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.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation under grant nos. DMR-1801993 (A.P.L., A.J.P., and F.S.B.) and DMR-1719692 (G.K.C., F.S.B., and K.D.D.) as well as a National Science Foundation Graduate Research Fellowship under grant no. 00039202 (A.P.L.). We thank Andreas Mueller and Mahesh Mahanthappa for helpful discussions concerning this work and extend our gratitude to the staff at sector 5 for their assistance in conducting these experiments during the pandemic. SAXS experiments were performed at sectors 5 and 12 of the Advanced Photon Source (APS). The DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at sector 5 is supported by Northwestern University, the Dow Chemical Company, and DuPont de Nemours, Inc. The APS is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Data were collected using an instrument at sector 5 funded by the National Science Foundation under award number 0960140. Parts of this work were carried out in the Characterization Facility at the University of Minnesota, which receives partial support from the NSF through the MRSEC program, DMR-2011401.
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