pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication of Monodisperse Polymer Nanoparticles by Membrane Emulsification Using Ordered Anodic Porous Alumina Takashi Yanagishita,†,‡,§ Ryoko Fujimura,† Kazuyuki Nishio,†,§ and Hideki Masuda*,†,§ †
Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-03, Japan, ‡JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan, and §Kanagawa Academy of Science and Technology, 5-4-30 Nishihashimoto, Sagamihara, Kanagawa 229-1131, Japan Received October 15, 2009. Revised Manuscript Received November 29, 2009
Uniformly sized droplets of photocurable monomer were obtained through membrane emulsification using highly ordered anodic porous alumina as a membrane. Subsequent polymerization of the monomer generated polymer particles, whose sizes could be controlled by changing the size of the pores in anodic porous alumina. The size distribution was very narrow owing to the uniformity of pore size in the anodic porous alumina used for the emulsification.
Introduction The fabrication of monodisperse polymer particles from the micrometer to the nanometer scale has attracted growing interest because of its applicability to various fields, such as supporters of molecules, biomarkers, and various types of sensors.1,2 Large numbers of processes, such as emulsification polymerization and suspension polymerization, have been adopted for the preparation of monodisperse polymer particles.3-7 However, in most cases, only specific kinds of polymers can be applied to these processes. Among these processes, membrane emulsification is promising for the preparation of emulsions containing monodisperse droplets of liquid.8,9 In this process, uniformly sized droplets can be obtained by injecting a liquid with a dispersed phase into a continuous phase through a porous membrane with uniformly sized pores. The advantages of this process are the uniformity and controllability of the size of the obtained droplets for a wide variety of liquids. The size of the droplets can be determined by the pore size in the membrane used for emulsification. In our previous work, we reported the preparation of SiO2 nanoparticles by membrane emulsification using anodic porous alumina as a membrane. The anodic porous alumina, which is formed by the anodization of Al in an acidic electrolyte, has a unique geometrical structure, that is, uniformly sized straight pores perpendicular to the surface.10 Because of its unique geometrical structure, this material is promising for use as a membrane in emulsification. The solidification of droplets containing a silicate source generated the uniformly sized SiO2 nanoparticles.11 In the present report, we describe the preparation of uniformly sized polymer particles using ordered anodic porous *Corresponding author. E-mail:
[email protected].
(1) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (2) Chen, Z.; Zhan, P.; Wang, Z.; Zhang, J.; Zhang, W.; Ming, N.; Chan, C. T.; Sheng, P. Adv. Mater. 2004, 16, 41. (3) Esen, C.; Schweiger, G. J. Colloid Interface Sci. 1996, 276, 113. (4) Gu, S; Mogi, T.; Konno, M. J. Colloid Interface Sci. 1998, 207, 113. (5) Zhang, H.; Cooper, A. I. Chem. Mater. 2002, 14, 4017. (6) Martin-Banderas, L.; Flores-Mosquera, M.; Riesco-Chueca, P.; RodriguezGil, A.; Cebolla, A.; Chavez, S.; Ganan-Calvo, A. M. Small 2005, 1, 688. (7) Shpaisman, N.; Margel, S. Chem. Mater. 2008, 20, 1719. (8) Nakashima, T.; Shimizu, M.; Kukizaki, M. Key Eng. Mater. 1991, 61, 513. (9) Kawakatu, T.; Kikuchi, Y.; Nakajima, M. J. Am. Oil Chem. Soc. 1997, 74, 317. (10) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (11) Yanagishita, T.; Tomabechi, Y.; Nishio, K.; Masuda, H. Langmuir 2004, 20, 554.
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alumina as a membrane for emulsification. Although the preparation of uniformly sized polymer particles based on membrane emulsification has been reported previously, the lower limit of the size of obtained polymer particles was on the order of micrometers.12 This was because of the difficulty of forming a membrane with small pores, allowing the formation of small droplets. In the present work, we prepare uniformly sized polymer particles from the submicrometer to nanometer scale by membrane emulsification using highly ordered anodic porous alumina. The photopolymerization of small droplets of a photocurable monomer formed by membrane emulsification generated uniformly sized polymer particles. The uniformly sized polymer particles obtained by the present process are expected to be applied in various fields, such as drug delivery systems, biomarkers, and functional optical devices.
