Development of Nanodomain and Fractal Morphologies in Solvent Annealed Block Copolymer Thin Films

Transcription

1 Communication Development of Nanodomain and Fractal Morphologies in Solvent Annealed Block Copolymer Thin Films Juan Peng, Yanchun Han,* Wolfgang Knoll, Dong Ha Kim* We have systematically studied the thin film morphologies of asymmetric polystyreneblock-poly(ethylene oxide) (PS-b-PEO) diblock copolymer subjected to solvent vapors of varying selectivity for the constituent blocks. Upon a short treatment in neutral or PS-selective vapor, the film exhibited a highly ordered array of hexagonally packed, cylindrical microdomains. In the case of PEO selective vapor annealing, such ordered cylindrical microdomains were not obtained. Instead, fractal patterns on the microscale were observed and their growth processes investigated. Furthermore, hierarchical structures could be obtained if the fractal pattern was exposed to neutral or PS selective vapor. Introduction Block copolymers have attracted significant attention as nanostructured materials, since they self-assemble into J. Peng, W. Knoll, D. H. Kim Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz, Germany Y. Han State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun , P. R. China Fax: þ ; ychan@ciac.jl.cn D. H. Kim Division of Nano Sciences and Department of Chemistry, Ewha Womans University, 11 1 Daehyun-Dong, Seodaemun-Gu, Seoul , Korea Fax: þ ; dhkim@ewha.ac.kr well defined nanoscale morphologies, with their shape, size and spacing being determined by the relative volume fraction of the constituent blocks and their respective molecular masses. [1 3] This versatility makes block copolymers ideal candidates for use as functional nanomaterials in recent nanoscience and nanotechnology. [4 6] Interfacial energies [7] and commensurability [8] determine the behavior of the block copolymer thin films. By applying external fields, such as electric fields, [9] controlled interfacial interactions, [10] shear, [11] chemically patterned substrates, [12] or crystallization, [13,14] the perpendicular orientation of microdomains can be readily achieved in thin block copolymer films. Increasingly, solvent induced ordering has been used to produce ordered nanodomain morphologies with controlled orientation. [15 24] The solvent vapor adjusts the interfacial energy of constituent blocks at the air/film interface, which allows the control of orientation normal 1422

2 Development of Nanodomain and Fractal Morphologies... to the film surface. For a given copolymer system, a particular solvent may be classified as neutral or selective, according to whether it is (i) a good solvent for both blocks, or (ii) a good solvent for one but a poor or non-solvent for the other. [25,26] There is growing interests in the effect of solvent selectivity on the resulting block copolymer morphologies. [18] In general, a neutral solvent distributes itself nearly equally between microdomains and can screen the unfavorable contact of blocks. By manipulating the solvent selectivity, the degree of microphase separation can be shifted. Previous work on the effects of solvent selectivity has focused on dilute solutions where the selectivity drives micellization. [27] In contrast, less attention has been devoted to solvent selectivity on solvent annealed block copolymer thin films. In the present paper, we adjust the interfacial energy at air/film surface by solvent vapor and explore the effect of solvent selectivity on solvent annealed PS-b-PEO thin films. In comparision to the classical polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) system, the longrange order in PS-b-PEO may arise from the stronger nonfavorable interactions between PS and PEO. Three solvents were chosen, from neutral to different block selective. We first compare the nanostructures in thin films exposed to solvent vapors of different selectivity, and the effect of mixed solvent vapors. Then, fractal patterns on the microscale are presented after a PEO selective solvent vapor treatment and morphological evolution is discussed. Finally, we report on hierarchical structures exhibiting fractal patterns and cylindrical nanodomains simultaneously. Since PEO is water soluble, the oriented arrays of nanocylinders have the potential application in water permeable membranes with nanoscopic channels. Experimental Part An asymmetric diblock copolymer of PS and PEO, denoted PS-b-PEO, with a molecular weight of g mol 1, a polydispersity of 1.05 and a PEO volume fraction of 0.25 was purchased from Polymer Source, Inc. and used as received. Solutions of the copolymer (1 wt.