Films solidified from a lyotropic mesophase which retain the cholesteric structure



Solidified cholesteric films of α-helical poly(γ-methyl L-glutamate) and poly(γ-benzyl L-glutamate) were prepared by casting from solutions of the lyotropic cholesteric mesophase. Colored films can be prepared in this manner, so the cholesteric structure is retained with a pitch corresponding to a visible wavelength. Their iridescent colors can cover the full range of the visible spectrum, and the colors remain unchanged for years. Although the films are similar in optical properties to those of fluid cholesteric phases, the temperature dependence of the color is quite different. On stretching, the film undergoes a permanent deformation, and the iridescent color is shifted toward the blue. If t0 is the initial film thickness and Δt is the change in thickness after stretching the film, the relative change in pitch, −ΔP/P0, of the cholesteric structure increases linearly with −Δt/t0 in the range −Δt/t0 > 0.10. This reduction of the pitch is attributed to a decrease in the number of pseudonematic layers in the span of one pitch, which may be interpreted in terms of delamination using an angle-ply model of the cholesteric structure.