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Bump-Surface Multicompartment Micelles from a Linear ABC Triblock Copolymer: A Combination Study by Experiment and Computer Simulation Zengwei Ma,† Haizhou Yu,† and Wei Jiang* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: December 19, 2008
Novel bump-surface multicompartment micelles formed by a linear amphiphilic ABC triblock copolymer via self-assembly in selective solvent were successfully observed both in simulation and experiment. The results revealed that the block A forms the most inner core, and the blocks B and C form the inner and outer layers, respectively, and the bumps were formed by block A and more likely to be born on curving surfaces. Moreover, the micelle shape could be controlled by changing the solvent selectivity of the blocks A and B. Spherical, cylindrical, and discoidal micelles with bumpy surfaces were obtained both in experiment and simulation. 1. Introduction Recently, multicompartment micelles composed of a watersoluble shell and a segregated hydrophobic core have attracted much attention due to their interesting properties for nanobiotechnology.1,2 Increasing attention is paid to their potential applications as nanocontainers to solubilize two or more incompatible agents within separate nanoscopic compartments.3 To our best knowledge, several strategies for preparing multicompartment micelles have been proposed and various multicompartment micelles have been designed and investigated.4-11 Lodge et al. reported a first convincing visualization by cryotransmission electron microscopy (cryo-TEM) of segregated domains in the hydrophobic core of multicompartment micelles.6 Through binary blending of µ-(polyethylethylene)-(poly(ethylene oxide))-(polyperfluoropropylene oxide) (µ-EOF) miktoarm star terpolymers with polyethylethylene-b-poly(ethylene oxide) (EO) diblock copolymers, they provide a promising strategy to tune the multicompartment micelle structure.7 Kubowicz et al. synthesized spherical multicompartment micelles with a core segregated into a nanometer-sized compartment in which many small, fluorocarbon-rich domains coexist with a continuous hydrocarbon-rich region.8 Besides the spherical micelles the cylindrical multicompartment micelles which may serve as models to simulate the properties of biological structures such as transport proteins were also investigated by Kubowicz et al.10,11 However, the lack of knowledge on the formation and detailed inner structure for multicompartment is still a big problem in this research field due to the limit of experimental methods. Computer simulation and theoretical methods are useful tools in the study of those complex morphologies. Various theoretical approaches, such as Brownian dynamic simulations,12 Monte Carlo (MC) simulation,13 and dissipative particle dynamics (DPD),14,15 have been used to investigate the self-assembly of amphiphilic molecules. In addition, a self-consistent field has been successfully applied to study the formation of mesophases of an amphiphilic copolymer in dilute solution in two* To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +86-431-85262151. Fax: +86-431-85262126. † Equal contribution to this work.
dimensional (2D) space through a real-space SCFT approach as developed by Fredickson’s group.16-21 The SCFT simulation results21 are in good agreement with the experimental findings.6,7 Compared to MC and DPD simulation methods, SCFT can deal with the simulative system at a larger scale. Therefore, it is more suitable for multicompartment micelle study. In this study, self-assembly of a linear amphiphilic ABC triblock copolymer into multicompartment micelles with bump surfaces in dilute solution was studied by combining experimental methods and a real-space SCFT in three-dimensional space. The purpose is to reveal the forming condition and the inner structure for bump-surface multicompartment micelles. 2. Theoretical Model In this section, the SCFT model for the linear ABC triblock copolymer dilute solution system is briefly described. The linear amphiphilic ABC triblock copolymer and solvent molecules S are involved in volume V. The length fractions of the ABC triblock copolymers are lA, lB, and lC ) 1 - lA - lB. The volume fractions of the ABC triblock copolymer and solvent in the dilute solution are fp and fS ) 1 - fp. As a result, the volume fraction of segment i (i ) A, B, and C) in the system is fi ) fp · li. Incompressibility is assumed in this mixture system. For such system, the free energy is given by
