Dual mechanochromism of cellulosic cholesteric liquid-crystalline films: wide-ranging colour control and circular dichroism inversion by mechanical stimulus

Abstract

We prepared structurally coloured films composed of a cellulosic cholesteric liquid crystal (ChLC) and investigated their mechanochromic properties. The obtained ethylcellulose/poly(acrylic acid) (EC/PAA) films exhibited wide-ranging mechanochromism over the wavelength range of visible light upon compression at 130 °C, which was higher than the glass transition temperature (∼120 °C) of the composites. The compressed films displayed the ability to recover their shape and colour to a significant extent after thermal treatment at 130 °C for 10 s. Interestingly, we found that compressive stress inverted the cholesteric helix in the EC/PAA films, and their original helical handedness could be restored by thermal treatment at 130 °C. Compressive stress can thus alter not only the wavelength but also the handedness of circularly polarized light reflected from the EC/PAA films. This concept of dual mechanochromism, i.e., the mechanical control of these two optical properties, can be applied for displays, sensing, and so on.