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A Facile Photochemical Surface Modification Technique for the Generation of Microstructured Fluorinated Surfaces J. D. Jeyaprakash S. Samuel and Ju¨rgen Ru¨he* Chemistry and Physics of Interfaces, Institute for Microsystems Technology, University of Freiburg, Georges Ko¨ hler Allee 103, D-79110 Freiburg im Breisgau, Germany Received March 4, 2004. In Final Form: June 30, 2004 We describe a simple photochemical process which allows fluoropolymers to be chemically bound at room temperature onto SiO2 surfaces. To achieve this, at first a benzophenone silane is used to form a selfassembled monolayer on the surface of the substrate, which is subsequently coated with the fluoropolymer and irradiated with UV light of wavelength 365 nm. Using this very simple approach, we have been able to create ultrahydrophobic surfaces with very low surface free energies together with a good degree of control in thickness and composition as well as strong adhesion to the monolayer. The use of a UV-based process to attach the films on SiO2 surfaces opens the door for photopatterning of surfaces with fluorinated and nonfluorinated compounds to yield well-defined microstructures with spatial control of the wetting properties of the substrates.
Introduction Formation of ultrahydrophobic surfaces using fluoropolymer thin films has been a topic of substantial interest owing to their attractive properties such as low surface energy, heat resistance, chemical inertness, low dielectric constant, and blood compatibility.1-3 Several pathways have been reported to generate such fluoropolymer thin films. They include (i) deposition of fluoropolymer thin films using diverse plasma techniques,4-6,10,11 (iii) pulsed laser deposition techniques,7 (iv) hot filament chemical vapor deposition of fluorinated precursors,8 and (v) direct liquid injection in an ultraviolet (UV) light assisted rapid isothermal processing.9 Despite their versatility, the laborious natures of the processes involved to form fluoropolymer thin films limit their use in various applications. Additionally, none of these processes offer a good degree of control in varying the thickness as well as composition of the films due to the harsh treatment of the substrates. Moreover, the films formed by physical deposition using most of these processes4-9 suffer from poor adhesion to the substrates due to the extremely low surface energy of the fluoropolymers. * To whom correspondence should be addressed. E-mail: ruehe@ imtek.uni-freiburg.de. (1) Scheirs, J. Modern Fluoropolymers; John Wiley & Sons: New York, 1997. (2) Masaaki, Y.; Masashi, M. Newest Aspects of Fluoro Functional Materials; CMC: Tokyo, 1994. (3) Ho, J. Y.; Matsuura, T.; Santerre, J. P. J. Biomed. Sci. Polym. Ed. 2000, 11, 1085. (4) Zhang, Y.; Yang, G. H.; Kang, E. T.; Neoh, K. G.; Huang, W.; Huan, A. C. H.; Lai, D. M. Y. Surf. Interface Anal. 2002, 34, 10-18. (5) Zou, X. P.; Kang, E. T.; Neoh, K. G.; Huang, W. J. Adhes. Sci. Technol. 2001, 15, 1655-1672. (6) Durrant S. F.; Mota R: P.; de Moraes, M. A. B. Thin Solid Films 1992, 220, 295. (7) Li, S. T.; Arenholz, E.; Heitz, J.; Bauerle, D. Appl. Surf. Sci. 1998, 125, 17-22. (8) Lau, K. K. S.; Gleason, K. K. J. Phys. Chem. B 2001, 105, 23032307. (9) Sharangpani, R.; Singh, R.; Drews, M.; Ivey, K. J. Electron. Mater. 1997, 26, 402-409. (10) Youngblood, J. F.; McCarthy T. J. Macromolecules 1999, 32, 6800-6806. (11) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J. F.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399.
