Size selection and stability of thick-walled vesicles

TL;DR

This paper uses mean-field theory to analyze the size selection and stability of thick-walled vesicles formed from copolymers, revealing preferred sizes and conditions for vesicle instability.

Contribution

It provides a theoretical model that explains vesicle size selection and predicts instability conditions based on copolymer composition and interactions.

Findings

Vesicles have a preferred size at a minimum free energy when N_B is several times larger than N_A.

Vesicle radius diverges near a critical N_B, indicating instability.

Changing solvent interaction strength can trigger vesicle instability.

Abstract

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.