Inward template synthesis of intact hollow spheres

Inward template synthesis of intact hollow spheres

Polymer 52 (2011) 4418e4422 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Polymer Communicati...

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Polymer 52 (2011) 4418e4422

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Polymer Communication

Inward template synthesis of intact hollow spheres Ke Shen a, Fuxin Liang a, *, Jiguang Liu a, Xiaozhong Qu a, Chengliang Zhang a, Jiaoli Li a, Qian Wang a, Wei Wei a, Yunfeng Lu b, Zhenzhong Yang a, * a b

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chines Academy of Sciences, Beijing 100190, China Chemical and Biomolecular Engineering Department, University of California at Los Angeles, Los Angeles, CA 90095, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2011 Received in revised form 20 July 2011 Accepted 23 July 2011 Available online 28 July 2011

A facile and general one-step approach is presented to synthesize hollow spheres with varied composition by an aerosol-assisted solvent evaporation process. The monomer of ethyl-2-cyanoacrylatemide contained in the aerosol droplets can form an outer shell by a fast polymerization around the droplets. Materials inside the droplets further grow inwardly against onto the interior surface of the first shell forming another shell forming composite hollow spheres. The hollow spheres are derived by dissolution of the outer shell, therefore the intact shell can be well preserved. Many approaches can be exploited forming the second shell for example sol-gel process of oligomers and phase separation from polymer solutions. Microstructure of the hollow spheres can be tuned from smooth to porous. The methodology is general. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: Inward template synthesis Hollow spheres Spray drying

1. Introduction Hollow spheres have attracted great attention in recent years for their potential applications in controlled release, artificial cells, light weight fillers and catalysis for confined reaction [1e8]. Template synthesis has been widely used toward a diversity of hollow spheres with varied compositions. Hard colloids such as silica or polystyrene latexes are commonly used as sacrificial cores in the template method, and a variety of materials ranging from polymer, inorganic materials to metal oxides, are coated onto the template surface [917]. Hollow spheres are achieved by removal of the hard cores either by dissolution or calcination at high temperature. The methods are essentially based on an outward growth from exterior surface of the template spheres and removal of the core materials which penetrates through the shell. The resulted osmotic pressure usually causes apertures or fragmentation in the shell. In order to preserve integrity of the shell of hollow spheres, an alternative approach by inward growing materials against the shell interior surface of a hollow sphere template has been proposed. For example, a kind of virus mimetic polymer hollow latex cages with hydrophilic transverse channels connecting to the interior surface are utilized as templates to fabricate hollow spheres with intact shells [18,19]. Precursors can diffuse inwardly through the channels and favorably grow onto the interior surface * Corresponding authors. Fax: þ86 10 62559373. E-mail address: [email protected] (Z. Yang).

forming a composite hollow sphere. Since the template is removed from the exterior surface of the hollow spheres, the shell integrity can be preserved better. Nevertheless, the prerequisite of this route is that, pre-designed functional groups should be distributed on the interior surfaces of the templates to induce the favorable growth of other materials therein. Herein, we report a simple approach toward hollow spheres by inward template synthesis against the interior surface of a shell as illustrated in Scheme 1. Different from the previous reports [18,19], the hollow sphere template is in situ constructed and the inward diffusion of precursors from the external surroundings is unnecessary. Multiple-component liquid droplets containing ethyl-2cyanoacrylatemide (ECA) form in air by spraying the mixture with an aerosol apparatus. When ammonia gas is introduced in the flowing stream (environment pH is about 13), spherical contour of the droplet thus size is immediately fixed by a fast polymerization of ECA on the droplet outer surface forming a first shell. Meanwhile, solvent evaporation induced dynamic phase separation and further condensation drive the other components inside the droplets to aggregate onto the interior surface of the first shell, forming the second shell subsequently. The second shell further grows inwardly during solvent evaporation out of the first shell. Eventually, a hollow cavity forms after all the solvent is evaporated and doubleshelled hollow spheres are achieved. Many routes can be utilized to create the second shell of varied compositions. Besides an easy formation of a polymer shell from a polymer solution directly by phase separation thereby, some precursors for example inorganic

0032-3861/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.07.036

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Scheme 1. Schematic fabrication of intact hollow spheres by inward template synthesis.

silica gel or organic phenolic resin (PF) can also deposit onto the first shell. The corresponding shell forms by a further crosslinking. After a simple dissolution of the first PECA shell from the exterior surface of the second shell, hollow spheres of desired compositions are derived. It is noticed that the second shell is not perturbed during removal of the PECA shell, and integrity of the shell can be well preserved. Surface microstructure of the hollow spheres can be tuned from smooth to porous.

