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Boundary Effect of Relief Structure on Crystallization of Diblock Copolymer in Thin Films Fajun Zhang,*,† Yongzhong Chen, Haiying Huang, Zhijun Hu, and Tianbai He*,‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun. Jilin 130022, China Received January 29, 2003. In Final Form: May 12, 2003 In this Letter, crystal growth of a symmetric crystalline-amorphous diblock copolymer, poly(styreneb--caprolactone) (PS-b-PCL), in thin films was investigated by atomic force microscopy (AFM). Relief structures of holes and islands were formed during annealing the film at the molten state, and the in situ observation of subsequent crystal growth at room temperature indicated that the crystals were preferred to occur at the edge of holes or islands and grew into the interior area. It was concluded that the stretched PCL blocks at the edge of relief structures, caused by material transportation or deformation of the interface, could act as nucleation agents during polymer crystallization. The crystal growth rate of individual lamellae varied both from lamellae to lamellae and in time, but the area occupied by crystals increased constantly with time. At 22 °C, the growth rate was 1.2 × 10-2 µm2/min with the scan size 2 × 2 µm2.
Introduction When copolymers are confined to the surface of a solid substrate, surface energy differences between the block components or the chemical affinity of one block for the substrate forces the orientation of lamellae or cylinders parallel to the film surface.1-5 For the symmetric diblock copolymer, this preferred orientation causes a quantization of the film height to (n + 1/2)L or nL, where L is the bulk value of the copolymer lamellar period and n is an integer. For films with thickness which do not initially satisfy this constraint, relief structures of islands or holes with a step height of L are formed on the surface.1-5 If one of the blocks is crystalline, it is of main interest whether block copolymers crystallize within a microdomain or not, that is, whether the microphase separation structure in the melt is destroyed by crystallization. Although the bulk properties of the crystalline block copolymer have been extensively studied6,7 and reviewed,8 far less is known * To whom correspondence should be addressed. † E-mail:
[email protected]. Fax: + 86-431-5262126. Telephone: +86-431-5262123. ‡ Present address: Unite ´ de Physique et de Chimie des Hauts Polyme`res, Universite´ Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium. E-mail:
[email protected]. Fax: (32 10) 45 15 93. Telephone: (32 10) 47 30 88. (1) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600. (b) Bassereau, P.; Brodbreck, D.; Russell, T. P.; Brown, H. R.; Shull, K. R. Phys. Rev. Lett. 1993, 70, 1716. (c) Mayes, A. M.; Russell, T. P.; Bassereau, P.; Baker, S. M.; Smith, G. S. Macromolecules 1994, 27, 749. (d) Cai, Z.; Huang, K.; Montano, P. A.; Russell, T. P.; Bai, J. M.; Zajac, G. W. J. Chem. Phys. 1993, 98, 2376. (2) Grim, P. C. M.; Nyrkova, I. A.; Semenov, A. N.; ten Brinke, G.; Hadziioannou, G. Macromolecules 1995, 28, 7501. (3) Maaloum, M.; Ausserre, D.; Chatenay, D.; Gallot, Y. Phys. Rev. Lett. 1993, 70, 2577. (4) Sikka, M.; Singh, N.; Karim, A.; Bates, F. S.; Satija, S. K.; Majkrzak, C. F. Phys. Rev. Lett. 1993, 70, 307. (5) Coulon, G.; Daillant, J.; Collin, B.; Benattar, J. J.; Gallot, Y. Macromolecules 1993, 26, 1582. (b) Coulon, G.; Russell, T. P.; Deline, V. R.; Green, P. F. Macromolecules 1989, 22, 2581. (6) Zhu, L.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Liu, L.; Lotz, B. Macromolecules 2001, 34, 1244. (b) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. J. Am. Chem. Soc. 2000, 122, 5957; Polymer 2001, 42, 5829. (7) Loo, Y. L.; Register, R. A.; Ryan, A. J.; Dee, G. T. Macromolecules 2001, 34, 8968. (b) Loo, Y. L.; Register, R. A.; Ryan, A. J. Phys. Rev. Lett. 2000, 84, 4120; Macromolecules 2002, 35, 2365. (c) Loo, Y. L.; Register, R. A.; Adamson, D. H. Macromolecules 2000, 33, 8361. (8) Hamley, I. W. Adv. Polym. Sci. 1999, 148, 113.
