Microstructure and Microtribology of Polymer Surfaces - American

Chapter 19

Molecular Alignment and Nanotribology of Polymeric Solids Studied by Lateral Force Microscopy

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G. Julius Vancso and Holger Schönherr Faculty of Chemical Technology, Polymer Materials Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands

Lateral force microscopy (LFM) was used to study the tribological behavior of polymer surfaces exhibiting morphological order. Anisotropic friction was observed (a) on extended-chain crystals obtained either by channel die compression or by friction transfer deposition (HDPE, PTFE); (b) on lamellar crystals (HOPE); and (c) on PEO obtained by transcrystallization. For extended-chain crystals the friction was investigated at various relative scan angles with respect to the polymer chain direction. Maximum friction was observed perpendicular to and minimum friction for scans parallel to the chain direction, respectively. The anisotropic friction for lamellar crystals was explained by the occurrence of chain-folding oriented preferentially in planes parallel to the corresponding crystal face. The magnitude of the friction force, as well as the friction anisotropy, increased with scan velocity. For all polymer systems discussed in this paper, the friction anisotropy can be rationalized in terms of the cobblestone model (interlocking asperity model) of interfacial friction described e.g. by Israelachvili and coworkers.

Friction and lubrication are phenomena which play a crucial role in various aspects of our daily life (7). Despite its importance the physics of friction is only partly

© 2000 American Chemical Society

Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

317

318 understood. Even conceptually simple systems, such as highly ordered monolayers, are often too complicated to allow one to predict the magnitude of friction, and to identify which parameter (e.g. molecular conformation, dispersion, packing arrangement, and chemical composition) is most important for the corresponding tribological behavior (2). Friction measurements on the nanometer scale have only recently become possible with the invention of the atomic force microscope (3). By scanning the specimen perpendicular to the long axis of the AFM cantilever, the lever will be twisted around its long axis as result of the friction forces acting between surface and tip (lateral force microscopy, LFM). Thereby LFM allows one to investigate the wear and frictional behavior of polymer surfaces from the 100 μηι to the submicrometer scales. In recent years it has been demonstrated that also adhesion (or adhesion hysteresis) plays an important role in friction. Israelachvili and coworkers could show that friction and adhesion hystereses are, in general, directly correlated if certain assumptions are fulfilled. These authors have proposed models based on data obtained by surface forces apparatus (SFA) experiments, e. g. the cobblestone model of interfacial friction (4). In addition, several groups described the application of continuum contact mechanics (e.g. Johnson-Kendall-Roberts (JKR) theory (5)) to describe friction data measured between flat surfaces and nanometer sized contacts

(6). Much effort is being recently devoted to bridge the gap between macroscopic (multiple asperities) and microscopic (single asperity) friction studies. In general, nanometer scale studies of friction (nanotribology) by LFM, combined with adhesion measurements (e. g. by measuring the pull-off force curves with an AFM), can be helpful to develop microscopic models of friction and to establish relationships between friction and adhesion (adhesion hysteresis) (7). Nanotribological studies resulted in a number of interesting observations such as molecular scale stick-slip motions on graphite (8), or dependence of friction on adhesion due to different functional groups exposed in highly ordered monolayers (9). For polymers, anisotropic friction (10) was observed on lamellar crystals (11 13), as well as on extended-chain crystals obtained by friction transfer deposition (14). The anisotropic friction for lamellar crystals was explained by the occurrence of chain-folding oriented preferentially in planes parallel to the corresponding crystal face. For extended-chain crystals friction anisotropy observed parallel and perpendicular to the chain direction was interpreted by the interlocking asperity model. In this paper we give an overview regarding our nanotribological observations on oriented high density polyethylene (HDPE), poly(tetrafluoroethylene) (PTFE) and on crystals of polyethylene (PE) and transcrystallized poly(ethylene oxide) (PEO).

