Frank-Kasper Phases in Block Polymers: Stable or Metastable?

Block polymers are a class of soft materials that self-assemble at mesoscopic length scales to form a variety of ordered structures. In recent years, several experimental studies witnessed the formation of complex low-symmetry structures in sphere-forming block polymers. These structures, commonly referred to as Frank-Kasper (FK) phases, contain particles of disparate sizes packed in a tetrahedral closed-packed arrangement with multiple local coordination environments. In this work, we use self-consistent field theory (SCFT) to examine the stability of different FK phases in AB diblock copolymers. We performed extensive SCFT calculations and observed that several FK phases exist on the free energy surface of diblock copolymers. More importantly, the associated free energies differ only marginally (10^-3 k_BT). Recently, our group experimentally discovered two new FK phases in poly(isoprene-b-lactide) diblock copolymers. Using SCFT, we will discuss the thermodynamic stability of these phases and demonstrate the importance of various molecular parameters that govern the relative stability of these complex phases in AB diblocks.