2.4. PS/PECA composite hollow spheres

2. Experimental

A very dilute dispersion of the spheres in ethanol was dropped onto carbon coated copper grids for transmission electron microscopy (TEM) characterization (JEOL 100CX operating at 100 kV). The composite hollow spheres were embedded in epoxy resin for ultramicrotomed sliced samples about 30e50 nm thick using Leica ultracut UCT ultramicrotome at room temperature. Scanning electron microscopy (SEM) measurements and energy dispersive X-ray (EDX) analysis were performed on a HITACHI S4300 apparatus operated at an accelerating voltage of 15 kV. The samples were ambient dried and vacuum sputtered with Pt. Thermal stability of the spheres was observed under an Olympus optical microscopy equipped with a Linkam LTS350 hot stage. Particle size and size distribution were measured by a Malvern MasterSizer 2000 Particle Size Analyzer. Thermogravimetric analysis (TGA) experiments were performed on the PerkineElmer Pyris 1 TGA under nitrogen at a heating rate of 10  C/min. Environment pH of spray drying system was tested by putting the pH test paper infiltrated with water into the inside of aerosol apparatus.

2.1. Silica/PECA composite hollow spheres A typical silica sol was prepared by adding TEOS to a solution of acetone and HCl (molar ratio of TEOS:HCl:water ¼ 1:0.3:4) and refluxing at 60  C for 4 h. ECA monomer and acetone was added to this sol (weight ratio of TEOS:ECA:acetone ¼ 1:1:18) under stirring as a spray solution. The aerosol process was carried out on an apparatus as depicted in Fig. S1, and the pressure to drive the process was generated by a flowing air at a speed of 30 L/min. The inlet temperature is 25  C. The feed flow rate of the solution is 15 mL/min. Ammonia was mixed with gas in the inlet by flowing the inlet air through aqueous ammonia in order to catalyze a fast polymerization of ECA in air. After the silica/PECA composite hollow sphere powder was collected, it was heated at 60  C for 4 h in air for a further polycondensation of silica. The silica hollow spheres were obtained after dissolving PECA from the composite hollow spheres with acetone. PECA hollow spheres were obtained after etching silica form the composite hollow spheres with 5% aqueous HF. 2.2. PF/PECA composite hollow spheres PF and ECA monomer were added to acetone (weight ratio of PF:ECA:acetone ¼ 1:3:36) under stirring as a spray solution. The aerosol process and collection of samples are similar to the silica/ PECA composite hollow spheres. The PF/PECA composite hollow spheres were obtained after the sample was treated in aqueous HCl (0.2 mol/L) for 24 h at room temperature to drive a further crosslinking of PF. 2.3. Silica/PF/PECA composite hollow spheres A desired amount of PF, ECA and acetone were added to the as-prepared silica sol (weight ratio of PF:TEOS:ECA:ace tone ¼ 1:9:10:180) under stirring as a spray solution. The silica/PF/ PECA hollow spheres were obtained by heating at 100  C for 24 h in air for a further crosslinking of phenolic resin and polycondensation of silica. PF/silica composite hollow spheres were obtained after PECA was dissolved with acetone. Silica hollow spheres were obtained after PF was removed from the silica/PF composite hollow spheres by calcination at 550  C for 4 h in air. PF hollow spheres were obtained after silica was etched from the silica/PF composite hollow spheres with 5% aqueous HF.

