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Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Coating Architects: Manipulating Multiscale Structures To Optimize Interfacial Properties for Coating Applications Chao Wang,† Gerald O. Brown,‡ David L. Burris,§ LaShanda T. J. Korley,†,# and Thomas H. Epps, III*,†,# †

Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States The Chemours Company, Chemours Discovery Hub, 201 Discovery Boulevard, Newark, Delaware 19713, United States § Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States # Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States

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‡

ABSTRACT: Polymeric coatings are ideal materials for the surface modification of many solid substrates. The interfacial properties of these coatings, i.e., surface energy and tribological features, impact a surface’s resulting wettability, repellency, friction, wear, and other performance metrics. To design polymer coatings for various applications, numerous studies have been dedicated to elucidating key structure− property relationships. This review highlights recent efforts to develop such relationships for the various length-scales across which different phenomena arise. For example, the surface energies of polymer coatings have been correlated to molecular aggregation (0.5−10 nm) and nanoscale phase separation (1 nm−1 μm), along with microscale phase separation (1−100 μm) and surface chemistry patterning (10−500 μm). In a similar vein, the low friction, ultra-low wear performance of polymer composites is highlighted, along with links to microscale phase separation, with emphasis on several studies that have connected micrometer features to improved tribological characteristics, such as reduced friction and wear. The limitations of current structure−property studies with respect to surface coatings also are discussed, and potential methods to overcome these limitations are presented. Finally, an outlook for future structured polymer coatings is provided, along with proposed opportunities for the next generation of polymer-based surfaces. KEYWORDS: polymers, coatings, surface energy, tribology, interfacial structure, structure−property relationships

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INTRODUCTION Polymer coatings play a pivotal role in modulating the interfacial properties of solid materials. They can be applied to solid substrates for mechanical purposes, such as anticorrosion protection,1−4 wear resistance,5−9 adhesion,10−13 and lubrication (i.e., reducing friction).14−16 These polymer coatings impart surfaces with desirable performance characteristics, such as water/oil repellency,17−19 biocompatibility,20,21 and self-healing22,23 or self-cleaning capabilities,24,25 etc. Because of these attractive features, polymer coatings have been used widely in applications ranging from antifogging,26−29 anti-icing,30,31 anti-fouling,28,32−37 scratch-resistant,22,38,39 anti-marring,40−45 friction/drag-reducing,46−49 nonstick,50−53 and oil/water separating54,55 surfaces. When preparing polymer coatings that meet the requisite criteria for the above purposes, surface energy and tribological attributes are of critical importance.17,56−58 Surface energy affects the interfacial interactions of polymer coatings. For instance, high-surface-energy coatings have strong surface interactions with contacted materials that are beneficial for applications that require adhesion,59−61 but these materials are easily fouled.62−65 On the other hand, low-surface-energy © XXXX American Chemical Society

polymer coatings typically have weak interfacial interactions with an opposing material/substance; such coatings usually possess water/oil repellent, anti-fouling, anti-fog, anti-icing, and/or drag-reducing properties. Commercially viable (i.e., products that can compete effectively in terms of performance and cost) low surface energy coatings have been developed and patented for use as water and/or oil repellent finishes on textiles, leather, and other substrates.66−70 While surface energy studies mostly focus on static surfaces and interfaces, tribology studies focus on the physics behind interacting surfaces in relative motion.71 In tribomechanical analyses, parameters of primary interest include contact mechanics of rough surfaces, adhesion, friction, and wear. Friction is a major factor impacting motor torques and loads for cases in which sliding surfaces are involved,15 and wear often affects service life of a product as unwanted wear leads to excess debris generation, binding, and slop;15 both are fundamentally related to the long-range interactions that Received: March 30, 2019 Accepted: May 29, 2019 Published: May 29, 2019 A

DOI: 10.1021/acsapm.9b00302 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

description of tribological features will follow, with considerations of critical metrics, such as friction coefficient and wear rate. Then, a variety of polymer composites with desired tribological characteristics will be highlighted, followed by recent advances in manipulating phase separation to optimize performance. Finally, current challenges in the development of next-generation polymeric surfaces and potential solutions for these challenges will be discussed.

govern adhesive forces between surfaces. In many cases, plastic deformation and wear from tribological interactions can alter surface topography to an extent that degrades visual qualities (e.g., marring, dulling, fogging, etc.) and changes wettability.72−78 A long-held goal of the coatings industry has been to create compounds with desired surface energy and tribological features.17,56−58 Traditionally, the development of new products depends upon trial-and-error synthesis and screening of materials, which is both time- and resource-intensive. Alternatively, a growing interest in the de novo development of functional surfaces and interfaces has motivated more systematic studies with model systems to clarify the underlying structure−property relationships. In this article, we describe recent efforts to correlate the structure of polymer coatings across various length scales to their surface energy and tribological attributes. The key multiscale features and interfacial characteristics of polymer coatings to be discussed are outlined in Figure 1. The surface energy, contact angle

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SURFACE ENERGY: THEORETICAL AND MODELING APPROACHES Macromolecules at a surface can have a significantly different energetic landscape from chains in the bulk, due to different enthalpic and entropic considerations associated with molecules residing at an interface. In a coating system, this additional free energy term is normally captured by the surface energy (γ), which has dimensions of energy per unit area or force per unit length. In the following sections, the definition, modeling, and measurement of the surface energy of solids will be discussed in greater detail. Surface Energy of an Ideal Solid. The basic rules that describe interactions near the interfaces of solid, liquid, and vapor were first established on ideal solids, i.e., solids without roughness or chemical heterogeneities.90 Copper and Nuttall quantified the spreading of a liquid on a solid by the spreading factor91 S = γsv − (γsl + γlv)

(1)

in which the combination of subscripts indicates the interface at which the surface energy is determined; s, l, and v correspond to solid, liquid, and vapor, respectively. As shown in Figure 3a, when S > 0 so that γsv > γsl + γlv, the solid/vapor interaction is less favorable than the sum of the solid/liquid and liquid/vapor interactions; hence, the liquid completely wets the solid. On the contrary, the liquid only partially wets the solid when S < 0, Figure 3b. The balance at this state can be described by Young’s equation92

Figure 1. Multiscale structures and their relationship to interfacial properties of polymer coatings. The molecular aggregation image is adapted with permission from ref 145. Copyright 2005 American Chemical Society. The multilayer structure image is adapted with permission from ref 146. Copyright 2015 American Chemical Society. The surface chemistry patterning image is adapted with permission from ref 164. Copyright 2017 American Chemical Society. The microscale phase separation image is adapted with permission from ref 48. Copyright 2006 Elsevier.

