Titanium Dioxide−Polymer Core–Shell Particles Dispersions as

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Titanium Dioxide-Polymer Core–Shell Particles Dispersions as Electronic Inks for Electrophoretic Displays M. P. L. Werts, M. Badila, C. Brochon,* A. Hébraud, and G. Hadziioannou* Laboratoire d’Ingénierie des Polymères pour les Hautes Technologies, UMR 7165, UniVersité Louis Pasteur, Ecole Européenne de Chimie, Polymères et Matériaux, 25, rue Becquerel, 67000 Strasbourg, France ReceiVed May 3, 2007. ReVised Manuscript ReceiVed NoVember 19, 2007

The preparation of inorganic–organic core–shell particles is presented. These particles, composed of a titanium dioxide core and a polymer shell, are prepared via precipitation polymerization and inverse microsuspension polymerization. The electrical and optical properties of dispersions of these particles in a paraffin oil are measured in view of the formulation of electronic inks for electrophoretic displays. Encapsulation of TiO2 by precipitation polymerization is improved by pretreating the pigments with 3-(trimethoxysilyl)propyl methacrylate, making it possible to prepare particles with a TiO2-to-polymer ratio varying over a wide range. This ratio has a considerable influence on the optical properties of the dispersion but also on the interactions between pigments and electrodes. The polymer shell can then be further functionalized by introducing acidic groups at the particle’s surface. Encapsulation of the TiO2 can also be achieved by inverse microsuspension polymerization of poly(sodium acrylate), allowing the introduction of the acidic groups in one step only. Finally, dispersions of TiO2-polymer particles in black dyed paraffin oil have successfully been applied in an A4-sized segmented electrophoretic display panel.

Introduction Since the development of the first electrophoretic particles image displays (EPIDs) in the 1970s,1–3 the progress in the field has been at a low level for over 20 years, until the explosive restart in the search for cheap flexible electronic paper displays in the late 1990s.4 In the past few years, the research has progressed so rapidly that the first products are being marketed at this moment. An EPID is a reflective-type panel based on electrophoresis. Electrophoresis is the movement of charged pigment particles, suspended in a liquid, under the influence of an electric field. In an electrophoretic display, the particles are required to migrate repeatedly between electrodes by changing polarites of the applied field without sticking to the electrode surface, sedimenting, or changing electrostatic properties. In our setup, the particles suspension, also called the electronic ink, is composed of charged white pigment in a black-dyed paraffin oil. When the white pigment is at the front of the display, it scatters back the incoming light (white state), and when it is at the back of the display, the light is absorbed by the black dye present in the medium (black state). The obtained contrast in an EPID, defined as the ratio between reflected light in the white state and black state, is mainly dependent on the electronic ink composition. Important criteria are the pigment particles (composition, size, light * Corresponding authors. E-mail: [email protected]; brochonc@ ecpm.u-strasbg.fr.

(1) Ota, I.; Onishi, J.; Yoshiyama, M. Proc. IEEE 1973, 61, 832. (2) Dalisa, A. L. IEEE Trans. Electron DeVices 1977, 24, 827. (3) Sheridon, N. K.; Berkovitz, M. A. Proc. Soc. Inf. Display 1977, 18, 289. (4) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. Nature (London) 1998, 394, 253.

scattering properties, density) and the concentration of the different compounds (dye and pigments) as a function of the thickness of the device. Therefore, much research has focused on the modification of pigment particles to alter the scattering properties, surface charges, steric stabilization, interactions with the electrode surface, and interparticle interactions. In many cases, titanium dioxide (TiO2) is used as pigment because of its excellent scattering properties. However, it has the disadvantage of having a very high density, making it susceptible to gravitational settling. In addition, TiO2 shows a strong interaction with metallic surfaces, such as the displays’ electrodes, which results in an irreversible adsorption of part of the pigment particles on the two electrodes; hence, a decrease in contrast is observed in time. Because of these two drawbacks, TiO2 needs to be coated with a layer of polymeric material (core–shell) to lower the average density and reduce the interactions. In the past, the preparation of TiO-polymer core–shell particles has been done differently by several companies. Brown, Boverie, & Cie5 described the coating of TiO2 with paraffinic waxes by using the so-called “thermosolution effect”. The pigment is dispersed in a solution of wax in an apolar solvent. Upon cooling, the wax precipitates on the pigment surface, forming a uniform shell. In order to improve the chemical stability, Xerox6 used a thermoplastic resin that hardens after it has precipitated on the pigment. Instead of direct physical adsorption from solution, the polymer shell can also be prepared in situ in the presence of (5) Mueller, K.; Zimmermann, A. US Patent 4,298,448, Nov 3, 1981. (6) Bush, A.; Pan, D. H.; Cheng, C.-M.; Pin, P. US Patent 6,525,866, Feb 25, 2003.

