Durable Antifog Films from Layer-by-Layer Molecularly Blended

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Durable Antifog Films from Layer-by-Layer Molecularly Blended Hydrophilic Polysaccharides Nurxat Nuraje,† Ramazan Asmatulu,†,§ Robert E. Cohen,*,‡ and Michael F. Rubner*,† †

Department of Materials Science and Engineering, and ‡Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States, and §Department of Mechanical Engineering, Wichita State University, Wichita, Kansas 67260, United States Received September 19, 2010. Revised Manuscript Received November 10, 2010 Mechanically durable, long-lasting antifog coatings based on polysaccharides were developed using a layer-by-layer (LBL) assembly process. The unique properties of these coatings are a result of a molecular-level blending of the polysaccharides, with multilayers containing chitosan and carboxymethyl cellulose providing the best overall properties. The antifog properties resulted from a strong interaction between the polar and H-bonding elements of the assembled polymers and water molecules and the concomitant formation of thin films of water. Environmental scanning electron microscopy (ESEM) studies confirmed that fogging coatings are decorated with light scattering, micrometer-sized droplets of water whereas antifogging coatings remain droplet free. To improve the mechanical durability of the multilayer films on substrates, the surface was modified via self-assembly of epoxy-functionalized silane molecules. Cross-linking chemistry was then applied to improve the mechanical robustness of the LBL films on various surfaces. These films were characterized using several techniques: optical profilometery (PL), spectroscopic ellipsometry (EL), contact angle goniometry (CA), and atomic force microscopy (AFM). The antifog properties of the films were evaluated by several tests under different environmental conditions. This work demonstrates that the unique water-adsorbing properties of polysaccharides can be exploited to create permanent antifog properties, which may be useful for various applications.

1. Introduction The fogging of surfaces due to the condensation of light-scattering microscopic droplets of water under a variety of environmental conditions continues to be problematic both in day-to-day life and in many technological applications.1-3 Currently, there remains an unmet need for long-lasting and mechanically robust antifog coatings that can operate under a variety of different fogging challenges2,4,5 (e.g., temperature and humidity). A number of different approaches have been put forth to alleviate the fogging problem, including the exploitation of surfaces with extreme wetting characteristics such as superhydrophobicity5,6 or superhydrophilicity.2,3 More often than not, however, these textured, nanoporous, or light-activated surfaces suffer from issues such as short lifetimes, excessive fouling, susceptibility to high-humidity environments, and poor mechanical durability.2,4,7 Hydrophilic surfaces with advancing water droplet contact angles of less than 40° are often explored as antifog coatings due to their potential ability to rapidly spread condensing water droplets into a uniform, non-light-scattering film of water.1,8,9 In this case, *Corresponding authors. E-mail: [email protected] (M.F.R.), recohen@ mit.edu (R.E.C.). (1) Howarter, J. A.; Youngblood, J. P. Macromol. Rapid Commun. 2008, 29, 455–466. (2) Cebeci, F. C.; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856–2862. (3) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431–432. (4) Gemici, Z.; Schwachulla, P. I.; Williamson, E. H.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2009, 9, 1064–1070. (5) Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. Adv. Mater. 2007, 19, 2213–2217. (6) Cheng, Y.-T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 144101. (7) Gemici, Z.; Shimomura, H.; Cohen, R. E.; Rubner, M. F. Langmuir 2008, 24, 2168–2177. (8) Grosu, G.; et al. J. Phys. D: Appl. Phys. 2004, 37, 3350. (9) Briscoe, B. J.; Galvin, K. P. Sol. Energy 1991, 46, 191–197.

782 DOI: 10.1021/la103754a

although condensation still occurs, the surface remains optically clear. The key to this approach is the use of materials that strongly interact with water molecules and/or have a high capacity for adsorbing water. Hydrophilic polymeric systems containing poly(ethylene glycol) and poly(vinyl alcohol), for example, are often utilized in antifog formulations.10 It is well established that many naturally occurring polymers such as cellulose and various hydrophilic polysaccharides11-13 strongly adsorb water in various forms, including nonfreezable forms of water. Given the superior water-absorbing characteristics of these materials and their ability to support multiple forms of water (freezing and nonfreezing), they are ideal candidates for antifog coatings. The processing of these materials into optical-quality thin-film coatings, however, is difficult, particularly if they are to be blended with other synthetic polymers or polysaccharides to achieve optimum coating characteristics and mechanical durability. In this work, we demonstrate that hydrophilic polysaccharides such as chitosan, alginate, hyaluronic acid, and carboxymethyl cellulose can be blended at the molecular level to produce opticalquality thin films with excellent, long-lasting antifog performance. Molecular-level blending of the polysaccharides, including blends with synthetic polymers containing ethylene glycol segments, was achieved via the well-established layer-by-layer assembly technique.14 Environmental scanning electron microscopy (ESEM) studies (10) See US Patents 4478909 (1984), 5075133 (1991), and 5804612 (1998). (11) Schartel, B.; Wendling, J.; Wendorff, J. H. Macromolecules 1996, 29, 1528– 1534. (12) Fringant, C.; Desbrieres, J.; Milas, M.; Rinaudo, M.; Joly, C.; Escoubes, M. Int. J. Biol. Macromol. 1996, 18, 281–286. (13) Radloff, D.; Boeffel, C.; Spiess, H. W. Macromolecules 1996, 29, 1528– 1534. (14) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2003.

