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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 hydrophobic when wet. A difference of ∼25° in advancing contact angles for dry (83°) and wet (108°) states is found by sessile drop and dynamic methods. The term “contraphilic” is suggested for this reversible change opposite customary amphiphilic behavior. Contraphilic behavior results from a soft block containing semifluorinated and 5,5-dimethyhydantoin segmers. Amide inter/intramolecular hydrogen bonding is proposed 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 to applications for easily switched wetting, such as in microfluidics.
Improved understanding of interfacial surface properties is both essential and fundamental to progress in the development of advanced concepts such as “intelligent” or “responsive” coatings for switching, sensing, protection, or improved biocompatibility. Because the design and architecture of the outermost layers control surface behavior, subtle surface modifications such as altering chemical structure, composition, topology, or architecture, can be used to manipulate surface responses to external cues.1-4 By tuning the interfacial properties, improved understanding of interfacial phenomena is developed.5-10 For applications, responsive surfaces must undergo facile rearrangements to stimuli. Recently, much attention has focused on cleverly designed nanofilms that predispose amphiphilic behavior, that is, switching between hydrophilic and hydrophobic response.11,12 Amphiphilic polymer brush nanofilms13,14 and “Y-shaped” molecules with polar and nonpolar arms1 become hydrophilic upon exposure to water and hydrophobic upon exposure to organic solvents. Nanofilms with an LCST phase transition15,16 thermally switch between hydrophilic and hydrophobic states above and below the transition temperature.2 Combined with * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Julthongpiput, D.; Lin, Y. H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125, 15912-15921. (2) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. 2004, 43, 357-360. (3) Carey, D. H.; Grunzinger, S. J.; Ferguson, G. S. Macromolecules 2000, 33, 8802-8812. (4) Khongtong, S.; Ferguson, G. S. J. Am. Chem. Soc. 2002, 124, 7254-7255. (5) Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635-698. (6) Russell, T. P. Science 2002, 297, 964-967. (7) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458-1460. (8) Genzer, J.; Efimenko, K. Science 2000, 290, 2130-2133. (9) Crevoisier, G. B.; Fabre, P.; Corpart, J. M.; Leibler, L. Science 1999, 285, 1246-1249. (10) Huber, D. L.; Manginell, R. P.; Samara, M. A.; Kim, B. I.; Bunker, B. C. Science 2003, 301, 352-354. (11) Lewis, K. B.; Ratner, B. D. J. Colloid Interface Sci. 1993, 159, 77-85. (12) Andrade, J. D.; Tingey, K. G. Langmuir 1991, 7, 2471-2478. (13) Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790-2796. (14) Boyes, S. G.; Granville, A. M.; Baum, M.; Akgun, B.; Mirous, B. K.; Brittain, W. J. Surf. Sci. 2004, 570, 1-12. (15) LCST ) lower critical solution temperature; above the LCST, water is expelled and a considerable volume contraction usually ensues. (16) Bae, Y. H.; Okano, T.; Kim, S., W. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 923-936.
microstructural patterning, these films show thermal switching between “ultrahydrophilic” and “ultrahydrophobic” behavior.2 In response to an electrical potential, molecularly designed nanofilms exhibit dynamic changes between hydrophilic and moderately hydrophobic states.17 All of these new nanoscale-designed amphiphilic switching phenomena have potential for surface-responsive devices. Unlike amphiphilic nanofilms, we describe herein the serendipitous discovery of coatings that have an opposite response to immersion in water. We describe dry polyurethane (PU) coatings that become more hydrophobic when exposed to water and hydrophobic coatings that become more hydrophilic when dried. To indicate that the surface response is contrary to the expected thermodynamic result, we suggest this new phenomenon be termed “contraphilic”. Interestingly, as described in more detail below, contraphilic surfaces are characterized by changes in advancing contact angles (θadv) of as much as 25°. The contraphilic effect was observed during the characterization of intermediates prepared for antimicrobial coatings.18 We outline below the course that led to the synthesis of a polyurethane characterized by contraphilic wetting behavior. A goal for research in surface modifying additives (SMAs) is to bring to the surface of macroscopically thick coatings some of the exquisite architecture and physical behavior that characterizes nanofilms. In contrast to nanofilms, SMAs are meant to modify a polymer surface while retaining desirable bulk coating properties. Success for the SMA approach has been mostly associated with surface-concentration of poly(dimethylsiloxane) or semifluorinated functions to change wetting or improve biocompatibility.19,20 SMA’s have also been employed to surface-concentrate UV-absorbing chromophores.21 In favorable cases, only small SMA addition (∼1 wt%) is necessary to modify surface behavior. It is well known that soft blocks, which have low glass transition temperatures (Tg), surface-concentrate because (17) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374. (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.
