Dissipative Particle Dynamics Simulation on Vesicles Self-Assembled

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Cite This: J. Phys. Chem. B 2018, 122, 10607−10614

Dissipative Particle Dynamics Simulation on Vesicles Self-Assembly Controlled by Terminal Groups Muhan Wang,† Shuai Pei,† Timing Fang,† Youguo Yan,† Jiafang Xu,*,‡ and Jun Zhang*,† †

College of Science and ‡School of Petroleum Engineering, China University of Petroleum, 266580 Qingdao, Shandong, China

J. Phys. Chem. B 2018.122:10607-10614. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/23/18. For personal use only.

S Supporting Information *

ABSTRACT: Block copolymer vesicles have been widely used in the field of drug delivery, microreactors, and cell membrane mimetics. Introducing terminal groups to the block copolymer can control the structures of vesicles, which is important for improving the application of vesicles. In this paper, the effects of terminal groups on the structure of vesicles were studied by dissipative particle dynamics simulation. We considered different locations, hydrophobicity, and numbers of terminal groups. When the terminal group located at the end of a hydrophobic block, the increase of wall thickness and the decrease of cavity size of vesicles were observed with the hydrophobicity of the terminal group increasing. In contrast, when the terminal group located at the end of the hydrophilic block, with the hydrophobicity of terminal groups increasing, the vesicular cavity size increased but the wall thickness of vesicles remained nearly unchanged. Finally, increasing the number of terminal groups lead to a decrease of cavity size and an increase of wall thickness of vesicles. The hydrophobic changes of polymer molecules are regarded as the origin of the structural changes of vesicles. This simulation study supplies a potential approach that controls the structures of vesicles and is expected to facilitate its further applications. on. Eisenberg and his co-workers25 summarized a wide range of vesicles that are prepared by poly(acrylic acid)-b-polystyrene (PAA-b-PS) at different conditions and obtained various morphologies of vesicles, such as spherical vesicles, entrapped vesicles, onion-like vesicles, large compound vesicles, and so on. They revealed that as the block length ratio of PAA to PS decreased, the vesicles size increased.26 Thereafter, Shen and Eisenberg reported that the initial polymer concentration is an important factor in vesicle formation and the diameter of the vesicles is increased with the increasing concentration.27,28 Lian et al.29 found that the graft copolymer with PEO and PSb-PNIPAM grafted onto PGMA backbone can self-assemble into vesicles and the temperature caused a significant influence on the structure and morphology of vesicles. At a temperature above lower critical solution temperature of PNIPAM, the size of the vesicles decreased due to the shrinking of PNIPAM blocks in the corona. Recently, researchers have drawn their attention to the importance of small changes in chain terminal chemistry for controlling the self-assembly of polymer. KitaTokarczyk and his co-workers30 studied the influence of terminal groups with different polarizabilities on the selfassembly of amphiphilic block copolymer. The result shows that the terminal groups not only facilitate the polymers self-

1. INTRODUCTION Vesicles,1−3 possessing a hollow structure separated from the exterior solution by a membrane composed of amphiphilic molecules, have been vastly investigated in biology,4,5 cytology,6,7 medicine,8,9 and artificial intelligence.10−12 Vesicles are usually classified by the monomeric amphiphilic molecules of self-assembly, such as surfactant vesicles, lipid vesicles, as well as polymer vesicles. The polymer vesicles have low permeability, superior stability, and toughness,13 which have attracted significant interest in the design and modification of polymer vesicles for applying in such wide fields as drug delivery,5,7 microreactor,6,8 and cell membrane mimetics.12 Vesicle application mostly depends on the structure information such as cavity size and membrane thickness.14,15 For example, in the area of the drug delivery, the hydrophilic components are stored in the aqueous cavities and the hydrophobic components are mainly stored in the membranes. 16−18 Therefore, the larger cavity vesicles can encapsulate more hydrophilic drugs and the thicker membrane vesicles can encapsulate more hydrophobic drugs.19,20 How to control the vesicles’ cavity size and membrane thickness is an important problem in the area of drug delivery. Due to the fact that this problem is particularly important, many efforts have been made to investigate the influencing factors on vesicular structure.21−24 In general, the structure properties of vesicles can be controlled by many traditional factors, such as block ratio, concentration, temperature, and so © 2018 American Chemical Society

