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Surface Deformation Properties of Polystyrene as Evaluated from the Morphology of Surfaces Scratched by Using the Tip of a Scanning Force Microscope Taku Aoike,* Takahiro Yamamoto, Hiroki Uehara, Takeshi Yamanobe, and Tadashi Komoto Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan Received February 23, 2001. In Final Form: June 11, 2001
Introduction The use of polymeric materials has been proposed for the fields of nanotechnology and microelectromechanical systems technology.1,2 Polymeric materials are softer than metallic and inorganic materials and are deformable during thermal and mechanical processes. Micro- and nanofabrications of polymeric surfaces have been attempted by using a scanning force microscope (SFM). Scratching with the tip of an SFM and thermomechanical modification using a heated tip have been proposed as a surface fabrication technique.3,4 However, there is an inherent problem in macromolecules for micro- and nanofabrications of polymeric surfaces: the influence of molecular weight on the surface deformation properties of polymeric materials. Leung et al.5 first reported that SFM tip multiline scanning produced a persistently periodic patterned structure perpendicular to the scanning direction on an atactic polystyrene (PS) surface. Their results demonstrated that there was no molecular weight dependence of the morphology of this structure for PS within a molecular weight range of 32K-573K. On the other hand, Meyers et al.,6 who studied the molecular weight dependence of this nanoperiodic pattern by using repeated multiline scanning on the same area of PS surfaces, reported that repeated scanning changed the morphology of this patterned structure depending on the number of scans and that the change in morphology depended on the molecular weight. It is well-known that plastic deformation of polymeric bulk depends on the molecular weight. We therefore consider that deformation and fracture of a polymeric surface due to scratching with an SFM tip might also depend on the molecular weight. Studies on surface compositions of low-molecular-weight protonated PS (h-PS)/high-molecular-weight deuterated PS (d-PS) blends have suggested that surface segregation of a low-molecular-weight component would occur in the case of athermal binary PS blends with disparate molecular weights.7,8 Thus, the existence of a lower-molecularweight component might influence the surface deformation * To whom correspondence should be addressed. E-mail: aoike@ polymer.chem.gunma-u.ac.jp. (1) Quake, S. R.; Scherer, A. Science 2000, 290, 1536. (2) Soper, S. A.; Ford, S. M.; Qi, S.; McCarley, R. L.; Kelly, K.; Murphy, M. C. Anal. Chem. 2000, 72, 643A. (3) Jin, X.; Unertl, W. N. Appl. Phys. Lett. 1992, 61, 657. (4) Mamin, H. J. Appl. Phys. Lett. 1996, 69, 433. (5) Leung, O. M.; Goh, M. C. Science 1992, 255, 64. (6) Meyers, G. F.; DeKoven, B. M.; Seitz, J. T. Langmuir 1992, 8, 2330. (7) Hariharan, A.; Kumar, S. K.; Russell, T. P. J. Chem. Phys. 1993, 98, 4163. (8) Hariharan, A.; Kumar, S. K.; Russell, T. P. J. Chem. Phys. 1993, 99, 4041.
properties in the case of polydisperse PS and binary PS blends. Tanaka et al.9 reported similar results using scanning force microscopy. Their results revealed that the surface molecular motions of binary PS blends and polydisperse PS containing a lower-molecular-weight component with a number-average molecular weight, Mn, of less than ca. 30K were dominated by the lowermolecular-weight component segregated at the surfaces of these samples.9 On the other hand, it has also been reported that the existence of a lower-molecular-weight component in bulk hardly influences the surface properties. Adhesion measurement of polydisperse poly(tert-butyl acrylate) by Tsui et al.10 using scanning force microscopy has demonstrated that molecular relaxation at the free polymeric surface where the lower-molecular-weight component might be segregated is not noticeably different from that of the bulk. Gracias et al.11 also reported that no discernible differences between the glass transition temperatures, Tg’s, of the bulks and surfaces of polydisperse isotactic and atactic polypropylenes were observed. These conflicting results indicate that the influence of molecular weight on surface mechanical properties of a polymeric material is still not clearly understood. We report here our findings from our investigation of the surfaces of PS films that were scratched by the tip of an SFM. The molecular weight dependence of the surface deformation properties of PS was evaluated from the morphology of scratched surfaces of monodisperse PS films having different Mn’s (8K-984K). Furthermore, the surfaces of athermal binary PS blend films with mixed Mn of 8K and 164K were also scratched to investigate the influence of the existence of a low Mn component in bulk on surface deformation properties. Experimental Section The atactic PSs used in this study were purchased from Scientific Polymer Products Inc. (SPP). Their Mn’s (the molecular weight distributions, Mw/Mn’s, where Mw denotes the weightaverage molecular weight) were 8K (1.05), 15.8K (1.05), 58K (1.06), 164K (1.04), and 984K (1.03) from data sheets supplied by SPP. The films were prepared by polymer solution casting. The solutions of monodisperse PSs and binary PS blends prepared by mixing Mn of 8K and 164K (8K/164K (wt/wt) ) 0/100, 10/90, 30/70, 50/50, 70/30, 90/10, and 100/0) in toluene were cast onto mica substrates. These films were dried at room temperature (RT) in ambient atmosphere for 24 h, then kept at RT and at 423 K in a vacuum for 24 h, respectively, and finally cooled to RT at ca. 1 K/min. The films were ca. 1-2 µm thick. Scratching on the PS surface was done with a tip of an SFM in ambient atmosphere (in air, 18-20 °C, relative humidity of ∼30%). The SFM used in this study was an SPA 400 with an SPI 3800N controller (Seiko Instruments Industry Co., Ltd.). A commercial triangle 200 µm cantilever having a spring constant of 0.16 N/m with an Si3N4 integrated tip (Olympus Optical Co., Ltd.; spring constant as specified by the manufacturer) was used.12 Scratches were made in a 1 × 1 µm2 area by scanning with the SFM tip at 1 µm/s under an applied load of 5-30 nN. Each scratching included 256 line scratches perpendicular to the scan direction. Topographical images were obtained at 10 (9) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1997, 30, 6626. (10) Tsui, O. K. C.; Wang, X. P.; Ho, J. Y. L.; Ng, T. K.; Xiao, X. Macromolecules 2000, 33, 4198. (11) Gracias, D. H.; Zhang, D.; Lianos, L.; Ibach, W.; Shen, Y. R.; Somorjai, G. A. Chem. Phys. 1999, 245, 277. (12) The radius of the tip used in this study was ca. 20 nm as measured by scanning electron microscopy. The applied load required for surface deformation changes with the tip radius.
10.1021/la0102962 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
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min after scratching by scanning over a wider area under an attractive or weakly repulsive force of