about crystallization of crystalline block copolymers in thin films. Up to now, only few studies on the behavior of crystalline block copolymers in thin films have been reported.9-13 They indicated that the geometry of a thin film may allow for morphologies which are impossible in the bulk. Reiter et al.9 investigated the morphology of PBh-PEO diblock copolymers in thin films; they observed that, for a copolymer containing 45% PEO, lamellae orientated parallel to the substrate in the melt but perpendicular to the substrate upon crystallization at a large undercooling. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase contact line. Opitz et al.10 studied the confined crystallization of ethylene oxide-butadiene diblock copolymers in lamellar films. They found that crystallization of the PEO block led to an increase in the lamellar thickness of both blocks. Hong et al.13 investigated the evolution of the morphology of poly(ethylene oxide-b-1,4 butadiene) upon crystallization in thin films. It was found that multiple parallel layers of crystalline PEO were in orientational registry even though they were separated by approximately 10 nm thick layers of amorphous PBD. Films of copolymers in the studies9-13 were characterized by smooth islands and holes at the surface due to incommensurability between the film thickness and an integral number of lamellae. The relief structure was retained upon crystallization. However, the effect of relief structure on crystallization of diblock copolymer in a thin film has not been reported. In this Letter, a symmetric diblock copolymer, poly(styrene-b--caprolactone) (PS-b-PCL), was used to study the boundary effect of relief structure. In this system, the crystallization of the PCL block could overwhelm the microphase-separated structure because of the weak segregation.14 The crystal growth process was observed (9) Reiter, G.; Castelein, G.; Hoerner, P.; Riess, G.; Blumen, A.; Sommer, J. U. Phys. Rev. Lett. 1999, 83, 3844. (b) Reiter, G.; Castelein, G.; Hoerner, P.; Riess, G.; Sommer, J. U.; Floudas, G. Eur. Phys. J. E 2000, 2, 319. (c) Reiter, G.; Castelein, G.; Sommer, J. U. Phys. Rev. Lett. 2001, 87, 226101. (10) Opitz, R.; Lambreva, D. M.; de Jeu, W. H. Macromolecules 2002, 35, 6930. (11) De Rosa, C.; Park, C.; Lotz, B.; Wittmann, J. C.; Fetters, L. J.; Thomas, E. L. Macromolecules 2000, 33, 4871. (12) Balsamo, V.; Collins, S.; Hamley, I. W. Polymer 2002, 43, 4207. (13) Hong, S.; MacKnight, W. J.; Russell, T. P.; Gido, S. P. Macromolecules 2001, 34, 2398; Macromolecules 2001, 34, 2876.
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in situ by atomic force microscopy (AFM); it was found that the nucleation process preferred to occur at the edge of holes or islands. Experimental Section An approximately symmetric PS-b-PCL diblock copolymer was obtained from Polymer Source, Inc., Canada. The number averaged molecular weight Mn ) 18 500 with a PS volume fraction of 54%, and the polydispersity was 1.13. The melting point, Tm, of the sample was determined using a Perkin-Elmer DSC-7 at a scanning rate of 10 °C/min; Tm was 48.6 °C, and no apparent glass transition temperature (Tg) was observed due to the overlapping between Tm and Tg.14 In the subsequent cooling and heating scans, both exotherm and endotherm were absent. However, holding the sample at room temperature (22 °C) for 3 days, an endotherm with peak temperature 48.3 °C appeared, which indicated that the PCL blocks could slowly crystallize at room temperature. Thin polymer films were prepared by spincoating a copolymer xylene solution (5 mg/mL) onto silicon wafers, the film thickness measured by AFM after scratching with a blade was around 18 nm, and the surface morphology and crystal growth were observed directly by AFM. The samples were annealed at 120 °C for several minutes and then quickly transferred to the scanner. Tapping mode AFM images were obtained at ambient conditions using a SPA-300HV with a SPI 3500N controller (Seiko Instruments Industry Co., Ltd.). Both height and phase images were recorded simultaneously. Etched Si tips with a resonance frequency of approximately 250-300 kHz and a spring constant of about 42 N m-1 were used, and the scan rate was in the range 1.0-2.0 Hz.
