Letter pubs.acs.org/macroletters
Versatile Method to Expand the Morphology Library of Block Copolymer Solution Self-Assemblies with Tubular Structures Xiao-Li Sun, Dong-Ming Liu, Shuai Pei, Kang-Kang Li, and Wen-Ming Wan* State Key Laboratory of Heavy Oil Processing and Centre for Bioengineering and Biotechnology, China University of Petroleum (East China), 66 Changjiang West Road, Qingdao Economic Development Zone, Qingdao, Shandong 266580, People’s Republic of China S Supporting Information *
ABSTRACT: Self-assembly of block copolymers (BCPs) in solution is a powerful technology to achieve a broad range of structures, such as spheres, cylinders, vesicles, and other hierarchical structures. However, the BCP self-assembly library is limited, especially with respect to tubular structures. Here we show a versatile strategy to expand the morphology library of block copolymer solution self-assemblies with tubular structures (including tubular dumbbells and tubules) via self-assembly of the most common diblock copolymers P4VP-b-PS BCPs in methanol. No special chemistry is needed in this strategy, which proves the universality of this method. The novelty of the strategy is to keep the BCPs both highly asymmetric and with very high molecular weight. The underlying formation mechanism and kinetics of these tubular structures were elucidated. The prepared tubular structures expand the structure library of BCP solution self-assemblies and open up a new avenue for the further applications of a variety of tubular materials.
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morphologies are limited and rare with respect to tubular structures. Yan et al. reported macroscopic tubules by selfassembly of a hyperbranched multiarm copolymer (HBPO-starPEG) consisting of a hydrophobic, hyperbranched poly(3ethyl-3-oxetanemethanol) core (HBPO) and hydrophilic poly(ethylene glycol) (PEG) arms.23 However, tubular structures based on well-defined diblock copolymer self-assembly have yet to be explored. Here, we present a simple strategy to achieve tubular structures via self-assembly of one of the most common diblock copolymer poly(4-vinylpyridine)-b-polystyrenes (P4VP-b-PS) in methanol. P4VP-b-PS BCPs are well-known BCPs comprehensively investigated in the BCP solution self-assembly individually24 or by supramolecular complexation with other molecules containing poly(acrylic acid),25 phenol moiety,26 or inorganic compound.27 No special chemistry is therefore needed in this strategy, which proves the universality of this method. The novelty of the strategy used here is that the P4VPb-PS diblock copolymer is both highly asymmetric and with a very high molecular weight, enabling the formation of welldefined tubular assemblies. For P4VP-b-PS BCP synthesis, in the theory of BCP solution self-assembly, the resulting self-assembly morphologies are mainly determined by a force balance involving the stretching
ne-dimensional (1D) nanostructures, such as wires, belts, and tubules, have become the focus of intensive research recently due to their potential applications in mesoscopic physics and fabrication of devices for separation, sensors, ion transport, catalysts, etc.1 The tubular structures have attracted particular interest since the discovery of carbon nanotubes by Iijima in 1991,2 owing to the versatile features of a large surface area and high porosity. Tubular structures have also been well developed in inorganic materials (metal, metal sulfides, and metal oxides, such as Te,3 Au,4 Ag,5 Pd,6 NbS2,7 MoS2,8 CdS,9 TiO2,10 SiO2,11 MoO3,12 ZnO,13 etc.), metal−organic framework,14 biomolecules (such as peptide,15 DNA,16 lipid,17 carbohydrate,18 etc.), and organic molecules19 in the past two decades. However, polymeric tubules have rarely been developed and then mainly by either templating20 or conjugation with cyclic peptide.21 To find an easy method to achieve polymeric tubular structure is an important goal for polymer scientists. Block copolymer (BCP) self-assembly in solution is well studied and has been a powerful technology to achieve a broad range of structures, such as spheres, cylinders, vesicles, lamellae, and other hierarchical structures.22 Generally, the morphology is mainly controlled by a force balance involving three factors, the core-chain stretching, the interfacial energy between the core and solvent, and repulsion between corona chains. Morphologies are therefore tuned by adjusting solvent type, solvent/selective solvent ratio, copolymer type and concentration, additives, temperature, etc.22 Even though BCP selfassembly in solution has gained considerable progress, © XXXX American Chemical Society
