Morphology and Structure Control of Multicompartment Micelles from

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Morphology and Structure Control of Multicompartment Micelles from Triblock Copolymer Blends Jing Xin, Dahuan Liu, and Chongli Zhong* Laboratory of Computational Chemistry, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China ReceiVed: March 15, 2009; ReVised Manuscript ReceiVed: May 18, 2009

Control of the overall morphology and inner structure of multicompartment micelles from binary blends of triblock copolymers in solution was studied by dissipative dynamics simulations. The effects of the block sequence, block ratio, block length, and chain architecture on the morphology and structure of mixed multicompartment micelles were investigated systematically. This work shows that by changing the block composition or chain architecture of one copolymer in the binary blends, the mixed degree, relative distance, and degree of participation of core-forming blocks from two triblock copolymers can be tuned, and diverse morphologies of mixed micelles with alterable domain arrangements and overall size can be obtained. This work shows that blending of copolymers is an effective way to control the morphology and inner structure of multicompartment micelles, providing useful information on the preparation of mixed micelles with unique properties for practical applications. 1. Introduction Micellization of amphiphilic block copolymers in a selective solvent is a well-known phenomenon and attracts widespread interest in polymer science and technology. Traditional micelles usually provide only two compartments: a hydrophilic corona and a hydrophobic core. Multicompartment micelles, consisting of a hydrophilic corona and a microphase-separated hydrophobic core, have received increasing attention due to their fascinating structures and wide applications in the fields of medicine, nanotechnology, and catalysis, etc.1-11 Tailoring the overall morphology and the inner structure of multicompartment micelles is the key to controlling their properties for potential applications. According to recent works on multicompartment micelles, the common approach to morphology control is to change the architecture and composition of the building block copolymers, and various polymers with complex architecture have been explored.3,4,12-16 Compared to the design and synthesis of new complex copolymers, cooperative self-assembly of copolymer blends in solution is considered as a simple and substitutable strategy to prepare multicompartment micelles. For example, multicompartment micelles formed from the blends of star triblock and diblock copolymers were investigated by experiment,8 selfconsistent field theory (SCFT) calculation,17 and dissipative particle dynamics (DPD) simulation.18 Cui and co-workers obtained wormlike multicompartment cylinders using a binary blend of linear triblock copolymers ABC and ABD,19 while Zhu and Hayward obtained wormlike micelles from blends of amphiphilic AB and hydrophobic BC diblock copolymers.20 Although these studies show that blending is an efficient approach to tune the morphology of multicompartment micelles, the formation mechanism of mixed micelles at the molecular level is not very clear, and the systematic understanding of the structure control of multicompartment micelles by copolymer blends, especially copolymers with complex chain architectures, is very limited to date. * To whom correspondence should be addressed. E-mail: zhongcl@ mail.buct.edu.cn.

In our previous work,18 the DPD method was used to study the formation of multicompartment micelles from star and linear triblock copolymer blends, focusing on the influence of the blending ratio and the evolution dynamics of a ring/cogwheel multicompartment micelle. The simulations of that work illustrate that a slight change in the blending ratio can produce various multicompartment micelles with new morphologies and structures and also shows that the length of the middle block of the linear triblock copolymers may play important roles in the morphology control of mixed micelles. Therefore, in the present work it was extended to investigate the effects of several structural parameters of copolymer chains on the cooperative aggregation of copolymer blends systematically, such as the block sequence, block ratio, block length, and molecular architecture. 2. Method and Simulation Details 2.1. Dissipative Particle Dynamics Method. The DPD method was first introduced by Hoogerbrugge and Koelman21,22 to simulate complex fluids at a coarse-grained level. It is a particle-based mesoscopic simulation technique, particularly suitable for complex fluids over large length and time scales. It can complement experimental work by direct access to fine structure, especially the inner microphase-separated cores, and has been proven as an effective method to provide valuable understanding of multicompartment micelles.18,23-28 Only a brief introduction of the theory is given here, and details of the DPD method are given elsewhere.29,30 The DPD method considers a series of soft particles, whose momenta and position vectors are governed by Newton’s equations of motion:

dri ) vi, dt

mi

dvi ) fi dt

(1)

The total force acting on a particle in the DPD method is composed of three parts, conservative, dissipative, and random forces, each of which is pairwise additive:

10.1021/jp902300g CCC: $40.75  2009 American Chemical Society Published on Web 06/18/2009

Morphology and Structure Control of Micelles

fi )

∑ (FijC + FijD + FijR)

