A new synthesis exploiting cross-linkable dyes and supercritical carbon dioxide has created well-defined polymethylmethacrylate microparticles. The particles can switch between coloured and white states, an essential process in refreshing e-reader pages.

Electrophoretic displays, the black-and-white technology used in e-readers, are very low energy compared to liquid crystal displays, which are used in tablets. Despite demand for electronic magazines and children’s books, full colour electrophoretic displays have not yet been commercialised due to difficulties in devising a scalable polymer synthesis. This is because large amounts of environmentally harmful volatile organic compounds are usually required. However, in this process ‘the technology is very clean’ says Steven Howdle from the University of Nottingham, UK, who led the study. ‘We can do all of the polymerisations in carbon dioxide, you don’t need any other solvent, only a small amount of dodecane in the e-reader to disperse the particles’. Marloes Peeters, who investigates polymeric sensors at Newcastle University, UK, notes the large-scale potential of the reaction, ‘they use supercritical carbon dioxide, which from a sustainability point of view is great and the one-pot synthesis makes a big difference’.

The researchers used free radical dispersion polymerisation in supercritical carbon dioxide to make the cheap commodity polymer polymethylmethacrylate, more commonly known as Perspex. Adding a second dye-containing monomer gave the usually white polymethylmethacrylate particles a distinctive colour, but these dye-containing monomers only polymerised around the microparticles surface. ‘This is what makes the chemistry important,’ says Howdle, as the materials are brightly coloured even at low dye concentrations.

Source: Steven Howdle/University of Nottingham

Electrophoresis test cells showing the effective colour switching needed for a full colour e-reader. The red cell was prepared in supercritical carbon dioxide using the sustainable polymerisation route described here

The dyes contain two acrylate units so are able to cross-link to the polymethylmethacrylate microparticles surface. This avoids issues with previous methods where the dye was not chemically bound to the polymer and so was susceptible to leaching out.

‘We had to learn how to inject the dyes at the right rate at the right time to get particles of the right size containing the right amount of colour.’ continues Howdle. ‘The synthesis itself opens up so many possibilities that it goes beyond electronic inks,’ adds Peeters.

Whilst this work achieves magenta, cyan, yellow and black colour switching, Howdle says a full colour display is a long way off. ‘To get the right colour mix you would need all the different pixels to operate at the same time. We would have to decide how much charge each coloured particle would carry, which ones would switch colour and how.’