www.elsevier.com/locate/susc

Surface Science 600 (2006) L148–L152

Surface Science Letters

Electrochemical control of surface wettabilityof poly(3-alkylthiophenes)

Linda Robinson, Joakim Isaksson *, Nathaniel D. Robinson, Magnus Berggren

Organic Electronics, Department of Science and Technology (ITN), Linkoping University, Campus Norrkoping, SE-601 74 Norrkoping, Sweden

Received 27 January 2006; accepted for publication 30 March 2006Available online 27 April 2006

Abstract

The effect of n-alkyl side-chain length on water contact angle with films in neutral and electrochemically doped states are studied.Increasing the side-chain from butyl to hexyl to octyl increases the contact angle of water on conjugated polymer films in both electro-chemical states, but decreases the difference in angle between the states in the same film. Devices based on these films have potentialapplication in, for example, guiding water and other liquids through microfluidic channels in lab-on-a-chip and micro-electro-mechanical(MEM) applications.� 2006 Elsevier B.V. All rights reserved.

Keywords: Water contact angle; Electrochemical phenomena; Surface energy; Wetting; Conducting polymer; Solid–liquid interfaces; Organic semi-conductors

1. Introduction

The affinity a liquid has for a solid surface, often calledthe wettability, is used by both nature and man to controladhesion and motion of fluids in countless applications.Classic examples in nature include the lotus leaf, the sur-face of which repels water so that the leaf is almost alwaysdry, and the proverbial ‘‘duck’s back’’, the feathers onwhich have the same effect. In technology, precise controlof wettability represents the key function in an array of dif-ferent applications. Along the cylindrical surface of offset-printing plates hydrophobic and hydrophilic patterns guidethe oil ink to the paper surface to form graphic art. Inmicrofluidic channels, used in various lab-on-a-chip analy-sis approaches, capillary forces draw the aqueous analytethrough channel systems. Hydrophobic gates can be usedto guide the analyte to mix and react with a cascade ofother reagents.

0039-6028/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2006.03.039

* Corresponding author. Tel.: +46 11 363481; fax: +46 11 363270.E-mail address: joais@itn.liu.se (J. Isaksson).

Surfaces that repel water are called hydrophobic. At theother end of the spectrum are hydrophilic surfaces such asglass. Given the choice between sitting on a hydrophobicand a hydrophilic surface, water will always choose thehydrophilic surface. A common way to measure hydropho-bicity is the contact angle formed when a drop of water isplaced on a surface. An example is shown in Fig. 1. Theangle h between the solid–water and water–air interfaceindicates how much the water ‘‘likes’’ the solid surface(compared to the air). A small contact angle (much lessthan 90�) indicates a hydrophilic surface, while a contactangle approaching or larger than 90� indicates a hydropho-bic surface.

Creative use of hydrophobic and hydrophilic surfacescan be used for sophisticated applications. The ability tocontrol or switch the hydrophobicity of a surface offerseven more potential. Electrowetting [1] is a process inwhich the affinity of a water drop for a surface is changedby applying an electrostatic potential between the drop andthe surface. With this technique, water droplets can be di-rected on a surface of electrodes. Applications for this tech-nology include dispensing and handling small volumes of

Fig. 1. Water drop on a P3HT/Si surface with surface tensions indicatedas arrows originating at the 3-phase interface line. The reflection of thedrop can be seen under the base line.

L. Robinson et al. / Surface Science 600 (2006) L148–L152 L149

SURFA

CESCIEN

CE

LETTERS

liquids (where interfacial tension is a dominant force) andmicrofluidics, where water is driven through micron-sizedchannels such as in biological lab-on-a-chip analyzers. Afantastic example of the variety of uses for electrowettingis the reflective ‘‘electronic paper’’ display developed byHayes et al. [2].

Local electronic control of the surface energy of specificlocations along a surface, for instance in a matrix ad-dressed system, could be a breakthrough in a wide varietyof applications. This technology, for instance, applied inthe printing industry could result in truly digital offsetplates, allowing each revolution in the press to transferexclusive and updated images and information. In micro-fluidics, electronic wettability switches can act as gates orlocks for liquid packages within fluidic channels, offeringus the opportunity to mix drops of various fluids in alab-on-a-chip analysis or chemical manufacturing system.

