BAB Block Copolymers Prepared

Article pubs.acs.org/Macromolecules

Self-Assembled Blends of AB/BAB Block Copolymers Prepared through Dispersion RAFT Polymerization Chengqiang Gao,† Jiaping Wu,§ Heng Zhou,† Yaqing Qu,† Baohui Li,*,‡,§ and Wangqing Zhang*,†,‡ †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China § School of Physics, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Synthesis of ingenious nanoassemblies is pursued in materials science. Herein, the in situ synthesis of the self-assembled blends of AB/BAB block copolymers of poly(ethylene glycol)-block-polystyrene/polystyrene-blockpoly(ethylene glycol)-block-polystyrene (PEG-b-PS/PS-bPEG-b-PS) via two-macro-RAFT agent comediated dispersion polymerization is reported. The synthesis strategy combines the advantages of polymer blending and polymerizationinduced self-assembly. Following this strategy, various nanoassemblies of PEG-b-PS/PS-b-PEG-b-PS blends such as high-genus compartmentalized vesicles, multilayer and bicontinuous nanoassemblies, and porous nanospheres are prepared. The parameters, such as PEG-b-PS/PS-b-PEG-b-PS molar ratio, polymerization degree of the PS block, and fed monomer concentration, affecting morphology/structure of PEG-b-PS/PS-bPEG-b-PS self-assembled blends are revealed. Computer simulations of self-assembly of the AB/BAB blends are performed, and nanoassemblies similar to those observed in our experiments are obtained, indicating that these morphologies are close to thermodynamical equilibrium. The formation mechanism of compartmentalized vesicles is investigated. The proposed strategy of two-macro-RAFT agent comediated dispersion polymerization is considered to be an efficient approach to construct selfassembled blends of block copolymers. defined by a dimensionless packing parameter p.9 In these years, which benefited from a wide variety of block copolymers and the adjustable preparation procedures, amphiphilic block copolymer nanoassemblies with novel morphologies, such as high-genus vesicles,10−12 multilayer vesicles,13−15 large compound vesicles,16−18 multicompartment micelles,19−23 toroidal micelles,24−26 and bicontinuous micelles,27−30 have been prepared. Besides nanoassemblies constructed with AB diblock copolymers,29−45 ABA triblock copolymers48−50 or ABC triblock copolymers,23−25,28,47 these nanoassemblies constructed with block copolymer blends such as the AB/B blends, 15,51−56 the AB/CB blends, 57−63 the AB/AC blends22,64,65 or the AB/BAB blends66 have also been reported, in which A denotes the solvophilic block and B and C denote the different solvophobic blocks throughout the article. For example, Lodge et al. prepared multicompartment micelles of the AB diblock copolymer and ABC miktoarm star terpolymer blends,21 and Pochan et al. prepared multicompartment micelles of the AB and AC diblock copolymer blends.22 Guo and co-workers prepared vesicles with different morphologies through micellization of the AB/CB blends of polystyrene-

1. INTRODUCTION Nanoassemblies of amphiphilic block copolymers have drawn great attention in recent years because of either versatile structure/morphology or their promising application in many fields.1−5 Generally, there are two routes to produce these block copolymer nanoassemblies. The first is traditionally through self-assembly of amphiphilic block copolymers in a selective solvent.1 In this approach, amphiphilic block copolymers are first dissolved in a good solvent and then a selective solvent is added, and finally the good solvent is removed usually through dialysis to freeze nanoassemblies. This approach encounters inconvenience of multistep procedures and disadvantage of diluted polymer concentration generally below 1 wt %. The other is recently developed in situ preparation of concentrated block copolymer nanoassemblies through polymerizationinduced self-assembly (PISA) especially via dispersion polymerization mediated by macromolecular RAFT (macro-RAFT) agents.6 This PISA method seems very reliable,6−8 since it affords convenient in situ production of block copolymer nanoassemblies with polymer concentration up to 30%. Of all the nanoassemblies, the most common structures prepared from amphiphilic block copolymers are spherical micelles, rodlike micelles, or vesicles. It is generally deemed that morphology of nanoassemblies formed under thermodynamical equilibrium is primarily as a result of the inherent molecular curvature and how it affects packing of copolymer chains as © XXXX American Chemical Society

Received: April 13, 2016 Revised: June 2, 2016

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DOI: 10.1021/acs.macromol.6b00771 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

