Fabrication of Ellipsoidal Mesostructures in Block Copolymers via a Step-Shear Deformation

Ellipsoids have attracted abiding attention because of their shape-dependent, anisotropic properties. In some applications, e.g., photonic crystals, both positional and orientational order of the ellipsoidal packing are required. We propose a versatile, facile, and efficient strategy to fabricate positionally and orientationally ordered crystals of soft ellipsoids in block copolymers via a step-shear deformation. Starting from the thermodynamically stable, equilibrium spherical mesophase of the copolymer material, the step-shear deformation provides an instantaneous, anisotropic stimulus to deform the spherical domains into ellipsoids and simultaneously stretches the macromolecular conformations. Subsequently, at fixed strain, the molecular stress relaxes to an equilibrium where the shape and orientation of the obtained ellipsoids are dictated by the packing frustration. Since the residual molecular stress is minuscule, the lattice relaxation via slippage in the absence of external stress is protracted, i.e., the crystal of soft ellipsoids with positional and orientational order is pseudometastable. Our strategy also allows for low volume fractions of ellipsoids (compared to colloidal systems). Both single-chain-in-mean-field (SCMF) simulations and self-consistent field theory (SCFT) calculations are employed to demonstrate the pseudometastability of the obtained ellipsoids. Varying the magnitude of the step-shear strain and the composition of the block copolymer, we can control the asphericity and orientation of the ellipsoidal domains independently. Our study provides a new concept for fabricating soft, positionally and orientationally ordered crystals of ellipsoids with potential applications in engineering functional materials