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Research Papers

X-Ray Diffraction Analysis of Kraft Lignins and Lignin-Derived Carbon Nanofibers OPEN ACCESS

[+] Author and Article Information
Azadeh Goudarzi

Department of Materials Engineering,
University of British Columbia,
6350 Stores Road,
Vancouver, BC V6T 1Z4, Canada
e-mail: goudarzi@mail.ubc.ca

Li-Ting Lin, Frank K. Ko

Department of Materials Engineering,
University of British Columbia,
6350 Stores Road,
Vancouver, BC V6T 1Z4, Canada

Manuscript received April 9, 2014; final manuscript received July 17, 2014; published online September 4, 2014. Assoc. Editor: Hsiao-Ying Shadow Huang.

J. Nanotechnol. Eng. Med 5(2), 021006 (Sep 04, 2014) (5 pages) Paper No: NANO-14-1032; doi: 10.1115/1.4028300 History: Received April 09, 2014; Revised July 17, 2014

Lignin is a renewable material and it is abundantly available as low priced industrial residue. Lignin-based carbon fibers are economically attractive and sustainable. In addition, remarkably oxidized molecule of the lignin decreases the required time and temperature of the thermostabilization process compared to other carbon fiber precursors such as polyacrylonitrile (PAN); and thus, decreases the processing cost of carbon fiber production. The fraction 4 of softwood Kraft lignin (SKL-F4) was previously shown to be spinnable via electrospinning to produce carbon nanofibers. In this paper, we characterized different Kraft lignin powders through X-ray diffraction (XRD) analysis to measure the mean size of the ordered domains in different fractionations of softwood and hardwood samples. According to our results, SKL-F4 has largest ordered domains among SKLs and highest hydroxyl content according to Fourier transform infrared (FTIR) analysis. In addition, variations in the XRD patterns during carbon nanofiber formation were studied and the peak for (101) plane in graphite was observed in the carbon nanofiber carbonized at 1000 °C.

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Lignin is one of the most abundant renewable materials on earth. Kraft lignin is available as an industrial residue in large quantities, which is usually burnt as low efficiency fuel [1,2]. One of the promising potential applications for lignin valorization is production of advanced material, such as lignin-based carbon fibers (CFs). The low cost of lignin, and the fact that it is a renewable resource, make it an attractive material to fabricate CFs. The predominant CF precursor is polyacrylonitrile (PAN, [C3H3N]n) followed by petroleum pitch, and rayon [3]. The demand for CF is estimated as 70,000 tons/yr by 2015, if the current 10–15% growth rate is maintained [4-6]. The price of CF has dropped temporarily in recent years, but is still too high for widespread industrialization. A low cost alternative like lignin would expand the range of applications. The cost of PAN is over 50% of the total manufacturing cost of the CF and commercial grade PAN-based CF is around $20/kg. With lignin-based CF, the manufacturing cost could be reduced to less than $10/kg, making lignin an attractive alternate precursor for CF production [2]. In addition to lower cost of lignin as the precursor, lignin molecule is substantially oxidized compared to PAN, therefore oxidative thermostabilization process of lignin fibers requires shorter stabilization time and lower stabilization temperature than PAN fibers, which translates to lower processing cost [2,7,8].

Lignin is an amorphous phenyl propylene polymer, which does not hold most of the CF forming requisites of a polymer (e.g., linear and flexible structure, high degree of symmetry, high molecular mass > 1000 Å, high degree of crystallinity, high degree of orientation, and high carbon content) [9]. However, lignin CFs have successfully been prepared from different types of lignins through different methods (electrospinning, melt-spinning, and wet spinning) [10,11]. Yet, the challenge remains preparing a consistent and uniform lignin substrate from such complex and variable compound [7,8]. The reported mechanical properties of the lignin-based CFs are relatively lower than the PAN-based CFs (e.g., tensile strength of 0.5–1 GPa compared to 3–7 GPa) [8,12-16]. Understanding the process in which the complex molecule of lignin forms fiber and the process of carbonization of such fiber is not clearly known. Understanding such process might assist improving the properties in the future. Therefore, in this paper we studied XRD analysis of different Kraft lignin powders to examine the mean size of the ordered domains. We also studied the carbonized lignin fibers produced from electrospinning process. XRD analysis previously was reported for lignins [17-19], but to our knowledge the size of the ordered domains and diffraction angel of the Kraft lignins were not reported.

