pubs.acs.org/Langmuir © 2009 American Chemical Society
Effects of Ionic Side Groups Attached to Polydimethylsiloxanes on Lubrication of Silicon Oxide Surfaces Erik Hsiao, Don Kim,† and Seong H. Kim* Department of Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802. † On sabbatical leave from Pukyong National University, Korea. Received March 16, 2009. Revised Manuscript Received May 1, 2009 The boundary film formation and lubrication effects of low molecular weight silicone lubricant molecules with cationic side groups were studied. Poly(N,N,N-trimethylamine-3-propylmethylsiloxane-co-dimethylsiloxane) iodide was synthesized and deposited on silicon oxide surfaces to form a bound-and-mobile lubricant film. The bound nature was investigated with ellipsometry, water contact angle, and X-ray photoelectron spectroscopy for the polymers with cationic mole percent of 6, 15, and 30 mol % (monomer based). The bound layer thickness decreased as the cationic content increased. The quaternary ammonium cations in this layer were electrostatically bound to the substrate surface. The mobile nature of the multilayers was explored with scanning polarization force microscopy. The multilayer films exhibited characteristic topographic features due to ionic interactions within the polymer film. Contact scratching of these films altered the multilayer topography within the contact scanned area. Even after high load contact scanning, the bound layer was not removed from the scanned region. These results implied that the molecules in the first layer are strongly bound and the molecules in the multilayers are mobile. Both nanoscale and macroscale tribological tests of these films revealed that the polymer with 15 mol % cationic groups gives lower friction and adhesion than the 6 and 30 mol % cationic polymers as well as the polydimethylsiloxane control sample. This seems to be due to a synergistic effect between the bound and the mobile layers.
I. Introduction No matter how small the mechanical devices are, lubrication is needed. In fact, the need for good lubrication becomes even larger as the physical size decreases. This is because the surface areato-volume ratio becomes larger at smaller scales so that surface forces such as adhesion and friction are more significant than body forces such as gravity. Microelectromechanical systems (MEMS) fall in this category. MEMS are usually fabricated from silicon-based materials using lithographic techniques.1 However, silicon has poor tribological properties such as high friction, high adhesion, and low wear resistance.2 Without proper lubrication methods being developed yet, commercially available MEMS are presently limited to the ones that do not contain sliding or rubbing parts. In order to enable a full spectrum of mechanical motions in MEMS, a good boundary lubricant is needed to reduce adhesion, lower friction, and prevent wear. Various lubrication approaches have been investigated to prolong the operation lifetime of MEMS. They include self-assembled monolayers (SAMs),3-7 *Corresponding author. E-mail:
[email protected]. (1) Kim, S. H.; Asay, D. B.; Dugger, M. T. Nano Today 2007, 2, 22. (2) Komvopoulos, K. Wear 1996, 200, 305. (3) Ashurst, W. R.; Yau, C.; Carraro, C.; Maboudian, R.; Dugger, M. T. J. Microelectromech. Syst. 2001, 10, 41. (4) Jun, Y.; Zhu, X. Y. J. Adhes. Sci. Technol. 2003, 17, 593. (5) Romig, A. D.; Dugger, M. T.; McWhorter, P. J. Acta Mater. 2003, 51, 5837. (6) Salmeron, M. Tribol. Lett. 2001, 10, 69. (7) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (8) Eapen, K. C.; Patton, S. T.; Smallwood, S. A.; Nainaparampil, J. J. Surf. Coat. Technol. 2005, 197, 270. (9) Smallwood, S. A.; Eapen, K. C.; Patton, S. T.; Zabinski, J. S. Wear 2006, 260, 1179. (10) Kasai, P. H. J. Info. Stor. Proc. Syst. 1999, 1, 23. (11) Ma, X.; Gui, J.; Smoliar, L.; Grannen, K.; Marchon, B.; Bauer, C. L.; Jhon, M. S. Phys. Rev. E 1999, 59, 722. (12) Waltman, R. J.; Khurshudov, A.; Tyndall, G. W. Tribol. Lett. 2002, 12, 163. (13) Xu, L.; Ogletree, D. F.; Salmeron, M.; Tang, H. A.; Gui, J.; Marchon, B. J. Chem. Phys. 2000, 112, 2952.
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perfluoropolyethers (PFPEs),8-17 bound-plus-mobile phase lubricants,17-21 and ionic liquids.22-27 Although these approaches show some improvements in MEMS lubrication, they do not work sufficiently enough to allow the operation of MEMS with sliding parts for an extended lifetime. As an alternative to the coating-based boundary film lubrication approach, the vaporphase lubrication (VPL) has recently been proven to be very promising.28-30 The main difference of VPL from other lubricant coating approaches is that it allows continuous replenishment of lubricant molecules from the vapor phase, rather than relying on an one-time loaded coating layer. However, there still exists the need to develop efficient boundary lubrication films that can work without the replenishment from the vapor phase and be incorporated into the forthcoming MEMS technology. (14) Waltman, R. J.; Tyndall, G. W.; Wang, G. J.; Deng, H. Tribol. Lett. 2004, 16, 215. (15) Waltman, R. J.; Zhang, H.; Khurshudov, A.; Pocker, D.; Karplus, M. A.; York, B.; Xiao, Q. F.; Zadoori, H.; Thiele, J. U.; Tyndall, G. W. Tribol. Lett. 2002, 12, UNSP 1023. (16) Bowles, A. P.; Hsia, Y. T.; Jones, P. M.; Schneider, J. W.; White, L. R. Langmuir 2006, 22, 11436. (17) Eapen, K. C.; Patton, S. T.; Zabinski, J. S. Tribol. Lett. 2002, 12, 35. (18) Choi, J. H.; Kawaguchi, M.; Kato, T. Tribol. Lett. 2003, 15, 353. (19) Eapen, K. C.; Patton, S. T.; Smallwood, S. A.; Phillips, B. S.; Zabinski, J. S. J. Microelectromech. Syst. 2005, 14, 954. (20) Nainaparampil, J. J.; Eapen, K. C.; Zabinski, J. S. J. Vac. Sci. Technol., B 2004, 22, 2337. (21) Satyanarayana, N.; Sinha, S. K. J. Phys. D: Appl. Phys. 2005, 38, 3512. (22) Fox, M. F.; Priest, M. Proc.;Inst. Mech. Eng. 2008, 222, 291. (23) Liu, W. M.; Ye, C. F.; Gong, Q. Y.; Wang, H. Z.; Wang, P. Tribol. Lett. 2002, 13, 81. (24) Phillips, B. S.; John, G.; Zabinski, J. S. Tribol. Lett. 2007, 26, 85. (25) Ye, C. F.; Liu, W. M.; Chen, Y. X.; Yu, L. G. Chem. Commun. 2001, 2244. (26) Liu, W.; Ye, C.; Chen, Y.; Ou, Z.; Sun, D. C. Tribol. Int. 2002, 35, 503. (27) Ye, C. F.; Liu, W. M.; Chen, Y. X.; Ou, Z. W. Wear 2002, 253, 579. (28) Asay, D. B.; Dugger, M. T.; Kim, S. H. Tribol. Lett. 2008, 29, 67. (29) Asay, D. B.; Dugger, M. T.; Ohlhausen, J. A.; Kim, S. H. Langmuir 2008, 24, 155. (30) Strawhecker, K.; Asay, D. B.; McKinney, J.; Kim, S. H. Tribol. Lett. 2005, 19, 17.
