Electroresponsive Polyelectrolyte Brushes Studied by Self-Consistent Field Theory.

Boris M Okrugin,Oleg V Borisov,Ekaterina B Zhulina,Igor M Neelov,Frans A M Leermakers,Ralf P Richter

doi:10.3390/polym12040898

Abstract

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.

Highlights

Layers of ionically charged macromolecules tethered by terminal segments onto solid-liquid interfaces find multiple technological applications because they allow for efficient controlling of adhesive, tribological andinteractive properties of surfaces operating in aqueous environment [1,2,3,4].Because of the presence of ionically charged monomer units, polyelectrolyte brushes are capable to undergo drastic conformational changes in response to variation in environmental conditions, such as solution pH or ionic strength
The theory implements the strong-stretching approximation to account for conformational entropy of the brush-forming chains introduced earlier by Semenov [33] combined with the Poisson-Boltzmann framework to describe Coulomb interactions between ionic species
As long as chain ends are distributed throughout the brush, which is the case for planar brushes of monodisperse linear chains in the absence of external fields, the self-consistent molecular potential acting on a monomer unit has the parabolic shape, U (x)

Summary

Introduction

Layers of ionically charged macromolecules (polyelectrolytes) tethered by terminal segments onto solid-liquid interfaces (so-called polyelectrolyte brushes) find multiple technological applications because they allow for efficient controlling of adhesive (wettability), tribological and (bio)interactive properties of surfaces operating in aqueous environment [1,2,3,4]. Because of the presence of ionically charged monomer units, polyelectrolyte brushes are capable to undergo drastic conformational changes in response to variation in environmental conditions, such as solution pH or ionic strength. Modification of solid surfaces exposed to liquid (aqueous) environment by layers of tethered polyelectrolytes is considered as a promising way for the fabrication of smart, stimuli-responsive materials. Polymers 2020, 12, 898 brushes and their response to external chemical stimuli were rationalized on the basis of the existing theories [6,7,8,9,10,11,12,13,14,15,16,17,18,19].

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