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Visualization of Active Surface Molecular Motion in Polystyrene Film by Scanning Viscoelasticity Microscopy Keiji Tanaka, Kohsuke Hashimoto, and Tisato Kajiyama* Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan
Atsushi Takahara Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 812-8581, Japan Received March 29, 2003. In Final Form: June 4, 2003 A monodisperse polystyrene (PS) film was spin-coated on a silicon wafer with native oxide layer and was in part scratched by a blade. Then, the surface modulus on a given area, in which PS and bared silicon were present, was two-dimensionally mapped as a function of temperature by scanning viscoelasticity microscopy. We visually present evidence that the PS surface started to soften up at a temperature much lower than the bulk glass transition temperature.
Introduction Since the 1990s, thermal properties at surfaces in polymer films have been extensively explored with the advent of modern spectroscopic and microscopic methods. Restricting ourselves to the discussion of surface molecular motion in polystyrene (PS) films for brevity, conclusions that emerged so far can be roughly divided into two notions: active1-10 and nonactive11-14 compared to the internal bulk one. Also, peculiar thermal properties of PS thin and ultrathin films have been accounted for by the notion of surface mobile layer.15-19 We have used scanning * To whom correspondence should be addressed. Fax: +81-92651-5606. Tel: +81-92-642-3560. E-mail: kajiyama@ cstf.kyushu-u.ac.jp. (1) Meyers, G. F.; DeKoven, B. M.; Seitz, J. T. Langmuir 1992, 8, 2330. (2) (a) Kajiyama, T.; Tanaka, K.; Takahara, A. Macromolecules 1997, 30, 280. (b) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2000, 33, 7588. (c) Satomi, N.; Tanaka, K.; Takahara, A.; Kajiyama, T.; Ishizone, T.; Nakahama, S. Macromolecules 2001, 34, 8761. (3) Jean, Y. C.; Zhang, R.; Cao, H.; Huang, B.; Nielsen, P.; AsokaKumar, P. Phys. Rev. B 1997, 56, R8459. (4) Hammerschmidt, J. A.; Gladfelter, W. L.; Haugstad, G. Macromolecules 1999, 32, 3360. (5) Schwab, A. D.; Agra, D. M. G.; Kim, J. H.; Kumar, S.; Dhinojwala, A. Macromolecules 2000, 33, 4903. (6) Fryer, D. S.; Nealey, P. F.; de Pablo, J. J. Macromolecules 2000, 33, 6439. (7) Zaporojtchenko, V.; Strunskus, T.; Erichsen, J.; Faupel, F. Macromolecules 2001, 34, 1125. (8) Kerle, T.; Lin, Z.; Kim, H. C.; Russell, T. P. Macromolecules 2001, 34, 3484. (9) Wallace, W. E.; Fischer D. A.; Efimenko, K.; Wu, W. L.; Genzer, J. Macromolecules 2001, 34, 5081. (10) Bliznyuk, V. N.; Assender, H. E.; Briggs, G. A. D. Macromolecules 2002, 35, 6613. (11) Xie, L.; DeMaggio, G. B.; Frieze, W. E.; DeVries, J.; Gidley, D. W.; Hristov, H. A.; Yee, A. F. Phys. Rev. Lett. 1995, 74, 4947. (12) Hamdorf M.; Johannsmann, D. J. Chem. Phys. 2000, 112, 4262. (13) (a) Ge, S.; Zhang, W.; Rafailovich, M.; Sokolov, J.; Buenviaje, C.; Buckmaster, R.; Overney, R. M. Phys. Rev. Lett. 2000, 85, 2340. (b) Pu, Y.; Ge, S.; Rafailovich, M.; Sokolov, J.; Duan, Y.; Pearce, E.; Zaitsev, V.; Schwarz, S. Langmuir 2001, 17, 5865. (c) Pu, Y.; Rafailovich, M. H.; Sokolov, J.; Gersappe, D.; Peterson, T.; Wu, W. L.; Schwarz, S. A. Phys. Rev. Lett. 2001, 87, 206101. (14) Weber, R.; Zimmermann, K. M.; Tolan, M.; Stettner, J.; Press: W.; Seeck, O. H.; Erichsen, J.; Zaporojtchenko, V.; Strunskus, T.; Faupel, F. Phys. Rev. E 2001, 64, 061508. (15) (a) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (b) Kawana, S.; Jones, R. A. L. Phys. Rev. E 2001, 63, 021501.
