Order-Order Transition from Ordered Bicontinuous Double Diamond to Hexagonally Packed Cylinders in Stereoregular Diblock Copolymer/Homopolymer Blends

Our previous works have disclosed a thermodynamically stable ordered bicontinuous double diamond (OBDD) structure and the order-order transition (OOT) between OBDD and ordered bicontinuous double gyroid (OBDG) phases in a stereoregular diblock copolymer, syndiotactic polypropylene-block-polystyrene (sPP-b-PS). Here we investigate the effect of blending with a small amount of sPP homopolymer (h-sPP) on the stability of the OBDD phase, where h-sPP formed the dry-brush mixture with the minority sPP block chains in the network microdomains due to their comparable molecular weights. The OBDD structure developed in the blend was found to exist over a broader temperature window than that formed in the neat sPP-b-PS. Nevertheless, the OBDD-OBDG transition displayed by the neat diblock was no longer observed in the blends; instead, the OBDD phase transformed into the hexagonally packed cylinder (HEX) structure on heating, indicating that HEX superseded OBDG upon blending with h-sPP. According to the experimentally observed conservation of the lattice parameter across the OBDD-HEX transition, we propose a possible kinetic pathway through which the OBDD tetrapods may reorganize and merge into a cylinder along the direction of the space diagonal in a unit cell; the cylinders developed from different unit cells subsequently shifted and fused at their ends for reducing the interfacial free energy and the packing frustration of the majority blocks, thereby turning into the hexagonal packing.