Disassembly of Block Copolymer Vesicles into Nanospheres through

Nov 26, 2014 - Block copolymer vesicles have aroused great interest, whereas how vesicles .... Matthew J. Derry , Oleksandr O. Mykhaylyk , Anthony J. ...
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Disassembly of Block Copolymer Vesicles into Nanospheres through Vesicle Mediated RAFT Polymerization Fei Huo,† Shentong Li,† Xin He,† Sayyar Ali Shah,‡ Quanlong Li,† and Wangqing Zhang*,† †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China ‡ Department of Chemistry, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: Block copolymer vesicles have aroused great interest, whereas how vesicles being formed is not well clarified. In this contribution, the vesicle mediated RAFT polymerization was performed, and the disassembly of block copolymer vesicles during the RAFT polymerization was investigated. It was found that when a solvophilic block of poly(N,N-dimethylacrylamide) (PDMA) was introduced into the vesicles of the poly[N-(4-vinylbenzyl)-N,N-diethylamine]block-polystyrene (PVEA-b-PS) diblock copolymer, the PVEAb-PS vesicles were gradually disassembled. With the increasing polymerization degree of the PDMA block, vesicles were first flattened to form tubules, then tubules were broken to form the jellyfish-like morphology, and last jellyfish converted into worms and worms minced into nanospheres of poly[N-(4-vinylbenzyl)-N,N-diethylamine]-block-polystyrene-block-poly(dimethylacrylamide) (PVEA-b-PS-b-PDMA). It is believed that the present vesicle mediated RAFT polymerization affords a real-time observation of vesicle disassembly, which is helpful to clarify the assembly of block copolymer vesicles.

1. INTRODUCTION

Despite the widely documented successful preparation of AB or ABC block copolymer vesicles,9−31 little is known on how vesicles are formed. Now, it is 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 are involved in the vesicle formation.32−38 This mechanism seems sound for vesicle formation at general conditions, whereas it fails to explain complex vesicular morphologies such as multilayered vesicles and vesicles partly filled with a network of channels and islands.39 He and Schmid assumed that vesicles could be formed via the nucleation of hydrophobic molecules in spherical micelles, and they suggested that closing bilayer membrane was not an exclusive pathway for vesicle formation.40,41 Up to now, the mechanism of block copolymer vesicle formation is generally based on simulation or assumption,36−41 and rare or no experimental results are available.32−35 The possible reason is due to the fast micellization of amphiphilic block copolymers in the block selective solvent, which makes great difficulty to check the vesicle formation by real-time monitoring. Besides, it is also difficult to monitor the intermediate morphology during the vesicle formation, since the block copolymer chains are generally frozen in the block selective solvent due to the relatively high molecular weight of block copolymers compared with the small surfactant molecules.35,42,43 In the pioneering studies by Chen and Eisenberg,32 the kinetics of block

1

Of all the block copolymer nanoassemblies, vesicles have become more attractive in recent years due to their special structure and potential applications in many fields.2,3 Similarly with the phospholipid vesicles and cells, vesicles of amphiphilic block copolymer have an enclosed bilayer structure, in which the solvophobic block forms the middle-layer and the solvophilic block locates at both the inner and the outer sides.2 It is generally deemed that the morphology of the amphiphilic block copolymer nanoassemblies is determined mainly by the block copolymer composition, e.g., the ratio of the solvophilic block to the solvophobic block.4−28 For AB diblock copolymer nanoassemblies in the block-selective solvent for the A block (note: A represents the solvophilic block and B represents the solvophobic block through this article), the nanoassembly morphology generally follows the evolution of spheres-to-worms-to-vesicles with the decreasing solvophilic/solvophobic ratio.4−24 That is, vesicles of AB diblock copolymers are usually formed at low solvophilic/ solvophobic ratio when the AB diblock copolymer contains a short solvophilic A block and a long solvophobic B block.9−24 For vesicles of ABC triblock terpolymer containing a solvophobic central B block and two solvophilic A and C blocks,29−31 their vesicles are identified by their unsymmetrical structure, in which the solvophilic block with short chain length or with low steric repulsion is generally located at the inner surface of the bilayer structure, and the long solvophilic block is on the outer surface. © XXXX American Chemical Society

