Photoresponsive Polymersomes Formed by Amphiphilic Linear

Aug 27, 2012 - Since DPD simulation utilizes soft-repulsive potentials, the systems studied are allowed to evolve much faster than the “brute-forceâ...
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Photoresponsive Polymersomes Formed by Amphiphilic Linear− Dendritic Block Copolymers: Generation-Dependent Aggregation Behavior Yung-Lung Lin, Hung-Yu Chang, and Yu-Jane Sheng* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C.

Heng-Kwong Tsao* Department of Chemical and Materials Engineering, Department of Physics, National Central University, Jhongli, Taiwan 320, R.O.C. ABSTRACT: Azobenzene-containing linear−dendritic block copolymers (LDBC) with varied generation numbers were synthesized recently. This photosensitive LDBC consists of a linear solvophilic block (R) and solvophilic dendrons of which the periphery is attached with a solvophobic coil−rod diblock (B−Y). The self-assembly and its photoresponsive transformation are explored by dissipative particle dynamics. Dependent on the generation number, polymer concentration, block lengths, and π−π interaction between rod blocks, the aggregate exhibits a rich variety of morphological conformations, including spherical micelle, worm-like micelle, cylindrical micelle, hamburger-like micelle, nanosheet, nanobowl, and vesicle. In general, polymersomes take shape for LDBCs with large generation number while nanofibers and nanosheets develop for LDBCs with small generation number. Upon UV illumination, the photoinduced trans-to-cis isomerization of azobenzene takes place and the membrane becomes distorted, wrinkled, and even ruptured. A significant increase in water permeation through the polymersome membrane is also observed. These simulation findings are consistent with experimental observations. By varying polymer concentration and lengths of R- and B-blocks, morphological phase diagrams and internal structures of the resulting aggregates are obtained. Transformation from nanosheet or nanobowl to polymersome is observed as polymer concentration grows. When the R-block length decreases, the generation number at which polymersomes start to form declines. On the other hand, nanosheets tend to form for long R-block length. This prediction is also consistent with experimental observations. Polymersomes can also be formed for long enough B-block length at lower generation number. The membrane resistance to water permeation grows with increasing B-block length because of the increment of the hydrophobic layer thickness. Finally, as the π−π strength is increased, the overall morphologies vary from polymersome, nanobowl, to nanosheet. Therefore, polymersomes are formed only when the π−π strength is weak enough due to the hindrance associated with high degree of alignment among azo-rods.

I. INTRODUCTION In recent years nanostructural copolymers with various macromolecular architectures are of increasing interest owing to their distinctive physical properties.1 Examples include linear multiblock, comb-like, star-like, dendritic, hyperbranched, and brush-like polymers, all of which have different topological structures. Among those, dendrimers are repetitively branched molecules which are monodisperse and usually highly symmetric, spherical compounds.2 They can be classified by generation, i.e., the number of repeated branching cycles that are performed during its synthesis. Higher generation dendrimers have more exposed functional groups on the surface, which can later be used to customize the dendrimer for a given application. Typical dendrimers have very strong potential for drug delivery as well as conducting chemical reactions in their interiors.3 Lately, new types of copolymers © 2012 American Chemical Society

combining linear and dendritic architectures have been developed due to the progress in controlling polymerization processes.4−6 The so-called linear−dendritic block copolymers (LDBCs) possess both the highly branched architecture and multifunctionality of a dendrimer and the conventional amphiphilic characteristics of a linear−linear block copolymers. This intriguing macromolecules have been exploited for various possible applications such as surface modification,7 drug delivery carriers,4,8 stimuli-responsive devices,9 and biomimetic mineralization.10 The concept of macromolecules with linear−dendritic hybrid structures was proposed by Fréhet et al.6 A family of LDBCs Received: June 20, 2012 Revised: August 15, 2012 Published: August 27, 2012 7143

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The polymersomes formed with azobenzene-containing (AZO) polymers should also possess this photoresponsive quality.4,5 It was found that the vesicle membrane becomes distorted, wrinkled, and even ruptured upon UV irradiation. As a consequence, they can serve as a model system for photoresponsive polymersomes to study feasible means to release the encapsulated active substances. Self-assembly behavior of amphiphilic block copolymers in a selective solvent has attracted intensive studies in recent years. The supramolecular aggregates including micellar aggregates and polymersomes have found numerous applications such as micro/nanoreactors, drug delivery systems, and microcapsule technology. Since the constituent blocks of LDBCs possess different degree of solvophobicity, they can exhibit amphiphilic properties and form self-assembled nanostructures. Typically, LDBC combines a linear hydrophilic coil with dendritic hydrophobic blocks or a linear hydrophobic chain with dendritic hydrophilic blocks. Those LDBCs have been proven to give rise supramolecular aggregates in solution. Recently, a series of amphiphilic LDBCs bearing hydrophobic AZO on the periphery of the polyester dendron and hydrophilic PEG as linear coil block were developed.4,5 The self-assembled supramolecular structures have been systematically investigated by varying the generation number and molecular weights of PEG block. The morphology of the supramolecular aggregates of the LDBCs in water was studied by transmission electron microscopy, cryo-electron microscopy, and atomic force microscopy. A generation-dependent aggregation behavior was observed for those LDBCs (denoted as PEGm-dendr[AZO]n where m represents the degree of polymerization of PEG and n the number of azobenzene units at the periphery of dendron). Core−shell structured nanofibers, sheet-like aggregates, and vesicles were formed for different m and n values. The membrane of the sheet-like aggregates, tubular micelles, and polymersomes was shown to have a bilayer structure. UV irradiation of the aqueous polymersome dispersion induced the formation of wrinkles in the vesicle membrane, indicating that this type of polymeric aggregate is photoresponsive. It appears that the resulting morphologies of linear−dendritic macromolecules in selective solvents are more diverse and easily modified than those of their linear−linear counterparts. Evidently, the self-assembled aggregates of LDBCs can be manipulated by adjusting specific polymer properties such as the generation number of the dendron or coil length of the linear block. However, previous studies are still at an initial exploratory stage. Because of the complicated generationdependent structures, amphiphilic LDBCs possess the additional complexity in their self-assembly behaviors. Detailed investigations about the effect of architecture on the resulting aggregative morphologies and characteristic photosensitive properties are still needed. This information is of great importance for the understanding of the connection between structure and properties and for the design of potential drug delivery vehicle. Experimental studies of the self-assembly behaviors of LDBCs are restricted owing to the synthetic difficulties of well-defined polymers with specific architectural configurations and controlled molecular weights. The theoretical approach based on molecular simulation may provide valuable microscopic insights and complement the deficiency of experimental studies on the self-assembly behavior of LDBCs. Mesoscopic simulations, such as dissipative particle dynamics (DPD), allow in silico experiments to be easily and inexpensively performed on complex, soft materials. Further-

