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Synthesis of functional microcapsules containing suspensions responsive to electric fields
Journal of Colloid and Interface Science 284 (2005) 646–651 www.elsevier.com/locate/jcis
Synthesis of functional microcapsules containing suspensions responsive to electric fields Huilin Guo, Xiaopeng Zhao ∗ , Jianping Wang Institute of Electrorheological Technology, Department of Applied Physics, Northwestern Polytechnical University, 710072 Xi’an, People’s Republic of China Received 24 April 2004; accepted 20 October 2004 Available online 15 December 2004
Abstract A sort of functional microcapsules, which contain a suspension responsive to electric fields, is prepared by in situ polymerization of urea and formaldehyde. The suspension is made up of pigment phthalocyanine green (PPG) and tetrachloroethylene. In order to solve the particles’ separation from the suspension during the microencapsulation and to obtain microcapsules applying to electronic ink display, the dispersibility of the particles, the contact angles between the particles and the tetrachloroethylene, and the influences of different emulsifiers on the microencapsulation are investigated. It is found that the dispersion extent and lipophilicity of the PPG particles are improved due to their surface modification with octadecylamine. The contact angles between the modified PPG particles and the tetrachloroethylene increase, and the PPG particles modified with 2 wt% octadecylamine have the best affinity for tetrachloroethylene. The interfacial tension between C2 Cl4 and H2 O with urea–formaldehyde prepolymer descends from 43 to 35 mN/m, which indicates that the polymer has certain surface activity. However, water-soluble emulsifiers have an important influence during the microencapsulation because they can absorb on the surfaces of internal phase and prevent the resin of urea–formaldehyde from depositing there. From the SEM images of shell surface and cross section, the microcapsules have relatively smooth surfaces and the average thickness is about 4.5 µm. When the microcapsules are prepared with agitation rates of 1000 and 600 rpm, the mean diameters of the obtained microcapsules are 11 and 155 µm, respectively. The particles in the capsules move toward positive electrode with a responsive time of several hundred milliseconds while providing an electric field. 2004 Elsevier Inc. All rights reserved. Keywords: Microcapsules; Suspension; Contact angle; Interfacial tension; Electric response
1. Introduction Microcapsules, whose diameters are from 5 to 2000 µm, can encapsulate various substances, such as gases, liquids, and solids, to form composite materials on a scale of millimeters. The substances can not only retain their own respective attributes, such as optics, magnetism, electricity, and other properties, but also combine to react to various stimuli. Thus, core–shell microcapsules exhibit significant promise for providing new “smart” functionality for applications related to the general field of intelligent microstructures and microsystems [1–5]. Recently, core–shell microcapsules have been investigated widely for utilization * Corresponding author. Fax: +86-29-88491000.
E-mail address: [email protected] (X.P. Zhao). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.10.056
in controlled-release systems, especially in drug delivery, in which the polymeric wall has a permeable part with a certain porosity that can determine the release behavior of core materials. Many environmental stimulus-responsive microcapsules have been reported to act in response to the changes in temperature [6–9], pH [10–13], light [14,15], external electric field [16–18], magnetic field [19], acoustic field [20], redox conditions [21], ionic strength [22–24], and other stimuli. These “smart” capsules continue to gather increasing attention. Here, it is worth mentioning a sort of core–shell microcapsules which are successfully applied in the field of electric display. Since the 1960s, electrophoretic image displays (EPDs) have developed as a kind of reflective displays and their images can be electrically written or erased repeatedly. This technology has several advantages, such as wide
H.L. Guo et al. / Journal of Colloid and Interface Science 284 (2005) 646–651
viewing angle and high contrast ratio, which are similar to those of paper-based hardcopy. So EPDs are considered to be a potential candidate to serve as electric paper. However, the industrialization of this technology is restrained because EPDs have serious problems with reliability and lifetime resulting from particle clustering, agglomeration, and lateral drift [25]. In 1997 Comiskey et al. contrived a conception of electrophoretic ink, based on microencapsulating the electrophoretic materials into individual microcapsules. This not only solves the problems of particle agglomeration and lateral drift on a scale larger than the capsule size, but also realizes bistable display into the microcapsules [26]. Today, microencapsulated EPDs are becoming one of the most appealing applications in electronic paper because electric ink can be printed onto a suitable substrate to realize flexible display [27,28]. However, most microencapsulation only takes monophase substances, either liquid or solid, as the internal phase [29,30]. On the other hand, the microcapsules containing multiphase substances, particularly suspension composed of organic solution and particles have rarely been studied. The encapsulation of suspension is different from that of monophase substances, especially the compatibility of all components must be considered. The primary problem during the microencapsulation is that particles transfer from the organic solution into the outer phase or adhere to the wall of the microcapsules [31,32]. The surface treatment of particles is the key to successful microencapsulation, for it can not only improve the lipophilicity of particles and the stability of suspensions, but also prevent the particles from agglomerating on the wall of the microcapsules, as well as endowing the particles with charges to respond to the electric field. In this study, a sort of functional microcapsules containing pigment phthalocyanine green (PPG) and tetrachloroethylene is prepared by in situ polymerization of urea and formaldehyde. In order to solve the particles’ separation from the suspension during the microencapsulation and to obtain microcapsules applying to electronic ink display, we focused on the dispersibility of the particles, the contact angles between the particles and the tetrachloroethylene, and the influence of different emulsifiers on the microencapsulation. Furthermore, the surface morphology of the microcapsules, prepared by the optimized process, was characterized by scanning electron microscopy (SEM) and its response behavior to electric field was also studied.
