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Effect of Substrate and Molecular Weight on the Stability of Thin Films of Semicrystalline Block Copolymers Guo-Dong Liang,† Jun-Ting Xu,*,† Zhi-Qiang Fan,† Shao-Min Mai,‡ and Anthony J. Ryan‡ Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang UniVersity, Hangzhou 310027, China, and Department of Chemistry, The UniVersity of Sheffield, S3 7HF, Sheffield, U.K. ReceiVed October 30, 2006. In Final Form: December 19, 2006 The thermal stability of the thin film morphology of two symmetric oxyethylene/oxybutylene block copolymers (E76B38 and E114B56) on mica and silicon was investigated via atomic force microscopy (AFM). It is found that morphological transition of EmBn thin films during melting is strongly dependent on the molecular weight of the diblock copolymers and their interaction with the substrate. For E76B38 on mica, a single-layered structure transforms into a double-layered structure upon melting, but the same polymer on silicon retains a single-layered structure after melting and spreads quickly to wet-out the silicon surface. Conversely a longer polymer, E114B56, has a thin film on mica that does not change much after melting of the crystalline E block. A mechanism was proposed to explain the relative stability of E76B38 and E114B56 thin films upon melting. Internal stress is produced during melting and can be released along two directions. The release along the vertical direction is restricted by the energy barrier related to the segregation strength, and the release along the horizontal direction is dependent on the mobility of block copolymer related to the interaction between the block copolymer and the substrate. Domain size affects the release rate of the internal stress along the horizontal direction and thus the thermal stability of EmBn thin films. Switching between horizontal and vertical releases can be realized by controlling the domain size of the thin films.
Introduction Block copolymers exhibit abundant hierachical textures in thin films depending on the composition of block copolymers, substrate, and preparation conditions and are widely used as templates to fabricate nanodevices.1,2 A reversible morphological transformation can significantly simplify the fabrication processes.3,4 The reversible morphological transformation usually corresponds to a reversible physical process, such as phase separation/phase mixing, absorption/desorption of solvent, or melting/crystallization. These reversible physical processes can be triggered by a change in temperature or exposure to solvent.5-8 Such reversible physical processes are, however, often accompanied by irreversible physical processes such as wetting/ dewetting.9 Due to these irreversible physical processes, the thin film morphology of block copolymers may not be recovered after an otherwise reversible cycle. As a result, the stability of thin film morphology before and after morphological transformation is key for a reversible morphological transformation. Since temperature is usually used to trigger many reversible physical processes, lowering the temperature to enhance stability of morphology is not a good choice. In microphase separated * Correspondence author. E-mail:
[email protected]. Fax: +86-57187952400. † Zhejiang University. ‡ The University of Sheffield. (1) Fasolka, M. J.; Mayes, A. M. Annu. ReV. Mater. Res. 2001, 31, 323. (2) Segalman, R. A. Mater. Sci. Eng., R 2005, 48, 191. (3) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (4) Cheng, J. Y.; Ross, C. A.; Chan, V. Z. H.; Thomas, E. L.; Lammertink, R. G. H.; Vancso, G. J. AdV. Mater. 2001, 13, 1174. (5) Xu, T.; Stevens, J.; Villa, J. A.; Goldbach, J. T.; Guarim, K. W.; Black, C. T.; Hawker, C. J.; Russell, T. R. AdV. Funct. Mater. 2003, 13, 698. (6) Peng, J.; Xuan, Y.; Wang, H. F.; Yang, Y. M.; Li, B. Y.; Han, Y. C. J. Chem. Phys. 2004, 120, 11163. (7) Cui, L.; Xuan, Y.; Li, X.; Ding, Y.; Li, B. Y.; Han, Y. C. Langmuir 2005, 21, 11696. (8) Reihs, K.; Voetz, M. Langmuir 2005, 21, 10573. (9) Green, P. F. J. Polym. Sci. Part B: Polym. Phys. 2003, 41, 2219.