Published on Web 12/16/2010

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reveal directly that these coatings are able to effectively spread condensing water droplets into uniform sheets. The mechanical durability of the resultant coatings was further improved with easily introduced surface modification and cross-linking chemistry.

2. Experimental Section 2.1. Materials. The following materials were purchased from Sigma-Aldrich: carboxymethyl cellulose (CMC) (Mw=250 000), low-molecular-weight chitosan (CHI, Mw = 50 000), glutaraldehyde (25% solution, Mw=100.12), poly(ethylene imine) (PEI) (branched, Mn =10 000, Mw =25 000), 2-(N-morpholino)ethanesulfonic acid monohydrate (MES), and sodium chloride. Poly(acrylic acid) (PAA) (25% aqueous solution, Mw = 90 000 g/mol) was supplied by Polysciences (Warrington, PA). Gelest was the source for (3-glycidoxypropyl)trimethoxysilane (GPTMS). Glass microscopy slides with 7.52.50.1 cm dimensions were obtained from VWR (Cat. No. 48300-047). Bare polycarbonate was provided by Teijin-Kasei Corp. Thermo Scientific supplied 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimidehydrochloride (EDC) and Nhydroxysulfosuccinimide (NHS). Asylum Research was the source for AFM (MFP3D) and tips (AC240TS, AC200TS), which were utilized to scratch the film using the nanolithography mode.15 A polishing cloth (DP-NAP) with adhesive backing was purchased from STRUERS Inc. Our laboratory synthesized poly(acrylic acid)-graft-poly(ethylene glycol) (PAA-g-PEG), following procedures in the literature.16,17 Deionized (DI) water was used in all aqueous solutions and rinsing procedures. 2.2. Methods. 2.2.1. Thin-Film Assembly. The LBL assembly of polymer/polymer systems on glass and polycarbonate slides was performed using a Stratosequence VI spin dipper (Nanostrata Inc.) controlled by StratoSmart v6.2 software. Dipping time for polymers was 10 min, followed by three rinses (one 2 min rinse followed by two 1 min rinses) using DI water of the same pH as the preceding polymer solution. The concentration of CHI and CMC was 0.1 wt %. The pHs of the polymer solutions and rinsing water were adjusted with either 0.1 M HCI or 1 M NaOH. Glass substrates were degreased in a 3% solution of Micro90 under sonication for 20 min, and the substrates were subsequently sonicated in 1 M NaOH for another 20 min. Finally, the glass substrates were sonicated in deionized water (DI) water for 5 min and blown dry with air. Polycarbonate substrates were pretreated with oxygen plasma (PDC-32G, Harrick Scientific Products, Inc.) for 30 s at 400 mTorr on both sides before building the multilayers. 2.2.2. Characterization. Thickness measurements of assembled multilayers on substrates were performed by a Tensor P16 surface profilometer (PL) using a 2 μm stylus tip and 2 mg stylus force. The topographical image of the multilayer was collected using a Nanscope IIIa, Dimension 3000 AFM microscope (Digital Instruments, Santa Barbara, CA) in the tapping mode. Variableangle spectroscopic ellipsometry (EL) was employed to measure the thickness and refractive index of the film on polycarbonate substrates. Contact angle measurements were carried out with a VCA-2000 contact angle system (AST Products, Inc., Billerica, MA). Contact angle values were calculated from dynamic video files captured at 60 frames/s using the software VCA Optima XE Version 1.90 provided by the manufacturer. 2.2.3. Antifog Tests. Antifog properties were evaluated through four separate testing protocols: (a) humidity chamber test, (b) aspiration test, (c) boiling test, and (d) cold-fog test. In test (a), the humidity within the chamber was controlled with an ultrasonic humidification/ventilation system. A slide coated with an antifog multilayer was evaluated by recording an image of the slide at 37 °C and 80% humidity in the chamber at various times. (15) Nuraje, N.; Banerjee, I. A.; MacCuspie, R. I.; Yu, L.; Matsui, H. J. Am. Chem. Soc. 2004, 126, 8088–8089. (16) Asatekin, A.; Kang, S.; Elimelech, M.; Mayes, A. M. J. Membr. Sci. 2007, 298, 136–146. (17) Irvine, D. J.; Mayes, A. M.; Griffith, L. G. Biomacromolecules 2001, 2, 85–94.

