Preparation of Microcapsules Containing Two-Phase Core Materials

Nov 9, 2004 - surfactant in the capsule core on the dispersity of β-CuPc particles in ... particles on the internal surface of capsule wall were expe...
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Langmuir 2004, 20, 10845-10850

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Preparation of Microcapsules Containing Two-Phase Core Materials Jian-ping Wang, Xiao-peng Zhao,* Hui-lin Guo, and Qing Zheng Institute of Electrorheological Technology, Department of Applied Physics, Northwestern Polytechnical Uuniversity, Xi’ an, 710072, People’s Republic of China Received April 9, 2004. In Final Form: July 22, 2004 Urea-formaldehyde (UF) microcapsules containing two-phase core materials in which phthalocyanine blue BGS (β-CuPc) particles were homodispersed in tetrachloroethylene (TCE) were prepared by in situ polymerization. The effects of the various process parameters, including the type of surface modifier, the viscosity of UF prepolymer, the type of water-soluble surfactant, and the concentration of oil-soluble surfactant in the capsule core on the dispersity of β-CuPc particles in TCE and the properties of the capsule wall and the adsorption of β-CuPc particles on the internal surface of capsule wall were experimentally investigated. It was shown that using octadecylamine (ODA) to modify β-CuPc particles resulted in a significant increase of the dispersing extent (DE) and the electrophoresis velocity of the particles in TCE (about 4 and 20 times more than that of unmodified). In addition, the optimal reaction conditions of the synthesis UF prepolymer were obtained by the orthogonal test. On the other hand, as the oil/water interfacial tension of emulsion was big enough, the microcapsule formed. The concentration of Span-80 in TCE was no less than 0.062 mM; the adsorption of β-CuPc particles on internal surface of wall were restrained. Finally, the microcapsules in which β-CuPc particles possess reversible response to dc electric field were obtained.

Introduction Electronic paper, a new type of the display technology, has recently gained much attention due to its advantages with flexibility, ultrathinness, low power consumption, and convenient portability.1-5 Electronic paper made of the electronic ink microcapsule is one of the display technologies with a great potential. Key to preparing such electronic paper is how to achieve the microcapsule containing two-phase materials. Comiskey et al. reported a sort of microcapsule of electronic ink with white particles dispersing in a blue dyed fluid that was microencapsulated by means of an in situ polymerization of urea and formaldehyde.1 Up to now, electronic ink display has been focused on the preparation of color electronic ink based on the three basic colors RGB (red-green-blue). However, there is rarely a report on the preparation method with regards to color electronic ink microcapsule containing functional two-phase core materials. Microencapsulation of two-phase materials is more difficult than that of the single-phase material. First, the problems that must be solved are the stability and the responsiveness to an electric field of the two-phase materials. Second, it must be prevented that the particles in the core material depart from the oil phase to the water phase during the encapsulation process. Third, the absorption of the particles on the internal surface of the capsule must be restrained. If these problems cannot be solved, the electric responsibility and functionality of the prepared microcapsules will become poor. Furthermore, * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Comiskey, B.; Albert, J.; Yoshizawa, H. Nature 1998, 394, 253255. (2) Wlsnleff, R. Nature 1998, 394, 225. (3) Rogers, J. A.; Bao, Z.; Baidwin, K. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4835. (4) Sheridon, N. K.; et al. In Proc. IDRC97; Morreale, J., Ed.; Society for Information Display: Toronto, 1997; L82-L85. (5) Podojil, G. M.; et al. In SID98 Digest; Morreale, J., Ed.; Society for Information Display: Anaheim, CA, 1998; 51.

