Vesicle Formation and Microphase Behavior of Amphiphilic ABC

Jul 20, 2010 - Bram van Hoof , Albert J. Markvoort , Rutger A. van Santen , and Peter A. J. Hilbers. The Journal of Physical Chemistry B 2014 118 (12)...
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Vesicle Formation and Microphase Behavior of Amphiphilic ABC Triblock Copolymers in Selective Solvents: A Monte Carlo Study Jie Cui†,‡ 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 University of Chinese Academy of Sciences Received June 1, 2010. Revised Manuscript Received June 29, 2010 w This paper contains enhanced objects available on the Internet at http://pubs.acs.org/Langmuir. n

Using Monte Carlo simulation, we studied the vesicle formation and microphase behavior of ABC triblock copolymers in selective solvent for A and C blocks. Simulation results show that the hydrophilicity of Blocks A and C determines not only the vesicle formation but also the microphase behavior. If Blocks A and C are of equal length, then Block A (with lower hydrophilicity) is likely to aggregate on the inner surface, whereas Block C (with higher hydrophilicity) tends to move to the outer surface, forming the ABC (from inside to outside) three-layer vesicle. Simulation results reveal that if the hydrophilicity difference between the two blocks is sufficiently low, then the ABC three layers are formed after the membrane closes (i.e., after the formation of vesicle profile). Otherwise, the ABC three layers are formed before the membrane closes. Furthermore, the effect of chain length and incompatibility between the two amphiphilic blocks (i.e., A and C) is studied and discussed in this article. The shorter block A is much more likely to aggregate on the inner surface, and the incompatibility between A and C must be sufficiently strong to ensure that the ABC copolymer forms an ABC (from inside to outside) three-layer vesicle.

I. Introduction The vesicle, a closed membrane, is attracting growing interest because of its unique hollow structure and significant potential applications in biomedical sciences and industries. These include applications in microcapsules,1 mimetic of cell membrane,2-4 drug delivery, and targeting release,5-7 and so forth. In the past decade, the formation of vesicles has been widely and comprehensively studied, both theoretically8-14 and experimentally.15-21 In general, vesicle membrane can be divided into three layers: two hydrophilic layers (i.e., the inside and outside hydrophilic surface) and one hydrophobic layer between the two hydrophilic surfaces. If the curvature effects are neglected, then most vesicles have the *Corresponding author. E-mail: [email protected]. Tel: þ86-431-85262151. Fax: þ86-431-85262126.

(1) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (2) Stoenescu, R.; Graff, A.; Meier, W. Macromol. Biosci. 2004, 4, 930. (3) Zhou, Y. F.; Yan, D. Y. J. Am. Chem. Soc. 2005, 127, 10468. (4) Zhou, Y. F.; Yan, D. Y. Angew. Chem., Int. Ed. 2005, 44, 3223. (5) Savic, R.; Luo, L. B.; Eisenberg, A.; Maysinger, D. Science 2003, 300, 615. (6) Allen, C.; Maysinger, D.; Eisenberg, A. Colloids Surf., B 1999, 16, 3. (7) Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728. (8) He, X. H.; Liang, H. J.; Huang, L.; Pan, C. Y. J. Phys. Chem. B 2004, 108, 1731. (9) Wang, Z. L.; He, X. H. J. Chem. Phys. 2009, 130, 094905. (10) He, X. H.; Schmid, F. Phys. Rev. Lett. 2008, 100, 137802. (11) Uneyama, T. J. Chem. Phys. 2007, 126, 114902. (12) Yamamoto, S.; Maruyama, Y.; Hyodo, S. A. J. Chem. Phys. 2002, 116, 5842. (13) Han, Y. Y.; Yu, H. Z.; Du, H. B.; Jiang, W. J. Am. Chem. Soc. 2010, 132, 1144. (14) Huang, J. H.; Wang, Y.; Qian, C. J. J. Chem. Phys. 2009, 131, 234902. (15) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (16) Chen, L.; Shen, H. W.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9488. (17) Luo, L. B.; Eisenberg, A. Langmuir 2001, 17, 6804. (18) Choucair, A.; Lavigueur, C.; Eisenberg, A. Langmuir 2004, 20, 3894. (19) Ma, L.; Eisenberg, A. Langmuir 2009, 25, 13730. (20) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745. (21) Bang, J.; Jain, S.; Li, Z. B.; Lodge, T. P.; Pedersen, J. S.; Kesselman, E.; Talmon, Y. Macromolecules 2006, 39, 1199.