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(2)

j*i

where the sum runs over all other particles within a certain cutoff radius rc. The conservative force FC is a soft repulsion acting along the line of centers and is given by

FijC )

{

aij(1 - rij/rc)rîj (rij < rc) (rij g rc) 0

(3)

The dissipative force FD and the random force FR are given by

FijD ) -γωD(rij)(rîj · vij)rîj

(4)

FijR ) σωR(rij)θijrîj

(5)

where aij is the maximum repulsion between particles i and j, rij ) ri - rj, rij ) |rij|, rîj ) rij/|rij|, vij ) vi - vj, ωD and ωR are weight functions vanishing for r > rc, γ is the friction coefficient, σ is the noise amplitude, and θij is a randomly fluctuating variable with Gaussian statistics. The two weight functions can be taken simply as

ωD(r) ) [ωR(r)]2 )

{

(1 - rij/rc)2 (rij < rc) (rij g rc) 0

σ2 ) 2γkBT

(6)

(7)

2.2. Model and Simulation Details. The aggregation behavior of star and linear ABC triblock copolymers to form multicompartment micelles in water has been studied in our previous work,18 and the DPD repulsion parameters adopted in that work were determined on the basis of reproducing the experimental works of Lodge and co-workers,8 in which A, B, and C represent the weakly hydrophobic polyethylethylene, the hydrophilic poly(ethylene oxide), and the strongly hydrophobic poly(perfluoropropylene oxide), respectively. As the subsequent study of our previous work,18 various ABC triblock copolymers were considered by keeping DPD repulsion parameters unchanged in this work as shown in Table 1. The systems studied in this work are composed of binary copolymer blends and a selective solvent. The solvent molecule (S) is still modeled as a single DPD bead, while the linear, star, and π-shaped triblock copolymers were modeled as spring-bead chains denoted as BNBANACNC, BNB(ANA)CNC, and ANAE(BNB)A(NA-2NAE)(CNC)ANAE, respectively, where Ni (i ) A, B, C) is the length of the i block and NAE is the segment length of the A block from the end to the graft point. A cubic cell of size 30 × 30 × 30 rc3 containing about 81 000 DPD beads and periodic boundary conditions were applied. The cutoff radius rc, the particle mass m, and kBT were all taken as unity for convenience. The time step ∆t and the harmonic spring constant were taken as 0.05 and 4.0. The bead density was set to 3.0. The overall volume fraction of the copolymer blend in solution was set to be 0.15, and a blending ratio of 1:1 was adopted for all the blends. A total of (2-5) × 105 DPD steps were carried out for a DPD simulation to guarantee the equilibration of the system through the indication of stability of properties including the conservative energy and the rootmean-square radius of gyration of copolymer chains.

TABLE 1: DPD Repulsion Parameters aij (DPD Units) Used in This Work A B C S

A

B

C

S

25 45 75 50

45 25 90 27

75 90 25 120

50 27 120 25

3. Results and Discussion In our previous work,18 DPD simulations were performed on copolymer blend systems and compared with the experimental observations of Lodge and co-workers.8 Therefore, the reliability of this method for describing multicompartment micelles from copolymer blends has been validated. As a result, the DPD method was adopted in this work directly without further validation. In blend systems there are two kinds of copolymers. Due to the different chain architectures, the two copolymers tend to locate at different places of the micelles formed. By the cooperative self-assembly of two copolymers, more complicated structures can be obtained in blend systems than in pure systems. We carried out DPD simulation on pure B12A10C2 and pure B12(A10)C2 to compare with the B2A6C1 + B10(A4)C1 blend system to illustrate this. The simulation results are shown in Figure 1. A “core-shell” two-layer structure of the hydrophobic core was obtained in the pure B12A10C2 system (Figure 1a), and a “sphere-on-sphere” two-layer structure of the hydrophobic core was obtained in the pure B12(A10)C2 system (Figure 1b). While in the B2A6C1 + B10(A4)C1 blend system (Figure 1c) B10(A4)C1 aggregated at the exterior of the micelle due to its starlike topology, B2A6C1 aggregated at the interior of the micelle, and a more complicated structure called “sphere-on-onion” was obtained, with the hydrophobic core separated into three layers. The results show that cooperative self-assembly of copolymer blends in solution is an effective strategy to prepare multicompartment micelles with complicated structures. Therefore, in the present work the effects of several structural parameters of copolymer chains on the cooperative aggregation of copolymer blends will be investigated systematically. 3.1. Effects of the Block Sequence. Copolymers with different block sequences can self-assemble into various morphologies,5,6,27,28,31 indicating that the block sequence is an important factor to influence micelle structures. Therefore, we first assess this effect in binary copolymer blends by DPD simulations. Multicompartment micelles from binary blends of linear B2A6C1 and star triblock copolymers were obtained in our previous work,18 and in this work the morphology transitions caused by changing the block sequence of linear B2A6C1 are presented. Three linear triblock copolymers (B2A6C1 (L1), B2C1A6 (L2), A6B2C1 (L3)) were chosen to blend with star triblock copolymers (B10(A4)C1 (S1), B10(A4)C2 (S2), B19(A4)C2 (S3)). The micelle morphologies are shown in Figure 2 in a matrix form. The micelles from the three star triblock copolymers are shown in the first line and those from the three linear copolymers in the first column, while those mixed micelles from the corresponding binary blends are given in the intersections. Three linear triblock copolymers have the same chemical composition, but their aggregate morphologies blending with the same star triblock copolymers are different. The linear B2A6C1 series forms spherical micelles (L1 + S1, L1 + S3) and sheetlike micelles (L1 + S2), the linear B2C1A6 series provides discrete disklike micelles (L2 + S1, L2 + S2, L2 +