the optical behaviors of the microcapsules are remarkably relied on their morphologies, particle distribution, surface roughness, and mechanical strength. A smooth surface and a good morphology of the capsule wall are favorable to the transmittancy of the material. Liquid cores, solid shells, oil phases, and UFs used in encapsulation may be prepared by a number of methods.6-8 The in situ polymerization of encapsulation in this case relies on monomers in the oil phase reacting at the oil/ water (o/w) interface with monomers from the aqueous phase.9-11 In this process, the prepolymers are incorporated only into the oil phase and they are polymerized interfacially by increasing temperature.12 Furthermore, the formation of UF microcapsules also depends on the presence of negatively charged polyelectrolyte material, such as ethylene maleic anhydride copolymer, poly (acrylic acid), methylvinyl ether maleic anhydride copolymer, and styrene maleic anhydride copolymer.13 Although a capsule prepared by such a method has a smooth inner surface, its outer surface is rough.14 This certainly will influence the optical behaviors of the electronic ink microcapsules. To help solve some of these problems, we suggested a method that could improve the properties of electronic ink microcapsule by managing process parameters in this paper. We prepared UF microcapsules containing twophase core materials, β-CuPc homodispersed in TCE, by an in situ polymerization technique. Key process param(6) Ichikawa, K. J. Appl. Polym. Sci. 1994, 54, 1321. (7) Mahabadi, H. K.; Ng, T. H.; Tan, H. S. J. Microencapsulation 1996, 13, 559. (8) Arshady, R. J. Controlled Release 1990, 14, 111. (9) Dobashi, T.; Yeh, F. J.; Ying, Q. C.; Ichikawa, K.; Chu, B. Langmuir 1995, 11, 4278. (10) Park, S. J.; Brendl’e, M. J. Colloid Interface Sci. 1997, 188, 336. (11) Park, S. J.; Donnet, J. B. J. Colloid Interface Sci. 1998, 206, 29. (12) Kim, A.; Park, S. J.; Lee, J. R. J. Colloid Interface Sci. 1998, 197, 119. (13) Foris, P. L.; Brown, R. W.; Phillips, P. S., Jr. US Patent 4,001,140, 1977. (14) Brown, E. N.; Sottos, N. R.; White, S. R. Exp. Mech. 2002, 42, 372-379.

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eters, involving modifier type, the viscosity of prepolymer, the type of surfactant in water phase, and the surfactant content in the oil phase, affecting the formation and the properties of microcapsules, were investigated in detail. Furthermore, we proposed a preliminary mechanical model for the formation of the capsule wall in the absence of negative-charged polyelectrolyte and investigated the reversible response of microcapsules containing β-CuPc particles under electric field. Experimental Section Surface Modification of β-CuPc and Preparation of the Core Material. Anhydrous alcohol (100 mL) and 0.3 g of cetyl trimethylammonium bromide (CTAB) were added in a 250 mL three-necked flask that contained 5.0 g of β-CuPc. The mixture was refluxed under stirring at 80 °C for 1.5 h, and then the alcohol was removed through a rotary evaporator in a vacuum at 50 °C. Thus, β-CuPc modified with CTAB was obtained. β-CuPc modified with octadecylamine (ODA) was gained with the same procedure as that of CTAB. Unmodified and modified β-CuPc (0.1 g) were placed in three test tubes, respectively. Span80 (30 mg) and 15 mL of TCE were added in each test tube. The suspensions were ultrasonicated for 20 min, and then the two-phase core materials were obtained. Preparation of Microcapsules. First, a mixture was prepared by mixing 0.25 mol of urea and 0.468 mol of formaldehyde (37%). The mixture was adjusted to pH 9.0 with triethanolamine and stirred until the urea was dissolved completely. After reacting at 75 °C for 1.5h, the product was rapidly cooled to room temperature, and then 30 mL of distilled water were added to the product to obtain a UF prepolymer solution. Second, 10 mL of the UF prepolymer solution were added into a 100 mL beaker containing 20 mL of distilled water. The pH of the solution was adjusted to approximately 3.0 with a buffer solution made of citric acid and sodium hydroxide. Under stirring, 2 mL of the core material was added into this solution to form an o/w emulsion. The polymerization was performed at room temperature for 1h. Then, the mixture was diverted into a 150 mL three-necked flask and further treated for 1.5 h at 60 °C in a water bath. Finally, the mixture was cooled, filtered, washed, and dehydrated to obtain resultant capsules.15 Characterization. The β-CuPc was characterized with a FTIR (EQUINO × 55 type) spectrophotometer. The particle size and the size distribution were determined by image analysis software (Data translation, Inc., global lab image/2 v3.5). The measurement of the dispersing extent (DE) of β-CuPc in TCE was effected by literature procedures.16 The optical micrographies was recorded by a biomicroscope (Nikon corporation, Alphaphot - 2 YS2 - H) with a CCD (Fujitsu corporation). The surface/ interfacial tension of emulsion was determined by the drop volume method at 25 °C((0.5 °C).