(3.50 mg, 0.0213 mmol), and methanol/water mixture (80:20 w/w, 13.3 g) were weighed into a 25 mL Schlenk flask with a magnetic stir bar. The solution was first degassed with nitrogen at 0 °C, and then the flask was placed in a prethermostated oil bath at 70 °C. After a predetermined time, polymerization was quenched through quick cooling, and styrene conversion could be measured by UV−vis at 245 nm.47 The resultant PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends were dialyzed against methanol at room temperature (molecular weight cutoff: 3.5 kDa), and removal of styrene was judged by UV−vis at 245 nm. To check morphology of block copolymer self-assembled blends, a drop of colloidal dispersion was deposited onto a piece of copper grid, dried in air at room temperature, and finally observed by transmission electron microscope (TEM). To collect synthesized polymers, colloidal dispersion of PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends was subjected to rotary evaporation to remove solvent and dried in vacuo at room temperature, and finally the mixture of PEG45-b-PS/PS-b-PEG45-b-PS was obtained. 2.3. Computer Simulation Details. The computer simulations are carried out using a simulated annealing method67 in combination with the single-site bond fluctuation model.68−70 The combination of the model and method has been proved to be an efficient methodology for investigating the self-assembly of block copolymers under confinement or in solutions,71,72 and details of the model and algorithm can be found therein.71 The model system is composed of four componentsthe AB diblock copolymer, the BAB triblock copolymer, and the solvents of methanol and waterand they are modeled on lattice sites of a simple cubic box of volume V = L × L × L (L = 60). One model diblock copolymer chain used in the simulations is of the type A2Bn and the corresponding triblock copolymer chain of the type BnA2Bn. Each repeated unit of a chain occupies one lattice site, and two consecutive bonded repeated units are connected through a length of 1 or 2 . The initial configuration is generated by randomly creating ND diblock copolymer chains and NT triblock copolymer chains on the lattices of the box, with a fixed chain number ratio of α = ND/NT. After the block copolymer chains have been generated, the unoccupied sites are assigned as solvent molecules. Throughout the simulations, each species (a repeated unit or a solvent molecule) occupies one lattice site, and double occupation of any one lattice site is not allowed. Periodic boundary conditions are applied to all the X, Y, and Z directions. Starting from the initial configuration, the ground state of the system is obtained by performing a set of Monte Carlo simulations at decreasing temperatures. The trial moves used in the simulations are the exchange movement between chain monomers and solvent molecules.71 The objective function in the simulated annealing procedure is the energy of the system. In our simulations only pair interactions between species on the nearest-neighbor sites are considered. These interactions are calculated by assigning an energy Eij = εijkBTref to each nearest-neighbor pair of components i and j, where i, j = A, B, M and W, representing the A unit, the B unit, the methanol molecule, and the water molecule, respectively. Here εij is the reduced interaction energy, kB the Boltzmann constant, and Tref a reference temperature. In our simulations, the volume ratio of methanol/water is 80/20, and the interactions are fixed as εii = 0 with i = A, B, M, and W, εAB = 1, εAM = −0.50, εBM = 1.0, εAW = −0.40, εBW = 4.0, and εMW = 0. The total number of block copolymer chains is fixed at ND + NT = 1296. Two sequences of simulations are performed. In one case, the chain length of the solvophobic B block in the blend A2Bn/BnA2Bn is fixed at n = 8, whereas the number ratio of diblock/triblock copolymer chains, α, is decreased from 6:0 to 0:6. In the other case, the ratio α is fixed at 6:3, whereas n is varied from 1 to 12. 2.4. Characterization. 1H NMR spectra was recorded on a Bruker Avance 400 MHz using CDCl3 as solvent. Polymer molecular weight and its distribution (Đ, Đ = Mw/Mn) were measured by a Waters 600E gel permeation chromatograph (GPC) at 30.0 °C using nearmonodisperse polystyrene standards with the THF flow rate at 0.6 mL/min, in which the system was equipped with a Waters 2414 refractive index detector and TSK-GEL columns. TEM observation

block-poly(ethylene oxide) and polystyrene-block-poly(acrylic acid) just by tuning the block copolymer ratio of AB/CB.59 Computer simulations also suggest that polymer blending is an efficient approach to tune morphology and/or size of block copolymer nanoassemblies.56,62,66 However, compared to numerous nanoassemblies constructed with individual block copolymers,10−14,16−18,23−50 nanoassemblies constructed with block copolymer blends are relatively limited.51−66 The reason is possibly due to the coexisting mixed micelles constructed with polymer blends and nonergodic micelles constructed with individual block copolymers. More recently, we have proposed a new strategy to prepare AB/CB self-assembled blends via two-macro-RAFT agent comediated dispersion polymerization.19,20 This approach combines the advantages of polymer blending and PISA to afford block copolymer self-assembled blends. Since synthesis of the two block copolymers, their blending, and self-assembly take place almost at the same time, formation of nonergodic micelles constructed with individual block copolymers is restricted, and therefore well-defined self-assembled blends such as multicompartment block copolymer nanoparticles of the poly(4-vinylpyridine)-block-polystyrene and poly(Nisopropylacrylamide)-block-polystyrene blends can be prepared.20 In this paper, synthesis of AB/BAB self-assembled blends of poly(ethylene glycol)-block-polystyrene/polystyrene-block-poly(ethylene glycol)-block-polystyrene (PEG-b-PS/PS-b-PEG-bPS) with novel structure and morphology such as lamellae (bilayer sheets), high-genus compartmentalized vesicles, multilayer and bicontinuous nanoassemblies, and porous nanospheres are discussed, and the typical high-genus compartmentalized vesicles and porous nanospheres are further obtained with simulations. The PEG-b-PS/PS-b-PEG-b-PS self-assembled blends have the ingenious structure and morphology beyond the nanoassemblies of individual block copolymers, which will afford new insights into self-assembly of block copolymer blends.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St, >98%, Tianjin Chemical Company, China) was purified by distilling under reduced pressure before use. 2,2′-Azobis(isobutyronitrile) (AIBN, >99%, Tianjin Chemical Company, China) was purified by recrystallization from ethanol prior to use. S-1-Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonateterminated poly(ethylene glycol) monomethyl ether [PEG45-TTC, the subscript denotes polymerization degree (DP) or repeated units of corresponding monomers and TTC denotes RAFT terminal of trithiocarbonate throughout the article] and bis(S-1-dodecyl-S′-(α,α′dimethyl-α″-acetic acid) trithiocarbonate-terminated poly(ethylene glycol) (TTC-PEG45-TTC) were synthesized as described elsewhere.45 Deionized water was employed in the present study. All other chemicals were analytical grade and purified by standard procedures or used as received. 2.2. Dispersion RAFT Polymerization and Synthesis of PEG45-b-PS/PS-b-PEG45-b-PS Self-Assembled Blends. The dispersion RAFT polymerization was conducted in the solvent of methanol/water mixture (80/20 w/w) at 70 °C under [St]0:[PEG45TTC + TTC-PEG45-TTC]0:[AIBN]0 = 100−600:1:1/3 with weight ratio of monomer/solvent at 10−25%. The molar ratio of PEG45TTC/TTC-PEG45-TTC was changed as needed, while their total molar amount was kept fixed. Herein, a dispersion polymerization under [St]0:[PEG45-TTC]0:[TTC-PEG45-TTC]0:[AIBN]0 = 300:3/ 4:1/4:1/3 at 15% weight ratio of monomer/solvent was typically conducted as follows: TTC-PEG45-TTC (0.0430 g, 0.0159 mmol), PEG45-TTC (0.110 g, 0.0478 mmol), St (2.00 g, 19.2 mmol), AIBN B


