J. Phys. Chem. B 2000, 104, 10329-10331
10329
Switching Liquid Repellent Surfaces R. A. Lampitt, J. M. Crowther, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham UniVersity, Durham DH1 3LE, England, U.K. ReceiVed: June 21, 2000; In Final Form: August 17, 2000
Cationic fluorosurfactants complexed to maleic anhydride plasma polymer layers readily undergo surface reconstruction in response to their local liquid environment. Repetitive alternation between oleophobic and hydrophilic behavior is observed.
1. Introduction
SCHEME 1: Structures
With the ever-growing demand for more sophisticated surfaces, one of the current challenges is to produce substrates whose interfacial properties are governed by their local environment (smart surfaces). In this article, a universal method for making solid surfaces that display both oleophobicity and hydrophilicity is described. This entails the formation of surfactant-polyelectrolyte complexes on structurally welldefined plasma polymer surfaces. Self-assembly between polyelectrolytes and oppositely charged surfactants in the solution phase to produce polyelectrolytesurfactant complexes1,2 has been extensively studied and understood to be driven by electrostatic and interchain packing interactions in accordance with the strict 1:1 stoichiometry needed for charge neutralization.3-6 Typical examples include complexation between polystyrenesulfonate and cationic alkyltrimethylammonium counterions,6 and poly(acrylic acid) with dodecyltrimethylammonium counterions.7 The final product tends to be amorphous on the local molecular scale (as shown by wide-angle X-ray scattering and differential scanning calorimetry); however, a very high degree of order can occur on a length scale of a few nanometers (i.e., ordered mesophases). Fluorinated surfactants8 have been utilized in this context to form self-assembled polyelectrolyte-fluorosurfactant complexes which display good oil and water repellency. This behavior can be attributed to the packing and alignment of the low surface energy perfluorocarbon tails away from the underlying substrate.9,10 Rather than making such polyelectrolyte-surfactant complexes beforehand in the solution phase, followed by solvent casting onto a substrate (thereby forming a multilayer system), a more direct and interfacially-specific approach is outlined in this article, where the fluorinated surfactant undergoes direct complexation onto the surface of a plasma-deposited polyelectrolyte coating. A comparison is made between the surface complexation of cationic fluorosurfactant with pulsed maleic anhydride plasma polymer surfaces and conventional analogues prepared in the solution phase from poly(ethylene-alt-maleic anhydride) copolymer (Scheme 1). The plasma polymer layer was deposited by pulsing the electrical discharge on the submicrosecond time scale,11-13 in order to achieve high structural retention of the anhydride groups. 2. Experimental Section Poly(ethylene-alt-maleic anhydride) copolymer (Zeeland Chemicals, ∼100% purity) films were spin-coated onto a glass * To whom correspondence should be addressed.
substrate from a 2 wt %/vol acetone solution. Fourier transform (FT) infrared spectroscopy was used to check the purity of the polymeric film and the absence of anhydride group hydrolysis.14 For the pulsed plasma polymerization experiments, briquettes of maleic anhydride (Aldrich, 99% purity) were ground into a fine powder and loaded into a monomer tube. This was connected to an electrodeless cylindrical glass reactor (5 cm diameter) enclosed in a Faraday cage. The system was continuously pumped by a two-stage rotary pump fitted with a liquid nitrogen cold trap (base pressure of 1 × 10-3 mbar). An L-C matching network was used to minimize the standing wave ratio (SWR) of the power transmitted from a 13.56 MHz rf generator to the inductively coupled electrical discharge. The rf source was triggered by a signal generator, and an oscilloscope was used to monitor the pulse width and amplitude. Prior to each deposition, the reactor was cleaned using a 30 W continuous wave air plasma ignited at a pressure of 0.2 mbar for 10 min. Glass slides were then placed into the center of the coil region. Maleic anhydride vapor at 0.1 mbar pressure was introduced into the chamber, and the glow discharge ignited for 10 min (ton ) 20 µs, toff ) 1200 µs, Pp ) 5 W). Upon completion of the deposition, the rf power supply was switched off, and the system was flushed with monomer for a further 5 min and then vented to air. The cationic fluorinated surfactant employed for complexation comprised an ammonium ion headgroup separated by an alkyl spacer from the perfluorinated tail and a chloride counterion (RfCH2CH2CH2(CH3)3N+Cl-, where n ) 6-20 for the Rf chain, Dupont, FSD). Glass substrates coated with either the poly(ethylene-alt-maleic anhydride) copolymer or maleic anhydride plasma polymer were immersed in a dilute aqueous solution of the surfactant for 1 h (at less than the cmc for the surfactant6), followed by rinsing with distilled water and drying in air. The conventional bulk polyelectrolyte-surfactant complex was prepared by adding fluorosurfactant solution to poly(ethylene-alt-maleic anhydride) copolymer dissolved in acetone. The precipitated product was separated from the liquid phase and rinsed several times in distilled water. Finally, it was dissolved in methanol and spin coated onto a clean glass substrate.
