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Azadeh GoudarziDepartment of Materials Engineering,

University of British Columbia,

6350 Stores Road,

Vancouver, BC V6T 1Z4, Canada

e-mail: [email protected]

Li-Ting LinDepartment of Materials Engineering,


6350 Stores Road,


Frank K. KoDepartment of Materials Engineering,


6350 Stores Road,


X-Ray Diffraction Analysis ofKraft Lignins and Lignin-DerivedCarbon Nanofibers

Lignin is a renewable material and it is abundantly available as low priced industrialresidue. Lignin-based carbon fibers are economically attractive and sustainable. In addi-tion, remarkably oxidized molecule of the lignin decreases the required time and temper-ature of the thermostabilization process compared to other carbon fiber precursors suchas polyacrylonitrile (PAN); and thus, decreases the processing cost of carbon fiber pro-duction. The fraction 4 of softwood Kraft lignin (SKL-F4) was previously shown to bespinnable via electrospinning to produce carbon nanofibers. In this paper, we character-ized different Kraft lignin powders through X-ray diffraction (XRD) analysis to measurethe mean size of the ordered domains in different fractionations of softwood and hard-wood samples. According to our results, SKL-F4 has largest ordered domains amongSKLs and highest hydroxyl content according to Fourier transform infrared (FTIR) anal-ysis. In addition, variations in the XRD patterns during carbon nanofiber formation werestudied and the peak for (101) plane in graphite was observed in the carbon nanofibercarbonized at 1000 C.[DOI: 10.1115/1.4028300]

Keywords: lignin, XRD, carbon nanofiber, electrospinning

Introduction

Lignin is one of the most abundant renewable materials onearth. Kraft lignin is available as an industrial residue in largequantities, which is usually burnt as low efficiency fuel [1,2]. Oneof the promising potential applications for lignin valorization isproduction of advanced material, such as lignin-based carbonfibers (CFs). The low cost of lignin, and the fact that it is a renew-able resource, make it an attractive material to fabricate CFs. Thepredominant 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 1015%growth rate is maintained [46]. The price of CF has dropped tem-porarily in recent years, but is still too high for widespread indus-trialization. A low cost alternative like lignin would expand therange of applications. The cost of PAN is over 50% of the totalmanufacturing cost of the CF and commercial grade PAN-basedCF is around $20/kg. With lignin-based CF, the manufacturingcost could be reduced to less than $10/kg, making lignin an attrac-tive alternate precursor for CF production [2]. In addition to lowercost of lignin as the precursor, lignin molecule is substantially oxi-dized compared to PAN, therefore oxidative thermostabilizationprocess of lignin fibers requires shorter stabilization time andlower stabilization temperature than PAN fibers, which translatesto 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., lin-ear and flexible structure, high degree of symmetry, high molecu-lar mass> 1000A, high degree of crystallinity, high degree oforientation, and high carbon content) [9]. However, lignin CFshave successfully been prepared from different types of ligninsthrough different methods (electrospinning, melt-spinning, andwet spinning) [10,11]. Yet, the challenge remains preparing aconsistent and uniform lignin substrate from such complex andvariable compound [7,8]. The reported mechanical properties ofthe lignin-based CFs are relatively lower than the PAN-based CFs(e.g., tensile strength of 0.51 GPa compared to 37 GPa)

[8,1216]. Understanding the process in which the complex mole-cule of lignin forms fiber and the process of carbonization of suchfiber is not clearly known. Understanding such process mightassist improving the properties in the future. Therefore, in thispaper we studied XRD analysis of different Kraft lignin powdersto examine the mean size of the ordered domains. We also studiedthe carbonized lignin fibers produced from electrospinning pro-cess. XRD analysis previously was reported for lignins [1719],but to our knowledge the size of the ordered domains and diffrac-tion angel of the Kraft lignins were not reported.

Materials and Methods

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 mole-cules, either through dissolving/precipitating the lignin in differ-ent organic solvents or using membranes [20,21]. Dallmeyerstudied electrospinning of softwood lignins and showed that thehigher molecular weight fraction, fraction 4, is spinnable throughelectrospinning [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.% concentra-tion. The polymer solution was electrospun using a number

25-gauge needle spinneret at a spinning distance of 18 cm, at20 kV applied voltage, and a 0.03 ml/min flow rate. The fiber matwas hold in a stretched position using Teflon coated glass sheetsin a glass petri-dish and thermostabilized for 1 h at 200 C withheating rate of 3 C/min in air. Then, the fiber mat is carbonizedin N2environment in a tube furnace at 1000

C for 1h.The XRD analysis instrument was a Rigaku model MultiFlex

diffractometer, running on Cu-Ka(k 1.542 A). The lignin pow-der data were collected at standard 2 deg/min scan rate. Thehighly porous nature of fiber mats required longer data collectiontimes to obtain sufficient amount of diffracted X-rays, thereforescan rate of 0.05 deg/min was applied. A Perkin Elmer spectrum100 was used to perform FTIR analysis, using attenuated totalreflection sampling technique and a diamond coated Tl-Br-I plate.

