Langmuir 2002, 18, 2949-2951
2949
Notes Control of Tribological Properties with a Series of Random Copolymers Taku Aoike,* Tomokazu Ikeda, Hiroki Uehara, Takeshi Yamanobe, and Tadashi Komoto Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan Received November 5, 2001. In Final Form: January 3, 2002
Introduction Tribological phenomena such as friction, wear, and lubrication on micro- and nanometer scales are becoming important in the fields of data storage and microelectromechanical systems technologies.1 Polymeric and organic materials have been employed to improve the tribological properties. Changes in the composition of polymer blends and block and graft copolymers enable us to control the mechanical properties of the bulk material.2,3 In contrast, use of these polymeric materials seems to be unable to control the surface physical properties since there is compositional heterogeneity normal and/or parallel to the surface due to the surface enrichment of one or more components and also to the phase separation. In particular, the heterogeneity may exert considerable influence on the surface characteristics on a nanometer scale due to the smaller area and depth required to evaluate the characteristics at that scale compared to those of the heterogeneous structures of the surface region. Consequently, the surface analyses for these polymeric materials with a scanning force microscope (SFM) having a nanoscopic probe tip have revealed that various properties on the surfaces are reflected by the heterogeneous structures.4-7 Russell and colleagues8,9 succeeded in controlling the surface static properties such as the interfacial energies and wetting behavior of polymers in contact with the surfaces by anchoring random copolymer chains to the surfaces of the substrates. This success is attributed to the fact that the size scale of any heterogeneity in a random copolymer is much smaller than that of the morphological structure in other copolymers and polymer blends. * To whom correspondence should be addressed. E-mail: aoike@ polymer.chem.gunma-u.ac.jp. (1) Handbook of Micro/Nanotribology, 2nd ed.; Bhushan, B., Ed.; CRC Press: Boca Raton, FL, 1999. (2) Polymer Blends; Paul, D. R., Bucknall, C. B., Eds.; WileyInterscience: New York, 2000; Vol. 2: Performance. (3) Thermoplastic Elastomers; Legge, N. R., Holden, G., Schroeder, H. E., Eds.; Hanser Publishers: New York, 1987. (4) Tsukruk, V. V.; Sidorenko, A.; Gorbunov, V. V.; Chizhik, S. A. Langmuir 2001, 17, 6715. (5) Kopp-Marsaudon, S.; Lecle`re, Ph.; Dubourg, F.; Lazzaroni, R.; Aime´, J. P. Langmuir 2000, 16, 8432. (6) Eaton, P. J.; Graham, P.; Smith, J. R.; Smart, J. D.; Nevell, T. G.; Tsibouklis, J. Langmuir 2000, 16, 7887. (7) Motomatsu, M.; Nie, H.-Y.; Mizutani, W.; Tokumoto, H. Thin Solid Films 1996, 273, 304. (8) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (9) Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J. Nature 1998, 395, 757.
Table 1. Molecular Characteristics, Tensile Properties, and Contact Angle of Random Copolymers Used in This Study sample M0 M16 M34 M50 M66 M84 M100
f
Mw/103
Mw/Mn
E/GPa
σ/MPa
θ/deg
0.00 0.16 0.34 0.50 0.66 0.84 1.00
234 244 227 184 203 176 203
1.3 1.6 1.8 2.0 2.1 2.1 1.9
0.58 1.0 1.3 1.5 1.9 2.0 2.4
13 21 29 35 47 49 58
85.9 85.2 83.8 83.2 80.4 78.7 79.0
Tribological phenomena are intricately related to both surface and bulk characteristics of materials. The extent of contacting and deforming regions during relative sliding depends on the tribological conditions such as the shape and size of the slider, the applied load, and the contact pressure. Therefore, materials whose bulk characteristics are similar to those of the surface region are better suited for controlling the surface dynamic properties with surface deformation regardless of measurement scales. In this paper, we report that the tribological properties occurring when a stiff slider moves on a polymer surface under conventional macro- and nanotribological conditions can be controlled by using a series of random copolymers. Experimental Section The samples used in this study were a series of random copolymers composed of methyl methacrylate (MMA) and n-butyl methacrylate with the MMA molar fractions in the copolymers varying from 0 to 1. These were synthesized by a free-radical polymerization in benzene solutions using 2,2′-azobisisobutyronitrile as an initiator. The polymerization