Self-assembly of gradient copolymers in a selective solvent. New structures and comparison with diblock and statistical copolymers

Abstract

We perform comparative studies of self-assembly of equivalent statistical, diblock and gradient AB copolymers in a selective solvent using dissipative particle dynamics simulations. Both effects of the fraction of soluble and insoluble groups, selectivity of the solvent and degree of immiscibility of A and B groups on self-assembled structures are studied. With a simplified model of the statistical copolymer we predict macroscopic phase segregation, when non-aggregated chains coexist with precipitate. The later can be dense or swollen (physical gel) depending on degree of immiscibility of the A and B groups. For the case of diblock copolymer solutions, we predict thermodynamic stability of few structures: spherical and worm-like micelles, rings, vesicles and precipitate. We develop a simple theory, which allows calculating free energies of worms and rings (closed worms) and explaining transition between the structures. A number of diagrams of states is constructed. In case of gradient copolymers, two scenarios of self-assembly are predicted. The spherical micelles of gradient copolymer can aggregate with each other forming worms, rings and vesicles through aggregation of coronae (so-called sticky corona), which contain insoluble groups, if the fraction of insoluble groups per chain is not high. Herewith, the core of the assembled structures remains multidomain despite a high interfacial tension. The second (“diblock-like”) scenario is realized when the fraction of insoluble groups is high enough. In this case the core of worms, rings and vesicles is practically a monodomain, however, solvophilic groups can be intercalated into the core due to the primary structure of the copolymer. A new structure of a multi-compartment vesicle on the basis of gradient copolymers is predicted.