Even without mesogen-groups, the anisotropic structure of some polymer chains
can form the liquid crystal phase under specific conditions. For instance, under high
pressures, the hexagonal phase of polyethylene is a condis crystal formed by the
flexible chains without any mesogen-groups, where the term “condis” comes from
first three characters of the words ‘conformational disorder’ (Wunderlich and
Grebowig 1984 ). The conformational-disordered chain can perform one-
dimensional motion along the column structure, similar to the structure of the
columnar liquid crystal. Further cooling can generate the common orthorhombic
crystal.
The mesophase state of liquid crystals is normally opaque due to relatively large
sizes of ordered domains. Its transition point to the isotropic melt state is called the
clear pointTi. The DSC scanning curves of liquid crystals can exhibit either
enantiotropic or monotropic phenomena. For the thermodynamically stable
mesophases of liquid crystals, they occur between the melt and the crystal states
during both cooling and heating processes, as illustrated in Fig.10.5. When both the
cooling and heating curves show two symmetric consecutive phase transitions, it is
known as the enantiotropic phenomenon. In contrast, for the metastable mesophase
Fig. 10.4Top-down illustration of the main-chain liquid crystal polymers, side-chain liquid
crystal polymers and mesogen-jacketed liquid crystal polymers
Fig. 10.5Illustration of enantiotropic phenomenon on the cooling and heating curves of the stable
mesophases. (a) Free energy curves; (b) DSC heating and cooling curves
10.1 Thermodynamics of Polymer Crystallization 191