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Macromolecules was carried out at 200 kV on a Tecnai G2 F20 electron microscope. Atomic force microscope (AFM) observation was carried out in the tapping mode on a Bruker MultiMode atomic force microscope. Scanning electron microscope (SEM) observation was carried out on a JEOL JSM-7500F field emission-gun microscope.

critical value. Compared to general blending of presynthesized block copolymers,51−66 the present AB/BAB self-assembled blends are formed just during the RAFT synthesis of the di- and triblock copolymers, and therefore AB/BAB self-assembled blends can be tuned by simply changing the initial conditions for the dispersion RAFT polymerization. As discussed elsewhere,57,63,66 the morphology and/or size of self-assembled block copolymer blends is firmly dependent on molar ratio of the ingredients in the blends. Herein, to prepare AB/BAB self-assembled blends with diffident molar ratio of PEG45-b-PS/PS-b-PEG45-b-PS and to check the influence of the molar ratio on morphology and/or size of the self-assembled blends, the dispersion polymerization under fixed ratio of [St]0: [PEG45-TTC + TTC-PEG45-TTC]0:[AIBN]0 = 300:1:1/3 but with different PEG45-TTC/TTC-PEG45-TTC molar ratio was performed. After 24 h polymerization with the similarly high monomer conversion at 93.3−96.2%, the PEG45-b-PS/PS-bPEG45-b-PS self-assembled blends were in situ synthesized and then checked by TEM. Under these synthetic conditions, two conclusions are made. First, as depicted in Figure 2, DP values of the PS block, ∼285, in the di- and triblock copolymers are very close to each other (see GPC analysis of these PEG45-bPS/PS-b-PEG45-b-PS blends in Figure S1). Second, the molar ratio of PEG45-b-PS/PS-b-PEG45-b-PS in the self-assembled blends is just equal to the ratio of PEG45-TTC/TTC-PEG45TTC, which is indicated in Figure 2, by assuming that the two macro-RAFT agents are all block-extended. Figure 2 summarizes TEM images of AB/BAB self-assembled blends with different molar ratio of PEG45-b-PS/PS-b-PEG45-bPS. As depicted in Figure 2A, individual PEG45-TTC results in the 160 ± 30 nm PEG45-b-PS289 vesicles with the membrane thickness at 35 ± 2 nm. Whereas when two PEG45-TTC/TTCPEG45-TTC macro-RAFT agents are used and the PEG45TTC/TTC-PEG45-TTC molar ratio decreases from 6/1 to 6/4, AB/BAB self-assembled blends with different morphology are formed (Figures 2B−F). By checking the TEM images, it is observed that morphology of AB/BAB self-assembled blends is firmly dependent on the PEG45-b-PS/PS-b-PEG45-b-PS molar ratio. For example, at the molar ratio of 6/1, 6/1.5, 6/2, 6/3, and 6/4, 690 ± 190 nm vesicles (Figure 2B), 420 ± 120 nm compartmentalized vesicles (Figure 2C), 410 ± 60 nm compartmentalized vesicles (Figure 2D), 350 ± 60 nm compartmentalized vesicles containing a thick wall (Figure 2E), and 330 ± 50 nm porous nanospheres (Figure 2F) are formed. Herein, the term of the compartmentalized vesicles is named, since vesicles as shown in Figures 2C,D contain a compartmentalized cavity with the hole genus at about 100. Note: the genus is a quantity from differential geometry,11 and it represents the number of handles or holes within a particle. With the PEG45-TTC/TTC-PEG45-TTC molar ratio further decreasing below 6/4, some gel-like precipitates are formed, and therefore their TEM images are not checked. The gel-like precipitates are attributed to formation of branched block copolymer aggregates at the low PEG45-TTC/TTC-PEG45TTC molar ratio similarly within the individual TTC-PEG45TTC mediated dispersion RAFT polymerization as discussed previously.45 By carefully checking the morphology of PEG45-b-PS/PS-bPEG45-b-PS self-assembled blends, the content or fraction of PS-b-PEG45-b-PS in the self-assembled blends is found to play profound effect on blend morphology, which undergoes from bilayer vesicles, to high-genus compartmentalized vesicles, and finally to porous nanospheres with increasing PS-b-PEG45-b-PS

3. RESULTS AND DISCUSSION 3.1. Synthesis of PEG45-TTC and TTC-PEG45-TTC. PEG45-TTC and TTC-PEG45-TTC were synthesized through esterification of hydroxyl in PEG45-OH or HO-PEG45-OH with carboxyl in DDMAT as discussed elsewhere.45 As shown by the 1H NMR spectra (Figure 1A), all the proton signals of PEG45-TTC and TTC-PEG45-TTC are nicely

Figure 1. 1H NMR spectra (A) and GPC traces (B) of PEG45-TTC and TTC-PEG45-TTC.

