Effect of Rubbing-Induced Polymer Chain Alignment on Adhesion and

temperature by means of atomic force microscopy in argon. ... cross-linked polystyrene surface in which the covalent cross-linking prevents chain real...
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Langmuir 2005, 21, 682-685

Effect of Rubbing-Induced Polymer Chain Alignment on Adhesion and Friction of Glassy Polystyrene Surfaces Melvina Leolukman and Seong H. Kim* Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received August 29, 2004. In Final Form: October 28, 2004 The friction and adhesion properties of polystyrene surfaces are studied below the glass transition temperature by means of atomic force microscopy in argon. Even at a temperature far below the glass transition, the repeated sliding of a polystyrene bead tip on the non-cross-linked polystyrene surface causes significant reduction of friction and adhesion forces. There is no measurable wear of the polystyrene surface due to repeated sliding. These decreases are associated with the alignment of the outermost polymer segments induced by repeated rubbing. There are only little changes in friction and adhesion on the cross-linked polystyrene surface in which the covalent cross-linking prevents chain realignment.

Introduction The fundamental understanding of the friction behavior of polymer surfaces at the molecular level is becoming an increasingly crucial factor in applications such as polymerpolymer microbearings, micromechanical devices, and nanotribology.1-3 The molecular-level studies of friction of polymeric materials have been focused on lubrication behaviors of low-Tg polymers, low molecular weight polymeric surfactants, and polymer melts.4,5 These studies found that the friction of polymer-on-mica systems is significantly affected by the relaxation dynamics of the shear stress at the polymer interface6,7 and the friction of polymer-on-polymer systems is strongly dependent on chain interdigitation at the interface.8-11 Compared to these rubbery or fluidic polymer systems, the molecularlevel studies for adhesion and friction between glassy polymers are found much less in the literature. Recently, Maeda et al. reported that the glassy polymer surfaces also undergo significant chain interdigitations upon contact, which increases adhesion hysteresis and friction.12 However, how these properties of the glassy polymer change upon repeated cycles of sliding action is not well studied yet. Understanding these changes will be critical to predicting the tribological properties of polymeric materials in extended operation. This paper reports the effect of repeated rubbing on polystyrene (PS) friction and adhesion at room temperature, which is far below its glass transition temperature * Corresponding author. E-mail: [email protected]. (1) Rymuza, Z.; Kusznierewicz, Z.; Solarski, T.; Kwacz, M.; Chizhik, S. A.; Goldale, A. V. Wear 2000, 238, 56. (2) Nanotribology, Critical Assessment and Research Needs; Hsu, S. M., Ying, Z. C., Eds.; Kluwer Academic Publishers: Boston, 2003. (3) Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V. V., Wahl, K. J., Eds.; ACS Symposium Series 741; American Chemical Society: Washington, DC, 2000. (4) Klein, J. Annu. Rev. Mater. Sci. 1996, 26, 581. (5) Kumacheva, E. Prog. Surf. Sci. 1998, 58, 75. (6) Heuberger, M.; Luengo, G.; Israelachvili, J. N. J. Phys. Chem. B 1999, 103, 10127. (7) Luengo, G.; Heuberger, M.; Israelachvili, J. N. J. Phys. Chem. B 2000, 104, 7944. (8) Luengo, G.; Pan, J.; Heuberger, M.; Israelachvili, J. N. Langmuir 1998, 14, 3873. (9) Yamada, S.; Israelachivili, J. N. J. Phys. Chem. B 1998, 102, 234. (10) Ruths, M.; Granick, S. J. Phys. Chem. B 1998, 102, 6056. (11) Dhinojwala, A.; Cai, L.; Granick, S. Langmuir 1996, 12, 4537. (12) Maeda, N.; Chen, N.; Tirrell, M.; Israelachvili, J. N. Science 2002, 297, 379.

