Inorganic Hybrid from Gelable

May 29, 2008 - As a result, novel organic/inorganic hybrid nano-objects such as arched ... Hairy Core–Shell Polymer Nano-objects from Self-Assembled...
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Langmuir 2008, 24, 6542-6548

Mesostructured Spheres of Organic/Inorganic Hybrid from Gelable Block Copolymers and Arched Nano-objects Thereof Ke Zhang, Xianglin Yu, Lei Gao, Yongming Chen,* and Zhenzhong Yang* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Sciences and Materials, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China ReceiVed January 11, 2008. ReVised Manuscript ReceiVed March 27, 2008 Mesostructured microspheres formed by aerosol-assisted self-assembly of a gelable block copolymer, poly(3(triethoxysilyl)propyl methacrylate)-block-polystyrene (PTEPM-b-PS), were studied by a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). When the copolymer composition was changed, the spheres with different internal patterns, such as onion-like lamella and curved cylinder, were obtained. Through a self-gelation process of PTEPM domains, novel organic/inorganic hybrid spheres with an internal tunable patterned structure were prepared. Since only PTEMP domains were cross-linked, the hybrid spheres could be further disintegrated by dispersion in a good solvent of PS. As a result, novel organic/inorganic hybrid nano-objects such as arched plates and cylinders were prepared.

Introduction Recently, well-defined spherical polymer particles have attracted great attention since they have many applications in advanced technologies. If a patterned structure can be generated inside these spheres, some new properties, for example, stepwise release, which cannot be obtained from simple spheres, are expected. It is well-known that self-assembly of block copolymers enables one to generate well-defined patterned structure in bulk,1 in solution,2 on a surface,3 and under a confined environment.4 Therefore, the microphase separation of block copolymers in a microdroplet should be an efficient and facile way to produce the internal patterned polymer spheres. However, very few reports on block copolymer microphase separation in a droplet have been found.5–9 Liu and co-workers have dispersed a block copolymer oil solution in water by an emulsion method and the polymer spheres with an intricate internal structure were obtained.6 Okubo et al. have obtained the spheres of an onion lamellar structure by dispersing the toluene solution of polystyrene-bpoly(methyl methacrylate) block copolymer in aqueous solution.7 The toluene evaporates from the polymer oil droplets in which the patterns are formed in the spheres by phase separation of the block copolymer. Also, onion-like polymer spheres have been formed by a two-step emulsion polymerization during synthesis of block copolymers.8 More recently, the formation of lamellar polymer particles has been reported by evaporation of a volatile * Corresponding author: phone, 0086-10-62659906; fax, 0086-1062559373; e-mail, [email protected]; [email protected]. (1) Abetz, V.; Simon, P. F. W. AdV. Polym. Sci. 2005, 189, 125–212. (2) Gohy, J. F. AdV. Polym. Sci. 2005, 190, 65–136. (3) Segalman, R. A. Mater. Sci. Eng. R 2005, 48, 191–226. (4) (a) Yu, B.; Sun, P. C.; Chen, T. H.; Jin, Q. H.; Ding, D. T.; Li, B. H.; Shi, A. C. Phys. ReV. Lett. 2006, 96, 138306. (b) Zhu, Y. T.; Jiang, W. Macromolecules 2007, 40, 2872–7335. (c) Chen, P.; Liang, H. J.; Shi, A. C. Macromolecules 2007, 40, 7329–7335. (5) He, X. H.; Song, M.; Liang, H. J.; Pan, C. Y. J. Chem. Phys. 2001, 114, 10510–10513. (6) (a) Lu, Z. H.; Liu, G. J.; Liu, F. T. Macromolecules 2001, 34, 8814–8817. (b) Zheng, R. H.; Liu, G. J.; Yan, X. H. J. Am. Chem. Soc. 2005, 127, 15358– 15359. (7) (a) Okubo, M.; Saito, N.; Takekoh, R.; Kobayashi, H. Polymer 2005, 46, 1151–1156. (b) Saito, N.; Takekoh, R.; Nakatsuru, R.; Okubo, M. Langmuir 2007, 23, 5978–5983. (8) (a) Okubo, M.; Izumi, J.; Takekoh, R. Colloid Polym. Sci. 1999, 277, 875–880. (b) Kagawa, Y.; Minami, H.; Okubo, M.; Zhou, J. Polymer 2005, 46, 1045–1049. (9) Yabu, H.; Higuchi, T.; Shimomura, M. AdV. Mater. 2005, 17, 2062–2065.