Experimental Materials and Orientation. The HDPE samples used in this study were obtained by (a) compression in a channel die (75, 76) and (b) by orienting a slider (77) in friction transfer (18). PTFE surfaces were unidirectionally oriented by sliding a block of


319 polymer over a heated (220°C) glass slide (7 7). In our study the worn surface of the slider (rather than the transferred film) (14) was investigated. The specimen orientation was adjusted manually in the force microscope such that SFM scans could be performed at preselected angles with respect to the orientation direction which coincides with the polymer chain axis. Single crystals of PE (Polysciences, M = 52000 g/mol, M / M = 2.9) were grown as reported earlier (77 - 75). The PEO samples were prepared by electrospinning of 1% aqueous solutions of PEO (average molar mass 2 χ 10 g/mol, Aldrich) onto glass substrates as described previously (19). Onto the electrospunfiberssmall droplets of aqueous PEO solution were placed. Crystallization of PEO took place at room temperature during the evaporation of the solvent. w

W

N

6

Scanning Force Microscopy / Lateral Force Microscopy. The SFM experiments were performed on a NanoScope III (Digital Instruments, (DI) Santa Barbara, CA) using triangular shaped cantilevers (DI) with a nominal spring constant of 0.12 N/m and 0.38 N/m, respectively. The torsional spring constant was calibrated as described in reference (20). In addition to unmodified tips, chemically modified tips were also used. The procedure for self-assembled monolayer deposition for the chemical modification of the tips is described in reference (20). Results and Discussion Anisotropic friction was observed for all samples discussed in this paper. The anisotropy is directly correlated with the directionality of the polymer molecules at the surface of the specimens. The directionality was either confirmed experimentally by AFM (for uniaxially oriented polymers and the transcrystallized PEO) or reported in the literature in numerous diffraction studies for extended-chain polymer single crystals. The existence of regularly packed folds at the surface of lamellar polymer crystals is still a matter of discussions. Uniaxially Oriented Polymer Fibers. For the crystal facets of PTFE and HDPE imaged, the uniaxially oriented arrangement of the polymer chains results in anisotropic friction. In Figure 1, a typical SFM image of the morphology of oriented PTFE is shown. The fibril orientation coincides with the macroscopic orientation direction. Figure 2 shows an LFM image of PTFE with molecular resolution. The image was acquired on top of one of the fibrils exposed at the surface of the oriented specimen depicted in Figure 1. The interchain distance of 5.6 Â corresponds well with the reported literature value of 5.55 Â of the phase IV hexagonal structure of PTFE (21). The mismatch betweenfibriland polymer chain orientation was found to be smaller than 10°. Therefore the polymer chain direction with respect to the fixed scanning direction of the LFM (90° with respect to the cantilever's long axis) could be easily adjusted manually by turning the specimen (22). These images were acquired in a liquid cell,filledwith ethanol, using -CF modified tips. 3


320

Figure 1. AFM height image of oriented PTFE with a -CF tip in ethanol (ζ = 250 nm). 3

Figure 2. LFM image of oriented PTFE obtained with a -CF tip in ethanol (ζ = 0.1 V). Molecular resolution is a result of a stick-slip process. 3