(∼100 °C). Atomic force microscopy (AFM) was used to measure friction and adhesion forces between high molecular weight polystyrene surfaces. The repeated scanning of a PS bead attached to an AFM cantilever causes the decrease of adhesion and friction of the non-crosslinked PS surface without any wear, while it does not alter adhesion and friction of the cross-linked PS surface. The rubbing-induced decrease of the adhesion and friction force of the non-cross-linked surface implies that restructuring of polymer chain segments occurs even in the glassy state. Experimental Method The PS films were prepared by spin coating a solution of 2.54 wt % atactic polystyrene (MW ) 223 200 g/mol, polydispersity ) 1.11) dissolved in high purity toluene on a silicon wafer at 3000 rpm. The resulting polymer films were dried and heated at 100 °C in vacuum for 20 h. The typical thickness of the spincast films was ∼150 nm. The cross-linking of the PS film was carried out by exposing the PS thin film to UV irradiation from a mercury arc lamp in an argon environment. The volume fraction of the cross-linked PS chains was calculated by comparing the thickness of the cross-linked PS film left after toluene washing with that of the initial PS film before UV irradiation.13 It was found that about 60% of the PS film was cross-linked under our UV exposure conditions. Polystyrene particle tips (BioForce Nanoscience Inc.) were used for measurements of adhesion and friction between PS surfaces. The PS particle tip consisted of a commercially available latex bead of a diameter of 5 µm attached to a V-shaped cantilever with a spring constant of 0.58 N/m. The initial adhesion and friction forces of the as-purchased tip were too large to be measured probably due to low molecular weight polymer chains loosely entangled at the latex bead surface or surfactant molecules (used in the emulsion polymerization making latex beads) remaining at the bead surface. The PS particle tips were preconditioned by repeated rubbing on a PS film surface in an argon environment until there were no changes in adhesion and friction. The preconditioning of the PS particle tip was needed to make sure that the adhesion and friction changes measured with the tip on a freshly prepared thin film substrate are due to the changes in the substrate surface, not mostly due to the changes in the tip surface. The friction and adhesion changes of non-cross-linked and cross-linked PS film surfaces were studied in contact mode with applied normal forces from 14 to 54 nN in an argon environment at room temperature. At these conditions, the Johnson-Kendall(13) Yan, M.; Harnish, B. Adv. Mater. 2003, 15, 244.

10.1021/la0478503 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/14/2004

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Figure 1. (a) Topography and (b) friction force images of a 10 × 10 µm2 region of the polystyrene film after 3072 line scans of the top region. The applied normal force is 27 nN. Roberts (JKR) theory predicts that the contact diameter is 0.9-1 µm and the contact pressure is 5-18 kPa.14 One line of the polymer film surface was scanned repeatedly for 3072 times with a linescan mode, and the lateral force signal was recorded as a function of scan cycles. Here, one line scan is defined as a cycle of trace and retrace scans. The force-distance curve was taken every 1024 line scans. A typical scan speed was 8-30 µm/s. The friction data of the polymer films were obtained from the lateral signal difference, and the adhesion data were obtained from the pulloff force of the force-distance curve. The friction and adhesion forces of each substrate were normalized with the initial data before repeated scanning to highlight the rubbing-induced changes.

Results and Discussion There are recurring features that are observed in the adhesion and friction measurements of polymer surfaces with newly purchased PS bead AFM tips. When a freshly prepared PS thin film surface was scanned with an aspurchased PS tip, the adhesion and friction force were always too high to be measured with AFM. When the tip is continuously used for contact mode imaging, the adhesion and friction forces decrease to the region that can be measured with a reasonable reproducibility. One could simply consider that these decreases are caused by rubbing-induced cleaning of the scanned area, wear of the polymer tip, or restructuring of surface segments at the interface. An optical microscope imaging did not find any visible wear of the PS tip, but optical microscopy cannot detect surface wear of a few nanometers. The use of scanning electron microscopy (SEM) to image the used tip did not reveal any discernible wear of the 5 µm PS tip because the nonconducting polymer surface is coated with a thin gold film to avoid charging problems in SEM. Because of these uncertainties, one could not understand exactly what caused the friction and adhesion behavior (14) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London 1971, A324, 301.

of the rubbed polymer surface. This can be resolved with the AFM results shown here, which rule out the first two cases and indicate the restructuring of the surface segments as the main source for the rubbing-induced adhesion and friction decreases. Figure 1 shows the three-dimensional images of topography and lateral force signals of the non-cross-linked PS surface. The top part of the image was repeatedly linescanned for 3072 times. The topography image does not show any hint of line scan history or wear marks, while the lateral force image clearly reveals significantly lower friction at the line-scanned region compared to the region which was not scanned previously. Most of the small asperities shown in both images appear to be noises. Their size is much smaller than the tip-surface contact diameter. Therefore, it can be said that the rubbing-induced friction decrease is not related to the polymer surface wear at all. Since the tip is made of the same material, it is not unreasonable to assume that the same changes occur on the PS tip during the scanning, especially during the preconditioning. The cross-linked PS film also shows no wear in the topographic image, but it makes only minor changes in the friction over the same repeated line scans. The friction and adhesion force changes as a function of the scanning cycles for the non-cross-linked and crosslinked PS substrate are shown in Figure 2. The initial adhesion and friction forces of the non-cross-linked samples were about 30% higher than those of the crosslinked samples. During the repeated line-scan test, the friction and adhesion between the preconditioned PS tip and the non-cross-linked PS film gradually decrease, while those between the preconditioned PS tip and the crosslinked film remain fairly constant. The fact that the adhesion force does not increase after the repeated rubbing also supports the absence of the polystyrene tip wear. If there was any tip wear due to rubbing, the adhesion force would be increased after the line-scan experiment.14 The