Results and Discussion The surface of the initial block copolymer thin film is rather smooth with roughness (1 nm as measured by AFM. After annealing at 120 °C for several minutes, islands or holes were observed, depending on the initial film thickness. Figure 1a shows the resulting surface morphology of diblock copolymer thin films measured immediately after annealing. The fraction of holes area is about 40%, and the period L is about 16 nm, as measured by AFM section analysis. For PS-b-PCL diblock copolymers, the polar PCL block prefers to wet the surface of a silicon wafer, while the PS block wets the air interface.12 Figure 1b is the resulting surface morphology after further crystallization at room temperature (22 °C) for about 10 h. It is obvious that the crystallization of the PCL block destroys the lamellar structure formed via microphase separation and dendritelike crystals fill both the holes and the plateau. It seems that the crystals are formed at the edges of holes and propagate throughout the film. To further investigate the nucleation process of a crystalline diblock copolymer thin film, the formation of initial crystals and their growth have been observed in situ by AFM. Figure 2 shows a sequence of AFM images of crystal growth in a large area. The time interval between each consecutive image is approximately 10 min. The time interval between quenching the film to room temperature and getting the first image is about 20 min. The left column of Figure 2 displays the height images, and the right column shows the corresponding phase images. In Figure 2a, three typical areas in various growth stages can be seen: in area A, the crystallization does not occur yet; area B crystals have been well developed; and in area C, the crystal growth is at the beginning stage. In Figure 2b, after further crystallization for 10 min, initial crystals can be observed at area A, while little change is found in area B and edge-on crystals form at area C. Continuing crystallization at room temperature as seen in Figure 2c (14) Heuschen, J.; Jerome, R.; Teyssie, P. H. J. Polym. Sci., Part B: Polym. Phys. 1989, 27, 523.
Figure 1. Surface morphology of a PS-b-PCL diblock copolymer thin film annealed at 120 °C for 5 min: (a) immediately measured by AFM after annealing; (b) stored at room temperature (22 °C) for 10 h.
and d, the initial crystals formed at the edges of holes grow constantly toward both the holes and the plateau. In all experiments, we find that the initial crystals are always formed at the edges of holes or islands. This result further proves that the relief structure really affects the nucleation process of a copolymer thin film, but why? The formation of islands or holes is obviously associated with material transport.1-5 It is believed that the surface distortion can be induced by a single edge dislocation under the monolayer.15,16 This situation for a two-layer film is illustrated in Figure 3. Because the PS layer is stiffer
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Figure 3. Schematic of an edge dislocation along the boundary of a hole at the surface of the diblock copolymer film. The bright layer presents the PCL block, and the gray layer presents the PS block.
Figure 2. Series of AFM images of crystal growth in a large area. The film was annealed at 120 °C for 5 min and then crystallized at room temperature; scan size 20 × 20 µm2.
than the PCL layer, it tends to form the dislocation core, while the PCL layer is forced to bend around it.16 From Figure 3, it can be seen that in order for the PCL chains which originate at the PS interface to reach the substrate surface, a significant amount of stretching must occur. Furthermore, formation of islands and holes requires the interface to deform and stretch (increase area). Thus, the edges of islands or holes can act as nucleating agents during crystallization. It is conceivable that the microdomain interface could act as a locus for nucleation, so that the process would not be true homogeneous nucleation. However, we expect the PCL blocks to be homogeneously nucleated at the edge of the relief structure because no crystals have been found (15) Maaloum, M.; Ausserre, D.; Chatenay, D.; Coulon, G. Phys. Rev. Lett. 1992, 68, 1575. (16) Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A.; Bahal, S. Macromolecules 1996, 29, 899.
