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Langmuir 2006, 22, 3428-3433
SEBS Aggregate Patterning at a Surface Studied by Atomic Force Microscopy Xia Han, Jun Hu, Honglai Liu,* and Ying Hu Department of Chemistry, State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed May 15, 2005. In Final Form: December 7, 2005 The morphologies of films spin coated from dilute block copolymer solution onto a mica substrate were studied by atomic force microscopy (AFM). Variables of interest were the polymer concentration, solvent, heating temperature, aging, and ultrasonic effect. It is shown that the solution concentration is the predominant factor in determining the shape of the aggregates displayed from spheres and rods to irregular patches with increasing concentration. The solubility parameter of the solvent plays an important role in modifying the distribution and the size of clusters at the surface. The structures of the aggregates at the surface are metastable, which could evolve with temperature from rodlike aggregates into regular stripes when annealed at a temperature higher than the order-disorder transition temperature of SEBS, whereas those in solution could evolve with aging and ultrasonic treatment into a more stable network structure.
1. Introduction Structures at surfaces on the nanometer scale are needed in many application such as microelectronics systems, filter technology, and biosensors. Self-assembly1 is a powerful principle for fabricating organized structures and devices on the mesoscale, bridging the gap between molecular and macroscopic scales.2 When the copolymer is near a surface, it self-assembles into an ordered structure with a specific microdomain orientation.3 The patterning achieved from these polymer assemblies seems to be a nice route for tuning the aggregation pattern of the polymers at a surface. Block copolymers consisting of chemically different and terminally connected segments usually form regularly ordered microdomain structures in the thermodynamic equilibrium state because of the repulsive energetic interaction between constituent block chains.4 Phase separation happens on the nanometer scale because of the unique structure of block copolymers. The different microphase separation morphologies5-7 of thin films of block copolymers have been observed in detail using many methods,8-12 especially atomic force microscopy in recent years.13-18 However, * Corresponding author. E-mail:
[email protected]. Tel: +86-2164252921. Fax: +86-21-64252845. (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (2) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823. (3) Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J. Nature 1998, 395, 757. (4) Bates, F. S.; Fredrickson, G. H. Annu. ReV. Phys. Chem. 1990, 41, 525. (5) Rehse, N.; Knoll, A.; Magerle, R.; Krausch, G. Macromolecules 2003, 36, 3261. (6) Sota, N.; Sakamoto, N.; Saijo, K.; Hashimoto, T. Macromolecules 2003, 36, 4534. (7) Fukunaga, K.; Hashimoto, T.; Elbs, H.; Krausch, G. Macromolecules 2002, 35, 4406. (8) Konrad, M.; Knoll, A.; Krausch, G.; Magerle, R. Macromolecules 2000, 33, 5518. (9) Zhao, Y. Macromolecules 1992, 25, 4705. (10) Jeong, U.; Lee, H. H.; Yang, H.; Kim, J. K.; Okamoto, S.; Aida, S.; Sakurai, S. Macromolecules 2003, 36, 1685. (11) Mackay, M. E.; Hong, Y.; Jeong, M.; Tande, B. M.; Wagner, N. J.; Hong, S.; Gido, S. P.; Vestberg, R.; Hawker, C. J. Macromolecules 2002, 35, 8391. (12) Zhu, Y.; Gido, S. P.; Iatrou, H.; Hadjichristidis, N.; Mays, J. W. Macromolecules 2003, 36, 148. (13) Motomatsu, M.; Mizutani, W.; Tokumoto, H. Polymer 1997, 38, 1779. (14) Wang, Y.; Song, R.; Li, Y.; Shen, J. Surf. Sci. 2003, 530, 136. (15) Scott McLean, R.; Sauer, B. B. Macromolecules 1997, 30, 0, 8314.
