Carboxylation of Cellulose Nanocrystals for Reinforcing and Toughing Rubber Through Dual Cross-linking Networks

The surface functionalization of cellulose nanocrystals (CNCs) and the construction of strong interfacial adhesion between CNCs and the rubber matrix are effective ways to achieve high-performance rubber/CNCs nanocomposites. Herein, carboxylation of sulfonic CNCs (CNC–OSO3H) was conducted in an aqueous medium by using citric acid (CA) as the modifier. A large number of carboxyl groups were successfully grafted on the surface of CNC–OSO3H, and most of the sulfonic groups were removed, which endows the carboxylated CNC–OSO3H (abbreviated as CNC–CA) with higher chemical reactivity and thermal stability. Subsequently, carboxylated styrene-butadiene rubber (XSBR)/CNC–CA nanocomposites with a dual cross-linking network were prepared by using polyethylene glycol diglycidyl ether (PEGDE) as the cross-linking agent and CNC–CA as reinforcing fillers. Fourier transform infrared spectroscopic investigation found that in the obtained nanocomposites, the carboxyl groups on CNC–CA and XSBR formed hydrogen bonds (physical cross-linking) with each other, and the carboxyl groups formed covalent bonds with the epoxy group on PEGDE simultaneously. The coexistence of physical and chemical cross-linking improved the interface compatibility between CNC–CA and the XSBR matrix, accelerated the homogeneous dispersion of CNC–CA, and realized the cross-linking of the matrix itself. As expected, XSBR/CNC–CA nanocomposites with a dual cross-linking network showed a remarkable enhancement in tensile strength (up to 500%), modulus (up to 151%), and work of fracture (up to 348%). This work provides both a facile and green approach to obtain carboxylated CNCs and a convenient method for the preparation of high-performance rubber nanocomposites with dual cross-linking networks.