Received: September 1, 2016 Accepted: September 20, 2016
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DOI: 10.1021/acsmacrolett.6b00672 ACS Macro Lett. 2016, 5, 1180−1184
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ACS Macro Letters of the core chains, the surface tension between the core and solvent, and repulsion among the corona chains, of which the stretching of the core chains contributes significantly and straightforward. In the molecular design of BCP, the molecular weight of the core-forming block has high impact on the stretching of the core chains. Untraditional structures, rather than micelles, vesicles, and large compound vesicles, are therefore highly anticipated with highly increased molecular weight. In this study, a series of P4VP-b-PS diblock copolymers of required asymmetry and high molecular weights were therefore synthesized to verify our hypothesis via reversible addition−fragmentation chain transfer polymerization induced self-assembly (RAFT PISA).28 S-1-Dodecyl-S-(α,α′-dimethylα″-acetic acid) trithiocarbonate (TC) and poly(4-vinylpyridine) (P4VP-TC) were reported previously (Mn,NMR = 10 400 g mol−1, ĐGPC = 1.08).28d,29 RAFT PISA of styrene was carried out in methanol by using P4VP-TC as the macrochain transfer agent, with a desired feed ratio of P4VP-TC:St:AIBN = 10:X:1 (styrene: 1.04 g, methanol: 1.0 mL), as shown in Table S1. Representative 1H NMR spectra and GPC curves of P4VP-b-PS are shown in Figures S1 and S2, respectively. All protons of P4VP-b-PS are well appointed, and the degree of polymerization (DP) of PS is calculated by comparing integrals of P4VP at ca. 8.3 ppm with PS at ca. 6.4 ppm (minus P4VP contribution). The unimodal and symmetric GPC curves in Figure S2 indicate well-controlled molecular weight and molecular weight distribution (lower than 1.26, as shown in Table S1) of BCPs via RAFT PISA. BCPs with 96 repeating units of 4-vinylpyridine and 341 to 1834 units of styrene were synthesized, exhibiting high asymmetry (P4VP % ranging from 22% to 4.97%) and high molecular weight (ranging from 45.9 to 201.2 kDa). For P4VP-b-PS BCP self-assembly in methanol, further BCP self-assembly was carried out by dissolving P4VP-b-PS in THF overnight, followed by the addition of methanol as a selective precipitant for PS and then the dialysis in methanol. These BCPs have a constant P4VP96 block (subscripts give the number of repeating units) and an adjustable PS341−1834 block, which leads to structures with a PS core and a P4VP shell. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the morphologies of the formed particles, as shown in Figure 1 and Table S1. Spherical micelles with a diameter of ∼110 nm were formed via self-assembly of BCP with PS block length of 341 (Figure 1A). Vesicles with a diameter and wall thickness of ∼400 nm and ∼70 nm, respectively, were prepared with PS block length of 663 (Figure 1B). When the PS block length increased to 1112, dumbbell-shaped tubular structures were formed with one big vesicle of ∼600 nm at each end joined by 6−10 tubular beads with a diameter of 230 nm and a length of up to 6 μm (tubular bead dumbbells (TBDB)) (Figure 1C). Further increase of the PS to 1834 gave end-capped tubules with an outer diameter of ∼300 nm, a wall thickness of ∼70 nm, an inner diameter of ∼160 nm, and a length of up to 15 μm (Figure 1D and Figure S3). In comparison with results from other asymmetric BCP self-assembly in solution, the molecular weights here are much higher. Thus, both highly asymmetric composition and very high molecular weight are important in the preparation of these tubular structures. The formation kinetics of the tubules were tracked by TEM and SEM. For BCPs with chemical composition P4VP96-bPS1834, spherical micelles with a diameter of ∼60 nm were observed when 10% of methanol was added to P4VP-b-PS
Figure 1. Morphologies of P4VP-b-PS aggregates formed by BCP selfassembly in methanol: A for P4VP96-b-PS341, B for P4VP96-b-PS663, C for P4VP96-b-PS1112, and D for P4VP96-b-PS1834. Starting BCP concentration is 10 mg/mL. Scale bar is 500 nm for A and B, 1 μm for insets, and 2 μm for C and D.
