Chinese Chemical Letters  2015, Vol.26 Issue (06):657-661   PDF    
pH responsive Janus polymeric nanosheets
Peng Zhou, Qian Wang , Cheng-Liang Zhang, Fu-Xin Liang, Xiao-Zhong Qu, Jiao-Li Li, Zhen-Zhong Yang     
State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Abstract: pH responsive polymeric Janus nanosheets with poly(maleic acid) moiety and crosslinked PS onto the corresponding sides have been synthesized by free radical polymerization. The Janus nanosheets can serve as solid emulsifier to stabilize an oil/water emulsion, whose stability is easily triggered by changing pH across pKa of the poly(maleic acid).
Key words: pH responsive     Janus     Nanosheets    
1. Introduction

Janus materials with two different compositions compartmentalized onto the same surface,have gained growing academic and industrial concerns [1, 2, 3, 4, 5, 6, 7]. Their anisotropic characteristics in composition or (and) shape endow the materials additional performances of such as amphiphilic [8, 9],magnetic [10, 11, 12] and catalytic [12, 13]. They can serve as building blocks toward complex structures [14, 15, 16, 17]. Those Janus particles with polar and apolar sides integrated have been extensively investigated as solid surfactants [18, 19, 20, 21]. They are capable to stabilize emulsions more effectively than the homogeneous counterparts arisen from the Pickering effect [22, 23]. Responsive Janus particles are becoming attractive since the Janus performances can be triggered by external stimuli such as pH [24, 25, 26],temperature [27],electric [28, 29] and ion [30]. In comparison with the spherical shape,Janus nanosheets are more advantageous in stabilizing emulsions since their rotation at an interface can be greatly restricted owing to highly anisotropic characteristic in shape [31, 32]. It is important to precisely tune thickness of the nanosheets. Lower amount of the Janus nanosheets is required to emulsify oil/water mixture using thinner nanosheets [33]. It becomes a key concern to develop methods to synthesize the responsive Janus nanosheets with tunable thickness. Multiple-step etching silicon substrate has been reported to achieve inorganic Janus nanosheets,which can stabilize liquid droplets more effectively even in air [34]. However, yield of the Janus nanosheets is extremely low. We have previously reported on the synthesis of inorganic Janus nanosheets and the corresponding inorganic-polymer bi-layered composite ones by crushing the corresponding polymeric hollow spheres [35, 36]. The Janus hollow spheres are prepared based on materialization of an emulsion interface using self-organized sol-gel process of silane precursors in the oil phase. Although the inorganic composite nanosheets can tolerate organic solvents,they are usually rigid and difficult to fold. Alternatively,polymeric Janus nanosheets are more flexible and foldable. Some guest species can be encapsulated thereby. Self-assembly approach from block copolymers is proposed to synthesize polymeric Janus nanosheets [37, 38, 39, 40, 41]. The block copolymers should possess sufficiently narrow molecular weight distribution in order to form uniform lamellar structure. Controlled polymerization methods are usually employed to synthesize such copolymers,which should be carried out under strict conditions. The polymeric Janus nanosheets lack of responsive performance. It is highly required to develop a general and facile approach toward Janus polymeric nanosheets which are responsive.

Herein,we report a facile approach to synthesize pH responsive Janus polymeric nanosheets as illustrated in Scheme 1. Four key steps are involved: (1) Maleic anhydride (MA) forms a selfassembly layer onto a sucrose particle template. MA is preferentially adsorbed onto the sucrose particle surface via hydrogen bonding between MA and hydroxyl group onto sucrose particle surface. Meanwhile,the particles become dispersible in toluene. (2) A polymer layer is subsequently grown onto theMAlayer. Since MA is not self-polymerizable,a polymer layer for example polystyrene (PS) is further grown by free radical polymerization after the initial copolymerization between MA and styrene (St). (3) Janus hollow spheres are derived after washing the sucrose particle with water,meanwhile the MA groups is hydrolyzed into maleic acid group (PMa). (4) Janus polymeric nanosheets are achieved after crushing the shell into pieces. Since one side of the nanosheets contain maleic acid group and the other side is composed of PS,the Janus nanosheets should be pH responsive. The PS layer will be strengthened to tolerate organic solvents rather than dissolution when a crosslinker for example divinylbenzene (DVB) is added in St.

