Escalating plastic waste and the limited property retention of mechanically recycled polymers motivate the development of bio-based reinforcements that enable higher-value reuse. This thesis targets reinforcing recycled polyethylene terephthalate glycol (R-PETG) and addresses the incompatibility between the hydrophobic matrix and hydrophilic cellulose nanocrystals (CNCs). CNCs were esterified with dodecenylsuccinic anhydride (DDSA) to introduce aliphatic succinyl chains at controllable grafting levels. The modified CNCs (mCNCs) were then incorporated into R-PETG via solution blending and film casting to develop biocomposite. Chemical modification was verified by Fourier-transform infrared spectroscopy. Colloidal stability and nanoparticle dispersion were examined by dynamic light scattering in aqueous and ethanolic media and by transmission electron microscopy in chloroform. The thermal property of the biocomposite was evaluated by thermogravimetric analysis and differential scanning calorimetry. The optical performance of the mCNCs suspension and the biocomposite was quantified by UV-Vis spectroscopy, and the mechanical properties were investigated using tensile testing. DDSA grafting increased CNC hydrophobicity, improved dispersion in semi-polar media and in R-PETG, and reduced aggregation relative to pristine CNCs at comparable contents (wt%). As a result, mCNCs/R-PETG films maintained higher visible transmittance than their unmodified counterparts, while providing effective reinforcement. Meanwhile, mCNCs incorporation converts R-PETG from brittle to ductile, maintaining comparable strength while markedly increasing the extensibility and toughness. Thermal analyses indicated that incorporating mCNCs did not compromise stability within the processing window and preserved the glass-transition characteristics of R-PETG. Overall, this work demonstrates a tunable, chemistry-guided route to compatibilize CNCs with R-PETG, delivering optically clear, mechanically robust, and potentially re-recyclable biocomposites. The findings support circular-economy pathways by coupling waste-plastic valorization with renewable nanofillers and provide design principles for next-generation sustainable polymer composites.