Results and Discussion 1. Effect of Surface Modification on Properties of β-CuPc. 1.1. The DE of Modified β-CuPc in TCE. β-CuPc has a higher irregularity surface with a dimensional scale 2.45 ( 0.10 according to Mather.17 ODA (or CTAB) is a liner molecule containing a hydrophobic group and a hydrophilic group, so it is suggested that ODA (or CTAB) is adsorbed on β-CuPc surface by single contact adsorption. Figure 1 shows the dispersion curves of β-CuPc modified with different surfactant in TCE. It shows that the DE of β-CuPc modified with ODA is excellent and the DE of β-CuPc after modifying with ODA is enhanced about 4 times that before modifying. This indicates that the β-CuPc modified with ODA would possess an outstanding compatibility in TCE. The DE of β-CuPc modified with CTAB is also improved, but its DE is slightly higher than that (15) Zhao, X. P.; Wang, J. P. CN Patent 02139592.6, 2002. (16) Zhang, T. Y.; Zhou, C. L. Dyes Pigments 1997, 35, 123-130. (17) Mather, R. R. Dyes Pigments 1990, 14, 249-274.

Figure 1. Dispersion curves of β-CuPc modified with different surfactants in TCE: (a) β-CuPc modified with 6 wt% ODA; (b) β-CuPc modified with 6 wt% CTAB; and (c) unmodified β-CuPc.

of unmodified β-CuPc. This shows that its modification effects were much behind those when β-CuPc was modified with ODA. 1.2. The Particle Size and the Size Distribution of Modified β-CuPc. Surface modification of particles can reduce the tendency of particles to flocculate during drying and when used in the dispersion medium. Figure 2 shows the particle size and the distribution of β-CuPc modified with ODA and CTAB and unmodified β-CuPc particles. The unmodified β-CuPc has a wider distribution and a bigger average diameter (up to 2.39 µm) mainly due to the aggregation of fine particles. However, the size distribution of modified β-CuPc obviously becomes narrow and the average diameter is smaller (1.29 µm for β-CuPc modified with 6 wt% ODA and 1.66 µm for β-CuPc modified with 6 wt% CTAB). The smaller average diameters, narrower particle size distribution, and softer texture aggregation of modified β-CuPc merit increasing the dispersion stability of the particles in TCE. 1.3. Electrophoresis Velocity of β-CuPc Particles. In general, a solid dispersed in a liquid may have a negative charge usually located at the interface, whereas the surrounding liquid may contain the positive charges necessary to make the total charge equal to zero, in the form of an excess of positive ions. These are distributed as an “ion cloud” around the solid.18,19 Electrophoresis gives an indication of the distribution of electrical charges at the interfaces studied,20 and this is important for understanding the behavior of dispersions of solids in liquids with regard to their stability against aggregate formation. For the system of β-CuPc particles dispersed in TCE, we used an electrophoresis apparatus (homemade electrophoretic cell) with 10 mm of gap between electrodes to determine the electrophoresis velocity of particles under an external electric field. A dc electric field about 10 V/mm is applied between electrodes. The consumed time of particles migration from negative to positive electrode is used to characterize the response to electric field. Furthermore, the charge property of particles is obtained by its migration direction under dc electric field. Experimental results show that they have all negative charge and the electrophoresis speed is 0.33, 2.8, and 7.7 mm/ min for unmodified β-CuPc, modified with CTAB, and modified with ODA, respectively. Such result may be due (18) Conway, B. E. Electrical Double Layer and Ion Adsorption at Solid/Solution Interfaces. In Encyclopedia of Surface and Colloid Science; Hubbard A. F., Ed.; Marcel Dekker Press: New York, 2002; pp 16581681. (19) Janusz, W. Electrical Double Layer at Oxide-Solution Interfaces. In Encyclopedia of Surface and Colloid Science, Hubbard A. F., Ed.; Marcel Dekker Press: New York, 2003; pp 1-17. (20) Hunter, R. J. Electrokinetics of Particles. In Encyclopedia of Surface and Colloid Science; Hubbard, A. F., Ed.; Marcel Dekker Press: New York, 2002; pp 1907-1919.

