Computational Study of Optical Properties of Cellulose Triacetate Film
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Computational Study of Optical Properties of Cellulose Triacetate Film Daichi Hayakawa and Kazuyoshi Ueda* Department of Chemistry, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan ABSTRACT The 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. INTRODUCTION Cellulose is known as the most abundant hydrocarbon in the world. Chemical derivatives of cellulose are actively researched based on their renewable and biodegradable properties. Among the known cellulose derivatives, cellulose triacetate (CTA), which is obtained by the acetylation of cellulose, is one of the most common members. CTA has been used in a variety of commercial products such as films and fibers for many years. Recently, the widespread use of CTA has been further expanded to new high-technology application fields [1]. Optical films with highly controlled optical properties represent a typical example of the application of CTA in a new field. However, the relationship between the molecular structure of CTA in the film and their optical properties has not been fully elucidated. In a real system, both crystalline and amorphous phases are present in a CTA film. Consequently, the influence of both phases should be considered in the evaluation of the optical properties of CTA films. As the starting point of the present study, a computational evaluation of the relationship between the molecular structure of CTA and the optical properties of an amorphous CTA film model oriented to one axial was undertaken. Particular focus was placed on the birefringence of the polymer film of CTA using density functional theory (DFT) and molecular dynamics (MD) simulation. Density functional calculation is one of the most powerful the
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