The Outermost Atomic Layer of Thin Films of Fluorinated

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The Outermost Atomic Layer of Thin Films of Fluorinated Polymethacrylates R. D. van de Grampel,†,‡ W. Ming,*,† A. Gildenpfennig,§ W. J. H. van Gennip,| J. Laven,† J. W. Niemantsverdriet,| H. H. Brongersma,§ G. de With,† and R. van der Linde† Laboratory of Coatings Technology, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Calipso BV, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Schuit Institute of Catalysis and Dutch Polymer Institute, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received February 24, 2004. In Final Form: May 12, 2004 In this paper, we investigate the surface properties of a series of copolymers of perfluoroalkyl methacrylate (CH2dC(CH3)COOCH2CnF2n+1, n ) 1, 6, or 10) and methyl methacrylate (MMA) and of blends of perfluorooctyl-end-capped poly(methyl methacrylate) (PMMA) and pure PMMA. The introduction of perfluoroalkyl groups significantly lowers the polymer surface energy as determined by the acid-base approach. X-ray photoelectron spectroscopy (XPS) confirms a higher fluorine concentration in the surface region (the outer 3.8 nm) as compared to in the bulk. The fluorine density in the outermost atomic layer is quantitatively determined by low-energy ion scattering (LEIS). A linear relationship is found between the fluorine density in the outermost atomic layer and the surface energy of the partially fluorinated polymethacrylates, irrespective of the length of the perfluoroalkyl chain. This linearity confirms Langmuir’s “principle of independent surface action”. Deviation from this linear relationship exists for both highly and sparsely fluorinated polymethacrylates and can be ascribed to the local (surface) ordering of the fluorinated tails and MMA units, respectively. This study may offer one further step toward a deeper understanding of the correlations between macroscopic surface properties and microscopic surface chemical composition.

1. Introduction Fluorinated surfaces of polymers have attracted much interest because of their exceptional properties, originating from the C-F bond. The low surface energy, low wettability, low adhesion, and low coefficient of friction associated with fluorinated surfaces play essential roles in microelectronic, antifouling, and medical applications. These properties of fluorinated surfaces depend not only on the coverage of the surface by fluorocarbons but also on the degree of order. For instance, a close-packed, uniformly organized array of perfluorolauric groups creates a surface with the lowest critical surface energy 6 mN/m.1 Understanding therefore the correlation of the physical chemical surface properties with the chemical surface composition is of paramount importance, in particular, when control of surface properties is desired. In this respect, numerous papers on polymer systems have been published dealing with the correlation of surface energy with surface structure, composition, and functionality.2 A wide variety of techniques are nowadays available for studying the molecular structures and chemical composition of fluorinated surfaces. X-ray photoelectron spectroscopy (XPS) is the most frequently used technique * Corresponding author. Email: [email protected]. Fax: +31-402445619. † Laboratory of Coatings Technology. ‡ Present address: General Electric Plastics, P.O. Box 117, 4600 AC Bergen op Zoom, The Netherlands. E-mail: [email protected]. § Calipso BV. | Schuit Institute of Catalysis and Dutch Polymer Institute. (1) Zisman, W. A. In Contact Angle, Wettability, and Adhesion; Fowkes, F. M., Ed.; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; Chapter 1.