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The anisotropy of the friction force as measured by LFM with unmodified tips is shown in Figure 3a (ethanol) and Figure 3b (air). The magnitude of friction is significantly higher for scans performed perpendicular to the polymer chain direction as compared to the parallel direction. Furthermore, the friction force vs. normal force dependence is not always linear. Linearity was reported earlier (23) although a single asperity contact is predicted to show a non-linear dependence (6). In contrast to our previous results, the ratio of friction measured at an angle with the polymer chain direction to friction measured parallel to the polymer chain direction was found to be higher than that for HDPE (vide infra). This discrepancy with data reported earlier can be attributed (a) to a lower degree of perfection of the oriented PTFE crystal surface (e.g. occurrence of chain ends and entanglements) and (b) inaccuracies in the adjustment of the specimen for LFM measurements performed parallel to the exposed polymer chains. The degree of structural perfection is not accessible by AFM as we obtain only lattice resolution. Careful analysis of the crystal facet orientation in the angle-dependent measurements shown in Figure 3 can minimize the error in relative scan direction. As seen in Figures 3a and 3b, the friction force measured in scans parallel to the PTFE chains is almost constant and its magnitude is close to the noise level of our experiment. A small change in relative scan angle of 7°, however, leads to a significant increase of friction (Figure 3a). Similar results could be obtained on the HDPE specimens (Figure 4). With reference to the HDPE images it is worthwhile mentioning that the repeat distances observed in the LFM scans correspond very well with the known repeat length in the (be) facet of (orthorhombic) PE. For measurements in air, we observed a reduction in pull-off forces for -CF tips as compared to unmodified, or differently modified tips. The friction anisotropy observed on HDPE is summarized in Figure 5a. For the -CF terminated tip a smaller magnitude of friction is measured than that for a -COOH tip. The ratio of friction (F[perpendicular/parallel] « 4) is lower than that for PTFE (vide supra). The friction anisotropy was measured for HDPE with unmodified tips in air as a function of relative scan angle (Figure 5b). Similar to PTFE (vide supra) the magnitude of the friction force was smallest for scans parallel to the polymer chain direction and increased with increasing relative scan angle. 3

3

Polymer Lamellar Crystals Grown from Solution. Similarly to the regular array of polymer chains exposed at extended-chainfibrillarcrystals, the folded sections of polymer molecules at the fold plane of solution-grown "single" crystals can cause anisotropy of friction (Figures 6). This has been observed for a variety of materials, such as POM, PE, and poly(4-methyl-l-pentene) (11 -13). The explanation proposed for this observation is based on the presence of oriented folds at the surface of the fold plane. In the present study, solution grown lamellar crystals of PE on mica were studied by LFM in air, as well as in water. The friction anisotropy shown in the corresponding micrographs was well pronounced. The relative friction measured by LFM on PE crystals in different orientations, as well for the same crystal after


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Figure 4. LFM image of HDPE (scan size: 17.3 nm χ 17.3 nm) obtained with a -CF tip in air (image reproduced with permission from reference 20 Copyright 1997 American Chemical Society.) 3


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Normal Force [nN] Figure 5b. Friction force vs. normal force plot for H D P E measured for different relative scan directions, obtained with a Si N tip in air. 3

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325

Figure 6a. LFM image of a solution grown PE lamellar crystal obtained in air (z = 0.2 V).

Figure 6b. The directions of the chain folds are schematically indicated in the LFM image of the solution grown PE lamellar crystal shown in Figure 6a (not to scale).


326

manual rotation of the specimen, was in all cases related to the orientation of the crystal edges (which coincide with the fold directions as depicted in Figure 6b) with respect to the fixed scan direction. By performing line scans in dependence on the normal force (24) and the scan velocity on the crystal shown in Figures 6, we could construct a friction "phase diagram" (4) between silicon nitride and PE lamellar crystals (Figure 7). Friction is found to be higher in the sectors denoted with A in Figure 6a which have the growth edge predominantly parallel to the scan direction. The difference in friction between low and high friction sectors (A and B, respectively), was found to increase with normal force and scan velocity. The adhesion, however, is the same on all different sectors on the sample (Figure 8). The observed friction behavior can be described in first approximation assuming a time (or sliding velocity) dependence of the friction signal similar to linear polymer viscoelasticity. The relative values of the dissipative and the conservative energy contributions during sliding friction vary as a function of scan velocity. According to the timetemperature superposition principle, increasing velocity corresponds to decreasing temperature. The friction forces observed in our experiments increase with increasing sliding velocity. This increase corresponds, for the given materials, to an increase of dissipative energy loss (related in thefirstapproximation to the loss tangent, tan δ, of the polymer surface). In other words, the polymer surface behaves more "solid-like" with increasing sliding velocity. The friction force (similar to tan δ) according to this model should possess a maximum if either the temperature is decreased, or the sliding velocity is increased. This surface "solidification" temperature is below ambient for the PE lamellar crystals for the sliding velocities and load values one can reach in the LFM experiments. The friction and adhesion of PE in air have lower values compared to mica (high capillary forces on the hydrophilic mica dominate), while in water the friction and adhesion values are higher (hydrophobic force on PE is dominant). In Figure 9 the corresponding images of single crystals in water are shown. Transcrystallized Polymer Crystals. A third example of friction anisotropy that is caused by the regular arrangement of polymer molecules was observed in transcrystallized samples of PEO grown on PEO fibers, as shown in Figure 10. Two electrospunfibers(19), which make an angle of ca. 90°, can be seen. Lamellae of PEO grown from aqueous solution, which was deposited on the fibers, exhibit sectored growth. Growth starts simultaneously and proceeds only until the lamellar features impinge. This process leads to sectors of the crystallizing material. As can be concluded from the lateral force image (right), the friction is clearly different in the sectors with different orientation.

