Effect of Charge Density on the Viscoelasticity and Underwater Adhesion of Entangled Complex Coacervates from Semi-Rigid Polysaccharides

Abstract

We analyzed the effect of charge density (CD) on the phase behavior, viscoelasticity, and underwater adhesion of complex coacervates formed from high-molecular-weight, semi-flexible, bio-sourced polyelectrolytes. For this, hyaluronic acid (HA) was complexed with chitosan (CHI) of different degrees of deacetylation (DD) at pH 5. It was found that increasing CHI deacetylation enhanced macroion pairing, expanding the two-phase region of the phase diagram. Time-Salt Superposition (TSS) was successfully applied, allowing to rescale the linear viscoelastic response of all the HA-CHI series onto individual master curves, indicating that the relaxation dynamics of all series are controlled by macroion pairing. The TSS curves were further collapsed onto a universal master curve via a so-called time-salt-charge density superposition (TSCDS). This first report of TSCDS for entangled complex coacervates revealed that the salt sensitivity of the dynamics depends on the charge density, which is in contrast with reports on flexible polyelectrolytes. It is proposed that this difference is due to the interplay between the persistence length of the semi-flexible polyelectrolytes and kinetic trapping in these entangled systems. The underwater adhesion strength (σ_max) of HA-CHI reached 74 kPa at 0.2 M NaCl. Replacing CHI’s acetyl moiety with a less polar butyryl group (but-CHI) at a given DD had a slight effect on the composition and viscoelastic properties. However, HA-but-CHI had the highest underwater adhesion near physiological salinity (σ_max of 110 kPa and an adhesion energy of 18 J·m⁻²), placing it among the most performant coacervate-based underwater adhesives without an external trigger.