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SAMs are one of the most extensively studied forms of boundary lubricant.3-7 SAMs typically consists of well-packed alkyl chains that are covalently bonded to the substrate surface. They can be synthesized with varying chain lengths and different functional groups. This allows researchers to study various aspects of chain packing and chemistry.7 For silicon surfaces, the most studied SAMs are long chain alkyl or fluoroalkyl groups attached to the surface via the silane linkage chemistry. It has been reported that the friction response depends on the degree of packing of the SAMs.6 When SAMs are not well packed, they are subject to high friction and wear. The well-packed SAMs seem to support higher contact pressures than the poorly packed ones.6 However, even the best SAM films wear off during repeated mechanical contacts. Once the SAM film is worn off, the bare surface gets exposed, leading to high friction and wear. The critical property that is missing in the SAM approach is that the worn layer cannot be healed during the operation of MEMS devices. Because of the self-limiting capability of the film thickness in the SAM deposition chemistry, there are no additional molecules in the film that can diffuse to the worn surface region and heal it. PFPEs are widely used in the magnetic recording industry. One of the desired properties of PFPEs is that they have the lateral mobility to diffuse and recover the worn surface regions. PFPEs interact with the substrate mostly via van der Waals interactions. PFPEs with hydroxyl terminal groups, such as Fomblin Z-dol, can have additional hydrogen-bonding interactions with the substrate, which helps the boundary film formation. By annealing them at a high temperature (typically 100-175 °C), the hydrogenbonded fraction can be increased. The unbound molecules will readily be available for diffusion and self-healing.17 One difficulty of applying the PFPE-based boundary film approach to MEMS is that PFPEs degrade on bare silicon surfaces over time, losing their lubricious properties.8,9 In order to prevent or delay this degradation process, a protective carbon coating could be applied to the silicon-based MEMS surface.8 However, this can pose additional challenges in device fabrications.1,8 There have been a significant amount of efforts to develop boundary lubrication films that have the bound and mobile natures at the same time.17-21 This is the bound-plus-mobile two-phase lubrication concept. The main idea is that the bound phase strongly interacts with the substrate to protect the surface, and the mobile phase can diffuse and heal the worn bound layer region. Examples are to use long chain alcohols and diols, such as 1-decanol and 1,7-heptanediol, as a bound phase and pentaerythritol ester (PETH), poly(alkylcyclopentane) (Penzanne), and polysilane as a mobile phase.19 As in the Fomblin Z-dol case, the bound phase is annealed to form hydrogen bonds with the surface oxygen species. The mobile phase is coated subsequently over the bound phase to form additional lubricant layers. The best bound-plus-mobile phase combination reported so far is 1,7-heptanediol and PETH.19 The 1,7-heptanediol can interact with the substrate via one hydroxyl group and with the ester of PETH via the other hydroxyl group. This prevents the separation of the mobile phase from the bound phase. Another boundplus-mobile phase lubricant example uses SAMs as a bound phase and PFPEs as a mobile phase.18,21 In this approach, if the bound phase and mobile phase do not have favorable interactions with each other, then the synergy effect of having these two phases is not guaranteed. As an effort to develop a more efficient boundary film lubrication method, a new bound-and-mobile lubricant molecule is proposed. Instead of physically adding two phases (one bound and the other mobile), one can combine these two Langmuir 2009, 25(17), 9814–9823
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Figure 1. Conceptual illustration of the bound-and-mobile CPL film. The x and Q symbols represent cations and anions, respectively.