few papers show that polymer aggregation from dilute solution leads to the formation of clusters, especially multiscale aggregates at a free surface. Grohens et al.19 studied the multiscale aggregation of poly(methyl methacrylate) (PMMA) stereo complexes at a surface using AFM observation. Effects of the nature of the solvent, PMMA concentration, isotactic/syndiotactic ratio, and the nature of the surface on the morphology of the stereo complex layer at a surface were addressed. It was found that the aggregate size increases with PMMA concentration and a 2D network of connecting aggregates is formed when the concentration reaches 1 g/L in acetone. The aggregates are always 3 to 6 times larger than those in solution measured by dynamic light scattering (DLS), owing to the further aggregation of the stereo complex particles during the spin-coating process at the silicon surface. Stange et al.20 used scanning tunneling microscopy (STM) and AFM to follow the surface morphology evolution of spin-coated polystyrene (PS) films from individual isolated molecules to a continuous film as well as the effects of polymer molecular weight and concentration. At low polymer concentrations (0.0005 wt %), individual polymer molecules are visualized; their apparent size increases with molecular weight. At intermediate concentrations, polymer molecules aggregate to form 2D Voronoi tessellation-like networks that are discussed in terms of a specific failure mechanism leading to film rupture in spin-coating processes. Recently, 2D Voronoi tessellation-like networks were also observed by Li et al.21 using poly(styrene-b-ethylene/ butylene-b-styrene) (SEBS) triblock copolymer. Detailed investigations21 have found that the size of the network grows with the increase in radius from the center to the edge of the substrate, leading to the conclusion that a shearing and stretching field can cause flexible polymer coils or aggregates to orientate during the spin-coating process. Besides, the periodically orientated stripe (16) Rasmont, A.; Lecle`re, Ph.; Doneux, C.; Lambin, G.; Tong, J. D.; Je´roˆme, R.; Bre´das, J. L.; Lazzaroni, R. Colloids Surf., B 2000, 19, 381. (17) Van Dijk, M. A.; Van den Berg, R. Macromolecules 1995, 28, 8, 6773. (18) Stocker, W.; Beckmann, J.; Stadler, R.; Rabe, J. P. Macromolecules 1996, 29, 7502. (19) Grohens, Y.; Castelein, G.; Carriere, P.; Spevacek, J.; Schultz, J. Langmuir 2001, 17, 86. (20) Stange, T. G.; Mathaw, R.; Evans, D. F. Langmuir 1992, 8, 920. (21) Li, X.; Han, Y.; An, L. Langmuir 2002, 18, 5293.
10.1021/la051293i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/01/2006
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Figure 1. AFM images of the films spin coated from (a) 0.005, (b) 0.06, (c) 0.3, (d) 1.0, (e) 1.5, and (f) 2.0 g/L toluene solutions. Insets in f are the highly magnified height image (top) and the phase image (bottom).
morphology of the as-prepared thin film produced in the spincoating process was first reported in the literature.21 The evolution of the orientation structures of these ordered strips can be explained by schematic drawings in accord with that introduced by Stange.20 References 20 and 21 were devoted to the aggregation evolution induced by a shearing force at a surface with particular focus on the effect of solution concentration. There are a lot of other factors that influence the aggregation patterning that have not attracted enough attention, such as the nature of the solvent, thermal annealing treatment, solution aging, and ultrasonic treating. However, multiscale aggregates in solution assembling into multipatterned clusters at the surface and the aggregate evolutions both at the solid substrate and in solution under these conditions were seldom touched. The aim of this article is to obtain a better understanding of the aggregation patterning of SEBS at surfaces. Various factors influencing the surface patterning were examined, and some new phenomena were detected. These multiscale patterned polymer thin films will find use in the design of block copolymer templates.22-25
to minimize solvent evaporation. Film specimens were periodically spin coated at 4000 rpm for 30 s on freshly cleaved mica substrates (atomically smooth surface) from these solutions. To estimate the effect of the ultrasonic treatment on the solution, a 1.0 g/L toluene solution was treated ultrasonically for about 2 or 10 min, and the film was immediately prepared on a mica substrate by spin coating from this solution. All of the as-prepared films were dried in vacuum at room temperature for several days to eliminate the last trace of solvent. 2.3. Tapping Mode Atomic Force Microscopy (AFM). The AFM topography images were obtained in constant repulsive force mode by an AFM (AJ-III, Aijian nanotechnology Inc., China) with a triangular microfabricated cantilever (Mikro Masch Co., Russia) with a length of 100 µm, a Si pyramidal tip, and a spring constant of 48 N m-1. A resonance frequency in the range of 240-400 kHz was used, and resonance peaks in the frequency response of the cantilever typically at 330 kHz were chosen for the tapping mode oscillation. The AFM images were obtained with a maximum scan range of 20 × 20 µm2, and scanning frequencies were usually in the range of 0.6 and 2.5 Hz per line. The measurements were carried out in air under normal conditions.
