Microphase Separation and Temperature-Dependent Structural Transitions of Mixed-Side-Chain Comb-like Copolymers

Introducing charged groups into comb-like copolymers significantly enhances the incompatibility between the main chain and side chains, enabling sub-5 nm microphase separation. In this work, a series of charged comb-like copolymers with imidazolium ionic junctions and mixed alkyl side chains (C18/C8, C20/C12, C22/C12) were synthesized. The regulation of microphase separation behavior via side-chain composition and length were investigated. The crystallization of long alkyl side chains could disrupt pre-formed microphase-separated structures, rendering a disordered state at low temperatures. The systems underwent a lower disorder-order transition (LDOT) that was closely correlated with the melting of side-chain crystallites, forming lamellar (LAM) or hexagonally packed cylindrical (HEX) nanostructures. By adjusting the side-chain length and feed ratio (r) of long/short chains, both the LDOT and closed-loop sequence of transitions (LDOT at low temperatures and upper order-disorder transition (UODT) at high temperatures) were achieved, with transition temperatures shifting accordingly. Notably, the SAXS data at a specific composition are consistent with the Frank-Kasper (F-K) A15 complex spherical phase. Moreover, the domain spacing showed an approximately linear dependence on the nominal long-chain feed ratio within the investigated range. This work demonstrates that statistical side-chain functionalization is an effective strategy for tuning microphase separation in charged comb-like copolymers with ultra-small self-assembled dimensions.