Biaxial nematics composed of flexible molecules: a molecular field theory

Abstract

The key requirement for the formation of a biaxial nematic phase is clearly biaxiality in the shape or interactions of the constituent molecules. However, in addition to this feature mesogenic molecules are invariably flexible so that their anisotropy and biaxiality can change. The orientational order of the system is also coupled to the conformational distribution and this coupling may stabilise the biaxial nematic phase. Here a molecular field theory of nematics composed of flexible biaxial mesogenic molecules is assembled from its essential elements. Of special significance is the use of the variational approach employed by de Gennes to formulate a molecular field theory for uniaxial nematics. This has the distinct advantage that it avoids the use of a pair potential of unknown reliability. Instead it requires the identification of the dominant orientational order parameters for the phases involved. Invariants constructed from these and an interaction supertensor then give the free energy for the system. This powerful variational approach has been employed for the biaxial nematic phase composed of flexible molecules taking into account the relationship between the interaction supertensors for the numerous conformers. The way now seems to be open to explore the influence of molecular flexibility on the stability of the biaxial nematic phase for a molecularly realistic model.

Keywords:

• biaxial nematics
• phase symmetry
• flexible mesogenic molecules
• molecular field theory
• variational approach