3. Results and Discussion 2. Experimental Section 2.1. Materials. The sample used in this study was a purchased SEBS triblock copolymer (Kraton G-1650; Shell Development Co.) without further treatment. The number-average molecular weight, Mw, the polydispersity index for the molecular weight distribution, Mw/Mn, and the weight fraction of polystyrene (PS) were determined to be 112 000, 1.27, and 29%, respectively. They are measured relative to polystyrene standards by gel permeation chromatography (GPC) and 1H nuclear magnetic resonance spectroscopy (NMR), respectively. Toluene, chloroform, cyclohexane, and n-heptane were analytical-grade reagents purchased from Shanghai Chemical Reagent Co., China. 2.2. Sample Preparation. Kraton G-1650 SEBS was dissolved in toluene to prepare 0.005, 0.06, 0.3, 1.0, 1.5, and 2.0 g/L solutions, respectively, at room temperature, followed by room-temperature storage in the dark for different periods of time in sealed glass vials (22) Li, R. R.; Dapkus, P. D.; Thompson, M. E.; Jeong, W. G.; Harrison, C.; Chaikin. P. M.; Register, R. A.; Adamson, D. H. Appl. Phys. Lett. 2000, 76, 1689. (23) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 4, 241. (24) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker C. J.; Russell T. P. AdV. Mater. 2000, 12, 787. (25) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126.
3.1. Effect of Solution Concentration. The concentration of the copolymer in solution is expected to play a major role in the AFM pattern on the mica substrate. A wide range of initial polymer concentrations (from 0.005 to 2.0 g/L in toluene) at room temperature was used. Figure 1 shows the effect of the concentration of SEBS in toluene solution on the patterning of the aggregates at the mica surface. The observed morphologies of the spin-coated films are on the order of single coil (a), spheres (b), rods (c), patches (d), continuous (e), and microphase separation (f) with increasing solution concentration. This trend is in accordance with that in the solution. In the extremely dilute solution, polymer molecules are separated by a large number of solvent molecules; they coil into a spherical morphology at the solid surface during the spin-coating process. With increasing concentration, netlike and patchlike morphologies appear because of the merging of coils. For the copolymer used in this work, the overlap concentration c* defined as a continuous film formation concentration was estimated to be ∼1.5-2.0 g/L. In this range, a semidilute solution is formed. Polymer chains are tangled with each other, and phase separation can be initiated. Figure 1 leads to the suggestion that the morphology of the film surface reflects the structure of the cast solution because of the rapid solvent evaporation during the spin coating.
