PHOTOREVERSIBLY TUNABLE WETTABILITY ON NANOSTRUCTURED SURFACES Kilwon Cho, Ho Sun Lim, Joong Tark Han and Donghoon Kwak Department of Chemical Engineering, Polymer Research Institute Pohang University of Science and Technology Pohang, 790-784, Korea (kwcho@postech.ac.kr ) Introduction Smart surfaces having reversibly switchable wettability have recently attracted much attention in both fundamental research and practical application. Conventionally, the wettability of a solid surface can be controlled by both the chemical composition and the geometrical microstructure of the surface; the wettability can be greatly changed by creating a local geometry with a large geometric area relative to the projected area. This effect can be observed in nature on the leaves of the sacred lotus, in which the leaves utilize superhydrophobicity as the basis of a mechanism to control the surface morphology for the protection and self-cleaning of that surface. 1-6 The layer-by-layer (LbL) processing of organic-inorganic nanocomposite films, comprising of polyelectrolytes and inorganic nanoparticles, is a promising candidate for protective coatings with greater toughness and strength than simple ceramic or polymeric coatings. 7 Azobenzene and its derivatives are known to exhibit large changes in both geometry and dipole moments as a result of UV/visble irradiation due to photoisomerization between the cis and trans conformations, which means that the wettability of azobenzene-modified surfaces can be altered with UV/visible irradiation. Especially, azobenzene surfaces prepared on rough substrates exhibit prodigious changes in reversible photoswitching wettability. In this study, we fabricate stable superhydrophobic surfaces by use of microporous polyelectrolyte multilayers, silica nanoparticles, and a hydrophobic surface treatment. 8 Further, by combining LbL deposition and photosensitive fluorinated azobenzene molecules, novel smart surface with wettability that can be reversibly switched between superhydrophobicity and superhydrophilicity with UV/visble exposure is materialized. 9 This approach can be used to make substrates with erasable and rewritable patterns of extreme wetting properties as a result of selective UV irradiation. Experimental Materials. Poly(allylamine hydrochloride) (PAH, mol. wt. 70,000), poly(acrylic acid) (PAA, mol. wt. 240,000), silica nanoparticles (SiO 2 nanoparticles, d~11 nm) and 6-bromohexanoic acid were provided from Aldrich. ZrO 2 nanoparticle (d~100 nm) colloidal dispersion was obtained from Alfa Aesar. 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDS) was purchased from Lancaster. 4-(Trifluoromethoxy)aniline was purchased from Acros. Phenol, sodium nitrite, sodium carbonate, acetic acid, 3-(aminopropyl) triethoxysilane (APTES) and N-Ethyl-N-(3-dimethylaminopropyl)carbodi imide hydrochloride (EDC) were supplied by Sigma-Aldrich and used as received. Deionized water (18 MΩ/cm, Millipore Milli-Q) was exclusively used in all aqueous solutions and rinsing procedures. All chemicals were used directly without further purification. Preparation of Superhydrophobic Organic-Inorganic Films. Organic-inorganic hybrid films were easily prepared by using the electrostatic self-assembly for PAA-coated ZrO 2 nanoparticles and PAH deposition. Prior to the first deposition of PAA-coated ZrO 2 , five bilayers of PAH and PAA ((PAH/PAA) 5 ) were deposited onto the cleaned Si wafer to prepare the dense first ZrO 2 layer. The (PAH/PAA) 5 -coated Si wafer was alternately immersed in 40 mM PAH solution and PAA-coated ZrO 2 colloid solution for 5 min. Then, 1.5 bilayers of silica nanoparticles/PAH were deposited on the (PAH/PAA-coated ZrO 2 ) n films using a 0.05 wt% nanosilica aqueous solution and pH 7.0 PAH solution. After the deposition process was complete, the (PAH/PAA-coated ZrO 2 ) n films were heated to 215 for 2 h to induce amidation between PAH and PAA chains. The hydrophobization of these organic-inorganic films was performed by dipping films in FDS/n-hexane solution for 1 h, and baking at 120 for 20 min. Preparation of Photoreversibly Switchable Surface. 4-(4-Trifluoro methoxyphenylazo)phenol was prepared via the diazo-coupling of 4-trifluoro methoxyaniline with phenol, and converted into 7-[(trifluoromethoxyphenyl azo)phenolxy]pentanoic acid (CF3AZO) with conventional procedures. Organic-inorganic hybrid films were easily prepared by using the electrostatic self-assembly for silica nanoparticles and PAH deposition. The cleaned Si wafer was alternately immersed in 40 mM pH 7.0 PAH solution and 0.05 wt% silica nanoparticles solution for 5 min, Then each substrate were incubated in 0.5 vol% of APTES in dry toluene solution for about 1 h. The amine-modified substrates were placed in an ethanolic solution of CF3AZO (1 mM) in the presence of EDC (10 mM) for 10h. Characterizations. Scanning electron microscopy (SEM, Hitachi 4200) was used for the observation of the morphologies on the fabricated films. The surface wettability on nanostructured substrate was confirmed by water contact angle (CA) measurement using 3 μl water droplet. Light source obtained from A 500 W high pressure Arc-Xe lamp (Oriel) with cut-off filter (λ = 365 nm for UV or λ > 420 nm for visible light irradiation). To pattern hydrophilic domains on the superhydrophobic surface, the surface was irradiated selectively with UV light through an aluminum mask (2.5 x 2.5 mm square patterns with a 2 mm pitch). Results and Discussion Stable Superhydrophobic Organic-Inorganic Hybrid Films. As shown in Figure 1, the SEM images of the organic-inorganic nanocomposite films constructed using 100 nm ZrO 2 particles have porous net structures even after over 10 deposition cycles. The morphology of films was controlled by the number of deposition cycles of PAH and PAA-coated ZrO 2 particles; specifically, the surface roughness and porosity after 20 deposition cycles were greater than that after five deposition cycles. The water CA increased gradually with increasing number of deposition cycles (Figure 2). After 20 deposition cycles, where a PAH layer was the outermost layer, the advancing and receding water CAs were 139 ± 3° and 97 ± 3°, respectively, whereas after 10 deposition cycles, the advancing water CA was 110°. When the outermost layer was PAA-coated ZrO 2 , the maximum water CA was about 95°, which was also larger than that of the flat film (48°). Our results agree well with the model of Cassie and Baxter for non-wetted surfaces. The hydrophilicity decrease of the outermost PAH layer implies that the NH 3 + pendant groups are compensated by the negative charges of PAA, and consequently, most of aliphatic groups in PAH chains are exposed at the surface, which would minimize the surface free energy of the system. Figure 1. SEM images of organic-inorganic nanocomposite films; (a) (PAH/PAA-coated ZrO 2 ) 10 film, (b) (PAH/PAA-coated ZrO 2 ) 20 film, and (c) (PAH/PAA-coated ZrO 2 ) 10 film after deposition of silica nanoparticles. Inset shows a water droplet profile. Figure 2. The relationships between the number of deposition cycles and the water contact angles. Inset is the shape of a water droplet on the surface of (PAH/PAA-coated ZrO 2 ) 20 film when PAH is the outmost layer. Polymer Preprints 2007, 48(1), 747