good solvent from a block copolymer solution containing a nonvolatile poor solvent.9 Above patterned particles are all generated in oil droplets dispersed in water phase. Another facile way to prepare such patterned spheres is the evaporation-induced self-assembly of block copolymer in gas, i.e., the so-called aerosol-assisted synthesis. This method has several advantages: (1) Self-assembly of a block copolymer occurs in droplets dispersed in gas rather than in bulk; (2) various mesostructures are expected dependent on composition; (3) it is simple, continuous, and scalable; (4) nano-objects with different shapes can be obtained easily by destroying the particles if one domain is fixed by cross-linking. The method has been used to synthesize mesostructured spheres by a combination of the assembly of amphiphilic molecules and sol-gel process of inorganic precursors.10 However, to the best of our knowledge, only one example of block copolymer self-assembly in aerosols without other additives has been reported so far. Thomas et al. have reported aerosol-assisted self-assembly of both polystyreneb-polybutadiene and polystyrene-b-polyisoprene to produce spheres with different internal structures.11 They have found that the block copolymers, which can exhibit lamellar, cylindrical, and spherical morphologies in bulk, result in concentric packing of lamellae, layers of curved cylinders, and irregularly packed spheres, respectively. Since the used block copolymers are purely organic, stability of the obtained spheres is relatively low. If one segment of a block copolymer contains reactive groups, further cross-linking may stabilize the preformed structures. This approach has been extensively applied to the block copolymer aggregates produced not only in solution12–15 but also in bulk;16–18 various stable materials of nanostructures have been prepared (10) (a) Lu, Y. F.; Fan, H. Y.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223–226. (b) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11, 579–585. (11) Thomas, E. L.; Reffner, J. R.; Bellare, J. Colloq. Phys. 1990, 51, C7, 363–374. (12) (a) Guo, A.; Liu, G.; Tao, J. Macromolecules 1996, 29, 2487–2493. (b) Yan, X. H.; Liu, G. J.; Li, Z. J. Am. Chem. Soc. 2004, 126, 10059–10066. (13) (a) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1996, 118, 7239–7240. (b) Ma, Q.; Remsen, E. E.; Clark, C. G.; Kowalewski, T.; Wooley, K. L. Proc. Natl. Acad. Sci. 2002, 99, 5058–5063. (14) Maskos, M.; Harris, J. R. Macromol. Rapid Commun. 2001, 22, 271–273. (15) (a) Du, J. Z.; Chen, Y. M.; Zhang, Y. H.; Han, C. C.; Fischer, K.; Schmidt, M. J. Am. Chem. Soc. 2003, 125, 14710–14711. (b) Du, J. Z.; Chen, Y. M. Macromolecules 2004, 37, 5710–5716. (c) Chen, Y. M.; Du, J. Z.; Xiong, M.; Zhang, K.; Zhu, H. Macromol. Rapid Commun. 2006, 27, 741–750.

10.1021/la800096w CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

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Scheme 1. Self-gelation of Block Copolymer Assembly in Aerosols To Produce Mesostructured Spheres of Organic/Inorganic Hybrida

a

Yellow stands for the PTEPM segment and its gelated domain; blue stands for the PS segment (not shown in onion).

