A New Type of Display Device Based on Remote Swelling and Collapse of a pH-Responsive Microgel
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A New Type of Display Device Based on Remote Swelling and Collapse of a pH-Responsive Microgel Joseph P. Cook and D. Jason Riley Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, U.K. ABSTRACT Microgel particles of poly(2-vinylpyridine) cross-linked with 0.5 wt% divinylbenzene were synthesised. Due to differences in the scattering of light, a dispersion of the particles appears opaque milky white above pH 4.5 and transparent below pH 4.5. The pH of a 0.15 wt% dispersion was modified using a poly(aniline) film as a source/sink of protons and a Ag/AgCl electrode as a source/sink of chloride counterions. The absorption of the sample at 400 nm was typically reduced from c.a. 3.5 to c.a. 0.5 in less than 20 minutes and increased to c.a. 3.2 in less than 20 minutes. This system could form the basis for a new type of display device, but it is insufficiently fast or consistent in its current form. INTRODUCTION A microgel particle is a colloidally sized cross-linked polymer particle that can respond to an external stimulus such as pH, temperature or ionic strength [1]. The most widely studied microgels are based on N-isopropylacrylamide, but other common monomers are acrylic acid, methacrylic acid and vinylpyridine, which impart pH-sensitivity. Concentrated dispersions of microgel particles can exhibit a vast difference in turbidity between the collapsed and swollen states. This arises because both the size and composition of the particles is different in these two states. Swollen particles have a high solvent content and so provide a lower degree of contrast with the surrounding solvent than the collapsed particles and as a result they scatter light less efficiently. Strong scattering from the collapsed particles results in a milky white dispersion and weak scattering from the swollen particles results in a more transparent dispersion. This difference in turbidity could form the basis for a new type of display device, where partitioned sections containing dispersions of the particles are placed in front of a black background and different sections are set in the collapsed and swollen states to produce an image. In order for this to be realised, a remote method would be required for switching between the two states. Potential benefits of such a device are similar to those of electrophoretic displays: there is no energy input requirement to maintain a static image and the mechanical flexibility of the device is not limited by the microgel. As such, it is likely that microgels would be most suited to devices such as electronic paper as is the case for electrophoretic displays. Sawai et al. [2] achieved remote changes in turbidity of a dispersion of a microgel consisting of acrylic acid and methylmethacrylate cross-linked with ethyleneglycol dimethacrylate by cycling the potential of a poly(pyrrole) coated platinum mesh electrode. Poly(pyrrole) releases or incorporates hydroxide ions depending on the potential applied to the electrode and so allows control over the solution pH. Sa
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