2. Experimental 2.1. Surface modification of the PPG particles A 50-mg sample of octadecylamine (analytical reagent) is dissolved in 100 ml of anhydrous ethanol (analytical reagent) at ambient temperature. One gram of PPG particles (specific gravity = 2.2, approximately 1 µm in diameter) is sonicated for several minutes in the above solution and
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is refluxed at about 80 ◦ C for 0.5–1 h. Then the resulting suspension is vaporized at 50 ◦ C with a rotary evaporator (RE-52A, Yarong, China) to remove the solvent. Thus, PPG particles modified with octadecylamine are obtained. 2.2. Preparation of the suspension The internal phase is a kind of suspension containing 30 mg of modified particles and 6 ml of tetrachloroethylene (specific gravity = 1.62, 20 ◦ C) with some sorbitan monooleate (Span 80) as an emulsifier. And the coefficient of viscosity of the suspension is 0.82 mPa s (20 ◦ C). 2.3. Microencapsulation of the suspension The multiphase composite microcapsules are fabricated by in situ polymerization, with urea and formaldehyde as wall materials [33]. First, 6 g of urea is dissolved in 12.5 g of 37 wt% formaldehyde aqueous solution at 1:1.5 molar ratio in a 100-ml flask, and then, after the mixture is adjusted to a pH of about 7–8 with triethanolamine, it is stirred and allowed to react at 70–85 ◦ C for 1 h. Next, it is cooled down and diluted with a double volume of distilled water to obtain the prepolymer. The internal phase described above is poured into the mixture of 8 ml of prepolymer and 3–5 multiples of distilled water under rapid stirring rate for 5–10 min. The emulsion reacts at ambient temperature with a moderate stirring rate for about 1–1.5 h while maintaining the pH at about 2–4 adjusted by dropwise addition of a 2 wt% solution of hydrochloric acid (HCl). Last, the slurry is heated to about 60 ◦ C for another 1–2 h to obtain the microcapsules containing the suspension. The resulting capsule slurry is then filtered, washed, dried, and sieved, to obtain the microcapsules (specific gravity 1.61, 20 ◦ C) in definite size. The procedures of the microencapsulation are shown in Fig. 1. 2.4. Measurement The C2 Cl4 /H2 O interfacial tension is measured by drop volume method at 25 ◦ C and the interfacial tension is given by the expression γ12 =
(ρ1 − ρ2 )V gF , r1
(1)
where V is the volume of the drop, ρ1 and ρ2 the densities of C2 Cl4 and H2 O, respectively, r1 the radius of the tube, g the gravity acceleration, and F the correction factor. Because C2 Cl4 in water does not wet the tip of the tube, the internal diameter is used. The contact angles between the PPG particles and the tetrachloroethylene are measured with the method developed by Bartell based on displacement pressures [34]. A 6-mm inside diameter glass tube, bottom closed with a porous glass sieve, is filled with 1.5 g of powder which is pressed to a fixed height. The tube bottom is then in close contact with tetrachloroethylene that penetrates up into the PPG particles
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Fig. 1. Schematic of the preparation of microcapsules containing tetrachloroethylene and PPG particles.
Fig. 2. Schematic diagram of measuring contact angle of powder.
column by capillarity rising. The height (h) of the penetrating liquid was measured as a function of time (t) after contact of the glass sieve with the liquid. According to Washburn’s conclusion, the square of the height is in direct proportion to the time needed. The Washburn equation is h2 =
cr2 γ cos θ t, 2η
(2)
where r2 is the capillarity radius, which depends on the particles size and degree of packing; γ and η the surface tension and viscosity of the fluid, θ the contact angle, c the factor. The contact angle can be calculated through the slope of the straight line of h2 –t. The measuring diagram is shown in Fig. 2. The dispersing extent (D.E.) of the PPG particles in tetrachloroethylene is expressed by the formula T 0 − Ts × 100%, (3) T0 where T0 and Ts are the transmissions of tetrachloroethylene and suspension, respectively [35]. The transmission is measured by a spectrophotometer (UV-1800, Puxi, China) at 600 nm in different time intervals such as 0, 2, 4, 8, and 30 h. As for the samples, the particles are dried previously at 60 ◦ C for 2 h in a vacuum drying oven (ZK-82A, China). Then 20 mg of modified and unmodified particles is respectively sonicated into different tubes, each containing 20 ml of tetrachloroethylene. One milliliter of the mixture at the surface is taken with pipettes and is measured. The organic functional groups of the particles are characterized by a FT-IR (scan 400–4000 cm−1 , Equinox55, Bruker, Germany) spectrophotometer. The surface morphology and optical photographs of microcapsules are observed under an optical microscope (Alphaphot-2 YS2-H, Nikon, Japan) with CCD (Fijitsu, Japan). Scanning electron microscopy (SEM) images are obtained on a scanning electron D.E. =
Fig. 3. IR spectra of the PPG particles (a) unmodified and (b) modified by octadecylamine.