Received: August 4, 2018 Revised: October 22, 2018 Published: October 31, 2018 10607

DOI: 10.1021/acs.jpcb.8b07567 J. Phys. Chem. B 2018, 122, 10607−10614

Article

The Journal of Physical Chemistry B

experimental resources but also provide guidance for the vesicles’ applications. Apart from these influencing factors studied in simulation, few simulations investigate the influence of the terminal group to the vesicles self-assembly and structures. DPD method is appropriate to investigate the self-assembly of amphiphilic polymer under the effects of terminal groups. Moreover, DPD simulation also can provide the information of cavity size and membrane thickness of the vesicles. In this paper, we systematically investigate the influence of the terminal group on the self-assembly of amphiphilic polymer by using the DPD approach. We have taken into account three factors: the hydrophilicity of the terminal groups, the location of the terminal groups, and the number of the terminal groups. We found that the structure of vesicles can be effectively controlled by adjusting the terminal groups. The vesicles of different cavity sizes and wall thickness were generated. The simulation results can provide a reference for vesicle structure design and regulation.

assembly into vesicles but also affect the vesicle size. The size of vesicles from ABA with carboxylic terminal groups is larger than that of vesicles from ABA with amine terminal groups. Du and his co-workers31 found that the alkyl terminal group can drive full hydrophilic homopolymers to self-assemble into vesicles. When the terminal groups connected to hydrophilic homopolymers POEGMA475 changed to PIB, PMPA, and DiPMPA, the average diameter of the vesicles were 68, 184, and 126 nm. These recent experimental investigations proved that the terminal group is a crucial factor to control the vesicle sizes. Compared with other approaches, terminal groups can change the vesicle size by molecule structure without changing the system environment, such as concentration, pH, and so on. This is beneficial for obtaining stable vesicles. From many experimental results obtained so far, it is recognized that it is significant to investigate the effects of terminal groups on vesicle size especially cavity and membrane thickness. This can provide a guidance to design the structure of vesicles and extend their potential application in the area of drug delivery. Although many valuable efforts have been made by experiments, the difficulties in characterization limit the fine study of the vesicle structure. Compared with experiments, computer simulation and theoretical methods have emerged as vital tools for studying the microscopic information of micelles, which can provide more details of vesicle structure at a molecular level.32−37 Therefore, many researchers devote to studying the polymer self-assembly by computer simulations, such as molecular dynamics (MD) simulation and dissipative particle dynamics (DPD) simulation. For example, Srinivas and group used molecular dynamic simulation to build a coarse-grained (CG) model to study the amphiphilic BCPs’ self-assembly in water and vesicles’ cylindrical and spherical micelle morphologies assembly.38 Recently, Eikerling39 applied CG MD simulations to study the cylindrical bundlelike aggregation of ionomer chains in dilute solution. In comparison with the MD simulation, the DPD method presents more predominance in the mesoscopic simulations and it can provide a wider range of length and time scales by many orders of magnitude than MD simulation. There are some classical investigations that employed this method to study the vesicle self-assembly and the structure change. For example, Zeng et al.40 built a minimalist Janus oligomer model and studied the self-assembly behavior of amphiphiles in selective solvent by a DPD method. The aggregate exhibits a rich variety of the morphological conformations, including onion-like vesicles that depend on temperature, concentration, and block ratio. Wang et al.41 studied the spontaneous vesicle formation of ABABA-type amphiphilic multiblock copolymers bearing thermos-sensitive hydrophilic A-block by DPD approach. The result shows that the copolymers of shorter hydrophobic block length or higher hydrophilicity are likely to form vesicles with larger aqueous cavity size and thinner wall thickness. Sheng42 used the DPD method to build a complex comblike graft copolymer model. The aggregates exhibit a rich variety of the morphological conformations depending on pH, concentration, side-chain length, and grafting density. As pH increases, bilayer plate vesicles can be formed at low polymer concentration and the vesicle size grows with increasing concentration. According to the above researches, it is apparent that DPD simulation is an appropriate method to investigate the amphiphilic polymer self-assembly under different conditions and to analyze the vesicular structure. DPD simulation can not only save the

