Enhancement of the mechanical properties of electrospun lignin-based nanofibers by heat treatment

Unique composite nanofibers were produced by electrospinning solvent-fractionated lignin using a small percentage of polyethylene oxide mixed with nanocrystalline cellulose (NCC) for fiber formation and reinforcement, respectively. The materials were characterized and the mechanical properties analyzed as a function of NCC content and thermal exposure (oxidative stabilization and carbonization). At the electrospinning conditions studied, uniform fibers were produced at all NCC concentrations. FTIR analysis of the samples showed shifts in the hydroxyl stretching regions, suggesting intermolecular interactions between the NCC and lignin. After thermal treatment, the mechanical properties of the materials were greatly enhanced for both the lignin and lignin composite fiber mats. The tensile strength and tensile modulus of the mats more than doubled, while the strain at break slightly increased when the materials were exposed to temperatures of 250 °C in an oxidative environment. The tensile strength, tensile modulus and elongation at break values were found to be similar to that of other glassy bioplastics such as polyhydroxybutyrate providing a path to significantly enhance the mechanical properties of technical lignin. NCC loading did not have a significant impact on the performance of the lignin mat for most conditions. Further carbonization of the mats greatly enhanced the stiffness of the mat up to 8 GPa with the inclusion of NCC at the highest loading level. Overall, this work highlights alternative route for the utilization of technical kraft lignin through simple means of heat treatment.