Softwood Kraft lignins (SKL) and hardwood Kraft lignin (HKL) were from FPInnovation, HP-L lignin was from Lignol. Lignins are composed of a mixture of different sized molecules. Lignin fractionation is method to separate different sized molecules, either through dissolving/precipitating the lignin in different organic solvents or using membranes [20,21]. Dallmeyer studied electrospinning of softwood lignins and showed that the higher molecular weight fraction, fraction 4, is spinnable through electrospinning [22]. Therefore, we used fractionation 4 SKL (SKL-F4) for electrospinning. SKL-F4:PEO with the 99:1 (w/w) ratio was dissolved in dimethylformamide at 30 wt.% concentration. The polymer solution was electrospun using a number 25-gauge needle spinneret at a spinning distance of 18 cm, at 20 kV applied voltage, and a 0.03 ml/min flow rate. The fiber mat was hold in a stretched position using Teflon coated glass sheets in a glass petri-dish and thermostabilized for 1 h at 200 °C with heating rate of 3 °C/min in air. Then, the fiber mat is carbonized in N2 environment in a tube furnace at 1000 °C for 1 h.

The XRD analysis instrument was a Rigaku model MultiFlex diffractometer, running on Cu-Kα (λ = 1.542 Å). The lignin powder data were collected at standard 2 deg/min scan rate. The highly porous nature of fiber mats required longer data collection times to obtain sufficient amount of diffracted X-rays, therefore scan rate of 0.05 deg/min was applied. A Perkin Elmer spectrum 100 was used to perform FTIR analysis, using attenuated total reflection sampling technique and a diamond coated Tl-Br-I plate. Microstructure analysis was conducted by a Hitachi S3000N Tungsten hairpin filament source SEM. To calculate the fiber diameter, the average of 100 measurements in four different SEM image was used. A Q500 TA instrument was used for thermo gravimetric analysis (TGA). The tests were performed in N2 and air, by heating rate of 10 °C/min.

Lignins have three precursors: p-coumaryl alcohol, coiferyl alcohol, and sinapyl alcohol [1,23]. The polymerization of these three primary monomers results in formation of structural units in lignin: p-hydroxyphenyl (H), guaiacyl (G), and syringyl, respectively [24]. Approximately, 90% of SKL units are G units [1,23]. HKL units contain both G and S units. FTIR spectroscopy, according to Popescu et al. is an important tool for structural characterization between hardwood and softwood samples [25]. Figure 1 shows FTIR spectra of the lignin powders. The hydroxyl region of the spectra (3000 cm−1–3700 cm−1) shows that SKL-F4 had the highest amount of hydroxyl groups available. A distinctive difference between HKLs and SKLs is the ratio of the peaks at 1130 cm−1 and 1030 cm−1: in softwoods 1130 cm1 < 1030 cm−1 and in hardwoods 1130 cm−1 > 1030 cm−1. The peak at 1030–1035 cm−1 is allocated to Aromatic C–H in plane deformation, where amount of G units are higher than S units, this region is also allocated to C–O deformation in primary alcohols, and C = O stretch (unconjugated) [26,27]. Peaks at the wavenumber range of 1266–1270 cm−1 are associated with G ring vibrations and as shown in Fig. 2, they are not present in HKL samples. Twin peak at 855 cm−1 and 815 cm−1 is assigned to softwoods and single peak at 835 cm−1 to hardwoods [26]. The presence of more methoxyl groups attached to the aromatic rings in HKLs (S unit) inhibits formation of 5–5 or dibenzodioxocin linkages [1], where for SKLs β-O-4 linkages are more predominant. Differences in the available functional groups might affect stacking of the molecules and affect size of the ordered domains in the samples. To calculate the size of the ordered domains, XRD analysis was acquired. Figure 2 shows XRD patterns of the three fractions of HKLs, three fractions of SKLs and HP-L powder samples. For each sample, peak fitting was performed for four different refinements: PearsonVII, Pseudo-Vigot, Lorentzian, and Gaussian. Gaussian character was predominant in overall peak shape for softwood samples and Lorentzian character was predominant in hardwood samples and HP-L. After fitting the curves, the maximum diffraction angel was determined for each sample. The average peak for hardwoods is located at 2θ = 21.2 ± 0.15 deg and for softwood it is located at 2θ = 19.35 ± 0.18 deg (Table 1 shows the values for each sample). Kubo et al. reported the diffraction angel of 22.7 deg for hardwood acetic acid lignin [18] and Ansari and Gaikar reported 2θ = 22.37 deg for lignin from sugar mill [19]. Such differences could be caused by the difference in the lignin type. The Scherrer's equation is usually used for calculating the mean size of the ordered domains (crystallite): d=Bλ/βcosθ, where d is the mean size of the ordered domains, B = dimensionless shape factor (value of 0.9 is used [28,29]), λ = X-ray wavelength (0.154 nm for Cu K-α), and β = full width at half maximum (FWHM) [30]. To measure the FWHM, instrumental broadening effect should be subtracted from the data. Table 1 shows the results of calculated mean size of the ordered domains (“d” in Scherrer's equation) for lignins. Among SKLs, the fraction 4 had the highest size of the ordered domains. According to literature, SKL-F4 has fiber forming ability by electrospinning method [22]. Among HKLs, the unfractionated sample (HKL) had the highest size of the ordered domains. However, because unfractionated sample is composed of different sized molecules in its structure (higher polydispersity index), when spinning parameters were adjusted for one portion of the molecular weight (e.g., viscosity of the solution), the other portions were not spinnable. In other words, although some fiber formation was observed in electrospinning, spraying also occurred. Therefore, SKL-F4 was selected for nanofiber production via electrospinning and further analysis of the nanofibers.

