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Effect of charge control agent on electrophoretic characteristics of polymer encapsulated titania nanoparticle
Materials Chemistry and Physics 135 (2012) 259e263
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Effect of charge control agent on electrophoretic characteristics of polymer encapsulated titania nanoparticle B.J. Park a, S.Y. Hong a, H.H. Sim a, H.J. Choi a, *, Y.S. Yoon b a b
Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Chemical Research Institute, Kolon Industries Inc., Yongin, Kyunggi-Do 446-797, Republic of Korea
h i g h l i g h t s < Hydrophilic titania nanoparticles were encapsulated with PMMA via dispersion polymerization. < Electrophoretic characteristics were studied using two different charge control agents. < Electrophoretic movement of the nanoparticles was observed using a prototype device manufactured with ITO glass.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2011 Received in revised form 9 April 2012 Accepted 1 May 2012
Titania nanoparticles with hydrophilic surface characteristics were encapsulated with poly(methyl methacrylate) via a dispersion polymerization method for their electronic-ink application. In addition, to enhance their electrophoretic properties, a dielectric functional group was introduced using a comonomer of ethylene glycol methyl ether acrylate. Encapsulation on particle surface was characterized using SEM and dynamic light scattering. Electrophoretic characteristics of the nanoparticles were studied using two different charge control agents via an electrophoretic mobility analyzer. In order to observe electrophoretic movement of the nanoparticles in medium oil, a prototype device was manufactured using ITO glass, in which various movements of electrophoretic nanoparticles were observed with different charge control agents. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanoparticle Electrophoretic Encapsulation Electronic ink
1. Introduction Recently, electrophoretic displays based on an electrophoretic phenomenon of the nanoparticles have attracted a lot of attention due to their potential applications as an electronic ink in various areas such as E-book, wearable computer screens, electronic newspaper, electronic signs and smart identity cards [1e7]. Especially, microcapsule type electrophoretic displays with titanium dioxide (TiO2) nanoparticles as white color particles have been widely investigated because of their reflectivity, great whiteness and electrophoretic properties [8e11]. However, in their industrial application, the titania nanoparticles need to be modified to improve their dispersion stability against sedimentation and to enhance the electrophoretic properties [12e14]. Beside the particle modification, the charge control additive should be used to enhance the electrophoretic mobility of the nanoparticles in a medium oil [15]. In other words, electrophoretic properties of the * Corresponding author. Tel.: þ82 32 860 7486; fax: þ82 32 865 5178. E-mail address: [email protected] (H.J. Choi). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.001
nanoparticles in a low dielectric medium such as electrophoretic direction, mobility and active voltage, depend on the charge control agent and the nanoparticle surface [16]. While the modification methods of electrophoretic nanoparticles to overcome the density mismatch of the suspension and enhance long term stability have been reported by many researchers [17e19], the charge control agents have been seldom studied. In a previous study, TiO2/P(St-co-DVB)-MAA hybrid composite and P(MMA-co-EGDMA) particle were investigated for applying to E-ink particle. And these green and white e-ink particles showed good movement in gelatin Microcapsule on electric field. Referring to this study, we coated TiO2 with P(MMA-co EGMEA) and investigated its electrophoretic characteristic [20]. In this study, the effect of polymer encapsulation and the charge control agent addition on the electrophoretic properties of TiO2 nanoparticles were investigated, in which the TiO2 nanoparticles with hydrophilic surface characteristics were encapsulated with poly(methyl methacrylate) (PMMA) via the dispersion polymerization technique. In addition, to enhance the electrophoretic properties, the dielectric functional group was introduced using
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B.J. Park et al. / Materials Chemistry and Physics 135 (2012) 259e263
a comonomer, ethylene glycol methyl ether acrylate (EGMEA) [21]. The density and morphology of the nanoparticles were characterized by using gas pycnometer and SEM, respectively. In order to enhance the electrophoretic mobility, a charge control additive was added. The electrophoretic characteristics of nanoparticles were studied with different charge control agents. It was found that the charge enhancement from the dielectric group was effective when the charge control agent was added to the suspending fluid which was prepared by dispersing the fabricated nanoparticles into the low dielectric medium. In order to observe the electrophoretic movement of the nanoparticles in a medium oil, the prototype device was manufactured using an ITO glass, and the movement of the electrophoretic nanoparticles was observed using two different charge control agents.
near infrared light source (880 nm) and two synchronous detectors. The backscattering detector receives the light scattered by the sample at 135 from the incident beam. 1 g of the encapsulated TiO2 particles was dispersed in the medium oil along with 0.1 g of the stabilizer, Span85 (Sorbitan Trioleate, Aldrich). The suspension was treated with sonicator and homo mixer before measuring. In order to investigate the real operation condition, the prototype device using ITO glass and adhesive film was prepared. The electrophoretic behavior of the as prepared particles dispersed in low dielectric oil was observed, in which the electric field was applied using voltage source (487, Keithley). The blue organic pigment and charge control agent were used for the observation of particle migration. Two different charge control agents, OLOA1200 (polyisobutylene succinimide, Chevron Oronite) and Span85 (Sorbitan Trioleate), were used to investigate the effect of charge control agent and polymer.
