Letter pubs.acs.org/macroletters
Janus Mesoporous Nanodisc from Gelable Triblock Copolymer Fan Jia, Fuxin Liang, and Zhenzhong Yang* State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: The polymer/inorganic composite trilayered Janus mesoporous nanodiscs are synthesized by coassembly of gelable poly(ethylene oxide)-blockpoly(3-triethoxysilylpropyl methacrylate)-block-polystyrene (PEO-b-PTEPM-b-PS) and PEO-b-PS via a sol−gel process at an emulsion interface. Bimodal phase separations are responsible for the formation of isolated nanodiscs and mesopores within the discs. The Janus mesoporous nanodiscs are amphiphilic to form superstructures in dispersions. They can serve as a solid surfactant to stabilize emulsions.
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tures. Taking good advantage of amphiphilic performance of the copolymers, they are capable to self-organize at an emulsion interface. The hydrophilic and hydrophobic blocks are synchronously segregated toward water and oil phases, respectively. It is expected to achieve a Janus robust membrane after a sol−gel process at the emulsion interface. In our previous report, a Janus silica shell can form at an emulsion interface by a self-organized sol/gel process of silane mixtures in the internal oil.5 The Janus shell can evolve into porous one even individual particle when another cosurfactant is added at increasing amount. Phase separation of binary surfactants at the emulsion interface leads to the patchy structure. Inspired by this finding, the microstructure of the Janus robust membrane from gelable block copolymers will be tunable. Herein, we present an easy approach toward PEO/silica/PS composite trilayered Janus mesoporous nanodisc from the gelable triblock copolymer poly(ethylene oxide)-block-poly(3triethoxysilylpropyl methacrylate)-block-polystyrene (PEO-bPTEPM-b-PS), as illustrated in Scheme 1. PEO-b-PTEPM-bPS is synthesized by RAFT polymerization. An oil/water emulsion forms in the presence of PEO-b-PTEPM-b-PS. Under acidic conditions, a trilayered composite shell forms at the interface via a self-organized sol−gel process. Another copolymer such as PEO-b-PS can drive a bimodal phase separation to form a major continuous phase at the emulsion interface and dispersed domains within isolated composite nanodiscs during sol−gel process. After treatment with tetrahydrofuran (THF) to dissolve PEO-b-PS, mesopores are generated within the Janus nanodiscs. The Janus mesoporous nanodiscs are amphiphilic to form superstructures. They can serve as a solid surfactant to stabilize emulsions.
anus materials with two different compositions and functions compartmentalized onto the same object can display diversified performances, which are promising in many fields including colloidal surfactants, optical biosensors, and medical probes.1 Among many methods to prepare nanoscale Janus materials, dis-assembly of copolymeric superstructures has been extensively used to tune shape of the Janus nanomaterials.2 Especially, highly asymmetric Janus sheets were synthesized from the polystyrene-block-polybutadiene-block-poly(tert-butyl mathacrylate) (PS-b-PB-b-PtBMA) lamellar microstructure. While the Janus sheets were crushed into smaller ones by sonication, irregular protrusions of the sheets could be preferentially shaved off the edge. Eventually, the sheets contour appeared round, for example, Janus discs. However, the discs are not strictly circular in shape. We have recently reported on synthesis of Janus nanodiscs by dis-assembly of the polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) disc alternatively stacked ellipsoids with a selective solvent.3 However, the approach involves many steps including formation of supramolecular structures by assembly, partial cross-linking, and disassembly. Additional reagents should be introduced to cross-link desired domains of the superstructures prior to disassembly. Sufficiently narrow molecular weight distribution is required to ensure uniform supramolecular structures. It remains challenging to precisely control the microstructure of the Janus nanodiscs. Transverse pores within the Janus nanodiscs are highly desired, which will greatly facilitate an interfacial mass transportation across the nanodiscs. Gelable block copolymers with inorganic silane side groups are interesting, which can self-cross-link the supramolecular structures by a simple sol−gel process. A series of robust organic/inorganic hybrid nanomaterials with tunable shape and composition have been derived by self-assembly of the poly(3triethoxysilylpropyl methacrylate) block contained gelable copolymers.4 The approach also requires narrow molecular weight distribution to ensure uniform supramolecular struc© XXXX American Chemical Society
Received: October 24, 2016 Accepted: November 18, 2016
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DOI: 10.1021/acsmacrolett.6b00812 ACS Macro Lett. 2016, 5, 1344−1347
Letter
ACS Macro Letters Scheme 1. Illustrative Synthesis of the Trilayered Composite Janus Mesoporous Nanodisca
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(a) PEO-b-PTEPM-b-PS/PEO-b-PS mixture at the emulsion interface; (b) isolated nanodiscs form after the co-assembled sol−gel process, in which some PEO-b-PS are present as dispersed domains; (c) Janus porous nanodiscs achieved after dissolving PEO-b-PS from the nanodiscs with THF.