microscope (Hitachi-570, Japan) operated at 15 kV with Au sprayed prior to examination. The response behavior of the capsules under an electric field is also investigated. And the microcapsules are placed into a 10 × 1 × 1 mm sample cell.
3. Results and discussion 3.1. Investigation of the surface treatment of PPG particles The IR spectra both modified and unmodified particles are shown in Fig. 3. Comparing these two spectra, the three peaks of the C–H band of modified particles at 2920, 2849, and 1458 cm−1 are increased and a peak of N–H band at 3447 cm−1 is weakened. The possible reason of this contrast can be that –H of octadecylamine is combined with –N of PPG to form a hydrogen bond. And the octadecylamine is thus coated on the surface of the PPG particles to form a membrane with the alkane chains. Therefore, it can be concluded that the membrane can prevent the particles from agglomerating and provide superior chemical compatibility for organic solvent. 3.2. Interfacial properties between the PPG particles and the tetrachloroethylene 3.2.1. Dispersion extent of the particles in suspension The PPG particles are modified because of their poor dispersity in tetrachloroethylene and their easy transfer from tetrachloroethylene during the microencapsulation. Compared with the non-modified PPG particles (Fig. 4), the dispersion extent of the modified PPG particles is obviously improved, so that the modified PPG particles can remain
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Fig. 4. Dispersing extent of the PPG particles in C2 Cl4 at 25 ◦ C: (a) unmodified, (b) modified by 2 wt% octadecylamine, (c) 5 wt%, (d) 8 wt%.
Fig. 5. h2 –t curves for the PPG particles wetted by C2 Cl4 at 25 ◦ C: (a) unmodified, (b) modified by 2 wt% octadecylamine, (c) 5 wt%, (d) 8 wt%.
in the internal phase during the microencapsulation. And among the PPG particles, those with 2 wt% octadecylamine are especially superior. 3.2.2. Contact angles between the PPG particles and the tetrachloroethylene To further investigate the chemical compatibility of C2 Cl4 with modified and unmodified PPG particles, the relations between wetted height h and time t are measured. The curves of h2 as a function of t are presented in Fig. 5, in which the curves show excellent linear relations since all the correlation coefficients (R 2 ) are near to 1. The slopes increase due to the PPG particles modified with octadecylamine. Besides, the contact angles and the slopes have the same changing tendency. As a result of the increasing slopes, the lipophilicity of the particles improves thanks to the alkane chains formed on the surface. The experiment demonstrates that the PPG particles modified with 2 wt% octadecylamine have the widest contact angles and thus have superior affinity for tetrachloroethylene, which is similar to Fig. 4. 3.3. Interfacial tension of water/tetrachloroethylene Shown in Fig. 6 is the interfacial tension between tetrachloroethylene and aqueous solution containing different
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Fig. 6. Interfacial tensions of C2 Cl4 /H2 O with different emulsifiers at 25 ◦ C: (a) prepolymer of urea–formaldehyde, (b) Tween 80, (c) OP-10, (d) Span-80, (e) C12 H25 SO4 Na, (f) C12 H25 –C6 H4 –SO3 Na.
concentrations of urea–formaldehyde prepolymer, polyoxyethylene (10) octylphenol ether (OP-10), dodecylbenzene sulfonate sodium (DBS), dodecyl sulfate sodium (SDS), Tween 80, and Span 80 surfactant. Because the interfacial tension descends from 43 to 35 mN/m, the prepolymer shows certain surface activity. Consequently, during the microencapsulation, the urea–formaldehyde resin deposits on the surface of the internal phase droplets to form the shell of the capsules. In contrast, no capsules will be produced if water-soluble emulsifier is added and occupies the surface of the droplets, since the surface activity of prepolymer is less effective. 3.4. Influence of emulsifiers on the microencapsulation When 0.25 wt% OP-10 is dissolved into the water and 0.25 wt% Span-80 in the tetrachloroethylene, the internal phase is emulsified into small droplets and the particles included separate from the internal phase. And the surface of the dispersed droplets is occupied by the molecules of emulsifiers and the resin deposits randomly in the aqueous solution but not on the interface between water and internal phase (Fig. 7a). In contrast, the microcapsules are formed when 0.25 wt% Span-80 exists in the tetrachloroethylene but no emulsifier exists in the water (Fig. 7b). So, the existence of oil-soluble emulsifier not only has no side effects on the microencapsulation, but even promotes the stability of the internal phase. Fig. 8 shows the SEM images of the outer surface and cross-sectional views of the microcapsules. The surfaces of the microcapsules are relatively smooth, and the thickness of the shell wall is about 4.5 µm. 3.5. Size distribution of the microcapsules The average microcapsule diameter is controlled by the agitation rate. As the agitation rate increases, the dispersed droplets turn finer, thus the average microcapsule diameter descends. Fig. 9 presents the microcapsule size distributions at different agitation rates. When the microcapsules are prepared with an agitation rate of 1000 and 600 rpm, the mean
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Fig. 7. Microencapsulation with different emulsifiers: (a) 0.25 wt% OP-10 in water and 0.25 wt% Span 80 in core; (b) only 0.25 wt% Span 80 in core.
