Letter pubs.acs.org/macroletters
Solution-Based Thermodynamically Controlled Conversion from Diblock Copolymers to Janus Nanoparticles Zhen Zhang, Haodong Li, Xiayun Huang,* and Daoyong Chen* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, 200433, Shanghai, China S Supporting Information *
ABSTRACT: Nanosized polymeric Janus particles (NPJPs) have important applications in a variety of theoretical and practical research fields. However, the methods that are versatile and can prepare NPJPs highly efficiently are very limited. Herein, we reported a two-step thermodynamically controlled preparation of NPJPs with a high yield using a diblock copolymer as the precursor. At the first step, A-b-B coassembled in the solution with a partner diblock copolymer C-b-B to form the mixed shell micelles (MSMs) with B core and A/C mixed shell. Then, intramicellarly covalently cross-linking the A block chains resulted in the complete phase separation of A and C chains in the mixed shell, leading to the direct conversion of the MSMs into NPJPs. The first step, diblock copolymer micellization, is known as a thermodynamically controlled process, and we also made the second step, conversion from MSMs to NPJPs, be thermodynamically controlled due to the application of covalent cross-linking. As the result, the conversion efficiency is close to 100%. Besides, it was further confirmed that the method can be applied to different systems and used to tune the Janus balance. Therefore, this conversion should be very significant for the fabrication and application of the NPJPs.
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triblock copolymers.13 This is also an elegant method that can prepare NPJPs of various compositions with a high efficiency. The method includes micellization into mixed shell micelles with B core and A/C mixed shell, self-assembly of the mixed shell micelles in the selective solvent of A (or C) to form multicompartment micelles, cross-linking of the B core, and dissociation of the as-cross-linked multicomponent micelles. Four steps are required and triblock copolymers were used. Herein we report a two-step approach for preparing NPJPs starting from A-b-B diblock copolymer chains in solutions with C-b-B diblock copolymer chains as the partner. At the first step, A-b-B coassembled with the partner to form mixed shell micelles (MSMs) with B core and A/C mixed shell. Then, intramicellarly covalently cross-linking the A chains resulted in the complete phase separation of A and C chains in the shell, leading to the direct conversion of the MSMs into NPJPs. From another perspective, the method can be imagined as the coassembly in the first step divided the random coils into groups (the micelles), and then the intramicellar cross-linking asymmetrically bundled the random coils in a group to form a NPJP (asymmetrical bundling forced all the A chains to aggregate together at one side of B core and exclude all the C chains in the group to the other side). The first step, the micellization of block copolymer into micelles, is a
anus particles (JPs), the noncentrosymmetric particles, have drawn widespread attention owning to their potential applications in the fields such as bionics, controlled drug release, interfacial regulation sensors, and so on.1−6 Besides, due to the intrinsic anisotropic interactions, JPs are capable of self-assembly into superparticles that are unique in structures and properties, compared with molecular assemblies and any assemblies from isotropic particles.2,5,7−11 Among JPs, nanosized polymeric Janus particles (NPJPs) are mostly capable of constructing superparticles that may have complex but regular nanostructures.12−20 Usually, NPJPs are flexible, and the flexibility enables them to fuse together during their selfassembly to gain the enthalpy necessary for stabilization of the resultant superstructures.21 During the past decade, although great efforts were devoted to the preparation of JPs, the efficient and well-controllable methods for preparing NPJPs are very limited. In our previous study, we reported that copolymerization of one hydrophilic monomer and one hydrophobic monomer in water in the presence of the hybrid nanotubes as desymmetrization tool produced NPJPs efficiently in solution.19 In most cases, NPJPs were prepared from block copolymers as the precursors, specifically triblock copolymers.13,15,17,18,20,22−29 In addition to the early elegant approach for preparing NPJPs via chemically cross-linking spherical microdomains in the “sphereon-the-lamellar” morphology of self-assembled A-b-B-b-C triblock in the bulk samples,30 Müller and co-workers recently reported a solution-based method preparing NPJPs by dissociating multicompartment micelles formed by A-b-B-b-C © XXXX American Chemical Society
Received: April 19, 2017 Accepted: May 16, 2017
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DOI: 10.1021/acsmacrolett.7b00296 ACS Macro Lett. 2017, 6, 580−585
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ACS Macro Letters
