Improvement of the Tunable Wettability Property of Poly(3

May 4, 2009 - The surface wettability properties of poly(3-alkylthiophene) (P3AT) can be reversibly switched between hydrophobic and hydrophilic state...
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Improvement of the Tunable Wettability Property of Poly(3-alkylthiophene) Films Peng Lin, Feng Yan,* and Helen L. W. Chan Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Received January 31, 2009. Revised Manuscript Received March 10, 2009 The surface wettability properties of poly(3-alkylthiophene) (P3AT) can be reversibly switched between hydrophobic and hydrophilic states by a low-voltage electrochemical approach. The switching processes corresponding to the oxidation and deoxidation processes of the polymer films have been systematically studied. The tunable region of the contact angle of the polymer films is dependent on the type of dopant, and the biggest effect was induced by SO24 doping, which changes the contact angle for about 30°. The tunable amplitude of the contact angle has been substantially enlarged by coating the polymer film on a substrate patterned with micrometer-sized posts, which shows a static contact angle of 147.4° at neutral state and 62.2° at oxidized state. It is envisaged that the polymer films have potential applications as low cost disposable wettability switches in microfluidics and contact-printing technology.

1. Introduction Films with tunable wettability properties have broad applications in microfluidics, self-cleaning surfaces, displays, and printing technologies. A variety of smart solid surfaces with such properties have been developed, in which wettability can be switched by light,1-3 temperature,4,5 electrical field,6 magnetic field,7 ultrasound,8 adsorption of biopolymer, treatment of selective solvent, or electrochemical process.9 Among these approaches, the switch based on electrochemical process is very important because it is simple, reversible, and controllable with electricity. More importantly, the electrochemical switching is readily individually addressable when the electrodes are patterned as an array of small surfaces, and therefore it can be easily integrated into a microfluidic system to drive liquid or operate as a wettability switch. Poly(3-alkylthiophene) (P3AT) is a type of organic semiconducting polymer and already shows very important applications in organic thin film transistors and organic solar cells due to its good solubility, processability, chemical stability, and excellent electronic properties.10-13 Recently, Robinson et al. reported that *Corresponding author. E-mail: [email protected]. (1) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624–1626. (2) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. B. Langmuir 2004, 20, 5659–5661. (3) Wan, P.; Jiang, Y.; Wang, Y.; Wang, Z.; Zhang, X. Chem. Commun. 2008, 44, 5710–5712. (4) De Crevoisier, G.; Fabre, P.; Corpart, J.-M.; Leibler, L. Science 1999, 285, 1246–1249. (5) Liang, L.; Rieke, P. C.; Liu, J.; Fryxell, G. E.; Young, J. S.; Engelhard, M. H.; Alford, K. L. Langmuir 2000, 16, 8016–8023. (6) Prins, M. W. J.; Welters, W. J. J.; Weekamp, J. W. Science 2001, 291, 277–280. (7) Katz, E.; Sheeney-Haj-Ichia, L.; Basnar, B.; Felner, I.; Willner, I. Langmuir 2004, 20, 9714–9719. (8) Wu, J. C.; Yi, T.; Shu, T. M.; Yu, M. X.; Zhou, Z. G.; Xu, M.; Zhou, Y. F.; Zhang, H. J.; Han, J. T.; Li, F. Y.; Huang, C. H. Angew. Chem., Int. Ed. 2008, 47, 1063–1067. (9) Isaksson, J.; Tengstedt, C.; Fahlman, M.; Robinson, N.; Berggren, M. Adv. Mater. 2004, 16, 316–320. (10) Sirringhaus, H. Adv. Mater. 2005, 17, 2411–2425. (11) Mok, S. M.; Yan, F.; Chan, H. L. W. Appl. Phys. Lett. 2008, 93, 023310. (12) Yan, F.; Mok, S. M.; Yu, J. J.; Chan, H. L. W.; Yang, M. Biosens. Bioelectron. 2009, 24, 1241–1245. (13) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197–203.

