Surface switching of mixed polyelectrolyte brushes made of 4-arm stars and linear chains: MD simulations

Using Langevin dynamics simulation, we study the surface switching properties of mixed polyelectrolyte brushes made of 4-arm stars and linear chains. The length and average charge fraction of the linear chains, Bjerrum length (a characteristic length to quantify the strength of the electrostatic interaction), the solvent quality as well as external electric fields were used as controlling parameters to induce sharp transitions of molecular conformations, leading to brush surfaces dominated by end monomers of linear chains or stars. Phase diagrams in terms of the length and charge fraction of linear chains demarcating different regimes of the composition of brush surfaces were constructed under different external electric fields. Besides the different regimes in the phase diagram of mixed brushes under a stretching electric field or in the absence of electric fields, a new regime, which is featured by the linear chains with very low charge fraction sandwiched between the collapsed and un-collapsed stars, emerges in the phase diagram under a collapsing electric field. The stratification within the brush layer of the mixed brushes under external electric fields was also investigated.