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Frictional Force Microscopy of Oxidized Polystyrene Surfaces Measured Using Chemically Modified Probe Tips C. Ton-That, P. A. Campbell, and R. H. Bradley* Materials Surfaces and Interfaces Group, School of Applied Sciences, The Robert Gordon University, St. Andrew Street, Aberdeen AB25 1HG, United Kingdom Received July 1, 1999. In Final Form: December 31, 1999 Chemical force microscopy (CFM) using probe tips that have been chemically functionalized to give hydroxylated (polar) or methylated (apolar) surfaces has been used to investigate frictional properties of polystyrene (PS) films that have been oxidatively modified to varying degrees using an ultraviolet-ozone (UVO) treatment. Oxygen chemisorption levels and functional group chemistry of the PS films have been followed using X-ray photoelectron spectroscopy (XPS), and the resulting data have been correlated with lateral (frictional) force measurements for each surface type. XPS analysis showed that UVO treatments introduced polar species at the film surfaces via the formation of C-O, CdO, and O-CdO functional groups. CFM was performed on the native and treated films using tips that were either hydroxyl- or methyl-terminated. The treated films exhibit substantially higher friction than untreated films when imaging with the hydroxyl-terminated tips. Friction is reduced when nonpolar methyl-functionalized tips are employed. The higher frictional forces and coefficients measured for the oxidized surfaces using the hydroxyl-terminated tip are due to hydrogen bonding between the polar hydroxyl groups of the tip and the oxygen functional groups of the PS surfaces.
Introduction Many industrial and technological processes benefit from a capability to tailor the chemical and mechanical nature of polymer surfaces.1,2 Of the various techniques that may be employed to perform such modifications, ultraviolet-ozone (UVO) treatment represents a particularly effective method for controlling precisely the level of oxygen-bearing chemical groups at the surface.2-8 We have recently reported on UVO treatment procedures on poly(ethylene terephthalate) (PET),7 and also on polystyrene (PS) dishes used for tissue culture purpose8 systems. In those studies, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) techniques were used to characterize and correlate changes in surface topography and surface chemistry, respectively. In this paper we present results from work in which we have employed chemically functionalized AFM tips and investigated their lateral (or frictional) force response with untreated and oxidized surfaces of PS films that have been spin-cast from chloroform. Oxidation has been achieved by UVO treatment for varying exposure times. In parallel, we measured the surface chemical composition of the UVO-treated PS samples using XPS. This has allowed correlation to be drawn relating the observed friction characteristics with evolving surface oxygen * Corresponding author. Phone: (44.1224) 262822. Fax: (44.1224) 262828. Email:
[email protected]. (1) Liston, E. M.; Martinu, L.; Wertheimer, M. R. J. Adhes. Sci. Technol. 1993, 7, 1091-1127. (2) Mathieson, I.; Bradley, R. H. Inter. Adhes. J. Adhes.,1996, 16, 29-31. (3) Mathieson, I.; Bradley, R. H. J. Mater. Chem. 1994, 4, 11571157. (4) Bradley, R. H.; Mathieson, I. J. Colloid Interface Sci. 1997, 194, 338-343. (5) Callen, B. W.; Ridge, M. L.; Lahooti, S.; Neumann, A. W.; Sodhi, R. N. S. J. Vac. Sci. Technol., A, 1995, 13, 2023-2029. (6) Hill, J. M.; Karbashewski, E.; Lin, A.; Strobel, M.; Walzak, M. J. J. Adhes. Sci. Technol. 1995, 9, 1575-1591. (7) Ton-That, C.; Teare, D. O. H.; Campbell, P. A.; Bradley, R. H. Surf. Sci. 1999, 433-435, 278-282. (8) Teare, D. O. H.; Emmison, N.; Ton-That, C.; Bradley, R. H. Langmuir 2000, 16, 2818-2824.