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Figure 2. Changes of friction (lateral signal difference) and adhesion (pull-off force) forces of (a) non-cross-linked and (b) cross-linked polystyrene films. The friction signal is shown with continuous (scattered) lines, and the adhesion signal is shown with open circles. The dashed line is drawn to guide the eye. The applied normal force is 14 nN, and the scan speed is 16 µm/s. The error bar of the adhesion force, calculated from 5 measurements per location, is smaller than the symbol size.

exact amount of the decrease varies from sample to sample, but the decrease over the 3072 line scans is typically 30-45% for the non-cross-linked sample and 3000 sliding cycles at room temperature. In the case of polystyrene, the glass-rubber transition occurs at ∼100 °C. This means that the polystyrene chains are essentially frozen into their entangled conformation in the experiment time scale at room temperature. The fact that the rubbing-induced segment alignment at the interface occurs without any measurable surface wear might indicate the presence of loose loops and dangling chain ends at the polymer surface that are more mobile than chains in the bulk.21,22 These mobile segments will be more susceptible to mechanical force or transient temperature rise caused by rubbing and can be aligned even at a temperature below the bulk Tg. Since the experiment temperature is much lower than Tg (∼100 °C), the polystyrene chains in the bulk cannot move or reorient with simple rubbing at the interface. The temperature rise due to friction is negligible. Even if all frictional energy is assumed to be used to heat the top 5 nm thick region (which is close to the radius of gyration of the polystyrene molecule), the calculated maximum temperature rise is only ∼5 K under our experiment conditions. Without the displacement of the bulk polymer chains (at least the polymer chains in the subsurface), the degree of rubbing-induced segment alignments at the interface would not be near completion. Instead, there will be a distribution of small domains in which all or some fraction of polymer chain segments are oriented to a certain direction.23 Once the surface density of these domains increases to a certain level, there seem (15) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. J. Phys. Chem. B 2001, 105, 2785. (16) Gautam, K. S.; Schwab, A. D.; Dhnojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854. (17) Clancy, T. C.; Jang, J. H.; Dhinojwala, A.; Matiice, W. L. J. Phys. Chem. B 2001, 105, 11493. (18) Greary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl. Phys. 1987, 62, 4100. (19) Oh, M.; Hong, S.-C.; Shen, Y. R. Appl. Phys. Lett. 2002, 80, 784. (20) Lee, S. W.; Chae, B.; Kim, H. C.; Lee, B.; Choi, W.; Kin, S. B.; Chang, T.; Ree, M. Langmuir 2003, 19, 8735. (21) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2000, 33, 7588. (22) Sasaki, T.; Shimizu, A.; Mourey, T. H.; Thurau, C. T.; Ediger, M. D. J. Chem. Phys. 2003, 119, 8730. (23) Kim, J.-H.; Rosenblatta, C. J. Appl. Phys. 2000, 87, 155.