block copolymers, the unfavorable interaction between the two unlike blocks is usually involved during morphological transformation. Therefore, the segregation strength may have a great effect on restricting morphological transformations. For example, it has been found that in bulk, crystallization behavior of crystalline/rubbery block copolymers is strongly dependent on segregation strength.10,11 For a crystalline/rubbery block copolymer, when the segregation is weak, breakout crystallization occurs and the morphology in the melt is destroyed upon crystallization. In contrast, confined crystallization is observed at a stronger segregation strength and the morphology in the melt is retained after crystallization.10,11 The thin film morphology of block copolymer is also dependent on the surface properties of the substrate.12-16 We have found that substrate surface has an influence on the thin film crystalline morphology of oxyethylene/oxybutylene block copolymers.17 It is expected that the interaction between the block copolymer and the substrate may also affect the stability of the molten thin film. In this paper, the thin film morphologies of two symmetric oxyethylene/ oxybutylene block copolymers with different molecular weights (thus different segregation strengths) were investigated, before and after melting, on mica and silicon. We found that both the molecular weight and substrate exerted great influence (10) Loo, Y. L.; Register, R. A.; Ryan, A. J. Macromolecules 2002, 35, 2365. (11) Xu, J. T.; Fairclough, J. P. A.; Mai, S. M.; Ryan, A. J.; Chaibundit, C. Macromolecules 2002, 35, 6937. (12) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (13) Mansky, P.; Russell, T. P.; Hawker, C. J.; Mays, J.; Cook, D. C.; Satija, S. K. Phys. ReV. Lett. 1997, 79, 237. (14) Huang, E.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 1998, 31, 7641. (15) Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J. Nature 1998, 395, 757. (16) Huang, E.; Pruzinsky, S.; Russell, T. P.; Mays, J.; Hawker, C. J. Macromolecules 1999, 32, 5299. (17) Liang, G. D.; Xu, J. T.; Fan, Z. Q.; Mai, S. M.; Ryan, A. J. Macromolecules 2006, 39, 5471.
10.1021/la0631724 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
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Figure 1. AFM height image of the annealed E76B38 thin film on mica.
on stability of the melt morphology with respect to the crystalline morphology. Experimental Section Materials. The synthesis and characterization of oxyethylene/ oxybutylene block copolymers, E76B38 and E114B56 (where E and B denote oxyethylene and oxybutylene units, respectively, and denoted as EmBn, the subscripts refer to the average degree of polymerization) have been described elsewhere.18-20 Both block copolymers have narrow molecular weight distributions (Mw/Mn < 1.05) by GPC and have lamellar morphology in the bulk. The segregation strengths, expressed by χN, at 20 °C are 19.9 and 29.7 for E76B38 and E114B56, respectively.18-20 The order-disorder transition temperature (TODT) is 114 °C for E76B38 and 210 °C for E114B56,18-20 and the melting temperatures of both polymers are around 55 °C. The long periods of the lamellar structure in the bulk are 16.7 and 19.8 nm for E76B38 and E114B56 (20 °C, after crystallization), respectively, which are measured by small-angle X-ray scattering. Preparation of Block Copolymer Thin Films. Block copolymer thin films were prepared by spin-coating EmBn block copolymer/ dichloromethane solution (0.5 w/v%) on substrates. When mica was used as substrate, the first several layers on the top of mica were stripped and the block copolymers were spin-coated on the clean and fresh mica surface. The single-crystal silicon wafers (p-type, 6 in. in diameter), supplied by Shanghai Institute of Ceramics, China, were cut into chips of about 10 mm × 10 mm in size, and then treated with “piranha” solution, a mixture of 70 vol % concentrated sulfuric acid and 30 vol % hydrogen peroxide, for about 30 min at 60 °C to generate a clean, hydrophilic oxide surface. The silicon wafers were then rinsed with a large volume of distilled water and dried in a vacuum oven at 80 °C for 8 h. The as-spun thin films of (18) Mai, S. M.; Fairclough, J. P. A.; Viras, K.; Gorry, P. A.; Hamley, I. W.; Ryan, A. J.; Booth, C. Macromolecules 1997, 30, 8392. (19) Mai, S. M.; Fairclough, J. P. A.; Terrill, N. J.; Turner, S. C.; Hamley, I. W.; Matsen, M. W.; Ryan, A. J.; Booth, C. Macromolecules 1998, 31, 8110. (20) Ryan, A. J.; Mai, S. M.; Fairclough, J. P. A.; Hamley, I. W.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 2961.
