ARTICLE pubs.acs.org/JPCB
Selective Self-Assembly of Surface-Functionalized Fullerenes in PS-PEO on Different Substrates Jing Wang, Guang-Xin Chen,* Jianli Sun, and Qifang Li* State Key Laboratory of Chemical Resource Engineering and College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ABSTRACT: Polystyrene-functionalized C60 (C60-PS) was synthesized by atom-transfer radical polymerization. The structure of the hybrid was characterized by gel permeation chromatography and thermal gravimetric analysis. The self-assembly of polystyrene-blockpoly(ethylene oxide) (PS-PEO)/C60-PS film in annealing solvents was studied on a silicon wafer and at the air/water interface by transmission electron microscopy. The method is an easy route to produce arrays of ordered nanostructures. The addition of C60-PS has a great effect on the self-assembly of PS-PEO. Treating the film under solvent vapor can modulate the orientation and ordering of PSPEO microdomains. The C60-PS enhanced the formation of lamellae microstructure, and the C60-PS entered the PS phase, expanding the scale of PS domains. Nevertheless, it becomes more complex when it refers to the self-assembly at the air/water interface under solvent vapor for a long time. The selectivity of solvent to the polymer chains plays an important role as the annealing time increases.
1. INTRODUCTION In the past decades, nanoscience has become one of the most influential and active research fields of new materials science. From the microcosmic and macroscopical scale in terms of physicochemical properties, nanomaterials are diverse because the scale of the nanostructure unit is distinctive to the characteristic length of matter, such as de Broglie wavelength, superconducting coherent length, and the ferromagnetism critical scale of electrons.1 Fullerenes, e.g., C60, a kind of natural nanometer-sized molecule, have been investigated extensively because of their unique electronic properties that are interesting for the design of optoelectronic and electronic devices, such as organic light emitting diodes and organic field effect transistors.2 Accordingly, one of the requirements for an electronic device is focused on the possibility of organizing fullerene-based molecule domains at the nanoscopic scale. Self-assembly, which has also attracted much attention recently, provides an elegant and inexpensive bottom-up method. Incompatible copolymers are a prominent example of this class of materials.3 Block copolymers, which comprise chemically distinct polymers covalently joined at one end, can self-assemble into well-defined, ordered arrays of nanoscopic domains ranging from spheres to cylinders to lamellae.4,5 The morphology of the domains depends on the molecular weight of the copolymer, the strength of the segmental interaction within the block, and the external environment, such as solvent, substrate, and temperature. Polystyrene-block-poly(ethylene oxide) (PS-PEO) is especially interesting due to the strong segregation force between the PS and PEO blocks. Moreover, PEO is a hydrophilic polymer and has a low tendency for protein adsorption.6 Cooper-White r 2011 American Chemical Society
et al. recently showed that PS-PEO self-assembly surfaces have a significant reduction in protein adsorption,7,8 which is interesting in the fields of biotechnology and biomedical sciences. Ordered microstructure in bulk block copolymers is usually induced by annealing at elevated temperatures or annealing in solvent vapor. Hahm et al. and Kimura et al. reported that arrays of nanoscopic cylindrical domains with a high degree of in-plane orientation and lateral order could be produced in spin-coated block copolymer films through solvent annealing.9,10 Fukunaga et al. also showed that solvent evaporation is beneficial to the formation of ordered copolymer morphologies in thin films.11,12 Compared with annealing at elevated temperatures, solvent annealing provides polymer chains sufficient mobility to move without the danger of degradation. The solvent also affects the final microphase-separated structure in various ways, such as the nature of the solvent, relative solvent vapor pressure, and solvent evaporation rate. Hence, the microstructures obtained this way depend not only on the composition but also on many other parameters.13 Solvent evaporation has been extensively investigated in recent years. The current knowledge on the ordering behavior of block copolymer thin film and their potential technological use has been summarized in recent review articles.14,15 One of the most interesting issues is the ordering in thin block copolymer/ nanoparticle hybrid films, specifically the effects of nanoparticles such as Au, Ag, Pd, CdSe, and C60 on the microstructure of the Received: November 21, 2010 Revised: February 23, 2011 Published: March 10, 2011 2824
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Figure 1. Synthesis scheme of PS-Br and C60-PS-Br.