10.1021/la050357m CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005
Letters Scheme 1. 5,5-Dimethylhydantoin Substitution Reaction
of their more favorable air/interface energetics.22 To leverage this thermodynamic tendency, our SMA strategy begins with functionalizing soft blocks and using these to make polyurethane SMA’s. Our route to functional cotelechelics began with ring opening co-polymerization of semifluorinated and -CH2Br-functionalized oxetane monomers to poly(2-substituted-1,3-propylene oxide) diols following known procedures.23 For results reported herein, the starting co-telechelic with semifluorinated (1,1,1,2,2pentafluoropropoxy, 5FOx) and -CH2Br (BrOx) repeat units is of key interest (1, P(5FOx-BrOx)). PU 2 (Scheme
1) was synthesized conventionally using isophorone diisocyanate (IPDI) and 1,4-butanediol (BD) for the hard segment.24 The molecular design for SMA 2 incorporates semifluorinated groups that enhance the tendency of the soft block to surface-concentrate25-27 bringing along CH2Br or other functional groups that replace -Br. After the preparation of 2, substitution of the reactive -CH2Br group with 5,5-dimethylhydantoin (DMH), was carried out giving PU 3 (Scheme 1). SMA PU 3 is a precursor for antimicrobial SMAs.18,28 Because wetting behavior is important to biointeractions, coatings were characterized by dynamic contact angle (DCA) measurements.29-31 Here we report contraphilic behavior for PU 3, that is, IPDI-BD(40)/P(5FOx/ HyOx/BrOx ) 2.0:0.7:0.3)(5000), with IPDI-BD hard block followed by wt% in parentheses; soft block segmer ratios p ) 2, y ) 0.7, and remaining -CH2Br ) 0.3; HyOx, the hydantoin substituted repeat; and Mn following in (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, 63536359. (23) Malik, A. A.; Archibald, T. G. Aerojet-General Corporation: US Patent 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, 23422347. (26) Vogl, O.; Bartus, J.; Qin, M.; Zarras, P. J. Macromol. Sci. Pure Appl. Chem. 1994, 31, 1329-1353. (27) Thanawala, S. K.; Chaudhury, M. K. Langmuir 2000, 16, 12561260. (28) Worley, S. D.; Sun, G. Trends Polym. Sci. 1996, 4, 364-370.