PEO-b-PTEPM-b-PS was synthesized by two-step RAFT polymerization.4 Starting from a PEO macromolecular chain transfer agent, the PEO-b-PTEPM diblock copolymer was synthesized by RAFT-mediated radical polymerization of TEPM. Using PEO-b-PTEPM as a new macro-CTA to further polymerize styrene, the PEO-b-PTEPM-b-PS triblock copolymer was achieved. Number-average molecular weight (Mn) of PEO is measured 3.1k (curve a, Figure 1a). The PEO
Figure 2. (a) SEM image of the sphere with a paraffin core and a composite shell; (b) SEM and (c) TEM images of the Janus hollow sphere; (d) SEM image of the Janus hollow sphere after loading paraffin inside the cavity, inset POM image; (e) cross-sectional TEM image of the Janus hollow sphere embedded in PMMA, inset the schematic structure; (f) cross-sectional TEM and inset (f1) magnified HAADF-STEM image after staining with PTA, and inset (f2) TEM image after another staining with RuO4.
is obtained. The hollow sphere is collapsed and wrinkled (Figure 2b). Under TEM, the shell is rather thin (Figure 2c). After adding the hollow sphere in a melt paraffin/water mixture, the sphere is well dispersible in water. Eventually, no paraffin is found in water phase. Upon cooling, the sphere becomes spherical in contour under SEM rather than collapsed (Figure 2d). This implies that the cavity is filled with paraffin, which is confirmed by polarizing optical microscopy image (inset Figure 2d). The PEO block is present at exterior surface of the sphere to ensure good dispersion in water, while the PS block at the interior surface responsible for the hydrophobic cavity. The cross sectional TEM image of the hollow sphere after being embedded in PMMA shows that the silica layer thickness is ∼3.5 nm (Figure 2e). After selectively staining PEO block with phosphotungstic acid (PTA), two dark layers are discerned at the exterior surfaces (Figure 2f). Particularly, the gap in between the silica layers was observed under high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) at a magnified scale (inset, Figure 2f1). The gap appears dark while the exterior surfaces are bright. This indicates that the gap contains no PTA. After another staining with ruthenium tetraoxide (RuO4), the PS blocks are distinguished within the gap (inset, Figure 2f2). In order to adjust microstructure of the Janus shell of PEO45b-PTEPM35-b-PS110, another diblock copolymer PEO318-bPS120 is added to stabilize the paraffin/water emulsion. When the PEO318-b-PS120/PEO45-b-PTEPM35-b-PS110 weight ratio is fixed at 3:1, the core/shell sphere is also achieved (Figure 3a). Differently, individual nanodiscs with a circular contour are achieved after treatment with THF (Figure 3b). There are mesopores within the nanodiscs under TEM (Figure 3c). The
Figure 1. (a) GPC traces of (a) PEO; (b) PEO-CPADB; (c) PEO-bPTEPM; (d) PEO-b-PTEPM-b-PS; (b) 1H NMR spectrum of PEO45b-PTEPM35-b-PS110 in CDCl3.