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Figure 1. Morphologies and structures of micelles obtained by (a) pure B12A10C2, (b) pure B12(A10)C2, and (c) B2A6C1 + B10(A4)C1 (B blocks and solvent were omitted for clarity; A, red; C blocks of linear copolymers, green; C blocks of star copolymers, blue). Views with and without A blocks are both given.

Figure 2. Morphologies and structures of multicompartment micelles obtained by blending linear copolymers (L1, L2, L3) with star copolymers (S1, S2, S3) (B blocks and solvent were omitted for clarity; A, red; C blocks of linear copolymers, green; C blocks of star copolymers, blue). Views with and without A blocks are both given.



Figure 3. Schematic illustrations of the shape of multicompartment micelles described by “the lengths of the three principal axes”. (a) Definition of dimensions: x and y axes lie within the paper plane, while the z axis is vertical into the paper plane. (b-d) Views of the sheetlike micelle in L1 + S2 along three defined orthogonal planes with the labeled lengths of the three principal axes of this micelle.

Figure 4. Slice views of the detailed structure of a multicompartment micelle obtained by blending linear copolymers with star copolymers: (a) L1 + S1; (b) L2 + S1; (c) L3 + S1 (solvent was omitted for clarity; B, light blue; A, red; C blocks of linear copolymers, green; C blocks of star copolymers, blue). Views with and without A and B blocks are both given.

S3), and the linear A6B2C1 series forms spherical micelles (L3 + S1) and much larger compound micelles (L3 + S2, L3 + S3). To tell these micelles apart more clearly, their lengths of the three principal axes were measured. In Figure 3, we take the sheetlike micelle (L1 + S2) as an example to explain the definition of the dimensions of the x, y, and z axes, and the lengths of the three principal axes of this micelle are given in DPD units (25.5 × 21.0 × 7.5). According to this definition, the morphology of the above micelles can be described quantitatively, for example, an average size of 9.0 × 9.0 × 9.0 for spherical micelles (L1 + S3), 18.0 × 15.0 × 9.0 for disklike micelles (L2 + S2), and 28.0 × 11.0 × 11.0 for large compound micelle (L3 + S2). The length of the C block was set to 1 in L1, L2, L3, and S1 in our simulations. To prove that the structural features of the copolymers are not lost when the length of the C block is set to 1, we carried out additional simulations on star B20(A8)C2 and compared the final micellar morphology obtained with that from S1 (star B10(A4)C1). The simulation results show that the two copolymers formed similar micellar morphologies. Therefore, the results with L1, L2, L3, and S1 are reliable. The block sequence also influences the inner structure of the multicompartment micelle. Taking the star B10(A4)C1 series as

