Solution Self-assembly of T-shaped Sphere-Rod Rigid "Block Copolymers" into Strictly defined Onion-like, Possible Liquid Crystalline Structures

A series of T-shaped sphere-rod amphiphilic macromolecules are studied to reveal the self-assembly behaviors of rigid block copolymers. The hydrophilic polar group is composed of Keggin, Anderson, or Dawson type of polyoxometalate (POM) clusters, linked with the hydrophobic domain in a T-shaped way. Onion-like spherical structures with multiple concentric layers are formed in the water/acetonitrile mixed solvents of Keggin T-shape-linked oligo fluorene (Keggin-TOF₄). Amazingly, the interlayer distance is strictly constant for different layers, and it does not change with the external coniditions. Instead, the assemblies respond to external changes by changing their total number of layers. Interestingly, in the mixture of two hybrid macromolecules with different polar head groups (Anderson-TOF₄ and Dawson-TOF₄), they can accurately self-recognize with each other by forming individual assemblies instead of mixed ones. An interdigitation model, supported by 2D-NMR, is proposed to explain how the rigid amphiphiles pack and assemble into highly ordered onion-like structures. The counterion-mediated attraction between charged hydrophilic clusters is expected to play an important role in the inter-layer attraction. Overall, the size, shape, charge, and specific geometry of rigid building blocks lead to the unique final assembled structures. This study demonstrated that block copolymers with fully rigid domains possess completely different assembly rules from common block copolymers.