with a Fréhet-type poly(benzyl ether) dendron was developed. However, in recent studies, compounds with distinct functionalizations are often attached on the periphery of the dendron for various applications. For example, a kind of LDBC combining linear poly(ethylene glycol) (PEG) and dendritic polyester functionalized with carboxylic acid groups as a crystal growth modifier was developed for the biomimetic mineralization of calcium carbonate.10a Self-assembled supramolecular structure of a series of LDBCs composed of linear PEG and polyether dendrons functionalized at the periphery with alkyl chains have been systematically explored.10b−g Well-ordered ultrathin Langmuir−Blodgett films have been formed by attaching stearic alkyl chains to PEG-dendr(PAMAM).10h−j LDBCs tethered with azobenzene or azobenzene derivatives were shown to be a promising photoresponsive material for purposes of stimuli-responsive drug carriers.4,5 In comparison with amphiphilic linear−linear copolymers, the interfacial curvature of LDBCs is facilely tunable by changing the generation number and peripheral groups of dendritic blocks. In addition to the experimental investigations of the linear− dendritic block copolymer (LDBC), there were also some computational studies on LDBCs. These simulations mainly focused on polymer melt behaviors and micelle formation. For example, Liu and Zhong11a have performed DPD simulations to systematically investigate their microphase separation and properties under shear flow. It was found that the microstructures of the LDBCs depend not only on shear rate but also on the degree of branching of the dendritic part. Also, they have smaller shear viscosities than linear−linear diblock copolymers of a similar molecular weight. Using molecular dynamics simulations techniques, Jang and coauthors11b determined the structure and dynamics of the LDBC formed by secondgeneration Fréhet polyaryl ethereal dendrimer as the hydrophilic component and linear polytetrafluoroethylene (PTFE) as the hydrophobic polymer, with 5 and 10 wt % of water. They found that this material produces a well-developed nanoscale structure in which water forms a continuous nanophase, making this new family of compounds promising candidates for applications in fuel cell membranes. Langevin dynamics simulations were performed by Suek and Lamm11c to investigate the assembly of amphiphilic LDBCs in solution. The critical micelle concentration (CMC), micelle size distribution, and shape are examined as a function of dendron generation and architecture. A related work of Georgiadis et al.12 studied the micellization behavior of hybrid dendritic-star copolymers with solvophilic dendritic units by means of Brownian dynamics simulations. The critical micelle concentration and the micelle size and shape are examined for different solvophobic/solvophilic ratios as a function of the number of the dendritic and linear arms. Appropriate functionalization of the dendron periphery can result in stimuli-responsive LDBCs, such as pH-, solvent-, or thermal-sensitive copolymers. Light is an advantageous stimulus as compared to other stimuli, since it is a rapid and remote stimulus that can be applied locally and requires no change in the chemical environment.4 Azobenzene (and derivatives) molecules possess a unique light-responsive property which is the photoisomerization of trans and cis isomers.5,13 Upon irradiation with particular wavelengths of light, the two isomers can be switched: ultraviolet light for trans-to-cis conversion and blue light for cis-to-trans isomerization. The cis isomer is less stable than the trans, and therefore cis-azobenzene can thermally relax back to the trans via cis-to-trans isomerization. 7144

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where aij is a maximum repulsion between particles i and j. rij and r̂ij denote the distance and unit vector between beads i and j, respectively. The dissipative force FD is proportional to the relative velocity, vij = vi − vj, FDij = −γwD(rij)(r̂ij·vij)r̂ij, where γ is the coefficient controlling the magnitude of the dissipative force and wD is a r-dependent weight function. The random force FR denotes the thermal motion of unresolved scales, such as the molecules inside each particle, FRij = −σwR(rij)θijr̂ij, where σ = (2γkBT)1/2 represents the noise amplitude, wR is a r-dependent weight function, and θij is a randomly fluctuating variable whose average number is zero. The dissipative force acts to reduce the relative momentum between beads i and j, while the random force is to impel energy into the system. For applying DPD to our linear−dendritic block copolymers system, the spring force (FSij) is added to bind together the connected beads of the polymer, FSij = −∑jCS(rij − req)r̂ij, where CS is the spring constant and req denotes the equilibrium bond length. To simulate the azobenzene blocks, two additional bonding forces are adopted: the bending force (Fθ) and the θ angle spring force (FSθ ij ). The bending potential is defined as U = kθ(θ − θeq)2 and Fθ = −∇Uθ. This force tends to compel the bond angle between two consecutive bonds to be close to the value of θeq. Since the azobenzene undergoes a trans-to-cis isomerization during UV exposure, θeq is set as 180° or 70°, corresponding to azobenzene before or after UV illumination. When azobenzene takes the trans conformation, it is rather rodlike, and the bending force alone cannot provide satisfactory stiff conformation of the azobenzene. Consequently, an additional spring force is also employed for the azobenzene S θ block, FSθ i = −∑jC (rij − req)r̂ij. Note that this angle spring force represents the spring force between the first and third beads of the azobenzene block, and it does not exist after UV illumination. We have chosen CS = Cθ = 100, req = 0.7, kθ = 20, and rθeq = 2req = 1.4. These choices of CS and req will not affect the qualitative behavior of the systems studied in this work. In addition to the intrinsic rigid nature associated with trans conformation, the azobenzene also possess π-conjugated electronic structures.4,5,22,23 It is known that the π-conjugated property of the azobenzene block can greatly affect the selfassembling morphologies and internal structures of the systems. We have used an attractive potential to mimic π−π interactions of rod segments similar to the one proposed by Polimeno et al.24,25

more, DPD simulation is currently the only viable simulation method that can be employed to study important macromolecular morphological expressions on the relevant length and time scales. It has been successfully applied to explore a wide variety of complex phenomena such as polymer-protected nanoparticle self-assembly14 and self-assembly behaviors of amphiphilic asymmetric macromolecular brushes.15 Also, DPD has been employed to study the anchoring transitions of nematic liquid crystals in the presence of a rod−coil amphiphilic monolayer at the aqueous−liquid crystal interface.16 The size-dependent behavior and the swelling of small unilamellar vesicles including the membrane characteristics and mechanical properties were also explored by DPD.17,18 The success in these studies indicates that DPD simulation is a very promising tool to study the self-assembly behaviors of amphiphilic LDBCs. As a consequence, the main objective of the current work is to achieve a qualitative understanding of the influences of generation number of the dendritic block, UV irradiation, polymer concentration, various block lengths, and π−π interactions on the morphological outcomes of the selfassembled, amphiphilic LDBCs by employing DPD. In this study, we shall focus on the supramolecular aggregates formed by azobenzene-containing linear−dendritic block copolymers, which have been synthesized by del Barrio et al.4

II. MODEL AND SIMULATION METHOD The dissipative particle dynamics (DPD) is an office-lattice particle-based mesoscopic simulation technique introduced by Hoogerbrugge and Koelman in 1992.19 Particles represent whole molecules or fluid regions rather than single atoms. Solvents are explicitly included in DPD. The particles’ internal degrees of freedom are integrated out and replaced by simplified pairwise dissipative and random forces, so as to conserve momentum locally and ensure correct hydrodynamic behavior. Like molecular dynamics, DPD beads obey Newton’s equation of motion20,21 dri = vi, dt

dvi = fi/mi dt

(1)

where ri is the position vector, vi is the velocity vector, mi is the mass of beads, and fi denotes the total forces acting on bead i with mass mi. These DPD beads are subject to soft potentials and governed by predefined collision rules. Consequently, this mesoscale method allows the simulation of hydrodynamic behavior for much larger length and longer time scales. Simulations of complex systems in volumes up to 100 nm in linear dimension for tens of microseconds are now common. A. Interactions between DPD Beads. The force fij exerted on bead i by bead j consists of a conservative force (FCij ), a dissipative force (FDij ), and a random force (FRij ). Thus, the total force acting on bead i is given by fi =

U π = −ε cos2 θ(1 − r ) and F π = −∇U π

where r and θ are the distance between center of mass and the included angle between the two rigid segments, respectively. The constant ε measures the force of the orienting potential. In this work, ε = 2 is chosen to demonstrate the presence of the π−π attraction. However, in the last part of the work, ε = 0−7 is adopted to explore the influence of the π−π strength, while ε = 0 depicts the absence of the interaction. Note that the main function of eq 4 is to increase the alignment tendency between rigid segments of different azobenzene units. In this work, weak π−π attraction is introduced to elucidate the effect of AZO groups. In fact, elimination of π−π interaction (turning off eq 4) does not influence the leading order behavior of our simulation results. However, the inclusion of this orienting potential between portions of rod-like segments allows a more versatile modeling. B. System Parameters. In this study, the amphiphilic azobenzene-containing linear−dendritic block copolymer synthesized by del Barrio et al.4 serves as a reference for the

∑ (FCij + FijD + FijR ) (2)

j≠i

The sum acts over all beads within a cutoff radius rc, beyond which the forces vanish. These forces conserve net momentum and all acts along the line joining two interacting particles. The conservative force FC for nonbonded beads is described by a soft-repulsive interaction FCij = aij(rc − rij)riĵ ,

rij ≤ rc ;

0, rij > rc

(4)

(3) 7145

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Figure 1. Schematic diagrams of the realistic linear dendritic block copolymer PEG27-dendr[AZO]4 and its corresponding model linear dendritic block copolymer Rx-dendr[By-Y3]4. This linear dendritic block copolymer is with two generations (g = 2) and thus has four azobenzene mesogens (n = 4).