γsv = γsl + γlv cos θ

(2)

The contact angle θ defines a cone (or a cylinder when θ = 90°) with an apex on the liquid side if θ < 90° or on the solid side if θ > 90°.93 When a liquid droplet rests on a surface, and the three-phase boundary is not moving, θ is defined as the equilibrium contact angle. θ is commonly determined by drop shape analysis upon deposition of a liquid droplet onto a solid surface. The shape of the liquid droplet is typically fitted with a geometrical model that is chosen on a case-by-case basis to obtain θ. For example, an ideal sessile drop oblated by its own weight is usually fitted by a Young−Laplace model,94 while a droplet with a spherical shape can be fitted by a circular arc. Although the surface energy of a solid is related to the measured contact angle of a droplet on its surface, no consensus has yet been reached for the model used to convert this contact angle data to surface energy values.95 Different theories and corresponding test liquids can emphasize the diverse aspects of a solid surface.95−99 For instance, the Owens−Wendt two-fluid theory can be applied to decouple the dispersive (induced dipole) and polar (permanent dipole) components of the surface energy,98 while a model including the Hamaker constant further considers the effect of longrange attractive or repulsive interactions on wetting behavior.100,101

hysteresis, friction, and wear will be emphasized for coatings based on polymers or polymer composites with inorganic nanoparticles, especially materials with high omniphobicity, zwitter-wettability, anisotropic surface energy, low friction, and/or ultra-low wear. The relationship between these properties and the design, structure, and experimental parameters shown in Figure 2 will be elaborated through a review of relevant studies. Although paint and applied coatings technologies,79−82 green coatings,83−86 and phase-change coatings87−89 are not the major focus of this Review, we have provided herein relevant references for readers that are interested in these important topics in the coatings field. The next section will start with a review of the theories and models of surface energy on ideal solids and heterogeneous surfaces, followed by a presentation of more recent arguments about the limitations of these models. Subsequently, studies on coating structures at subnanometer to micrometer length scales and their impact on surface energy will be emphasized. A B


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Figure 2. Factors that influence the interfacial properties of materials. Microstructure, surface chemistry, and surface topography/patterning are affected by both molecular structure and processing conditions. These structures of materials and experimental (testing) parameters combine to impact the measured values of interfacial properties. Additionally, evolution of structure and chemistry and, hence, properties can occur during experimentation/evaluation.

removal of liquid droplets from a surface. Similarly, surface topography and/or chemical heterogeneities of a surface can be designed to direct the liquid droplet transportation along a desired path through directional tuning of the contact angle hysteresis.103 When predicting the changes in contact angle that result from topographical heterogeneities, two commonly used models are those proposed by Wenzel104 and Cassie.105 Wenzel’s approach assumes that homogeneous wetting is achieved, for which the contact area of the sessile drop conforms to all topographical variations. On the basis of this assumption, Wenzel modified Young’s equation by including a roughness factor r, which is the ratio between the actual contact area and the apparent contact area on a rough surface.104 The Wenzel model suggests that the surface roughness leads to a smaller contact angle when θ < 90° and a larger contact angle when θ > 90°.90 This relationship is not always satisfied on rough surfaces because the homogeneous wetting assumption is not always valid.90 For example, on a superhydrophobic rough solid surface, the contact area of a water droplet may not follow all topographical variations due to air pockets trapped between the liquid and the solid.106 As an alternative to Wenzel, Cassie proposed that the contact angle on such a composite surface is defined by the contact angle of each component on the basis of the area fraction.105 According to the Cassie model, the air pockets on a rough surface lead to a larger contact angle in comparison to the contact angle on a smooth surface, as θ equals 180° for a liquid

Figure 3. (a) When γsv > γsl + γI, the liquid (blue) completely wets the solid (yellow) and spreads as a film. (b) When γsv < γsl + γlv, the liquid partially wets the solid, and a contact angle θ is established.

Heterogeneous Surfaces and Contact Angle Hysteresis. In practice, most solid surfaces have some roughness and/or chemical heterogeneities that can pin the three-phase contact line. This pinning normally results in a metastable liquid contact angle.93 As illustrated in Figure 4, a liquid droplet must increase its liquid/vapor interfacial area to move from one metastable state to another.102 During this process, the shape of the droplet is altered so that maximum and minimum contact angles, i.e., advancing (θA) and receding (θR) angles, are established. The difference between θA and θR, or the contact angle hysteresis, is related to the activation barrier for motion resulting from the growth of the liquid/ vapor interfacial area.102 For applications involving water/oil repellency, anti-corrosion, anti-fogging, anti-icing, etc., a low contact angle hysteresis is preferred to facilitate the easy

Figure 4. To move from one metastable state to another, a droplet must increase its liquid/vapor interfacial area. The shape change during this process leads to maximum and minimum contact angles, i.e., the advancing (θA) and receding (θR) angles. Adapted with permission from ref 102. Copyright 2006 American Chemical Society. C


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on or drop off the leaves.34,111,125,126 Other plants, such as Nepenthes raf f lesiana, combine slippery wax crystals and anisotropic epidermal cells to capture prey.127 Likewise, inhomogeneous surface energy coatings are used by spiders128,129 and desert beetles130 for directional water transportation. By mimicking the structural features found in nature, a variety of materials with desired interfacial properties have been developed.131−137 For instance, self-cleaning and omniphobic slippery liquid infused porous surfaces (SLIPs) were prepared by replacing the layer of air between lotus-leaflike structures with liquids or lubricants.131−135 Benefiting from the incompressibility and flowability of the fluid, SLIPs are less easily damaged under pressure and are able to self-heal.135 One other family of bioinspired systems are gecko-feet-mimetic materials, in which micro- and nanostructured fibrillar surfaces were designed for reversible adhesive properties.136,137 The examples mentioned above underscore the importance of understanding structure−interfacial property relationships to impart polymer coatings with desirable attributes. The next section highlights the design of multiscale features to impact wetting behavior, including molecular aggregation (0.5−10 nm),138−145 phase separation (1 nm−100 μm),26,146−155 and patterning of surface chemistry (10−500 μm).103,156−164 Molecular Aggregation. At the solid−vapor interface, the surface molecular aggregation state (e.g., crystalline vs amorphous) of polymeric chains can be dissimilar to that in bulk due to a difference in free energy.165 The interfacial characteristics of a coating are dominated by this aggregation state. For instance, an amorphous polyethylene (PE) surface has a surface energy of 35.4 mJ/m2, which is lower than that for a highly crystalline PE surface, 66.1 mJ/m2.166 The increased wettability of the latter polymer was attributed to a higher density of methanediyl groups (−CH2−) at the air/ solid interface.166 A similar phenomenon was demonstrated when the crystallinity of n-alkane substrates were reduced from 67 to 29%, during which a more disordered surface was developed with a greater concentration of −CH2−.167 As a result, the receding water contact angle decreased from 96.4° to 72.0°, while the advancing water contact angle remained relatively stable (at ∼111°), resulting in larger contact angle hysteresis.167 In comparison to their hydrogenated counterparts, aggregates of polymers with fluorine-containing backbones or side chains tend to have lower surface energies, and therefore, imbue their constituent coatings with better oleophobicity,168 lower friction,143 and lower adhesion.50 The representative chemical structures of the aforementioned substances are shown in Figure 6a,b,e. These fluorinated molecules have comparatively low polarizabilities that lead to smaller intermolecular forces, and thus, lower surface energies.169 Notably, a polymer with a fluorocarbon backbone used in numerous products, polytetrafluoroethylene (PTFE), has a native surface energy of 19 mJ/m2.170,171 However, the general application of PTFE is limited by its high sintering temperature (>350 °C) and poor solubility in solvents that are integral to many coating processes.171 Numerous attempts have been made to prepare alternative fluorinated substances that can be deposited under mild conditions.138−145,171−176 Among these materials, side-chain fluorinated polymers (SCFPs) that contain pendant perfluoroalkyl (CnF2n+1) chains attached to hydrocarbon polymer backbones have shown great promise, as

droplet that is in contact with air only. A liquid droplet in the Cassie state can be metastable due to the existence of a Wenzel state with lower energy, and a perturbation of the droplet may bring about a transition to the Wenzel state, see Figure 5.107

Figure 5. A 50 μL water droplet (a) in the Cassie state and (b) in the Wenzel state on a polystyrene (PS) micrometrically scaled honeycomb template. The schematics of each state are shown next to the photos. To trigger a Cassie−Wenzel wetting transition, the droplet was exposed to a vertical vibration with an increasing amplitude from 0.02 to 2 mm and a constant frequency between 30 and 50 Hz. Once the transition took place, the corresponding amplitude was fixed and recorded. Adapted with permission from ref 107. Copyright 2007 American Chemical Society.

