Entropy-enthalpy competition controls self-assembly of functionalized star polymers under cylindrical confinement.

Self-assembly under cylindrical confinement is typically governed by geometric matching between the building-block size and the confinement diameter, with structural transitions generally achieved by modifying the confinement geometry. Here, we demonstrate that interaction design provides an alternative mechanism for structural selection under fixed confinement. Using functionalized star polymers, whose effective interactions arise from the competition between corona-mediated entropic repulsion and sticky-end-induced enthalpic attraction, we achieve temperature-controlled structural transitions from a disordered liquid to a zigzag structure and ultimately to a double helix at a constant cylindrical diameter. Combining theoretical analysis, umbrella sampling simulations, and free-energy calculations, we reveal that these transitions originate from temperature-dependent effective interactions, which stabilize the zigzag and double-helix structures in distinct temperature regimes. Furthermore, the zigzag-to-double-helix transition proceeds through a finite free-energy barrier associated with the formation of a complete helical turn. These findings establish entropy-enthalpy competition as a mechanism for controlling confined self-assembly and provide a general strategy for programming soft-matter assembly through interaction design.