2. Material and methods 4. Results and discussion Titanium dioxide (TiO2, MT500B, Tayca, Japan) nanoparticles were adopted as the white electrophoretic ink particles after grinding them using an agate mortar. Monomers of methyl methacrylate (MMA, Aldrich, USA), ethylene glycol methyl ether acrylate (EGMEA, Aldrich, USA) and ethylene glycol dimethacrylate (EGDMA, Aldrich, USA), and a stabilizer, poly(vinyl pyrrolidone) (PVP, Aldrich, USA, Mw ¼ 55,000 g mol1), were used as received without further purification, while 2,20 azobis(isobutyronitrile) (AIBN, Junsei, Japan) was purified via re-crystallization prior to use. Halocarbon oil (polychlorotrifluoroethylene, Halocarbon) and Isopar G (isoparaffinic hydrocarbon, Exxon Mobile) were used as low dielectric medium oil. 3. Experimental Titanium dioxide nanoparticles were encapsulated with crosslinked PMMA-co-EGMEA using a two-step dispersion polymerization. Before coating process, the surface of TiO2 was treated with HCl (hydrochloric acid) via sonication to form eOH chemical composition on the surface of TiO2. This eOH on the surface of TiO2 was thought to be the one which possess an effect on forming hydrogen bonding with eOH chemical composition of PMMA-coEGMEA. Moreover, Hþ induced by HCl on the surface of TiO2 could make a dipoleedipole interaction with oxygen of C ] O in PMMA-co-EGMEA. And then 3 g of surface treated TiO2 nanoparticles were dispersed in 300 ml of methanol in which 3 g of dispersion stabilizer, PVP, was dissolved. On the other hand, 0.2 g of AIBN initiator was dissolved in a mixture of 9 g of MMA and 1 g of EGDMA. Thereafter, the above-mentioned TiO2 suspended in methanol was heated at 60 C, and the mixture of monomer and initiator was slowly added dropwise into the solution with stirring and kept at the same temperature for 12 h. On the other hand, a solution of 0.1 g of AIBN and 1 g of EGMEA as comonomer was prepared separately. After 12 h, the solution containing AIBN was added dropwise into the reactor. The reaction was progressed for another 8 h. The reaction products were then washed with methanol and di-water. The solid was separated from the mixture by means of centrifugation at 2500 rpm for 30 min followed by a freezing drying process. The final product was obtained in a powder form. Morphology and shape of the particles were examined by scanning electron microscopy (SEM) (Hitachi S-4300, 10 kV). The zeta potential and electrophoretic mobility was measured using an electrophoretic light scattering spectrophotometer (Photal ELS8000, Otsuka Electronics) [22]. Dispersion stability of the encapsulated TiO2 particles in the medium oil, the mixture of Halocarbon oil and Isopar G, was observed using Turbiscan (Classic MA2000, Formulaction). The Turbiscan lab reading head consists of pulsed
Encapsulation was performed via a dispersion polymerization [23,24], and the polymerization was followed by the seed and growth mechanism. Note that Li et al. [25] recently encapsulated titania cores with copolymer using a different method of distillation precipitation polymerization. Radical polymerization method was also applied for the encapsulation by polystyrene [26] in addition to mini-emulsion polymerization [27]. At the start of the process, all of the seed particles, monomer, stabilizer comonomer (co-stabilizer) and initiator are present in a homogeneous solution in the continuous phase (a). Upon heating, the initiator decomposes and the free radicals react with solute monomer to form oligomeric radicals (b). At a critical chain length, the oligomers are adsorbed on the seed particles with both stabilizer and co-stabilizer to form stable particle (c). Once the oligomers have been adsorbed, they absorb monomer from the continuous phase (d). Because the adsorbed phase on particle surface is still soluble to the medium, the washing step and freezing drying is very important to obtain stably encapsulated particles. On the other hand, especially regarding adsorption mechanism of coating polymer onto titania, in addition to the above-mentioned bonding and interaction mechanism, electrostatic interaction between TiO2 and polymer could be also considered because surface treated TiO2 would have plus charge while PMMA-coEGMEA has minus charge, based on zeta potential measured by Photal