  In recent experiments, small, thick-walled vesicles with a narrow size
distribution were formed from copolymers where the degree of polymerisation of
the hydrophobic block, N_B, was significantly greater than that of the
hydrophilic block, N_A. Using a mean-field theory, we reproduce several aspects
of the behaviour of these vesicles. Firstly, we find a minimum in the free
energy of the system of vesicles as a function of their radius, corresponding
to a preferred size for the vesicles, when N_B is several times larger than
N_A. Furthermore, the vesicle radius diverges as N_B is increased towards a
critical value, consistent with the instability of the vesicles with respect to
further aggregation seen in the experimental work. Finally, we find that this
instability can also be triggered in our model by changing the interaction
strength of the copolymers with the solvent.
