Effect of Magnetic Nanoparticles on the Morphology of Polystyrene

Article pubs.acs.org/JPCC

Effect of Magnetic Nanoparticles on the Morphology of Polystyrene‑b‑Poly(methyl methacrylate) Diblock Copolymer Thin Film Ping Yang,† Shuchao Wang,† Xue Teng,† Wei Wei,† Vinayak P. Dravid,‡ and Ling Huang*,† †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, 637457 Singapore Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, United States

‡

ABSTRACT: We report the effect of oleic acid-capped magnetic nanoparticles (mNPs, γ-Fe2O3) on the morphology and phase separation behavior of diblock copolymer thin film, polystyrene-bpoly(methyl methacrylate), PS-b-PMMA, under various experimental conditions such as the casting solvent, mNPs concentration, and the annealing time. If chloroform (CHCl3, slightly selective for PMMA) is selected as the casting solvent, the morphologies of the PS-bPMMA thin film are affected by the embedded mNPs, while there will be no such effect when toluene (slightly selective for PS) is used. In toluene systems, at lower mNP concentration, lateral microphase separation occurred faster than that without mNPs, but at higher concentration, the underlying self-assembled diblock copolymer structure is no longer correlated with the mNPs assembly. Increasing the annealing time of the PS-b-PMMA/ mNPs nanocomposite thin film causes a similar effect as that of increasing the concentration of mNPs.

1. INTRODUCTION Under proper experimental conditions, block copolymers, which are composed of chemically different blocks, can selfassemble at the nanometer scale, into various well-defined morphologies corresponding to the molecular dimensions of each block.1−6 Recent studies have found that incorporation of nanoparticles into self-assembled block copolymers can greatly improve the mechanical strength, electrical conductivity, and optical properties of this composite.7−13 Among various methods to make polymer−nanoparticle composites, the most common approach is to use cooperative self-organization of nanoparticles and block copolymers.14 However, simply mixing bare nanoparticles with polymer matrixes is always problematic, and nanoparticles are commonly inclined to microphase-separate or aggregate within the polymer matrixes.15 This is because, in the absence of repulsive interaction, van der Waals attraction between nanoparticles favors the nanoparticle clustering and aggregation. Therefore, the appropriate surface modification on the presynthesized nanoparticles by according surfactant molecules is usually required to achieve stable dispersion in polymer matrixes. More recently, there are reports using polymer brushes to modify nanoparticles where the key is to choose the right type of polymer brush and polymer chain length, as it determines whether the modified nanoparticles will have good dispersion in polymer matrixes or not.16−18 For example, direct assembly of block copolymers with presynthesized magnetic nanoparticles (mNPs) has become a straightforward method to control the arrangement of mNPs,19 where the basic idea is to tailor the surface chemistry of the mNPs so that ordered structures of © 2012 American Chemical Society

mNPs can be produced during the phase separation process of the block copolymer. mNPs can form clusters or extensive aggregations due to the magnetic interaction among them.20,21 Thus, the addition of strongly interacting mNPs may change the characteristic dimensions or even the morphology of the block copolymer structure. Herein, taking advantage of the interactions between block copolymers and the surfactant molecules on the mNPs, as well as the controllable phase separation behavior of the PS-bPMMA, we report a facile method for generating ordered spatial arrangements of presynthesized mNPs (γ-Fe2O3) within the polymer matrixes, by changing the casting solvent, mNPs concentration, and the annealing time of the polymer/mNPs composite thin film. We believe the fundamentals gained from this study will be instructive to other types of nanoparticle− polymer pairs for their applications in data storage, energy harvesting, printable electronics, and smart coatings for optical and electronic devices.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solvents. Diblock copolymer, PS-bPMMA, with molecular weight of Mw = 263 kg/mol, polydispersity of 1.10, and the volume fraction of the PMMA block 0.51, was purchased from Polymer Source Inc. (Canada). Oleic acid-capped Fe2O3 NPs (about 9 nm in diameter, Figure 1h) were synthesized by the reaction of iron pentacarbonyl and Received: June 14, 2012 Revised: September 24, 2012 Published: September 26, 2012 23036

