Effect of propionyl group on birefringence and wavelength dispersion of propionylated cellulose acetate optical compensation films

Propionylated cellulose acetate (PCA) with different propionyl substitution degrees ( DS Pr ) was synthesized, and the corresponding optical films were prepared by solution casting. The birefringence and its wavelength dispersion of PCA films stretched at 10°C above and below the glass transition temperature ( T g ) with different draw ratios ( DR ) were studied. The introduction of propionyl group at different substitution sites present different contribution to birefringence and its wavelength dispersion. The propionyl group at C-2 and C-3 sites have a larger positive orientation birefringence with stronger inverse wavelength dispersion, while that at the C-6 site shows a higher negative orientation birefringence with weaker normal wavelength dispersion compared with the acetyl group. Compared with CA film, the introduction of the propionyl group weakens the orientation birefringence of PCA film. With the increase of DS Pr from 0.023 to 0.303, the occurrence of propionyl substitution only at the C-6 site turns to C-2, C-3 and C-6 sites and the wavelength dispersion of in-plane birefringence (Δ n in ) decreases. When DS Pr is further increased to 0.537, the Δ n in and out-of-plane birefringence (Δ n th ) of PCA films at different DR show a very weak wavelength dispersion with the relative horizontal curves. Current results indicate that when the substitution degree of the propionyl group and acetyl group is suitable, the wavelength dispersion of PCA film can be eliminated.