Body-centered cubic phase in 3-arm star mesogens: a torsional tapping AFM and GISAXS study

Abstract

The mode of liquid crystal (LC) self-assembly of asymmetric three-arm star oligobenzoate mesogens has been investigated by torsional tapping mode AFM imaging, and by bulk and grazing-incidence X-ray diffraction. It was confirmed that the cubic liquid crystal phase has Imm symmetry and established that it consists of spherical aggregates arranged on a body-centered lattice, rather than having a bicontinuous structure. Molecular simulation suggests that, in spite of their perceived rigidity, the oligobenzoate mesogens are folded, often sharply, within the supramolecular spheres, and that they act effectively as conical objects. The recently introduced torsional tapping AFM technique has allowed high resolution and contrast to be obtained from the soft mobile surface of the samples, showing the (110) plane of highly ordered supramolecular spheres. Notably the fine structure of the observed steps reveals that nearly isolated micelles still preserve their integrity. Finally, the equilibrium habits of the cubic LC droplets were shown by optical microscopy to be polyhedral with crystallographic facets, rather than spherical. This suggests that the air–LC interface is below the roughening transition temperature, which is attributed to the whole micelles rather than the isolated molecules acting as the interacting units.