contracted during drying and the edge of the MSMs should be of a low contrast and thus invisible in the TEM images. The P4VP blocks in the mixed shell were cross-linked by using 1.4-dibromobutane (DBB) as the cross-linker (Supporting Information, S3).34,35 The ⟨Rh⟩ of the cross-linked MSMs was ∼50 nm (Figure 1a, blue line), which was slightly smaller than that of the un-cross-linked MSMs (∼54 nm). The Mw of the cross-linked MSMs was 1.20 × 107 g/mol in DMF, which is slightly larger than the Mw of the un-cross-linked MSMs (1.11 × 107 g/mol). Therefore, the cross-linking of the P4VP block chains occurred intramicellarly; the slight increase in Mw was attributed to the incorporation of the cross-linker into the MSMs (Supporting Information, S4). It is conceivable that if there had the considerable intermicellar cross-linking in the system, both ⟨Rh⟩ and Mw of the aggregates would have been significantly increased. Since the PS block chains in the mixed shell had 720 repeating units, which was about six times that of the P4VP block chains (115 repeating units),the position of the P4VP block chains in the MSMs was similar to that of the shell in the core−shell-corona micelles.36 The relatively long PS block chains effectively protected the P4VP block chains from the intermicellar cross-linking (Supporting Information, S5). The as-cross-linked SB/BV MSMs were stained by AgNO3 and then observed by TEM. It is clearly exhibited in the TEM images that the as-cross-linked MSMs are NPJPs. Each of the SB/BV NPJPs showed distinguishable two domains (Figure 1c,d): a high contrast domain and a low contrast spherical domain. The former should be assigned to the cross-linked P4VP domain since it is sensitive to the staining of AgNO337,38 and the latter the PS chains; the PS chains are visible since the benzene rings can interact with AgNO339,40 due to the cation-π interaction (Supporting Information, S6). The PB core, which is insoluble in DMF, should be wrapped within the NPJPs and surrounded by PS block chains and the cross-linked P4VP domain. Different from the previous studies in which intramicellarly cross-linking the shell (core−shell−corona or core−shell micelles) resulted in a centrosymmetric network covering the whole surface of the core,36,41 in the present study, cross-linking one component in the mixed shell led to NPJPs. It is attributed to the large and dynamic core, the existence of the SB partner in the MSMs and application of the strong interaction (covalent cross-linking) for aggregation of the P4VP block chains. It is obvious that the cross-linked P4VP cannot cover the whole surface of the PB core since the weight ratio of P4VP to PB is only 1:14. P4VP chains contracted remarkably since the cross-linking forced them to aggregate together. Enabled by the flexibility of the dynamic core, P4VP chains migrated on the surface and aggregated into a single domain at the one side of the core; thus the NPJP were formed. The existence of SB partner was also necessary. As mentioned, the longer PS block chains prevented the intermicellar crosslinking. Besides, the PB block chains of SB increase the size of the core. During the migration and aggregation of the P4VP block chains to form a single cross-linked P4VP domain at the one side of the core, the PS block chains were gradually excluded from the cross-linked P4VP network and moved to the other side of the core to stabilize the surface originally occupied by the P4VP chains. The NPJPs were similar to the precursor MSMs in both molecular weight and particle size, indicating one MSM was converted to one NPJP. Particularly, such conversion efficiency is nearly 100% (Figure 1c and Supporting Information, S7).
thermodynamically controlled process, and the second step is also thermodynamically controlled in our system as explained later. As the result, the conversion efficiency is close to 100% and the method can be applied to different systems and get NPJPs with different Janus balance. In the first example, poly(butadiene(1,4 addition)-block-4vinylpyridine) (PB1923-b-P4VP115, BV) was used as precursor of NPJPs, and poly(styrene-block-butadiene(1,4 addition)) (PS720b-PB1682, SB) as the partner. An equal-weight mixture of SB and BV was first molecularly dissolved in the common solvent THF. The SB/BV random coils in THF have the hydrodynamic radius (⟨Rh⟩) of ∼16 nm and a polydispersity index (PDI) of 0.2 (Figure 1a, black line). Then, the coassembly was
Figure 1. Conversion of PS720-b-PB1682/PB1923-b-P4VP115 (SB/BV) random coils to NPJPs. Top: schematic illustration of the conversion from SB/BV random coils to MSMs and finally to NPJPs via intramicellar covalent cross-linking; (a) Hydrodynamic radius distributions of SB/BV in THF (black line) and DMF (red line), and the cross-linked SB/BV MSMs in DMF (blue line); (b) TEM image of the SB/BV MSMs in DMF without staining; (c) TEM image of the cross-linked SB/BV MSMs (the SB/BV NPJPs) stained by AgNO3; and (d) Magnified TEM image of the selected area of (c).