On the basis of the interaction models described in section A and LDBC polymer described above, the dynamics of 648 000 DPD particles was performed in a cubic box (603) under periodic boundary conditions. The system density is set to 3. Note that in DPD simulations all the units are scaled by the bead mass m, cutoff distance rc, and thermal energy kBT. The equation of motion is integrated with a modified velocity-Verlet algorithm20 with a time step of Δt = 0.04 and λ = 0.65. Since DPD simulation utilizes soft-repulsive potentials, the systems studied are allowed to evolve much faster than the “brute-force” molecular dynamics. A typical simulation required 1 × 106 steps, and the first 2 × 105 steps are for equilibration. The concentration of model LDBC in the solution (φp) is defined as

parametrization of the model LDBC. In order to correspond qualitatively to the realistic LDBC, i.e. PEGm-dendr[AZO]n, a model LDBC is proposed, i.e., Rx-dendr[By-Y3]n. Both realistic and model LDBCs are shown in Figure 1. In our model LDBC, R-block (the red bead) denotes the solvophilic PEG-block, Gdendrite (the green bead) denotes the solvophilic dendritic polyester, B-block (the blue bead) stands for the solvophobic aliphatic block, and Y-block (the yellow bead) corresponds to the solvophobic azobenzene-block. The LDBC has different generation g (g = 1−5), and the number of azobenzene mesogens can be calculated, i.e., n = 2g. Note that the total number of G beads varies with g and is equal to ∑gz=02z. Figure 1 demonstrates the LDBC with two generations (g = 2), and thus it has four azobenzene mesogens (n = 4). Evidently, there are five different species of DPD beads which are solvent (S), two kinds of solvophobic beads (B and Y), and two solvophilic beads (R and G). The mapping between the realistic LDBC and the coarse-grained R x-dendr[B y -Y 3 ]n in our DPD simulations can be performed as follows. First, an isolated PEGm-dendr[AZO]n was constructed by using Materials Studio’s polymer and simulation modeling software.26 It was found that the azobenzene-block can be represented by three DPD beads with diameter equal to 5 Å. The radius of gyration for PEG27 is about 60 Å and therefore can be represented by 12 DPD beads. The water molecule has the size of ∼1.51 Å, and therefore one DPD bead contains roughly three water molecules. Figure 1 shows how PEG27-dendr[AZO]4 is mapped onto the model LDBC.

φp =

nLDBClLDBC total beads of polymer = total beads in the system nLDBClLDBC + nsol

(5)

where nsol is number of solvent bead and nLDBC is the number of LDBCs in the system; lLDBC is the total number of beads associated with a LDBC (Rx-dendr[By-Y3]n), and lLDBC = x + ∑gz=02z + 2g(y + 3). Various values of φp are used to study the effect of LDBC concentration on the resulting morphological structures of the aggregates. The interaction parameters chosen are shown in Table 1. These repulsive interaction parameters (aij) used in this work are not randomly selected. If species i and j are fairly compatible, one has aij ≈ 25. As the incompatibility between i and j rises, aij increases. According to Groot and Warren,20 the interaction parameter can be estimated from the Flory− 7146

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tive behaviors of the azobenzene-containing LDBCs in selective solvents. First, the comparisons between the experimental findings and our simulation results are presented to demonstrate the validity of our model LDBC polymer and simulation approach. A change in the generation number of the LDBCs affects the morphology of the aggregates. Cylindrical, sheet-like, bowl-like aggregates, or polymersomes are observed. The detailed morphological changes of the polymersomes upon UV irradiation are investigated. The dynamics process of the water permeating through the membrane of polymersome is also monitored. Second, the morphological phase diagram of our model LDBC, Rx-dendr[By-Y3]n, is displayed to demonstrate the overall picture regarding the effects of generation number (g) and polymer concentration (φ p ) on the morphologies of the LDBCs’ self-assembly aggregates. The representative morphological snapshots are also shown. Third, the dependence of the aggregative outcomes on PEG-block length (R-block) and solvophobic, aliphatic block length (Bblock) are illustrated by morphological snapshots and crosssectional slices. Finally, the effect of π−π strength between azobenzenes (Y-blocks) on the self-assembled behaviors of the LDBCs is discussed. A. Self-Assembly of LDBCs: Comparison between Experiments and Simulations. The azobenzene-containing LDBC polymer, PEGm-dendr[AZO]n, synthesized by del Barrio et al.4 is the archetype for the model LDBC studied in this work. As pointed out by del Barrio and co-workers, due to the unique dendritic structure, the azobenzene-containing LDBCs can self-assemble into aggregates with different morphologies, such as nanofibers, sheet-like aggregates, tubular micelles, or polymersomes by changing the generation number of the dendritic block or the length of the PEG-block. Accordingly, our model LDBC is denoted as Rx-dendr[By-Y3]n as shown in Figure 1. If one regards LDBC as a typical surfactant which forms micelles due to its amphiphilic nature, then the R-block (PEG chain) and G-block (polyester dendron) function as the hydrophilic head-group while the B-block (aliphatic chain) and Y-block (AZO-block) serve as the hydrophobic tail-group. Certainly, the molecular architectures associated with both head-group and tail-group are much complicated than those of the typical surfactant made of amphiphilic diblock copolymer. As shown in Figure 2, our simulation results of R12-dendr[B2Y3]n in selective solvents confirm that the generation number (g) of the dendritic structure is one of the main factors affecting the outcome of the self-assembled conformations. Here the lengths of R-block and B-block are fixed at x = 12 and y = 2. When g = 1, one has n = 2g = 2, which indicates that there are two azobenzene units at the periphery of the dendritic block. R12-dendr[B2-Y3]2 in selective solvent tends to self-assemble into short worm-like micelles, long cylindrical micelles, or nanofiber depending on the overall polymer concentration. The internal structures of the cylindrical micelle as shown in Figure 2a agree with the schematic representation of the self-assembly and packing of PEG45-dendr[AZO]2.4 The diameters of the cylindrical micelles obtained in our simulations were about 10− 15rc, i.e., 5−7.5 Å, which are compatible with the experimental findings (∼8 nm). When g = 2 (i.e., n = 4), there are four azobenzene units at the outside edge of the dendritic block. The increase in the generation number designates that both hydrophilic (G-block) and hydrophobic (B- and Y-blocks) contributions in LDBC grow. However, the hydrophilic/ hydrophobic block ratio of a LDBC molecule declines and the LDBC becomes more hydrophobic as g or n increases.

Table 1. Interactions Parameters (aij) for the Polymer Rxdendr[By-Y3]n in Aqueous Solutions before UV Illuminationa aij

S (water)

S R G B Y

25

R (block) solvophilic

G (dendrite) solvophilic

B (block) solvophobic

Y (rod) solvophobic

26 25

26 26 25

40 40 30 25

60 (45) 50 (40) 50 (30) 50 25

a

The numbers shown in the parentheses are the interaction parameters used after UV exposure.