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P3AT films can be tuned from hydrophobic to hydrophilic by electrochemical doping (oxidation),9,14,15 and thus it can be used as a wettability switch in microfluidic systems.16 A neutral P3AT film is hydrophobic, which is believed to arise from the alkyl side chains, while an oxidized one can be hydrophilic due to dipoles induced by anions near the backbones of polymer chains. Therefore, the wettability can be tuned by controlling the oxidation level on the polymer film, and the tunable region depends greatly on the type of dopant. However, the tunable amplitude that has been reported is rather small (∼10°), which is achieved with a sandwich structure of P3AT/electrolyte/metal electrode.15 Although the semiconductor properties of P3AT have been well studied, its electrochemical property is still not clear and needs to be carefully investigated. The importance of studying the electrochemical doping process in P3AT is multifold. A controllable doping process also can be used to modulate the electric and optical properties of the semiconducting polymer films.17 Mixed ionic conjugated polymers have many applications in lightemitting electrochemical cells,18,19 polymer electrochromic devices,20,21 artificial muscles,22,23 organic electrochemical transistors (OECTs),24 biosensors,25,26 etc. The aim of this work is to systematically study the electrochemical doping (oxidation) process in the P3AT polymer films (including poly(3-hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT), and poly(3-decylthiophene) (P3DT)) with different anions and to achieve a broader

(14) Robinson, L.; Isaksson, J.; Robinson, N. D.; Berggren, M. Surf. Sci. 2006, 600, L148–L152. (15) Robinson, L.; Hentzell, A.; Robinson, N. D.; Isaksson, J.; Berggren, M. Lab Chip 2006, 6, 1277–1278. (16) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621–633. (17) Leger, J. M. Adv. Mater. 2008, 20, 837–841. (18) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086–1088. (19) Edman, L. Electrochim. Acta 2005, 50, 3878–3885. (20) Sonmez, G. Chem. Commun. 2005, 42, 5251–5259. (21) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27, 2–18. (22) Baughman, R. H. Synth. Met. 1996, 78, 339–353. (23) Ashley, S. Sci. Am. 2003, 289, 53–59. (24) Nilsson, D.; Chen, M.; Kugler, T.; Remonen, T.; Armgarth, M.; Berggren, M. Adv. Mater. 2002, 14, 51–54. (25) Mabeck, J. T.; Malliaras, G. G. Anal. Bioanal. Chem. 2006, 384, 343–353. (26) Berggren, M.; Richter-Dahlfors, A. Adv. Mater. 2007, 19, 3201–3213.

Published on Web 05/04/2009

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range of tunable contact angle of the P3AT polymer films by various approaches.