Scheme 2. Disintegration of the Spheres with Internal Pattern into Individual Arched Organic/Inorganic Hybrid Nano-objectsa

a

Yellow stands for the gelated PTEPM domain while the hairy PS chains are not shown.

based on cross-linking. It is well-known that the sol-gel process of alkoxylsilyl groups has been applied to prepare the organic/ inorganic hybrid materials.19 Recently, we have studied the solution property of an amphiphilic block copolymer of poly(ethylene oxide)-b-poly(3-(trimethoxysilyl)propyl methacrylate) (PEO-b-PTMSPMA), which has a gelable segment. After the block copolymer self-assembles into micelles and vesicles in solution, self-gelation of PTMSPMA domain has resulted in robust organic/inorganic hybrid spheres and capsules.15 We have also studied the phase structure of poly(3-(triethoxysilyl)propyl methacrylate)-b-polystyrene (PTEPM-b-PS) block copolymers in the melt.20a After the gelation process occurred only in PTEPM domains, the organic/inorganic hybrid bulk materials with well-defined domain structures were obtained. Furthermore, the continuous PS phase can be dissolved in organic solvent and the novel hybrid hairy nano-objects like plates and cylinders have been produced. If a functional segment like poly(vinyl pyridine) was applied, the functional nano-objects with different morphology have been obtained.20b Herein, we report the synthesis of organic/inorganic hybrid mesostructured spheres from block copolymer PTEPM-b-PS of different composition by aerosol-assisted self-assembly. As shown in Scheme 1, the self-gelation inside these spheres produces the hybrid particles with different patterned structure dependent on polymer composition. By disintegration and dispersion in a good solvent of PS, curved lamellae and cylinders are derived (Scheme 2). To the best of our knowledge, this is the first example of the preparation of these hybrid nano-objects of curved morphologies from block copolymers.

Experimental Section Materials. The synthesis method of the block copolymers PTEPM88-b-PS408, PTEPM66-b-PS758, and PTEPM46-b-PS1009 and their bulk microphase separation morphologies were presented in (16) Liu, G. J.; Qiao, L. J.; Guo, A. Macromolecules 1996, 29, 5508–5510. (17) Ishizu, K.; Ikemoto, T.; Ichimura, A. Polymer 1999, 40, 3147–3151. (18) Liu, Y.; Abetz, V.; Mu¨ller, A. H. E. Macromolecules 2003, 36, 7894– 7898. (19) (a) Schottner, G. Chem. Mater. 2001, 13, 3422–3435. (b) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83–114.

a previous report.20a Tetrahydrofuran (THF) (>99%, Beijing Chemical Reagent Co.) was used as received. Preparation of Hybrid Spheres and Nano-objects. The aerosol device used for the preparation of the polymer spheres was similar to the device described elsewhere,10 which consists of a medical atomizer (Model YC-Y800, Beijing YA DU Science and Technology Co., Ltd.), a nitrogen trap, a drying chamber, a membrane filter, and a vacuum pump set in-line in the given order (Figure S1 in the Supporting Information). Aerosols were generated from the PTEPMb-PS polymer THF solution (0.2%, w/v) using the aerosol generator. The droplets were carried by a nitrogen stream through a homemade spiral glass drying tube with a heating length of 10 m. The glass tube inside diameter is 2.0 cm, which provides a mean residence time of approximately 90 s for droplets in the drying chamber at 200 °C. The powder product was collected on a Teflon filter (0.2 µm), carboncoated copper grid, or silicon substrate at a temperature of 60 °C to avoid THF condensation. For the gelation process, the powders were exposed to HCl atmosphere for about 1 h. After being crosslinked, the spheres were milled under liquid nitrogen and then immersed in THF under stirring. The particles were disintegrated into the corresponding nano-objects. Measurements. Scanning electron microscopy (SEM) was carried out on Hitachi S-4300 at 15 kV and 10 mA. The images were recorded by a digital camera. All samples were coated with platinum. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai F-30 instrument operated at an accelerating voltage of 300 kV, a JEM-2011 instrument operated at an accelerating voltage of 200 kV, and a Hitachi H-800 instrument operated at an accelerating voltage of 100 kV. Samples were embedded in epoxy and cured at 40 °C overnight. Thin sections (50-100 nm) were cut using Leica Ultracut UCT ultramicrotome and a diamond knife at room temperature. No staining was performed for the microtomed sections. SAXS data were collected on a Rigaku RU300 copper rotating anode (λ ) 1.54 Å) operated at 40 kVand 50 mA. X-rays were monochromatized with a Ni filter and focused using orthogonal Franks mirrors. SAXS patterns were collected with a 1 K × 1 K pixel CCD detector. The samples were loaded as thin slices into a 1.5 mm capillary. For the cross-linked spheres, 10 µL of toluene was added into the capillary.

