Salt Softening of Polyelectrolyte

Salt softening of polyelectrolyte multilayers.

Heuvingh J., Zappa M., Fery A.

Max-Planck Institute for Colloids and Interfaces, Department Interfaces, Am Muhlenberg, D14424 Potsdam, Germany. Julien.heuvingh@mpikg-golm.mg.de marco.zappa@mpikg-golm.mpg.de andreas.fery@mpikg-golm.mpg.de

Abstract

Mechanical properties of polyelectrolyte multilayers immersed in salt solutions were investigated by measuring the elastic response to deformation of multilayer capsules with a colloidal probe Atomic Force polyelectrolyte Microscope.

Poly(styrenesulfonate)/ Poly(allylaminehydrochloride) capsules softened when immersed in a NaCl solution in concentration above 3M, but kept their original mechanical properties when immersed in a NaCl solution below 2.7M. These changes were not due to the dissolution of the multilayers, as capsules were stable for weeks in 5M NaCl. The Young modulus of the polyelectrolyte multilayer decreased from a value of 2.1 GPa in water to 0.22GPa in 4.1M NaCl. This softening is attributed to the reduction of ionic links between oppositely charged polyelectrolytes upon adding small salt ions as competitors.

Introduction

The formation of thin polymer films by layer-by-layer deposition of polyelectrolytes has received great interest in the last ten years as a highly versatile method for producing thin films with well defined thickness from a broad range of materials with little requirements

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for the substrate.1-3 The process is based on the fact that adsorption of a polyelectrolyte on a charged surface can result in surface charge reversal. Thus, a coated surface can serve as a substrate for the adsorption of an oppositely charged polyelectrolyte and for many systems the process of alternating adsorption can be continued to any desired layer number. Due to the self-repulsion of the charged polymers, the thickness increase per adsorbed layer is typically in the nanometer range and well defined. This versatility is expected to lead to promising...