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An experimental description of this test was discussed in a previous study.2 In test (b), for quick evaluation of the antifog performance of the coatings, a simple aspirating/breathing test was conducted on the sample. The boiling test (c) was established mimicking the European Standard test (EN168). In this test, an Erlenmeyer flask containing boiling water was placed on top of written letters. The written letters were observed through the other side of the film by the naked eye after the film was exposed to the steam of the Erlenmeyer flask. The visibility of the letter at the bottom of the Erlenmeyer flask was evaluated for the degree of antifog. In the cold-fog test (d), the substrates coated with multilayers were placed in a refrigerator (4 °C) for several hours and then placed in a humidity chamber (37 °C and 80% humidity) or on the top of a Erlenmeyer flask containing steaming water. 2.2.4. Cross-Linking of the Multilayers. The glass substrate was first sonicated in water for 5 min and cleaned in oxygen plasma for 2 min at 120 mTorr or treated in a piranha solution for 30 min at 80 °C. The cleaned substrate was immediately incubated with an anhydrous toluene solution of 1% (3-glycidoxypropyl)trimethoxysilane overnight and then rinsed with pure toluene. Toluene can be replaced by other anhydrous organic solvents such as chloroform and hexane. All of the above experiments were conducted in a dry glovebox. The dried substrate was immersed in a 0.01 M aqueous solution of branched PEI (pH=9) for at least 4 h and subsequently rinsed with water. Next, multilayers of CHI/ CMC were assembled onto the PEI-modified substrate at pH=4. The substrate with the assembled multilayer of CHI/CMC was then immersed into a 0.05 M MES buffer (pH = 5), including 200 mM EDC and 50 mM NHS,18 for 30 min and subsequently immersed into a 1X PBS buffer for 20 min. The cross-linked multilayer of CHI/CMC on the glasslike substrate was then rinsed with water. The antifog coatings were not only chemically bonded between layers but also chemically bonded to the functionalized substrate surface. To further cross-link the multilayers, the coating was immersed in an aqueous solution of 2.5% glutaraldehyde at pH=9 and 30 °C for 45 min. The substrate was then rinsed with DI water. 2.2.5. Mechanical Properties. Mechanical integrity of the films was evaluated by several methods, including the following: (a) ASTM D3363 pencil hardness test, (b) home-built rotary cloth abrasion test,7 (c) KIMWIPE test, (d) qualitative cloth-sponge rubbing test, (e) crosshatch adhesion test (i.e., Scotch tape test), and (f) AFM nanolithography-controlled scratching test. In the pencil hardness test, the coated glass substrate was placed under the tip of a pencil, and the pencil holder was moved in one direction. The force applied to the pencil tip came from a 750 g static load. The scratched regions were evaluated by optical microscopy. In the abrasion test, changes in optical transmittance of the film after rotary wiping with a standardized cloth were correlated with mechanical damage using experimental methods. Data analysis protocols were explained in detail in the previous study.7 In the present study, we challenged the coatings with the maximum available level of normal stress (100 kPa) in the abrasion testing. The cloth-sponge rubbing test included three different levels of challenges. The coating was first wetted with water or soap solution (2% MICRO-90 solution from International Products Corp.) and rubbed with a cloth (ANTICON, Lot #5562) or a cellulose sponge under three different conditions. In the first test, the wet coating was rubbed with a wet sponge 10 times. In the second test, the wet coating was rubbed with a wet cloth 10 times. In the third test, the wet coating was rubbed with a dry cloth 10 times. The last test was considered to be the most rigorous test of mechanical strength among the cloth-sponge tests. In the crosshatch adhesion test, 3M tape was first placed on the top of the film for a couple of seconds and then removed from the film. The damaged areas were evaluated. This technique shows the adhesion degree of the film onto the substrate. (18) Nuraje, N.; Mohammed, S.; Yang, L.; Matsui, H. Angew. Chem., Int. Ed. 2009, 48, 2546–2548.

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Nuraje et al. Table 1. Antifog Properties of Various Multilayers on Glass Slides multilayer

climate chamber test

Erlenmeyer steam test

Huff test

cold-fog test

thickness (nm)

refractive index

[CHI/CMC]15 antifog antifog antifog antifog ∼100 1.54 antifog antifog antifog antifog ∼100 1.44 [CHI/CMC]15 Cross-linked fogging fogging fogging fogging >100 1.54 [CHI/PAA]15 fogging fogging fogging fogging >100 1.54 [CHI/PAA-co-PEG]10 25% PEG in copolymers antifog antifog antifog antifog ∼100 1.54 [CHI/PAA-co-PEG]7 PEG ∼37% in copolymers a fogging fogging fogging fogging ∼100 1.54 [PAH/SPS]60 fogging fogging fogging fogging ∼100 1.54 (PAH/PAA)10b antifog antifog antifog antifog >100 1.54 [CHI/AG]15c d antifog antifog antifog antifog >100 1.54 [CHI/HA]15 a [PAH/SPS] assembled at pH=3. b pHs of PAH and PAA were respectively 7.5 and 3.5 for the assembly of the films. c Concentration and pH of AG are 0.02% and 4. d Concentration and pHs of HA(Mw: 650K) and CHI(medium Mw) respectively are 0.1% and 4.

