topology, the concentration of nanoparticles is large enough that the nanoscale features of the particles are retained and exposed. Fig. 3D shows the surface profile of the composite coated substrate at the micro-scale (using surface profilometry) with spike-like protrusions and valleys with a width of 25 μιτι and average roughness of 4.0 μιτι. Herein, the composite coated substrate shows a surface structure of hierarchical multiscale (micro and nano) roughness. This surface structure directly determines its hydrophobicity and anti-icing capability as discussed below.

Scheme 2. Covalent Attachment of Copolymer/Nanoparticle Composite to C-H Containin Surfaces

Surface hydrophobicity. The hydrophobicity of the composite coated surface

(m oiymer/rriNP = 2) is characterized by a static CA and CAH of 138° and 5°, respectively. When a droplet resides on top of a rough surface, two wetting states, the Wenzel state and Cassie-Baxter state, can occur. In the Wenzel state, the droplet impales the grooves and asperities and the contact angle is defined as follows:

cos Θ* = Tf cos Θ (3)

where Θ * and Θ are the CA on rough and flat surfaces, respectively. r_f , known as the roughness factor, is the ratio of the actual area of the solid surface to the apparent area. In Cassie-Baxter state, the droplet sits on top of the asperities, forming "air pockets" between the liquid and solid surface. In this case, the contact angle is defined by the following equation:

cos e^* = (l + cos e) - 1 (4)

where is the area fraction of liquid-solid interface. A superhydrophobic surface that includes hierarchical micro- and nano-scale roughness combined with a low surface energy moiety satisfies the Cassie-Baxter model.^{9, 50} Based on the surface image and profile shown above, Figure 3D demonstrates the wetting behavior of the water droplet on our

polymer/nanoparticle composite. The composite dual-scale roughness prohibits water droplets to wet the spaces between spikes of both micro- and nanostructure. Consequently, the water-solid contact area is small, due to an extremely small value (f_hi_erarchicai = fmirco ^x fnano) - According to Equation (4), the CA of such surface is large. Because of the air entrapment inside the groove texture, a heterogeneous interface composed of solid and air is generated. As a result, the adhesion force between water and solid is extremely low, which accounts for the observed low hysteresis.

Ice Prevention towards Impact of Supercooled Water. One of the methods to examine the anti-icing capability of surfaces involves with impacting the tested surface with supercooled water. ^{0, 5, 5} In our study, the uncoated and composite coated silicon wafers were tilted at an angel of 5° from horizontal and impinged by a stream of supercooled water. The effectiveness was evaluated by observation of ice generation on substrate surface. The photos of resulting substrates from the icing experiment in the freezer are shown in Figure 4. In Figure 4, the left substrate is the control (bare Si0₂) and the right substrate is the wafer coated with the polymer/particle composite after UV irradiation, where the opaque coating is apparent. On the uncoated substrate, ice formed instantly when the supercooled water impacted the surface. A full layer of ice is shown on the control whereas no ice was formed on the coated substrate. Only some accumulated water remains on the surface due to the low tilt angle in the experiment. The substrates were warmed back to room temperature and air-dried. The process of striking substrate with supercooled water, warming and drying was considered as one icing/deicing cycle. The icing/deicing process was repeated three times on the same substrate and no ice was formed on the composite coated surface each time. According to previous reports, multiple icing/deicing cycles caused gradual loss of effective icing prevention, as a result of damage of nano-asperities over the expansion of water upon freezing¹³. This is not the case with the crosslinked composite, which has reliable anti-icing capability towards striking of supercooled water after multiple icing/deicing cycles, due to its excellent mechanical durability as described in the following section.