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oleic acid.22 The solvents used including acetone, toluene, and CHCl3 were ordered from Aldrich (Singapore). 2.2. Sample Preparation. PS-b-PMMA/mNPs nanocomposites were prepared by adding different amount of mNPs into solutions of PS-b-PMMA (5 mg/mL) in CHCl3 and toluene, respectively. The weight fraction of mNPs in the nanocomposite ranges from 0 to 20 wt %. The thin films were prepared by spin-coating the nanocomposite in toluene and CHCl3 solution, respectively, onto the SiOx wafer at 5000 rpm for 50 s. Prior to spin-coating, the wafer was treated with piranha solution (H2SO4/H2O2 7/3, v/v) at 80 °C for 30 min. (Caution: piranha solution is highly corrosive and reacts violently with organic matters!) Then, the wafer was thoroughly rinsed with deionized water and dried in air. Before removing the residual solvent after spin coating, the samples were exposed to saturated acetone vapor in a closed vessel at room temperature (20 °C) to anneal for different time periods as designed and were then removed from the vessel quickly for fast drying. 2.3. Characterization. The surface morphology of the nanocomposite thin film was characterized by atomic force microscopy (AFM, Asylum Research, model MFP-3D) under tapping mode in air. Transmission electron microscopy (TEM, JEOL, model JEM-1400) operated at 100−120 kV was used to study the mNPs distribution within the polymer thin film. Thickness of the thin film was measured by Ellipsometer (Jobin Yvon S.A.S Co., France), at a fixed incident angle of 70°, and the wavelength range was 550−800 nm.

3. RESULTS AND DISCUSSION 3.1. Solvent Effect. The addition of selective solvent to a block copolymer can greatly expand the range of accessible selfassembled morphologies.23,24 Taping mode AFM images in Figure 1 show the surface morphology changes of PS-b-PMMA diblock copolymer thin films spin-coated onto the SiOx wafer from toluene solution with different amounts of mNPs. As reported, the brighter regions in the phase contrast AFM image correspond to the PMMA block because PMMA has a higher modulus than PS chain at room temperature.25,26 Before adding mNPs, lamellae with very short-range order appear at the thin film surface (Figure 1a,b). From the AFM images of thin films containing 5 wt % (Figure 1c,d) and 9 wt % (Figure 1e,f) mNPs, we can see not only the similar short-range order lamellae but also the well-dispersed mNPs randomly incorporated into the PMMA lamellae. The TEM images (insets in Figure 1d,f) of the nanocomposite thin films also clearly prove the existence of mNPs. The difference between these two samples is that, further increasing the mNPs concentration results in the formation of small aggregates containing 2 to 3 mNPs, which can also be seen inside the yellow circles in Figure 1c,e. Figure 1g shows that the aggregation happens at an even higher extent when the concentration of mNPs increases to 20 wt %, and higher density of mNPs can be seen compared to those of 5 wt % and 9 wt %, as shown in the insets of Figure 1d,f, respectively. The insets in Figure 1c,e,g depict the schematic distribution of the mNPs on the copolymer blocks, with the green and yellow areas representing the PS and PMMA domains, respectively. When the concentration of mNPs increases, the aggregation of mNPs started and the phase separation occurred between the polymer and the aggregated mNPs. However, different phenomenon was observed when CHCl3 is used as the casting solvent. At zero mNPs concentration, randomly distributed nanodomains with wide transition

Figure 1. Topographic (a,c,e) and phase contrast (b,d,f) AFM images of the nanocomposite thin films spin-coated from 5 mg/mL toluene solution onto the SiOx wafer. (g; insets in d and f) TEM images of mNPs in the thin film. (h) TEM image of the as-synthesized mNPs. The weight fractions of mNPs in the nanocomposites are (a,b) 0%, (c,d) 5%, (e,f) 9%, and (g) 20%, respectively. Insets in panels c, e, and g are the schematic representation of the mNPs and polymers blocks, and the green and yellow areas represent PS and PMMA domains, respectively.