assigned and therefore indicate successful synthesis of the two macro-RAFT agents. Based on the peak area ratio of the signals at δ = 1.10−1.45 ppm (b) and δ = 3.64 ppm (g or g′), molecular weight Mn,NMR of the two macro-RAFT agents, 2.3 kDa for PEG45-TTC and 2.7 kDa for TTC-PEG45-TTC, is calculated. From the GPC traces (Figure 1B), the molecular weight Mn,GPC of two macro-RAFT agents with low Đ values is obtained. Mn,NMR of the macro-RAFT agents is slightly lower than Mn,GPC, and reason is probably attributed to the PS standards employed in GPC analysis. 3.2. Synthesis of PEG45-b-PS/PS-b-PEG45-b-PS SelfAssembled Blends and Effect of AB/BAB Molar Ratio on Morphology of Self-Assembled Blends. Different from dispersion polymerization employing one macro-RAFT agent,14,18,36−47 herein two macro-RAFT agents, PEG45-TTC and TTC-PEG45-TTC, are simultaneously employed in the present dispersion polymerization. During dispersion polymerization, PEG45-TTC results in synthesis of PEG45-b-PS and TTC-PEG45-TTC results in synthesis of PS-b-PEG45-b-PS. In the polymerization medium of the 80/20 methanol/water mixture, the PEG block is soluble and the PS block is insoluble; thus, co-micellization of the in situ synthesized PEG45-b-PS and PS-b-PEG45-b-PS occurs to form PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends, when DP of the PS block reaches a C


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Figure 2. TEM images of PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends synthesized with molar ratio of PEG45-TTC/TTC-PEG45-TTC at 6/ 0 (A), 6/1 (B), 6/1.5 (C), 6/2 (D), 6/3 (E), and 6/4 (F). Note: insets show the composition of the block copolymers, and A means the PEG block and B means the PS block.

Scheme 1. Schematic Morphological Evolution of PEG45-b-PS/PS-b-PEG45-b-PS Self-Assembled Blends with Increasing Content of the BAB Triblock Copolymer

content as summarized in Scheme 1. Since BAB triblock copolymer usually tends to form gels or bridged networks in a selective solvent for A block,45,73−76 it is supposed that (1) PSb-PEG45-b-PS as well as few of PEG45-b-PS forms the spongelike cavity of the compartmentalized vesicles, in which the PS block forms the sponge-like body and the solvated PEG block is

swollen in the holes, and (2) PEG45-b-PS as well as few of PS-bPEG45-b-PS forms the main body of the vesicle membrane, which are schematically shown in Scheme 1. In the subsequent simulations, this hypothesis is further confirmed, although exact location of the di- and triblock copolymers in the high-genus compartmentalized vesicles cannot be detected experimentally D


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PEG45-b-PS288 and PS288-b-PEG45-b-PS288 in the PEG45-b-PS/ PS-b-PEG45-b-PS self-assembled blends (Figure S3). From TEM images of self-assembled blends synthesized at different monomer conversion (Figure 4 and Figure S4), it is found that

due to very similar chemical composition of the di- and triblock copolymers of PEG45-b-PS and PS-b-PEG45-b-PS. Based on this hypothesis, the morphological evolution of PEG45-b-PS/PS-bPEG45-b-PS self-assembled blends can be reasonably explained. That is, in the absence of the PS-b-PEG45-b-PS triblock copolymer, PEG45-b-PS289 vesicles are formed (Figure 2A); in the case of low PS-b-PEG45-b-PS content, just low content of the gel-like flakes are encapsulated in the vesicle cavity (Figure 2B; and interestingly some of the encapsulated gel-like flakes are embedded with 19 ± 2 nm pores as indicated by the white cycles); in the case of moderate PS-b-PEG45-b-PS content, compartmentalized vesicles are formed (Figures 2C,D); in the case of high PS-b-PEG45-b-PS content, the encapsulated PS-bPEG45-b-PS content further increases, and finally the gel-like PS-b-PEG45-b-PS infills the cavity to form porous nanospheres (Figure 2F). As far as we know, the compartmentalized vesicles formed with PEG45-b-PS/PS-b-PEG45-b-PS molar ratio at 6/2, e.g., Figure 2D, may be one of interesting block copolymer nanoassemblies. This aroused our great interest to investigate how the compartmentalized vesicles was formed. As depicted in Figure 3, during dispersion RAFT polymerization, the styrene

Figure 4. TEM images of PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends prepared at 8 (A), 12 (B), 14 (C), and 24 h (D). Polymerization conditions: [St]0:[PEG45-TTC]0:[TTC-PEG45-TTC]0: [AIBN]0 = 300:3/4:1/4:1/3, 15 wt % monomer concentration, 70 °C.

48 ± 5 nm nanospheres at 8 h, lamellae at 12 h, then immature compartmentalized vesicles at 14 h, and finally compartmentalized vesicles at 24 h are formed during dispersion RAFT polymerization. For general bilayer vesicles, it was deemed that two procedures of (1) assembling block copolymer into a bilayer membrane and (2) closing the membrane into vesicles upon changing the interfacial curvature were involved in the vesicle formation.77 The intermediate morphology of the PEG45-b-PS142/PS142-b-PEG45-b-PS142 lamellae suggests that formation of the PEG45-b-PS288/PS288-b-PEG45-b-PS288 compartmentalized vesicles possibly follows the procedures similar to those in general bilayer vesicles. The PEG45-b-PS288/PS288-b-PEG45-b-PS288 compartmentalized vesicles were also characterized by AFM (Figure S5), and they were further stained either with CuSO4 or with RuO4 vapor and then checked by TEM. In the first staining, the PEG 45 -b-PS 288 /PS 288 -b-PEG 45 -b-PS 288 compartmentalized vesicles were dispersed in methanol (1 mg/mL); then an equal volume of the methanol solution of CuSO4 (10 mg/mL) was added, the mixture was stirred magnetically for 7 days at room temperature, immersed in the ultrasonic bath set at 25 °C for around 1 h (KQ-200 KDE, 40 kHz, 200 W, Zhoushan, China), and finally checked by TEM. (Note: CuSO4 depositing on surface of vesicles was removed by washing with MeOH, and then the excess solvent was split with filter paper.) Following these procedures, CuSO4 can diffuse into the vesicles, and the solvophilic PEG microdomains may be contaminated. As shown in Figure 5A, CuSO4 is deposited on both the membrane of the broken vesicle and the inner surface of the holes in the vesicle cavity as indicated by red arrows. Therefore, the holes in the vesicle cavity are confirmed, and the PEG block is supposed to locate at the inner surface of the holes. In the second staining, RuO4 vapor was used to stain PEG as discussed elsewhere.78 As shown in Figures 5B and 5C,