Experimental Sections Figure 1 shows a schematic for the preparation of uniformly sized polymer particles by membrane emulsification using anodic porous alumina. The ordered porous alumina used for emulsification was prepared by a similar process reported previously.13 In this process, the arrangement of pores was precisely controlled through pretexturing by imprinting using a mold with ordered convexes. Anodization of the pretextured Al generates anodic porous alumina with an ideally ordered pore arrangement. In the experiment, the periods of the pretextured Al were 200 and 500 nm. In the case of the 200 nm period, the tetxtured Al was anodized in 0.05 M oxalic acid solution at a constant voltage of 80 V at 0 °C for 90 min. For the 500 nm period, the textured Al was anodized at 200 V in 0.1 M phosphoric acid at 0 °C for 1 h. After the selective etching of Al in saturated I2 solution in methanol, a through-hole membrane was obtained by removing the bottom part of anodic porous alumina by an Ar ion milling apparatus. The size of the pores in the membrane was adjusted by a postetching treatment in 5 wt % phosphoric acid solution. The porous membrane was then set in the holder used for emulsification using epoxy resin. As a dispersed phase, a solution of photocurable monomer (PAK-02, Tokyo Gosei) and oleic acid (1:1 v/v) was used, and as a continuous phase, water with 0.3 wt % SDS as a detergent, was used. Although the details of the composition of (12) Chu, L.; Park, S.; Yamaguchi, T.; Nakao, S. Langmuir 2002, 18, 1856. (13) Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.; Tamamura, T. Appl. Phys. Lett. 1997, 71, 2770.
Published on Web 12/09/2009
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Figure 1. Schematic of the preparation of monodisperse polymer particles by membrane emulsification using anodic porous alumina. photocurable monomer (PAK-02) are not disclosed, it is a kind of monomer for acrylic resins. Emulsification was carried out at a constant pressure supplied by N2 gas. The pressure was usually from 10 to 40 kPa. Polymer particles were obtained by photopolymerization using a high-pressure Hg UV lamp. The size of the droplets obtained by emulsification was determined by a dynamic light scattering particle size analyzer (FPAR-1000, Otsuka Electronics Co.). The size of the polymer particles was determined by scanning electron microscopy observations (SEM, JSM-6700F, JEOL). The number of particles observed for the size analysis was 2000 for each sample.
Figure 2. Monodisperse nanoparticles prepared by membrane emulsification: (a) low-magnification and (b) high-imagination SEM images.
Results and Discussion Figure 2 shows SEM images of the obtained polymer particles. The polymer particles were observed after photopolymerization on the porous alumina membrane used for emulsification. The pressure used for the emulsification in the experiment was the lowest pressure under which the droplets could be formed through pores of each size. The increase in pressure caused the decrease in the uniformity of the obtained particles. The throughput of emulsion was ca. 0.1 μL/min 3 cm-2 under a pressure of 15 kPa for a pore size of 220 nm. The interfacial tension of the solution of monomer, which contains the detergent, was 0.6 mN/m. The calculated value of the pressure based on the Laplace equation was in good agreement with the experimental values. From the low-magnification SEM image in Figure 2a, it can be confirmed that uniformly sized polymer particles were formed by this process. In Figure 2a, each particle was dispersed without the formation of aggregates. As shown in the enlarged SEM image in Figure 2b, each polymer particle was almost perfectly spherical with a very smooth surface. Figure 3 shows the size distribution of the obtained polymer particles. In this Figure, the size distribution of the droplets in the liquid phase before polymerization is also shown for comparison. For the size analysis, 2000 particles were sampled from the high-resolution SEM images. From the results in Figure 3, the average size of the polymer particles was evaluated to be ca. 500 nm. The size distribution is confirmed to be very narrow. The relative standard deviation obtained from this analysis was 9.6% in the case of the sample in Figure 3. The average size of the polymer particles was almost the same as that of the monomer droplets. This implies that the droplets were solidified without any volume shrinkage during the polymerization. The wider size Langmuir 2010, 26(3), 1516–1519
Figure 3. Size distributions of (a) polymer particles and (b) droplets in an emulsion.