2. Experimental Section 2.1. Reagents and Solvents. Three poly(3-alkylthiophene)s (P3ATs) (Rieke Metals Inc.) with different alkyl chain length including poly(3-hexylthiophene) (P3HT, regioregularity: 90%-93%), poly(3-octylthiophene) (P3OT, regioregularity: 90%-93%), and poly(3-decylthiophene) (P3DT, regioregularity: 90%-93%), polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning), and chloroform (International Laboratory) were used as received without any purification. To study the effect of different anions, solutions of sodium chloride (NaCl, Aldrich), sodium nitrate (NaNO3, Aldrich), sodium sulfate (Na2SO4, Aldrich), sodium carbonate (Na2CO3, Aldrich), and sodium phosphate (Na3PO4, Aldrich) dissolved in DI water were prepared with the concentration of 0.1 M. All of the salts were used as received without further purification. 2.2. Polymer Films and Cells. The P3AT polymers were dissolved in chloroform with a concentration of 10 mg/mL, and the solutions were spin-coated on Pt/Ti/SiO2/Si, ITO/SiO2, or Au/Cr/PDMS substrates at 3500 rpm for 30 s. Then the P3AT films were annealed at 80 °C for 1 h in a glovebox filled with high-purity N2. The thicknesses of the P3HT, P3OT, and P3DT films characterized by a scanning probe microscopy (SPM, Digital Instruments NanoScope IV) were 30, 30, and 35 nm, respectively. The surface roughness of the films spin-coated on Pt/Ti/SiO2/Si characterized by SPM were less than 5 nm. To amplify the tunable contact angle of the polymer films, P3HT and TiO2 nanoparticles (crystal phase: anatase, particle size: 20 nm, Aldrich) were dissolved in chloroform and mixed by ultrasonic agitation before being spin-coated on Pt/Ti/SiO2/Si substrates. Solutions with different weight ratio between P3HT and TiO2 were prepared. The concentration of P3HT in the solution was kept at an identical value of 10 mg/mL. The surface roughness was greatly enhanced as observed under SPM. The P3AT films were electrochemically oxidized in five different electrolytes;NaCl, NaNO3, Na2SO4, Na2CO3, and Na3PO4;dissolved in deionized (DI) water by applying bias voltages on the electrodes beneath the polymer films. The counter electrode was a platinum (Pt) wire. The schematic diagram of the electrochemical cell is shown in Figure 1. P3AT films were oxidized for 3 min at each bias voltage. 2.3. Fabrication of Au/Cr/PDMS Substrate. PDMS substrates patterned with micrometer-sized posts were fabricated by soft lithography. First, a mold was prepared by spin-coating a negative photoresist SU-8 50 with a thickness of 30 μm on a silicon wafer prior to patterning with UV photolithography and antisticking treatment with trimethylchlorosilane in vapor phase. Then, PDMS mixed with cross-linker (ratio 5:1) was poured onto the photoresistor patterns. After 1 h annealing at 100 °C, the PDMS layer was peeled off from the mold, and micrometer-sized posts were formed on the surface of PDMS layer. The thickness of the PDMS layer was about 2 mm. Thin metal films;typically 50 nm thick layer of gold (Au) with a 5 nm adhesion interlayer of chromium (Cr);were deposited on PDMS substrates by a multihead magnetron sputtering system. The Au/Cr/PDMS substrate was used as an electrochemical electrode to oxide a P3HT layer coated on top of it. 2.4. Contact Angle Measurement. Water contact angles were measured using a standard goniometer (Rame-Hart Inc.) equipped with a microscope and illumination system to visualize both the drop deposited on a sample and the measurement marker. The samples were placed on a flat, horizontal support. The humidity in the measurement chamber was about 70%. Experiments were carried out at room temperature (24 ( 2 °C) and pressure. Contact angle from both ends (left and right) were considered and usually found equal within standard error. 7466 DOI: 10.1021/la900387m

Figure 1. Schematic diagram of the electrochemical cell for oxidizing a P3AT film coated on a Pt electrode. The films coated on flat substrates showed little hysteresis of the contact angles (3 V) may destroy the polymer film due to the large tension induced by highly doped anions; therefore, the applied bias voltage in the experiment is normally lower than 3 V. On the other hand, it is not necessary to apply too high voltage since the most sensitive tuning voltage is below 1.5 V and the contact angle tends to saturate above 2.5 V. Figure 2a shows that SO24 can induce the biggest effect among the five anions. The minimum water contact angle on the oxidized P3HT film (doped with SO24 ) is 76.7 ( 1.5°, and thus the tunable amplitude of the contact angle is about 30° by the electrochemical doping. Small polarons (holes) with positive charge are induced in the backbones of polymer chains due to the doping of anions; therefore, dipoles formed by holes and anions increase the surface Langmuir 2009, 25(13), 7465–7470

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Article Scheme 1.

a

a

A; = anion, R = alkyl side chain.

Figure 3. Time-dependent contact angle of oxidized P3HT films (doped with different anions) after being dipped into DI water (a) with no bias voltage applied on the sample and (b) with a bias voltage of -2.0 V applied on the P3HT/Pt electrode.