Figure 3. Polymerization kinetics (A) for dispersion RAFT polymerization and evolution of molecular weight of PEG45-b-PS and PS-bPEG45-b-PS in the AB/BAB self-assembled blends (B). Polymerization conditions: [St]0:[PEG45-TTC]0:[TTC-PEG45-TTC]0:[AIBN]0 = 300:3/4:1/4:1/3, 15 wt % monomer concentration, 70 °C.

conversion increases slowly in the initial homogeneous stage of 8 h, and then it quickly increases to 96.0% in the subsequent heterogeneous stage of 16 h. With styrene conversion increasing, molecular weight of the synthesized di- and triblock copolymers by GPC analysis, Mn,GPC, linearly increases just as that in individual macro-RAFT agent mediated dispersion polymerization36−47 (see GPC traces of these PEG45-b-PS/PSb-PEG45-b-PS blends in Figure S2). The GPC analysis of the separated block copolymers confirms the low Đ values of E


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Figure 5. TEM images of the PEG45-b-PS288/PS288-b-PEG45-b-PS288 compartmentalized vesicles stained with CuSO4 (A) and RuO4 vapor (B, C) and schematic structure of the compartmentalized vesicles (D).

porous nanospheres with the average sized at 190 ± 10 nm (Figure 6F). In these PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends, some specials are remarkable. The first is the broken flake as indicated by white arrow in the PEG45-bPS194/PS194-b-PEG45-b-PS194 vesicles (Figure 6B), which is very similar to that in Figure 2B; therefore, it confirms the spongelike gel of PS-b-PEG45-b-PS in the vesicle cavity. The second is the bicontinuous structure as indicated by red cycle in the PEG45-b-PS383/PS383-b-PEG45-b-PS383 cavity-lowly filled vesicles shown in Figure 6D. The bicontinuous morphology of block copolymer nanoassemblies was initially reported by Eisenberg,29 whereas no further exploration was perused. To date, just very few bicontinuous aggregates based on AB diblock copolymers,29,30 linear ABC triblock copolymers,28 and comblike block copolymers27 are observed. The present bicontinuous morphology of AB/BAB self-assembled blends, which will be further discussed subsequently, demonstrates a new way to prepare these novel nanoassemblies. The third is the porous structure shown in Figure 6F. Compared to the PEG45-b-PS280/ PS280-b-PEG45-b-PS280 porous nanospheres shown in Figure 2F, the PEG45-b-PS571/PS571-b-PEG45-b-PS571 porous nanospheres contain much fewer holes, despite that the size of the holes is very close to each other (13 ± 2 nm vs 13 ± 2 nm). The similar hole size is probably attributed to the PEG blocks in the di- and triblock copolymers having the same DP of 45, which contributes to the hole formation, and the decreased number of the holes is possibly ascribed to the relatively decreasing content of the PEG block in self-assembled AB/BAB blends with increasing DP of the PS block. For nanoassemblies of individual amphiphilic block copolymers, their morphology undergoes from nanospheres to worms and finally to vesicles, when DP of solvophobic block increases.33−42,46 It seems that this rule fails to explain the morphological evolution of PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends, and an exact reason needs further study. 3.4. Effect of Monomer Concentration on Morphology of AB/BAB Self-Assembled Blends. Compared to numerous reports on preparation of block copolymer nanoassemblies via PISA,14,18−20,36−47 those on checking how monomer concentration affects morphology of block copolymer nanoassemblies are rather limited.36,46 However, these rare experimental results have demonstrated the significant influence of monomer concentration on structure/morphology of block copolymer nanoassemblies. For example, Armes and co-workers as well as Xiao et al. have demonstrated that block copolymer nanoassemblies changes from nanospheres to worms and finally to vesicles with monomer concentration increasing.36,46 The concentration of the fed monomer exerting strong influence on morphology of block copolymer nanoassemblies can be ascribed to concentrated monomer, e.g., 15 wt % St monomer, which changes greatly solvent character and

a dark gray ring corresponding to the stained PEG as indicated by the red cycle is observed in the inner surface of the holes, and therefore the PEG block is believed to locate at the inner surface of the holes. Based on these results, the structure of the PEG 45 -b-PS 288 /PS 288 -b-PEG 45 -b-PS 288 compartmentalized vesicles is schematically shown in Figure 5D. That is, (1) the majority of the PS block in PEG45-b-PS as well as few of PS-bPEG45-b-PS forms the membrane of compartmentalized vesicles, and the linear PEG chains as well as few of the looped PEG chains stabilize the vesicles in the solvent; (2) the majority of PS-b-PEG45-b-PS as well as few of PEG45-b-PS forms the porous cavity; and (3) the PEG block is swollen in the holes of the cavity of the compartmentalized vesicles. 3.3. Effect of DP of the PS Block on Morphology of AB/BAB Self-Assembled Blends. For nanoassemblies of individual block copolymers, their morphology could be tuned by the hydrophobic/hydrophilic ratio as discussed above.9 In the present AB/BAB self-assembled blends with fixed molar ratio of PEG45-b-PS/PS-b-PEG45-b-PS at 6/2, the morphology can also be tuned by DP of the PS block in the di- and triblock copolymers. As depicted in Figure 6 and Figure S6, with

Figure 6. TEM images of PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends prepared via dispersion RAFT polymerization under [St]0:[PEG45-TTC]:[TTC-PEG45-TTC]0:[AIBN]0 = m:3/4:1/ 4:1/3 with target DP (m) of the PS block at 100 (A), 200 (B), 300 (C), 400 (D), 500 (E), and 600 (F).