Carbon content is an important factor in a CF precursor. Figure 3 shows the results of elemental analysis of the lignin powders by energy-dispersive X-ray spectroscopy (EDS) method to measure carbon and sulfur content. EDS is considered a qualitative method for elemental analysis, specially, when the energy region in below 3 keV. A procedure in which corrections for atomic number effects (Z), absorption (A), and fluorescence (F) are calculated, are called “ZAF” correction and usually available in instrument settings [31]. The “ZAF” correction was selected for better accuracy of the results. However, we are aware that the results presented in Fig. 3 are relative rather than absolute values. The presented EDS results are the average of at least six analyses. Relatively, HP-L sample was a sulfur-free lignin and the sulfur content of the SKL-F4 was the highest among SKLs. SKL-F4 was previously shown to be spinnable via electrospinning [22]. As a summary from the lignin powder characterization, SKL-F4 had the highest hydroxyl and sulfur content, and the highest size of the ordered domain among SKLs. We produced and electrospun SKL-F4 fibers and characterized the produced carbon nanofibers. Figure 4 shows (a) as-spun fiber, (b) thermostabilized fiber, and (c) the carbonized fiber with their EDS analysis results for carbon and sulfur content. The sulfur content was decreased by thermal treatment, which probably could be due to the release of SO2 gas during the heat treatment. Further investigation such as analyzing the gas content of the sample is required to confirm release of SO2 gas. The decomposition temperature in air was higher than decomposition temperature in N2 environment for the SKL-F4 lignin powder (Fig. 5, Td in air: 475 °C and in N2 was 376 °C, SD < 1 °C for three tests). The reason for oxidative stabilization (thermostabilization) process is to increase stability of the molecule prior the carbonization process and avoid fusion of the fibers. Figure 6 shows XRD patterns of empty sample holder, SKL-F4 powder, carbonized fiber mat, and grinded carbonized fiber mat. The top right hand side of Fig. 6 shows the SEM image of carbonized fiber mat, a schematic of carbonized fiber structure, and a schematic of sample preparation method. For fiber mat sample, we used a similar method to XRD method for thin film characterization. For analyzing thin films with XRD, smaller slits in the receiving detector should be applied. However, our results showed using smaller slit to increase the resolution of the recording peak was not effective for the CF mat sample, possibly due to the nano-sized/amorphous nature of the CF structure and broadness of the peak(s). Therefore, the only effective parameter was decreasing the scanning rate (increasing the data collection time). The graphite peak for (101) plane (marked with * in Fig. 6(d) pattern) was not detected for fiber mat sample. However, the peak for (101) plane is a distinctive characteristic of graphite formation and was available in the grinded CF mat XRD pattern. The EDS analysis showed that although the sulfur content of the as spun fiber mat was reduced significantly during carbonization process, but still some sulfur is available in the CF structure after carbonizing at 1000 °C. EDS analysis of the PAN based CFs do not show the presence of any sulfur in their structure [32,33]. However, Hwang et al. manually added elemental sulfur to the PAN based nanofibers and suggested dehydrogenation process during reaction of PAN with sulfur “facilitates intermolecular cross-linkage” and therefore resulted in higher degree of graphitization [34]. In addition, they concluded that the sulfurization reaction affects configuration of the turbostratic carbon and increases efficiency of π–π stacking [34]. Kraft lignin precursors have remarkably higher oxidized molecules than PAN [2] and they contain sulfate groups in their structure.