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When a very dilute polymer solution of 0.005 g/L, in which the polymer-polymer interactions can be neglected and polymer chains are separated from each other by large amount of solvent, is dried by spin coating, individual polymer molecules are visualized. We can calculate from the single-molecule diameter in Figure 1a that the average diameter and the average height are 40 ( 2 nm and 2.0 ( 0.2 nm in our experiment, respectively. The particle size of the SEBS aggregates in the dilute solution, as determined by DLS, is 12 ( 2 nm, which is smaller than that on the solid substrate, which is calculated from the AFM image. The discrepancies in the size of the objects, in the solution, and after solvent evaporation on a solid substrate are significant, which can be explained by the change from the hydrodynamic radius of 3D polymer coils swollen by solvent molecules in solution to the radius of the 2D dried aggregates at the surface and by the widening effect of the AFM tip.26 When the solution concentration increased to 0.06 g/L, the spherical morphology was formed as shown in Figure 1b. The observed spheres are aggregates of polymer molecules, with an aggregation number that is considered to be 3-8 in this concentration by assuming that no dramatic change from solution to the solid state due to the fast spin-coating process occurs. The main feature of the AFM images observed in Figure 1c is similar to those investigated by Li:21 both are 2D network structures made of rods. These rods are probably composed of spherical agglomerates as indicated in Figure 1b and c, with the latter showing spherical and rodlike agglomerates at the film surface. All of the figures indicate a structure-formation sequence (i.e., spheres aggregated to form rods and rods cross linked to form networks). The morphologies at the same surface in regions other than the center were also investigated, and the size of the network grows with the increase in radius from the center to the edge of the substrate. Larger aggregates appearing as large patches are formed from the 1.0 g/L solution. These very dilute solutions produce inhomogeneous deposits of polymer aggregates dispersed on the substrate surface. This allows for better AFM imaging of individual aggregates. The deposits are not monolayers because they are very inhomogeneous. When more concentrated solutions, 1.5 and 2.0 g/L, were used, the films formed were homogeneous on a larger lateral scale with a small number of pinholes only for the former and featureless for the latter. The thickness of these homogeneous films, measured by the scratching method using AFM, is about 8.5 ( 0.5 nm, indicating an ultrathin film. From the highly magnified images in Figure 1f, we can observe microphase separation of SEBS with wormlike morphology. 3.2. Effect of the Nature of the Solvent. Figure 2 shows the morphologies of SEBS aggregates on the surface of films spin coated onto mica from chloroform (δ ) 9.3), toluene (δ ) 8.9), cyclohexane (δ ) 8.2), and n-heptane (δ ) 7.45) solutions, indicating the significant effect of the nature of the solvent, where δ is the solubility parameter. The corresponding δ for polymers PEB and PS are 8.0 and 8.6-9.0, respectively. Sizes, volumes, and heights of the aggregates obtained from bearing analysis and section analysis by image treatment software from the AFM images are listed in Table 1, where average aggregate diameters denote those of the spherical, rodlike aggregates or the average width of the networks. For the films spun from chloroform and toluene solution, it shows a similar spherical, rodlike morphology but with different sizes and volumessa larger size and smaller volume in the former case. Besides, more spheres and rods are joined to each other in the latter case. When cyclohexane and n-heptane were used, network morphology formed by spheres (26) Vesenka, J.; Manne, S.; Giberson, R.; Marsh, T.; Henderson, E. Biophys. J. 1993, 65, 992.
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Figure 2. AFM images of the films spin coated from 0.35 g/L (a) chloroform solution, (b) toluene solution, (c) cyclohexane solution, and (d) n-heptane solution onto mica. Table 1. Sizes and Volumes of Aggregates at the Surface and Solubility Parameters of Solventsa
solvent chloroform toluene cyclohexane n-heptane
average average surface- volume of solubility aggregate aggregate bearing aggregates/ parameter 3 1/2 δ/(cal/cm ) diameter/nm height/nm area/% 107 nm3 9.3 8.9 8.2 7.45
230 ( 8 160 ( 5 90 ( 15
14.1 ( 0.2 9.2 ( 0.4 8.5 ( 0.8 7.2 ( 0.6
26.3 48.4 52.1 67.1
3.03 3.60 3.93 5.45
a Solubility parameters of PEB and PS are 8.0 and 8.6-9.0, respectively.
