Ultrastrong and flame-resistant freestanding films from nanocelluloses, self-assembled using a layer-by-layer approach

Abstract Nanosized cellulose nanofibrils (CNF) prepared from phosphorylated pulp fibers (P-CNF) are combined with CNF prepared from aminated fibers (cationic CNF) through a layer-by-layer (LbL) assembly to prepare a freestanding, transparent all-cellulose film. It is shown that the thermal stability and flame-retardant properties of the all CNF film are significantly improved when phosphorylated CNF is combined with cationic fibrils in an LbL assembled structure. The freestanding films also show a tensile strength of 160 MPa and a Young's modulus of 9 GPa, placing it among strongest freestanding LbL films fabricated so far, showing large promise for the use of these types of ultrathin films in advanced applications. The LbL build-up of the cationic CNF/P-CNF multilayer film is carefully studied by quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). Hydrophobized silicon substrates are used for the LbL deposition and it is shown that the (cationic CNF/P-CNF)300 film, 2.3 μm thick, can be easily detached from the substrate using tweezers. The thermal stability, combustion behavior and mechanical properties of the films are further studied by thermogravimetric analysis, combustion and tensile tests respectively.