-%) in benzene were spin coated onto silicon substrates. The film thickness was 40 nm determined by a surface profiler (Tencor P-10, KLA, Tencor). The spin-coated films were exposed to three different solvent vapors (benzene, toluene, water) in closed vessels at room temperature (22 8C). In addition, some samples were also exposed to mixed benzene/water or toluene/water vapors with an initial volume of 50/50 for comparison. After a certain exposure time, the sample was removed to an ambient atmosphere. Atomic force microscopy (AFM) images were obtained in both height and phase contrast modes using a Digital Instruments Dimension 3100 scanning force microscope in the tapping mode. Etched silicon tips on a cantilever (Nanoprobe) with spring constants ranging between 33.2 N m 1 and 65.7 N m 1 (as specified by the manufacturer) were used. Optical images were obtained using an optical microscope (Axioskop, Zeiss, Germany) in reflection mode. Results and Discussion This section is divided into two parts: first, we describe the different nanostructures that are formed in PS-b-PEO thin films after annealing in solvents with varying selectivity. Second, we focus on the fractal growth process after PEO selective solvent annealing and a possible mechanism for the fractal growth is proposed. Finally, a hierarchical structure is also demonstrated by exposing the fractal pattern to neutral or toluene vapor. Nanostructures Formed in PS-b-PEO Films by Solvent Vapor with Varying Selectivity For a given system, a solvent that is good for one block can be classified as neutral or selective, according to whether it is good or a non-solvent for the other block. The relative affinity of solvents for each block is governed by the polymer-solvent interaction parameter, x P-S (P denotes polymer and S denotes solvent). [28] For non-polar systems: x P S ¼ V S ðd S d P Þ 2 =RT þ 0:34 (1) where V s is the molar volume of the solvent, R is the gas constant, T is the temperature and d S and d P are the solubility parameters of the solvent and polymer, respectively. For polar systems: x P S ¼ V S ½ðd ds d dp Þ 2 þðd ps d pp Þ 2 Š=RT (2) where d d is the dispersion solubility parameter and d p is the polar solubility parameter. In the present work, the solubility parameters of PS and PEO are d ps ¼ 18.6 (J cm 3 ) 1/2 and d PEO ¼ 20.5 (J cm 3 ) 1/2, respectively. [28] The calculated x P-S values for different polymer-solvent pairs at room temperature (22 8C) are listed in Table 1. Using the Flory-Huggins criterion for complete polymer-solvent miscibility, i.e., x P-S < 0.5, benzene is a good solvent for both PS and PEO blocks. Toluene is a selective solvent for Table 1. Polymer-solvent interaction parameters (x P-S ) calculated from different polymer-solvent pairs. Benzene Toluene Water PS a) PEO a) Obtained from Polymer Handbook (T ¼ 435 K). [28]

3 J. Peng, Y. Han, W. Knoll, D. H. Kim PS. Though x PEO-water > 0.5, water is a good solvent for PEO at room temperature. [28] Neutral Benzene Vapor Annealing The as-cast PS-b-PEO thin films show a disordered wormlike pattern [Figure 1(a)] due to fast solvent evaporation, giving the chains insufficient time to rearrange to attain an equilibrium morphology. Upon exposure to benzene vapor for 10 min, the film showed a highly ordered array of cylindrical domains [Figure 1(b)]. A fast fourier transform (FFT) of the AFM image shown in the inset of Figure 1(b) indicates hexagonally packed cylinders with few defects. According to the volume ratio of 25% of the PEO segment, it is concluded that the darker areas represent the PEO nanodomains. As reported by Lin and coworkers, [29] the cylindrical PEO domains span the entire film thickness and the orientation normal to the film surface can be achived in films with thickness many times the period of the bulk copolymer. a The average diameter of 20.1 nm and a lattice spacing of 27.1 nm were derived from this AFM image. During Figure 1. AFM phase images of PS-b-PEO thin films prepared under different conditions: (a) as-cast film; (b) annealed in neutral benzene vapor for 10 min; (c) annealed in PS selective toluene vapor for 5 min; (d) annealed in PEO selective water vapor for 167 h; (e) annealed in mixed toluene/ water vapor for 5 h. Fast Fourier Transform (FFT) pattern in the inset of Figure 1(b) indicates a perfect hexagonal arrangement of nanocylinders. The phase scale is shown in the inset. Image sizes are on a 1 mm 1 mm scale. a By varying the solution concentration and the spinning speed, films with four different thicknesses (16, 68, 135, and 900 nm) were also achieved. The former three films showed similar morphologies after subsequent solvent annealing compared to 40 nm-thick films, while the latter showed cylindrical domains of PEO were oriented parallel to the surface of the film. It indicates nanoscopic cylindrical PEO domains can be produced normal to the PS-b-PEO film with thickness several times the period of the copolymer. However, when the film was very thick, the external field (solvent vapor) is not strong enough to orient cylindrical domains normal to the film surface. 1424