Received: October 16, 2014

A

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and the vesicles mediated RAFT polymerization is anticipated to be a valid method to prepare triblock terpolymer nanoobjects.

copolymer micellization is monitored by adjusting the rate of adding block selective solvent. Jiang and co-workers further investigated the kinetics of vesicle formation of ABA amphiphilic triblock copolymer by adding the block selective solvent into the homogeneous solution of the ABA triblock copolymer.33 In 30 days of monitoring, it was found that the vesicle formation depended greatly on the addition rate of the block selective solvent. Besides, some intermediate morphologies such as worms and bended membrane were detected in the vesicle formation. Recently, Winnik explored the vesicle formation by using a thermosensitive AB diblock copolymer as typical model, and they found that several metastable states such as spheres and worm-like micelles were involved in vesicle formation.34 Armes and co-workers had detected some novel intermediate diblock copolymer morphologies, e.g. worms, octopi, and jellyfish, in the worm-to-vesicle transition during the polymerization induced self-assembly.35 These abundant intermediate morphologies suggest much more subtleties than expected being involved in the vesicle formation. Seeded polymerization is a convenient method to synthesize polymeric nano-objects. 44 By employing the reversible addition−fragmentation chain transfer (RAFT) polymerization technology in seeded polymerization, block copolymer nanoobjects can be in situ prepared.45−54 Recently, we have employed this seeded RAFT polymerization to prepare various block copolymer nano-objects with targeted morphology.50−54 In these seeded RAFT polymerizations, the core−corona nanoparticles of AB diblock copolymer including a suitable RAFT terminal were used as seeded nucleus. During the seeded RAFT polymerization, the newly formed C block extends with monomer conversion, and ABC triblock terpolymer nanoobjects form; the morphology of the ABC triblock terpolymer nano-objects changes with the increasing DP of the newly formed C block. Since the DP of the newly formed C block increases gradually with the monomer conversion, the morphology of the in situ synthesized ABC triblock terpolymer nano-objects at different polymerization time can be easily monitored by real-time observation with transmission electron microscope (TEM). In this contribution, to detect how block copolymer vesicles being formed, the disassembly of vesicles, which is just the inverted procedure of the vesicle formation, by seeded RAFT polymerization was performed, and the intermediate morphologies of the in situ synthesized block copolymer nano-objects during the vesicle disassembly were checked by TEM. Initially, the model vesicles of poly[(N-(4-vinylbenzyl)-N,N-diethylamine)]-block-polystyrene (PVEA-b-PS) were in situ prepared by the macro-RAFT agent mediated dispersion polymerization as discussed elsewhere.18−23 Then, the vesicle mediated RAFT polymerization of N,N-dimethylacrylamide (DMA) was performed, and the in situ synthesized triblock terpolymer nano-objects of poly[(N-(4-vinylbenzyl)-N,N-diethylamine)]block-polystyrene-block-poly(N,N-dimethylacrylamide) (PVEAb-PS-b-PDMA) were checked by TEM. The introduction of the poly(N,N-dimethylacrylamide) (PDMA) block into the PVEAb-PS vesicles is due to the good solubility of the newly formed PDMA block in the polymerization medium, which is essential for the disassembly of the PVEA-b-PS vesicles. It was found that, during the vesicle mediated RAFT polymerization, the PVEA-b-PS vesicles initially stretched into distorted tubules, then tubules were broken to form the jellyfish-like morphology, and last jellyfish converted into worms and nanospheres. We believe these results afford new insight for vesicle formation,

