Material design of retardation films with extraordinary wavelength dispersion of orientation birefringence: a review

Orientation birefringence and its wavelength dispersion for various types of cellulose esters are reviewed. Cellulose esters such as cellulose acetate propionate and cellulose acetate butyrate show positive orientation birefringence with extraordinary wavelength dispersion, which is determined mainly by the ester groups rather than the main chains. The acetyl group provides negative orientation birefringence with strong ordinary wavelength dispersion, whereas the propionyl and butyryl groups give positive orientation birefringence with weak wavelength dispersion. Although all groups show ordinary wavelength dispersion, the summation of their orientation birefringences gives extraordinary dispersion. Moreover, the wavelength dispersion is dependent on the stretching ratio due to the difference in the orientation relaxation of each group. On the contrary, cellulose triacetate (CTA) shows negative birefringence with ordinary wavelength dispersion because it has no positive contribution. However, doping a plasticizer having positive orientation birefringence changes the orientation birefringence of CTA from negative to positive, and the wavelength dispersion from ordinary to extraordinary. This is attributed to the cooperative orientation of plasticizer molecules to the stretching direction with CTA chains, known as nematic interaction upon a hot drawing process.