10.1021/jp002234a CCC: $19.00 © 2000 American Chemical Society Published on Web 10/11/2000
10330 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Figure 1. XPS C(1s) stack plot of (a) spin-coated poly(ethylene-altmaleic anhydride) copolymer; (b) spin-coated poly(ethylene-alt-maleic anhydride) copolymer reacted with cationic fluorosurfactant; and (c) bulk poly(ethylene-alt-maleic anhydride) copolymer-cationic fluorosurfactant complex.
A Vacuum Generators ESCALAB system was used for chemical characterization of the polyelectrolyte-surfactant complex surfaces. Photoelectrons generated by an unmonochromated X-ray source (Mg KR1,2 ) 1253.6 eV) were collected at 30° takeoff angle from the substrate normal using a concentric hemispherical analyzer (CHA) operating in constant analyzer energy mode (CAE, pass energy ) 20 eV). Instrumentally determined sensitivity factors for unit stoichiometry were taken as C(1s):F(1s):O(1s):N(1s):Si(2p) ) 1.00:0.23:0.36:0.72:0.97, respectively. A Marquardt minimization computer program was used to fit the core level spectra with Gaussian peak shapes possessing equal full width at half-maximum (fwhm).15 Complete coverage of the substrate was verified by the absence of any Si(2p) signal showing through from the underlying glass substrate. Hexadecane (oleophobicity) and water (hydrophilicity) probe liquid contact angles were measured by the sessile drop technique at 20 °C using a motorized microsyringe and video capture apparatus (A.S.T. Products VCA2500XE). 3. Results and Discussion The C(1s) XPS envelope of spin coated poly(ethylene-altmaleic anhydride) copolymer as shown in Figure 1, was fitted to the following carbon functionalities:16 hydrocarbon (CHx ∼ 285.0 eV), carbon adjacent to an anhydride group (C-CO(dO) ∼ 285.8 eV), and an anhydride group (OdC-O-Cd O ∼ 289.4 eV). Good agreement with the theoretical copolymer structure was found (a slight amount of hydrocarbon contamination is present at the surface). The XPS O:C ratio of 0.46 ( 0.02 as well as infrared absorption analysis were consistent with this assignment. In the case of the maleic anhydride pulsed plasma polymer layer, the C(1s) envelope could be fitted to five different carbon functionalities:11 hydrocarbon (CHx ∼ 285.0 eV), carbon singly bonded to an anhydride group (C-C-O(dO) ∼ 285.7 eV), carbon singly bonded to oxygen (-C-O ∼ 286.6 eV), carbon doubly bonded to oxygen (O-C-O/-CdO ∼ 287.9 eV), and anhydride groups (OdC-O-CdO ∼ 289.4 eV). Optimum electrical pulsing conditions during plasma deposition (in terms of duty cycle and peak power) gave rise to anhydride group retention corresponding to approximately 58% of the surface carbon environments being associated with maleic anhydride
Lampitt et al.