2. EXPERIMENTAL SECTION 2.1. Materials. The monomer of N-(4-vinylbenzyl)-N,N-diethylamine (VEA) was synthesized by the nucleophilic substitution reaction of chloromethylstyrene (CMS) with the secondary amine of diethylamine (DEA) as discussed elsewhere.55 The monomers of DMA (99.5%, Alfa) and styrene (St, >98%, Tianjin Chemical Company) were distilled under reduced pressure prior to use. 4Cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid (CDTPA) was synthesized as discussed elsewhere,56 and its 1H NMR spectra are shown in Figure S1. The initiator of 2,2′azobis(isobutyronitrile) (AIBN, >98%, Tianjin Ruijinte Chemical Reagent Co.) was recrystallized from alcohol and dried under vacuum. The internal standard of 1,3,5-trioxane (98%, Alfa) for the 1H NMR analysis, and other reagents were analytic grade and were used as received. 2.2. Synthesis of the PVEA-b-PS Vesicles. The PVEA-b-PS vesicles were prepared by the macro-RAFT agent mediated dispersion polymerization, in which (1) the synthesis of poly[N-(4-vinylbenzyl)N,N-diethylamine] trithiocarbonate [PVEA68-TTC, herein and in the subsequent discussion the subscript represents the polymerization degree (DP) of the monomer and TTC represents the RAFT terminal of trithiocarbonate] by solution RAFT polymerization and (2) the PVEA-TTC macro-RAFT agent mediated dispersion polymerization to prepare the PVEA-b-PS vesicles are included. Into a 100 mL Schlenk flask with a magnetic bar, VEA (30.00 g, 0.158 mol), CDTPA (0.532 g, 1.321 mmol), and AIBN (72.3 mg, 0.440 mmol) dissolved in 1,4-dioxane (10.00 g) were added. The solution was initially degassed with nitrogen at 0 °C, and then the flask content was immersed into preheated oil bath at 70 °C for 20 h. The polymerization was quenched by rapid cooling upon immersion of the flask in iced water, in which the monomer conversion at 56.8% was determined by 1H NMR analysis in the presence of the internal standard of 1,3,5-trioxane following eq S1. The synthesized PVEA68TTC was precipitated into the ethanol/water mixture (6/5 by weight) and then dried at room temperature under vacuum for the next use. Into a 100 mL Schlenk flask with a magnetic bar, PVEA68-TTC (3.64 g, 0.274 mmol), St (10.00 g, 96.0 mmol), and AIBN (15.0 mg, 0.091 mmol) dissolved in the ethanol/water mixture (40.92 g, 95/5 by weight) were added. The oxygen dissolved in the solution was removed by nitrogen purging, and then the polymerization was performed at 70 °C for 20 h. The polymerization was quenched by rapid cooling upon immersion of the flask in iced water, and the monomer conversion at 65.9% was detected by UV−vis analysis as discussed elsewhere.53 The colloidal dispersion of the in situ synthesized PVEA-b-PS vesicles was dialyzed against the ethanol/ water mixture (95/5 by weight) at room temperature for 3 days (molecular weight cutoff: 7000 Da) to remove the residual monomer in the vesicle dispersion and then diluted with the ethanol/water mixture (95/5 by weight) to afford 10.0 wt % dispersion of the PVEAb-PS vesicles for the next use. 2.3. Vesicle Mediated RAFT Polymerization. The vesicle mediated RAFT polymerization was performed under [DMA]0: [PVEA-b-PS-TTC]0:[AIBN]0 = 600:3:1 and with the weight percent of the fed monomer plus the PVEA-b-PS-TTC vesicles at 12%. Typically, into the freshly prepared dispersion of the PVEA68-b-PS231TTC vesicles (5.65 g, containing 0.565 g or 0.0151 mmol of PVEA68b-PS231-TTC and 5.04 g of the 95/5 ethanol/water mixture), DMA (0.300 g, 3.026 mmol), and AIBN (0.83 mg, 0.0050 mmol) dissolved in the 95/5 ethanol/water mixture (1.24 g) were added. The flask content was vigorously stirred, degassed with nitrogen at 0 °C, and then the polymerization was initiated by immersing the flask into preheated oil bath at 70 °C. After a given time, the polymerization was quenched, and the monomer conversion of DMA was determined by 1 H NMR analysis employing 1,3,5-trioxane as the internal standard, and the synthesized PVEA-b-PS-b-PDMA nano-objects dispersed in B