attributes into one molecule. Then, the synergy effect of having these two phases is guaranteed. Low molecular weight polymers with ionic side groups can be used for this purpose. Figure 1 schematically illustrates the bound and mobile nature of the proposed molecule. The use of ionic groups is inspired by the recent success of using room temperature ionic liquids as lubricants. Ionic liquids reportedly provide high load carrying capabilities and good lubricities.22-27 A part of the reason for this success is that the anions can react with the substrate to form surface films, and the lubricious cationic groups are bound to this film via electrostatic interactions. The strong electrostatic interactions prevent the lubricant molecules from being squeezed out of the contact area. However, the ionic liquids are highly toxic and subject to decomposition and degradation under tribological conditions.24 By using ions with much simpler structures than the ones used in ionic liquids, one can produce less toxic and more stable lubricants that can form a strongly bound film via electrostatic interactions. The polymer backbones carrying ionic groups can provide the flexibility needed for good lubricity. The flexible polymer backbone also can render the lateral mobility needed for self-healing of the bare exposed surfaces nearby. In this paper, low molecular weight polydimethylsiloxanes with cationic side groups are reported as a bound-and-mobile lubricant molecule. The base structure of the lubricant is poly(N,N,Ntrimethylamine-3-propylmethylsiloxane-co-dimethylsiloxane) iodide. The mole percentage of monomer units containing cationic groups are 6%, 15%, and 30%. Hereafter, these molecules will be called cationic polymer lubricants (CPLs) with their ionic mole fractions. The bound nature of CPL was investigated with water DOI: 10.1021/la900921m
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Figure 2. Synthesis of CPL by hydrosilylation reaction of poly(methylhydrosiloxane-co-dimethylsiloxane) followed by reaction with methyl iodide.
contact angle, ellipsometry, and X-ray photoelectron spectroscopy (XPS). The mobile CPL layer was probed with atomic force microscopy (AFM) and scanning polarization force microscopy (SPFM). In the friction test at both nano- and macroscales, the CPL films show an interesting ionic content dependence. The 15% CPL multilayer exhibited the lowest friction and adhesion and worked much better than the pure polydimethylsiloxane lubricant.
II. Experimental Details The CPL molecules were synthesized in a two-step process (Figure 2).31 The first step is the hydrosilylation reaction of random copolymer poly(methylhydrosiloxane-co-dimethylsiloxane) with N,N-dimethylallylamine in the presence of the PtO2 catalyst at 50 °C. The second step is to react the tertiary amine groups of the polymer with methyl iodide to form quaternary ammonium salts. The poly(methylhydrosiloxaneco-dimethylsiloxane) random copolymers with a molecular weight of 2000 g/mol were obtained from Gelest with methylhydrosiloxane mole percents of 6%, 15%, and 30%. The N,Ndimethylallylamine was obtained from Sigma-Aldrich. The methyl iodide and the PtO2 catalyst were obtained from Alfa Aesar. A 2000 g/mol molecular weight polydimethylsiloxane (PDMS) was purchased from Gelest as a control sample for friction tests. All chemicals were used without further purification. 1H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy were used to analyze the synthesized and purified products. The thermal stability of the synthesized polymers was analyzed with differential scanning calorimetry (DSC). These data are presented in the Supporting Information. The substrates used were 500 μm thick silicon (100) wafers (elastic modulus = 160 GPa, Poisson ratio = 0.27) with amorphous native SiO2 layer. The silicon wafers were cleaned with RCA-1 process (mixture of 5:1:1 Millipore water:30% hydrogen peroxide:30% ammonium hydroxide at 70 °C for 30 min). The RCA-1 cleaning ensures that organic contaminants are removed from the native SiO2 layer. The CPL films were deposited on the substrate by spin-coating. Since CPL is not readily soluble in water, CPL was first dissolved in ethanol and then mixed with water. The solutions of 0.5-1.5 wt % CPL in a 9:1 water:ethanol mixture were used for spin-coating. PDMS was spin-coated from a chloroform solution. The deposited CPL films were characterized with ellipsometry, water contact angle, and XPS. An ellipsometer (Stokes LSE, wavelength = 632.8 nm, incidence angle = 70°) was used to measure the film thickness. The refractive index of PDMS (1.45) was used to calculated the film thickness. For every film thickness measurement, at least five different spots were measured and averaged. An automated goniometer (First Ten Angstrom) was (31) Sabourault, N.; Mignani, G.; Wagner, A.; Mioskowski, C. Org. Lett. 2002, 4, 2117.
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used to measure the water contact angle. A droplet of 10 μL of Millipore water was viewed with a magnifying camera to measure the water contact angles of the films. For every water contact angle measurement, at least three different spots were measured and averaged. XPS (Kratos Analytical Axis Ultra X-ray) was used to investigate the chemical status of N and I in the CPL film. The photon energy was 1486.7 eV from a monochromatic Al KR source. Survey scans were obtained with a pass energy of 80 at 0.5 eV scan step size. High-resolution scans were conducted with a pass energy of 40 eV at 0.3 eV scan step size. The C 1s from the survey scan was used to compensate for the charging by normalizing it to 284.5 eV. AFM (a Molecular Imaging Pico-SPM microscope with a RHK SPM 100 controller) was used to measure nanoscale friction coefficients during contact scanning and obtain topographic and surface potential difference images in SPFM. SPFM imaging and contact scratch testing used AFM tips on rectangular silicon cantilevers (MikroMasch CSC12/15, spring constants = 0.08 -0.13 N/m, resonance frequencies = 18-23 kHz). The AFM tips used to obtain friction-load curves were rectangular silicon cantilevers (MikroMasch CSC37/AlBs, spring constants ranging = 0.5-0.8 N/m, resonance frequencies = 30-35 kHz). All AFM tips used were calibrated with the modified direct force balance method (DFBM)32 for lateral force and the Sader method33 for normal force. Friction-load curves were collected during 1 μm line scans at 2 μm/s while the applied load was decreased from ∼150 nN until the tip was snapped off the surface. The friction-load curves were measured while the applied load was ramped down and up as well. In order to avoid crosscontamination, different tips were used for each film. The highest applied load corresponds to a Hertzian pressure of 5.5 GPa. SPFM provided noncontact images of the topography and surface potential difference between the tip and the surface. An ac electric field in a frequency range (ω) between 11 and 15 kHz was applied to the AFM tip. The cantilever deflection due to electrostatic interaction between the tip and the sample was monitored with two lock-in amplifiers. One lock-in monitored the second harmonic signal (2ω) which was governed by the surface topography and dielectric constants of the film. This signal was used for feedback control of the tip-sample distance. During the SPFM imaging, the tip was positioned at 10-15 nm above the sample. The other lock-in monitored the magnitude and phase of the first harmonic signal (1ω). This signal was proportional to the surface potential difference between the tip and the sample. More details of SPFM can be found in the literature.34 SPFM images were obtained for a 4 μm 4 μm area at a 2 μm/s scan speed. Contact mode scanning at high applied loads (100-300 nN) over 1 μm 1 or 2 μm 2 μm areas at 2 μm/s was used to scratch the CPL film in the mobility test. The drift of AFM during the imaging mode change was negligible. (32) Asay, D. B.; Kim, S. H. Rev. Sci. Instrum., in press. (33) Sader, J. E.; Chon, J. W. M.; Mulvaney, P. Rev. Sci. Instrum. 1999, 70, 3967. (34) Hu, J.; Xiao, X. D.; Salmeron, M. Appl. Phys. Lett. 1995, 67, 476.
