Molecular Weight Dependence of Polymersome Membrane Structure, Elasticity, and Stability

Vesicles prepared in water from a series of diblock copolymers “polymersomes” are physically characterized and compared to lipid vesicles. With increasing molecular weight M̄n, the hydrophobic core thickness d for the self-assembled bilayers of poly(ethylene oxide)-polybutadiene (PEO-PBD) increases up to ≃20 nm considerably greater than any previously studied lipid system. The mechanical responses of these membranes, specifically, the area elastic modulus Ka and maximal areal strain αc are measured by micromanipulation. As expected for interface-dominated elasticity, Ka (≃100 pN/nm) is found to be independent of M̄n, but lower than the usual values for zwitterionic lipid membranes. Experiments on polymersomes show αc increases in a nearly linear fashion with M̄n, approaching a limiting value predicted by mean-field ideas which is universal and about 10-fold above that typical of lipids. Nonlinear responses and memory effects generally emerge with increasing M̄n, indicating the onset of chain entanglements at higher M̄n. The effects of M̄n thus suggest a compromise between stability and fluidity for biomembranes. More generally, the results highlight the interfacial limits of self-assemblies at the nanoscale.