diameters of the obtained microcapsules are 11 and 155 µm, respectively. 3.6. Responsive behavior of microcapsules to electric field To investigate the response behavior of the microcapsules to electric field, the dried microcapsules are placed into the sample cell. Then the sample is shifted to the middle of two parallel copper electrodes, and each of the electrodes welded a wire to provide a DC electric field (120 V/mm). Fig. 10 shows the response behavior of a typical microcapsule before and after the electric field is applied. When there is no electric field, the particles are distributed randomly in the capsule because of their Brown motion (Fig. 10a). But when the electric field is applied, the particles in the capsules moved obviously toward the positive electrode due to the presence of the electroosmotic and electrophoretic forces (Fig. 10b). The responsive time is about several hundred milliseconds. This indicates the particles possessing negative charges and good response behavior to the electric field.
4. Conclusions A sort of functional microcapsules responsive to electric fields and consisting of suspension core materials has been
Fig. 8. SEM images of the microcapsules: (a, c) outer surface; (b) cross section.
successfully synthesized. The capsules are prepared by in situ polymerization of urea and formaldehyde, while the suspension is made up of pigment phthalocyanine green (PPG) and tetrachloroethylene. The interfacial properties during the encapsulation are investigated. It is found that the dispersing extent and lipophilicity of the PPG particles are improved due to its surface modification with octadecylamine. The contact angles between the modified PPG particles and the tetrachloroethylene increase, and the PPG particles modified with 2 wt% octadecylamine have the best affinity for tetrachloroethylene. Because urea–formaldehyde prepolymer presents certain surface activity invalid, the water-
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant 90101005) and the National Science Foundation of China for Distinguished Young Scholar (Grant 50025207).
References
Fig. 9. Size distribution of the microcapsules at different agitation rates: (a) 1000 rpm, (b) 600 rpm.
Fig. 10. The typical microcapsule under the electric field: (a) E = 0, (b) E = 120 V/mm.
soluble emulsifiers can absorb on the surfaces of the internal phase and prevent the resin from depositing there. SEM images of shell surface and cross section show that the microcapsules have relatively smooth surfaces and the average thickness is about 4.5 µm. When the microcapsules are prepared with an agitation rate of 1000 and 600 rpm, the mean diameters of the obtained microcapsules are 11 and 155 µm, respectively. When an electric field is provided, the particles in the capsules moved toward the positive electrode with a response time of several hundred milliseconds.
[1] K. Takahashi, S. Nozaki, IEEE Trans. Compon. Pack. T. A 18 (1995) 245. [2] M. Taya, Composites A 30 (1999) 531. [3] M.R. Sheen, J.C. MacBryde, Technovation 15 (1995) 99. [4] W. Ehrfeld, Electrochim. Acta 48 (2003) 2857. [5] A. Manz, N. Graber, H.M. Widmer, Sens. Actuat. B 1 (1990) 244. [6] L.Y. Chu, S.-H. Park, T. Yamaguchi, S. Nakao, Langmuir 18 (2002) 1856. [7] L.Y. Chu, S.-H. Park, T. Yamaguchi, S. Nakao, J. Membrane Sci. 192 (2001) 27. [8] K. Kono, H. Okabe, K. Morimoto, T. Takagishi, J. Appl. Polym. Sci. 77 (2000) 2703. [9] X.C. Xiao, L.Y. Chu, W.M. Chen, et al., Adv. Funct. Mater. 13 (2003) 847. [10] T. Kidchob, S. Kimura, Y. Imanishi, J. Appl. Polym. Sci. 63 (1997) 453. [11] X. Li, Y. Huang, J. Xiao, Ch. Yan, J. Appl. Polym. Sci. 55 (1995) 1779. [12] K. Kono, K. Kawakami, K. Morimoto, T. Takagishi, J. Appl. Polym. Sci. 72 (1999) 1763. [13] L. Zhang, Y. Jin, H. Liu, Y. Du, J. Appl. Polym. Sci. 82 (2001) 584. [14] A. Suzuki, T. Tanaka, Nature 346 (1990) 345. [15] S.J. Chang, C.H. Lee, Y.J. Wang, J. Biomater. Sci. Polym. Ed. 10 (1999) 531. [16] Y. Okahata, S. Hachiya, K. Ariga, T. Seki, J. Am. Chem. Soc. 108 (1986) 2863. [17] I.C. Kwon, Y.H. Bae, S.W. Kim, Nature 354 (1991) 291. [18] M. Yoshida, T. Mori, T. Matsui, et al., J. Chem. Eng. Jpn. 35 (2002) 398. [19] A. Voigt, N. Buske, G.B. Sukhrukov, et al., J. Magn. Magn. Mater. 