The effect of ion pairs on coacervate-driven self-assembly of block polyelectrolytes

The incorporation of oppositely charged polyelectrolytes into a block copolymer system can lead to formation of microphase separated nanostructures driven by the electrostatic complex between two oppositely charged blocks. It is a theoretical challenge to build an appropriate model to handle such coacervate-driven self-assembly, which should capture the strong electrostatic correlations for highly charged polymers. In this paper, we develop the self-consistent field theory considering the ion paring effect to predict the phase behavior of block polyelectrolytes. In our model, two types of ion pairs, the binding between two oppositely charged monomers and the binding between charged monomers and counterions, are included. Their strength of formation is controlled by two parameters K-aa and K-ac, respectively. We give a detailed analysis about how the binding strength K-ac and K-aa and salt concentration affect the self-assembled nanostructure of diblock polyelectrolyte systems. The results show that the binding between two oppositely charged blocks provides driven force for microphase separation, while the binding between charged monomers and counterions competes with the polyion pairing and thus suppresses the microphase separation. The addition of salt has a shielding effect on the charges of polymers, which is a disadvantage to microphase separation. The phase diagrams as a function of polymer concentration and salt concentration at different situations are constructed, and the influence of K-aa, K-ac, and charged block composition f(a) is analyzed in depth. The obtained phase diagrams are in good agreement with currently existing experimental and theoretical results.

Automated summary

This study investigates the self-assembly process of incorporating oppositely charged polyelectrolytes into a block copolymer system, predicting phase behavior through self-consistent field theory considering the ion pairing effect. The results show that the pairing between oppositely charged blocks drives microphase separation, while pairing between charged monomers and counterions competes and suppresses this phenomenon. The addition of salt shields polymer charges, hindering microphase separation.