Study of Controllable Aggregation Morphology of ABA Amphiphilic

Bump-Surface Multicompartment Micelles from a Linear ABC Triblock Copolymer: A Combination Study by ... Zheng Wang , Yuhua Yin , Run Jiang , Baohui Li...
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J. Phys. Chem. B 2007, 111, 1938-1945

Study of Controllable Aggregation Morphology of ABA Amphiphilic Triblock Copolymer in Dilute Solution by Changing the Solvent Property Hongbo Du,†,‡,§ Jintao Zhu,†,§ and Wei Jiang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Department of Physics, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China ReceiVed: NoVember 2, 2006; In Final Form: December 20, 2006

We have studied, both experimentally and theoretically, the aggregation morphology of the ABA amphiphilic triblock copolymer in dilute solution by changing the solvent property. Experimental results showed that the micellar morphology changed from spheres to rods and then to vesicles by changing the common solvent from N-N-dimethylformamide (DMF) to dioxane and then to tetrahydrofuran (THF). These controllable aggregates were also obtained by Monte Carlo simulation. The simulative results showed that the solvent property is a key factor that determines the copolymer aggregation morphology. The morphology changed from spheres to rods and then to vesicles by increasing the solvent solubility, corresponding to the change of stretched of the copolymer chains in the micellar cores. This result is in good agreement with the experimental one. Moreover, the simulative results revealed that the end-to-end distant of the ABA triblock copolymer in the vesicle was larger than that in the spheres and rods, indicating that the copolymer chains were more stretched in vesicles than in the spheres and rods. Furthermore, we gave the distribution of the fraction of the chain number with the end-to-end distance. The results indicated that the amount of folded chains is almost the same as that of stretched chains in the vesicle. Although most chains were folded, stretched chains could be found in the rod and sphere micelles.

1. Introduction Self-assembly of amphiphilic block copolymers in selective medias generally results in aggregates of core-shell structures.1,2 Compared to small molecular weight amphiphilies, aggregates from amphiphilic block copolymers are more thermodynamically and kinetically stable, broadening their applications.2,3 Therefore, aggregates of block copolymers in solution have potential applications in the field such as drug delivery, cosmetic industry, catalysis, separations, microelectronics, advanced materials formation, etc.4-7 Great efforts have been given to the study of self-assembly of amphiphilic block copolymers in selective solvents and the applications of polymeric micelles in a broad range.1,8-11 Depending on the block copolymer composition, copolymer micelles can be distinguished by star-like and “crew-cut”-like micelles.12 The term “crew-cut” was proposed by Halperin et al.13 based on an earlier theoretical work of de Gennes.14 Crewcut aggregates represent a new type of aggregates formed in solutions by the self-assembly of highly asymmetric amphiphilic block copolymers.12 The aggregates are termed “crew-cut” because the dimensions of the core are much larger than those of the corona, as opposed to the star micelles, in which the core is small and the corona is relatively large.15 Due to the high hydrophobicity of the PS block (most of the crew-cut micelles including PS block, so name it as an example), the aggregates are prepared by first dissolving block copolymers in a common * Author to whom correspondence should be addressed. Tel: +86431-5262151. Fax: +86-431-5262126. E-mail: [email protected]. † Changchun Institute of Applied Chemistry. ‡ Northeast Normal University. § These authors contributed equally to this work.

