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Controlled morphological transition of ABC triblock copolymer aided by oleic acid via hydrogen bonding
Colloids and Surfaces A 581 (2019) 123839
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Controlled morphological transition of ABC triblock copolymer aided by oleic acid via hydrogen bonding
T
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Xiaobo Niea, , Wei Jiangb a b
School of Chemistry and Chemical Engineering, University of South China, Hengyang, 421001, PR China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, Jilin, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Triblock copolymer Self-assembly Multicompartment micelles Supramolecular polymer Hydrogen bonding
A facile method is introduced to tune the aggregate morphologies of ABC triblock copolymer in selective media by combining the self-assembly and hydrogen bonding. Poly(styrene)-block-poly(1,4-butadiene)-block-poly(2vinyl pyridine), abbreviated for PS-b-PBd-b-P2VP and used as an ABC triblock copolymer, self-assembles in toluene and methanol mixture to form discoid micelles with PBd as the disk containing part of PS domains in the core, other PS as bumps and P2VP as corona, respectively. When oleic acid (OA) is added in the assembly system, supramolecular polymer PS-b-PBd-b-P2VP(OA) is prepared by the hydrogen bonding between OA and P2VP of triblock copolymer. As a result, biscuit-like and mushroom-like micelles are formed with assistance of hydrogen bonding. Interestingly, the biscuit-like and mushroom-like micelles can transform reciprocally by fission and fusion mechanism through varying the volume ratio of toluene and methanol. Thus, it provides a simple and convenient approach to control the aggregate morphologies of block copolymers by tuning the hydrogen bonding and selective solvent content. The multicompartment micelles from ABC triblock copolymer may present potential applications in drug delivery, targeting, catalysis and others.
1. Introduction Block copolymers, consisting of two or more chemically different polymer segments or blocks connected by covalent linkages, have been
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widely recognized as particularly important materials because they can form a wide range of periodically nanoscopic structures through microphase separation [1]. Various morphologies, mainly spherical micelles, cylindrical micelles and vesicles, have been observed for
Corresponding author. E-mail addresses: [email protected], [email protected] (X. Nie).
https://doi.org/10.1016/j.colsurfa.2019.123839 Received 5 July 2019; Received in revised form 6 August 2019; Accepted 19 August 2019 Available online 20 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 581 (2019) 123839
X. Nie and W. Jiang
delivery, targeting, catalysis and other aspects.
amphiphilic diblock copolymers [2–8]. Additionally, more complex macromolecules, such as ABC triblock copolymers, have attracted much interest in recent years [9–17]. ABC triblock copolymers show additional complexity, relative to diblock copolymers, resulting from three polymer-polymer Flory-Huggins interaction parameters (instead of one in diblock copolymer), and other effects from the conditions utilized for solution-state assembly and the block copolymer architecture. Thus, some complicated, yet intriguing and variable morphologies may be expected in the solution assembly of triblock copolymers. For example, Wang [9] reported pure polymeric nanotubes with uniform diameter and high aspect ratio self-assembled by PS-b-P4VP-b-PEO triblock copolymer in N,N-dimethylformamide/ethanol solvent mixtures with assistance of solution thermal annealing. Pochan’s group [11,12] prepared the toroidal micelles by self-assembly of poly(acrylic acid)-blockpoly(methyl acrylate)-block-polystyrene (PAA-b-PMA-b-PS) triblock copolymer via interaction with organic diamines in mixed THF/water solution. Moreover, other morphological structures including helices [13], disks [14] and striped cylinders [15] were produced by tailoring the ratio of THF to water, the type and amount of amino counterion as well as the solution preparation procedure. The major driving forces to such multicompartment micelles [18,19] originate from the incompatibility between the blocks, and between the polymer and solvents. It has been widely pursued to achieve well controlled self-assembly structures and convenient control of the morphologies and their transformations. In nature, hydrogen bonds which widely exist in most of biomolecules such as nucleic acids, proteins and polysaccharides can participate in the structure formation and induce explicit functions. Similarly, hydrogen bonds can be used as moderate interactions between small molecules and block copolymer to construct supramolecular polymers [20–22]. The supramolecular assemblies can be controlled by readily varying the molar ratio of small molecules to block copolymers which can regulate the volume fraction of the supramolecules [23]. In addition, after disassembly