In the AFM nanolithography scratching test,15,18 various spring constant tips were applied to scratch the films with both horizontal and vertical lines via a nanolithography program. The scratched lines were evaluated using section analysis software. All of the experiments were performed using an Asylum MFP AFM. The applied force for scratching the films was found by calibrating the force curve of specific tips to bare glass substrate. After the spring constant of the specific tip was determined, voltage equal to the force needed was set as the set point for the AFM nanolithography program. Thus, this program directed the tip to move in certain directions with specific applied forces. 2.2.6. Environmental Scanning Electron Microscopy. ESEM connected to a DVD recording system (Philips) was used to observe in situ water droplet growth on the surface of the LBL films. The temperature and humidity of an environment were controlled during ESEM studies. The experiments were conducted at 15 °C with a humidity range of 90-95%. The movies were recorded as soon as the environment in the ESEM chamber began getting humid.

3. Results and Discussion 3.1. CHI/CMC Multilayer Assembly. In order to create multilayer thin films with permanent antifog properties, we explored the use of various hydrophilic polysaccharides, with the idea that such polymers have a number of functional elements that have the ability to interact strongly with water molecules through, for example, the formation of hydrogen bonds. The blending of these materials at the molecular level was achieved by using a layer-by-layer assembly technique. Table 1 summarizes the various polymer combinations examined including polyelectrolyte multilayers assembled with typical synthetic polyanions and polycations. As will be discussed, the best antifog capability was realized when specific oppositely charged polysaccharides, such as hyaluronic acid (HA), alginate (AG), chitosan, and carboxymethyl cellulose, were assembled into thin-film coatings. Notably, it was found that multilayer films assembled from chitosan (CHI) and carboxymethyl cellulose (CMC) produced the best overall properties; hence, we will focus on the assembly and physical properties of this multilayer system. In initial screening studies, the film growth of the CHI/CMC multilayer system was evaluated at different pH solutions (pH = 1.5, 4, and 6). Because of the instability of chitosan at pH=7 and above, we only investigated the film growth behavior up to pH 6. The total film thickness of a 30-bilayer film of CHI/CMC assembled at pH=6 (215 nm) was less than that of a film assembled at pH=4 (530 nm). With a solution of pH=1.5, no film formation occurred on the substrates. To understand this pH dependence, the turbidity of mixed solutions of these two polymers was measured as a function of pH and salt concentration. The data in Figure 1 reveal that turbid solutions, an indication of the formation of insoluble polymer complexes, only occur when the solution pH is 784 DOI: 10.1021/la103754a

Figure 1. Turbidity results of CHI/CMC system at various salt concentrations and pH values. Turbidity was measured 10 min after mixing the polyanion and the polycation.

∼2.5 or higher. At pH = 1.5, essentially only soluble polymer chains/complexes exist in solution (no turbidity), suggesting that multilayer assembly at this pH would be prohibited.19 As the solution pH increases, turbidity increases significantly as a result of an increased ionization of the CMC chains and the formation of insoluble polymer complexes. As pH = 7 is approached, the level of turbidity further increases due to polymer solubility problems with the CHI. Thus, the multilayer assembly of this system is best accomplished in the pH range of 3-6, with pH=4 being the preferred assembly pH. To determine how salt concentration influences the growth behavior of the CHI/CMC system, solutions with between 0 and 0.25 M NaCl were used. The thicknesses of 30-bilayer coatings assembled on glass substrates at different salt concentrations were investigated. Salt concentrations of 0, 0.1, and 0.25 M resulted in total film thicknesses of 531, 1630, and 19 nm, respectively ((10%). The much lower bilayer thickness of the film prepared in 0.25 M salt solutions suggests that multilayer assembly under these conditions is restricted. Sukhishvili and co-workers have shown that soluble polymer complexes can be formed during multilayer assembly due to local concentration effects.19-21 Alternatively, Mjahed et al.22 demonstrated that turbidity diagrams can be used (19) Sukhishvili, S. A.; Kharlampieva, E.; Izumrudov, V. Macromolecules 2006, 39, 8873–8881.