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Figure 1. SEM image of the aerosol-assisted spheres formed by PTEPM88-b-PS408 collected on a silicon substrate.

Results Threediblockcopolymerswithdifferentcompositions,PTEPM88b-PS408 (Mw/Mn 1.20), PTEPM66-b-PS758 (Mw/Mn 1.09), and PTEPM46-b-PS1009 (Mw/Mn 1.10), were applied to prepare the block copolymer aerosols. Previously, it has been shown that PTEPM88-b-PS408 block copolymer in the bulk exhibits lamellar morphology, whereas PTEPM66-b-PS758 and PTEPM46-b-PS1009 display hexagonally packed cylinders and liquid-like micellar morphologies, respectively.20a An aerosol in a carrying nitrogen stream was generated from a block copolymer solution of THF, for example, PTEPM88-bPS408 (0.2%, w/v), using an aerosol generator. The solvent in aerosols was evaporated in a drying tube and the dried powder materials were collected by a filtering membrane. To make an electron microscopy analysis, the block copolymer particles were collected with a silicon substrate or a copper grid coated by carbon film. Figure 1 gives a representative SEM micrograph of the block copolymer particles prepared by PTEPM88-b-PS408 collected on a silicon substrate. The particles collected by filtering membranes are similar to those collected on the silicon, whose SEM micrograph is shown in Figure S2 in the Supporting Information. The aerosols are found to be spheres and most of them have the size of 100-1000 nm. The appearance of particles prepared by PTEPM66-b-PS758 and PTEPM46-b-PS1009 is similar to that obtained by PTEPM88-b-PS408. The structure of the particles, gelation, and derived individual nano-objects from each block copolymer were demonstrated separately in the following sections. PTEPM88-b-PS408: Onion Spheres, Organic/Inorganic Hybrids, and Curved Plates. Figure 2A shows a representative TEM image of a sphere ca. 500 nm without staining from PTEPM88-b-PS408. Onion-like internal structure with a period length of 31 nm is confirmed from the TEM image. To further explore the internal structure, an ultramicrotomed slice was obtained by embedding the spheres in epoxy resin. The TEM image (Figure 2B) displays a more clearly curved lamellar structure, and all the circular lamellae have the same center. The darker layer is corresponding to the PTEPM domain since this silica-rich domain scatters more electron beam. It is noticed that the thickness of the outmost dark layer is half the inner counterparts, indicating that the surface layer is PTEPM domain. This is consistent with the fact that the PTEPM domain has a lower surface energy than the PS domain. The semithickness of this layer is attributed to the A-BB-AA-BB- structure. Further(20) (a) Zhang, K.; Gao, L.; Chen, Y. M. Macromolecules 2007, 40, 5916– 5922. (b) Zhang, K.; Gao, L.; Chen, Y. M. Macromolecules 2008, 41, 1800–1807.

Figure 2. TEM images and SAXS characterization of the spheres obtained by aerosol-assisted self-assembly of PTEPM88-b-PS408: (A) the asprepared spheres, (B) the microtomed slice, and (C) SAXS curves of (a) the as-prepared spheres and (b) the cross-linked spheres in the presence of toluene.

more, this structure is universal for all the obtained particles with different sizes. The SAXS curve of the sphere powders (curve a, Figure 2C) is dominated by 1/q3 scattering and only a weak peak is observed. The d-spacing from this peak is ca. 30 nm, close to the TEM data but smaller than the equilibrium periodic length in the bulk. Due to the large surface area of the powders, the scattering from the surface may interfere with the scattering intensity of the lamellae. Because the PTEPM segment contains many ethoxysilyl groups that may undergo the sol-gel process, it provides us a chance to cross-link the PTEPM domains by forming a silica-based hybrid network similar to our previous reports in polymer vesicles in solution15 and ordered structure in bulk.20 The mesostructured powders from the block copolymer aerosols were exposed to

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Figure 4. (A) SEM and (B) TEM images of the hybrid curved nanoplates obtained by dispersing the disintegrated hybrid spheres in THF; the inset in B is a single plate (bar: 500 nm).

