Bilayer structures in cholesteric, cyclic-siloxane liquid crystals

Abstract

X-ray diffraction has been used to investigate a series of commercially available liquid crystals based on a cyclic penta(methylsiloxane) with combinations of cholesteryl-4′-allyloxybenzoate and biphenyl-4′-allyloxybenzoate mesogens pendant on the siloxane ring. A lamellar, (smectic-like) structure is indicated in the mesophase and quenched glassy solid state. Extended molecular dimensions of the mesogens generally account for the observed d spacings. The mesogens may interdigitate within lamellae but the packing and extent of interdigitation is dependent on the ratio of cholesteryl to biphenyl mesogens. There is probably also a structural contribution from the nano-aggregated (nano-phase separated), immiscible, siloxane-rich layer delineating the lamellae interfaces as this mesophase shows unusually good definition; diffraction patterns exhibit high order reflections (up to 6th order). The lamellae have a macroscopic helicoidal twist about an axis lying in the plane of the lamellae, a cholesteric supramolecular structure, which derives from the chiral, steroid mesogenic component. Surprisingly, this mesophase composed of low molar mass cyclic siloxanes may be drawn into fibres tens of metres in length, to give a morphology having the lamellae normal to the fibre axis.