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parentheses.32 The presence of residual BrOx segmers reflects a limit for 5,5-dimethylhydantoin substitution under our reaction conditions. Coatings were prepared by dipping glass coverslips into THF solutions of 3 (15-20 wt%), carefully manipulating the coverslip to obtain a smooth, optically transparent coating, and removing solvent at 60 °C in vacuo. Results on SMAs having different semifluorinated groups and segmer ratios, all of which exhibit the contraphilic effect to varying degrees, will be reported in due course. Figure 1 shows both DCA and sessile drop contact angle measurements for 3. In DCA, the contact angle is determined by extrapolation of the linear portion of the force distance curve (fdc) to the maximum or minimum mass value at immersion or emersion.29,30,33 During DCA cycle 1 (1A-d), the coating was immersed to 10 mm prior to emersion. From the first advancing fdc, fdc1A-d, where “d” indicates water advancing on a dry surface, the hydrophilic surface character is evident by the apparent gain in mass (θ1A-d ) 82.3 ( 2.1°). During DCA cycle 2Aw, where w ) previously wetted (10 mm), negative apparent force readings are obtained, indicating a switch to hydrophobic character (θ2A-w ) 109.6 ( 1.5°). After crossing the wet-dry boundary (2A-d), the apparent force readings suddenly become positive, indicating a change from hydrophobic to hydrophilic character (θ2A-d ) 82.3 ( 2.1°). These readings remain positive until the coated slide is withdrawn (another 10 mm). Apparent force readings are then obtained from which the receding contact angle (θR ) 41.2 ( 1.3°) is obtained. On removal of the coated slide from water, a return to initial mass is observed, signaling that water is absorbed only to the surface of the coating. A parallel sessile drop experiment was carried out on the same PU coating.34 The results are shown as image inserts in Figure 1. Image 1A-d shows a drop emerging onto dry, pristine 3. The contact angle θ1A-d is 81.3 ( 0.6°. The drop was then withdrawn into the syringe providing an image of the receding contact angle 1R. Reversing flow, the drop impinged on the same previously wetted spot as shown in 2A-w. Now, the contact angle θ2A-w is 104.7 ( 0.7° reflecting the hydrophobic wetted surface. Water flow was continued and the drop passed through the wet-dry boundary. At this point, the perimeter of the drop suddenly “jumped” and spread to give image 2A-d. After crossing the wet-dry boundary, the advancing contact angle on the dry surface, θ2A-d ) 84.3 ( 0.6° is the same within experimental error as that for the dry, pristine surface shown in image 1A-d. In summary, θA changes from 83° for the dry surface to 105-110° for the wet surface. Once the surface is wetted, (29) Dynamic contact angle (DCA) analysis based on the Wilhelmy plate method was carried out with a Cahn Model 312 Analyzer (Cerritos, CA). The surface tension quantification limit of the instrument is 0.1 dyn/cm. The probe liquid was ∼18 MΩ‚cm deionized water from a Barnstead (Dubuque, IA) nanopure system. The surface tension of the probe 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 with hot tap water, and then rinsed another 30 s with nanopure water. Water contact angles are reported with a high degree of confidence because the interrogation water was uncontaminated after each DCA cycle series.30, 31 (30) Uilk, J. M.; Mera, A. E.; Fox, R. B.; Wynne, K. J. Macromolecules 2003, 36, 3689-3694. (31) Fujiwara, T.; Wynne, K. J. Macromolecules 2004, 37, 84918494. (32) Materials and methods (synthesis and analytical data) are available 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-Hart advanced automated Gonoimeter (Model 500).
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Letters
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 water drop after hitting wet-dry boundary; (1R, 2R) receding of the water drop. Table 1. Advancing and Receding Contact Angles From Dynamic Contact Angle (DCA) and Sessile Drop Measurements Using Water and Hexadecanea water
hexadecane
cycle
θ (°) DCAb
θ (°) sessile dropc
cycle
θ (°) DCAb
θ1A-d θ2A-w θ2A-d θ1R θ2R
82.3 109.6 83.3 40.0 41.8
81.3 104.7 84.3 -d 38.7
θA-de θA-wf θR-d θR-w
35.4 45.0 26.0 30.4
a Experimental error (2°. b Average of five measurements. Average of three measurements. d Not determined. e Advancing contact angle for dry “d” surface. f Advancing contact angle for wet “w” surface.
c
the material continues to display a high θA. If the coating is dried at 60 °C in vacuo, the surface displays θA ) 83°, which is characteristic of a hydrophilic coating. This cycle can be repeated many times. If 3 is left at ambient humidity overnight, an intermediate initial θA is observed; wetted cycles are identical to those described above. To examine whether contraphilic behavior would be reflected in wetting by an organic medium, 3 was examined using DCA in hexadecane. The hexadecane advancing contact angle for the dry surface, θA-d ) 35.4 (0.9°, is less than that for the wet surface, θA-w ) 45.0 ( 0.7°. The hexadecane DCA measurements confirm that the wet coating is more oleophobic than when dry. Table 1 shows the advancing and receding contact angles from DCA measurements and sessile drop measurements for water and hexadecane. Coating 3 switches in a way opposite to specially designed nanofilms that become hydrophilic when exposed to water and hydrophobic when exposed to organic solvents. Thus, we suggest the term “contraphilic” for the unexpected wetting behavior of 3. Coating 3 displays opposite wetting behavior compared to reports on polymers that are modestly hydrophilic and become more so on water immersion, such as PUs.12,35 Our results also stand in contrast with wetting behavior of other polymers with polar functionality that become (35) Pike, J. K.; Ho, T.; Wynne, K. J. Chem. Mater. 1996, 8, 856-860.
