Influence of Hardwood Lignin Blending on the Electrical and Mechanical Properties of Cellulose Based Carbon Fibers

Carbon fibers (CFs) are fabricated by blending hardwood kraft lignin (HKL) and cellulose. Various compositions of HKL and cellulose in blended solutions are air-gap spun in 1-ethyl-3-methylimidazolium acetate (EMIM OAc), resulting in the production of virtually bead-free quality fibers. The synthesized HKL–cellulose fibers are thermostabilized and carbonized to achieve CFs, and consequently their electrical and mechanical properties are evaluated. Remarkably, fibers with the highest lignin content (65%) exhibited an electrical conductivity of approximately 42 S/cm, surpassing that of cellulose (approximately 15 S/cm). Moreover, the same fibers demonstrated significantly improved tensile strength (∼312 MPa), showcasing a 5-fold increase compared to pure cellulose while maintaining lower stiffness. Comprehensive analyses, including Auger electron spectroscopy and wide-angle X-ray scattering, show a heterogeneous skin–core morphology in the fibers revealing a higher degree of preferred orientation of carbon components in the skin compared to the core. The incorporation of lignin in CFs leads to increased graphitization, enhanced tensile strength, and a unique skin–core structure, where the skin’s graphitized cellulose and lignin contribute stiffness, while the predominantly lignin-rich core enhances carbon content, electrical conductivity, and strength.

Figure S8.a) Two-dimensional WAXS scattering results.For a bundle of fibers, the crystals are typically ordered along two main directions, the meridional direction along the fiber axis and the equatorial direction perpendicular to it, whose diffraction pattern is made by arcs mainly localized along these 2 axes 1 .The intensity of diffraction spots in the cellulose's map corresponds to the number of scattered X-rays and relates to its degree of crystallinity.The lower intensity or diffusing scattering with increasing lignin may suggest the presence of more and more amorphous or disordered regions.b to e) One-dimensional WAXS scattering of fibers at different strains and from azimuthal angle integration of the 2D WAXS data for the different fibers.The intensity distribution across the scattering angle reveals information about the preferred orientation of cellulose nanofibrils, and the crystalline structures within the fibers.Anisotropic features in the intensity profile may suggest preferential alignment of crystallographic planes.The absence of distinct peaks that is the presence of a broad, diffuse scattering pattern in the WAXS plot for the samples with higher lignin probably indicates the presence of amorphous or disordered regions within the carbon fibers.The intensity of peaks in these regions remains distinct but decreases as the lignin content increases for the carbon fibers.Quantitative analysis of the intensity peaks involves extracting information such as crystallite size, interplanar spacing, and orientation distribution.This analysis is often performed using advanced techniques like peak fitting and modeling which is not the scope of this work but would be interesting for future studies.Especially considering the need for understanding the quality of graphitization.Table S2.When conducting calculations on lignin leaching in the coagulation bath, the following results were obtained: Ultraviolet-visible (UV/Vis) absorbance measurements at 280 nm were conducted on coagulation baths using SPECORDE® 200 PLUS (Analytik Jena AG, Jena, Germany) to determine the amount of leached lignin.The absorbance values were converted to lignin concentration utilizing an extinction coefficient of 24.6 L/g.cm 2 .Corrections were made for the contribution to the absorbance from EMIMAc, estimated to be approximately 1/10th of that of lignin.The EMIMAc concentration in the coagulation bath was determined by conductivity measurements at 23°C using inoLab Cond 720 Benchtop Conductivity Meter (Thomas Scientific, Swedesboro, NJ, USA), based on a linear calibration curve.
Please note that the actual lignin yield is likely lower due to washing after the process.However, the trend remains consistent, indicating relatively more lignin is leached when the initial lignin amount in the fibers is lower.

Figure S1 .
Figure S1.The viscosity of the solution measured using small amplitude oscillatory shear.The graph illustrates the viscosity profile of the solution under varying shear rates, providing insight into the rheological behavior of the different lignin-cellulose blend solutions.

Figure S2 .Figure
Figure S2.Thermostabilization and carbonization profile for precursor fibers blending hardwood kraft lignin with cellulose.

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Figure S7.a) Conductivity of 1 cm long segments of longer carbon fibers with ranging error bars.b) Conductivity of 4 cm long segments.