The validity of the Wenzel and Cassie models has been questioned over the years.93,102,108−112 One of the first arguments was made by Pease, who stated that both equilibrium and dynamic contact angles are related to the one-dimensional system consisting of the junction of the air− liquid interface with the solid surface, i.e., the contact line.108 Bartell et al. and Extrand later showed that the contact angle of a liquid droplet is not affected by the roughness of a surface within the contact line.109,110 Following these ideas, McCarthy and co-workers contended that θA, θR, and contact angle hysteresis are dependent solely on the interaction between the solid and liquid at the contact line.93,102,111,112 Models and numerical simulations also have been implemented to explain the dynamics of the moving contact line.113−121 The validity of the models proposed thus far was dependent on the physical properties of the liquid and the solid, while the applicability of the numerical simulations was restricted to certain lengthscales.122 For example, a continuum approach solves for the macroscopic flow field, but the microscopic interactions are omitted.122 On the other hand, most molecular simulation routes are valid only in the nanoscale regime, in which thermal fluctuations can dominate the predictions.122

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MULTISCALE STRUCTURES IMPACT THE SURFACE ENERGY OF POLYMER COATINGS Although the theories for surface energy introduced above were developed only in the last two centuries, their key tenets have been apparent over the long history of natural selection, during which time many organisms have evolved to incorporate surfaces with wetting properties that enhance their survival.34,111,123−128 For example, the multiscale structures on gecko feet provide high adhesive forces toward solids and water, while still maintaining superhydrophobicity.123,124 Similar hierarchical micro- and nanotextures on the leaves of certain plants (e.g., rose and lotus) create superhydrophobic surfaces, such that water droplets either impinge D


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Figure 7. (a) Perfluoroalkyl groups with high mobility rearrange upon contact with water and expose the more polar carbonyl groups to the water interface leading to reduced retention of surface hydrophobicity. (b) Perfluoroalkyl groups with low mobility do not rearrange upon contact with water resulting in enhanced retention of a stable hydrophobic surface. Adapted with permission from ref 145. Copyright 2005 American Chemical Society. Figure 6. Representative chemical structures of polymeric repellent chemistries with (a) PTFE backbones, (b) acrylic perfluoroalkyl side chains, (c) acrylic n-alkyl side chains, (d) polysiloxane backbones, and (e) acrylic oligomeric PFPE (perfluoropolyether) side chains. R is typically a hydrogen atom or a methyl group.

repellency and anti-adhesion.50,66,67,178−189 However, sidechain perfluoroalkyl groups can be severed from the hydrocarbon polymeric chain to which they are attached (largely by ester bonds) by hydrolysis followed by oxidative degradation to release perfluoroalkyl acids (PFAAs), including perfluoroalkyl sulfonic acids (CnF2n+1SO3H, PFSAs) and perfluoroalkyl carboxylic acids (CnF2n+1COOH, PFCAs).178,190 Among these substances, the “long-chain” PFAAs (i.e., PFSAs with n ≥ 6 and PFCAs with n ≥ 7) have become a matter of focus because they are more bioaccumulative than their short-chain analogues.178,190−195 Thus, evolving consumer preferences are driving innovations toward replacement of long-chain SCFPs with nonfluorinated substances,196−199 short-chain SCFPs,66,67,200−205 or perfluoropolyethers (PFPEs).206−213 In the next section, major advances in these areas will be examined. Alternatives for Perfluoroalkyl Groups for Low Surface Energy Coatings. Hydrocarbons and siliconebased chemistries have been identified as promising replacements for SCFPs as water repellents for a variety of surface protection applications.174,196,214−220 One of the first approaches used to improve the hydrophobicity of textiles was to treat them with paraffin wax-based emulsions, which imparted low surface energies to the textile surface through the crystallization and packing of the linear alkyl chains.174,221 Most modern hydrocarbon repellents include acrylic and urethane-based chemistries with n-alkyl groups extending from their respective backbones, as indicated by Figure 6c.196 The side-chain crystallization in these materials affords large contact angles against water.197,198 Alternatively, siliconebased repellents deliver excellent hydrophobicity, which is facilitated by the orientation of the methyl groups away from the substrate.218,222,223 The directional alignment of molecules results from the flexibility of the polysiloxane backbone,199 as shown in Figure 6d, which also gives the coated material a soft feel to the hand.220 Unfortunately, in comparison to many hydrocarbon polymers, silicones are less environmentally friendly and less durable.218 Also, neither of the two nonfluorinated alternatives readily offer the desirable oil repellency provided by SCFP products.224 To introduce oleophobicity to the aforementioned substitutes, SCFPs based on short-chain chemistry with improved

their uniformly organized arrays of CF3 terminal groups can reduce coating surface energies to as low as 6.7 mJ/m2.177 The interfacial properties of SCFP coatings are strongly influenced by the packing, orientation, and motion of the sidechain groups in molecular aggregates.138−141,143 For instance, Pittman et al. examined the wetting behavior of a series of poly(fluoroalkyl acrylate)s with different perfluoroalkyl chain lengths and concluded that polyacrylates with higher crystallinity were less wettable by n-alkane droplets.138 Systematic studies on the effect of perfluoroalkyl chain molecular aggregate structures on surface energies were conducted by Honda et al., for which a family of poly(fluoroalkyl acrylate)s with different side chain lengths were prepared, and the crystal structures, surface chemistries, and contact angles of these polymers as spin-coated films were characterized.145 For polyacrylates with ≤6 carbons per perfluoroalkyl side group, the amorphous fluorinated units rearranged upon contact with water to expose the carbonyl groups and reduced the hydrophobicity of the coatings, Figure 7a.145 On the contrary, longer perfluoroalkyl (≥8 carbons) chains arranged in highly ordered smectic phases139−141 with CF3 groups oriented toward the polymer−air interface.142 The low mobility of this liquid crystalline state retained a stable surface with a large contact angle against water, Figure 7b.145 Honda et al. further examined short-chain (≤6 carbons) poly(fluoroalkyl methacrylate)s with high glass transition temperatures (Tgs) and reported that they have larger θR and more stable hydrophobicity in comparison to poly(fluoroalkyl acrylate)s with the same number of perfluorinated carbons but significantly lower Tgs.144 On the basis of these studies, it was suggested that the key to maintaining low surface energy was restricting molecular motion of perfluoroalkyl chains, as noted by the behavior of longer chain and/or higher Tg SCFPs.144 Benefiting from the ability to form molecular aggregates with low surface energy as discussed above, SCFPs have been extensively used in coating applications that involve water/oil E


