[
    {
        "id": 1,
        "chunk": "# Design of Abrasion-Resistant, Long-Lasting Antifog Coatings  \n\nBrian Macdonald, Fan-Wei Wang, Brian Tobelmann, Jing Wang, Jason Landini, Nipuli Gunaratne, Joseph Kovac, Todd Miller, Ravi Mosurkal, and Anish Tuteja\\*  \n\nCite This: ACS Appl. Mater. Interfaces 2024, 16, 13018−13028",
        "category": " Introduction"
    },
    {
        "id": 2,
        "chunk": "# ACCESS  \n\nMetrics & More  \n\nArticle Recommendations  \n\nSupporting Information  \n\nABSTRACT: Fog formation is a common challenge for numerous applications, such as food packaging, mirrors, building windows, and freezer/refrigerator doors. Most notably, fog forms on the inner surfaces of prescription glasses and safety eyewear (particularly when used with a mask), face shields, and helmet lenses. Fogging is caused by the distortion of light from condensed water droplets present on a surface and can typically be prevented if the surface static water contact angle $\\mathbf{\\eta}(\\theta)$ is less than ${\\sim}40^{\\circ}$ . Such a low contact angle can be readily achieved by either increasing the substrate surface energy or by engineering surface nanotexture. Unfortunately, such nanotexture can be readily damaged with use, while high surface energy substrates get covered with low surface energy foulants over time. Consequently, even with numerous ephemeral antifog coatings, currently there are no commercially available, durable, and permanent antifog coatings. Here we discuss the development of a new class of high-performance antifog coatings that are abrasion-resistant and long-lasting. These polyvinylpyrrolidone-based coatings, designed based on the classical Ratner−Lancaster wear model, dramatically outperform the base polymer, as well as all tested commercially available antifog coatings. Specifically, these coatings exhibit $\\mathrm{\\a>400\\%}$ increase in fogging time compared to base polymer, $\\mathfrak{a}>50,000\\%$ increase in wear resistance, and excellent long-term antifog performance. The developed coatings also significantly outperformed all tested commercially available antifog coatings in terms of their antifog performance, wear resistance, and long-term cyclical performance. Additionally, the key design strategies employed here\u0001 incorporation of toughening agents and hydrophilic slip additives\u0001offer a new approach to developing high-performance, durable antifog coatings based on other well-known antifog polymers.  \n\n![](images/b726223f3c8464c51dbf3ac599e770be77de68dac2bfaf8860a72b3886a115fd.jpg)  \n\nKEYWORDS: surface science, polymers, antifog, tribology, interfacial phenomena",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# 1. INTRODUCTION  \n\nSurface fogging is typically caused by the condensation of water microdroplets on cold surfaces which scatter light, rendering ordinarily transparent surfaces hazy.1 Fogging typically occurs when the static water contact angle on the surface $\\theta~>~40^{\\circ}$ .1 Fogging adversely affects a variety of transparent surfaces such as prescription glasses,2 safety eyewear,3 swimming goggles,4 camera lenses,5 Hazmat face shields,6 and soldier eyewear.7,8 Additional negative impacts of fogging include the reduction of efficient solar energy conversion,9 and the contamination of medical devices.10 To mitigate surface fogging, highly hydrophilic coatings (water contact angle ${<}40^{\\circ}$ ) are preferable, though some studies have demonstrated antifog performance with hydrophilic coatings that possess somewhat higher contact angles.11,12 Such coatings can maintain optical transparency and clarity,13 and can dry faster via convection than hydrophobic surfaces.14  \n\nThe design of hydrophilic antifog coatings typically employs one or both of the following strategies: modifying the surface chemistry or surface texture. A common approach to increase surface wetting is by increasing the coatings surface energy via plasma treatment.15,16 However, plasma-treated materials typically succumb to “hydrophobic recovery,” where the material loses its hydrophilic properties due to the deposition of lower surface energy molecules from the environment (such as different silicones, waxes, and other organic molecules), and/or the diffusion of low energy molecules from within the coating to the air interface.16,17  \n\nMicro- and/or nanotextured surfaces have also been utilized for developing hydrophilic antifog coatings. Such surfaces can significantly reduce the apparent contact angles on the surface as the water droplets fully wet the porous texture and are in the Wenzel state.18 Notable examples of this approach include microfabricated nanocones and nanopillars, which display good antifogging properties.19,20 S uch textured surfaces typically require highly precise and expensive fabrication/ processing conditions, making scalability and cost a significant concern. The durability of the fabricated textures is also an issue, as the precisely produced micro- or nanostructures are highly susceptible to damage and wear during use.  \n\n![](images/41287e19a542adc88e312862c921fa22da4bdb4dc584b87e0660efa921165e60.jpg)  \n\n![](images/5943363775684c557351030d3717bba4bd687fce537bfdf9af2d02b34b2f6d78.jpg)  \nFigure 1. (A) A schematic illustrating the different components and the fabrication methodology to produce a durable, high-performance antifog coating. (B) Optical images highlighting the enhanced antifog performance due to the addition of a hydrophilic slip additive. (C) A schematic and scanning electron microscopy images illustrating the friction and wear reduction induced via the addition of a hydrophilic slip additive.  \n\nCeramic coatings such as titania and zinc oxide have also been investigated for antifog applications. On these coatings, water contact angle can decrease dramatically upon UV exposure.1,21−24 However, despite their performance and inherent durability, ceramic coatings often require high temperature calcination, eliminating the possibility of using plastic substrates such as polycarbonate, commonly utilized for producing safety glasses or goggle lenses. Additionally, such coatings are also vulnerable to the deposition of lower surface energy molecules from the environment (due to their high solid surface energy) and as such cannot provide long-term antifog performance.  \n\nAntifog coatings can additionally be fabricated by utilizing relatively high surface energy polymers such as polyethylene glycol,25 poly(vinyl alcohol),11 polyvinyl acetate,11 polyvinylpyrrolidone,26 as well as the recently developed zwitterionic and amphiphilic polymers.27−29 These hydrophilic polymers have low water contact angles, and most are able to absorb moisture to further reduce the water contact angles over time.26,29 However, such polymeric antifogging coatings can typically be easily damaged via mechanical abrasion and may lose their performance after prolonged exposure to a fogging environment.  \n\nFinally, layer-by-layer self-assembly of cationic and anionic polymers has also been utilized to form highly effective antifog coatings.30−33 This methodology, however, can suffer from the need for extensive surface preparation and long dip- and/or cure-times. Additionally, while many of the developed coatings can display high pencil hardness, most of the coatings can still be readily damaged by mechanical abrasion. Overall, the current strategies for the fabrication of antifog coatings suffer from either poor mechanical durability and/or loss of performance over the long-term due to their inherently high surface energy. Consequently, despite numerous temporary, commercial antifog coatings, there are no high-performance permanent, durable antifog coatings available in the market.  \n\nWith the benefits of easier fabrication, processing, lower weight, feasible application to plastic substrates, and lower cost, polymer coatings are well-suited for the development of durable, long-term, antifog coatings. The mechanical durability of polymeric coatings during abrasion can be described by the Ratner−Lancaster relation as  \n\n$$\nW=C\\mu{\\left({\\frac{1}{H\\varepsilon_{\\mathrm{u}}\\sigma_{\\mathrm{u}}}}\\right)}\n$$  \n\nwhere $W$ is the wear rate (defined as the abraded material volume per unit travel distance of the abraser), $C$ is a constant, $\\mu$ is the friction coefficient, $H$ is the coating hardness, and $\\varepsilon_{\\mathrm{u}}$ and $\\sigma_{\\mathrm{u}}$ are the strain and stress at tensile break, respectively.34,35 This relation illustrates that the polymer wear coefficient is proportional to the interfacial friction coefficient and inversely proportional to the film hardness, and the strain and stress at tensile break.34 Therefore, increasing the coating hardness, tensile stress, and strain at break and reducing the surface frictional coefficient can increase the polymer wear resistance.  \n\nThough eq 1 provides important material design criteria for developing durable polymeric coatings, there are two major challenges specific to the design of hydrophilic antifogging coatings. First, hydrophilic polymers tend to have a narrow range of miscibility, limiting the number of additives that can be used to enhance their hardness and tensile properties. Second, the high surface energy of the hydrophilic antifogging polymeric coatings inherently results in a high work of adhesion, which can lead to a high friction coefficient.36,37 Although several attempts have been made to engineer wearresistant polymeric antifog coatin g,26,38−40 thus far it has not been possible to simultaneously overcome these two material design challenges.  \n\nHerein, we report the systematic design and fabrication of durable polymeric antifogging coatings that simultaneously demonstrate high surface hardness, stress, strain at break, and a low friction coefficient. These coatings are fabricated by utilizing a cross-linked polyvinylpyrrolidone (PVP) matrix, which is modified with two critical additives for the simultaneous enhancement of antifogging performance, as well as mechanical durability. We have also developed a facile two-step spray-coating process to apply these coatings onto a wide variety of plastic surfaces as shown in Figure 1A. This fabrication process can be readily scaled up, and as shown in Figure 1A, the developed coating is highly effective at preventing fogging. As described in detail below, the fabricated coatings are over $400\\%$ more effective at preventing the appearance of fog than unmodified PVP, and readily surpass the long-term performance of different commercially available antifog coatings (Figure 1B). In addition, our antifogging coatings display a higher toughness and a friction coefficient that is $300\\%$ lower than that of PVP alone. Consequently, they can survive more than 500-fold $(50,000\\%$ increase) additional mechanical abrasion cycles before failure when compared with unmodified PVP coatings (Figure 1C). Overall, our work shows that the Ratner−Lancaster relation can be used to systematically design high-performance antifog coatings that are simultaneously abrasion-resistant and long-lasting.",
        "category": " Introduction"
    },
    {
        "id": 4,