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symmetric layer structure with respect to their hydrophobic midlayer (i.e., the inner and outer hydrophilic surfaces are formed by the same block species). This feature is evident in nearly all of the AB- and ABA-type block copolymers or ABC-type block copolymers with a hydrophilic Block A and other two consecutive hydrophobic blocks. This symmetric property leads to essentially the same solution environment both inside and outside the vesicles. However, in some practical application areas, vesicles with asymmetric layers in which the inner and outer surfaces of vesicles are formed by different species of hydrophilic blocks are more advantageous. Meier et al.2,22 pointed out that many potential technical applications of reconstitutions of cell membranes inserted with proteins depend on the correct orientation of the protein. The correct orientation of the inserted protein further depends on the presence of asymmetric properties on the vesicle membrane. These properties help protein insert into the vesicle membrane with a preferred direction. Therefore, the study of the vesicle microphase structure and its formation is of fundamental and practical importance. A relevant feature of vesicles with asymmetric layers is their tunable structures. Luo and Eisenberg investigated the vesicles formed by a diblock copolymer mixture of PS300-PAA11/PS310P4VP33.23 The PAA block chains in PS300-PAA11 are segregated inside the vesicles, whereas the outside corona of the vesicles consists of P4VP chains. This phenomenon can be attributed to the fact that the PAA chains are much shorter than the positively charged P4VP chains, implying a repulsive interaction in solution. In contrast, the PAA chains are neutral under the experimental conditions. Stoenescu and Meier showed that PEO45-PDMS65PMOXA346 triblock copolymers can form vesicles with the longer hydrophilic block PMOXA pointed outward the vesicles, whereas (22) Stoenescu, R.; Meier, W. Chem. Commun. 2002, 3016. (23) Luo, L. B.; Eisenberg, A. Angew. Chem., Int. Ed. 2002, 41, 1001.

Published on Web 07/20/2010

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the shorter hydrophilic PEO block is pointed inward.22 Later, they described a new approach to induce a directed insertion of membrane proteins into asymmetric membranes formed by amphiphilic ABC triblock copolymers with two chemically different water-soluble A and C blocks.2 Liu et al. studied the aggregate morphologies of the amphiphilic triblock copolymer PAA26-PS890-P4VP40 as a function of pH in DMF/THF/H2O mixtures.24 They found that the inner (or outer) surface of vesicle membranes is controllable via the adjustment of pH value. The corresponding transition process was also discussed. Recently, Njikang et al. studied the self-assembly morphologies of PtBA107-PCEMA193-PGMA115 triblock copolymer in solution.25 Various morphologies with segregated hydrophilic coronas have been obtained. Vesicles, with surfaces predominantly occupied by one of the hydrophilic blocks, have been investigated in detail. Although significant progress has been made in the study of the vesicles with asymmetric layers, some questions still remain unclear because of the limitations of experimental technique. These limitations have affected efforts at clarifying the dynamic formation processes of both the vesicle and its layer structure. These have also affected the initiatives at identifying the factors, aside from the relative chain length of different hydrophilic blocks, which, in turn, can affect the vesicle layer structure. In general, computer simulations are more advantageous in studying the self-assembly of polymers.26-28 However, to the best of our knowledge, this method has not been applied to such asymmetric assemblies up to now.