A given amount of PS and ECA monomer were added to CH2Cl2 under stirring as a spray solution. During aerosol process, the solvent was easily evaporated, PS/PECA composite hollow spheres were fabricated. 2.5. Characterization

3. Results and discussion In the presence of ammonia in the gas stream, ECA monomer is polymerized rapidly from the gaseliquid interface of the aerosol droplets, forming a first shell to encapsulate the whole droplet. Meanwhile, size of the droplets is also fixed. While ECA monomer in the droplet diffuses outwardly, the shell becomes thicker and robust within a short time. Precursors start to settle onto the interior surface of the first shell during the solvent evaporation. By a further sol-gel process at high temperature, a silica/PECA composite hollow sphere forms (Fig. 1a). The average diameter of the silica/PECA sphere is about 9.3 mm measured by Malvern Mastersizer 2000 laser diffractometer (Fig. S2). The characteristic band at 1074 cm1 is assigned to SiO2, and the characteristic bands of PECA are at 1749 and 2993 cm1 (FTeIR spectra as shown in Fig. S3). From image of the deliberately broken sphere, the presence of the internal cavity is verified and both surfaces of the shell are smooth (Fig. 1b). After a selective removal of PECA with acetone, silica hollow spheres were derived. They are robust and the spherical contour is retained (Fig. 1c). The characteristic bands of PECA disappear, consistent with the removal of PECA (Fig. S3). Average diameter of the silica hollow spheres is smaller about 8.4 mm. The exterior surface of the silica shell is porous, whilst the interior surface is smooth (Fig. 1d, e). Interestingly, there exists a gradient of porosity evolution from the exterior to interior surface, indicating the

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Fig. 1. SEM images of some representative hollow spheres from silica/PECA composite hollow spheres at a typical recipe (ECA:TEOS:acetone ¼ 1:1:18). a) silica/PECA hollow spheres; b) the broken silica/PECA hollow spheres and inset cross-section of the spheres; c) the silica hollow spheres; d) exterior surface of the silica hollow spheres; e) crosssection of the silica spheres; f) the PECA hollow spheres; g) cross-section of the PECA hollow spheres; h) interior surface of the PECA hollow spheres.

presence of an intermediate layer of bi-continuous phase network. In order to further elucidate the microstructure, silica was selectively etched by 5% aqueous HF, which is confirmed by the disappearance of SiO2 peaks in FTeIR spectra (Fig. S3). The resultant PECA hollow spheres preserve spherical contour, indicating PECA shell is continuous (Fig. 1f). Especially, the exterior

surface is smooth (Fig. 1f, g), and the interior surface is porous (Fig. 1h). This finding further verifies that there exists the intermediate silica/PECA interpenetrating network layer. The composite shell is sandwiched with the exterior PECA, PECA/ silica interpenetration network (IPN) and the interior silica triple layers coexisted. ECA monomer polymerizes rapidly in ammonia

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Fig. 2. SEM images of two representative hollow spheres. a) Cross-section and interior surface of the silica hollow spheres at a lower TEOS content (ECA:TEOS ¼ 9:1); b) the PECA hollow spheres and inset cross-section at a lower ECA content (ECA/TEOS ¼ 1:9); c) schematic microstructure of the hollow spheres at three typical ratios of ECA/TEOS (9:1; 1:1; 1:9).

gas forming the exterior PECA layer. During acetone evaporation, the silane precursors experience a sol-gel process and aggregate onto the interior surface of the PECA layer forming the IPN intermediate layer. When the precursors are excess, additional silica interior layer forms achieving multiple-layered composite hollow spheres. Withholding the common multi-layered geometry, the as-prepared hollow sphere shell is still tunable with regard to its delicate microstructure (Fig. S4).

At a given concentration of the spray solution, the silica becomes perforative across the whole shell at a relatively higher content of ECA (for example ECA/TEOS ¼ 9:1) (Fig. 2a). At a relatively lower content of ECA (for example ECA/TEOS ¼ 1:9), only PECA shell becomes fully perforative (Fig. 2b). Thus, the hollow sphere shell structure is significantly influenced by the ECA/TEOS ratio as illustrated in Fig. 2c. At high content of PECA, the as-formed silica/PECA hollow sphere possesses PECA outer layer and silica/PECA IPN

Fig. 3. Morphologies of some representative composite hollow spheres. a, b) the PF/PECA hollow spheres and the corresponding PF hollow spheres; c) SEM image and inset crosssection TEM image of the silica/PF/PECA hollow spheres; d) SEM image and inset TEM image of the silica/PF hollow spheres.

