Weakly nematic–highly nematic phase transitions in main-chain liquid-crystalline polymers

Abstract

A mean field theory is introduced to describe the nematic-isotropic phase transitions (NIT) of main-chain liquid-crystalline polymers (MLCPs) which consist of rigid mesogens and spacers with various degrees of flexibility. We here assume that two neighboring bonds on the spacers have either bent or straightened conformations and the straightened conformation gives rise to a rigid rodlike shape. The theory takes into account not only the nematic ordering of mesogens but also the partial ordering of spacer segments in the nematic phase. On the basis of the Onsager type excluded volume interactions and the Maier-Saupe model for orientational-dependent attractive interactions between rigid segments, we derive the free energy of the MLCPs in melts. The NIT temperature, the order parameter of mesogens, and that of spacers are examined as a function of the spacer length, the flexibility of spacers, and the strength of the anisotropic interactions. We also derive the Landau\char21{}de Gennes expansion of our free energy. We find that the NIT temperature ${T}_{\mathrm{NI}}$ of a MLCP with semiflexible spacers has a minimum as a function of the spacer length ${n}_{s}.$ At small values of ${n}_{s},$ we have a weakly nematic phase which is mostly formed by the ordering of the mesogens and the spacers play a softening role. At large values of ${n}_{s},$ we have a highly nematic phase where the straightened segments on the spacers and the mesogens are highly ordered and the spacers play a stiffening role. The two different nematic phases are discussed in the phase diagrams of the temperature-spacer length plane.