10.1021/cm071197y CCC: $40.75  2008 American Chemical Society Published on Web 01/09/2008

TiO2-Polymer Core–Shell Particles Dispersions

pigment. In a precipitation polymerization, the solvent and monomer are chosen so that the polymer precipitates on the particles’ surface during polymerization.7,8 To guarantee a long-term stability, it is preferable to chemically link the polymer chains to the particle surface. This can be achieved by the radical polymerization of a monomer in the presence of pigment particles bearing polymerizable groups or initiator groups on their surfaces, as has been shown for example with ceramic fillers,9 Al2O3,10 silica,11 or TiO2.12 Alternatively, bifunctional monomers have been polymerized as comonomers in the presence of pigment particles to form a cross-linked shell via precipitation polymerization.13 These cross-linked core–shell polymeric particles have a highly improved heat and solvent resistance compared to non-cross-linked particles. Here, we describe the combination of the two methods mentioned above, as well as a different approach in an inverse polymerization system, to synthesize TiO-polymer core–shell particles for the preparation of an electronic ink for EPIDs. In a first part, we present the preparation of such materials by precipitation polymerization of several monomers, one of them being a cross-linker such as divinylbenzene. Mixtures of styrene and divinylbenzene are polymerized in a water-ethanol (5/95 v/v) medium with poly(N-vinylpyrrolidone) (PVP) as stabilizer, in the presence of hydrophobic TiO2 (RCL-188) pigments or hydrophilic TiO2 (RCL-11A) pigments functionalized with 3-(trimethoxysilyl)propyl methacrylate (TPM). In our system, charging of the particles is accomplished by adding a basic surfactant, the polyisobutylene succinimide OLOA 1200, which exchanges a proton with acidic groups on the surface of the particle.14 To improve the charging, we functionalize the polymer shell by addition of acrylic or sulfonic acid groups. The effects of the TiO2-to-polymer ratio, of the shell functionalization, and of the different synthetic routes on the optical and electrical response of the ink have been studied. In a second part, we propose a simplified approach in which the polystyrene shell is directly replaced by a functional polyelectrolyte shell of acrylic acid, cross-linked with ethylene glycol dimethacrylate. In this case, the particles are prepared in one step by inverse microsuspension polymerization in the presence of hydrophilic TiO2 (RCL-11A) pigments. Finally, our studies have led to the development of a new electronic ink with excellent optical properties, which has (7) Schubert, F. E.; Chen, J. H.; Hou, W.-H. US Patent 5,783,614, July 21, 1998. (8) Kim, J.-W.; Shim, J.-W.; Bae, J.-H.; Han, S. H.; Kim, H.-K.; Chang, I.-S.; Kang, H.-H.; Suh, K.-D. Colloid Polym. Sci. 2002, 280, 584. (9) Abboud, M.; Turner, M.; Duguet, E.; Fontanille, M. J. Mater. Chem. 1997, 7, 1527. (10) Duguet, E.; Abboud, M.; Morvan, F.; Mahue, P.; Fontanille, M. Macromol. Symp. 2000, 151, 365. (11) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293. (12) Moran, E. A.; Pratt, E. J.; Herb, C. A.; King, M. A.; Zang, L.; Honeyman, C. H.; Houde, K. L.; Paolini, R. J.; Pullen, A. E. WO Patent 02,093,246, Nov 21, 2002. (13) Li, W.-H.; Stöver, H. D. H. Macromolecules 2000, 33, 4354. (14) Kornbrekke, R. E.; Morrison, I. D.; Oja, T. Langmuir 1992, 1211.