chemistry on the UVO-treated surfaces. The application of AFM to obtain chemical information is relatively recent, and until now studies have been confined to model systems. In this paper we present data for the application of the technique to the characterization of chemically and energetically heterogeneous polymer surfaces, which extends the application of AFM into a novel field. Experimental Section Preparation of Polymer Samples. The polystyrene (molecular weight of 280K, from Sigma Aldrich) films were spin-cast under STP conditions from 1% w/v chloroform solutions. Mica was chosen as the substrate owing to the ease with which large atomically flat areas can be obtained by cleaving. All films were cast onto freshly cleaved (about 1 cm2 area) substrates using 60 µL aliquots of the polymer solution, which were rotated at ∼3000 rpm for 2 min. Chloroform solvent was completely evaporated from the films, as no chlorine signal was detected by XPS. UVO treatments of the films were carried out in a modular benchtop UVO reactor.9 The reactor contains a high-intensity low-pressure mercury vapor grid lamp that emits UV light at 184.9 and 253.7 nm wavelengths, which excites oxygen to form ozone and atomic oxygen4 and also photosensitizes polymer surfaces.10 Preparation of AFM Tips. Cleaning and hydrophilization of the AFM tips was carried out in accordance with the procedure of Ito et al.11 The Si3N4 cantilevers with integrated tip (Digital Instruments) were exposed to UVO for 15 min using the UVO reactor as described above. The UVO treatment is known to effectively remove contaminants such as hydrocarbons12 and probably further oxidizes the native oxide layer on the Si3N4 tip surface. After UVO treatment, the Si3N4 tips were immersed in 0.5 M NaOH for 20 min, in 0.1 M HCl for 10 min, and in 0.5 M NaOH for 10 min (in this order). Subsequently, they were rinsed in Milli-Q deionized water (resistivity ) 18.3 MΩ cm) and dried in a vacuum for 10 min at 100 °C. This procedure has been (9) Bradley, R. H.; Mathieson, I.; Byrne, K. M. J. Mater. Chem. 1997, 7, 2477-2482. (10) Walzak, M. J.; Flynn, S.; Foerch, R.; Hill, J. M.; Karbashewski, E.; A. Lin, A.; Strobel, M. J. Adhes. Sci. Technol. 1995, 9, 1229-1248. (11) Ito, T.; Namba, M.; Buhlmann, P.; Umezawa, Y. Langmuir 1997, 13, 4323-4332. (12) Vig, J. R.; J. Vacuum Sci. Technol., A 1985, 3, 1027-1034.
10.1021/la9908586 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000
CFM of Oxidized Polystyrene Surfaces reported to make the tip surface become much more hydrophilic by increasing the surface concentration of silanol groups.11,13 Hydrophobic AFM tips were made by adsorption of alkylsilane monolayers of carbon chains terminated with methyl groups. After treatment with UVO/NaOH/HCl according to the above procedure, the tips were then immersed into a 5 mM solution of n-dodecyltrichlorosilane (CH3(CH2)11SiCl3 (Bas Technicol, U.K.)) dissolved in 4:1 hexadecane/chloroform for 3 h. This procedure allows the silane to form self-assembled monolayers (SAMs) chemically anchored to the Si3N4 surface as a result of hydrolysis at the terminal SiCl3 group.14,15 Upon removal from the silane solution, the silanated tips were washed with chloroform and then rinsed in Milli-Q deionized water to remove unreacted silane materials. The tips were then dried in a vacuum at ∼80 °C for an hour and used immediately thereafter for SPM imaging. Surface Analysis. Surface chemical compositions of the native and oxidized PS films were measured by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis 5-channel spectrometer with monochromated Al KR (1486.6 eV) X-rays operated at 150 W. The main chamber of the XPS instrument was maintained at a pressure of ∼10-9 Torr. Charge neutralization was used for all the samples to offset charge accumulation on surfaces. Elemental compositions were calculated from the areas of C 1s and O 1s peaks in survey spectra, collected at a pass energy of 80 eV, using appropriate relative sensitivity factors (errors approximated at (5% by analysis of reference materials). Detailed surface chemical composition was obtained by analysis of carbon 1s envelopes collected at a pass energy of 20 eV. Chemical shift peaks are charge referenced to the C-C/C-H peak at 284.6 eV. Peak analysis was carried out using Kratos software and also version 1.5 of the Spectral Data processor (XPS International). A Digital Instruments Multimode SPM system, with incorporated fluid cell, was used to obtain topography and friction data on the PS films. Lateral force microscopy (LFM) was performed simultaneously with topographical imaging in contact mode and using 200 µm long V-shaped Si3N4 cantilevers with narrow legs (nominal spring constant 0.06 N m-1, nominal tip radius 20-60 nm16). Individual spring constants were determined using the method described previously by Cleveland et al.17 The samples were scanned in the direction orthogonal to the long cantilever axis in order to record lateral signals. A constant scan rate of 6 µ ms-1 was used in all LFM experiments since friction forces were found to dependent on scan