The cryo-ET profile in Figure also shows a shoulder forming on the outer edges of these peaks at approximately ± 80 Å. Although the position of this shoulder does not align with the outer peak at ± 59 Å in the SPA profile, Figure , this outer peak in the SPA profile is directly between the peak and shoulder positions in the cryo-ET profile. The same observation can be made when comparing the SAXS profile with the cryo-ET profile, where the SAXS peak position at ± 54 Å is directly between the peak and shoulder in the cryo-ET profile. As mentioned, the 4-Gaussian model, used here for 100 mol% PBd22-PEO14 membranes, does not account for these two additional positive electron density contributions in this structural region, which is why this particular feature cannot be reproduced, and may require application of an asymmetric membrane model. Comparing the SPA profile of thick membranes, these outer peaks have a greater intensity in 100 mol% PBd22PEO14 membranes than in thick 50 mol% PBd22PEO14 membranes. The lower intensity of the outer peaks in the thick 50 mol% PBd22PEO14 membrane profile is consistent with there being less polymer in these membranes, than the 100% polymer membranes. The inner peaks of the SPA profile in Figure could represent the hydrophobic/hydrophilic interface between the hydrophobic polybutadiene (PBd)