Stability of the Orthorhombic Single Network <i>Fddd</i> Phase in Pentablock Copolymer Melts

Among the block copolymer (BCP) self-assembled microstructures, the Fddd (O70) phase is recognized as a unique orthorhombic single network phase that is useful in many applications that require advanced network morphologies. In this work, we propose that a stable Fddd (O70) phase can be accessed via self-assembly of symmetric BxAyBzAyBx pentablock copolymers (pentaBCPs) over a broad range of temperatures when the volume of the middle B2 block is 10% of the total volume of B blocks (τB2=0.1)). Our self-consistent field theory (SCFT) calculations predict that a considerable stability window of the Fddd (O70) phase is located at unconventionally low A block volume fractions fA ≈ 0.18, which is driven by the tendency of BxAyBzAyBx pentaBCPs of τB2=0.1 to form A–B interfaces with little to no curvature. This is partially confirmed by coarse-grained (CG) molecular (MD) simulations of the BxAyBzAyBx pentaBCPs exhibiting O70-like networks or perforated lamellae in those pentaBCP designs. CG MD simulations also show that analogous BxAyBx triBCPs with the same fA as the BxAyBzAyBx pentaBCPs of τB2=0.1 do not form O70-like networks and, instead, exhibit the hexagonally packed cylinders phase. The SCFT free energy calculations and block density distribution analyses reveal that the sizable Fddd (O70) stability window for the BxAyBzAyBx pentaBCPs of τB2=0.1 is a result of a reduction in packing frustration when forming the Fddd (O70) phase rather than the double gyroid phase.