A Method To Preserve the Chiral Nematic Order of Lyotropic Ethylcellulose and (Acetyl)(ethyl)cellulose Mesophases in Solid Films

Solid films that reflect visible light due to chiral nematic order can be prepared from lyotropic ethylcellulose (EC) and (acetyl)(ethyl)cellulose (AEC) mesophases by a three-step casting method. A thin uniform film of desired thickness is cast from dilute solution onto a substrate. The film and substrate are then equilibrated with a mesophase of the same polymer in an appropriate solvent. The mesophase is a convenient source of solvent vapor at an appropriate composition and vapor pressure for the equilibration process. After equilibration, the solvent is allowed to evaporate, leaving a film that reflects visible light. The reflection wavelength of the EC film depends on the activity of the equilibrating vapor and the degreee of polymerization. Reflection intensity is governed by the degree of polymerization and the length of the equilibration period. The EC films showed good selectivity in reflection of left-circularly polarized light. Acetylation of EC gives AEC, which forms mesophases that reflect right-circularly polarized light. The three-step casting process was used to produce right-circularly polarizing films of AEC. The effects of degree of polymerization, equilibration concentrations, and temperature on the wavelength and intensity of the AEC reflection peak were measured by apparent circular dichroism and UV−visible spectrometry.