How to broaden the light reflection band in cholesteric liquid crystals? Anew approach based on polymorphism

Cholesteric liquid crystals (CLCs) may selectively reflect light when the h elicoidal pitch is of the order of the wavelength of the incident beam prop agating along the helix axis. The reflection bandwidth is dependent on the birefringence and is limited to a few tens of nanometers, which is insuffic ient for applications such as white-on-black reflective displays. Recent st udies have shown that CLC polymer networks with a pitch gradient induce a b roadening of the reflectio bandwidth over several hundreds of nanometers. M ost related processes rely on photocrosslinking reactions with a UV-gradien t in a mixture made of chiral and achiral monomers with different UV-reacti vities. Here a new experimental route exploiting the polymorphism of the mi xture is presented. The basic concept lies in a thermally-induced pitch var iation simultaneously carried out with the UV-crosslinking reaction. The op tical behaviour is investigated in parallel with the cross-sectional micros tructure as observed by transmission electron microscopy.