Discussion with Respect to the Cobblestone Model of Interfacial Friction (4). Let us first discuss the friction problem of the fold plane of lamellar crystals and the AFM tip asperity, as described by the Cobblestone model of interfacial friction (4). Ideally, in adhesional contact at rest, the outermost atom(s) of the tip fit snugly into


327

Figure 7. Friction vs. normal force vs. scan velocity for (a) high friction sector,

(b) low friction sector,


328

(c) difference in friction between sectors measured on PE lamellar crystal in air.


329

Figure 8. Adhesion image of the lamellar PE crystal shown in Figure 6. Bright tones corresponds to low adhesion (50 nN), dark tones to high adhesion (80 nN). The contrast in the adhesion images is defined on the basis of the convention that attractive forces are negative.



Figure 9. Friction force image (left, z-scale: dark 0.0 V - bright 0.5 V) and adhesion force image (right, z-scale: bright 1 nN - dark 60 nN) on PE lamellar crystal obtained in water. The contrast in the adhesion images is defined on the basis of the convention that attractive forces are negative.

Figure 10. Dual height (left) and friction (right) image of PEO transcrystallized on electrospun PEO fibers. Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

332 the grooves defined by the folds of the polymer crystal. Upon applying a lateral force on the tip, it will move laterally and in the normal direction by the small distances d and d, respectively, as shown in Figure 11. The general equation for both static and kinetic friction force F can be given in the following form: 0

F = ε ( 2EqA/D + L) d/d

(1)

0

where ε is the energy fraction dissipated during the kinetic friction process, E is the energy per unit area needed to separate the tip from contact to infinity, A is the contact area, and L is the load on the tip. For the fold surface of the solution-grown lamellar crystal the ratio of d/d , and thus the friction force, will depend on the direction of the lateral movement of the sample under the tip. The effective d in the fold direction is expected to be lower as compared to the value of the effective d in the fold-perpendicular direction as the outermost atom(s) of the tip can "slide" in the grooves defined by the chain folds. During the friction process, which occurs perpendicular to the folds, the tip apex will "bump" into the folds (provided that the chain folds are predominantly directional within each sector, compare Figure 6). In the case of the uniaxially oriented HDPE and PTFE surfaces consisting of fibrillar extended-chain crystals, the ratio of d/d and thus the friction force, is expected to depend on the relative angle between chains and fixed scan direction. For the highly oriented fibers depicted in Figures 1, 2, and 4, rough estimates for d values can be given. The value of d in the chain-perpendicular direction is equal to the known van der Waals radius of the polymer chains. The quantity d depends on the relative scan angle γ (angle between fast scan axis and polymer chain direction) while d can be assumed to be constant. From geometrical considerations it can be shown that: 0

0

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0

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d (r)=d (90y/sinr 0

0

(2)

Thus, if we assume that E , A, and ε are independent of the relative scan direction and L is constant, the friction should vary systematically with the scan angle. The friction data measured for PTFE and HPDE (vide supra) can be reasonably well described by a sin γ / d (90°) dependency as predicted by equations 1 and 2. In Figure 12 an example for PTFE measured in air is shown. In the chain-parallel direction, only the HDPE showed a periodic stick slip behavior with a repeat distance of ca. 2.5 Â. This distance is equal to the repeat unit of polyethylene (16, 20). For PTFE the LFM friction loops in our experiments did not reveal any stick slip behavior. Thus, in this case, we cannot determine the value of d in the chain-parallel direction. In this case we can, however, assume that the value of d is close to 0. Based on equation 1, the friction anisotropy is therefore expected to be larger for PTFE, than that for HDPE. For a semi-quantitative comparison of friction forces predicted by equation (1) on one hand and experimental results obtained on highly oriented polymer surfaces on the other hand, one should be 0

0

0


333

Figure 11. Schematic representation of the events occurring during sliding motion of the AFM tip over an ordered surface (Adapted from ref. 4).

