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Recombination of dimers as a mechanism for the formation of several nematic phases.

A mixture of nonelongated monomers A and B capable of formation of three kinds of mesogenic dimers AA, BB, and AB is investigated. We show the possibility of a temperature-induced transition from one nematic phase consisting mostly of dimers AB composed of dissimilar monomers to a different nematic phase consisting mostly of dimers AA and BB composed of similar monomers. The binding energy of dimers AB is supposed to be lower (the binding of monomers A and B is more preferable) than that of dimers AA and BB (the binding of monomers A with each other and monomers B with each other is less preferable). On the contrary, the interactions of dimers AA themselves with each other, dimers BB with each other, and dimers AA with dimers BB are supposed to be stronger (the interaction energy is lower) than those of dimers AB with each other and with dimers AA and BB. If inequality of binding energies is stronger than inequality of interactions between various dimers, but the corresponding energy difference is small, the entropy can play a crucial role in the formation of particular dimer kinds and can drive the transitions between two nematic phases. The theoretical prediction is well reproduced experimentally.