In summary, analyzing different Kraft lignin samples, HKL (unfractionated hardwood Kraft lignin) and SKL-F4 (fraction 4 of SKL) had the highest size of ordered domain according to XRD analysis. SKL-F4 also showed highest hydroxyl content according to FTIR analysis. Due to the challenges in electrospinning of unfractionated HKL sample, we chose SKL-F4 for electrospinning. As-spun fibers from SKL-F4 precursor were thermostabilized and carbonized to produce CFs. XRD analysis of the carbon nanofibers indicated that the graphite peak for (101) plane was available in the grinded sample. According to literature, for PAN fibers sulfur facilitates graphite formation. SKL-F4 showed highest sulfur content among SKL samples. Our proposed hypothesis based on the presented results is that the available sulfate groups in Kraft lignins might facilitate graphite formation in carbon nanofiber production process. Further investigations are required to confirm such hypothesis.

The authors acknowledge the financial support from Genome British Columbia (Genome BC) and Genome Canada.

 

 Nomenclature
  • CF =

    carbon fiber

  • DMF =

    dimethylformamide

  • FTIR =

    Fourier transform infrared

  • HKL =

    hardwood Kraft lignin

  • PAN =

    polyacrylonitrile

  • PEO =

    poly ethylene oxide

  • SKL =

    softwood Kraft lignin

  • SKL-F4 =

    fraction 4 of SKL

Zakzeski, J., Bruijnincx, P. C., Jongerius, A. L., and Weckhuysen, B. M., 2010, “The Catalytic Valorization of Lignin for the Production of Renewable Chemicals,” Chem. Rev., 110(6), pp. 3552–3599. [CrossRef] [PubMed]
Baker, D. A., and Rials, T. G., 2013, “Recent Advances in Low-Cost Carbon Fiber Manufacture From Lignin,” J. Appl. Polym. Sci., 130(2), pp. 713–728. [CrossRef]
Morgan, P., 2005, “Precursors for Carbon Fiber Manufacture,” Carbon Fibers and Their Composites, Vol. 121, Taylor & Francis Group, LLC, Boca Raton, FL. [CrossRef]
Hüttinger, K. J., Figueiredo, J. L., Bernardo, C. A., Baker, R. T. K., and Hüttinger, K. J., 1990, Carbon Fibers, Filaments and Composites, Vol. 177, Springer, The Netherlands.
Frank, E., Hermanutz, F., and Buchmeiser, M. R., 2012, “Carbon Fibers: Precursors, Manufacturing, and Properties,” Macromol. Mater. Eng., 297(6), pp. 493–501. [CrossRef]
Boehm, H. P., 1994, “Some Aspects of the Surface Chemistry of Carbon Blacks and Other Carbons,” Carbon, 32(5), pp. 759–769. [CrossRef]
Kadla, J. F., Kubo, S., Gilbert, R. D., and Venditti, R. A., 2002, “Lignin-Based Carbon Fibers,” Chemical Modification, Properties, and Usage of Lignin, Springer, New York, pp. 121–137.
Kubo, S., and Kadla, J. F., 2006, “Carbon Fibers From Lignin-Recyclable Plastic Blends,” Encyclopedia of Chemical Processing, Taylor & Francis, UK, pp. 317–331.
Broughton, J. R., R. M., and Brady, P., 1995, “Fiber Forming Polymers,” Wellington Sears Handbook of Industrial Textiles, Vol. 31, CRC Press.
Baker, D. A., Gallego, N. C., and Baker, F. S., 2012, “On the Characterization and Spinning of an Organic ‐Purified Lignin Toward the Manufacture of Low‐Cost Carbon Fiber,” J. Appl. Polym. Sci., 124(1), pp. 227–234. [CrossRef]
Maradur, S. P., Kim, C. H., Kim, S. Y., Kim, B. H., Kim, W. C., and Yang, K. S., 2012, “Preparation of Carbon Fibers From a Lignin Copolymer with Polyacrylonitrile,” Synth. Met., 162(5), pp. 453–459. [CrossRef]
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Kubo, S., Uraki, Y., and Sano, Y., 1996, “Thermomechanical Analysis of Isolated Lignins,” Holzforschung, 50(2), pp. 144–150. [CrossRef]