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been successfully incorporated in a flexible electrophoretic display prototype. Experimental Section Materials. Marcol 52 (a light paraffin oil) was obtained from ESSO. OLOA 1200 (polyisobutylene succinimide) was offered by Chevron Texaco, and TiO2 (rutile, RCL-188 and RL-11A) was obtained from Millenium Chemicals. 3-(Trimethoxysilyl)propyl methacrylate (TPM) and Sudan Red 7B were purchased from Acros, Solvent Blue 35 was obtained from Aldrich, and Sudan Yellow 146, Sudan Blue, and poly(N-vinylpyrrolidone) (Kollidon 90) were obtained from BASF. Styrene and divinylbenzene were passed through a column of basic aluminum oxide before use to remove the inhibitor. Acrylic acid, ethylene glycol dimethacrylate, Span 80, Tween 80, and potassium persulfate were obtained from Aldrich and used as received. Grafting of TPM on TiO2. TiO2 (12 g) and EtOH (150 mL) were mixed in a one-neck 250 mL flask and ultrasonicated/stirred for 60 min. Ammonia (9 mL) and 3-(trimethoxysilyl)propyl methacrylate (1.9 mL) were added, and the reaction mixture was stirred for 24 h at 50 °C. The product was centrifuged (3000 rpm, 15 min), washed two times with EtOH, and subsequently redispersed in EtOH (∼80 mL). Synthesis of TiO2-Polymer Core–Shell Particles via Precipitation Polymerization. A three-neck 500 mL flask, equipped with a mechanical Teflon stirrer and nitrogen inlet, was charged with a known amount of grafted or ungrafted TiO2 (0.5-10 g), PVP (1 g), EtOH (94.5 mL), and water (5.5 mL). When ungrafted TiO2 was used, the reaction mixture was ultrasonicated for 60 min. After bubbling nitrogen through the medium for 10 min, the reaction mixture was heated to 70 °C, and styrene (5 mL), divinylbenzene (5 mL), and AIBN (0.6 g) were added. After 24 h, the reaction mixture was centrifuged (4000 rpm, 15 min) and the precipitate was washed with EtOH three times. The product was dried overnight under vacuum, yielding a white powder. This method allows for the synthesis of core–shell particles with a functionalized shell by adding in the second part of the polymerization a second functional monomer such as acrylic acid or sodium styrenesulfonate and AIBN and continuing the polymerization for another 8 h. One-Step Synthesis of TiO2-Polymer Core–Shell Particles. In a three-neck 500 mL flask, equipped with a mechanical Teflon stirrer and nitrogen inlet, TiO2 (10 g) was dispersed in EtOH (90 mL) and ultrasonicated for 60 min. TPM (0.4 mL) and ammonia (2.0 mL) were added, and the reaction was stirred for 24 h at 50 °C. Subsequently, PVP (1 g), water (3.5 mL), styrene (2 mL), divinylbenzene (2 mL), and AIBN (0.25 mg) were added, and the reaction mixture was stirred at 70 °C. After 24 h, the reaction mixture was centrifuged (4000 rpm, 15 min), and the precipitate was washed with EtOH three times. The product was dried overnight under vacuum, yielding a white powder. Synthesis of TiO2-Polymer Core–Shell Particles via Inverse Microsuspension. In a three-neck 250 mL flask, equipped with a mechanical Teflon stirrer and nitrogen inlet, was placed a mixture of cyclohexane (110 mL) and surfactants Span 80 (2.8 mL) and Tween 80 (1.9 mL). Separately, an aqueous phase was formed containing TiO2 (2 g), which was dispersed in an aqueous solution of acrylic acid (28 mL), 80% neutralized using a 5 M NaOH solution and the cross-linker, ethylene glycol dimethacrylate (0.5 mL). The mixture was ultrasonicated for 60 min, and then it was added to the oil phase in the reactor. After homogeneization, potassium persulfate (0.1 g) was added to initiate the polymerization. The reaction was heated at 60 °C. After 3 h, the reaction mixture was filtered and washed with cyclohexane and EtOH. The

