|Lattermann, G.; Schaz, A.: ABC mesogens: a novel 'hexagonal columns-in-lamellae' morphology of low molar mass partially fluorinated 'three-chain' benzoic acids, Liquid Crystals 32(4), 407-415 (2005)|
Two homologous series of 3,4,5-tris(perfluoroalkylalkyloxy) benzoic acids ( K3S[ n, m]), with alkyl chain lengths m=8 and 11 and perfluoroalkyl chain length n=6,8 and 10, have been investigated with respect to their liquid crystalline behaviour and, in particular, the structure of the mesophases exhibited. The mesophase characterization was performed by means of polarising optical microscopy, differential scanning calorimetry and X-ray diffraction of powder-like samples. At a first glance, the results for each compound indicate the presence of a classical hexagonal columnar mesophase. However, a comparative study of the small angle diffractograms of the two series with alkyl chain lengths m=8 and 11 reveal an increasing lamellar arrangement within the hexagonal columnar lattice on increasing the perfluoroalkyl chain length n. A conclusive explanation leads to a mesophase structure combining both a hexagonal arrangement of the carboxylic groups and a layer arrangement of the perfluoroalkyl chains. The model has been proved by simulation of the X-ray diffractograms using corresponding molecular models. This is the first evidence for the existence of a 'hexagonal columns-in-lamellae' morphology, realized by low molar mass compounds, and is analogous to a similar phase structure of linear ABC triblock copolymers.