Diblock Copolymer Blends Do Not Mimic Metal Alloys

Periodic\nparticle packings are ubiquitous across many forms of\ncondensed matter including colloids, metals, and block polymers. We\nreport the morphologies of binary blends of compositionally asymmetric\npoly(ethylene-<i>alt</i>-propylene)-<i>block</i>-poly(ethylene oxide) (PEP-PEO) and PEP-<i>block</i>-poly(dimethylsiloxane)\n(PEP-PDMS) diblock copolymers based on small-angle X-ray scattering\nmeasurements. Strong thermodynamic incompatibility of the minority\nblocks results in discrete particles containing PEO and PDMS cores\nwith common PEP coronae in the mixtures. Nearly identical particle\nradii, i.e., within ∼10% including the core and corona segments,\nlead to rapid formation of solid solutions with BCC symmetry at all\ncompositions when the disordered mixtures are cooled below the order-to-disorder\ntransition temperature. Increasing the pure component particle radius\nratio to <i>R</i><sub>PEP‑PEO</sub>/<i>R</i><sub>PEP‑PDMS</sub> ≈ 1.2 greatly retards nucleation\nand growth of order, restricting the development of BCC packing, with\nlimited formation of the Laves C14 phase anticipated by self-consistent\nmean-field theory. Surprisingly, the added degrees of freedom in the\ntwo-component system, naively expected to favor complex tetrahedral\npackings, detract from the well-established behavior of single component,\nsphere-forming diblock copolymers, where interparticle chain exchange\nleads to various Frank–Kasper phases. This striking and unanticipated\nbehavior is attributed to frustrated dynamics associated with the\ncombined effects of restricted chain exchange and jammed space-filling\nsoft particles.