Facile synthesis of phosphorylated lignocellulosic nanofibril/graphene oxide composite film via co-milling method towards enhancing mechanical and flame-retardant performance

Achieving both excellent flame retardancy and high mechanical performance is essential for the potential application of composite films in industrial application. However, designing and fabricating film materials that produce a feasible balance between the aforementioned features presents a daunting challenge. Herein, as a proof-of-concept, phosphorylated lignocellulosic nanofibrils (P_LCNF) and ammonium polyphosphate (APP) were well integrated into a densified large/small graphene oxide-based network using a simple evaporation-induced self-assembly method. Benefiting from the incorporation of hydrogen bonding (P_LCNF and APP) and nano-reinforcement (P_LCNF), the mechanical and structural reliability of the interconnected networks gain a discernible improvement. Correspondingly, the optimized films exhibits markedly improved mechanical performance (i.e., tensile strength of 269.4 MPa, elongation at break of 7.92%, and toughness of 11.17 MJ⸱m^-3, ∼2.0, 1.8 and 7.3 times surpass those of the virgin one) and good structural stability in aqueous solutions with different pH value and different reagents. Additionally, the resultant films demonstrated exceptional flame resistance attributed to the synergistic effect of P_LCNF (phosphorous-containing group and lignin molecules) and APP (phosphorous- and nitrogen-containing group), as evidenced by their structural integrity after repeated exposure to a high-temperature flame. The key parameters for micro-scale combustion calorimetry testing, i.e., peak heat release rate and total heat release, dramatically decreased, and the corresponding value was lowered to 12.8 W⸱g⁻¹ and 0.40 kJ⸱g⁻¹, respectively. The reinforcing and flame-retardant mechanisms were elucidated through an analysis of structural evolution and performance variation. The strategy developed in this study offers a novel approach for the design and development of advanced film materials for fireproof coatings.