Using a bioinspired approach, we prepare poly(3,4-ethylenedioxypyrrole) (PEDOP) films with parahydrophobic properties, characterized by high apparent water contact angle and strong water adhesion. The films are made by electropolymerization and the influence of substitution by an alkyl chain of various length (from C4H9 to C14H29) on the 3,4-ethylenedioxy-bridge is reported. More precisely, the best properties are obtained from a length of C12H25 due to the formation of spherical nanoparticles.
Bioinspiration presents a unique way to find novel innovative ideas in various fields , . This is particularly true in the surface wettability , , . In Nature, various species have developed surface with both high hydrophobicity but low water adhesion, called superhydrophobicity , , , . Thanks to these properties, species are able for example to move faster in air, on the water surface or even underwater. Other species such as red roses, peanuts and geckos have surfaces with both high hydrophobicity but also strong adhesion to water droplets , , . This property, also called parahydrophobicity by Marmur , is extremely interesting to develop water harvesting systems, particularly in hot and arid environments. As observed in Nature, two key parameters are very important to control the surface hydrophobicity and the water adhesion: the surface roughness/structures and the surface energy , , .
Many processes have been implemented to change both the surface structures and the surface energy , , , . Conducting polymers are very interesting for creating nanostructured materials by self-assembly. Various 1D-, 2D- and 3D-structures such as nanofibers, nanosheets, nanotubes or flower-like structures could be obtained , , . Structured conducting polymer films can also be produced directly on substrates by electropolymerization , , , . In this process, a monomer is electrochemically oxidized to produce a conducting polymer film of a working electrode. The surface structures can be tuned by changing the electrochemical parameters and the monomer structure. The surface energy can also be modified by grafting a substituent of various hydrophobicity.
The monomers of the 3,4-alkylenedioxypyrrole family such as 3,4-ethylenedioxypyrrole (EDOP) and 3,4-propylenedioxypyrrole (ProDOP) are extremely versatile monomers with unique opto-electronic properties, easily to functionalize, making them candidates for various applications. For example, PEDOP-carbon nanotube-modified electrodes were used as electrochemical biosensor for the selective determination of serotonin . Reynolds and al. prepared various PEDOP and ProDOP polymers substituted with different substituents and studied their optoelectronic properties , , . The substituents were grafted either on the 3,4-alkylenedioxy-bridge either on the nitrogen. Our research group also reported PEDOP and ProDOP films but with hydrophobic chains such as fluorocarbon chains, hydrocarbon chains or aromatic groups, but in the aim the control the surface morphology and the resulting hydrophobicity/oleophobicity , , , .
Following this strategy, here, we study for the first time, the morphology and wetting properties of PEDOP films with alkyl chains on the 3,4-ethylenedioxy-bridge. Different alkyl chain lengths from butyl to tetradecyl are tested. The monomers are given in Scheme 1.
1-benzyl-3,4-dihydroxy-1H-pyrrole-2,5-dicarboxylic acid (1) was synthesized in four steps from iminodiacetic acid (Scheme 2). Then, 1 (1.0 eq, 10.0 g) was reacted with the corresponding alkane-1,2-diol (2.0 eq) and triphenylphosphine (PPh3) (2.0 eq, 15.7 g) in 150 mL of anhydrous tetrahydrofuran (THF). The reaction was stirred and then diethyl azodicarboxylate (2.4 eq, 14.5 g) was added dropwise. The mixture was stirred for 48 h at reflux. THF was removed and 20 mL of diethyl ether was added to induce the precipitation of phosphorous compounds after 8 h in the fridge. Then, the filtrate containing the products (2–4 to 2–14) were purified by chromatography on silica gel (eluent: diethyl ether/petroleum ether 1/1).
2–4 to 2–14 (1 eq) were reacted with anisole (2.3 eq, 1.1 g), 1 mL of sulfuric acid (H2SO4) in 30 mL of trifluoroacetic acid. After stirring for 1 h at 90°C, the product was added to 400 mL of an aqueous solution saturated with sodium hydrogen carbonate (NaHCO3). The products 3–4 to 3–14 were extracted with ethyl acetate and purified by chromatography on silica gel (eluent: diethyl ether/petroleum ether 1/1).
Diethyl 2-butyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole-5,7-dicarboxylate (3–4): Yield 51%; δH(200 MHz, CDCl3): 8.56 (s, 1H), 4.33 (m, 5H), 4.18 (m, 1H), 3.96 (dd, J=11.4 Hz, J=7.6 Hz, 1H), 1.66 (m, 4H), 1.36 (m, 8H), 0.92 (t, 7.0 Hz, 3H).
