Dye induced great enhancement of broadband reflection from polymer stabilized cholesteric liquid crystals

A series of polymer stabilized cholesteric liquid crystals (PSCLCs) films were prepared from cholesteric liquid crystal (Ch-LC) mixtures containing different components such as non-reactive LC monomer, polymerizable monomer, chiral dopant, dye, and photoinitiator upon polymerization. The influence of the polymerizable monomer and dye of Ch-LC mixtures on the reflection properties was investigated. The reflection bandwidth for all the samples can be increased by photo-polymerization, and the network upon polymerization derived from two different polymerizable monomers with both one and two functional groups is more effective than that from one polymerizable monomer for broadening the reflection band. Especially, a dye-doped Ch-LC film can reflect incident light with the bandwidth over the wavelength range of 550-2350 nm, which is due to a greater pitch gradient formed inside of Ch-LC film. The gradient pitch network structure was firstly demonstrated by scanning electron microscopy (SEM) with the film prepared from high diacrylate monomer concentration and subsequently proved by using a wash-out/refill method. The nematic liquid crystals monomers was infiltrated into the polymer network that was prefabricated by removing the low molar weight LCs from the original PSCLCs film, and SEM exhibited the existence of a pitch gradient across the film thickness. The refilled nematic liquid crystals film showed broadband reflection after polymerzition, too. Copyright (C) 2011 John Wiley & Sons, Ltd.