Previous studies have been focused on two aspects to design an anti-icing coating: elimination of ice adhesion and delay or prevention of ice nucleation. It also has been shown that wetting hysteresis directly relates to ice adhersion.¹¹ In this work, the composite coated surface with a CA of -140° and a CAH of 5° is speculated to be wetted in the Cassie-Wenzel transition state, where water is partially pinned in the grooves or asperities. This mixed state explains the fact that the composite coated surface was slightly wetted after striking with supercooled water (Fig. 4). Thus, instead of reaching a perfectly non-wetting Cassie-Baxter state in which the frozen droplets are easily removed with a small input of thermal energy,¹⁵ the delay of ice nucleation is likely the more important factor that leads to the icephobic nature of this coating. Cao et al. systematically investigated the effect of the size of the spherical roughness on icing probability.¹⁰ They successfully demonstrated that the ratio of the free energy barrier between heterogeneous and homogeneous nucleation, (values between 0 and 1), estimates the anti-icing capability of surfaces with nano-spherical roughness. The mathematic derivation to calculate and experimental parameters are found in the Supporting Information. Under our experimental conditions of icing, is plotted versus radius of nanoparticle, R in Fig. 5. is found to drop dramatically as R increases from 10 to 110 nm. When R is above 1 μι ι, the size of nanoparticle has little influence on f. This curve implies that within a 10 to 1 10 nm particle radius, the size of the surface exposed nano- spherical features is the defining parameter in terms of likelihood of preventing ice nucleation. Specifically, the smaller the nanoparticles, the greater the nucleation energy barrier, and lower tendency towards icing on the surface. The radius of our surface exposed nanoparticle is 35 nm based on SEM, and is 0.862 according to Fig. 5, suggesting a relatively high nucleation free energy barrier. For these reasons, the composite coating has excellent icephobicity towards impinging supercooled water.

Composites with different polymer/NP ratios. The effect of different

terpolymer/nanoparticle composition on morphology and anti-icing capability was also investigated by comparing the composites with a polymer (p(iBP/F/iBA)) nanoparticle mass ratio of 2 and 2.8 (w/w). Figures 6A-B illustrate the morphology of the two composites when coated on Si0₂ substrates. With the composite containing a polymer/nanoparticle ratio of 2 (Figure 6A), numerous spherical particles are uniformly exposed, which provides nanoscale roughness. However, as the polymer/nanoparticle composition ratio increases to 2.8 (Fig. 6B), the individual particles start to become buried and submerged in the polymer matrix, which leads to a smoother surface on the nanoscale. The morphology change due to the higher polymer content results in a low CA (122°) and high CAH (22°). The different morphologies of two composites lead to different behavior in icing experiments. Upon striking of the surface with supercooled water, the composite with polymer/NP ratio of 2 repells the water and prevents icing. In comparison, though inhibiting instant ice formation, the composite with ratio of 2.8 provide much less water repellancy. The remaining water on surfaces can nucleate in sub-zero conditions. Therefore, the proper composition of p(iBP/F/iBA)/NP is essential to enable the optimum amount of nanoscale roughness, which aids in water repellency and icephobicity.

Robustness of Polymer/Nanoparticle Composites against Abrasion. The robustness of BP induced covalent bonding to substrate surface was evaluated by conducting an eraser abrasion test (modified Taber testing) on BP containing and non-BP containing

polymer/nanoparticle composites. P(BP/F/iBA) was used as the polymer in the tested samples and a homopolymer that only contains the fluorinated hydrocarbon,

poly(hexafluorobutyl methacrylate), was used as the control. An eraser with 300 g load was rubbed across the two composite coated samples on silicon substrates 60 times. Optical pictures and water CA of the resultant coating after abrasion testing are shown in Figs. 7A-B. The control poly(hexafluorobutyl methacrylate)/nanoparticle composite without BP

crosslinker was removed after only one cycle of rubbing, which left the bare substrate with CA of 83°, whereas the BP containing copolymer composite remained, and the surface retained its superhydrophobicity with CA of 140°. This implies that BP moiety can act as a "photo-reactive glue" that combines the polymer and nanoparticles together with adherence to the substrate surface through crosslinking. In comparison, the physisorbed non-BP composite provides no mechanical resistance towards abrasion. Additionally, for the surface- bound composite, as the exterior of the coating is abraded and lost, more deeply embedded nanoparticles are exposed at the interface, and the nano-scale roughness remains intact. Hence, the BP-containing composite presents mechanical resistance to friction. Anti-icing capability of p(BP/F/iBA) nanoparticle composite was tested after 30 abrasion cycles. The resultant substrate is shown in Fig. 4 (right). No ice formation is observed, demonstrating that the composite not only has excellent anti-icing capability, but also a mechanical robustness that survives harsh abrasion. However, it was observed that some liquid droplets remained on the abraded surface due to the slight decline in surface hydrophobicity (-5° CA decrease after abrasion (Fig. 8C).