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to be no more measurable changes in the friction and adhesion forces for the polymer-on-polymer interface at room temperature. This might be due to depletion of loose loops and dangling chain ends or interlocking of these aligned domains. The difference in the magnitude of the friction force decrease for PnBMA and PS can be explained with the degree of chain segment alignment.23 The PnBMA surface shows a decrease by more than 90%, while the PS surface shows only 30-45% reduction. This could be interpreted as almost complete alignment in the PnBMA case and only partial alignment in the PS case. If the distribution of the aligned segment domains cannot be measured, it is difficult to make a quantitative correlation of the observed friction and adhesion changes to a specific origin that governs these changes. However, one can still make a qualitative interpretation of the effect of the chain segment alignment at the interface. When the polymer chains at the interface get aligned to some extent, (a) the entanglement of loops and chain ends at the interface can be reduced and (b) the dynamic relaxation of shear stress caused by sliding can be enhanced. These two factors will be discussed in the following paragraphs. The important fact discovered from the AFM results reported here is that the friction reduction of the glassy polymer is accompanied by the decrease of adhesion force. According to the JKR theory, the reduction of adhesion force implies the reduction of the contact area. Since the friction in microscale depends on the contact area, the reduced adhesion will lead to a lower friction.24-27 It is intuitively expected that when the polymer segments at the surface are oriented, the degree of chain packing increases and the surface density of loose loops and dangling chain ends decreases. This will result in less entanglement and thus lower adhesion force upon contact with other polymer surfaces.28-30 The effects of the reduced loops and chain ends on friction have been demonstrated by Maeda et al. on cross-linked poly(vinylbenzyl chloride) and polystyrene.12 Let us turn the focus to dynamic energy transfer and shear stress relaxation during the mechanical contact of two surfaces. One can assume that the bending vibration of the pendent group with respect to the polymer backbone aligned along the rubbing direction can reduce energy transfer upon collisions of two polymer chains moving opposite directions.31 The collective motion of the aligned pendent group can also effectively dissipate the energy transferred to the polymer chain.31 If these dynamic energy transfer and relaxation processes are dominant factors, (24) Chaudhury, M. K.; Owen, M. J. Langmuir 1993, 9, 29. (25) DePalmo, V.; Tillman, N. Langmuir 1989, 5, 868. (26) Briscor, D. C.; Evans, D. C. B. Proc. R. Soc. London 1982, A380, 389. (27) Makkonen, L. Proceedings, 27th Annual Meeting of the Adhesion Society, Wilmington, NC, Feb 15-18, 2004; The Adhesion Society: Blacksburg, VA, 2004; pp 399-401. (28) Schnell, R.; Stamm, M.; Creton, C. Macromolecules 1998, 31, 284. (29) Haire, K. R.; Windle, A. H. Comput. Theor. Polym. Sci. 2001, 11, 227. (30) Perez-Salas, U.; Briber, R. M.; Rafailovich, M. H.; Sokolov, J. J. Polym. Sci. Part B: Polym. Phys. 2003, 41, 1902. (31) Berman, A.; Israelachvili, J. N. Isr. J. Chem. 1995, 35, 85.

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one can expect that there should be friction anisotropy for the aligned polystyrene surface, as in crystalline polyethylene32 and ordered liquid crystals.33,34 In other words, the scanning orthogonal to the alignment direction would give a higher friction value than the scanning along the alignment direction. This can be studied best by taking contact mode images of the aligned segment domains with a sharp AFM tip made of silicon or silicon nitride. In this way, one could avoid the complication from the interpenetration effect of polymer chains at the interface. However, finding the repeatedly scanned location after changing the tip in AFM is practically impossible since there is no specific wear feature on the spin-cast polymer film that can be used as a guide. An alternative approach is to investigate the crystalline domains of isotropic polystyrene, which will be the subject of future study. Or it can be studied more thoroughly by using computational methods. The experiment results described in this paper caution that the adhesion and friction force measurement of polymer surfaces can be dynamically changing even at temperatures far below the glass-rubber transition. Since one image scanning in AFM typically consists of 512 line scans, the adhesion and friction forces can be changing up to 5-20% during the one full image scan. If one tries to measure the friction coefficient of polymer surfaces by repeated scanning of one area at varying loads, there could be a significant error due to dynamic changes of the polymer surface during the measurement. Conclusion The adhesion and friction of the non-cross-linked polystyrene surface decrease upon repeated rubbing. The rubbing history dependence of adhesion and friction is attributed to alignments of loose-packed chain segments at the surface. What is interesting is that these alignments occur in the glassy state and at a very low load sliding and lower the adhesion and friction behavior of the glass polymer surface. Compared to other polymers studied at temperatures near their glass-rubber transition, the polystyrene surface shows a much slower change and the magnitude of the decrease is much smaller, indicating only partial alignment of chain segments at the surface. The origin of the lower adhesion and friction of the partially aligned polymer surface could be attributed to the reduced interpenetration of the more closely packed chains at the interface and/or the reduced energy transfer and enhanced shear stress dissipation of the aligned polymer chains. Acknowledgment. This work was supported by Penn State Start-up funds and, in part, by the National Science Foundation (Grant No. DMI-0210229). Supporting Information Available: Topography and lateral force images of the pristine PS film surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA0478503 (32) Schonherr, H.; Vancso, G. J. Macromolecules 1997, 30, 6391. (33) Ruths, M.; Steinberg, S.; Israelachvili, J. N. Langmuir 1996, 12, 6637. (34) Jani, J.; Tadmor, R.; Klein, J. Langmuir 2001, 17, 5476.