Water Induced Hydrophobic Surface

Umit Makal and Kenneth J. Wynne*

Department of Chemical and Life Science Engineering, Virginia Commonwealth University(VCU), Richmond, Virginia 23284-3028

Received February 8, 2005. In Final Form: March 8, 2005

A polyurethane coating is described that has hydrophilic wetting behavior when dry and hydrophobicwhen wet. A difference of ∼25° in advancing contact angles for dry (83°) and wet (108°) states is foundby sessile drop and dynamic methods. The term “contraphilic” is suggested for this reversible changeopposite customary amphiphilic behavior. Contraphilic behavior results from a soft block containingsemifluorinated and 5,5-dimethyhydantoin segmers. Amide inter/intramolecular hydrogen bonding isproposed for the hydrophilic (dry) state, while surface-confined, amide-water hydrogen bonding “releases”semifluorinated groups, giving the hydrophobic state. Water-induced hydrophobic surfaces may lead toapplications for easily switched wetting, such as in microfluidics.

Improved understanding of interfacial surface proper-ties is both essential and fundamental to progress in thedevelopment of advanced concepts such as “intelligent”or “responsive” coatings for switching, sensing, protection,or improved biocompatibility. Because the design andarchitecture of the outermost layers control surfacebehavior, subtle surface modifications such as alteringchemical structure, composition, topology, or architecture,can be used to manipulate surface responses to externalcues.1-4 By tuning the interfacial properties, improvedunderstanding of interfacial phenomena is developed.5-10

For applications, responsive surfaces must undergofacile rearrangements to stimuli. Recently, much attentionhas focused on cleverly designed nanofilms that predisposeamphiphilic behavior, that is, switching between hydro-philic and hydrophobic response.11,12 Amphiphilic polymerbrush nanofilms13,14 and “Y-shaped” molecules with polarand nonpolar arms1 become hydrophilic upon exposure towater and hydrophobic upon exposure to organic solvents.Nanofilms with an LCST phase transition15,16 thermallyswitch between hydrophilic and hydrophobic states aboveand below the transition temperature.2 Combined with

microstructural patterning, these films show thermalswitching between “ultrahydrophilic” and “ultrahydro-phobic” behavior.2 In response to an electrical potential,molecularly designed nanofilms exhibit dynamic changesbetween hydrophilic and moderately hydrophobic states.17

All of these new nanoscale-designed amphiphilic switchingphenomena have potential for surface-responsive devices.

Unlike amphiphilic nanofilms, we describe herein theserendipitous discovery of coatings that have an oppositeresponse to immersion in water. We describe dry poly-urethane (PU) coatings that become more hydrophobicwhen exposed to water and hydrophobic coatings thatbecome more hydrophilic when dried. To indicate that thesurface response is contrary to the expected thermody-namic result, we suggest this new phenomenon be termed“contraphilic”. Interestingly, as described in more detailbelow, contraphilic surfaces are characterized by changesin advancing contact angles (θadv) of as much as 25°. Thecontraphilic effect was observed during the characteriza-tion of intermediates prepared for antimicrobial coatings.18

We outline below the course that led to the synthesis ofa polyurethane characterized by contraphilic wettingbehavior.

A goal for research in surface modifying additives(SMAs) is to bring to the surface of macroscopically thickcoatings some of the exquisite architecture and physicalbehavior that characterizes nanofilms. In contrast tonanofilms, SMAs are meant to modify a polymer surfacewhile retaining desirable bulk coating properties. Successfor the SMA approach has been mostly associated withsurface-concentration of poly(dimethylsiloxane) or semi-fluorinated functions to change wetting or improve bio-compatibility.19,20 SMA’s have also been employed tosurface-concentrate UV-absorbing chromophores.21 Infavorable cases, only small SMA addition (∼1 wt%) isnecessary to modify surface behavior.

It is well known that soft blocks, which have low glasstransition temperatures (Tg), surface-concentrate because

* Author to whom correspondence should be addressed. E-mail:kjwynne@vcu.edu.

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K.; Brittain, W. J. Surf. Sci. 2004, 570, 1-12.(15) LCST ) lower critical solution temperature; above the LCST,

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(18) Wynne, K. J.; Makal, U.; Fujiwara, T.; Ohman, D.; Wood, L.Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 2004, 45, 100.