To circumvent the problems described above, several techniques have been developed to chemically bind the films to the substrate using either chemisorption of preformed polymers (“grafting-to”) or growth of polymers from surfaces (“grafting-from”). In the grafting-to technique, polymer chains with specific functional groups along the chain or at the chain ends are made to bind to appropriate functional groups on the substrate surface.12,13 For example, carboxylic acid end group containing thiols are initially self-assembled on gold, subsequently amineterminated poly(tert-butyl acrylate) chains are grafted onto them, and finally amine-terminated oligomeric fluorocarbons are made to couple with the deprotected carboxylic acids of the main chain, resulting in a hydrophobic surface.13 The grafting-from technique14,15 has been recently used to grow fluorinated polymer “brushes” from surface-attached azo initiators on porous surfaces to yield ultrahydrophobic surfaces with a good degree of control in thickness and surface composition.16 Despite the versatility of the two approaches, the need for carrying out synthetic reactions in situ on the substrate surface, which is inherent to both approaches, might render alternative routes also attractive. Recently, a novel room-temperature UV-assisted process has been reported for the attachment of thin polymeric layers onto modified SiO2 surfaces.17 In this work we use the same process to attach fluoropolymers at room temperature to SiO2 surfaces. The silicon oxide surfaces are first modified with molecules carrying benzophenone groups, and in a second step, the polymer is attached using UV radiation as shown in Figure 1. Using this very simple approach, we demonstrate the possibility of creating surfaces with very low surface free energies while having a good degree of control in thickness, adhesion, and (12) Ru¨he, J.; Blackman, G.; Novotny, V. J.; Clarke, T.; Street, G. B.; Kuan, S. J. Appl. Polym. Sci. 1994, 53, 825-836. (13) Zhou, Y.; Bruening, M. L.; Liu, Y.; Crooks, R. M.; Bergbreiter, D. E. Langmuir 1996, 12, 5519. (14) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (15) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602. (16) Jung D-W.; Park I. J.; Choi Y. K.; Lee S-B.; Park H. S.; Ru¨he J. Langmuir 2002, 16, 6133. (17) Prucker, O.; Naumann, C. A.; Ru¨he, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121; 8766-8770.
10.1021/la049428s CCC: $27.50 © 2004 American Chemical Society Published on Web 10/05/2004
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Figure 1. Schematic depiction of the photochemical attachment of fluoropolymers (1) to a surface-anchored benzophenone (2).
composition of the coating. Benzophenone (BP) has been chosen as the photolinker over others such as diazo esters, aryl azides, and diazirines. Distinctive advantages of the BP system are thermal stability, weak sensitivity to ambient light, chemical inertness in the absence of light, and unspecific reactivity toward any C-H bonds even in the presence of solvent water and bulk nucleophiles.17,27 Finally, we show that UV-assisted photopatterning of fluorinated surfaces can be carried out, resulting in spatial control of the wetting behavior against hydrophilic and hydrophobic solvents. Experimental Section Materials. 1H,1H,2H,2H-Heptadecafluorodecyl acrylate (Aldrich, 97%) was purified chromatographically over neutral AlOx, distilled under vacuum from copper(I) chloride, and stored at -30 °C. Toluene was distilled from sodium after refluxing it overnight. Benzophenone was used as an indicator. Triethylamine was dried by distillation from CaH2, and 1,1,2-trichlorotrifluoroethane (TCTFE; Fluka) was used as received. Dimethlylchlorosilane was purified by distillation. The other chemicals were used as received. Surface Modification. Synthesis and Immobilization of Benzophenone Photolinker on SiO2 Surfaces. The synthesis of 4-allyloxybenzophenone (ABP) and 4-(3′-chlorodimethylsilyl)propyloxybenzophenone (Si-ABP) follows previously reported procedures.17 The immobilization of the silane onto the SiO2 surface was carried out overnight at room temperature under inert conditions in toluene as reported.17 Triethylamine was added to catalyze the surface attachment and to scavenge the acid produced during the condensation reaction. After being carefully rinsed with chloroform, the samples were dried under a stream of nitrogen. Ellipsometric measurements reveal a layer thickness of 1 ( 0.2 nm assuming a refractive index of the monolayer of n ) 1.5. Polymer Synthesis. Poly(1,1,2,2,heptadecafluorodecyl acrylate)s (fluoropolymers, FPs) with five different molecular weights were synthesized by varying the concentration of initiator. The polymerization reactions were carried out using AIBN as the initiator and TCTFE as the solvent at 60 °C. The reaction conditions are shown in Table 1. The fluoropolymers were finally precipitated in methanol and vacuum-dried overnight. Poly(Nisopropylacrylamide) (pNIPAAM) was synthesized as reported previously18 using 3.0 mmol of monomer and 3 × 10-3 mmol of AIBN as initiator in 40 mL of benzene at 50 °C for 14 h. The resulting polymer was precipitated in diethyl ether and vacuumdried overnight. Immobilization of Fluoropolymers on SiOx Surfaces. Fluoropolymer (FP1-5) films (40-50 nm thick) were formed on modified SiO2 surfaces by dip-coating from a 40 mg/mL solution of the (18) a. Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 665-671.