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Hsiao et al. A home-built pin-on-disk tribometer was used to measure friction at the macroscale. The system consisted of a 3 mm fused silica ball (elastic modulus = 70 GPa, Poisson ratio = 0.17) rubbing against a flat substrate. The ball was fixed to a stationary pin to which 50 g load was applied. This corresponds to an average Hertzian contact pressure of 0.36 GPa. The pin was moving in a linear motion at 0.5 mm/s, while the lateral force exerted to the pin was recorded with a strain gauge sensor. The strain gauge sensor was calibrated by placing known weights on top and measuring the voltage signal corresponding to a given load. The friction coefficient was calculated using Amonton’s law (friction force = friction coefficient normal load).
III. Results A good boundary lubricant film should have both bound and mobile characteristics. The bound nature helps the lubricant molecules to support high mechanical loads, while the mobile nature renders the good lubricity and self-healing capability, as illustrated in Figure 1. The bound layer formation will be discussed first, followed by the mobile property of the multilayer film. Then, the lubrication behavior of CPL films will be presented. III.a. Formation of the Bound CPL Layer. The formation of the bound layer was confirmed by checking the presence of the polymer layer with a constant thickness remaining on the substrate after thoroughly rinsing thick films with water or ethanol. For CPL, the bound layer can be formed through electrostatic interactions with the substrate. Since the isoelectric point of SiO2 is 2-3, the oxide surface will be negatively charged in neutral aqueous solution. The cations in CPL can directly interact with the negatively charged oxide surface. Once formed, the electrostatic interactions between the cations and the surface charge prevent the desorption of the polymer molecules. This layer will not be removed by simple washing with solvent unless ionexchange reactions take place. Another possibility is that quaternary ammonium salts react with surface oxygen species and form an amine oxide moiety, polymer-propyl-(CH3)2N+-O--surface. Ellipsometry was use to measure the thickness of the spincoated films and the bound layer remaining on the substrate after thorough rinsing with solvents. Both water and ethanol were used as a solvent. The results were the same regardless which solvent was used. Table 1 shows the thicknesses of the spin-coated CPL films before and after rinsing with the solvent. It should be noted that the spin-coated CPL films show characteristic topographic features in SPFM (details in section III.b). So the thickness measured for these films with ellipsometry is only a rough estimate. The bound layer is smooth and did not show any topographic features in SPFM; so, the bound layer thickness determined with ellipsometry is accurate. The bound layer thickness is constant irrespective of the initial spin-coated film thickness and remains unchanged even after immersion in the solvent for a few days. The CPL bound layer thickness varies with the ionic content in the polymer. It decreases from ∼1.2 nm to ∼0.8 nm and ∼0.7 nm as the ionic content increases from 6% to 15% and 30%, respectively. The average separations between ionic groups are 16, 6, and 2 monomer units for 6%, 15%, and 30%, respectively. Maximizing the interactions between the ionic groups and the surface seems to result in more bent loops of the polymer backbone at the lower ionic content CPL molecule, giving rise to a thicker film.35 In addition, as the (35) Claesson, P. M.; Dedinaite, A.; Rojas, O. J. Adv. Colloid Interface Sci. 2003, 104, 53.
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spin-coated
after rinsing with water or ethanol
98 ( 1°/3.4 ( 0.6 nm 96 ( 1°/3.4 ( 0.7 nm 93 ( 3°/3.9 ( 0.6 nm 96 ( 3°/3.5 ( 0.5 nm
51 ( 6°/1.6 ( 0.1 nm 98 ( 1°/1.2 ( 0.2 nm 93 ( 1°/0.8 ( 0.2 nm 99 ( 1°/0.7 ( 0.2 nm
cationic group mole fraction increases in the polymer, the CPL molecules will assume a more stretched conformation in the aqueous solution. So, the bound layer of the higher ionic content film would be thinner if the conformation in the solution phase is reserved during the deposition.35,36 As a control, rinsing of PDMS films with solvents was tested. When the PDMS films thicker than 3 nm were rinsed with water or ethanol, the film thickness decreased to ∼1.6 nm. This seems to be the consequence of incomplete rinsing or the insolubility of PDMS in water or ethanol, rather than the formation of strongly bound films. When the PDMS film was rinsed with chloroform, then the PDMS layer thickness decreased below the detection limit of the ellipsometry (0.2 nm). This comparison indicated that the pure PDMS molecules do not form the bound layer, but the CPL molecules (PDMS with cationic groups) can form the bound layer. When the bound CPL film was immersed in an aqueous solution of 1 M KCl, the bound layer thickness of 6% CPL significantly decreased from 1.2 to ∼0.4 nm, while those of 15% CPL and 30% CPL showed only marginal decreases. These results imply that the high ionic content CPLs are strongly attached to the SiO2 surface. The water contact angle can be used to probe the surface property of CPL films. The water contact angle is very sensitive to changes in the hydrophobicity/hydrophilicity of the surface. The hydrophobic PDMS surface gives a water contact angle of 98°. This is because the CH3 groups are exposed at the film surface.37 As more hydrophilic groups are exposed at the film surface, the water contact angle will be reduced from this value.37 So, if the ionic groups of CPLs were exposed at the film surface, then the water contact angle would be much lower than 90°. Table 1 shows the water contact angles measured for various CPL films. The water contact angle for all CPL bound layers is almost constant regardless of the film thickness. The water contact angle higher than 90° indicates that the methyl groups of CPL are exposed at the film surface and the ionic groups are imbedded in the film. It is noteworthy that the water contact angles of the CPL bound layers are similar to those of the thicker films, but that of the 1.6 nm thick PDMS film is only 51°. This implies that the CPL bound layer (0.7-1.2 nm depending on the ionic content) fully covers the silicon oxide surface, preventing water from directly interacting with the substrate. In contrast, the 1.6 nm thick PDMS film remaining after rinsing with water or ethanol appears to fail to protect the surface from water. They might exist in a patch form since they lack strong interactions with the surface and cannot form a bound layer. The direct electrostatic interaction of the cationic group of the bound CPL molecule with the substrate surface can be confirmed with XPS. If the ammonium cation of CPL is electrostatically coupled with the iodine counterion and the CPL molecule is physisorbed on the SiO2 surface, then there (36) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J.; Bohmer, M. R. Langmuir 1996, 12, 3675. (37) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. Rev. Phys. Chem. 2002, 53, 437.