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Table 1. Concentration of Compounds in Electrophoretic Dispersions compound pigment dye surfactant fluid medium

TiO2 Sudan Blau Flüssig OLOA 1200 Marcol 52

conc (mg mL-1) 40.0 20.0 15.0

product was dried overnight under vacuum, yielding a white powder. Particle Characterization. The zeta potential of the particles in water at pH 6 was measured using a zetameter (Zetasizer 2000 from Malvern Instruments). Dynamic light scattering (DLS) measurements were performed using Zetasizer 2000 from Malvern Instruments. Electrooptical Characterization. Electrophoretic dispersions are prepared by dispersion of the synthesized particles in Marcol, in the presence of a surfactant OLOA 1200 and a dye or a mixture of dyes. It is then introduced in an electrophoretic cell, composed of two ITO-covered glass slides (2 × 3 cm) separated by a 50 µm Kapton spacer (optical area 15 × 15 mm). The cell is operated using a Keithley 230 programmable power source. For the determination of a time-resolved optical response we have measured the light scattering under a 0° angle with a silicon photodiode. Illumination was under an angle of 30° with a 4 mW He/Ne laser (λ ) 664 nm).15 White state and black state lightness values (system CIE L*a*b* 1976) are also determined with a spectrophotometer (Minolta CM508d), with the specular component excluded, under simulated sunlight illumination (D65).

Results and Discussion We have started our investigations with the electrooptical characterization of dispersions (Table 1) of pure TiO2 (RL11A). Figure 1 shows the optical response (scattering intensity) of the display at a constant bias of 30 V. The direction of the applied field is changed every 90 s. When the display is switched from the dark state to the white state, we observe a fast initial increase of the scattering (at 90 and 270 s), followed by a smaller slow increase after ∼20 s, which continues until the next switch. Switching back from the white state to the black state, a fast initial change in scattering is observed, followed by a slow component which is not completed at the end of the switch. Furthermore, the reached minimum scattering intensity in the dark state at the first switch is significantly lower than the minimum dark state scattering in the subsequent switches. These results can be explained by the adsorption of the TiO2 particles to the electrodes. Starting from a homogeneous system, the particles are moved to the back in the first switch, leading to a good dark state. Because of the electrical field, the particles are brought in close contact with the back electrode and the first layer adheres strongly to the back electrode (Subsequent layers will be adsorbed less strongly since the distance between the particle and electrode surface is bigger. The effective electrical field is reduced even more because it is shielded by the first layer of particles.) When the field is reversed, the particles in the last layers on the (15) Groenewold, J.; Dam, M. A.; Schroten, E.; Hadziioannou, G. Proc. Soc. Inf. Display 2002, 33, 671.

Figure 1. Optical response of an electrophoretic cell containing TiO2 particles in Marcol with a blue dye at 30 V. Particles are charged with OLOA 1200. The polarity of the applied electrical field is reversed every 90 s.