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the polymerization medium of the 95/5 ethanol/water mixture were checked by TEM. To collect the polymer for GPC analysis and 1H NMR analysis, the synthesized PVEA-b-PS-b-PDMA nano-objects were separated by centrifugation, washed twice with ethanol/water (50/50 by weight), and finally dried at room temperature under vacuum to afford the PVEA-b-PS-b-PDMA triblock terpolymer in pale yellow. 2.4. Characterization. The 1H NMR analysis was performed on a Bruker Avance III 400 MHz NMR spectrometer using CDCl3 as solvent. To obtain the molecular weight and its distribution or the polydispersity index (PDI, PDI = Mw/Mn) of the synthesized polymers, the GPC analysis was performed on a Waters 600E GPC system equipped with the TSK-GEL columns and a Waters 2414 refractive index detector, where THF was used as eluent at flow rate of 1.0 mL/min at 30.0 °C and the narrow-polydispersity polystyrene was used as calibration standard. The UV−vis analysis was performed on a Varian 100 UV−vis spectrophotometer. The TEM observation was performed using a Tecnai G2 F20 electron microscope at an acceleration of 200 kV. To observe the morphology of the in situ synthesized block copolymer nano-objects dispersed in the polymerization medium of the 95/5 ethanol/water mixture, a drop of the colloidal dispersion (0.02 mL) was initially added into water (1 mL) at room temperature to freeze the block copolymer nano-objects; then a small drop of the dispersion of the diblock copolymer nano-objects was dripped onto a piece of copper grid until the solvent was evaporated at reduced atmosphere pressure and last detected by TEM.

Figure 1. Dispersion of the PVEA68-b-PS231-TTC vesicles in the 95/5 ethanol/water mixture (A), the TEM image (B), and the schematic structure (C) of the PVEA68-b-PS231-TTC vesicles.

dispersion polymerization at 65.9% monomer conversion in 20 h. From the optical image, it is concluded that the in situ synthesized PVEA68-b-PS231-TTC vesicles with block copolymer concentration at 10.0 wt % are uniformly distributed in the polymerization medium, which is essential for the vesicle mediated RAFT polymerization. Figure 1B shows the TEM images of the PVEA68-b-PS231-TTC vesicles, from which vesicles with size from 320 to 380 nm are observed. The average wall thickness of the PVEA68-b-PS231-TTC vesicles, 45 nm, which is calculated by statistical analysis of above 100 vesicles, is approximately 2 times the maximum length Lmax of the extended molecular chain of the PS231 block, 29 nm. Therefore, the structure of the PVEA68-b-PS231-TTC vesicles as shown in Figure 1C and Scheme 1 is concluded. The PVEA68-

3. RESULTS AND DISCUSSION 3.1. Synthesis of the PVEA-b-PS Vesicles. Recently, the macro-RAFT agent mediated dispersion polymerization has been demonstrated to be a valid method to in situ synthesize block copolymer nano-objects with block copolymer concentration up to 30 wt %.18−23 Herein, this macro-RAFT agent mediated dispersion polymerization is employed to prepare the PVEA-b-PS vesicles. Initially, the PVEA68-TTC macro-RAFT agent was first prepared by solution RAFT polymerization at 56.8% monomer conversion. The 1H NMR analysis (Figures S1 and S2) and GPC analysis (Figure S3) show the molecular weight of the PVEA68-TTC macro-RAFT agent at Mn,NMR = 12.5 kg/mol and Mn,GPC = 10.1 kg/mol with the molecular weight distribution index at PDI = 1.25, in which Mn,NMR of PVEA68-TTC is determined by eq S2. Subsequently, the PVEA68-TTC macro-RAFT agent mediated dispersion polymerization of styrene in the 95/5 ethanol/water mixture under [St]0:[PVEA68-TTC]0:[AIBN]0 = 1050:3:1 was performed. With the proceeding of the RAFT polymerization, the transparent solution, in which the styrene monomer, the AIBN initiator, and the PVEA68-TTC macro-RAFT agent were molecularly soluble, became bluish, indicating formation of block copolymer nano-objects at 12.3% monomer conversion in 9 h and last became milky when the dispersion RAFT polymerization was quenched in 30 h with the monomer conversion at 89.1%. It was found that the PVEA68-TTC macro-RAFT agent mediated dispersion polymerization of styrene afforded vesicles of the PVEA-b-PS diblock copolymer just at monomer conversion higher than 56.8%. Below this 56.8% monomer conversion, nanospheres or worms of the PVEA-b-PS diblock copolymer were formed (Figure S4). Clearly, this evolution of the PVEA-b-PS morphology with the extension of the PS block in the dispersion RAFT polymerization is similar as those reported previously.20−23 Figure 1A shows the optical image of the PVEA68-b-PS231-TTC vesicles dispersed in the polymerization medium of the 95/5 ethanol/water mixture, which were prepared by the PVEA68-TTC macro-RAFT agent mediated

