Phase behaviour of polyampholyte chains from grand canonical Monte Carlo simulations

Abstract

The phase behaviour of polyampholyte chains has been investigated by grand canonical Monte Carlo simulations. First, the effects of chain length and charge sequence on the critical parameters and phase diagrams of fully-charged but overall neutral polyampholytes up to a chain length of 16 beads were determined. As the chain length increases, the critical temperature increases while the critical volume fraction decreases. The critical temperatures of the diblock polyampholytes are much higher and increase more quickly with increasing chain length than the critical temperatures of random polyampholytes, consistent with current experimental observations and theoretical predictions. This difference is attributed to the localization of like charges in a diblock chain which leads to stronger attraction and aggregation between chains. Cluster size distributions also reveal a stronger tendency of diblock polyampholytes to form larger aggregates than random chains for the same conditions. The phase behaviour of end-charged polyampholytes up to a chain length of 18 beads was also investigated, and both the critical temperature and volume fraction were observed to decrease with increasing chain length. The formation of micelles was not observed up to the longest chain length considered. Instead, the chains form clusters which eventually aggregate into a percolating gel-like structure through chain bridges spanning across different clusters. The sol–gel density–temperature locus has also been determined for the longest chain length considered, and the vapour–liquid transition is found to be between two gel phases.

Acknowledgements

The authors acknowledge Dr Jianwen Jiang for helpful discussions, as well as for providing us with his unpublished numerical data for the phase behaviour of polyampholytes from his theory. Funding for this work was provided by the Department of Energy, Office of Basic Energy Sciences (DE-FG201ER15121). Additional support was provided by ACS-PRF (Grant No. 38165-AC9).