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Preparation of Surface Porous Microcapsules Templated by Self-assembly of Nonionic Surfactant Micelles Baoquan Xie,†,‡ Haifeng Shi,† Guoming Liu,†,‡ Yong Zhou,†,‡ Yang Wang,† Ying Zhao,† and Dujin Wang*,† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100080, China ReceiVed December 5, 2007. ReVised Manuscript ReceiVed February 9, 2008
Microcapsules with controllable porous surface morphology and good monodispersity were prepared using the one-step synthetic strategy by employing the self-assembly template of nonionic surfactant micelles above its cloud point. Both the pore size (from 100 to 400 nm) and pore density are tunable by changing the amount of core materials or the ratio of core material to shell material. This methodology provides a versatile and effective route for preparation of porous microsphere materials, which can encapsulate lipophilic functional compounds.
Introduction Polymeric microcapsules have recently attracted widespread interest as a novel type of smart carriers and microreactors with designed properties because of their controllable permeability and surface functionality which are strongly affected by the size and surface morphology.1,2 The porous microcapsules are known to have large surface area and confined internal space, which, if accessible, can be used in numerous scientific and technological applications for drug storage and controlled release, the food and cosmetic industries, biotechnology, catalytic supports, as well as the media of confined crystallization, and so on.3–7 The templatedirected approach has been utilized to prepare nanoporous and mesoporous spherical materials.8–10 Many types of templates have been used, including hard templates such as * Corresponding author. E-mail:
[email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
(1) (a) Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2003, 42, 4472. (b) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (c) Sukhorukov, G. B.; Donath, E.; Davis, S. A.; Lichtenfeld, H.; Caruso, F.; Popov, V. I. Polym. AdV. Technol. 1998, 9, 1. (2) Kreft, O.; Prevot, M.; Mo¨hwald, H.; Sukhorukov, G. B. Angew. Chem., Int. Ed. 2007, 46, 5605. (3) (a) Skrabalak, S. E.; Suslick, K. S. J. Am. Chem. Soc. 2006, 128, 12642. (b) He, X.; Ge, X.; Liu, H.; Wang, M.; Zhang, Z. Chem. Mater. 2005, 17, 5891. (c) Zhang, X.; Tu, K. J. Am. Chem. Soc. 2006, 128, 15036. (4) Flodstro¨m, K.; Wennerstro¨m, H.; Alfredsson, V. Langmuir 2004, 20, 680. (5) Han, J.; Song, G.; Guo, R. AdV. Mater. 2006, 18, 3140. (6) Ras, R. H. A.; Kemell, M.; Wit, J. de.; Ritala, M.; Brinke, G. ten.; Leskelä, M.; Ikala, O. AdV. Mater. 2007, 19, 102. (7) Xie, B.; Shi, H.; Jiang, S.; Zhao, Y.; Han, C. C.; Xu, D.; Wang, D. J. Phys. Chem. B 2006, 110, 14279. (8) Caruso, F.; Caruso, R.; Mo¨hwald, H. Science 1998, 282, 1111. (9) Khanal, A.; Inoue, Y.; Yada, M.; Nakashima, K. J. Am. Chem. Soc. 2007, 129, 1534. (10) Sun, D.; Riley, A. E.; Cadby, A. J.; Richman, E. K.; Korlann, S. D.; Tolbert, S. H. Nature 2006, 441, 1126. (11) Tahaharea, Y. K.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. J. Am. Chem. Soc. 2005, 127, 6271.