conducted by adding DMF into the SB/BV mixture in THF, and subsequently switching the solvent from the THF/DMF mixture to pure DMF (Supporting Information, S2). In DMF, ⟨Rh⟩ of SB/BV increased to ∼54 nm (Figure 1a, red line), which is much larger than that of the SB/BV random coils (∼16 nm). The remarkable increase in ⟨Rh⟩ demonstrates that SB/BV underwent self-assembly in DMF. Because DMF is the good solvent for P4VP and PS but the nonsolvent for PB block,18,31−33 the coassembly in DMF produced SB/BV MSMs with the aggregated PB block chains as the core, and the PS/ P4VP block chains as the mixed shells. The TEM observations showed that SB/BV MSMs were spherical nanoparticles with a diameter of ∼26 nm (Figure 1b), which is much smaller than the size measured by DLS because the micelles remarkably 581
DOI: 10.1021/acsmacrolett.7b00296 ACS Macro Lett. 2017, 6, 580−585
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ACS Macro Letters Furthermore, this approach can tune the Janus balance of NPJPs conveniently by changing the SB/BV weight ratio (Supporting Information, S8).When PB1923-b-P4VP115 and PS720-b-PB1682 were used and the SB/BV ratios were 1:1 (Figure 1c,d), 2:1 (Figure 2a) and 3:1 (Figure 2b), size ratios of
Figure 2. TEM images of NPJPs with different Janus balance prepared from (a) SB/BV-121; (b) SB/BV-131; (c) SB/BV-211; (d) SB/BV311. The total polymer concentration in a system was fixed at 0.5 mg/ mL, and the NPJPs were stained by AgNO3.
Figure 3. Free energy landscape of patchy particles with different patches. (a) Schematic illustration of the process for the conversion from AB/CB diblock copolymer random coils to NPJPs; (b) Free energy landscape of patchy particles with different patches where the phase separation was driven by weak interactions; and (c) Free energy landscape of patchy particles formed by intramicellarly covalently cross-linking one component in the mixed shell of the MSMs.
the respective kinds of the NPJPs are 6.63, 3.56, and 1.74 (SB/ BV-111, 121, and 131 in Table 1). Additionally, the Janus Table 1. Preparation of NPJPs with Different Janus Balance sample
polymera
wt ratiob
f P4VPc (wt %)
size ratio%d
SB/BV-111
PS720-b-PB1682 PB1923-b-P4VP115 PS720-b-PB1682 PB1923-b-P4VP115 PS720-b-PB1682 PB1923-b-P4VP115 PS720-b-PB1682 PB4105-b-P4VP96 PS720-b-PB1682 PB4105-b-P4VP172
1.0
5.17
6.63
2.0
3.45
3.56
3.0
2.59
1.74
1.0
2.16
1.32
1.0
3.75
2.09
SB/BV-121 SB/BV-131 SB/BV-211 SB/BV-311
shell is driven by noncovalent weak interactions between the two components. In these cases, the NPJPs are not global minima in energy except the Flory−Huggins interaction parameter between the two shell-forming components is unreasonably high;42,43 when the patch number is decreased, the enthalpy gain may be balanced or overbalanced by the entropy loss. Therefore, it is difficult to form NPJPs by intramicellar complete phase separation in the mixed shell between the two shell-forming components (Figure 3b). It should be mentioned that, in the method by Müller et al.,14 the enthalpy gain was enhanced by further self-assembly of MSMs; as aforementioned, additional steps were used and the structure of the NPJPs obtained is different from that prepared in the present work (Supporting Information, S9). Here, we conducted direct conversion from the MSMs to the NPJPs in solutions by utilizing strong interaction−covalent cross-linking. Under the conditions that the cross-linking reaction is unsaturated (which was realized by controlling the amount of the cross-linker) and intramicellar, the formation of the single cross-linked P4VP domain is the global minima in energy, because intramicellar fusion of the domains always gains a large enthalpy due to the formation of new covalent bonds, thus, the entropy loss caused by the fusion between domains can be neglected (Figure 3c and Supporting Information, S10). The micellization of block copolymer into micelles is a thermodynamically controlled process. In addition, the NPJPs induced by the intramicellar cross-linking are global minima in energy, indicating that the conversion from the MSMs to NPJPs is also a thermodynamically controlled process on the condition that any intermicellar cross-linking is sufficiently prevented by the longer chains of the partner diblock copolymer. Therefore, the solution-based conversion from the diblock copolymer random
a
Subscripts denote number-average degrees of polymerization of each block. bWeight ratio of SB to BV. cWeight ratio of the P4VP to whole polymers in the system. dSupposing that the domains are spherical, the size ratio is (rh/rl)3; rh and rl represent diameters of the high contrast domain and the low contrast domain, respectively. By unbiasedly averaging over more than 100 NPJPs, the rh and rl were obtained and the size ratio of that kind of the NPJPs was calculated.
