Orientation birefringence and its wavelength dispersion in blends of cellulose acetate propionate (CAP) and aromatic additive molecules having various alkyl chains were investigated, in order to evaluate the effect of the chemical structure on the optical properties. We found that the additive improved the orientation birefringence of CAP, meaning that the additive orientation is parallel to CAP chains via the "nematic interaction (NI)" reflecting the intermolecular orientation correlation. The NI strength was evaluated by using a theory concerning the molecular orientation. The result demonstrates that the alkyl chain length in the additive molecule strongly affects the orientation correlation. Although the intrinsic birefringence monotonically decreases with the alkyl chain length, the NI strength shows the highest value between the additive with propionyl group and CAP. From the result, the contribution of birefringence from the aromatic molecule, which is determined by the intrinsic birefringence and orientation function, needs to be considered to design the optical films with high performances.
The effect of flexible chains on the orientation dynamics of small additive molecules in cellulose acetate propionate (CAP) films is investigated during stretching using birefringence and polarized Fourier-transform infrared (FT-IR) spectra measurements. The orientation birefringence is enhanced by additives such as ethylene 2,6-naphthalate (C 2 Np) and hexamethylene 2,6-naphthalate (C 6 Np) oligomers, suggesting that the additives orient parallel to the CAP chain as a result of an intermolecular orientation correlation referred to as the nematic interaction (NI). At a high drawing temperature, the orientation of C 2 Np is stronger than that of C 6 Np, indicating that the flexible alkyl chain has reduced the intermolecular NI with CAP. By contrast, C 6 Np orients along the stretching direction at low temperatures. Polarized FT-IR spectra show that the orientation of the rigid portions of C 6 Np is delayed relative to those of the flexible groups and the matrix CAP chain. Consequently, the contribution of C 6 Np to birefringence and its wavelength dependence become stronger as the draw ratios increase.
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