In this work, we have highlighted the potential of the "bottom-up" design of a molecularly engineered water-borne reactive compatibilizer for the tuning of the polycaprolactone (PCL)/cellulose nanofibril (CNF) interface, with the aim to tailor the mechanical properties of the corresponding nanocomposite. This design of an amphiphilic random copolymer with hydrophilic quaternized ammonium and hydrophobic reactive groups (methacrylate moieties) allowed for self-assembly in aqueous medium into nanoparticles. The reactive compatibilizer adsorbed onto CNF in water dispersion forming a uniform core-shell nanohybrid due to electrostatic interactions as confirmed by AFM. The mixture of polymer matrix and the core-shell nanohybrid in water dispersion allowed for a wet feeding approach with an efficient evaporation of the water during the melt compounding,1, 2 as schematically shown in Figure 1. A remarkable improvement of the mechanical properties is demonstrated. The Young’s modulus and toughness of the polycaprolactone containing 10 wt.% CNF increased 80%, and an impressive 1000%, respectively, when a low amount of reactive compatibilizer (< 5 wt%) was used. Furthermore, the possibility of tuning of the mechanical properties (increased Young’s modulus, ductility and toughness) was demonstrated by increasing the amount of peroxide, resulting in a higher crosslink density at the interface. Our results may pave the way towards a deeper understanding of the interfacial design for future nanocellulose-based composite materials. References 1. Lo Re, G.; Spinella, S.; Boujemaoui, A.; Vilaseca, F.; Larsson, P. T.; Adås, F.; Berglund, L. A. ACS Sustainable Chemistry & Engineering 2018. 2. Lo Re, G.; Engstrom, J.; Wu, Q.; Malmström, E.; Gedde, U. W.; Olsson, R. T.; Berglund, L. A. ACS Applied Nano Materials 2018.