Glyoxalation of Kraft lignin and optimization of electrospinning process parameters for producing polyacrylonitrile/KL nanomats for potential applications as carbon material

Abstract

In this work, Kraft lignin (KL), glyoxalated KL and its blends with polyacrylonitrile (PAN/KL) were electrospun by solution electrospinning in order to investigate the morphology of the mats to be used in carbon material applications. The process parameters to obtain PAN/KL nanofibers with higher amount of KL and without defects were optimized. Three types of KL were used in this work: non-purified (KL), purified (KLp) and glyoxalated (KLg). The effects of the glyoxalation reaction time and the ratio of glyoxal-to-KL were, also, investigated in order to improve the molar mass of the KL. The results show that the molar mass of each glyoxalated KL increases compared to that of the standard KL. The molar mass remained unchanged with increasing the reaction time and ratio of glyoxal-to-KL. All obtained nanofiber mats were characterized by scanning electron microscopy (SEM) and their morphology showed to be dependent on the ratio of the PAN/KL used. Increasing the KL content decreases the diameter of the nanofibers. The maximum KL content in the PAN/KL blend, required to produce nanofibers without defects and with small fiber diameter, was 50 wt.%, making this material promising for a carbon material precursor. For the case of KLg, nanomats with beads were obtained with 70 and 80 wt.% KLg in the blend. Thermal properties of the nanomats showed a direct relationship to the content of KL used and its molar mass. In addition, these results point to a new approach for the application of KL and PAN/KL nanofibers.