To determine the structural change due to CO2-induced protonation, we examined PEI samples using small-angle X-scattering (SAXS, Figure ). SAXS results indicate the structural characteristic changes of PEI-C16 samples upon activation, acidification, CO2 capture, and ion exchange (desalination). The reflections observed in SAXS at high angles correspond to the ordering pattern of the C16 side chains. The low-angle scattering reflections indicate the spatial repetitions associated with the layered structure, with the main chains occupying distinct crystalline domains. A reflection at approximately q = 1.5 Å⁻¹ or d-space = ~ 4.2 Å appears in all samples, representing the periodicity of the C16 chain groups (Figure ). 8, The same spatial spacing was observed in the PXRD analysis (Figure ), suggesting a strong self-organization interaction between these side chain groups, possibly induced by hydrophobic effects during the synthesis and post-synthetic treatments in aqueous solutions. Figure summarizes the reflections observed by SAXS for various PEI-C16 samples.