ELS-8000, Otsuka Electronics. The encapsulation was confirmed by using SEM. Fig. 1 shows the SEM images of both raw TiO2 (a) and PMMA-co-EGMEA coated TiO2 pigment particles (b). The figure demonstrates that the raw TiO2 nanoparticles have a regular spherical shape with an average size of about 50 nm. In addition, it is represented that the polymer coated nanoparticles are larger and rounder than the raw TiO2 nanoparticles. The density of composite particles was measured to be 1.8 g cm3 via a pycnometer. Moreover, the particle aggregation was reduced through the polymer encapsulation on the TiO2 particles. However, it was also shown that several TiO2 nanoparticles coated together while most particles were encapsulated individually. The amount of polymer coating was confirmed by measuring the thermal degradation of particles using TGA. Fig. 2 displays the TGA thermograms of both PMMA-co-EGMEA-coated TiO2 and pure polymer particles with multi-step degradation from 150 C to 350 C due to oligomer phase and stabilizer. It shows that the polymer encapsulated TiO2 contains less oligomer as compared to pure polymer. The onset temperature of main chain degradation for the composite particles is slightly higher than that of the pure polymer particles, demonstrating that the polymer shell is compatible to the titania core. The total weight loss was detected to
B.J. Park et al. / Materials Chemistry and Physics 135 (2012) 259e263
261
Fig. 3. The back light scattering intensity as function of time for the electrophoretic suspension.
fraction of the shell, which herein equals 40%. Introduce other two equations for V1 and V2, as
4 3 pR 3
(2)
V2 ¼
i 4 h p ðR þ dÞ3 R3 3
(3)
To calculate the d value by substituting them into Eq. (1), r1 and r2 are measured through a pycnometer to be 3.24 and 1.2 g cm3,
Fig. 1. SEM image of (a) raw TiO2 and (b) PMMA-EGEMA coated TiO2.
be about 40%. In other words, the composite particles were composed of 40% of polymer segment and 60% of pigment segment, indicating that the yield of polymerization was about 20%. On the other hand, the coating thickness (d) of PMMA-coEGMEA to the TIO2 particles can be calculated approximately by using the following simple equation based on the assumption of monodisperse particle and uniform coating thickness:
V2 r2 100% ¼ m V1 r1 þ V2 r2
V1 ¼
(1)
where V1 and V2 are the volume of TiO2 core and PMMA-co-EGMEA shell respectively, similar to the density r1 and r2. m is the mass
respectively. From the SEM image, the radius (R) of the monodipersed TiO2 particle is measured as 25 nm. Finally the d value is determined to be 10.3 nm [28]. The polymer encapsulation is expected to enhance the dispersion stability of the titania particles, and the dispersion stability of the particles in the medium was measured using Turbiscan. Fig. 3 shows the change of backscattering intensity over time at a taken point, exhibiting that the intensity of polymer coated TiO2 is stronger than the raw TiO2 in whole range which indicates that the polymer coated TiO2 nanoparticles are widely distributed in the medium. The raw TiO2 shows sharper decrease of backscattering intensity over time, while there is a little decrease of intensity, representing that the polymer coated TiO2 hardly sediment while raw TiO2 settled due to their high density. Note that the backscattering (BS) of incident light is measured by calculating transport mean free path of photons (l*) throughout the medium. Based on Mie theory, the BS can be obtained for a concentrated suspension as follows:
BSz
1=2 1 [*
(4)
Here, r is an internal radius of a measurement cell. The photon transport mean free path (l*) is defined as:
2d [* z 34ð1 gÞQs
Fig. 2. TGA data of PMMA-EGEMA coated TiO2 and pure polymer.
(5)
where d, 4, g, and Qs denote a particle mean diameter, the volume fraction of a dispersed phase, asymmetry factor, and scattering efficiency factor, respectively [29]. The medium oil was prepared by mixing halocarbon oil and isoparaffin oil. In order to enhance dispersion stability and electrophoretic mobility of nanoparticles, charge control agents
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B.J. Park et al. / Materials Chemistry and Physics 135 (2012) 259e263
Table 1 Relative dielectric constant of low dielectric medium with charge control agents at 1 kHz.