block copolymer. Schmaltz et al.16 used the self-assembled structure of a host polystyrene-b-polyisoprene (PS-b-PI) copolymer as a scaffold to organize polystyrene stars with a C60 core and found that the molar mass and the functionality of the stars are two key parameters that control their solubility in the host copolymer matrix. Cheyne et al.17 demonstrated the first example of a controlled organization of semiconducting nanoparticles using amphiphilic block copolymer self-assemblies at the air water interface, which offers new routes to hierarchical hybrid assemblies with potential photonics applications. Extensive investigations have been conducted to control the location of nanoparticles in block copolymers in recent years. However, much less attention has been directed to the effects of the substrate materials on the microphase structure of the block copolymer and the location of nanoadditives in block copolymers under the condition of solvent vapor. In our previous work, we described the formation of unusually large lamellar structures in PS-PEO on varied substrates18 and responsive assemblies of gold nanoparticles19 through annealing in different solvent vapors. In this work, we utilized the self-assembly of PS-PEO to organize fullerene-based molecules on a large scale at the air/ water interface or on the silicone wafer. C60-PS, which has good solubility in organic solvent, was synthesized using atom-transfer radical polymerization (ATRP) and was used for self-assembly in the PS-PEO host matrix. To obtain well-ordered arrays of nanoscopic structures in thin films, a solvent annealing method was used to regulate the orientation of PS-PEO/C60 thin films both on the silicon wafer and the water surface.
2. EXPERIMENTAL METHODS 2.1. Materials. C60 was obtained from the Interuniversity Microelectronics Center. Reagent grade copper(I) bromide (CuBr), 2,2-dipyridyl, benzyl bromide, and all solvents were purchased from Beijing Chemical Reagents Corporation. THF, N,N-dimethylformamide (DMF), and 1,2-dichlorobenzene (DCB) were dried with sodium by refluxing and distillation. Styrene was purified by distillation under reduced pressure before use. The PEO-PS diblock copolymer was obtained from Polymer Source, Inc. The number-average molar masses (Mn) of the PEO and PS blocks are 71 000 and 58 600 g/mol, respectively. 2.2. Preparation of PS-Br. ATRP was used to prepare bromine end-capped polystyrene (PS-Br) using benzyl bromide as an initiator. In a 50 mL round-bottom flask, 0.625 g (4 mmol) of 2,2-dipyridyl, 0.287 g (2 mmol) of CuBr, and 2 mL of DMF were combined. After purging with dry argon for 10 min, 0.2379 mL (2 mmol) of benzyl bromide was added to the flask. The reaction system was degassed by three freezepumpthaw
cycles. Subsequently, 11.44 mL (0.1 mol) of styrene was added. The system was then allowed to react at 110 °C for 4 h. The crude polymer was obtained by precipitation from methanol and then was dissolved in THF. The solution was passed through an alumina column to remove the catalysts. A white, fine powdery polymer was obtained after the polymer was precipitated from a large excess of methanol and dried under a vacuum at 80 °C. The Mn and the polydispersity index (Mw/Mn) of PS-Br measured with GPC were 2812 g/mol and 1.114, respectively. 2.3. Preparation of C60-PS. A 50 mL round-bottomed flask was charged with C60 (0.0513 g), CuBr (0.0143 g), 2,2-dipyridyl (0.0445 g), and Ps-Br (0.420 g), degassed, and refilled with nitrogen three times. Afterward, 5 mL of DCB was added using a syringe, and the reaction mixture was degassed by three freeze pumpthaw cycles. The flask was placed in a thermostatted oil bath at 120 °C for 24 h. The solubility of C60 in THF is very low; thus, the reaction mixture was diluted with THF and was centrifuged at 3000 rpm for at least 30 min to separate the unreacted C60. After centrifugation, the solution was passed through an alumina column, poured into excess methanol, purified by repeated dissolutionprecipitation process three times, and then dried under a vacuum. Due to the different solubilities between C60-PS and PS (or PS-Br) in organic solvents, this process was effective enough to separate C60-PS (precipitate) from PS (in solution).20 Hence, 10 mL of benzene was added to dissolve the product, and methanol was slowly added to the solution until the precipitate no longer formed. The precipitate was separated by filtration and dried under a vacuum, obtaining C60-PS (brown powder). The product was analyzed by a thermal gravimetric analyzer. Approximately 93.4% of C60PS was incinerated, indicating an average of three or four PS chains, whose Mn of 2812 g/mol was grafted to one C60. The synthesis scheme of PS-Br and C60-PS-Br is shown in Figure 1. 