and provides quantitative information on the surface composition. Angle-dependent XPS measurements have revealed surface enrichment of fluorine in the outer 10 nm in many cases.3 The combination of XPS and secondary ion mass spectroscopy (SIMS) has been used to elucidate the chemical composition in the upper 2-10 nm of a polymer layer.3e,4 Application of depth-profiling SIMS has identified gradient5 and lamellar structures3d of the fluorinated species perpendicular to the polymer surface. Genzer et al.6,7 performed near-edge X-ray absorption fine structure (NEXAFS) measurements to study the molecular orientation of semifluorinated ‘fingers’ attached to a (2) (a) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (b) Olbris, D. J.; Ulman, A.; Shnidman, Y. J. Chem. Phys. 1995, 102, 6865. (c) Ulman, A. Thin Solid Films 1996, 273, 48. (d) Mach, P.; Huang, C. C.; Nguyen, H. T. Phys. Rev. Lett. 1998, 80, 732. (e) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759. (f) Castner, D. G.; Lewis, K. B., Jr.; Fischer, D. A.; Ratner, B. D.; Gland, J. L. Langmuir 1993, 9, 537. (g) Lu¨ning, J.; Sto¨hr, J.; Song, K. Y.; Hawker, C. J.; Ionidice, P.; Nguyen, C. V.; Yoon, D. Y. Macromolecules 2001, 34, 1128. (h) Vargo, T. G.; Gardella, J. A., Jr. J. Polym. Sci., Part A: Polym. Chem. 1991, 29, 555. (i) Wang, J.; Ober, C. K. Macromolecules 1997, 30, 7560. (3) (a) Kassis, C. M.; Steehler, J. K.; Betts, D. E.; Guan, Z.; Romack, T. J.; DeSimone, J. M.; Linton, R. W. Macromolecules 1996, 29, 3247. (b) Takahashi, S.; Kasemura, T.; Asano, K. Polymer 1997, 38, 2107. (c) Bongiovanni, R.; Beamson, G.; Mamo, A.; Priola, A.; Recca, A.; Tonelli, C. Polymer 2000, 41, 409. (d) Bo¨ker, A.; Reihs, K.; Wang, J.; Stadler, R.; Ober, C. K Macromolecules 2000, 33, 1310. (e) Affrossman, S.; Bertrand, P.; Hartshorne, M.; Kiff, T.; Leonard, D.; Pethrick, R. A.; Richards, R. W. Macromolecules 1996, 29, 5432. (4) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Sauer, B. B.; Stika, K. M.; Swartzfager, D. G. Macromolecules 1997, 30, 2883. (5) van de Grampel, R. D.; van Gennip, W. J. H.; Wassing, B.; Krupers, M. J.; Laven, J.; Niemantsverdriet, J. W.; van der Linde, R. Polym. Mater. Sci. Eng. 2000, 83, 305. (6) Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang, J.; Ko¨rner, H.; Xiang, M.; Char, K.; Ober, C. K.; DeKoven, B. M.; Bubeck, R. A.; Chaudhury, M. K.; Sambasivan, S.; Fischer, D. A. Macromolecules 2000, 33, 1882.

10.1021/la049519p CCC: $27.50 © 2004 American Chemical Society Published on Web 06/18/2004

Thin Films of Fluorinated Polymethacrylates

styrene-isoprene block copolymer at the surface. An average tilt angle with respect to the surface normal in the range 29-46° was found, depending on the total length of the side chain and the length of the hydrogen-carbon spacer between the perfluoroalkyl side chains and the backbone. All surface methods mentioned so far provide useful information on the chemical composition and orientation near the surfaces, but none of them can exclusively analyze the composition of the outermost atomic layer. In 1916, Langmuir8 already anticipated that surface properties are in principle determined by the composition and packing of the atoms or groups of atoms in the first atomic layer. Zisman et al.1,9 reported that the surface energy of hydrocarbon and fluorocarbon materials decreases in the order -CH2 > -CH3 > -CF2 > -CF3. More recently, Mach et al.2d observed a dramatic change in the surface energy by a single atom replacement within a submolecular length scale at the surface. Besides, from the nature of the forces involved, for example, van der Waals interactions, it is also expected that the outermost atomic layer of the surface predominately determines the wettability. Consequently, the wettability of a surface is not related to the overall chemical composition, but it rather depends on the chemical nature of the outermost atomic layer in particular. Therefore, the characterization of the outermost atomic layer is crucial. Low-energy ion scattering (LEIS) is a unique technique that exclusively probes the first atomic layer (even when the sample is not smooth).10 The principles of LEIS, also referred to as ion scattering spectroscopy (ISS), are well documented in literature.10,11 Most of the surface characterization studies involving LEIS have been carried out in the field of metals and semiconductors.12 The application of LEIS toward insulating samples such as organic polymer surfaces is limited and requires detection of functional groups of the polymer chain.13-16 Gardella et al.13,14 showed, for example, that LEIS can distinguish conformations between stereoregular poly(methyl methacrylate) (PMMA) and random copolymers of methacrylic acid and methyl methacrylate (MMA), whereas XPS or SIMS could not have resolved such information. Affrosman et al.15 showed the surface segregation phenomenon of perfluoroalkyl-end-capped polystyrene with a combination of LEIS, XPS, and SIMS analyses. Using LEIS, it is also possible to follow changes in the surface composition due to segregation, aging, etc. In this respect, Maas et al.16 presented a study of aging of polymer surfaces, where it was found that aging is mainly confined to the outermost atomic layer. (7) Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang, J.; Ko¨rner, H.; Xiang, M.; Char, K.; Ober, C. K.; DeKoven, B. M.; Bubeck, R. A.; Fischer, D. A.; Sambasivan, S. Langmuir 2000, 16, 1993. (8) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (9) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1954, 58, 236. (10) Brongersma, H. H.; Mul, P. M. Chem. Phys. Lett. 1972, 14, 380. (11) Vargo, T. G.; Gardella, J. A., Jr.; Schmitt, R. L.; Hook, K. J.; Salvati, L., Jr. In Surface Characterization of Advanced Polymers, 1st ed.; Sabbatini, L., Zambonin, P. G., Eds.; VCH: New York, 1993; Chapter 4. (12) (a) Viitanen, M. M.; Jansen, W. P. A.; van Welzenis, R. G.; Brongersma, H. H.; Brands, D. S.; Poels, E. K.; Bliek, A. J. Phys. Chem. B 1999, 103, 6025. (b) Jansen, W. P. A.; Ruitenbeek, M.; Denier van de Gon, A. W.; Geus, J. W.; Brongersma, H. H. J. Catal. 2000, 196, 379. (13) Hook, T. J.; Schmitt, R. L.; Gardella, J. A., Jr.; Salvati, L., Jr.; Chin, R. L. Anal. Chem. 1986, 58, 1285. (14) Hook, K. J.; Gardella, J. A., Jr.; Salvati, L., Jr. Macromolecules 1987, 20, 2112. (15) Affrosman, S.; Bertrand, P.; Hartshorne, M.; Kiff, T.; Leonard, D.; Pethrick, R. A.; Richards, R. W. Langmuir 1996, 12, 5432. (16) Maas, A. J. H.; Viitanen, M. M.; Brongersma, H. H. Surf. Interface Anal. 2000, 30, 3.

