Article pubs.acs.org/JPCC
Enhancement of the Superoleophobic Properties of Fluorinated PEDOP Using Polar Glycol Spacers Thierry Darmanin and Frédéric Guittard* CNRS, LPMC, UMR 7336, Université de Nice Sophia Antipolis, 06100 Nice, France S Supporting Information *
ABSTRACT: The formation of superoleophobic materials is extremely important for various potential applications, but these materials are extremely difficult to obtain due to the low surface tension of oils. Here, we synthesized original fluorinated (C4F9, C6F13, and C8F17) 3,4-ethylenedioxypyrrole (EDOP) derivatives with polar glycol spacers (diethylene glycol or triethylene glycol) to enhance the superoleophobic properties by electropolymerization. The mobility of the glycol spacers induces steric hindrance during electropolymerization and affects the presence of surface nanoporosity. However, the superoleophobic properties of fluorinated PEDOP can be improved, even if the presence of nanoporosity decreases, on condition that a high degree of nanostructures is preserved around the surface microstructures. This is possible by controlling the length of the fluorinated chain and the length of the glycol spacer. Using a diethylene glycol spacer and a C8F17 chain it is possible to obtain superoleophobic properties with θdiiodomethane = 153.0°, θsunflower oil = 148.3°, and θhexadecane = 148.0°, and with also low oil adhesion. The micro and nanostructures present on the surface and the high intrinsic oleophobic properties of the polymer allow the stabilization of the Cassie−Baxter state.
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state even with low surface tension liquids.24−26 Indeed, an important negative Laplace pressure difference can occur changing the liquid−vapor interface27 from concave to convex, for example, which can increase the energy barrier between the Wenzel and the Cassie−Baxter state.28 Very recently, the importance of re-entrant curvatures for superoleophobic properties was also found in Nature.29−32 For example, the group of Werner reported the superoleophobic properties of collembola, also called springtails, which are skin-breathing arthropods and live in soil environment.30−32 The authors showed that to survive in their environment they have developed on their cuticles robust superoleophobic properties due to hexagonal or rhombic comblike patterns with negative overhangs. Different processes can be employed to reach superoleophobic properties.1,33−35 Among them, the electrodeposition of conducting polymers is a special process with many advantages such as the reproducibility and the ease of implementation.36−39 Because the conducting polymers exist in different doping states, doping agents can be introduced to change the surface wettability.40 The surface wettability can also be modified by grafting substituents in the monomer structure before polymerization.41 One of the best ways to reach superoleophobic properties with conducting polymers is the use of fluorinated 3,4ethylenedioxypyrrole (EDOP) derivatives.42−48 These properties were due to the formation of microstructures with
INTRODUCTION The creation of super oil-repellent materials, also called superoleophobic, is a hot field of research from a theoretical point of view and for various potential applications1 such as for oil/water separation,2−4 in microfluidics devices,5,6 antisoil clothes,7−11 antifingerprint/antireflective touchscreen and glasses,12,13 anticorrosion coatings,14 or printing technologies.15 In the literature, two different superoleophobic properties have been studied: superoleophobicity in air (solid−liquid−vapor interface)1 and underwater superoleophobicity (solid−liquid− liquid interface).16,17 In this manuscript only materials with superoleophobic properties in air are concerned. Because of their extremely low surface tension ( 90°), intrinsically oleophobic with diiodomethane and sunflower oil (θYdiiodomethane and θYsunflower oil > 90°), but intrinsically oleophilic with hexadecane (θYhexadecane < 90°). To our knowledge, these intrinsic oleophobic properties are the highest in comparison to the literature for a fluorinated polymer. Now, the results can be explained using the Wenzel and Cassie−Baxter equations.56,57 In the Wenzel state, the interface between the droplet and the surface is only composed of solid− liquid interface, and a roughness parameter (r) increases this interface.56 The equation is cos θ = r cos θY. If θY > 90°, r increases the apparent contact angle and reversely if θY < 90°, r decreases the apparent contact angle. In this work, the increase in θhexadecane from 77.8° to 148.0° in the case of PEDOP-EG2-F8 cannot be explained with this equation. Only the Cassie−Baxter equation can explain these values.57 In the Cassie−Baxter state, the droplet is suspended on top of asperities but also on air trapped inside the surface roughness. The equation is cos θ = rf f cos θY+ f − 1 where rf is roughness ratio of the wet surface, f is the solid fraction, and (1 − f) is the air fraction, as described by Marmur.58−60 The Cassie−Baxter equation can induce an increase in the contact angle whatever θY. The presence of both micro- and nanostructures on PEDOP-EG2-F8 films led to a high pinning of the three-phase contact line23 and as a consequence a high increase in θoils and also in θhexadecane even if θYhexadecane < 90°. Moreover, the high θYhexadecane is also important for the stabilization of the Cassie−Baxter state.27,28
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ASSOCIATED CONTENT
S Supporting Information *
Table 1. Apparent Contact Angles of the Smooth Surfaces (Young Angles θY) polymer
Article
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank J.-P. Laugier of the Centre Commun de Microscopie Appliquée (CCMA, Univ. Nice Sophia Antipolis) for the realization of the SEM images.
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CONCLUSIONS
We have shown the possibility of enhancing the superoleophobic properties of electrodeposited fluorinated PEDOP using polar glycol spacers (diethylene glycol or triethylene glycol). Even if the mobility of the glycol spacers induces steric hindrance during electropolymerization and affected the presence of surface nanoporosity, the superoleophobic properties could be improved on condition that a high degree of nanostructures is preserved around the surface microstructures. Hence, it was necessary to control the length of the fluorinated chain and the length of the glycol spacer. Superoleophobic properties with θdiiodomethane = 153.0°, θsunflower oil = 148.3°, and θhexadecane = 148.0° and with also low oil adhesion were obtained using a diethylene glycol spacer and a C8F17 chain. The stabilization of the Cassie−Baxter state was possible thanks to micro and nanostructures on the surface and the high intrinsic oleophobic properties of the polymer. 26918
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