increasing DP of the PS block from 93 to 571 (see GPC analysis of these blends in Figure S7), the AB/BAB selfassembled blends change from the small-sized vesicles with average size at 110 ± 30 nm (Figure 6A) to the large-sized vesicles with average size at 500 ± 200 nm (Figure 6B), to the compartmentalized vesicles with average size at 310 ± 40 nm (Figure 6C), to the cavity-lowly filled vesicles (Figure 6D), to the cavity-highly filled vesicles (Figure 6E), and finally to the F


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Figure 7. TEM images of PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends prepared via dispersion RAFT polymerization under [St]0:[PEG45TTC]0:[TTC-PEG45-TTC]0:[AIBN]0 = 300:3/4:1/4:1/3 with monomer/solvent weight ratio at 10% (A), 15% (B), 20% (C), and 25% (D).

thus changes inherent molecular curvature of the in situ synthesized block copolymers and therefore leads to different morphologies of the AB/BAB self-assembled blends in the present dispersion RAFT polymerization. The TEM images of PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends synthesized under fixed ratio of [St]0:[PEG45TTC]0:[TTC-PEG45-TTC]0:[AIBN]0 = 300:3/4:1/4:1/3 and with 10−25% monomer concentration are depicted in Figure 7. It should be noted that the polymerization rate of the dispersion RAFT polymerization with different monomer concentration was slightly different, and therefore these dispersion RAFT polymerizations were quenched at slightly different time of 24−30 h with the similarly high monomer conversion at 94.4−96.0% to ensure the PS block having a similar DP in the PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends (Figure S8). As summarized in Figure 7, the mixture of the compartmentalized vesicles with genus at about 20 (1), the entrapped vesicles (2 and 3), and the general vesicles (4), all of which have the similar membrane thickness at 29 ± 2 nm, are formed at the 10 wt % monomer concentration (Figure 7A and Figure S9A); at 15 wt % monomer concentration, the 410 ± 60 nm compartmentalized vesicles with genus at about 150 are formed (Figure 7B); at 20 wt % monomer concentration, the mixture of the multilayer-bicontinuous joints (5), the bicontinuous nanospheres (6), the cavity-highly filled vesicles with a bicontinuous membrane (7), and porous nanospheres (8) are formed (Figure 7C and Figure S9B); and at 25 wt % monomer concentration, bicontinuous nanospheres are formed (Figure 7D, and see the SEM image in Figure S10). Herein, due to the complex morphologies of the PEG45-b-PS/PS-b-PEG45b-PS self-assembled blends, we cannot afford a clear explanation on why and how they are formed, which is just being pursued in our lab. Despite this, the present results as well as those mentioned above suggest that the present dispersion polymerization is an efficient approach to synthesize self-assembled block copolymer blends with ingenious structure and morphology. 3.5. Computer Simulations of the Self-Assembly of AB/BAB Blends. Typical morphologies obtained for the blends as a function of the chain number ratio of diblock/ triblock copolymers, α, are presented in Figure 8. It is noted

Figure 8. Typical morphologies for the self-assembled blends of A2B8/ B8A2B8 as a function of the diblock/triblock chain number ratio α. (A) α = 6:0, (B) α = 6:2.0, (C) α = 6:2.1, (D) α = 6:2.5, (E) α = 6:3, (F) α = 6:3.5, (G) α = 6:4, and (H) α = 0:6.

that a pure AB diblock copolymer system results in formation of vesicles (Figure 8A). With the increase of the amount of the BAB triblock copolymer in the system, the vesicles deform their shape to ellipsoidal vesicles when α = 6:2.0 (Figure 8B), and when α = 6:2.1, a small vesicle occurs inside the shell of the big vesicles to form 2-compartment vesicles (Figure 8C). With the further increase of the BAB triblock copolymer in the system, 2-, 3-, and 4-compartment vesicles are formed sequentially (Figures 8D−G). It is also noted that the number of solvent compartments increases whereas the size of each solvent compartment decreases with the increase of the amount of BAB triblock copolymers in the blend. It is noteworthy that the sequence of morphologies shown in Figures 8A−G is similar to that observed experimentally, as shown in Figure 2, considering that the porous nanospheres observed at a high PS-b-PEG-b-PS content (Figure 2F) are similar to the multicompartment vesicles with small-sized solvent compartments. These results indicate that the experimentally observed morphologies are or are close to thermodynamically stable structures. On the other hand, a pure BAB triblock copolymer system (Figure 8H, α = 0:6) leads to the formation of multicompartment vesicles with more solvent compartments than those in the blend systems. However, such multicompartment vesicles cannot be formed from pure PS-b-PEG-b-PS as discussed elsewhere,45 which may G