Diethyl 2-hexyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole-5,7-dicarboxylate (3–6): Yield 94%; δH(200 MHz, CDCl3): 8.52 (s, 1H), 4.33 (m, 5H), 4.18 (m, 1H), 3.96 (dd, J=11.3 Hz, J=7.5 Hz, 1H), 1.69 (m, 4H), 1.40 (m, 12H), 0.88 (t, 6.4 Hz, 3H).
Diethyl 2-octyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole-5,7-dicarboxylate (3–8): Yield 24%; δH(200 MHz, CDCl3): 8.54 (s, 1H), 4.34 (m, 5H), 4.19 (m, 1H), 3.97 (dd, J=11.6 Hz, J=7.7 Hz, 1H), 1.65 (m, 4H), 1.34 (m, 16H), 0.88 (t, 6.6 Hz, 3H).
Diethyl 2-decyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole-5,7-dicarboxylate (3–10): Yield 20%; δH(200 MHz, CDCl3): 8.54 (s, 1H), 4.34 (m, 5H), 4.19 (m, 1H), 3.97 (dd, J=11.4 Hz, J=7.5 Hz, 1H), 1.62 (m, 4H), 1.32 (m, 20H), 0.87 (t, 6.4 Hz, 3H).
Diethyl 2-dodecyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole-5,7-dicarboxylate (3–12): Yield 14%; δH(200 MHz, CDCl3): 8.54 (s, 1H), 4.33 (m, 5H), 4.19 (m, 1H), 3.97 (dd, J=11.3 Hz, J=7.5 Hz, 1H), 1.62 (m, 4H), 1.30 (m, 24H), 0.87 (t, 6.4 Hz, 3H).
Diethyl 2-tetradecyl-3,6-dihydro-2H-], dioxino[2,3-c]pyrrole-5,7-dicarboxylate (3–14): Yield 34%; δH(200 MHz, CDCl3): 8.54 (s, 1H), 4.33 (m, 5H), 4.19 (m, 1H), 3.97 (dd, J=11.3 Hz, J=7.6 Hz, 1H), 1.62 (m, 4H), 1.30 (m, 28H), 0.87 (t, 6.4 Hz, 3H).
3–4 to 3–14 were added to 15 mL of an aqueous solution of sodium hydroxide (2M) containing 7.5 mL of ethanol. After heating for 24 h at 90°C, the solution was cooled to room temperature and hydrochloride acid (HCl) was slowly added until pH≈3. Then, the solvent was removed by rotavapor and the products 4–4 to 4–14 were dried in an oven.
Finally, 40 mL of triethylamine was heated at 200°C with a high stirring speed. 4–4 to 4–14 were added until all the released gas (CO2) ceases (~30 s). The solution was cooled to room temperature and extracted with ethyl acetate. ~10 extractions were needed, as it was very difficult to fully extract the product. Both the aqueous and organic phases were controlled with ninhydrin to ensure that all the product passed through the organic phase. The products EDOP-C4 to EDOP-C14 were purified by chromatography on silica gel (eluent: diethyl ether/petroleum ether 1/1). Silica gel was treated before with 10% triethylamine in order to protect the very reactive monomers.
2-butyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C4): Yield 11%; δH(200 MHz, CDCl3): 7.07 (s, 1H), 6.18 (d, J=3.1 Hz, 2H), 4.10 (m, 2H), 3.86 (dd, J=11.6 Hz, J=8.4 Hz, 1H), 1.41 (m, 6H), 0.92 (t, J=7.1 Hz, 6H); MS (ESI LC/MS): 182.00.
2-hexyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C6): Yield 10%; δH(200 MHz, CDCl3): 7.02 (s, 1H), 6.19 (d, J=3.1 Hz, 2H), 4.10 (m, 2H), 3.86 (dd, J=11.6 Hz, J=8.4 Hz, 1H), 1.41 (m, 10H), 0.89 (t, J=6.5 Hz, 6H); MS (ESI LC/MS): 210.07.
2-octyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C8): Yield 51%; δH(200 MHz, CDCl3): 7.03 (s, 1H), 6.18 (d, J=3.1 Hz, 2H), 4.10 (m, 2H), 3.86 (dd, J=11.6 Hz, J=8.4 Hz, 1H), 1.41 (m, 14H), 0.88 (t, J=6.4 Hz, 6H); MS (ESI LC/MS): 238.07.
2-decyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C10): Yield 18%; δH(200 MHz, CDCl3): 7.09 (s, 1H), 6.18 (d, J=3.1 Hz, 2H), 4.10 (m, 2H), 3.86 (dd, J=11.6 Hz, J=8.4 Hz, 1H), 1.41 (m, 18H), 0.88 (t, J=6.3 Hz, 6H).
2-dodecyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C12): Yield 4.9%; δH(200 MHz, CDCl3): 7.00 (s, 1H), 6.18 (d, J=3.1 Hz, 2H), 4.10 (m, 2H), 3.86 (dd, J=11.6 Hz, J=8.4 Hz, 1H), 1.41 (m, 22H), 0.88 (t, J=7.1 Hz, 6H); MS (ESI LC/MS): 294.13.
