RHEOLOGICAL PROPERTIES BASED OF NA-CMC AND CHITOSAN POLYELECTROLYTE COMPLEXES

Abstract

 Complex coacervate hydrogels formed by sodium carboxymethyl cellulose (Na-CMC) and chitosan were prepared and analyzed for rheological behavior over broad ranges of concentration (1–4% w/v), pH (7–18), and temperature (30–60 °C). Viscosity measurements under steady shear revealed pronounced non-Newtonian (shear-thinning) flow behavior for all samples, with higher polymer concentrations yielding dramatically increased zero-shear viscosity and stronger shear-thinning character. Oscillatory tests showed that low concentrations (∼1%) behaved as viscous liquids (loss modulus G'' exceeding storage modulus G'), whereas at 4% a weak gel-like network formed with G' approaching or exceeding G'' at higher frequencies. The rheological properties were highly sensitive to pH: near-neutral pH enabled electrostatic intermolecular associations between the anionic Na-CMC and cationic chitosan, resulting in maximum viscosity and viscoelastic moduli. Increasing pH beyond ~8 caused a sharp drop in viscosity and G', as chitosan became deprotonated and the polyelectrolyte complex disintegrated, leaving a solution with properties approaching those of Na-CMC alone. Temperature elevations from 30 to 60 °C reduced solution viscosity by roughly 2–3 and slightly decreased viscoelastic moduli, consistent with thermal disruption of polymer interactions and decreased solvent viscosity. These results illustrate how polymer concentration, pH, and temperature co-modulate the flow and viscoelastic properties of Na-CMC/chitosan complexes. Mechanistically, the strong pH-responsiveness is attributed to the polyelectrolyte nature of the constituents: only within a certain pH window are both polymers sufficiently charged to form extended ionically-crosslinked networks. The findings deepen our understanding of biopolymer complexation and provide quantitative guidance for tuning the rheology of Na-CMC/chitosan systems in applications ranging from injectable hydrogels and controlled drug release matrices to sustainable thickening agents.