viscoelasticity microscopy (SVM),20 which is one of the scanning force microscopic techniques, to work on it. In SVM, a probe tip is indented into the sample surface and then sinusoidally modulated. By detection of an amplitude change in the response force signal as well as a phase lag between the stimulus displacement and the response force signals, surface viscoelastic properties can be extracted. Consequently, we have found that in the case of monodisperse PS, surface glass transition temperature, Tgs, was much lower than the corresponding bulk glass transition temperature, Tgb. The objective of this study is to present visually by SVM how thermal molecular motion at the surface in PS films is activated in comparison with the internal bulk phase. Experimental Section Monodisperse PS with the number-average molecular weight, Mn, of 1.03 × 106 was used as the material. The Tgb of this sample determined by differential scanning calorimetry (DSC) was 378 K. The PS film was spin-coated from a toluene solution onto a cleaned silicon wafer with native oxide layer. The film was dried at 296 K for more than 24 h and then annealed at 423 K for 48 h under vacuum. The film thickness evaluated by ellipsometric measurement was approximately 100 nm. Surface morphology of the film was observed by atomic force microscopy (AFM). The AFM image was obtained by an SPA 300HV with an SPI 3700 controller (Seiko Instruments Industry Co., Ltd.) at 293 K in air. Surface molecular motion was examined by SVM with the stage (16) (a) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, 2002. (b) Forrest, J. A.; Dalnoki-Veress, K.; Dutcher, J. R. Phys. Rev. E 1997, 56, 5705. (c) Dalnoki-Veress, K.; Forrest, J. A.; de Gennes, P. G.; Dutcher, J. R. J. Phys. IV 2000, 10, 221. (d) Dalnoki-Veress, K.; Forrest, J. A.; Murray, C.; Gigault, C.; Dutcher, J. R. Phys. Rev. E 2001, 63, 031801. (17) (a) Forrest, J. A.; Svanberg, C.; Re´ve´sz, K.; Rodahl, M.; Torell; L. M.; Kasemo, B. Phys. Rev. E 1998, 58, R1226. (b) Forrest, J. A.; Mattsson, J. Phys. Rev. E 2000, 61, R53. (c) Mattsson, J.; Forrest, J. A.; Bo¨rjesson, L. Phys. Rev. E 2000, 62, 5187. (18) (a) Tsui, O. K. C.; Zhang, H. F. Macromolecules 2001, 34, 9139. (b) Xie, F.; Zhang, H. F.; Lee, F. K.; Du, B.; Tsui, O. K. C.; Yokoe, Y.; Tanaka, K.; Takahara, A.; Kajiyama, T.; He, T. Macromolecules 2002, 35, 1491. (19) (a) Hall, D. B.; Torkelson, J. M. Macromolecules 1998, 31, 8817. (b) Ellison, C. J.; Kim, S. D.; Hall, D. B.; Torkelson, J. M. Eur. Phys. J. E 2002, 8, 155. (c) Ellison, C. J.; Torkelson, J. M. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2745. (20) Kajiyama T.; Tanaka, K.; Ohki, I.; Ge, S.-R.; Yoon, J.-S.; Takahara, A. Macromoelcules 1994, 27, 7932.
