Ionization-Induced Network Evolution in Chitosan-g-Poly(Acrylic Acid) Superabaorbent: Correlating Degree of Saponification with Swelling and Viscoelastic Transition

The structural evolution of chitosan-g-poly(acrylic acid) (Cs-g-PAA) superabsorbents induced by controlled saponification was systematically investigated to clarify the relationship between ionization degree, swelling behavior, and viscoelastic response. Post-synthetic treatment with NaOH solutions (0.1 N, 0.5 N, and 1 N) was employed to regulate the degree of saponification while preserving the original covalent network structure. The resulting materials were characterized by swelling analysis, kinetic modeling, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and oscillatory rheology. A non-monotonic structure–property relationship was observed. The sample treated with 0.5 N NaOH exhibited the highest swelling ratio (767 g/g) and an optimal degree of saponification (∼75%), together with enhanced viscoelastic properties (G′ = 479.57 Pa; G″ = 351.29 Pa). Spectroscopic and morphological analyses confirmed effective carboxylate formation and development of a well-defined porous network. Loss tangent analysis (tan δ ≈ 1) indicated a balanced viscoelastic state. In contrast, insufficient (0.1 N) or excessive (1 N) saponification produced viscosity-dominant (tan δ > 1.6) or rigidity-dominant (tan δ < 0.3) networks, respectively, both exhibiting reduced swelling performance. These findings demonstrate that controlled saponification governs ionization-driven network reorganization and modulates swelling and viscoelastic behavior in grafted polyelectrolyte systems.