Scheme 1. Vesicle Mediated RAFT Polymerization

b-PS231-TTC diblock copolymer was characterized by 1H NMR analysis (Figure S1) and GPC analysis (Figure S3). It is found that the molecular weight of the PVEA68-b-PS231-TTC diblock copolymer, Mn,GPC by GPC analysis at 32.0 kg/mol, Mn,NMR by 1 H NMR analysis at 38.2 kg/mol, and the theoretical molecular weight Mn,th = 37.3 kg/mol, are close to each other, and the molecular weight is narrowly distributed as indicated by the low PDI at 1.07. Note: Mn,NMR of PVEA68-b-PS231-TTC is calculated by comparing area ratio of the proton signal with the characteristic chemical shift at δ = 3.50 ppm corresponding C

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Figure 2. Monomer conversion−time plot and the ln([M]0/[M])−time plot (A) for the PVEA68-b-PS231-TTC vesicle mediated RAFT polymerization, the molecular weight and PDI (B), the 1H NMR spectra (C), and the GPC traces (D) of the synthesized PVEA-b-PS-b-PDMA triblock terpolymers. Polymerization conditions: DMA (0.300 g, 3.026 mmol), the ethanol/water mixture (6.34 g, 95/5 by weight), [DMA]0/ [PVEA-b-PS]0/[AIBN]0 = 600/3/1, 70 °C.

to the methylene group in the PVEA block and at δ = 7.22− 6.26 ppm in the PS and PVEA blocks following eq S3, and Mn,th is determined by the monomer conversion following eq 1 as discussed elsewhere,50 respectively. M n,th =

TTC vesicles gradually convert into the PVEA-b-PS-b-PDMA triblock terpolymer nano-objects as shown in Scheme 1. Since the present study is focused on the vesicle disassembly, therefore the solvophilic PDMA block but not a solvophobic block is inserted into the PVEA-b-PS-TTC vesicles to form the PVEA-b-PS-b-PDMA triblock terpolymer nano-objects, in which the two solvophilic blocks of PVEA and PDMA are soluble in the polymerization medium of the 95/5 ethanol/ water mixture whereas the solvophobic central block of PS forms the body of the triblock terpolymer nano-objects. With the increasing monomer conversion, it is optically observed that the milky colloidal dispersion becomes clearer and clearer, and last bluish colloidal dispersion is observed at the end of the polymerization. The polymerization kinetics of the PVEA68-b-PS231-TTC vesicle mediated RAFT polymerization are summarized in Figure 2A. The monomer conversion increases with the polymerization time and finally reaches at 90.1% in 20 h. The further increase in the polymerization time just leads to a very slight increase in the monomer conversion. From the linear ln([M]0/[M])−time plot shown in Figure 2A, the pseudo-firstorder kinetics of the PVEA68-b-PS231-TTC vesicle mediated RAFT polymerization just like a general homogeneous RAFT polymerization is concluded.58 The PVEA-b-PS-b-PDMA

[monomer]0 × conversion + M n,macro‐RAFT [macro‐RAFT]0 (1)

3.2. Vesicle Mediated RAFT Polymerization. The PVEA-b-PS-TTC vesicle mediated RAFT polymerization of DMA was carried out in the 95/5 ethanol/water mixture under [DMA]0:[PVEA-b-PS-TTC]0:[AIBN]0 = 600:3:1. In the 95/5 ethanol/water mixture, both the fed DMA monomer and the newly formed PDMA block are soluble, and therefore the present vesicle mediated RAFT polymerization looks somewhat similarly with the homogeneous RAFT polymerization, although the heterogeneous colloidal dispersion is clearly observed by the naked eye during the whole stage of the RAFT polymerization. With the proceeding of the RAFT polymerization, the solvophilic PDMA block with the Z-group at the outside extends with monomer conversion (note: Z-group is an activating group in the RAFT agent connected with the CS bond in a dithiocarbonate or with the C−S bond in a trithiocarbonate as discussed in ref 57), and the PVEA-b-PSD