solvent (N-N-dimethylformamide (DMF), for example), followed by the addition of a precipitant, such as deionized water, for the core-forming block and dialysis to remove the organic solvent.16 17 In the course of dialysis, the aggregates become frozen. As opposed to star-micelles (most of the aggregates are spherical structures), it is found that crew-cut aggregates can give rise to cornucopian morphologies,18-23 such as spheres, rods, vesicles, lamellae, large compound micelles, tubes, disks, onions, ring-shaped structures, and several others. Previous studies have extensively explored the crew-cut aggregates with various morphologies from amphiphilic diblock copolymers of polystyrene-b-poly(acrylic acid) (PS-b-PAA),15,24 polystyrene-b-poly(ethylene oxide) (PS-b-PEO),12 and other diblock copolymer systems.25 In contrast, few studies have been reported on the aggregates of triblock copolymer in dilute solution. Riegel and co-workers reported the crew-cut aggregates of triblock copolymer of styrene and quaternized 5-(N,Ndiethylamino) isoprene (PAI-b-PS-b-PAI) and found some complex morphologies.26 The self-assembly behaviors of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO), polystyrene-b-poly(2-vinyl pyridine)-bpoly(ethylene oxide)(PS-b-P2VP-b-PEO), and several other block copolymers in dilute solutions have also been studied.27-30 In our previous reports, we have studied the self-assembly of P4VP-b-PS-b-P4VP in aqueous solution. Some complex aggregate morphologies have been obtained through annealing the micellar solution, and building hydrogen bonding between one of the block and small molecular surfactant.31-32 Great control over micellar shape and size is very important to obtain the desired functions and properties in practical application possibilities. In the fabrication of nanostructurd materials based on the copolymer micelles template, for

10.1021/jp067221x CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007

ABA Amphiphilic Triblock Copolymer example, it would be highly desirable to control the morphology of the micelles. It is thus not surprising that great efforts have been devoted to the preparation of block copolymer micelles with various morphologies. It has been found that the size and shape of the aggregates are controllable according to some tunable parameters of the systems,8,24,33 which affect the force balance of three contributions to the free energy of the system. These parameters include the stretched (deformation) of the core-forming blocks in the core, the repulsive interaction among the corona chains, and the interfacial energy between the solvent and the micellar core. There are many factors that affect the above three terms and by variation of one or more of these, the morphologies can, in principle, be fine-tuned.24,34-36 Examples of such factors that have been studied previously include block length of the copolymer, initial polymer concentration in solutions, solvent used, precipitant, temperature, type and amount of the adding ions, added additives et al. Moreover, study on the kinetic and mechanism of the morphology transition of the aggregates will give insight into tuning the micellar size and morphology. On the other hand, computer simulation have played very important role in the study of the aggregation of block copolymers in both bulk state and solution. Various theoretical approaches, such as coarse-grained surface models,37,38 Browniandynamicsimulations,39self-consistentfieldtheory(SCFT),16,40,41 and dissipative particle dynamics (DPD), have been used to investigate the self-assembly of amphiphilic molecules in dilute solution.42 In addition, Monte Carlo simulation has been successful in the study of the self-assembly of amphiphilic molecules in dilute solution.43,44 Comparing with the mesoscale simulation method such as SCFT, DPD et al., Monte Carlo simulation can give not only the aggregation morphologies but also the microstructures in the micelles such as chain conformation. In this paper, Monte Carlo simulative and experimental methods were combined to study the morphology of selfassembled crew-cut aggregates of ABA amphiphilic triblock copolymer as a function of common solvent properties. Multiple morphologies can be obtained by changing the solvent property with the same of other conditions. Simulative results were in good agreement with the experimental one. Insights into the chain packing in the copolymer micelles with different shapes have been obtained through the Monte Carlo simulation. 2. Experimental Section 2.1 Sample Preparation. The copolymer used in this study is a triblock copolymer of P4VP43-b-PS260-b-P4VP43 (The numbers in the subscripts indicate the number of repeat units of the blocks) (Mn ) 36000 g/mol, PDI ) 1.09) which is purchased from Polymer Source Inc. Canada. The length ratio of P4VP block to PS block is close to 1/6. To prepare the aggregates in aqueous solution, the triblock copolymer was first dissolved in a common solvent, such as dioxane (DMF and THF respectively), which is a good solvent for both PS and P4VP block. Then, deionized water was added to the solution at a rate of 0.2 wt %/30 s with vigorously stirring to induce the segregation of the PS blocks. The initial copolymer concentration in organic solvent was 1 wt % for all of the samples. Blue tint or turbidity appearance indicated the appearance of micelles.20,21 After that, more water was added until water content reached 50 wt %. Subsequently, a large amount of water (ca. 500 wt %) was added to the resulting solution in order to quench the aggregate morphologies. At this water content range, the structures of the aggregates became kinetically locked over

