Accessing the Frank-Kasper Phase of Block Copolymer via Selective Incorporation of Metal Salt

The formation of Frank-Kasper (FK) phase in block copolymers (BCPs) has been achieved through delicate design of the chemical structure and architecture of the BCP, as well as by blending with homopolymer or another BCP component. Here, we report a new approach of generating FK phase via selective incorporation of a metal salt into a BCP. Specifically, the selective incorporation of lithium perchlorate (LiClO4) into the poly(ethylene oxide) (PEO) domain of a cylinder-forming poly(ethylene oxide)-block-poly(1,2-butadiene) (PEO-b-PB) was found to create a window for the spherical phase at the higher temperature, in which the micelles packed preferentially into FK sigma phase or dodecagonal quasicrystal (DDQC). The addition of a bivalent salt, magnesium perchlorate (Mg(ClO4)2), expanded the spherical phase window further, where the micelles still preferred packing in the sigma and DDQC phases. Nevertheless, with sufficiently high Mg(ClO4)2 concentration, the micelles were found to organize into the Laves C14 phase instead of recovering the DDQC and sigma phases during cooling from the disordered micelle phase. We proposed that the preferential solubilization of salt in the PEO core increased the effective interaction parameter of the BCP, leading to greater stretching of the constituent blocks due to the formation of a larger core. The increased tendency to reduce the conformational free energy penalty of the coronal blocks facilitated the access of spherical phase at higher core volume fractions, thereby promoting the micelle organization into FK phases. The present study highlights the effectiveness of metal salts in promoting the complex packing of BCP micelles, providing a simple alternative for modulating the structure of spherical phase.