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This study explores the CO₂ capture characteristics of hyperbranched poly(ethylenimine) (HB-PEI) and poly- (propyleneimine) (HB-PPI) through molecular dynamics simulations using density functional theory-calibrated force fields. Key features such as density, free volume, glass transition temperature, CO₂/H₂O distribution, and molecular diffusion are systematically investigated to elucidate structure−function relationships under dry and hydrated conditions. HB-PEI demonstrates a slightly higher density and lower free volume compared to HB-PPI yet shows superior CO₂ capture due to the high amine concentration. Glass transition analysis indicates a higher thermal mobility in HBPEI, enhancing the CO₂ diffusivity. Pair correlation and coordination analyses confirm a stronger affinity ofmore » CO₂ with primary and secondary amines, particularly in hydrated environments where water competes with CO₂ for binding sites. Despite its more compact structure, HB-PEI outperformed HB-PPI in CO₂ and H₂O transport, as confirmed by higher diffusion coefficients across all hydration levels. These findings highlight a critical balance among polymer architecture, amine accessibility, and hydration in designing next-generation solid amine sorbents for efficient direct air capture applications.« less