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Figure 8. (a) A safety goggle coated with 20 bilayers of anti-fogging chitosan/carboxymethyl cellulose. The yellow boxed area indicates where the coating was applied. Reprinted with permission from ref 147. Copyright 2011 American Chemical Society. (b) A zwitter-wettable film based on a poly(ethylene glycol methyl ether)-functionalized poly(vinyl alcohol)/poly(acrylic acid) multilayer. The right portion of the glass slide was coated with the multilayer film and could resist frost formation at −20 °C. The inset photograph shows the zoom-in image of a water droplet on the multilayer film with a contact angle greater than 90°. Reprinted with permission from ref 148. Copyright 2013 American Chemical Society. (c) Schematic representation of a zwitter-wettable film with added sessile drop (left) and during condensation (right). Reprinted with permission from ref 149. Copyright 2015 American Chemical Society.

product, leading to a low surface energy as a result of the surface-segregated PFPE groups.210 PFPEs also can be incorporated into the backbone of polyesters for better water and oil repellency.207,212 Notably, Wei et al. synthesized triblock polyesters, in which two identical fluorinated segments were separated by poly(ethylene isophthalate) blocks.212 When blended with poly(ethylene terephthalate), nylon-6, and poly(methyl methacrylate) (PMMA), these triblock chains readily migrated to the film surfaces and formed brush-like structures possessing an enhanced concentration of PFPE segments to deliver hydrophobicity and oleophobicity.212 In addition to the linear polymers discussed above, dendrimers, nanosized, nearly monodisperse macromolecules that contain tree-like units built around a core, have also been used as water/oil repellents.219,225 To impart hydrophobicity and/or oleophobicity, the surfaces of these hyperbranched molecules can be modified with fatty acids, polyalkylsiloxanes, or perfluoroalkyl groups, among others.218 Dendritic polymers are especially useful in the textile industry because they are able to form nanometer-sized crystals that do not reduce the air and vapor permeability of the fiber materials.219 However, the widespread application of dendrimers is limited by the significant synthetic costs226 and a lack of knowledge with respect to their associated environmental hazards.218 The above examples suggest that the surface-enriching segregation of side-chain fluorinated polymers that contain perfluoroalkyl chains (CnF2n+1) with n ≥ 6 in low surface energy coatings have been successful, and various substitutes with tuned capabilities have been developed to cater to the consumer’s need. Hazard rankings, such as those developed by Holmquist et al., indicate that systems based on hydrocarbons are the most environmentally benign, followed by silicones and fluorocarbons.218 However, the complete replacement of fluorinated substances still is not feasible in consumer and industrial products that require oleophobicity. Currently, the choice of chemistry for water/oil repellents in the consumer and industrial world is made on a case-by-case basis by

human health and environmental profiles have been employed.174 One family of substances that meets this criterion are SCFPs with ≤6 perfluorinated carbons.200−203 In practice, polymers containing both short-chain SCFPs and hydrocarbon side chains have been adopted in commercially viable products to incorporate both water and oil repellency.66,67,204 However, manufacturing challenges have arisen in the development of shorter perfluoroalkyl chain (e.g., 4 or 6 carbons) alternatives to incumbent long chain technologies over the past decade. Marked deficiencies in performance were observed upon direct “drop-in” substitution. Considerable R&D efforts from manufacturers have been required to optimize the fundamental structure−property relationships of these new polymeric systems toward benchmark performance. 143,174,205 One attempt to reveal the structure−property relationships in these materials was made by Zhang et al. using model systems based on spin-coated copolymers of stearyl acrylate and perfluorohexyl(meth)acrylates with different spacers between perfluoroalkyl chains and hydrocarbon polymer backbones.205 In these materials, perfluoroalkyl and hydrocarbon side chains could form mixed smectic phases that offered restricted mobilities to perfluoroalkyl chains and maintained low surface energies of the films.205 These results agreed with Honda et al. in that systems with decreased perfluoroalkyl side chain molecular mobility exhibited enhanced water/oil repellency.144,145 Another series of fluorinated alternatives are hydrocarbon polymers with oligomeric PFPE (perfluoropolyether) chains as pendant groups, as shown in Figure 6e, which have more favorable toxicological profiles178 and low surface energies.212 These substances usually serve as additives for the surface modification of polymeric materials.207,210−212 As an example, Wang et al. blended poly(butylene terephthalate) with a fluorinated multiblock copolyester containing PFPE side chains and then fabricated fibers from the resulting mixtures via melt-blowing.210 A 4-fold fluorine enrichment relative to the blend composition was found on the surface of the F


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Figure 9. Stable, no-loss transportation of water droplets along (a) a hydrophobic path (PDMS) on a superhydrophobic coating and (b) lowhysteresis guiding trails as straight lines, circular rings, curves, and sinusoidal shapes. The hydrophobic path image is reprinted with permission from ref 103. Copyright 2016 American Chemical Society. The low-hysteresis trails image is reprinted with permission from ref 164. Copyright 2017 American Chemical Society.

hydrogen-bonding-assisted poly(vinyl alcohol) and poly(acrylic acid) functionalized with poly(ethylene glycol methyl ether), Figure 8b.148 Near the surface of these zwitter-wettable coatings, a thin hydrophobic layer with high water permeability was assembled on a hydrophilic reservoir, Figure 8c.149 A sessile drop could retain a large contact angle due to the hydrophobicity of the film, while the permeability of the top layer promoted condensed water to imbibe into the reservoir without nucleating and growing on the surface.149,154 Following the work by Lee et al., several more zwitter-wettable materials were developed for anti-fogging and frost-resistant surface applications.149,150,154,155 The industry based on functional LbL assemblies is currently growing, and it is propelled by the low cost and the ease of film manufacture enabled by mild LBL fabrication conditions.231 Patterning of Surface Chemistry. For most of the materials discussed in earlier sections, a homogeneous surface energy is desired for consistent interfacial performance. Nevertheless, “defects” or gradients may be intentionally introduced, and the resulting anisotropic surface energy can be beneficial for certain applications. Notably, Julthongpiput et al. designed micropatterns that gradually and systematically change in their chemical contrast, i.e., the difference in surface energy between two domains.156 The gradient patterns could serve as a reference tool to calibrate the image contrast in surface-chemistry-sensitive, scanning probe microscopy approaches, such as chemical force and atomic force acoustic microscopy.156 Moreover, a polymer cast on the substrate would have its morphology vary along the wettability gradient, from which a library of dewetting behavior can be constructed, and high-throughput mapping of surface energy effects on interfacial phenomena can be achieved.156 Following a similar

