Reexamining the Emergence and Stability of the Square Cylinder Phase in Block Copolymers

• Published in: Chemistry : a European journal Vol. 29; no. 42; pp. e202301043 - n/a
• Main Authors: Yuan, Kangrui, Xu, Zhanwen, Huang, Xianbo, Li, Weihua
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 26.07.2023
• Subjects: Block copolymers; Chemistry; Copolymers; Field theory; free-energy landscape; self-consistent field theory; square cylinders; Stability analysis; Unit cell; block copolymers; self-consistent field theory; free-energy landscape; square cylinders
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202301043

Recently, a few AB‐type multiblock copolymers have been successfully designed to form stable square cylinder phase based on self‐consistent field theory (SCFT) calculations. The previous works only identify the stability region of the square phase but not analyzing its stability, which is closely related to the free‐energy landscape. In this work, we have reexamined the stability of the square phase in B1A1B2A2B3 ${{{\rm B}}_{1}{{\rm A}}_{1}{{\rm B}}_{2}{{\rm A}}_{2}{{\rm B}}_{3}}$ linear pentablock and (B1AB2)​5 ${({{\rm B}}_{1}{\rm A}{{\rm B}}_{2}{)}_{5}}$ star triblock copolymers by drawing the free‐energy landscape with respect to the two dimensions of a rectangular unit cell. Our results demonstrate that the square phase continuously transfers to the rectangular phase as the degree of packing frustration is gradually released. Moreover, the prolate contour lines of the free‐energy landscape indicate the weak stability of the square phase in the B1A1B2A2B3 ${{{\rm B}}_{1}{{\rm A}}_{1}{{\rm B}}_{2}{{\rm A}}_{2}{{\rm B}}_{3}}$ copolymer. In contrast, the stability of the square phase is notably improved in the (B1AB2)​5 ${({{\rm B}}_{1}{\rm A}{{\rm B}}_{2}{)}_{5}}$ copolymer due to its enhanced concentration of bridging configurations. Our work sheds light on the understanding of the stability of the square cylinder phase in block copolymers. Accordingly, we propose some possible strategies for further designing new AB‐type block copolymer systems to obtain more stable square phase. The transition from the square cylinder phase to the rectangular cylinder phase is a second‐order transition that accompanies the transition of the free‐energy landscape from one minimum to two minima. The prolate contour lines of the free‐energy landscape indicate the weak stability of the square phase in the B1A1B2A2B3 copolymer.