Self-Assembly of Star-Polyelectrolytes in Various Solution Conditions

The topological complexity of star-polyelectrolytes provides a larger probability for controlling self-assembly behaviors in solution to obtain plentiful morphologies. Therefore, we study the charge-driven self-assembly of oppositely charged star-polyelectrolytes with different arm numbers in solution with various ionic strengths and temperatures by performing coarse-grained molecular dynamics simulations. Morphological phase diagrams are constructed, and a rich variety of morphological structures are obtained with a sequence of unimolecular micelle, loose aggregate, amorphous aggregate, and cross-linked structure as the solution ionic strength decreases. The structural transitions of self-assembly depend on both the strength of electrostatic attraction between oppositely charged groups and long-range repulsion between the same charged groups, which increases the energy barriers for further aggregation. The competition of various interactions results in the alternating connection of polycations and polyanions. The tendency of the transition is also exhibited by increasing the arm numbers or lowering the temperatures, which increase the local charge density and reduce the thermal fluctuation, respectively. The potential of mean force analyses reveals that the complexation thermodynamics and structural transitions between opposite star-polyelectrolyte depend mainly on ionic strength. The formation pathways of amorphous and cross-linked aggregates show similar initial short worm-like structures with some cross-links; then, the different pathways are observed due to multiple free energy minima for the association of the worm-like structures. The simulation results may help to understand the self-assembly mechanism of star-polyelectrolytes in various conditions and provide useful references for fabricating target aggregate morphologies in potential applications.