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Figure 2. Particle distribution of (a) β-CuPc; (b) β-CuPc modified with 6 wt% CTAB; and (c) β-CuPc modified with 6 wt% ODA. Table 1. The Investigated Variables and Their Levels levels of each variable variables investigated A B C D

Figure 3. FTIR spectra of (a) β-CuPc; (b) β-CuPc modified with CTAB; and (c) β-CuPc modified with ODA.

Figure 4. Reaction equation for urea and formaldehyde.

to modified β-CuPc having a smaller particle size and a narrower size distribution. 1.4. FT-IR Spectra of β-CuPc. To compare surface modification effects of CTAB and ODA on β-CuPc, FT-IR was performed in the unmodified original sample, modified β-CuPc with 6 wt% of CTAB, and modified with 6 wt% of ODA. Figure 3 shows that peaks of a C-H (CH3- and -CH2-) stretching vibration at 2850 and 2917 cm-1 can be observed (Figure 3b, c). Moreover, a higher peak of C-H stretching vibration appears in Figure 3c. Therefore, it is obvious that β-CuPc has been coated with CTAB and ODA and the effect of coating with ODA is better than CTAB. 2. Influence of the Viscosity of Prepolymer on the Properties of the Microcapsules. 2.1. Polymerization Conditions of Prepolymer Investigated by Orthogonal Experiment. Figure 4 presents the suggested reaction mechanism of UF prepolymer. If there is an appropriate

reaction time (min) urea/formaldehyde (mol) reaction temperature (°C) pH

1

2

3

45 1/1.6 60 7.0

60 1/1.75 75 8.0

75 1/1.9 90 9.0

reaction condition at the polymerization, hydrotrope UF monomer (1-hydroxymethylurea and 1,3-dihydroxymethyl) will be produced. Otherwise, the cross linkage and polymerization between monomers will be continued to produce an insoluble white deposit of high-molecularweight UF resin in water. Thus, the viscosity of prepolymer solution will be markedly increased due to the production of linear prepolymer (1-hydroxymethylurea and 1,3dihydroxymethyl).21 Therefore, we selected the limiting viscosity number of prepolymer as a criterion of orthogonal design. The best condition for preparing UF prepolymer was studied by the orthogonal test. Four controllable variables, including reaction temperature (°C), reaction time (min), mole ratio of urea to formaldehyde (mol), and the pH value, were selected, each at three levels. The investigated variables and their test levels are listed in Table 1. According to the experimental design theory, the orthogonal array L9(34) was selected to arrange the test program. The limiting viscosity number of prepolymer solution was a criterion of each test. The test results are listed in Table 2. Obviously, the order of influence of each variable on the limiting viscosity number of prepolymer solution is A > D > B > C. The variance of reaction time is the greatest, (21) John A. D. In Lange’s Handbook of Chemistry,15th ed.; John, A. D., Ed.; CN Sci. Pub. Press: Beijing, 2003; Chapter 10, p 17.

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Figure 5. Result analysis curves of orthogonal layout. Table 2. Experimental Arrangement and Test Result experiment no.