A comparison of robustness between the terpolymers of p(BP/F/iBA) and p(BP/F/nBA) within the nanoparticle composite was also performed using eraser abrasion testing and subsequent SEM imaging (Figs. 8A-C). The morphology of abraded surfaces coated with p(BP/F/iBA) and p(BP/F/nBA) nanocomposites are presented in Fig. 8A and Fig. 8B, respectively. For p(BP/F/iBA) composite, only the top layer of the nanoparticles was removed after treated with 28 abrasion cycles. The embedded nanoparticles within the polymeric network were exposed and the surface nanoscale roughness was preserved. In contrast, p(BP/F/nBA) nancomposite was completely removed from the surface after undergoing the same number of abrasion cycles, indicating the adhesive failure of the composite. Furthermore, the CAs were measured over continuous abrasion cycles to characterize the changes of the hydrophobicity for two polymer/nanoparticle composites (Fig. 8C). It was observed that the CA of p(BP/F/nBA) composite drops by -30° (from 128° to 96°, which is close to the original contact angle of the isobutyl SAM on silicon wafer) after 28 abrasion cycles. On the other hand, the CA of p(BP/F/iBA) composite decreases by only 5° (from 134° to 129°), retaining its hydrophobicity. The robustness difference is likely explained by the occurrence of β-scission of the polymer backbone during cross-linking. It is suggested that β-scission is one of the pathways a carbon radical can undergo after hydrogen abstraction.⁵² If β-scission occurs on backbone, it causes chain scission in the polymer. For a polyacrylate that contains 3° hydrogens on the polymer backbone, hydrogen abstraction occurs most favorably on the polymer main chain, leading to higher amounts of chain scission and resultant loss in the mechanical integrity of the coating. However, if there are additional 3° hydrogens on the pendent groups (as is the case with an isobutyl side chain), the probability of hydrogen abstraction from the backbone is smaller, where the pendant 3° hydrogens "dilute" the β-scission probability. Additionally, it is likely that isobutyl groups are more accessible for abstraction, and hinder access to the polymer backbone more than n-butyl side chains. A similar conclusion was reached by Christensen et al. in a recent study on polymer gelation.⁵³

On Plastic. To investigate the versatility of the p(BP/F/iBA)/nanoparticle coating on commercially available plastics, we photochemically modified a polypropylene (PP) sheet with the composite by drop-casting using the same material composition described above. The PP substrates were irradiated to covalently graft the copolymer/nanoparticle to the plastic surface via hydrogen abstraction from the PP surface. After UV curing, the uncoated and coated PP substrates were challenged against the icing test using the icing

experimental method described above. Figs. 9A-B shows the anti-icing capability of composite coated PP before (Fig. 9A) and after (Fig. 9B) eraser abrasion (300 g, 30 cycles). In both (Fig. 9A) and (Fig. 9B), ice is observed on the pristine PP substrate (left). No ice formation occurred on the composite coated PP substrate (right) before and after undergoing 30 abrasion cycles. These results demonstrate that the covalent immobilization of durable copolymer/nanoparticle composite is applicable to a variety of substrates, including inert plastics. Conclusion

