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Impressive mechanical properties and reinforcing capability make cellulose nanocrystal (CN) a promising candidate as biomass nanofiller for the development of polymer-based nanocomposites. With the recent announcement of large-scale CN production, the use of industrial processing techniques for the preparation of CN-reinforced nanocomposites, such as extrusion, is highly required. However, low thermal stability of sulfuric acid-prepared CN limits the processing since most polymeric matrices are processed at temperatures close to 200 °C or above. It has been proved that surface adsorption of polymers on CN as compatibilizer, such as hydrophilic polyoxyethylene (PEO), can improve its thermal stability due to the shielding and wrapping of PEO. However, the weak combination between CN and PEO allows the free movement of surface polymer, which can induce the self-aggregation of CN and microphase separation in composites especially during melt processing. Using carboxylation?amidation reaction, short chains poly(ethylene glycol) (PEG) can be first grafted on the surface of the nanocrystals, and immobilize long PEO chains on modified nanocrystals through physical adsorption and entanglement. Two polymeric layers should further improve the thermal stability of CNs, and surface polymeric chains should provide significant dispersibility and compatibilization for extruded nanocomposites. Rheological analysis showed better PEO adsorption for PEGgrafted nanocrystals than pristine CN. Results from AFM and SEM revealed homogeneous dispersion and good compatibility of modified nanocrystals in PS matrix. Finally, the thermal, mechanical, and barrier properties of ensuing nanocomposites have been investigated to study the effect of physically and/or chemically modified nanocrystals.