Both lipids and amphiphilic block copolymers are now well known for assembling into vesicle and worm-like micelle morphologies, but only mixtures of lipids in vesicles have been directly seen to phase separate into meso-scale lateral domains. Here we visualize meso-scale spots in tough polymersomes and also micron-length stripes in stable worms that result from strong lateral segregation (SLS) of polyanionic and neutral diblock copolymers. With pure charged polymersomes, we demonstrate a fluid-to-solid transition that scales weakly across the pKa with effective rigidication scaling as ~[H⁺]^-0.36 and chain immobilization as ~[H⁺]^-0.9, whereas calcium rigidifies more strongly as ~[Ca²⁺]^2.5. The pure system studies suggest that SLS in mixed copolymer systems occurs for liquid-like, nascent gel states. The systematic phase diagram that is mapped out for domain formation demonstrates with increasing pH and comparatively small decreases in calcium a progression from budding on the vesicle surface to spotted vesicles followed by worm-attached vesicles, or ‘squids', and finally striped worm-like micelles. We present a model for understanding the crucial role of Ca²⁺ ions on segregation behavior, which incorporates counterion condensation and "crosslinking" (ion bridging). We find a tendency towards segregation near the isoelectric point as a result of competition among counterion entropy, repulsion due to the net charge, and attraction due to crosslinking.

These results portend new classes of robust membranes and cylinders that exhibit lateral patterns on a meso-scale.