Wavelength Dispersion of Orientation Birefringence for Cellulose Esters Containing Tricresyl Phosphate

Orientation birefringence and its wavelength dispersion are studied employing cellulose esters containing tricresyl phosphate (TCP) as a plasticizer. The addition of tricresyl phosphate (TCP) is found to increase the orientation birefringence of cellulose triacetate (CTA) and cellulose acetate propionate (CAP). In the case of CTA, which has negative birefringence with ordinary wavelength dispersion, the addition of TCP changes the sign of the birefringence to become positive. Moreover, it also provides extraordinary wavelength dispersion that is required for multiband retardation films. The origin of the phenomena is investigated by stress relaxation measurements and solvent treatment. It is found that after the removal of TCP from the stretched CTA/TCP film by methanol the film reverts to negative birefringence. This suggests that TCP molecules have positive birefringence associated with the polarizability anisotropy parallel to the long axis and are aligned to the stretching direction accompanying the chain orientation of CTA and CAP. However, as TCP is a liquidus compound with low molecular weight, the orientation relaxation will occur in a short time after stretching, leading to negative orientation birefringence with ordinary dispersion if not properly quenched.