Side‐Chain Nanophase Separation Broadens the Double‐Gyroid Stability Window in PS–PODMA Diblock Copolymers

Expanding access to bicontinuous network phases in block copolymers remains an important challenge because the double gyroid (DG) phase is usually stable only within a narrow composition window in conventional diblock copolymers. Here, we show that poly(styrene‐ block ‐octadecyl methacrylate) (PS‐ b ‐PODMA) exhibits an unusually broad DG window. Small‐angle X‐ray scattering measurements across a wide composition range reveal lamellar and hexagonally packed cylindrical phases at higher polystyrene fractions. In contrast, the DG phase appears over 0.22  ≤   f _PS   ≤  0.35, with DG/BCC and DG/HEX coexistence near f _PS  = 0.18 and 0.40, respectively. This broad DG window is much wider than those reported for neat diblock copolymers and is not readily explained by conformational asymmetry alone. Wide‐angle X‐ray scattering detects a characteristic signature of nanophase separation within the PODMA‐rich domains, indicating that side‐chain ordering introduces an additional internal length scale. These results suggest that hierarchical side‐chain nanophase separation modifies packing frustration and curvature selection, thereby broadening DG stability and providing a new molecular design strategy for stabilizing complex network morphologies.