silica spheres11–13 and polystyrene latices14,15 and soft templates, among which block copolymer latex16–18 and surfactant19–21 are the most commonly involved. In the past decade, porous materials made by use of self-assembled nonionic surfactants as templates have attracted extensive attentions for their nontoxicity, low cost profiles and hydrogen-bonding interactions with precursors.22–24 Nonetheless, the pore size of the porous materials made from nonionic surfactants template is generally less than 100 nm, mostly concentrated on mesoporous material, because of the limitation of molecular weight and aggregates formation below their cloud point temperatures (Tcp).13 For the synthesis of porous spheral materials with micro or macropores, the conventional templating method of self-assembling of nonionic surfactants seems to be helpless, because the synthesis temperature for such kind of materials generally can not be higher than the cloud point of nonionic surfactants. Therefore, the application of nonionic surfactant templating method is strongly limited in the field of microporous microcapsules (12) Polarz, S.; Smarsly, B.; Bronstein, L.; Antonietti, M. Angew. Chem., Int. Ed. 2001, 40, 4417. (13) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963. (14) He, X.; Ge, X.; Wang, M.; Zhang, Z. Polymer 2005, 46, 7598. (15) He, X.; Ge, X.; Wang, M.; Zhang, Z. J. Colloid Interface Sci. 2006, 299, 791. (16) Antonietti, M.; Berton, B.; Go¨ltner, C.; Hentze, H. AdV. Mater. 1998, 10, 154. (17) Deng, Y.; Yu, T.; Wan, Y.; Shi, Y.; Meng, Y.; Gu, D.; Zhang, L.; Huang, Y.; Liu, C.; Wu, X.; Zhao, D. J. Am. Chem. Soc. 2007, 129, 1690. (18) Fustin, C. A.; Lohmeijer, B. G. G.; Duwez, A. S.; Jonas, A. M.; Schubert, U. S.; Gohy, J. F. AdV. Mater. 2005, 17, 1162. (19) Berggren, A.; Palmqvist, A. E. C.; Holmberg, K. Soft Matter 2005, 1, 219. (20) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (21) Wang, J.; Xiao, Q.; Zhou, H.; Sun, P.; Yuan, Z.; Li, B.; Ding, D.; Shi, A. C.; Chen, T. AdV. Mater. 2006, 18, 3284. (22) Wan, Y.; Shi, Y.; Zhao, D. Chem. Commun. 2007, 897. (23) Kim, S.; Pauly, T. R.; Pinnavaia, T. J. Chem. Commun. 2000, 1661. (24) (a) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (b) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.
10.1021/cm7034618 CCC: $40.75 2008 American Chemical Society Published on Web 04/09/2008
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preparation. Although temperature-responsive reversible micellization behaviors of nonionic surfactants above the cloud point have been widely studied,25 as far as we know, there is no report on the preparation of microporous material with nonionic surfactant micelles as template above the cloud point. In the present investigation, we report an elegant and direct method for the preparation of surface porous monodispersed microcapsules with controllable surface morphology by using nonionic surfactant colloids as templates. The unique characteristic of the present preparation methodology is that the temperature for surface pore formation on the microcapsules is above the cloud point of the used nonionic surfactant. The size and density of the surface pores can be easily tuned from 100 to 400 nm by changing the core-to-shell material ratios. The microcapsules were prepared by in situ polymerization with melamine-formaldehyde resin (M-F) as the shell and normal alkanes as the core.26,27
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Figure 1. Distributions of hydrodynamic radius (Rh) for Triton X-100 micelles with the concentration of 0.5 wt % at different temperatures.
at 200 kV. The ultrathin section with a thickness of approximately 70-90 nm was obtained from samples embedded in an epoxy resin by ultramicrotoming under cryogenic conditions.
Experimental Section Materials. n-Octadecane was purchased from Aldrich Company (purity >99%). Melamine (purity 98%) and a nonionic surfactant, 4-(1, 1, 3, 3-tetramethylbutyl) phenyl-polyethylene glycol (Triton X-100) were purchased from Beijing Chemical Regents Inc. Formaldehyde solution (37 wt %) was purchased from Ji’nan Organic Chemical Plant. All the chemicals were used as received. Using melamine-formaldehyde resin as the shell material and n-octadecane as the core material, we prepared porous microcapsules by in situ polymerization, as described below. n-Octadecane (1 g) was mixed with a dilute solution of Triton X-100 (0.5 wt %) above the melting temperature of the alkane and the emulsion was prepared after being vigorously stirred for 1 h. At the same time, 1 g of melamine, 5 mL of formaldehyde solution, and 5 mL of deionized water were mixed and the pH of the mixture was then adjusted to 11 with sodium carbonate solution (10 wt %) and stirred at 60 °C till the suspension became transparent. Through this process, the prepolymer of shell material was synthesized, and then added to the n-octadecane containing emulsion. The pH value of the solution was adjusted to 4-5 by adding citric acid (5 wt %) and reacted for 3 h at 70 °C. At last, the reaction system was cooled down to 25 °C, filtered and washed with distilled water and dried overnight in a vacuum oven at 50 °C. Characterization. The temperature dependent dynamic light scattering (DLS) measurements were carried out with a temperature controllable LLS spectrometer (ALV/SP-125) which employs a multi-τ digital time correlator (ALV-5000). A solid-state He-Ne laser (output power ) 22 mW at λ ) 632.8 nm) was used as a light source. The scattering angle was 90°. The samples were introduced into the 7 mL glass bottle through a 0.45 µm filter prior to measurements. The autocorrelation function of scattering data was analyzed via the CONTIN method. The particle size and surface morphology of the microcapsules were examined on a Hitachi S-4300 scanning electron microscope (SEM), fitted with a field emission source and operated at an accelerating voltage of 15 kV. TEM imaging was performed using a JEM-2200FS transmission electron microscope (TEM), operated (25) (a) Ruiz, C. C.; Molina-Bolívar, J. A.; Aguiar, J. Langmuir 2001, 17, 6831. (b) Charlton, I. D.; Doherty, A. P. J. Phys. Chem. B 2000, 104, 8327. (26) Kumar, N.; Tilton, R. D. Langmuir 2004, 20, 4452. (27) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006.