Table 1. Stoichiometry and Reaction Conditions for the Free Radical Polymerization of Poly(heptadecafluorodecyl acrylate) sample code
amt of monomer (10-3 mol)
amt of AIBN (10-4 mol)
t (min)
conversion (%)
FP1 FP2 FP3 FP4 FP5
6.3 6.3 6.3 6.3 6.3
2.01 1.01 0.50 0.30 0.20
290 100 100 250 245
93 70 55 67 54
polymer in TCTFE at a typical withdrawal speed of 60 mm/min. After film deposition the samples were air-dried and subjected to UV irradiation for the desired periods of time using a Stratalinker 2400 UV illumination system equipped with 365 nm high-pressure mercury lamps with an energy density of 4.0 mW/cm2. After illumination, the samples were extracted with TCTFE for at least 48 h to remove any nonattached polymer chains. Micropatterning. Standard TEM grids (400 mesh) were used as contact masks for generating patterns on the SiO2 surfaces. Alternatively, masks were used which were generated through simply printing the desired pattern onto transparent foils using a laser printer. After dip-coating with a 40 mg/mL FP4 solution, the masks were placed on top of the substrate and irradiated for 60 min followed by extraction in TCTFE. Alternatively, the SiABP-modified surfaces were coated with a 100 nm thick pNIPAAM polymer from a 40 mg/mL solution and irradiated using the mask. After extraction of the unattached polymers using methanol, the same substrate was once again dip-coated in a 40 mg/mL solution of FP5 and irradiated for another 60 min. The nonattached polymers were once again extracted using TCTFE. Surface Characterization. The layer thickness of the polymeric monolayer was determined using ellipsometry and X-ray reflectivity (XRR). Ellipsometric measurements were performed on a DRE-XO2 C ellipsometer operating with a 638.2 nm He/Ne laser at an incident angle of 70°. For data analysis it was assumed that the refractive index of the fluoropolymer film is the same as that of the bulk material (n ) 1.337). Imaging ellipsometric measurements were obtained using an IElli2000 instrument from Nanofilm Technology equipped with a 633 nm He/Ne laser at an incident angle of 60°. XRR measurements were carried out on a Siemens/Bruker D5000 instrument with a 1.6 kW rotating anode using Cu KR radiation (λ ) 0.154 nm). The fitting was carried out using Parratt software based on Fresnel equations. The thickness and roughness of the SiO2 substrate were determined in separate experiments and included as constants in the calculation of the final layer thickness. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Physical Electronics 5600 ci spectrometer with an Al KR X-ray source (15 keV, filament current of 20 mA). Unless otherwise specified, the takeoff angle was set to 45° and the spot size was
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chosen as 800 µm. For line scans a step size of 15.6 µm and a spot size of 150 µm were selected. Dynamic contact angle measurements were performed using a Dataphysics OCA20 goniometer coupled with a CCD camera. The initial drop size was 10 µL, which was incremented at a rate of 0.06 µL/s for dynamic measurements. Optical images were obtained using an EHD vista video inspection system equipped with a color CCD camera and a computer with a frame-grabber and image analysis software.