Kubo, S., and Kadla, J. F., 2005, “Lignin-Based Carbon Fibers: Effect of Synthetic Polymer Blending on Fiber Properties,” J. Polym. Environ., 13(2), pp. 97–105. [CrossRef]
Compere, A. L., Griffth, W. L., Leitten, C. F., Jr., and Pickel, J. M., 2005, “Evaluation of Lignin From Alkaline-Pulped Hardwood Black Liquor,” http://www.ornl.gov/info/reports/2005/3445605475900.pdf
Wahyuni, E. T., Kunarti, E. S., and Sugiharto, E., “Performance of TiO2 Nanoparticle Prepared on Lignin Structure as Photocatalyst for Hazardous Mercury Removal Through Photoreduction Mechanism,” Available at http://mipa.ugm.ac.id/web/files/publikasi/paper-endangtw.pdf
Kubo, S., Yasumitsu, U., and Yoshihiro, S., 2003, “Catalytic Graphitization of Hardwood Acetic Acid Lignin With Nickel Acetate,” J. Wood Sci., 49(2), pp. 188–192. [CrossRef]
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Brodin, I., Sjöholm, E., and Gellerstedt, G., 2009, “Kraft Lignin as Feedstock for Chemical Products: The Effects of Membrane Filtration,” Holzforschung, 63(3), pp. 290–297. [CrossRef]
Wallberg, O., Jönsson, A. S., and Wimmerstedt, R., 2003, “Fractionation and Concentration of Kraft Black Liquor Lignin With Ultrafiltration,” Desalination, 154(2), pp. 187–199. [CrossRef]
Dallmeyer, J. I., 2013, “Preparation and Characterization of Lignin Nanofibre-Based Materials Obtained by Electrostatic Spinning,” Ph.D. thesis, https://circle.ubc.ca/bitstream/./ubc_2013_spring_dallmeyer_james.pdf
Laurichesse, S., and Avérous, L., 2014, “Chemical Modification of Lignins: Towards Biobased Polymers,” Prog. Polym. Sci, 39, pp. 1266–1290. [CrossRef]
Bugg, T. D., Ahmad, M., Hardiman, E. M., and Rahmanpour, R., 2011, “Pathways for Degradation of Lignin in Bacteria and Fungi,” Nat. Prod. Rep., 28(12), pp. 1883–1896. [CrossRef] [PubMed]
Popescu, C. M., Singurel, G., Popescu, M. C., Vasile, C., Argyropoulos, D. S., and Willför, S., 2009, “Vibrational Spectroscopy and X-ray Diffraction Methods to Establish the Differences Between Hardwood and Softwood,” Carbohydr. Polym., 77(4), pp. 851–857. [CrossRef]
Huang, Y., Wang, L., Chao, Y., Nawawi, D. S., Akiyama, T., Yokoyama, T., and Matsumoto, Y., 2012, “Analysis of Lignin Aromatic Structure in Wood Based on the IR Spectrum,” J. Wood Chem. Technol., 32(4), pp. 294–303. [CrossRef]
Boeriu, C. G., Bravo, D., Gosselink, R. J., and van Dam, J. E., 2004, “Characterisation of Structure-Dependent Functional Properties of Lignin With Infrared Spectroscopy,” Ind. Crops Prod., 20(2), pp. 205–218. [CrossRef]
Kumar, S., Negi, Y. S., and Upadhyaya, J. S., 2010, “Studies on Characterization of Corn Cob Based Nanoparticles,” Adv. Mater. Lett., 1(3), pp. 246–253. [CrossRef]
Monshi, A., Foroughi, M. R., and Monshi, M. R., 2012, “Modified Scherrer Equation to Estimate More Accurately Nano-Crystallite Size Using XRD,” World J. Nano Sci. Eng., 2, pp. 154–160. [CrossRef]
Cullity, B. D., 1957, “Elements of X-ray Diffraction,” Am. J. Phys., 25(6), pp. 394–395. [CrossRef]
Russ, J. C., 1984, Fundamentals of Energy Dispersive X-Ray Analysis, Butterworths, London, UK.
Chen, S., He, G., Carmona-Martinez, A. A., Agarwal, S., Greiner, A., Hou, H., and Schröder, U., 2011, “Electrospun Carbon Fiber Mat With Layered Architecture for Anode in Microbial Fuel Cells,” Electrochem. Commun., 13(10), pp. 1026–1029. [CrossRef]
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References

