Cubic Phase Formation and Interplay between Alkyl Chains and Hydrogen Bonds in 1,2-Bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n)

The phase transitions of a homologous series of 1,2-bis(4′- n -alkoxybenzoyl)hydrazines (BABH- n, where n is the number of carbon atoms in the alkyl chain and in this investigation varies from 4 to 22) were investigated by differential scanning calorimetric (DSC), polarizing optical microscopic (POM), X-ray diffraction (XRD), and infrared (FT-IR) measurements. The formation of bicontinuous type cubic (Cub) mesophases was observed enantiotropically for n ≥ 6 and only on cooling for n = 5. The structures were examined by XRD, which revealed the presence of two types with symmetries Ia 3 d and Im 3 m, depending on n, and for n = 13, 15, and 16, phase transitions between the two Cub phases were observed. The FT-IR studies elucidated the formation of intermolecular hydrogen bonding between the C═O bond of one molecule and the NH group of another molecule, whose binding strength was temperature-dependent. The temperature dependence of the Cub lattice parameter (d a /d T ) varied from a large positive to a large negative value with increasing alkyl chain length n, which can be well-explained in terms of the temperature-responsive shape change of the constituent molecules. The packing structures of two Ia 3 d -Cub phases formed by shorter and longer alkyl chain members are compared and discussed. It is revealed that two competitive mechanisms are mainly operated on the self-organization, i.e., the preferential orientation of the long axes of the aromatic core parts parallel to each other and microsegregation between the aromatic core and alkyl chain parts of the molecules; the former mechanism is effective in the shorter chain members, whereas the latter is predominant in the longer chain members.