Synthesis and characterization of novel chiral liquid crystalline polysiloxanes

Abstract

Several novel side chain liquid crystalline polysiloxanes have been prepared containing a chiral cyanohydrin moiety prepared using enantioselective synthesis. Homopolymers incorporating a side chain with the chiral group exhibited no liquid crystalline behavior. Copolymers of the side chain with the cyanohydrin group with an alkenyloxybenzoate yielded polymers exhibiting a N* phase. Increased amounts of the side chain with the cyanohydrin moiety led to a decrease in the transition temperatures of the resulting copolymers. Dielectric relaxation studies showed that all polymers exhibited δ motion due to relaxation of the longitudinal dipole of the mesogen around its short axis. One copolymer with a large amount of the chiral unit and a long flexible spacer exhibited distinct α and δ relaxations where the α relaxation is assigned to the motions of the dipoles perpendicular to the side chain. The fact that both are clearly seen as independent peaks rather than in a broad peak which must be deconvoluted is indicative of the fact that long, flexible spacer chains decouple the two motions from each other. The δ process for the polymers with a small amount of the cyanohydrin side chain tended to be dominated by the non-chiral pendant group. The motion of the polymer with an increased amount of the pendant group containing the chiral moiety was dominated by the cyanohydrin side chain and the δ relaxation was much slower (longer relaxation time) due to the bulky nature of the chiral moiety.