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

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Microstructure analysis was conducted by a Hitachi S3000NTungsten hairpin filament source SEM. To calculate the fiberdiameter, the average of 100 measurements in four different SEMimage was used. A Q500 TA instrument was used for thermogravimetric analysis (TGA). The tests were performed in N2 andair, by heating rate of 10 C/min.

Results and Discussion

Lignins have three precursors: p-coumaryl alcohol, coiferyl

alcohol, and sinapyl alcohol [1,23]. The polymerization of thesethree primary monomers results in formation of structural units inlignin: p-hydroxyphenyl (H), guaiacyl (G), and syringyl, respec-tively [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 char-acterization between hardwood and softwood samples [25]. Figure1shows FTIR spectra of the lignin powders. The hydroxyl regionof the spectra (3000 cm13700 cm1) shows that SKL-F4had thehighest amount of hydroxyl groups available. A distinctive differ-ence between HKLs and SKLs is the ratio of the peaks at1130 cm1 and 1030 cm1: in softwoods 1130 cm1< 1030 cm1

and in hardwoods 1130cm1> 1030 cm1. The peak at10301035 cm1 is allocated to Aromatic CH in plane deforma-tion, where amount of G units are higher than S units, this regionis also allocated to CO deformation in primary alcohols, andCO stretch (unconjugated) [26,27]. Peaks at the wavenumberrange of 12661270 cm1 are associated with G ring vibrationsand as shown in Fig. 2, they are not present in HKL samples.Twin peak at 855cm1 and 815 cm1 is assigned to softwoodsand single peak at 835 cm1 to hardwoods [26]. The presence ofmore methoxyl groups attached to the aromatic rings in HKLs (Sunit) inhibits formation of 55 or dibenzodioxocin linkages [1],where for SKLs b-O-4 linkages are more predominant. Differen-ces in the available functional groups might affect stacking of themolecules and affect size of the ordered domains in the samples.To calculate the size of the ordered domains, XRD analysis wasacquired. Figure 2 shows XRD patterns of the three fractions ofHKLs, three fractions of SKLs and HP-L powder samples. Foreach sample, peak fitting was performed for four different refine-ments: PearsonVII, Pseudo-Vigot, Lorentzian, and Gaussian.

Gaussian character was predominant in overall peak shape for

softwood samples and Lorentzian character was predominant inhardwood samples and HP-L. After fitting the curves, the maxi-mum diffraction angel was determined for each sample. The aver-age peak for hardwoods is located at 2h 21.26 0.15 deg and for

softwood it is located at 2h 19.356 0.18 deg (Table1shows thevalues for each sample). Kubo et al. reported the diffraction angelof 22.7 deg for hardwood acetic acid lignin [18] and Ansari andGaikar reported 2h 22.37 deg for lignin from sugar mill [19].Such differences could be caused by the difference in the lignintype. The Scherrers equation is usually used for calculatingthe mean size of the ordered domains (crystallite):dBk=bcosh, where dis the mean size of the ordered domains,B dimensionless shape factor (value of 0.9 is used [28,29]),kX-ray wavelength (0.154 nm for Cu K-a), and b full widthat half maximum (FWHM) [30]. To measure the FWHM, instru-mental broadening effect should be subtracted from the data.Table1 shows the results of calculated mean size of the ordereddomains (d in Scherrers 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

Fig. 1 FTIR spectra of lignin powders

Fig. 2 XRD patterns of the lignin powders

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electrospinning method [22]. Among HKLs, the unfractionatedsample (HKL) had the highest size of the ordered domains. How-ever, because unfractionated sample is composed of differentsized molecules in its structure (higher polydispersity index),when spinning parameters were adjusted for one portion of themolecular weight (e.g., viscosity of the solution), the other por-

tions were not spinnable. In other words, although some fiber for-mation was observed in electrospinning, spraying also occurred.Therefore, SKL-F4 was selected for nanofiber production viaelectrospinning and further analysis of the nanofibers.

Carbon content is an important factor in a CF precursor. Figure3shows the results of elemental analysis of the lignin powders byenergy-dispersive X-ray spectroscopy (EDS) method to measurecarbon and sulfur content. EDS is considered a qualitative methodfor elemental analysis, specially, when the energy region in below3 keV. A procedure in which corrections for atomic numbereffects (Z), absorption (A), and fluorescence (F) are calculated,are called ZAF correction and usually available in instrumentsettings [31]. The ZAF correction was selected for better

accuracy of the results. However, we are aware that the resultspresented in Fig. 3 are relative rather than absolute values. Thepresented EDS results are the average of at least six analyses.Relatively, HP-L sample was a sulfur-free lignin and the sulfurcontent of the SKL-F4was the highest among SKLs. SKL-F4waspreviously shown to be spinnable via electrospinning [22]. As a

summary from the lignin powder characterization, SKL-F4 hadthe highest hydroxyl and sulfur content, and the highest size of theordered domain among SKLs. We produced and electrospunSKL-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 carbonand sulfur content. The sulfur content was decreased by thermaltreatment, which probably could be due to the release of SO 2gasduring the heat treatment. Further investigation such as analyzingthe gas content of the sample is required to confirm release of SO2gas. The decomposition temperature in air was higher than decom-position temperature in N2 environment for the SKL-F4 ligninpowder (Fig.5, Td in air: 475 C and in N2was 376