Relative dielectric constant
Halocarbon þ Isopar G
Medium þ OLOA1200
Medium þ Span85
2.15
2.45
2.50
Table 2 Zeta potential and electrophoretic (EP) mobility of polymer encapsulated TiO2 with and without CCA (Avg. E. Field ¼ 157.65 (V/cm))
Zeta potential EP mobility
Without CCA
OLOA1200
Span85
0.56 mV 1.24110-7 cm2/Vs
17.4 mV 3.12 106 cm2 Vs1
18.3 mV 3.54 106 cm2 Vs1
were added. Here, the dispersants, Span85 and OLOA1200 were used as the charge control agents. The dielectric constant of the medium with charge control agent was measured using the LCR meter (4284A, HP) with HP 16452 liquid test fixture with AC electric field. Table 1 shows the relative dielectric constant, indicating that the dielectric constant of a medium was increased with the charge control agent. It was also displayed that the dielectric constant was similar for both charge control agents. The effect of charge control agent on the surface charge of particles was investigated by measuring the zeta potential. Table 2 shows the zeta potential of polymer coated TiO2 with each charge control agent. It indicated that the zeta potential and electrophoretic mobility increased with the addition of charge control agent. It represents the surface charge of dispersed particles become significant with charge control agent in oil medium. Also, the electrophoretic mobility of Span85 was higher than that of OLOA1200. The OLOA1200 control the charge of particle by being adsorbed on particle while Span85 control the charge valance by being dispersed in medium oil. Span85 might enhance surface charge density of particles by enhancing dielectric properties of medium oil [30,31]. Electrophoretic behavior was observed using prototype device. Each piece is attached using adhesive film with 170 micron of thickness. And each ITO glass has connection with electrode of high voltage and ground voltage. The devices contain the electrophoretic suspension with charge control agent. Fig. 4 shows a color change due to the electrophoretic migration of polymer coated TiO2 particles. When only Span85 was used as a charge control agent, the particle migration was irregular. When 10 V was applied, the suspension having both charge control agents displayed a clear color change, while the OLOA1200 suspension showed a little migration which cannot be noticed in the picture. It represents that the mixture of the OLOA1200 and Span85 helps
each colored particle to possess different charge valance while the OLOA1200 only makes them to possess similar charge valance. Therefore it can be considered that the Span85 is a good dispersant as well as a good charge control agent for this system. 5. Conclusions In this study, the white pigment, TiO2, nanoparticles were successfully encapsulated using a two-step dispersion polymerization, to be applied as electrophoretic ink particles of e-paper display. The PMMA-co-EGMEA coating enhances the dispersion of TiO2 nanoparticles in the dielectric medium because of reduced density and improved surface characteristics. The electrophoretic behavior was characterized via both electrophoretic light scattering and prototype device also demonstrated that the EGMEA was adaptable as a comonomer to enhance the electrophoretic mobility of polymer coated TiO2 particles in the dielectric medium with the charge control agent. In addition, the OLOA1200 and Span85 mixture was found to be a good additive for this system. Acknowledgment This work was supported by the Ministry of Knowledge Economy, Korea and Kolon (2011). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 4. The prototype display without Span85 (a) and with Span85 (b) under 10 V of applied external voltage.
[19] [20] [21]
S. Jahromi, Macromol. Chem. Phys. 200 (1999) 2230e2239. I. Satoshi, IEEE Trans. Electron Devices 49 (2002) 1532e1539. D.W. Wang, X.P. Zhao, J. Microencap. 26 (2009) 37e45. H.L. Guo, X.P. Zhao, Optic. Mater. 26 (2004) 297e300. T. Wen, X. Meng, Z. Li, J. Ren, F. Tang, J. Mater. Chem. 20 (2010) 8112e8117. W.C. Kao, J. Display Technol. 6 (2010) 625e632. I.H. Shen, K.K. Shieh, C.Y. Chao, D.S. Lee, Displays 30 (2009) 53e58. C.A. Kim, S.Y. Kang, G.H. Kim, S.D. Ahn, J. Oh, K.S. Suh, Mol. Cryst. Liq. Cryst. 499 (2009) 604e611. K.S. Kim, J.Y. Lee, B.J. Park, J.H. Sung, I. Chin, H.J. Choi, J.H. Lee, Colloid Polym. Sci. 284 (2006) 813e816. K. Kim, J.H. Sung, J.Y. Lee, J.K. Song, I. Chin, H.J. Choi, Mol. Cryst. Liq. Cryst. 445 (2006) 333e338. X. Fang, H. Yang, G. Wu, W. Li, H. Chen, M. Wang, Curr. Appl. Phys. 9 (2009) 755e759. I.B. Jang, J.H. Sung, H.J. Choi, I. Chin, J. Mater. Sci. 40 (2005) 3021e3024. D.G. Yu, J.H. An, J.Y. Bae, Y.E. Lee, S.D. Ahn, S.Y. Kang, K.S. Suh, J. Appl. Polym. Sci. 92 (2004) 2970e2975. D.G. Yu, J.H. An, J.Y. Bae, S.D. Ahn, S.Y. Kang, K.S. Suh, Macromolecules 38 (2005) 7485e7491. B.J. Park, J.H. Sung, H.J. Choi, J. Electroceram. 17 (2006) 1031e1033. C.A. Kim, M.J. Joung, S.D. Ahn, G.H. Kim, S.Y. Kang, J. Oh, H.J. Myoung, K.H. Baek, K.S. Suh, Synth. Met. 151 (2005) 181e185. I.B. Jang, J.H. Sung, H.J. Choi, I. Chin, Synth. Met. 152 (2005) 9e12. J.W. Kim, J.W. Shin, J.H. Bae, S.H. Han, H.K. Kim, I.S. Chang, H.H. Kang, K.D. Suh, Colloid Polym. Sci. 280 (2002) 582. T. Tan, S. Wang, S. Bian, X. Li, Synth. Met. 159 (2009) 1739e1743. D.G. Yu, S.H. Kim, J.H. An, J. Ind. Eng. Chem. 13 (2007) 438e443. J.H. Park, M.A. Lee, Y.H. Kim, B.J. Park, H.J. Choi, J. Korean Phys. Soc. 53 (2008) 50e54.