back electrode can readily move to the front, showing a fast initial increase in scattering. The subsequent layers from the back electrode have to be desorbed, which is a slow process. As on the back electrode, the first particles that reach the front electrode will also be adsorbed strongly at the surface. Thus, when the electrical field is reversed again to change from the white state to the dark state, the last (not or weakly adsorbed) layers will move toward the back, showing a fast initial decrease in scattering, while desorption of the first layers is again slow. This first experiment shows the necessity to reduce adsorption of TiO2 pigment to the electrodes. Encapsulation of the pigment in a polymer shell is a good solution to this problem as it provides for good sterical repulsion and decreases the particles-electrodes interactions. Furthermore, it also has the advantage to substantially decrease the density of the electrophoretic particles and hence prevent the rapid sedimentation of the pigments in the cell. 1. Synthesis of TiO2/Polymer Particles via a Direct, Precipitation Polymerization System. We have first studied the encapsulation of a TiO2 core by a polystyrene shell using dispersion polymerization. Two different methods have been used: (i) Direct coating of nonfunctionalized hydrophobic TiO2 (RCL-188). Since the polymeric shell is based on the hydrophobic monomers styrene and divinylbenzene, RCL188 is chosen as core pigment because of its hydrophobic nature. (ii) Coating of TPM-functionalized hydrophilic TiO2 (RL-11A). The TPM grafting procedure is based on the work described by Philipse and Vrij16 and Bourgeat-Lami11 and performed on RL-11A, an untreated, hydrophilic TiO2. These particles, bearing polymerizable groups on their surface, are subsequently used in the dispersion polymerization. 1.1. Effect of TiO2 Surface Treatment. Figure 2 shows SEM micrographs of (a) pure RCL-188 and the two types of core–shell particles, either prepared with (b) nonfunctionalized hydrophobic TiO2 or (c) TPM-functionalized TiO2. TiO2 is irregularly shaped with an average particle diameter of 250 nm. After the precipitation polymerization of styrene-divinylbenzene in the presence of nonfunctionalized TiO2, agglomerates of polymer particles of 100–200 nm are observed, which have precipitated on the surface of the TiO2. From the irregular shapes that are still present we have concluded that the TiO2 has not been encapsulated completely. Indeed, optical measurements of electrophoretic cells (16) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121.

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Figure 2. SEM micrographs of (a) pure TiO2 (RCL-188), (b) nonfunctionalized TiO2 (RCL-188), coated with styrene and divinylbenzene via precipitation polymerization, and (c) core–shell TiO2-polystyrene particles, prepared via precipitation polymerization in the presence of TPM-functionalized TiO2.

Figure 3. Optical response, at 30 V, of an electrophoretic dispersion of core–shell TiO2-polystyrene particles (63 wt % polymer), prepared via precipitation polymerization in the presence of TPM-functionalized TiO2.

show a poor off-state, similar to pure TiO2, as a result of particle adsorption on the front electrode. To improve the TiO2 encapsulation, we have grafted a layer of 3-(trimethoxysilyl)propyl methacrylate (TPM) on the hydrophilic TiO2 (RL-11A) particles.10,11,16 Precipitation polymerization with this modified TiO2-TPM under similar conditions as described before shows a complete encapsulation of the TiO2 particles with a shell of cross-linked polystyrene, as confirmed by SEM (Figure 2c). Polydisperse spherical particles with a diameter of 300–500 nm are obtained. Since the polymerization also takes place at the TiO2 surface, the TiO2 is well encapsulated with a polymer coating and becomes compatible with the precipitating polymer aggregates, forming a closed shell. Thermogravimetric analysis (TGA) showed a ratio TiO2:polymer of 37:63 in mass. As can be seen from the optical measurements in Figure 3, no particle adsorption on the electrode surface occurs, off-states with very low scattering are reached, and the switching is completely reversible. The obtained contrast is 25. The polydispersity in size of our particles induces a polydispersity in their electrophoretic mobility, which could have a detrimental effect on the white/dark transition. However, it also has the advantage of providing for a better packing at the electrodes surface, the smaller particles filling the voids in between the bigger particles, hence increasing the contrast. 1.2. Effect of TiO2:Polymer Ratio. The ratio of titanium dioxide to polymer can be controlled by varying the amount of TiO2 and monomers in the precipitation polymerization. Particles have been prepared with an amount of TiO2 ranging from 15 to 70%. The average densities can be calculated from TGA measurements taking the densities of pure TiO2

Figure 4. Core–shell particles of TiO2-TPM and polystyrene. The amount of TiO2 in the particles, measured by TGA, is (A) 28%, (B) 40%, and (C) 67%. The experiment was carried out at 30 V.