(19) Chen, Z.; Ward, R.; Tian, Y.; Malizia, F.; Gracias, D. H.; Shen,Y. R.; Somorjai, G. A. J. Biomed. Mater. Res. 2002, 62, 254-264.

(20) Ward, R. S.; White, K. A.; Hu, C. B. In Biomedical Engineering;Planck, H., Egbers, G., Syre, I., Eds.; Elsevier Science Publishers:Amsterdam, 1984.

3742 Langmuir 2005, 21, 3742-3745

10.1021/la050357m CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 03/31/2005

of their more favorable air/interface energetics.22 Toleverage this thermodynamic tendency, our SMA strategybegins with functionalizing soft blocks and using these tomake polyurethane SMA’s. Our route to functional co-telechelics began with ring opening co-polymerization ofsemifluorinated and -CH2Br-functionalized oxetane mono-mers to poly(2-substituted-1,3-propylene oxide) diols fol-lowing known procedures.23 For results reported herein,the starting co-telechelic with semifluorinated (1,1,1,2,2-pentafluoropropoxy, 5FOx) and -CH2Br (BrOx) repeatunits is of key interest (1, P(5FOx-BrOx)). PU 2 (Scheme

1) was synthesized conventionally using isophorone di-isocyanate (IPDI) and 1,4-butanediol (BD) for the hardsegment.24 The molecular design for SMA 2 incorporatessemifluorinated groups that enhance the tendency of thesoft block to surface-concentrate25-27 bringing along CH2-Br or other functional groups that replace -Br.

After the preparation of 2, substitution of the reactive-CH2Br group with 5,5-dimethylhydantoin (DMH), wascarried out giving PU 3 (Scheme 1). SMA PU 3 is aprecursor for antimicrobial SMAs.18,28

Because wetting behavior is important to biointerac-tions, coatings were characterized by dynamic contactangle (DCA) measurements.29-31 Here we report con-traphilic behavior for PU 3, that is, IPDI-BD(40)/P(5FOx/HyOx/BrOx ) 2.0:0.7:0.3)(5000), with IPDI-BD hardblock followed by wt% in parentheses; soft block segmerratios p ) 2, y ) 0.7, and remaining -CH2Br ) 0.3; HyOx,the hydantoin substituted repeat; and Mn following in

parentheses.32 The presence of residual BrOx segmersreflects a limit for 5,5-dimethylhydantoin substitutionunder our reaction conditions.

Coatings were prepared by dipping glass coverslips intoTHF solutions of 3 (15-20 wt%), carefully manipulatingthe coverslip to obtain a smooth, optically transparentcoating, and removing solvent at 60 °C in vacuo. Resultson SMAs having different semifluorinated groups andsegmer ratios, all of which exhibit the contraphilic effectto varying degrees, will be reported in due course.

Figure 1 shows both DCA and sessile drop contact anglemeasurements for 3. In DCA, the contact angle isdetermined by extrapolation of the linear portion of theforce distance curve (fdc) to the maximum or minimummass value at immersion or emersion.29,30,33 During DCAcycle 1 (1A-d), the coating was immersed to 10 mm priorto emersion. From the first advancing fdc, fdc1A-d, where“d” indicates water advancing on a dry surface, thehydrophilic surface character is evident by the apparentgain in mass (θ1A-d ) 82.3 ( 2.1°). During DCA cycle 2A-w, where w ) previously wetted (10 mm), negativeapparent force readings are obtained, indicating a switchto hydrophobic character (θ2A-w ) 109.6 ( 1.5°). Aftercrossing the wet-dry boundary (2A-d), the apparent forcereadings suddenly become positive, indicating a changefrom hydrophobic to hydrophilic character (θ2A-d ) 82.3( 2.1°). These readings remain positive until the coatedslide is withdrawn (another 10 mm). Apparent forcereadingsare thenobtained fromwhichtherecedingcontactangle (θR ) 41.2 ( 1.3°) is obtained. On removal of thecoatedslide fromwater,a returnto initialmass isobserved,signaling that water is absorbed only to the surface of thecoating.

