The smectogenity as a crucial factor of broadening of the selective light reflection peak in cholesteric photopolymerizable mixtures

Cholesteric liquid crystals reflecting circularly polarised light are promising materials for creating flexible and compact polarisers. One of the issues of these systems is the narrow width of the photonic stop band. The range of selective light reflection can be increased by creating a gradient of the cholesteric pitch in the process of photopolymerisation of reactive cholesteric mixtures. The possibility of broadening the peak of selective light reflection during UV polymerisation of nematogenic diacrylate mixed with chiral monoacrylates was studied. The monoacrylates used had a different number of aromatic rings. A broadening of the peak of selective light reflection was observed during the photopolymerisation of mixtures containing a chiral monomer with three benzene rings. While before irradiation the width of the selective reflection peak was about 100 nm, after irradiation it was broadened to about 300 nm. A mechanism is proposed for the formation of the helical pitch gradient in the cholesteric, in which the smectogenity of the system plays a crucial role. The obtained results open up new possibilities for creating flexible and thin circular polarisers operating in a wide spectral range.