Results and Discussion A nonionic surfactant, 4-(1, 1, 3, 3-tetramethylbutyl) phenyl-polyethylene glycol (Triton X-100), was used as emulsifier in the present investigation, and the cloud point of which is known as 59 ( 3 °C. Although extensive studies have been carried out on the Triton X-100 system to elucidate the micellar size and shape under various conditions below the cloud point,25 the micellar size and shape above the cloud point have not been well addressed. The temperaturedependent dynamic light scattering measurement of the Triton X-100 solution was carried out to investigate the micellar size above Tcp. The hydrophilic–lipophilic balance of nonionic surfactant containing polyoxyethylene chains is strongly affected by temperature, and the increase in temperature will lead to dehydration of the polyoxyethylene chains, thus leading to the decrease in surfactant solubility in water.20 Several nonionic surfactant molecules can aggregate into one micelle above the cloud point. The micellar size depends on the aggregation number of nonionic surfactant molecules. Figure 1 shows the distributions of micellar hydrodynamic radius (Rh) with the concentration of 0.5 wt % at different temperatures. With temperature increasing, the micelle grows up gradually below the Tcp and the hydrodynamic radius increases sharply from 14 to 193 nm with the temperature increased to the Tcp. At temperature above the Tcp two coexisted phases are water-rich micellar solutions in which the micelles may be spherical and fairly monodispersed.28 Although the amount of smaller micelle (Rh < 10 nm) decreases as the temperature elevates continually, the amount of bigger micelles increases because of the phase separation occurring in the nonionic surfactant solution. As the temperature is higher than 69.8 °C, just the big micelles with Rh of about 190 nm exist in the solution, the size of which does not increase with a further temperature increase. The final size of micelle depends on the amount of nonionic surfactant and the population of micelles. In the (28) Hall, D. G. Nonionic Surfactants Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Vol. 23, Chapter 5.
Microcapsules Templated by Nonionic Surfactant Micelles
Figure 2. Structure of Triton X-100 spherical micelle and schematic illustration detailing all major steps involved in the synthesis of surface porous microcapsules via self-assembly of the nonionic surfactant micelles as templates above the cloud point.
aggregation process, the micelle size grows from nanometer to submicrometer with the temperature increasing gradually. The aggregation process is temperature-responsive and reversible, i.e., the micelles can be redissolved as cooling the system below the cloud point temperature. Figure 2 shows the formation mechanism of the Triton X-100 spherical micelles and the synthesis schematic of the surface porous microcapsules by in situ polymerization of melamine with formaldehyde and succedent purification process. The stable n-octadecane O/W emulsion was first prepared using Triton X-100 (0.5 wt %) as emulsifier above the melting temperature of n-octadecane and below the cloud point of nonionic surfactant. With increasing temperature up to 70 °C, which is higher than the cloud point (59 ( 3 °C) of the surfactant, the monodisperse big micelles formed and were adsorbed on the surface of the n-octadecane emulsion droplets because increasing temperature leads to a weakening of the repulsive interaction between the micelles and oil droplets.29 At the same time, the prepolymer of melamine-formaldehydes, hydroxyl-methy melamine, was synthesized under alkaline condition and then added into the n-octadecane O/W emulsion; subsequently, the pH value of the system was adjusted to 4-5 by adding citric acid (5 wt %). The polymerization reaction continued for 3 h at 70 °C. Through this procedure, the M-F shell was formed, inside which n-octadecane was encapsulated, and the nonionic surfactant micelles deposited on the surface of the shell and were locked by the shell material. With the hypothesis that the size of absorbed-locked micelles can not further increase, the adsorption and aggregation processes are competitive as the nonionic surfactant concentration keeps constant. After the polymerization terminated, the emulsion was cooled down to 25 °C, filtered and washed with distilled water. During this process, the nonionic surfactant micelles were redissolved from the surface of the synthesized microcapsules and the holes were left. By the so-called in situ polymeri(29) Hough, D. B.; Thompson, L. Nonionic Surfactants; Schick, M. J., Eds.; Marcel Dekker: New York, 1987; Vol. 23, Chapter 11.