        "chunk": "# 2. MATERIALS AND METHODS  \n\n2.1. Antifog Coating Fabrication. 2.1.1. Solution Parameters. A $0.0265~\\mathrm{g/mL}$ solution of $1300\\mathrm{kDa}$ PVP (Sigma-Aldrich) and 0.001 $\\mathbf{g}/\\mathbf{m}\\mathbf{L}$ benzophenone (Sigma-Aldrich) was made by dissolving the polymer in a solution containing 1-propanol (Fischer Scientific), distilled water, and $15\\%$ Hydrogen peroxide (Fisher Scientific). The volumetric ratio of these three solvents was 0.931:0.029:0.040 respectively. The surfactants and cross-linker were later dissolved in solutions of 1-propanol in order to dilute and add to the PVP solution. Polysorbate 20, polysorbate 80, polysorbate 85, Span 20 or Span 80 (all purchased from Sigma-Aldrich) were dissolved in 1- propanol in varying concentrations. Pentaerythritol tetraacrylate (PETRA) (Sigma-Aldrich) was also dissolved in 1-propanol in varying concentrations. PETRA and surfactant solutions were added to the solvent mixture to achieve a volume ratio of 0.895 1-propanol: 0.027 DI water: 0 $.03915\\%\\mathrm{H}_{2}\\mathrm{O}_{2}$ : 0.019 PETRA/1-propanol solution: 0.019 surfactant/1-propanol solution (PVP now at $0.0254~\\mathrm{mg/mL}$ ). All contents were stirred in a glass septa-jar until fully dissolved $\\cdot{>}2$ h). Concentrations of PETRA or surfactants are with respect to the mass of PVP.  \n\n2.1.2. Coating Fabrication. Impact-resistant polycarbonate strips (purchased from McMaster-Carr) of 1 in. width and $1/16$ in. thickness were cut to 4 in. slides for testing. Polycarbonate strips were first cleaned with isopropyl alcohol and blown dry before exposure to $30~\\mathrm{W}$ oxygen plasma (Harrik Plasma Cleaner PDC-001) for $10~\\mathrm{min}$ . Once treated with plasma, the samples were coated using a homebuilt automated spray coating apparatus. The plasma-treated samples were placed at a distance of $20\\ \\mathrm{cm}$ from the spray gun. The automated spray coater with spray gun (HVLP mini touch up ATD-6903) maintained parameters set to a linear speed of ${\\sim}20\\ \\mathrm{cm/s},$ 2 spray passes, and spray pressure of 20 psi. The solutions were then dispensed into the spray reservoir and sprayed onto the plasmatreated polycarbonate. This allowed for the fabrication of coatings with a consistent thickness and surface texture. Immediately after, the coatings were subjected to UVA (UVP 100 W Longwave Mercury Spot Lamp) and UVC (UVP XX-40S Bench Lamp $2.6\\mathrm{\\mW/cm^{2}}$ ) exposure at a distance of $12\\mathrm{cm}$ from the lamps for a period of $10\\mathrm{min}$ . Commercially available antifog coatings: Optix 55 Anti Fog Treatment for Anti-Reflective Lenses (Optix) (purchased from Optix 55), Op Drops Anti-Fog and Lens Cleaner (Ops Drops) (purchased from Gear Aid), Revision wipes (Revision Military), and  \n\nExxene HCF-100 (Exxene Corporation) were fabricated following the instructions provided on the products.  \n\n2.1.3. Evaluation of Coating Properties and Antifog Performance. Infrared spectrum measurement was conducted by a Nicolet 6700 FTIR spectrometer (Thermo Scientific). The indentation hardness measurement and the tensile tests of PVP and PVP/ PETRA thin strips were conducted by utilizing the Hysitron TI 950 TriboIndenter (Bruker) and an RSA-G2 DMA (TA Instruments), respectively. The detailed sample fabrication for PVP and PVP/ PETRA thin strips and the raw stress−strain data for each coating formulation are provided in the Supporting Information.  \n\nThe Hansen solubility parameters were calculated by using the HSPiP software (version 5.3.09). The friction coefficients were measured with a Shimpo model FG-7005 force gauge. The coating’s mechanical durability was evaluated via both abrasion testing and wiping. The images of samples before and after abrasion were captured by a scanning electron microscope (SEM) Tescan Mira3 FEG SEM (Tescan). The surface roughness was measured by a Dektak 6M stylus profilometer. Fogging tests were conducted in a homemade fog tester, and a fog tester developed at the US Army Soldier Center, Natick, MA. The pass criterion for antifog substrates or eyewear was that the change in the measured contrast ratio before and after fogging be $<7\\%$ . Additional information on the fog tester developed at the US Army Soldier Center, Natick, MA, can be found elsewhere.41 See the Supporting Information for additional details.",
        "category": " Materials and methods"
    },
    {
        "id": 5,
        "chunk": "# 3. RESULTS AND DISCUSSION  \n\n3.1. Optimization of Coating Toughness. PVP can selfcross-link via the condensation of local succinimide groups in the presence of hydrogen peroxide and UVC radiation.26,42 The reaction results in a polymer network that is resistant to dissolution in humid or aqueous environments. Thus, PVP has been widely utilized previously as an antifog coating that can survive several fogging/defogging cycles.42−45 However, this self-cross-linking still leaves the coating vulnerable to adhesive failure from the underlying substrate. The addition of benzophenone can facilitate the strong adhesion of the PVP coating to different underlying polymeric substrates (such as the polycarbonate used here) via covalent bonding after exposure to UVA radiation.26 Unfortunately, this cross-linked PVP can still be brittle and therefore easily fractured. To overcome this limitation, we added a miscible tetrafunctional oligomer pentaerythritol tetraacrylate (PETRA) cross-linker to increase the toughness of PVP. The chemical structure of PETRA is shown in Figure 1A. PETRA can help generate a semi interpenetrating polymer network (S-IPN) through a selfreaction between the acrylate groups. The self-reaction of PETRA was confirmed via FTIR (Figure S1), which demonstrates a significant decrease in the acrylate peak after $10\\ \\mathrm{min}$ of UV exposure. Substrate UV exposure during the process was minimized to avoid degradation of the underlying polycarbonate.46  \n\nFigure 2A shows the hardness values for the different PVP coatings as a function of PETRA weight $\\%$ . The self-crosslinking and the cross-linking with PETRA both increased the coatings hardness by over $100~\\mathrm{{MPa}}$ . This increase in hardness is expected to reduce the coatings wear rate as predicted by eq 1. The hardness of PVP with varying amounts of PETRA was effectively unchanged at weight fractions of up to $40\\%$ . The toughness of the PVP coatings was, however, enhanced significantly via the addition of PETRA. Tensile test data from the cross-linked PVP and PVP−PETRA films (Figure 2B,C) showed that the coating ductility was notably increased with the addition of up to $20\\%$ PETRA. Figure 2B also illustrates an increase in the stress−strain product at the breaking point, from 0.3 to $1.3~\\mathrm{MPa}$ , for the self-cross-linked PVP and cross-linked PVP with $20\\%$ PETRA respectively. As described earlier, the increase in the product of the tensile stress and strain at break correlates inversely with the coating wear rate (eq 1).  \n\n![](images/0a7b752f122748e542099fe2778e5bd84e71fc4c591e81d904011558b92ddfb5.jpg)  \nFigure 2. (A) Indentation hardness values for polycarbonate, PVP, and cross-linked PVP with increasing concentration of PETRA. (B) Stress−strain product at break/tear onset for PVP and cross-linked PVP with increasing concentration of PETRA. (C) Stress vs strain curve for PVP and PVP with $20\\%$ PETRA highlighting the increase in PVP toughness through the addition of PETRA. (D) Optical images showing the brittle failure for PVP and the ductile failure for PVP with $20\\%$ PETRA.  \n\nFrom the data shown in Figure 2C, it can be observed that the self-cross-linked PVP exhibited a brittle failure, while PVP/ $20\\%$ PETRA exhibited a ductile tearing failure. The difference in the films fracture with and without the PETRA additive is also clear from the optical images shown in Figure 2D. Thus, the maximum enhancement of coating toughness was achieved via the addition of 20 wt $\\%$ PETRA, which simultaneously increased the coating hardness, stress−strain product at the breaking point, and thereby likely the overall coating durability, as may be predicted by eq 1.  \n\n3.2. Reducing the Coating’s Friction Coefficient. Based on the Ratner−Lancaster relation, further enhancement of the wear-resistant properties of the coating can be achieved by reducing the coating’s coefficient of friction (COF). To achieve this reduction, we introduced miscible slip additives into the PVP−PETRA network to form a lubricating layer. Slip additives are relatively low molecular weight macromolecules that are blended into a polymer matrix. Owing to the differences in surface energies between the PVP matrix and the free-slip agents, these macromolecules can partially migrate to the air interface via diffusion, forming an interfacial, ultrathin lubricant layer.47 Identifying slip additives that are miscible with PVP and other antifog polymers while maintaining antifogging properties presents a unique challenge. The majority of slip additives, for example, different oils and fatty amides,48 are not miscible with hydrophilic antifog polymers like PVP. We identified and utilized a class of amphiphilic macromolecular surfactants as slip additives for PVP. Specifically, the surfactants identified include sorbitan monooleate (SPAN80), sorbitan monolaurate (SPAN20), polyoxyethylene sorbitan trioleate (PS85), polyoxyethylene sorbitan monooleate (PS80), and polyoxyethylene sorbitan monolaurate (PS20). These amphiphilic macromolecules were selected because of their broad solubility in different polymer matrices, which results from their asymmetric polar molecular structure.  \n\n![](images/970f055e03ee6676b8e5b247d1d2158a28ee98c5f6fa74cbafe2746d30fe242e.jpg)  \nFigure 3. Hansen solubility spheres for PVP, PS20, PS80, and SPAN80, as well as the miscibility factor $(S^{*})$ values for each system.  \n\n![](images/9cccf35a1b6038234f4989d9c9755ff10ee90c29f3a9c3596a43e285827ae4d5.jpg)  \nFigure 4. (A) Miscibility factors and HLB values for the different surfactant additives. SEM micrograph insets demonstrate the surface phase separation of the surfactants from the PVP matrix. (B) Coefficient of friction (COF) values for PVP, PVP with PETRA and PVP with the different surfactant additives. The inset shows the variation of the COF with $S^{*}$ values. (C) The variation of the COF with surfactant HLB values.  \n\nThe surfactants and their miscibility with PVP were analyzed by utilizing Hansen solubility parameters. Hansen solubility parameters are related to the sum of cohesive energy densities, consisting of hydrogen bonding, polar forces, and dispersive forces intrinsically exerted by a given compound.49 By experimentally determining the good and poor solvents for the different slip agents and subsequently utilizing the HSPiP software (see the Materials and Methods section), we estimated their solubility parameters and generated a solubility sphere in Hansen space for each molecule (Table S1).  \n\nThe Hansen solubility spheres for each component enable calculation of the miscibility factor $(S^{*})$ . The miscibility parameter, $S^{*,50}$ can be defined as  \n\n$$\nS^{*}=\\frac{\\Delta R-R_{\\mathrm{{surfactant}}}+R_{\\mathrm{{matrix}}}}{2R_{\\mathrm{{matrix}}}}\n$$  \n\nwhere $\\Delta R$ is the distance in 3D solubility space between the centers of the surfactants and PVP’s solubility spheres, with their radii denoted by $R_{\\mathrm{surfactant}}$ and $R_{\\mathrm{matrix}},$ respectively.  \n\n$S^{*}$ can be conceptualized as the degree of spherical overlap between two Hansen spheres and indicates the level of miscibility between two molecules.50,51 A negative $S^{*}$ value implies that the two molecules are miscible, and the more negative the value for $S^{*}$ , the more miscible the two molecules. Figure 3 displays the Hansen solubility spheres and $S^{*}$ values between the different surfactants and PVP. The data shows that $S^{*}$ values decrease in the order of $\\mathrm{SPAN80}>\\mathrm{SPAN}20>$ $\\mathrm{PS}85>\\mathrm{PS}80>\\mathrm{PS}20.$ . Only PS80-PVP and PS20-PVP systems have negative $S^{*}$ values, indicating that both PS80 and PS20 are highly miscible with PVP (Figure 4A).  \n\nThe surfactant hydrophilic−lipophobic balance value (HLB) is another important dimensionless parameter for evaluating surfactant miscibility within the PVP matrix. HLB values relate to the hydrophilic or hydrophobic nature of a nonionic surfactant.52 In general, molecules with HLB values ${>}10$ are hydrophilic, while those with HLB values $<10$ are hydrophobic. For antifogging applications, hydrophilic chemistry is preferred, and therefore surfactants with higher HLB values should be preferred as slip additives. Based on its higher HLB values, and miscibility within the PVP matrix, we selected PS20 as the optimal slip additive for PVP in order to maximize the antifog performance of the developed coatings. The HLB values and $S^{*}$ values for the different surfactants are provided in Table S2.  \n\nNext, we measured the coefficient of friction (COF) values for PVP, and PVP with different added surfactants. The COF values are shown in Figure 4B, and the values are listed in Table S3. From the data, it is clear that all the surfactants can significantly reduce the friction coefficient for PVP. The red dashed line shows the friction coefficient for uncoated polycarbonate. Both PVP and toughened $\\mathrm{PVP}/20\\%$ PETRA coatings increase the surface friction coefficient relative to uncoated polycarbonate. This is consistent with previous work that shows that increasing the substrate surface energy typically increases the surface friction coefficient.36,37 The addition of $15\\%$ surfactant into the PVP/ $20\\%$ PETRA matrix induces a nearly threefold reduction in COF. Interestingly, the coefficient of friction was found to be independent of both $S^{*}$ (Figure 4B inset) and HLB values for the different surfactants (Figure 4C).  \n\n![](images/ef240c09ee020eea3e9fbdad210eb626f988e86051127cbd09db679fc8e55772.jpg)  \nFigure 5. (A) Optical and SEM images for polycarbonate and different coated polycarbonate substrates after sequential Taber abrasion with a CS-5 abrader. (B) Change in RMS roughness before and after sequential abrasion cycles.  \n\n![](images/f90b56807abc2b8a1f352d9920379e6229e805c3a736799f71343a2595c5fdaf.jpg)  \nFigure 6. (A) Static water contact angles for PC, PVP, and PVP with increasing mass $\\%$ of PETRA. (B) Static water contact angles for $\\mathrm{PVP}/20\\%$ PETRA with $15\\%$ of varying surfactant additives.  \n\n3.3. Abrasive Wear Resistance. The abrasive wear resistance of different antifogging coatings was tested by using a standard linear Taber Abrasion apparatus with a CS-5 abrader (Figure S2). The data in Table S4 shows that polycarbonate, PVP, and PVP−PETRA were easily abraded after a few abrasion cycles $(\\sim10)$ below $300~\\mathrm{g}$ of applied load within the Taber apparatus. In comparison, the cross-linked PVP with $15\\%$ PS80 was able to survive 5000 cycles of abrasion under the same applied load (i.e., $a>500\\times$ increase in wear resistance), and even remained intact after another 1000 cycles of abrasion under a higher $550\\ \\mathrm{g}$ load. Adding the toughening agent PETRA allowed the coatings to survive the same abrasion as PVP−PS80, and an additional two sequences of 1000 abrasion cycles with increasing loads of 800 and 1050 g. In addition, the enhanced wear resistance of PVP with $20\\%$ PETRA and $15\\%$ surfactant slip additive was found to be independent of the choice of slip additive (Figure 5 and Table S4). This observation agrees well with the data reported earlier which shows that the friction coefficient of the PVP coatings was independent of the choice of added surfactant. Figure 5A shows SEM micrographs for polycarbonate and polycarbonate coated with the different PVP films after Taber abrasion. Each film was abraded with the following protocol: 5000 cycles under a $300\\mathrm{g}$ load, 1000 cycles under a $550\\mathrm{g}$ load, 1000 cycles under $\\mathbf{800}~\\mathbf{g}$ load, and 1000 cycles under $\\boldsymbol{1050}\\mathrm{\\g}$ load. The macroscale damage from the abrasion cycles can be readily seen on polycarbonate, polycarbonate coated with PVP and $\\mathrm{PVP}/20\\%$ PETRA. The surface root-mean-square (RMS) roughness before and after abrasion is shown in Figure 5B. Polycarbonate, polycarbonate coated with PVP, and PVP/ PETRA yielded notably higher changes in surface roughness after abrasion, indicating significant abrasive wear during the Taber testing. Meanwhile, substrates coated with PVP− PETRA with $15\\%$ surfactant yielded minimal changes in roughness after all the sequentially harsh abrasion cycles described above.  \n\n3.4. Antifog Performance. Figure 6A displays the static water contact angle of polycarbonate, PVP, and PVP with increasing concentration of PETRA over a period of $5\\ \\mathrm{min}$ after water-surface contact. As expected, hydrophobic polycarbonate showed only minor changes in contact angle over a period of 5 min due to evaporation. PVP-based coatings on the other hand are water-absorbing, causing the contact angle to significantly decrease over time. PVP-coated polycarbonate displays a contact angle below the $40^{\\circ}$ threshold in about a minute, and the addition of up to $20\\%$ PETRA did not notably change the coating’s contact angle despite the cross-linker’s hydrophobic properties. This minimal effect on contact angle is advantageous due to the enhancement of wear resistance provided by PETRA.  \n\nNext, we evaluated the impact of adding different surfactants on the observed antifog properties. As described above, all of the surfactant containing coatings contain PVP, $20\\%$ PETRA, and $15\\%$ of the surfactant. For simplicity, we will denote the different surfactant containing coatings as PS20, PS80, PS85, SPAN20, and SPAN80 based on the surfactant utilized. Figure 6B shows the impact of the HLB values on the measured water contact angles for the different coatings. The addition of SPAN 80 notably increases the contact angle of the antifog coating beyond that of pure PVP. As the HLB values increase up to 16.7 for PS20, the contact angle is reduced to near that of unmodified PVP.  \n\nAntifog testing on the different coatings was conducted in an environmental chamber that enabled precise control of the environmental humidity and substrate temperature (Figures S3 and S4). Briefly, the surface temperature of all samples was reduced to $14\\pm0.5{}^{\\circ}\\mathrm{C}$ (below the dew point) and the relative humidity was maintained to $54~\\pm~1\\%$ . The analysis and quantification of fogging utilized a modification to a previously reported testing methodology developed by Lee et al.29 Our modified testing methodology (see the Materials and Methods section and the Supporting Information) allowed for the direct evaluation of long-term antifogging performance under controlled, sustained humidity, and surface temperature. To the best of our knowledge, such a methodology has not been previously utilized for evaluating the antifog performance of various coatings. Unlike most previously developed antifog testing methodologies, our testing can not only discriminate between antifogging and fogging surfaces but also measure the performance of different antifog coatings over long periods of sustained fogging conditions. This enables quantitative performance comparison between different high-performing antifog coatings based on the change in optical transmissivity over time.  \n\nFigure 7 shows optical images from the different fogging experiments conducted on pure polycarbonate and polycarbonate coated with PVP, PS20, PS80, PS85, SPAN20, and SPAN80 samples. At about the $10\\ \\mathrm{min}$ mark, the substrate reaches the set cooling temperature, and fogging begins on the polycarbonate control surface. Unmodified PVP coatings demonstrated antifogging properties for up to $20\\ \\mathrm{min}$ , while the PVP coatings incorporating the SPAN 20 and SPAN 80 surfactants fogged up almost immediately due to their low HLB values, which resulted high water contact angles. The higher HLB values for the different polysorbate surfactants enabled enhanced antifogging properties. Figure 8A,B show the changes in image correlation factor (ICF) over time for each coating tested within our fogging chamber. The ICF values at any time point during the fogging experiment are calculated by measuring the digital image correlation between two images of the same area before and after fogging. The ICF quantifies visual clarity and scales from 0 to 1, where 1 means no distortion (i.e., the image after fogging perfectly correlates with the image before fogging) and 0 means no correlation among the images. Usually, values of $\\alpha$ above 0.95 indicate no image distortion, while $\\alpha$ values below 0.5 indicate poor visual clarity.29 Figure 8A shows the ICF values for the polycarbonate (PC), PVP, and PS20 samples during the fogging experiment. The PC surface fogs after ${\\sim}10\\ \\mathrm{\\min}$ , and the ICF values decrease continuously for the duration of the experiment. The PVP surface was able to prevent fogging for ${\\sim}15\\ \\mathrm{min}$ $\\mathrm{\\hbar}\\sim5~\\mathrm{min}$ of antifogging as it takes ${\\sim}10~\\mathrm{min}$ for the sample to cool below the dew point) with the ICF dropping to below 0.90 until it fully fogs with an ICF value of ${\\sim}0.50$ . The PS20 coating was able to fully eliminate fogging throughout the duration of the entire experiment and maintained ICF values ${>}0.96$ . This demonstrates a significant enhancement in antifog properties when compared with unmodified PVP. Figure 8B compares the performance of the PS20 coating to that of the more hydrophobic SPAN 80 and SPAN 20 coatings. It can be observed that both SPAN 20 and SPAN 80 coatings are unable to prevent fogging and perform worse than unmodified polycarbonate.  \n\n![](images/df8b4ad74dda8d09b47ede1a7260d7ccc67e95e8f0ae17e71694eaa7e975d085.jpg)  \nFigure 7. Optical images for polycarbonate, and different coated polycarbonate substrates within a controlled environment, fogging chamber (relative humidity $\\mathbf{\\Phi}=\\sim54\\%$ and substrate temperature of ${\\sim}14~^{\\circ}\\mathrm{C}\\ '$ over a period of $40\\ \\mathrm{min}$ .  \n\nAn interesting phenomenon observable from the data shown in Figure 8B is the recovery of the ICF values (transparency) for PVP, and the SPAN coatings after extended fogging times. This somewhat surprising observation can be explained based on the transition from small-scale dropwise condensation to larger scale film-wise condensation on the fogged surfaces. The formation of a continuous water film, as opposed to discrete droplets (which scatter light), increases the transmissivity for these fogged surfaces. Although this recovery is observed, it does not occur rapidly enough to be considered to be sufficiently antifogging.  \n\nAll polysorbate-based coatings demonstrated good antifog performance as shown in Figure 8C. The differences in performance between each polysorbate additive can be related to their HLB values. PS85, having the higher HLB value fogs at around the $10\\ \\mathrm{min}$ mark (ICF values ${\\sim}0.90\\$ ), but recovers almost immediately. This rapid, but quickly dissipating fog can also be seen in optical images (Figure 7). For the higher HLB value PS80 and PS20 coatings, no fogging occurred throughout the duration of the experiment and minor differences in the data shown are likely due to image distortion. PS20 samples had minimal condensation and no fogging and proved to be exceedingly wear-resistant.  \n\n![](images/97e5412c7a5f23edaa98e7e0b6d239c0b13624785bd639187a477968044a40c0.jpg)  \nFigure 8. (A) The ICF values for the optimized PS20 coating compared with PVP and PC over a time period of $40~\\mathrm{min}$ in the fogging chamber. (B) ICF values for the PVP coatings with $20\\%$ PETRA and three different surfactant additives (PS 20, SPAN 20, and SPAN 80). (C) ICF values for PVP coatings with different polysorbate surfactants. (D) ICF values during cyclic fog testing. We report the ICF values after $20~\\mathrm{min}$ for each fogging cycle. PVP and polycarbonate reference lines indicate ICF values for these reference substrates in the first fogging cycle after $20~\\mathrm{min}$ . (E) $\\%$ transmission and (F) $\\%$ haze vs light wavelength for polycarbonate and polycarbonate coated with $\\mathrm{PVP}/20\\%$ PETRA/ $15\\%$ surfactant before fogging.  \n\n![](images/fc239bb06a921e48845bb5d8e2b7cbba1ef3346a558e86473a08185ebe3bbade.jpg)  \nFigure 9. (A) ICF values as a function of time for PC, optimized PS 20 coating, and different commercial coatings\u0001revision wipes, Optix, Exxene, and Ops drops. Optical images for the different samples after $40~\\mathrm{min}$ in the fogging chamber are also shown. (B) Fog testing after wiping each sample 500 times with a microfiber lens wipe. Optix was tested immediately after wiping and one PS20 coating was tested immediately after wiping, while another was tested after $24{\\mathrm{~h}}.