boundaries between PS and PMMA blocks were seen in AFM images (Figure 2a), which is a typical phenomenon for PS-b-PMMA thin film.26−28 When 5 wt % mNPs were added, the spinodal microphase separation pattern remains unchanged (Figure 2c), and mNPs are shown inconspicuously in the phase AFM image of the thin films (Figure 2d), especially when compared with that without mNPs (Figure 2b). However, when the mNPs concentration was increased to 6 wt %, the lateral microphase separation with disordered pattern becomes very obvious (Figure 2e,f), and clusters of mNPs appeared on the thin film surface, as shown inside the yellow circles in Figure 2e. From the above discussion, we conclude that, when CHCl3 is used as solvent, the thin film morphology of the nanocomposite is affected by mNPs at different levels according to the concentrations used. While in toluene system, mNPs bring no effect on the morphology of the PS-b-PMMA thin film at concentrations of up to 20 wt %. 23037

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assembly of the nanocomposite. The mNPs tend to aggregate at strong interactions; however, they prefer to disperse in the polymer matrix to reduce the conformational entropy of the system at weak interactions.31 In toluene system, when the concentration of mNPs increases, the aggregation of mNPs started and the phase separation occurred between polymer and the aggregated mNPs. Thus, the mNPs have limited effect on the morphology change of the nanocomposite thin film. By adding mNPs into toluene solution of PS-b-PMMA, the PS chains are mostly decorated with mNPs due to the higher degree stretch of PS chains in solution. However, the wellreported coordination interaction between the iron oxide core of the mNPs and PMMA via carboxyl and hydrogen bonds19,32 also helps to hold mNPs in the PMMA domain. So, it is interesting to see that mNPs locate within the PMMA domain but mostly at the interface between PS and PMMA blocks, as shown inside the circles in Figure 1c,e. When symmetric PS-b-PMMA is casted on the SiO x substrate, PMMA is preferentially attracted to the silica surface because of its polarity, while PS is preferentially exposed to the air due to its lower surface energy.33 CHCl3 has a high vapor pressure (18.6 kPa at room temperature34), which is the most volatile among the solvents used. Toluene has a vapor pressure of 2.8 kPa at room temperature,34 meaning a markedly lower volatility compared to chloroform.35 Since CHCl3 evaporates faster than toluene, mNPs will have insufficient time to rearrange. On the other side, the small amount of mNPs cannot induce strong lateral microphase separation, so mNPs locate in the PMMA domain near the substrate (Figure 2c). However, our experimental results at 6 wt % mNPs concentration (Figure 2e) indicate that the stronger lateral microphase separation and mNPs are exposed on the surface of the films due to the low surface tension.32 3.2. Solvent Vapor Effect. The preparation of block copolymer thin films under various solvent evaporation conditions has proven to be a reliable way to manipulate the microstructures.26,36−38 Since mNPs (up to 20 wt % concentration) do not affect the morphology of the nanocomposite thin film made from toluene solution, we choose the toluene system to study in detail the effect of mNPs on the thin film morphology evolution during the annealing process. Figure 3 shows AFM images of nanocomposite thin films annealed in acetone vapor for 1 h. Hexagonally packed cylinders can be clearly observed (Figure 3a,b) when no mNPs were added. However, at 5 wt % mNPs concentration, the mixture of cylinders and lamellae are formed (Figure 3c), and mNPs can be easily seen in AFM images (Figure 3c,d). When the concentration of mNPs was increased to 9 wt %, more PS chains appear on the surface of the thin film, and the size of PMMA cylinder and PMMA lamellae decreases (Figure 3e,f). Upon further increasing the mNP concentration to 20 wt %, disordered PMMA domains with diameters at about 50−60 nm started to form (Figure 3h). When the size of aggregates is greater than 1/2L0 (L0 is the bulk period of block copolymer, and L0 = 90 nm in our system),14,26 the PS assembles around the PMMA domain to form hole-like features. Again, with concentration increasing, more and more mNPs can be seen in the thin film surface, as shown in the circles in Figure 1e and TEM image in Figure 1g. Due to the carboxylate anion and hydrogen bonding between the Fe2O3 and PMMA,39 the mNPs will self-arrange inside the PMMA domains, which results in an increased domain size, but near the interface between the PMMA and PS domains, as shown in Figure 1e.