of the hydrogen-bonding by alcohol, isolated nano-objects or nanoporous films can thus be generated [22,24,25]. For instance, Deng and coworkers [22] construct comb-like PS-b-P4VP (PDP)x supramolecule through hydrogen bonding of 3-n-pentadecyphenol (PDP) and poly(4-vinyl pyridine) (P4VP). Particles with unique internal structures are prepared via supramolecular assembly and mesoporous nanoparticles with tailored porous structures can be generated through disassembly of the supramolecules. These nanostructures are useful for functional materials fabrication, such as well-ordered nanoscopic channels membranes and nanofabrication templates. Moreover, several attempts have been made to tune block copolymers aggregates morphologies through varying hydrogen-bonding interactions between the block copolymers and hydrogen bonding agents in dilute solution [6,13,14,20]. Herein, we introduce a facile method to tune the aggregate morphologies of ABC triblock copolymer in selective media by combining the self-assembly and hydrogen bonding. Poly(styrene)-blockpoly(1,4-butadiene)-block-poly(2-vinyl pyridine), abbreviated for PS-bPBd-b-P2VP and used as an ABC triblock copolymer, self-assembles in toluene and methanol mixture to form discoid micelles with PBd as the disk containing part of PS domains in the core, other PS as bumps and P2VP as corona, respectively. When oleic acid (OA) is added in the PSb-PBd-b-P2VP dilute solution, supramolecular polymer PS-b-PBd-bP2VP(OA) is prepared by the hydrogen bonding between OA and P2VP of triblock copolymer. In consequence, new aggregates of biscuit-like and mushroom-like micelles are obtained with assistance of hydrogen bonding. Interestingly, the biscuit-like and mushroom-like micelles can transform reciprocally by fission and fusion mechanism through varying the volume ratio of toluene and methanol. Therefore, it provides a simple and convenient approach to control the aggregate morphologies of block copolymers by tuning the hydrogen bonding and solvent composition. The multicompartment micelles from ABC triblock copolymer may present new opportunities for applications in drug
2. Experimental section 2.1. Materials Poly(styrene)-block-poly(1,4-butadiene)-block-poly(2-vinyl pyridine), abbreviated for PS-b-PBd-b-P2VP, was purchased from Polymer Source, Inc., Canada. The number-average molecular weights of PS, PBd and P2VP are 45,900 g·mol−1, 55,100 g·mol−1 and 12,000 g·mol−1, respectively. Accordingly, their number-average degrees of polymerization are 441, 1020 and 114, respectively. The molecular weight distribution index is 1.06. AR grade toluene, methanol and oleic acid (OA) were obtained from Sinopharm Chemical Reagent Co., Ltd., China. All of the materials were used after receiving without further purification. 2.2. Micellar solution preparation The self-assembly of PS-b-PBd-b-P2VP is carried out in the mixture of toluene and methanol. Toluene is a good solvent for PS and PBd blocks, whereas methanol is selective to P2VP. In a typical experiment, 10 mg of PS-b-PBd-b-P2VP copolymer and 60 μL of OA are added in toluene (0.6 mL) under stirring for overnight, then 0.4 mL of methanol is added in the above solution. The system is sealed by Teflon tapes and stirred for different time at room temperature (25 ℃). Aliquots from solution (10 μL) after finishing assembly are quenched in methanol (1 mL) for subsequent TEM tests. For comparison, the pure self-assembly of PS-b-PBd-b-P2VP without OA is done in the same conditions. Moreover, varying volume ratio of toluene and methanol with total volume of 1 mL, which has been recognized as a convenient strategy to control aggregate structures, is carried out for the block copolymer and OA assembly. 2.3. Characterization Micellar morphologies were characterized by TEM which was performed on a JEOL JEM-1011 transmission electron microscopy operated at an accelerating voltage of 100 kV. The sample was prepared by depositing one drop of micellar solution (mainly methanol) onto a 300 mesh copper grid coated with a carbon film. Then, the sample was allowed to dry in air at room temperature before observation. No staining was needed since the electron density difference among PS, PBd and P2VP provides adequate contrast for identification of these three phases. 3. Results and discussion PS-b-PBd-b-P2VP, as an ABC triblock copolymer, is used to assemble in selective solvents in this study. Selective solvents used here are toluene and methanol. They can blend with each other in any proportion to form a homogeneous solution. In dilute solution, the self-assembly of block copolymers in solvents is a result of the balance between polymer-solvent and polymer-polymer interactions. The Flory-Huggins interaction parameter between polymer and solvent [26,27], χP-S, can be calculated based on the solubility parameters of the solvents and the polymers as shown in Table 1. Table 2 gives χP-S of different blocks of copolymer with toluene and methanol. PS and PBd are miscible with Table 1 Solubility parameters of polymer blocks and solvents.