Figure 3. (A) TEM image of organic/inorganic hybrid spheres formed by PTEPM88-b-PS408 after being extracted with THF for 3 days; (B) SEM and (C) TEM images of the purely inorganic spheres obtained by calcining the hybrid spheres at 600 °C under air for 4 h.

hydrochloric acid for about 1 h to catalyze the sol-gel reaction. The completion of the sol-gel process was confirmed by FT-IR spectroscopy.20a SAXS analysis was also performed on the cross-linked particles by adding a small amount of toluene to decrease surface scattering. As shown in Figure 2C (curve b), the peak position ratio is 1:2, clearly revealing a lamellar structure. It is noteworthy that the scattering data was obtained in the presence of toluene, demonstrating the spheres have already been cross-linked and resisted the good solvent for the ungelated particle. The lamellar peaks of block copolymers indicate a d-spacing of ca. 37 nm. The slight change of the d-spacing can be attributed to a swelling of PS layers by toluene. The onion-like structure is well preserved in THF for 3 days (Figure 3A). To further confirm the hybrid structure, the spheres were calcined at 600 °C under air to remove

the organic component. As shown by the SEM image in Figure 3B, the spherical shape was retained with little shrinkage, which was caused by the loss of the PS layers. A TEM image (Figure 3C) displays the onion-like layers which are attributed to the silicon oxide structure. It should be mentioned that the white layers of the image are empty since the PS layers have been removed. Therefore, this can be an approach for preparing onionlike silica hollow particles. Thus produced onion particles are interesting since the gelation occurs only in silica-rich domains, which are isolated by PS layers. This structure looks like the Russia dolls. However, to split each layer by dispersion in good solvents for PS block is impossible since the gelated layers have a closed structure. We destroyed the onion particles by milling them in liquid nitrogen and then dispersed the residues in THF. The morphology of the destroyed hybrid particles dispersed in THF for 3 days is shown in Figures 4A and 4B. All the spheres were destroyed by the physical treatment and such plates with a curved appearance of uniform thickness, which look like the broken balloons, were obtained. The inset in Figure 4B is one piece of broken layer and a folded appearance is clearly observed, implying an arched character in the solution. Therefore, the molecular “Russia dolls” have been peeled off. Since the spheres were destroyed by physical treatment, the size and plate periphery are irregular. The curved morphology of the plates is inherited from the concentric lamella in the spheres, whose PTEPM layers are fixed by the gelation. This is in contrast to the plane layers from the block copolymers

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Figure 6. (A) TEM image of the hybrid curved nanocylinders obtained by dispersing the hybrid spheres in THF. (B) SEM image of the curved nanocylinders obtained by calcining the hybrid spheres at 600 °C under air.

Figure 5. SAXS and TEM characterization of the spheres obtained by aerosol-assisted self-assembly of PTEPM66-b-PS758: (A) TEM image of the as-prepared spheres; (B) SAXS curve of (a) the as-prepared spheres and (b) the cross-linked spheres in the presence of ethanol; (C) TEM image of the microtomed slice of the cross-linked spheres.

