Expanding the window of the Frank-Kasper σ phase of block copolymer/homopolymer blend by selective incorporation of metal salt

The formation of a large core is crucial for accessing the Frank-Kasper (FK) phase of block copolymer (bcp) micelle, which can be achieved through several methods that promote the formation of the spherical phase at a relatively high minority block composition. Recently, it has been discovered that solubilizing a selective solvent into the coronal region of the bcp micelle can shift the cylinder-sphere phase boundary to a higher core volume fraction, enabling access to the FK σ phase even in weakly conformationally asymmetric systems, such as poly(ethylene oxide)-block-polybutadiene (PEO-b-PB), within a narrow composition range. In this study, we reveal a facile approach for expanding the σ phase window of the PEO-b-PB/PB homopolymer blend along both temperature and composition coordinates by selectively incorporating a low concentration of lithium salt into the PEO core. The coordination interaction between Li+ and PEO increased the effective interaction parameter, leading to a broadening of the thermal window of the σ phase. Notably, the increase in segregation strength also expanded the composition window of the spherical phase, with an upshift of the cylinder-sphere boundary to higher PEO composition. Accessing the spherical phase at the even higher core volume fractions yielded the larger core that facilitated its deformation into the polyhedral shape templated by the geometry of the micelles, which in turn promoted their efficient packing in the FK σ phase. The c/a ratio, representing the aspect ratio of the tetragonal unit cell of the σ phase, was predominantly observed to be around 0.53, deviating from the value of 0.5176 of the ideal σ phase. The fact that the maximum average sphericity of the Voronoi cells comprising the σ unit cell was achieved at c/a ≈ 0.53 suggested that the polyhedral micelles in the σ lattice tended to adjust their shapes to minimize the overall deviation from their native spherical geometry.