weighing and assessing the desired performance level to the potential health and environmental risks throughout the product life-cycle.218 Phase Separation. In block copolymer and polymer blend systems, the surface chemistries and interfacial attributes of the composites can be controlled by careful manipulation of phase separation. As one example, Salunke et al. tuned the wettability of an amphiphilic film based on a triblock copolymer of poly(2-(N-ethylperfluorooctanesulfonamido)ethyl methyl acrylate) and poly(N,N′-dimethyl acrylamide) by varying the surface morphology.146 For films with hydrophilic domain densities larger than 1 domain/μm2, the contact angle of a water droplet on the copolymer surface was as large as 120° right after the droplet was deposited but then sharply decreased to ≈15°.146 This transition in wettability was explained by a “jump percolation”, in which the water droplet overcame the interconnected hydrophobic background and bridged the hydrophilic domains close to each other.146 Phase separation also can be leveraged to form segregated structures parallel to the polymer−air interface.26,147−153,227−230 For instance, layer-by-layer (LbL) assembly can be employed to fabricate multilayer thin films with anti-fogging properties.26,147−149,151−153 A seminal example used LbL assembly to prepare hydrophilic or superhydrophilic surfaces that promoted the condensation of a layer of optically clear water, Figure 8a.26,147,227 Although these materials had a satisfying performance in the Erlenmeyer steam test and the cold-fog test,147,151,228−230 water at their surfaces induced image distortion and/or frosting.148 To address this problem, Lee et al. demonstrated designs of hydrophobic but hygroscopic; i.e., zwitter-wettable LbL films with both anti-fogging and anti-frosting capabilities based on G


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topography also directly affects tribological attributes by changing the contact area between two bodies. Material attributes, such as viscosity, and modulus, along with experimental (testing) parameters and structure and property evolution during measurements, can add an additional layer of complexity to the aforementioned studies. For example, the heat generated during friction can change the viscosity of the polymer near the contact interface, high contact stresses deform asperities and change the mechanics of contact, and wear during sliding can quickly and completely consume the original surface, which can profoundly affect the surface and interface properties of the system. The following sections provide an overview of recent studies that have investigated the relationship between the structure and tribological properties of polymeric materials. To begin, the two primary tribology metrics of materials performance, friction and wear, will be reviewed. Subsequently, several design strategies for polymer composites with desired tribological attributes will be highlighted, followed by discussion of recent advances that leverage phase separation to uniquely tune structure and tribological characteristics, and ultimately, optimize performance. Quantifying Friction and Wear. Measurements of the friction and wear of materials provide insights into the fundamental interactions between two sliding surfaces and an empirical basis for the design of tribological systems.251 Friction can be characterized by a friction coefficient, μ, which is most often defined as the ratio between the force that resists motion and the applied normal force (the other common definition is the change in friction force per unit change in applied normal force). For rough surfaces in contact, μ is affected by the real contact area, i.e., the total area of all individual spots in contact between two surfaces.252 The other primary factor of concern in tribology studies, wear, can be characterized by examining a wear track or measuring of the volume/mass loss after sliding. A wear track is created when a counterface slides against another surface. After sliding (and in some cases, during sliding), the resulting surface structure, the mechanics of contact, and the wear resistance of the sample can be analyzed.253 The volume of the material removed per unit of normal load per unit distance of sliding is defined as the wear rate, k.15 When dimensional distortions caused by elasticity, plasticity, creep, and thermal expansions are likely to confound the measurement, the mass loss of the sample is instead measured.15 It is also important to consider that μ and k may be time-dependent for a given sample due to the evolution of surface structure during sliding caused by asperitylevel deformation,76 damage-free changes in topography,74 wear,254 and effects of third bodies that are either free to move within the contact interface (debris)255,256 or stick to the surface.257 Most systems exhibit two distinct friction and wear regimes. Run-in is usually a high wear rate regime in which surfaces change rapidly to accommodate the new tribological environment.257 Following reorganization of surfaces, which is usually accompanied by gross changes to the surface, liberation of wear debris from one or both surfaces, and the formation of a transfer film on one or both surfaces, the system usually achieves a lower and stable steady-state wear rate.257 Most measurements of polymer wear isolate and quantify the steadystate wear rate. Polymer Composites as Low-Friction, Ultra-Low Wear Materials. Polymers are the most tribological forgiving class of materials due primarily to their relatively low surface energy

strategy, Albert et al. generated a linear gradient of surface energy on a silicon wafer using functionalized chlorosilanes to offer a platform for studying copolymer thin film morphology on surfaces.157,158 These specimens were prepared by controlled vapor deposition, which has the advantage of facile implementation and enhanced gradient tunability.157,158 The gradients in interfacial characteristics also can be found in both natural and artificial structures for liquid droplet manipulation. In nature, spider silks,128 bird feathers (e.g., breast feathers of Alca torda and Podiceps cristatus),232 and plant leaves233,234 have inhomogeneous surface energies for directional collection or removal of water droplets. Inspired by these materials, Chaudhury et al. created a spatial gradient of hydrophobicity on a silicon wafer by systematically varying the concentration of decyltrichlorosilane on the surface.235 A water droplet deposited on the treated substrate moved from downhill to uphill on a 15° slope.235 Expanding this idea, Hu et al. fabricated a sinusoidal, water-repellent polydimethylsiloxane (PDMS) path on a superhydrophobic surface made from a commercially viable coating, Ultra-Ever Dry (Ultra Tech International).103 With a directional wind flow, water droplets moved along the PDMS path, as shown in Figure 9a.103 In comparison to previously reported designs using hydrophilic tracks on which small droplets could be left behind during transportation,162,163 Hu et al. achieved movement of water droplets without mass loss.103 Dong et al. posited that a defect-free track with low contact angle hysteresis was key to stable liquid transportation along the path.164 Surfaces prepared based on this strategy proved effective at guiding both water droplets and streams, Figure 9b.164 These tracks can be easily manufactured by the shadow mask method or photolithography.103,164 The capability of liquid transport along these paths can be used in applications, such as fog collection,236,237 oil−water separations,238 and microfluidics.239

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TRIBOLOGICAL PROPERTIES FROM A STRUCTURAL ASPECT Tribology, the science that relates to interacting surfaces in relative motion, is prevalent in every aspect of our daily lives and has an enormous economic impacts.240 According to a report by Holmberg et al., ∼23% (119 EJ) of the world’s total energy consumption originates from tribological contacts, and the implementation of new materials and technologies potentially could eliminate 40% of the energy losses.241 The tribological characteristics of polymers are critical to a wide range of applications involving direct contact with other surfaces (lubricants,240,242 electric motors,243 pumping systems,244 tires,245 touchscreens,246 automotive coatings,42−44 rotor blades,247 footwear soles,248 flooring,249 etc.). Although the history of tribology can be traced back to Leonardo da Vinci in 1493,250 few systematic studies were conducted on the relationship between structures and tribological attributes of polymeric substances over the following five centuries due primarily to the interdisciplinary nature of the research and the difficulty in directly probing the buried interface where the phenomena of interest occur. As suggested in Figure 2, the tribological properties are influenced by numerous factors. The structure of the specimen (e.g., microstructure, surface chemistry, and surface topography/ patterning from molecular structures and processing conditions) influences the surface energy, which in turn dominates the interfacial forces that alter friction and wear. Surface H


