Controlling the surface wettability is of a fundamental interest in an extremely large range of applications  such as in cookware coatings , anti-fingerprint screen protectors , microfluidic systems , gas and liquid separation membranes  or the anti-bioadhesion . The wettability of “smooth” surfaces is determined by the Young equation, which gives the apparent contact angle of a smooth surface (θY) as a function of three surface tensions . However, whatever the surface chemistry, θY of a water droplet deposited on a smooth surface does not exceed 130° but in nature many plants and animals display extreme wetting properties with apparent contact angle (θ) above 150° . Hence, biomimetic or bioinspired approaches have been developed to reproduce the wetting phenomena observed in nature. The most famous example is the superhydrophobic properties of the Lotus leaves characterized by θ above 150°, very low water adhesion or hysteresis and self-cleaning properties . However, other phenomena are also extremely important for fundamental applications [10–15]. For example, when a water droplet is deposited on Rosa montana leaves [10, 11], peanut leaves  or gecko feet , θ is extremely high (sometimes above 150°) but the water adhesion is extremely high. As a consequence, the water droplet remains stuck even if the leaf or foot is turned upside down. For these surface properties, Marmur proposed to use the term of parahydrophobic (as soon as θ>θY but with high adhesion) . For superhydrophobic or parahydrophobic properties, it is necessary to create surface structures.
Various processes have been implemented to induce the formation of surface structures with different surface morphologies (pillars, fibers, spheres, plates…) . The electrodeposition of conducting polymers is a unique method of formation of various surface structures with specific wettability properties [18–21]. Conducting polymers can, in their doping state, incorporate ions in their chemical structure with the possibility to tune the surface wettability . Moreover, various substituents can also be used to modify the surface morphology and wettability. One of the most important parameter was found to be the polymerizable core, which is necessary to induce the electrodeposition [23, 24]. The 3,4-ethylenedioxypyrrole (EDOP) heterocycle is an excellent core with exceptional polymerization capacity associated to excellent optical and electrical properties [25, 26]. Moreover, it was also shown the possibility to obtain very specific surface structures with exceptional wettability properties, such as nanoporous structures [27–34]. For example, superhydrophobic or superoleophobic properties were obtained using linear hydrocarbon and fluorocarbon chains, respectively. In order to produce parahydrophobic surfaces, it is preferable either to change the surface morphology or to reduce the intrinsic hydrophobicity of the materials. That can be done by introducing branching in hydrocarbon chains, for example. Here, we have elaborated five novel EDOP derivatives containing branched hydrocarbon chains of different size. The monomers are given in Scheme 1. We report their synthesis, characterization and the surface properties (surface wettability and roughness/morphology) of the corresponding conducting polymers.
The monomers were developed using a nine-step process (Scheme 2). The key intermediate EDOP was successfully reached in eight steps from iminodiacetic acid. The synthetic way includes the esterification of the two acid groups of iminodiacetic acid, the nitrogen protection, the formation of the pyrrole moiety using diethyl oxalate, the formation of the ethylenedioxy bridge using 1,2-dibromoethane, the deprotection of the nitrogen and the acid groups and the decarboxylation of the two resulting carboxylic groups. Finally, the monomer were obtained by nucleophilic substitution of the nitrogen of EDOP by the corresponding alkylbromide. The best conditions for the substitution was found to be the use of potassium hydroxide (KOH) in dimethylsulfoxide leading to excellent yield and an easy control of the reaction by thin-layer chromatography (TLC). For this reaction, 0.5 g of EDOP (4 mM, 1 eq.) and 0.45 g of KOH (8 mM, 2 eq.) were added to 20 mL of anhydrous DMF. After stirring for 30 min, 10 mL of anhydrous DMF containing 1.5 eq. of the corresponding alkylbromide were added. After 24 h at 50 °C, 100 mL of water were added and the product was extracted with ethyl acetate. The product was then purified by column chromatography using diethyl ether/cyclohexane 25:75 as eluent and silica gel as stationary phase.
6-isopropyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-br-C3 ): Yield 6 %; Slightly yellow oil; δH(200 MHz, CDCl3): 6.13 (2 H, s), 4.17 (4 H, s), 3.99 (1 H, m), 1.36 (6 H, d, J 6.7); δC(200 MHz, CDCl3): 131.50, 98.73, 65.85, 51.06, 23.56; MS (70 eV): m/z 167 (M+, 100), 152 (C8H10NO2+·, 52).
6-isobutyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-br-C4 ): Yield 15 %; Slightly yellow oil; δH(200 MHz, CDCl3): 6.04 (2 H, s), 4.18 (4 H, s), 3.43 (2 H, d, J 7.3), 1.92 (1 H, m), 0.85 (6 H, d, J 6.6); δC(200 MHz, CDCl3): 131.52, 101.39, 65.82, 58.05, 30.44, 19.97; MS (70 eV): m/z 181 (M+, 84), 138 (C7H8NO2+·, 52), 125 (C6H7NO2+, 42), 112 (C5H6NO2+·, 100).