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The sensitivity of LEIS differs from element to element. It depends on the mass and nuclear charge of the projectile ion, the mass of the target atom, and the energy levels of, in particular, the inner-shell electrons of the target atom in relation to the levels of the ion. Because of this complexity, an absolute calibration of the LEIS signal is only possible with a reference sample with a well-defined composition in the outermost atomic layer. Recently, we presented a new method for determining quantitatively the fluorine density in the first atomic layer, using a LiF single crystal as the calibration standard.17 In this paper, we investigate the surface properties of a series of copolymers of perfluoroalkyl MMA and MMA and of blends of perfluorooctyl-end-capped PMMA and pure PMMA by contact angle measurements and XPS. We determine quantitatively the fluorine density in the outermost atomic layer of these films by LEIS and attempt to correlate the surface energy with the fluorine density in the outermost atomic layer of these fluorinated films. 2. Experimental Section 2.1. Materials. MMA, 2,2,2-trifluoroethyl methacrylate (F1MA), methacrylic anhydride (94%), triethylamine (TEA, 99.5%), and 4-(dimethylamino)pyridine (DMAP, 99+%) were purchased from Aldrich and used as received. 1,1-Dihydroperfluoroheptyl methacrylate (F6MA, ACR Technologies B.V. Amsterdam) was used as received. 1H,1H-Perfluoroundecane1-ol (96%, Fluorochem) was sublimed prior to use. Azobisisobutyronitrile (AIBN, Merck) was recrystallized from methanol and stored in a refrigerator prior to use. All solvents (analytical grade) were used as received. 2.2. Monomer and Polymer Synthesis. 2.2.1. 1H,1HPerfluorodecyl Methacrylate (F10MA). A 250 mL three-neck round-bottom flask, equipped with a magnetic stirrer, a reflux condenser, an additional funnel, and a septum, was charged with 20.2 g (36.7 mmol) of 1H,1H-perfluoroundecane-1-ol and 0.5 g (4.2 mmol) of DMAP. With a syringe, 100 mL of freshly distilled dichloromethane and 22 mL of 1,1,2-trichlorotrifluoroethane was added. After the perfluoro alcohol and DMAP were dissolved, the solution was cooled with ice. Slowly, 5.3 mL (34.2 mmol) of methacrylic anhydride was added to the cooled solution, followed by the addition of 4.7 mL (38.5 mmol) of TEA. The solution was stirred for 18 h at 30 °C and subsequently washed with deionized water, diluted hydrochloric acid, saturated sodium bicarbonate solution, and deionized water. After drying the organic phase with anhydrous MgSO4, the solvent was removed, resulting in a yellow liquid. The monomer was further purified by passing it over a short column of neutral Al2O3 with dichloromethane as eluent. The solvent was removed under reduced pressure, resulting in a white waxy compound, which was stored under nitrogen at 4 °C prior to use. Yield, 3 g (45%); purity, >99.5%; GC/MS, m/z ) 618. 1H NMR (CDCl3): δ 2.0 (s, 3H, sCH3), 4.7 (t, 2H, sOsCH2s), 5.7 (s, 1H), 6.2 (s, 1H). 13C NMR (CDCl3): δ 18.1 (s, sCH3), 60.0 (t, sCH2sCF2s), 100-125 (fluorinated carbon region), 127.8 (s, sCdCH2s), 134.8 (s, sCdCH2s), 165.6 (s, sCdO). 19F NMR (CDCl3): δ -126.6 (m, 2F, sCF2sCF3), -123.7 (m, 2F, sCF2sCF2sCF3), -123.1 (m, 2F, sCF2sCF2s CF2sCF3), -122.3 (m, 10F, sCF2sCF2sCF2s), -119.8 (m, 2F, sCF2sCH2s), -81.2 (m, 3F, sCF3). 2.2.2. Homopolymers and Copolymers. Copolymerizations of the perfluoroalkyl methacrylate (F1MA, F6MA, or F10MA; see Figure 1) with MMA were carried out under “starved” conditions as described before.18 Homopolymers of F1MA and F6MA were synthesized via a bulk polymerization. PMMA and perfluorooctyl-end-capped PMMA (C8F17-PMMA, Figure 1) were synthesized by atom transfer radical polymerization (ATRP) with ethyl 2-bromoisobutyrate and 2-(perfluorooctyl)ethyl 2-bromoisobutyrate as initiators, respectively; details (17) van de Grampel, R. D.; Ming, W.; Gildenpfennig, A.; Laven, J.; Brongersma, H. H.; de With, G.; van der Linde, R. Langmuir 2004, 20, 145. (18) van de Grampel, R. D.; van Geldrop, J.; Laven, J.; van der Linde, R. J. Appl. Polym. Sci. 2001, 79, 159.

