Computational study to evaluate the birefringence of uniaxially oriented film of cellulose triacetate

•We evaluated birefringence of CTA film using DFT calculation and MD simulation.•CTA with gt and gg conformation show negative intrinsic birefringence.•CTA with tg conformation shows positive intrinsic birefringence.•Orientation birefringence of CTA film shows negative because of gt conformers.

The intrinsic birefringence of a cellulose triacetate (CTA) film is evaluated using the polarizability of the monomer model of the CTA repeating unit, which is calculated using the density functional theory (DFT). Since the CTA monomer is known to have three rotational isomers, referred to as gg, gt, and tg, the intrinsic birefringence of these isomers is evaluated separately. The calculation indicates that the monomer CTA with gg and gt structures shows a negative intrinsic birefringence, whereas the monomer unit with a tg structure shows a positive intrinsic birefringence. By using these values, a model of the uniaxially elongated CTA film is constructed with a molecular dynamics simulation, and the orientation birefringence of the film model was evaluated. The result indicates that the film has negative orientation birefringence and that its value is in good agreement with experimental results.

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