Huggins χ-parameter by their χ−a relation: χij = (0.286 ± 0.002)(aij − aii), where aii = 25 for i = Y, B, G, R, and S. “Blends”, which is the module of Materials Studio from Accelrys Inc., provides a way to obtain χ by estimating the miscibility behavior of binary mixtures.26 It predicts the thermodynamics of mixing directly from the chemical structures of the five kinds of beads. Therefore, only their molecular structures and a force field are required as inputs. Here, the COMPASS force field is adopted, and the interaction parameters can then be calculated by the χ−a relation. Note that the interaction parameters are rounded to “cleaner” numbers. Nevertheless, the simulation outcomes should remain qualitatively unaffected. Compared to all-atom simulations, the DPD approach seems to be less refined. Nevertheless, our intention is to develop a coarse-grained model that possesses characteristic physical and structural features of solvent and LDBC polymer system. In order to analyze the degree of alignment of the azobenzene mesogen aggregates, a short-ranged order parameter (s) is defined as Np Ni *

s=

∑∑ i=1 j=1

(3 cos2 θij − 1) 2

Np

∑ Ni* i=1

(6)

where θij is the included angle between two neighboring rods i and j. Np is the total number of azobenzene rods in an aggregate. Ni* means the total number of neighboring rods for rod i and is determined according to the schematic demonstration shown in ref 27. Ni* can be determined by counting the total number of azo-rods within the cylindrical domain of a centered azo-rod. The cylindrical domain is with radius Rg and length Re, where Rg and Re are the radius of gyration and end-to-end distance of the rod block. For a rod, Rg ≈ (1/2√3)Re. Note that any rod with one or more beads situating within the cylindrical domain is counted as a neighboring rod of the centered rod. As a consequence, the value of s = 1 represents that the neighboring rods are perfectly aligned in parallel. Complete rotational disorder is indicated by s = 0. Note that the initial states of our systems were random distributions of the model LDBCs. Several initial random states were employed, and the resulting aggregates were reproducible, indicating that the systems were not trapped in metastable states. Also, in this work each time step corresponds roughly to 10 ps, and the simulation proceeds for about 10 μs.

III. RESULTS AND DISCUSSION In this study, dissipative particle dynamics is applied to investigate the morphological conformations and thermosensi7147

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the LDBC has a very low hydrophilic/hydrophobic ratio, and therefore polymersomes are formed in our simulation, as depicted in Figure 2d. This result is consistent with the experimental findings of ref 4. Since the thickness of the membrane was found to be about twice the length of the dendritic block, they inferred that the membrane of the vesicle has a bilayered structure in a way similar to that of the sheetlike aggregate. The cross-sectional slice taken from our simulation agrees with the conjecture of a bilayer-like packing. However, note that the azobenzene mesogens are also interdigitated significantly. In our simulation, we have created a LDBC with g = 5 (i.e., n = 32) which does not appear in the experimental work. Such a high-generation LDBC possesses an even lower hydrophilic/hydrophobic block ratio and a fan-like hydrophobic tail-group. As a consequence, only small-sized polymersomes are formed. It is worth mentioning that the order parameter (s) for the azo-rods increases slightly as the generation number rises. However, all the values of s are less than 0.3. This consequence indicates that in the AZO phase the rod packing is only slightly anisotropic since s is on the order of 0.3−0.8 for a typical ordered liquid crystal sample. Upon increasing the generation number of our model LDBC, R12-dendr[B2−Y3]n, the morphological transition of the polymeric aggregate in a selective solvent from cylindrical micelles (nanofibers) to sheet-like micelles (nanosheets) and eventually to vesicles has been illustrated in Figure 2. This simulation result agrees with the experimental declarations.4 That is, micellar-like aggregates develop for LDBCs with small generation number while polymersomes take shape for LDBCs with large generation number. By simply regarding LDBC as a surfactant, the driving force for its self-assembly behavior is the consequence of solvophilic head-groups formed by R-chain and G-dendrite and solvophobic tail-groups constituted by B-chain and Y-rod. The aforementioned outcomes validate our proposed LDBC model and interaction parameters, which allow us to explore the behavior of the LDBC systems further. As a consequence, a qualitative understanding of the influences of UV irradiation, polymer concentration, various block lengths, and π−π interactions on the morphological outcomes of the self-assembled, amphiphilic LDBCs can be achieved. B. Photoresponsive Polymersome upon UV Irradiation. As mentioned, azobenzene is a photoresponsive compound. Photoisomerization of trans and cis isomers takes place when azobenzenes are illuminated with particular wavelengths of light. For example, irradiation of the ultraviolet light will trigger trans-to-cis conversion of azobenzene and its derivatives. A cis-to-trans conversion takes place upon visible light irradiation. The polymersomes formed with azobenzenecontaining polymers were found to also possess this photosensitive quality and can be considered as a model system for control release of the encapsulated active substances when and where they are needed.4,22,28,29 In the experimental work of del Barrio et al.,4 the polymersomes formed by the azobenzenecontaining LDBCs were exposed to UV light. During UV exposure, the azobenzene groups go through a trans-to-cis isomerization in the middle of the membrane, and finally the vesicle membrane becomes distorted, wrinkled, and even ruptured after illumination. A change of the membrane structure due to trans-to-cis conversion may be responsible for the morphological change in polymersomes. According to our analyses based on Materials Studio,26 it is found that the trans-to-cis conformation change is not the only factor which alters the membrane structure. The photoinduced

Figure 2. Generation-dependent aggregation behavior of amphiphilic linear−dendritic block copolymers, R12-dendr[B2-Y3]n. The resulting aggregates are represented by the red (solvophilic PEG-block), green (solvophilic dendritic polyester), blue (solvophobic aliphatic block), and yellow (solvophobic azo-block) beads. The solvent beads are omitted.

Sheet-like aggregates emerge in our simulation for R12dendr[B2-Y3]4 as shown in Figure 2b. In the experimental study of del Barrio et al., it was found that the thickness of the hydrophobic part is around twice the length of trans azobenzene, and therefore a bilayer organization for the mesogen packing in the hydrophobic domains was proposed. The enlarged cross-sectional slice of the sheet-like aggregate obtained from the simulation is also demonstrated in the third column of Figure 2b. As one can see, a bilayer-like structure is formed within the nanosheet in which the hydrophobic azobenzenes are packed in the centered domain and PEG chains form the corona. However, the bilayer is not perfectly aligned and azobenzene mesogens are clearly interdigitated. Sheet-like aggregate and tubular micelles were observed for PEG45-dendr[AZO]8 in del Barrio et al’s work.4 Nevertheless, stable bowl-like micelles (Figure 2c) develop for our model LDBC, R12-dendr[B2−Y3]8, in addition to the sheet-like aggregates. The bowl-like micelle appears to be a curled sheet-like aggregate and seems to be an intermediate structure between sheet and vesicle. As the generation number increases to g = 4, 16 azobenzene units exist in the LDBC. In this case, 7148

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Figure 3. Snapshots of the vesicles formed by R9-dendr[B2-Y3]16 (a) before UV irradiation and (b) after UV irradiation for 78 000 DPD time steps. The polymersomes are represented by the red (solvophilic PEG-block), green (solvophilic dendritic polyester), blue (solvophobic aliphatic block), and yellow (solvophobic azo-block) beads. The solvent beads are omitted.

