Superhydrophobic and superoleophobic poly(3,4-ethylenedioxypyrrole) polymers synthesized using the Staudinger-Vilarrasa reaction

Synthesis of 1-azido-10-bromodecane

To 50 mL of N,N-dimethylformamide (DMF), were added 1,10-dibromodecane (16 mmol, 1 equiv.) and sodium azide (16 mmol, 1 equiv.). The mixture was stirred and heated at 95°C for 1 day. Then, the solvent was evaporated and purified by column chromatography using silica gel and cyclohexane as eluent for obtained the 1-azido-10-bromodecane.

Synthesis of the precursor monomer by nucleophilic substitution of 3,4-ethylenedioxypyrrole

To 20 mL of anhydrous N,N-dimethylformamide, were added 3,4-ethylenedioxypyrrole (3,3 mmol, 1 equiv.) and potassium hydroxide (6,7 mmol, 2 equiv.). The mixture was stirred and heated during 30 min at 45°C. Then, 1-azido-10-bromodecane (5 mmol, 1,5 equiv.) was added droplet by droplet to the mixture diluted in 10 mL of anhydrous N,N-dimethylformamide. The solution was stirred and heated at 45°C for 3 h. Then, The mixture was evaporated and purified by column chromatography using gel silica and cyclohexane/ethyl acetate as eluent to obtain the precursor monomer 6-(10-azidodecyl)-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C₁₀-N₃).

Synthesis of the monomers by Staudinger-Vilarrasa reaction

To 20 mL of anhydrous tetrahydrofuran (THF), were added 4-dimethylaminopyridine (0,72 mmol, 2,2 equiv.), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (0,59 mmol, 1,8 equiv.) and 4,4,5,5,6,6,7,7,7-nonafluoroheptanoic acid or 4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononanoic acid or 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoic acid (0,5 mmol, 1,5 equiv.). The mixture was stirred at room temperature during 30 min to activate the carboxylic acid. Then, EDOP-C₁₀-N₃ was added diluted in 10 mL of anhydrous THF droplet by droplet. The solution was stirred at room temperature during 3 h. After the solution was purified by column chromatography using gel silica and a melt of cyclohexane/ethyl acetate as eluent.

Electrochemical conditions

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 for cyclic voltammetry experiments, a carbon rod as counter-electrode while a saturated calomel electrode (SCE) was used as reference electrode. For the surface characterization, the polymers were electrodeposited on gold plates purchased from Neyco and consisting in a deposition of chromium (20 nm) and gold (150 nm) on silicon wafer. In a glass cell, connected to the potentiostat with the three-electrode system, 10 mL of anhydrous acetonitrile containing 0.1 M of electrolyte (tetrabutylammonium perchlorate) and 0.01 M of monomer were introduced under argon. First, the monomer oxidation potential (0.86–1.08 V vs. SCE following the alkyl chain length of the monomer) was determined by cyclic voltammetry with the platinum tip as working electrode. Multiple potential scans were performed to study the polymer growth and to determine the polymer oxidation and reduction potentials. Then, polymer films were electrodeposited by cyclic voltammetry (CV) or at imposed potential (IP) on larger gold plates. Six depositions by imposed potential were performed for each monomer at several deposition charge during the experiments: 12.5, 25, 50, 100, 200, 400 mC cm⁻². Depositions at 1 mC cm⁻² were also performed to prepared smooth polymer films.

Surface characterization

Polymers were characterized by goniometry, scanning electron microscopy (SEM) and optical profilometry. The SEM images were recorded using a 6700F microscope (JEOL) in the CCMA (Centre Commun de Microscopie appliquée, Univ. Nice Sophia Antipolis). The mean arithmetic (Ra) and quadratic (Rq) roughness were determined using a Wyko NT 1100 optical microscope (Bruker) the objective 50×, and the field of view (FOV) 0.5×. VSI mode was used for smooth surfaces and PSI mode was used for very rough surfaces. The apparent contact angles (θ) were measured using a DSA30 goniometer (Krüss) by taking the tangent at the triple point contact line with a droplet of 2 μL. For the dynamic contact angles, a 4 μL water droplet was put on the substrate, and the substrate was inclined until the droplet moves. The advanced and receding contact angles are taken just before the droplet moving. The maximum inclination angle is called sliding angle (α). If the droplet does not move even for α>90°, the hysteresis is extremely high and the substrate is called sticky (parahydrophobic) [10]. The surface free energy (γ_SV) of the smooth surfaces and its dispersive (γ^D_SV) and polar (γ^P_SV) parts were determined using the Owens–Wendt equation: γ_LV(1+cos θ)=2(γ^D_LV.γ^D_SV)^1/2+2(γ^P_LV.γ^P_SV)^1/2. Using three different liquids (water, diiodomethane, hexadecane, for which γ_LV, γ^D_LV and γ^P_LV are known, γ^D_SV and γ^P_SV can be calculated by drawing the function y=ax+b where y=γ_LV(1+cos θ)/2(γ^D_LV)^1/2 and x=(γ^P_LV)^1/2/(γ^D_LV)^1/2. Then, γ^D_SV=b² and γ^P_SV=a² are determined. In our case, γ^D_SV and γ^P_SV were directly obtained using the software “Drop Shape Analysis” of our goniometer.

Deposition experiments

The electropolymerization experiments were performed in anhydrous acetonitrile containing 0.01 M monomer and 0.1 M tetrabutylammonium perchlorate (Bu₄NClO₄) used as electrolyte. First, it was necessary to determine the monomer oxidation potential (E^ox_m) by cyclic voltammetry to know the intervals of tension where the monomer is reactive (oxidized). The measurements of the monomer oxidation potential of each monomer (E^ox_m), also called sometimes reduction potentials of the monomer oxidation, [32] are reported on Table 1. The potential is close the same for all the polymer which means that the length of the fluorinated chains has not a significant effect on the oxidation of the monomer because of the presence of the very long decyl spacer.

