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Membrane gas separation processes represent a crucial platform for achieving carbon neutral climate goals. CO 2 -philic membranes offer excellent solubility selectivity due to their high affinity, thereby allowing the separation of gas fractions with similar molecular sizes or smaller than CO 2 molecules. This level of separation is difficult to achieve with conventional diffusion-dominated separation membranes. However, the gas permeability of PEO membranes is often hampered by dense chain packing and crystallization behavior. To overcome this challenge, we employed a molecular engineering approach to develop a unique PEO membrane with a hyperbranched chemical topology connection. The packing density of the polymer chains was effectively regulated by employing cross-linker molecules with distinct spatial configurations, while the free volume fraction was successfully reinforced. This unique molecular engineering strategy led to the creation of a PEO membrane exhibiting superior performance that surpassed the limit of the 2019 upper-bound line. This work demonstrated a molecular design strategy for the precise control of structural attributes in crosslinked PEO networks, offering an alternative solution for developing CO 2 capture membranes and enabling further opportunities for rational membrane material design. [Display omitted] • Cross-linked PEO membranes with various topological structure were constructed. • Molecular engineering optimized the free volume of cross-linked network. • The gas diffusion in cross-linked PEO membranes was promoted substantially. • The CO 2 /N 2 separation performance exceeded the 2019 upper bound. [ABSTRACT FROM AUTHOR]