We modulate precisely the surface morphology of polymer microcapsules containing perfluorooctyl bromide prepared by the solvent-evaporation technique by varying the percentage of PLGA-PEG copolymer in the formulation. As copolymer percentage increases, protrusions start to appear, become more numerous and finally fuse to yield sponge-like capsules. It has been postulated that the shell morphology arises from an interfacial tension instability. We show here that if an interfacial tension instability develops, it happens during the final step of polymer desolvation, when the interface is no longer fluid and interfacial tension changes cannot be monitored. The formation of protrusions occurs in order to accommodate all the PEG chains of the copolymer in a hydrophilic environment, since PEG and PLGA moieties are not miscible in the solid state.