A printable H-bonded cholesteric liquid crystal (CLC) polymer film has been fabricated that, after conversion to a hygroscopic polymer salt film, responds to temperature and humidity by changing its reflection color. Fast-responding humidity sensors have been made in which the reflection color changes between green and yellow depending on the relative humidity. The change in reflection band is a result of a change in helix pitch in the film due to absorption and desorption of water, resulting in swelling/deswelling of the film material. When the polymer salt was saturated with water, a red-reflecting film was obtained that can potentially act as a time/temperature integrator. Finally, the films were printed on a foil, showing the potential application of supramolecular CLC materials as low-cost, printable, battery-free optical sensors.
Herzer, N., Guneysu, H., Davies, D. J. D., Yildirim, D., Vaccaro, A. R., Broer, D. J., ... Schenning, A. P. H. J. (2012). Printable optical sensors based on H-bonded supramolecular cholesteric liquid crystal networks. Journal of the American Chemical Society, 134(18), 7608-7611. https://doi.org/10.1021/ja301845n