225 (2001) 59. [20] D. Koyama, N. Barbier, Y. Watanabe, Jpn. J. Appl. Phys. Part 1 42 (2003) 6700. [21] T. Nagaoka, H. Nakao, T. Suyama, K. Ogura, Analyst 112 (1997) 1399. [22] C.H.Y. Gao, S. Leporatti, S. Moya, et al., Chem. Eur. J. 9 (2003) 915. [23] J.K. Park, Y.B. Jin, H.N. Chang, Biotech. Bioeng. 63 (1999) 116. [24] L.Y. Chu, T. Yamaguchi, S. Nakao, Adv. Mater. 14 (2002) 386. [25] P. Murau, B. Singer, J. Appl. Phys. 49 (1978) 4820. [26] B. Comiskey, J.D. Albert, H. Yoshizawa, et al., Nature 394 (1998) 253. [27] Y. Chen, J. Au, P. Kazlas, et al., Nature 423 (2003) 136. [28] J.A. Rogers, Z. Bao, K. Baldwin, et al., Proc. Natl. Acad. Sci. USA 98 (2001) 4835. [29] E.N. Brown, M.R. Kessler, N.R. Sottos, S.R. White, J. Microencapsulation 20 (2003) 719. [30] S.J. Park, Y.S. Shin, J.R. Lee, J. Colloid Interface Sci. 241 (2001) 502. [31] H.L. Guo, J.P. Wang, X.P. Zhao, Chinese J. Mater. Res. 18 (2004) 46. [32] H.L. Guo, X.P. Zhao, Opt. Mater. 26 (2004) 297. [33] X.P. Zhao, H.L. Guo, Chinese Patent CNP 02139591.8, 2002. [34] F.E. Bartell, Ind. Eng. Chem. 19 (1927) 1277. [35] T.Y. Zhang, C.L. Zhou, Dyes Pigments 35 (1997) 123.
Synthesis of functional microcapsules containing suspensions responsive to electric fields Huilin Guo, Xiaopeng Zhao ∗ , Jianping Wang Institute of Electrorheological Technology, Department of Applied Physics, Northwestern Polytechnical University, 710072 Xi’an, People’s Republic of China Received 24 April 2004; accepted 20 October 2004 Available online 15 December 2004
Abstract A sort of functional microcapsules, which contain a suspension responsive to electric fields, is prepared by in situ polymerization of urea and formaldehyde. The suspension is made up of pigment phthalocyanine green (PPG) and tetrachloroethylene. In order to solve the particles’ separation from the suspension during the microencapsulation and to obtain microcapsules applying to electronic ink display, the dispersibility of the particles, the contact angles between the particles and the tetrachloroethylene, and the influences of different emulsifiers on the microencapsulation are investigated. It is found that the dispersion extent and lipophilicity of the PPG particles are improved due to their surface modification with octadecylamine. The contact angles between the modified PPG particles and the tetrachloroethylene increase, and the PPG particles modified with 2 wt% octadecylamine have the best affinity for tetrachloroethylene. The interfacial tension between C2 Cl4 and H2 O with urea–formaldehyde prepolymer descends from 43 to 35 mN/m, which indicates that the polymer has certain surface activity. However, water-soluble emulsifiers have an important influence during the microencapsulation because they can absorb on the surfaces of internal phase and prevent the resin of urea–formaldehyde from depositing there. From the SEM images of shell surface and cross section, the microcapsules have relatively smooth surfaces and the average thickness is about 4.5 µm. When the microcapsules are prepared with agitation rates of 1000 and 600 rpm, the mean diameters of the obtained microcapsules are 11 and 155 µm, respectively. The particles in the capsules move toward positive electrode with a responsive time of several hundred milliseconds while providing an electric field. 2004 Elsevier Inc. All rights reserved. Keywords: Microcapsules; Suspension; Contact angle; Interfacial tension; Electric response
1. Introduction Microcapsules, whose diameters are from 5 to 2000 µm, can encapsulate various substances, such as gases, liquids, and solids, to form composite materials on a scale of millimeters. The substances can not only retain their own respective attributes, such as optics, magnetism, electricity, and other properties, but also combine to react to various stimuli. Thus, core–shell microcapsules exhibit significant promise for providing new “smart” functionality for applications related to the general field of intelligent microstructures and microsystems [1–5]. Recently, core–shell microcapsules have been investigated widely for utilization * Corresponding author. Fax: +86-29-88491000.