Relative Scan Angle [deg.] Figure 12. Experimentally determined friction force vs. relative scan angle for PTFE. The corresponding fits are based on equations 1 and 2. For details see text.


334 able to measure or calculate the adhesion energy, as well as the contact area from AFM data. Corresponding research is currently in progress in our laboratory.

Conclusions Anisotropic friction and molecular stick-slip friction were observed on a number of polymer systems by LFM using non-fiinctionalized and functionalized tips. The observations described in this paper can be understood within the frame of the cobblestone (interlocking asperity) model. For a quantitative description, however, important parameters such as the energy fraction dissipated during the kinetic friction process (ε) are still lacking.

Acknowledgment The authors thank Dr. Rob Pearce for the donation of the PE lamellar crystal sample. This work has been supported by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (CW-NWO) in the priority program materials (PPM).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15.

Fundamentals of Friction; Singer, I. L.; Pollock, Η. M., Eds.; Kluwer: The Netherlands, 1992. Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192. For a recent review see: Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VHC: Weinheim 1996. Israelachvili, J. N.; Chen, Y.-L.; Yoshizawa, H. In Fundamentals of Adhesion and Interfaces; Rimai, D. S.; DeMejo, L. P.; Mittal, K. L., Eds.; VSP, 1995, pp. 261 - 279. Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, A. 1971, 324, 301. Carpick, R. W.; Agraït, N.; Ogletree, D. F.; Salmeron, M. Langmuir 1996, 12, 3334. For a recent review see: Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. Mate, C. M.; McCelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. Frisbie, C. D.; Rozsnyai, L. F.; Noy, Α.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. Examples of anisotropic friction were reported for a variety of systems, see reference 7, and: Schönherr, H.; Kenis, P. J. Α.; Engbersen, J. F. J.; Harkema, S.; Hulst, R.; Reinhoudt, D. N.; Vancso, G. J. Langmuir 1998, 14, 2801 and references therein. Nisman, R.; Smith, P. F.; Vancso, G. J. Langmuir 1994, 10, 1667. Smith, P. F.; Nisman, R.; Ng, C.; Vancso, G. J. Polym. Bull. 1994, 33, 459. Pearce, R.; Vancso, G. J. Polymer 1998, 39, 6743. Vancso, G. J.; Förster, S.; Leist, H. Macromolecules 1996, 29, 2158. Galeski, Α.; Bartczak, Z.; Argon, A. S.; Cohen, R. E. Macromolecules 1992, 25, 5705.


335 16. 17. 18. 19. 20. 21. 22. 23. 24.

Schönherr, H.; Vancso, G. J.; Argon, A. S. Polymer 1995, 36, 2115. Schönherr, H.; Vancso, G. J. Polymer 1998, 39, 5705. Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. Jaeger, R.; Schönherr, H.; Vancso, G. J. Macromolecules 1996, 29, 7634. Schönherr, H.; Vancso, G. J. Macromolecules 1997, 30, 6391. Polymer Handbook; Brandrup, J.; Immergut, E. H., Eds.; 3rd ed.; Wiley: New York, 1989; p V/37. The accurate determination of the angle (error ca. ± 2°) was based on the orientation of the crystal facet as measured by in situ LFM scans (see Figure 2). Koutsous, V. Ph.D. Thesis, State University of Groningen, The Netherlands, 1997. Normal force is defined a the sum of experimentally determined pull-off force and external load.


Microstructure and Microtribology of Polymer Surfaces - American

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