J. Phys. Chem. B, Vol. 111, No. 8, 2007 1939 the experimental time scale.12,24 The resulting solution was placed in dialysis tubes (DM-16 (MD-25) EI9002, molecular weight (Mw) cutoff of 12 000-14 000, USA) and dialyzed against distilled water for a few days to remove all of the organic solvent from the solution. Although the P4VP block is not soluble in pure water, it is soluble in DMF (dioxane or THF) solutions containing up to 50 wt % water and is also soluble in water below a pH of 4.8 because Poly (4-vinyl pyridine) is protonated and soluble in water at low values of pH and deprotonated and fairly hydrophobic at pH higher than 4.8.20,31 Thus, for dialysis the solution of the aggregates of the triblock copolymer, the pH of the distilled water was adjusted to 4 to keep the colloid solutions from precipitating.20 The morphology of the aggregates was stable during the dialysis process and after the dialysis. 2.2Transimission Electron Microscopy. The resulting aggregate morphologies were visualized with a combination of regular transmission electron microscopy (TEM) and tappingmode atomic force ,icroscopy (TM-AFM). TEM was performed on a JEOL JEM-2000FX transmission electron microscopy operating at an acceleration voltage of 160kv. The dialyzed colloidal solutions were diluted by a factor of 10-20 in order to prepare the TEM samples. A drop of the very dilute solution was placed onto TEM copper grid which had been covered by a holey polymer film and then coated with thin carbon film. After 15 min, excess solution was blotted away using a strip of filter paper. The samples were allowed to dry in air and at room temperature for 1 day before observation. 2.3 Atomic Force Microscopy. The atomic force microscopy (SPI-3800 atomic force microscopy (AFM)) was operated at the tapping mode (TM-AFM) with an SPI3800 controller (Seiko instruments Industry Co. Ltd.). The tip type was sharpened tetrahedral (R < 10 nm, tip height 14 µm) and the cantilever used was fabricated from silicon with a spring constant of 2 N/m and a resonance frequency of 70 kHz. To prepare the samples for TM-AFM, a drop of the very dilute solution after dialysis were spin-coated onto the freshly cleaved mica substrates. All of the samples were dried in air and at room temperature for 1 day before observation. The experiments were all performed in air and at room temperature. 3. Simulation Section In this paper, Monte Carlo simulation of self-avoiding chains was carried out in a simple cubic lattice system. The length, width and height of the cubic are all 40. The periodic boundary condition was applied in all three directions. A Larson-type bond fluctuation model with the permitted bond length of 1 or x2was used.45,46 Each lattice site was occupied by either a bead or a vacancy (a solvent molecule). Excluded volume interactions were enforced to assume that no more than one bead existed per lattice site. No bond crossing was allowed. If the attempted move violated the excluded volume condition, or the no bond-crossing, or bond length restrictions, then it was rejected. Attempted moves that satisfy the excluded volume condition the no bond-crossing and the bond length restrictions were accepted or rejected according to Metropolis rules,47 The partial-reptation algorithm43,48 has been applied in our simulation. This algorithm enhances the efficiency of the simulation by introducing a cooperative motion of beads, and has been proved suitable to study the dynamic process. The microrelaxation modes are defined as follows: A bead is randomly chosen to exchange with one of its 18 nearest and next-nearest neighbors. If the neighbor is a vacancy, exchange with the bead is attempted. If the exchange does not break the chain, it is

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Du et al.

Figure 1. The TM-AFM topological images (A-C) and TEM micrographs (A′-C′) of P4VP43-b-PS260-b-P4VP43 triblock copolymer aggregates prepared from first dissolving in DMF (A/A′), dioxane (B/B′), and THF (C/C′), respectively.

allowed to do so. This process constitutes a single movement. If the exchange would break two chain connections, it is not allowed. If the exchange creates a single break in the chain, the vacancy will continue to exchange with subsequent beads

along the chain until reconnection of a bond. This process constitutes a cooperative movement. The total number of the beads in the whole simulated system is fixed at 6000, corresponding to a volume fraction of 9.375%.

