Phase diagrams of conformationally asymmetric pentablock copolymer melts: a theory and simulation study

Conformational asymmetry, a difference in Kuhn length between two monomers, is known to shift the order-order transition (OOT) boundaries of block copolymer (BCP) melts and stabilize complex morphologies (e.g., Frank-Kasper phases) with enticing optical and transport properties. In this work, we investigate the influence of conformational asymmetry on the self-assembly of A_xB_yA_zB_yA_x pentablock copolymers (pentaBCPs) by adopting our previously developed workflow involving self-consistent field theory (SCFT) calculations and coarse-grained (CG) molecular dynamics (MD) simulations aided by the RAPSIDY protocol. For varying conformational asymmetry ratios, CAR ≡ β_A/β_B, where β_i is related to the Kuhn length of each block, SCFT shows shifts in the OOTs as CAR changes. For example, as CAR increases, the stability window widths of the hexagonally closed-packed cylinder and body-centered cubic spherical phases expand. To further understand the effect of CAR on chain conformations, we conduct MD simulations using two CG models - the 'semiflexible' chain model and the 'unequal-bead-diameter' (UBD) model. To vary CAR, in the 'semiflexible' chain model we vary the stiffness of A and B blocks, and in the UBD chain model we vary the A bead diameter with respect to the B bead diameter. The simulated phase behavior is qualitatively similar with both models and agrees to a large extent with the predicted phase diagram from SCFT. The normalized chain end-to-end distance and A-B interface width are consistent between both models and generally insensitive to CAR. However, the normalized lamellae periodicity expands with increasing conformational asymmetry (i.e., as CAR moves away from 1) for the semiflexible chain model, and the opposite is observed with the UBD chain model.