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.