F ) -fp ln(Qp /fpV) - Nfs ln(Qs /fsV) - 1/V
∫ dr[wAφA +
wBφB + wCφC + wSφS + ξ(b)(1 r - φA - φB - φC φS)] + 1/V
∫ dr[χABNφAφB + χBCNφBφC + χACNφAφC +
χASNφAφS + χBSNφBφS + χCSNφCφS] (1)
where N is the length of the block copolymer chain, χij is the Flory-Huggins interaction parameter between species i and j, and φA, φB, φC, and φS are the densities of the blocks A, B, C, and the solvent S, respectively. V is the volume of the system, ξ(r b) is the known Lagrange multiplier which enforces the bq(r b,1) is the partition function of a incompressibility, Qp ) ∫ dr single polymer chain in an effective potential field ωA, ωB, ωC, b(r b,1) is the partition function of the solvent in an and Qs ) ∫ dr
10.1021/jp8089775 CCC: $40.75 2009 American Chemical Society Published on Web 02/25/2009
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effective potential field ωS, q(r b,s) is the end-segment distribution function that gives the probability of finding segment s at position b, r and q(r b,s) is the function that satisfies the following modified diffusion equation:
∂q(b, r s) r s) - ω(b, r s) q(b, r s) ) ∇2q(b, ∂s
(2)
The two ends of the polymer chains are distinct, so a second end-segment distribution function q+(r b,s′) is needed, which also satisfies equation (2). The density of each component is obtained by
φA(b) r )
fpV Qp
∫0l
φB(b) r )
fpV Qp
∫l l +l
dsq(b, r s) q+(b, r 1 - s)
(4)
φC(b) r )
fpV Qp
∫l 1+l
dsq(b, r s) q+(b, r 1 - s)
(5)
φS(b) r )
fsV exp(-ws(b)/N) r Qs
A
dsq(b, r s) q+(b, r 1 - s)
A
B
A
A
B
(3)
(6)
From the equilibrium condition, that is, the minimization of free energy to density and the Lagrange multiplier, we have the following five equations:
wA ) χABN(φB - fB) + χACN(φC - fC) + χASN(φS - fS) + ξ (7) wB ) χABN(φA - fA) + χBCN(φC - fC) + χBSN(φS - fS) + ξ (8) wC ) χACN(φA - fA) + χBCN(φB - fB) + χCSN(φS - fS) + ξ (9) wS ) χASN(φA - fA) + χBSN(φB - fB) + χCSN(φC - fC) + ξ (10) φA + φB + φC + φS ) 1
SCHEME 1: Molecular Structure of the Triblock Copolymer
(11)
The numerical simulation was carried out in three-dimensional space with periodic boundary in all three directions. The grid size is ∆x ) 0.333. The simulation is carried out until the morphologies are stable and the free energy difference between two iterations is smaller than 10-5. The same initial fluctuation amplitude in the order of 10-2 is used in our work. The volume fraction of the ABC triblock copolymer is set as fp ) 0.1. 3. Experiment Method Polystyrene-b-polybutadiene-b-poly(2-vinylpridine) (Scheme 1) is used to prepare multicompartment micelles. The photocross-linkable polybutadiene (PB) is chosen as the middle blocks of the copolymer. Blocks PB can be cross-linked by UV
irradiation stabilizing the structures. Polystyrene (PS) and poly (2-vinylpridine) (P2VP) were chosen as the end blocks which are strongly incompatible. A convenient strategy to control aggregate structures is altering the ratio of the two blockselective solvents.22 Herein, toluene and methanol are used as the block selective solvents. The copolymer used in this study was PS440-b-PBd1020-b-P2VP114 (wherein the subscripts indicate the number of repeat units of the blocks) (Mn ) 112 900 g/mol, PDI ) 1.06), which was purchased from Polymer Source Inc., Canada. To prepare the micelles, the copolymer (1 wt %) was directly dissolved in 1 mL of solvent mixtures of toluene and methanol with differential volume proportions. After stirring the mixture for one day the solution was irradiated using a UV generator with the wavelength of 254 nm for 10 h, in order to stabilize the aggregates. The resulting aggregate morphologies were visualized with transmission electron microscope (TEM) and atomic force microscope (SPA-300HV atomic force microscope (AFM)). TEM was performed on a JEOL JEM-1011 transmission electron microscope operated at an acceleration voltage of 100 kV. A drop of the micellar solution was placed onto a TEM copper grid covered by a polymer support film precoated with carbon thin film in a liquid nitrogen environment to freeze the samples. Experiments performed at room temperature (20 °C) yielded lesser quality specimens due to the fast evaporation of toluene and methanol. Excess solution was blotted away using a strip of filter paper. The frozen samples were subsequently freeze-dried under vacuum. The micellar structures appear to be indistinguishable before and after being cross-linked by UV irradiation. To enhance the electron density contrast between the three blocks, uncross-linked samples were exposed to OsO4 vapor to highlight the PB microdomains. Three minutes later the solution was blotted with a filter paper. AFM observation was operated at the tapping mode with an SPI3800 controller (Seiko instruments Industry Co. Ltd.). To prepare the samples for AFM, a drop of the micellar solution was placed on the silicon wafers in liquid nitrogen environment to freeze the samples. These were freeze-dried in vacuum and stored before observation. 