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Figure 2. Film growth of the CHI/CMC system assembled at pH = 4 on glass substrates (units of the slope are nm/bilayer).

to predict polyelectrolyte multilayer deposition on surfaces. To test this hypothesis, the two polymers were mixed at a 25:1 ratio in a 0.25 M NaCl aqueous solution. The resultant clear solution demonstrated the formation of a water-soluble complex. In contrast, turbid solutions were formed when the polymers were mixed at this same ratio in solutions containing no salt or 0.1 M salt. On the basis of the above screening results, CHI/CMC films were assembled at pH = 4 without the addition of salt (similar antifog properties resulted when 0.1 M salt was included in the LbL production of the films) and characterized by profilometer, AFM, contact angle measurements, and ellipsometry. Figure 2 shows the film growth of the CHI/CMC system on a glass substrate as a function of the number of deposited bilayers (polycation plus polyanion). Two linear regions can be seen: the first region is between 0 and 15 bilayers, while the second region is between 15 and 30 bilayers. The thicknesses of 10- and 30-bilayer films were around 23 and 570 nm, respectively. Similar growth trends were found when polycarbonate was used as the supporting substrate. When the number of bilayers of CHI/CMC was lower than 15, the growth behavior of the multilayer film was different from that of CHI/CMC with 15 or more bilayers. Similar film growth behavior has been observed with other polysaccharide multilayers.23,24 Picart et al.23 explained that the multilayer buildup of hyaluronan/chitosan is comprised of two distinct stages. The first stage is characterized by isolated islet growth, whereas the second stage involves a more continuous film construction. They also reported that film thickness increased with increasing salt concentration in the deposition solutions. AFM analysis revealed that the multilayers were relatively smooth with rms roughness values of 20-, 30-, 40-, and 50-bilayer films being less than 8 nm. 3.2. Antifog Properties of Multilayer Films. The antifog properties of various polyelectrolyte multilayers were examined under a variety of different fogging conditions. These included the following: (a) exposing the film to 80% humidity at 37 °C in an (20) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607–5612. (21) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343. (22) Mjahed, H.; Voegel, J.-C.; Chassepot, A.; Senger, B.; Schaaf, P.; Boulmedais, F.; Ball, V. J. Colloid Interface Sci. 2010, 346, 163–171. (23) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531–12535. (24) Richert, L.; Lavalle, P.; Payan, E.; Shu, X. Z.; Prestwich, G. D.; Stoltz, J.-F.; Schaaf, P.; Voegel, J.-C.; Picart, C. Langmuir 2004, 20, 448–458.

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Figure 3. Photo images of CHI/CMC coatings on glass and polycarbonate taken after exposure to high humidity in climate chamber for 10 s; 20 bilayers of CHI/CMC on (A) glass, (B) polycarbonate, (C) left lens of a safety goggle. Yellow lined area indicates where coating was applied.

environmental chamber after incubation at room temperature, (b) using a basic room temperature breath test (Huff test), and (c) placing the sample over boiling water after incubation at room temperature. The results of these various test are summarized in Table 1. Similar results were obtained when samples were first cooled to T=4 °C and then subjected to these various fogging challenges. First, it should be noted that although typical multilayers fabricated from synthetic polyelectrolytes, such as SPS, PAA, and PAH, are generally hydrophilic, they do not exhibit acceptable antifogging behavior: extensive fogging occurs within seconds of exposure to a fogging environment. In contrast, multilayers fabricated from hydrophilic polysaccharides, such as hyaluronic acid, alginate, chitosan, and carboxymethylcellulose, provide excellent antifogging capability under a variety of different fogging challenges. In the case of the optimized CHI/CMC multilayers (assembly pH=4.0, no added salt), excellent antifog ability was observed on films assembled on both glass and polycarbonate substrates (see Figure 3). In addition, similar antifog performance was observed in multilayers with either CMC or chitosan as the last deposited layer. To achieve this high level of antifog performance, a minimum thickness of about 20 nm is required for the multilayer film. For multilayer films of lower thickness, incomplete coverage of the substrate surface compromises the antifog effect. 3.3. Structural/Molecular Requirements Needed for Antifog Multilayers. On the basis of the excellent antifog behavior of CHI/CMC multilayer coatings, we hypothesized that specific polar groups in the backbone and side chains of these hydrophilic polymers interact strongly with water molecules via hydrogenbonding and related dipole interactions, ultimately leading to the formation of a continuous, non-light-scattering layer of water. DOI: 10.1021/la103754a

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Figure 4. Humidity chamber tests of 10-bilayer films of chitosan/PAA-g-PEG (25% PEG) at 37 °C and 80% relative humidity for (A) 10 s and (B) 20 s.

