Emergence and Stability of a Hybrid Lamella-Sphere Structure from Linear ABAB Tetrablock Copolymers

The self-assembly of linear A₁B₁A₂B₂ tetrablock copolymers is studied using the self-consistent field theory, aiming to target the formation of stable hybrid structures composed of lamellar and spherical domains of the same component, i.e., the lamella-sphere (LS) phase. Two types of lamellar morphologies, regular (L) and sandwich-like (L'), are observed, and their transition is identified as first-order. The formation of L' is a prior condition for the formation of LS because the disordered short A₂-blocks sandwiched in the B domain in L' aggregate into spheres as χ increases, leading to the formation of LS. The separation of A₂-blocks from A₁-blocks in L' or LS causes extra interfacial energy, which is compensated by the gain of configurational entropy. The tail B₂-block is demonstrated to play a critical role in enlarging the gain of configurational entropy. In a word, the formation of L' is driven by entropy, while the transition from L' to LS is driven by enthalpy.