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ACS Applied Polymer Materials and relatively high tolerance to strain without failure.254 Nonetheless, the overall tribological performance of any single component polymer system is objectively poor. As an example, PTFE is frequently used as a solid lubricant because of its low friction,143 yet the polymer suffers from a high wear rate, 10−3−10−5 mm3/(N·m).258 Engineering polymers like polyetheretherketone (PEEK) offer improved wear performance at the expense of increased friction. In contrast to the previous single-component examples, polymer composites with two or more constituents often exhibit a favorable combination of tribological attributes from each constituent.15,240,254 For instance, hard, micrometer-sized particles and fibers can be introduced into PTFE to prevent crack formation and delamination.259−261 The disadvantage of these microscale fillers is that they tend to abrade the counterface and the transfer film, a protective layer of wear fragments that normally adheres to the relatively high surface energy counterface.15,257,259,262−274 Under these conditions, transfer films form and reform in perpetuity, which inhibits interfacial stability, low friction, and low wear at steady state.15 The adverse effect of the aforementioned microparticles and fibers can be alleviated when nanoscale or otherwise nonabrasive fillers are used.275−277 Nanofillers have been successfully integrated into various polymer matrices to significantly reduce either friction or wear; in the most successful examples, the authors note that tribological improvements are accompanied by improvements in the quality and apparent stability of the transfer films.15,259,262−267,277−282 Inspired by this design strategy, Burris et al. saw a decrease in the wear rate for PTFE by orders of magnitude after filling the polymer with 5 wt % alumina nanoparticles.283,284 In subsequent work, Burris and others demonstrated that similar materials were capable of wear rates below 10−7 mm3/(N·m).268,270,272,285−288 Interestingly, other similar alumina nanoparticle-filled PTFEs created and tested under exactly the same conditions produced ∼100× greater steady-state wear rates.279,284,289 The discrepancy was explained in a recent article, which showed that the most successful nanoparticles were micrometer agglomerates of lightly sintered nanoparticles; these agglomerates were big and strong enough to arrest subsurface damage but soft enough to disband into their nanoparticle constituents without damaging the protective polymeric transfer film once reaching the tribological interface.289 Polymer composites with reduced friction and wear also can be achieved by blending one polymer with another.14,48,240,253,290 McCook et al. impregnated expanded PTFE with epoxy for improved wear resistance.14 In this system, the cocontinuous structure was proposed to compartmentalize subsurface damage while promoting the formation of stable low shear strength transfer films over a relatively stiffer and stronger composite bulk.14 Adopting a similar methodology, Burris et al. developed PEEK-filled PTFE composites with among the lowest dry sliding wear rates reported in the literature.48 Scanning electron microscopy studies of failure surfaces indicated that homogeneous and phase-separated PTFE and PEEK domains were interconnected via interwoven fibrils, Figure 10a.48 As with the epoxy-PTFE system from McCook et al., it was proposed that this entanglement between filler and matrix helped compartmentalize debris while promoting transfer film stability and lubricity.48 Similar structures were reported by Jones et al. for polyamide-imide/ PTFE composites, for which similar ultra-low wear rates were

Figure 10. (a) An illustration of the PEEK/PTFE composite structure, in which the major phase (PTFE) exists as neat regions interconnected via PTFE fibrils. The PTFE running films thought to be responsible for lubrication are shown as originating from neat PTFE. Reprinted with permission from ref 48. Copyright 2006 Elsevier. (b) PI/PS blends have an μ lower than homopolymers, along with the predicted trend on the basis of standard mixing rules. The minimum value of μ is comparable to that of PTFE. Reprinted with permission from ref 253. Copyright 2017 American Chemical Society.

attributed to the mechanical interlocking of the two components in the blend.290 Despite the successful iterative development of exceptional tribological polymer composites and blends over the last two decades, dramatic improvements in performance often are attributed to improved transfer films with little regard for the underlying structure−property relationships that enabled those stable low friction transfer films to form. One significant insight into this topic was made by Emerson et al. during a study of model polyisoprene (PI) and PS blend coatings.253 Several of the PS−PI blends produced substantially lower μ’s than those of either pure constituent, Figure 10b; in fact, the 0.1 friction coefficient of the optimum blend (75 wt % PS) is competitive with that of neat PTFE.291 This result was unusual in that neither constituent was a lubricating or low friction polymer. Furthermore, the authors saw no obvious evidence of transfer films, which suggested tribological synergy independent of the usual effects from protective or lubricating transfer films. While the size scale of the phase-separated microstructure is expected to vary significantly with changes in composition, this work was perhaps the first study to isolate the specific correlation between polymer composite microstructure and the tribological results. They isolated this potential microstructural size effect with optimal 75 wt % PS samples by annealing them for varying times to systematically increase their domain sizes; increasing domain size by 100% systematically increased friction by up to 200%. They proposed that the phaseseparated microstructure effectively limited the size of real contacts and disrupted the shear-induced growth and I


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ACS Applied Polymer Materials coalescence of contact junctions; in this context, the observed correlation between microstructure size and tribology is consistent with recent theoretical work showing how the size of each real contact area directly controls adhesive friction, wear, and transfer mechanisms.255,256 This study provided the only direct evidence (to the authors’ knowledge) that the microstructural size scale of these polymer composites and blends directly and significantly affects tribological performance independently of transfer film effects.253 Additionally, the work of Aghababaei et al. predicted (via simulations) a critical size of the contact junction (d*) that separates two wear mechanisms.255 Wear from junctions with diameter d > d* would produce loose particles, while that from d < d* would produce films.255 Both the above experimental study and the simulation work are consistent with the hypothesis that domain size is the primary variable that controls transfer film quality. It is interesting to note, that the ultra-low wear PEEK−PTFE from Burris and Sawyer48 used a proprietary cryo-ground PEEK, which likely reduced the microstructural size scale relative to those of more typical PEEK−PTFE composites using larger particles and exhibiting greater wear rates.240,273,274 In the light of this recent work, it is important to investigate other model systems, especially the ones with low friction constituents, to further elucidate the relationship between polymer blend morphology and tribological performance.