6-isopentyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-br-C5 ): Yield 28 %; Slightly yellow oil; δH(200 MHz, CDCl3): 6.06 (2 H, s), 4.17 (4 H, s), 3.67 (2 H, t, J 7.1), 1.57 (3 H, m), 0.90 (6 H, d, J 6.3); δC(200 MHz, CDCl3): 131.61, 100.92, 65.83, 48.35, 40.15, 25.36, 22.35; MS (70 eV): m/z 195 (M+, 100), 139 (C7H8NO2+, 95), 112 (C5H6NO2+·, 50), 83 (C4H5NO+, 93).
6-(4-methylpentyl)-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-br-C6 ): Yield 32 %; Slightly yellow oil; δH(200 MHz, CDCl3): 6.06 (2 H, s), 4.18 (4 H, s), 3.63 (2 H, t, J 7.2), 1.67 (2 H, m), 1.54 (1 H, m), 1.17 (2 H, m), 0.87 (6 H, d, J 6.6); δC(200 MHz, CDCl3): 131.60, 100.94, 65.83, 50.50, 35.76, 29.22, 27.71, 22.48; MS (70 eV): m/z 209 (M+, 100), 194 (C11H16NO2+·, 8), 166 (C9H12NO2+·, 6), 152 (C8H10NO2+·, 8), 138 (C7H8NO2+·, 52), 125 (C6H7NO2+, 42), 112 (C5H6NO2+·, 35), 83 (C4H5NO+, 38).
6-(5-methylhexyl)-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-br-C7 ): Yield 35 %; Slightly yellow oil; δH(200 MHz, CDCl3): 6.06 (2 H, s), 4.18 (4 H, s), 3.64 (2 H, t, J 7.1), 1.66 (2 H, m), 1.51 (1 H, m), 1.21 (4 H, m), 0.85 (6 H, d, J 6.6); δC(200 MHz, CDCl3): 131.60, 100.95, 65.83, 50.22, 38.47, 31.57, 27.85, 24.47, 22.54; MS (70 eV): m/z 223 (M+, 100), 208 (C11H16NO2+·, 4), 180 (C11H16NO2+·, 15), 166 (C9H12NO2+·, 6), 152 (C8H10NO2+·, 23), 138 (C7H8NO2+·, 53), 125 (C6H7NO2+, 15), 112 (C5H6NO2+·, 31), 83 (C4H5NO+, 42).
An Autolab potentiostat purchased from Metrohm was used for the electrodeposition experiments. The connection was realized via a three-electrode system. A platinum tip was used as working electrode, a carbon rod as counter-electrode while a saturated calomel electrode (SCE) was used as reference electrode. The three electrodes were introduced inside a glass cell containing 10 mL of anhydrous acetonitrile with 0.1 M of tetrabutylammonium hexafluorophosphate (Bu4NPF6) and 0.01 M of monomer. The solution was degassed before each experiment.
Generalities for surface characterization
The contact angles were measured using a DSA30 goniometer (Krüss). The advanced contact angles (θ) were determined with the sessile drop method by taking the contact angle at the triple point between the water droplet, the substrate and the air. The dynamic contact angles were obtained with the tilted-drop method. This method allows the determining of the advanced and receding contact angles and as a consequence the hysteresis (H) after deposition of a 6 μL water droplet and surface inclination. The hysteresis is taken at the maximum surface inclination called sliding or tilting angle (α) just before the droplet moving. If the water droplet remained stuck whatever the surface inclination, the surface is called sticky.
The SEM images were obtained with a 6700F microscope (JEOL).
Results and discussion
Polymer growth study by cyclic voltammetry
The cyclic voltammetry is a very useful tool in order to study the growth of conducting polymers. Indeed, the redox properties of conducting polymers can be observed by cyclic voltammetry. Here, the monomer oxidation potential (Eox) was found to be 0.88–0.91 V vs SCE following the branched hydrocarbon chain. Then, the polymer growth was observed by cyclic voltammetry (–1 V to a potential slightly lower to Eox and at a scan rate of 20 mV/s). First of all, the cyclic voltammogram of EDOP-br-C3 displayed no significant peak corresponding to the polymer because the polymer formed was extremely soluble in solution. The presence of an isopropyl group on the nitrogen of EDOP induced very huge steric hindrance, especially with the adjacent units (Scheme 3). As a consequence, the polymer chain length is highly reduced and the polymer solubility highly increased. Indeed, a signal of low intensity was observed using EDOP-br-C4 but lowest steric hindrances were observed with EDOP-br-C5 and EDOP-br-C7 (Fig. 1).
Previously, it was shown the possible formation of nanoporous structures by electrodeposition of EDOP derivatives. It was demonstrates that this formation is possible only if the planarity of the PEDOP backbone in highly rigid or planar and the presence of a substituent can affect this rigidity. Here, we expected no porosity in the electrodeposited films due a distortion of the PEDOP backbone by the branched alkyl chains.
