Non-additive impacts of covalent cross-linking on the viscoelastic nanomechanics of ionic polyelectrolyte complexes

This study elucidates the influences of adding covalent cross-linking on the nanomechanical viscoelasticity of ionically cross-linked polyelectrolyte networks. Using layer-by-layer (LbL) assembled PAH/PAA networks, we report how adding covalent amide cross-links changes the degree of swelling, indentation modulus, and force relaxation behaviors with varying solution conditions such as pH and ionic strength, which modulate the ionic cross-link density and fixed charge density. The addition of covalent cross-linking increases both the modulus and degree of elasticity through providing permanent anchorage to the ionically linked networks. Such addition also strongly increases the relaxation times at net neutral, less swollen states. These results together underscore the synergistic interplay between the covalent and ionic cross-links in the viscoelastic nanomechanics of polyelectrolyte networks, potentially enabling the use of these stimulus-responsive materials in mechano-sensitive biomedical and engineering applications.