Results and Discussion The synthesis of the benzophenone silane (Si-ABP) was carried out as reported previously.17 The surface attachment of Si-ABP onto clean SiO2 surfaces was verified using ellipsometry, which yielded a thickness of around 1 nm. FP monomer was polymerized by free radical polymerization reactions using various AIBN concentrations to obtain a bandwidth of different molecular weights. The purified fluoropolymers were dip-coated from 1-40% solutions to result in uniform films. Five ellipsometric measurements were taken on each slide and gave the same film thickness within 1 nm. The coated SiO2 surfaces were then irradiated using UV light (λ ) 365 nm) for desired time periods and subsequently extracted overnight using TCTFE as a solvent to remove any unbound polymer chains. The absorption of UV light at a wavelength of around 365 nm triggers an n,π* transition in the carbonyl functionality of the benzophenone groups attached to the SiO2 surface, resulting in the formation of a biradicaloid triplet excited state.19,20 This then abstracts a hydrogen atom from a polymer segment in close proximity, which results in the formation of a benzophenone ketyl radical and polymer radical. These can recombine in the monolayer to form stable carbon-carbon bonds21,22 (Figure 1). So far no mechanistic details are known for the photochemical reaction in the thin film, and it is assumed that the behavior is somewhat similar to that in the bulk.27 Further experiments are required to allow a more detailed understanding of the system. Anyway, as a consequence of the photochemical process a single polymer chain becomes covalently attached to the benzophenone moiety by means of C-C bond formation. The BP moiety in turn had been covalently attached to the SiO2 surface. As several BP units from the surface can bind to the same polymer, it is assumed that each polymer chain is typically attached to the surface through several anchor points. Although other photochemical reactions such as disproportionation, recombination of two respective ketyl radicals or polymer radicals, and electron transfer cannot be ruled out during this process, they do not contribute toward the attachment of the polymer chain to the surface. Additionally, adverse effects that might be caused by the presence of dissolved oxygen have been ruled out by performing illumination reactions both in air and under an argon atmosphere using a degassed polymer solution. Both sets of experiments gave identical results concerning polymer attachment. To characterize the surfaces from a qualitative point of view, XPS studies of unmodified, benzophenone-modified, and fluoropolymeranchored SiO2 surface were taken and are shown in Figure 3a. The appearance of a F(1s) peak along with a C(1s) peak due to C-F in addition to that of C-H and the attenuation of the Si(2s) and Si(2p) peaks confirm the presence of the fluoropolymer monolayer. The spectra are (19) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Mill Valley, CA, 1991. (20) Bra¨uchle, Chr.; Burland D. M.; Bjorklund G. C. J. Phys. Chem. 1981, 85, 123. (21) Horie, K.; Morishita, K.; Mita, I. Macromolecules, 1984, 17, 1746. (22) Horie, K.; Ando H.; Mita, I. Macromolecules 1987, 20, 54.
Figure 2. X-ray reflectivity curve of a 4 nm thick fluoropolymer film (FP2) attached to a Si-ABP-modified surface (O, data; s, fit). The inset shows the electron density profile for the same film. Fit parameters: fluoropolymer layer, d ) 42.5 Å, 1.45E-5 Å-2, σ ) 4.5 Å; Si-ABP layer, d ) 11.5 Å, 6.0E-6 Å-2, σ ) 4.5 Å.