4 Development of Nanodomain and Fractal Morphologies... solvent annealing, the high mobility of both the PS and the PEO, along with the strong repulsion between the two blocks leads to a high degree of long range lateral order. The cylindrical PEO domains appear dark in the AFM phase images, indicating that PEO is softer than PS matrix. Consequently, PEO has not crystallized upon solvent annealing. PS Selective Toluene Vapor Annealing water soluble, some amount of water is contained in the PEO domains during the annealing process, leading to an increase in the diameter of the nanoscopic cylindrical domains and the repeat period of the lattice to 31.4 and 41.2 nm, respectively. In addition, the phase contrast seen in the image (508) is much higher than those in Figure 1(b) and 1(c), which is due to the softer PEO domains swollen by water. With the mixed benzene/water vapor treatment, Next, we present nanostructures of the PS-b-PEO films annealed by toluene vapor. Cylindrical nanodomains with a hexagonal packed array were also observed, similar to the nanostructure induced by benzene vapor [Figure 1(c)]. Since toluene is a good solvent for PS but a poor solvent for PEO, this indicates that the movement of the major PS block has a critical effect on the formation of cylindrical domains with long range order. PEO Selective Water Vapor Annealing The above results indicate that benzene or toluene vapor confers enough chain mobility to the PS block in the cylindrical morphology formation. Therefore, it is not surprising to see that upon PEO selective water vapor annealing, regardless of the length of solvent vapor treatment, the thin film nanostructure remains almost unchanged compared to the as-cast film [Figure 1(d)]. It was interesting to find that, during water vapor annealing, some polymer droplets were formed which acted as nuclei for the following development of the complex fractal pattern. The growth process and formation mechanism will be discussed later. Mixed Benzene/Water or Toluene/ Water Vapor Annealing The nanostructures in thin PS-b-PEO films induced by mixed benzene/ water or toluene/water vapor were also investigated. Ordered arrays of hexagonally packed cylinders are observed by exposure of the films to mixed toluene/water vapors similar to those formed by pure benzene or toluene vapor annealing, as shown in Figure 1(e). Since the PEO block is Figure 2. Optical micrographs in reflection mode for PS-b-PEO thin films after (a) water vapor annealing for 24 h, removal of water vapor, and storage in air for (b) 48 and (c) 960 h, respectively. (a ), (b ) and (c ) are AFM height images of PS-b-PEO thin films after (a ) water vapor annealing for 117 h and removal of water vapor, and storage in air for (b ) 24 h and (c ) 84 h, respectively

5 J. Peng, Y. Han, W. Knoll, D. H. Kim we could also get results similar to Figure 1(e) (images not shown). Compared to pure benzene or toluene vapor annealing, the stability of PS-b-PEO films during mixed vapor annealing is much higher. For example, if the PS-b-PEO film was annealed in benzene vapor for 5 h, the film dewetted, some large droplets formed, and cylindrical domains were destroyed. In contrast, the PS-b- PEO film was still stable and cylindrical domains remained unchanged by benzene/water mixed vapor annealing. Nanostructure Formation Mechanisms We further investigated the differences in surface morphologies observed after benzene, toluene or water vapor annealing. This difference can be understood by considering the polymer-solvent interaction parameter (x P-S ) values for different polymer-solvent pairs. If asymmetric PS-b-PEO is cast on a Si substrate with a SiO x surface layer, due to the preferential interaction of the minor PEO block with the substrate and the lower surface energy of the major PS block (g PS ¼ 33 mn m 1 < g PEO ¼ 43 mn m 1 ), most PS segregates to the air interface to form a PS-rich layer. The wormlike structure indicates uncompleted microphase