2. METHODOLOGY In this work, the large-scale atomic/molecular massively parallel simulator (LAMMPS) software43 was used to perform dissipative particle dynamics (DPD) simulations. DPD is a particle-based and mesoscopic simulation technique, which was originally proposed by Hoogerbrugge and Koelman.44,45 In DPD, a block or cluster of atoms is united into a single coarsegrained bead and the motion of all beads in the system obeys Newton’s equations of motion.46 The time integration of motion equations adopts the modified velocity-Verlet algorithm. In general, the force (fi) is between a pair of beads i and j, including the conservative repulsive forces FijC, dissipative forces FijD, and random forces FijR. fi can be written as formula 1 fi =

∑ (FijC + Fij D + Fij R ) (1)

j≠i

The conservative repulsive force is a soft repulsive potential, usually following formula 2 rij zy ji FijC = αijjjj1 − zzzriĵ j rc z{ k

(2)

The αij > 0 indicates that this force is always repulsive. The rij is the distance between beads i and j, r⃗ij = r⃗i − r⃗j, rij = |r⃗ij|, r̂ij = r⃗ij/rij, and rc is the cutoff radius. The dissipative forces FijD is a friction force that is proportional to the velocity of two beads approaching each other, given byformula 3 Fij D = −γωD(rij)(rij·vij)riĵ

(3)

where γ is the friction coefficient. The function ω(rij) determines the radial dependence of the force; the function is continuous, positive for rij < rc, and zero for rij ≥ rc; DPD uses a simple linear weight function for the conservative force: ωD(rij) =1 − (rij/rc) for rij < rc. The random force FijR is defined by formula 4 Fij R = σωR (rij)θij riĵ

(4)

where σ is the noise amplitude, ω (rij) are r-dependent weight functions vanishing for r > rc that describe the range of the R

10608


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The Journal of Physical Chemistry B dissipative and random forces, and θij is a randomly fluctuating variable with Gaussian statistics, defined by formula 5

Table 1. Polymer Systems of Different Terminal Group Hydrophilicity, Location, Number, and Hydrophobic Block Length

⟨θij(t )⟩ = 0; ⟨θij(t )θkl(t ′)⟩ = (δikδji + δilδjk)δ(t − t ′) (5)

A2By-T1

A2By-T2

A2By-T3

ByA2-T1a

To obey the fluctuation−dissipation theorem, only one of σ and γω can be chosen arbitrarily and the other is fixed by the relation, formula 6

A2B4-T1 A2B6-T1 A2B8-T1 A2B10-T1 A2B12-T1



B4A2-T1 B6A2-T1 B8A2-T1 B10A2-T1 B12A2-T1

σ 2 = 2γkBT

(6) a

The examples of B2A4-T1, A2B4-T1, and A2B4-T3 are shown in Figure 1.

where kB is Boltzmann’s constant and T is the temperature. The bonded interactions between the consecutive beads are the spring force FSij, defined by formula 7 FijS = C(1 − rij/req)riĵ

7.1 Å) and the energy scale kBT = 1. The DPD step was set as 0.05, and each simulation took at least 1 000 000 to achieve thermodynamic equilibrium. The morphology of aggregates at equilibrium is independent of the initial conditions. And the repulsive interaction parameters of the four types of beads are shown in Table 2. The interaction parameters are 25 to

(7)

To obtain flexible chains, in this work, the spring constant C = 10 and the equilibrium bond distance req is set as 0.8.47 In this work, we focus on the model of amphiphilic diblock copolymer AxBy-Tz. As shown in Figure 1a, the acrylic acid is

Table 2. Interaction Parameters for the Polymer in Water Solution W A B T

W

A

B

T

25 25 60 αTW

25 25 40 30

60 40 25 50

αTW 30 50 25

account for a good compatibility. The value of the interaction parameter between A and B is selected as αAB = 40.0, which implies that A and B components are incompatible. The interaction parameter between B and W is selected as αBW = 60.0, which implies that B component has high hydrophobicity.48 The parameter between the terminal group and water was represented by αTW, which can increase from 25 to 75 to reflect the changing solubility.