A

B

C

D

[η]

1 2 3 4 5 6 7 8 9 K1 K2 K3 variance

1 1 1 2 2 2 3 3 3 0.558 1.134 0.764 0.576

1 2 3 1 2 3 1 2 3 0.830 0.912 0.713 0.199

1 2 3 2 3 1 3 1 2 0.719 0.898 0.837 0.179

1 2 3 3 1 2 2 3 1 0.656 0.819 0.981 0.325

0.3074 0.7320 0.6334 1.3873 1.0835 0.9298 0.7953 0.9210 0.5760

the optimum levels of each variables is A2B2C2D3 (Figure 5). Thus, the optimum reaction condition is obtained as follows: reaction time (min), 60min; mole rate of urea to formaldehyde (mol), 1:1.75mol; reaction temperature (°C), 75 °C; and pH value, 9. 2.2. Influence of the Limiting Viscosity Number of Prepolymer Solution on the Properties of Capsules. During the preparation of prepolymer, the reaction urea and formaldehyde in the water phase will form a lowmolecular-weight prepolymer. As the molecular weight of the prepolymer increases, it deposits at the o/w interface.22 Thus, the UF microcapsules ultimately become highly cross-linked and form the microcapsule shell wall.23 Gelatin of bulk UF resin is attributed to the coalescence of a lyophobic colloidal sol,24 which are known to precipitate out of solution as the molecular weight increases.25 Here, the molecular weight of prepolymer is very important for the preparation of a microcapsule because the limiting viscosity number is the function of the molecular weight. Figure 6 shows the optical micrographs of the microcapsules prepared by using a prepolymer with the different limiting viscosity number. The microcapsules in Figure 6a and b have rough surfaces, and large numbers of precipitate of UF resin are produced in the water phase. However, the microcapsules prepared with prepolymer under the optimal conditions have smooth surfaces and the average diameter of the capsule reduce (see Figure 6c). There is also not much UF precipitate in the water phase, which were caused by the different molecular weight of the prepolymer. (22) Brown, E. N.; Kessler, M. R.; Sottos, N. R.; White, S. R. J. Microencapsulation 2003, 20, 719-730. (23) Thies, C. Microencapsulation. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N., Overberger, C. G., Menges, G., Kroschwitz, J. I., Ed.; Wiley Press: New York, 1987; Vol. 9. (24) Pratt, T. J.; William, E. J.; Rammon, R. M.; Plagemann, W. L. J. Adhes. 1985, 17, 275-295. (25) Dunker, A. K.; William, E. J.; Rammon, R.; Farmer, B.; Johns, S. J. J. Adhes. 1986, 19, 153-176.

3. The Influence of Surfactants. 3.1. Influence of Surfactant in the Water Phase on the Formation of Microcapsule. 3.1.1. Emulsion Droplet Size Distributions. In this experiment, five emulsifiers, including poly(acrylic acid), sodium dodecyl sulfate, OP-10, NaCl, and an intermixture of NaCl-glycerol, were used to improve the dispersion of the oil phase in the water phase. The concentration of emulsifiers in prepolymer hydrous solution is 1, 1, 1, 10, and 10 wt% of NaCl + 0.1 mL of glycerol, respectively. Under the same experimental conditions, they are all emulsified by stirring (800 rpm) for 10 min. The average droplet diameters for these systems containing 10 mL of prepolymer are presented in Table 3. It is found that using NaCl and NaCl + glycerol as emulsifiers yields large droplets with a broad size distribution, while using poly(acrylic acid), OP-10, and sodium dodecyl sulfate as emulsifiers yields small droplets with a narrow size distribution. Among the emulsions, those prepared by using sodium dodecyl sulfate and OP-10 as emulsifier have a much smaller average droplet size and a narrower size distribution compared with any of the other systems. These smaller droplet sizes are directly related to the fact that the o/w interfacial tension is decreased by a much greater extent when they are employed (see Table 4). The microcapsule formed from these emulsions will be discussed later. 3.1.2. Microcapsules Morphologies. Table 4 gives the o/w interfacial tension of different systems and the morphologies of prepared microcapsules in poly(acrylic acid), sodium dodecyl sulfate, OP-10, NaCl, and NaCl + glycerol. Figure 7 shows the corresponding optical micrograph of them. It is found that the influence of surfactant on the preparation of microcapsules is obvious. There is a bigger o/w interfacial tension in the systems with NaCl, NaCl + glycerol, and without surfactant, and the corresponding microcapsules have a smooth surface and a regular shape. When the system contains sodium dodecyl sulfate or OP-10, the o/w interfacial tension is small and the capsules will not be formed. Meanwhile, many UF precipitates are found in the water phase. These results indicate that UF polymer deposits at the o/w interface to form a microcapsule only when the o/w interfacial tension is large enough. The morphology of the microcapsules prepared from a surfactant containing poly(acrylic acid) is shown in Figure 7a. The microcapsules have the clear “hedgehog” morphology (as shown by the arrow in Figure 7a.). It can be explained as follows: when the emulsion is formed, the hydrophilic end of a poly(acrylic acid) molecule stretches out its carboxyl group toward the water phase but its oleophilic end is dissolved in the oil droplet interior. A chain of poly(acrylic acid) molecules is very long and has a lot of carboxyl groups (-COOH) on it. With the polymerization, the carboxyl on a poly(acrylic acid) molecule will catalyze polymerization and make UF polymer produce around its chain. Thereby, the “hedgehog” morphology around the microcapsule will be produced. In sodium dodecyl sulfate or OP-10, their hydrophilic groups, dissolved in the water phase, have a dissociable group (-SO3Na) or a nonionic surface-active agent. These groups either have no catalytic ability or reduce the surface energy of the oil droplet. Thus, UF polymer produced in the water phase cannot congregate on the o/w interface to form a capsule and many precipitates of the polymer will be produced in the water phase (as shown by the arrow in Figure 7b, e). In contrast, for the system containing NaCl or NaCl + glycerol, there is a bigger o/w interfacial tension. UF polymer can be deposited at the o/w interface to form a regular morphologic microcapsule (see Figure 7c, d).

