Green and Efficient Synthesis of Dispersible Cellulose Nanocrystals in Biobased Polyesters for Engineering Applications

Despite attractive properties of cellulose nanocrystals (CNCs) such as high natural abundance, inherent biodegradability and high modulus, CNCs tend to degrade and aggregate when exposed to high temperatures during melt processing. In the present work, the surface of CNCs was modified with PMMA to take advantage of the miscibility with various biobased polymers including PLLA when melt-blended. Particular attention was paid to grafting techniques in water medium using two different redox initiators: Fe2+/H2O2 (Fenton's reagent) and ceric ammonium nitrate (CAN). The successful synthesis of CNC-g-PMMA was verified by gravimetric analysis, FTIR, CP-MAS 13C NMR and suspension tests. A high grafting efficiency of 77% was achieved using CAN as the redox initiator. Increasing the PMMA content on CNC surfaces led to higher CNC thermal stability. As a consequence of PMMA grafting in water, modified CNCs were found to be predispersed in a PMMA network. PLLA/CNC nanocomposites were then prepared by melt-blending, i.e., in the absence of solvent, and the quality of the dispersion was confirmed by dynamic rheology, TEM and DMA. The presence of a high amount of PMMA grafts on CNC surfaces reduced CNC aggregation and favors the percolation of CNCs with the development of a weak long-range 3D network. Miscibility between PMMA grafts and PLLA as well as the predispersion of CNCs was found to play a key role in the dispersion of CNCs in PLLA. Thermomechanical analysis revealed that PMMA grafts on CNC surfaces significantly enhanced elastic moduli in the glassy and rubbery state. The high dispersion state (related to high PMMA grafting) also showed a positive effect on O2 permeability of PLLA and a strong beneficial effect on heat deflection temperature (HDT) reaching outstanding temperatures higher than 130 °C. Thus, free-radical grafting of PMMA in water provides an efficient and green route to dispersible (bio)nanofillers by solvent-free extrusion techniques with PMMA-miscible matrices such as PLLA for high-performance applications.