2
Polymer/solvent
PS
PBd
P2VP
Toluene
Methanol
Solubility parameter δ (J·cm−3)1/2
17.4-19.0
16.6-17.6
25.0
18.3
29.5
Colloids and Surfaces A 581 (2019) 123839
X. Nie and W. Jiang
convenient approaches to tune micellar structures in solution, such as regulating time or temperature [30], adding homopolymer [16,29], adjusting pH [30–32], adding small molecule or salt [7,8,12–14], and so forth. Here, OA is introduced in the PS-b-PBd-b-P2VP assembly system containing toluene and methanol with volume ratio of 3:2. Compared to discoid micelles in Fig. 1, morphological transformation occurs after addition of OA. We can see from Fig. 2(a) and (b) that PS-bPBd-b-P2VP assembles into dominant mushroom-like micelles aided by OA. The statistical histogram in Fig. 2(c) demonstrates that the size measured from bottom to top of mushroom-like micelle is 66.2 ± 5.2 nm. The size distribution is narrow and agrees with a “Gauss Fit”, suggesting a good monodispersity of mushroom-like micelles, as also evidenced by Fig. 2(a). Besides, some discoid micelles still appear, but the rim bumps become more visible and sparser relative to discoid micelles self-assembled by PS-b-PBd-b-P2VP without OA. For comparison, the discoid micelles with the help of OA are called biscuitlike micelles which will also be seen in Figs. 3 and 4. At the top-right corner of Fig. 4(b), real picture of biscuit is used as a visual illustration. The biscuit-like micelles have the varied number of bumps, from two to a dozen. The biscuit-like micelles with the large number of bumps are obviously bigger than those with small amount of bumps. Therefore, we speculate that the big biscuit-like micelles may be divided into small ones with less bumps, even to mushroom-like micelles. In order to figure out the morphological transition and the stability of mushroom-like micelles, we prolong the assembly time until 5 days. Aliquots of solution at different time were quenched by methanol and then characterized by TEM. We find in Fig. 3(a) that mushroom-like micelles are still dominant (more than 70 percent) when assembly time is 24 h. Within 1 day, the evaporation of methanol is slight and the methanol content in the mixture was close to 40%. However, after 2 days, there are only few mushroom-like micelles, yet biscuit-like micelles become dominant in Fig. 3(b). Because methanol inevitably evaporates even in a sealed vial as time extends, this leads to evident decrease of methanol content (< 40%) in assembly solution and consequently results in the change of micellar morphologies. Therefore, we consider methanol content plays an important role on the self-assembly of PS-b-PBd-b-P2VP with or without OA. In other words, the preparation of discoid micelles or mushroom-like micelles in assembly process is sensitive to the methanol content. Owing to the evaporation of methanol, it can be inferred that the morphological transition from mushroom-like micelles in Fig. 3(a) to biscuit-like micelles in Fig. 3(b) may ascribe to the fusion of mushroom-like micelles into biscuit-like micelles. In the following experiments, we further decrease methanol content to illustrate the fact that formation of biscuit-like micelles in Fig. 3(b) is attributed to the evaporation of methanol owing to long time stirring. In Fig. 4, mainly biscuit-like micelles although with a relatively wide size distribution (real picture of biscuit inserted at the top-right corner)
Table 2 Polymeric block-solvent interaction parameters in this worka. Block-solvent interaction parameters
χP-S (toluene) χP-S (methanol)
Block PS
PBd
P2VP
0.34 2.43
0.41 2.90
2.26 0.67
a The interaction parameter (χP-S) is obtained according to the following equation [27].
toluene over the entire composition range since χP-S < 0.5. But P2VP is hardly miscible in toluene. Methanol is selective to P2VP but non-solvent to PS and PBd because the χP-S for P2VP, PS and PBd are 0.67, 2.43 and 2.90, respectively. Thus, toluene is a good solvent for PS and PBd but a precipitate for P2VP, whereas methanol is a precipitate for PS and PBd but a good solvent for P2VP. Additionally, the hydrophobic PS and PBd blocks are incompatible with P2VP. Such sutle differences allow for a delicate tuning of self-assemblies of the triblock copolymer. When the volume ratio of toluene and methanol is 3:2, the block copolymer mainly self-assembles into discoid micelles, as seen in Fig. 1(a) and (b). Besides, there are occasionally few vesicles. The discoid micelles, rims of which grow many small bumps, have been observed by Jiang’s group both in simulation and experiments [10,28]. Before TEM measurements, aliquots from mixture of toluene and methanol (10 μL) were quenched in methanol (1 mL). Thus, the micelles can be recognized fully disperse in methanol. The PS and PBd aggregate into visible domains in TEM images, whereas the P2VP blocks well stretched toward methanol are not seen in TEM images. According to the reports by Jiang’s group [10] and our finding, we infer PBd blocks with the longest chains in triblock copolymers aggregate into disk and PS blocks assemble into bumps on the edge of disk. In addition, some PS domains also exist in the inner body of the disk and are covered by PBd layer. The apparent structure of discoid micelle is simply presented in Fig. 1(c). Meanwhile, we can see the PBd domains appear gray and PS bumps show black in Fig. 1(b). χP-S = VS(δS -δP)2/RT + 0.34 where, χP-S is interaction parameter of polymeric block and solvent, VS is the mole volume of solvent, T is the temperature, and R is the gas constant. Here, VS for toluene and methanol are 105.9 and 40.6 cm3 mol−1, respectively. δS for toluene and methanol from Table 1 are 18.3 and 29.5 (J cm−3)1/2, respectively; δP for PS, PBd and P2VP from Table 1 are 18.2, 17.0 and 25.0 (J cm−3)1/2, respectively. It was reported that micellar morphologies can be controlled by changing solvent selectivity to different blocks [3,29], which avoids the synthesis of series of block copolymers in pursuit of varying their composition and architecture. Additionally, there are many other
Fig. 1. Global (a) and magnified (b) TEM images of discoid micelles self-assembled by PS-b-PBd-b-P2VP in the mixture of toluene and methanol with volume ratio 3:2, t =4 h. (c) Schematic diagram of the discoid micelle. 3
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Fig. 2. Global (a) and magnified (b) TEM images of mushroom-like micelles assembled by PS-b-PBd-b-P2VP with OA in the mixture of toluene and methanol with volume ratio 3:2, t =4 h. (c) Size distribution of mushroom-like micelles. The statistical histogram is from TEM image by counting at least 200 micelles.
