The SPA profile of 100 mol% PBd22-PEO14 in Figure also shows a small electron dense peak within the centre of the trough region. Comparing the SPA profile of thick membranes, this central peak has a greater intensity in 100 mol% PBd22PEO14 membranes than in thick 50 mol% PBd22PEO14 membranes. As this peak appears indicative of interdigitation within the membranes, perhaps the low peak intensity seen in the thick 50 mol% PBd22PEO14 membrane profile is due to an increased free volume in the hydrophobic core of these membranes due to the presence of shorter lipid molecules, reducing the extent of polymer interdigitation in the membrane mid-plane.