2.4. Preparation and Annealing of Thin Films on the Silicon Wafer. C60 or C60-PS was added to a solution of PSPEO in toluene to a concentration of 10 mg/mL. The solution was left to stand for 24 h prior to use. Thin films were generated by spin-coating at 3000 rpm from the solution. An open weighing bottle filled with solvent was placed in a glass chamber for a short period before the films were placed inside, allowing it to be saturated with solvent vapor. The films were annealed at different times. Dichloromethane and chloroform were selected as the annealing solvents. After annealing, the solvent was removed, and the films were left in the chamber for 24 h. 2.5. Preparation and Annealing of Thin Films at the Air/ Water Interface. The solution was prepared as above. A clean culture dish filled with 60 mL of distilled water was placed in a closed container, where an open weighing bottle filled with solvent was also placed. An hour later, a 17 μL polymer solution 2825
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Figure 2. UVvis spectra of (a) C60 and (b) C60-PS.
was added at the solvent vapor/water interface; the solvent was allowed to evaporate to the confined space. The toluene evaporated immediately in contact with the solventwater interface, and the droplet spread at the interface. PS-PEO film formed gradually and in solvent vapor, allowing the macromolecular chains to move and solvent anneal. Chloroform was used as solvent vapor. The annealing time was controlled in 26 h. 2.6. Transmission Electron Microscopy (TEM) Imaging. The morphology of PS-PEO films was characterized by TEM. To get the film from the silicon wafer, the silicon wafer was placed on the water surface including 5% hydrofluoric acid (HF). After a while, the silica layer on the silicon wafer was corroded and the silicon wafer sank into the water. Then, the annealed film, which floated on the water surface, was obtained with carboncoated copper grids. The annealed films at the air/water interface were directly collected with carbon-coated copper grids on the water surface. Both the annealed films on the silicon wafer and at the air/water interface were stained with RuO4 at room temperature for 20 min. A transmission electron microscope, Hitachi 800, was used to observe the microstructure at an accelerating voltage of 200 kV.
3. RESULTS AND DISCUSSION Figure 2 shows the comparison of the ultravioletvisible (UVvis) spectra of C60-PS in toluene and C60 in cyclohexane. C60 has characteristic absorption peaks at 283 and 335 nm. C60 absorbs at approximately 335 nm and is known to be sensitive to chemical reaction. Its absorption intensity weakens upon chemical modification to the molecular structure of the C60 cage.2126 The C60-PS shows a weak but readily discernible absorption shoulder at 330 nm, confirming that the molecular structure of C60 has been modified by the polymerization reaction.27 The color of the toluene solution of C60 and C60PS differed markedly from purple black to brown, strongly supporting our conclusion. Herein, we report the first results of the spatial organization of C60 at varied substrates. Our method combines the self-assembling properties of classical PS-PEO block copolymers with the high solubility of C60-PS in organic solvents such as toluene and its miscibility with polystyrene. Figure 3 shows the TEM images of microphase separated PSPEO, PS-PEO/C60, and PS-PEO/C60-PS thin films on a silicon wafer under solvent vapor annealing. The bright regions in the image correspond to PEO, and the dark regions correspond to PS stained preferentially with RuO4. Solvent vapor imparts mobility
Figure 3. TEM images of thin films on a silicon wafer after annealing for 2 h under chloroform vapor: (a) PS-PEO; (b) PS-PEO þ 10 wt % C60; (c) PS-PEO þ 10 wt % C60-PS. Bar = 400 nm.
