Ancient Material Shows High Degree of Birefringence for Use in Displays

When researchers at Osaka University measured the optical birefringence of highly aligned cellulose nanofibers, they found that cellulose’s intrinsic birefringence could be powerful enough to be used in optical displays.

For its experiments, the team used unidirectionally aligned cellulose nanofiber films that it created by stretching hydrogels from nata de coco (a jelly-like food produced from coconut water) at different rates. The nata de coco nanofibers allowed a straight line of cellulose chains to be formed at the molecular level, which helped the researchers precisely determine the maximum birefringence of fully extended polymer chains. Using the extended chain crystals of cellulose, the researchers fabricated relatively small orientation degrees and measured the in-plane retardation maps.

Control of optical retardation by the aligned cellulose film. Courtesy of Osaka University.

The researchers said that they were able to measure the birefringence more accurately by using improved methods. “Using high-quality samples and methods, we were able to reliably determine the inherent birefringence of cellulose, for which very different values had been previously estimated,” said researcher Masaya Nogi.

The researchers estimated the intrinsic birefringence of cellulose to be 0.09. The resulting eigenvalue was greater than those previously reported for cellulose and many petroleum-based polymers.

The primary application the researchers envision for cellulose is for use in light compensation films for LCDs, since LCDs operate by controlling the brightness of pixels with filters that allow only one orientation of light to pass through. Potentially, any smartphone, computer, or television that has an LCD screen could see improved contrast, along with reduced color unevenness and light leakage, with the addition of cellulose nanofiber films.

“Cellulose nanofibers are promising light compensation materials for optoelectronics, such as flexible displays and electronic paper, since they simultaneously have good transparency, flexibility, dimensional stability, and thermal conductivity,” said researcher Kojiro Uetani. “So look for this ancient material in your future high-tech devices.”

The research was published in ACS Macro Letters (https://pubs.acs.org/doi/10.1021/acsmacrolett.9b00024).