Synthesis and characterization of liquid-crystalline ionomers with pendant cholesteryl pyridinium salt mesogens

Abstract

Novel chiral liquid-crystalline (LC) bromo-polysiloxanes (IP, IIP and IIIP) were graft copolymerized in a one-step hydrosilylation reaction by use of polymethylhydrogenosiloxane, cholest-5-en-3-ol(3β)-10-undecenoate and 4-bromobut-1-ene. A pyridyl-containing LC monomer cholesteryl isonicotinate (CIN) was synthesized and characterized. Some LC ionomers [IP–IIIP-Py][X] (X = Br) were synthesized by use of CIN and the bromo-polysiloxanes. Other LC ionomers [IP–IIIP-Py][X] (X = BF₄, PF₆, Tf₂N) were prepared by metathesis of the pyridinium bromides. The chemical structure and liquid-crystalline properties were investigated by various experimental techniques. All the bromo-polysiloxanes display a smectic A (S_A) mesophase when they are heated and cooled, while all these LC ionomers show a chiral smectic C (S^*_C) phase on heating, and exhibit S_A and S^*_C phases on cooling. The LC ionomers show a narrower range of LC temperature than the corresponding bromo-polysiloxanes. For the LC ionomers bearing homologous pyridinium cations and different anions, the glass transition temperature, mesophase–isotropic phase transition temperature and LC temperature range tend to reduce, and the spontaneous polarization value increases slightly in the sequence of Br⁻, BF₄⁻, PF₆⁻, and Tf₂N⁻. Rearrangement of smectic layers due to electrostatic attraction and ion aggregation of cholesteryl pyridinium salt mesogens lead to the formation of the S^*_C phase for these LC ionomers.