Morphological Transformations of Diblock Copolymers in Binary Solvents: A Simulation Study

TL;DR

This simulation study explores how amphiphilic diblock copolymers change their morphology in binary solvents, revealing a sequence from dissolved to vesicle structures influenced by solvent composition and interactions.

Contribution

It provides detailed simulation-based insights into the morphological transformations of diblock copolymers in mixed solvents, highlighting the effects of solvent selectivity and interactions.

Findings

Morphological sequence from dissolved to vesicle observed with increasing selective solvent.

Selectivity of the solvent significantly influences the microstructure formation.

Vesicle size depends on polymer concentration and solvent composition, but not strongly on interaction parameters.

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 CS2, the concentration of the polymer CP, and the polymer-solvent interactions {\epsilon}ij (i = A, B; j = S1, S2). Morphology diagrams are constructed as functions of CP, CS2, and/or {\epsilon}AS2. The copolymer morphological sequence from dissolved -> sphere -> rod-> ring/cage-> vesicle is obtained upon increasing CS2 at a fixed CP. 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 {\epsilon}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 CP or with decreasing CS2, but remains almost unchanged with variations in {\epsilon}AS2.