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experimental results show no signs of the secondary aggregation, and our simulations support that the multicompartment vesicles are or are close to stable structures. With increase of the B block length, the sequence of morphologies from spherical micelles to rod-like micelles and vesicles, as shown in Figure 9, is the same as that obtained from pure diblock copolymer solution when the volume fraction of the solvophilic block decreases.1 The formation mechanism of spherical micelles, rod-like micelles, and vesicles should be similar to that in the pure diblock copolymer solution. Our further simulation result shows that for the pure diblock copolymers even with n = 16, that is, A2B16, the morphology remains vesicles. The volume fraction fA of the pure diblock copolymers A2B16 is much smaller than that in the blends A2B8/ B8A2B8 considered in our simulations. Therefore, the decrease of the volume fraction fA is excluded as the main factor of forming multicompartment vesicles. The formation of multicompartment vesicles should be mainly due to the chain architecture and conformation of the BAB triblock copolymer. It has been demonstrated that the microdomain structures of BAB (or ABA) type symmetrical triblock copolymers in bulk are almost identical to those of AB diblock copolymers, produced through cutting BAB (or ABA) in half.80,81 Hence, the thickness of the solvophobic shell in a vesicle formed by the AnBm/BmAnBm blends is expected to be nearly identical to that formed by AnBm diblock copolymers if vesicles remain in both systems. In a vesicle structure, the volume of the solvophobic shell of the B block (B-shell) should be proportional to the number of the solvophobic repeated units in the system, and the surface area (including both inside and outside surfaces) of the B-shell should be approximately proportional to the number of the solvophilic repeated units of the A block in the system. For AB/BAB blends shown in Figure 8, the number of total chains is fixed; hence, the number of solvophilic repeated units keeps constant whereas the number of solvophobic repeated units increases when the amount of BAB triblock copolymer increases. This means that with increasing the amount of BAB triblock copolymer in AB/BAB blends, the volume of B-shell should become larger whereas the thickness of the B-shell remains unchanged if a vesicle structure remains. This results in that the surface area of the B-shell becomes larger with increasing the amount of BAB in the blends. In this case, however, the amount of the solvophilic A repeated unit keeps unchanged. Therefore, the traditional vesicle structure becomes unstable with increasing the amount of the BAB triblock copolymer in the AB/BAB blends. In such a case, the increased B-shell constitutes partitioning walls, which spans the internal solvent space of a vesicle, to form multicompartment vesicles. Figure S11 shows the chain number ratio of the inside (or outside) triblock chains out of the total inside (or outside) chains, rtri,in (or rtri,out), as a function of α. It is noted that both curves of rtri,in and rtri,out increase when the amount of the triblock copolymer chains in the blends increases, while rtri,in is much larger than rtri,out. This result suggests that more BAB triblock copolymer chains are distributed inside than on the outside, whereas more AB diblock copolymer chains are distributed on outside of a vesicle or multicompartment vesicle as shown in Scheme 1 and Figure 5D. From the radical density profiles shown in Figure S12, it is noted there are more BAB triblock copolymers locating inside the B-shell than the AB diblock copolymers. Both Figures S11 and S12 suggest that the BAB triblock copolymers prefer to distribute inside a vesicle or

be attributed to large volume fraction of the solvophobic PS block in the samples, so that the system is trapped to be kinetically frozen, instead of thermodynamically stable. Figure 9 shows typical morphologies obtained for the blends of A2Bn/BnA2Bn as a function of the solvophobic block length n

Figure 9. Typical morphologies for the self-assembled blends of A2Bn/ BnA2Bn as a function of the block length n at a fixed molar ratio of AB/ BAB block copolymer of α = 6:3. (A) n = 1, (B) n = 2, (C) n = 3, (D) n = 4−6, (E) n = 7, (F) n = 8, (G) n = 10, and (H) n = 12.

at a fixed AB/BAB ratio of α = 6:3. It is noted that when n = 1, spherical micelles are formed (Figure 9A). When n = 2, the mixture of spherical micelles and rod-like micelles are produced (Figure 9B). When n = 3, bilayer lamellae are formed (Figure 9C). When n = 4−6, vesicles are formed (Figure 9D). When n = 7−12, multicompartment vesicles are formed (Figures 9E− H). It is noted that the sequence of morphologies depicted in Figure 9 is similar to that observed experimentally (Figure 4). As the simulated annealing method, used in our simulation, is a well-known procedure for obtaining the lowest-energy “ground states” in complex systems,79 similar morphologies from simulations (Figures 8 and 9) and experiments (Figures 2 and 4) demonstrate that the multicompartment vesicles observed in our experiments are close to the thermodynamically stable structures. Herein, possible reason is discussed. In the present dispersion RAFT polymerization, the PS block of the AB and BAB di- and triblock copolymers has a glass transition temperature (Tg) higher than polymerization temperature, and therefore PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends should be kinetically frozen in the polymerization solvent. However, the following three facts rescue the PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends from kinetically frozen. The first is because of the excess of styrene monomer, which can plasticize the solvophobic PS block and therefore helps to shift the PEG45-b-PS/PS-b-PEG45-b-PS selfassembled blends to thermodynamic equilibrium. The second is because of the gradual increase of the PS block with proceeding of the RAFT polymerization, which affords time or possibility for the PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends to tune their morphology with growth of the PS block. The third is because of PEG45-b-PS/PS-b-PEG45-b-PS self-assembled blends being dispersed in the alcohol-rich solvent at relatively high temperature of 70 °C, which also helps to plasticize the solvophobic PS block and thus to form the self-assembled blends close to thermodynamic equilibrium. The multicompartment vesicles observed in our experiments and simulations appear to be a new morphology for block copolymer aggregates. They should be quite different from the large compound vesicles formed from diblock copolymer solution by the addition of ions.16,17 The large compound vesicles were thought as involving the secondary aggregation of individual vesicles and a subsequent fusion process. Our H


