University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Thursday, February 6th 2020
3:35 pm:
Speaker: Frank Bates, University of Minnesota (CEMS)
Subject: Diblock Copolymer Melts Mimic Metallurgy
Refreshments in atrium after the Colloquium.

Block copolymers have captured the interest of scientists and engineers for more than half a century. In general, the phase behavior of diblock copolymers, the simplest category of such self-assembling macromolecules, has been accepted as thoroughly understood. Until several years ago, sphere forming diblock copolymers seemed particularly simple, with universal ordering on a body-centered cubic (BCC) lattice as predicted 40 years ago. Experiments with low molecular weight poly(isoprene)-b-poly(lactide) (PI-PLA) diblock copolymer melts have disrupted this picture, revealing remarkable phase complexity in the limit of asymmetric compositions, 0.15 < fL < 0.30, where fL signifies the volume fraction of PLA. Small-angle X-ray scattering (SAXS) measurements conducted in the vicinity of the order-disorder transition have demonstrated the formation of several topologically close-packed Frank-Kasper (FK) phases, including the , C14 and C15 phases, and a dodecagonal quasicrystal (QC), in addition to BCC packing. This lecture will explore the underlying molecular and geometric factors that drive this fascinating complex phase behavior. A central feature is the tendency for self-assembled micelles to be spherical, which is frustrated by the necessity for soft materials to fill space at constant density leading to the formation of space filling polyhedral particles. A surprising analogy will be drawn with the underlying competition between electronic structure and ordered state symmetry that contributes to the formation of FK and QC phases in various metals and alloys. This work highlights extraordinary opportunities for uncovering the principles governing symmetry breaking in soft and hard materials by exploiting the unparalleled control over molecular structure associated with block copolymers.

Faculty Host: Rafael Fernandes

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