Benzoate-Substituted Cellulose Films with Tunable Birefringence and Anomalous Optical Anisotropy

Developing sustainable biobased alternatives is imperative not only to address mounting environmental concerns but also to overcome the intrinsic limitations of conventional petroleum-derived optical films in next-generation displays. However, conventional biobased materials like cellulose triacetate suffer from restricted optical tunability (typically negative C-plate behavior). Herein, we fabricate high-performance optical films from naturally sourced cellulose by benzoate substitution with controlled degrees of substitution (DS) from 0.89 to 3.00, using a homogeneous ionic liquid platform. By systematically investigating the “structure-processing-property” relationships, we reveal a distinct nonmonotonic “V-shaped” dependence of birefringence on DS. Crucially, we identify a steric-hindrance-driven orientation flip of the benzoyl side groups, which enables the precise tailoring of optical properties: from a “zero-birefringence” state at DS ≈ 1.55 to specific Negative Retro-Zero characteristics (nx > nz > ny) for 1.55 < DS < 2.59, and finally to positive C-plate behavior at DS > 2.59. The precise control over birefringence achieved in this work broadens the applicability of cellulose derivatives and paves the way for the design of advanced materials for next-generation display technologies.