Spontaneous Curvature of Mixed-Copolymer Bilayers

We investigate the effect of mixing two diblock copolymers of identical chemical structure but different molecular weight on self-assembled bilayers, using theoretical concepts of curvature elastic energy. Entropy and chain-chain interactions are found to favour ideal mixing in the two monolayers composing the bilayer, while curvature energy favours an inhomogeneous distribution of chains among the layers and, therefore, vesicle formation. The vesicle phase boundaries are determined as a function of mixture composition and molecular weight. We find that adding a small fraction of shorter chains to a copolymer bilayer is much more effective in destabilizing lamellar bilayers than the addition of a small fraction of longer chains.