balance can also be modulated by changing the structure parameter of BV, but using the same SB as the partner and the same SB/BV weight ratio (SB/BV-211 and 311 in Table 1 and Figure 2c,d). In each of the system, the size ratio is close to the weight ratio of the P4VP to whole polymers in the system. We believe that the application of covalent cross-linking, a strong interaction, is essential for the complete conversion from MSMs into NPJPs (Figure 3a). Theoretically, MSMs could be converted into different kinds of patchy particles with different patch numbers, and the concentration of each kind of patchy particles is determined by their free energy.14 Conventionally, the phase separation between the components in the mixed 582
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ACS Macro Letters coils to the NPJPs is thermodynamically controlled. This accounts for the ∼100% one-to-one conversion from MSM to NPJP repeatedly (Figure S1). Furthermore, we confirmed that this straightforward, thermodynamically controlled and partner polymer assisted conversion could be successfully applied to other diblock copolymer systems. In the second system, polystyrene-blockpoly(4-vinylpyridine) (PS480-b-P4VP124, SV) and the partner polystyrene-block-poly(methyl methacrylate) (PS 528 -bPMMA220, SM) were selected. Similarly, an equal-weight mixture of the two diblock copolymers was dissolved in the good solvent, THF. Then, methanol was added to desired volume ratios of methanol/THF. In the methanol/THF mixed solvent at a relatively high methanol/THF volume ratio (1.5:1.0), SV/SM MSMs was formed with PS as the core and P4VP/PMMA as the mixed shell (Figure 4a,b and Supporting Information, S11). We cross-linked the P4VP block chains in the mixed shell and found that, when the methanol/THF is 1.5:1.0 the intramicellarly cross-linking could not lead to the conversion from the MSMs to NPJPs and instead lead to the isolated domains of the cross-linked P4VP dotted on the surface of the core (Figure 4c, d). Obviously, the PS core in this mixed solvent is not dynamic enough for the large range migration of all the P4VP block chains, which is necessary for the formation of NPJPs. For MSM-to-NPJP conversion, the core must be dynamic enough to allow the large range migration of the cross-linked P4VP block chains. To make the PS core more dynamic, THF (the good solvent for PS) was added into the system to decrease methanol/THF volume ratio from 1.5:1.0 to 1.0:1.5 (Supporting Information, S12). As expected, intramicellarly cross-linking the P4VP successfully induced the conversion from the SV/SM MSMs to the SV/SM NPJPs (Figure 4e, Supporting Information, S13). In summary, through the two simple steps, the mixture of the precursor diblock copolymer (A-b-B) and a partner diblock copolymer (C-b-B) that are initially molecularly solubilized in solutions were converted to NPJPs in solutions. At the step 1, the two block copolymers coassembled to form MSMs with B as the core and A/C as the mixed shell, and at the step 2, crosslinking A chains in the mixed shell induced complete phase separation between A block chains and C block chains in the mixed shell. The two steps can be imagined as grouping the random coils and then asymmetrically bundling each of the groups. The preconditions for the successful conversion are the core is dynamic and relatively large and the C block chains are relatively long. Because a partner diblock copolymer was used, the relatively large core and long C block chains can be realized by separately controlling the structure parameters of the partner, and the dynamic core is obtainable by addition of common solvent into the system if the core is originally not dynamic enough. Under the preconditions, the two steps are both thermodynamically controlled and dynamically allowed. Besides, to a certain extent, the method can tune Janus balance of the NPJPs conveniently through changing the structure parameters of the precursor or the weight ratio of the crosslinkable block chains. Therefore, the present study presents a well-controllable, highly efficient and widely applicable method for the preparation of NPJPs.
Figure 4. Conversion of PS480-b-P4VP124/PS528-b-PMMA220 (SV/SM) random coils to either dotted nanoparticles or NPJPs, depending on the flexibility of the core. Top: schematic illustration of the conversion; (a) Hydrodynamic radius distributions of SV/SM in THF (black line) and methanol/THF (1.5:1.0, V/V; red line), as well as the cross-linked SV/SM MSMs in methanol/THF (1.5:1.0, V/V; blue line); (b) TEM image of the SV/SM MSMs in methanol/THF (1.5:1.0, V/V) without staining; inset: magnified image of the selected area; (c) TEM image of the cross-linked SV/SM MSMs in methanol/THF (1.5:1.0, V/V) without staining; (d) Magnified TEM image of selected area of (c); inset: the assignment of the domains in a cross-linked SV/SM MSM; and (e) TEM image of the cross-linked SV/SM MSMs (the SV/SM NPJPs) in methanol/THF (1.0:1.5, V/V) stained by AgNO3; insets: the magnified image of the selected area and the assignment of the domains in a SV/SM NPJP.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00296. Materials, instruments and all experiments (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xiayun Huang: 0000-0003-3053-7354 583
DOI: 10.1021/acsmacrolett.7b00296 ACS Macro Lett. 2017, 6, 580−585
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ACS Macro Letters
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Daoyong Chen: 0000-0001-6776-6332 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the programs of NSFC (21334001 and 21574025), MOST (2016YFA0203302), STCSM (16JC1400702), and SKLSSM (201737).
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