macromolecular chain transfer agent PEO−CPADB was synthesized via the reaction between the ended hydroxyl group of PEO and the carboxyl group of CPADB in the presence of EDC·HCl using DMAP as a catalyst. Only one GPC peak of the product is present, implying that all PEO molecules are terminated with CPADB (curve b, Figure 1a). Mn is slightly increased to 3.7k. PEO45-CPADB was determined by 1 H NMR (Figure S1a). PEO-b-PTEPM was synthesized using PEO-CPADB as a macro-CTA and AIBN as an initiator (5:1 molar ratio) at 60 °C. Molecular weight of PEO-b-PTEPM is dramatically increased to 15.8k with a small PDI of 1.09 (curve c, Figure 1a). PEO45-b-PTEPM35 was determined by 1H NMR (Figure S1b). Based on PEO-b-PTEPM, a triblock copolymer of PEO-b-PTEPM-b-PS was synthesized. Molecular weight is further increased to 32.8k with a PDI of 1.20 (curve d, Figure 1a). PEO45-b-PTEPM35-b-PS110 was determined by 1H NMR (Figure 1b). PEO45-b-PTEPM35-b-PS110 is amphiphilic and can form a self-organized film at an oil/water emulsion. When adding acid in the aqueous phase to adjust pH = 3, a sol−gel process occurs at the paraffin/water emulsion interface at 70 °C for 8 h. The paraffin core is covered with a composite shell (Figure 2a). A strong broad peak appears around 1104 cm−1, which is assigned to the asymmetric Si−O−Si stretching vibration (Figure S2). After dissolution of the paraffin core with THF, a hollow sphere 1345
DOI: 10.1021/acsmacrolett.6b00812 ACS Macro Lett. 2016, 5, 1344−1347
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ACS Macro Letters
ratio 6:1, number of the mesopores increases while the Janus nanodiscs are the same in size (Figure S6a). At a higher ratio of 10:1, the mesopores become smaller (Figure S6b). Meanwhile, the disc contour becomes irregular. On the other hand, at a lower ratio of 2:1, a Janus hollow sphere is obtained (Figure S6c). Some pores are present within the shell (Figure S6d). The Janus mesoporous nanodiscs are amphiphilic and well dispersible both in water and oil. A bilayered superstructure is found after drying the aqueous dispersion (Figure 4a). Since
Figure 3. (a) SEM image of the sphere with a paraffin core and a composite shell; (b, c) SEM and TEM images of the trilayered composite Janus mesoporous nanodisc; (d) AFM height image and inset phase image of the Janus mesoporous nanodisc; (e, f) TEM and inset SEM images of the Janus nanodisc after treatment with n-hexane and the derived Janus mesoporous nanodisc after further washing with THF.
nanodiscs are thick, ∼5 nm, measured by AFM (Figure 3d). When paraffin selective solvent n-hexane is used to dissolve the core, the nanodiscs are adhered together rather than individual. No pores are found within the nanodiscs (Figure 3e). It is understood that PEO318-b-PS120 is not dissolved with n-hexane. After a further treatment with THF, the discs become isolated and mesopores appear within the nanodisc (Figure 3f). PEO318b-PS120 is present in the THF supernatant (Figure S3). This implies that PEO318-b-PS120 acts as a porogen to form the mesopores within the nanodiscs. Besides, the discs are dispersed in the PEO318-b-PS120 continuous phase at the paraffin particle surface. This morphology is analogous to an oil/water/oil triple emulsion structure. Hydrogen bonding between Si-OH group during PTEPM hydrolysis and PEO segment of PEO318-b-PS120 is conducive to the phase separation at the emulsion interface forming the mesopores.6 When the sol−gel process is faster performed at pH = 10, isolated nanodiscs are also achieved. However, the mesopores are irregular and disordered (Figure S4). It is explained by an incomplete phase separation at the emulsion interface during the faster sol−gel process. In order to demonstrate the method general, another triblock copolymer PEO45-b-PTEPM35-b-PS448 is used to achieve the corresponding Janus mesoporous nanodiscs (Figure S5a). Moreover, Janus mesoporous nanodiscs can be also obtained from the PEO45-b-PTEPM35-bPS110/PEO45-b-PTEPM35-b-PS448 mixtures (Figure S5b). This method is robust to avoid strict requisites of narrow molecular weight distribution and specific range of block fraction of the copolymers.2 PEO318-b-PS120/PEO45-b-PTEPM35-b-PS110 weight ratio is significant to affect formation of the Janus nanodiscs. At a high
Figure 4. (a) SEM and (b) AFM images of the bilayered superstructure by the Janus mesoporous nanodiscs in water; (c) left: a cyclohexane/water mixture; right: cyclohexane/water emulsion stabilized with the Janus mesoporous nanodiscs; oil soluble dye dilC18 is added into cyclohexane for easy observation; (d) fluorescence microscopy image of the cyclohexane/water emulsion droplet; (e) SEM image of the frozen paraffin/water emulsion; (f) a magnified SEM image of the paraffin sphere surface.
