To investigate the crystallinity of PEI and PEI-C16 powder, we employed PXRD analysis to corroborate the presence of regions of coherent diffraction within the samples. PEI (after drying the commercial aqueous solution of PEI, i.e., PEI in Figure , black) shows an amorphous state with a broad diffraction peak around 2θ ≤ 20°, resembling the liquid state with no ordered structure. Upon alkylation, the polymers (PEI-C16-33% and PEI-C16-20%) exhibited a sharp diffraction peak at 2θ ≤ 21.39° with a 4.2 Å d-spacing (Figures 2A). It indicated the formation of coherently diffracting regions, e.g., crystallites or domains of hexadecyl (C16) chains within the polymer, potentially due to the hydrophobic interaction between two C16 chains (Figure ). Further analysis of PEI-C16-33% using inelastic neutron spectroscopy (INS) coupled with DFT calculations revealed that C16 hydrophobic chain interactions may exist as comb- like domain structures within alkylated PEI (Figures and). The theoretical vibrational bands conformed with the experimental spectra. The prominent bands corresponding to symmetric and asymmetric C-H stretching were distinctly observable at ~ 2900 cm⁻¹ in the experimental spectra, accompanied by an N-H stretching mode at ~ 3100 cm⁻¹. The strong intensity in the 1000-1700 cm -1 frequency range can be ascribed to vibrations consisting of coupled angular deformations involving hydrogens in the C16 chains (CH2 twisting, CH2 wagging, and CH2-CH3 scissoring, Figure and Table ). Additionally, vibrations between 700 and 1000 cm -1 can be associated with CH2 -rocking of the C16 chains. Low-frequency modes from 150 to 700 cm -1 were assigned to CH2 -torsions of the C16 chains, CH3 torsions, and C-C scissoring, thus causing displacements between the carbons. The signal in this region is mainly due to the hydrogens that participated in these skeletal displacements. The sharp peaks at 187 cm -1 (PEI-C16-33%) and 252 cm -1 (mode 1, DFT simulated, gray) were due to methylene (CH2) torsions. Table provides the attribution of the primary hydrogen vibrations calculated by DFT and observed through INS. Similar observations were made for all selected modes, and differences of up to 90 cm -1 were observed between the experimental and theoretical peaks, except for mode 4, which has a difference of 113 cm -1 . In the DFT calculations, the proposed model considered two units of PEI with their C16 chains, thus representing a condition with a high degree of ordering and rigidity. Consequently, the theoretical and experimental spectra (~ 2900 cm -1 ) agreed excellently, owing to the nature of these vibrational peaks. However, a slight discrepancy existed between the experimental and calculated modes for regions of low wavenumbers. This discrepancy was more profound at ~ 1500 cm -1 , as these vibrations are related to the sample's stiffness. These findings agreed with 1 H NMR analyses of PEI-C16-33% . The coherent length of molecularly ordered regions and a possible comb-like orientation of C16 chains may be responsible for the integrity of alkylated PEIs, enabling their applications in CO2-mediated desalination. Interestingly, these crystalline domains remained stable in the presence and absence of CO2 and aqueous solutions (Figure ). PEI-C16 has low water solubility, which is crucial to its application in desalination, ensuring high performance and easy recovery post-desalination. Therefore, we decided to follow up on the crystalline domain step-by-step by mimicking the desalination procedure (Scheme S1C) and analyzing their PXRD patterns.