A parallel sessile drop experiment was carried out onthe same PU coating.34 The results are shown as imageinserts in Figure 1. Image 1A-d shows a drop emergingonto dry, pristine 3. The contact angle θ1A-d is 81.3 ( 0.6°.The drop was then withdrawn into the syringe providingan image of the receding contact angle 1R. Reversing flow,the drop impinged on the same previously wetted spot asshown in 2A-w. Now, the contact angle θ2A-w is 104.7 (0.7° reflecting the hydrophobic wetted surface. Water flowwas continued and the drop passed through the wet-dryboundary. At this point, the perimeter of the drop suddenly“jumped” and spread to give image 2A-d. After crossingthe wet-dry boundary, the advancing contact angle onthe dry surface, θ2A-d ) 84.3 ( 0.6° is the same withinexperimental error as that for the dry, pristine surfaceshown in image 1A-d.

In summary, θA changes from 83° for the dry surface to105-110° for the wet surface. Once the surface is wetted,

(21) Stoeber, L.; Sustic, A.; Simonsick, W. J.; Vogl, O. J. Macromol.Sci. Pure Appl. Chem. 2000, 37, 943-970.

(22) Garrett, J. T.; Runt, J.; Lin, J. S. Macromolecules 2000, 33, 6353-6359.

(23) Malik, A. A.; Archibald, T. G. Aerojet-General Corporation: USPatent 5,703,194, 1995.

(24) Makal, U.; Uilk, J.; Kurt, P.; Cooke, R. S.; Wynne, K. J. Polymer,in press.

(25) Chen, W.; McCarthy, T. J. Macromolecules 1999, 32, 2342-2347.

(26) Vogl, O.; Bartus, J.; Qin, M.; Zarras, P. J. Macromol. Sci. PureAppl. Chem. 1994, 31, 1329-1353.

(27) Thanawala, S. K.; Chaudhury, M. K. Langmuir 2000, 16, 1256-1260.

(28) Worley, S. D.; Sun, G. Trends Polym. Sci. 1996, 4, 364-370.

(29) Dynamic contact angle (DCA) analysis based on the Wilhelmyplate method was carried out with a Cahn Model 312 Analyzer (Cerritos,CA). The surface tension quantification limit of the instrument is 0.1dyn/cm. The probe liquid was ∼18 MΩ‚cm deionized water from aBarnstead (Dubuque, IA) nanopure system. The surface tension of theprobe liquid was checked daily and was typically 72.6 ( 0.5 dyn/cm.Beakers used for DCA analysis were cleaned by soaking in a 2-propanol/potassium hydroxide base bath for at least 24 h, rinsed for 30 s withhot tap water, and then rinsed another 30 s with nanopure water. Watercontact angles are reported with a high degree of confidence becausethe interrogation water was uncontaminated after each DCA cycleseries.30, 31

(30) Uilk, J. M.; Mera, A. E.; Fox, R. B.; Wynne, K. J. Macromolecules2003, 36, 3689-3694.

(31) Fujiwara, T.; Wynne, K. J. Macromolecules 2004, 37, 8491-8494.

(32) Materials and methods (synthesis and analytical data) areavailable as supporting material at http://pubs.acs.org.

(33) Hogt, A. H.; Gregonis, D. E.; Andrade, J. D.; Kim, S. W.; Dankert,J.; Feijen, J. J. Colloid Interface Sci. 1985, 106, 289-298.

(34) Static contact angle values were obtained with a Rame-Hartadvanced automated Gonoimeter (Model 500).

Scheme 1. 5,5-Dimethylhydantoin SubstitutionReaction

Letters Langmuir, Vol. 21, No. 9, 2005 3743

the material continues to display a high θA. If the coatingis dried at 60 °C in vacuo, the surface displays θA ) 83°,which is characteristic of a hydrophilic coating. This cyclecan be repeated many times. If3 is left at ambient humidityovernight, an intermediate initial θA is observed; wettedcycles are identical to those described above.

To examine whether contraphilic behavior would bereflected in wetting by an organic medium, 3 was examinedusing DCA in hexadecane. The hexadecane advancingcontact angle for the dry surface, θA-d ) 35.4 (0.9°, is lessthan that for the wet surface, θA-w ) 45.0 ( 0.7°. Thehexadecane DCA measurements confirm that the wetcoating is more oleophobic than when dry. Table 1 showsthe advancing and receding contact angles from DCAmeasurements and sessile drop measurements for waterand hexadecane.

Coating 3 switches in a way opposite to speciallydesigned nanofilms that become hydrophilic when exposedto water and hydrophobic when exposed to organicsolvents. Thus, we suggest the term “contraphilic” for theunexpected wetting behavior of 3.