and rods was observed. In the latter case, the net grows thicker. The figures and the table indicate that the decreasing solubility parameter of solvent from chloroform to heptane favors the linking of spheres and rods to form networks. This is because chloroform and toluene are more selective to terminal PS blocks that form stiff phases whereas the other two, cyclohexane and heptane, are more selective to soft PEB blocks. The middle PEB blocks are more stretched in the solvents with smaller δ that facilitates the linking. From the bearing analysis shown in Table 1, from chloroform to heptane, the volume of the aggregates at the surface ranges from 3.03 × 107 to 5.45 × 107 nm3 along with an increase in the surface coverage of the aggregates from 26.3 to 67.1%. Because both the extents of the increase for the volume and surface coverage are of the same order of magnitude, the surface aggregation indicated from the morphology mainly proceeds in 2D on the surface.19 Although these aggregates were assumed to be 2D patterns, they protruded from the substrate surface to different extent, and the average heights of the aggregates in these figures were different. From chloroform to heptane, the average height of the aggregates deceases from 14.1 to 7.2 nm. It is suggested that although the same amount of the material was spin coated onto the surfaces the aggregate height and the amount of aggregate coverage were different. The lower the surface coverage, the higher the aggregate height. The discrepancy in surface coverage is due to their different morphologies with different extents of aggregation and linking. It is interesting when comparing Figure 1c and Figure 2b that the concentrations of SEBS in toluene solutions are similar but the latter is a little higher, 0.35 g/L, and the former is 0.3 g/L. Although the surface patterns are similar, some interesting differences regarding the possible composition of the structures formed on the surface were detected. Both samples showed network morphologies. The film sample from the lower-concentration solution (Figure
SEBS Aggregate Patterning at a Surface
Figure 3. AFM images of the films spin coated from a 1.0 g/L toluene solution after (a) 30 days and (b) 60 days of aging at room temperature. Figure 1d is that from a solution without aging.
1c) gave a thin network with some separated spheres or rods, whereas that from the higher-concentration solution (Figure 2b) exhibited a thicker one. It can be assumed that both networks are composed of spheres and rods, but with different densities, and that the network formation concentration for SEBS in toluene is about ∼0.3 g/L. The solvent effect on the morphology is analogous to the concentration effect. We can believe that the same morphology will appear sooner or later at different concentrations in different solvents. They have different overlap concentration due to different viscosities, which is comparable to that for polymers with different molecular weights; the latter has been experimentally supported by a previous study on PS.27 3.3. Effect of Aging the Solution. Because the relaxation of polymer molecules is relatively slow, the aging of the solution is expected to have significant influence on the self-association of polymer chains leading to aggregation. We therefore prepared a polymer thin film by spin coating the SEBS solution in toluene that had been aged for 1 and 2 months at room temperature. Different morphologies were obtained. Figure 3 shows the results; the solution used is the same as that of Figure 1d (i.e., a 1.0 g/L toluene solution). When the fresh solution was used as shown in Figure 1d, large isolated patches were formed on the film surface. However, when the solution had been aged, a network structure appearedsan incomplete network when aged for 1 month and a well-developed network with a slightly decrease in the height scale when aged for 2 months. The results indicate that the solution used is metastable; spontaneous rearrangement of the SEBS chains may take place because of the van der Waals attractions between polymer chains. The reorganization of the aggregates was strongly influenced by time. This is also supported by the DLS measurement by which the aggregate sizes in a fresh 0.3 g/L toluene solution and a solution aged for 2 months were obtained. A polydisperse size distribution from 10 to 3960 nm was detected in fresh solution with peaks at 10, 26, 150, 1050, and 3960 nm, indicating a multimode aggregation, whereas the size distribution in the aged solution was narrowed with double peaks at 22 and 1680 nm. It is suggested that polymer chains form different kinds of aggregates such as coils, spheres, rods, patches, and/or networks of different sizes in freshly prepared solution, in accord with the AFM image in Figure 1c. The chains were relaxed and equilibrated with solvent molecules as the solution was aged. Rearrangement of the polymer chains was carried on, inducing the aggregation of single coils into larger aggregates separated by solvent molecules. A more stable and uniform state was then achieved. It is apparent that the prolonged storage of SEBS solutions at room temperature may result in a rearrangement of aggregates and the formation of cross links. Thus, we may say that the aging of solutions, in addition to other parameters, must be considered when we use these solutions for the processing of solid aggregates. (27) Zhao, J.; Jiang, S.; Wang, Q.; Liu, X.; Ji, X.; Jiang, B. Appl. Surf. Sci. 2004, 236, 131.