Thus, the typical polymer-paired, hydrophilic ionic groups present in electrostatically assembled multilayers are not sufficient to impart antifogging behavior; strong hydrogen-bonding groups, such as hydroxyl and/or ether groups, are also needed. To test this hypothesis, we assembled and tested multilayers containing chitosan and PAA molecules with different amounts of a comonomer containing poly(ethylene glycol) segments. PEG is nonionic and is known for its ability to interact strongly with water molecules.25 PEG-co-PAA copolymers were synthesized with an increasing amount of PEG content in the copolymer and assembled with chitosan into multilayers. Multilayers assembled from CHI/PAA and CHI/PAA-co-PEG (25%) did not exhibit antifog behavior (Table 1 and Figure 4). However, when the PEG amount in the copolymer reached about 37%, excellent antifog behavior was observed (Table 1 and Figure 5). Likewise, multilayer films assembled with PAA copolymers containing 50% and 75% PEG content exhibited excellent antifog performance. These results are consistent with the hypothesis that strong waterinteracting functional groups are needed in addition to ionic bonds to create a multilayer coating with antifogging capability. Hydrophilic polysaccharides and synthetic polymers rich in alcohol groups and ethylene glycol units are known to exhibit the ability to imbibe large amounts of water and are often used in applications that exploit this effect.26,27 In addition, solid-state NMR and DSC studies11,12 of water interactions in cellulose and poly(vinyl alcohol) blends have shown that water molecules form hydrogen bonds with the hydroxyl, oxygen, and carboxyl groups of the polysaccharide repeat unit. In the case of CMC, one repeating unit can form hydrogen bonds with five to nine water molecules.12 One possible measure of the level of interaction of water with a material is the advancing contact angle a water droplet makes € (25) Ozdemir, C.; G€uner, A. J. Appl. Polym. Sci. 2006, 101, 203–216. (26) Mikolajczyk, T.; Woowska-Czapnik, D.; Bogun, M. J. Appl. Polym. Sci. 2008, 107, 1670–1677. (27) Dai, M.; Zheng, X.; Xu, X. J. Biomed. Biotechnol. 2009, 2009, 1–8.

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with the surface of the material. It has been suggested that hydrophilic surfaces with advancing contact angles less than 40° should exhibit antifog behavior.1,9 In recent work, however, we found that for nanoporous coatings contact angles less than about 7° were needed to achieve acceptable antifog behavior.2 Thus, a clear correlation between the antifog capability of a coating and its water droplet advancing contact angle does not always exist. The advancing contact angles of the antifogging CHI/CMC and CHI/PAA-co-PEG (37%, 48%) multilayers were measured to be between 20° and 40°. In a previous paper, the contact angle reported for multilayers of HA/CHI24 was 25°. The advancing contact angles of the fogging multilayers of PAH/SPS, PAH/ PAA, CHI/PAA, and CHI/PAA-g-PEG (25%) were in the range of 30°-90°. For all multilayers, including those that did not exhibit antifog properties, the receding contact angles were always close to zero. For the most part, for these dense (not nanoporous) multilayers, it appears that the general rule of needing a contact angle less than 40° to produce antifog behavior holds true. We do note, however, that exceptions to this rule were observed. 3.4. Kinetics of Water Droplet Formation. It is generally accepted that hydrophilic coatings inhibit fogging by preventing lightscattering water droplets from forming on a surface (Figure 6). Water in this case spreads across the surface forming a “transparent sheet”; hence, although water has condensed onto the surface, its presence is not visually detectable. Experimental proof of this effect is oftentimes provided by a simple visual observation of a substrate exposed to steaming water. In this experiment, rapid uniform wetting of the surface is taken as proof that water sheeting has occurred. To provide a more direct measurement of water droplet formation on surfaces, we developed an in situ ESEM technique that allows for the probing of a surface at a much higher resolution. Grosu et al.8 also studied fog-droplet formation using optical microscopy and examined the relationship between the size of fog droplets and contact angles. ESEM measurements were made by exposing samples held at 15 °C to humidity levels between 90% and 95%. Images were Langmuir 2011, 27(2), 782–791

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Figure 5. Humidity chamber tests of 10-bilayer films of chitosan/PAA-g-PEG (37% PEG) at 37 °C and 80% humidity for (A) 10 s and (B) 20 s.

Figure 6. Scheme illustrating antifog and fogging mechanisms on different polyelectrolyte films: (A) ESEM image of water droplets formed on PAH/SPS film; (B) simple cartoon of water droplets on regular polyelectrolyte films such as PAH/SPS; (C) ESEM image of CHI/CMC 20bilayer antifog coating after exposure to 90% humidity (note the lack of discernible water droplets); (D) simple cartoon showing a water film forming on antifog film; (E) water molecules interacting with dense polar groups on polysaccharide.

recorded after the introduction of humid air for half an hour. As shown in Figure 7 (and movies in Figure S.1, Supporting Information), no water droplets were observed to form on any multilayer films that exhibited excellent antifog properties. In these cases, samples remained free of detectable features throughout the entire experiment. Sometimes, however, large water droplets Langmuir 2011, 27(2), 782–791

were observed to form around defect areas. This underscores the importance of creating uniform, defect-free coatings for antifog applications. In sharp contrast, water droplet formation was clearly observed on all multilayer films that were not antifogging (PAH/ SPS, PAH/PAA, CHI/PAA, etc.). The growth of water droplets DOI: 10.1021/la103754a

