Strong and tough cellulose nanopapers enabled by imidazolium ionic liquids

The development of high-performance cellulose nanopaper is essential to enable its emerging application as an alternative eco-friendly substrate in electronics, photonics, and energy storage devices. Herein, we propose a novel chemical strategy to substantially improve the strength and toughness of cellulose nanopaper by introducing cations and anions from imidazolium ionic liquids (ILs) into it, thereby forming additional hydrogen bonds between the hydroxy groups of cellulose nanofibrils (CNFs) and the IL ions. At an IL content of 0.5 wt%, the strength, toughness, and fracture toughness of the IL/cellulose nanopapers are 210.5 MPa, 5.1 MJ/m³, and 4.48 MPa·m^1/2, respectively, which are 2.5, 2.1, and 1.25 times greater than those of the unmodified nanopapers. Interfacial structure characterization and mechanistic analyses demonstrate that a moderate IL content facilitates the formation of extensive hydrogen bonds between the hydroxy groups on the CNF surface and both the imidazolium cations and anions. This substantially strengthens the interfacial bonding of the CNFs, effectively enhancing the tensile strength and toughness of the cellulose nanopaper while preserving its failure strain.