cis−trans isomerization causes a different orientation of the dipole moment of the azobenzene which results in a distinctive hydrophilicity for the azobenzene. In fact, the result that the cisazobenzene is more hydrophilic is consistent with previous experimental studies regarding the wettability of azobenzenecontaining monolayer. Contact angles of the azobenzenecontaining films decreased as the films were under 1 h UV irradiation.30−32 As a consequence, certain interaction parameters are adjusted in this work (as shown in Table 1) to mimic the behavior of the azobenzene-containing polymersomes after UV illumination. A polymersome with trans-azobenzene is obtained for R9-dendr[B2-Y3]16 at φp = 0.05. When the system is exposed to UV light, θeq is set as 70° instead of 180°, and the interaction parameter aSY is reduced to 45 instead of 60. Figure 3 shows snapshots of the vesicles containing azobenzene groups (a) before UV irradiation and (b) after UV irradiation for 78 000 DPD time steps. Before UV illumination, the hydrophobic azobenzene mesogens are bound due to rod-like configuration and π−π interaction, which lead to stable, spherical vesicles as in Figure 3a. On irradiation with UV light, the azobenzene (Yrod) goes through trans-to-cis isomerization. When the rod-like

trans conformation of azobenzene converts to a cis (bent) arrangement within the vesicle, these spherical vesicles become unstable and undergo deformation to the irregular vesicles, as shown in Figure 3b. Also, a perforation occurs in the azobenzene layer. The dynamic process associated with the shape change of the Y-block can disclose the change of the membrane structure and release of the lumenal content due to the photoinduced transformation. The evolutions of the hydrophobic azobenzene layer and overall morphology after UV illumination are demonstrated in the second and third columns of Figure 4a. For comparison, a series of snapshots of an isolated vesicle without UV irradiation at different time steps are also displayed in the fourth column. As one can see, without UV illumination, the polymersome exists stably as time proceeds and the permeation of water molecules is not noticeable. Note that only the azobenzene layer (yellow) and water beads (dark blue) originally encapsulated within the polymersome core are shown in the snapshots. After UV exposure, however, the vesicle membrane becomes distorted, wrinkled, and even ruptured. This result is consistent with experimental observation.4 In 7149

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of Figure 4a, the inner water region enclosed by the membrane can still be recognized even with the hole in the azobenzene layer. Our UV-photoresponsive simulation performs under the assumption that the trans-to-cis isomerizations are completed for all LDBCs instantly after the UV irradiation. It is justified if the characteristic time of the dynamic process associated with the shape change is larger compared to that of the photoinduced trans-to-cis isomerization. However, the final structure of the polymersome upon UV illumination is independent of the isomerization kinetics. Our results clearly display the effect of the trans-to-cis isomerization of the AZOs on the morphology variation of the polymersome. The quantitative behavior of lumenal content release after UV illumination can be realized by following the variation of the residual water beads with simulation time steps, Nw(t), as shown in Figure 4b. Nw is defined as the number of water beads that are initially located inside the polymersome. Because of permeation through the membrane, there exists water exchange between interior and exterior water domains. Evidently, the permeation of the water beads increases significantly for polymersomes exposed to UV light. The residual water within the vesicle can be described quite well by exponential decay, Nw(t) = Nw(t = 0) exp(−t/τp), where τp denotes the characteristic permeation time that is inversely proportional to the permeability of the membrane. Two features can be drawn from this consequence. It is found that τp ≃ 105Δt for polymersomes with UV irradiation, but τp ≃ 107Δt for polymersomes without UV irradiation. Obviously, the permeability is dramatically increased due to UV illumination. The behavior of simple exponential decay also reveals that the rupture of the azobenzene layer does not enhance the permeation suddenly, and the permeability increment is mainly caused by the structural change of the azobenzene layer. That is, for hydrophilic active substances encapsulated within the interior water region, the release process starts from the instant when the polymersome is under UV irradiation. Therefore, it is a feasible way to use azobenzene-containing polymersomes as a control release drug delivery system due to their photosensitive characteristics. C. Phase Diagram of R12-dendr[B2-Y3]n: Effect of Polymer Concentration. Both experiments and DPD simulations reveal that the generation number of the dendritic block has a significant impact on the overall morphologies and internal structures of the aggregates formed by LDBCs. Aggregates with various conformations can be developed by changing g. For typical surfactants, it is well-known that micelle type and size can vary with surfactant concentration. Previous studies on self-assembly of block copolymers in a selective solvent with various architectures24 also indicate that the aggregate morphology varies with polymer concentration as well for a specified molecular structure. In this study, the effect of polymer concentration on the self-assembled behaviors of LDBCs is investigated by constructing the morphological phase diagram, i.e., the variation of the aggregate morphology with φp and g. The polymer concentration φp is varied from 0.04 to 0.18. Without loss of generality, the hydrophilic block is fixed as R12 and hydrophobic block as B2 in our model LDBC, i.e., R12dendr[B2-Y3]n. The number of generation (g) is varied from 1 to 5, and thus n changes from 2 to 32. As pointed out before, the increase in the generation number corresponds to the decrease in the hydrophilic/hydrophobic block ratio of a LDBC molecule. That is, the LDBC becomes more hydrophobic as n grows.

Figure 4. (a) Evolution of the photoinduced transformation of the azo-containing polymersome formed by R9-dendr[B2-Y3]16. The dark blue beads represent the inner water beads originally encapsulated within the polymersome at time = 0. In the second column, only azoblocks of the polymersome are displayed. For comparison, the polymersome conformations as a function of simulation time steps for R9-dendr[B2-Y3]16 without UV illumination are also shown in the fourth column. (b). Nw(t) as a function of simulation time steps for R9dendr[B2-Y3]16 with and without UV illumination.

addition, the water molecules permeate through the membrane significantly as displayed by the spread of the dark blue beads in the exterior water region. As illustrated in Figure 4a, at time steps equal to 68 000, the azobenzene layer ruptures, and thus one may anticipate that substantial amount of water comes out through the ruptured hole. Since the overall shape of the polymersome remains identifiable as shown in the third column 7150

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et al.,4 PEG45-dendr[AZO]16 self-assembled into a polymersomes; however, sheet-like aggregates were found for PEG114dendr[AZO]16. The hydrophilic/hydrophobic ratio of PEG114dendr[AZO]16 is about 2.5 times that of PEG45-dendr[AZO]16. On the other hand, macroscopic phase separation takes place for PEG25-dendr[AZO]16 and lamellar-type bulk nanostructures are observed by XRD and TEM. These results reveal that in addition to the generation number the length of the PEG block also plays a crucial role in the morphological conformation of the self-assembly aggregates. The R-block of our model LDBC Rx-dendr[By-Y3]n corresponds to the PEG block in the PEGmdendr[AZO]n. Since the length of the R-block (x) affects the hydrophilicity of the LDBC at a fixed g, it is anticipated that the morphological conformations vary significantly with x. The phase diagram for the variation of the morphology with the generation number (g) and hydrophilic R-block length (x) is shown in Figure 6 for φp = 0.10. The influence of the

Figure 5a is the morphological phase diagram of our model LDBC, R12-dendr[B2-Y3]n, as a function of the generation

Figure 5. (a) Morphological phase diagram of aggregates formed by model linear−dendritic block copolymer, R12-dendr[B2-Y3]n. (b) The corresponding characteristic morphological snapshots for R12-dendr[B2-Y3]n where n = 2g.

number (g) and the polymer concentration (φp). Seven distinct types of aggregates are observed: spherical micelle, worm-like micelle, cylindrical micelle, hamburger-like micelle, sheet-like micelle, bowl-like micelle, and polymersome. Generally speaking, polymersomes take shape for LDBCs with large generation number and micellar-like aggregates develop for LDBCs with small generation number. As one can see from Figure 5, the polymer concentration also plays an important role in the morphological outcomes of the self-assembled aggregates. The characteristic morphological snapshots for R12-dendr[B2-Y3]2 at different φp are illustrated in Figure 5b. For g = 1, only spherical micelles are formed at low φp. As polymer concentration increases, worm-like micelles take shape. Only at sufficient high φp can the fibrous micelles develop. For g = 2, hamburger-like micelles form at low φp, and nanosheets are observed at high φp. For g = 3 and 4, transformation from sheet-like or bowl-like aggregate to polymersome is observed as φp grows. For g = 5, polymersome is formed even at very low φp. Also, the size of the polymersome rises with increasing φp. These results indicate that the generation number plays a more effective role in polymersome formation than polymer concentration does. However, polymer concentration can be manipulated for controlling the shape and size of the LDBC aggregate. D. Phase Diagram of Rx-dendr[B2-Y3]n: Effects of Solvophilic R-Block. In the experimental work of del Barrio

