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 g 6 and only on cooling for n ) 5. The structures were examined by XRD, which revealed the presence of two types with symmetries Ia3d and Im3m, 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 CdO 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 (da/dT) 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 Ia3d-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.

The cubic (Cub) phase structure of a thermotropic mesogen 1,2‐bis(4′‐n‐alkoxybenzoyl)hydrazine (BABH‐n, where n is the number of C atoms in the aliphatic tail) was examined by small‐angle X‐ray scattering. The BABH‐n system exhibits two types of Cub phases, Ia3d type and Im3m type. In the phase diagram as a function of the tail length (n) the Im3m‐Cub phase region (n = 13–16) was sandwiched by two Ia3d‐Cub phase regions with n < 13 and n > 16. On the basis of the triply periodic minimal surface (TPMS) picture, the Ia3d and Im3m structures are described by the gyroid (G) and doubled‐P (PP) surfaces, respectively. In this paper we focused attention on the internal structure of the Ia3d‐Cub phase at the molecular level. By examining how the relative intensity of the 220 reflection with respect to the 211 reference peak varies with n, we have successfully determined the position of the aliphatic tails, which are located on the G‐TPMSs. As far as we know, this is the first approach to clarifying the internal structure of the thermotropic Ia3d‐Cub phase. The usefulness of the systematic studies with respect to the tail length n has also been demonstrated.

The structure type of bicontinuous cubic phase formed by 1,2-bis(4′-n-alkoxybenzoyl)hydrazines (BABH) was strongly dependent on the alkoxy chain length, showing the phase sequence of Ia3d–Im3m–Ia3d with lengthening the chain.