While consistent interfacial performance is desired for the aforementioned structures, gradients can be intentionally introduced to a substrate to provide a platform for the highthroughput analysis of polymer thin film behavior.156 Additionally, anisotropic surface energies can be used to guide the movement of water droplets and streams.235 Such systems have potential applications in fog collection, oil−water separation, and microfluidics.236−239 As the development of structure−interfacial property relationships over multiple length scales is ongoing, challenges remain in the design of coatings with desirable performance characteristics. For instance, the phase-out of hazardous chemicals (e.g., long-chain perfluoroalkyl substances293) in low surface energy products has been driven by policy actions, including the Stockholm Convention that restricted the use of perfluorooctanesulfonic acids,294 the ECHA REACH legislation that identified perfluorooctanoic acid as a substance of very high concern,295 and the Zero Discharge of Hazardous Chemicals Roadmap that aimed to eliminate application of priority chemicals that can cause pollution in the global textile, leather, and footwear value chain, etc.296 Materials that meet the requirements of these regulations have been fabricated from nonfluorinated substances, such as paraffin waxes, silicones, and alkyl urethanes, among others.174,218,221−223 However, the oleophobicities of the above alternatives are not satisfying, and less environmentally benign fluorocarbons are still needed in oil-repellent applications.218 To develop products that are compliant with the policies without sacrificing the performance, it is important to exploit the diversity of candidates by examining the structure−interfacial property relationships of a variety of model systems, during which the critical variables for the design of the next generation of low surface energy coatings can be identified. A similar strategy can be applied to promote the creation of substances with desired tribological characteristics. As an example, although PTFE-based composites with low friction and ultra-low wear have been reported,48,268,270,272,283−285,290 their applications can be hindered by the high sintering temperature and poor solubility of PTFE in solvents under many coating conditions.171 To design alternatives without these issues, it is necessary to unravel the structural basis of the friction- and wear-reduction mechanisms. However, a lack of interdisciplinary expertise is currently hindering the essential tribological studies. In the next section, the challenges highlighted above and their potential solutions will be discussed in greater detail. Outlook. Toward Environmentally Friendly, Low Surface Energy Coatings. During the lifetime of a coating, surface components can be released by wear, tear, laundering, photooxidation, and/or hydrolysis.218 These diffuse-emission pollutants are a major concern in the hazard rankings of coating materials. For example, side-chain fluorinated polymers used in low surface energy applications may degrade into PFAAs, and long-chain PFAAs, such as perfluorooctanesulfonate, which are associated with potential adverse effects to human health and the environment. Policy, along with industry action, has led major global manufacturers to severely limit the manufacture and use of the long-chain fluorinated substances.178,218 In comparison to side-chain fluorinated polymers, alternative polymers that deliver the needed function and do not release potentially harmful pollutants are preferred for the next generation of durable water-repellent coatings. Among a series of candidates, polymers sourced from

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SUMMARY AND OUTLOOK Summary. Due to their ease of application, costeffectiveness, and design flexibility, polymer coatings remain some of the most widely used tools for imparting desired interfacial characteristics to solid substrates. To develop the next generation of polymer films with optimized performance, extensive research efforts have focused on elucidating the relationship between structures at different length-scales and their resulting interfacial properties. Subnanometer- to nanometer-sized molecular aggregates have been employed to tune the wettability of coatings, in which low surface energies were achieved by manipulating two factors, (1) the concentration and nature of the hydrophobic/oleophobic groups at the surface of the coating, and (2) the molecular motion of the aggregate structure. At larger dimensions, nanoscale or microscale phase separation delivers synergistic effects from each of the composite’s constituents to the interfacial attributes of the coatings. Notably, nanometer-sized hydrophilic and hydrophobic layers in LbL assemblies can be manipulated to create zwitter-wettable surfaces for anti-fogging and anti-frosting applications.148−150,154,155 Materials synergies from phase separation also can be found at the micrometer scale.48,292 For instance, in PTFE/PEEK blends, the mechanical interlocking of the two neat regions prevents delamination of PTFE to reduce wear, and the running and transfer films originated from PTFE provide lubrication to decrease friction.48 These two mechanisms together contribute to achieving μ and k values that are smaller than either of the homopolymer constituents; these values also were much smaller than predictions based on standard mixing rules.48 As another option, running/transfer-film-free systems made from high friction materials can benefit from the structural synergies that change the sizes of real contact areas between two surfaces, hinder the growth and coalescence of contact junctions during sliding, and in turn, decrease μ and k of the composites.253 J


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tribological studies on this topic require interdisciplinary knowledge from physics, chemistry, mechanical engineering, and materials science.242 However, as stated in a review article by Burris et al., “Often, tribologists lack the materials science background to conduct thorough nanocomposite characterization, and materials scientists lack the expertise required to conduct detailed tribological investigations on their wellcharacterized nanocomposites.”15 Up to now, the problem revealed by this statement has not been fully resolved, and the deconvolution of key mechanisms remains challenging. Looking forward, there are two major research directions that can be greatly facilitated by the collaboration of materials scientists and tribologists, (1) elucidating the formation and growth process of polymeric transfer films, and (2) determining the structural basis of the friction/wear reduction in running/transfer-film-free materials. A stable, continuous, and uniform transfer film is crucial for the low friction and ultra-low wear behavior of PTFE-based polymer composites.15,259,262−267 However, no structure-based design is currently available to impart transfer films that meet these criteria, because the formation mechanism of these tribomaterials is not fully elucidated. Harris et al., among others, described a tribochemistry processes that is applicable to PTFE-nanoparticle composites sliding with metal counterfaces in humid environments.268−272 More recently, Onodera et al. and Haidar et al. proposed a physical interaction between PEEK and the counterface that is responsible for the transfer film formation in dry environments.273,274 While no consensus has been reached, tribology tests on PTFE-based polymer composites combining in situ studies of transfer film chemistry and structure are encouraged to enhance the understanding of these tribomaterials. A common in situ tribology approach is to perform measurements (e.g., optical microscopy, interferometry, Raman microscopy, attenuated total reflectance Fouriertransform infrared spectroscopy, etc.) on the surface of the sample within the environment but outside the contact.303 This method, when associated with a cycle-by-cycle analysis, can be implemented to connect the evolution of tribofilms and surface topography with friction and wear data.303 Additionally, optics combined with a transparent counterbody can be used to monitor the change of the buried interfaces within the contact.303 For polymer composites that do not form running and transfer films, small μ and k values that deviate from the mixing rules can still be achieved due to the development of a phaseseparated microstructure. Up to now, this tribological synergy has only been isolated using the PS/PI blends reported by Emerson et al.253 Thus, investigations on other systems are required to determine if this structural effect is a general phenomenon. Polymer composites with low friction transfer film forming constituents are especially interesting given recent theoretical evidence that the same contact scaling effects likely control the quality of the transfer films formed by the adhesive wear process.255 At the moment, these new ideas suggest that domain size is one of the most influential parameters available for the design of next generation tribomaterials. To summarize, understanding the key structure−interfacial properties remains crucial to facilitating the realization of new products with desired surface performance. Scientists with different synergistic expertise are encouraged to collaborate with each other and exploit a diversity of systems at multiple length-scales. In this way, the commercialization of more