and PS found in the literature: respectively 4.2617 and 1.05.18 They are comprised between 2.2 and 1.2 for the prepared particles. The encapsulation of the pigment in polystyrene thus is a good mean to decrease the sedimentation of the particles in the display. The optical response at 30 V of three dispersions with a blue contrasting dye is plotted for three different TiO2 content in Figure 4. The total concentration of particles in the dispersions is kept constant at 40 mg mL-1. A larger amount of TiO2 inside the particles clearly gives a higher white state scattering. This is not only because the total amount of TiO2 is higher but also because the separation between the TiO2 cores is smaller. Polystyrene has a refractive index close to that of Marcol; hence, no scattering occurs at the Marcol-polymer interface. Particles with a higher TiO2 content lead to a better packing of the highly scattering cores and thus higher overall scattering. 1.3. Simplification of the Synthetic Route. In the procedure described above for the synthesis of the TiO2-polymer core–shell particles, the preparation occurs in two steps: (i) the preparation and purification of TPM-functionalized TiO2 and (ii) the precipitation polymerization in the presence of the modified TiO2. Attempts have been made to eliminate the purification step after the TPM functionalization of TiO2. This would have as a disadvantage that ammonia and unreacted TPM remain in the reaction mixture during the polymerization process, which might lead to inhibition of the polymerization or aggregation of particles. It is indeed claimed by Bourgeat-Lami and Lang11 that the excess TPM needs to be removed to prevent aggregation. However, an (17) Handbook of Chemistry and Physics, 49th ed.; CRC Press: Cleveland, pp 1968–1969. (18) Polymer Handbook, 4th ed.; John Wiley and Sons: New York, 1999.

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Table 2. Formulation of an Electrophoretic Dispersion compound

concentration (mg mL-1)

TiO2-polymer core–shell particle OLOA 1200 (surfactant) black dyea Marcol

40 15 18.5

Scheme 1. Direct Multilayered System in Two Polymerization Steps

a The black dye is a mixture of four dyes with different absorption maxima that give, at the optimum concentrations, an average black color.

Table 3. Zeta Potential Values at pH 6 Measured in Water for Three Types of Particles with the Same Pigment:Polymer Ratio

Figure 5. Lightness L*, measured with a spectrophotometer (Minolta CM508d) at 30 V, as a function of the PS content, for TiO2-PS core–shell particles prepared via a one-step process (open symbols) or a two-step process (plain symbols). Square: on-state lightness; circle: off-state lightness. Lines are guides to the eye.

advantage would be the reduction in the number of purification steps and the amount of used chemicals. Besides the core–shell particles, prepared by the abovedescribed method (in two steps), particles have also been prepared in one step. After the grafting of TPM on the TiO2 at 50 °C, the monomers, PVP and initiator are added, and the reaction mixture is stirred for an additional 24 h at 70 °C, followed by purification. To prevent coagulation, lower concentrations of ammonia and TPM were used. Via both methods, particles with an amount of polymer ranging from 1 to 70% have been synthesized. Dispersions of these particles with a surfactant and a contrasting dye have been prepared according to the concentrations in Table 2. The influence of the TiO2:polymer ratio on the optical properties is studied by measuring the black and white state lightness of the dispersions at 30 V, with a Minolta CM508d spectrophotometer under simulated sunlight illumination (D65, Figure 5). As a comparison, particles made in one and two steps are shown. For both preparation methods, the white state lightness decreases slightly with an increasing amount of polymer. This corresponds to the TiO2 packing effect already described above. Moreover, an even stronger decrease of the black state lightness is observed with an increasing amount of polymer. This can be attributed to incomplete encapsulation of the TiO2 cores for low polymer amount, resulting in a strong adsorption of the particles at the electrodes. On the contrary, if the TiO2 is completely covered with polymer (above (15%), the black state lightness is low and becomes more or less independent of the polymer amount. Finally, the particles prepared in the one-step process show (as compared to the two-step process) a lower scattering in both the white state and the black state. Most likely, particles are aggregated together due to the excess TPM present in the reaction mixture. This would lead both to a less efficient packing at the electrodes, resulting in a lower white state lightness, and to a weaker adsorption of the particles to the electrodes

particle

zeta potential (mV)

TiO2-PS core–shell particle TiO2-PS/sulfonic acid core–shell particle TiO2-PS/acrylic acid core–shell particle