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TABLE 1: Polymer/Solvent Solubility Parameter (δ) and Dielectric Constant (E) polymer/solvent

solubility parameter [MPa]1/2

dielectric constant

PS P4VP THF Dioxane DMF H2O

16.6-20.2 9.9 18.6 20.5 24.8 46.7

7.5 2.2 38.2 78.5

There are 750 A1B6A1 amphiphiles in the simulated system. Pairwise nearest-neighbor (NN) and next-nearest-neighbor (NNN) interactions are considered for the amphiphile solution. Only BB interaction is considered, and the strength of one pair interaction is set as BB ) ′/κBT (κB is the Boltzmann constant and ′ is the BB interaction energy), while all other interaction energes are set to be zero. The inverse temperature 1/T is used and equals zero at the athermal state. Metropolis importance algorithm47 is employed in sampling. In this study the time t is measured in units of Monte Carlo step (MCS). One MCS means that on average every bead has attempted exchange once. From the relation between the interaction  and the FloryHuggins interaction parameter χ, we can obtain: χAB ) 1/kBT[AB - 1/2(AA + BB)], and χAB ) - (1/ 2kBT)BB in the case of AB ) 0 and AA ) 0. Therefore, it is phase separated between A and B when BB < 0. Similarly, we can obtain the χBS between solvent molecule (S) and B: χBS ) - (1/2kBT)BB in the case of BS ) 0 and SS ) 0. If BB < 0, it is known that χBS increases with the increase of the absolute value of BB. This means the solvent quality become poorer for block B with the increase of the absolute value of BB in our model system. 4. Results and Discussion In our experimental studies, the triblock copolymer P4VP43b-PS260-b-P4VP43 cannot be dissolved in water directly.20,32 Thus, the obtained microstructures can be classified as “crewcut” aggregates, combined with the lower volume fraction (24.8%) of hydrophilic blocks (the P4VP blocks). The selfassembly of the triblock copolymer in solutions (indicated by the blue tint or cloudiness of the solution) occurred at a water content of ca. 5 wt % in DMF, ca. 8 wt % in dioxane, and ca. 14 wt % in THF. Figure 1 shows the typical morphologies of spheres (A/A′), rods (B/B′), and vesicles (C/C′) formed from the triblock copolymer by first being dissolved in the common solvent of DMF, dioxane, and THF, respectively. The diameter of the spheres and the vesicles is about 30 and 90 nm, respectively. The vesicular nature (Figure 1 (C/C′)) was evidenced from TEM micrograph demonstrating a higher transmission in the center of the aggregates than around their periphery, coupling with TM-AFM images, which show the aggregates to be spherical.42 Figure 1 (B/B′) shows that the rods are very uniform in the diameter (ca.31 nm). Also, branched rods can occasionally be observed in the experiment. As can be seen from Figure 1 (B/ B′) in arrows, branches can be unambiguously distinguished from overlaps by measuring the optical densities at the junctions; i.e., the overlaps appear darker than other parts of the micelles while the optical densities are uniform through the branches.5 The rod-like micelles occasionally contain branches which have been captured in the TEM micrograph in Figure 1 (B′). Moreover, typical branched micelles can also be seen in the TM-AFM topological image (see Figure 1 B). Copolymer aggregate morphologies change from spheres to rods and then to vesicles in order to reduce entropic penalty

Figure 2. Monte Carlo simulation result showing the aggregation morphology of A1B6A1 amphiphilic block copolymer at equilibrium state for BB )0. This result was employed as initial state for all simulations in this study. A and B are marked by green and red color, respectively.