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Figure 2. AFM height image of the annealed E114B56 thin film on mica. the block copolymers were annealed at 35 °C for 30 h under vacuum (10 Torr).17,21 AFM images of the annealed thin films were recorded. The annealed thin films were then subject to melting and kept molten for various times, and then the thin films were quenched to room temperature for AFM observation. Atomic Force Microscopy (AFM). The thin film morphology of EmBn block copolymers was investigated by tapping mode AFM. A commercial atomic force microscopy (SPA 300HV/SPI3800N Probe Station, Seiko instruments Inc., Japan) in tapping mode was used with a silicon microcantilever (spring constant 16 N/m and resonance frequency ∼138 kHz). The scan rate ranged from 1 to 2.0 Hz to optimize the image quality. The set-point ratio, the ratio between the set-point amplitude and the free vibration amplitude (the lowest amplitude when tip and sample are not in contact), was chosen to be ∼0.8. Parameters characterizing the thin films such as thickness of the thin film were obtained directly from cross-sectional profiles. In order to ensure the reliability of the data, at least ten values were obtained for every parameter and the average value and standard deviation are presented.
Results Morphology of the Annealed Thin Films. Figures 1 and 2 show the AFM height images of the annealed thin films of E76B38 and E114B56 on mica. Annealing was performed at 35 °C for 30 h in a vacuum system. The annealed thin films of both block copolymers exhibit a densely branched structure comprising a single polymer layer with thickness of 1/2L0 (L0 is the long period of EmBn diblock copolymers in the bulk).22 Such a densely branched structure is usually formed via a diffusion limited aggregation (DLA) mechanism during crystallization,23-27 which (21) Liang, G. D.; Xu, J. T.; Fan, Z. Q. Chin. J. Polym. Sci. 2006, 24, 341. (22) Xu, J. T.; Turner, S. C.; Fairclough, J. P. A.; Mai, S. M.; Ryan, A. J.; Chaibundit, C.; Booth, C. Macromolecules 2002, 35, 3614. (23) Witten, T. A.; Sander, L. M. Phys. ReV. Lett. 1981, 47, 1400. (24) Witten, T. A.; Sander, L. M. Phys. ReV. B 1983, 27, 5686. (25) Reiter, G.; Sommer, J. U. Phys. ReV. Lett. 1998, 80, 3771. (26) Wang, M. T.; Braun, H. G.; Meyer, E. Macromolecules 2004, 37, 437.
Thin Films of Semicrystalline Block Copolymers
Figure 3. AFM height image of the annealed E76B38 thin film on silicon.
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Figure 4. AFM height image of the melted E76B38 thin film on mica. The thin film was held at 70 °C for 30 min.
means that the mobility of EmBn block copolymers on mica is low under such crystallization conditions. The AFM image of annealed E76B38 thin film on silicon is shown in Figure 3. In contrast to the densely branched structure on mica, the morphology of annealed E76B38 thin films on silicon is rounded plaques comprising a single polymer layer which is relatively featureless, even though the surfaces of both mica and silicon are hydrophilic. That a densely branched structure was not observed for the annealed E76B38 thin film on silicon shows that the mobility of E76B38 on silicon is high and crystallization is not controlled by diffusion. Effect of Substrate. Figure 4 shows the AFM image of the melted and quenched E76B38 thin film on mica. The molten E76B38 thin film is held at 70 °C, higher than the melting temperature of E block but lower than the order-disorder transition temperature, followed by quenching to room temperature (20 °C) and subsequent recrystallization for 30 min prior to imaging. It is found that, upon melting, the morphology is transformed into double-layered structure for the melted E76B38 thin film from the single-layered structure for the annealed thin film. The cross-sectional profile in Figure 4 shows that the thickness of the first polymer contacting mica surface is about 1/2L0, and the thickness of the upper layer is L0. The distinct structures before and after melting show that the thin film of E76B38 on mica is not stable upon melting. Figure 5 shows the AFM image of the melted and quenched E76B38 thin film on silicon. One can see from Figure 5 that the single-layered structure is retained after melting for the thin film of E76B38 on silicon. Comparing with the annealed E76B38 thin film on silicon (Figure 3), melting leads to spreading of the thin
Figure 5. AFM height image of the melted E76B38 thin film on silicon. The thin film was held at 70 °C for 30 min.
(27) Zhai, X. M.; Wang, W.; Zhang, G. L.; He, B. L. Macromolecules 2006, 39, 324.
film on silicon. As a result, the thin film of E76B38 on silicon is not stable upon melting either. A similar phenomenon is observed
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Figure 6. AFM height images of the melted E114B56 thin films on mica. The thin films were held at 70 °C for 30 min (a) and for 30 h (b), respectively.
for the thin film of E114B56 on silicon (Figures S1-S2 in the Supporting Information). Effect of Molecular Weight. Figure 6 shows the AFM images of the E114B56 thin film on mica after holding at 70 °C for different times followed by quenching to room temperature. It is observed that, for short holding time (
