Novel nematic bis-1,5-hexadiene monomers: Synthesis and photopolymerization to cholesteric polymer networks

Abstract

The synthesis of two novel nematic monomers 1 a and 1 b with two polymerizable 1,5-hexadiene groups is described. Their photopolymerization in the nematic mesophase has been investigated by photo-DSC-measurements and IR-spectroscopy. The hexadienes photopolymerize within 30 min to a final conversion of 87% and thus slower than the corresponding acrylates. Mixtures of hexadienes and acrylates copolymerize and photopolymerization is finished after two minutes of UV-irradiation. By mixing a nematic bis-1,5-hexadiene compound with a cholesterol bisacrylate, enantiotropic cholesteric phases are obtained. Photopolymerization of such mixtures in the liquid crystalline phase leads to crosslinking. Networks with reflection colors from red to blue are obtained by variation of the amount of the chiral component. The helical twisting power (HTP) for networks of the bishexadiene 1a and the cholesterol derivative 3 is HTP/n = 5.6 μm–1 (n: average refractive index) which is almost by 50% larger than the value for the corresponding bisacrylate network.