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Figure 3. High-resolution N 1s and I 3d XPS spectra of (A) the bound-only layer of 6% CPL (1.2 ( 0.2 nm), 15% CPL (0.8 ( 0.2 nm), and 30% CPL (0.7 ( 0.2 nm) prepared by solvent washing and (B) 6% CPL (1.4 ( 0.1 nm), 15% CPL (1.7 ( 0.1 nm), and 30% CPL (2.0 ( 0.1 nm) samples prepared by depositing 2.5 nm thick films followed by outgassing for 2 h in UHV. The film thicknesses were measured with ellipsometry after solvent rinsing and outgassing.
will be I- detected along the N+ species in XPS. If the quaternary ammonium ion is reduced to amine, then there will be a N0 component in XPS and no I- peak will be detected. When the CPL molecule is chemisorbed via electrostatic interaction between the ammonium ion and the surface or through formation of amine oxide species, then the N+ component will be detected without the I-counterion in XPS. Figure 3A shows the N 1s and I 3d high-resolution XPS spectra for the CPL bound layers remaining on the surface after rinsing with the solvent. There is no I- species detected in the I 3d region. In the N 1s region, two peaks are detected. The peak at 397.9 eV is assigned to the amine (N0) species, and the peak at 402.7 eV is assigned to the ammonium (N+) species. These results strongly support that the cations of CPL are electrostatically bound to the silicon oxide surface. For all the CPL bound layers, ∼70% of the nitrogen atoms are in the ammonium form and ∼30% are in the neutral amine form. It is interesting to note that the NMR analyses of all CPLs indicated that the nitrogen in the CPL molecule were 100% in the ammonium form before the deposition. It seems like that 30% reduction of the ammonium ions take place during the deposition process. It is not clear what causes this reduction at this moment. For comparison, the CPL multilayers were also analyzed with XPS. The spin-coated CPL samples (∼2.5 nm thick) were first placed in an ultrahigh-vacuum (UHV) chamber for 2 h to outgas any volatile components. During this outgassing process, the weakly bound molecules are desorbed, causing a decrease in the film thickness. The 6% CPL film showed the largest decrease from 2.6 to 1.4 nm. The final thickness is very close to the 9818 DOI: 10.1021/la900921m
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bound layer thickness for 6% CPL. The 15% and 30% CPL film thicknesses decreased from 2.5 to 1.7 nm and from 2.3 to 2.0 nm, respectively, during the same outgassing process. These results may indicate that the 6% CPL vapor pressure is not low enough to keep the multilayer film in the UHV condition. Figure 3B shows the XPS spectra collected for the CPL films remaining after 2 h outgassing in UHV. In the case of 6% CPL, no I- peak was detected, and only the bound layer seems to remain on the surface. The 15% and 30% CPL multilayer films clearly show the I- 3d peaks in addition to the N0 1s and N+ 1s peaks. In summary, ellipsometry, water contact angle, and XPS analysis results support the formation of the electrostatically bound CPL layer on the silicon oxide surface. The bound layer thickness appears to be determined by the amount of ionic group attached to the polymer. As the ionic content increases, the thickness of the bound layer decreases down to ∼0.7 nm. This thin CPL bound layer is able to keep the hydrophobicity of the surface. III.b. Mobile Nature of the CPL Multilayer. In order to test the mobility of the multilayer, scratch tests were performed. A 4 μm 4 μm area was imaged at 2 μm/s with noncontact SPFM to obtain the surface topography and first harmonic signal images. This was followed by a high load AFM contact scan (1 μm 1 μm) at the center of the original image. Finally, the same 4 μm 4 μm area was reimaged with SPFM to observe any changes in the surface topography and first harmonic signals. If the bound molecules in the contact scan region is scratched off and the CPL molecules in the nearby multilayer film are not mobile, then the bare SiO2 surface would be exposed and the scratched molecules would be piled up outside the contact scan region. If the CPL molecules in the multilayers are mobile, then they can diffuse onto the exposed bare oxide surface in order to reduce the surface energy. Even if the bare substrate is not exposed at the highpressure contact scan, they could be dragged by the tip into the contact scan region. In this experiment it is important to be able to distinguish between the bare oxide surface and the CPL layers to determine whether the CPL molecules are squeezed out from the contact scan region. This can be done by measuring the first harmonic signal of SPFM which is governed by the surface potential difference between the tip and the substrate. The surface potential difference is sensitive to the surface chemistry differences such as bare oxide versus polymer-covered surfaces. In order to interpret the first harmonic signal data, the surface potential difference between CPL and oxide must be determined first. In this measurement, the tip was positioned at 50 nm above the sample, and the dc bias to the sample was scanned from +1 to -1 V. The magnitude of the first harmonic signal varied linearly with the dc bias and became zero when the dc bias nullified the potential difference between the tip and substrate surfaces.38 The phase of the first harmonic signal changed by 180° depending on whether the dc bias was larger or smaller than the nullifying point. Figure 4A shows the first harmonic signal as a function of dc bias for the 15% CPL sample. All the surface potential difference data are shifted such that the surface potential difference for a CPL-coated tip probing a CPL-coated substrate is 0 mV. The CPL-coated tips were prepared by touching or contactscanning the CPL-coated substrate, which allowed materials transfer from the substrate film to the tip.16,39,40 Figure 4B (38) Thomas, R. C.; Tangyunyong, P.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. J. Phys. Chem. 1994, 98, 4493. (39) Mate, C. M. Phys. Rev. Lett. 1992, 68, 3323. (40) Mate, C. M.; Lorenz, M. R.; Novotny, V. J. J. Chem. Phys. 1989, 90, 7550.
