外文翻译原文:Chem. Mater. 2008,1292–1298_人人文库网

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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 paraffi n oil are measured in view of the formulation of electronic inks for electrophoretic displays. Encapsulation of TiO2by 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 infl uence 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 paraffi n oil have successfully been applied in an A4-sized segmented electrophoretic display panel. Introduction Since the development of the fi rst electrophoretic particles image displays (EPIDs) in the 1970s,1–3the progress in the fi eld has been at a low level for over 20 years, until the explosive restart in the search for cheap fl exible electronic paper displays in the late 1990s.4In the past few years, the research has progressed so rapidly that the fi rst products are being marketed at this moment. An EPID is a refl ective-type panel based on electrophore- sis. Electrophoresis is the movement of charged pigment particles, suspended in a liquid, under the infl uence of an electric fi eld. In an electrophoretic display, the particles are required to migrate repeatedly between electrodes by chang- ing polarites of the applied fi eld 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 paraffi n 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, defi ned as the ratio between refl ected light in the white state and black state, is mainly dependent on the electronic ink composition. Impor- tant criteria are the pigment particles (composition, size, light 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 modifi cation 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, TiO2shows a strong interaction with metallic surfaces, such as the displays’ electrodes, which results in an irreversible adsorp- tion of part of the pigment particles on the two electrodes; hence, a decrease in contrast is observed in time. Because of these two drawbacks, TiO2needs 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, 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. (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. 1292Chem. Mater. 2008, 20, 1292–1298 10.1021/cm071197y CCC: $40.75 2008 American Chemical Society Published on Web 01/09/2008 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 fi llers,9Al2O3,10 silica,11or TiO2.12Alternatively, bifunctional monomers have been polymerized as comonomers in the presence of pigment particles to form a cross-linked shell via precipitation polymerization.13These 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 fi rst 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-vinylpyrroli- done) (PVP) as stabilizer, in the presence of hydrophobic TiO2(RCL-188) pigments or hydrophilic TiO2(RCL-11A) pigments functionalized with 3-(trimethoxysilyl)propyl meth- acrylate (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.14To 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 simplifi ed 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 poly- merization 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 been successfully incorporated in a fl exible electrophoretic display prototype. Experimental Section Materials. Marcol 52 (a light paraffi n 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 fl ask 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 redis- persed in EtOH (∼80 mL). Synthesis of TiO2-Polymer Core–Shell Particles via Precipi- tation Polymerization. A three-neck 500 mL fl ask, equipped with a mechanical Tefl on 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 fl ask, equipped with a mechanical Tefl on 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 fl ask, equipped with a mechanical Tefl on 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 polymeriza- tion. The reaction was heated at 60 °C. After 3 h, the reaction mixture was fi ltered and washed with cyclohexane and EtOH. The (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. 1293Chem. Mater., Vol. 20, No. 4, 2008TiO2-Polymer Core–Shell Particles Dispersions 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 CM- 508d), 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(RL- 11A). Figure 1 shows the optical response (scattering intensity) of the display at a constant bias of 30 V. The direction of the applied fi eld 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 fi rst switch is signifi cantly lower than the minimum dark state scattering in the subsequent switches. These results can be explained by the adsorption of the TiO2particles to the electrodes. Starting from a homogeneous system, the particles are moved to the back in the fi rst switch, leading to a good dark state. Because of the electrical fi eld, the particles are brought in close contact with the back electrode and the fi rst 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 fi eld is reduced even more because it is shielded by the fi rst layer of particles.) When the fi eld is reversed, the particles in the last layers on the 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 fi rst particles that reach the front electrode will also be adsorbed strongly at the surface. Thus, when the electrical fi eld 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 fi rst layers is again slow. This fi rst experiment shows the necessity to reduce adsorption of TiO2pigment 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 fi rst studied the encapsulation of a TiO2core 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, RCL- 188 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 Vrij16and Bourgeat-Lami11and 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 TiO2Surface Treatment. Figure 2 shows SEM micrographs of (a) pure RCL-188 and the two types of core–shell particles, either prepared with (b) nonfunc- tionalized hydrophobic TiO2or (c) TPM-functionalized TiO2. TiO2is 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 TiO2has not been encapsulated com- pletely. Indeed, optical measurements of electrophoretic cells (15) Groenewold, J.; Dam, M. A.; Schroten, E.; Hadziioannou, G. Proc. Soc. Inf. Display 2002, 33, 671.(16) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121. Table 1. Concentration of Compounds in Electrophoretic Dispersions compoundconc (mg mL-1) pigmentTiO240.0 dyeSudan Blau Flüssig20.0 surfactantOLOA 120015.0 fl uid mediumMarcol 52 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 fi eld is reversed every 90 s. 1294Chem. Mater., Vol. 20, No. 4, 2008Werts et al. show a poor off-state, similar to pure TiO2, as a result of particle adsorption on the front electrode. To improve the TiO2encapsulation, we have grafted a layer of 3-(trimethoxysilyl)propyl methacrylate (TPM) on the hydrophilic TiO2(RL-11A) particles.10,11,16Precipitation polymerization with this modifi ed TiO2-TPM under similar conditions as described before shows a complete encapsula- tion of the TiO2particles with a shell of cross-linked polystyrene, as confi rmed by SEM (Figure 2c). Polydisperse spherical particles with a diameter of 300–500 nm are obtained. Since the polymerization also takes place at the TiO2surface, the TiO2is well encapsulated with a polymer coating and becomes compatible with the precipitating polymer aggregates, forming a closed shell. Thermogravi- metric 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 fi lling 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 TiO2and monomers in the precipitation polymerization. Particles have been prepared with an amount of TiO2ranging from 15 to 70%. The average densities can be calculated from TGA measurements taking the densities of pure TiO2 and PS found in the literature: respectively 4.2617and 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. 展开阅读全文