solution in THF, as shown in Figure 2A. As the methanol content increased to 20%, the spherical micelles changed to
Figure 2. TEM images of P4VP96-b-PS1834 aggregates formed by BCP self-assembly in methanol with different content of methanol/THF (v/v): (A) 10%, (B) 20%, (C) 30%, (D) 40%, (E) 50%, and (F) after dialysis, with starting BCP concentration of 10 mg/mL. Scale bar is 500 nm for A and B and 2 μm for the rest.
vesicles with coexistence of spherical micelles and vesicles. The fresh vesicles had a diameter of ∼600 nm and a wall thickness of ∼60 nm. The wall thickness of vesicles is similar to the size of the parent spherical micelles, which indicates that the vesicles were formed by fusion of spherical micelles. When the methanol content reached 30%, the vesicles started to fuse, resulting in coexistence of fused vesicles and individual vesicles. TBDBs were formed with a length of ∼4 μm, end vesicles with a diameter of ∼500 nm, and up to 18 tubular beads in between, at 40% methanol content, as shown in Figure 2D. This structure formed by P4VP96-b-PS1834 at 40% methanol content is morphologically similar to the one by P4VP96-b-PS1112 after 1181
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ACS Macro Letters dialysis (Figure 1C), indicating both chemical composition of BCP and selective solvent content are very important to the morphology control of the BCP aggregates via BCP selfassembly in solution. It is clear that the diameter of vesicles becomes smaller during the process of vesicle fusion, which results in the tubular structure formation. Further increase of methanol to 50% leads to smoother tubular beads with increased radius of curvature of the vesicular beads. The length of the tubules increased up to 6 μm. Finally, tubules were formed after dialysis with a length of up to 15 μm, a diameter of ∼300 nm, and end-capped vesicles with a diameter of ∼500 nm (Figure 2D). For a better understanding of the tubule formation process tracked by TEM imaging above, the corresponding morphology evolution for tubule formation from molecular BCP to micelle, vesicle, TBDB, and finally tubule is illustrated in Scheme 1, and the tubule formation
Figure 3. Morphologies of P4VP-b-PS aggregates formed by PISA technique with feed ratio of P4VP:St = 1:500 (A), 1:1000 (B), 1:2000 (C), and 1:4000 (D), respectively (styrene: 1.04 g, methanol: 1.0 mL). Scale bar is 500 nm for all.
Scheme 1. Illustration of Morphology Evolution of P4VP96b-PS1834 upon Increasing of Methanol Content
79.4 kDa), only traditional morphologies, including micelles and vesicles, are prepared, while BCPs with high molecular weight (ranging from 126.1 to 201.2 kDa) result in untraditional tubular structures. The molecular weight of the coreforming block, which affects the stretching of the core chains, therefore plays important roles in the formation of these tubular structures. Geometric packing parameters are commonly adopted in the molecular-level guidance for the morphologies of self-assembly in theory. For a better understanding of how molecular weight of core-forming block affects the final morphology, molecularlevel geometric structure of P4VP-b-PS BCP is therefore illustrated in Scheme 2, which is mainly composed of ashell and
through fusion of vesicles is illustrated as a cartoon in the Supporting Information. The aspect ratio of these tubules reaches 50. In comparison with other inorganic tubules, these polymer tubules do not have an open structure but are endcapped with vesicles or even fused vesicles on the end and in the middle (blue arrow in Figure 1D and Figure S4). PISA has gained considerable research attention since our pioneering reports 7 years ago,28a,b which have become a powerful tool nowadays with easy preparation of a variety of highly concentrated morphologies, including spherical micelles, nanowires, vesicles, and large compound vesicles.30 Both PISA and BCP solution self-assembly are powerful tools for the preparation of micro/nanostructures. It is therefore highly worthy to compare these two techniques here. As mentioned above, tubular structures were prepared through the BCP solution self-assembly method. In comparison, morphologies of block copolymers through PISA were a series of micelles and vesicles, as shown in Figure 3. Apparently, these two techniques exhibit the same tendency of forming bigger and bigger particles with increasing complexity, as core-forming block length increases. The BCP solution self-assembly technique enables particles with more intriguing morphologies, e.g., tubular structures here. In the theory of BCP solution self-assembly, the resulting self-assembly morphologies are mainly determined by a force balance involving the stretching of the core chains, the surface tension between the core and solvent, and repulsion among the corona chains. In the molecular design of BCP, the molecular weight of the core-forming block has a high impact on the stretching of the core chains. In this study, a series of BCPs with 96 repeating units of 4-vinylpyridine and 341 to 1834 units of styrene were synthesized, exhibiting wide molecular weight range (from 45.9 kDa to 201.2 kDa) with corresponding asymmetry (P4VP % ranging from 22% to 4.97%). Apparently, when BCPs exhibit low molecular weight (ranging from 45.9 to