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Figure 3. TEM images of the synthesized PVEA-b-PS-b-PDMA triblock terpolymer nano-objects prepared at the polymerization time of 0 (A), 1 (B), 2 (C), 3 (D), 5 (E), 8 (F), 12 (G), and 20 h (H).

order of Mn,NMR > Mn,th > Mn,GPC. The slight underestimation of Mn,GPC is possibly due to the interaction of the N-containing block copolymer with the GPC columns and the hydrophobic PS standards used in the GPC analysis. It is also found that all the PVEA-b-PS-b-PDMA triblock terpolymers synthesized at different polymerization times have a narrow molecular weight distribution with PDI at 1.1−1.2, though a slight shoulder at the higher molecular weight side is detected at the case of low monomer conversion. This shoulder is possibly ascribed to the slight bimolecular radical termination in the initial stage of the RAFT polymerization, which is also observed in homogeneous RAFT polymerizations.58 All together, these results suggest that the PVEA68-b-PS231-TTC vesicle mediated RAFT polymerization undergoes a polymerization kinetics similarly with a homogeneous RAFT polymerization, and the good control both in the triblock terpolymer molecular weight and in the molecular weight distribution is achieved. 3.3. Morphology Evolution of the Block Copolymer Nano-Objects in the Vesicle Mediated RAFT Polymerization. The in situ syntheszied PVEA-b-PS-b-PDMA triblock terpolymer nano-objects in the PVEA68-b-PS231-TTC vesicle mediated RAFT polymerization at different polymerization times are checked by TEM, and the results are summarized in Figure 3. To clearly evaluate the DP of the newly formed PDMA block affecting the triblock terpolymer morphology, the

triblock terpolymers synthesized at different polymerization times are characterized by 1H NMR analysis and GPC analysis (Figure 2C,D and Figure S5). From the 1H NMR spectra shown in Figure 2C, the signal at δ = 2.91 ppm (a, as indicated by the green square corresponding to the methyl group in the PDMA block) increasing with the monomer conversion is observed, indicating the chain extension of the PDMA block in the PVEA-b-PS-b-PDMA triblock terpolymer. The molecular weight Mn,NMR of the triblock terpolymer is calculated by comparing the area ratio of the characteristic chemical shift at δ = 2.91 ppm (a) of the methyl group in the PDMA block to that of the protons of methyl at δ = 1.03 ppm (b) in the PVEA block, and the results are summarized in Figure 2B. Based on the GPC analysis, the molecular weight Mn,GPC of the PVEA-bPS-b-PDMA triblock terpolymer and its distribution index of PDI are obtained and summarized in Figure 2B. The molecular weight of the PVEA-b-PS-b-PDMA triblock terpolymer, whether Mn,NMR by 1H NMR analysis or Mn,GPC by GPC analysis, increases linearly with the monomer conversion, which is just like those in a homogeneous RAFT polymerization.58 The theoretical molecular weight Mn,th of the PVEA-b-PS-bPDMA triblock terpolymer is calculated by the monomer conversion following eq 1. It is found that the three values of the triblock terpolymer molecular weight, Mn,NMR, Mn,th, and Mn,GPC, are close to each other, and they are generally in the E

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Figure 4. Disassembly of the PVEA-b-PS vesicles into the PVEA-b-PS-b-PDMA nanospheres through the vesicle mediated RAFT polymerization. Insets: schematic structure of the block copolymer nano-objects during the disassembly of vesicles.