Zakzeski, J., Bruijnincx, P. C., Jongerius, A. L., and Weckhuysen, B. M., 2010, “The Catalytic Valorization of Lignin for the Production of Renewable Chemicals,” Chem. Rev., 110(6), pp. 3552–3599. [CrossRef] [PubMed]
Baker, D. A., and Rials, T. G., 2013, “Recent Advances in Low-Cost Carbon Fiber Manufacture From Lignin,” J. Appl. Polym. Sci., 130(2), pp. 713–728. [CrossRef]
Morgan, P., 2005, “Precursors for Carbon Fiber Manufacture,” Carbon Fibers and Their Composites, Vol. 121, Taylor & Francis Group, LLC, Boca Raton, FL. [CrossRef]
Hüttinger, K. J., Figueiredo, J. L., Bernardo, C. A., Baker, R. T. K., and Hüttinger, K. J., 1990, Carbon Fibers, Filaments and Composites, Vol. 177, Springer, The Netherlands.
Frank, E., Hermanutz, F., and Buchmeiser, M. R., 2012, “Carbon Fibers: Precursors, Manufacturing, and Properties,” Macromol. Mater. Eng., 297(6), pp. 493–501. [CrossRef]
Boehm, H. P., 1994, “Some Aspects of the Surface Chemistry of Carbon Blacks and Other Carbons,” Carbon, 32(5), pp. 759–769. [CrossRef]
Kadla, J. F., Kubo, S., Gilbert, R. D., and Venditti, R. A., 2002, “Lignin-Based Carbon Fibers,” Chemical Modification, Properties, and Usage of Lignin, Springer, New York, pp. 121–137.
Kubo, S., and Kadla, J. F., 2006, “Carbon Fibers From Lignin-Recyclable Plastic Blends,” Encyclopedia of Chemical Processing, Taylor & Francis, UK, pp. 317–331.
Broughton, J. R., R. M., and Brady, P., 1995, “Fiber Forming Polymers,” Wellington Sears Handbook of Industrial Textiles, Vol. 31, CRC Press.
Baker, D. A., Gallego, N. C., and Baker, F. S., 2012, “On the Characterization and Spinning of an Organic ‐Purified Lignin Toward the Manufacture of Low‐Cost Carbon Fiber,” J. Appl. Polym. Sci., 124(1), pp. 227–234. [CrossRef]
Maradur, S. P., Kim, C. H., Kim, S. Y., Kim, B. H., Kim, W. C., and Yang, K. S., 2012, “Preparation of Carbon Fibers From a Lignin Copolymer with Polyacrylonitrile,” Synth. Met., 162(5), pp. 453–459. [CrossRef]
Kadla, J. F., Kubo, S., Venditti, R. A., Gilbert, R. D., Compere, A. L., and Griffith, W., 2002, “Lignin-Based Carbon Fibers for Composite Fiber Applications,” Carbon, 40(15), pp. 2913–2920. [CrossRef]
Kadla, J. F., and Kubo, S., 2004, “Lignin-Based Polymer Blends: Analysis of Intermolecular Interactions in Lignin–Synthetic Polymer Blends,” Composites Part A, 35(3), pp. 395–400. [CrossRef]
Kubo, S., Uraki, Y., and Sano, Y., 1996, “Thermomechanical Analysis of Isolated Lignins,” Holzforschung, 50(2), pp. 144–150. [CrossRef]
Kubo, S., and Kadla, J. F., 2005, “Lignin-Based Carbon Fibers: Effect of Synthetic Polymer Blending on Fiber Properties,” J. Polym. Environ., 13(2), pp. 97–105. [CrossRef]
Compere, A. L., Griffth, W. L., Leitten, C. F., Jr., and Pickel, J. M., 2005, “Evaluation of Lignin From Alkaline-Pulped Hardwood Black Liquor,” http://www.ornl.gov/info/reports/2005/3445605475900.pdf
Wahyuni, E. T., Kunarti, E. S., and Sugiharto, E., “Performance of TiO2 Nanoparticle Prepared on Lignin Structure as Photocatalyst for Hazardous Mercury Removal Through Photoreduction Mechanism,” Available at http://mipa.ugm.ac.id/web/files/publikasi/paper-endangtw.pdf
Kubo, S., Yasumitsu, U., and Yoshihiro, S., 2003, “Catalytic Graphitization of Hardwood Acetic Acid Lignin With Nickel Acetate,” J. Wood Sci., 49(2), pp. 188–192. [CrossRef]
Ansari, K. B., and Gaikar, V. G., 2013, “Green Hydrotropic Extraction Technology for Delignification of Sugarcane Bagasse by Using Alkybenzene Sulfonates as Hydrotropes,” Chem. Eng. Sci., pp. 157–166.