10.1021/la034542g CCC: $25.00 © 2003 American Chemical Society Published on Web 07/17/2003
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Figure 1. PS film in part scratched by a blade: (a) surface morphology by AFM and (b) modulus mapping by SVM. modulation mode. A cantilever tip used for the measurement was microfabricated from Si3N4, and its spring constant was 0.75 N m-1. Also, the radius of curvature of the tip was approximately 20 nm. The normal force was fixed to be 1 nN. The modulation frequency and amplitude were 1 kHz and 1.0 nm, respectively. The probe depth of the measurement was roughly 70 nm,21 provided that viscoelastic properties closer to the surface predominate in results obtained. Also, an ultrathinning effect on the results might be trivial, if any.22
Results and Discussion At first, a PS film was in part scratched by a blade so that the silicon substrate was exposed to the air. Figure 1a shows a topographic image of the partly scratched PS
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film. The height difference between the unscratched and valley regions was approximately 100 nm and was in good accordance with the film thickness evaluated by ellipsometry. Hence, it seems reasonable to postulate that the higher and lower regions in the image correspond to the PS and silicon wafer surfaces, respectively. Part b of Figure 1 shows two-dimensional mapping of surface modulus in the film at room temperature, which was simultaneously obtained with the topographic image. Brighter and darker areas correspond to higher and lower modulus regions, respectively. Since the PS and silicon moduli at room temperature are 4.5 and 280 GPa, respectively, it is quite reasonable that the PS surface was observed as the lower modulus region in the SVM image. To gain access to the absolute value of surface modulus by SVM, it is necessary to evaluate how a tip contacts with the surface. This has been experimentally carried out at a given temperature.2a However, the temperature varies in this experiment, and this leads to technical difficulties to examine the temperature dependence of tip contact manner with the surface. Hence, the apparent surface modulus is here expressed in an arbitrary unit. Figure 2 shows the SVM images collected at various temperatures from 200 to 400 K. Since the surface modulus of the silicon substrate should be invariant with respect to temperature in the employed range, the contrast enhancement between the PS and Si surfaces with temperature reflects that the modulus of the PS surface starts to decrease. In the case of a lower temperature, the image contrast was trivial, as shown in the top line of Figure 2. On the other hand, as the temperature went beyond 330 or 340 K, the contrast between the PS and Si surfaces became remarkable with increasing temperature, meaning that the PS surface reached glass-rubber transition state at around these temperatures. Here, it should be recalled that the Tgb of the PS by DSC was 378 K. Hence, we visually present in Figure 2 that surface mobility in the PS film was enhanced in comparison with the bulk one. To discuss quantitatively about a temperature at which the PS surface started to become softer, the modulus difference between the PS and Si surfaces, ∆E, was plotted as a function of temperature. Figure 3 shows such a plot. Also, for comparison, our previous data, surface phase lag between imposed displacement signal and detected re-
Figure 2. SVM images of the film at various temperatures. The left- and right-hand side regions in each image correspond to PS and Si surfaces, respectively.
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of the two tangents was determined to be 341 K, being much smaller than the Tgb of 378 K. This result is in good accordance with the temperature-surface phase lag relation marked by open circles. In conclusion, we directly observed by SVM that the PS surface started to soften up at a temperature lower than the corresponding Tgb. The results undoubtedly show that surface mobility in the PS film was much enhanced in comparison with the bulk one.
Figure 3. Temperature dependence of surface modulus difference between PS and Si, ∆E. For a comparison, the temperature-surface phase lag, δ, relation is cited from ref 2c.
sponse force, for the PS film with Mn of 1.5 × 106 is shown in Figure 3.2c The dashed tangents denote linear regions of ∆E in glassy and transition regions. The crossing point
Acknowledgment. This was in part supported by Grant-in-Aids for Scientific Research (A) (#13355034) and for the 21st century COE program “Functional Innovation of Molecular Informatics” from the Ministry of Education, Culture, Sports, Science and Technology, Japan. LA034542G (21) Satomi, N.; Tanaka, K.; Takahara, A.; Kajiyama T. Macromoelcules 2001, 34, 6420. (22) Akabori, K.; Tanaka, K.; Kajiyama T.; Takahara, A. Macromolecules 2003, 36, 4937.
