Molecular Packing in Two Bicontinuous Ia3d Gyroid Phases of Calamitic Cubic Mesogen BABH(n); Roles in Structural Stability and Reentrant Behavior
1,2-Bis(4’-n-alkoxybenzoyl)hydrazine [BABH(n), n is the number of carbon atoms in the alkyl chain] exhibits two different Ia3d cubic phases depending on the chain length (5 <= n <= 13 and 15<= n <= 22). The molecular packing modes, not only of molecular cores but also of alkyl-chains, are investigated through a maximum entropy method (MEM) and an analysis of the chain-length dependence of the body diagonal of the unit cell. The analyses revealed the difference in the molecular packing. The short-chain Ia3d structure of the BABH(n) (5 <= n <= 13) is constructed by single-layered core motifs and bilayers of alkyl-chains, of which the orientation is tilted from the normal to the layer. The long-chain Ia3d structure (15 <= n <= 22) is formed by double-layered core motifs and monolayers of alkyl-chains, of which the orientation is normal to the layer. Based on the molecular packing modes, the reentrant behavior between the two Ia3d phases was clarified. It was revealed that the alkyl-chains of BABH(n) molecules play an important role in the formation and stability of the two Ia3d structures.