Figure 2. XPS C(1s) stack plot of (a) electrically pulsed maleic anhydride plasma polymer; and (b) maleic anhydride plasma polymercationic fluorosurfactant complex.
repeat units. This high level of structural retention was confirmed by infrared analysis.11 Both the poly(ethylene-alt-maleic anhydride) copolymer and the plasma polymer layers were reacted with a dilute aqueous solution of cationic fluorinated surfactant followed by rinsing in water. This gave rise to the appearance of fluorinated carbon functionalities in the C(1s) spectra: CF2 (291.4 eV) and CF3 (293.6 eV), at the expense of the anhydride group (289.4 eV) (Figures 1 and 2 and Table 1). Nitrogen from the cationic headgroup of the surfactant was also present. No signal from the chloride counterion could be detected, which is consistent with the formation of a monolayer polyelectrolyte-surfactant complex rather than just entrappment of the surfactant or the formation of a bilayer.17,18 Similar XPS spectra were obtained for the solvent cast bulk poly(ethylene-alt-maleic anhydride)fluorinated surfactant complex prepared in solution (Figure 1). The fluorinated tails were most prominent in the C(1s) spectra of the surface polyelectrolyte-surfactant complex prepared from maleic anhydride pulsed plasma polymer. Fluorosurfactant complexed to the poly(ethylene-alt-maleic anhydride) copolymer film displayed high contact angles toward both oil and water probe liquids in a fashion similar to that observed for the bulk fluorosurfactant-poly(ethylene-alt-maleic anhydride) complex cast from solution and other conventional polyelectrolyte-fluorosurfactant systems.9 Table 2. This is consistent with the perfluoroalkyl chains remaining oriented outward in the presence of both types of liquid, as previously exemplified by the adsorption of cationic fluorocarbon surfactant monolayers onto negatively charged mica surfaces19,20 (a much lower water contact angle would be expected if the hydrophilic surfactant headgroups were exposed21). However, the fluorosurfactant-plasma polymer layer was found to repel oil but allow the spreading of water (Table 2). This ability to switch between oleophobicity and hydrophilicity could be repeated at least 20 times. The liquid-specific response of the fluorosurfactant-plasma polymer system can be attributed to complexation occurring at just the very outer surface. Plasma-induced cross-linking during deposition prohibits penetration of the large surfactant moieties into the subsurface region. In turn this can be expected to suppress interdigitation, cooperative binding, and layering of the surfactant tails.22,23 This ultrathin array of aligned perfluorocarbon chains at the liquid-solid interface effectively repels nonpolar liquids, whereas polar molecules such as water either
Switching Liquid Repellent Surfaces
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10331
TABLE 1: XPS Analysisa elemental composition
C(1s) FIT
substrate
%C
%O
%F
%N
% CF2
% CF3
bulk poly(ethylene-alt-maleic anhydride) copolymer-fluorosurfactant complex spin-coated poly(ethylene-alt-maleic anhydride) copolymer followed by reaction with fluorosurfactant maleic anhydride plasma polymer reacted with fluorosurfactant
47 ( 1
17 ( 1
33 ( 1
3(1
22 ( 1
6(1
42 ( 3
9(1
46 ( 2
3(1
32 ( 3
9(2
40 ( 1
11 ( 2
45 ( 3
4(2
35 ( 4
8(0
a
The surfactant chloride counterion was not detected.
TABLE 2: Contact Angle Measurements contact angle (deg) substrate
water
hexadecane
clean glass glass dipped into fluorosurfactant and then washed poly(ethylene-alt-maleic anhydride) copolymer maleic anhydride plasma polymer bulk poly(ethylene-alt-maleic anhydride) copolymer-fluorosurfactant complex spin-coated poly(ethylene-alt-maleic anhydride) copolymer followed by reaction with fluorosurfactant maleic anhydride plasma polymer reacted with fluorosurfactant
38 ( 3 28 ( 4
20 ( 3 49 ( 5
31 ( 2
58 ( 3
53 ( 1 107 ( 6
< 20 78 ( 2
105 ( 1
81 ( 1
22 ( 2
79 ( 1
penetrate into the hydrophilic subsurface or cause the perfluorocarbon tails to reorganize in such a way as to vacate hydrophilic regions. This is in marked contrast to the coupling of cationic fluorosurfactant species onto the surface of spin cast poly(ethylene-alt-maleic anhydride) copolymer, where subsurface swelling can be expected to occur during exposure to the aqueous surfactant solution, thus enabling complexation to extend below the surface in an analogous fashion to what happens in solution during the formation of conventional bulk fluorosurfactant-copolymer material. Such oleophobicity/hydrophilicity behavior is potentially attractive for applications such as micromechanical devices and antifogging surfaces. In the latter case, the spreading of water droplets in combination with the hindrance toward oily substances is highly sought after.24 Another area of interest is soil release, where the substrate is required to repel oily substances in the dry state while allowing solvent molecules access to the surface in the wet state, e.g., to assist with the removal of any adhered soil moieties.25,26 4. Conclusions Coupling of cationic fluorosurfactants to the surface of welldefined maleic anhydride pulsed plasma polymer coatings produces a shallow complexed layer. This is found to exhibit reversible repellency/wetting behavior toward oil/water probe liquids.