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van de Grampel et al. Table 1. Surface Energy Components for the Test Liquids Used wetting liquid deionized water diiodomethane 1,2-ethylenediol

Figure 1. Chemical structures of the methacrylate monomers used and C8F17-PMMA synthesized by ATRP. were given elsewhere.19 The number-average molecular weight (Mn) of PMMA and C8F17-PMMA was 3300 and 4000 g/mol, respectively, with a polydispersity of 1.3. Their glass transition temperatures were determined to be 83.6 and 84.4 °C, respectively. 2.3. Film Preparation. Thin polymer films were spin-coated on silicon wafers using solutions of the polymers with concentrations of 100 mg in 1 g of toluene, 1,1,2-trichlorotrifluoroethane, or 1,1,1,3,3,3-hexafluoropropan-2-ol depending on the fluorine concentration. The solutions obtained were filtered through a 0.2 µm Teflon filter prior to spin-coating (5000 rpm for 60 s). After spin-coating, the polymer films were annealed for 2 h at 125 °C in an oven. By this procedure, the silicon wafer was completely covered with the polymers, resulting in a film free of cracks with a layer thickness of 150-200 nm as determined by atomic force microscopy. For the films from the blends of PMMA and C8F17-PMMA, fluorine concentrations were adjusted by varying the ratio between PMMA and C8F17-PMMA. 2.4. Bulk Characterization Techniques. NMR spectra were recorded on a Varian 400 spectrometer at 25 °C. Operating frequencies were 400.16 MHz for 1H, 100.63 MHz for 13C, and 376.49 MHz for 19F. Tetramethylsilane (TMS) (1H and 13C) was used as an internal standard, and CFCl3 (19F) was used as an external reference. Elemental analyses (C, H, and F) of the copolymers were carried out at the Microanalytical Department of the University of Groningen. Molecular weights were obtained by means of gel permeation chromatography (GPC) using four PLGel (Mixed-C) columns (Polymer Laboratories) at 40 °C. The injection volume was 100 µL, and tetrahydrofuran (THF) was used as eluent at a flow rate of 1 mL/min. A Waters 410 differential refractometer was applied for detection. Narrow-polydispersity polystyrene standards with molecular weights ranging from 580 to 7.1 × 106 g/mol were used to calibrate the GPC setup. Glass transition temperatures were measured with a Perkin-Elmer Pyris 1 differential scanning calorimetry (DSC) instrument using a scan rate of 10 K/min with sample masses of 5-10 mg. Transition temperatures were taken from the second heating cycle. 2.5. Surface Characterization Techniques. 2.5.1. Contact Angle Measurements and Surface Energy Calculations. Advancing contact angles were obtained by means of the sessile drop method, using a Kru¨ss G10 setup. During the measurement of advancing contact angles, the needle remained inside the drop. The droplet was monitored by a charge-coupled device (CCD) camera and analyzed by drop shape analysis software (DSA, version 1.0, Kru¨ss). The complete profile of the sessile droplet was fitted by the tangent method to a general conic section equation. The derivative of this equation at the baseline gives the slope at the three-phase contact point and thus the contact (19) (a) Snijder, A. Ph.D. Dissertation, Eindhoven University of Technology, 2002. (b) Ming, W.; van de Grampel, R. D.; Gildenpfennig, A.; Snijder, A.; Brongersma, H. H.; van der Linde, R.; de With, G. Polym. Mater. Sci. Eng. 2003, 88, 517.

