Synthesis and properties of low-molar-mass liquid-crystalline siloxane derivatives

Abstract

A number of liquid-crystalline siloxane derivatives based on the 4-(ω-alkenyloxy)-4′-cyanobiphenyl mesogenic unit have been prepared and their phase-transitional properties characterised. The compounds were synthesized by the addition of the 4-(ω-alkenyloxy)-4′-cyanobiphenyl moiety to pentamethyldisiloxane catalysed by a platinum divinyltetramethyldisiloxane complex. The materials are denoted ABs where A refers to the number of methylene groups in the 4-(ω-alkenyloxy)-4′-cyanobiphenyl and B refers to the number of silicon atoms in the siloxane unit. With the exception of 3/2 and 4/2, which exhibit only monotropic phase transitions, all these materials exhibit an enantiotropic smectic A phase. Preliminary X-ray results also support this. The thermal properties of this new series of thermotropic liquid-crystalline siloxanes were studied by differential scanning calorimetry (DSC) and polarising microscopy. The transitional properties for the AB series exhibit a pronounced odd–even effect with the length of the alkyl segment.