Figure 6. (a) Morphological phase diagram of aggregates formed by model linear−dendritic block copolymer, Rx-dendr[B2-Y3]n. (b) The corresponding characteristic morphological snapshots for Rx-dendr[B2Y3]n where n = 2g and φp = 0.10.

macromolecular architecture on the self-assembly morphology may be realized through the packing parameter defined as P = vB/(lBae), where vB is the volume of the solvophobic tail-group, lB is the length of the tail-group, and ae is the equilibrium area per polymer at the aggregate surface.33 Amphiphiles with a packing parameter of P < 1/3 appear to have a cone-like shape which will pack together to form spherical micelles. Amphiphiles with a packing parameter of P = 1/3−1/2 appear to have a truncated cone-like shape and will aggregate together to form cylindrical micelles. Amphiphiles with a packing parameter of P = 1/2−1 appear to have a cylinder-like shape and 7151

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pack together to form a vesicle or bilayer. The morphological transformation associated with the R-block length can be explained by the change of P. The increase of the R-block length (x) can correspond to the increase of the effective area (ae), and thereby the packing parameter declines accordingly. For the generation number g = 1, for x ≲ 3, P approaches unity. P decreases and becomes smaller than 1/2 but greater than 1/3 for 6 ≲ x ≲ 9. P turns out to be smaller than 1/3 for x ≳ 12. In contrast, at a given x, the packing parameter rises with increasing the generation number owing to the increment of the hydrophobic volume vB. As a consequence, for higher generation number with small enough x, nanosheet and vesicle are formed. For example, as depicted in Figure 6 for g = 2, vesicles are developed as x ≲ 3 and sheetlike aggregates are observed as 6 ≲ x ≲ 12. The above results clearly point out the significant effect of R-block on the morphological outcomes of the self-assembled aggregates of our model LDBCs. According to experiments, polymersomes are generally observed for LDBCs with large generation number, e.g., g = 4 for PEG45-dendr[AZO]n. Figure 6 has shown that as the Rblock length is increased, the generation number at which polymersomes start to form grows. For x = 12, polymersomes occur only for g ≥ 4, and this result is consistent with the experimental observation. However, for short R-block length, e.g. x = 3, polymersomes take shape as early as g = 2. On the other hand, the sheet-like aggregate tends to form for high Rblock length. This prediction is also consistent with the experimental observation. Moreover, the size of spontaneously formed polymersome decreases with increasing g. Note that the fusion events among polymersomes are insignificant during our simulation. The radius of the smallest vesicle33 that may be formed can be estimated from the critical radius below which a bilayer cannot curve, Rc ≈ lB/(1 − P). As a result, it is anticipated that the packing parameter declines as the generation number is increased. The packing parameter associated with a polymersome can be directly obtained through the evaluation of the membrane characteristics, particularly the hydrophobic layer, from simulations.17,18,33 For R3-dendr[B2-Y3]n, the packing parameters are determined as P ≃ 0.96, 0.95, 0.94, and 0.92 for g = 2−5. This result agrees with the observation of vesicle sizes and indicates that the radius of spontaneous curvature is smallest for g = 5. Despite of the fact that the packing parameter is close to unity, i.e. P ≈ 1, the conformations of LDBCs in inner and outer leaflets are quite different. Because of geometrical asymmetry between the inner and outer layers, the number of polymer in the outer leaflet is significantly greater than that in the inner leaflet. According to DPD simulations, it is found that the equilibrium area per LDBC in the outer leaflet is larger than that in the inner layer, i.e. (ae)o > (ae)i. On the contrary, the length of the tail-group in the inner leaflet exceeds that in the outer leaflet, i.e. (lB)i > (lB)o. The stretching in the outer leaflet and compression in the inner leaflet lead to smaller ae and longer lB in the inner layer. Note that the change in lB comes mainly from the hydrophobic B-chain, not the Y-rod. Although vesicles with smaller aggregation number can be easily formed at higher generation number, they are liable to vesicle fusion. As shown in Figure 7, it takes about 2.4 × 104Δt for two neighboring vesicles of g = 4 to fuse into a larger one but two vesicles of g = 2 remain intact after 5 × 105Δt. Our simulation results show that a large, spherical polymersome of g = 5 spontaneously formed from a prearranged sheet possesses

Figure 7. Cross-sectional images of the fusion processes for LDBCs with various generation numbers. The polymersomes are represented by the red (solvophilic PEG-block), green (solvophilic dendritic polyester), blue (solvophobic aliphatic block), and yellow (solvophobic azo-block) beads. The solvent beads are omitted.

the packing parameter close to unity. This consequence reveals that fusion of polymersomes can reduce the asymmetry between the inner and outer leaflets and thereby lower the free energy of the system. E. Effects of B-Block Length on Phase Diagram and Photoresponse. As shown in Figure 1, the azobenzene block is attached to a solvophobic, aliphatic chain which is represented by By, the blue block. The hydrophobic layer within the vesicle membrane are thus constructed by the azobenzene block and aliphatic chain. The azobenzene block is rod-like and tends to align due to the effects of π−π interaction and excluded volume effect while the aliphatic chain is coil-like. The connection of a coil-block to a rod-block may influence the arrangement among rod blocks.24 In our simulation of R12dendr[By-Y3]16, it is found that the aggregate is inclined to form polymersomes as shown in Figure 8. However, as the B-block length increases (y changes from 1 to 4), the polymersome increases in size as well as in membrane thickness. This result can be attributed to the fact that the growth in y intensifies the hydrophobicity of the LDBC and thus increases the aggregation number of an aggregate. Nevertheless, the growth in B-block length seems to disrupt the alignment of the azo-rods, and thus the order parameter (s) associated with the azo-rods layer declines with increasing y, e.g., s = 0.24, 0.22, and 0.19 for y = 1, 2, and 3, respectively. This result is consistent with our previous 7152

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Figure 8. (a) Morphological phase diagram of aggregates formed by model linear−dendritic block copolymer, R12-dendr[By-Y3]n. (b) The corresponding characteristic morphological snapshots for R12-dendr[By-Y3]n where n = 2g and φp = 0.08.

Figure 9. (a) Variation of layer thickness for R12-dendr[By-Y3]16 at different y. (b) Nw(t)/Nw(0) as a function of simulation time steps for R 12 -dendr[By -Y 3] 16 after UV illumination for various y. For comparison, Nw(t)/Nw(0) as a function of simulation time steps for R12-dendr[B2-Y3]16 without UV illumination is also shown.

study,24 which shows that the increase in the coil length evidently reduces the order parameter of the systems with the same rod length. This finding demonstrates that it is feasible for us, at the expense of azo-rod entropy, to control the size of the interior water region for encapsulating hydrophilic active substances in the drug delivery application. The phase diagram for the variation of the morphology with the generation number (g) and hydrophobic B-block length (y) is shown in Figure 8 for R12-dendr[By-Y3]n at φp = 0.08. For the generation number g = 1, the increment of the B-block length leads to the morphology change from spherical, worm-like, to cylindrical micelles. This consequence reveals that the packing parameter grows with increasing y because of the increment of the hydrophobic volume vB. For g ≥ 2, polymersomes can be formed for high enough B-block length. For example, at g = 2, polymersomes can be observed for y ≥ 4, but sheet-like aggregates are developed for y ≤ 3. For g = 4, polymersomes are always formed regardless of the B-block length, and the thickness of the hydrophobic layer within the membrane grows with y, as shown in Figure 9a. Similar to the effect of R-block, the morphological transformation associated with the B-block length can also be explained by the change of P. The above results demonstrate the significant influence of the B-block length on the thickness of the hydrophobic layer, order parameter of rod blocks, and the morphological outcomes of the self-assembled aggregates. It has been shown in Figure 4b that water permeation through the polymersome membrane formed by R9-dendr[B2Y3]16 is significantly enhanced after UV exposure. Since the

