Regulation of orientation birefringence for cellulose acetate film: The role of crystallization and orientation

Cellulose acetate (CA) based films are widely used in liquid crystal displays due to their outstanding transparency, and a certain orientation birefringence of CA films is required when they are used as retardation films. The regulation of orientation birefringence is usually from the perspective of stretch-induced orientation, while the effects of crystallization behaviors of CA films remain obscure. In this study, the roles of crystallization and orientation on the orientation birefringence of CA films were elucidated. For cellulose diacetate films, the orientation birefringence is dominated by the orientation degree. In comparison, apart from the orientation degree, crystallinity is another key variable to regulate the orientation birefringence of cellulose triacetate and plasticized cellulose triacetate films, originating from the birefringence heterogeneity of the crystalline and amorphous phases. These results provide valuable guidelines for the production of CA-based optical films with excellent optical performance.