In order to study the growth of the polymers, 10 scans of cyclic voltammetry were performed between −1 and E^ox_m on the platinum working electrode. The cyclic voltammograms are shown in Fig. 1. The voltammograms of each polymer shows very intense oxidation and reduction polymer curves between a range of 0.23 V and the E^ox_m with a perfect superposition after each scan indicating that the polymerization proceeds perfectly and that the amount of electrodeposited polymer is quite constant after each scan. More precisely, two oxidation and reduction peaks are observed in the voltammograms. As it was done in first experiments, the peak at around 0.90 V during the forward scans corresponds to the oxidation of the monomer. The other peaks observed during the forward scans correspond to the polymer oxidations and the other peaks observed during the back scans corresponds to the polymer reductions. Usually, the size, the position and electronic effect of the substituent can affect the polymer growth. Here, the position of the peaks is close the same for all polymers indicating that the decyl spacer allows to reduce the steric hindrance induce by the azide or the fluorinated chains. This is in agreement with previous works showing that due to very high van der Waals interactions, a decyl spacer is suitable to functionalize EDOP monomers on the nitrogen [33]. It was also shown in the literature that the presence of the second peak, [21] which seems to be due to a complex doping process, is important in the formation of surface nanoporosities/nanostructuration. Here, the second peak is extremely low with azide groups but extremely intense with the fluorinated chains.

Fig. 1:

Ten scans of cyclic voltammetry of different monomers. Scan rate 20 mV.s⁻¹.

Then in order to study the wettability of this polymers, different samples were performed on gold working substrates by cyclic voltammetry with 1, 3 and 5 deposition scans and at imposed potential with different deposition charges: 1 mC cm⁻² for the smooth samples (with a roughness <10 nm) and 12.5, 25, 50, 100, 200 and 400 mC cm⁻² for the rough samples. The depositions at imposed potential lead to conductive polymers in the oxidized state while by cyclic voltammetry the polymers are obtained in the reduced state.

Surface wettability and surface morphology

In order to evaluate the wetting behavior of the PEDOP derivatives, different probe liquids were studied (water, diiodomethane and hexadecane) to better characterize the surface hydrophocity and oleophobicity. The comparison between rough and smooth polymers shows the impact of the surface roughness on the wettability. First, at imposed potential (100 mC cm⁻²), the contact angles are higher than by cyclic voltammetry (3 scans) (Fig. 2). As expected, the higher properties were obtained with C₈F₁₇, for which superhydrophobic and superoleophobic properties are obtained.

Fig. 2:

Graphic of the contact angle with water of rough polymers deposited by imposed potential at 100 mC cm⁻² (left) and CV with 3 scans of deposition (right) compared to the smooth polymers.

Indeed, it was observed higher surface structuration at imposed potential than by cyclic voltammetry (Figs. 3 and 4), especially for PEDOP-C₁₀-N₃, explained the differences measured on these surfaces.

Fig. 3:

SEM images of PEDOP-C₁₀-N₃ and PEDOP-C₁₀-NHCO-EC₄F₉ deposited by imposed potential at 100 mC cm⁻² (left) and CV with 3 scans of deposition (right).

Fig. 4:

SEM images of PEDOP-C₁₀-NHCO-EC₆F₁₃ and PEDOP-C₁₀-NHCO-EC₈F₁₇ deposited by imposed potential at 100 mC cm⁻² (left) and CV with 3 scans of deposition (right).

The largest contact angles were obtained at imposed potential and using a deposition charge of 200 mC·cm⁻² (Fig. 5 and Table 2), for which superhydrophobic and superoleophobic properties are obtained with C₈F₁₇, and C₆F₁₃. These two polymers also displayed ultra-low water adhesion compared to with C₄F₉, which is parahydrophobic.

Fig. 5:

Wettability of PEDOP-C₁₀-NHCO-EC₈F₁₇, PEDOP-C₁₀-NHCO-EC₆F₁₃, PEDOP-C₁₀-NHCO-EC₄F₉ deposited at 200 mC cm⁻² by imposed potential with different liquid.

SEM images at higher magnification (Fig. 6) show a relatively similar surface morphology with both micro and nanostructures. Only PEDOP-C₁₀-NHCO-EC₈F₁₇ is more nanostructured than the other polymers, explaining the exceptional results obtained with this polymer. Hence, PEDOP-C₁₀-NHCO-EC₄F₉ is less hydrophobic and oleophobic especially because the fluorinated chain is shorter and as a consequence the polymer surface energy is higher.

Fig. 6:

SEM images of polymers deposited by imposed potential (Qs=200 mC·cm⁻²) with a magnification at ×5000 (left side) and ×25 000 (right side).

Other experiments

In order to do post-functionalization treatment with PEDOP-C₁₀-N₃ as a platform polymer, depositions of this polymers have been done at 12.5, 25, 50, 100, 200 and 400 mC·cm⁻². The protocol of post-functionalization on substrates with azide function are described in the literature. The substrate need to be immerged during 3 h under vigorous agitation into a solution of activated fluorinated carboxylic acid with DMAP, EDC and triphenylphosphine in anhydrous THF medium. A few second after the immersion, the polymeric films of PEDOP-C₁₀-N₃ (at any charge of deposition) dissolved in the THF medium meaning that the bonds between the film and the gold substrate are broken during this treatment.