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Figure 6. Micrograph of the microcapsules (a) prepared by experiment no. 1 in Table 2, (b) prepared by experiment no. 7 in Table 2, and (c) prepared by the optimal conditions. Table 3. The Diameter of an Emulsified Droplet Containing Different Surfactants surfactant

average diameter of droplet (µm)

poly(acrylic acid) sodium dodecyl sulfate OP-10 NaCl NaCl + glycerol

12.3 10.4 11.8 40.8 35.6

Table 4. Interfacial Tension and the Morphology of Microcapsules surfactant

γow

morphology of microcapsules

without surfactant poly(acrylic acid) sodium dodecyl sulfate OP-10 NaCl NaCl + glycerol

7.85 6.52 0.97 0.53 2.69 4.34

regular hedgehog shape without capsule without capsule regular regular

On the basis of these results above, we propose a model about the formation of the capsule wall, as shown schematically in Figure 8. With the formation of an emulsified droplet and the proceeding of the polyreaction, great deals of tiny polymer particles are formed in the water phase. When the o/w interfacial tension is bigger, these tiny particles will be congregated at the o/w interface so that the surface energy of the oil droplet is reduced and form the base of the capsule. With the proceeding of the reaction, not only is the capsule wall thickened but also the cross-link is performed between particles. Thus, the capsule wall will be further hardened and microcapsules with good airtightness are formed. In contrast, when the

o/w interfacial tension is smaller, the surface energy of the oil droplet is small enough. The system is in a stable state. The formed UF polymer particles cannot be congregated on the o/w interface. Furthermore, the UF polymer particle has a very strong hydrophilicity, which results in the polymer particle being retained in the water phase and forming a single large polymer particle rather than a capsule. 3.2. Influence of Surfactant in the Oil Phase on the Dispersibility of β-CuPc Particles Inside the Capsule. It is found that most of the β-CuPc particles in the core are adsorbed on the internal surface of the wall when an oilsoluble surfactant is not added in TCE. The prepared dried capsules have no response to a dc electric field. However, when 0.05 mL of Span80 (10 w/v% solution of TCE) are added in 15 mL of the core liquid, the adsorption phenomenon of β-CuPc particles on the internal surface of the wall disappear. To understand the action of Span80, we determine the o/w interfacial tension by the drop volume method. The Span-80 concentration in TCE is varied from 0 to 0.16 mM. The results are showed in Figure 9. It is found that the interfacial tension is 7.85 mNm-1 when Span80 is not added. With the increase of content of Span-80, the interfacial tension reduces gradually. When its content is over 0.062 mM (10 w/v% 0.04 mL TCE solution), the interfacial tension tends to saturate. Furthermore, the experiment also shows that not only is the dispersibility of β-CuPc in TCE improved but also the particles inside the capsule are not adsorbed on the internal surface of the wall when the concentration of Span80 is larger than 0.062 mM. This may be because the Span-80 molecule forms a protective layer on the interface

Figure 7. Microcapsules prepared from systems containing different surfactants: (a) poly(acrylic acid); (b) sodium dodecyl sulfate; (c) NaCl; (d) NaCl + glycerol; and (e) OP + 10.