due to PS-chain stretched. However, the increase of the core size is restricted by an entropic penalty resulting from the stretched of the PS-chains.30,32 Generally, PS blocks have better solubility in THF, dioxane and their mixture with water than that in DMF and its mixture with water, respectively. As can be seen from their solubility parameters (see Table 1).49,50 The higher degree of swelling of the homo-PS in THF and dioxane than in DMF reflects their solubility parameter values. The solubility parameters (δ) of THF (δ ) 18.6) and dioxane (δ ) 20.5) are closer to that of homo-PS (δ ) 16.6-20.2) than that of DMF (δ ) 24.8). Similar swelling ratios are likely to be found in the cores of the PS-b-PAA aggregates.21,24,53 Therefore, dimensions of the aggregates cores, at the onset of micellization, must be much larger in THF, dioxane and their mixtures with water than that in DMF and its mixture with water. As a result, the degree of stretched of PS chains has to increase correspondingly, which is entropically unfavorable. The penalty for the larger stretched of PS chains in core forces the aggregates to adopt vesicle morphology (in THF) in order to decrease the free energy. The formation of mainly spherical aggregates in DMF can be attributed to the high dielectric constant ( ) 38.2).21,51 In DMF, the repulsion among the corona chains is high, and consequently the degree of stretched of the PS chains is decreased, leading to the formation of spherical micelles. In our simulative study, we first set all interaction energies as zeros. The copolymer chains can be dissolved in the solution in this case as shown in Figure 2. We set Figure 2 as initial state for all cases in this study. Changing BB to a desirable value, the system can evolve from Figure 2 based on the Monte Carlo algorithm as introduced in section 3. In order to probe whether the systems were at the equilibrium state, we introduced contact number (NBS) into this work. The definition of NBS is the average pair number between B segment and solvent molecule (S) within the distance of x2. Figure 3 shows the variation of NBS with Monte Carlo time (MCT) when the BB interaction energy BB changed form 0 to -0.7 with a step of -0.05. It is clearly seen that NBS decreases considerably with increasing Monte Carlo time up to about 40 000; thereafter, it almost remains unchanged. The result shows that the selfassembled microstructures can be considered as an equilibration state after 30 thousand MCT. All results given below are at

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Figure 3. Variation of the contact numbers of BS pairs of segments with Monte Carlo Time when the BB interaction energy BB annealed form 0 to -0.7 with a step of -0.05.

Figure 4. Vesicular micelles of A1B6A1 amphiphilc copolymer at equilibrium state when the BB interaction energy BB annealed form 0 to -0.7 with a step of -0.05. Panels b-d are the cross sections of the vesicle. Typical stretched chain and folded chain are marked by special color in panels c and d, respectively.

equilibration state, i.e., the NBS remains unchanged with increasing Monte Carlo time. The simulative results reported in this paper were obtained by annealing method;52 i.e., the energy was changed by several steps to a predetermined value. At each step 5000 MCS was performed. When BB changed from 0 to -0.7 with the step of -0.05, vesicular micelles were obtained at equilibrium state as shown in Figure 4. Here, we would like to mention that the simulation was continued for 3 million MCS in order to ensure that the system could reach equilibrium state when BB reached -0.7. In Figure 4, the hydrophobic and hydrophilic parts (A and B) are marked by red and green spheres, respectively, while the solvents are not shown. We can see that the block copolymers aggregated into a big sphere (see Figure 4a) covered by A segments on the outer surface. From the cross sections in Figure 4b, it is quite clear that the sphere is with hollow structure. The inner surface is also coved by A segments. Therefore, we can conclude that the copolymers have aggregated

Du et al.

Figure 5. The variation of the densities of segment A, B, and solvent with r (r is the radii around the mass center of the vesicle).

Figure 6. Rod-like micelles of A1B6A1 amphiphilc copolymer at equilibrium state when the BB interaction energy BB annealed form 0 to -1.0 with a step of -0.1. Panels b-d are the cross sections of the rods. Typical stretched chain and folded chain are marked by special color in panels c and d, respectively.

into typical vesicles. Figure 5 displays the variation of the densities of segment A, B, and solvent with r (r is the radii around the mass center of the vesicle). Two peaks for the curve of segment A are corresponding to the densities of segment A at outer surface and inner surface of the vesicle, respectively. It is thus obtained that the thickness of the vesicle is ca. 5 from the distance between the two peaks. The unit for “ca.” is the lattice spacing which is set as “1” in our simulation model. Moreover, we can find that the density of segment A at outer surface is obvious lower than that at inner surface. When the BB interaction energy BB was changed form 0 to -1.0 with the step of -0.1, rod-like micelles were obtained. Similarly, the simulation was continued for 3 million MCS in order to ensure that the system could reach equilibrium state when BB reached -1.0. Figure 6a is the snapshot of the rodlike micelles. Figure 6b is the cross-section of the rod-like micelles, indicating that the A segments form the corona of the micelles while B segments form the core. Moreover, there are also some spherical micelles coexisted with rod-like micelles in the system. However, spherical micelles would be always existed in the system when the BB interaction parameter BB was changed form 0 to -2.0 with the step of -0.5 (Figure 7).

