Design of birefringence and its wavelength dispersion for cellulose derivatives using substitution, low-mass additives, and porous structures

Abstract

Birefringence and its wavelength dependence are important properties for optical applications, such as liquid crystal displays and pick-up lenses. Cellulose esters, which are eco-friendly materials from biomass resources, have been used in optical materials due to their transparency and heat resistance. This review summarized our recent works on birefringence control of cellulose esters by substitution of multiple ester groups, addition of low-mass molecules, and formation of a porous structure. Cellulose acetate propionate (CAP), which has acetyl and propionyl groups in a pyranose unit, exhibited positive birefringence with extraordinary wavelength dispersion owing to contributions from its two esters, while cellulose triacetate (CTA) showed negative birefringence with ordinary dispersion. Moreover, the effect of substitution sites (C-2, C-3, and C-6) on birefringence and its wavelength dependence was investigated by comparing birefringence data of CTA and xylan acetate. The addition of low-mass molecules improved the value of birefringence due to the intermolecular orientation correlation with the matrix cellulose esters. The anisotropic porous structure in CTA films, which was prepared by thermoinduced phase separation, generated extraordinary wavelength dispersion of birefringence. We successfully controlled birefringence and its wavelength dispersion of cellulose esters by choosing different ester substitution groups and sites, controlling the orientation of the crystal, adding low-mass molecules, and forming microscale porous structures. The substitution of two ester groups could provide extraordinary wavelength dispersion of birefringence, which is important for wide-range retardation films. The low-mass additives enhance birefringence values due to the intermolecular orientation correlation with matrix cellulose esters. The anisotropic microporous structure produced by thermoinduced phase separation generates form birefringence, which improves the total birefringence of cellulose esters.