separation between PS and PEO. If exposed to solvent vapor, the film is covered with solvent molecules. The interfacial energy at vapor/film is different from that in air or in vacuum and it can be adjusted by varying the solvent vapor used for annealing. During benzene or toluene vapor annealing, the solvent molecules induce sufficient chain mobility for the PS chain and the strong non-favorable interactions between PS and PEO blocks lead to rapid further microphase separation. The ordering is initiated both at the surface and the bottom of the film and propagates laterally throughout the entire film. If treated in PEO selective water vapor, two factors prevent the formation of ordered cylindrical nanostructures. Firstly, if the PEO blocks are swollen by water, there would be no direct contact between most of the underlying PEO blocks and the water vapor due to the presence of a PS-rich layer near the air/film interface. Thus, the solvent must diffuse through the upper PS-rich layer in order to reach the underlying PEO blocks, resulting in a delay in the system s response. Secondly, during water vapor annealing, the major PS blocks do not have enough mobility to reconstruct themselves and the mobility of the minor PEO blocks is not enough to induce the rearrangement of the entire film. between 7.0 and 9.0 mm [Figure 2(a)]. The droplets remain unchanged during further annealing in water vapor for a long time. However, it is interesting to find that the droplets develop into fractal patterns after storing the film in air for different lengths of time [Figure 2(b) and 2(c)]. The typical size of the fully grown fractal patterns exceeds 300 mm. AFM measurements were also carried out in order to follow the evolution of the fractal structure and to obtain more accurate information on the profile of the film. Figure 2(a ) shows a nearly round droplet with a uniform front on the film surface, with a diameter of 7.1 mm and a height of 468 nm. As time lapsed, the round droplet became anisotropic [Figure 2(b )]. Some small drops or fingers were generated near the contact line, giving rise eventually to a fractal pattern [Figure 2(c )]. By AFM Fractal Pattern Growth after PEO Selective Water Vapor Annealing Fractal Pattern Growth Process As mentioned above, some polymer droplets are formed on the film surface during water vapor annealing with sizes Figure 3. (a) Analysis of the fractal dimension of a fully developed fractal pattern (inset) using the box count method. The slope yields the fractal dimension d f. (b) Time evolution of the fractal dimension of fractal patterns in PS-b-PEO films after water vapor annealing and storage in air. 1426

6 Development of Nanodomain and Fractal Morphologies... sectional profile analysis, the height of the fractal pattern was found to be about 9 nm. Fractals are generally observed in far from equilibrium growth phenomena. In our systems, the fractals are quite similar in shape to nonequilibrium patterns observed in other systems, such as dendritic crystallization, electrode position, viscous fingering and dielectric breakdown. [30] As a result, we performed the fractal dimension analysis for an isolated fully developed fractal pattern using the box count method, [30] with the results shown in Figure 3(a). The fractal dimension d f can be obtained from the scaling relationship of NðlÞ l d f, where N(l) is the number of boxes of size l that contain at least part of the fractal pattern region. From the slope in Figure 3(a), a fractal dimension of d f ¼ 1.66 was obtained. This result is in exact agreement with the theoretical fractal dimension result of Muthukumar: [31] d f ¼ðd 2 þ 1Þ=ðd þ 1Þ (3) where d ¼ 2 is the space dimension. Image analysis was then performed in order to obtain quantitative information on the growth process of fractal patterns. Figure 3(b) shows the time evolution of the fractal dimensions of the fractal patterns after water vapor annealing and being stored in air. It can be seen that the fractal dimension gradually increases from 0.85 at short storage time to 1.67 at long storage time. The increase of the fractal dimension is an indication of gaining fractality. Fractal Pattern Growth Mechanism Based on our observations, we present a tentative mechanism to interpret the formation of fractal patterns. The water contained in the film is believed to play a critical role in the fractal pattern formation. Microscopically heterogeneous stress is believed to trigger the formation of fractal patterns and the heterogeneity comes from the water contained in the film. Since