primary resistance to water permeation comes from the hydrophobic layer, it is anticipated that the release of the lumenal content is influenced by the length of the B-block in addition to the trans-to-cis transformation of the azobenzene layer. Figure 9b illustrates the evolution of the residual water beads within the vesicle after UV illumination with respect to DPD time steps for R12-dendr[By-Y3]16 with different B-block lengths, y = 1, 2, and 4. Again, the fraction of residual water beads can be well depicted by exponential decay. Nw(t)/Nw(0) decays fastest for y = 1 and slowest for y = 4. The characteristic permeation times are τp ≈ 0.4 × 105Δt for y = 1 and τp ≈ 1.5 × 105Δt for y = 4. The snapshots in Figure 9b show that the rupture of the azobenzene layer is observed for y = 4 but not seen for y = 1. This result indicates again that the increase in permeability is due to the structural change of the azobenzene layer, and the rupture is not relevant. Note that without UV irradiation water permeation is very much slower than those with UV illumination. Therefore, the membrane resistance to water permeation grows with increasing the thickness of the hydrophobic layer, which is raised by the B-block length as pointed out in Figure 9a. The aforementioned results disclose the fact that the photoresponsive behavior of polymersomes in terms of permeation is also influenced by the length of the hydrophobic block attached to the azobenzene block. F. Effect of π−π Strength Associated with Y-Rods. In addition to the intrinsic rigid nature, many of the rod-like polymers possess π-conjugated electronic structures as well. The π-conjugated property of the rod blocks provides additional attraction between rod blocks and thus is expected 7153

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interdigitated structure lowers the bilayer thickness. Therefore, the π−π attraction can enhance the alignment between neighboring rods and thus reduce the extent of interdigitation within the bilayer. The above results indicate that LDBCs tend to self-assemble into sheet-like structure when π−π interaction is dominant. Thereby, the azobenzene blocks can only possess weak to moderate π−π strength for the formation of polymersome.

to affect the self-assembly morphologies and structures. It is believed that the trans-azobenzene also possesses π−π interaction; however, the exact π−π strength remains unknown.4,22,28,29 Figure 10 demonstrates the comparison

IV. CONCLUSION The self-assembly behavior of amphiphilic linear−dendritic block copolymer Rx-dendr[By-Y3]n which corresponds to PEGm-dendr[AZO]n in a selective solvent is explored by DPD simulations. The LDBC consists of a solvophilic R-block and solvophilic dendrons (G-block). However, the periphery of the dendron is attached with a hydrophobic diblock made of coil-like B-block and rod-like Y-block. The aggregate of these LDBCs exhibits a rich variety of morphological conformations. Dependent on the generation number (g), the block lengths of R- and B-blocks (x and y), and π−π interaction between Yblocks, the morphology of the self-assembly includes seven distinct types: spherical micelle, worm-like micelle, cylindrical micelle, hamburger-like micelle, sheet-like micelle, bowl-like micelle, and vesicle. In order to confirm the validity of our DPD model of LDBC, we compare our simulation findings with the experimental results by del Barrio et al. As the generation number is increased from 1 to 4, the morphology of polymeric aggregates spontaneously formed by R12-dendr[B2-Y3]n changes from nanofibers, nanosheets, nanobowl, to polymersomes. This result is in agreement with the experimental observation. Upon exposure to UV irradiation, azobenzene group (Y-block) exhibits the trans-to-cis conformational change leading to a change in the orientation of the dipole moment. Thus, the hydrophilicity of azobenzene is increased. The photoinduced isomerization of azobenzene-containing LDBC is found to have a significant influence on the morphology of the polymersome structure. Based on the changes of conformation and hydrophilicity for Y-block, our simulation of the model LDBCs shows that after UV illumination the spherical polymersome undergoes deformation to an irregular vesicle, and the membrane (Y-block layer) becomes distorted, wrinkled, and even ruptured. This finding is also consistent with the experimental observation. Furthermore, it is found that the membrane permeability is dramatically increased due to the structural change of the azobenzene layer, and the release of lumenal content after UV irradiation simply follows an exponential decay. The influences of the block lengths (x and y) on the morphological conformations of the self-assembly formed by our model LDBCs are investigated as well. The morphological phase diagram of Rx-dendr[B2-Y3]n is obtained as a function of the generation number (g) and the R-block length (x). As the length of PEG (R-block) decreases, the generation number at which polymersomes start to form declines. For x = 12, polymersomes occur only for g ≥ 4, but for short R-block length, e.g. x = 3, polymersomes can be formed for g ≥ 2. On the other hand, the sheet-like aggregate tends to form for long R-block length. This prediction is also consistent with the experimental observation. Since the connection of a coil-block to a rod-block may affect the alignment among rod blocks, the length of hydrophobic, aliphatic chain (B-block) connected to the azobenzene can affect the supramolecular structure. The

Figure 10. Comparison between the aggregative morphologies of the model LDBC (R9-dendr[B2-Y3]16) with different π−π strengths.

between the aggregative morphologies of the model LDBC (R9-dendr[B2-Y3]16) with different π−π strengths. Note that the π−π attraction has been simply modeled by eq 4. Since the strength of π−π interaction ranges between 0 and 50 kJ/mol,34 we choose the constant ε to be 0−7, corresponding to the weak to strong interaction. It seems that the π−π strength does affect the overall morphologies, varying from polymersome, bowl-like, to sheet-like micelles. It is interesting to note that polymersomes are formed only when the π−π strength is weak enough. This consequence reveals that high degree of alignment among azo-rods may hinder the formation of polymersomes. In order to understand the change in the overall morphologies for R9-dendr[B2-Y3]16 at different π−π strengths, the arrangements of the azo-rods within the azobenzene domains are explored. The characteristics of the photoresponsive layer can be expressed in terms of the local order parameter and thickness of the azo-rod layer. The calculated order parameters listed in Figure 10 manifest the discrepancy between bilayers and illustrates the enhancement of orientation alignment due to π−π interactions. The significant increase in s with π−π interaction is especially astonishing from 0.02 to 0.8 as π−π strength varies from 0 to 7. Figure 10 also demonstrates the enlarged snapshot of the azobenzene domains. These results unmistakably point out that as π−π strength grows, the neighboring rods within the azobenzene domains become more coherently oriented. As a consequence, the thickness of azo-rod layers rises from l ≈ 2.8 for ε = 0 to l ≈ 4.1 for ε = 7. Note that the length of the azo-rod is about 2.1, and significantly 7154

