Zero–Zero Birefringence Cellulose Acetate-Based Optical Films by Benzoylation

Zero–zero birefringence film is one of the key components in optical displays, and its design and preparation remain a challenge nowadays. Here, we report the synthesis of benzoylated cellulose acetate (BCA), the preparation of BCA films by solution casting, and the birefringent properties of unstretched and stretched BCA films. It is found that zero–zero birefringence films can be fabricated using both unstretched and stretched BCA films. As the degree of substitution of benzoyl (DSBz) increases, the out-of-plane birefringence (Δnth) gradually decreases and changes from positive to negative. Particularly, for DSBz = 0.511, both the in-plane birefringence (Δnin) and out-of-plane birefringence (Δnth) of stretched BCA films are close to zero. The orientations of the acetyl and benzoyl groups under stretching are measured to interpret the underlying compensation mechanisms. Finally, we show that the wavelength dispersion of BCA films with DSBz = 0.511 varies significantly with the increase in draw ratio, and formulize the normalized birefringence with the relative orientation degree of the benzoyl and acetyl groups. Our results suggest that the birefringence and wavelength dispersion of cellulose acetate-based optical films can be well regulated by a combination of chemical modification and stretching, and have potential significance to the industrial production of zero–zero birefringence films.