reconnection of the links occurs. The acceptance or rejection of the attempted move is further governed by the Metropolis rule: if the energy change ΔE is negative, then the exchange is accepted. Otherwise, the exchange is accepted P with a probability of p = exp[-ΔE/(kBT)], where ΔE = ijΔNijεij is the energy change caused by the attempted move; ΔNij is the number difference of the nearest neighbor pairs between components i and j before and after the movement, where i,j = A, B, C, and S (solvent), respectively; εij is the reduced interaction intensity between i and j; and kB is the well-known Boltzmann constant and assumed to be 1 in the whole simulation. The parameter T is the reduced temperature. The ABC-type triblock copolymer studied in this simulation consists of one hydrophobic block, B, and two hydrophilic blocks, A and C. The interaction parameter εBS was set as positive, whereas the parameters εAS and εCS were set as negative, corresponding to their amphiphilic nature. The parameters εAB, εBC, and εAC were set as positive to mimic their incompatibilities. In addition, all other self-interaction parameters between the same components (i.e., εAA, εBB,, εCC, and εSS) were set as 0 for all of the results reported in this article. In contrast, to obtain the vesicular structures, we implemented the annealing method. In other words, the inverse temperature 1/T changed gradually from 0 (representing the athermal state of T = ¥) to a given positive value with 1/T = 0.07 (representing a lower temperature). The annealing process was achieved by 350 time steps. In other words, the annealing rate (i.e., the increasing rate of 1/T) was set as 0.0002 per time step. At each time step, 9000 Monte Carlo steps (MCSs) were performed. In one MCS, on the average, each monomer has attempted one exchange move. After 350 time steps, keeping 1/T = 0.07 unchanged, another 200 time steps were carried out to confirm that the final structures are in equilibrium state.

II. Model and Method Lattice Monte Carlo simulation method, as proposed by Carmesin et al.29 and Larson et al.,30-32 was used in this study. The system is embedded in a simple cubic lattice of volume V = L  L  L with L = 40. Periodic boundary conditions are imposed in all three directions. The concentration of the system is 8%, and the studied triblock copolymer is denoted as An1Bn2Cn3. Each copolymer chain contains n1 þ n2 þ n3 monomers. Each monomer occupies one lattice site, and the monomers are self-avoiding and mutually avoiding, which insures that only one monomer occupies the same lattice site. The √ permitted bond length in our simulation is confined within 1 and 2. Therefore, each lattice site has 18 nearest-neighbor sites in a rectangular 3D space. The evolution of the chain configuration is achieved through the exchange move between monomer and vacancy (a solvent molecule). To enhance the efficiency, partial-reptation algorithm, which has been proven suitable for studying the dynamic process, is applied in our simulation. The microrelaxation modes are defined as follows. A monomer is randomly chosen to exchange with one of its 18 nearest neighbors. If the neighbor is a vacancy, then exchange with the bead is attempted. If the exchange does not violate the bond length restriction, the exchange is allowed. This process constitutes a single movement. If the exchange would break two chain connections, then it is disallowed. If the exchange creates a single break in the chain, then the vacancy will continue to exchange with subsequent monomers along the chain until (24) Liu, F. T.; Eisenberg, A. J. Am. Chem. Soc. 2003, 125, 15059. (25) Njikang, G.; Han, D. H.; Wang, J.; Liu, G. J. Macromolecules 2008, 41, 9727. (26) Jiang, Y.; Chen, T.; Ye, F. W.; Liang, H. J.; Shi, A. C. Macromolecules 2005, 38, 6710. (27) Kong, W. X.; Li, B. H.; Jin, Q. H.; Ding, D. T.; Shi, A. C. J. Am. Chem. Soc. 2009, 131, 8503. (28) Kong, W. X.; Li, B. H.; Jin, Q. H.; Ding, D. T.; Shi, A. C. Langmuir 2010, 26, 4226. (29) Carmesin, I.; Kremer, K. Macromolecules 1988, 21, 2819. (30) Larson, R. G.; Scriven, L. E.; Davis, H. T. J. Chem. Phys. 1985, 83, 2411. (31) Larson, R. G. J. Chem. Phys. 1988, 89, 1642. (32) Larson, R. G. J. Chem. Phys. 1989, 91, 2479.

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III. Results and Discussion In this section, we mainly focused on the vesicles formed by A2B5C2 triblock copolymers in selective solvent for A and C. The interaction parameters εAS, εCS, and εAC are variables in the simulations. All other interaction parameters between different components are kept constant and are set as εAB = εBC =1 and εBS =6. The hydrophilicity difference of Blocks A and C is achieved by setting εAS and εCS with different negative values. In addition, a variable δAC is defined to measure the hydrophilicity difference between different hydrophilic blocks, where δAC  |εAS - εCS| and εAS are fixed as -1 unless specified. A. Effect of the Hydrophilicity Difference between Blocks A and C. To study the effect of the hydrophilicity, we consider the Blocks A and C as having the same hydrophilicity and set εAS =εCS = -1 (i.e., δAC = 0) and εAC = 5 to ensure that Blocks A and C can be fully separated. Figure 1 shows the final equilibrium vesicle structure formed by A2B5C2 triblock copolymers in selective solvent for A and C. Blocks A, B, and C are drawn represented by red, blue, and green, respectively. In Figure 1a, the area occupied by Block A is larger than the area occupied by Block C, indicating that more of Block A is located on the outer surface of the vesicle because the total number of A and C blocks is equal. This can be clearly observed in the cross sections of the vesicle. In Figure 1b, the outer surface is mainly occupied by Block A, and the inner surface is completely occupied by Block C, forming the ABC three-layer vesicle. Notably, depending on the initial state, the CBA three-layer vesicle (where the inner surface was formed by Block A) can also appear in the simulation. We randomly selected more than 10 initial states and found that the possibilities for forming ABC and CBA three-layer vesicles are equal. This results from the same characteristics (including chain length, hydrophilicity, and interactions with other components) of Blocks A and C. DOI: 10.1021/la102211d