in good agreement with that of films of the same polymer reported previously.16 A detailed XPS curve for the C(1s) region of the fluoropolymer-modified surface is shown in Figure 3b. The deconvolution divided the C(1s) spectra into several features using multiple Gaussian fit functions. Each of these features were identified according to the reported chemical shifts23,24 to correspond to -CHx, -COO, -CF2, and -CF3 carbons. To obtain information about the internal structure of the perfluoropolymer films, angle-dependent XPS experiments were carried out. It is clearly evident that with decreasing takeoff angle the signal intensity of the peak attributed to the CF3 groups increases, while that of the peak attributed to the CF2 groups decreases. For a takeoff angle of 45°, for example, the CF3/CF2 ratio was determined to be 0.19, which is 1.3 times higher than that of the original precursor molecule. This indicates that the -CF3 groups are aligned toward the outer surface, thus imparting significant hydrophobicity.25 To prove that covalent attachment of the polymers takes place, control experiments were performed using FP films coated on unmodified SiO2 substrates. After irradiation under the same conditions as for the BP-coated layers, the polymer layer was easily washed away in these reference experiments by gentle solvent rinses, implying that the surface-anchoring process on the photoactive monolayer was indeed caused by formation of covalent bonds. A typical XRR curve for FP2 is shown in Figure 2. From the slope of the reflection curve it can be concluded that the roughness of the polymer layer closely resembles that of the SiO2 substrate. The thicknesses of the attached polymer layers as measured by XRR and ellipsometry matched each other within experimental error. The thickness of the fluoropolymer surface-attached films after careful extraction, which removes all physisorbed polymer, exhibits a nonlinear dependency on the irradiation time as shown in Figure 4 for two different fluoropolymers with different molecular weights. Initially, the thicknesses of the fluoropolymer films increase with irradiation time as more and more benzophenone molecules become activated. However, the film growth reaches a limiting value after (23) Goto, T.; Chen, J.; Wakida, T. Chem. Express 1991, 6, 711. (24) Clark, D. T.; Shuttleworth, D. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 27. (25) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600.
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Figure 4. Thickness of the fluoropolymer monolayers as a function of the irradiation time (b, FP5; O, FP4).
Figure 5. Thickness of the fluoropolymer monolayer after irradiation and extraction as a function of the thickness of the dip-coated layer. (b, FP5; x, FP4).
Figure 3. (a) XPS spectra of an unmodified SiO2 surface (1) and the same surface after Si-ABP immobilization (2) and after attachment of a monolayer of the fluorinated polymer (FP5) (3). (b) C(1s) XPS detail spectrum of the FP5-modified SiO2 surface (0, experimental data; dotted lines, deconvolutions; full line, sum of all deconvolutions; peaks A and B correspond to -CHx, peak C corresponds to -COO, peak D corresponds to -CF2, and peak E corresponds to -CF3 groups (more details are given in the text). (c) Angle-dependent XPS spectra of the same sample: takeoff angles (s) 15°, (---) 45°, and (‚‚‚) 90°.
approximately 60 min of irradiation time. This is in good agreement with previous work on other polymeric systems, where the half-life time τ1/2 for the depletion of benzophenone absorption has been estimated to be around 30 min.26 That the photochemical reaction slows and eventually no further polymer becomes attached to the substrate is a (26) Chang, B. J. Control of Cell Adhesion on Heart Valve Implants through Ultrathin Surface-attached Polymer Layers. Ph.D. Thesis, Albert-Ludwigs University of Freiburg, 2002. (27) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (28) Dorman, G.; Prestwich, G. D. Biochemistry 1994, 33, 5661.
