Chemistry Reference
In-Depth Information
4.1.4. Textures of polymeric liquid crystals
Texture or the orientation pattern refers to the picture observed in ortho-
scopic observation of a liquid crystal film, which is a combination of sample
image and polarizing effect. Defects of material and orientation as well as
interference colors are shown in texture. An ideal monodomain of a uni-
form thickness and orientation (“liquid single crystal”) gives out only the
contour of the domain in certain monotonic color. The texture of a liquid
single crystal is termed the “homogeneous texture” and is a result of per-
fect orientation. A liquid single crystal of homeotropic orientation will be
extinct between crossed polars and will remain such with rotation of the
sample stage. The liquid single crystals with other orientations will change
in color with rotation of the stage.
Often there is a variety of defects in liquid crystals (Chapter 1). This is so
particularly in polymeric liquid crystals because in polymers the molecular
structures are often very complex and the melt viscosity is very high. The
complexity in molecular structure often prevents perfect orientation or even
results in phase separation. For example in a side-group type polymer the
non-mesogenic main chain segment may not join the mesogenic side groups
in orientation. In a main-chain type polymer containing flexible or non-
linear segments the molecular orientation in the liquid crystal phase is also
imperfect. On the other hand because of the often very high viscosity of
polymer melts, the molecular motions of polymer segments have a high
activation energy and are slow in comparing with that of low mass liquid
crystals. Much longer experimental time is needed for the formation of a
polymer liquid crystal in thermodynamic equilibrium. Thus while a low
mass liquid crystal enters its liquid crystal phase as soon as it melts, a
period of time may have to be waited for a polymer sample to finish this
process.
Nevertheless in polymeric liquid crystals the same types of orienta-
tional defects and thus the same types of textures as present in the low
mass counterparts have been observed. The textures often formed by
polymers are the threaded texture, the schlieren texture and the focal
conic texture of smectics. As is for low mass liquid crystals, the tex-
ture is a consequence of defects (disclinations and dislocations, refer to
Chapter 1) present in the liquid crystal and is characteristic of a spe-
cific type of the phase. The texture examination has become a very useful
tool in the determination of the type and nature of the polymeric liquid
crystals.