to the polymeric chains, which helps in the dynamic evolution and improves the ordered structures. After annealing under chloroform vapor (a nonselective solvent vapor to PS and PEO) for 2 h, the PS-PEO films showed a multiple microstructure, and the PS assembled slightly into a lamellar structure. This may be attributable to the great difference in hydrophilicity between PS and PEO. In recent years, much research has been conducted on PS-PEO diblock copolymers that can self-organize to a variety of morphologies from spheres, cylindrical, and lamellae. As shown in Figure 3b, a microstructure similar to pure PSPEO was found on PS-PEO/C60 thin films with aggregations of C60 (the black spots), indicating the poor interaction between C60 and two blocks of PS-PEO copolymers. The lamellae grew when 10 wt % C60-PS was introduced in the PS-PEO film compared with PS-PEO/C60 film (Figure 3c). At the same time, there was no aggregation of C60-PS on the surface, indicating that the C60 moieties are confined in the PS lamellae. As nanoparticles were added, new microphase structures appeared. C60-PS seems beneficial to the formation of the ordered lamellar structure. 2826
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Figure 5. TEM images of thin films on a silicon wafer after annealing for 6 h under chloroform vapor: (a) PS-PEO þ 5 wt % C60-PS, bar = 60 nm; (b) PS-PEO þ 10 wt % C60-PS, bar = 100 nm.
Figure 4. TEM images of thin films on a silicon wafer after annealing for 6 h: (a) PS-PEO; (b) PS-PEO þ 10 wt % C60; (c) PS-PEO þ 10 wt % C60-PS and 24 h; (d) PS-PEO under chloroform vapor. In parts a, b, and c, bar = 400 nm, and in part d, bar = 200 nm.
After annealing under chloroform vapor for 6 h, different morphologies were obtained, as shown in Figure 4. The PS-PEO thin films (Figure 4a) self-organized into reticular structures with about 40 nm width of PS strips, as there was increased time for polymer chains to move when the annealing time increased to 6 h. Figure 4b shows a disordered microstructure. C60 aggregated after 10 wt % C60 was added, similar to the morphologies of 2 h
treatment with chloroform. However, PS-PEO/C60-PS films (Figure 4c) show an ordered lamellae microstructure lying parallel to the silicon wafer surface without aggregation of C60-PS, indicating that the C60 moieties are preferentially confined in the PS lamellae and that the introduction of C60PS is favorable in the formation of lamellar microstructure. Note that this type of morphology was observed throughout the whole sample. The transition from disordered to ordered structure seems to occur due to the introduction of C60-PS, which increases the volume fraction of PS. The increased volume fraction of PS can accelerate the formation of a lamellar microstructure; the lamellae emerge when the volume fraction of PS reaches the limit.2830 Longer annealing time is another important parameter that enables PS and PEO chains to move adequately to form ordered microstructures, such as lamellar microstructures. To demonstrate the role of C60-PS in the PS-PEO film, we prepared a long-time annealing pure PS-PEO film placed under chloroform vapor for 1 day. After long-time annealing, a lamellar microstructure appeared, as shown in Figure 4d. This further confirmed our conclusion that the C60-PS can decrease the annealing time for PS-PEO to form the ordered microstructure. In other words, the addition of C60-PS is propitious for diblock copolymers to form lamellar microstructures. The amount of C60-PS introduced into PS-PEO was also changed to investigate its effect on the microstructure of the diblock copolymers. Figure 5 shows the TEM images of thin films with different wt % of C60-PS, which is 5 and 10 wt %, respectively. As shown in Figure 5a, after annealing under chloroform vapor for 6 h, the film self-assembled to a lamellar microstructure, with the width of the PS phase at about 30 nm 2827
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Figure 6. Schematic depiction of the self-assembly of C60-PS in PSPEO on a silicon wafer.