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Figure 8. Schematic illustration of the forming mechanism of capsule (A) emulsified oil droplet, (B) emulsified oil droplet and UF polymer particle, (C) base of capsule, (D) regular capsule, (C′) the same as B, and (D′) the same as B.

Figure 10. Typical response images of capsules containing β-CuPc modified with ODA to applied dc electric field at different stages.

β-CuPc, the same phenomenon has been observed. Nevertheless, their response time is over 2.6 s and the clusters cannot disperse again in TEC when the field is turned off. This also indicates that modified β-CuPc particles possess better dispersion ability, which merits a quick response to electric field.

Figure 9. Interfacial tension of the Span-80 system and its action. (a) Interfacial tension of TCE containing Span-80/water. (b) Schematic illustration of Span-80 action.

between the core and the wall, which restrained the adsorption of β-CuPc particles (Figure 9b). 4. Response Behavior of the Microcapsules Under a dc Electric Field. In this experiment, the dried microcapsules containing two-phase core materials are placed in a microscopic electrophoresis cell, and a dc electric field of about 0.1 V/µm is applied. Figure 10 shows the response behavior of a typical capsule containing β-CuPc modified with ODA before and after applying the electric field. When the field is applied, the particles move to one side of the microcapsule quickly due to the presence of the electroosmotic and electrophoretic forces. The response time is about 0.6 s. If the field is turned off, β-CuPc particles disperse again in TEC. In this case, the dispersion of the particles almost completely depends on Brownian motion so that the dispersion rate is relatively low. When the direction of the field is reversed, it is found that the particles move quickly to another sidewall of the capsule. This shows that the response is as well reversible as the change of the applied electric field. Furthermore, The modified β-CuPc particle clusters are stable only when the field is applied, and they disperse as the field is turned off.26-29 For the microcapsules containing unmodified

Conclusions UF microcapsules containing β-CuPc fine particles homodispersed in TCE were prepared by an in situ polymerization technique. The dispersibility and migration speed of β-CuPc fine particles in TCE were found to be strongly influenced by the type of surface modifier. The experiments showed that the DE and migration speed of a β-CuPc particle modified with ODA in TCE were enhanced about 4 and 20 times more than that of unmodified β-CuPc, respectively. The formation of a microcapsule showed a dependence on the limiting viscosity number of prepolymer and the interfacial tension of o/w system. The higher limiting viscosity number of prepolymer and adequate o/w interfacial tension merited formation of a microcapsule. Moreover, the adsorption of β-CuPc particles on the internal surface of the wall was also related to the interfacial tension of oil droplet/water. When the concentration of Span80 is larger than 0.062 mM, the particles in the capsule were not adsorbed on the internal surface of the wall. The microcapsule, which was prepared on the basis of the optimal process parameters and conditions, could quickly and reversibly response to applied dc electric field. Acknowledgment. The authors are thankful to the National Natural Science Foundation of China (Grant No. 90101005), the National Natural Science Foundation of China for Distinguished Young Scholar (Grant No. 50025207), and Graduate Starting Seed Fund of Northwestern Polytechnical University (Grant No. Z20030095) for financial support. LA0490902 (26) Bohmer, M. Langmuir 1996, 12, 5747. (27) Trau, M.; Saville, D. A.; Aksay I. A. Science 1996, 272, 706. (28) Trau, M.; Saville, D. A.; Aksay, I. A. Langmuir 1997, 13, 6375. (29) Yuri, S.; Scott, A. G.; Michael, B.; John, L. A. Langmuir 2000, 16, 9208-9216.