Fig. 3. TEM images of micelles assembled by PS-b-PBd-b-P2VP with OA in the mixture of toluene and methanol with volume ratio 3:2 for different time: (a) 24 h, (b) 48 h.
Fig. 4. Low-magnified (a) and magnified (b) TEM images of biscuit-like micelles assembled by PS-b-PBd-b-P2VP and OA with methanol volume fraction of 33%, t =4 h.
Fig. 3 results from the evaporation of methanol undoubtedly. The pure PS-b-PBd-b-P2VP self-assembled into discoid micelles in mixture of toluene and methanol, while biscuit-like or mushroom-like micelles relevant to methanol content formed after addition of OA. This is due to the hydrogen bonding between OA and P2VP of triblock copolymer. The structure of supramolecular polymer, denoted as PS-bPBd-b-P2VP(OA), is shown in Fig. 5(a). There is only one H-bond-
assembled by PS-b-PBd-b-P2VP and OA indeed appear when methanol content lowered to 33%. There are almost no individual mushroom-like micelles. This indicates that formation of biscuit-like micelles is definitely affected by the volume ratio of toluene and methanol. The morphologies of biscuit-like micelles in Fig. 4(b) are almost identical with those in Fig. 3(b), which suggests that the fusion of mushroom-like micelles into biscuit-like micelles with prolonging assembly time in
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Colloids and Surfaces A 581 (2019) 123839
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mushroom-like micelles appear in the same solvent media after introducing OA. Hence, hydrogen bonding plays an important role in inducing the morphological transition that presumably results from the increases of repulsion and distance between adjacent P2VP(OA) corona chains. Additionally, biscuit-like micelles of PS-b-PBd-b-P2VP(OA) are formed when the volume fraction of methanol lowers to 35%. Interestingly, the biscuit-like and mushroom-like micelles can transform reciprocally by fission and fusion mechanism through varying the volume ratio of toluene and methanol. Therefore, it provides a facile way to control the aggregate morphologies of block copolymers by tuning the hydrogen bonding and selective solvent content. The multicompartment micelles from ABC triblock copolymer may present new opportunities for applications in drug delivery, targeting, catalysis and other aspects. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Fig. 5. (a) Supramolecular structure of PS-b-PBd-b-P2VP(OA) via hydrogen bonding of P2VP block and OA. (b) Morphological transformations from discoid micelles to biscuit-like and mushroom-like micelles induced by hydrogen bonding and different methanol content.
This work was supported by Natural Science Foundation of Hunan Province (2017JJ3264). We are grateful to Professor Zhen Fang of Anhui Normal University for the help in some TEM testing.
donating group (carboxyl) in a single OA molecule. The H-bonding interaction between OA and P2VP leads to the enrichment of OA in the micellar corona and swelling of the corona, which may further cause stress inside the corona. The balance of solvent swelling, entropic effect, and volume gain by oleic acid binding on P2VP domains give rise to the curvature change of micelles. In consequence, the repulsions among corona chains which are invisible in TEM images become stronger, and the adjacent distances of corona chains become larger accordingly. This phenomenon agrees with the fragmentation of cylindrical micelles assembled by poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) block copolymer and phenol through H-bonding interaction [6]. As a result, PS bumps nearby to P2VP(OA) domains become sparser and more obvious in biscuit-like and mushroom-like micelles, as seen in Fig. 5(b). The formation of biscuit-like or mushroom-like micelles is sensitive to the methanol content. When the volume fraction of methanol is below 35%, the supramolecular PS-b-PBd-b-P2VP(OA) assembles into vast majority of biscuit-like micelles. However, the most of mushroomlike micelles appear when the methanol content is above 35%. It’s demonstrated that the transition between biscuit-like and mushroom-like micelles takes place by fission and fusion mechanism. With increasing methanol content, the conformational entropy of P2VP(OA) chains on corona increases, which make the PS bumps in biscuit-like micelles become sparser and sparser and finally disassemble into mushroom-like micelles by fission mechanism. On the contrary, the mushroom-like can assemble into biscuit-like micelles by fusion when methanol lessens to critical content. Therefore, it provides a facile way to change the morphologies of PS-b-PBd-b-P2VP(OA) supramolecule through assembly and disassembly by varying the content of selective solvent.