In conclusion, we have demonstrated an efficient and easy-to-implement approach to covalently attach an anti-icing composite on to any plastic substrate or surface that contains C-H bonds. The copolymer contains pendant benzophenone, hexafluorobutyl, and isobutyl side chains. When the polymer is combined with silica nanoparticles, a composite that consists of a densely cross-linked network is immobilized on a substrate surface via covalent bonds upon UV irradiation. The optimized copolymer/nanoparticle composite exhibits fast attachment and good mechanical durability when subjected to abrasion. No ice formation occurred on the coated surface when subjected to supercooled water for both abraded and non-abraded coatings. This simple, one step photochemical attachment of icephobic coatings is promising and scalable for domestic and industrial applications. In addition, we have observed that incorporation of 3° hydrogens on pendent groups of BP containing polymer reduces the backbone chain scission occurrence and improves the overall crosslinking possibility and mechanical strength of the coatings after irradiation. This study provides some design principles for polymer/nanoparticle composites that have excellent anti-icing capability along with robust attachment and mechanical durability.

Calculation of icing possibility, f

Based on the study of three-dimensional heterogeneous nucleation on a foreign particle,⁵⁴ the free energy of formation of an embryo on the foreign particle is

AG_C = AGc ^07nof(m, x) (1)

Here AG_C is the free energy barrier of heterogeneous nucleation, and AGj ⁰⁷⁷¹⁰ corresponds to the free energy barrier of homogeneous 2D nucleation. f(m, x) is a dimensionless factor that determines AG_C. It can be calculated by as

f(m, X) = - + - ( V +

' ^J 2 2 2 - 3 (¾ + (^)³M-²(^- ⁱ) P⁾

with

m = cos e_flat (3)

x = R/r_c (4)

and

where d_flat is the static CA of the flat surface and is 105° for our cross-linked p(BP/F/iBA) surface. R is the radius of the foreign particle, r_c is critical radius of ice embryos and only determined as following

r_c = 2Ά_Ύοί/Αμ (6) where y_cf is crystal-fluid (ice-water) surface energy (0.034 J m^"2, Ketcham and Hobbs, 1969)⁵⁵, Ω is water molar volume (1.8 x 10⁵ m³ mol^" ), and Δμ is difference in chemical potential of ice crystal structural units and water growth units, which approximates to

C_pT(\n{T/T_m) + ^T -l) (7)

Under our icing experimental condition, T is 255 K, T_m is 273 K, and C_p is approximately 75.3 J mol^"1 K^" herefore, Δμ is calculated to be 45.66 J mol^"1 and r_c is 26.8 nm.

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It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of "about 0.1 % to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1 %, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1 %, 2.2%, 3.3%, and 4.4%) within the indicated range. In an embodiment, "about 0" can refer to 0, 0.001 , 0.01 , or 0.1. In an embodiment, the term "about" can include traditional rounding according to significant figures of the numerical value. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about y.

It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations, and are set forth only for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiments of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure.

Claims

We claim:

1. A substrate comprising:

a film of a glaciophobic polymer-nanoparticle composite bound to the surface of the substrate, wherein the glaciophobic polymer-nanoparticle composite in a first state includes a polymer and nanoparticles, wherein the polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety, wherein prior to exposure to UV radiation the substrate contains C-H bonds on its surface and the nanoparticle contains C-H bonds on its surface, wherein after exposure to UV radiation, the glaciophobic polymer-nanoparticle composite in the second state includes a first portion of the photo cross-linkable moieties that are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross-linkable moieties that are covalently bonded to the nanoparticle through C-C bonds upon H abstraction.

2. The substrate of claim 1 , wherein the glaciophobic polymer-nanoparticle composite in a first state prior to exposure to UV irradiation, the polymer has the following structure:

, wherein R1 is the photo cross-linkable moiety, wherein R2 is the fluorinated moiety, wherein R3 is H or a first alkyl group, wherein R4 is the alkyl moiety, and R5 is H or a second alkyl group, wherein each of m, n, and o are independently 1 to 10,000.

3. The substrate of claim 1 , wherein the glaciophobic polymer-nanoparticle composite in iation, the polymer has the following structure:

, wherein each of m, n, and o are independently 1 to 10,000.

4. The substrate of claim 1 , wherein the glaciophobic polymer-nanoparticle composite in a first state prior to exposure to UV irradiation, the photo cross-linkable moiety is

benzophenone.