at the other parts of the films. The idea of homogeneous nucleation in block copolymers was first proposed by Lotz and Kovacs17 and supported by the work of Robitaille et al.,18 which showed that very high undercoolings were needed to crystallize the PEO minority blocks in a triblock with a vitreous matrix. In our case, the prealignment of PCL blocks at the edge of a relief structure leads the nucleation process to be easy to occur, and no high undercoolings are needed to crystallize the PCL blocks in such a thin layer. Figure 4 shows three consecutive AFM height images of crystal growth from the edge of a hole. Initially (Figure 4a), the edge-on lamellae are observed near the edge. Three edge-on lamellae have been labeled in Figure 4a; straight lamella 1 and 2 are parallel to each other with similar lengths. Lamella 3 shows a curvature near its tip. After 5 min (Figure 4b), lamella 1 grows about 300 nm, but lamella 2 only lengthens about 20 nm. The tip of lamella 1 becomes widened. Branches have been formed at the tip of lamella 3. After further crystallization for 5 min (Figure 4c), several branches have formed at the tip of lamella 1, and the growth of lamella 2 seems stopped, which may be caused by the screening effect of fast growth of lamella 1. Lamella 3 continues to grow and branch with time. This result indicates that the growth rates of the edge-on lamellae vary both from each other and in time. A number of AFM studies of lamellar scale growth have reached the same conclusion.19-21 Very recently, Frank and co-workers reported the crystal growth of a PEO thin film on a silicon wafer observed in situ by hot-stage AFM.22 Their observation of a constant linear growth rate suggested that the previously reported discontinuous growth of lamellae including our case is caused by competition among the different lamellae at the growth front. Further observations on the early stage of crystal growth indicate that the edge-on lamellae cannot grow longer than 2 µm before branching and coarsening. Thus, it is difficult to determine the growth rate of individual lamellae. In fact, in the growth front of well-developed crystals (see onset images in Figure 5), one cannot distinguish the individual lamellae anymore. In this case, (17) Lotz, B.; Kovacs, A. J. ACS Polym. Prepr. 1969, 10 (2), 820. (18) Robitaille, C.; Prud’homme, J. Macromolecules 1983, 16, 665. (19) Li, L.; Chan, C. M.; Yeung, K. L.; Li, J. X.; Ng, K. M.; Lei, Y. Macromolecules 2001, 34, 316. (b) Lei, Y. G.; Chan, C. M.; Li, J. X.; Ng, K. M.; Jiang, Y.; Li, L. Macromolecules 2002, 35, 6751. (20) Hobbs, J. K.; Humphris, A. D. L.; Miles, M. J. Macromolecules 2001, 34, 5508. (b) Hobbs, J. K.; Miles, M. J. Macromolecules 2001, 34, 353. (21) Pearce, R.; Vancso, G. J. Macromolecules 1997, 30, 5843. (22) Scho¨nherr, H.; Frank, C. W. Macromolecules 2003, 36, 1188 and 1199. (b) Scho¨nherr, H.; Bailey, L. E.; Frank, C. W. Langmuir 2002, 18, 490.
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Figure 4. Three consecutive AFM images for edge-on lamellae grown from the edge of a hole at high magnification. The scale bar is 1 µm.
Figure 5. Plots of the area occupied by crystals as a function of time for the scan size 2 × 2 µm2. Onsets are the crystal growth montages at different times.
the crystal growth front is characterized by clusters and no preferred growth direction is observed. So we use the time dependence of the ratio of occupied area by crystals to characterize the growth rate (Figure 5). It is shown that at the early and later stages the area of crystal increases constantly with time. A fast increase at the
intermediate time stage is not due to the acceleration of crystal growth, but the incorporation of other crystals at the left region in onset image b of Figure 5. So the crystal growth rate of PCL blocks in phase-separated thin films can be presented by the time dependence of the area occupied by crystals. It is about 1.2 × 10-2 µm2/min at room temperature (22 °C) with the scan size 2 × 2 µm2. In summary, we reported here a novel crystallization behavior of diblock copolymer in thin films: the crystal growth always started at the edge of relief structures formed during annealing. It was concluded that the stretched PCL blocks at the edge of relief structures, caused by material transportation or deformation of the interface, could act as nucleation agents during polymer crystallization. Kinetics results indicated that the growth rate of individual lamellae varied both from lamellae to lamellae and in time, but the area occupied by crystals increased constantly with time. At 22 °C, the growth rate was 1.2 × 10-2 µm2/min with the scan size 2 × 2 µm2. Acknowledgment. This work is subsidized by the Special Funds for Major State Basic Research Projects of China and supported by the National Science Foundation of China. The authors thank Professor Gu¨nter Reiter (Institute de Chimie des Surfaces et interfaces, CNRS, Mulhouse Cedex, France) for helpful discussions. LA034162V