4. Results and Discussions In this study, we focus on the effect of the solvent selectivity and the fraction of blocks A and B on the micelle structure formed by the linear amphiphilic ABC triblock copolymer in selective solvent. We set χCSN with -7.65 (the C block is solvophilic), the total length of the triblock copolymer is 26 (N ) 26) and lC ) 0.08 (end block NC ) 2). Three mutually immiscible blocks in simulation can be realized by setting χABN ) χACN ) χBCN ) 30. These values remain unchanged in the simulations. 4.1. Effects of the Solvent Selectivity and Length Fraction of Blocks A and B. First, the length fractions of the blocks are fixed at lA ) 0.30 (head block NA ) 8), lB ) 0.62 (middle block NB ) 16), and lC ) 0.08 (end block NC ) 2). The effects of the solvent selectivity of blocks A and B on the micelle structure was studied by changing χASN, χBSN. The simulation results
Bump-Surface Multicompartment Micelles
Figure 1. Morphologies of linear ABC copolymers in dilute solution with different solvent selectivity of blocks A and B: (a) χASN > χBSN; (b) χASN ) χBSN; (c) χASN < χBSN. Blue represents the blocks A (the bumps); green represents the blocks B.
for the three cases, that is, χASN > χBSN, χASN ) χBSN, and χASN < χBSN are given in Figure 1. It can be seen that cylindrical micelles with smooth surfaces are more likely to be formed when χASN > χBSN (Figure 1,a1-a3). Increasing χASN and χBSN synchronously has less effect on the micelle structure. When χASN ) χBSN, a few bumps appear on the micelle. Moreover, the micelle roughly remains cylinder shaped with increasing χASN and χBSN synchronously. However, bump-surface multicompartment micelles appear clearly when χASN < χBSN. More interesting is that the micelle shape changes from cylinder to disk then to sphere with increasing χASN and χBSN synchronously. It is noteworthy that discoidal micelles have been reported in surfactants23 and block copolymer24 systems. In surfactant solutions, ellipsoidal shape of a micelle is also intermediate between spherical and cylindrical shapes.25,26 Oblate ellipsoids prove to be thermodynamically favored over prolate ellipsoids for most micelles.27 In this study the oblate micelles are also intermediate between spherical and cylindrical shapes. With the increase of χAS,χBS, more blocks A and B are incorporated into the micelle in order to reduce the surface of the solvophobic core and the solvent, leading to the distortion in the micelle shape.25 Second, the case of χASN < χBSN is considered only, whereas three cases for the length fraction of blocks A and B, that is, lA:lB:lC ) 16:8:2 (lA > lB), lA:lB:lC ) 12:12:2 (lA ) lB), and lA: lB:lC ) 8:16:2 (lA < lB), are studied. The simulation results are given in Figure 2. It can be found that the micelle shape tends to change from cylinder to disk in all three cases, while from disk to sphere further in the third case with increasing χASN and χBSN synchronously. Moreover, the micelles become asymmetric in the case of higher χASN and χBSN. These asymmetry structures should be the metastable states with topological defects because the diffusion for both block A and block B is difficult when the repulsive interactions become extremely strong. From Figure 2, we can see that the well bumpsurface multicompartment micelles are more likely to be formed in the case of lA:lB:lC ) 8:16:2 (lA < lB). Small uniform bumps are locked at the surfaces of cylinder, sphere, and the edge of disk. Therefore, the following studies are focused on the bumpsurface multicompartment micelles.
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Figure 2. Morphologies of linear ABC copolymers in dilute solution with different length fractions of the blocks A and B in a selective solvent with χASN < χBSN. (a) lA < lB; (b) lA ) lB; (c) lA > lB. Blue represents the blocks A (the bumps); green represents the blocks B.
