Clear Nanofiber Paper

KYOTO UNIVERSITY researchers have made an optically transparent paper from nanosized cellulose fibers. The renewable material's transparency, strength, and thermal stability give it potential advantages over glass or polymers for use in electronic devices.

Masaya Nogi, a postdoc in professor Hiroyuki Yano's lab at the Research Institute for Sustainable Humanosphere, presented the work before the Division of Cellulose & Renewable Materials on March 26 at the ACS national meeting in Salt Lake City.

Nogi and colleagues made nanofiber paper with cellulose, the material used to make traditional paper. But differences in fiber width and size of internal cavities give the papers different physical properties, Nogi said. Nanofiber paper appears transparent because the fibers pack densely and create tiny spaces that avoid light scattering. Conventional paper is opaque because the interstices between the fibers scatter light.

The new material bridges the gap in desirable properties for electronic applications between traditional paper and glass or clear polymer films, says Margaret Frey, an associate professor of fiber science at Cornell University and session coorganizer.

Glass has thermal stability but lacks the flexibility desired in electronic displays, for example, whereas many flexible plastics are not thermally stable.

"Transparent substrates are needed for transparent electronics," says Zhenan Bao, an associate professor of chemical engineering and an electronics expert at Stanford University. The nanofiber paper has thermal expandability comparable with that of glass, so it would be a good substrate for inorganic materials but probably not for organics because they tend to have high thermal expansion coefficients and hence are a large mismatch, she says.

However, Frey adds, all cellulose would be flammable and hydrophilic without further treatment.

To make the densely packed nanofiber paper, Nogi and coworkers pressed a wood flour and water slurry into sheets that were dried and then polished with fine-grit sandpaper (Adv. Mater., DOI: 10.1002/adma.200803174). Hydrogen bonding of hydroxyl groups holds the cellulose fibers together.

Other researchers have created materials like transparent paper, but they were not as flexible or required complicated processing, says Derek G. Gray, a professor in the chemistry department at McGill University, in Montreal. He says the Kyoto group has developed a relatively simple processing method. The new method "is a practical route to novel materials based on a key renewable resource," he adds.