Control of optical anisotropy of cellulose esters and their blends with plasticizer

Orientation birefringence of various cellulose esters, such as cellulose triacetate, cellulose acetate propionate and cellulose acetate butyrate, and their blends with a plasticizer are studied. It is found that propionyl and butyryl groups show positive orientation birefringence, whereas the acetyl group exhibits negative birefringence. These results indicate that a cellulose ester with an appropriate composition of ester groups shows no orientation birefringence, irrespective of the molecular orientation. Further, photoelastic birefringence of cellulose esters in the glassy state is also investigated, and is found to be more pronounced with increasing butyryl group content. Moreover, the addition of the plasticizer affects both the orientation and photoelastic birefringences. It should be noted that the magnitude of these birefringences is dependent on the plasticizer species. Plasticizers with a large solubility parameter enhance the birefringence, whereas those with a small solubility parameter decrease it. Moreover, infrared dichroic measurements suggest that the orientation birefringence is mainly determined by the polarizability anisotropy of the ester groups, which is affected by the plasticizer.