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Scheme 1.Illustrative synthesis of the pH responsive Janus polymeric nanosheets. (1) Adsorption of MA (pink dots) onto the sucrose particle (blue droplet) surface. (2) A free radical polymerization of crosslinked PS (blue-black shell) conjugated onto the MA layer forming a PS-PMA bi-layered shell (orange shell). (3) Removal of the sucrose to achieve the polymeric hollow spheres whilst PMA is hydrolyzed into acidic PMa (red chains). (4) Janus polymeric nanosheets derived by crushing the shell into pieces.
2. Experimental

2.1. Materials

Maleic anhydride (MA),styrene (St) and azodiisobutyronitrile (AIBN) were purchased from Sinopharm Chemical Reagent Beijing Co. Divinylbenzene (DVB) was purchased from Sigma-Aldrich. Monomers of St and DVB were purified by passing through Al2O3 to remove the inhibitor. Sucrose particles were purchased from Sinopharm Chemical Reagent Beijing Co. and mechanically crushed into small particles. All the other reagents were used as received without further purification.

Synthesis of pH responsive Janus polymeric nanosheets: 4.0 g of sucrose particle was added into 50.0 mL of toluene containing 0.2 g of MA under stirring for 10 min. The sucrose particles were coated with a layer of MA to ensure a good dispersion. Free MA was washed away with 50.0 mL of toluene for three times after centrifugation. The MA coated sucrose particles were dispersed in 50.0 mL of toluene again. After degassing with nitrogen for 30 min, the solution of 0.12 g of St,0.12 g of DVB and 0.05 g of AIBN was added into the dispersion. The system was heated to 70 ℃ to initiate the polymerization and stood for 4 h to form the sucrose/ polymer core/shell particles. After centrifugation,free polymers were removed from the dispersion. Polymer hollow spheres were obtained after washing the sucrose with water at ambient temperature. After crushing the hollow spheres with ultrasonic cell crusher,the Janus nanosheets were achieved.

2.2. Characterization

Morphology of the samples was characterized using transmission electron microscopy (JEOL 100CX operating at 100 kV) and scanning electron microscopy (S-4800 at 15 kV). The samples for SEM observation were prepared by vacuum sputtering with Pt after being ambient dried. The samples for TEM observation were prepared by spreading very dilute dispersions in ethanol onto carbon-coated copper grids. FT-IR spectroscopy was performed using a Bruker EQUINOX 55 spectrometer with KBr pressed pellets with the horizontal attachment. Morphology of the emulsions was characterized using Olympus BX 51 microscope. Fluorescence microscopy images were taken using confocal laser scanning microscope (CLSM,Leica TCS-sp2,Germany) with an excitation wavelength of 488 nm. The absorption spectra of dispersions were collected in the range of 200-800 nm on a TU-1901 UV-vis spectrophotometer. AFM images were recorded under ambient conditions using a Digital Instrument Multimode Nanoscope IIIA at tapping mode. The Zeta potential of the Janus nanosheets was performed in aqueous dispersion at varied pH using a Zeta-sizer (Nano Series,Malvern Instruments,UK) at 25 ℃ and repeated for three times.