Surface characterization of polymers electrodeposited by cyclic voltammetry
Then, the polymers were electrodeposited on gold plates using the same technique. Two electrolytes were studied: Bu4NPF6 and tetrabutylammonium perchlorate (Bu4NClO4) as well as different numbers of deposition scan (1, 3 and 5). Only PEDOP-br-C5, PEDOP-br-C6 and PEDOP-br-C7 could by deposited in agreement with the study of the polymer growth.
The water apparent contact angles (θ) as a function of the number of deposition scans and the electrolyte are given in Fig. 2 while the SEM images of the films are displayed in Fig. 3. Using Bu4NPF6, The θ of the three polymers were relatively low (80° < θ < 100°) whatever the numbers of deposition scans. Indeed, the SEM images for 5 scans revealed that the films were not really structured. However, higher θ were measured on PEDOP-br-C6 and PEDOP-br-C7 electrodeposited in Bu4NClO4 (until 135.2° for PEDOP-br-C7). Moreover, water droplets deposited on these surfaces remained stuck even after a substrate inclination of 90°, as shown in Fig. 4, indicating extremely high water adhesion. These higher θ were due to the presence of fibrous structures forming large agglomerates on the substrates (Fig. 3). Indeed, large agglomerates were also reported on the surface of parahydrophobic R. montana leaves [10, 11]. However, these leaves were composed of large (16 μm in diameter and 7 μm in height) conical cells (micropapillae) while the agglomerates of our surfaces are more porous and irregular.
In order to improve the surface properties, the polymers were also electrodeposited at constant potential.
Surface characterization of polymers electrodeposited at constant potential
This technique allows an easier control of the electrodeposited polymer amount using various deposition charges (Qs). Moreover, at constant potential, the polymers are obtained in their doping state, which means than PF6– or ClO4– anions are present in the polymer structure and can influence the surface wettability.
The θ as a function of the Qs and the electrolyte are given in Fig. 5 while the SEM images of the films are displayed in Fig. 6. Extremely high θ above 135° (until 153.7° for PEDOP-br-C7) were obtained with the three polymers and the two electrolytes. This time, a water droplet deposited on these surfaces remained also stuck except for PEDOP-br-C7 for which the hysteresis H = 62° and the sliding angle α = 42° using Bu4NPF6, and H = 22° and the sliding angle α = 17° using Bu4NClO4. The difference in water adhesion are due to difference in the surface morphology. Using Bu4NPF6, fibrous structures forming large agglomerates were also observed but using Bu4NClO4, the structures were more spherical.
It is possible to explain these results with the Wenzel and Cassie–Baxter equations [35, 36]. These equations are dependent on the apparent contact angle of a smooth surface (θY), called Young angles . Hence, to know which equation should be used, it is necessary to determine the θY of each polymer. The smooth surfaces were obtained using an extremely short Qs (1 mC/cm2) to avoid the formation of surface structures. The θY are given in Table 1. Interestingly, whatever the polymer or the used electrolyte, the smooth polymer surfaces were intrinsically hydrophilic (θY < 90°). These hydrophilic properties were expected because of the presence of short alkyl chains and because also of the presence of branching.
Apparent water contact angles for the different smooth (θY) substrates.
The Wenzel and Cassie–Baxter equations can now be used. If a water droplet follows is in the Wenzel state , the droplet is in full contact with all the surface roughness. The Wenzel equation is cos θ = rcos θY where r is a roughness parameter. Using this equation, θ can be above θY only if θY > 90°. Hence, the results obtained with our structured surfaces cannot be explained using the Wenzel equation because θY < 90° (intrinsically hydrophilic polymers). These results can be explained if the water droplet follows the Cassie–Baxter state . The Cassie–Baxter equation is cos θ = rffcos θY+ f – 1 where rf corresponds to the roughness ratio of the substrate wetted by the liquid, f corresponds to the solid fraction and (1 – f) corresponds to the air fraction, as described by Marmur . This time, the water droplet is only in contact with the top of the asperities while there is trapped air present between the droplet and the surface. The Cassie–Baxter equation can lead to superhydrophobic properties (high θ and low H) if the contact between the surface and the water droplet is extremely low. The Cassie–Baxter equation can also lead to parahydrophobic properties (θ>θY and high H), as described by Marmur , if the contact between the surface and the water droplet is extremely important. For our surfaces, the presence of intrinsically hydrophilic polymer assembled in fibrous structures forming large agglomerates is responsible of both high θ and extremely high water adhesion.
Here, in order to reproduce the parahydrophobic properties (θ>θY and high water adhesion) of natural surfaces such as the R. montana petals or the gecko foot, we have studied original 3,4-ethylenedioxypyrrole monomers containing branched alkyl chains in order to have intrinsically hydrophilic polymers. Controlling the electrochemical conditions, it was possible to obtain parahydrophobic properties by electropolymerization. These properties were due to the presence fibrous structures forming large agglomerates. On such surfaces, a water droplet deposited on them remained stuck even after a substrate inclination of 180°. Such properties can be interesting for applications in water harvesting, for example.
The group thanks Jean-Pierre Laugier of the Centre Commun de Microscopie Appliquée (CCMA, Univ. Nice Sophia Antipolis) for the realization of the SEM images.
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