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 (DS Bz ) increases, the out-of-plane birefringence (Δn th ) gradually decreases and changes from positive to negative. Particularly, for DS Bz = 0.511, both the in-plane birefringence (Δn in ) and out-of-plane birefringence (Δn th ) 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 DS Bz = 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.

By physical blending of the rigid-rod-like 4-cyano-4′-pentylbiphenyl (5CB) molecule with the cellulose triacetate (CTA), we have prepared the CTA/5CB blend films with different weight fractions of 5CB molecules (ω 5CB ) ranging from 0 to 15%. Even if only a small amount of 5CB molecules are added to the CTA matrix, the orientation birefringence of the stretched blend films becomes positive. With the increasing ω 5CB and the strain ε, the orientation birefringence gets significantly increased. The orientation degrees of CTA chains and 5CB molecules in the stretched blend films are acquired from the infrared dichroism measurements. Then, the nematic interaction strength between CTA chains and 5CB molecules is evaluated to be about 0.43. As ω 5CB ≥ 10%, aggregation of 5CB molecules occurs in the stretched blend films, while it does not affect the transmittance of films. We further show that the aggregation can improve the orientation birefringence of blend films significantly because of the strong nematic interaction between 5CB molecules. A physical picture at the molecular level is proposed to explain why the orientation birefringence of the stretched blend films gets increased. Finally, the orientation birefringence diagrams of the pure CTA films and the CTA/5CB blend films stretched at two different temperatures (170 and 200 °C) in the strain weight fraction of 5CB molecule (ε-ω 5CB ) space are established. We expect that these diagrams would provide a great potential guiding significance for the fabrication of CTA retardation films with specific values of orientation birefringence (Δn in ), such as quarter-wave film and half-wave films.

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