Figure 2. (a) Contact angles of P3HT films oxidized at different bias voltages in five different electrolytes;NaCl, NaNO3, Na2SO4, Na2CO3, and Na3PO4;dissolved in DI water. The oxidation time is 3 min for each voltage. The concentration of each electrolyte is 0.1 M. (b) Change of the adhesion tension γad of P3HT films oxidized in the five electrolytes. (c) Change of the contact angle of P3HT as a function of oxidation time.

energy of the P3HT film. We consider that the change of the surface energy is related to both the charge and the density of doped anions. Since SO24 has more charge than Cl and NO3 23and more active than CO3 and PO4 , the obtained results are reasonable. The relationship between the contact angle and the surface energy of the oxidized P3HT films is given by the Young’s equation27 γ -γ cos θ ¼ SV SL ð1Þ γLV where θ is the contact angle and γSV, γSL, and γLV represent the interfacial tensions of solid-vapor, solid-liquid, and liquidvapor interfaces, respectively. γSV - γSL is always called adhesion tension, and γLV is ∼ 72 mN/m for the water-gas interface at (27) Quere, D. Rep. Prog. Phys. 2005, 68, 2495–2532.

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room temperature. Figure 2b shows the adhesion tension γad = γSV - γSL of P3HT calculated with eq1. The electrochemical doping process in a P3HT film can be completed in a very short time at any bias voltage. Figure 2c shows the contact angle of P3HT film as a function of oxidation time at a bias voltage of 2.5 V in the five different electrolytes. The contact angle decreases very rapidly and saturates within several seconds. The advantage of using a conjugated polymer as a wettability switch is its “memorial” behavior, i.e., the ability of keeping an oxidation state for a certain period of time. Figure 3a shows the change of the contact angle of the P3HT films doped with different anions at 2.5 V when they are dipped into DI water for different periods of time. It can be found that the contact angles increase with increasing time and arrive at stable values in 3 h. Therefore, not all of the anions can induce a stable hydrophobic surface when the film is kept in DI water for long enough time. The recovery of the contact angle in DI water is due to the diffusion of doped anions from the P3HT film to DI water. But the P3HT film cannot be recovered to its neutral state in DI water without applying a negative bias voltage on the film. Figure 3b shows the recovery of the contact angle and the adhesion tension when a negative bias voltage (-2 V) is applied on an oxidized P3HT film (doped with SO24 ) in DI water. The contact angle can be recovered back to 102 ( 2.0° in 3 h, which is a relatively slow process. The recovery of the adhesion tension can be fitted with an exponential function: γad ðtÞ ¼ γ0 þ Δγe -t=τ

ð2Þ

where γ0 is the adhesion tension of a neutral P3HT film, Δγ is the change of the adhesion tension due to the oxidation, and τ is the relaxation time of the recovery process. The best-fitting curve shows that the relaxation time τ = 38.7 min, which may reflect the average diffusion time of SO24 from P3HT to DI water. Although the recovery process is relatively slow, the film can be reversibly switched for several times. Since the film can be easily fabricated DOI: 10.1021/la900387m

