ordering structures. The common liquid crystalline mesophases of small molecules
are nematic, smectic, cholestic and columnar liquid crystals, as illustrated in
Fig.10.2. The nematic liquid crystal pocesses only long-range orientational order
without long-range positional order. The smectic liquid crystal shows long-range
positional order along the orientation of the nematic liquid crystal, and appears
close to the crystalline order, as suggested by its name. The cholestic liquid crystal
contains the nematic liquid crystal layers formed by the specific cholestic
molecules, but their orientations vary periodically along one normal direction.
The periodic size can approach the wavelength of visible lights, which causes
scattering. Therefore, the complementary color to the scattered wavelength
displays, which has been widely applied in display devices. The columnar liquid
crystal exhibits hexagonal structures formed by the stacking of disk-like molecules,
which allows one-dimensional motion along the long axis of the column.
The phase transitions of liquid crystals in solutions occur normally through two
mechanisms, i.e. lyotropic and thermotropic transitions. The lyotropic liquid crystal
occurs upon addition of solvent into the crystalline phase, while the thermotropic
liquid crystal occurs upon heating the crystalline phase, as illustrated by the two
arrows in Fig.10.3, respectively. The phase diagrams for the transition from the
homogeneous solution to the liquid crystal are formed by two almost parallel
curves, reflecting the concentration gap between the two coexisting phases.
The liquid crystal molecules, for example, the rigid rod mesogens, exhibit
characteristic anisotropy of shapes, which is an essential feature in their driving
forces for the phase transition from the disordered state to the liquid crystalline
ordered state. In 1949, Onsager considered the anisotropic volume-exclusion
hydrodynamic interactions between rod-like molecules and thus explained the
lyotropic liquid crystal ordering in solutions (Onsager 1949 ). This transition occurs
when some rod-like molecules perform parallel compact packing to release a part of
their occupied space to gain higher translational entropy of the other molecules.
Such a situation is favorable to fill more rod-like molecules into limited space at
high concentrations. This effect is also called the entropy-driven ordering effect,
which is one of the characteristics of soft matter. In 19581960, Maier and Saupe
realized that most rod-like molecules contain a stable conjugated chemical struc-
ture, such as O 2 N–f–C¼C–f–NH 2 (Maier and Saupe 1958 , 1959 ). The electron
Fig. 10.2 Illustration of the structure characters of the nematic, smectic, cholestic and columnar
liquid crystals
10.1 Thermodynamics of Polymer Crystallization 189