in bulk.20 Therefore, by combining the gelation process the block copolymer aerosols can be used to prepare the nano-objects with unusual morphologies. PTEPM66-b-PS758: Spheres Embedding with Cylinders, and Curved Hybrid Rods. The PTEPM66-b-PS758 block copolymer, which adopts hexagonally packed cylinders in bulk, was used to prepare spheres by an aerosol-assisted process. The periphery is a thin gray layer of a few nanometers (Figure 5A). Similar to the onions in the previous section, the outer surface of the sphere should be PTEPM chains.21 Figure 5B (curve a) is the corresponding SAXS curve of the sphere powders. Because of the large surface area of the powders, the scattering peak was not observed. In a previous section, we demonstrated that the self-gelation inside the onion particles does not alter the particle structure. Therefore, the particles of PTEPM66-b-PS758 were treated with hydrochloride acid to induce the internal gelation. After cross-linking, the gelated particles were mixed with ethanol (21) The “light gray shell” was confirmed to be the deformed sphere by gravity through tilting of the TEM sample.

to eliminate the influence of the large surface for SAXS analysis as shown in Figure 5B (curve b). One peak is found, corresponding to a d-spacing of ca. 34 nm, consistent with the bulk data. However, the typical ordered arrays are not observed. The internal structure of the gelated particles was studied by TEM analysis of microtomed slices. The outer region of the spheres displays a concentric layer and the inner region demonstrates some dark dots and short cylinders coexisting in the PS domain (Figure 5C). It is noteworthy that the outermost black shell is half as thick as the inner black dots, indicating that the PTEPM layer enriched on the outer surface of the spheres. This observation demonstrates that the surface of aerosol forces the PTEPM66-b-PS758, which gives a cylindrical structure in melt, to organize into a curved layer periphery. As for the detailed internal structure, the observation of morphology of destroyed spheres is more informative. Since the minor PTEPM segment forms the core of the cylinders, it is possible to get individual cylinders by dispersing the cross-linked particles in a good solvent for the PS block. The cross-linked particles of PTEPM66-b-PS758 were dispersed in THF under slow stirring and, after 3 days, a transparent dispersion was obtained, a hint of formation of small size particles. However, no closed cylindrical rings were observed from the TEM image (Figure 6A). Instead, a large amount of short wormlike uniform nanorods was obtained. The diameter of the silica core is ca. 14 nm, which is the same as the cylinders in bulk. It is interesting to notice that those nanorods adopt an arched morphology, reflecting that the cylinders are bent concentrically in the aerosols formed by PTEPM66-b-PS758. Also, it reveals that the gelated

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Scheme 3. Possible Cross-section Structure of Spheres from Sample PTEPM66-b-PS758a

a Gold color, PTEPM core; blue color, continuous PS domain; arch, curved cylinders of block copolymer; sphere, standing cylinders or spheres.

cylinders become rather stiff in solution. The calcination of these particles was also carried out to remove the continuous PS phase. The SEM image of calcined particles (Figure 6B) demonstrates the nanorods of silicon oxide. From the results of microtomed slice and dispersed cylinders, it can be confirmed that the sphere’s internal structure takes the shape of the short curved cylinders with low packing order. Scheme 3 illustrates a sphere cross section. To study the influence of molecular weight on the cylindrical morphology confined in the spheres prepared by this aerosolassisted process, we have also applied PTEPM38-b-PS371 and PTEPM38-b-PS497 block copolymers, which all form hexagonal cylindrical morphology in the bulk, to prepare the aerosols. The results obtained were similar to that from PTEPM66-b-PS758. In contrast to the long dispersed cylinders from the melt of PTEPM66b-PS758,20a only short cylinders with a curved conformation using the same block copolymer were obtained, demonstrating that the time of the solvent evaporation in the aerosols could be too short for the block copolymers organized into an ordered closed concentric cylinder. However, the arched hybrid rod is still very interesting. PTEPM46-b-PS1009: Spheres Embedding with Disordered PTEMPM Dots. The block copolymer PTEPM46-b-PS1009, adopting a spherical structure in the melt, was used to prepare polymer particles. Because the silica content is low, the internal structure of the particles cannot be visualized by TEM analysis even in the microtomed slice. Figure 7A (curve a) is the corresponding SAXS curve of the powders. Because of the large surface area of the powder, the scattering peak is not observed. After the PTEPM domains were cross-linked, the organic/ inorganic hybrid particles were prepared. Spherical gelated PTEPM domains are irregularly distributed in the PS matrix (Figure 7B). There exists a black shell at the outermost surface, indicating that the PTEPM domains are enriched on the surface. The SAXS curve of the gelated particles immerged in ethanol was obtained and a weak scattering peak was observed (Figure 7A, curve b). Thus, the particles hold a liquid-like spherical structure. The morphology of hairy spheres obtained by dispersing the cross-linked particles in THF is similar to the structure from PTEPM46-b-PS1009 in bulk, but the nanospheres were released from the microspheres (Figure 7C).