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Figure 4. Surface potential difference measurements using SPFM. (A) First harmonic signal versus surface potential difference for 15% CPL films. There is a 180° phase shift when the applied bias voltage is scanned across the nullified surface potential value. (B) Surface potential differences measured between silicon oxide surfaces and 6% CPL, 15% CPL, and 30% CPL films.
Figure 5. Representative surface topography images (4 μm 4 μm) of SPFM for (A) 6% CPL (4.3 nm thick), (B) 15% CPL (4.0 nm thick), and (C) 30% CPL (2.9 nm thick) films.
compares the surface potential differences measured for 6%, 15%, and 30% CPL films. When the CPL-coated tip probes the clean substrate, there is a ∼500 mV positive shift in the surface potential difference. When the CPL-coated substrate is probed with the clean tip, then there is a shift to the opposite direction by almost same magnitude. The surface potential difference seems to be independent of the cationic content or thickness of the CPL film. This might be due to the fact that all CPL films are highly hydrophobic, exposing the CH3 groups at the film surface (Table 1). The surface chemistry dependence in the first harmonic signal shown in Figure 4A enables the distinction between the bare substrate and CPL-coated surfaces. In typical SPFM experiments, both tip and substrate were initially coated with CPL, and the effective bias to the sample was around -500 mV Langmuir 2009, 25(17), 9814–9823
(marked with a circle in Figure 4A). If the CPL layer is removed from the substrate but not from the tip after the scratch test and the bare SiO2 surface is exposed, then there will be a large increase (+600 mV) in the first harmonic signal magnitude and no shift in the phase. If CPL is removed preferentially and completely from the tip, there will be a significant decrease in the first harmonic signal (-400 mV) and a -180° shift in the phase. Note that the surface potential difference for the clean tip against the clean substrate is different from the CPL-coated tip against the CPL-coated substrate. So, the removal of CPL from both tip and substrate surfaces can also be distinguished. Although the surface potential difference is independent of the ionic content in the polymer, the surface topography of the CPL multilayer film seems to vary with the ionic content. Figure 5 DOI: 10.1021/la900921m
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compares typical topographic images of the 6%, 15%, and 30% CPL multilayer films. All CPL images consist of regions with two different thicknesses. The width of the low and high regions is large for 6% and decreases as the ionic content increases to 15% and 30%. It should be noted that the lowest regions in surface topography are not the bare SiO2 surface but the bound CPL layer. The first harmonic signal of the topographically lower region is the same as that of the higher region. Note that SPFM can tell the height of the topographical features but is unable to measure the thickness of the bound layer which covers the substrate surface uniformly. When the bound layer thickness determined from the ellipsometry is considered the base of the SPFM topography and added to the mean value of the SPFM topographic height, the total film thickness becomes comparable to the value estimated with the ellipsometry. The topographical features observed for CPL multilayers appear to be similar to the ones reported for Fomblin Z-dol.11-13 In a film thickness range of 2-4 nm, Fomblin Z-dol does not form a homogeneous thickness film. Instead, they tend to form a film with a bimodal thickness distribution. This is due to hydrogen-bonding interactions of two hydroxyl groups at the chain ends inside the hydrophobic polymer film. When the bimodal thickness film is to be spread to form a homogeneous thickness film, the number of hydrogen bonding in the film should be reduced; thus, the homogeneous thickness film is less stable than the bimodal thickness film. In the CPL film, the cationcounterion pairs can form dipoles and interact with other ion pairs. These dipole interactions between ionic groups attached to the hydrophobic polymer backbone might be responsible for the bimodal distribution of the CPL film thickness. The abundance of these ion pairs may control the shape or size of the bimodal thickness domains. The quantitative relation between the ionic content and the topographic feature is not known yet and beyond the scope of this paper. A contact scratch test was performed to check whether the topographical domains of the multilayer film are easily deformable, i.e., the molecules in the domain are readily mobile during or after the tribological contact sliding. The contact scratch scan was made at an applied load of 100-300 nN, corresponding to a Hertzian contact pressures of 4.8-6.9 GPa. Figure 6A shows the SPFM images of the 6% CPL multilayer film (4.8 nm thick) before and after the contact scratch. From the comparison of two images taken before and after scratching, one can notice that there is a change in the thicker region of the CPL film in the scratched region. Initially, there is a ∼3.0 nm hillock in the top left region of the scan region. After the contact scan, this is moved to the right side. There is a 4% decrease in the first harmonic signal in the contact scan region compared to the outside region. This decrease is most likely due to some changes or rearrangements in the 6% CPL bound layer, rather than removal of the CPL film. If the CPL film was removed from the contact scanned region, then the first harmonic signal would increase significantly (Figure 4A). This decrease is not due to the removal of CPL from the tip either. If so, the phase of the first harmonic signal would have changed by -180° but was not observed. Note that the shift in the entire image signal after contact scan is due to a slight change in the distance between the tip and the surface. Figure 6B illustrates the same scratch test performed for the 15% CPL multilayer film (3.4 nm thick). The contactscan-induced change in the topography of the 15% CPL multilayer is more apparent. During the contact scan, the tip appears to drag and pull the mobile molecules from the 9820 DOI: 10.1021/la900921m
