Solubility of kraft lignin-g-polyacrylonitrile copolymer in various ionic liquids and characterization of its solution

초록

Lignin is a potential precursor for low-cost carbon fiber production, but it is difficult to spin and spool lignin because of its complex and interconnected molecular structure. This disadvantage can be overcome by introducing g-polyacrylonitrile (PAN) to lignin. However, the resulting copolymer is insoluble in common organic solvents. In this study, kraft lignin (KL)-g-polyacrylonitrile copolymers with different KL/PAN proportions were prepared via atom transfer radical polymerization (ATRP) method and their solubility in ionic liquids (ILs) was investigated at different temperatures. 1-Ethyl-3-methylimidazolium acetate ([EMIM] Ac), 1,3-dimethylimidazolium methyl sulfate ([MMIM] MeSO4), 1-butyl-3-methylimidazolium chloride ([BMIM] Cl), and 1-butyl-3-methylimidazolium bromide ([BMIM] Br) were used as the ILs. At all investigated temperatures, the highest solubility of KL-g-PAN was observed in [EMIM] Ac, with the order of [MMIM] MeSO4 > BMIM] Br > [BMIM] Cl. The solubility in BMIM] Br and [BMIM] Cl was remarkably low, reaching values of less than 4 g/Kg. The viscosity and surface tension of the KL-g-PAN/[EMIM] Ac solution increased and decreased, respectively, with increasing amounts of PAN and further by the addition of dimethylformamide (DMF) to the solution. FTIR spectra of KL-g-PAN copolymers before and after dissolution suggested that PAN was partially depolymerized from the copolymer during the dissolution process.

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