Optical anisotropy in solution-cast film of cellulose triacetate

The out-of-plane birefringence and its wavelength dispersion are studied employing solution-cast films of cellulose triacetate (CTA). In solution-cast process, CTA molecules are induced to align in the film plane. Although refractive index is the lowest in the oriented direction for the CTA films stretched more than 110 %, refractive index is found to be the lowest in the normal direction for the unstretched cast film. Attenuated total reflection measurements reveal that in-plane alignment of the acetyl group which provides strong polarizability anisotropy is responsible for the phenomenon. Furthermore, the out-of-plane birefringence is found to increase with increasing wavelength, i.e. extraordinary wavelength dispersion, whereas a stretched CTA film shows ordinary wavelength dispersion. The level of the out-of-plane birefringence in cast films depends on the preparation conditions, which is predictable considering the evaporation rate. Moreover, it is demonstrated for the first time that the out-of-plane birefringence and its wavelength dispersion can be modified by addition of a certain plasticizer such as tricresyl phosphate (TCP). During the evaporation, TCP molecules orient in the film plane accompanying the orientation of CTA chains by intermolecular orientation correlation, called nematic interaction. This technique will widen the scope of material design of retardation films because there are numerous liquid compounds having strong polarizability anisotropy.