Effect of acetyl substitution on the optical anisotropy of cellulose acetate films
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The effect of acetyl substitution on the optical properties of cellulose acetate (CA) was investigated in the present study, because a strong demand for advanced retardation films increases greatly these days. The hot-stretched films with high acetyl substitution had negative orientation birefringence, whereas those with low acetyl substitution had positive orientation birefringence with extraordinary wavelength dispersion. It should be noted that orientation birefringence hardly relaxed even after cessation of hot-stretching. The slow relaxation of crystal orientation was responsible for the anomalous optical anisotropy, as confirmed by two-dimensional X-ray diffraction. Moreover, the slow relaxation of orientation birefringence would greatly benefit the preparation of CA optical retardation films by hot-stretching, because it would simplify the precise control of retardation. The stress-optical coefficient in the glassy state was also evaluated, and was found to decrease with the degree of acetyl substitution. This is an attractive property for optical film applications.
KeywordsCellulose acetate Birefringence Orientation Refractive index
A part of this work was supported by JSPS Grant-in-Aid for Scientific Research (B) Grant Number 16H04201. The authors would like to express their sincere gratitude to Daicel Corporation for their valuable suggestions and the kind supply of the samples employed in this study.
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