Classification of the smectic phases

The notation of Sackmann and Demus is according to the order of the discovery of the different phases and bears no relation to the molecular packing. The broad structural features of these phases are summarized in table 5.1.1. A more detailed description of these structures may be found in the excellent reviews by Pershan and by Lead better.  

Liquid-like layers with the molecules upright on the average (fig. 1.1.5(a)) negligible in-plane and interlayer positional correlations. Thus the structure may be described as an orientationally ordered fluid on which is superimposed a onedimensional density wave. A number of polymorphic types of smectic A have been discovered (see 5.6). 

Liquid-like layers, as in S,, but with the molecules inclined with respect to the layer normal (fig. 1.1.5 (6)). 

Chiral S, with twist axis normal to the layers. 

Cubic lattice with about 10 molecules per unit cell. The detailed molecular arrangement is not known, but is generally assumed to be of the micelle type. Thus this phase should probably be labelled as D rather than Smectic D. At present only four compounds are known to exhibit this phase, 4 -n-hexadecyloxy- and 4 -n-octadecyloxy-3 -nitrobiphenyl-4-carboxylic add and two similar acids with CN replacing NO. Interestingly, D occurs between S, and S, or between S and the isotropic phase. The manner in which such a structural rearrangement takes place is yet to be resolved. Three-dimensional crystal, orthorhombic with interlayer herringbone arrangement of the molecules.  

---