Diverse Frank-Kasper phases in High-χ and Conformationally Asymmetric PDMS-b-PTFEA Copolymers

The Frank-Kasper (FK) phases which were originally observed in metal alloys have emerged with single-component block copolymers (BCPs) self-assembly providing a new potential in soft matters since the early 2000s. To directly impact these low-symmetry structures, here, we introduce a new-type of BCPs with high-χ and high-ε, where χ and ε are the Flory−Huggins interaction parameter and conformational asymmetry between the two blocks, respectively. The compositional asymmetric Polydimethylsiloxane-b-poly(2,2,2-triflouroethyl acrylate) (PDMS-b-PTFEA) copolymers were precisely synthesized via atom transfer radical polymerization using the same PDMS block. As a result, using small-angle X-ray scattering, we systematically explore the self-assembled packing structures of PDMS-b-PTFEAs in the space of temperature versus the volume fraction of core-forming PTFEA blocks (f_PTFEA). As f_PTFEA decreases, the sphere-packing phase behavior exhibits the composition dependence of the FK phases (A15, σ, and C14) and classical body-centered cubic (BCC) phase. The experimental sequence of sphere-packing phases agrees with the free-energy density calculation of A15−σ−C14−BCC with decreasing f_PTFEA, which are subject to both high-χ and high-ε nature of PDMS-b-PTFEAs.