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[27] L.J. Borthakur, D. Das, S.K. Dolui, Mater. Chem. Phys. 124 (2010) 1182e1187. [28] Y.D. Liu, J.E. Kim, H.J. Choi, Macromol. Rapid Commun. 32 (2011) 881e886. [29] F.F. Fang, Y.D. Liu, H.J. Choi, Y. Seo, ACS Appl. Mater. Interf. 3 (2011) 3487e3495. [30] S.K. Sainis, V. Germain, C.O. Mejean, E.R. Dufresne, Langmuir 24 (2008) 1160e1164. [31] M.E. Parent, J. Yang, Y. Jeon, M.F. Toney, Z.L. Zhou, D. Henze, Langmuir 27 (2011) 11845e11851.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Effect of charge control agent on electrophoretic characteristics of polymer encapsulated titania nanoparticle B.J. Park a, S.Y. Hong a, H.H. Sim a, H.J. Choi a, *, Y.S. Yoon b a b
Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Chemical Research Institute, Kolon Industries Inc., Yongin, Kyunggi-Do 446-797, Republic of Korea
h i g h l i g h t s < Hydrophilic titania nanoparticles were encapsulated with PMMA via dispersion polymerization. < Electrophoretic characteristics were studied using two different charge control agents. < Electrophoretic movement of the nanoparticles was observed using a prototype device manufactured with ITO glass.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2011 Received in revised form 9 April 2012 Accepted 1 May 2012
Titania nanoparticles with hydrophilic surface characteristics were encapsulated with poly(methyl methacrylate) via a dispersion polymerization method for their electronic-ink application. In addition, to enhance their electrophoretic properties, a dielectric functional group was introduced using a comonomer of ethylene glycol methyl ether acrylate. Encapsulation on particle surface was characterized using SEM and dynamic light scattering. Electrophoretic characteristics of the nanoparticles were studied using two different charge control agents via an electrophoretic mobility analyzer. In order to observe electrophoretic movement of the nanoparticles in medium oil, a prototype device was manufactured using ITO glass, in which various movements of electrophoretic nanoparticles were observed with different charge control agents. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanoparticle Electrophoretic Encapsulation Electronic ink
1. Introduction Recently, electrophoretic displays based on an electrophoretic phenomenon of the nanoparticles have attracted a lot of attention due to their potential applications as an electronic ink in various areas such as E-book, wearable computer screens, electronic newspaper, electronic signs and smart identity cards [1e7]. Especially, microcapsule type electrophoretic displays with titanium dioxide (TiO2) nanoparticles as white color particles have been widely investigated because of their reflectivity, great whiteness and electrophoretic properties [8e11]. However, in their industrial application, the titania nanoparticles need to be modified to improve their dispersion stability against sedimentation and to enhance the electrophoretic properties [12e14]. Beside the particle modification, the charge control additive should be used to enhance the electrophoretic mobility of the nanoparticles in a medium oil [15]. In other words, electrophoretic properties of the * Corresponding author. Tel.: þ82 32 860 7486; fax: þ82 32 865 5178. E-mail address: [email protected] (H.J. Choi). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.001
nanoparticles in a low dielectric medium such as electrophoretic direction, mobility and active voltage, depend on the charge control agent and the nanoparticle surface [16]. While the modification methods of electrophoretic nanoparticles to overcome the density mismatch of the suspension and enhance long term stability have been reported by many researchers [17e19], the charge control agents have been seldom studied. In a previous study, TiO2/P(St-co-DVB)-MAA hybrid composite and P(MMA-co-EGDMA) particle were investigated for applying to E-ink particle. And these green and white e-ink particles showed good movement in gelatin Microcapsule on electric field. Referring to this study, we coated TiO2 with P(MMA-co EGMEA) and investigated its electrophoretic characteristic [20]. In this study, the effect of polymer encapsulation and the charge control agent addition on the electrophoretic properties of TiO2 nanoparticles were investigated, in which the TiO2 nanoparticles with hydrophilic surface characteristics were encapsulated with poly(methyl methacrylate) (PMMA) via the dispersion polymerization technique. In addition, to enhance the electrophoretic properties, the dielectric functional group was introduced using