5. The substrate of claim 1 , wherein the nanoparticles are selected from the group consisting of: silica nanoparticles, metal nanoparticles, metal oxide nanoparticles, core/shell nanoparticles, and a combination thereof.

6. The substrate of claim 1 , wherein the glaciophobic polymer-nanoparticle composite has a polymer: nanoparticle ratio of about 1.5-2.5.

7. The substrate of claim 1 , wherein the particle size of the nanoparticles in the glaciophobic polymer composite is about 50-80 nm.

8. The substrate of claim 1 , wherein the film has a thickness of about 200 nm to 2 millimeters.

9. A composition comprising:

a glaciophobic polymer-nanoparticle composite in a first state, wherein the glaciophobic polymer-nanoparticle composite includes a polymer and nanoparticles dispersed in the polymer, wherein the polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety.

10. The composition of claim 9, wherein the polymer has the following structure:

, wherein R1 is the photo cross-linkable moiety, wherein R2 is the fluorinated moiety, wherein R3 is H or a first alkyl group, wherein R4 is the alkyl moiety, and R5 is H or a second alkyl group, wherein each of m, n, and o are independently 1 to 10,000. rein the polymer has the following structure:

, wherein each of m, n, and o are independently 1 to 10,000.

12. The composition of claim 9, wherein the photo cross-linkable moiety is

benzophenone.

13. The composition of claim 9, wherein the fluorinated moiety includes hexafluorobutyl.

14. The composition of claim 9, wherein the nanoparticles are selected from the group consisting of: silica nanoparticles, metal nanoparticles, metal oxide nanoparticles, core/shell nanoparticles, and a combination thereof.

15. The composition of claim 9, wherein the alkyl moiety is a C4 hydrocarbon.

16. The composition of claim 9, wherein the glaciophobic polymer-nanoparticle composite has a polymer: nanoparticle ratio of about 1.5-2.5.

17. The composition of claim 9, wherein the particle size of the nanoparticles in the glaciophobic polymer composite is about 50-80 nm.

18. A method of making a film on a substrate, comprising:

exposing the substrate containing C-H bonds on its surface to a glaciophobic polymer-nanoparticle composite in a first state, wherein the glaciophobic polymer- nanoparticle composite includes a polymer and nanoparticles dispersed in the polymer, wherein the polymer includes a hydrocarbon backbone that contains a photo cross-linkable moiety, a fluorinated moiety, and an alkyl moiety, wherein the nanoparticles contains C-H bonds on its surface; and

forming a film of the glaciophobic polymer-nanoparticle composite in a second state that is bound to the surface of the substrate, wherein after exposure to UV radiation, a first portion of the photo cross-linkable moieties are covalently bonded to the substrate through C-C bonds upon H abstraction and a second portion of the photo cross-linkable moieties are covalently bonded to the nanoparticle through C-C bonds upon H abstraction.

19. The method of claim 18, wherein the glaciophobic polymer-nanoparticle composite in a first state prior to exposure to UV irradiation, the polymer has the following structure:

, wherein R1 is the photo cross-linkable moiety, wherein R2 is the fluorinated moiety, wherein R3 is H or a first alkyl group, wherein R4 is the alkyl moiety, and R5 is H or a second alkyl group, wherein each of m, n, and o are independently 1 to 10,000.

20. The method of claim 18, wherein the glaciophobic polymer-nanoparticle composite in iation, the polymer has the following structure:

, wherein each of m, n, and o are independently 1 to 10,000.

21 The method of claim 18, wherein the photo cross-linkable moiety is benzophenone.

22. The method of claim 18, wherein the fluorinated moiety includes hexafluorobutyl.

23. The method of claim 18, wherein the nanoparticles are selected from the group consisting of: silica nanoparticles, metal nanoparticles, metal oxide nanoparticles, core/shell nanoparticles, and a combination thereof.

24. The method of claim 18, wherein the alkyl moiety is a C4 hydrocarbon.

5. The method of claim 18, wherein the film has a thickness of about 200 nm to 2