C, SD< 1 Cfor three tests). The reason for oxidative stabilization (thermosta-bilization) process is to increase stability of the molecule prior thecarbonization process and avoid fusion of the fibers. Figure 6shows XRD patterns of empty sample holder, SKL-F4 powder,carbonized fiber mat, and grinded carbonized fiber mat. The topright hand side of Fig.6shows the SEM image of carbonized fibermat, a schematic of carbonized fiber structure, and a schematic ofsample preparation method. For fiber mat sample, we used a simi-lar method to XRD method for thin film characterization. For ana-lyzing thin films with XRD, smaller slits in the receiving detectorshould be applied. However, our results showed using smaller slitto increase the resolution of the recording peak was not effectivefor the CF mat sample, possibly due to the nano-sized/amorphousnature of the CF structure and broadness of the peak(s). Therefore,the only effective parameter was decreasing the scanning rate

Table 1 Location of the maximum peak for different Kraft ligninsamples in XRD patterns, peak fitting method, and mean size ofthe ordered domain

Samplename

Peak fittingmethod 2h

Mean size of theordered domain (nm)

HKL Lorentzian 21.25 0.75HKL-F2 Lorentzian 21.22 0.73HKL-F4 Lorentzian 20.99 0.63HPL Lorentzian 21.35 0.69SKL Gaussian 19.42 0.67SKL-F2 Gaussian 19.48 0.68SKL-F4 Gaussian 19.14 0.71

Fig. 3 Carbon and sulfur content of the lignin powder basedon EDS analysis (error bars: standard deviation, n>6)

Fig. 4 SEM images of the SKL-F4 nanofibers: (a) as-spun, (b) thermostabilized, and (c) car-bonized sample. Scale bar is 10 lm, dis the average diameter of the fibers.

Fig. 5 TGA analysis of the SKL-F4 (filled line: in air, dashedline: in nitrogen)

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(increasing the data collection time). The graphite peak for (101)plane (marked with * in Fig. 6(d) pattern) was not detected forfiber mat sample. However, the peak for (101) plane is a distinc-tive characteristic of graphite formation and was available in thegrinded CF mat XRD pattern. The EDS analysis showed thatalthough the sulfur content of the as spun fiber mat was reducedsignificantly during carbonization process, but still some sulfur isavailable in the CF structure after carbonizing at 1000 C. EDSanalysis of the PAN based CFs do not show the presence of anysulfur in their structure [32,33]. However, Hwang et al. manuallyadded elemental sulfur to the PAN based nanofibers and suggesteddehydrogenation process during reaction of PAN with sulfur

facilitates intermolecular cross-linkage and therefore resulted inhigher degree of graphitization [34]. In addition, they concludedthat the sulfurization reaction affects configuration of the turbos-tratic carbon and increases efficiency ofpp stacking [34]. Kraftlignin precursors have remarkably higher oxidized molecules thanPAN [2] and they contain sulfate groups in their structure.

Conclusion

In summary, analyzing different Kraft lignin samples, HKL(unfractionated hardwood Kraft lignin) and SKL-F4(fraction 4 ofSKL) had the highest size of ordered domain according to XRDanalysis. SKL-F4also showed highest hydroxyl content accordingto FTIR analysis. Due to the challenges in electrospinning ofunfractionated HKL sample, we chose SKL-F4 for electrospin-ning. As-spun fibers from SKL-F4 precursor were thermostabi-

lized and carbonized to produce CFs. XRD analysis of the carbonnanofibers indicated that the graphite peak for (101) plane wasavailable in the grinded sample. According to literature, for PANfibers sulfur facilitates graphite formation. SKL-F4 showed high-est sulfur content among SKL samples. Our proposed hypothesisbased on the presented results is that the available sulfate groupsin Kraft lignins might facilitate graphite formation in carbonnanofiber production process. Further investigations are requiredto confirm such hypothesis.

Acknowledgment

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

Nomenclature

CF carbon fiberDMF dimethylformamideFTIR Fourier transform infraredHKL hardwood Kraft ligninPAN polyacrylonitrilePEO poly ethylene oxideSKL softwood Kraft lignin

SKL-F4 fraction 4 of SKL

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) grinded carbonized sample. The SEM image of car-bonized fiber mat, a schematic of carbonized fiber structure, and a schematic ofsample preparation method are shown in the top right-hand side.

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