New fluorinated monomers containing an ester function in the spacer, precursors of side chain liquid crystalline polysiloxanes

Two series of monomers, 2-(perfluoro-n-alkyl)ethyl-4-[4-(allyloxy)phenyl]benzoates and 2-(perfluoro-n-alkyl)ethyl-4-[4-(allyloxy)benzoyloxy]benzoates have been synthesized. They contain a fluorinated chain obtained from 2-(perfluoro-n-alkyl)ethanol, an aliphatic chain containing a double bound on ω and a central mesogenic moiety. Their hydrosilylation by polymethylhydrogenosiloxane give the corresponding polysiloxanes. All of the compounds, characterized by a combination of techniques consisting of differential scanning calorimetry (DSC) and thermal optical polarized microscopy, exhibited high smectogen properties. Monomers, which differ only by the nature of the rigid mesogenic core, showed enantiotropic smectogen mesophases over a wide temperature range. It was shown that the peculiar effect of the biphenyl group leads to a thermotropic polymorphism, while phenyl benzoate group induces a monomorphic character. Transition temperatures are also dependent of the length of the fluorinated moiety.