A bioinspired approach to produce parahydrophobic properties using PEDOP conducting polymers with branched alkyl chains

Monomer synthesis

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.

Scheme 2:

Synthesis way to produce the monomers.

6-isopropyl-3,6-dihydro-2 H -[1,4]dioxino[2,3- c ]pyrrole (EDOP-br-C ₃ ): Yield 6 %; Slightly yellow oil; δ_H(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); δ_C(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-2 H -[1,4]dioxino[2,3- c ]pyrrole (EDOP-br-C ₄ ): Yield 15 %; Slightly yellow oil; δ_H(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); δ_C(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-2 H -[1,4]dioxino[2,3- c ]pyrrole (EDOP-br-C ₅ ): Yield 28 %; Slightly yellow oil; δ_H(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); δ_C(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-2 H -[1,4]dioxino[2,3- c ]pyrrole (EDOP-br-C ₆ ): Yield 32 %; Slightly yellow oil; δ_H(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); δ_C(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-2 H -[1,4]dioxino[2,3- c ]pyrrole (EDOP-br-C ₇ ): Yield 35 %; Slightly yellow oil; δ_H(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); δ_C(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).

Electrochemical parameters

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).

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 (E^ox) 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 E^ox 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).

Scheme 3:

Schematic representation of steric hindrance induced during polymerization by the substitution of branched hydrocarbon chains.

Fig. 1:

Cyclic voltammogram of different monomers in 0.1 M Bu₄NPF₆/anhydrous acetonitrile; scan rate: 20 mV/s; number of scans: 10.

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.

Fig. 2:

Apparent contact angle (θ) of the polymers at the function of the number of deposition scans and the electrolyte (Bu₄NPF₆ or Bu₄NClO₄).

Fig. 3:

SEM images of different polymers electrodeposited by cyclic voltammetry (number of deposition scans: 5; scan rate: 20 mV/s) in two different electrolytes (Bu₄NPF₆ or Bu₄NClO₄); Magnification: 5000x.

Fig. 4:

Water droplet deposited on a parahydrophobic substrates after a surface inclination of 90°.

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.

Fig. 5:

Apparent contact angle (θ) of the polymers at the function of the deposition charge (Qs) and the electrolyte (Bu₄NPF₆ or Bu₄NClO₄).

Fig. 6:

SEM images of different polymers electrodeposited at constant potential (deposition charge: 100 mC/cm²) in two different electrolytes (Bu₄NPF₆ or Bu₄NClO₄); Magnification: 5000x.

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 (θ^Y), called Young angles [8]. Hence, to know which equation should be used, it is necessary to determine the θ^Y of each polymer. The smooth surfaces were obtained using an extremely short Qs (1 mC/cm²) to avoid the formation of surface structures. The θ^Y are given in Table 1. Interestingly, whatever the polymer or the used electrolyte, the smooth polymer surfaces were intrinsically hydrophilic (θ^Y < 90°). These hydrophilic properties were expected because of the presence of short alkyl chains and because also of the presence of branching.

The Wenzel and Cassie–Baxter equations can now be used. If a water droplet follows is in the Wenzel state [35], the droplet is in full contact with all the surface roughness. The Wenzel equation is cos θ = rcos θ^Y where r is a roughness parameter. Using this equation, θ can be above θ^Y only if θ^Y > 90°. Hence, the results obtained with our structured surfaces cannot be explained using the Wenzel equation because θ^Y < 90° (intrinsically hydrophilic polymers). These results can be explained if the water droplet follows the Cassie–Baxter state [36]. The Cassie–Baxter equation is cos θ = r_ffcos θ^Y+ 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 [37]. 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>θ^Y and high H), as described by Marmur [16], 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.