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by solution process and tuned by the electrochemical method, it is suitable for the applications as low-cost disposable wettability switches. It has been reported that the length of alkyl chain can influence the hydrophobicity of the P3AT film.14 P3OT and P3DT that have longer alkyl chains than P3HT have been studied in the same process. Figure 4 shows the contact angles of the three polymer films oxidized with SO24 at different bias voltages. Although the contact angles of P3DT and P3OT at neutral state are bigger than that of P3HT, their tunable amplitudes are smaller than that of P3HT. Therefore, P3HT is optimum for the application in wettability switches between hydrophobic and hydrophilic states. The color of a P3HT film changes from pink to transparent when it is oxidized with increasing bias voltage. Figure 5 shows the light absorbance of a P3HT film doped with SiO24 at different bias voltages. For the neutral P3HT film, the peak at ∼2.1 eV (A1) is usually attributed to an interchain absorption while the other two peaks (B1, B2) at higher energy levels are normally described in term of the intrachain π-π* transition and the transition coupled to a phonon.28 The three peaks become lower in an oxidized P3HT film since some polarons are induced in the polymer chains by anion doping. The polarons have higher energy level than the highest occupied molecular orbital (HOMO), and therefore several absorption peaks appear at the lower energy region. The transition of electrons from the polaron level will induce an absorption peak (C1) at about 1.5 eV,29 as shown in Figure 5. It has been found that the peak height of C1 increases with an increase of doping level, reaches the maximum intensity when the polymer is saturated with independent polarons, and decreases at higher doping level for the formation of bipolarons, as predicted by theoretical models.30 Figure 5 shows that the peak height of C1 has the maximum value at a bias voltage of 2.5 V, which corresponds to a doping level of about 0.2 per monomer unit (one charge accommodated by at least five or six monomers),29 and bipolarons are induced in the polymer film at higher bias voltage. Peak C1 cannot be observed when the bias voltage is lower than 1.0 V, indicating a very low bulk doping level under this condition, which is consistent with the cyclic voltammetry (CV) measurement of P3HT films. Figure 6 shows the CV diagrams for P3HT/Pt/Ti/SiO2/Si electrodes measured in an electrolyte with 0.1 M Na2SO4 for continuous two cycles. The oxidation current only can be observed at bias voltages higher than 1 V in the first cycle. It is obvious that the contact angle is mainly influenced by the doped anions at the surface of the polymer film. Therefore, the contact angle shows a different relationship with the bias voltage compared to that of the bulk doping level. P3HT film oxidized at 2.5 V can be deoxidized in DI water by applying a negative bias voltage (-2 V) as confirmed by the measurement of contact angle. However, the light absorption spectrum of the film shows that it cannot be completely recovered. C1 disappears after the deoxidation process, indicating that polarons and thus anions have been removed from the polymer film. However, A1, B1, and B2 peaks cannot be recovered to their original heights, which may be due to a decrease in the conjugation length or disorder of polymer chains induced by the electrochemical doping.28 Figure 5 shows that the P3HT film cannot be deoxidized when it is doped with SO24 at a bias voltage of 3.0 V, (28) Brown, P. J.; Thomas, D. S.; Kohler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Phy. Rev. B 2003, 67, 064203. (29) Apperloo, J. J.; Janssen, R. A. J.; Nielsen, M. M.; Bechgaard, K. Adv. Mater. 2000, 12, 1594–1597. (30) Fesser, K.; Bishop, A. R.; Campbell, D. K. Phys. Rev. B 1983, 27, 4804–4825.

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Figure 4. Contact angles of P3HT, P3OT, and P3DT films oxidized at different bias voltages in 0.1 M Na2SO4 solution. The oxidation time is 3 min for each voltage.

Figure 5. Evolution of the UV-vis/near-IR spectra of P3HT films being oxidized with SO24 at different bias voltages. Film A and B were deoxidized at -2 V in DI water for 3 min after being oxidized at 2.5 and 3.0 V in Na2SO4 solution for 3 min, respectively.

Figure 6. Cyclic voltammogram for the P3HT/Pt/Ti/SiO2/Si electrode in 0.1 M Na2SO4/H2O solution at room temperature at a scan rate of 0.5 V/s for continuous two cycles. Cycle 1 and 2 correspond to the CV curve of neutral and oxidized P3HT film.

which is normally called the overoxidization process. This effect can be correlated with the appearance of bipolarons in the polymer chains oxidized at the bias voltage, which may induce stronger Coulombic interaction between polymer chains and the anions. The tunable region of the contact angle of the P3HT film can be amplified by increasing the surface roughness of the film. The contact angle on a rough surface is given by Wenzel’s relation:27 cos θ ¼ γ cos θ

ð3Þ

where θ* and θ are contact angles on a rough and a flat surface, respectively; γ (g1) is the ratio of the actual surface area (taking into account the asperities at the solid surface) over its apparent one. Hybrid films of P3HT and TiO2 nanoparticles (weight ratio of P3HT:TiO2 is 1:2) have been coated on Pt/Ti/SiO2/Si electrodes, and the surface roughness has been increased to Langmuir 2009, 25(13), 7465–7470

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Figure 7. (a) Scanning electron microscopy (SEM) image of a PDMS substrate patterned with micrometer-sized posts. (b) Schematic drawing of the cross-sectional view of the P3HT/Au/Cr/PDMS plate. (c) Photos of water droplets on pristine (I) and oxidized (II, III) P3HT/ Au/Cr/PDMS plates. II and III represent advancing and receding drops, respectively.

