Unlocking the response of lignin structure for improved carbon fiber production and mechanical strength
We successfully obtained soda lignin and kraft lignin with high purity and quality by the mild soda pulping process and the harsh kraft process combined with the sequential membrane separation and purification processes. Soda lignin had excellent linear structure with 88% β-O-4 aryl ethers among all the lignin interunit linkages and relatively less phenolic –OH groups as compared with kraft lignin (32% linear β-O-4 and 64% branched C–C), which provide soda lignin with better alignment along polyacrylonitrile (PAN), reduce the entanglement of the fibers, and thus double the tensile stress of the derived carbon fibers when it was blended and electrospun with PAN in the weight ratio of 1/0.25. In particular, higher aromaticity of soda lignin was effectively retained during thermostabilization as compared with kraft lignin, which enabled higher yield of the derived carbon fiber as well as enhanced mechanical performance. The improved mechanical performance of soda lignin-derived carbon fibers could also be evidenced through the increased pre-graphitic turbostratic carbon and crystallite size in the carbon fibers, as revealed by XRD and Raman spectroscopy.