Liquid Crystal States of Matter - Liquid Crystalline Polymers - page 17

of crystallography even though the correlation between layers in these two
smectic phases is weak.
The smectic M phase, discovered in 1990, may be classified as a hexatic
phase. The molecules are tilted within layers but are not miscible with the
S F and S I phases. The structural details are under investigation.
1.4.4. Three-dimensional translational order but
correlation is weaker than that in crystals
This class of liquid crystals includes the smectic liquid crystal L, G, and J
phases, and the higher ordered ones: the smectic liquid crystal E, H, and
K phases.
Compared with the previously mentioned class of liquid crystals,
S B , S F , S I , and S M , this class of liquid crystals exhibits a stronger corre-
lation between smectic layers, the correlation length ranging from tens to
hundreds of layers. The way of stacking of successive layers may be either
AAA ..., ABABAB .... or ABCABC .... These six smectic liquid crystals
are actually very similar to real crystals, but they do exhibit a significant
disorder in both translation and orientation. Dynamic experiments demon-
strated that they exhibit rather different characteristics from real crystals,
such as the dielectric relaxation and the Mossbauer spectrum. The X-ray
diffraction of these liquid crystals shows reflections of only less than five
orders. In addition, as the transition to a real crystal happens, a relatively
large amount of transition enthalpy and volume change occurs. These vari-
ations are in general one order of magnitude greater than those transitions
occurring between liquid crystals.
In the S L phase, the molecules are perpendicular to layers and are
arranged in a hexatic lattice within layers. This symmetry is the same
as that of the S B phase. Both of the S L and S B phases are optically uni-
axial, their molecules within smectic layers rotate more freely around their
long axes or rotate in groups simultaneously. In some of the literature, the
S B phase is called the hexatic B phase while the S L phase is named the
S B phase or crystal B phase. The molecules within the S G and S I phases
are tilted with respect to the layers while the molecules of the S L phase are
aligned more or less perpendicularly to the layers. The stacking of molecules
within the layer of the S L ,S G and S J phases is similar to those shown in
Figure 1.7 as well. The crystallography classifications of these three liquid
crystal phases are the same as those of the S B ,S F and S I phases.