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surrounding area (especially from the region below the contact scan region in this image), creating a uniform and thick film in the contact scan region. Unlike the 6% CPL, there is no measurable change to the first harmonic signal. This suggests that during the high load contact scan there is no damage or change to the CPL film. Figure 6C illustrates the scratch test result for the 30% CPL multilayer film (3.8 nm thick). Unlike the 6% and 15% CPL films, there seems to be little changes in the surface topography in the 30% case. The only noticeable change is a small protrusion at the center of the contact scan region. Note that this was the spot where the AFM tip was separated from the surface after the contact scan and the AFM mode was changed to SPFM. This means that the 30% CPL film can be stretched to some degree by the tip but they are not readily spreadable. In fact, the 30% CPL bulk sample looks like a solid instead of viscous liquid. It might be the consequence of the fact that the 30% CPL has too much electrostatic interaction among the ion pairs within the film. Once again, the first harmonic signal remains unchanged. In summary, the topographical features and their contact-scan induced changes are found to vary with the ionic content in the CPL molecule. As the ionic character increases, smaller topographical domains are formed in the film with a thickness of ∼3 nm. The multilayer features of the 6% and 15% CPL films are readily mobile or deformable during the contact scan, while those of the 30% CPL film are not. The high load contact with a Hertzian contact pressure up to several gigapascal range cannot remove the CPL molecules from the contact scan region. III.c. Lubrication Behavior of the Bound-and-Mobile CPL Coating. The effects of the ionic groups in CPL on the lubrication of silicon oxide surfaces were tested in both nanoand macroscales. AFM makes it possible to measure the friction and adhesion of the nanoscale single-asperity contact. The contact pressure in AFM is in the gigapascal range. This allows one to test whether the molecules are removed from the contact region under high load conditions. But, this pressure is too high compared to the MEMS conditions. The pin-on-disk tribo-test with a 3 mm ball diameter provides the contact pressure much closer to the realistic pressure in the MEMS operation conditions. But, its contact area is much larger than those in MEMS.28,29 By interpolating between the nano- and macroscale lubrication effects, one could predict the MEMS scale lubrication effects. The nanoscale adhesion force and friction coefficients were estimated from the AFM measurements for the CPL films coated on silicon wafers. The CPL films were tested in two thickness conditions: bound-only and bound-and-mobile layers. The bound-and-mobile samples were 3-4 nm thick films produced by spin-coating (details in section III.b). The bound-only samples were the ones rinsed with ethanol after spin-coating (details in section III.a). As a control for the mobile-only layer, the PDMS films were also included. Table 2 summarizes the forces measured with AFM for these films. The adhesion forces were obtained from the retraction portion of the force-distance curves. The critical snap-off forces and the friction force at 0 nN applied load were obtained from the friction-load curves. The critical snap-off force is the force required to snap-off from the surface while the tip is scanning laterally. It should be noted that the adhesion force could not be measured for the boundand-mobile multilayer films. When the mobile layer was present, a meniscus was formed and stretched as the tip was retracted from the surface to the free-standing position.16 This meniscus stretching had a film thickness dependence. Quantitative analyses of the Langmuir 2009, 25(17), 9814–9823
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Figure 6. Topography and first harmonic signal images (4 μm 4 μm) of SPFM for (A) 6% CPL (3.2 nm), (B) 15% CPL (3.4 nm), and (C) 30% CPL (3.8 nm) films taken before and after contact scratching of a 1 μm 1 or 2 μm 2 μm area at the center (marked with dotted lines) at a contact load of at least 150 nN (Hertzian pressure of >5.5 GPa).
meniscus stretching behaviors can be used to calculate the disjoining pressure of the film, which will be the subject of a future publication. Among the CPL bound films tested in this study, the 6% film has the highest adhesion force (∼500 nN) and the 15% film gives the lowest (∼20 nN). When the ionic content is increased to 30%, then the adhesion force increases to ∼70 nN. Langmuir 2009, 25(17), 9814–9823
The critical snap-off force values measured during the contact scan are similar to the adhesion forces determined from the force-distance curve. A similar ionic content dependence of the adhesion force has been reported for other polyelectrolytes.35 The increased adhesion for 30% CPL must be due to the increased electrostatic bridges. At the nanoscale, the adhesion force is added to the applied load increasing the net DOI: 10.1021/la900921m
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Hsiao et al. Table 2. Adhesion Force Data (in nN) polymer samplea
adhesion force measured during vertical retraction
critical snap-off force measured during lateral scanning
friction force measured at zero applied load
6% CPL (1.2 ( 0.2 nm) B 527 ( 23 506 ( 11 50 ( 6 15% CPL (0.8 ( 0.2 nm) B 21 ( 7 39 ( 7 9( 2 30% CPL (0.7 ( 0.2 nm) B 74 ( 10 69 ( 4 18 ( 2 52 ( 6 5( 1 6% CPL (3.4 ( 0.7 nm) B + M -b 25 ( 3 2(1 15% CPL (3.9 ( 0.6 nm) B + M -b b 65 ( 10 14 ( 2 30% CPL (3.5 ( 0.5 nm) B + M 31 ( 7 4(2 PDMS (3.4 ( 0.6 nm) M -b a B: bound only; B + M: bound and mobile; M: mobile only. b The adhesion force could not be measured due to the meniscus stretching of the polymer film during the tip retraction.16
Figure 7. (A, B) Friction-load curves measured with AFM for (A) the bound-only films of 6% CPL (1.2 ( 0.2 nm), 15% CPL (0.8 ( 0.2 nm), and 30% CPL (0.7 ( 0.2 nm) and (B) the bound-and-mobile films of 6% CPL (3.4 ( 0.7 nm), 15% CPL (3.9 ( 0.6 nm), 30% CPL (3.5 ( 0.5 nm), and the PDMS mobile-only film (3.4 ( 0.6 nm). (C, D) Comparison of friction coefficient values measured with AFM (nanoscale) and pin-on-disk tribometer (macroscale) for (C) the bound-only and (D) bound-and-mobile films of 6%, 15%, and 30% CPL as well as the PDMS mobile-only film.