Coating 3 displays opposite wetting behavior comparedto reports on polymers that are modestly hydrophilic andbecome more so on water immersion, such as PUs.12,35

Our results also stand in contrast with wetting behaviorof other polymers with polar functionality that become

more hydrophilic as a function of water immersiontime.36,37 In water, surface reorganization for polarpolymers has a strong thermodynamic driving force.Hydrogen bonding of near surface polar groups is drivenenthalpically, easily overcoming unfavorable entropy informing a more-ordered molecular arrangement. Uponexposure to water, the surface rearranges, opening pathsto more polar hydrophilic moieties so that hydrophilicityoften increases with time.38,39 The migration of hydrophilicpolar groups to the polymer surface gradually decreasesthe advancing contact angle. What we have observed isthe exact opposite of the anticipated wetting behavior:the surface of 3 becomes more hydrophobic when wet andupon dehydration returns to its original hydrophiliccharacter.

We believe that like amphiphilic systems mentionedabove, contraphilic behavior is also driven enthalpically.An unusual feature in our soft block is the presence of anamide group in the hydantoin moiety. Amides are stronglyhydrogen bonding groups and are responsible for theswitching of wetting behavior between hydrophilic andhydrophobic states above and below the LCST.2 Thepresence of semifluorinated and amide (hydantoin)-containing side chains on a flexible backbone clearlyfacilitates a rapid hydration-driven surface reorganizationof a sort not previously observed. A proposed mechanismfor contraphilic behavior is shown in Scheme 2. For the“dry” surface state 3-d, we propose enthalpically drivenhydrogen bonding of the amide carbonyls to one or moreavailable acceptor sites. Shown is hydrogen bonding ofthe amide carbonyl to the acidic protons of the semiflu-orinated group. In the dry coating, inter- or intramolecularamide hydrogen bonding disrupts the usual surfaceconcentration of semifluorinated moieties, resulting inhydrophilic character. Upon surface hydration, surfaceamide groups prefer to hydrogen bond with water ratherthan sites such as -CH2CF3. With the introduction of

(35) Pike, J. K.; Ho, T.; Wynne, K. J. Chem. Mater. 1996, 8, 856-860.

(36) Tretinnikov, O. N.; Ikada, Y. Langmuir 1994, 10, 1606-1614.(37) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch,

T. J.; Whitesides, G. H. Langmuir 1985, 1, 724-740.(38) Senshu, K.; Yamashita, S.; Mori, H.; Ito, M.; Hirao, A.; Naka-

hama, S. Langmuir 1999, 15, 1754-1762.(39) Lemieux, M.; Usov, D.; Minko, S.; Stamm, M.; Shulha, H.;

Tsukruk, V. V. Macromolecules 2003, 36, 7244-7255.

Figure 1. Dynamic contact angle measurements and drop images for polyurethane 3. (1A-d) first advancing force distance curve(fdc), dry; (1R) first receding fdc; (2A-w) second advancing fdc, wet; (2A-d) second advancing fdc, dry; (2R) second receding fdc.Insert images: (1A-d) water droplet placed on dry 3 surface; (2A-w) water droplet on the same wetted spot; (2A-d) advanced waterdrop after hitting wet-dry boundary; (1R, 2R) receding of the water drop.

Table 1. Advancing and Receding Contact Angles FromDynamic Contact Angle (DCA) and Sessile Drop

Measurements Using Water and Hexadecanea

water hexadecane

cycle θ (°) DCAb θ (°) sessile dropc cycle θ (°) DCAb

θ1A-d 82.3 81.3 θA-de 35.4

θ2A-w 109.6 104.7 θA-wf 45.0

θ2A-d 83.3 84.3 θR-d 26.0θ1R 40.0 -d θR-w 30.4θ2R 41.8 38.7

a Experimental error (2°. b Average of five measurements.c Average of three measurements. d Not determined. e Advancingcontact angle for dry “d” surface. f Advancing contact angle for wet“w” surface.

3744 Langmuir, Vol. 21, No. 9, 2005 Letters

water, semifluorinated groups are “released” and thesurface becomes hydrophobic (3-w). If the surface isdehydrated, it returns to its initial hydrophilic state.Multiple cycles between hydrophobic and hydrophilicstates are observed with hydration and dehydration,respectively.