Figure 1. Coarse-grained mapping of (a) styrene and acrylic acid. (b) The chain structure of polymer A2B4-T1. (c) The chain structure of polymer B4A2-T1. (d) The chain structure of polymer A2B4-T3.

represented by hydrophilic bead (A) and the styrene is represented by hydrophilic bead (B), where x is the number of the hydrophilic beads and y is the number of the hydrophobic beads; the terminal group beads (T) are connected to the final hydrophobic bead. The number of terminal groups is defined as z. The T beads have variable nature and their hydrophilicity is between A bead and B bead. The model A2B4-T1 was chosen to describe the chain structure in Figure 1b. In Figure 1c, the terminal group is connected to the hydrophilic bead and this model is used to investigate the effects of the terminal group location. In Figure 1d, the increasing number of terminal group model is shown. At the same time, we also investigate the effects of hydrophobic block length on the vesicle structure and the polymer systems are shown in Table 1. On the basis of the model of the amphiphilic diblock copolymer-tethered terminal groups, we performed the simulation of the total 81 000 DPD particles in a cube box of 30 × 30 × 30 rc3 (which represents that the size of the simulation box is approximately 213 Å) under the periodic boundary conditions. In initial configurations, all molecules were randomly placed in the simulation box. The number of the polymer chain is kept at 900; if the beads in the polymer increased, the water beads (W, contains 4 water molecules) will be decreased to ensure that total beads number is 81 000. The number density of the beads is set to 3. The cutoff radius for polymer and water beads was set to rc = 1 (approximately

3. RESULTS AND DISCUSSION 3.1. Effect of the Hydrophilicity of the Terminal Groups. First, we investigated the effect of the hydrophilicity of the terminal group and the hydrophobic block length on the polymer self-assembly morphology. Figure 2 shows the phase diagram of morphologies as a function of the interaction parameter αT‑W and the length of the hydrophobic block NB, where αT‑W is the interaction parameter between the terminal group and the water bead. The higher αT‑W presents the stronger hydrophobicity.49 In the phase diagram, there are four distinct types of aggregates observed as rods, plate, vesicles, and microcavity micelles (the microcavity micelle is the aggregate that has the minimal vesicular cavity, and the hydrophilic blocks are wrapped up by the hydrophobic blocks). Depending on the diagram, the short hydrophobic block polymer A2B4-T1 will self-assemble into rods at the αT‑W from 25 to 55. Then, at high αT‑W between 65 and 75, the aggregates form plates. When the length of the hydrophobic block exceeds 4, the aggregates are vesicles or microcavity micelles. When the hydrophobic block length is 6, the vesicles are formed at low αT‑W between 25 and 35. The area of the microcavity micelles is larger than that of vesicles. The area of the vesicles and microcavity micelles is equal when the aggregates are formed by polymer A2B6-T1. Under the situation of the hydrophobic block between 10 and 12, the morphology 10609


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calculated wall thickness of the vesicular member from the width of the valley and the cavity size from the origin point of valley.20,51 To investigate the effects of the terminal group on the vesicle structure (wall thickness and cavity size), we calculated water radial density profile curve of the vesicles from αT‑W = 25 to 75 for A2B10-T1, as shown in Figure 4. The membrane

Figure 2. Phase diagram of morphologies obtained from polymer A2By-T1 at different hydrophobic lengths NB and αT‑W. For clarity, the water molecules are hidden and the same strategy is adopted in the following figures.

