(191e) Block Copolymer Blending Strategy for Creating Alternating Gyroid Morphology

Alternating gyroid, composed of two independent chiral networks, is an attractive self-assembled morphology for optical applications. It has been predicted that the alternating gyroid possesses a complete photonic band gap, subject to the constraints that there is sufficient optical matching between one of the networks and the matrix, and simultaneously a large optical contrast between the individual networks. For block polymers, a photonic band gap material should be realizable by first self-assembling an alternating gyroid network, extracting one network, and then backfilling with a metal to create single-gyroid-structured nanomaterials. The conventional method to stabilize alternating gyroid in block copolymers is using strategically designed ABC triblock terpolymers. Unfortunately, this approach is synthetically challenging and theoretically predicted to produce a very narrow stability window for alternating gyroid phase.

In this talk, we propose a new method to produce an alternating gyroid phase by blending double-gyroid-forming AB and BC diblock polymers with relatively immiscible A and C blocks. Using self-consistent field theory (SCFT), we compared the free energies of various phases, such as alternating lamellae, cylinders, and spheres, and alternating gyroid, along with various hybrid morphologies. We predict that stoichiometric blends of the AB and BC diblocks form an alternating gyroid that is nearly degenerate in free-energy with two phase-separated AB-rich and BC-rich double gyroid states, which implies that solvent casting could be a potential method for capturing the metastable alternating gyroid phase. We further investigated the effect of adding a small amount of ABC triblock terpolymer on the stability of the alternating gyroid in ternary blends by performing grand canonical SCFT calculations. We found that even a minuscule amount (<1%) of ABC triblock terpolymer can open an alternating gyroid stability window in the ternary phase diagram, acting as a surfactant to bridge between the A-rich and C-rich gyroid networks. Surprisingly, the ability to stabilize the alternating gyroid phase in the blends was remarkably insensitive to the amount of the surfactant and the composition of the constituent blocks, provided that the middle block is sufficiently long to bridge between the two network domains. We anticipate this strategy will provide an effective and practical route to creating advanced optical materials with complete photonic band gaps when combined with solvent extraction to remove one of the networks in the alternating gyroid phase.