Figure 2. Topographic (a,c,e) and phase (b,d,f) AFM images of the nanocomposite thin film spin-coated from 5 mg/mL CHCl3 solution onto the SiOx wafer. The weight fractions of mNPs in the nanocomposite are (a,b) 0%, (c,d) 5%, (e,f) 6%, respectively.

For a certain type of diblock copolymer, a solvent may be neutral (good for both blocks) or selective (good for one block but not for the other). Therefore, the dissolution and phase behavior of diblock copolymers varies dramatically based on the solvent used. The miscibility between a polymer and a solvent is governed by the polymer−solvent interaction parameter, χP−S (P = polymer and S = solvent). Using the Flory−Huggins criterion, the complete solvent−polymer miscibility can be realized when χP−S < 0.5, and the smaller the value is, the stronger the affinity between solvent and polymer will be. From the values of χP−S (χPS−CHCl3 = 0.45; χPMMA−CHCl3 = 0.39; χPS−toluene = 0.34; χPMMA−toluene = 0.45),26 we know that CHCl3 and toluene are slightly selective for PMMA and PS, respectively. For oleic acid-capped γ-Fe2O3 mNPs, the nonpolar alkyl chains pointing outside. Both CHCl3 and toluene can be used as solvent where PS and mNPs and PMMA and mNPs are readily dissolved, respectively. For mNPs, the effect of the magnetic dipole interaction is huge, and it can significantly expand the range of interparticle interaction,29 while the oleic acid ligands bound on the surface of γ-Fe2O3 nanoparticles work as a shell to prevent macroscopic aggregation of the particles (Figure 1h). It is reported that the strong magnetic interaction among mNPs can cause clustering or extensive aggregation because block copolymers are often weakly associated with mNPs.29,30 In other words, a change in the ordered morphology can be realized by controlling the competitive interactions between mNP−mNP and mNP−block. The morphology of the PS-bPMMA/mNPs thin film depends on the free energy of mixing of mNPs and PS-b-PMMA. In addition, van der Waals and magnetic dipole−dipole interactions also contribute to the self23038

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Figure 4. Topographic (a,c,e) and phase (b,d,f) AFM images of (95 wt %) PS-b-PMMA/(5 wt %) mNPs thin films spin-coated from 5 mg/ mL toluene solution onto the SiOx wafer and then annealed in acetone vapor for (a,b) 40 min, (c,d) 80 min, and (e,f) 180 min.

PS and PMMA blocks induces the increase of enthalpy in the system and PS and PMMA blocks can easily reconstruct themselves, which cause the swollen in both PS and PMMA domains when annealed in acetone vapor. Ellipsometer measurement also proved that the thickness of the film changed from 16.7 ± 1 nm in air to 36.2 ± 1 nm in acetone vapor, indicating ∼53.9% swelling in volume of the films. When the annealing time was fixed, the transitions from nanopores to mixture of nanopores and lamellae and then nanopores are observed as the concentration of mNPs increases (Figure 3a−d). The migration of the mNPs can occur only when the film is not glassy and the polymer chains are mobile.40 During the annealing process, the polymer chains get sufficient thermal energy to become mobile; therefore, mNPs are allowed to migrate to the PMMA domain. When the concentration of mNPs is low, the mNPs cannot self-assemble themselves, which is why there is no more hexagonal packing presented in Figure 3b. In order to form upright cylinders, the surface tension of both domains at the substrate and air interface should be the same. For that reason, the oleic acid-capped mNPs also prefer the PMMA domain in order to have a balanced interfacial tension since the surface tension of PS, PMMA, and hydrocarbons are 39, 41, and 29 mN/m, respectively.32,41 The mNPs have lower surface tension and incline to expose to the surface. In the meanwhile, PMMA has a higher swelling tendency in acetone vapor. Therefore, the driving force that attracts PMMA upward is stronger, and more PMMA moves to the surface of the film.42,43 As a result, the mixture of nanopores and lamellae formed. When the mNPs concentration continually increases, the diameter of PMMA cylinder becomes larger (Figure 3c) because more mNPs will require more space

