Computational Study of Optical Properties of Cellulose Triacetate Film

ABSTRACTThe birefringence of a cellulose triacetate (CTA) polymer film was evaluated based on density functional theory and molecular dynamics (MD) simulation. The polarizability of the monomer unit of CTA was initially calculated to determine the intrinsic properties of the birefringence of CTA. The most important conformational freedom of the CTA monomer unit is derived from the C-6 acetyl methyl groups. This exocyclic group is known to have three low energy conformers referred to as gg, gt, and tg according to the rotation of the different torsion angles. Because the polarizability can be viewed as dependent on the conformation of CTA, the polarizability of these three conformers was evaluated. The results demonstrated that negative intrinsic birefringence was associated with the CTA repeating units having gg or gt structures, whereas the monomer units with tg structures were characterized by positive intrinsic birefringence. A model of the polymer film was constructed based on MD simulation and the birefringence of the model was evaluated using the calculated monomer birefringence values. The birefringence of the CTA film was found to be negative because most of the CTA repeating units adopt the gg conformation in the film. The negative value of the simulated birefringence is in good agreement with the result obtained by the experiment.