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Figure 7. ESEM study of two different coatings at 15 °C and humidity 90%-95% after exposure for 15 s: (A) 80 bilayers of PAH/SPS coating on glass; (B) magnified region of PAH/SPS coating; (C) 20 bilayers of CHI/PAA-co-PEG(37%) on glass; (D) magnified region of CHI/PAA-co-PEG(37%) film.

with time on fogging multilayer films is presented in Figure 8. In the case of the PAH/SPS multilayer film, water droplets ranging in size from a few hundred nanometers to several micrometers formed within about 10-20 s of exposure to the humid environment. These sized water droplets are capable of scattering visible light. The average water droplet size was found to be typically less than 10 μm after the full 30 min exposure to the humid environment. A statistical analysis of the average number of droplets per mm2 observed after 10 s exposure to the humidity is found in Figure 9. The water droplet density, at least 10 000 droplets per mm2, was found to be high in all of the fogging polyelectrolyte multilayers. From the ESEM study, it can be concluded that fogging is indeed the result of the formation of a densely packed collection of water droplets of sufficient size to scatter visible light. 3.5. Durability of CHI/CMC Multilayer Films. The utility of any antifog coating is ultimately determined by the stability of the antifogging effect and the mechanical durability of the coating. In the former case, we found that antifog coatings based on multilayers of CHI/CMC retain their antifog properties in a normal laboratory environment for at least a few years. In addition, CHI/CMC films can be stored in a humidity chamber (37 °C, 80% humidity) for weeks at a time without losing their antifogging capability. Previously, we reported that excellent antifog coatings can be obtained from nanoporous polyelectrolyte multilayers containing silica nanoparticles.2 These coatings, 788 DOI: 10.1021/la103754a

Figure 8. Water droplet growth (observed for 10 min) versus time (seconds): (A) [PAH/SPS]80, (B) [CHI/CMC]15, (C) [PAH/PAAco-PEG(25%)]20, and (D) [PAH/PAA]16.

however, do not retain their antifogging functionality after exposure to the humidity chamber for times as short as a few days. The most likely cause of the loss of antifogging properties is capillary condensation of water into the nanopores and subsequent reaction Langmuir 2011, 27(2), 782–791

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of the silica nanoparticles with water.4 Thus, the CHI/CMC antifog films offer greater stability in those applications that require prolonged exposure to warm, highly humid environments. Five different tests were performed to determine the mechanical durability of CHI/CMC multilayer films on glass substrates as described in the Experimental Section. As-assembled CHI/ CMC films with either 15 or 30 bilayers were tested. In the case of the KIMWIPE test, no visible scratches were observed after testing. However, as-assembled films were easily removed by rubbing with sponges and cloths soaked in a soap solution. In the pencil hardness test, all of the as-assembled CHI/ CMC films achieved a 1H pencil hardness, which is comparable to a polymer like PMMA.28 Abrasion testing (Figure 10) revealed that as-assembled CHI/CMC films on glass substrates exhibit no loss in transmission after abrasion testing under a 100 kPa load.

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The films retained excellent optical properties, even after being abraded for 60 min at this load. For a comparison, identical abrasion tests were also conducted on bare polycarbonate substrates, and the results are shown in Figure 10. Bare polycarbonate substrates are readily damaged by this testing protocol. The CHI/CMC films are mechanically more robust than as-prepared nanoparticle containing multilayers and bare polycarbonate systems.7 In the same abrasion test (100 kPa and 60 min duration), as-assembled nanoparticle multilayers lost more than 4% transmittance. In contrast, the as-assembled polymer system showed no loss of transmittance.

Figure 10. Optical transmittance of CHI/CMC films on polycar-

Figure 9. Water droplet population (number of water droplets per mm2) on different films: (A) [PAH/PAA]16, (B) [CHI/PAA]20, (C) [PAH/PAA-co-PEG(25%)]20, (D) [PAH/SPS]80, and (E) [CHI/ CMC]15.

bonate before and after grinding at 100 kPa stress. Blue line represents the transmittance of polycarbonate before abrasion testing. Yellow line represents the transmittance of polycarbonate after abrasion testing. Green line represents the transmittance of 30-bilayer CHI/CMC-coated polycarbonate before abrasion testing. Red line represents the transmittance of this film after abrasion testing.

Figure 11. Chemical modification scheme of glass substrate using self-assembly process involving epoxysilane and subsequent cross-linking scheme for the multilayer. Langmuir 2011, 27(2), 782–791

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Figure 12. AFM and SEM images of 20-bilayer as-assembled and cross-linked CHI/CMC films on a glass substrate. (A) AFM image of 20bilayer as-assembled CHI/CMC film. (B) AFM image of 20-bilayer cross-linked CHI/CMC film. (C) Side-view SEM image of 20-bilayer asassembled CHI/CMC film. (D) Side-view SEM image of 20-bilayer cross-linked CHI/CMC film.