Acknowledgment. J.M.C. thanks DERA and EPSRC for financial support. Also, we are grateful to DuPont for the generous gift of fluorosurfactant solutions. References and Notes (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC: Boca Raton, FL, 1993; Chapter 5. (3) Antonietti, M.; Burger, C.; Effing, J. AdV. Mater. 1995, 7, 751. (4) Macdonald, P. M.; Tang, A. J. Langmuir 1997, 13, 2259. (5) Antonietti, M.; Radloff, D.; Wiesner, U.; Spiess, H. W. Macromol. Chem. Phys. 1996, 197, 2713. (6) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (7) Antonietti, M.; Conrad, J. Angew. Chem. 1994, 106, 1927. (8) Szonyi, S.; Watzke, H. J.; Canbon, A. Thin Solid Films 1996, 284/ 285, 769. (9) Antonietti, M.; Henke, S.; Thunemann, A. F. AdV. Mater. 1996, 8, 41. (10) Thunemann, A. F.; Lochhaas, K. H. Langmuir 1999, 15, 4867. (11) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996, 8, 37. (12) Hynes, A. M.; Badyal, J. P. S. Macromolecules 1996, 29, 4220. (13) Rinsch, C. L.; Panchalingam, V.; Savage, C. R.; Wang, J. H.; Eberhart, R. E.; Timmons, R. B. ACS Polym. Prepr. 1995, 36, 95. (14) Trivedi, B. C.; Culbertson, B. M. Maleic Anhydride; Plenum: New York, 1982. (15) Evans, J. F.; Gibson, J. H.; Moulder, J. F.; Hammond, J. S.; Goretzki, H. Fresenius Z. Anal. Chem. 1984, 319, 841. (16) Evenson, S. A.; Badyal, J. P. S. J. Phys. Chem. B 1998, 102, 5500. (17) Lens, J. P.; Terlingen, J. A. C.; Engbers, G. H. M.; Feijen, J. Langmuir 1998, 14, 3214. (18) Surfactant Science Series Volume 37: Cationic Surfactant Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker Inc.: New York, 1991. (19) Claesson, P. M.; Herder, P. C.; Berg, J. M.; Christenson, H. K. J. Colloid Interface Sci. 1990, 136, 541. (20) Christenson, H. K.; Claesson, P. M.; Berg, J. M.; Herder, P. C. J. Phys. Chem. 1989, 93, 1472. (21) Lai, C.-L.; Harwell, J. H.; O’Rear, E. A.; Komatsuzaki, S.; Arai, J.; Nakakawaji, T.; Ito, Y. Colloids Surf. A: Physicochem. Eng. Aspects 1995, 104, 231. (22) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 4554. (23) Ober, C. K.; Wegner, G. AdV. Mater. 1997, 9, 17. (24) Ueno, M.; Ugajin, Y.; Horie, K.; Nishimura, T. J. Appl. Polym. Sci. 1990, 39, 967. (25) Sherman, P. O.; Smith, S.; Johannessen, B. Textile Res. J. 1969, 39, 449. (26) Ellzey, S. E.; Connick, W. J.; Drake, G. L.; Reeves, W. A. Textile Res. J. 1969, 39, 809.