3. Results and discussion

The sucrose particles are large ranging from 100 to 101 of micrometers (Fig. 1a). It is easy to separate the particles by simple filtration or low speed centrifugation. The particle surface is smooth. Hydroxyl groups are present on the surface which is conducive to a favorable absorption of MA forming a self-assembly layer by hydrogen bonding. The sucrose particles precipitate from toluene. After coating with a layer of MA,the particles become well dispersible. After the dispersion is heated to high temperature for example 70 ℃ in the presence of AIBN,no any particles are found after washing with water. This implies that that MA is not self polymerized onto the particle surface. When monomers St and DVB are added in the dispersion,the polymer layer is formed [42, 43]. At an initial stage of the polymerization,MA should be copolymerized with the monomers. Upon consumption of MA,St is polymerized forming a crosslinked PS layer. The particle surface remains smooth after the polymerization (Fig. 1b). After washing with water,the particles are achieved which can keep their original shape without collapse (Fig. 1c). TEM image indicates they are hollow (Fig. 1d). This is resulted from removal of the sucrose core. The irregular contour of the hollow spheres is duplicated from the irregular sucrose particles. The polymeric hollow spheres are well dispersible in toluene but not in water. This indicates that the exterior surface is hydrophobic. A water/toluene immiscible mixture is used to demonstrate a selective capture of water by the hollow spheres (Fig. 2a,left). A trace amount of FITC is added into water phase for easier observation. Both top oil and bottom water phases are transparent. After addition of the hollow spheres, the top oil phase becomes opaque (Fig. 2a,middle). Upon stirring, the bottom water is absorbed inside the hollow spheres. As results, the whole system becomes opaque (Fig. 2a,right). CLSM image reveals that cavity of the hollow spheres is filled with the dyed water (Fig. 2b). This implies that the interior surface of the hollow spheres is hydrophilic. The hollow spheres are Janus. Besides the immiscible mixture,the Janus hollow spheres can capture water from the water-in-toluene emulsion droplets which are stabilized with a non-ionic surfactant Span-80. The droplets of the water-intoluene emulsion are spherical in shape (Fig. 2c) and large about tens of micrometers in diameter. In comparison,the Janus hollow spheres are irregular in shape. It is easy to discern the Janus hollow spheres from the water-in-toluene droplets. At early stage,no water is captured inside the Janus hollow spheres. The hollow spheres are invisible under CLSM,and only the droplets are visible. After 10 min,the irregular hollow spheres are visible (Fig. 2d, inset),which are coexistent with the water droplets. After standing for 20 min,no spherical droplets are visible (Fig. 2e,inset). This indicates that all water has been captured by the Janus hollow spheres (Fig. 2e). After filtration of the water absorbed Janus spheres,the continuous oil phase is separated. Interestingly,the oil phase becomes transparent. This implies that all water in the emulsion has been captured inside the Janus hollow spheres.

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Fig. 1.(a) SEM image of the sucrose particles; (b) SEM image of the sucrose/polymer core/shell composite particles; (c,d) SEM and TEM images of the polymeric hollow spheres after washing the sucrose particle core with water.

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Fig. 2.Water captured by the Janus polymeric hollow spheres. (a) Left: a water/toluene mixture,a trace amount of FITC is added into water phase; middle: the Janus polymeric hollow spheres dispersible in the oil phase; right: water is captured inside the hollow spheres which are dispersible in the oil phase; (b) the corresponding fluorescence microscopy image of water contained polymeric hollow spheres; (c) fluorescence microscopy image of the water droplets in the water-in-toluene emulsion stabilized with Span-80 (inset); (d) at the early stage (10 min) after the polymeric hollow spheres are added into the emulsion under stirring; (e) all water is captured inside the polymeric hollow spheres after 20 min.

Janus nanosheets are easily achieved by crushing the asprepared Janus hollow spheres. Janus nanosheets become smaller with crushing time. Both sides of the Janus nanosheet are smooth (Fig. 3a). The nanosheets are thick ~50 nm. It is rational that one side is crosslinked PS while the other side contains PMa moiety. Amine-group capped silica nanoparticles about 25 nm in diameter are used to favorably label the maleic acid terminated side by electrostatic interaction. Only one side of the nanosheets is covered with the silica particles,which is corresponded to the hydrophilic layer (Fig. 3b). Thickness of the Janus nanosheets measured by AFM (Fig. S1 in Supporting information) is consistent with the SEM measurement. Thickness of the Janus nanosheets is tunable from submicrons to several nanometers with decreasing the monomer feeding amount (Fig. 3c,Fig. S2 in Supporting information). Thicker nanosheets (~200 nm) can preserve sheet shape (Fig. 3d),implying they are more rigid. The thinner Janus nanosheets (5-10 nm) are flexible and easily foldable (Fig. 3e).

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Fig. 3.(a) SEM image of the Janus nanosheets from the hollow spheres prepared at a monomer feeding amount of 1%; (b) SEM image of the Janus nanosheets after a favorable absorption of amine-group capped silica nanoparticles; (c) dependence of thickness of polymeric Janus nanosheets (■) and relative amount of PS and poly(maleic acid) (▲) measured by FT-IR characteristic peak intergation on varied monomer/sucrose ratio; (d,e) SEM images of two representative polymeric Janus nanosheets obtained at two different monomer feeding amount levels of 10% and 0.005%,respectively.

