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 di pyridyl and 4-(oligodimethylsiloxyl)alkoxybenzoic acids was investigated by X-ray diffraction and deuterium NMR. These H-bonded mesogens exhibit a cub ic thermotropic liquid crystalline phase at a temperature below a smectic A phase. Moreover, above the smectic A phase two optically isotropic, low vi scosity phases (I-1 and I-2) exist with the lower temperature I1 phase comp rising aggregated mesogens. Two broad, temperature-independent, wide angle X-ray reflections suggest that the aggregation is driven by nanophase-separ ation; the aggregate adopts a structure with one domain siloxane rich and t he other hydrocarbon rich. In the isotropic melt near the I-1-to-I-2 phase transition, a very weak small angle reflection, indicates incipient lamella r-like clustering of the aggregated mesogens. With decreasing temperature t he intensity of the small angle reflection increases and in the smectic A p hase, 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, lamellar-li ke structure, respectively. In the cubic phase, the collapse of quadrupolar splittings is consistent with an isotropic average of the quadrupole inter action caused by mesogen translational diffusion through a cubic supramolec ular structure.