vesicles, the width of the worms, and the diameter of the final nanospheres of PVEA68-b-PS231-b-PDMA180 are very similar to each other (40−45 nm). This is possibly due to the body of these block copolymer nano-objects is composited of the PS block with the same DP at 231, and it also suggests that the PVEA-b-PS-b-PDMA nanospheres are originated from the disassembly of the PVEA-b-PS-TTC vesicles. Note: the solvophobic PS block in the block copolymer nano-objects is visible and the solvophilic PVEA or PDMA block is invisible in the TEM images. Clearly, the morphology evolution of vesiclesto-tubules-to-jellyfish-to-worms-to-spheres in the present vesicle disassembly is somehow reverse to the spheres-toworms-to-vesicles occurring in the vesicle formation as discussed elsewhere.4−24 The PVEA68-b-PS313-TTC vesicle mediated polymerization of styrene was also performed and the in situ synthesized PVEA-b-PS-b-PDMA triblock terpolymer nano-objects with different DP of the PDMA block were checked. As shown in Figure S7, the average size of the PVEA68-b-PS313-TTC vesicles is about 360 nm, which is close to the PVEA68-b-PS231 vesicles mentioned above. Compared with the PVEA68-b-PS231 vesicles, the PVEA68-b-PS313-TTC vesicles have a slightly thicker wall thickness (55 nm vs 45 nm) due to the large DP of the PS313 block. It is found that similar morphology evolution of vesiclesto-tubules-to-worms-to-spheres during the PVEA68-b-PS313TTC vesicle mediated polymerization is observed (Figure S7). However, due to the larger DP of the PS313 block or the thicker wall of the PVEA68-b-PS313-TTC vesicles, insertion of a longer PDMA block is needed to disassemble the PVEA68-bPS313-TTC vesicles into the PVEA-b-PS-b-PDMA nanospheres. For example, the PVEA68-b-PS231-b-PDMA180 nanospheres with a shorter PDMA180 block and the PVEA68-b-PS313-b-PDMA285 nanospheres with a longer PDMA285 block were detected in the two cases of the vesicle mediated RAFT polymerization, respectively. This is not surprised, since the PVEA-b-PS-TTC vesicles with a thick PS wall are more substantial than those

detail composition of the triblock terpolymers at different polymerization times is indicated out as insets in the TEM images. From Figure 3, it is observed that five dominated morphologies including vesicles (Figure 3A,B), tubules (Figure 3C,D), jellyfish (Figure 3E,F), and worms and spheres (Figure 3G,H) are involed in the disassembly of the PVEA68-b-PS231TTC vesicles with the increasing DP of the PDMA block during the vesicle mediated RAFT polymerization. When the solvophilic PDMA block is introduced into the PVEA68-b-PS231 vesicles, the inherent molecular curvature gradually increases, which leads to the deformation of vesicles and the vesicles-to-tubes transition, and therefore tubules are formed. Based on the average size of the vesicles shown in Figure 3A and the tubules shown in Figure 3C, the surface area of the PVEA68-b-PS231 vesicles and the PVEA68-b-PS231-bPDMA53 tubules, 3.8 × 105 and 4.4 × 105 nm2, are approximately calculated (see Figure S6 for the calculation). The similar surface area of the vesicles and the tubules suggests that the deformation of vesicles justly leads to the tubular morphology of the PVEA-b-PS-b-PDMA triblock terpolymer. By comparing Figures 3C and 3D, it is found that, with the increasing DP of the solvophilic PDMA block, part of the tubules are deformed and broken to form tube−lamellae jointed complex as shown in Figure 3E,F. With the DP of the solvophilic PDMA block further increasing, the tube−lamellae jointed complex converts into jellyfish. It is supposed that the tubule structure in the tube−lamellae jointed complex converts into the body of the jellyfish and the lamellae structure in the tube−lamellae jointed complex converts into the tentacles of the jellyfish. When DP of the solvophilic PDMA block further increases, the jellyfish converts into mixture of worms and nanospheres containing the dominated morphology of worms (Figure 3G) and last into mixture of nanospheres and worms containg the dominated morphology of 40 nm nanospheres (Figure 3H). From the TEM images shown in Figure 3, it is also observed that the wall thickness of the PVEA68-b-PS231 F

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mediated RAFT polymerization undergoes a similar polymerization dynamics with a general homogeneous RAFT polymerization as indicated by the pseudo-first-order polymerization kinetics and the linear increase in polymer molecular weight with the monomer conversion, and good control both in the polymer molecular weight and its distribution is achieved. With the increasing DP of the solvophilic PDMA block, the vesicles are first flattened to form tubules, then tubules are broken to form the jellyfish-like morphology, and last jellyfish converts into worms and worms mince into nanoparticles. It is believed that the present vesicle mediated RAFT polymerization affords a real-time observation of the disassembly of block copolymer vesicles, which is helpful to illuminate how vesicles being formed.