The Janus nanosheets are pH responsive. PMa exhibits two step dissociations corresponding to pK1 of 3.2 and pK2 of 7.9 [44]. Zeta potential of the nanosheets dispersed in water is monitored at varied pH (Fig. S3 in Supporting information). A subtle increase of pH from 3.0 to 3.2 results an abrupt decrease in Zeta potential from neutral to negative. This means that the acid group has been highly ionized. With increase in pH,Zeta potential becomes further negative progressively. Another abrupt decrease in Zeta potential is observed after a subtle increasing pH from 7.5 to 8.0. When pH of the dispersion is at very low level below pK1 = 3.2,the PMa moiety preserves undissociated. The nanosheets precipitate from the aqueous dispersion (Fig. 4a,pH 3.0). When pH is slightly increased to 3.2,PMa is partially dissociated and becomes negatively charged. The side becomes hydrophilic,while the other side remains hydrophobic. The Janus nanosheets start to disperse in water (Fig. 4a,pH 3.2). When pH is further increased above 4,the Janus nanosheets are well dispersible in water (Fig. 4a,pH 4). On the other hand,the Janus nanosheets can be also well dispersible in toluene (Fig. 4b). This implies that the Janus nanosheets are amphiphilic and thus can serve as a solid emulsifier. At pH 8.0,the Janus nanosheets are stacked into a face-to-face bi-layered superstructure with the PMa-group terminated side exposed to the aqueous phase (Fig. 4c). Similarly,the Janus nanosheets are stacked into a back-to-back bi-layered superstructure in toluene. In the presence of 0.2 wt% of the Janus nanosheets (~200 nm thick),a toluene-in-water (1:2,v/v) emulsion forms (Fig. 4d,left). The droplets are 10-100 μm in diameter (Fig. 4d,inset). The emulsion is stable over weeks (Fig. 4e,left). The spherical contour is preserved after the emulsion droplets are dried. A magnified SEM image shows that the Janus nanosheets are stacked parallel onto the droplet surface (Fig. 4e,inset). In comparison,although a water-in-toluene emulsion forms in the presence of crosslinked PS nanosheets (Fig. 4d,right) due to Pickering effect,the emulsion is easily de-stabilized after 2 h (Fig. 4e,right). The Janus nanosheets are pH responsive,stability of the emulsion can be triggered by changing pH. At high pH level for example 8,the emulsion is rather stable (Fig. 4f,left). When pH is decreased to 2 after adding aqueous HCl,the emulsion starts to de-emulsify progressively. Two immiscible oil and water phases are separated with a distinct boundary. Eventually,the nanosheets are present in the top toluene phase after 2 h (Fig. 4f,right).

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Fig. 4.Emulsification of a representative immiscible liquid mixture with the nanosheets. (a) pH responsive behavior of the Janus nanosheets in water at varied pH; (b) the Janus nanosheets dispersed in toluene; (c) SEM image of the Janus nanosheets after drying the aqueous dispersion at pH 8,two Janus nanosheets are face-to-face stacked; (d) the emulsions stabilized with the Janus nanosheets (left) and crosslinked PS nanosheets (right),inset optical image of the emulsion stabilized with the Janus nanosheets; (e) 2 h later after the emulsions stabilized by the Janus nanosheets (left) and the crosslinked PS nanosheets (right),the dried droplets of the emulsion stabilized by the Janus nanosheets (inset); (f) the emulsion stabilized by the Janus nanosheets at pH 13 (left) and 2 h later after changing pH to 2 of the emulsion by adding aqueous HCl (right).
4. Conclusion

To summarize,we have proposed a facile approach to large scale fabricate pH responsive Janus polymeric nanosheets. The nanosheets are highly crosslinked and tolerant against organic solvents. When the Janus nanosheets serve as a solid emulsifier to stabilize an oil/water emulsion,de-emulsification is easily triggered by decreasing pH below pKa. Since the synthetic method is mainly based on a favorable absorption of functional monomers onto the template particles surface and free radical polymerization, many responsive Janus nanosheets of varied composition and performance are expected when other monomers are used.

Acknowledgments

This work was supported by Ministry of Science and Technology of China (No. 2012CB933200),the NSF of China (Nos. 51233007 and 51203169). Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.04.006.

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