Uniaxial tensile strain-insensitive and zero-birefringence cellulose acetate films via atom-economic isocyanate
Zhouhang Lei1, Wenhao Guo1, Xingyue Fang1
1National Synchrotron Radiation Laboratory, Department of Polymer Science and Engineering, State Key Laboratory of Advanced Glass Materials, Anhui Provincial Engineering Research Center for Advanced Functional Polymer Films, Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui, 230029, China.
Abstract:
Zero-birefringence protective films are essential for polarizers in flexible displays, preventing light leakage and color shifts. However, conventional films, such as cellulose triacetate, often suffer from residual birefringence induced by solvent evaporation and mechanical stress. Here, we employ a facile, atom-economical carbamate reaction between cellulose acetate and p-tolyl isocyanate (PTSI) to produce a uniaxial tensile strain-insensitive, zero-birefringence cellulose ester film without generating small-molecule by-products. Density functional theory calculations reveal that the introduced p-tolylcarbamoyl group exhibits large negative polarizability anisotropy, enabling efficient intramolecular compensation to offset the birefringence of the backbone. Guided by these theoretical calculations, we systematically investigate birefringence evolution as a function of both the p-tolylcarbamoyl substitution degree (DS) and the draw ratio under uniaxial stretching. Experimental results reveal that, at an optimal DS of 0.332, the modified films achieve exceptional optical isotropy, maintaining near-zero in-plane (∣Δnin∣ < 0.0005) and out-of-plane (∣Δnth∣ < 0.001) birefringence regardless of the applied draw ratios. This work provides a green synthetic route for modifying cellulose esters and offers a robust material design strategy for next-generation protective films demanding uniaxial tensile strain-insensitive zero-birefringence.