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Figure 7. Spherical micelles of A1B6A1 amphiphilic copolymer at equilibrium state when the BB interaction energy BB annealed form 0 to -2.0 with a step of -0.5. Panels b and c are the cross sections of the spheres. Typical stretched chain and folded chain are marked by special color in panels c and d, respectively.

For the same reason as mentioned previously, the simulation was continued for 3 million MCS when BB reached -2.0. From Figure 7, it can be obtained that the size of the sphere in this system is about ca. 7. In the simulation, all interaction energies are set to be zero except BB. In this case, the B blocks are more likely to be coalescence for lower BB and the solvent solubility becomes poorer as mentioned in simulation section. The simulation results show that the triblock copolymer tends to form vesicle, rod, and sphere micelles with decreasing BB from -0.7 to -1.0, then to -2.0. This tendency is in good agreement with the experimental results. One main reason for the morphological transition is believed to relate to the changes of the degree of stretched of the middle blocks in the core regions.20,21,24 However, this view point has not been directly supported by simulative and experimental investigations. An advantage for Monte Carlo simulation is that it can give some microstructure information such as end-toend distance of polymer chains. It is generally known that endto-end distance is a main parameter to characterize the conformation for a linear chain. The average end-to-end distance is defined as

Figure 8. Variation of the fraction of chain number with the end-toend distance for various micelles.

N

R ) 1/N

[(xi2 - xi1)2 + (yi2 - yi1)2 + (zi2 - zi1)2]1/2 ∑ i)1

(1)

where N is the total chain number and xi1, yi1, zi1, and xi2, yi2, zi2 are the coordinates for the two ends of the ith copolymer chain. The calculated average end-to-end distance of the copolymer A1B6A1 is 4.1, 3.9, and 4.0, for vesicle (Figure 8a), rod (Figure 8a), and sphere (Figure 8a), respectively. The results indicate that the copolymer chains are more stretched in the vesicle than that in rod (or in sphere) micelles. In order to investigate the chain conformation in the micelles, we calculated the chain number distribution with the end-toend distance as shown in Figure 8. From Figure 8a, it is known that the fraction of the chain number increases first with

Figure 9. Schematic diagrams showing the P4VP-b-PS-b-P4VP copolymer chain packing in aqueous solution. (a) Vesicles; (b) rods; (c) spheres.

increasing the end-to-end distance up to 3, then remains almost unchanged with increasing end-to-end distance from 3 to 5; thereafter, it decreases rapidly for the sphere micelles. For convenience, we use folded chain and stretched chain to describe chain conformation. The folded chain can be briefly judged by the angle constructed by the center point and the two end beads