with a thin PS wall, and therefore insertion of a long hydrophilic PDMA block is needed to disassemble the vesicles. The direct formation of block copolymer vesicles and the block copolymer morphology transition of spheres-to-vesicles have been well-documented.9−24 Herein, the disassembly of vesicles, which is just the inverted procedure of the vesicle formation, through the vesicle mediated RAFT polymerization is investigated. Compared with the block copolymer morphology transitions including spheres-to-vesicles or vesicles-tospheres discussed previously,9−14,22−30 two differences exist in the present disassembly of vesicles. First, the starting vesicles of PVEA-b-PS-TTC were prepared by the in situ synthesis strategy of the PVEA-TTC macro-RAFT agent mediated dispersion polymerization in the ethanol-rich solvent of the 95/5 ethanol/ water mixture. Because of the gradual increase in the DP of the PS block and the ethanol-rich solvent, the in situ synthesis strategy favorably affords the near-equilibrium PVEA-b-PSTTC vesicles instead of frozen block copolymer nano-objects,1 which were usually prepared by the general self-assembly strategy through the initial dissolution of an amphiphilic block copolymer in a common solvent, then addition of a blockselective solvent, and last the removal of the common solvent usually by dialysis, although we cannot make a neat distinction between dynamically equilibrium vesicles and frozen nanoobjects of block copolymer due to the very slow exchange dynamics of block copolymer compared with small surfactants.59−61 Clearly, this is also valid in the cases of the intermediate morphologies of tubules, jellyfish, worms, and nanospheres synthesized during the vesicle mediated RAFT polymerization. Second, here a new solvophilic PDMA block is insrted into the vesicles to increase the interfacial curvature of the polymer chains to accomplish the disassembly of vesicles into nanospheres, which is different from the morphology transition by tuning the solvent character such as solvent composition, pH, and temperature or by changing the DP of the solvophilic/solvophobic block.9−14,22−30 Our finding on the block copolymer morphology in the vesicle disassembly is summarized in Figure 4A−G. That is, during the disassembly of vesicles, vesicles are first flattened to form tubules, then tubules are broken to form the jellyfish-like morphology, and last jellyfish converts into worms and worms mince into nanospheres of the PVEA-b-PS-b-PDMA triblock terpolymer. The disassembly of vesicles is ascribed to the introduced solvophilic PDMA block, which increases the interfacial curvature of the polymer chains as shown in Figure 4H. Therefore, despite the disassembly of vesicles being the inverted procedure of the vesicle formation, the in situ synthesized intermediate morphologies during the vesicle disassembly is believed to be useful to discover how block copolymer vesicles being formed. Besides, the complex intermediate morphologies of the triblock terpolymer nano-objects such as tubules and jellyfish during the vesicle disassembly suggest that the present vesicle mediated RAFT polymerization is a valid method of in situ synthesis of block copolymer nano-objects with abundant morphologies.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3 showing the 1H NMR spectra and GPC traces of PVEA68-TTC and the PVEA68-b-PS231-TTC, Figure S4 showing the polymerization kinetics of the PVEA68-TTC macro-RAFT agent mediated dispersion polymerization of styrene and the TEM images of the PVEA68-b-PS-TTC nanoobjects at different monomer conversions, Figure S5 showing the GPC traces of the PVEA-b-PS-b-PDMA triblock terpolymers, Figure S6 showing the approximate calculation of the surface area of the PVEA68-b-PS231-TTC vesicles and the PVEA68-b-PS231-b-PDMA53 tubules, Figure S7 showing the polymerization kinetics of the PVEA68-b-PS313-TTC vesicle mediated polymerization and the TEM images of the PVEA68b-PS313-b-PDMA nano-objects at different monomer conversions, and eqs S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-22-23509794; Fax +86-22-23503510 (W.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by National Science Foundation of China (No. 21274066 and 21474054) and PCSIRT (IRT1257) is gratefully acknowledged.

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REFERENCES

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4. CONCLUSIONS The disassembly of vesicles of the PVEA-b-PS diblock copolymer through the vesicle mediated RAFT polymerization is investigated. In this vesicle mediated RAFT polymerization, a solvophilic PDMA block is introduced into the PVEA-b-PS vesicles, the disassembly of vesicles occurs with the monomer conversion, and the PVEA-b-PS-b-PDMA triblock terpolymer nano-objects are in situ formed. It is found that, the vesicle G

dx.doi.org/10.1021/ma5021165 | Macromolecules XXXX, XXX, XXX−XXX

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