more hydrophilic as a function of water immersion time.36,37 In water, surface reorganization for polar polymers has a strong thermodynamic driving force. Hydrogen bonding of near surface polar groups is driven enthalpically, easily overcoming unfavorable entropy in forming a more-ordered molecular arrangement. Upon exposure to water, the surface rearranges, opening paths to more polar hydrophilic moieties so that hydrophilicity often increases with time.38,39 The migration of hydrophilic polar groups to the polymer surface gradually decreases the advancing contact angle. What we have observed is the exact opposite of the anticipated wetting behavior: the surface of 3 becomes more hydrophobic when wet and upon dehydration returns to its original hydrophilic character. We believe that like amphiphilic systems mentioned above, contraphilic behavior is also driven enthalpically. An unusual feature in our soft block is the presence of an amide group in the hydantoin moiety. Amides are strongly hydrogen bonding groups and are responsible for the switching of wetting behavior between hydrophilic and hydrophobic states above and below the LCST.2 The presence of semifluorinated and amide (hydantoin)containing side chains on a flexible backbone clearly facilitates a rapid hydration-driven surface reorganization of a sort not previously observed. A proposed mechanism for contraphilic behavior is shown in Scheme 2. For the “dry” surface state 3-d, we propose enthalpically driven hydrogen bonding of the amide carbonyls to one or more available acceptor sites. Shown is hydrogen bonding of the amide carbonyl to the acidic protons of the semifluorinated group. In the dry coating, inter- or intramolecular amide hydrogen bonding disrupts the usual surface concentration of semifluorinated moieties, resulting in hydrophilic character. Upon surface hydration, surface amide groups prefer to hydrogen bond with water rather than sites such as -CH2CF3. With the introduction of (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.; Nakahama, 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.
Letters Scheme 2. Proposed Mechanism for Contraphilic Wetting Behavior
Scheme 3. Conversion of N-Amide to N-Chloramide with Hypochloride and Reduction of N-Chloramide to N-Amide with Thiosulfate
water, semifluorinated groups are “released” and the surface becomes hydrophobic (3-w). If the surface is dehydrated, it returns to its initial hydrophilic state. Multiple cycles between hydrophobic and hydrophilic states are observed with hydration and dehydration, respectively. Wetting in hexadecane was examined to determine if the contraphilic effect would be observed using a solvent other than water. The hexadecane advancing contact angle for the dry surface (36°) is less than the corresponding angle for the wet surface (46°). Consistent with the proposed mechanism, hexadecane DCA measurements demonstrate that the hydrated (wet) coating is more oleophobic than when dry. Attempts to investigate wetting by liquids of intermediate polarity were thwarted by swelling or solubility. According to the proposed mechanism, if the hydrogen bonding capability of the hydantoin moiety is removed, the contraphilic effect should be attenuated or disappear. Therefore, the conversion of near-surface amide 3a (3) to chloramide 4a was carried out according to Scheme 3. This conversion of near-surface amide is carried out by immersing 3 in dilute hypochlorite for 1 h. This surface reaction has received considerable study, as the chloramide form 4a lends antimicrobial character to the surface.40,41 The wetting behavior of the resulting chloramide functionalized coating 4 was investigated by DCA analysis: θ1A-d ) 101.3 ( 1.7°, θ1R ) 41.6 ( 0.6°. This high advancing/low receding contact angle behavior is characteristic of polymers with side-chain semifluorinated groups.42-46 A second DCA cycle and successive ones (40) Lin, J.; Winkelmann, C.; Worley, S. D.; Kim, J. H.; Wei, C. I.; Cho, U. C.; Broughton, R. M.; Santiago, J. I.; Williams, J. F. J. Appl. Polym. Sci. 2002, 85, 177-182. (41) Sun, Y.; Sun, G. J. Appl. Polym. Sci. 2002, 84, 1592-1599. (42) Katano, Y.; Tomono, H.; Nakajima, T. Macromolecules 1994, 27, 2342-2344. (43) Wang, J. G.; Mao, G. P.; Ober, C. K.; Kramer, E. J. Macromolecules 1997, 30, 1906-1914. (44) Xiang, M. L.; Li, X. F.; Ober, C. K.; Char, K.; Genzer, J.; Sivaniah, E.; Kramer, E. J.; Fischer, D. A. Macromolecules 2000, 33, 6106-6119.