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Figure 4. AFM images of the films spin coated from a 1.0 g/L toluene solution after (a) 2 min and (b) 10 min of ultrasonic treatment.
If solution aging can influence polymer aggregation patterns, then what is the effect of prepared film aging? A comparison between the fresh-prepared film and the same film aged for 2 months at room temperature was carried out. No difference was found from the AFM images. Although the driving forces for the aggregation of colloidal particles on a surface are known to be mainly capillary forces,28 we believe that the hydrodynamic forces generated in a rapid solvent evaporation process, the van der Waals particle/particle interaction, and aggregate/surface interaction are predominant.19 3.4. Effect of Ultrasonically Treating the Solution. Generally, the ultrasound treatment may induce heat, cavitation, agitation, acoustic streaming, interface instabilities and friction, diffusion, and mechanical rupture.29 Ultrasound has been successfully applied to enhance reaction rates30 and to monitor network formation31 as well as to synthesize32 and assist the degradation of polymers.33 To our knowledge, there is no report concerning the control of surface aggregation patterns by ultrasonic treatment of dilute polymer solutions. After using ultrasound with a frequency of 40 kHz for about 2 and 10 min, we spin coated the solution onto the mica substrate to form the SEBS thin film. The surface morphology is significantly different from that of film spin coated from the untreated solution. For the same 1.0 g/L toluene solution, compared with Figure 1d, when the solution was slightly ultrasonically treated before spin coating, the morphology turns into a network structure with some dispersed elliptical holes as shown in Figure 4a. Figure 4b shows the morphology of the sample coated from the same solution after a long treatment time. A network morphology that is different from the former one (Figure 4a) but similar to the aged one (Figure 3b) is detected. The network is thicker, with a small number of larger and smoother rim holes in the slight ultrasonic treatment case. The ultrasonic wave makes the polymer chains move faster and tend to be entangled, favoring aggregation. Because the solution concentration is low, a continuous film cannot form; a network with a thicker net is therefore obtained. Another possibility that could account for the ellipselike network is the cavitation induced by ultrasound34 because there are a large number of microbubbles in the solution and they oscillate under ultrasound. When sound pressure reaches a critical degree, these bubbles grow rapidly and collapse. The released shockwave energy on bubble collapse gives rise to a number of eddies that interact with the macromolecules in solution, causing the formation of sharp elliptical holes embedded in the network. It is known that degassing these gas bubbles is dominant at the lower frequency because of the effects of rectified diffusion and (28) Denkov, N. D.; Velev, O. D.; Kralchevesky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (29) Li, J.; Liang, M.; Guo, S.; Lin, Y. Polym. Degrad. Stab. 2004, 86, 323. (30) Suslick, K. S. Science 1990, 3, 1439. (31) Lionetto, F.; Sannino, A.; Maffezzoli, A. Polymer 2005, 46, 1796. (32) Xia, H.; Wang, Q.; Liao, Y.; Xu, X. Ultrason. Sonochem. 2002, 9, 151. (33) Tayal, A.; Khan, S. A. Macromolecules 2000, 33, 9488. (34) Chemistry with Ultrasound; Mason, T. J., Ed.; Elsevier Applied Science: London, 1991; p 119.
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Figure 6. (a, b) FFT images and (c, d) autocovarianced images of Figure 5c and d.