Figure 3. Topographic (a,c,e,g) and phase (b,d,f,h) AFM images of nanocomposite thin films spin-coated from 5 mg/mL toluene solution onto the SiOx wafer and annealed in acetone vapor for 1 h. The weight fractions of mNPs in the nanocomposite are (a,b) 0%, (c,d) 5%, (e,f) 9%, and (g,h) 20%, respectively.

3.3. Annealing Time Effect. In order to discover the effect of mNPs on the morphology of the nanocomposite thin film during the annealing process, we chose the PS-b-PMMA (95 wt %)/mNPs (5 wt %) system, which has similar initial morphology as the PS-b-PMMA thin film. AFM images in Figure 4 record the morphology change of the nanocomposite thin films, after annealing in acetone vapor for different time periods. At 40 min annealing, PMMA cylinders formed, but the degree of order is low (Figure 4a,b), and only very few inconspicuous mNPs can be seen. At 80 min, only few mNPs can be seen on the surface. However, the morphology of the thin film changed to the mixture of lamellae and cylinders (Figure 4c,d), indicating that phase separation between mNPs and PS-b-PMMA gradually occurred. When annealed for 180 min, the random PMMA cylinders with dramatically decreased sizes are formed (Figure 4e,f), and more mNPs appear at the interface between PMMA and PS domains, as can be seen inside the yellow circles in Figure 4e. When exposed to acetone solvent vapor, the film is covered with solvent molecules, and the interfacial energy at acetone vapor/film is different from that of air/film or vacuum/film. The gain of free volume of the 23039

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in PMMA domain, and the structure has to swell more to accommodate the mNPs.12 In this situation, the underlying selfassembled diblock copolymer structure is no longer correlated with the mNPs assembly. Comparison of Figures 3 and 4 shows that increasing the concentration of mNPs results in the similar effect as increasing the annealing time in the same solvent system. The mNPs contribute to the microphase separation due to its lower surface tension, so that the lateral microphase separation in the nanocomposite thin film occurs faster than that without mNPs. At prolonged annealing time, the fusion of pores occurs with sufficient chain mobility when the system eventually overcomes the activation energy barrier. However, thermodynamic stable lamellae parallel to the substrate cannot be formed since mNPs in PMMA domain prefer to float on the surface.

4. CONCLUSIONS We have investigated, at the nanoscale, the interactions between oleic acid-capped Fe2O3 mNPs and PS-b-PMMA in the thin film format under various experimental conditions. Because of the evaporation rate difference, the morphologies of the block copolymer are affected by the mNPs in CHCl3 (slightly selective for PMMA) but not in toluene (slightly selective for PS). In the toluene system, at low concentration of mNPs, lateral microphase separation occurred faster than that without mNPs, and at high concentration, the structures of the underlying self-assembled diblock copolymer are no longer correlated with the mNPs. Increasing the annealing time of the nanocomposite thin film generates a similar effect as that of increasing the concentration of mNPs in the acetone vapor system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support of the Tier 1 grant (RG20/09) from MOE, Singapore, and the INSIST Programme between NTU−Northwestern University.

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REFERENCES

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