Magnetic polymer beads promise color e-paper

Magnetic polymer beads promise color e-paper

Capable of producing different bistable color states, beads change color instantly and reversibly using a magnetic field

A research team led by Yadong Yin, a chemist at the University of California, Riverside (UCR), has fabricated microscopic polymer beads that change color instantly and reversibly when applied external magnetic fields change orientation. These stable photonic materials with tunable colors can be fabricated on a large scale, promising the advent of full-color electronic paper, as well as rewritable signage and posters.

The beads, or magnetochromatic microspheres, are said to have excellent structural stability and can be manufactured in diameters from 10 to 100 μm. They also are highly compatible with various types of dispersion media such as water, alcohol, hexane, and even polymer solutions, allowing them to retain magnetically tunable colors in a variety of chemical environments. Embedded in a viscous material, the beads would maintain their orientation without an external field, and could be erased and rewritten when the field is applied.

In their lab experiments, researchers from UCR and Seoul National University embedded arrays of spatially ordered magnetic iron oxide (Fe3 O4 ) nanostructures within each polymer microsphere, enabling its colors to be switched on and off simply by changing the microsphere’s orientation or more precisely the orientation of the array (see Fig. 1 ). This produces bistable color states, required for making rewritable displays.

Fig. 1. At the top, when the applied magnetic field is perpendicular (vertical) to the surface, the beads’ color is switched on. When the field is horizontal to the surface, the color is switched off.

“Unlike many conventional approaches, the instantaneous color change occurs with no change in the structure or intrinsic properties of the microspheres themselves,” Yin points out. “What changes instead are the magnetic fields acting externally on the orientation of these microspheres, these photonic crystals.”

The color observed in the new materials is “structural color,” caused by interference effects rather than pigments. Such color effects, as often seen in bird feathers, butterfly wings, and beetle shells, are produced when microstructures in these objects are aligned in periodic arrays (see Fig. 2 ). ”Within a certain range [about 80 to 90 nm],” Yin adds, “it is possible also to tune the color of the material by simply rotating the microspheres,”

Fig. 2. Polymer microspheres with 10 to 100-μm diameters can create a variety of colors produced when microstructures are aligned in periodic arrays.

“Conventional methods to produce tunable structural color rely on changing the periodicity of the array or the refractive index of the materials changes that are difficult to achieve or involve slow processes,” Yin said. “In our method, the color is tuned by changing the relative orientation of the periodic arrays in the microspheres by conveniently using external fields. The use of magnetic fields as external stimuli has the additional benefits of instant action, contactless control, and easy integration into electronic devices already in the market.”

Sunghoon Kwon, a leading biophotonics and nanoengineering expert whose Seoul National University lab collaborated with Yin, notes that “the new technology has a great potential for a wide range of photonic applications, because the on/off switching of the diffraction color by the rotating photonic sphere is fast, greatly simplifying the pixel structures.”

To fabricate the microspheres, researchers mixed magnetic iron oxide particles into a curable resin in liquid phase; the resin can be made solid by exposure to ultraviolet light. The resin solution was then dispersed in oil (mineral oil or silicon oil), whereupon the resin transformed into spherical droplets suspended in the oil.

The researchers next applied an external magnetic field to organize the iron oxide particles into periodically ordered structures that display a reflective color if viewed along the direction of the magnetic field. Finally, the liquid system is exposed to ultraviolet radiation to polymerize the resin droplets and make them solid microspheres.

The group now plans to work on specific applications for the magnetochromatic microspheres rewritable display units such as e-papers and e-posters are their main interest — as well as to develop similar materials for use in chemical and biological sensors. The UCR Office of Technology Commercialization has filed a patent application on the technology and is currently seeking industry partners interested in its commercial development.

For more information, contact Yadong Yin by e-mail at yadong.yin@ucr.edu or by phone at 951-827-4965, or visit the site http://faculty.ucr.edu/~yadongy/index.html.

Richard Comerford