Self-Organization of Rod−Coil Molecules with Layered Crystalline States into Thermotropic Liquid Crystalline Assemblies

The synthesis and characterization of the rod−coil molecules of ethyl 4-[4-[oxypoly(propyleneoxy)propyloxy]-4‘-biphenylcarboxyloxy]-4‘-biphenylcarboxylate with poly(propylene oxide) coil of 7 ( 7-P-4 ), 8 ( 8-P-4 ), 10 ( 10-P-4 ), 12 ( 12-P-4 ), 15 ( 15-P-4 ), 17 ( 17-P-4 ), and 20 ( 20-P-4 ) propylene oxide units are described. The introduction of poly(propylene oxide) (PPO) coils with different lengths into the rodlike mesogen gives rise to a rich self-assembled microstructures. In the crystalline state, the rod−coil molecules with 7−12 repeating units of PPO organize into a microphase-separated monolayer lamellar structure in which rods are fully interdigitated. In contrast, the rod−coil molecules with 15−17 repeating units of PPO exhibit a lamellar structure with rod tilt relative to the layer normal. A dramatic mesophase change is observed with the variation of the coil length. The rod−coil molecules 7-P-4 and 8-P-4 with short PPO coils display layered smectic C and smectic A mesophases, while the rod−coil molecules 10-P-4, 12-P-4, and 15-P-4 with medium-length coils exhibit a biconticuous cubic mesophase with Ia 3 d symmetry. Further increasing the length of coils as in the cases of 15-P-4, 17-P-4, and 20-P-4 induces a hexagonal columnar mesophase. Estimations based on the lattice parameters and densities have shown that the organization of the rod−coil molecules into a cross sectional slice of a cylinder for the cubic and columnar phases can give rise to an aromatic core with a square cross section. This unique behavior in the rod−coil molecules is believed to originate from the anisotropic aggregation of rod segments and consequent entropic penalties associated with coil stretching.