2-tetradecyl-3,6-dihydro-2H-[1,4]dioxino[2,3-c]pyrrole (EDOP-C14): Yield 2%; δH(200 MHz, CDCl3): 7.02 (s, 1H), 6.18 (d, J=3.1 Hz, 2H), 4.10 (m, 2H), 3.86 (dd, J=11.6 Hz, J=8.4 Hz, 1H), 1.41 (m, 26H), 0.86 (t, J=7.1 Hz, 6H); MS (ESI LC/MS): 322.27.
The electropolymerizations were performed with a potentiostat (Autolab, Metrohm). Three electrodes were used: a saturated calomel electrode (SCE) as a reference electrode, a carbon rod as a counter-electrode. As a working electrode, a platinum tip was first used for electrochemical characterization, and replaced after by 2 cm2 gold-coated silicon wafers for the surface characterization. For the electrolyte, anhydrous acetonitrile was used with 0.1 M of tetrabutylammonium perchlorate (Bu4NClO4) and 0.01 M of monomer.
For the apparent contact angle (θ), a water droplet is placed on the surface and θ is taken at the triple point using a goniometer (DSA30, Krüss). For the hysteresis, a water droplet is placed on the surface and the surface is inclined until the water droplet moves (sliding angle). The advanced and receding contact angles, and as a consequence the hysteresis, are taken just before the water droplet moves. If the droplet does not move, the surface is called sticky or parahydrophobic.
The surface morphology was determined by scanning electron microscopy (6700F microscope, JEOL). The arithmetic (Ra) and quadratic (Rq) surface roughness were determined by optical profilometry (WYKO NT1100, Bruker) with the working mode High Mag Phase Shift Interference (PSI), the objective 50X, and the field of view 0.5X.
Results and discussion
The electropolymerizations were performed in anhydrous acetonitrile with Bu4NClO4 as supporting electrolyte. The oxidation potential of each monomer (Eox) was determined by cyclic voltammetry and found to be around 0.90–1.05 V vs. SCE. Then, the polymer growth was studied by cyclic voltammetry (10 scans from −1 V to Eox at 20 mV s−1) (Fig. 1). Whatever the alkyl chain length, the polymer oxidation and reduction potentials were much below 0 V vs. SCE indicating of extremely long polymer chains. This is due to the exceptional polymerization capacity of EDOP , , . Moreover, a constant increase in the curve intensity is observed after each scan which indicates that the growth is homogeneous, and that alkyl chain has not a significant influence. It was just observed much lower intensity with EDOP-C14.
Then, polymer films were performed at constant potential (Eox) and using different deposition charge from 12.5 to 400 mC cm−2. SEM images were performed to determine the surface morphology of the films (Fig. 2). For an alkyl chain length n<8, the films are relatively smooth. In contrast, for longer alkyl chains n≥10, the formation of spherical particles is observed leading to rough and porous surfaces. For n=10, the size of the particles is nanometric while for n≥12, their size is micrometric. The change in the surface morphology from smooth to spherical particles can be easily explained. Then the polymer is substituted with a relatively short alkyl chain (n<8), the polymer solubility in the solvent (acetonitrile) is important leading to smooth films, as observed in the literature , . However, when the alkyl chain increases the polymer solubility decreases leading to spherical particles (3D-growth).
The wetting properties of the films was also investigated by contact angle (θ) measurement (Table 1). For n<8, the films are hydrophilic or slightly hydrophobic whatever the deposition charge because the films are relatively smooth. In contrast, for n≥10, extremely high hydrophobic properties are reached due to both lower surface energy and the presence of surface roughness made of spherical particles. The higher θw up to ≈143° are obtained with EDOP-C12 and EDOP-C14, and for a deposition charge of 400 mC cm−2. Moreover, the water adhesion of these films is extremely strong (Fig. 3). Water droplets in contact to these surfaces remain completely stuck on them whatever the surface inclination (parahydrophobic).
|Polymer||Deposition charge (mC cm−2)||Ra (nm)||Rq (nm)||θw (deg)||–θdiiodo (deg)||θhexa (deg)|
Here, we reported the possibility of obtaining parahydrophobic properties by electrodeposition of PEDOP with alkyl chains. The best properties were obtained from an alkyl chain length of C12H25 due to the formation of spherical nanoparticles. Such materials could be in the future excellent candidates for applications in water harvesting system.
A collection of invited papers based on presentations at the 4th International Conference on Bioinspired and Biobased Chemistry & Materials (NICE-2018), Nice, France, 14–17 October 2018.
©2020 IUPAC & De Gruyter. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information, please visit: http://creativecommons.org/licenses/by-nc-nd/4.0/