As the PBd22-PEO14 mole fraction increases, the membrane thickness increases (Figure ). A large increase in membrane thickness (from 57 to 78 Å and 83 to 97 Å) and hydrophobic core (from 36 to 50 Å and 53 to 66 Å) is observed between 25 and 50 mol% and 75 and 100 mol% PBd22-PEO14, respectively. From 0 to 25 mol% PBd22-PEO14 samples, we observe only a small increase in membrane thickness, suggesting that at low ratios, the polymer largely adapts to the intrinsic lipid bilayer thickness. In hybrid vesicles with >25 mol% PBd22-PEO14, the lipids instead appear to adapt to the thicker polymer membrane structure. 50 and 75 mol% PBd22-PEO14 compositions have a broad distribution in membrane thickness measurements, with some thicknesses comparable with that of a lipid membrane, and others close to that of a polymer membrane, as shown in Figure . The hydrophobic core thickness (peak-to-peak measurements) follows the same general trend as the membrane thickness: there is an increase in hydrophobic core thickness as the polymer fraction increases (see SI Section 2). The hydrophobic core measurement delivers particularly useful information, when setting up an appropriate membrane model for fitting the SAXS pattern (see below).