Dynamic High Contrast Reflective Coloration From Responsive Polymer/Cholesteric Liquid Crystal Architectures10/24/2011
We report on high reflectivity CLC structures (R > 50%) whose wavelength can be thermally tuned reversibly by a de-swelling/re-swelling transition unique to ordered solvent-gel systems. The system contains no chiral dopant and the coloration is completely induced by a responsive chiral structured gel. The de-swelling transition, leading to blue tuning, occurs at the nematic-isotropic transition of the liquid crystal, which is a result of a mismatch in the orientational energy of the isotropic liquid crystal and the anisotropic gel. The re-swelling transition subsequently occurs at the nematic-isotropic transition of gel, due to the miscibility of the isotropic liquid crystal and the isotropic gel, which induces a red-shift in the coloration. Examination of varying clearing point liquid crystal solvents, contact angle measurements, and white light optical profilometry localized thickness measurements shed light on this de-swelling/re-swelling transition. A dynamic, high reflectivity cell was demonstrated by combining both a left-handed chiral gel with a right handed chiral gel, both of whose initial periodicities were equal. Heating of this so-called hyper-reflective cell drove reversible and large scale wavelength changes (100's of nm) while maintaining large reflectivity (R similar to 90%).