Discussion The above observations indicate that the aerosol-assisted microphase separation of the gelable block copolymers by an atomization process could be used to obtain the organic/inorganic hybrid spherical materials with a patterned inner structure and

Figure 7. SAXS curves and TEM characterization of the spheres obtained by aerosol assisted self-assembly of PTEPM46-b-PS1009: (A) SAXS curve of (a) the as-prepared spheres and (b) the cross-linked spheres in the presence of ethanol; TEM image of (B) the microtomed slice of crosslinked spheres and (C) the hybrid curved nanospheres obtained by dispersing the hybrid spheres in THF.

further to generate nano-objects with curved shapes. The atomizer produces tiny droplets of PTEPM-b-PS solution and the THF evaporates gradually in a N2 stream, accompanied by a dramatic decrease of particle size. The solvent evaporation resulted in a radial gradient polymer concentration in each droplet, whose surface has a higher polymer concentration. At a certain onset concentration, the block copolymer experiences a phase separation by forming supramolecular structures inwardly. The structures are fixed in the glassy PS domains after all the solvent is evaporated completely. Because of the lower surface energy, the PTEPM domains are preferentially located onto the outer surface of the spheres. For the block copolymers that organize into a lamellar morphology in the melt, the concentric layered onion structure is produced in the spheres. In terms of PTEPM66-b-PS758, forming short cylinders is due to a nonequilibrium metastable phase of

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the spheres formed in a short time. When the particles with different internal structures are produced, the sol-gel process in the PTEPM domains is carried out to stabilize the structures to give patterned hybrid spheres. Meanwhile, by physically destruction of the spheres and dispersion in a solvent, those nanoplates and nanocylinders with arched conformation are achieved (Scheme 2). It should be mentioned that the PS chains are densely grafted from those gelated nano-objects. This hairy structure is important for the dispersion and functionalization.

Conclusions In summary, mesostructured spheres of organic/inorganic hybrid with different internal structure have been synthesized by self-gelation of the preformed aerosols of a gelable block copolymer PTEPM-b-PS. The internal structure is dependent on polymer composition and onion-like multiple layered spheres and short concentrically cylinder spheres have been obtained. By destruction of the spheres, curved nanoplates and nanorods are prepared, respectively. This simple approach produces not only hybrid polymer spheres with patterned internal structure (22) Zhang, K.; Gao, L.; Chen, Y. M.; Yang, Z. Z. Chem. Mater. 2008, 20, 23–25.

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but also different novel arc-shaped nano-objects thereafter. These novel hybrid materials may find application as the supports of catalysts,22 the carriers of functional molecules for stepwise release, building blocks for self-assembly, and templates for preparation of novel nanomaterials. Acknowledgment. Financial support from the NSF China (50473056, 20534010, 20625412, 50573083, 50325313, and 50521302) and the 973 program of MOST (G2003CB615605) is gratefully acknowledged. The authors would like to thank Professor M. Schmidt and Dr. M. Maskos at University of Mainz, Prof. S. Gruner’s group at Cornell University for valuable discussion and SAXS measurement, Mr. S. Warren at Cornell University for discussion and language polishing, and Prof. Y. F. Lu at the Chemical and Biomolecular Engineering Department of UCLA for discussion on the aerosol process. Supporting Information Available: Scheme of the homemade aerosol apparatus and SEM image of the aerosol-assisted spheres formed by PTEPM88-b-PS408 and collected by a filtering membrane. This material is available free of charge via the Internet at http://pubs.acs.org. LA800096W