Figure 5. AFM images and their sectional views along the line shown in the images of the films spin coated from a 0.3 g/L toluene solution on mica (a) as-prepared and after annealing at (b) 80 °C, (c) 180 °C for 10 h, and (d) 200 °C for 4 h.
the fact that the larger resonant radii of the cavitation bubbles result in larger-diameter gas bubbles that are easier to remove by buoyancy forces.35 Because the frequency used in this experiment is low, we think that the solution treated by ultrasound can be considered to be a degassed solution. As expected, it shows a lacy network structure that is the same as for the one aged for a long period of time. It is suggested from our experiment that the lacy network is the stable morphology of this solution. The 1.0 g/L SEBS/toluene solution tends to form a network morphology in solution and at a solid surface after aging at room temperature and after ultrasonic treatment because of chain aggregation and rearrangement. The network is a stable structure at this concentration. It took about 2 months of aging to reach this stable structure, whereas it took only a few minutes under ultrasonic treatment to obtain a similar structure. Compared to other complex and expensive techniques, the ultrasonic technique can be a valuable tool for monitoring polymer chain cross linking in solution. The results suggest a potential application to the preparation of filter membranes. 3.5. Stability of Aggregates on the Surface. The aggregate patterns on the solid surface are stable at room temperature as described above. The thermal stability of the aggregates on the film surface was addressed as shown in Figure 5. Films were prepared by spin coating a 0.3 g/L toluene solution onto mica. Surface patterns were studied after maintaining temperatures of 80 and 180 °C for 10 h and 200 °C for 4 h. The original pattern without annealing is also shown for comparison (i.e., Figure 5a). No modification of the size and shape of the aggregates was (35) Weavers, L. K.; Hoffmann, M. R. EnViron. Sci. Technol. 1998, 32, 2, 3941.
observed with annealing below 80 °C, although it is higher than the glass-transition temperature of the PEB block, Tg,PEB ) -40 °C. After 10 h of annealing at 80 °C, which is lower than the glass-transition temperature of the PS block, Tg,PS ) 105 °C, the aggregates begin to collapse at the surface, and their height decreases slightly as shown in Figure 5b. From the particle analysis, the average heights of the aggregates in two images are 20.5 and 17.2 nm for Figure 5a and b, respectively, which is direct evidence of the aggregate collapsing after annealing at 80 °C. From the roughness analysis, the root-mean-square (rms) roughness values of the two images are 6.38 and 5.88 nm, respectively, which gives a supplementary proof. This probably means that a partial transition from the glass state to a rubbery state is developed. When the sample is annealed at a temperature higher than the glass-transition temperature of the PS block (i.e., at 120°C as well as at 180 °C, close to the order-disorder transition (ODT) temperature for the SEBS copolymer, 195 °C36), the film morphology is deeply modified at the surface, exhibiting large disperse drops with a significant increase in their height as shown in Figure 5c. This is probably because the copolymer softens and swells to form larger aggregates at these temperatures. Maintaining the sample at 200 °C dramatically modifies the assembly pattern of SEBS at the mica surface by changing the spherical or rodlike aggregates to periodic stripes as shown in Figure 5d. Although the number of SEBS aggregates on the surface in Figure 5d is similar to that on the surface in Figure 5c as shown by the AFM-bearing measurement, the stripes in Figure 5d are more continuous than the spherical aggregates in Figure 5c. These discontinuous stripes are parallel, appearing with the same orientation. The diameter and the height of the stripes are not uniform, ranging from 180 to 390 nm in diameter and from 5 to 45 nm in height. From the cross-section profile, it can be seen that the distance between the two adjacent stripes is 1120 ( 40 nm, which is in the middle of the literature-reported condensed (780 nm) and sparse stripes (1450 nm) prepared from a 0.025% SEBS/toluene solution without annealing treatment.21 Generally, a shearing and stretching field occurring during the spin coating is considered to be an important factor in the formation mechanism of the ordered strips, whereas in this work, formally, it is formed because of a heating treatment. As we know, heating treatments can be used to control the long-rangeorder structure,37 and the morphology will change dramatically at temperatures higher than the phase-transition temperature.38 It is probable that the oriented aggregates have already been formed in the spin coating process,21 and the preferential orientation suggests that further molecular aggregation by thermal treatment might take place along this direction.38 To explain the evolution of a thin film into ordered strips in this work, parts c and d of Figure 5 are associated with fast Fourier transform (FFT) and autocovariance function as shown in Figure 6. The (36) Krishnamoorti, R.; Modi, M. A.; Tse, M. F.; Wang, H.-C. Macromolecules 2000, 33, 3810. (37) Segalman, R. A.; Yokoyama, H.; Kramer, E. J. AdV. Mater. 2001, 13, 1152. (38) Lecle`re, Ph.; Hennebicq, E.; Calderone, A.; Brocorens, P.; Grimsdale, A. C.; Mu¨llen, K.; Bre´das, J. L.; Lazzaroni, R. Prog. Polym. Sci. 2003, 28, 55.