260
B.J. Park et al. / Materials Chemistry and Physics 135 (2012) 259e263
a comonomer, ethylene glycol methyl ether acrylate (EGMEA) [21]. The density and morphology of the nanoparticles were characterized by using gas pycnometer and SEM, respectively. In order to enhance the electrophoretic mobility, a charge control additive was added. The electrophoretic characteristics of nanoparticles were studied with different charge control agents. It was found that the charge enhancement from the dielectric group was effective when the charge control agent was added to the suspending fluid which was prepared by dispersing the fabricated nanoparticles into the low dielectric medium. In order to observe the electrophoretic movement of the nanoparticles in a medium oil, the prototype device was manufactured using an ITO glass, and the movement of the electrophoretic nanoparticles was observed using two different charge control agents.
near infrared light source (880 nm) and two synchronous detectors. The backscattering detector receives the light scattered by the sample at 135 from the incident beam. 1 g of the encapsulated TiO2 particles was dispersed in the medium oil along with 0.1 g of the stabilizer, Span85 (Sorbitan Trioleate, Aldrich). The suspension was treated with sonicator and homo mixer before measuring. In order to investigate the real operation condition, the prototype device using ITO glass and adhesive film was prepared. The electrophoretic behavior of the as prepared particles dispersed in low dielectric oil was observed, in which the electric field was applied using voltage source (487, Keithley). The blue organic pigment and charge control agent were used for the observation of particle migration. Two different charge control agents, OLOA1200 (polyisobutylene succinimide, Chevron Oronite) and Span85 (Sorbitan Trioleate), were used to investigate the effect of charge control agent and polymer.
2. Material and methods 4. Results and discussion Titanium dioxide (TiO2, MT500B, Tayca, Japan) nanoparticles were adopted as the white electrophoretic ink particles after grinding them using an agate mortar. Monomers of methyl methacrylate (MMA, Aldrich, USA), ethylene glycol methyl ether acrylate (EGMEA, Aldrich, USA) and ethylene glycol dimethacrylate (EGDMA, Aldrich, USA), and a stabilizer, poly(vinyl pyrrolidone) (PVP, Aldrich, USA, Mw ¼ 55,000 g mol1), were used as received without further purification, while 2,20 azobis(isobutyronitrile) (AIBN, Junsei, Japan) was purified via re-crystallization prior to use. Halocarbon oil (polychlorotrifluoroethylene, Halocarbon) and Isopar G (isoparaffinic hydrocarbon, Exxon Mobile) were used as low dielectric medium oil. 3. Experimental Titanium dioxide nanoparticles were encapsulated with crosslinked PMMA-co-EGMEA using a two-step dispersion polymerization. Before coating process, the surface of TiO2 was treated with HCl (hydrochloric acid) via sonication to form eOH chemical composition on the surface of TiO2. This eOH on the surface of TiO2 was thought to be the one which possess an effect on forming hydrogen bonding with eOH chemical composition of PMMA-coEGMEA. Moreover, Hþ induced by HCl on the surface of TiO2 could make a dipoleedipole interaction with oxygen of C ] O in PMMA-co-EGMEA. And then 3 g of surface treated TiO2 nanoparticles were dispersed in 300 ml of methanol in which 3 g of dispersion stabilizer, PVP, was dissolved. On the other hand, 0.2 g of AIBN initiator was dissolved in a mixture of 9 g of MMA and 1 g of EGDMA. Thereafter, the above-mentioned TiO2 suspended in methanol was heated at 60 C, and the mixture of monomer and initiator was slowly added dropwise into the solution with stirring and kept at the same temperature for 12 h. On the other hand, a solution of 0.1 g of AIBN and 1 g of EGMEA as comonomer was prepared