Vesicles prepared in water from a series of diblock copolymers and termed polymersomes are physically characterized. With increasing molecular weight $bar{M}_n$, the hydrophobic core thickness $d$ for the self-assembled bilayers of polyethyleneoxide - 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 $K_a$ and maximal areal strain $alpha_c$ are measured by micromanipulation. As expected for interface-dominated elasticity, $K_a$ ($simeq$ 100 $pN/nm$) is found to be independent of $bar{M}_n$. Related mean-field ideas also predict a limiting value for $alpha_c$ which is universal and about 10-fold above that typical of lipids. Experiments indeed show $alpha_c$ generally increases with $bar{M}_n$, coming close to the theoretical limit before stress relaxation is opposed by what might be chain entanglements at the highest $bar{M}_n$. The results highlight the interfacial limits of self-assemblies at the nano-scale.
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