remains as vesicles. The self-assembly process of vesicles driven by the hydrophobic interaction is shown in Supporting Information S1. This demonstrates that the long hydrophobic chain and the hydrophilic terminal group facilitate the generation of vesicles. The hydrophilicity of the terminal group can influence the vesicle formation, but it is less powerful than the length of hydrophobic chains. Therefore, introducing a terminal group is a useful method that controls vesicle structure without transforming the morphology of selfassembly. Therefore, we focused on the investigation of the structure information of these vesicles. The radial density profiles were used to analyze the structures of these vesicles.50 The vesicle self-assembled from A2B10-T1 at αT‑W = 35 was chosen as the sample, and the curves of A beads, T beads, and water beads with the vesicle mass center are shown in Figure 3. Intuitively,

Figure 4. Radial density profiles g(r) of water beads, the polymer A2B10-T1 at different αT‑W, as a function of distance from the vesicular center of mass.

thickness of vesicles is increased, and the cavity size of vesicles is decreased with the hydrophobicity (αT‑W) increasing. The main reason is that more hydrophobic terminal groups stay closely to each other and form a denser hydrophobic membrane to prevent the water permeating. In addition, the water radial density profiles of the vesicles from A2By-T1 (y = 6, 8, 10, and 12) at αT‑W = 35 are shown in Figure 5. These

Figure 5. Radial density profiles g(r) of water beads, the different hydrophobic block lengths of polymer A2By-T1 at αT‑W = 35, as a function of distance from the vesicular center of mass.

Figure 3. Radial density profiles g(r) of water beads, terminal group (B beads), and hydrophilic beads (A beads) as a function of distance from the vesicular center of mass.

curves were calculated to investigate the effects of the hydrophobic block length on the vesicles’ structure. The result shows that with the hydrophobic block length increasing, the membrane thickness is increased but cavity size is unchanged. This demonstrates that the membrane thickness is also controlled by hydrophobic block length. Therefore, membrane thickness is critically influenced by hydrophilicity of the terminal group and length of hydrophobic

there are 2 peaks for A beads and 2 peaks for T beads, which demonstrates that both the hydrophilic block A and terminal group T are located at the outer surface of the vesicle and the inner surface of the cavity to protect the hydrophobic layer of block B. The curve of the water shows a flat-bottomed valley, which represents the area that is fully filled with hydrophobic blocks. Depending on these radial density profile curves, we 10610


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terminal group changes. This indicates that the long hydrophobic block chain has a more powerful effect on the vesicle formation than the location of the terminal groups. To analyze the structural changes of the vesicles caused by the hydrophilicity terminal groups, we took the polymer B10A2T1 as an example. As shown in Table 3, with the αT‑W

block. The longer hydrophobic block will cause the thicker membrane. When the terminal groups become more hydrophobic, they are more similar to hydrophobic group and become a part of the membrane. Therefore, the thickness of the membrane is also increased. The cavity size is mainly affected by terminal group hydrophilicity. When the terminal group is hydrophobic, they pack closer so the cavity size become smaller. In other words, tuning terminal group is an effective way to control the cavity size and membrane thickness of vesicles. 3.2. Effect of the Location of the Terminal Groups. Apart from the hydrophilicity of terminal groups, their location in a polymer chain also influenced the self-assembly morphologies and structure of vesicles. We built a model ByA2-T1 to study the effect of the location of the terminal group via comparing with the results from model A2By-C1. We intend to investigate the effects of the interaction parameter αT‑W and the number of the hydrophobic block bead NB on the self-assembled behaviors of the polymer ByA2T1. As shown in Figure 6, four distinct types of aggregates are

Table 3. Cavity Size and Wall Thicknessa of Polymer B10A2T1 at Different αT‑W

a

αT‑W

25

35

45

55

65

75

cavity size wall thickness

4.55 2.7

4.75 2.8

5.05 2.6

5.15 2.5

5.15 2.7

5.25 2.7

It was calculated by the radial density profiles g(r) of water beads.

