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 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.
guo et al. (Tue,) studied this question.