TABLE 1: Polymer/Solvent Solubility Parameter (δ) polymer
solubility parameter [Mpa]1/2
PS PB toluene methanol
17.3-19.0 16.5-17.5 18.2 29.6
4.2. Bump-Surface Multicompartment Micelles from the Linear ABC Triblock Copolymer. In this section, the bumpsurface multicompartment micelles are studied by combining experimental and simulation methods. From the above simulation results, it is known that well bump-surface multicompartment micelles are more likely to be formed in the case of lA: lB:lC ) 8:16:2 and χASN < χBSN. In experiment, the copolymer used in this study is PS440-b-PBd1020-b-P2VP114, which is basically consistent with the block length ratio of lA:lB:lC ) 8:16:2 in the simulation. Selective solvents used here are toluene and methanol, which can blend with each other in any proportion, forming a homogeneous solution. Toluene is a good solvent for PS and PB but a precipitate for P2VP, whereas methanol is a precipitate for PS and PB but a good solvent for P2VP. When the ratio of toluene to methanol is 50/50 (v/v), the mixed solvent quality is poor for the large fraction of the blocks PS and PB which are methanol-insoluble. From Table 1 we can see that the solubility parameter of the PS is smaller than that of the PB, which is consistent with the simulation condition of χASN < χBSN. Figure 3a shows the TEM image exhibiting dominant spherical particles (average diameter is 81 nm) with bumpy surfaces from PS440-b-PBd1020-b-P2VP114 in selective solvent mixtures of toluene and methanol (50/50, v/v). The bumps have an average size of ca. 24 nm. When volume proportion of toluene to methanol is adjusted to 60/40, v/v, dominating discoidal micelles can be seen as shown in Figure 3b (TEM image) and Figure 4a (AFM height image). The discoidal structures (ca. 126 nm of the average diameter collected from TEM images) are round with small bumps (ca. 23 nm) distributed on the edge of the disks. However, they exhibit a broad size distribution. This can be attributed to the existence
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Figure 3. Bumpy-surface micelles of experimental (a-c) formed in toluene/methanol mixtures with different ratios: (a) 50/50, v/v; (b) 60/40, v/v; (c) 70/30, v/v. SCFT simulative results (d-f): (d) χASN ) 45 and χBSN ) 50, (e) χASN ) 40 and χBSN ) 45, and (f) χASN ) 30 and χBSN ) 35. Blue represents the blocks A (the bumps); green represents the blocks B.
Figure 4. (a, b) AFM height images obtained from a sample of discoidal micelle. The sample was spin-coated from 10 mg/mL solution of toluene and methanol (60/40, v/v) onto silicon wafers. Image b is the enlarged part of the area highlighted in image a. Graphs c and d are section analyses corresponding to the lines shown in the height image b.
of many metastable states in the system. It was proposed that such polymorphism results from the existence of multiple metastable states rather than the polydispersity of the triblock copolymer.28,29 The bumpy edge can be further illustrated by Figure 4c which shows a fluctuant line corresponding to the pink-signed line in Figure 4b. In contrast, there are no bumps on the flat surfaces of the disks as observed from Figure 4a. This special feature can be further confirmed by Figure 4d which shows a flat line of the section analysis corresponding to the red-signed line in Figure 4b. When the toluene proportion in the selective solvent mixtures is further increased to 70 vol %, cylinders (>500 nm) with bumpy surfaces can be seen from Figure 3c. From the TEM images displayed in Figure 3a-c, we can find that the bumps are mostly found on the surfaces of the spheres, cylinders, and on the edge of the disks. These are well consistent with the simulation results. In the simulation, the blocks A, B, and C are corresponding to the PS, PB, and P2VP
blocks, respectively. Increasing the volume proportion of toluene to methanol means the solvent quality becomes good for both PS and PB blocks synchronously. This can lead to the synchronous decrease of χASN and χBSN. For the purpose of comparison, the simulation results (Figure 3d-f) are given in Figure 3. More interesting is that both experiment and simulation results indicate the bumps are more likely to be born on curving surfaces. It is clear that the uniform bumps appear on the edge rather than on the flat surfaces of the disk. To further illustrate the structures of the bump-surface multicompartment micelle, the particles are exposed in OsO4 vapor in order to highlight the PB blocks in the micelles. Representative TEM images of spherical, discoidal, and cylindrical micelles are shown in Figure 5a-c. After exposure to OsO4 vapor the blocks PS become gray and the blocks PB become black. Figure 5a indicates the two separated compartments in the core of the spherical micelle clearly. The bumps of the spherical micelle are composed of the blocks PS (gray
Bump-Surface Multicompartment Micelles
J. Phys. Chem. B, Vol. 113, No. 11, 2009 3337
Figure 5. TEM images of spherical, discoidal, and cylindrical micelles after exposure to OsO4 vapor and density distributions of three blocks of ABC triblock copolymers in spherical, discoidal, and cylindrical micelles from simulation. Blue represents the blocks A, green represents the blocks B, and red represents the blocks C.