separately. After 12 h, the solution containing AIBN was added dropwise into the reactor. The reaction was progressed for another 8 h. The reaction products were then washed with methanol and di-water. The solid was separated from the mixture by means of centrifugation at 2500 rpm for 30 min followed by a freezing drying process. The final product was obtained in a powder form. Morphology and shape of the particles were examined by scanning electron microscopy (SEM) (Hitachi S-4300, 10 kV). The zeta potential and electrophoretic mobility was measured using an electrophoretic light scattering spectrophotometer (Photal ELS8000, Otsuka Electronics) [22]. Dispersion stability of the encapsulated TiO2 particles in the medium oil, the mixture of Halocarbon oil and Isopar G, was observed using Turbiscan (Classic MA2000, Formulaction). The Turbiscan lab reading head consists of pulsed
Encapsulation was performed via a dispersion polymerization [23,24], and the polymerization was followed by the seed and growth mechanism. Note that Li et al. [25] recently encapsulated titania cores with copolymer using a different method of distillation precipitation polymerization. Radical polymerization method was also applied for the encapsulation by polystyrene [26] in addition to mini-emulsion polymerization [27]. At the start of the process, all of the seed particles, monomer, stabilizer comonomer (co-stabilizer) and initiator are present in a homogeneous solution in the continuous phase (a). Upon heating, the initiator decomposes and the free radicals react with solute monomer to form oligomeric radicals (b). At a critical chain length, the oligomers are adsorbed on the seed particles with both stabilizer and co-stabilizer to form stable particle (c). Once the oligomers have been adsorbed, they absorb monomer from the continuous phase (d). Because the adsorbed phase on particle surface is still soluble to the medium, the washing step and freezing drying is very important to obtain stably encapsulated particles. On the other hand, especially regarding adsorption mechanism of coating polymer onto titania, in addition to the above-mentioned bonding and interaction mechanism, electrostatic interaction between TiO2 and polymer could be also considered because surface treated TiO2 would have plus charge while PMMA-coEGMEA has minus charge, based on zeta potential measured by Photal ELS-8000, Otsuka Electronics. The encapsulation was confirmed by using SEM. Fig. 1 shows the SEM images of both raw TiO2 (a) and PMMA-co-EGMEA coated TiO2 pigment particles (b). The figure demonstrates that the raw TiO2 nanoparticles have a regular spherical shape with an average size of about 50 nm. In addition, it is represented that the polymer coated nanoparticles are larger and rounder than the raw TiO2 nanoparticles. The density of composite particles was measured to be 1.8 g cm3 via a pycnometer. Moreover, the particle aggregation was reduced through the polymer encapsulation on the TiO2 particles. However, it was also shown that several TiO2 nanoparticles coated together while most particles were encapsulated individually. The amount of polymer coating was confirmed by measuring the thermal degradation of particles using TGA. Fig. 2 displays the TGA thermograms of both PMMA-co-EGMEA-coated TiO2 and pure polymer particles with multi-step degradation from 150 C to 350 C due to oligomer phase and stabilizer. It shows that the polymer encapsulated TiO2 contains less oligomer as compared to pure polymer. The onset temperature of main chain degradation for the composite particles is slightly higher than that of the pure polymer particles, demonstrating that the polymer shell is compatible to the titania core. The total weight loss was detected to
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Fig. 3. The back light scattering intensity as function of time for the electrophoretic suspension.
fraction of the shell, which herein equals 40%. Introduce other two equations for V1 and V2, as
4 3 pR 3
(2)
V2 ¼
i 4 h p ðR þ dÞ3 R3 3
(3)
To calculate the d value by substituting them into Eq. (1), r1 and r2 are measured through a pycnometer to be 3.24 and 1.2 g cm3,
Fig. 1. SEM image of (a) raw TiO2 and (b) PMMA-EGEMA coated TiO2.
