Alternating Gyroid Stabilized by Surfactant-like Triblock Terpolymers in IS/SO/ISO Ternary Blends

Abstract

Block polymers self-assembled into chiral alternating-gyroid (GA) morphologies are desirable templates for fabricating photonic materials. Recent self-consistent field theory (SCFT) calculations on a model system predicted that adding a minuscule amount of ABC triblocks into a blend of thermodynamically symmetric double-gyroid-forming AB and BC diblocks with immiscible A and C blocks stabilizes GA. Here, we use SCFT to investigate the feasibility of this method in practice by computing the phase behaviors of ternary mixtures of poly(isoprene-b-styrene) (IS), poly(styrene-b-ethylene oxide) (SO), and poly(isoprene-b-styrene-b-ethylene oxide) (ISO). Addition of ISO indeed stabilizes GA; however, the phase window is relatively narrow, due to the packing frustration induced by the mismatch in preferred domain sizes between IS and SO diblock copolymers. Designing the system to have comparable diblock domain sizes significantly enhances the relative stability of GA against the competing alternating lamellae, and this stability is also sensitive to the degree of polymerization of the ISO midblock. Moreover, the stabilization of alternating morphologies is sensitive to temperature, and less ISO is needed at lower temperatures. The effect of some other design parameters, such as diblock volume fraction, is also investigated. These computational results provide insights into possible experimental strategies for producing GA and highlight the sensitivity of network phase formation to asymmetries in the thermodynamic parameters.