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4. Conclusion Supramolecular polymer PS-b-PBd-b-P2VP(OA) is prepared by the hydrogen bonding between OA and P2VP of triblock copolymer. The multicompartment micelles from ABC triblock copolymer can be controlled by varying the hydrogen bonding and solvent composition. The pure PS-b-PBd-b-P2VP self-assembles into discoid micelles when the volume fraction of methanol is 40%. The PBd, PS and P2VP form the disk, dense bumps and corona, respectively. However, predominant 5
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Controlled morphological transition of ABC triblock copolymer aided by oleic acid via hydrogen bonding
T
⁎
Xiaobo Niea, , Wei Jiangb a b
School of Chemistry and Chemical Engineering, University of South China, Hengyang, 421001, PR China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, Jilin, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Triblock copolymer Self-assembly Multicompartment micelles Supramolecular polymer Hydrogen bonding
A facile method is introduced to tune the aggregate morphologies of ABC triblock copolymer in selective media by combining the self-assembly and hydrogen bonding. Poly(styrene)-block-poly(1,4-butadiene)-block-poly(2vinyl pyridine), abbreviated for PS-b-PBd-b-P2VP and used as an ABC triblock copolymer, self-assembles in toluene and methanol mixture to form discoid micelles with PBd as the disk containing part of PS domains in the core, other PS as bumps and P2VP as corona, respectively. When oleic acid (OA) is added in the assembly system, supramolecular polymer PS-b-PBd-b-P2VP(OA) is prepared by the hydrogen bonding between OA and P2VP of triblock copolymer. As a result, biscuit-like and mushroom-like micelles are formed with assistance of hydrogen bonding. Interestingly, the biscuit-like and mushroom-like micelles can transform reciprocally by fission and fusion mechanism through varying the volume ratio of toluene and methanol. Thus, it provides a simple and convenient approach to control the aggregate morphologies of block copolymers by tuning the hydrogen bonding and selective solvent content. The multicompartment micelles from ABC triblock copolymer may present potential applications in drug delivery, targeting, catalysis and others.
1. Introduction Block copolymers, consisting of two or more chemically different polymer segments or blocks connected by covalent linkages, have been
⁎
widely recognized as particularly important materials because they can form a wide range of periodically nanoscopic structures through microphase separation [1]. Various morphologies, mainly spherical micelles, cylindrical micelles and vesicles, have been observed for
Corresponding author. E-mail addresses: [email protected], [email protected] (X. Nie).
https://doi.org/10.1016/j.colsurfa.2019.123839 Received 5 July 2019; Received in revised form 6 August 2019; Accepted 19 August 2019 Available online 20 August 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 581 (2019) 123839
X. Nie and W. Jiang
delivery, targeting, catalysis and other aspects.
amphiphilic diblock copolymers [2–8]. Additionally, more complex macromolecules, such as ABC triblock copolymers, have attracted much interest in recent years [9–17]. ABC triblock copolymers show additional complexity, relative to diblock copolymers, resulting from three polymer-polymer Flory-Huggins interaction parameters (instead of one in diblock copolymer), and other effects from the conditions utilized for solution-state assembly and the block copolymer architecture. Thus, some complicated, yet intriguing and variable morphologies may be expected in the solution assembly of triblock copolymers. For example, Wang [9] reported pure polymeric nanotubes with uniform diameter and high aspect ratio self-assembled by PS-b-P4VP-b-PEO triblock copolymer in N,N-dimethylformamide/ethanol solvent mixtures with assistance of solution thermal annealing. Pochan’s group [11,12] prepared the toroidal micelles by self-assembly of poly(acrylic acid)-blockpoly(methyl acrylate)-block-polystyrene (PAA-b-PMA-b-PS) triblock copolymer via interaction with organic diamines in mixed THF/water solution. Moreover, other morphological structures including helices [13], disks [14] and striped cylinders [15] were produced by tailoring the ratio of THF to water, the type and amount of amino counterion as well as the solution preparation procedure. The major driving forces to such multicompartment micelles [18,19] originate from the incompatibility between the blocks, and between the polymer and solvents. It has been widely pursued to achieve well controlled self-assembly structures and convenient control of the morphologies and their transformations. In nature, hydrogen bonds which widely exist in most of biomolecules such as nucleic acids, proteins and polysaccharides can participate in the structure formation and induce explicit functions. Similarly, hydrogen bonds can be used as moderate interactions between small molecules and block copolymer to construct supramolecular polymers [20–22]. The supramolecular assemblies can be controlled by readily varying the molar ratio of small molecules