$ . Optical images for the different samples after $10\\ \\mathrm{min}$ in the fogging chamber are also shown.  \n\nThe best performing antifog formulation, PS20, was also examined in terms of repeated fog exposure. For this experiment, $20~\\mathrm{min}$ fog cycles were used, and an initial heating step was utilized to ensure similar levels of condensation for each cycle (see the Materials and Methods section and the  \n\nSupporting Information). Figure 8D reports the ICF values for the PS20 coating at the final 20th minute of each cycle. The dashed lines for polycarbonate and PVP indicate the ICF values at fog onset on those samples after about $10~\\mathrm{min}$ in the fogging chamber. The data shows that even after 15 cycles, the highly durable PS20 coating maintains its excellent antifog performance. It is important to note that the fogging procedure employed here is significantly more aggressive than what has been typically utilized in previous work, such as directly breathing on a cold surface. Figure 8E,F demonstrate the optical properties of uncoated PC and different coated substrates. As shown in Figure 8E, no discrimination can be made between the coatings based on UV−vis transmittance data alone since all curves overlap. However, $\\%+\\mathrm{Iaze}$ spectra for each coating in Figure 8F yielded notable differences. The data in Figure 8F show that the haze caused by the PS 20 coating is essentially identical to the uncoated polycarbonate and that the amount of haze caused by the coated surface seemed to correlate with the $S^{*}$ values, with the lowest $S^{*}$ values causing the least amount of haze. This is likely due to the increase in the miscibility between the surfactant and the PVP matrix with decreasing $S^{*}$ values.  \n\n3.5. Comparison to Commercial Antifogging Coatings. To validate the wear and antifog performance of the optimized PS20 antifog coating formulation, it was compared against three commonly sold temporary antifog treatments and one commercial polyurethane-based antifog coating. Figure 9A demonstrates the antifog performance of the PS20 coating against different temporary coatings such as Optix, Ops Drops, Revision wipes, and an Exxene HCF-100 commercial antifog coating. The samples were fog tested in the same manner as previously described. The tests reveal that Optix and the PS20 coating demonstrated excellent, sustained antifogging properties despite Optix being a temporary solution. Ops Drops performed to a lesser degree, while Revision appeared to yield negligible antifogging benefits. Exxene was able to delay fogging for ${\\sim}20~\\mathrm{min}$ but fogged soon after as indicated by a decline in ICF values. Water contact angle measurements and abrasion testing on the different commercial coatings are provided in Figures S5 and S6. Since the temporary antifog agents exist as free macromolecules on the surface, they impart a lubricating effect on the surface and therefore provide a degree of wear resistance. However, unlike our coating which contains embedded slip additive within the film network, these ephemeral antifog solutions can only offer wear resistance for a short duration and can be easily removed mechanically. Exxene does not contain any slip additives but is composed of a durable polyurethane and therefore proved to be highly wear resistant. However, as described earlier, the coating demonstrates relatively poor antifog performance.  \n\nTo confirm that the excellent antifog performance of our coatings is not an artifact of our antifog testing methodology, we also conducted fog testing of the fabricated PS20 coatings and a few commercial coatings (Revision and Ops Drops) in a fog tester developed by the US Army Soldier Center (see the Materials and Methods section). Images from that test are shown in Figure 10. Only the PS 20 coatings passed the US Army antifog test, while the other commercial coatings failed, validating the excellent antifog performance of the PS 20 coating.  \n\nNext, we evaluated the regenerative properties of our optimized PS 20 coating. Figure 9B shows the antifogging performance of the Optix and PS20 coatings after 500 Taber abrasion cycles with a microfiber lens wipe. Fog testing was conducted both immediately after the wiping cycles, and a day later. As can be seen from the data, the antifog properties of PS20 declined slightly after 500 wiping cycles, but eventually improved as the test progressed. When the fogging tests were conducted 1 day after the 500 microfiber wipes, the coating maintained ICF values $>0.9$ throughout the duration of the test. In comparison, the antifog properties of Optix coating are completely depleted after 500 microfiber wipes, and the surfaces displayed immediate, and nonrecoverable fogging.  \n\n![](images/affe69d1487b9ad851fcb8d9c8ab43d1faa3775a2cd5e6f329a989aab0633cdb.jpg)  \nFigure 10. Comparative antifog performance for uncoated polycarbonate, wear-resistant PS20 coating, and two different commercial antifog coatings\u0001revision wipes, Ops drops within a fog tester developed by US Army soldier research center (see the Materials and Methods section). Visual pictures from the fog tester camera show the haze in each sample at intervals of $20~\\mathrm{s}$ .  \n\nAdditionally, we measured the long-term water contact angle values on our coating, as well as plasma-treated polycarbonate, Optix, Exxene, and cross-linked PVP. Over the course of 5 weeks, our coating maintained its low water contact angles while all other formulations experienced a gradual increase (Figure S7) likely due to surface fouling from low surface energy molecules present in the environment. This data highlights how the use of surfactants can overcome one of the major challenges associated with antifog coatings−degradation in performance over the long-term due to fouling from low surface energy molecules.",
        "category": " Results and discussion"