increasing, the membrane thickness changed slightly and the cavity size increased slightly. This phenomenon is explained by the changes of molecular structure. The terminal group is separated by the hydrophilic block A. This will hinder the terminal group from closing to the hydrophobic membrane. Therefore, the changes of membrane thickness are not clear. However, with the αT‑W increasing, the terminal groups become more hydrophobic and tend to close to each other; then, the connected area with water decreases. This causes the cavity size to increase slightly. Then, we calculated the radial density profiles of the vesicles that self-assemble by polymer ByA2-T1 (y = 6, 8, 10, 12) at αT‑W = 35 to investigate the effects of the length of hydrophobic blocks. The tendency of ByA2T1 is consistent with that of A2B10-T1. In summary, the morphology distributions are different between ByA2-T1 and A2By-T1. The microcavity micelles easily appear under the condition that the terminal groups are connected to the hydrophilic block. In addition, the vesicular structure self-assembled by ByA2-T1 is also different from that of A2By-T1. Because the terminal group and hydrophobic block are separated by the hydrophilic block A and the hydrophobic block is the main influence factor of membrane, the terminal groups have almost no effect on the membrane thickness. But, the cavity size will increase. Because when the terminal groups become hydrophobic, they will avoid contact with water and expose the hydrophilic block to the water. Changing the location of the terminal group can provide a new way to design the vesicle structure. 3.3. Effect of the Number of the Terminal Groups. To investigate the effects of the number of the terminal groups on the self-assembled morphology of vesicle structure, we built two type of models: A2By-T2 and A2By-T3. In these models, all terminal groups are connected to the end of block B. The selfassembly diagrams of A2By-T2 and A2By-T3 are shown in Figures 7 and 8. Comparing the two diagrams, there are four same morphologies that are wormlike micelles, plates, porous micelles, and vesicles. However, the aggregates of A2By-T3 are more complicated than those of A2By-T2. From Figure 7, when the hydrophobic block chain is short (numbers of beads are 4 and 6), the polymers formed wormlike micelles. For the plate aggregation, we suggested that the plate will wrap up to a sphere and generate a bilayer vesicle structure if the simulation system is big enough. But, DPD system cannot be expanded infinitely. The system expansion was performed eight times, shown in Supporting Information S2. In Figure 8, in the system of the polymer A2B4-T3 and A2B6-T3, the morphologies are vesicles when the αT‑W exceeds 45 and 55. This indicates

Figure 6. Phase diagram of morphologies obtained from polymer ByA2-T1 at different hydrophobic lengths and different αT‑W.

observed as spheres, plate, vesicles, and microcavity micelles. Comparing the formation conditions, the phase diagram of ByA2-T1 is different from that of A2By-T1 in Figure 2. In the shortest hydrophobic block polymer B4A2-T1, the morphology is of microcavity micelles when the αT‑W is larger than 25. This demonstrates that the microcavity micelles of ByA2-T1 can appear at the shorter hydrophobic block compared with those of A2B4-T1. As the hydrophobic block length is 6, with αT‑W increasing, the vesicles are gradually transformed into microcavity micelles and these changes of regulation are consistent with the polymer A2B6-T1. In contrast, by increasing the hydrophobic block length to 8, the microcavity micelles diminished and the morphologies are vesicles from αT‑W = 35 to 75. Compared to the A2By-T1, the microcavity micelles selfassembled by ByA2-T1 are easily diminished with the hydrophobic block increasing. Finally, when the hydrophobic block lengths are 10 and 12, the morphologies are vesicles. This phenomenon is consistent with that in the polymer A2ByT1. In conclusion, when the terminal group T is connected to hydrophilic chain A, the microcavity micelles are easily formed and diminished at the short hydrophobic block chain. Nevertheless, at the long hydrophobic block chain, the morphologies are still vesicles when the location of the 10611


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A2B10-T1 have the biggest cavity 6.55 and the thinnest wall 1.70. In contrast, the polymer A2B10-T1 vesicles have the smallest cavity 5.75 and the thickest wall 2.30. This reflects that, the cavity size decreased and the wall thickness increased with the number of terminal groups increasing. The main reason is that more terminal groups occupy more cavity space, which forbids the water to permeate. Therefore, the vesicles self-assembled by A2B10-T3 have the thickest wall. In conclusion, the increasing number of terminal groups can not only result in the complicated morphologies but also change the vesicular structure. For studying the stretching52 of the vesicles, plates, microcavity micelles, and wormlike micelles, we performed a stretching simulation. In Figure 9, the stretched force of

Figure 7. Phase diagram of morphologies obtained from polymer A2By-T2 at different hydrophobic lengths and different αT‑W.