to block copolymers which can regulate the volume fraction of the supramolecules [23]. In addition, after disassembly of the hydrogen-bonding by alcohol, isolated nano-objects or nanoporous films can thus be generated [22,24,25]. For instance, Deng and coworkers [22] construct comb-like PS-b-P4VP (PDP)x supramolecule through hydrogen bonding of 3-n-pentadecyphenol (PDP) and poly(4-vinyl pyridine) (P4VP). Particles with unique internal structures are prepared via supramolecular assembly and mesoporous nanoparticles with tailored porous structures can be generated through disassembly of the supramolecules. These nanostructures are useful for functional materials fabrication, such as well-ordered nanoscopic channels membranes and nanofabrication templates. Moreover, several attempts have been made to tune block copolymers aggregates morphologies through varying hydrogen-bonding interactions between the block copolymers and hydrogen bonding agents in dilute solution [6,13,14,20]. Herein, we introduce a facile method to tune the aggregate morphologies of ABC triblock copolymer in selective media by combining the self-assembly and hydrogen bonding. Poly(styrene)-blockpoly(1,4-butadiene)-block-poly(2-vinyl pyridine), abbreviated for PS-bPBd-b-P2VP and used as an ABC triblock copolymer, self-assembles in toluene and methanol mixture to form discoid micelles with PBd as the disk containing part of PS domains in the core, other PS as bumps and P2VP as corona, respectively. When oleic acid (OA) is added in the PSb-PBd-b-P2VP dilute solution, supramolecular polymer PS-b-PBd-bP2VP(OA) is prepared by the hydrogen bonding between OA and P2VP of triblock copolymer. In consequence, new aggregates of biscuit-like and mushroom-like micelles are obtained with assistance of hydrogen bonding. Interestingly, the biscuit-like and mushroom-like micelles can transform reciprocally by fission and fusion mechanism through varying the volume ratio of toluene and methanol. Therefore, it provides a simple and convenient approach to control the aggregate morphologies of block copolymers by tuning the hydrogen bonding and solvent composition. The multicompartment micelles from ABC triblock copolymer may present new opportunities for applications in drug
2. Experimental section 2.1. Materials Poly(styrene)-block-poly(1,4-butadiene)-block-poly(2-vinyl pyridine), abbreviated for PS-b-PBd-b-P2VP, was purchased from Polymer Source, Inc., Canada. The number-average molecular weights of PS, PBd and P2VP are 45,900 g·mol−1, 55,100 g·mol−1 and 12,000 g·mol−1, respectively. Accordingly, their number-average degrees of polymerization are 441, 1020 and 114, respectively. The molecular weight distribution index is 1.06. AR grade toluene, methanol and oleic acid (OA) were obtained from Sinopharm Chemical Reagent Co., Ltd., China. All of the materials were used after receiving without further purification. 2.2. Micellar solution preparation The self-assembly of PS-b-PBd-b-P2VP is carried out in the mixture of toluene and methanol. Toluene is a good solvent for PS and PBd blocks, whereas methanol is selective to P2VP. In a typical experiment, 10 mg of PS-b-PBd-b-P2VP copolymer and 60 μL of OA are added in toluene (0.6 mL) under stirring for overnight, then 0.4 mL of methanol is added in the above solution. The system is sealed by Teflon tapes and stirred for different time at room temperature (25 ℃). Aliquots from solution (10 μL) after finishing assembly are quenched in methanol (1 mL) for subsequent TEM tests. For comparison, the pure self-assembly of PS-b-PBd-b-P2VP without OA is done in the same conditions. Moreover, varying volume ratio of toluene and methanol with total volume of 1 mL, which has been recognized as a convenient strategy to control aggregate structures, is carried out for the block copolymer and OA assembly. 2.3. Characterization Micellar morphologies were characterized by TEM which was performed on a JEOL JEM-1011 transmission electron microscopy operated at an accelerating voltage of 100 kV. The sample was prepared by depositing one drop of micellar solution (mainly methanol) onto a 300 mesh copper grid coated with a carbon film. Then, the sample was allowed to dry in air at room temperature before observation. No staining was needed since the electron density difference among PS, PBd and P2VP provides adequate contrast for identification of these three phases. 3. Results and discussion PS-b-PBd-b-P2VP, as an ABC triblock copolymer, is used to assemble in selective solvents in this study. Selective solvents used here are toluene and methanol. They can blend with each other in any proportion to form a homogeneous solution. In dilute solution, the self-assembly of block copolymers in solvents is a result of the balance between polymer-solvent and polymer-polymer interactions. The Flory-Huggins interaction parameter between polymer and solvent [26,27], χP-S, can be calculated based on the solubility parameters of the solvents and the polymers as shown in Table 1. Table 2 gives χP-S of different blocks of copolymer with toluene and methanol. PS and PBd are miscible with Table 1 Solubility parameters of polymer blocks and solvents.
