Morphological transformations of diblock copolymers in binary solvents: A simulation study

Abstract

Morphological transformations of amphiphilic AB diblock copolymers in mixtures of a common solvent (S1) and a selective solvent (S2) for the B block are studied using the simulated annealing method. We focus on the morphological transformation depending on the fraction of the selective solvent C_S2, the concentration of the polymer C_p, and the polymer–solvent interactions ε_ij (i = A, B; j = S1, S2). Morphology diagrams are constructed as functions of C_p, C_S2, and/or ε_AS2. The copolymer morphological sequence from dissolved→sphere→rod→ring/cage→vesicle is obtained upon increasing C_S2 at a fixed C_p. This morphology sequence is consistent with previous experimental observations. It is found that the selectivity of the selective solvent affects the self-assembled microstructure significantly. In particular, when the interaction ε_BS2 is negative, aggregates of stacked lamellae dominate the diagram. The mechanisms of aggregate transformation and the formation of stacked lamellar aggregates are discussed by analyzing variations of the average contact numbers of the A or B monomers with monomers and with molecules of the two types of solvent, as well as the mean square end-to-end distances of chains. It is found that the basic morphological sequence of spheres to rods to vesicles and the stacked lamellar aggregates result from competition between the interfacial energy and the chain conformational entropy. Analysis of the vesicle structure reveals that the vesicle size increases with increasing C_p or with decreasing C_S2, but remains almost unchanged with variations in ε_AS2.

Keywords

self-assembly / diblock copolymers / binary solvents / morphological transformation / simulated annealing

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