New mesogens with cubic phases: hydrogen-bonded bipyridines and siloxane-containing benzoic acids II. Structural studies

The supramolecular structure of new hydrogen-bonded mesogens composed of dipyridyl and 4-(oligodimethylsiloxyl)alkoxybenzoic acids was investigated by X-ray diffraction and deuterium NMR. These H-bonded mesogens exhibit a cubic thermotropic liquid crystalline phase at a temperature below a smectic A phase. Moreover, above the smectic A phase two optically isotropic, low viscosity phases (I₁ and I₂) exist with the lower temperature I₁ phase comprising aggregated mesogens. Two broad, temperature-independent, wide angle X-ray reflections suggest that the aggregation is driven by nanophase-separation; the aggregate adopts a structure with one domain siloxane rich and the other hydrocarbon rich. In the isotropic melt near the I₁-to-I₂ phase transition, a very weak small angle reflection, indicates incipient lamellar-like clustering of the aggregated mesogens. With decreasing temperature the intensity of the small angle reflection increases and in the smectic A phase, the reflection becomes very sharp. In the cubic phase the small angle reflection splits into two peaks which can be indexed with a face-centred cubic (FCC) structure. The deuterium NMR measurements in the isotropic and smectic A phases are indicative of aggregation and an oriented, lamellarlike structure, respectively. In the cubic phase, the collapse of quadrupolar splittings is consistent with an isotropic average of the quadrupole interaction caused by mesogen translational diffusion through a cubic supramolecular structure.