and the width of PEO at about 50 nm. In contrast, the difference in width between the PS and PEO phase disappeared when the 10 wt % C60-PS was added, with a repeat spacing of 40 nm (Figure 4b). George et al.31 illustrated a simple method to control the domain size of self-assembled block copolymers. They introduced homopolymer (PS) to asymmetric (PS-PEO) diblock copolymers to self-assemble and found that increasing the addition of PS could reduce the diameter of the PEO domains due to the kinetically constrained phase separation of the system. The presence of the solvent mediates the surface energies of the copolymer and imparts a high mobility to the PEO chains and PS chains, including PS grafted to C60. The copolymer chains are removed from the surface to form regular microdomains that propagate through the film during long-time solvent annealing. Due to the driving effect of PS chains, C60 nanoparticles were located in the PS microphase of PS-PEO diblock polymer neatly. The TEM images confirm that lamellar microstructures parallel to the silicon wafer and the C60 moieties are preferentially confined in the PS lamellae due to the effect of PS chain modification on the surface of C60. When the addition of C60PS is increased, more C60 nanoparticles appear in the PS microdomain to enlarge the size of PS block. This can increase the wt % of PS and make PS lamellae swell by reducing the effective diffusion of PS-PEO. This is the reason why the size of the PS phase became larger when the wt % of PS-C60 was increased from 5 to 10%, as shown in Figure 5. Therefore, the transition here is proposed to be caused by the increasing wt % of C60-PS, which mediates the assembly of PEO domains by reducing the effective diffusion of PS-PEO and induces the swelling of PS lamellae. We propose the following formation mechanism of the block copolymer/nanoparticle film, as illustrated in Figure 6: the selfassembly process of PS-PEO/C60-PS through solvent annealing on a silicon wafer with varied C60-PS contents. In this figure, blue and red represent the PS and PEO blocks, respectively, in the diblock copolymer, whereas gray stands for C60 to which three or four PS chains are attached by covalent bond. As the solvent evaporates, the concentration of the solvent at the free surface
Figure 7. TEM images of thin films at the at air/water interface after annealing for 2 h under chloroform vapor: (a) PS-PEO; (b) PS-PEO þ 10 wt % C60-PS. Bar = 900 nm.
decreases, and the ordered microstructure such as lamellae microstructure parallel to the substrate is formed. The same phenomenon occurs for different kinds of annealing solvents. This phenomenon arises from two factors: the solvent and the addition of C60-PS. The fact that the PS-PEO copolymer can be charged up to more than 10 wt % while preserving the lamellar structure implies that we can organize spatially significant amounts of C60. The morphologies of PS-PEO and PS-PEO/C60 films by water as a substrate were also investigated (Figure 7). In our previous work, a novel route to orient PS-PEO diblock copolymer films was produced at the air/water interface.18 The selfassembly of diblock copolymers at the air/water interface mainly depends on the interaction between two polymer blocks, annealing solvent, and water substrate. When a droplet of polymer deposits on the water surfaces, the stock solvent evaporates quickly and an irregular microstructure is presented. After 2 h of annealing under chloroform vapor, the irregular lamellar structure is formed for the pure PS-PEO film and the PS blocks with about 100 nm width in a PEO matrix (Figure 7a). PEO is hydrophilic, whereas PS is not. Thus, the PEO blocks swell quickly to occupy the main domains when the film is placed under the organic solvent. Instead, the size of the PS domain increased to about 140 nm when PS-PEO/C60-PS was annealed for 2 h under chloroform vapor (Figure 7b). The addition of C60-PS could have increased the volume fraction of PS, causing the PS domains to swell and the diameter of the PEO domains to decrease. Figure 8 shows the TEM images of the microphase separated PS-PEO and PS-PEO/C60-PS thin films at the air/water interface annealed for 6 h under chloroform vapor. After annealing for 6 h under chloroform vapor (Figure 8a), the microstructure of 2828
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Figure 9. Schematic depiction of the self-assembly of C60-PS in PSPEO at the air/water interface.
Figure 8. TEM images of thin films at the air/water interface after annealing for 6 h under chloroform vapor: (a) PS-PEO; (b) PS-PEO þ 10 wt % C60-PS. Bar = 900 nm.
