Diblock Copolymer Blends Do Not Mimic Metal Alloys

Periodic particle packings are ubiquitous across many forms of condensed matter including colloids, metals, and block polymers. We report the morphologies of binary blends of compositionally asymmetric poly(ethylene-alt-propylene)-block-poly(ethylene oxide) (PEP-PEO) and PEP-block-poly(dimethylsiloxane) (PEP-PDMS) diblock copolymers based on small-angle X-ray scattering measurements. Strong thermodynamic incompatibility of the minority blocks results in discrete particles containing PEO and PDMS cores with common PEP coronae in the mixtures. Nearly identical particle radii, i.e., within ∼10% including the core and corona segments, lead to rapid formation of solid solutions with BCC symmetry at all compositions when the disordered mixtures are cooled below the order-to-disorder transition temperature. Increasing the pure component particle radius ratio to R_PEP-PEO/R_PEP-PDMS ≈ 1.2 greatly retards nucleation and growth of order, restricting the development of BCC packing, with limited formation of the Laves C14 phase anticipated by self-consistent mean-field theory. Surprisingly, the added degrees of freedom in the two-component system, naively expected to favor complex tetrahedral packings, detract from the well-established behavior of single component, sphere-forming diblock copolymers, where interparticle chain exchange leads to various Frank-Kasper phases. This striking and unanticipated behavior is attributed to frustrated dynamics associated with the combined effects of restricted chain exchange and jammed space-filling soft particles.