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showed slightly different results (θ1A-d ) 104.5 ( 0.6°, θ1R ) 43.5 ( 1.3°), suggesting conversion to chloramide was not quite complete, but conversion of amide to chloramide virtually eliminates the contraphilic effect. Immersing 4/chloramide 4a in thiosulfate for 1 h reduces the chloramide back to the amide (Scheme 3). Contraphilic wetting behavior is restored and is indistinguishable from the pristine coating. Cycling between amide and chloramide as shown in Scheme 3 may be repeated at least four times with identical results. In summary, a new and unexpected kind of wetting behavior has been observed whereby a polymer surface becomes more hydrophobic on immersion in water. We suggest the term “contraphilic” for this enthalpically driven phenomenon. Contraphilic behavior may be compared with Fergusson’s report of an entropically driven process, whereby an oxidized, cross-linked 1,4-polybutadiene surface becomes more hydrophobic against hot water.47 The entropic loss due to stretched chains translates into elastic restoring force as the temperature is increased; the hydrophilic groups are pulled away from the polymer-water interface, resulting in increased hydrophobicity. In contrast to the observation that the change in wetting behavior for oxidized 1,4-polybutadiene surfaces is damped in successive cycling, the contraphilic effect for 3 is constant when cycled between dry and wetted surfaces or when cycled through oxidation-reduction cycles. The mechanism offered in Scheme 2 necessitates some change in surface topology, most likely on the order of 1 nm. The concomitant nanoscale change in chemical composition involves surface-confined water hydrogen bonding accompanied by semifluorinated side-chain “release”. We believe that the nanoscale change in chemical composition outweighs the contribution due to topological change. Previously, we have shown that nanoscale compositional changes (ran vs block soft blocks) effect wetting behavior.31 We suppose there is some “roughening” at the sub-nanometer-to-nanometer scale, but we doubt that this would result in the very large change in θadv (25°) that is observed. Water-induced hydrophobic surfaces may lead to applications requiring easily switched wetting behavior, such as in microfluidics. More importantly, exploiting enthalpic driving forces for surface responsiveness may be envisioned as a means to overcome entropically driven events. This opens up a new area of nanostructural surface design and may result in novel materials that exhibit waterrepelling responses to external cues. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. DMR 0207560. Partial support from the Defense Advanced Projects Agency and the VCU School of Engineering Foundation is also gratefully acknowledged. Supporting Information Available: Materials and methods for oxetane co-telechelic, polyurethane synthesis and characterization, procedure for 5,5-dimethylhydantoin substitution reaction, coating preparation, and 1H NMR spectra for the polyurethanes. This material is available free of charge via the Internet at http://pubs.acs.org. LA050357M (45) Johnston, E.; Bullock, S.; Uilk, J.; Gatenholm, P.; Wynne, K. J. Macromolecules 1999, 32, 8173-8182. (46) Bertolucci, M.; Galli, G.; Chellini, E.; Wynne, K. J.; Uilk, J. Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 2002, 43, 344. (47) Khongtong, S.; Ferguson, G. S. J. Am. Chem. Soc. 2001, 123, 3588-3594.