Wetting in hexadecane was examined to determine ifthe contraphilic effect would be observed using a solventother than water. The hexadecane advancing contact anglefor the dry surface (36°) is less than the correspondingangle for the wet surface (46°). Consistent with theproposed mechanism, hexadecane DCA measurementsdemonstrate that the hydrated (wet) coating is moreoleophobic than when dry. Attempts to investigate wettingby liquids of intermediate polarity were thwarted byswelling or solubility.

According to the proposed mechanism, if the hydrogenbonding capability of the hydantoin moiety is removed,the contraphilic effect should be attenuated or disappear.Therefore, the conversion of near-surface amide 3a (3) tochloramide 4a was carried out according to Scheme 3.This conversion of near-surface amide is carried out byimmersing 3 in dilute hypochlorite for 1 h. This surfacereactionhasreceivedconsiderablestudy,as thechloramideform 4a lends antimicrobial character to the surface.40,41

The wetting behavior of the resulting chloramidefunctionalized coating 4 was investigated by DCA analy-sis: θ1A-d ) 101.3 ( 1.7°, θ1R ) 41.6 ( 0.6°. This highadvancing/low receding contact angle behavior is char-acteristic of polymers with side-chain semifluorinatedgroups.42-46 A second DCA cycle and successive ones

showed slightly different results (θ1A-d ) 104.5 ( 0.6°, θ1R) 43.5 ( 1.3°), suggesting conversion to chloramide wasnot quite complete, but conversion of amide to chloramidevirtually eliminates the contraphilic effect. Immersing4/chloramide 4a in thiosulfate for 1 h reduces thechloramide back to the amide (Scheme 3). Contraphilicwetting behavior is restored and is indistinguishable fromthe pristine coating. Cycling between amide and chlora-mide as shown in Scheme 3 may be repeated at least fourtimes with identical results.

In summary, a new and unexpected kind of wettingbehavior has been observed whereby a polymer surfacebecomes more hydrophobic on immersion in water. Wesuggest the term “contraphilic” for this enthalpicallydriven phenomenon. Contraphilic behavior may be com-pared with Fergusson’s report of an entropically drivenprocess, whereby an oxidized, cross-linked 1,4-polybuta-diene surface becomes more hydrophobic against hotwater.47 The entropic loss due to stretched chains trans-lates into elastic restoring force as the temperature isincreased; the hydrophilic groups are pulled away fromthe polymer-water interface, resulting in increasedhydrophobicity. In contrast to the observation that thechange in wetting behavior for oxidized 1,4-polybutadienesurfaces is damped in successive cycling, the contraphiliceffect for 3 is constant when cycled between dry and wettedsurfaces or when cycled through oxidation-reductioncycles.

The mechanism offered in Scheme 2 necessitates somechange in surface topology, most likely on the order of 1nm. The concomitant nanoscale change in chemicalcomposition involves surface-confined water hydrogenbonding accompanied by semifluorinated side-chain “re-lease”. We believe that the nanoscale change in chemicalcomposition outweighs the contribution due to topologicalchange. Previously, we have shown that nanoscale com-positional changes (ran vs block soft blocks) effect wettingbehavior.31 We suppose there is some “roughening” at thesub-nanometer-to-nanometer scale, but we doubt that thiswould result in the very large change in θadv (25°) that isobserved.

Water-induced hydrophobic surfaces may lead to ap-plications requiring easily switched wetting behavior, suchas in microfluidics. More importantly, exploiting enthalpicdriving forces for surface responsiveness may be envi-sioned as a means to overcome entropically driven events.This opens up a new area of nanostructural surface designand may result in novel materials that exhibit water-repelling responses to external cues.

Acknowledgment. This material is based upon worksupported by the National Science Foundation underGrant No. DMR 0207560. Partial support from the DefenseAdvanced Projects Agency and the VCU School of Engi-neering Foundation is also gratefully acknowledged.

Supporting Information Available: Materials andmethods for oxetane co-telechelic, polyurethane synthesis andcharacterization, procedure for 5,5-dimethylhydantoin substitu-tion reaction, coating preparation, and 1H NMR spectra for thepolyurethanes. This material is available free of charge via theInternet at http://pubs.acs.org.

LA050357M

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Scheme 2. Proposed Mechanism for ContraphilicWetting Behavior

Scheme 3. Conversion of N-Amide to N-Chloramidewith Hypochloride and Reduction of N-Chloramide to

N-Amide with Thiosulfate

Letters Langmuir, Vol. 21, No. 9, 2005 3745