distribution of the droplets in Figure 3 was thought to be due to the lower resolution of the size measurement of the analyzer used to measure the droplets in the liquid phase. Figure 4 shows polymer particles of different sizes obtained by this process. One of the advantages of membrane emulsification is the controllability of the size of the obtained polymer particles. In the experiment, the pore size was varied between 130 and 320 nm. From the SEM images in Figure 4, it can be confirmed that uniformly sized polymer particles were obtained in all cases. The size of the obtained particles was dependent on the size of the pores in the membrane used for emulsification. Figure 5a shows the size distributions of the obtained polymer particles. The average sizes of the polymer particles were 196, 418, and 744 nm for pore sizes of 130, 220, and 320 nm, respectively. For each particle with a different size, the size distribution was very narrow. Figure 5b summarizes the relationship between the size of particles and the pore size in the membrane. In this Figure, the dependence of the standard deviation of DOI: 10.1021/la903913h
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Figure 5. (a) Size distribution of polymer particles prepared by membrane emulsification using anodic porous alumina with hole diameters of 130 (a), 220 (b), and 320 nm (c). (b) Relationship between the size of particles and the pore size in the membrane (a). Dependence of the standard deviation of particle size on the pore size in the membrane (b).
Figure 4. SEM images of monodisperse nanoparticles prepared using anodic porous alumina with hole sizes of (a) 130, (b) 220, (c) and 320 nm, respectively.
particle size on the pore size in the membrane is also shown. Particle size had an almost linear relationship to pore size. The relative standard deviation was ca. 10% for all samples and was independent of the pore size. During membrane emulsification, droplets were grown at the pores to a specific size and then became detached and diffused into the liquid phase. The results in Figure 5b imply that the growth and detachment of the droplets proceed in almost the same manner for the different pore sizes. Figure 6 shows an example of polymer particles with a size of less than 100 nm. For this sample, ordered anodic porous alumina with a 200 nm period was used for emulsification. The pore size in the membrane was 85 nm. According to the SEM image in Figure 6a, uniformly sized particles were formed even in this case. The sphericity of each particle was somewhat low compared to that of particles on the submicrometer scale. This is thought to be caused by the difficulty in photopolymerizing the small monomer droplets. Figure 6b reveals that the average size of the particles was 79 nm. The relative standard deviation was 9.1%, which is almost the same as that for particles on the submicrometer scale. From this result, it can be confirmed that uniformly sized polymer nanoparticles with a size of less than (14) Matsui, Y.; Nishio, K.; Masuda, H. Small 2006, 2, 522.
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Figure 6. SEM image of polymer particles with (a) a size of less than 100 nm and (b) a size distribution.
100 nm can be obtained by the present process. The lower limit of pore size in ordered anodic porous alumina is ca. 10 nm.14 Therefore, by adopting porous alumina with smaller pores for the emulsification, smaller polymer particles will be achieved.
Conclusions Uniformly sized droplets of photocurable monomer were obtained through membrane emulfication using highly ordered Langmuir 2010, 26(3), 1516–1519
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anodic porous alumina as a membrane. Subsequent polymerization of the monomer generated polymer particles whose size could be controlled from 80 to 320 nm by changing the pore size in the membrane. In each case, the size distribution was very narrow because of the uniformity of the pore size in the anodic porous alumina used for the emulsification. In the present experiment, the liquid with an interfacial energy of less than 1 mN/ m can
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reproducibly produce the droplets via our system. If the droplet can be solidified by the appropriate process, then this process can be used for the preparation of various kinds of polymer particle from the submicrometer to nanometer scale. The obtained monodisperse polymer particles are expected to be used for a wide range of application fields that require uniformly sized polymer particles.
DOI: 10.1021/la903913h
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