Optical and mesomorphic properties characterisation of chiral liquid crystalline copolysiloxane exhibiting cholesteric and blue phases

Abstract

A series of liquid crystalline polymers (LCPs) have been synthesised by two cholesteric monomers M1, M2 and a nematic monomer M3. The chemical structures and liquid crystalline properties of the monomers and polymers have been characterised by FTIR, 1H-NMR, differential scanning calorimetry, thermogravimetric analyses, X-ray diffraction measurements and polarising optical microscopy. All LCPs show a high thermal stability with wide mesophase temperature ranges. For polymer P1 bearing only cholesteric LC monomers component, it shows a cholesteric phase, whereas others display a blue phase besides a cholesteric phase. The formation of the blue phase is based on the structures of the polymers and the produced biaxial helix. The glass transition temperature and isotropic temperature of the polymers decrease on heating cycle with increasing the content of M3 in the polymers. The specific rotation values of the polymers are temperature-sensitive. The reflection spectra of polymers P1–P6 show that the maximum reflected wavelengths shift to long wavelength with increasing the content of M3 in the polymer systems. The frequency and intensity of the bands change sharply at the temperature where cholesteric phase changes to blue phase, but they show a weak dependence on temperature in the blue phase.