Systematic variation of length ratio and the formation of smectic A and smectic C phases

The phase diagrams of four binary mixtures of chemically similar smectogenic mesogens differing only in molecular length are investigated. In these bidisperse systems the length ratio varies systematically. The phase diagrams show the stabilization of the smectic A and the destabilization of the smectic C phase with increasing length ratio as a general trend. Detailed small-angle X-ray diffraction and electro-optic measurements revealed a decrease in smectic translational order and a continuous reduction of the tilt angle with increasing length difference. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases. The remarkably strong impact of the length difference on the smectic layer structure and the phase behavior is discussed from a mechanistic point of view taking into account sterical interactions. For the observed structural changes in these bidisperse smectics we propose pronounced out-of-layer fluctuations with increasing length difference as driving force, causing neighboring molecules within nearest layer into a smectic A-like packing.