PS-PEO thin film was more regular due to the sufficient time for assembly. The size of the PS became slightly smaller (with a width of about 95 nm) compared with that of the microstructure shown in Figure 7a. In Figure 8b, the PS blocks shrank markedly (with a width of about 90 nm), and the PEO blocks occupied the main domains again compared with the microstructure shown in Figure 7b. Chloroform as the annealing solvent could modulate copolymer chains to form the ordered microstructure. Nevertheless, the interaction between two blocks and water becomes much more important because it results in the self-assembly of amphiphilic diblock copolymers at the air/water interface. Chloroform is nonselective to both PS and PEO chains, whereas water is selective to PEO. When a droplet of polymer is deposited on the water surface, the condition is more favorable for PEO chains to form main domains. Interestingly, the addition of C60PS complicates the phenomenon. When the annealing time is short, such as 2 h, the addition of C60-PS plays a key role, and the microstructure occurs at a higher PS projected area than at PEO (Figure 7b). As the annealing time is increased, such as 6 h, the selectivity of water to PEO is the key factor because the PEO chains have enough time to move and stretch out. This is the reason why the diameter of the PS domains is much smaller in Figure 8b. The size of the microdomains on the water surface is larger than that on solid substrates. The periodic size of PEO and PS is about 50 nm on the silicon wafer, whereas the thin film made from the same PS-PEO solution self-assembled to lamellae on the water surface has a larger periodic size of approximately 100 nm. In addition, different morphologies are obtained under the same conditions when the annealing time is changed (compare Figure 3a with Figure 4a). The results indicate that morphology ordering is strongly dependent on the time of film
annealing. This conclusion has also been corroborated by Kim.32 When spin-coated PS-PEO was placed under benzene for 24 h, morphologies changed dramatically. However, half of the defects were reduced in the first hour. Moreover, they became much more complex in the self-assembly on the water surface as a result of the more complicated environment. The interactions among the two polymer blocks, annealing solvent, air, and water should be considered, and the dominant factor should also be defined. These observations prompted us to propose the following formation mechanism of self-assembly of composite films at the air/water interface, as illustrated in Figure 9. Water, a soft substrate, is beneficial to polymer chains in stretching out to obtain nanoscopic domains at a large scale. As shown in Figure 9, the PS and PEO chains are more extended than those on a silicon wafer. However, the size of the PEO phase is much larger than the PS phase in pure PS-PEO film. When the thin film is placed in solvent vapor, the PEO chains can stretch out quickly due to the selectivity of water to the PEO chains to form a larger phase, as displayed in Figure 9a. The size of the PS microdomain increases after the C60-PS is introduced, and C60-PS enters the PS phase (Figure 9b). In addition, when the annealing time is increased, the PEO phase occupies the main microdomain again. Figure 9c can be used to describe the formation mechanism. Water is the main vapor after a long period, and its selectivity to PEO enables the polymer chains to stretch out adequately. Furthermore, it also imposes restrictions on the extension of PS chains, making the hydrophobic chains draw back. This robust and simple methodology is expected to apply to other nanoscale organizations of other electron-acceptor molecules in copolymer host matrix. In further research, more investigation will be done to study the mechanism of selfassembly of diblock copolymers with other nanoparticles/nanorods on a silicon wafer or at the air/water interface.
4. CONCLUSIONS The selective self-assembly of surface-functionalized fullerenes in PS-PEO films either on a silicon wafer or on the water surface was investigated. The process illustrates an interesting route to organize C60 by taking advantage of the self-assembly of 2829
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’ AUTHOR INFORMATION Corresponding Author
*Phone/Fax: 86-10-64445680 (G.-X.C.); 86-10-64421693 (Q.L.). E-mail:
[email protected] (G.-X.C.); qfl
[email protected]. edu.cn (Q.L.).
’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support of this work coming from Natural Science Foundation of China (NSFC) (No. 21074009) and Polymer Chemistry and Physics, Beijing Municipal Education Commission (BMEC, No. XK100100640).
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