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Figure 1. Vesicle structure formed at t = 550 by A2B5C2 triblock

copolymers in selective solvent for A and C in the case of δAC = 0: (a) overall perspective of the vesicle structure for εAS = εCS = -1 and (b) the corresponding cross sections of part a. The red, blue, and green colors represent blocks A, B, and C.

Figure 3. Variations of contact numbers (NBS) between block B and solvent S and between block A and C (NAC) with time for δAC = 0 and εAS = εCS = -1. Typical morphologies at different times are also given in this diagram for the purpose of comparison: (a) t = 212, (b) t = 230, (c) t = 240, and (d) t = 320.

Figure 2. n w Snapshots showing the formation of the vesicle formed by the amphiphilic triblock copolymers A2B5C2 for δAC = 0 and εAS = εCS = -1: (a) t = 212, (b) t = 230, (c) t = 240, and (d) t = 320. A video in .mpg format of the dynamic formation process of the vesicle shown in a-d is available in the HTML version this article. (e-h) Cross sections corresponding to a-d, respectively. A video in .mpg format of the dynamic formation process of the vesicle cross section shown in e-h is available in the HTML version this article.

The kinetic process of the vesicle formation is shown in Figure 2. The vesicle is formed via the bending and the subsequent closing of the membrane. Furthermore, we examined the microphase separation process of the ABC block copolymer during the vesicle formation. In Figure 2e,f (t = 212 and 230, respectively), Blocks A and C are mixed with each other on both the inner and outer surfaces. This means that the phase segregation was not completed before the membrane closed. With further increase in time, more A blocks migrate to the outer surface, and more C blocks move to the inner surface. The blocks finally form the ABC threelayer vesicle (Figure 2g,h). To study the microphase separation process of the ABC block copolymer during the vesicle formation quantitatively, we introduced the contact numbers between Block B and solvent S (NBS) and between Blocks A and C (NAC) in this study. Figure 3 shows the variations of NBS and NAC with time. As time increases, NBS drops rapidly before the membrane closes. The result indicates that the B blocks aggregate quickly in the solvent to form the middle layer of the membrane. However, Figure 3 shows that NAC increases considerably with increasing time, up to t = 170, then remains mostly unchanged until t = 240, leading to the appearance of a flat region in NAC-t curve. The results indicate the tendency of Blocks A and C to enclose each other during the copolymer aggregation to form the membrane in the solvent. The membrane is formed, bent, and closed only in the flat regions, thereby forming the vesicle. Furthermore, Figure 3 clearly shows the decrease in NAC with the increase in time from t = 240 to 320 (orange color). Thereafter, it remained unchanged with further increases in time. A decrease in NAC means Blocks A and C have 13674 DOI: 10.1021/la102211d

Figure 4. Variations of mean-square distance GCM and the meansquare values gCM with time. t0 = 240 is defined as initial time.

parted (i.e., phase separation between Blocks A and C occurred after the membrane closed). The result is consistent with the observation in Figure 2. To study the kinetics of phase separation further, we calculated the variations of two mean-square parameters as GCM(t) and gCM(t) with time. The variations are defined as GCM(t) = and gCM(t) = 240). Therefore, we focus on the separation after this time and set t0 = 240 as the initial time. Figure 4 shows that GCM(t) is mostly unchanged as time increases. This means that once the vesicle is formed, it neither expands nor shrinks. The profile of the vesicle remains unchanged. However, Figure 4 also shows that gCM(t) has an apparent increase with the increase in time before t = 320. This indicates that the immigration of the polymer chains occurs within the vesicle layers after t = 240. Therefore, we can conclude that the vesicle size and shape remain unchanged once formed, whereas the microphase separation between Blocks A and C can occur within the vesicle layers. Langmuir 2010, 26(16), 13672–13676

