Molecular Insights into Miscibility and Mechanical Modulation in PHBV/Chitosan Blends via Atomistic Simulations

Chitosan (CS) is a biodegradable, biocompatible, and low-cost biopolymer with broad applications; however, its intrinsic backbone rigidity severely limits its processability. While small-molecule plasticizers have been explored to enhance the flexibility of CS, polymeric blending strategies using bio-based components remain less understood at the molecular scale. In particular, the role of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biodegradable biopolymer, as a blending modifier in PHBV/CS blends remains insufficiently understood, particularly at the molecular level. In this study, we employ atomistic molecular dynamics simulations to systematically investigate the miscibility behavior and mechanical modulation mechanisms of PHBV/CS blends. Molecular-level miscibility of CS and PHBV is quantified using the Flory-Huggins interaction parameter across a series of blend compositions at both 300 K and 500 K, revealing favorable miscibility at both temperature regimes, particularly at low and high PHBV volume ratios. Steered molecular dynamics simulations demonstrate that CS chains possess substantially higher backbone stiffness and torsional resistance compared to PHBV, underscoring the mechanical modulation potential of PHBV. Two representative compositions (10:90 and 90:10 PHBV/CS) are further examined to evaluate chain mobility, mechanical response, and conformational energetics. Results show that incorporation of PHBV enhances the mobility and ductility of CS under tensile deformation, while CS imposes modest confinement on the dynamics of PHBV. Conformational energetic analysis further confirms that PHBV facilitates conformational transitions in CS chains, lowering the energetic barriers for deformation. Notably, these softening and mobility-enhancing effects persist in initially phase-separated morphology. Overall, our findings provide molecular-level insights into how contrast in polymer stiffness and conformational flexibility governs mechanical modulation in PHBV/CS blends. The findings offer mechanistic guidance for the design of fully biodegradable, mechanically adaptable polymer materials.