Stepwise phase transitions of chain molecules: Crystallization/melting via a nematic liquid-crystalline phase

Segmented chain molecules involving mesogenic units along the backbone chain, conventionally called mainchain liquid crystals, often exhibit an enatiotropic nematic liquid-crystalline phase over a certain temperature range between the crystal and the isotropic melt. The phase transitions involved are usually of first order. Spectroscopic analyses such as X-ray and neutron diffraction, IR and Raman absorption, and H-1 and H-2 NMR demonstrated that the orientational ordering of mesogenic cores in the mesophase are strongly coupled with the spatial arrangement of the spacer. While the orientation of the molecular axis (i.e., the whole molecule) varies as a function of temperature, the well-defined nematic conformation of the spacer remains almost unaltered over the entire range of the liquid-crystalline state. In these compounds, the conformational transition takes place in the order isotropic random coil <-> nematic conformation <-> extended crystalline form, or vice versa during the crystallization/melting process. In this article, we first present a brief summary of the entropy of fusion of chain molecules. In the second part, conformational entropy changes associated with the melting via a nematic phase, or the crystallization via a nematic mesophase, are reviewed. The experimental data on liquid-crystal transitions were mostly collected from studies on oligomeric compounds such as dimer and trimer mainchain liquid crystals with neat chemical structures. Finally it is suggested that further studies on the phase transition of chain molecules via a nernatic mesophase may provide some valuable information regarding the embryonic stage of the polymer crystallization mechanism.