clear indication that the film thickness increase is indeed due to consumption of the benzophenone molecules and not due to photochemical reactions in the polymer as in that case the polymer layer would grow without bounds during the irradiation. Fluoropolymer films of a wide range of film thicknesses for two polymers with different molecular weights (FP4 and FP5) were spin cast onto modified SiO2 surfaces and irradiated with UV light for 60 min. The thickness of the extracted surface-attached fluoropolymer film increased linearly with the thickness of the original spin-cast films at lower thicknesses as shown in Figure 5. However, they reach a limiting value if very thick films are deposited and eventually become independent of the thickness of the spin-cast films. This behavior is expected for a monolayer as all molecules which are to become attached to the surface have to be in close proximity to the surface so that the surface-attachment reaction is possible and the further piling up of more and more molecules on top of them will only result in more material to be washed away during extraction.17 The influence of the molecular weight of the deposited polymer on the final thickness was studied by dip-coating 40-50 nm thick films for all five different polymers and irradiating them for 1 h. The film thickness as a function of the molecular weight is shown in Figure 6. It is clearly evident that higher molecular weight polymers yield higher film thicknesses of the surface-attached monolayers than the lower molecular weight ones17 and that the film thickness can easily be controlled through this parameter. Dynamic water contact angle studies performed on the fluoropolymer-modified SiO2 substrates displayed
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Figure 6. Thickness of the surface-attached monolayer as a function of the molecular weight of the fluoropolymer.
Figure 7. Zisman plot of various alkane test liquids (dodecane, hexadecane, decane, octane, heptane, hexane, cyclohexane) on FP5.
advancing and receding contact angles of 124° and 103°, respectively. This reveals the highly hydrophobic nature of the surfaces. The comparably low hysteresis proves that the films are stable and rather smooth, as roughness always induces a significant hysteresis. However, much more interesting than the surface tension against a single test liquid is to obtain information about the surface free energy of the modified surfaces. A good measure of the surface energy of the fluorinated substrate can be obtained from a Zisman plot, where the cosine of the contact angle of nonpolar test liquids is plotted against their surface tensions. Extrapolating the data obtained to complete wetting (cos θ ) 1) yields the “critical” surface tension of the surface (below which all liquids are expected to wet the substrate). The Zisman Plot for the fluorinated silicon substrate is shown in Figure 7. The critical surface tension was found to be close to 8 dyn/cm, which agrees well with previously reported values for films of the same polymer prepared by other techniques of film deposition. Such low values of the critical surface tension are possible when the CF3 groups of the fluoropolymer films segregate to the surface as discussed above. The use of UV light to bind the fluoropolymers to surfaces is also advantageous as it opens the door for facile surface patterning. Figure 8 shows an ellipsometric image of a patterned fluoropolymer film of 7 nm thickness obtained by irradiating the dip-coated substrate through a mask. Only those areas exposed to the UV radiation remain attached to the surface, while the fluorpolymer in the unexposed regions gets washed away during extraction. Thus, a surface is obtained in which the surface properties vary strongly from place to place, and substrates
Jeyaprakash S. Samuel and Ru¨ he
with spatially controlled wetting properties can be obtained, which was demonstrated by dipping the patterned substrate into heptadecane solvent.27 Heptadecane selectively wets only those parts of the surface where no fluoropolymer is present (Figure 8). The ability to precisely tailor microstructures through simple photochemical means makes this technique highly attractive for microfluidic applications where drag reduction and nonwetting of the side walls by solutions having a wide range of surface tensions play a very important role. Another major advantage of the photochemical process is the ability to create in a very simple way chemical patterns on the surface of the substrate. In the first step a polymer can be covalently attached to the surface by illumination through a mask as described above. During such a procedure, the polymer becomes attached only in the illuminated areas and the photoactive molecules are consumed, while in the shaded areas they remain unchanged and can be used for further functionalization steps. If the substrate is now coated with a different polymer and exposed to UV light once again, the second polymer becomes attached only in those areas where the photoactive moieties are still active. In the second step no exposure through a mask is required, and a simple flood exposure gives the desired structure as the photoactive molecules have been patterned in the first step and the noncoated areas are backfilled with the second polymer. In Figure 9 the generation