PEO is water soluble, it is swollen in the presence of water vapor with a high mobility after some time and PS is mobile to some degree. PS and PEO aggregate with each other to form droplets like a melt which contain much water inside. If the film is stored in the ambient atmosphere, since the T g of PEO is Figure 4. A typical hierarchical pattern in a PS-b-PEO thin film after annealing the fractal pattern using toluene vapor for 10 min. lower than room temperature, the PEO block is also mobile in the air. The water contained in the film gradually evaporates and causes an anisotropic stress throughout the film, thereby destabilizing the initially uniform front of the droplets and inducing heterogeneity of the polymer mobility. The evolution of the droplet front becomes anisotropic and spreading fronts develop faster than the adjacent regions, forming fractal patterns. While in the water vapor environment, the water contained in the film does not evaporate. Therefore, droplets remain unchanged and no fractal patterns form. Hierarchical Micro/Nanostructure Hierarchical structures are important forms among various unconventional morphologies. These structures are made up of building units at different length scales. From the above results it is concluded that upon neutral benzene or PS selective toluene vapor annealing, the film exhibits a highly ordered array of hexagonally packed, cylindrical microdomains, while after PEO selective water vapor annealing, no such cylindrical microdomains are formed. Instead, fractal patterns on the micron scale are observed. Several questions naturally arise: (i) What would happen if one put the film with a fractal pattern into benzene or toluene vapor? (ii) Can the fractal pattern on the microscale and cylindrical domains on the nanoscale be combined

7 J. Peng, Y. Han, W. Knoll, D. H. Kim to form a hierarchical pattern at multiple length scales? To answer these questions, the PS-b-PEO films with fractal patterns were exposed to benzene or toluene vapor and examined by both optical microscopy and AFM. A typical hierarchical pattern is shown in Figure 4. Hexagonally packed cylindrical microdomains appear both inside and outside of the fractal pattern. It should be noted that the fractal pattern was not destroyed by the short benzene or toluene vapor treatment employed in this experiment. Such a complex evolution of micro/nanostructured morphology is of fundamental scientific interest, and may offer useful templates for unconventional functional nanostructures. Conclusion Thin film morphologies of asymmetric PS-b-PEO diblock copolymer were investigated after annealing by solvent vapors with neutral and different block selectivity. After neutral benzene and PS selective toluene vapor annealing, highly ordered arrays of hexagonally packed cylindrical PEO domains were produced in a PS matrix. Ordered cylindrical nanostructures could not be obtained by PEO selective water vapor annealing, indicating that the movement of the PS block has a major effect on the formation of ordered arrays of cylindrical nanodomains. Mixed benzene/water or toluene/water vapor annealing proved to be a strategy for manipulating the cylindrical domain size and the repeat period of the lattice. It was interesting to observe the development of a fractal pattern after PEO selective water vapor annealing. The polymer droplets formed during water vapor act as nuclei and the anisotropy of the polymer mobility induced by the water contained in the film plays a critical role in fractal pattern formation. Further, by exposing the film with a fractal pattern to benzene or toluene vapor annealing, hierarchical patterns with well defined cylindrical microdomains could be produced both within and outside the fractal pattern. Our results open up new possibilities for the construction of complex controllable structures by a simple bottom up approach. Acknowledgements: J. Peng is grateful to the Science and Technology Cooperative Program between the Chinese Academy of Sciences and the Max Planck Gesellschaft and Alexander von Humboldt Foundation. This work was supported by the Korea Research Foundation and grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF D00138). The authors are indebted to Dr. Jun Fu and King Hang Aaron Lau for helpful discussion. Received: March 18, 2007; Revised: April 27, 2007; Accepted: April 30, 2007; Keywords: atomic force microscopy; fractal patterns; hierarchical nanostructures; PS-b-PEO block copolymers; solvent annealing [1] F. S. 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