The morphology of a H‐shaped block copolymer (poly(ethylene glycol) backbone and polystyrene branches (PS)2PEG(PS)2) in a thin film has been investigated. A peculiar square lamella that has a phase‐separated microdomain at its surface is obtained after spin coating. The experimental temperature plays a critical role in the lamellar formation. The copolymer first self‐assembles into square lamellar micelles with an incomplete crystalline core due to the crystallizability of PEG. In the subsequent process of solvent evaporation, phase separation between PS and non‐crystalline PEG attached at the chain‐folded PEG surface takes place, which results in phase‐separated microdomains being formed at the lamellar surface.
