The H-bond evolution of cellulose nanofibrils treated with choline chloride/oxalic acid

The fracture and recombination of its H-bond network have significant influence on the pore structure of nanocellulose and its materials. Deep eutectic solvents (DESs) have been widely used in the cellulose dissolution and nanocellulose treatment because of their efficient H-bond cleavage in cellulose; however, the H-bond evolution mechanism is still not fully understood. The main objective of the present work was to treat cellulose nanofibrils (CNFs) using a DES system based on choline chloride and oxalic acid. The effect of DES treatment on the H-bond, micro-structure, and properties of CNFs were analyzed. The results confirmed that choline chloride/oxalic acid effectively destroyed the intermolecular and intramolecular H-bonds in CNFs, thereby reducing the H-bond energy. This resulted in a decrease in the intramolecular H-bond contents, and an increase in the intermolecular H-bond contents of the CNFs. The changes in the three types of H-bond were independent, and their fracture sequence was of the order v(O(3)H…O(5) intramolecular H-bond) > v(O(6)H…O(3′) intermolecular H-bond) > v(O(2)H…O(6) intramolecular H-bond). Moreover, H-bond destruction improved the crystallinity index and crystallite size of the CNFs. The changes in the crystal parameters further led to an increase in the thermal stability. This work provides a theoretical basis for the treatment of CNFs by DESs, which is critical for the H-bond evolution principle of CNFs.Graphic Abstract