Smectic behavior and molecular structure: On the mesomorphism of a series of hydrocarbons

The polymorphism is discussed of a series of hydrocarbons that show, besides a smectic-A phase, a strong tendency towards the formation of smectic-B. The powder x-ray diffraction patterns show at large Bragg angles an unusual double reflection with a very small separation, which indicates a 3D hexagonal closed packing. It is shown that, quite generally, nonpolar molecules form orthogonal smectic phases. The implications are discussed for the relationship between molecular structure and smectic behavior.