domains), and the blocks PB appear among the bumps (black domains). Figure 5b shows that the bumps on the edge of the disk become gray indicating that they are composed of the blocks PS, while the flat surfaces of the disk composed of the blocks PB become dark. We can also see that the bumps of the cylinders in Figure 5c become gray, and the residual parts of the surfaces become dark. However, the blocks which spread around the periphery forming the shell of these particles can not be seen directly. The inner components of the micellar core still cannot be distinguished exactly through the staining method. Thus, the simulation method is utilized to further analyze the inner components of the three kinds of micelles. Figure 5 is the simulation result showing the inside structure for the three kinds of bump-surface multicompartment micelle. Blue, green, and red represent the blocks A, B, and C, respectively. It can be seen that the block A forms the most inner core, and block B and C form the inner and outer layers, respectively. It is clearly shown that the bumps are formed by block A and most of them are grown from the inner core. This can be further confirmed by that the bumps (Figure 5,a1, b1, c1) and the holes (Figures 5,a2-3, b2-3, and c2-3) appear at the same places in the micelles. More important is that the micelle shape can be controlled by changing the solvent selectivity of the blocks A and B.
Figure 6. Variation of various energies with χBSN when χBSN - χASN is fixed at 5.
Finally, we calculate the free energy, the interaction energy, and the energy contributed from entropy in units of kBT based on eq 1.20 Figure 6 shows the variations of the free energy, the interaction energy and the energy from entropy with χBSN in the case of χASN < χBSN. It can be seen that the free energy and the energy from entropy increase, whereas the interaction energy decreases with increasing χASN and χBSN synchronously. The energy from entropy of the sphere-shaped micelle (χBSN ) 50) is considerably higher than that of the cylinder micelle (χBSN ) 35). However, the interaction energy of the sphere shape micelle is markedly lower than that of the cylinder micelle. This
3338 J. Phys. Chem. B, Vol. 113, No. 11, 2009 indicates that the micelle morphology is a competition result between the interaction energy and the entropy within the system. 5. Conclusions The bump-surface multicompartment micelles were successfully obtained via the self-assembly of the linear ABC triblock copolymer in selective solvent. The simulation and experimental results revealed that the block A forms the most inner core and the blocks B and C form the inner and outer layers, respectively. The bumps were formed by block A and more likely to be born on curving surfaces. More interesting was that the micelle shape can be controlled by changing the solvent selectivity of the blocks A and B. It changed from cylinder to disk and then to sphere with increasing repulsive interactions between the solvent and the blocks A and B. Acknowledgment. Financial support was provided by the National Natural Science Foundation of China for General Program (20874099), Creative Research Groups (50621302), Outstanding Young Investigators (50725312), and the National Basic Research Program (2007CB808000) of China. References and Notes (1) Laschewsky, A. Curr. Opin. Colloid Interface Sci. 2003, 8, 274– 281. (2) Lutz, J.-F.; Laschewsky, A. Macromol. Chem. Phys. 2005, 206, 813–817. (3) Lodge, T. P.; Rasdal, A.; Li, Z.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 17608–17609. (4) Sta¨hler, K.; Selb, J.; Candau, F. Langmuir 1999, 15, 7565–7576. (5) Kotzev, A.; Laschewsky, A.; Adriaensens, P.; Gelan, J. Macromolecules 2002, 35, 1091–1101. (6) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98–101. (7) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2006, 39, 765–771.
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