Asymmetric AB diblock copolymers form self-assembled point particles generally assumed to be spherical and ordered on a body centered cubic (BCC) lattice. Recent small-angle x-ray scattering (SAXS) experiments with low molecular weight diblocks containing minority poly(lactide) (PLA) and larger hydrocarbon blocks have revealed the formation of Frank-Kasper (FK) and quasicrystal phases below the order disorder transition temperature. Whereas the BCC phase is characterized by one particle per lattice site the FK phases contain large unit cells with 2 or more crystallographic sites resulting in multiple particle sizes and shapes, mimicking the packing behavior of metal alloys. Transitions from the disordered to ordered states and between ordered phases requires mass transfer resulting in long-lived metastable structures that can be accessed through different thermal processing procedures, again analogous to the wealth of solid state materials produced using techniques common in the field of metallurgy. This presentation will relate our recent discoveries to the current theoretical understanding of ordering in block polymer melts.
This research has been conducted in collaboration with Kyungtae Kim, Akash Arora, Morgan Schulze, Ron Lewis, Tim Gillard, Sangwoo Lee, Chris Leighton, Marc Hillmyer, and Kevin Dorfman.