Synclinic-anticlinic phase transition in tilted organosiloxane liquid crystals

In this paper, we considered the case of low molecular weight bimesogenic l iquid crystals containing a siloxane moiety as the central part of their mo lecular architecture. For some of these compounds, both ferro- and antiferr oelectric mesophases are present. Two distinct smectic structures can devel op as a function of temperature, the first one at high temperature correspo nding to a synclinic molecular arrangement with elongated molecules, and th e second one at lower temperature corresponding to an anticlinic organisati on with V-shaped molecules. Numerical calculations of the energy of differe nt conformations of these bimesogenic molecules presented here indicate tha t there is no difference in energy between V-shaped and linear conformation s regardless of the number of silicon atoms in the siloxane moiety. Thus a microscopic model of the synclinic-anticlinic phase transition is developed where the driving force is indeed a free energy difference between the two phases, and not a difference of energy between the V-shaped and linear con formations. The model explains why the anticlinic SmCA phase is more stable than the synclinic SmC one, why the synclinic SmC phase is generally the h igher temperature one, and why in some organosiloxane materials the anticli nic SmCA phase is not present.