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

The self-assembly of linear A 1 B 1 A 2 B 2 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 A 2 -blocks sandwiched in the B domain in L' aggregate into spheres as χ N increases, leading to the formation of LS. The separation of A 2 -blocks from A 1 -blocks in L' or LS causes extra interfacial energy, which is compensated by the gain of configurational entropy. The tail B 2 -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.