2
Polymer/solvent
PS
PBd
P2VP
Toluene
Methanol
Solubility parameter δ (J·cm−3)1/2
17.4-19.0
16.6-17.6
25.0
18.3
29.5
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convenient approaches to tune micellar structures in solution, such as regulating time or temperature [30], adding homopolymer [16,29], adjusting pH [30–32], adding small molecule or salt [7,8,12–14], and so forth. Here, OA is introduced in the PS-b-PBd-b-P2VP assembly system containing toluene and methanol with volume ratio of 3:2. Compared to discoid micelles in Fig. 1, morphological transformation occurs after addition of OA. We can see from Fig. 2(a) and (b) that PS-bPBd-b-P2VP assembles into dominant mushroom-like micelles aided by OA. The statistical histogram in Fig. 2(c) demonstrates that the size measured from bottom to top of mushroom-like micelle is 66.2 ± 5.2 nm. The size distribution is narrow and agrees with a “Gauss Fit”, suggesting a good monodispersity of mushroom-like micelles, as also evidenced by Fig. 2(a). Besides, some discoid micelles still appear, but the rim bumps become more visible and sparser relative to discoid micelles self-assembled by PS-b-PBd-b-P2VP without OA. For comparison, the discoid micelles with the help of OA are called biscuitlike micelles which will also be seen in Figs. 3 and 4. At the top-right corner of Fig. 4(b), real picture of biscuit is used as a visual illustration. The biscuit-like micelles have the varied number of bumps, from two to a dozen. The biscuit-like micelles with the large number of bumps are obviously bigger than those with small amount of bumps. Therefore, we speculate that the big biscuit-like micelles may be divided into small ones with less bumps, even to mushroom-like micelles. In order to figure out the morphological transition and the stability of mushroom-like micelles, we prolong the assembly time until 5 days. Aliquots of solution at different time were quenched by methanol and then characterized by TEM. We find in Fig. 3(a) that mushroom-like micelles are still dominant (more than 70 percent) when assembly time is 24 h. Within 1 day, the evaporation of methanol is slight and the methanol content in the mixture was close to 40%. However, after 2 days, there are only few mushroom-like micelles, yet biscuit-like micelles become dominant in Fig. 3(b). Because methanol inevitably evaporates even in a sealed vial as time extends, this leads to evident decrease of methanol content (< 40%) in assembly solution and consequently results in the change of micellar morphologies. Therefore, we consider methanol content plays an important role on the self-assembly of PS-b-PBd-b-P2VP with or without OA. In other words, the preparation of discoid micelles or mushroom-like micelles in assembly process is sensitive to the methanol content. Owing to the evaporation of methanol, it can be inferred that the morphological transition from mushroom-like micelles in Fig. 3(a) to biscuit-like micelles in Fig. 3(b) may ascribe to the fusion of mushroom-like micelles into biscuit-like micelles. In the following experiments, we further decrease methanol content to illustrate the fact that formation of biscuit-like micelles in Fig. 3(b) is attributed to the evaporation of methanol owing to long time stirring. In Fig. 4, mainly biscuit-like micelles although with a relatively wide size distribution (real picture of biscuit inserted at the top-right corner)
Table 2 Polymeric block-solvent interaction parameters in this worka. Block-solvent interaction parameters
χP-S (toluene) χP-S (methanol)
Block PS
PBd
P2VP
0.34 2.43
0.41 2.90
2.26 0.67
a The interaction parameter (χP-S) is obtained according to the following equation [27].
toluene over the entire composition range since χP-S < 0.5. But P2VP is hardly miscible in toluene. Methanol is selective to P2VP but non-solvent to PS and PBd because the χP-S for P2VP, PS and PBd are 0.67, 2.43 and 2.90, respectively. Thus, toluene is a good solvent for PS and PBd but a precipitate for P2VP, whereas methanol is a precipitate for PS and PBd but a good solvent for P2VP. Additionally, the hydrophobic PS and PBd blocks are incompatible with P2VP. Such sutle differences allow for a delicate tuning of self-assemblies of the triblock copolymer. When the volume ratio of toluene and methanol is 3:2, the block copolymer mainly self-assembles into discoid micelles, as seen in Fig. 1(a) and (b). Besides, there are occasionally few vesicles. The discoid micelles, rims of which grow many small bumps, have been observed by Jiang’s group both in simulation and experiments [10,28]. Before TEM measurements, aliquots from mixture of toluene and methanol (10 μL) were quenched in methanol (1 mL). Thus, the micelles can be recognized fully disperse in methanol. The PS and PBd aggregate into visible domains in TEM images, whereas the P2VP blocks well stretched toward methanol are not seen in TEM images. According to the reports by Jiang’s group [10] and our finding, we infer PBd blocks with the longest chains in triblock copolymers aggregate into disk and PS blocks assemble into bumps on the edge of disk. In addition, some PS domains also exist in the inner body of the disk and are covered by PBd layer. The apparent structure of discoid micelle is simply presented in Fig. 1(c). Meanwhile, we can see the PBd domains appear gray and PS bumps show black in Fig. 1(b). χP-S = VS(δS -δP)2/RT + 0.34 where, χP-S is interaction parameter of polymeric block and solvent, VS is the mole volume of solvent, T is the temperature, and R is the gas constant. Here, VS for toluene and methanol are 105.9 and 40.6 cm3 mol−1, respectively. δS for toluene and methanol from Table 1 are 18.3 and 29.5 (J cm−3)1/2, respectively; δP for PS, PBd and P2VP from Table 1 are 18.2, 17.0 and 25.0 (J cm−3)1/2, respectively. It was reported that micellar morphologies can be controlled by changing solvent selectivity to different blocks [3,29], which avoids the synthesis of series of block copolymers in pursuit of varying their composition and architecture. Additionally, there are many other
Fig. 1. Global (a) and magnified (b) TEM images of discoid micelles self-assembled by PS-b-PBd-b-P2VP in the mixture of toluene and methanol with volume ratio 3:2, t =4 h. (c) Schematic diagram of the discoid micelle. 3
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Fig. 2. Global (a) and magnified (b) TEM images of mushroom-like micelles assembled by PS-b-PBd-b-P2VP with OA in the mixture of toluene and methanol with volume ratio 3:2, t =4 h. (c) Size distribution of mushroom-like micelles. The statistical histogram is from TEM image by counting at least 200 micelles.