    },
    {
        "id": 6,
        "chunk": "# 4. CONCLUSIONS  \n\nIn this work, we have engineered different abrasion-resistant antifog coatings that can maintain their properties over the long-term. These coatings were developed based on the classic Ratner−Lancaster relation wear model for polymers and utilized the introduction of coating toughening agents and low-friction slip additives within a previously well studied polymer with antifog properties\u0001PVP. The additives identified in this work enhanced the abrasion resistance of the coating by over 500-fold when compared with unmodified PVP. Additionally, HLB values, and a unique miscibility factor that utilizes the Hansen solubility parameters $(S^{*})$ , were investigated as criteria for enhancing antifogging performance and coating durability. We illustrated that friction reduction is independent of additive HLB values and $S^{*}$ , while the optical clarity and the antifog ability can be improved by lower $S^{*}$ and higher HLB values, respectively. Our optimized PVP antifog coating demonstrates continuous antifogging performance and strong wear resistance, and significantly outperformed all tested commercially available antifog coatings in terms of antifog performance, wear resistance, and long-term cyclical performance. Overall, the incorporation of toughening agents and hydrophilic slip additives to existing hydrophilic antifog polymers offers a new strategy for developing high-performance, and durable antifog coatings.",
        "category": " Conclusions"
    },
    {
        "id": 7,
        "chunk": "# ASSOCIATED CONTENT",
        "category": " References"
    },
    {
        "id": 8,
        "chunk": "# $\\bullet$ Supporting Information  \n\nThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.3c17117.  \n\n(A) Change in $\\scriptstyle{\\mathrm{C}}={\\mathrm{O}}$ bond for a PVP antifog coated polycarbonate as a function of UVC exposure time, (B) change in acrylate peak of pure PETRA coated polycarbonate with increasing UVC exposure, Hansen solubility parameters calculated by HSPiP software, select Surfactants and corresponding HLB values and $S^{*}$ values, average steady-state friction coefficients of polycarbonate and antifog coated polycarbonate, linear Tabor abrasion setup for antifog coated polycarbonate and the SEM image of the microstructure of the CS-5 felt tip abrasion tip, list of experiments demonstrating material and additive effects on sequential abrasion resistance to CS-5 Tabor Abrasion under increasing mass, a schematic diagram of the test chamber, the humidity and temperature evolution during the experiments, static water contact angles for different commercial antifog coatings, as well as the optimal PS20 coating developed in our work, change in roughness of commercial antifog formulations and, polycarbonate (PC), and the PS20 antifog coating after 8000 CS-5 abrasion cycles, longevity of static water contact angle performance with extended exposure time to normal environmental conditions (PDF)",
        "category": " Results and discussion"
    },
    {
        "id": 9,
        "chunk": "# AUTHOR INFORMATION",
        "category": " References"
    },
    {
        "id": 10,
        "chunk": "# Corresponding Author  \n\nAnish Tuteja − Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States; Department of Chemical Engineering, Department of Macromolecular Science and Engineering, and Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan 48109, United States; orcid.org/0000-0002- 2383-4572; Email: atuteja@umich.edu",
        "category": " References"
    },
    {
        "id": 11,
        "chunk": "# Authors  \n\nBrian Macdonald − Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States   \nFan-Wei Wang − Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States; $\\circledcirc$ orcid.org/0000-0002-6401-4255   \nBrian Tobelmann − Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States   \nJing Wang − Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States; $\\circledcirc$ orcid.org/0000-0002-7757-1261   \nJason Landini − Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States   \nNipuli Gunaratne − Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States   \nJoseph Kovac − Department of Aerospace Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States  \n\nTodd Miller − Protection Materials Division, Soldier Protection Directorate, US Army DEVCOM Soldier Center, Natick, Massachusetts 01760, United States Ravi Mosurkal − Protection Materials Division, Soldier Protection Directorate, US Army DEVCOM Soldier Center, Natick, Massachusetts 01760, United States; $\\circledcirc$ orcid.org/ 0000-0002-2769-2292  \n\nComplete contact information is available at: https://pubs.acs.org/10.1021/acsami.3c17117",
        "category": " References"
    },
    {
        "id": 12,
        "chunk": "# Author Contributions  \n\nThe manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.",
        "category": " References"
    },
    {
        "id": 13,
        "chunk": "# Funding  \n\nThis work was financially supported by the US Army DEVCOM Soldier Center, Natick, MA, (Contract#: FA9300- 20-F-9801)# which is gratefully acknowledged and approved for Public Release (PAO #PR2023-204).",
        "category": " References"
    },
    {
        "id": 14,
        "chunk": "# Notes  \n\nThe authors declare no competing financial interest.",
        "category": " References"
    },
    {
        "id": 15,
        "chunk": "# ACKNOWLEDGMENTS  \n\nB.M thanks the National Science Foundation for the NSF GRFP Fellowship in support of this work. We thank Andrea Poli for the instruction and usage of the RSA-G2 DMA. R.M. thanks Joseph Palomba and David Ziegler for the help in the Antifog testing. The authors acknowledge the financial support of the University of Michigan College of Engineering and technical support from the Michigan Center for Materials Characterization.",
        "category": " References"
    },
    {
        "id": 16,
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]