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(7) Fréhet, J. M. J.; Gitsov, I.; Monteil, T.; Rochat, S.; Sassi, J. F.; Vergelati, C.; Yu, D. Chem. Mater. 1999, 11, 1267. (8) (a) Wood, K. C.; Little, S. R.; Langer, R.; Hammond, P. T. Angew. Chem. 2005, 44, 6704. (b) Namazi, H.; Adeli, M. Eur. Polym. J. 2003, 39, 1491. (9) (a) Zhu, L. Y.; Zhu, G. L.; Li, M. Z. Eur. Polym. J. 2002, 38, 2503. (b) Gillies, E. R.; Jonsson, T. B.; Fréhet, J. M. J. J. Am. Chem. Soc. 2004, 126, 11936. (10) (a) Wang, L.; Meng, Z.; Yu, Y.; Meng, Q.; Chen, D. Polymer 2008, 49, 1199. (b) Cho, B. K.; Jain, A.; Gruner, S. M.; Wiesner, U. Science 2004, 305, 1598. (c) Cho, B. K.; Jain, A.; Mahajan, S.; Ow, H.; Gruner, S. M.; Wiesner, U. J. Am. Chem. Soc. 2004, 126, 4070. (d) Cho, B. K.; Jain, A.; Gruner, S. M.; Wiesner, U. Chem. Mater. 2007, 19, 3611. (e) Chung, Y. W.; Lee, J. K.; Zin, W. C.; Cho, B. K. J. Am. Chem. Soc. 2008, 130, 7139. (f) Lee, E.; Lee, B. I.; Kim, S. H.; Lee, J. K.; Zin, W. C.; Cho, B. K. Macromolecules 2009, 42, 4134. (g) Cho, B. K.; Jain, A.; Gruner, S. M.; Wiesner, U. Chem. Commun. 2005, 16, 2143. (h) Iyer, J.; Fleming, K.; Hammond, P. T. Macromolecules 1998, 31, 8757. (i) Johnson, M. A.; Iyer, J.; Hammond, P. T. Macromolecules 2004, 37, 2490. (j) Nguyen, P. M.; Hammond, P. T. Langmuir 2006, 22, 7825. (11) (a) Liu, D.; Zhong, C. Macromol. Rapid Commun. 2005, 26, 1960. (b) Jang, S. S.; Lin, S. T.; Ç ağ1n, T.; Molinero, V.; Goddard, W. A., III J. Phys. Chem. B 2005, 109, 10154. (c) Suek, N. W.; Lamm, M. H. Langmuir 2008, 24, 3030. (12) Georgiadis, C.; Moultos, O.; Gergidis, L. N.; Vlahos, C. Langmuir 2011, 27, 835. (13) (a) Boiko, N.; Zhu, X.; Bobrovsky, A.; Shibaev, V. Chem. Mater. 2001, 13, 1447. (b) Bobrovsky, A.; Ponomarenko, S.; Boiko, N.; Shibaev, V.; Rebrov, E.; Muzafarov, A.; et al. Macromol. Chem. Phys. 2002, 203, 1539. (c) Bobrovsky, A. Y.; Pakhomov, A. A.; Zhu, X. M.; Boiko, N. I.; Shibaev, V. P.; Stumpe, J. J. Phys. Chem. B 2002, 106, 540. (d) Shibaev, V.; Bobrovsky, A.; Boiko, N. Prog. Polym. Sci. 2003, 28, 729. (14) Spaeth, J. R.; Kevrekidis, I. G.; Panagiotopoulos, A. Z. J. Chem. Phys. 2011, 135, 184903. (15) Chang, H. Y.; Lin, Y. L.; Sheng, Y. J.; Tsao, H. K. Macromolecules 2012, 45, 4778. (16) Zhang, Z.; Guo, H. Soft Matter 2012, 8, 5168. (17) Lin, C. M.; Li, C. S.; Wu, D. T.; Tsao, H. K.; Sheng, Y. J. Langmuir 2012, 28, 689. (18) Lin, C. M.; Wu, D. T.; Tsao, H. K.; Sheng, Y. J. Soft Matter 2012, 8, 6139. (19) Hoogerbrugge, P. J.; Koelman, J. M. V. A. Europhys. Lett. 1992, 19, 155. (20) Groot, R. D.; Warren, P. B. J. Chem. Phys. 1997, 107, 4423. (21) Espanol, P.; Warren, P. B. Europhys. Lett. 1995, 30, 191. (22) Hu, J.; Yu, H.; Gan, L. H.; Hu, X. Soft Matter 2011, 7, 11345. (23) Ma, L.; Jia, J.; Yang, Y.; Yin, G.; Liu, Y.; Sun, X.; Tao, X. RSC Adv. 2012, 2, 2902. (24) Chou, S. H.; Tsao, H. K.; Sheng, Y. J. J. Chem. Phys. 2011, 134, 034904. (25) Polimeno, A.; Gomes, A.; Martins, A. Computer Simulations of Liquid Crystals and Polymers; Springer: Dordrecht, The Netherlands, 2005; pp 135−147. (26) Accelrys Software Inc. (2008) Materials Studio Release Notes, Release 4.4; Accelrys Software Inc., San Diego, CA. (27) Chou, S.-H.; Wu, D. T.; Tsao, H.-K.; Sheng, Y.-J. Soft Matter 2011, 7, 9119. (28) Mabrouk, E.; Cuvelier, D.; Brochard-Wyart, F.; Nassoy, P.; Li, M.-H. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 7294. (29) Su, W.; Han, K.; Luo, Y.; Wang, Z.; Li, Y.; Zhang, Q. Macromol. Chem. Phys. 2007, 208, 955. (30) Delorme, N.; Bardeau, J. F.; Bulou, A.; Poncin-Epaillard, F. Langmuir 2005, 21, 12278. (31) Dietrich, P.; Michalik, F.; Schmidt, R.; Gahl, C.; Mao, G.; Breusing, M.; Raschke, M. B.; Priewisch, B.; Elsäser, T.; Mendelsohn, R.; et al. Appl. Phys. A: Mater. Sci. Process. 2008, 93, 285.

phase diagram for the variation of the morphology with the generation number (g) and hydrophobic B-block length (y) is determined for R12-dendr[By-Y3]n. It is found that polymersomes can be formed for long enough B-block length at lower generation number. The membrane resistance to water permeation grows with increasing y because the thickness of the hydrophobic layer is raised accordingly. The effect of the macromolecular architecture on the self-assembly morphology may be realized through the packing parameter P = vB/(lBae). The decrement of the R-block length (x) corresponds to the decrease of the effective area (ae), and thereby the packing parameter rises accordingly. On the other hand, the packing parameter grows with increasing y because of the increment of the hydrophobic volume vB. Previous studies on self-assembly of block copolymers indicate that the aggregate morphology varies with polymer concentration (φp). Generally speaking, polymersomes take shape for LDBCs with large g and micellar-like aggregates develop for LDBCs with small g. However, as polymer concentration is increased, nanosheets or polymersomes can form at lower generation number. Moreover, polymer concentration can be manipulated for controlling the shape and size of LDBC aggregates. In addition to the effects of block length and polymer concentration, π−π interaction (ε) is also an important factor that influences the self-assembled morphological conformation of LDBCs in selective solvents. As the π−π strength is increased, the overall morphologies vary from polymersome, bowl-like, to sheet-like micelles. It is interesting to note that polymersomes are formed only when the π−π strength is weak enough. This consequence reveals that high degree of alignment among azo-rods may hinder the formation of polymersomes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.-J.S.); [email protected] (H.-K.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work is supported by National Science Council of Taiwan. Computing times, provided by the National Taiwan University Computer and Information Networking Center and National Center for High-performance Computing (NCHC), are gratefully acknowledged.

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REFERENCES

(1) Ishizu, K.; Tsubaki, K.; Mori, A.; Uchida, S. Prog. Polym. Sci. 2003, 28, 27. (2) (a) Tomalia, D. A.; Naylor, A. N.; Goddard, W. A., III Angew. Chem. 1990, 102, 119. (b) Hawker, C. J.; Fréhet, J. M.J. J. Chem. Soc., Chem. Commun. 1990, 15, 1010. (3) Kaanumalle, L. S.; Ramesh, R.; Maddipatla, M. V. S. N.; Nithyanandhan, J.; Jayaraman, N.; Ramamurthy, V. J. Org. Chem. 2005, 70, 5062. (4) del Barrio, L.; Oriol, C.; Sáchez, J. L.; Serrano, A.; Di Cicco, P.; Keller, P.; Li, M.-H. J. Am. Chem. Soc. 2010, 132, 3762. (5) Shi, Z.; Lu, H.; Chen, Z.; Cheng, R.; Chen, D. Polymer 2012, 53, 359. (6) (a) Gitsov, I.; Wooley, K. L.; Hawker, C. J.; Ivanova, P. T.; Fréhet, J. M. J. Macromolecules 1993, 26, 5621. (b) Gitsov, I.; Fréhet, J. M. J. Macromolecules 1993, 26, 6536. 7155

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Macromolecules

Article

(32) Hamelmann, F.; Heinzmann, U.; Siemeling, U.; Bretthauer, F.; Vor der Brügen, J. Appl. Surf. Sci. 2004, 222, 1. (33) Israelachvili, J. Intermolecular and Surface Forces; Academic: New York, 1985. (34) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491.

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