Here, we report the dynamic and electronic control ofthe water contact angle along the surface of alkyl-substi-tuted polythiophenes in electrochemical solid-state devices,see Fig. 2. As the oxidation state in the polymer is updated,dipoles in the polymer backbone are formed, which we be-lieve affect the interfacial energy between the polymer and awater drop at the polymer surface. By choosing the lengthof the side-chains on the polythiophene derivative, we wereable to tune the initial water contact angle, causing the

Fig. 2. (a) Chemical structures of P3OT, P3HT and P3BT. (b) Cross-section offilm with a scalpel and applying a potential as shown.

swing to cross 90�. For practical applications, such as inoffset printing, it is desired to achieve active control ofthe wetting (contact angle <90�) and non-wetting (contactangle >90�) regions, in order to efficiently separate thewater phase from the oil ink [3]. Similarly, surface texturingcan be used to amplify the switching effect dramatically ifthe contact angle crosses the 90� threshold [4,5].

The energy per unit area c of an interface between twophases, called the surface tension for liquid–vapor inter-faces, is a function of the chemistry of the two phases. Justas chemists say ‘‘like dissolves like’’, meaning, for example,that polar solvents usually dissolve polar materials, solidsand liquids (or solids and gases or liquids and gases) whichare similar have a low interfacial energy. Materials whichdiffer tremendously from a chemistry perspective (oil andwater, for example) do not mix, and have a rather highinterfacial energy. This, and the difference in density be-tween the two phases, is what causes traditional oil andvinegar salad dressing to separate into two phases. In real-ity it is more than just the polarity or chemical dipoles ofthe two materials that matters (hydrogen bonding, and dis-persive effects also contribute), but the polarity is the majorpart of this interaction. The idea in this paper is to studyhow changing the polarity of the backbone of a conjugatedpolymer material by doping (or undoping) it affects thewater contact angle of the surface. We go further to studythe ‘‘shielding’’ effect of non-polar side-chains on the poly-mer by measuring on polymers with differing side-chainlengths. On such an alkyl-substituted polymer surface weexpect the surface energy to include contributions fromboth the conjugated backbone of the polymer, to the extentthat it is visible at the outermost surface, and the alkylchain. By measuring the water contact angle on polymerswith varying alkyl chain length in both the doped and un-doped states, we gain insight into the magnitude of theseindividual contributions. This process also allows us totune the alkyl chain length so that the water contact angleis greater than 90� when undoped and less than 90� whendoped.

Each phase boundary has its own interfacial energy(or surface tensions): solid–liquid csl, solid–vapor csv andliquid–vapor clv. These balance at the 3-phase contact line,

a wettability switch. The area to be oxidized is defined by cutting the P3AT

L150 L. Robinson et al. / Surface Science 600 (2006) L148–L152SU

RFACE

SCIENCE

LETTERS

determining the contact angle h per the Young-Dupreequation [6]:

csv ¼ csl þ clv cos h ð1Þas shown by the arrows superimposed on the drop photo-graph in Fig. 1.

Conjugated polymers (CP) are quasi-one-dimensionalmacromolecules, with a delocalized p-orbital system thatprovides electronic conduction when doped by adding orremoving an electron from the system, electrochemically,via charge injection from electrodes or via chemical doping.Small ions can also migrate within the fairly soft and flex-ible bulk of most CP films. This combination means thatfilms of CPs are electrochemically active, in the sense thatthey can be oxidized and reduced reversibly. Polymers froma particular class of conjugated systems, the polythioph-enes, are soluble (increasing with increasing side-chainlength), processible, and environmentally stable [7,8]. Poly-thiophenes have been used in a wide variety of applica-tions, such as field-effect transistors [9,10], flexible solarcells [11] and light emitting diodes [12].

Previously, we have shown that films of polyaniline(PANI) doped with dodecylbenzenesulfonate (DBSA) andpoly(3-hexylthiophene) (P3HT) can be electrochemicallydoped to control the contact angle of a water drop placedon the film [13]. Polypyrrole is also capable of such switching[14]. In this article, we explore the alkyl-substituted polythio-phene chemical system in more detail, particularly the effectof the side-chain length on the absolute contact angle and thedifference in contact angle between the neutral and dopedstates of the polymer film. We present results from threedifferent poly(3-alkylthiophene)s (P3ATs): poly(3-butyl-thiophene) (P3BT), poly(3-octylthiophene) (P3OT) andP3HT as reported previously. Each of the three structuresis shown in Fig. 2a.

