Liquid crystals are states of matter with symmetries intermediate between those of isotropic liquids and crystalline solids. The role of permanent dipoles in determining the stability of the nematic phase was discussed soon after the discovery of liquid crystals. Thought it is now well known that polar interactions are not essential for the stability of the mesophases, strong longitudinal dipoles give rise to many unusual phenomena like reentrant phases, polymorphism of smectic A and nematic liquid crystals, etc. The article gives a summary of these phenomena and phenomenological as well as molecular theories which have been developed to describe them.
