Over the last decade, it has become increasingly clear that amphiphilic block copolymers composed of strongly segregating blocks will self-direct their assembly in water into the same basic aggregate structures found for small surfactants: spheres, cylinders, and vesicular membranes. [1][2][3] For each of these morphologies, the idealized packing parameter p 4 is readily determined by a comparison of the core volume per molecule to its interfacial area: p < 1/3 for spherical micelles, p ¼ 1/3 À 1/2 for cylindrical worm micelles, and p ¼ 1/2 À 1 for membrane-bound vesicles. This simple and widely used shape parameterization serves to show that worm micelles not only are an intermediate microphase but also occupy the narrowest sliver of the phase diagram: Dp sph ¼ 0.333 versus Dp wm ¼ 0.167 versus Dp ves ¼ 0.50. In other words, worm micelles not only require more carefully proportioned hydrophilic-to-hydrophobic block ratios to achieve stability but also are likely to be less stable than other morphologies. Indeed, although spherical micelles and vesicles of lipids and surfactants have been extensively characterized, there are relatively few studies and applications of lipid-or surfactantbased worm micelles, in part because stresses and thermal defects accumulate over the length of any embedded linear object and tend to disrupt it. Block copolymers lead to more robust assemblies and thus open up new opportunities.What block copolymers provide, of course, is a broadened choice of both the composition and molecular weight versus small amphiphiles. 2,5 The simplest strong segregation theory balances the interfacial tension against the chain elasticity and predicts that core diameter d of worm micelles will scale with molecular weight N of the hydrophobic block. Within experimental error, 6 the scaling iswhere a is the monomer size. To understand the implications of such scaling for chemical stability as well as mechanical stability, particularly in stirring, extrusion, and sonication, old and new theories of surfactancy and soft-matter stability can be invoked. First, recent studies of polymer vesicles and polymersomes have taught us that interfacial tension c, established at the semisolvated interface between the core and corona, is independent of N. 7 For a series of diblock copolymers composed of poly(ethylene oxide)