Brodin, I., Sjöholm, E., and Gellerstedt, G., 2009, “Kraft Lignin as Feedstock for Chemical Products: The Effects of Membrane Filtration,” Holzforschung, 63(3), pp. 290–297. [CrossRef]
Wallberg, O., Jönsson, A. S., and Wimmerstedt, R., 2003, “Fractionation and Concentration of Kraft Black Liquor Lignin With Ultrafiltration,” Desalination, 154(2), pp. 187–199. [CrossRef]
Dallmeyer, J. I., 2013, “Preparation and Characterization of Lignin Nanofibre-Based Materials Obtained by Electrostatic Spinning,” Ph.D. thesis, https://circle.ubc.ca/bitstream/./ubc_2013_spring_dallmeyer_james.pdf
Laurichesse, S., and Avérous, L., 2014, “Chemical Modification of Lignins: Towards Biobased Polymers,” Prog. Polym. Sci, 39, pp. 1266–1290. [CrossRef]
Bugg, T. D., Ahmad, M., Hardiman, E. M., and Rahmanpour, R., 2011, “Pathways for Degradation of Lignin in Bacteria and Fungi,” Nat. Prod. Rep., 28(12), pp. 1883–1896. [CrossRef] [PubMed]
Popescu, C. M., Singurel, G., Popescu, M. C., Vasile, C., Argyropoulos, D. S., and Willför, S., 2009, “Vibrational Spectroscopy and X-ray Diffraction Methods to Establish the Differences Between Hardwood and Softwood,” Carbohydr. Polym., 77(4), pp. 851–857. [CrossRef]
Huang, Y., Wang, L., Chao, Y., Nawawi, D. S., Akiyama, T., Yokoyama, T., and Matsumoto, Y., 2012, “Analysis of Lignin Aromatic Structure in Wood Based on the IR Spectrum,” J. Wood Chem. Technol., 32(4), pp. 294–303. [CrossRef]
Boeriu, C. G., Bravo, D., Gosselink, R. J., and van Dam, J. E., 2004, “Characterisation of Structure-Dependent Functional Properties of Lignin With Infrared Spectroscopy,” Ind. Crops Prod., 20(2), pp. 205–218. [CrossRef]
Kumar, S., Negi, Y. S., and Upadhyaya, J. S., 2010, “Studies on Characterization of Corn Cob Based Nanoparticles,” Adv. Mater. Lett., 1(3), pp. 246–253. [CrossRef]
Monshi, A., Foroughi, M. R., and Monshi, M. R., 2012, “Modified Scherrer Equation to Estimate More Accurately Nano-Crystallite Size Using XRD,” World J. Nano Sci. Eng., 2, pp. 154–160. [CrossRef]
Cullity, B. D., 1957, “Elements of X-ray Diffraction,” Am. J. Phys., 25(6), pp. 394–395. [CrossRef]
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Figures

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Fig. 1

FTIR spectra of lignin powders

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Fig. 2

XRD patterns of the lignin powders

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Fig. 3

Carbon and sulfur content of the lignin powder based on EDS analysis (error bars: standard deviation, n>6)

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Fig. 4

SEM images of the SKL-F4 nanofibers: (a) as-spun, (b) thermostabilized, and (c) carbonized sample. Scale bar is 10 μm, d is the average diameter of the fibers.

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Fig. 5

TGA analysis of the SKL-F4 (filled line: in air, dashed line: in nitrogen)

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Fig. 6

XRD patterns of the empty sample (a) empty holder, (b) SKL-F4 powder, (c) carbonized fiber mat, and (d) grinded carbonized sample. The SEM image of carbonized fiber mat, a schematic of carbonized fiber structure, and a schematic of sample preparation method are shown in the top right-hand side.

Tables

Table Grahic Jump Location
Table 1 Location of the maximum peak for different Kraft lignin samples in XRD patterns, peak fitting method, and mean size of the ordered domain

Errata

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