cloud in these molecules distributes non-locally along the long axis, and the
corresponding polarizability is strongly anisotropic. Therefore, the dispersion
forces also show anisotropic features, i.e. the parallel-packed rod-like molecules
display the lowest potential of the attractive interactions. Such strong anisotropic
dispersion attractions between rod-like molecules can explain the thermotropic
liquid crystal phase transition in concentrated and bulk systems.
Liquid crystal polymers conventionally carry anisotropic mesogen groups that
can form a liquid crystal phase. Depending on the locations of the mesogen groups
on the polymer chains, liquid crystal polymers can be categorized into two groups,
i.e. main-chain liquid crystal polymers and side-chain liquid crystal polymers, as
demonstrated in Fig.10.4. The main-chain liquid crystal polymers often exhibit the
feature of rigid chains, and are suitable for high strength and high modulus
materials. The side-chain liquid crystal polymers often exhibit the feature of
flexible chains, suitable for soft functional materials, for instance liquid crystal
display. Zhou and his coworkers invented mesogen-jacketed liquid crystal
polymers (Zhou et al. 1987 ). They synthetically controlled the side-chain rod-like
mesogen groups to orient in parallel with the backbone chain. Thus, the semi-
flexibility of the main chains can be continuously adjusted by the length of the
spacers. When the spacers are rather short, the mesogen-jacketed polymers appear
similar to the rigid main-chain liquid crystal polymers. When the spacers are long
enough, the polymers appear as flexible side-chain liquid crystal polymers. Since
the common main-chain liquid crystal polymers are prepared by condensation
polymerization, their molecular weights are relatively low. The mesogen-jacketed
polymers can be prepared by other methods, which provide high strength and high
modulus materials with high molecular weights.
Fig. 10.3 Illustration of
typical phase diagrams of
liquid crystal solutions.
ISOmeans isotropic
solutions,LCmeans the
liquid crystal phase, andCR
means the crystalline phase.
Thearrowsindicate the
directions of lyotropic
(horizontal) and thermotropic
(vertical) liquid crystal phase
transitions
190 10 Polymer Crystallization