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Figure 3. Distribution of hydrodynamic radius (Rh) for the octadecane emulsion system at room temperature. The sample was directly measured after emulsification without filtration.
Figure 4. SEM images of porous microcapsules prepared by in situ polymerization via self-assembly of Triton X-100 micelles as template above its cloud point. The amount of core material is 1 g, with the ratio of core material to shell material of 1:4. The inset is the high-magnification image.
zation process, the surface pore size and density are directly related to size and density of micelles absorbed on the oil droplet. The n-octadecane droplet distribution of the emulsion was investigated by dynamic light scattering before the prepolymer of shell materials was added. The sample was directly measured after emulsification without filtration. As shown in Figure 3, two peaks emerge in the emulsion system. The big peak with the average hydrodynamic radius of 670 nm and the relative peak width of 0.15 is related to the size distribution of n-octadecane droplet, and the small peak corresponds to the small amount of nonionic surfactant micelle with a hydrodynamic radius about 52 nm. These results are consistent with the SEM measurements of prepared microcapsules, which will be introduced as follows. Scanning electron microscopy (SEM) images of the product were displayed in Figure 4, where monodispersed microcapsules with an average diameter of about 2.0 µm can be observed. As predicted in the above section, the microcapsules display rough surfaces and possess many pores on the surface with the pore size of about 150 nm. To investigate the compositional effect on the synthesis of porous microcapsules, a series of experiments were carried out by changing the amount of n-octadecane or the ratio of core material to shell material with all the other conditions fixing.
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Figure 5. SEM images of porous microcapsules prepared by fixing the ratio of core material to shell material at 1:3 and changing the amount of core material: (A) 0.50, (B) 0.67, (C) 1.00, and (D) 1.50 g, respectively. The insets are the high-magnification images.
Figure 5 shows the SEM images of porous microcapsules prepared by in situ polymerization with templating of Triton X-100 micelles self-assembly above its cloud point with fixing the ratio of core material to shell material while changing the amount of n-octadecane. The prepared monodispersed microcapsules show different surface porous structure with n-octadecane content. From Figure 5A to Figure 5D, with increasing amount of core material, the diameter of the microcapsules increases, accompanied by an increase in the pore density on the microcapsules but a decrease in the pore size from 400 to 100 nm. The pore size is well-accorded with the micelle size obtained from the temperature dependent dynamic light scattering measurements (Figure 1). As the n-octadecane content increased to higher values such as 1.0 and 1.5 g (images C and D in Figure 5), honeycomblike porous microcapsules were obtained. Such morphology may be attributed to the crowded aggregation of the micelles on the prepared microcapsules. The possible mechanism for the variation of both microcapsule size and surface morphology is proposed below. When keeping a constant stirring speed and ratio of core to shell material, the diameter of a single droplet increases with increasing n-octadecane content, and the amount of micelles adsorbed on the droplet surface increases, correspondingly, resulting in bigger microcapsules and densely piling of micelles on the microcapsule surface. However, the size of micelles locked by the M-F shell decreases with increasing n-octadecane content, because the total amount of nonionic surfactant molecules is constant. On the basis of the above discussion, a conclusion can be drawn that increasing the amount of core material (n-octadecane) will result in bigger microcapsule, higher pore density, and smaller pore size on the microcapsule surface.
