The important role of cosolvent in the amphiphilic diblock copolymer self-assembly process

Abstract

A common strategy to obtain self-assembly structures of amphiphilic block copolymers resorts to using solvent mixtures in which one solvent is a selective solvent while the other is a cosolvent for both components of the diblock copolymer. In this study, the noticeable influence of cosolvent on determining the block copolymer self-assembly structures as well as the corresponding kinetic pathways is studied via dissipative particle dynamics simulations. The preferential adsorption of cosolvent in polymer aggregate is found to alter the local solvent environment, which then results in various self-assembly structures in cases of different contents of cosolvent in the system. Based on Flory-Huggins theory, the preferential adsorption of cosolvent in different domains is analyzed to corroborate the influence of cosolvent on the self-assembly structures. Taking vesicles with similar size and shape as examples, we find that the preferential adsorption of cosolvent also affects their formation kinetic pathways. If there is less cosolvent in local environment of the polymer aggregate, the vesicle formation takes spherical micelle–wormlike micelle–membrane–vesicle pathway, in which the characteristic step is the bending and close of the membrane to form the vesicle. If there is more cosolvent in local environment of the polymer aggregate, the small micelles aggregate continuously till the vesicle forms. By comparing the simulations taking two solvent components explicitly with those using one solvent that has averaged solvation capability based on the two solvent components, we find the second simulation strategy may result in incorrect equilibrium self-assembly structures.