Thermal Processing of Diblock Copolymer Melts Mimics Metallurgy

Small-angle x-ray scattering experiments conducted with compositionally asymmetric low
molar mass poly(isoprene)-b-poly(lactide) diblock copolymers reveal an extraordinary
thermal history dependence. The development of distinct periodic crystalline or aperiodic
quasicrystalline states depends on how specimens are cooled from the disordered state to
temperatures below the order-disorder transition temperature. Whereas direct cooling
leads to the formation of documented morphologies, rapidly quenched samples that
are then heated from low temperature form the hexagonal C14 and cubic C15 Laves phases
commonly found in metal alloys. Self-consistent mean-field theory calculations show
that these, and other associated Frank-Kasper phases, have nearly degenerate free
energies, suggesting that processing history drives the material into long-lived metastable
states defined by self-assembled particles with discrete populations of volumes and
polyhedral shapes.