The SEM images of porous microcapsules obtained by changing the ratio of core material to shell material were given in Figure 6. Morphology controllable porous microcapsules with good monodispersity were obtained. The diameter of the microcapsules can be easily tunable from 1 to 4 µm with alerting the ratio of core to shell material. From Figure 6A to Figure 6C, with an increase in shell material content, the microcapsule size increases, whereas the diameter of pores decreases. Similar to the above explanation, as the content of core material keeps constant, the droplet size increases with the increase of shell material content, and the surface area of the droplets increase correspondingly. All these behaviors lead to the increase in microcapsule diameter and a decrease in surface pore size. The TEM images of microcapsule prepared with the amount of core material of 0.5 g and the ratio of core material to shell material of 1:1.53 were shown in Figure 7. The size of the porous microcapsule is about 1µm and there are two pores on each microcapsule (Figure 7A), which is in good agreement with the results of Figure 6A. As shown in Figure 7B, the microcapsule has a typical core–shell structure and the shell thickness is about 100 nm, which indicates that the n-octadecane has been successfully encapsulated inside the microcapsules. To confirm that the formation of the surface pore is due to the self-assembly of nonionic surfactant micelles above Tcp, we synthesized the microcapsules using Triton X-100 as emulsifier at the temperature (50 °C) below Tcp (59 ( 3 °C) of Triton X-100. As shown in Figure 8, the surface of the microcapsules is smooth and there are no pores on the surface. This confirms that the nonionic surfactant molecules do not aggregate into big micelles below the cloud point of Triton X-100; therefore, the templating effect of nonionic
Microcapsules Templated by Nonionic Surfactant Micelles
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Figure 6. SEM images of porous microcapsules prepared by changing the ratio of core material to shell material with fixing the amount of core material (0.5 g): (A) 1:1.5, (B) 1:6, and (C) 1:12, respectively. The insets are the high-magnification images.
Figure 7. TEM images of microcapsule obtained with the amount of core material 0.5 g and the ration of core material to shell material 2:3. (A) The sample was prepared by the ultramicrotomy method (inset is SEM image of the same sample. (B) The high-magnification TEM image of the microcapsule (scale bar: 200 nm).
surfactant on the pore formation at 50 °C is neglectable. Above Tcp, however, the self-assembly of micelles provides the template for the formation of surface pores on the microcapsules. The presently obtained porous microcapsules have a plenty of current and future possible applications, as core–shell particles often exhibit improved physical and chemical properties over their single-component counterparts, and hence are potentially useful in a broader range of applications.30–32 The monodispersed porous microcapsules with controllable core–shell structure can also provide an ideal (30) Caruso, F. AdV. Mater. 2001, 13, 11. (31) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206. (32) Aden, A L.; Kerker, M. J. Appl. Phys. 1951, 22, 1242.
confined inner space for the study of confined crystallization of normal alkanes and other small molecules. Such surface porous microcapsules could also find applications as drug storage and controlled release, selective separation, catalytic supports, and so on. This preparation method of surface porous microcapsules is versatile in that it can be readily extended to other nonionic surfactant systems. On the basis of the prepared porous microcapsules, porous hollow microspheres can be easily obtained by heating to 250 °C or solvent etching to remove the core material. On the other hand, with the porous microcapsules as a new kind of template, hollow spheres can be obtained first using layer-by-layer methods and then exposing the coated particles to an acidic solution (pH < 1.6)
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Figure 8. SEM images of the microcapsule prepared with Triton X-100 as emulsifier at reaction temperature of 50 °C. The recipe of the reactant is the same as that in Figure 5C. The inset shows the high-magnification SEM image of the microcapsules surface. The inset scale bar is 500 nm.
to dissolve the acid-sensitive melamine-formaldehyde colloidal core.33,34 Conclusions
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monodispersity by employing the template of nonionic surfactant micelles self-assembly above the cloud point of the surfactant. Both the pore size (from 100 to 400 nm) and pore density are tunable by changing the amount of core materials or the ratio of core material to shell material. This method can provide an effective route for the preparation of porous microsphere materials encapsulating lipophilic functional compounds. The method is versatile in that it can be readily extended to other nonionic surfactant systems. Furthermore, similar preparations could be applied to a variety of other shell materials such as phenol formaldehyde resin, silica, carbon, and inorganic shell materials. We foresee that templating of nonionic surfactant micelles self-assembly above its cloud point can be used as a general route for the synthesis microporous (size >100 nm) materials with controlled morphologies and sizes. Acknowledgment. We thank the National Natural Science Foundation of China (50573086) for financial support.
We have demonstrated a novel simply synthetic strategy to prepare surface microporous microcapsules with good
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(33) Donatch, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; MÖhwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201.
(34) Sukhorukov, G. B.; Donath, E.; Davis, S. A.; Lichtenfeld, H.; Caruso, F.; Popov, V. I. Polym. AdV. Technol. 1998, 9, 1.