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(3) Holder, S. J.; Sommerdijk, N. A. J. M. New micellar morphologies from amphiphilic block copolymers: disks, toroids and bicontinuous micelles. Polym. Chem. 2011, 2, 1018−1028. (4) Blanazs, A.; Armes, S. P.; Ryan, A. J. Self-assembled block copolymer aggregates: from micelles to vesicles and their biological applications. Macromol. Rapid Commun. 2009, 30, 267−277. (5) Du, J.; O’Reilly, R. K. Advances and challenges in smart and functional polymer vesicles. Soft Matter 2009, 5, 3544−3561. (6) Derry, M. J.; Fielding, L. A.; Armes, S. P. Polymerization-induced self-assembly of block copolymer nanoparticles via RAFT non-aqueous dispersion polymerization. Prog. Polym. Sci. 2016, 52, 1−18. (7) Sun, J.-T.; Hong, C.-Y.; Pan, C.-Y. Recent advances in RAFT dispersion polymerization for preparation of block copolymer aggregates. Polym. Chem. 2013, 4, 873−881. (8) Rieger, J. Guidelines for the synthesis of block copolymer particles of various morphologies by RAFT dispersion polymerization. Macromol. Rapid Commun. 2015, 36, 1458−1471. (9) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Elsevier: Amsterdam, 2011; pp 536, 539. (10) Zhao, W.; Chen, D.; Hu, Y.; Grason, G. M.; Russell, T. P. ABC triblock copolymer vesicles with mesh-like morphology. ACS Nano 2011, 5, 486−492. (11) Haluska, C. K.; Gózd́ ź, W. T.; Döbereiner, H.-G.; Förster, S.; Gompper, G. Giant hexagonal superstructures in diblock-copolymer membranes. Phys. Rev. Lett. 2002, 89, 238302. (12) Zhu, H.; Geng, Q.; Chen, W.; Zhu, Y.; Chen, J.; Du, J. Antibacterial high-genus polymer vesicle as an “armed” drug carrier. J. Mater. Chem. B 2013, 1, 5496−5504. (13) Pietsch, C.; Mansfeld, U.; Guerrero-Sanchez, C.; Hoeppener, S.; Vollrath, A.; Wagner, M.; Hoogenboom, R.; Saubern, S.; Thang, S. H.; Becer, C. R.; Chiefari, J.; Schubert, U. S. Thermo-induced selfassembly of responsive poly(DMAEMA-b-DEGMA) block copolymers into multi- and unilamellar vesicles. Macromolecules 2012, 45, 9292− 9302. (14) Zhang, W.-J.; Hong, C.-Y.; Pan, C.-Y. Fabrication of spaced concentric vesicles and polymerizations in RAFT dispersion polymerization. Macromolecules 2014, 47, 1664−1671. (15) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Ordered structures of block copolymer/homopolymer mixtures. 5. Interplay of macro- and microphase transitions. Macromolecules 1994, 27, 6532−6540. (16) Zhang, L.; Yu, K.; Eisenberg, A. Ion-induced morphological changes in “crew-cut” aggregates of amphiphilic block copolymers. Science 1996, 272, 1777−1779. (17) Zhang, L.; Eisenberg, A. Morphogenic effect of added ions on crew-cut aggregates of polystyrene-b-poly(acrylic acid) block copolymers in solutions. Macromolecules 1996, 29, 8805−8815. (18) Zhao, W.; Gody, G.; Dong, S.; Zetterlund, P. B.; Perrier, S. Optimization of the RAFT polymerization conditions for the in situ formation of nano-objects via dispersion polymerization in alcoholic medium. Polym. Chem. 2014, 5, 6990−7003. (19) Li, S.; He, X.; Li, Q.; Shi, P.; Zhang, W. Synthesis of multicompartment nanoparticles of block copolymer through two macro-RAFT agents co-mediated dispersion polymerization. ACS Macro Lett. 2014, 3, 916−921. (20) Shi, P.; Li, Q.; He, X.; Li, S.; Sun, P.; Zhang, W. A new strategy to synthesize temperature- and pH-sensitive multicompartment block copolymer nanoparticles by two macro-RAFT agents comediated dispersion polymerization. Macromolecules 2014, 47, 7442−7452. (21) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Control of structure in multicompartment micelles by blending μ-ABC star terpolymers with AB diblock copolymers. Macromolecules 2006, 39, 765−771. (22) Zhu, J.; Zhang, S.; Zhang, K.; Wang, X.; Mays, J. W.; Wooley, K. L.; Pochan, D. J. Disk-cylinder and disk-sphere nanoparticles via a block copolymer blend solution construction. Nat. Commun. 2013, 4, 1−7. (23) Marsat, J.-N.; Heydenreich, M.; Kleinpeter, E.; Berlepsch, H.; Böttcher, C.; Laschewsky, A. Self-assembly into multicompartment micelles and selective solubilization by hydrophilic-lipophilic-fluorophilic block copolymers. Macromolecules 2011, 44, 2092−2105.

a multicompartment vesicle. As the formation of multicompartment vesicles is due to the existence of the BAB triblock copolymers, and in our experiments, the chain is growing from the end of the B block in the AB and BAB block copolymers; the above result can explain why the increased B-shell forms partitioning walls inside the vesicle instead of outside the vesicles.

4. CONCLUSIONS In this contribution, a strategy combining the advantages of polymer blending and polymerization-induced self-assembly is proposed to prepare the AB/BAB self-assembled blends of poly(ethylene glycol)-b-polystyrene/polystyrene-b-poly(ethylene glycol)-b-polystyrene (PEG-b-PS/PS-b-PEG-b-PS). The present PEG-b-PS/PS-b-PEG-b-PS self-assembled blends exhibit various morphologies such as lamellae, high-genus compartmentalized vesicles, multilayer and bicontinuous nanoassemblies, and porous nanospheres. The parameters, such as PEG-b-PS/PS-b-PEG-b-PS molar ratio, DP of the PS block, and fed monomer concentration, affecting structure/morphology of PEG-b-PS/PS-b-PEG-b-PS self-assembled blends were revealed. The formation of the typical high-genus compartmentalized vesicles was investigated, and it was suggested that the formation of the vesicle membrane following the procedures similar to those in the general bilayer vesicles and the formation of the sponge-like compartmentalized cavity were involved in the formation of the high-genus compartmentalized vesicles. The formation mechanism of compartmentalized vesicles is investigated based on computer simulations of the self-assembly of AB/BAB blends. Our results demonstrate a valid and in situ preparation of the self-assembled block copolymer blends with ingenious structure and morphology, and some of which, such as the high-genus compartmentalized vesicles, are expected to be new block copolymer morphology and may have potential application.

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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.macromol.6b00771. Experimental details and Figures S1−S12 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (B.L.). *E-mail [email protected] (W.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support by National Science Foundation for Distinguished Young Scholars (No. 21525419) and National Science Foundation of China (Nos. 21274066, 21474054, 21574071, and 91227121) and PCSIRT (IRT1257).

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REFERENCES

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BAB Block Copolymers Prepared

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