SYNTHESIS AND PROPERTIES OF LOW-MOLAR-MASS LIQUID-CRYSTALLINE SILOXANE DERIVATIVES

A number of liquid-crystalline siloxane derivatives based on the 4-(om ega-alkenyloxy)-4'-cyanobiphenyl mesogenic unit have been prepared and their phase-transitional properties characterised. The compounds were synthesized by the addition of the 4-(omega-alkenyloxy)-4'-cyanobiphe nyl moiety to pentamethyldisiloxane catalysed by a platinum divinyltet ramethyldisiloxane complex. The materials are denoted ABs where A refe rs to the number of methylene groups in the 4-(omega-alkenyloxy)-4'-cy anobiphenyl and B refers to the number of silicon atoms in the siloxan e unit. With the exception of 3/2 and 4/2, which exhibit only monotrop ic phase transitions, all these materials exhibit an enantiotropic sme ctic A phase. Preliminary X-ray results also support this. The thermal properties of this new series of thermotropic liquid-crystalline silo xanes were studied by differential scanning calorimetry (DSC) and pola rising microscopy. The transitional properties for the AB series exhib it a pronounced odd-even effect with the length of the alkyl segment.