Symmetric Diblock Copolymers in Cylindrical Confinement: A Way to Chiral Morphologies?

We investigate the confinement-induced formation and stability of helix morphologies in lamella-forming AB diblock copolymers via large-scale, particle-based, single-chain-in-mean-field simulations. Such helix structures are rarely observed in bulk or thin films. Structure formation is induced by quenching incompatibility, χN, from a disordered morphology. If the surfaces of the cylindrical confinement do not prefer one component over the other, we observe that stacked lamellae, with their normals along the cylinder axis, are the preferred morphology. Kinetically, this morphology initially forms close to the cylinder surface, whereas the spontaneous, spinodal microphase separation in the cylinder's interior gives rise to a microemulsion-like morphology, riddled with defects and no directional order. Subsequently, the ordered morphology on the cylinder surface progresses inward, pervading the entire volume. In case that the cylindrical pore is only partially filled, the additional confinement along the cylinder axis generally gives rise to incommensurability between the equilibrium spacing of stacked lamellae and the cylinder height. To accommodate this mismatch, the lamella normals will tilt away from the cylinder axis and generate helices of lamellae on the surface of the cylinder. Again, this order progresses from the cylinder surface inward, generating a chiral morphology. Because the spacing between the internal AB interfaces decreases upon approaching the helix center, the concomitant stress results in a decrease in the number of lamellae and the formation of unique dislocation defects. This type of chiral defect morphology is reproducibly formed by the kinetics of structure formation in partly filled cylindrical pores with nonpreferential surfaces and may find applications in photonic applications.