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intermediate inner layer. Therefore, the perforative porous silica shell could be obtained from the silica/PECA IPN layer after removing PECA. At high content of silica content, the shell of silica/PECA hollow sphere is composed of the silica/PECA IPN outer layer and silica inner layer. Perforative PECA shell is left after removing silica. At a decreased concentration of ECA/TEOS mixture (for example 2 wt.-% and 0.5 wt.-%), the hollow spheres can not preserve spherical contour and the shell becomes wrinkled (Fig. S5). Besides inorganic materials, crosslinked phenolic resin hollow spheres can also be synthesized similarly. PF oligomer can be acidic crosslinked forming the second shell against the interior surface of the first PECA shell. Average diameter of the PF/PECA composite hollow spheres is 10 mm (Fig. 3a). After the PECA shell was dissolved with acetone, the corresponding PF hollow spheres were derived (Fig. 3b). With an increased PF content in the mixture solution, a PF oligomer capsule with the PECA shell was achieved when PF was not further treated for crosslinking. The capsules appear isolated powder at room temperature. At higher temperature (for example 120  C), the PECA shell was destroyed and PF oligomer was released (Fig. S6). This performance is promising for self-healing in composites. Besides, the idea works for synthesizing inorganicorganic composite hollow spheres. The silica/PF/PECA composite hollow spheres were obtained (Fig. 3c). Exterior surface of the asprepared silica/PF/PECA hollow spheres is smooth (Fig. S7a). When the PECA shell was dissolved with acetone, the corresponding silica/PF composite hollow spheres were achieved (Fig. 3d). The surface of the silica/PF hollow spheres is porous. Cross-section image indicates that there exists a gradient of porosity from the exterior to the interior surface (Fig. S7b). Similar to the previous silica/PECA hollow sphere, the composite shell of silica/PF/PECA is also sandwich-like with the exterior PECA, PECA/ silica/PF interpenetration network (IPN) and the interior silica/PF triple layers coexisting. The interior surface of the as-prepared silica/PF hollow sphere is smooth. After selective removal of either PF or silica from the silica/PF composite hollow spheres, the corresponding silica or PF hollow spheres were derived. Crosssection images of the silica hollow spheres (Fig. S7c) and PF hollow spheres (Fig. S7d) reveal that both silica and PF are continuous in the silica/PF composite shell. Pore diameter of the PF shell is 10e80 nm, while the pore diameter of the silica shell is 50e150 nm. Linear polymer for example polystyrene (PS) hollow spheres can be synthesized similarly from the solution containing PS. PS will enrich onto the interior surface of PECA shell while solvent evaporates. Eventually, the second PS shell forms while the internal cavity is achieved after all the solvent evaporates. It is noticed that surface of the PS/PECA hollow spheres becomes highly wrinkled since PS concentration is very low (Fig. S8a,b). The solution becomes increasingly viscoelastic with an increasing PS concentration. Hollow spheres and fibers coexist initially (Fig. S8c), and eventually PS fibers dominate at higher PS concentration (Fig. S8d). With the increase of PS concentration in the spray solution, solution viscosity increases, it is difficult for the sprayed solution to be split into liquid droplets from the nozzle. The polymer should be kept at

a low concentration level in the spray solution in order to obtain spherical shape. 4. Conclusion We have demonstrated a general approach toward hollow spheres by an aerosol-assisted solvent evaporation process. The solution contains ECA which can rapidly solidify forming the outer PECA shell in the presence of ammonia. A second inner shell forms subsequently against the interior surface of PECA shell. A diversity of chemistry could be exploited to generate the inner shell with desired compositions, for example sol-gel process of oligomers and phase separation from polymer solutions. Microstructure of the shell can be controlled from smooth to porous. After removing the exterior PECA shell, hollow spheres of the second shell can be easily derived without sacrificing shell integrity. The method is facile and scalable to synthesize hollow spheres with a variety of compositions. Acknowledgments This work was supported by the NSF of China (50733004, 20720102041, and 50973121) and CAS (KJCX2-YW-H20). Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.polymer.2011.07.036. References [1] [2] [3] [4] [5] [6]

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