pH-dependent structural and mechanical properties in chitosan and its stabilization by oxidized chitin doping

Chitin is the second most abundant biopolymer on Earth, yet its applications largely depend on its conversion into chitosan (CS). However, due to its pK_a of 6.3, CS becomes positively charged under mildly acidic conditions, weakening intermolecular interactions and leading to dissolution. Here, we investigated the structural and mechanical differences between CS films in acidic and neutral states, and examined the behavior of composites with C6-oxidized chitin (OC). The novelty of this study lies in stabilizing CS over an extended pH range through ionic bridges with OC, a polymeric polyanion derived from the same biopolymer source. This approach preserves the mechanical properties and pH-dependent swelling of CS without covalent cross-linking and ensures chemical and biological homogeneity within the system. In all materials studied, film neutralization induced stabilization of the polymer backbone and structural reorganization, particularly involving the acetamide groups, with changes in crystallinity along the b-axis. In pure CS, these effects enhanced mechanical strength (6-fold increment in stiffness and maximum stress) and reduced water uptake. Incorporating 15 wt% OC prevented CS dissolution at pH 3, stabilized its mechanical properties as in the neutral state, while retaining acidic swelling behavior (3 times higher at pH 3). Conversely, adding 15 wt% CS to OC reduced its brittleness, improved processability, and yielded a material with increased swelling at neutral pH (2 times higher). This study provides new insights into CS pH-dependent behavior and introduces two chitin-derived materials with complementary pH-responsive properties and extended pH stability, expanding opportunities in biomedicine, agrochemistry, and sustainable packaging.