1944 J. Phys. Chem. B, Vol. 111, No. 8, 2007 of a chain. If the angle is smaller than 90°, such chain is called folded chain. Otherwise, it is called stretched chain. As the average diameter of the spheres is about ca. 7, it can be known that most copolymer chains existed in the spheres as folded chain. From Figure 8b it is known that the fraction of the chain number increases considerably first with increasing the end-toend distance up to 3; thereafter, it decreases rapidly with increasing end-to-end distance for the rod micelles. The result suggests that most copolymer chains also existed as folded chain in the rod micelles. However, the fraction of the chain number increases first, and then reduces considerably with increasing end-to-end distance up to near ca. 4; thereafter, it increases up to a peak value and then decreased with further increasing endto-end distance in the vesicle. The two peaks are almost symmetric. The results imply that both folded and stretched are dominating chain conformation in the vesicle. In fact, typical stretched and folded chains have been found in the abovementioned simulative results (in Figure 4c-e, Figure 6c,d, and Figure 7c,d, respectively) although the stretched chains are quite few in spheres and rods. We used special color to mark these chains in the figures. For the vesicle micelle, it has two surfaces, i.e., inner and outer surfaces. The chain is more folded if its two ends are at same surface. Otherwise, the chain is more stretched. Therefore, the folded conformation and stretched conformation in vesicles are more topologically distinct, while they are not in spheres and rods. This leads to the differences in Figure 8. On the basis of the simulative results discussed above, we can give the schematic representation of the P4VP-PS-P4VP chain packing in the spheres, rod, and vesicles in the experiment (displayed in Figure 9). For this block copolymer system, rodlike and spherical aggregates involved looping of the hydrophobic PS middle block into the core of the aggregates and tailing two hydrophilic P4VP end blocks to form the corona of aggregates (as shown in Figure 9b,c). In the vesicles, the insoluble blocks constitute the vesicle wall, while the chains of soluble block extend from the inner and outer surfaces into the solvent system (as can be seen in Figure 9a). Stretched chains are appeared in these microstructures. Therefore, the aggregates are very stable after dialysis because of the strong interactions between the P4VP blocks or the aqueous corona-forming P4VP blocks and the water (under the pH of 4.8), as well as the glassy nature of the PS-cores at room temperature. This result is in good agreement with the previously proposed chain packing in the micelles for the similar system.20,26,28,42 5. Conclusion The effect of solvent property on the aggregation morphology of the ABA amphiphilic triblock copolymer in dilute solution was studied by combining experimental method and Monte Carlo simulation. Experimental results showed that the aggregate morphology changed from spheres to rods and then to vesicles by changing the common solvent from DMF to dioxane, and then to THF, respectively. The simulative results showed that the solvent property is a key factor that determines the aggregation morphology. The morphology changed also from spheres to rods and then to vesicles by increasing the solvent solubility, which is in good agreement with the experiment. Moreover, the simulative results revealed that the end-to-end distant of the ABA triblock copolymer in vesicles was larger than that in the spheres and rods. Moreover, we gave the distribution of the fraction of the chain number with the endto-end distance. The results indicated that the amount of stretched chains is almost same as that of folded chains in the

