Electroresponsive Polyelectrolyte Brushes Studied by Self-Consistent Field Theory

End-grafting of polyelectrolyte chains to conducting substrates offers an opportunity to fabricate electro-responsive surfaces capable of changing their physical/chemical properties (adhesion, wettability) in response to applied electrical voltage. We use a self-consistent field numerical approach to compare the equilibrium properties of tethered strong and weak (pH-sensitive) polyelectrolytes to applied electrical field in both salt-free and salt-containing solutions. We demonstrate that both strong and weak polyelectrolyte brushes exhibit segregation of polyions in two populations if the surface is oppositely charged with respect to the brush. This segregation gives rise to complex patterns in the dependence of the brush thickness on salt concentration. We demonstrate that adjustable ionization of weak polyelectrolytes weakens their conformational response in terms of the dependence of brush thickness on the amplitude of the applied voltage

Structure and properties of polydisperse polyelectrolyte brushes studied by self-consistent field theory.

Two complementary self-consistent field theoretical approaches are used to analyze the equilibrium structure of binary and ternary brushes of polyions with different degrees of polymerization. Stratification in binary brushes is predicted: the shorter chains are entirely embedded in the proximal sublayer depleted of end-points of longer chains while the peripheral sublayer contains exclusively terminal segments of longer chains. The boundary between sublayers is enriched with counterions that neutralize the residual charge of the proximal sublayer. These analytical predictions for binary brushes are confirmed and extended to ternary brushes using the numerical Scheutjens-Fleer approach

Brownian dynamics simulation of linear polymers under elongational flow: Bead–rod model with hydrodynamic interactions

Brownian dynamics (BD) simulations of a linear freely jointed bead–rod polymer chain with excluded volume (EV) interaction have been performed under elongational flow with and without the use of fluctuating hydrodynamic interactions (HI). The dependence of the chain size, shape and intrinsic elongational viscosity on the elongational rate are reported. A sharp coil–stretch transition is observed when exceeds a critical value, c. The inclusion of the HI leads to a shift in the coil–stretch transition to higher flow values. Chain deformation due to elongational flow is observed to first consist of the alignment of the chain with the direction of flow without significant chain extension followed by additional alignment of the bond vectors with the flow direction and chain extension as flow rate is increased further. The distribution function for the chain's radius of gyration becomes significantly broader within the transition region which implies an increase in fluctuations in the chain size in this region. The structure factors parallel and perpendicular to the flow direction illustrate different elongational rate dependencies. At high rates, the structure factor in the direction of the flow exhibits an oscillating dependence which corresponds to the theoretically predicted shape for a rigid-rod model. The mean squared orientation of each bond within the chain with respect to the flow direction as function of bond number is nearly parabolic in shape with the highest degree of orientation found within the chain's interior. The dependence of the critical elongational rate, c, on the chain length, N, is observed to be c~N–1.96 when hydrodynamic interactions are not employed and c~N–1.55 when they are invoked. These scaling exponents agree well with those obtained in previous BD simulations of bead-FENE (i.e., finitely extensible nonlinear elastic) spring chains as well as with the theoretical predictions of c~N–2 and c~N–1.5 without and with hydrodynamic interactions based on the Rouse and Zimm models, respectivel