Alternating Gyroid Stabilized by Surfactant-Like Triblock Copolymers in SI/SO/ISO Ternary Blends*

Block copolymers self-assembled into chiral alternating-gyroid (AG) 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 triblock into a blend of double-gyroid-forming AB and BC diblocks with immiscible A and C blocks stabilizes AG. Here, we use SCFT to investigate the feasibility of this method in practice by computing the phase behaviors of ternary mixtures of poly(styrene-block-isoprene) (SI), poly(styrene-block-ethylene oxide) (SO), and poly(isoprene-block-styrene-block-ethylene oxide) (ISO). The addition of ISO successfully stabilizes AG, however, the phase window is relatively narrow, due to the packing frustration induced by the mismatch in preferred domain sizes between SI and SO diblock copolymers. Designing the system to have comparable diblock domain sizes significantly enhances the relative stability of AG against the competing alternating lamellae. Moreover, the stabilization of alternating morphologies is sensitive to temperature, and less ISO is needed at lower temperatures. The effect of other design parameters, such as diblock volume fraction, will be provided. These computational results provide insights into possible experimental strategies for producing AG.