Controlled acetylation of kraft lignin for tailoring polyacrylonitrile-kraft lignin interactions towards the production of quality carbon nanofibers

Abstract Lignin/polyacrylonitrile (PAN) precursors can provide a green solution for lowering the cost while adequately maintaining the mechanical performance of carbon fibers, opening the gates for energy efficient lightweight materials to reach price-sensitive markets. We anticipated that improving the interactions between hydrophobic PAN chains and lignin, abundant in hydrophilic -OH groups, is important issue for obtaining a more aligned polymeric network, advantageous for the mechanical performance of carbon fibers. A controlled acetylation using acetic anhydride or isopropenyl acetate (IPA; in DMSO/EmimOAc) allowed us to step-by-step increase the degree of substitution of lignin and fine-tune the thermal characteristics as well as the miscibility of the blends, thus tailoring the mechanical properties of the resulting electrospun carbon fiber mats, along with the morphological evolution of the fibrous network. Differential scanning calorimetry and rheometry results indicated that by increasing the amount of substituents, the complex hydrogen bonding network connecting lignin molecules gradually breaks down leading to an increase in lignin/PAN polymer blend miscibility. However, it also became apparent that by masking hydroxyl groups important free radical mediated processes during the stabilization step might be inhibited, this phenomenon appeared to be more pronounced when aliphatic hydroxyl groups are selectively acetylated (IPA in DMSO/EmimOAc). Carbon fiber mats with the best mechanical performance were obtained only via controlled partial acetylation of kraft lignin with an almost 3-fold improvement in tensile strength (about 2-fold increase in elastic modulus) compared to the unmodified lignin-based fibrous materials. Our study holds promise for the realization of potentially low-cost quality carbon fibers.