Extraordinary Wavelength Dispersion of Orientation Birefringence for Cellulose Esters

Orientation birefringence and its wavelength dispersion are studied for hot-drawn films of cellulose esters such as cellulose triacetate (CTA), cellulose diacetate (CDA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB). The orientation birefringence of the cellulose esters is not proportional to the orientation function, indicating that the stress-optical law is not applicable for the cellulose esters. Furthermore, CTA shows negative birefringence, and the magnitude of the absolute value decreases with increasing the wavelength. On the contrary, CAP, CAB, and CDA show positive orientation birefringence that increases with the wavelength. The extraordinary wavelength dispersion of the orientation birefringence for CAP, CAB, and CDA is attributed to the difference in polarizability anisotropy of ester groups. Furthermore, the wavelength dependence of the orientation birefringence for CAP is dependent on the draw ratio and draw temperature, demonstrating that the contribution of polarizability anisotropy from each ester group to the orientation birefringence varies with the draw ratio and temperature. Moreover, the molecular weight and film-processing method also affect the orientation birefringence