-24.3 -41.0 -46.5

surface, resulting in a lower black state lightness. However, evidence of the presence of aggregates has not been found in SEM images. 1.4. Effect of Surface Functionalization. The particles are then functionalized by grafting acidic moieties on the particles surface. The increased amount of acidic groups should lead to more charges, hence a higher pigment velocity during switching (faster switch speed) and a better packing at the electrodes (higher white state scattering). In particular, sodium styrenesulfonate and acrylic acid are added to the reaction mixture at the end of the dispersion polymerization process (as shown in Scheme 1). The zeta potential is measured as a function of pH for three types of particles containing the same TiO2:polymer ratio. The values at pH 6, which is the pH of the electrophoretic dispersion at equilibrium, are given in Table 3. The nonfunctionalized particles already have a negative zeta potential. This could be due to the presence of anionic sulfate groups at the surface of the particles, coming from the decomposition of the persulfate photoinitiator. Moreover, it was shown that hydroxyl anions preferentially adsorb onto hydrophobic surfaces in water, providing for a negative charge.19 However, adding charged groups such as sodium acrylate or sulfonate to the particle surface further increases the zeta potential of the particles. Both functionalized and nonfunctionalized core–shell particles with different TiO2:polymer ratios have been prepared. The white state scattering is plotted as a function of the amount of TiO2 in the particles (Figure 6). Once more, it is clearly visible that the white state scattering is related to the TiO2:polymer ratio, but more important, this dependency is the same for the nonfunctionalized, acrylic acid and sulfonic acid functionalized pigment particles. However, the added charges are not sufficient, and the switch speeds (not shown) do not change significantly when the acidic groups are introduced. In the electronic ink, OLOA 1200 charges the particles by exchanging a proton with the introduced acid groups. We believe that free acid groups are already present on the surface of the nonmodified polystyrene particles. We describe hereafter the encapsulation of TiO2 with poly(acrylic acid) alone in order to increase more signifi(19) Beattie, J. K.; Djerdjev, A. M. Angew. Chem., Int. Ed. 2004, 43, 3568.

TiO2-Polymer Core–Shell Particles Dispersions

Figure 6. White state scattering, at 30 V, as a function of the TiO2 content in the core–shell particles, for dispersions containing nonfunctionalized ([), styrenesulfonic acid (2), and acrylic acid (9) functionalized TiO2-polymer core–shell particles.

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Figure 8. SEM micrographs of (a) pure TiO2 (RL-11A) and (b) nonfunctionalized TiO2, coated with poly(acrylic acid) and ethylene glycol dimethacrylate via inverse microsuspension polymerization.

Scheme 2. Inverse Single Polyelectrolyte Layered System

cantly the number of acid groups on the particles and improve the switch speed. 2. Inverse Microsuspension Polymerization Approach. An inverse microsuspension procedure is employed in order to obtain better surface charge control by using a polyelectrolyte as unique polymer coating layer on pigment surface, as represented in Scheme 2. The synthesized particles are characterized by dynamic light scattering (DLS, Figure 7) and scanning electron microscopy (SEM, Figure 8). DLS measurements show an increase of the particle diameter from 250 nm (pure TiO2) to about 430 nm for the coated particles and a broadened size distribution. The successful encapsulation of TiO2 in poly(acrylic acid) is finally reflected in the optical measurement in Figure 9. An electrophoretic dispersion of the TiO2-PAA particles is prepared according to the formulation given in Table 2. The optical response times of the electrophoretic dispersion are improved from 3 s for the TiO2-PS-PAA particles previously prepared to 2 s for the TiO2-PAA particles. The obtained contrast is higher than 40. However, electrode adsorption of the TiO2-PAA particles occurs, which is manifested by a higher off-state scattering intensity. This could be the effect of the higher affinity of the polyelectrolyte layer toward the electrode due to insufficient particle stabilization. The stabilization of the particles could be

Figure 9. Optical response, at 30 V, of a dispersion with core–shell TiO2-poly(acrylic acid) particles, prepared via inverse microsuspension polymerization in the presence of TiO2. The direction of the applied field is changed every 90 s.