E-mail address: [email protected] (X.P. Zhao). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.10.056
in controlled-release systems, especially in drug delivery, in which the polymeric wall has a permeable part with a certain porosity that can determine the release behavior of core materials. Many environmental stimulus-responsive microcapsules have been reported to act in response to the changes in temperature [6–9], pH [10–13], light [14,15], external electric field [16–18], magnetic field [19], acoustic field [20], redox conditions [21], ionic strength [22–24], and other stimuli. These “smart” capsules continue to gather increasing attention. Here, it is worth mentioning a sort of core–shell microcapsules which are successfully applied in the field of electric display. Since the 1960s, electrophoretic image displays (EPDs) have developed as a kind of reflective displays and their images can be electrically written or erased repeatedly. This technology has several advantages, such as wide
H.L. Guo et al. / Journal of Colloid and Interface Science 284 (2005) 646–651
viewing angle and high contrast ratio, which are similar to those of paper-based hardcopy. So EPDs are considered to be a potential candidate to serve as electric paper. However, the industrialization of this technology is restrained because EPDs have serious problems with reliability and lifetime resulting from particle clustering, agglomeration, and lateral drift [25]. In 1997 Comiskey et al. contrived a conception of electrophoretic ink, based on microencapsulating the electrophoretic materials into individual microcapsules. This not only solves the problems of particle agglomeration and lateral drift on a scale larger than the capsule size, but also realizes bistable display into the microcapsules [26]. Today, microencapsulated EPDs are becoming one of the most appealing applications in electronic paper because electric ink can be printed onto a suitable substrate to realize flexible display [27,28]. However, most microencapsulation only takes monophase substances, either liquid or solid, as the internal phase [29,30]. On the other hand, the microcapsules containing multiphase substances, particularly suspension composed of organic solution and particles have rarely been studied. The encapsulation of suspension is different from that of monophase substances, especially the compatibility of all components must be considered. The primary problem during the microencapsulation is that particles transfer from the organic solution into the outer phase or adhere to the wall of the microcapsules [31,32]. The surface treatment of particles is the key to successful microencapsulation, for it can not only improve the lipophilicity of particles and the stability of suspensions, but also prevent the particles from agglomerating on the wall of the microcapsules, as well as endowing the particles with charges to respond to the electric field. In this study, a sort of functional microcapsules containing pigment phthalocyanine green (PPG) and tetrachloroethylene is prepared by in situ polymerization of urea and formaldehyde. In order to solve the particles’ separation from the suspension during the microencapsulation and to obtain microcapsules applying to electronic ink display, we focused on the dispersibility of the particles, the contact angles between the particles and the tetrachloroethylene, and the influence of different emulsifiers on the microencapsulation. Furthermore, the surface morphology of the microcapsules, prepared by the optimized process, was characterized by scanning electron microscopy (SEM) and its response behavior to electric field was also studied.
2. Experimental 2.1. Surface modification of the PPG particles A 50-mg sample of octadecylamine (analytical reagent) is dissolved in 100 ml of anhydrous ethanol (analytical reagent) at ambient temperature. One gram of PPG particles (specific gravity = 2.2, approximately 1 µm in diameter) is sonicated for several minutes in the above solution and
647
is refluxed at about 80 ◦ C for 0.5–1 h. Then the resulting suspension is vaporized at 50 ◦ C with a rotary evaporator (RE-52A, Yarong, China) to remove the solvent. Thus, PPG particles modified with octadecylamine are obtained. 2.2. Preparation of the suspension The internal phase is a kind of suspension containing 30 mg of modified particles and 6 ml of tetrachloroethylene (specific gravity = 1.62, 20 ◦ C) with some sorbitan monooleate (Span 80) as an emulsifier. And the coefficient of viscosity of the suspension is 0.82 mPa s (20 ◦ C). 2.3. Microencapsulation of the suspension The multiphase composite microcapsules are fabricated by in situ polymerization, with urea and formaldehyde as wall materials [33]. First, 6 g of urea is dissolved in 12.5 g of 37 wt% formaldehyde aqueous solution at 1:1.5 molar ratio in a 100-ml flask, and then, after the mixture is adjusted to a pH of about 7–8 with triethanolamine, it is stirred and allowed to react at 70–85 ◦ C for 1 h. Next, it is cooled down and diluted with a double volume of distilled water to obtain the prepolymer. The internal phase described above is poured into the mixture of 8 ml of prepolymer and 3–5 multiples of distilled water under rapid stirring rate for 5–10 min. The emulsion reacts at ambient temperature with a moderate stirring rate for about 1–1.5 h while maintaining the pH at about 2–4 adjusted by dropwise addition of a 2 wt% solution of hydrochloric acid (HCl). Last, the slurry is heated to about 60 ◦ C for another 1–2 h to obtain the microcapsules containing the suspension. The resulting capsule slurry is then filtered, washed, dried, and sieved, to obtain the microcapsules (specific gravity 1.61, 20 ◦ C) in definite size. The procedures of the microencapsulation are shown in Fig. 1. 2.4. Measurement The C2 Cl4 /H2 O interfacial tension is measured by drop volume method at 25 ◦ C and the interfacial tension is given by the expression γ12 =
(ρ1 − ρ2 )V gF , r1
(1)
where V is the volume of the drop, ρ1 and ρ2 the densities of C2 Cl4 and H2 O, respectively, r1 the radius of the tube, g the gravity acceleration, and F the correction factor. Because C2 Cl4 in water does not wet the tip of the tube, the internal diameter is used. The contact angles between the PPG particles and the tetrachloroethylene are measured with the method developed by Bartell based on displacement pressures [34]. A 6-mm inside diameter glass tube, bottom closed with a porous glass sieve, is filled with 1.5 g of powder which is pressed to a fixed height. The tube bottom is then in close contact with tetrachloroethylene that penetrates up into the PPG particles
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Fig. 1. Schematic of the preparation of microcapsules containing tetrachloroethylene and PPG particles.