2. Experimental details

Devices were manufactured as follows: Silver (Ag 5000,DuPont) was painted on the plastic substrate and annealedfor at least 10 min at 80 �C. Next, a polymer electrolytelayer, approximately 370 lm thick, was cast on top of thesilver, followed by a 7 min anneal at 60 �C. The conductingpolymer (3 mg P3AT/1 ml CHCl3) was spin-coated at1200 rpm for 20 sec on the electrolyte or on a Si waferand heated at 60 �C for 1 min to evaporate the chloroformsolvent, resulting in a 170 A thick film on Si. All threeP3ATs used in this work (from Sigma Aldrich) are regioreg-ular: P3OT and P3HT greater than 98.5% and P3BT 97%head-to-tail regiospecific conformation. P3HT and P3OTwere easily dissolved in chloroform, P3BT needed tobe heated to about 75 �C in order to dissolve most of thepolymer. The electrolyte was made of 25.6 wt.% poly-(sodium 4-styrenesulfonate) (MW 70,000, Aldrich), 8 wt.%D-sorbitol (97% Lancaster), 8 wt.% glycerol (87% Merck),20 wt.% Magnesium sulphate, MgSO4 (Merck) and38.4 wt.% deionized water.

Static water–air contact angles were measured with agoniometer (CAM 200, KSV) both on neutral and oxidizedareas of the polymer.

The area to be oxidized (5 V potential) was defined byscratching an electronically isolated square in the polymerfilm with a scalpel. A typical oxidized area of 1.5 cm ·1.5 cm takes about 3–5 min to fully switch, as observedby the electrochromic color change. The static water con-tact angles were measured after the electrode surfaces werefully switched, i.e. the electrode surfaces were not biasedduring the measurements, and within the first few secondsafter deposition of each water droplet onto the surface. Thewettabilility switches are not long-term stable (days) inambient atmosphere and were used less than 1 h after spincoating of the P3AT film.

3. Results and discussion

A summary of the contact angles measured on films ofthe three P3ATs studied is found in Table 1 and Fig. 3.The difference between the measurements and our interpre-tation of the results follow.

It has been reported that the water contact angle of non-substituted plasma-polymerised polythiophene is between80� and 90� [15]. By adding alkyl chains to the polymer,the water contact angle of the neutral polymer surface in-creases to between 102� and 110�, depending on the chainlength. The contact angle of water droplets on pristine(neutral) P3ATs spin-coated on Si show that increasingside-chain length results in increased contact angle. Thisis an expected result, as the relatively hydrophobic alkylchains effectively shield the polar water from the p-elec-trons in the neutral polythiophene backbone. However,the effect is rather small – the difference in contact anglebetween the polymers with longest (P3OT) and shortest(P3BT) side-chains is only about 8�. A similar result hasbeen found for (non-conjugated) polymethacrylates [16].

In order to electrochemically dope the polymer film, itmust be in contact with an electrolyte. This was achievedby casting a layer of an electrolyte on plastic foil, and thenspin-coating the P3AT film on the annealed electrolyte.The resulting structure is shown in Fig. 2b. The contactangle of water droplets on pristine (undoped) films on theelectrolyte is lower than that of the same film on Si. Thereason for this is unclear, but differences in the structureor surface morphology of the polymer film or pinholes thatallow some wetting of the electrolyte are two possibilities.

When the polythiophene film is doped through electro-chemical oxidation the water contact angle decreasesdramatically. Anodically doping the polymer introducespositive charges (polarons and bipolarons) in the thiophenegroups which are usually counterbalanced by anions thatmove into the film from the adjacent electrolyte [17,18].Electrochemically doping the polymer effectively intro-duces a large dipole in the backbone, thus one wouldexpect doped polythiophenes to be more hydrophilic thanthe same materials in the neutral state. The difference

Fig. 4. Water droplet on pristine (a) and oxidized (b) P3BT surfaces.

Fig. 3. (a) Water–air static contact angle of P3OT, P3HT and P3BT in the pristine (neutral) state on Si and electrolyte and after electrochemical oxidationon electrolyte. (b) The difference in contact angle between pristine and oxidized P3AT on electrolyte.