improved by grafting an alkyl chain that would provide for sterical repulsive interactions. This can be easily done by replacing the surfactant in the inverse microsuspension by a polymerizable surfactant, also called a surfmer. This work will be described in a future paper. Finally, it is visible that the white state scattering remains constant during switching, and like in the case of the multilayered particles, its level could be related to the TiO2: polymer ratio. 3. Applications. On the basis of the above-described findings, an electrophoretic ink is formulated using TiO2-PS-PAA particles and a contrasting black dye, as described in Table 2. The particles, having a polymer content of 14%, were prepared by precipitation polymerization followed by functionalization with acrylic acid, as described in section 1. The dispersion shows a contrast of 25 and a white state lightness of 51. Figure 10 shows an A4-sized direct drive display panel filled with this black-white dispersion. The display has 14 × 17 pixels and operates at 50 V. A pixelated foil structure with a thickness of 50 µm is used to encapsulate the ink, hereby preventing settling of the TiO2 particles. Multiple panels can be stacked in horizontal and vertical direction to any desired size. Conclusions

Figure 7. Particle size distribution profiles measured by DLS. Solid line: pure TiO2 (RL-11A); dashed line: TiO2 coated with poly(acrylic acid) and ethylene glycol dimethacrylate via inverse microsuspension polymerization.

In this paper, we describe the successful preparation of titanium dioxide-polymer core–shell particles, their dispersion in a dyed media, and use in electrophoretic displays. Both a precipitation polymerization and an inverse microsuspension polymerization process have been studied. To prevent adsorption of titanium dioxide on the electrode surface and decrease particle density, the particles are first coated with a shell of cross-linked polystyrene via a precipitation polymerization of styrene and divinylbenzene

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Figure 10. A4-sized segmented 50 µm thick electrophoretic display panel using white TiO2-PS-PAA core–shell particles containing 14% polymer and prepared by direct precipitation polymerization followed by functionalization with acrylic acid as described in section 1.4. The formulation of the electrophoretic ink is given in Table 2. The display operates at 50 V.

in the presence of TiO2. A better encapsulation is achieved by pretreating the TiO2 particles with 3-(trimethoxysilyl) propyl methacrylate. Electrophoretic dispersions are prepared by dispersing the particles in a dyed paraffin oil. The polymer content has a considerable influence on the optical properties of the electrophoretic ink. Increasing the polymer content of the particles results in a lower black state lightness as irreversible adsorption of TiO2 on electrodes is completely

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hindered by the polymer shell above a polymer content of 15%. However, a high polymer content also results in a lower white state due to less effective packing of the TiO2 cores on the front electrode. On the other side, the density of the pigments can be decreased from 4.26 to 1.2, making the sedimentation of the particle slower. Charging of the particles is accomplished by addition of a basic surfactant (OLOA 1200) that exchanges a proton with acid groups on the surface of the particle. Functionalization of the polymer shell with acrylic acid or styrenesulfonate to increase the amount of chargeable groups does not have a significant effect on the switch speed of the electrophoretic ink. Finally, by eliminating cleaning steps in the precipitation process, the synthetic route could be simplified without a significant alteration of the optical properties. In a second part, we describe the encapsulation of TiO2 with a shell of cross-linked poly(acrylic acid) via an inverse microsuspension polymerization of acrylic acid and ethylene glycol dimethacrylate. This time, the switch speed is improved but adsorption of the particles on electrodes occurs. Improvement of the particle stabilization by addition of a polymerizable surfactant will be described in a future paper. Finally, we have used the acrylic acid-functionalized TiO2-PS particles to successfully assemble an A4-sized segmented electrophoretic display panel. Acknowledgment. We are grateful to Jacques Faerber from IPCMS Strasbourg and Josianne Widmaier from Institut Charles Sadron (ICS, Strasbourg, France) for their expert assistance with different particle characterizations. Also, we acknowledge the members of the company Papyron for the framework we used in the display assembling and the Dutch Technology Foundation (STW) for financially supporting the project. CM071197Y