Fig. 2. Schematic diagram of measuring contact angle of powder.
column by capillarity rising. The height (h) of the penetrating liquid was measured as a function of time (t) after contact of the glass sieve with the liquid. According to Washburn’s conclusion, the square of the height is in direct proportion to the time needed. The Washburn equation is h2 =
cr2 γ cos θ t, 2η
(2)
where r2 is the capillarity radius, which depends on the particles size and degree of packing; γ and η the surface tension and viscosity of the fluid, θ the contact angle, c the factor. The contact angle can be calculated through the slope of the straight line of h2 –t. The measuring diagram is shown in Fig. 2. The dispersing extent (D.E.) of the PPG particles in tetrachloroethylene is expressed by the formula T 0 − Ts × 100%, (3) T0 where T0 and Ts are the transmissions of tetrachloroethylene and suspension, respectively [35]. The transmission is measured by a spectrophotometer (UV-1800, Puxi, China) at 600 nm in different time intervals such as 0, 2, 4, 8, and 30 h. As for the samples, the particles are dried previously at 60 ◦ C for 2 h in a vacuum drying oven (ZK-82A, China). Then 20 mg of modified and unmodified particles is respectively sonicated into different tubes, each containing 20 ml of tetrachloroethylene. One milliliter of the mixture at the surface is taken with pipettes and is measured. The organic functional groups of the particles are characterized by a FT-IR (scan 400–4000 cm−1 , Equinox55, Bruker, Germany) spectrophotometer. The surface morphology and optical photographs of microcapsules are observed under an optical microscope (Alphaphot-2 YS2-H, Nikon, Japan) with CCD (Fijitsu, Japan). Scanning electron microscopy (SEM) images are obtained on a scanning electron D.E. =
Fig. 3. IR spectra of the PPG particles (a) unmodified and (b) modified by octadecylamine.
microscope (Hitachi-570, Japan) operated at 15 kV with Au sprayed prior to examination. The response behavior of the capsules under an electric field is also investigated. And the microcapsules are placed into a 10 × 1 × 1 mm sample cell.
3. Results and discussion 3.1. Investigation of the surface treatment of PPG particles The IR spectra both modified and unmodified particles are shown in Fig. 3. Comparing these two spectra, the three peaks of the C–H band of modified particles at 2920, 2849, and 1458 cm−1 are increased and a peak of N–H band at 3447 cm−1 is weakened. The possible reason of this contrast can be that –H of octadecylamine is combined with –N of PPG to form a hydrogen bond. And the octadecylamine is thus coated on the surface of the PPG particles to form a membrane with the alkane chains. Therefore, it can be concluded that the membrane can prevent the particles from agglomerating and provide superior chemical compatibility for organic solvent. 3.2. Interfacial properties between the PPG particles and the tetrachloroethylene 3.2.1. Dispersion extent of the particles in suspension The PPG particles are modified because of their poor dispersity in tetrachloroethylene and their easy transfer from tetrachloroethylene during the microencapsulation. Compared with the non-modified PPG particles (Fig. 4), the dispersion extent of the modified PPG particles is obviously improved, so that the modified PPG particles can remain
H.L. Guo et al. / Journal of Colloid and Interface Science 284 (2005) 646–651
Fig. 4. Dispersing extent of the PPG particles in C2 Cl4 at 25 ◦ C: (a) unmodified, (b) modified by 2 wt% octadecylamine, (c) 5 wt%, (d) 8 wt%.
