Stabilize a Nonclassical Square Array of Cylinders by Tailoring ABA/AB Block Copolymer Blends

Uniform cylinders self-assembled by AB-type block copolymers usually pack into a hexagonal array, which aligns with intuition or common sense. How to design simple block copolymer systems for forming an unconventional square array of cylinders in both thermodynamics and kinetics remains an unsolved problem, which is of great interest for both fundamental research and applications. In this work, we propose the design of a simple binary blend, composed of symmetric ABA triblock and AB diblock copolymers, to stabilize the tetragonal structure. Using self-consistent field theory (SCFT), we have predicted a stable tetragonal structure in a considerable parameter region. It is revealed that the tetragonal structure primarily results from the swelling of the longer AB diblock to the ABA triblock, enabling the effect of a stretched bridging block that reduces the packing coordination number of the cylinders. Simultaneously, the local separation of the long B-block in AB and the short B-block in ABA releases the high packing frustration of chains within the matrix. These two synergistic effects substantially reduce the interfacial area of the tetragonal structure relative to the hexagonal structure and thus lower its interfacial energy even though they also widen the interface. Since the considered blend comprises two simple block copolymers, our work is expected to facilitate the experimental discovery of the unconventional tetragonal structure.