Figure 9. Stretching of vesicles, plates, microcavity micelles, and wormlike micelles. The simulation is performed in 2 000 000 step and saved 200 frames.

vesicles, plates, and microcavity micelles is around 0, which indicates that these structures are less affected by stretching. In Supporting Information S3, we can see that the vesicles in the stretched system are lengthened. But the stretched force of wormlike micelles is larger than 0, which indicates that the wormlike micelles are against system stretching. Moreover, we analyzed the stretching of periodic boundary condition plate. We found that the stretched force of the plate is larger than 0 but smaller than that of the wormlike micelles. Therefore, the wormlike RMs are significantly affected by the stretching, even more than the plate. The different structured vesicles show the same phenomenon.

Figure 8. Phase diagram of morphologies obtained from polymer A2By-T3 at different hydrophobic lengths and different αT‑W.

that with the number of terminal groups increasing, the vesicles appear more easily while the hydrophobic block chain is short. In contrast, for long hydrophobic block chains with numbers from 8 to 12, the vesicles also easily disappear with the hydrophobicity of the terminal groups increasing. As shown in Figure 8, the vesicles are transformed into multicompartment vesicles as the αT‑W increases from 35 to 75. But, in Figure 7, the aggregates can be kept as vesicle with the αT‑W increasing. This phenomenon reflected that the distribution of the vesicles is narrow and the vesicle structures are not stable with larger numbers of terminal groups. The various vesicular structures of polymer A2B10-T1, A2B10T2, and A2B10-T3 were intuitively calculated by the radial density profile curves of the vesicles at αT‑W = 55. The result is shown in Table 4. The vesicles self-assembled by polymer

4. CONCLUSIONS In conclusion, we employed the dissipative particle dynamics method to investigate the effects of the terminal group on the self-assembly of the amphiphilic block copolymers. The different locations, hydrophobicity, and number of terminal groups were considered: When the terminal group located at the end of the hydrophobic block, the increase of wall thickness and the decrease of cavity size of vesicles were observed with the hydrophobicity of terminal group increasing. In contrast, when the terminal group located at the end of hydrophilic block, with the hydrophobicity of terminal group increasing, the cavity size of vesicles increases but the wall thickness of vesicles is nearly unchanged. Moreover, increasing the number of terminal groups will lead to a decrease in cavity size and an increase in wall thickness of vesicles.

Table 4. Cavity Size and Membrane Thicknessa of Polymers A2B10-T1, A2B10-T2, and A2B10-T3 cavity size wall thickness

A2B10-T1

A2B10-T2

A2B10-T3

6.55 1.70

6.15 1.90

5.75 2.30

a

It was calculated by the radial density profiles g(r) of water beads. 10612


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This investigation leads to a possible new direction for designing the vesicle structures, which is possible if changing the terminal group of block copolymers. In the future, it will be desirable if the terminal group could induce the generation of compound vesicles. These works of the DPD simulations provide an excellent approach for observing structure of vesicles. This strategy of designing vesicle structures may facilitate broadening of the potential applications of vesicles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.8b07567. Section 1, self-assembly process of vesicle; Section 2, additional simulations of plates; Section 3, simulation of stretching system (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 0532-86983366. ORCID

Jun Zhang: 0000-0001-7786-4825 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (2014CB239204), the National Natural Science Foundation of China (U1663206 and U1762212), the Climb Taishan Scholar Program in Shandong Province (tspd20161004), the Fundamental Research Funds for the Central Universities (15CX08003A), and the Key Laboratory of Tectonics and Petroleum Resources (TPR-2016-16).

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REFERENCES

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Dissipative Particle Dynamics Simulation on Vesicles Self-Assembled

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