an example (Figure 4), the morphologies of mixed micelles change from spherical ones (Figure 4a,c) to disklike ones (Figure 4b), and the middle slices of these micelles are shown to give a detailed observation of their fine structures. Due to the long hydrophilic B blocks and starlike architecture, star copolymers cannot fully enter the interior of the hydrophobic core, and they tend to aggregate at the hydrophilic-hydrophobic interface of mixed micelles. As a result, different structures of the hydrophobic core are mainly induced by different block sequences of linear triblock copolymers. For the B2A6C1 system, hydrophilic B blocks connect to A blocks and then to C blocks; therefore, spherical micelles with coronal B, shell A, and core C are formed. Due to the hydrophobic shell A, C blocks of star copolymers are separated from core C formed by linear copolymers (Figure 4a). For the B2C1A6 system (Figure 4b), hydrophilic B connects to C and then to A; therefore, disklike micelles with coronal B, shell C, and core A are found. C blocks of star copolymers are coexistent with the C domain formed by linear copolymers at the hydrophilic-hydrophobic interface of the micelle, and they form the hydrophobic shell C cooperatively. For the A6B2C1 system (Figure 4c), hydrophilic B blocks are confined by A and C blocks. If core C was formed and surrounded by layer B and then by layer A, there would not be sufficient B blocks to protect the micelle from the solvent.

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Figure 5. Morphologies and structures of multicompartment micelles obtained by blending linear copolymers B2AxCy (x/y changes from 6/1 to 5/4 to 1/6) with star copolymers B10(A4)C1 (B blocks and solvent were omitted for clarity; A, red; C blocks of linear copolymers, green; C blocks of star copolymers, blue). Views with and without A blocks are both given.

As a result, some linear copolymers together with star copolymers stretch to the exterior of the micelle. B blocks of them form the coronal B, while C blocks of them form the outer layer C. Other linear copolymers enter the interior to form core C. Different from the foregoing sphere-on-onion structure (Figure 4a), the outer layer C in this multilayer structure is formed from both star copolymers and linear copolymers (Figure 4c). On the other hand, a few linear A6B2C1 chains are almost fully extended and situated within the core; the middle hydrophilic B blocks, surrounded by strongly hydrophobic C blocks and hydrophobic A blocks, are confined in the hydrophobic core as shown in Figure 4c. With different block sequences, hydrophobic C domains from star copolymer B10(A4)C1 and three linear copolymers can achieve various mixed degrees: nonmixed (Figure 4a), fully mixed (Figure 4b), and partly mixed (Figure 4c). As shown above, star copolymers tend to aggregate at the hydrophilic-hydrophobic interface of the micelle, and the internal structures of the hydrophobic core are mainly influenced by linear copolymers. Different block sequences of linear copolymer lead to different domain arrangements in the micelles, as well as various mixed degrees of C blocks from star and linear copolymers. 3.2. Effects of the Ratio of Hydrophobic Blocks. The microphase-separated core of multicompartment micelles is formed by different hydrophobic blocks; therefore, the ratios of these hydrophobic blocks may have a non-negligible effect on the structure of the core. To investigate this factor, we studied the selfassembly of binary blends of linear B2AxCy and star B10(A4)C1.

Figure 6. Conservative energy between C blocks of star copolymers and linear copolymers in a binary blend corresponding to Figure 5 as a function of the ratio of A and C blocks of linear copolymers x/y.

For linear B2AxCy, x and y denote the lengths of the hydrophobic A and C blocks, respectively. The value of x/y was varied from 6/1 to 5/2 and then to 1/6 gradually. Six typical morphologies of multicompartment micelles are shown in Figure 5. Keeping the ratio of hydrophilic and hydrophobic blocks unchanged, the equilibrium overall morphologies cannot be changed obviously just by varying the ratio of hydrophobic A and C blocks in linear copolymers. As shown in Figure 5, only dispersed spherical or disklike micelles exist in the six systems. However, the inner structures of these micelles changed greatly.



Figure 7. Morphologies and structures of multicompartment micelles obtained by blending L4, π1, π2, and S4 with star copolymers (S1, S2, S3) (B blocks and solvent were omitted for clarity; A, red; C blocks of linear copolymers, green; C blocks of star copolymers, blue). Views with and without A blocks are both given.

With the increase of the ratio of C blocks in linear polymers, the inner C core formed by linear copolymers occupies more space of the hydrophobic core, and gets closer to the outer C domains formed by star copolymers. C domains from two kinds of copolymers get contacted gradually, and outer C domains formed by star copolymers change from the spherical aggregates at the hydrophilic-hydrophobic interface to the clusters dispersed in core C. The core structures change from “sphere-ononion” (x/y ) 6/1) to “cluster-on-disk” (x/y ) 5/2, 4/3, 3/2) and then to mixed spheres (x/y ) 2/5, 1/6). To further understand the structures of the hydrophobic cores in these six systems mentioned above, the conservative energy estimated by the conservative interactions between different parts of the blend system was calculated from the following equation:

VC )

∑ 21 aij(rc - rij)2 i

Morphology and Structure Control of Multicompartment Micelles from

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