about ∼100 nm. The contact angle of the neutral hybrid film is ∼117.8° and can be tuned to 76.4° by electrochemical doping with SO24 . But it is difficult to further increase the tunable region of the contact angle of the film by mixing more nanoparticles. On the other hand, the electrochemical doping cannot be uniformly controlled in the hybrid film due to the nonuniform conductivity of the film. Another feasible way is to coat a P3HT film on a surface patterned with micrometer-sized posts. It has been reported that the contact angle of a hydrophobic surface can be easily increased to be superhydrophobic by controlling the size and density of the micrometer-sized posts.27 In this case, a drop sits on the patterned surface consisting of solid and air. The equilibrium contact angle is then given by the Cassie-Baxter equation:27 cos θ ¼ -ε þ φs ðcos θ þ εÞ

ð4Þ

where θ* and θ are contact angles on a patterned and a flat surface, respectively, φs is the fraction of area of the solid tops, and ε = 1. Figure 7 shows the surface of a patterned PDMS substrate. The square micrometer posts have a size of 58.5 μm  58.5 μm on the top and a height of 30.0 μm. The side faces of the posts tilt for about 20° from the normal direction of the substrate. The horizontal distance between the tops of the neighboring posts is 81.5 μm, and thus φs ≈ 0.175. According to eq 4, Langmuir 2009, 25(13), 7465–7470

assuming θ ≈ 105.9° for neutral P3HT film, the contact angle on such surface can be increased to θ* ≈ 150.8°, which can be regarded as a superhydrophobic surface.16,31 Figure 7 shows the contact angle θn* ≈ 147.4 ( 2° of a neutral P3HT film coated on a patterned Au/Cr/PDMS substrate, which is slightly smaller than the theoretical value. Then the polymer film was oxidized in Na2SO4 solution at a bias voltage of 2.5 V for 3 min. The advancing and receding contact angles of the polymer film are θA = 86.5° and θR = 29.5°, respectively, as shown in Figure 7c. Therefore, a big hysteresis of the contact angle on the film can be observed, which may be attributed to an energy barrier in front of the drop.32 Assuming the static contact angle θr is given by cos θr ¼ ðcos θA þ cos θR Þ=2

ð5Þ

we can get θr = 62.2°, which is close to the value given by eq 3 assuming γ = 1.4 and θ = 76.7°. It is obvious that the tunable region of the contact angle can be further improved by carefully designing the size and density of the posts. According to eq 4, smaller size of posts will lead to lower (31) Wang, S. T.; Jiang, L. Adv. Mater. 2007, 19, 3423–3424. (32) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 911–918.

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fraction of area of the solid tops and thus bigger contact angle of neutral P3HT film. On the other hand, we can increase the height of the posts and thus increase the ratio of the actual surface area over its apparent one (γ, given in eq 3), which will decrease the contact angle of the oxidized P3HT film. In real application, a systematic work is still needed to find the optimum design of the patterned surface.

4. Conclusions In conclusion, the contact angle of P3AT film can be tuned from hydrophobic to hydrophilic by a low-voltage electrochemical doping process. The tunable region of the contact angle of the polymer films is dependent on the type of dopant, and SO24 can induce the biggest effect in the several used anions. The contact angle can be recovered from hydrophilic to hydrophobic when the oxidation voltage is lower than 2.5 V, which corresponds to the voltage that can induce the maximum density of independent

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polarons in the polymer chain (∼0.2 per monomer). The P3HT film is overoxidized at the bias voltage higher than 3.0 V and cannot be recovered to its neutral state, which may be due to the bipolarons induced in the polymer chain. The tunable amplitude of the contact angle can be substantially enlarged by coating a P3HT film on a substrate patterned with micrometer-sized posts, which shows a static contact angle of 147.4° at neutral state and 62.2° at oxidized state. It is envisaged that the polymer films have potential applications as low-cost disposable wettability switches in microfluidics and contact-printing technologies. Acknowledgment. The authors acknowledge Mr. S. M. Mok and Dr. F. T. Chang from the same department for their help on experiments. This work is financially supported by the Research Grants Council (RGC) of Hong Kong, China (B-Q10T and E-RD38), and the Hong Kong Polytechnic University (1-ZV46 and 1-BB95).

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