Poly( <i>o</i> -phenylenediamine) as an Organic Filler for Enhancing the Mechanical and Antibacterial Performance of Chitosan Films

Understanding structure-property relationships is essential for designing multifunctional biopolymer composites that integrate mechanical robustness, barrier performance, and antimicrobial activity in sustainable materials. Chitosan (CS) exhibits excessive hydrophilicity, limited mechanical strength, and poor moisture stability, which restrict its long-term performance in packaging applications. With the aim to enhance the mechanical strength, moisture absorption, and overall performance of CS, an organic aromatic polymer, poly(<i>o</i>-phenylenediamine) (PoPD), was introduced into the matrix through in situ oxidative polymerization. Incorporation of PoPD improved the properties of CS by introducing aromaticity and electron delocalization, thereby limiting water uptake and molecular diffusion without relying on petroleum-derived additives. Remarkably, a low filler concentration (0.15 wt % of PoPD) produced drastic enhancement, in a tensile strength of 27.98 ± 1.40 MPa (as compared to 9.28 ± 0.46 MPa in neat CS) and an elongation at break value of 5.44 ± 0.27%. Moisture absorption studies confirmed a marked reduction at low filler levels, whereas higher PoPD contents generated compact morphologies that further restricted diffusion. Antibacterial evaluations revealed pronounced inhibition of <i>Bacillus subtilis</i> across all filler concentrations. Molecular docking analyses attributed this behavior to π-π-stacking, hydrogen bonding, and electrostatic interactions between PoPD and bacterial residues. The properties can be tuned by adjusting the filler content, producing multifunctional composites suitable for smart, sustainable packaging applications.