renewable resources, such as carbon dioxide, terpene, vegetable oil, lignin, and cellulose, are of primary interest for two reasons, (1) these materials can reduce the dependence of the manufacturing process on petrochemicals, which will decrease the impact on the environment; and (2) these substances may be more sustainable, recyclable, and/or biodegradable.83,297−301 Furthermore, certain desired performance, such as hydrophobicity, must not be compromised during substitution with the polymers described above. One notable product in this vein is a renewably sourced, plant-based alkyl urethane fabric finish, Zelan R3, which has excellent water repellency, high durability, and has been approved by the “Bluesign” standard for the health and safety in the production of textiles.302 Although the aforementioned materials have high hydrophobicity and reduced potential risk to the environment, their lack of oil repellency limits their ability to completely replace side-chain fluorinated polymer coatings. This problem was emphasized in a recent article by Schellenberger et al., in which a series of nonfluorinated water repellents based on wax, PDMS, silicone functionalized polyurethane, fatty acidmodified saccharide, or silicone-modified saccharide were assessed for their performance on textiles against perfluoroalkyl-containing alternatives.224 Schellenberger et al. concluded that samples with minimal or no fluorocarbon content can have comparable hydrophobicity to the perfluoroalkyls and moderate repellency toward liquids with intermediate polarity, such as red wine or synthetic blood.224 However, only polymers with perfluoroalkyl groups can provide sufficient protection against nonpolar liquids with low surface tension (e.g., olive oil or gastric fluid).224 To achieve a complete phase-out of fluorinated substances, it is essential to explore a variety of model systems and identify the key structure−property relationships that promote the manufacturing of coatings with desired performance. One advance on this topic was made by Emerson et al., for which functionalities and interfacial characteristics were connected to a representative library of polymers inspired by lignin, an abundant, inexpensive, and renewable resource with comparable thermal and mechanical properties as PS and PMMA.86 Upon examining the interfacial characteristics of these bioderived materials, Emerson et al. concluded that the surface energy of the coatings was affected by the number and chain length of substituents on the aromatic ring, and the friction coefficient was not strongly influenced by the chemistry of the polymers.86 Future work on renewable substances in coatings could target the connection between interfacial properties and systematically varied features in model systems to facilitate the structure-based design of desired attributes (e.g., oleophobicity) in the next generation of coatings. Unraveling the Friction and Wear Reduction Mechanisms in Polymer Composites. As discussed above, PTFE-based blends or nanocomposites with low friction, ultra-low wear behavior have been fabricated,48,268,270,272,283−285,290 but the application of these materials can be limited by poor compatibility with many coating processes.171 To develop polymer materials with better processability without compromising the tribological characteristics, it is necessary to elucidate the mechanisms behind the reduction of friction and wear in soft materials composites. The interplay between design, structure, properties, and experimental parameters shown in Figure 2 indicates that K


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(13) Gu, M.; Jiang, C.; Liu, D.; Prempeh, N.; Smalyukh, I. I. Cellulose nanocrystal/poly (ethylene glycol) composite as an iridescent coating on polymer substrates: Structure-color and interface adhesion. ACS Appl. Mater. Interfaces 2016, 8, 32565−32573. (14) McCook, N.; Burris, D.; Bourne, G.; Steffens, J.; Hanrahan, J.; Sawyer, W. Wear resistant solid lubricant coating made from PTFE and epoxy. Tribol. Lett. 2005, 18, 119−124. (15) Burris, D. L.; Boesl, B.; Bourne, G. R.; Sawyer, W. G. Polymeric nanocomposites for tribological applications. Macromol. Mater. Eng. 2007, 292, 387−402. (16) Ye, J.; Khare, H. S.; Burris, D. L. Quantitative characterization of solid lubricant transfer film quality. Wear 2014, 316, 133−143. (17) Das, S.; Kumar, S.; Samal, S. K.; Mohanty, S.; Nayak, S. K. A Review on Superhydrophobic Polymer Nanocoatings: Recent Development and Applications. Ind. Eng. Chem. Res. 2018, 57, 2727−2745. (18) Steele, A.; Bayer, I.; Loth, E. Inherently superoleophobic nanocomposite coatings by spray atomization. Nano Lett. 2009, 9, 501−505. (19) Xue, Z.; Liu, M.; Jiang, L. Recent developments in polymeric superoleophobic surfaces. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 1209−1224. (20) Smith, J. R.; Lamprou, D. A. Polymer coatings for biomedical applications: a review. Trans. Inst. Met. Finish. 2014, 92, 9−19. (21) Wei, Q.; Haag, R. Universal polymer coatings and their representative biomedical applications. Mater. Horiz. 2015, 2, 567− 577. (22) Cho, S. H.; White, S. R.; Braun, P. V. Self healing polymer coatings. Adv. Mater. 2009, 21, 645−649. (23) van Benthem, R. A.; Ming, W. M.; de With, G. B. Self healing polymer coatings. In Self Healing Materials; van der Zwaag, S., Ed.; Springer, 2007; pp 139−159. (24) Howarter, J. A.; Youngblood, J. P. Self Cleaning and Anti Fog Surfaces Via Stimuli Responsive Polymer Brushes. Adv. Mater. 2007, 19, 3838−3843. (25) Ganesh, V. A.; Raut, H. K.; Nair, A. S.; Ramakrishna, S. A review on self-cleaning coatings. J. Mater. Chem. 2011, 21, 16304− 16322. (26) Cebeci, F. Ç .; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Nanoporosity-driven superhydrophilicity: a means to create multifunctional antifogging coatings. Langmuir 2006, 22, 2856−2862. (27) Tahk, D.; Kim, T. I.; Yoon, H.; Choi, M.; Shin, K.; Suh, K. Y. Fabrication of antireflection and antifogging polymer sheet by partial photopolymerization and dry etching. Langmuir 2010, 26, 2240− 2243. (28) Zhao, J.; Ma, L.; Millians, W.; Wu, T.; Ming, W. Dualfunctional antifogging/antimicrobial polymer coating. ACS Appl. Mater. Interfaces 2016, 8, 8737−8742. (29) England, M. W.; Urata, C.; Dunderdale, G. J.; Hozumi, A. Antifogging/self-healing properties of clay-containing transparent nanocomposite thin films. ACS Appl. Mater. Interfaces 2016, 8, 4318−4322. (30) Li, J.; Zhao, Y.; Hu, J.; Shu, L.; Shi, X. Anti-icing performance of a superhydrophobic PDMS/modified nano-silica hybrid coating for insulators. J. Adhes. Sci. Technol. 2012, 26, 665−679. (31) Peng, C.; Xing, S.; Yuan, Z.; Xiao, J.; Wang, C.; Zeng, J. Preparation and anti-icing of superhydrophobic PVDF coating on a wind turbine blade. Appl. Surf. Sci. 2012, 259, 764−768. (32) Banerjee, I.; Pangule, R. C.; Kane, R. S. Antifouling coatings: recent developments in the design of surfaces that prevent fouling by proteins, bacteria, and marine organisms. Adv. Mater. 2011, 23, 690− 718. (33) Chambers, L. D.; Stokes, K. R.; Walsh, F. C.; Wood, R. J. Modern approaches to marine antifouling coatings. Surf. Coat. Technol. 2006, 201, 3642−3652. (34) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (35) Genzer, J.; Marmur, A. Biological and synthetic self-cleaning surfaces. MRS Bull. 2008, 33, 742−746.

environmentally friendly, more feasible, and lower-cost materials can be promoted to unlock a safe, reliable, and cost-effective future toward the next generation of polymer coatings.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Chao Wang: 0000-0002-5205-9771 David L. Burris: 0000-0003-2687-7540 LaShanda T. J. Korley: 0000-0002-8266-5000 Thomas H. Epps, III: 0000-0002-2513-0966 Notes

The authors declare the following competing financial interest(s): G.O.B. is an employee of The Chemours Company.

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ACKNOWLEDGMENTS C.W., G.O.B., D.L.B., L.T.J.K., and T.H.E. acknowledge financial support from Chemours, LLC, and the feedback and contributions from Dr. Robert C. Buck, Dr. John C. Sworen, and Dr. Justin M. Hoerter during the writing of this manuscript. T.H.E. also thanks the Thomas & Kipp Gutshall Professorship for financial support.

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REFERENCES

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