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Figure 5. Vesicle structure formed at t = 550 by A2B5C2 triblock

copolymers in selective solvent for A and C for δAC = 0.5: (a) overall perspective of the vesicle structure for εAS = -1 and εCS = -1.5, (a1) the corresponding cross sections of part a, (b) overall perspective of the vesicle structure for εAS = -1.5 and εCS = -1, and (b1) the corresponding cross sections of part b.

Figure 6. n w Snapshots showing the vesicle formation by the amphiphilic triblock copolymers A2B5C2 for δAC = 0.5, εAS = -1 and εCS = -1.5: (a) t = 235, (b) t = 260, (c) t = 288, (d) t = 310. A video in .mpg format of the dynamic formation process of the vesicle shown in a-d is available in the HTML version this article. (e-h) Cross sections corresponding to a-d, respectively. A video in .mpg format of the dynamic formation process of the vesicle cross section shown in e-h is available in the HTML version this article.

Consequently, we consider that Blocks A and C have different hydrophilicity and set εAS = -1 and εCS = -1.5 (i.e., δAC =0.5). Likewise, we keep εAC = 5 to ensure that Blocks A and C can be fully separated. Figure 5 shows the equilibrium vesicle structures formed by A2B5C2 triblock copolymer in the selective solvent for A and C. In Figure 5a,a1, the inner surface of the vesicle is covered by A blocks (with lower hydrophilicity). Blocks with higher hydrophilicity (C blocks) are located on the outer surface of the vesicle. For comparison, we maintained δAC = 0.5 and exchanged the values of εAS and εCS. Thus, εAS = -1.5 and εCS = -1. Figure 5b,b1 shows that C blocks (with lower hydrophilicity) aggregate into the inner surface, whereas A blocks (with higher hydrophilicity) aggregate into the outer surface, forming the CBA three-layer (from inside to outside) vesicle when δAC =0.5. Unlike the case of δAC = 0 (Figure 1), the formation of CBA three-layer vesicle occurs definitely and independent of the initial state specific to this case. The probability of forming the ABC three-layer vesicle becomes lower, or becomes zero, with the increase in δAC from 0 to 0.2. Therefore, the hydrophilicity difference between Blocks A and C is a key factor that determines the vesicle membrane structure. This can be controlled by tuning Langmuir 2010, 26(16), 13672–13676

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Figure 7. Variations of the contact numbers between block B and solvent S (NBS) and between blocks A and C (NAC) with time for δAC = 0.5, εAS = -1 and εCS = -1.5. Typical morphologies at different times are also given in this diagram for the purpose of comparison: (a) t = 235, (b) t = 260, (c) t = 288, (d) t = 310.

the hydrophilicity of Blocks A or C. As to the reason the blocks with higher hydrophilicity locate on the outer surface of the vesicle, the blocks with higher hydrophilicity become more stretched in the selective solvent. Compared with the blocks with lower hydrophilicity, these blocks need greater effective volume to stretch. The outer surface of the vesicle can provide more volume than the inner surface. We also studied the vesicle formation kinetics and the microphase behavior of the ABC triblock copolymer. Similar to the case of δAC = 0 (Figure 2), the vesicle is formed via the bending and then closing of the membrane, as shown in Figure 6. However, the difference between the two cases is time of the microphase separation. In the case of δAC = 0.5 (Figures 6e-h), separation occurs before the membrane closes, whereas in the case of δAC = 0 (Figures 2e-h), separation occurs after the membrane closes. To confirm this result further, the variations of the contact numbers NBS and NAC with time are given in Figure 7, showing that there is no significant difference between the two curves of NBS versus time in Figures 3 and 7. However, for the curve of NAC versus time, the flat region in Figure 3 disappears. Instead the NAC decreases continuously from the peak point with the increase in time up to t = 310 after the membrane closes. Thereafter, it remains unchanged, as shown in Figure 7. Therefore, the fact microphase separation of Blocks A and C occurs before the membrane closes is further confirmed, particularly when δAC = 0.5. B. Effect of the Chain Length Ratio and Incompatibility between Blocks A and C. In this section, A2B5C3 triblock copolymer was introduced to study the effect of chain length. The vesicle formation condition is a function of chain structure. To ensure that the A2B5C3 triblock copolymers can form a vesicle, the interaction parameters have to be changed to εAB = εBC =1 and εBS =7. The simulation result when εAS = -1, εCS = -0.7 (δAC = 0.3), and εAC =4 is given in Figure 8. In Figure 8a,b, the longer C blocks are located on the outer surface of the vesicle, whereas the shorter A blocks form the inner surface because the outer surface of vesicle can provide more volume than the inner surface. The vesicles obtained for various values of εAS and εCS have the same membrane structure, as shown in Figure 8. From the results in Section IIIA, we observed that the blocks with lower hydrophilicity tend to form the inner surface of the vesicle if the two hydrophilic block chains have equal length. However, Figure 9 shows that the longer Block C is much more likely to aggregate on the outer surface of vesicle, even though its hydrophilicity (εCS = -0.1) is much lower than that of Block A (εAS = -1). The result indicates that the chain length ratio is a more dominant DOI: 10.1021/la102211d

