Jump to main content
Jump to site search

Issue 4, 2016
Previous Article Next Article

Nanomechanics of layer-by-layer polyelectrolyte complexes: a manifestation of ionic cross-links and fixed charges

Author affiliations

Abstract

This study investigates the roles of two distinct features of ionically cross-linked polyelectrolyte networks – ionic cross-links and fixed charges – in determining their nanomechanical properties. The layer-by-layer assembled poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) network is used as the model material. The densities of ionic cross-links and fixed charges are modulated through solution pH and ionic strength (IS), and the swelling ratio, elastic and viscoelastic properties are quantified via an array of atomic force microscopy (AFM)-based nanomechanical tools. The roles of ionic cross-links are underscored by the distinctive elastic and viscoelastic nanomechanical characters observed here. First, as ionic cross-links are highly sensitive to solution conditions, the instantaneous modulus, E0, exhibits orders-of-magnitude changes upon pH- and IS-governed swelling, distinctive from the rubber elasticity prediction based on permanent covalent cross-links. Second, ionic cross-links can break and self-re-form, and this mechanism dominates force relaxation of PAH/PAA under a constant indentation depth. In most states, the degree of relaxation is >90%, independent of ionic cross-link density. The importance of fixed charges is highlighted by the unexpectedly more elastic nature of the network despite low ionic cross-link density at pH 2.0, IS 0.01 M. Here, the complex is a net charged, loosely cross-linked, where the degree of relaxation is attenuated to ≈50% due to increased elastic contribution arising from fixed charge-induced Donnan osmotic pressure. In addition, this study develops a new method for quantifying the thickness of highly swollen polymer hydrogel films. It also underscores important technical considerations when performing nanomechanical tests on highly rate-dependent polymer hydrogel networks. These results provide new insights into the nanomechanical characters of ionic polyelectrolyte complexes, and lay the ground for further investigation of their unique time-dependent properties.

Graphical abstract: Nanomechanics of layer-by-layer polyelectrolyte complexes: a manifestation of ionic cross-links and fixed charges

Back to tab navigation

Supplementary files

Publication details

The article was received on 09 Jun 2015, accepted on 11 Nov 2015 and first published on 11 Nov 2015


Article type: Paper
DOI: 10.1039/C5SM01430A
Author version available: Download Author version (PDF)
Citation: Soft Matter, 2016,12, 1158-1169
  •   Request permissions

    Nanomechanics of layer-by-layer polyelectrolyte complexes: a manifestation of ionic cross-links and fixed charges

    B. Han, D. R. Chery, J. Yin, X. L. Lu, D. Lee and L. Han, Soft Matter, 2016, 12, 1158
    DOI: 10.1039/C5SM01430A

Search articles by author

Spotlight

Advertisements