velocity in previous studies.18,19 To allow meaningful comparisons of friction on each of the different films, the same tip was used for all the films tested. The laser alignment of each tip used was not altered throughout the experiments, as this would also introduce relative errors.20 Control images using tapping mode were also undertaken on the native and treated films to ensure that wear or plastic deformation did not occur on the films during contact mode imaging. To evaluate friction forces, we analyzed line profiles of the forward and reverse scans, i.e., friction loops. The scope mode of the AFM was utilized to provide friction loops, in the fashion of Overney et al .21 The friction at a given load was evaluated as half the difference between forward and reverse lateral signals. Except where specified, the loads in this paper refer to externally applied loads, i.e., the product of calibrated spring constant with cantilever displacement. Cantilever displacements were mea(13) Ogbuji, L. U. T.; Jayne, D. T. J. Electrochem. Soc. 1993, 140, 759-766. (14) Wenzler, L. A.; Moyes, G. L.; Raikar, G. N.; Hansen, R. L.; Harris, J. M.; Beebe, T. P.; Wood, L. L.; Saavedra, S. S. Langmuir 1997, 13, 3761-3768. (15) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775-3780. (16) Digital Instruments, Santa Barbara, CA. (17) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instr. 1993, 64, 403-405. (18) Haugstad, G.; Gladfelter, W. L.; Weberg, E. B.; Weberg, R. T.; Jones, R. R. Langmuir 1995, 11, 3473-3482. (19) Overney, R. M.; Takano, H.; Fujihira, M.; Meyer, E.; Guntherodt, H. J. Thin Solid Films 1994, 240, 105-109. (20) Hu, J.; Xiao, X. D.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358-370. (21) Overney, R. M.; Takano, H.; Fujihira, M.; Paulus, W.; Ringsdorf, H. Phys. Rev. Lett. 1994, 72, 3546.
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Figure 1. High-resolution C 1s spectra of PS films with fitted curves: (a) untreated; (b) 40 s UVO treatment. sured relative to the point where “jump to contact” occurs. The possible influence of capillarity was removed by undertaking all lateral force imaging under fluid (Milli-Q deionized water).
Results and Discussion XPS survey spectra of the PS films demonstrate the presence of only carbon for the untreated surfaces and only carbon and oxygen for the UVO-treated surfaces. There are no signals indicative of the mica substrate. The concentration of incorporated oxygen into the PS surfaces increases with UVO exposure time; after 60 s treatment the surface oxygen level is at 25.4 atom % as calculated from XP survey spectra. Longer treatment times continue to increase the oxygen concentrations up to a saturated level of ∼35 atom %, but at these higher levels, a significant amount of loosely bound low molecular weight oxidized material (LMWOM) is produced as a consequence of chain scission.5,8 This LMWOM may be mobile on the surface and therefore might be displaced by, or disrupt, the motion of the AFM tip when imaging in contact mode, which could result in an inaccurate friction measurement. In the work presented here films with low surface oxygen levels have been used in order to minimize effects resulting from surface mechanical change. The surface chemistry has been analyzed via interpretation of the carbon 1s envelopes. Figure 1a shows the carbon 1s spectrum for the untreated PS film, while Figure 1b illustrates the case after 40 s treatment. The spectrum of the untreated film matches that of pure polystyrene,22 which includes a hydrocarbon C-C/C-H peak at binding energy of 284.6 eV and π-π* shake-up satellites at shifts of 6-8 eV from the hydrocarbon peak. The envelopes of the treated films can be resolved into four components: a main hydrocarbon peak and three chemically shifted (22) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers; John Wiley & Sons: Chichester, 1992.
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Figure 2. Typical friction loop recorded on the 60 s UVOtreated film using the hydroxyl-terminated probe tip. Applied load was 41 nN. Arrows indicate the scanning directions. Static friction can be observed at the beginning of each scan line. Table 1. Results of the High Resolution XPS Analysis of the C 1s Envelopes from Both Untreated and UVO Treated PS Films exposure time (s) 0 20 40 60
percentages in C 1s envelope (atom %) C-C/C-H C-O CdO O-CdO π-π* 92.9 77.6 66.7 58.3
0 9.9 15.3 16.9
0 4.0 6.9 11.1
0 3.8 7.8 11.3
7.1 4.5 3.1 2.2
peaks due to alcohol/ether C-O, carbonyl CdO, and carboxyl/ester O-CdO at respective shifts of 1.5, 3.0, and 4.5 eV.5,22 A more detailed discussion of data resulting from XPS analysis of oxidized PS surfaces has been published elsewhere.8 Measurement of the relative intensities of these peaks as a function of exposure time (Table 1) indicates the proliferation of additional oxygen functional groups. Analysis of π-π* shake-up satellites from the carbon 1s envelopes reveals a decrease in intensity with treatment time, indicative of a loss in aromatic structure. Imaging the PS films in contact mode with low loading forces (