be about 40%. In other words, the composite particles were composed of 40% of polymer segment and 60% of pigment segment, indicating that the yield of polymerization was about 20%. On the other hand, the coating thickness (d) of PMMA-coEGMEA to the TIO2 particles can be calculated approximately by using the following simple equation based on the assumption of monodisperse particle and uniform coating thickness:
V2 r2 100% ¼ m V1 r1 þ V2 r2
V1 ¼
(1)
where V1 and V2 are the volume of TiO2 core and PMMA-co-EGMEA shell respectively, similar to the density r1 and r2. m is the mass
respectively. From the SEM image, the radius (R) of the monodipersed TiO2 particle is measured as 25 nm. Finally the d value is determined to be 10.3 nm [28]. The polymer encapsulation is expected to enhance the dispersion stability of the titania particles, and the dispersion stability of the particles in the medium was measured using Turbiscan. Fig. 3 shows the change of backscattering intensity over time at a taken point, exhibiting that the intensity of polymer coated TiO2 is stronger than the raw TiO2 in whole range which indicates that the polymer coated TiO2 nanoparticles are widely distributed in the medium. The raw TiO2 shows sharper decrease of backscattering intensity over time, while there is a little decrease of intensity, representing that the polymer coated TiO2 hardly sediment while raw TiO2 settled due to their high density. Note that the backscattering (BS) of incident light is measured by calculating transport mean free path of photons (l*) throughout the medium. Based on Mie theory, the BS can be obtained for a concentrated suspension as follows:
BSz
1=2 1 [*
(4)
Here, r is an internal radius of a measurement cell. The photon transport mean free path (l*) is defined as:
2d [* z 34ð1 gÞQs
Fig. 2. TGA data of PMMA-EGEMA coated TiO2 and pure polymer.
(5)
where d, 4, g, and Qs denote a particle mean diameter, the volume fraction of a dispersed phase, asymmetry factor, and scattering efficiency factor, respectively [29]. The medium oil was prepared by mixing halocarbon oil and isoparaffin oil. In order to enhance dispersion stability and electrophoretic mobility of nanoparticles, charge control agents
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Table 1 Relative dielectric constant of low dielectric medium with charge control agents at 1 kHz.
Relative dielectric constant
Halocarbon þ Isopar G
Medium þ OLOA1200
Medium þ Span85
2.15
2.45
2.50
Table 2 Zeta potential and electrophoretic (EP) mobility of polymer encapsulated TiO2 with and without CCA (Avg. E. Field ¼ 157.65 (V/cm))
Zeta potential EP mobility
Without CCA
OLOA1200
Span85
0.56 mV 1.24110-7 cm2/Vs
17.4 mV 3.12 106 cm2 Vs1
18.3 mV 3.54 106 cm2 Vs1
were added. Here, the dispersants, Span85 and OLOA1200 were used as the charge control agents. The dielectric constant of the medium with charge control agent was measured using the LCR meter (4284A, HP) with HP 16452 liquid test fixture with AC electric field. Table 1 shows the relative dielectric constant, indicating that the dielectric constant of a medium was increased with the charge control agent. It was also displayed that the dielectric constant was similar for both charge control agents. The effect of charge control agent on the surface charge of particles was investigated by measuring the zeta potential. Table 2 shows the zeta potential of polymer coated TiO2 with each charge control agent. It indicated that the zeta potential and electrophoretic mobility increased with the addition of charge control agent. It represents the surface charge of dispersed particles become significant with charge control agent in oil medium. Also, the electrophoretic mobility of Span85 was higher than that of OLOA1200. The OLOA1200 control the charge of particle by being adsorbed on particle while Span85 control the charge valance by being dispersed in medium oil. Span85 might enhance surface charge density of particles by enhancing dielectric properties of medium oil [30,31]. Electrophoretic behavior was observed using prototype device. Each piece is attached using adhesive film with 170 micron of thickness. And each ITO glass has connection with electrode of high voltage and ground voltage. The devices contain the electrophoretic suspension with charge control agent. Fig. 4 shows a color change due to the electrophoretic migration of polymer coated TiO2 particles. When only Span85 was used as a charge control agent, the particle migration was irregular. When 10 V was applied, the suspension having both charge control agents displayed a clear color change, while the OLOA1200 suspension showed a little migration which cannot be noticed in the picture. It represents that the mixture of the OLOA1200 and Span85 helps
each colored particle to possess different charge valance while the OLOA1200 only makes them to possess similar charge valance. Therefore it can be considered that the Span85 is a good dispersant as well as a good charge control agent for this system. 5. Conclusions In this study, the white pigment, TiO2, nanoparticles were successfully encapsulated using a two-step dispersion polymerization, to be applied as electrophoretic ink particles of e-paper display. The PMMA-co-EGMEA coating enhances the dispersion of TiO2 nanoparticles in the dielectric medium because of reduced density and improved surface characteristics. The electrophoretic behavior was characterized via both electrophoretic light scattering and prototype device also demonstrated that the EGMEA was adaptable as a comonomer to enhance the electrophoretic mobility of polymer coated TiO2 particles in the dielectric medium with the charge control agent. In addition, the OLOA1200 and Span85 mixture was found to be a good additive for this system. Acknowledgment This work was supported by the Ministry of Knowledge Economy, Korea and Kolon (2011). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 4. The prototype display without Span85 (a) and with Span85 (b) under 10 V of applied external voltage.
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