of a very simple test structure with stripes of 400 µm width through this technique is shown. In the first step the substrate with the surfaceattached BP-silane was coated with a 100 nm thick pNIPAAM layer as described above. As a mask for micropatterning, a simple transparency foil was used onto which the desired pattern was printed with a laser printer. After UV irradiation through this mask the nonattached polymer was removed by extraction with methanol. Then the substrate was coated once again through dipping into a 40 mg/mL solution of FP5, subjected to UV irradiation, and finally extracted using TCTFE as solvent. Due to the low surface tension of the fluorinated polymer the whole substrate is covered with the fluorinated polymer during dip-coating. This, however, poses no problem as in the first step the BP units in the illuminated areas were more or less consumed. If any residual groups should have “survived” the photoprocess and still be active, the pNIPAAM layer, which is not swellable in the solution of the FP5 polymer, will prevent any reaction between the FP5 polymer and the BP units as the polymer is in the glassy state and thus will work as an efficient buffer between the fluoropolymer and the surface. A micrograph of the obtained patterned substrate obtained by imaging ellipsometry is shown in Figure 9a. The imaging conditions were chosen so that the 12 nm thick layer of the fluoropolymer film fulfills the nulling conditions and no light from these areas reaches the detector. The pNIPAAM film in contrast was only 4 nm thick, so that, in all areas where this polymer was deposited, the nulling conditions were not fulfilled and accordingly appear bright in the image. To prove that, apart from topological differences, lines of different chemical composition had actually been generated, an XPS line scan was performed at two different energy ranges corresponding to the F(1s) (682.2-689.8 eV) and N(1s) (398.2-404.2 eV) binding energies. The presence of alternating stripes of FP5 and pNIPAAM is confirmed by an increase of the F(1s) signal intensity for all areas where the fluoropolymer is present and by the increase of the N(1s) intensity and a concurrent decrease of the F(1s) signal intensity on areas where the pNIPAAM
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Figure 8. Photochemical patterning of the fluoropolymer monolayers: (a) imaging ellipsometry micrograph of a 7 nm thick FP film obtained through irradiation of a 50 nm thick FP film through a mask and subsequent extraction (the mask was a TEM grid); (b) the same film as in (a) after immersion into a hexadecane solution and withdrawal (image taken by a CCD camera).
described here, while the step size with which the sample was scanned was 15.6 µm. Accordingly, the signal intensity of the line scans represents a convolution of the chemical pattern with the spot size during the experiments. In subsequent studies the pattern quality will be elucidated in more detail. Conclusions
Figure 9. (a) Imaging ellipsometric micrograph of pNIPAAM/ FP5 microstructures. Dark areas correspond to regions where fluoropolymer and bright areas to regions where pNIPAAM have been attached to the surface. (b) Line scan of the same microstructure (O, F(1s) signals; 0, N(1s) signals). Further details are described in the text.
is present. However, it should be noted that in the experiments shown here no special effort was made concerning the resolution and the spot size of the XPS experiment, which was 150 µm in the experiments
Ultrathin fluorinated polymer layers have been generated with well-defined composition and film thickness using a facile room temperature photochemical process through which the polymer molecules become covalently attached to the surface of the substrate. The film thickness of the surface-attached polymer monolayers can be precisely controlled by choosing an appropriate molecular weight of the polymer (or to be more precise radius of gyration of the polymer, which is of course directly related to the former). This process seems to have an edge over conventional methods for the deposition of fluorinated polymers as the process is very simple and yields welldefined uniform films with low hysteresis and critical surface tension. The use of UV light to anchor the films to the surfaces provides a direct route to the generation of surface patterns which display a remarkably selective wetting of hydrophilic and hydrophobic liquids. This could serve as an important tool in modifying the surface energy of microfluidic devices for effective control of fluid flow. In the experiments described in this work a relatively simple chemical pattern was generated. Although the height differences between the different areas on the surface of the substrate are marginal (and can be virtually zero if the appropriate molecular weights are selected), the chosen example shows that a chemical micropatterning of the substrate can be easily achieved. If more elaborate step-and-repeat procedures and more sophisticated masking processes are used, the photoattachment of polymers allows a complex spectrum of different polymers to be bound on one substrate so that rather complicated chemical micropatterns can be generated quite easily. Acknowledgment. Technical assistance from Friedjof Heuck is gratefully acknowledged. LA049428S