Table 1Summary of the water–air static contact angles on P3OT, P3HT and P3BT with calculated standard deviations r

P3AT Pristine on Si ± r Pristine on electrolyte ± r Oxidized on electrolyte ± r Difference betweenpristine and oxidized ± r

P3OT 109.8 ± 2.4 105.2 ± 3.0 96.3 ± 2.7 8.9 ± 2.3P3HT 106.5 ± 0.3 101.8 ± 1.9 89.1 ± 1.1 12.7 ± 1.3P3BT 102.1 ± 0.7 88.1 ± 2.4 71.1 ± 4.0 17.0 ± 4.3

L. Robinson et al. / Surface Science 600 (2006) L148–L152 L151

SURFA

CESCIEN

CE

LETTERS

between the contact angle on the doped and undopedP3ATs studied depends on the alkyl side-chain length(Fig. 3b). Longer octyl side-chains appear to shield thewater from the polar backbone better than shorter hexyland butyl side-chains. Photographs of water droplets onpristine and oxidized P3BT are shown in Fig. 4.

Thus, beyond the switching phenomenon, conjugatedpolymers also allow flexibility in situations where controlof contact angle is critical. Simply changing the side-groupon the polymer chain allows the starting contact angle tobe tuned for nearly any application. The range of contactangles available can likely be increased dramatically byreplacing the alkyl chains with, for example, oligoethylene-oxide chains (to decrease the water contact angle) or fluo-rinated systems (to increase the contact angle).

As mentioned in the introduction, the change in watercontact angles near 90� can be effectively amplified by tex-turing the surface [4,5]. Thus, P3HT is a very interestingcandidate for making devices employing surface texture toachieve superhydrophobic to superhydrophilic switching.

4. Conclusions

The previously proposed mechanism [13] for the changein water contact angle upon doping P3AT films appears tobe correct, namely that the polarity induced in the thio-phene ring in the backbone of the polymer increases theaffinity of water for the polymer. The side-chain onP3ATs affects both the contact angle of the neutral filmand the magnitude of the change in contact angle on the

L152 L. Robinson et al. / Surface Science 600 (2006) L148–L152SU

RFACE

SCIENCE

LETTERS

polymer film when the film is doped by shielding the waterfrom the polar doped backbone. Since seemingly unlimitedfunctionality can be built into the side-chains of conjugatedpolymers during synthesis, almost any initial contact anglecan be achieved. We expect these materials to be extremelyimportant in all aspects of microfluidics in the future.

Acknowledgements

We would like to thank VR and SSF for funding thisresearch.

References

[1] F. Mugele, J.-C. Baret, J. Phys.: Condens. Matter 17 (2005) 705.[2] R.A. Hayes, B.J. Feenstra, Nature 425 (2003) 383.[3] H. Kipphan, Handbook of Print Media Technologies and Production

Methods, Springer, New York, 2001.[4] A. Lafuma, D. Quere, Nature Mater. 2 (2003) 457.[5] T.L. Sun, G.J. Wang, L. Feng, B.Q. Liu, Y.M. Ma, L. Jiang, D.B.

Zhu, Angew. Chem. Int. Ed. 43 (2004) 357.

[6] F. MacRitchie, Chemistry at Interfaces, Academic Press, San Diego,1990.

[7] W.J. Feast, J. Tsibouklis, K.L. Pouwer, L. Groenendaal, E.W.Meijer, Polymer 37 (1996) 5017.

[8] T.A. Chen, X.M. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995)233.

[9] Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 (1996)4108.

[10] S. Scheinert, T. Doll, A. Scherer, G. Paasch, I. Horselmann, Appl.Phys. Lett. 84 (2004) 4427.

[11] M. Al-Ibrahim, H.K. Roth, U. Zhokhavets, G. Gobsch, S. Sensfuss,Sol. Energy Mater. 85 (2005) 13.

[12] L.S. Roman, O. Inganas, Synth. Met. 125 (2001) 419.[13] J. Isaksson, C. Tengstedt, M. Fahlman, N. Robinson, M. Berggren,

Adv. Mater. 16 (2004) 316.[14] J. Causley, S. Stitzel, S. Brady, D. Diamond, G. Wallace, Synth. Met.

151 (2005) 60.[15] Y.-J. Yu, J.-G. Kim, J.-H. Boo, J. Mater. Sci. Lett. 21 (2002) 951.[16] M. Wulf, K. Grundke, D.Y. Kwok, A.W. Neumann, J. Appl. Polym.

Sci. 77 (2000) 2493.[17] T. Johansson, N.K. Persson, O. Inganas, J. Electrochem. Soc. 151

(2004) E119.[18] G. Zotti, Synth. Met. 97 (1998) 267.