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Figure 8. Vesicle structure formed at t = 550 by A2B5C3 triblock copolymers in selective solvent for A and C: (a) overall perspective of the vesicle structure and (b) the corresponding cross sections of part a.

the inner surface of the vesicle is mainly covered by shorter A blocks. However, a few C blocks were found to aggregate on the inner surface, even though the repulsive interaction energy (εAC) between Blocks A and C is as high as 1.8. This result is in good agreement with the experimental result of Njikang and Liu et al. (Figures 5 and 6 in ref 25), who found that a few PGMA blocks located at the inner surface of the vesicles were formed by PtBA107-PCEMA193-PGMA115 (subscripts indicate the block lengths) triblock copolymer, although its chain length is longer than that of PtBA and the incompatibility between PGMA and PtBA is sufficiently high. Figure 9. Vesicle structures formed at t = 550 by A2B5C3 triblock

copolymers in selective solvent for A and C for εAS = -1 and various values of εCS: (a) εCS = -0.5, (b) εCS = -0.3, and (c) εCS = -0.1.

Figure 10. Vesicle surfaces formed by A2B5C3 triblock copolymers

for various values of εAC: (a) εAC = 0, (b) εAC = 1, and (c) εAC = 1.8.

factor in determining the vesicle structure compared with the hydrophilicity difference between Blocks A and C. Finally, we studied the effect of the incompatibility between Blocks A and C by changing the interaction parameter εAC. Figure 10 shows the vesicle surfaces for different values of εAC. These results were obtained at t = 550. The time was set to ensure that the system is at equilibrium state. From Figure 10a, when εAC =0, Blocks A and C distribute randomly on both sides of the vesicle. As εAC increases, more A blocks migrate to the inner surface of vesicle, and an increasing number of C blocks move to the outer surface accordingly (Figures 10b,c). From Figure 10c,

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IV. Conclusion The vesicle formation and microphase behavior of ABC triblock copolymers in selective solvent for A and C blocks were studied by Monte Carlo simulation. The simulation results show that the hydrophilicity difference between Blocks A and C is a key factor that determines the vesicle membrane structure. The block with lower hydrophilicity is likely to aggregate on the inner surface if Blocks A and C have equal length, whereas the block with higher hydrophilicity tends to move to the outer surface, forming the ABC three-layer vesicle. The simulation results likewise revealed that the ABC three layers are formed after the membrane closes (i.e., after the formation of vesicle profile) if the hydrophilicity difference between the two blocks is sufficiently low. Otherwise, the ABC three layers are formed before the layer closes. Furthermore, the simulation results indicate that compared with the hydrophilicity difference between Blocks A and C, the chain length ratio is the more dominant factor that determines the vesicle structure. The shorter hydrophilic block is much more likely to aggregate on the inner surface. The present study not only offers new insight into the microphase behavior of block copolymers during the vesicle formation but also provides a new clue for preparing structure-controlled polymer vesicles. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China for General Program (20874099), Major Program (50930001), Creative Research Groups (50921062), Outstanding Young Investigators (50725312), and the National Basic Research Program (2007CB808000) of China.

Langmuir 2010, 26(16), 13672–13676