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former (FFT) is related to the regularity of the patterns, whereas the latter (autocovariance function) provides a possible way to obtain information about the hidden regularities in the surface. The FFT image of Figure 5c indicates an almost random pattern with a little lamellar regularity, whereas that of Figure 5d indicates a perfect lamellar pattern. However, both of the autocovarianced images of Figure 5c and d show stripe regularity, but the latter is more notable. It is therefore suggested that the preorientation of the aggregates caused by the shearing field during the spincoating process exists in the low-temperature sample as shown in Figure 5c. Although no stripes are formed, it still shows the tendency of the oriented structures. Obviously, the dispersed spherical morphology of Figure 5c implicates a higher surface energy; the annealing treatment promotes the migration of the molecules resulting in a stripe morphology with a lower surface energy. In other words, lowering the surface energy is the driving force for forming more regular structures in this case.
4. Conclusions SEBS aggregate patterns on mica substrate surfaces spin coated from various solvents were studied. It is found that the solution concentration is the predominant factor in determining both the appearance of the surface and the homogeneity of the film. With the increase in solution concentration, the surface morphology varies from single coil, spheres, rods, and irregular patches to continuous and microphase separation. There is an overlap concentration to separate the dilute solution from the semidilute solution; the former shows different aggregates, whereas the latter shows a continuous film. For the SEBS used (Mw ) 112 000 g/mol), the overlap concentration is about 1.5-2.0 g/L. The solubility parameter does not modify the distribution but has an impact on the distribution, which proceeds in 2D on the surface plane. The aggregates at the surface could evolve with heat treatment from spherical and rodlike aggregates into regular stripes when annealed at a temperature higher than the order-disorder temperature of SEBS. The rearrangement of polymer aggregates
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was carried out in solution instead of at the surface at room temperature. Both DLS data and AFM images showed a different aggregate patterning, a more stable and uniform state, after the solution was aged. The prolonged storage of SEBS solutions at room temperature may result in a reorganization of the aggregates or cross-linking formation in the subsequently cast films. If the solution is slightly ultrasonically treated (with an exterior force) before film preparation, then the pitchlike morphology could change into well-defined network patterns with elliptical holes. The discrepancy in the network patterns between the short-term ultrasonically treated sample and the aging sample and the longterm ultrasonically treated sample can be elucidated by the solvent-polymer interaction and eddies induced by bubble collapse. The latter two, which can eliminate the cavitation effect of microbubbles in solution, show a stable network structure with lacy rims. Fascinating possibilities for tuning the surface morphology are provided by the SEBS aggregates’ reorganization both on surfaces and in solutions. Monolayers of block copolymers39,40 are attractive template materials that can be used to create a broad range of nanostructures such as spheres and cylinders with a characteristic length on the order of 10 nm.41 The mesosize morphologies with various shapes and distributions obtained in this work indicate wide potential application fields for microelectronics systems, filter devices, and biosensor fabrication. Acknowledgment. This work is supported by the National Natural Science Foundation of China (nos. 20236010, 20476025, and 20490200) and the Shanghai Municipal Education Commission of China. LA051293I (39) Thomas, E. L.; Kinning, D. J.; Alward, D. B.; Henkee, C. S. Macromolecules 1987, 20, 2934. (40) Mansky, P.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Yao, N. Appl. Phys. Lett. 1996, 68, 2586. (41) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32.