Fig. 3. TEM images of micelles assembled by PS-b-PBd-b-P2VP with OA in the mixture of toluene and methanol with volume ratio 3:2 for different time: (a) 24 h, (b) 48 h.
Fig. 4. Low-magnified (a) and magnified (b) TEM images of biscuit-like micelles assembled by PS-b-PBd-b-P2VP and OA with methanol volume fraction of 33%, t =4 h.
Fig. 3 results from the evaporation of methanol undoubtedly. The pure PS-b-PBd-b-P2VP self-assembled into discoid micelles in mixture of toluene and methanol, while biscuit-like or mushroom-like micelles relevant to methanol content formed after addition of OA. This is due to the hydrogen bonding between OA and P2VP of triblock copolymer. The structure of supramolecular polymer, denoted as PS-bPBd-b-P2VP(OA), is shown in Fig. 5(a). There is only one H-bond-
assembled by PS-b-PBd-b-P2VP and OA indeed appear when methanol content lowered to 33%. There are almost no individual mushroom-like micelles. This indicates that formation of biscuit-like micelles is definitely affected by the volume ratio of toluene and methanol. The morphologies of biscuit-like micelles in Fig. 4(b) are almost identical with those in Fig. 3(b), which suggests that the fusion of mushroom-like micelles into biscuit-like micelles with prolonging assembly time in
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mushroom-like micelles appear in the same solvent media after introducing OA. Hence, hydrogen bonding plays an important role in inducing the morphological transition that presumably results from the increases of repulsion and distance between adjacent P2VP(OA) corona chains. Additionally, biscuit-like micelles of PS-b-PBd-b-P2VP(OA) are formed when the volume fraction of methanol lowers to 35%. Interestingly, the biscuit-like and mushroom-like micelles can transform reciprocally by fission and fusion mechanism through varying the volume ratio of toluene and methanol. Therefore, it provides a facile way to control the aggregate morphologies of block copolymers by tuning the hydrogen bonding and selective solvent content. The multicompartment micelles from ABC triblock copolymer may present new opportunities for applications in drug delivery, targeting, catalysis and other aspects. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Fig. 5. (a) Supramolecular structure of PS-b-PBd-b-P2VP(OA) via hydrogen bonding of P2VP block and OA. (b) Morphological transformations from discoid micelles to biscuit-like and mushroom-like micelles induced by hydrogen bonding and different methanol content.
This work was supported by Natural Science Foundation of Hunan Province (2017JJ3264). We are grateful to Professor Zhen Fang of Anhui Normal University for the help in some TEM testing.
donating group (carboxyl) in a single OA molecule. The H-bonding interaction between OA and P2VP leads to the enrichment of OA in the micellar corona and swelling of the corona, which may further cause stress inside the corona. The balance of solvent swelling, entropic effect, and volume gain by oleic acid binding on P2VP domains give rise to the curvature change of micelles. In consequence, the repulsions among corona chains which are invisible in TEM images become stronger, and the adjacent distances of corona chains become larger accordingly. This phenomenon agrees with the fragmentation of cylindrical micelles assembled by poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) block copolymer and phenol through H-bonding interaction [6]. As a result, PS bumps nearby to P2VP(OA) domains become sparser and more obvious in biscuit-like and mushroom-like micelles, as seen in Fig. 5(b). The formation of biscuit-like or mushroom-like micelles is sensitive to the methanol content. When the volume fraction of methanol is below 35%, the supramolecular PS-b-PBd-b-P2VP(OA) assembles into vast majority of biscuit-like micelles. However, the most of mushroomlike micelles appear when the methanol content is above 35%. It’s demonstrated that the transition between biscuit-like and mushroom-like micelles takes place by fission and fusion mechanism. With increasing methanol content, the conformational entropy of P2VP(OA) chains on corona increases, which make the PS bumps in biscuit-like micelles become sparser and sparser and finally disassemble into mushroom-like micelles by fission mechanism. On the contrary, the mushroom-like can assemble into biscuit-like micelles by fusion when methanol lessens to critical content. Therefore, it provides a facile way to change the morphologies of PS-b-PBd-b-P2VP(OA) supramolecule through assembly and disassembly by varying the content of selective solvent.
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4. Conclusion Supramolecular polymer PS-b-PBd-b-P2VP(OA) is prepared by the hydrogen bonding between OA and P2VP of triblock copolymer. The multicompartment micelles from ABC triblock copolymer can be controlled by varying the hydrogen bonding and solvent composition. The pure PS-b-PBd-b-P2VP self-assembles into discoid micelles when the volume fraction of methanol is 40%. The PBd, PS and P2VP form the disk, dense bumps and corona, respectively. However, predominant 5
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