Multiple Metastable Micellar States in Disordered Diblock Copolymers

Low molar mass and compositionally asymmetric polyisoprene-b-polylactide (PI-PLA) diblock copolymers were ordered into multiple crystalline states, including BCC and the σ, C14, and C15 Frank-Kasper phases, using recently discovered thermal processing techniques. For a given pre-ordered phase, heating above the order-disorder transition temperature (T_ODT) followed by cooling below T_ODT returns the system to the same ordered state. Small angle x-ray scattering revealed that the number density of micellar particles, n/V, is retained in the highly structured disordered liquid. We hypothesize that the number of micelles per volume, n/V, imprinted on the liquid during the initial ordering stage, governs the symmetry breaking during subsequent crystallization, and that the metastable structured liquid cannot achieve the equilibrium particle density due to prohibitively large free-energy barriers for micelle fusion and fission. Once the n/V is fixed, facile chain exchange redistributes mass to meet the required particle sizes and packing associated with individual ordered phases. This work reveals the nature of the fluctuating disordered state in asymmetric diblock copolymers in the low molar mass limit.