Du et al. vesicle. However, most chains existed in the rod-like and spherical micelles as folded conformation. Acknowledgment. We are grateful for the financial support provided by the Major Program (20490220) of National Natural Science Foundation of China (NSFC), the National Basic Research Program (2007CB808000) of China. Jiang W. thanks the Fund (50621302) for Creative Research Groups (NSFC). References and Notes (1) Riess, G. Prog. Polym. Sci. 2003, 28, 1107-1170. (2) Liu, G. J. Curr. Opin. Coll. Interface Sci. 1998, 3, 200-208. (3) Won, Y. Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J. Phys. Chem. B 2002, 106, 3354-3364. (4) Massey, J. A.; Winnik, M. A.; Manners, I. J. Am. Chem. Soc. 2001, 123, 3147-3148. . (5) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589-592. (6) Bronich, T. K.; Vinogradov, S. V.; Kabanov, A. V. Nano. Lett. 2001, 1, 535-540. (7) Lodge, T. P.; Rasdai, A.; Li, Z.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 17608-17609. (8) Gohy, J. F. AdV. Polym. Sci. 2005, 190, 65-136. (9) Stewart, S.; Liu, G. J. Angew. Chem., Int. Ed. 2000, 39, 340-344. (10) Loos, K.; Mu¨ller, A. H. E. Biomacromolecules 2002, 3, 368-373. (11) Jain, S.; Bates, F. S. Science 2003, 300, 460-464. (12) Zhang, L. F.; Eisenberg, A. Polym. AdV. Technol. 1998, 9, 677699. (13) Halperin, A.; Tirrell, M.; Lodge, T. P. AdV. Poly. Sci. 1992, 100, 31-71. (14) de Gennes, PG. In Solid State Physics; Leibert, L.; Ed.; Academic Press: New York, 1978; Supplement 14, p1. (15) Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728-1731. (16) Jiang, Y.; Zhu, J. T.; Jiang, W.; Liang, H. J. J. Phys. Chem. B. 2005, 109, 21549-21555. (17) Desbaumers, L.; Eisenberg, A. Langmuir 1999, 15, 36-38. (18) Ding, J.; Liu, G. J. Macromolecule 1997, 30, 655-657. (19) Zheng, Y.; Won, Y. Y.; Bates, F. S.; Davis, H. T.; Scriven, L. E. J. Phys. Chem. B. 1999, 103, 10331-10334. (20) Zhu, J. T.; Liao, Y. G.; Jiang, W. Langmuir 2004, 20, 3809-3812. (21) Zhang, L. F.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 31683181. (22) Pochan, D. J.; Chen, Z. Y.; Cui, H. G. Hales, K.; Qi, K.; Wooley, K. L. Science 2004, 306, 94-97. (23) Zheng, R. H.; Liu, G. J.; Yan, X. H. J. Am. Chem. Soc. 2005, 127, 15358-15359. (24) Zhang, L. F.; Yu, K.; Eisenberg, A. Science 1996, 272, 17771779. (25) Shen, H. W.; Zhang, L. F.; Eisenberg, A. J. Am. Chem. Soc. 1999, 121, 2728-2740. (26) Riegel, C. I. C.; Samios, D.; Petzhold, C. I.; Eisenberg, A. Polymer 2003, 44, 2117-2128. (27) Talingting, M. R.; Munk, P.; Webber, S. E. Macromolecules 1999, 32, 1593-1601. (28) Schille´n, K.; Bryskhe, K.; Mel’ nikova, Y. S. Macromolecules 1999, 32, 6885-6888. (29) Zhu, J. T.; Jiang, W. Macromolecules 2005, 38, 9315-9323. (30) Liu, F.; Eisenberg, A. J. Am. Chem. Soc. 2003, 125. 15059-15064. (31) Zhu, J. T.; Yu, H. Z.; Jiang, W. Macromolecules 2005, 38, 74927501. (32) Zhu, J. T.; Jiang, W. Mater. Chem. Phys. 2006, Inpress. (33) Zhang, L. F.; Eisenberg, A. Macromolecules 1996, 29, 8805-8815. (34) Gohy, J. F.; Willet, N.; Varshney, S.; Zhang, J. X. Je´roˆme, R. Angew. Chem., Int. Ed. 2001, 40, 3214-3216. (35) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Nano. Lett. 2006, 6, 12451249. (36) Yan, X. H.; Liu, G. J.; Hu, J. W.; Willson, C. G. Macromolecules 2006, 39, 1906-1912. (37) Srinivas, G.; Nielsen, S. O.; Moore, p. b.; Klein, M. L. J. Am. Chem. Soc. 2006, 128, 848-853. (38) Massey, J.; Power, K. N.; Manners, I.; Wink, M. A. J. Am. Chem. Soc. 1998, 120, 9533-9540. (39) Noguchi, H.; Takasu, M. Phys. ReV. E 2001, 64. 041913. (40) He, X. H.; Liang, H. J.; Huang, L.; Pan, C. Y. J. Phys. Chem. B 2004, 108, 1731-1735. (41) Laradji, M.; Shi, A. C.; Noollandi, J.; Desi, R. C. Macromolecules 1997, 30, 3234-3241. (42) Yamamoto, S.; Maruyama, Y.; Hyodo, S. J. Chem. Phys. 2002, 116, 5842-5849. (43) Ji, S. C.; Ding, J. D. Langmuir 2006, 22, 553-559.

ABA Amphiphilic Triblock Copolymer (44) He, X. H.; Liang, H. J.; Pan, C. Y. Phys. ReV. E. 2001, 63, 031804. (45) Larson, R. G.; Scriven, L. E.; Davis, H. T. J. Chem. Phys. 1985, 83, 2411-2420. (46) Larson, R. G. J. Chem. Phys. 1988, 89, 1642-1650. (47) Metropolis, N.; Rosenbluth, A. W.; Rosenbluth, M. N.; Teller, A. H.; Teller, E. J. Chem. Phys. 1953, 21, 1087-1092. (48) Hu, W. B. J. Chem. Phys. 1998, 109, 3686-3690.

J. Phys. Chem. B, Vol. 111, No. 8, 2007 1945 (49) Lide, D. R. CRC Handbook of Chemistry and Physics, 77th ed.; CRC Press; New York, 1996-1997, p6-151. (50) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; WileyInterscience: New York, 1989, VII.526. (51) Yu, Y.; Eisenberg, A. J. Am. Chem. Soc. 1997, 119, 83838384. (52) Sun, P.; Yin, Y.; Li, B.; Chen, T.; Jin, Q. Ding, D.; Shi, A.-C. J. Chem. Phys. 2005, 122, 204905.