Hierarchical Structures with Double Lower Disorder-to-Order Transition and Closed-Loop Phase Behaviors in Charged Block Copolymers Bearing Long Alkyl Side Groups

In this work, poly(ionic liquid) (PIL)-containing block copolymers (BCPs), poly(ethylene oxide)-b-poly(1-(2-acryloyloxy)ethyl)-3-docosylimidazolium) with different counterions (PEO-b-P(AOEDIm-X), X = Br or PF6), were synthesized. Hierarchical structures, including the microphase-separated structure between the PEO and PIL blocks at the first level, the segregated substructure between the long alkyl side groups and the other parts of the PIL block at the second level, and the crystalline or liquid crystalline (LC) structure of the long alkyl side groups at the third level, are formed in these BCPs. The relative orientation of hierarchical structures and the interplay between hierarchical structures at different levels were investigated. In weakly segregated BCPs, the breakout crystallization of the long alkyl side groups can first disturb the second-level structure and then cause cascade disordering of the first-level structure, leading to double lower disorder-to-order transition (LDOT) phase behaviors at low temperatures. In addition, when the order-to-disorder transition (ODT) occurs for the first-level structure at high temperatures, it will force the second-level structure to switch into the disordered state, resulting in synchronous ODTs of hierarchical structures. The results of this work provide new insight into the design of hierarchical structures with special phase behaviors in BCPs.