reactions were carried out at 333 or 343 K for 12 h. Each copolymer was obtained by precipitation in hexane at room temperature, then filtered, and dried in an oven under a vacuum. Table 1 presents the sample name and the characteristics of the samples such as the MMA molar fraction, f, determined by 1H NMR, the weight-average molecular weight, Mw, and the molecular weight distribution, Mw/Mn, where Mn denotes the number-average molecular weight, determined by gel permeation chromatography with poly(methyl methacrylate) (PMMA) calibration. The number in the sample name corresponds to the MMA molar percentage in the copolymer. The mechanical properties of the bulk and the contact angle of water, θ, on the copolymer surfaces at room temperature are also listed in Table 1. The tensile modulus, E, and the yield stress, σ (the breaking strength was adopted when the sample broke before yielding), of each sample were evaluated by using a tensile tester at a strain rate of 1 × 10-1 min-1. The tribological properties were evaluated by macro- and nanotribological measurements, whose measurement scales were quite different. The films for tribological measurements were prepared by casting copolymers from 15 wt % toluene solutions onto glass slides (for macrotribological measurement) and stainless steel plates (for nanotribological measurement). These films were dried at 298 K in an ambient atmosphere for 24 h and then at 413 K in a vacuum for 24 h. The films for both measurements were ca. 40 µm thick. The macrotribological measurement was carried out with a ball-on-disk type friction tester. A 2.5 mm radius ball made of Si3N4 slid on the copolymer films rotating at a speed of 250 µm/s under an applied load of 1.96 N. The friction force generated by the sliding ball was determined by distortion of a strain gauge attached to the arm of the tester. The width of the wear track made by the sliding
10.1021/la011643a CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002
2950
Langmuir, Vol. 18, No. 7, 2002
Notes
Figure 1. (a) Friction force measured by using ball-on-disk type friction tester as a function of MMA molar fraction in copolymer. (b) Friction force measured by using scanning force microscope as a function of MMA molar fraction in copolymer. ball was characterized by reflected optical microscopy (OM). The nanotribological measurement was done by the use of a SFM, SPA 400 with SPI 3800N controller (Seiko Instruments Inc.). A commercial rectangular 200 µm cantilever having a spring constant of 0.05 N/m with a Si3N4 integrated tip (Olympus Optical Co., Ltd.; the spring constant as specified by the manufacturer) was used. The tip radius, measured by scanning electron microscopy, was approximately 10 nm. The frictional data were obtained in the SFM tip reciprocating scan on a single line at 1 µm/s for 1 µm perpendicular to the cantilever long axis. The applied load was maintained at 5 or 10 nN during the perpendicular scanning. The multiline scratch test was performed by the use of the SFM in order to investigate the surface deformation of copolymer films under the nanotribological conditions. Both measurements were carried out in a normal atmosphere (in air, 295-297 K, and relative humidity of 40-50%). The details of both measurements are described elsewhere.10,11
Results and Discussion First, the characteristics of the series of copolymers are explained. Mw values of the samples used in this study were adjusted to approximately 200 × 103, which was higher than the molecular weight of the critical entanglement, Mc, of PMMA, ∼27.5 × 103.12 For polystyrene (PS) with a molecular weight exceeding Mc of PS, ∼31.2 × 103,12 the surface deformation properties evaluated by using SFM were independent of their molecular weight.11,13 (10) Aoike, T.; Uehara, H.; Yamanobe, T.; Komoto, T. Langmuir 2001, 17, 2153. (11) Aoike, T.; Yamamoto, T.; Uehara, H.; Yamanobe, T.; Komoto, T. Langmuir 2001, 17, 5688. (12) Graessley, W. W. Adv. Polym. Sci. 1974, 16, 1.
Figure 2. (left) Optical micrographs of wear tracks on copolymer surfaces made by ball sliding. The arrow and scale bars represent the ball sliding direction and 200 µm, respectively. (right) SFM topographical images of copolymer surfaces after multiline scratching under an applied load of 10 nN. The scan dimensions and contrast variations of these images are 1.6 × 1.6 µm2 and 30 nm, respectively.
