Complex Phase Behavior in Particle-Forming AB/AB′ Diblock Copolymer Blends with Variable Core Block Lengths

Over the past decade, a wealth of complexity has been reported in the packing of compositionally asymmetric, particle-forming diblock copolymer melts, beginning with the discovery of the Frank-Kasper σ phase and continuing with subsequent discoveries of a dodecagonal quasicrystal and the C14, C15, and A15 phases. First identified by self-consistent mean-field theory (SCFT), blending diblock copolymers has proven to be a useful strategy in extending these packings to new chemistries and length scales. However, much of the immense phase space created on blending two copolymers remains unexplored. Herein, we expand on our previous work investigating binary blends of polystyrene-block-1,4-polybutadiene diblock copolymers, focusing on binary mixtures with a constant corona (majority) block length and a range of ratios of core (minority) block lengths. Small-angle X-ray scattering and transmission electron microscopy conducted with 5 narrow dispersity diblock copolymers and the associated blends uncovered a rich phase space including 12 distinct nanostructures. Notably, in agreement with SCFT predictions, we document a C14 Laves phase at low fractions of the larger copolymer in a mixture of high and low molecular weight components. However, experiments and SCFT calculations reveal that this window is truncated by close packing when the smaller copolymer is weakly segregated. Moreover, we find that even a modest difference in core block lengths is sufficient to stabilize the σ phase, highlighting the impact of core block dispersity in previous studies as well as the utility of blending in accessing these complex particle phases.