Simulations Of Finite Sized Planar Polyelectrolyte Brushes

As a highly promising polymer system,polyelectrolyte brushes are widely used in fields such as biology and material science.The research on polyelectrolyte brushes is of great scientific significance and application prospects.Using molecular dynamics method,the static and dynamic behavior of finite sized polyelectrolyte brushes grafted on two apposing substrates in good solvent is studied.Main contents and results are as follows:(1)the static behavior of polyelectrolyte brushesThe influence of counterion valence,grafting density and charge fraction of polyelectrolyte chains on the static behavior of polyelectrolyte brushes is studied with fixed substrate spacing.The results show that,the increase of the counterion valence leads to the collapse of the brush.Compared with monovalent counterions,the correlation between multivalent counterions and polyelectrolyte chains is more stronger.The counterion-chain correlation is the driving force of the collapse of polyelectrolyte brushes in multivalent counterion system.The increase of grafting density results in the linear increase of brush thickness,and the brush thickness increases more rapidly in trivalent counterion system.Moreover,when the charged fraction is small,the polyelectrolyte brush expands further with the increase of the charged fraction in trivalent counterion system.As the charged fraction continues to increase,the brush begins to collapse.(2)the dynamic behavior of polyelectrolyte brushesBy varying the subtrate spacing D,the dynamic behavior of polyelectrolyte brushes in trivalent counterion system is studied.Simulation results indicate that the dynamic behavior of polyelectrolyte brushes is decided by the competition between electrostatic interaction and excluded volume interaction.The bridging effect between multivalent counterions and polyelectrolyte chains grafted on different substrates plays an important role in brush thickness and the pressure.When the substrate spacing is large(32 < D/? < 44),the brush thickness reduces slightly and the pressure increases tinily with decreasing D.The two brushes do not overlap in this region,and the behavior of polyelectrolyte brushes is driven by long-range electrostatic repulsion between them.As the two brushes are close to each other(28 < D/? < 32),the brush thickness increases and the pressure decreases.By this time,the two brushes began to overlap,and the behavior of polyelectrolyte brushes is driven by the bridging effect between counterions and charged brushes.When the substrate spacing is small(10 < D/? < 28),the brush thickness reduces significantly and the pressure increases dramatically.The two brushes have a large degree of overlap in this region,and the excluded volume interaction is dominant.Compared to the infinite sized planar brush,the finite sized polyelectrolyte brush has a larger degree of freedom,which causes that the density profiles of end monomers,the brush thickness,the bridging coefficient and the pressure are different from the infinite sized system when substrate spacing is changed.The finite sized planar polyelectrolyte brush system is similar to the real system in the experiment,and the result will be instructive in some extent for the experiment study in future.