the superstructure is dispersible in water, it is reasonable that the PEO blocks are present at the outer surface while the PS blocks are sandwiched. Five nm and 10 nm heights are measured within the superstructure (Figure 4b), implying the superstructure is consisted of two nanodiscs stacked by partially overlapping. Some larger superstructures coexist (Figure S7). In cyclohexane, another bilayered superstructure coexists with some larger superstructures (Figure S8). It is reasonable that the PS blocks are present at the outer surface facing the oil phase. The Janus mesoporous nanodiscs can serve as a solid surfactant to stabilize emulsions. A cyclohexane/water immiscible mixture is used as a model immiscible system (left, Figure 4c). A trace of oil soluble dye dil-C18 is added into cyclohexane for easy observation. In the presence of the Janus mesoporous nanodisc (Figure 3c), a cyclohexane/water (1:5 v/v) emulsion forms (right, Figure 4c). The emulsion is stable over months. The emulsion droplets are 2−4 μm in diameter (Figure 4d). In order to observe orientation of the Janus mesoporous nanodiscs at the emulsion interface, a melt paraffin (Tm = 52−54 °C) is used to form a melt paraffin/water emulsion at 70 °C. Upon cooling to room temperature, the emulsion droplets 1346
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J. L.; Liang, F. X.; Yang, Z. Z. Macromolecules 2013, 46, 4126−4130. (c) Yang, H. L.; Liang, F. X.; Chen, Y.; Wang, Q.; Qu, X. Z.; Yang, Z. Z. NPG Asia Mater. 2015, 7, e176. (6) (a) Catauro, M.; Bollino, F.; Papale, F.; Gallicchio, M.; Pacifico, S. Mater. Sci. Eng., C 2015, 48, 548−555. (b) Yu, K.; Smarsly, B.; Brinker, C. J. Adv. Funct. Mater. 2003, 13, 47−52.
become solid (Figure 4e). Orientation of the Janus mesoporous nanodiscs is frozen. The paraffin core is covered with a single layer of the Janus mesoporous nanodiscs, which are lying on the sphere surface (Figure 4f). In summary, we have proposed a facile approach toward PEO/silica/PS composite trilayered Janus mesoporous nanodisc from a gelable triblock copolymer PEO-b-PTEPM-b-PS. It is significant to add another copolymer PEO-b-PS to induce a bimodal phase separation to generate isolated nanodiscs and mesopores within the nanodiscs during the coassembled sol− gel process of PEO-b-PTEPM-b-PS at an emulsion interface. The Janus mesoporous nanodiscs are amphiphilic to form superstructures in dispersions. They can serve as a solid surfactant to stabilize emulsions. It will be interesting to investigate mass transportation through Janus mesoporous nanodiscs when they are present at an interface.
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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.6b00812. Experimental details, and additional FT-IR spectra, SEM, and TEM images (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhenzhong Yang: 0000-0002-4810-7371 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by NSF of China (51233007, 51622308). We thank Prof. Ke Zhang of Institute of Chemistry (CAS) for helpful discussion about the copolymer synthesis, Prof. Dong Wang of BUCT for AFM measurement, and Prof. Lin Gu of Institute of Physics (CAS) for HAADF-STEM measurement.
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