Although the mechanical durability of as-assembled films may be sufficient for some applications, it is clearly not at the level needed for more demanding environments. To enhance the mechanical robustness of the CHI/CMC antifog coatings, chemistry was introduced to improve both the adhesion of the coating to the substrate and the mechanical durability of the multilayer (see Figure 11). In the former case, a glass substrate was first chemically modified by forming covalent bonds between an epoxysilane compound (3-glycidoxypropyl)trimethoxysilane and the hydroxyl groups on the substrate surface. Branched poly(ethylenimine) was then exposed to the epoxy-functionalized surface for reaction between the epoxy groups and the amino groups of the PEI. The net result is a surface replete with amino groups that are covalently anchored onto the surface. Multilayers of CHI/ CMC were then assembled onto this surface starting with CMC. In the latter case, the CHI/CMC multilayers were cross-linked postassembly with EDC/NHS chemistry, followed by reaction with glutaraldehyde. Following this procedure, it was found that the cross-linked multilayer films exhibited superhydrophilic wetting behavior and retained their good antifog performance. The change from hydrophilic to superhydrophilic wetting behavior after crosslinking suggested that significant morphological changes had been introduced by this chemistry. AFM and SEM images of the cross-linked and as-assembled CHI/CMC films (Figure 12) revealed that surface roughness increased from 6 to 25 nm and (28) Tsai, T.-Y.; Wen, C.-K.; Chuang, H.-J.; Lin, M.-J.; Ray, U. Polym. Compos. 2008, 30, 1552–1561.

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that the dense multilayer film had become nanoporous after cross-linking. Ellipsometry further confirmed the fact that cross-linking induced nanoporosity; the refractive index of the as-assembled CHI/CMC multilayer is 1.54 and drops to 1.44 after cross-linking. Thus, the superhydrophilicity of the cross-linked CHI/CMC multilayer is a result of a nanoporous state, as was observed with nanoporous nanoparticle multilayer coatings.2 Cross-linked nanoporous (CMC/CHI)15.5 coatings on glass substrates were evaluated by the mechanical tests previously mentioned. The KIMWIPE and wet cloth/sponge tests revealed no visible scratches. The pencil hardness test indicated the film now had a pencil hardness approaching 4H, at which hardness the coating exhibited some damage and delamination. To mimic the pencil hardness test and better explore the mechanical durability of the films on glass substrates, AFM was utilized to apply scratches to the films in certain directions using the nanolithography mode at different forces. The CHI/ CMC films, including films with 12.5, 14.5, 16.5, and 30.5 bilayers, with and without cross-linking were evaluated. The asassembled films were readily damaged, even with a 400 nN force (Figure S.2); however, the cross-linked films showed no damage even with a 3000 nN force (Figure S.2). Therefore, these results were consistent with pencil and sponge-cloth rubbing test results. From a sectional analysis of the non-cross-linked (CMC/CHI)14.5 coatings, it can be seen that the depth of scratched areas of the film was 23 nm at 1.5 μN and 12 nm at 1 μN. Because of the roughness of the (CMC/CHI)14.5 films after cross-linking, it was difficult to judge the exact depth of the scratch. On the basis of the average results, it seems that the depth of the scratch was around 20 nm. Langmuir 2011, 27(2), 782–791

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In order to further explore the mechanical enhancement of the cross-linked system, the adhesion of the multilayer film on glass substrates before and after cross-linking was evaluated via a crosshatch adhesion test (Scotch tape test). As-assembled films failed the Scotch tape test, whereas cross-linked films remained intact after the test. Thus, chemically anchoring the multilayer to the glass surface dramatically improved adhesion.

4. Conclusions This work demonstrates that layer-by-layer assembled thin films rich in polysaccharides can exhibit excellent antifog properties under a variety of environmental challenges. With suitable surface and cross-linking chemistry, thin-film coatings with very good mechanical durability and adhesion can be realized. ESEM studies provided direct evidence that the excellent antifog capabilities of the polysaccharide-containing multilayers were associated with the formation of water sheets as opposed to light-scattering

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water droplets observed in the fogging films. The long-lasting antifog properties of these coatings open up new possibilities for biomedical and optical devices that require easily applied and durable antifog coatings. Acknowledgment. This work was supported partially by the MRSEC Program of the National Science Foundation (DMR0819762 and DMR-0213282). The authors thank the Center for Materials Science and Engineering (CMSE) and the Institute for Soldier Nanotechnologies (ISN) for use of the characterization facilities. We also acknowledge the contribution of Dr. Junyoung Kim and Mr. Girma Endale for helpful discussions and some sample characterizations. The authors also thank Dr. Haipeng Zheng of Essilor company for valuable input discussions. Supporting Information Available: ESEM movies, optical images, and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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