Assessing the dynamics of symmetric and asymmetric hyaluronic acid–chitosan complex coacervates

Abstract

The mechanics of complex coacervates are governed not only by electrostatic interactions but also by polymer molecular weight (M_w) and chain stiffness, yet their combined role remains insufficiently resolved for biopolyelectrolytes. We investigate hyaluronic acid (HA)–chitosan (CHI) coacervates across nine symmetric and asymmetric M_w pairings, using rheology, van Gurp–Palmen analysis, and probe-tack adhesion to map salt responsiveness. At 0 M NaCl, when charges are not screened, entangled systems display solid-like behavior, while an unentangled pair behaves as a viscoelastic liquid. All systems show two regimes of plateau modulus (G⁰_N): a gradual decrease at low salt content, followed by an abrupt drop beyond a critical salt concentration. Symmetric systems consistently display pronounced modulus decay, whereas asymmetric systems show contrasting responses: long, flexible CHI amplifies salt-induced weakening, while stiff, hydrated HA confers greater resistance and slows modulus reduction. Adhesion measurements reflect the same molecular determinants, showing that chain flexibility enhances interfacial energy dissipation at low salt, whereas chain stiffness promotes adhesion retention at elevated ionic strength. These findings highlight M_w and stiffness, alongside charge density, as central design parameters for coacervates with programmable resilience and adhesion in underwater and biomedical settings.