Thus, it is reasonable to assume that the effect of molecular weight disparity among samples on tribological properties is considerably smaller than that of the molar fraction in the copolymer. As is evident from Table 1, the tensile modulus and yield stress for the series of copolymers increased linearly with increasing MMA molar fraction. The mechanical properties of M100 were nearly four times larger than those of M0. The contact angle of water on the copolymer (13) Leung, O. M.; Goh, M. C. Science 1992, 255, 64.
Notes
surfaces decreased with increasing MMA molar fraction. This decrease corresponds to the higher surface free energy of PMMA compared to that of poly(n-butyl methacrylate).14 This result of contact angle measurement shows that the composition of the surfaces of these copolymers is similar to that of the bulk. The friction forces generated by the Si3N4 sliders moving on the surfaces of copolymer films were measured as a function of the MMA molar fraction in the copolymer by using the ball-on-disk type friction tester and SFM. The contact pressures between the sliders and the copolymer surfaces, which were estimated based on the Hertz contact theory,15 were ca. 20-50 MPa under the macrotribological conditions and ca. 110-360 MPa under the nanotribological conditions.16,17 These values are at a comparable level with or higher than the yield stress of the sires of copolymers. The macro- and nanotribological results presented in Figure 1 show that the friction force decreased with increasing MMA molar fraction in both cases. Thus, it can be said that the friction force generated by a stiff slider moving on a copolymer surface under macro- and nanotribological conditions can be controlled by the MMA molar fraction in the copolymer that controls both bulk and surface characteristics. The copolymer surface deformations generated under both tribological conditions were investigated by OM observation of the wear tracks formed by the sliding ball and the SFM multiline scratch tests. The narrow single scratch line formed by the tip sliding during the friction force measurement was difficult to observe minutely by scanning force microscopy. Thus, the multiline scratch (14) Wu, S. J. Phys. Chem. 1970, 74, 632. (15) Landau, L. D.; Lifshitz, E. M. Theory of Elasticity, 2nd ed.; Pergamon Press: London, 1970. (16) The contact pressures were calculated by assuming Poisson’s ratio of 0.4 under all conditions, in consideration of a variety of reported values of PMMA (ref 17) and the fact that the values of these copolymers had not been reported. (17) (a) Winkelhahn, H.-J.; Pakula, T.; Neher, D. Macromolecules 1996, 29, 6865. (b) Koppelmann, J. Rheol. Acta 1958, 1, 20. (c) Yee, A. F.; Takemori, M. T. J. Polym. Sci., Polym. Phys. Ed. 1982, 20, 205.
Langmuir, Vol. 18, No. 7, 2002 2951
test was performed under an applied load of 10 nN to evaluate the deformation of the copolymer surface region from the air-solid interface to the depth comparable to the chain size occurring under the nanotribological conditions. Figure 2 shows the optical micrographs of the wear tracks on copolymer surfaces (left) and the SFM topographical images of copolymer surfaces after multiline scratching (right). It is found that all the copolymer surfaces were plastically deformed under both tribological conditions. Moreover, the magnitude of both surface deformations, which appeared as the width of the wear track and the surface roughness of the scratched area, decreased with an increase of MMA molar fraction. The root-mean-square value of the height distribution, which was evaluated from the topographical data of the central dimensions of 800 × 800 nm2 in the scratch area, decreased from ca. 2.1 nm to ca. 0.7 nm, and the width of the wear track also decreased from ca. 210 µm to ca. 120 µm with an increase of MMA molar fraction. This indicates that the mechanical properties of both bulk and surface regions are dominated by the MMA molar fraction in the copolymer. Therefore, it is reasonable to conclude that both friction forces generated under the macro- and nanotribological conditions examined in this study were closely related to the surface deformation of the copolymer. In summary, this study shows that tribological properties occurring when a stiff slider moves on a copolymer surface under both conventional macro- and nanotribological conditions can be controlled by the composition of a random copolymer. Random copolymer surfaces may also be applicable to controlling surface dynamic behavior such as indentation and adhesion on macroscopic to nanoscopic scales. Acknowledgment. This work was supported by a grant from Gunma UniversitysSatellite Venture Business Laboratory (GUsSVBL). LA011643A