Fig. 5. h2 –t curves for the PPG particles wetted by C2 Cl4 at 25 ◦ C: (a) unmodified, (b) modified by 2 wt% octadecylamine, (c) 5 wt%, (d) 8 wt%.
in the internal phase during the microencapsulation. And among the PPG particles, those with 2 wt% octadecylamine are especially superior. 3.2.2. Contact angles between the PPG particles and the tetrachloroethylene To further investigate the chemical compatibility of C2 Cl4 with modified and unmodified PPG particles, the relations between wetted height h and time t are measured. The curves of h2 as a function of t are presented in Fig. 5, in which the curves show excellent linear relations since all the correlation coefficients (R 2 ) are near to 1. The slopes increase due to the PPG particles modified with octadecylamine. Besides, the contact angles and the slopes have the same changing tendency. As a result of the increasing slopes, the lipophilicity of the particles improves thanks to the alkane chains formed on the surface. The experiment demonstrates that the PPG particles modified with 2 wt% octadecylamine have the widest contact angles and thus have superior affinity for tetrachloroethylene, which is similar to Fig. 4. 3.3. Interfacial tension of water/tetrachloroethylene Shown in Fig. 6 is the interfacial tension between tetrachloroethylene and aqueous solution containing different
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Fig. 6. Interfacial tensions of C2 Cl4 /H2 O with different emulsifiers at 25 ◦ C: (a) prepolymer of urea–formaldehyde, (b) Tween 80, (c) OP-10, (d) Span-80, (e) C12 H25 SO4 Na, (f) C12 H25 –C6 H4 –SO3 Na.
concentrations of urea–formaldehyde prepolymer, polyoxyethylene (10) octylphenol ether (OP-10), dodecylbenzene sulfonate sodium (DBS), dodecyl sulfate sodium (SDS), Tween 80, and Span 80 surfactant. Because the interfacial tension descends from 43 to 35 mN/m, the prepolymer shows certain surface activity. Consequently, during the microencapsulation, the urea–formaldehyde resin deposits on the surface of the internal phase droplets to form the shell of the capsules. In contrast, no capsules will be produced if water-soluble emulsifier is added and occupies the surface of the droplets, since the surface activity of prepolymer is less effective. 3.4. Influence of emulsifiers on the microencapsulation When 0.25 wt% OP-10 is dissolved into the water and 0.25 wt% Span-80 in the tetrachloroethylene, the internal phase is emulsified into small droplets and the particles included separate from the internal phase. And the surface of the dispersed droplets is occupied by the molecules of emulsifiers and the resin deposits randomly in the aqueous solution but not on the interface between water and internal phase (Fig. 7a). In contrast, the microcapsules are formed when 0.25 wt% Span-80 exists in the tetrachloroethylene but no emulsifier exists in the water (Fig. 7b). So, the existence of oil-soluble emulsifier not only has no side effects on the microencapsulation, but even promotes the stability of the internal phase. Fig. 8 shows the SEM images of the outer surface and cross-sectional views of the microcapsules. The surfaces of the microcapsules are relatively smooth, and the thickness of the shell wall is about 4.5 µm. 3.5. Size distribution of the microcapsules The average microcapsule diameter is controlled by the agitation rate. As the agitation rate increases, the dispersed droplets turn finer, thus the average microcapsule diameter descends. Fig. 9 presents the microcapsule size distributions at different agitation rates. When the microcapsules are prepared with an agitation rate of 1000 and 600 rpm, the mean
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Fig. 7. Microencapsulation with different emulsifiers: (a) 0.25 wt% OP-10 in water and 0.25 wt% Span 80 in core; (b) only 0.25 wt% Span 80 in core.
diameters of the obtained microcapsules are 11 and 155 µm, respectively. 3.6. Responsive behavior of microcapsules to electric field To investigate the response behavior of the microcapsules to electric field, the dried microcapsules are placed into the sample cell. Then the sample is shifted to the middle of two parallel copper electrodes, and each of the electrodes welded a wire to provide a DC electric field (120 V/mm). Fig. 10 shows the response behavior of a typical microcapsule before and after the electric field is applied. When there is no electric field, the particles are distributed randomly in the capsule because of their Brown motion (Fig. 10a). But when the electric field is applied, the particles in the capsules moved obviously toward the positive electrode due to the presence of the electroosmotic and electrophoretic forces (Fig. 10b). The responsive time is about several hundred milliseconds. This indicates the particles possessing negative charges and good response behavior to the electric field.
4. Conclusions A sort of functional microcapsules responsive to electric fields and consisting of suspension core materials has been
Fig. 8. SEM images of the microcapsules: (a, c) outer surface; (b) cross section.
successfully synthesized. The capsules are prepared by in situ polymerization of urea and formaldehyde, while the suspension is made up of pigment phthalocyanine green (PPG) and tetrachloroethylene. The interfacial properties during the encapsulation are investigated. It is found that the dispersing extent and lipophilicity of the PPG particles are improved due to its surface modification with octadecylamine. The contact angles between the modified PPG particles and the tetrachloroethylene increase, and the PPG particles modified with 2 wt% octadecylamine have the best affinity for tetrachloroethylene. Because urea–formaldehyde prepolymer presents certain surface activity invalid, the water-
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant 90101005) and the National Science Foundation of China for Distinguished Young Scholar (Grant 50025207).
References
Fig. 9. Size distribution of the microcapsules at different agitation rates: (a) 1000 rpm, (b) 600 rpm.
Fig. 10. The typical microcapsule under the electric field: (a) E = 0, (b) E = 120 V/mm.
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