Scheme 2. Illustration of Molecular-Level Geometric Structure of P4VP-b-PS BCP and the Relationship between Geometry and Morphology of BCP Solution Assemblies
lcore (refer to average shell area and core length). In geometry, micellar morphologies are highly desirable with cone-shaped geometric structures, while tubular morphologies are highly desirable with cylinder-shaped geometric structures. In this study, P4VP-b-PS BCPs have a fixed shell length of 96 repeating units, indicating similar ashell’s in geometry, while the core length of BCPs increases from 341 to 1834 repeating units, resulting in significantly increased lcore’s and a significant transition of geometric structure from cone-shaped to cylindershaped structures. When the core has 341 repeating units, the repeating unit ratio of shell to core is 1:3.6. Geometric structure is therefore cone-shaped, resulting in micellar morphologies, as shown in Scheme 2, where the relationship between geometry 1182
DOI: 10.1021/acsmacrolett.6b00672 ACS Macro Lett. 2016, 5, 1180−1184
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and morphology is illustrated. When repeating units of core increase to 663, the repeating unit ratio of shell to core decreases to 1:6.9, and the geometric structure therefore locates between cone-shaped and cylinder-shaped structures, resulting in bilayer structures, e.g., vesicular morphologies. As repeating units of the core increase further from 1112 to 1834, the repeating unit ratio of shell to core decreases from 1:11.6 to 1:19.1. Geometric structures are therefore with the tendency of forming cylinder-shaped structures, resulting in one-dimensional bilayer structures, e.g., tubular structures. Thus, the repeating units of the shell and core and their corresponding ratios, i.e., the molecular weight and the asymmetry of P4VP-bPS BCPs, play important roles in the molecular-level geometric structures and the resulting morphologies. The roles of molecular weight of P4VP-b-PS BCPs in the formation mechanism of tubular structures above are mainly based on geometry. To confirm the important roles of molecular weight, coarse-grained molecular dynamics (MD) simulations of P4VP-b-PS BCPs with the same asymmetry (1:10, 2:20, and 4:40) were carried out, and the snapshots of particles are shown in Figure 4. Obviously, the morphology of
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00672. Detailed experimental section and supporting data (PDF) Tubule formation through fusion of vesicles (ZIP)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Robert K. Thomas of University of Oxford for helpful discussion and funding support from NSFC 51503226, Shandong Provincial NSF ZR2015EQ018, China University of Petroleum (East China) starting funding and “the Fundamental Research Funds for the Central Universities”.
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Figure 4. Snapshots of particles formed through MD simulations of P4VP-b-PS BCPs in methanol. (A) P4VP1-b-PS10; (B) P4VP2-b-PS20; and (C) P4VP4-b-PS40. Red, green, and purple beads refer to pyridyl, phenyl, and CH2−CH moieties, respectively.
the particle relies highly on the molecular weight of BCPs. Spherical micellar structure was observed for P4VP1-b-PS10. As the molecular weight increased, ellipsoid-like micellar structure and bitubular structure were obtained for P4VP2-b-PS20 and P4VP4-b-PS40, respectively (detailed radial distribution function discussion in Supporting Information). These simulation results therefore confirm the importance of molecular weight on the resulting morphology. In conclusion, tubular structures, including tubular bead dumbbells and tubules, were successfully prepared to expand the morphology library of block copolymer self-assemblies by systematic self-assembly of P4VP-b-PS BCPs with highly asymmetric composition and very high molecular weight. P4VP-b-PS BCP is a common and even commercially available polymer. No special BCP chemistry is therefore needed in this method, and the novelty of this method is to keep BCP both highly asymmetric and with very high molecular weight. This method is therefore not limited to P4VP-b-PS BCP. Considering the functionality of P4VP, these tubular materials show potential applications in the field of ion exchange, water purification, inorganic−organic hybrid, catalyst etc. Thus, our work provides a new strategy for the preparation of tubular one-dimensional materials, which expands the structure library of BCP self-assembly and enables the further application of these tubular materials. 1183
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