total load.41-43 Thus, the friction force at the zero applied load is the highest for 6% (∼50 nN), lower for 30% (∼18 nN), and the lowest for 15% (∼9 nN). The 6% bound film is the thickest (∼1.2 nm) among the three CPLs, and the ionic content in the polymer is the lowest. If the AFM tip interacts with only the polymer layer during the load application and release process, then the adhesion force is expected to be the smallest for the 6% bound film. But, the opposite is observed. This might be related to the fact the amount of ionic group in the 6% CPL bound layer is too low (only one out of 17 monomers), and the long polymer chain segments could easily be displaced and allow some local interactions between the substrate and tip surfaces within the contact region. It seems that these few ionic anchoring points in the 6% CPL bound layer (1.2 nm thick) is not strong enough to support the load applied by the AFM tip. When the film thickness is increased allowing the formation of the bound-and-mobile structure, the critical snap-off force in the (41) Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. J. Colloid Interface Sci. 1975, 53, 314. (42) Grierson, D. S.; Flater, E. E.; Carpick, R. W. J. Adhes. Sci. Technol. 2005, 19, 291. (43) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301.
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friction-load curve (equivalent to the adhesion force on the force-distance curve) decreases significantly for 6% CPL while it is reduced marginally for 15% CPL. The critical snap-off force and zero-load friction force of the 6% CPL film are now only factor of 2 times larger than those of the 15% CPL film. In contrast, the 30% film does not show any discernible thickness dependence. Regardless of the film thickness, ∼70 nN adhesion force and 14-18 nN zero-load friction force are observed for the 30% CPL film. The high adhesion force of the 30% CPL film must originate from the high ionic content (one out of three monomers). The nanoscale friction coefficient reported in this paper is defined as the slope of the linear portion of the friction-load curve. Figure 7A,B displays representative data obtained for the CPL films as well as the PDMS control sample. The macroscale friction coefficient was calculated using Amonton’s law. The friction coefficients of the CPL bound-only and bound-andmobile films as well as the PDMS mobile-only film are compared in Figure 7C,D. Both nano- and macroscale friction data show the same trend as the film thickness and ionic content in the molecule vary. The small difference in the friction coefficient value between the nano- and macroscale measurements for the Langmuir 2009, 25(17), 9814–9823
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bound-only layer might be attributed to the surface roughness of the ball used in the macroscale pin-on-disk measurement. One can easily notice that the friction signals of the boundonly films are noisier than those of the bound-and-mobile films in Figure 7A,B. The noisiness could be an implication of insufficient amounts of the CPL molecule for good lubrication. Except the 30% CPL film, the presence of mobile layers lowers the friction coefficient (slope of the friction-load dependence). The insensitivity of the 30% CPL friction coefficient toward the film thickness must be related to the same insensitivity of the adhesion force for this sample (Table 2). In both nano- and macroscales, the friction coefficient is observed to be lowest for the 15% CPL and the highest for the 30% (Figure 7C,D). The friction coefficient of the 6% CPL bound-and-mobile film seems to be the same as the PDMS mobile-only case. These results imply that the 6% ionic content is too low to see the advantage of the bound layer and 30% is too high to keep the lubricity of the flexible polymer backbone. The 15% case appears to be the optimum to ensure the bound and mobile effects. In summary, the adhesion and friction behaviors of the CPL bound-only and bound-and-mobile films exhibit an interesting dependence on the amount of ionic groups attached to the polymer. When the ionic content is too low, then the CPL molecule behaves similar to the PDMS control sample. As the ionic content increases from 6% to 15%, there is a significant enhancement in the reduction lubrication performance. If the ionic content is too high, the lubrication performance seems to deteriorate.
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IV. Conclusion The effect of adding cationic groups to the flexible PDMS backbone was investigated. As the ionic content increase from 6% to 30%, the thickness of the electrostatically bound layer decreases from 1.2 to 0.7 nm. The topographical features of the multilayer films are also a function of the ionic content. The molecules in the 6% and 15% multilayer film are readily mobile, but not the 30% film. When the ionic content is only 6%, the lubrication efficacy is similar to the PDMS control. When the ionic content is increased to 30%, the molecule loses its lubricity. The 15% CPL film shows the lowest adhesion and friction behaviors. Acknowledgment. This work was supported by the National Science Foundation (Grants CMS-0528141 and CMS-0637028). The authors gratefully acknowledge Dr. D. V. Patwardhan for his valuable consultation at the beginning of this research, Dr. M. T. Dugger for his help with the construction of the pin-on-disk tribometer, Prof. E. A. Vogler for allowing us to use his contact angle goniometer, and Prof. D. L. Allara for allowing us to use his ellipsometer. E.H. and S.H.K. are grateful to Prof. N. D. Spencer for their stay and research at ETH Zurich. D.K. was supported by Pukyong National University Research Abroad Fund in 2007 (PS-2007-010). Supporting Information Available: FT-IR and 1H NMR spectra of two-step CPL synthesis and table of DSC measurements for methylhydrosiloxane-co-dimethylsiloxane macromers. This material is available free of charge via the Internet at http://pubs.acs.org.
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