γLW γ+ γγAB γlv lv lv lv lv (mN/m) (mN/m) (mN/m) (mN/m) (mN/m) 21.8 50.8 29.0

25.5 0 1.9

25.5 0 17.0

51.0 0 19.0

72.8 50.8 48.0

angle. In this way, angles were determined at both the right and the left side. Reproducibility was within 0.5°. Surface energies of the polymer films were evaluated using the surface energy component approach following the three-liquid Lifshitz-van der Waals acid-base (LWAB) method,20 using advancing angles for the calculation.21 The surface energies of the liquids (γlv’s) used as well as their Lewis acid components (γ+ lv’s) and Lewis base ’s) are listed in Table 1. In view of the discussions components (γlv associated with the use of hydrocarbon liquids for the evaluation of the surface energies of perfluorinated solid surfaces,2a,22 the surface energy of poly(tetrafluoroethylene) (PTFE) was always measured for a comparison. 2.5.2. XPS Measurements. XPS measurements were performed using a non-monochromized VG Mg KR X-ray beam operated at 20 mA emission at 12.5 kV and a VG Clam II hemispherical analyzer. As fluoropolymers can undergo degradation when irradiated by X-rays,23 XPS measurement time was limited to 10 min or less. Under these conditions, surface damage due to radiation is minimized. All C1s peaks corresponding to hydrocarbons were calibrated at a binding energy of 285.0 eV to correct for the energy shift caused by charging. For peak fitting, the program XPSPEAK, version 4.0, of Kwok24 was used. All peaks were fitted with a 30% Lorentzian and a 70% Gaussian peak shape. 2.5.3. LEIS Measurements. LEIS measurements were performed with the Calipso setup,25 which is an improved version of the ERISS setup. It has a base pressure of 2 × 10-10 mbar. All LEIS measurements were carried out using 3 keV 3He+ ions. In this way, a high sensitivity to light elements is obtained. The kinetic energy of the ions backscattered by 145° is analyzed by a double toroidal analyzer and a position-sensitive detector. A shower of low-energy electrons during the LEIS measurements prevents charging of nonconducting samples. To avoid crosscontamination, only one sample at a time was present in the LEIS ultrahigh vacuum (UHV) analysis chamber. All experiments were carried out under static conditions. This means that the ion and electron doses were kept sufficiently low to avoid damage of the sample. To obtain these static conditions, the ion beam was rastered over a sample area of 1.5 × 1.5 mm2. The total ion dose was kept below 3 × 1013 3He+/cm2 (typical beam current, ∼0.5 nA; typical measuring time, 99.5%. The synthesized F10MA, F1MA, and F6MA monomers were used in free radical homopolymerization and copolymerizations with MMA. 3.2. Partially Fluorinated Polymethacrylates. Copolymers of the perfluoroalkyl methacrylate (F1MA, F6MA, or F10MA; see Figure 1) with MMA were synthesized by free radical polymerization at 80 °C under nitrogen.18 Copolymerization parameters point to almost “ideally random” copolymers. Moreover, polymerizations were quenched with methanol directly after the complete addition of the two monomers in order to minimize composition drift. Therefore, a low conversion (