This thesis introduces innovative toughening techniques for brittle polymers such as styrene-acrylonitrile (SAN) and polylactic acid (PLA), aimed at enhancing mechanical properties while emphasizing environmental sustainability. A novel in situ fibrillation method was utilized for SAN, integrating thermoplastic polyurethane (TPU) nanofibers within the matrix through silane grafting onto TPU, forming nanofibrils sized 90 to 360 nm, followed by post-crosslinking. This technique significantly enhances tensile toughness by up to 350% with just 1 wt% nanofibril TPU, thereby reducing the reliance on higher rubber contents and preserving SAN's stiffness.

Nanofibril TPU proves to be an efficient toughening agent, outperforming traditional agents like butadiene rubber (BR). Introducing 0.6 wt% nanofibril TPU alongside 20 wt% BR amplifies tensile toughness and elongation at break by 239% and 218%, respectively, compared to 20 wt% BR alone. Against pure SAN, these increases reach 1040% and 1220%, respectively.

The thesis also explores innovative toughening strategies for PLA to reduce dependence on petroleum-based polymers. Two crosslinking methods are investigated:​ silane/moisture crosslinking and a tailored diisocyanate method that optimally balances chain extension and crosslinking. The latter method notably increases PLA's tensile toughness to 30-60 MPa and impact strength to 60-140 J/m, facilitating a brittle-to-ductile transition with only 3 wt% nanofibril TPU, while maintaining PLA’s high modulus, crystallinity, and transparency.

Additionally, this research explores the synergistic effects of TPU nanofibrils and cellulose nanocrystals (CNC) on PLA composites. Incorporating just 3 wt% TPU and 0.3 wt% CNC substantially enhances PLA's mechanical properties, crystallization kinetics, and barrier capabilities, while maintaining transparency. Isothermal crystallization studies show CNC's efficacy in accelerating PLA crystallization, reducing crystallization half-time to less than a minute at 100°C and significantly lowering oxygen transmission rates. This formulation yields composites with remarkable improvements in tensile strain—up to 3,900 times that of neat PLA—while maintaining tensile strength and Young’s modulus.

Overall, this research underscores the potential of TPU and CNC to significantly enhance the performance of PLA composites, offering a viable and sustainable alternative to conventional plastics. The advanced techniques developed in this thesis are poised to broaden the application horizons of bioplastics to high-value uses, promoting sustainable manufacturing practices.