Preparation of zwitterionic cellulose nanofibers with betaine-oxalic acid DES and its multiple performance characteristics

In this study, the zwitterionic cellulose nanofibers (Z-CNFs) were prepared by pretreating bleached softwood kraft pulp (BSKP) with a deep eutectic solvent (DES) composed of betaine and anhydrous oxalic acid and then high-pressure homogenization. Some carboxyl groups from oxalic acid and quaternary ammonium groups from betaine were introduced onto the surface of cellulose by esterification in this pretreatment process, which endows cellulose nanofibers (CNFs) with zwitterionic characteristic. A significant feature is that the zeta potential of as-prepared CNFs approaches − 4 eV, so that Z-CNFs are prone to aggregation. The Z-CNFs maintain cellulose Iβ crystal form with a width of 5–15 nm and a length of > 1 μm, and demonstrate good thermal stability (T_on = 188–190 °C, T_max = 359–363 °C). The steady rheological analysis and modulus analysis show that all Z-CNF suspensions are pseudoplastic fluid with shear-thinning behavior, and have weak fibril network structure but still exhibit solid like behavior under low shear conditions at a concentration of 1.2%. The Z-CNF films fabricated from Z-CNFs exhibit good light transmittance (76–83%), high haze (87–90%) and high tensile strength (96 MPa). Therefore, an efficient green pretreatment method was developed for preparation of zwitterionic cellulose nanofibers, which could be used in the field of various fluid and film materials.