Toughening of Poly(hydroxyurethane)-Epoxy Hybrid Networks by High-Aspect-Ratio Cellulose Nanocrystals

Nonisocyanate polyurethanes are sustainable alternatives to conventional polyurethanes but often exhibit limited mechanical performance. Here, poly(hydroxyurethane) (PHU)-epoxy hybrid networks were prepared to tune matrix stiffness through covalent cross-linking and reinforced with wood- and tunicate-derived cellulose nanocrystals (CNCs). Increasing epoxy content enhanced Young's modulus and tensile strength while reducing strain at break. In contrast, incorporation of CNCs into an intermediate PHU-epoxy matrix produced substantial mechanical enhancement at low loadings. Tunicate-derived CNCs, characterized by higher aspect ratios, provided greater reinforcement than wood-derived CNCs and achieved a maximum toughness of ∼27 MJ m-3 at 2 wt %, exceeding both wood-derived CNC nanocomposites (≤17 MJ m-3) and neat PHU-epoxy hybrids (≤16.5 MJ m-3). Scanning electron microscopy confirmed homogeneous CNC dispersion, while Fourier transform infrared spectroscopy indicated extensive hydrogen-bonding interactions. These results demonstrate that epoxy hybridization and high-aspect-ratio CNC reinforcement provide complementary routes to enhance the mechanical performance of biobased PHU materials.