Role of Physicochemical Structure of Organosolv Hardwood and

Research Article pubs.acs.org/journal/ascecg

Role of Physicochemical Structure of Organosolv Hardwood and Herbaceous Lignins on Carbon Fiber Performance Omid Hosseinaei,* David P. Harper,* Joseph J. Bozell, and Timothy G. Rials Center for Renewable Carbon, University of Tennessee, 2506 Jacob Drive, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Lignin was extracted from Alamo switchgrass (Panicum virgatum) and yellow polar (Liriodendron tulipifera) by organosolv fractionation at different pretreatment temperatures, and its chemical structure was studied by means of elemental analysis and spectroscopy. Thermal properties of lignins were investigated using thermogravimetric analysis and differential scanning calorimetry. Lignin fibers were produced via melt-spinning by a twin-screw extruder with a custom spinneret. Fibers were thermostabilized at different rates and finally carbonized. In both species, lignin obtained from higher severity organosolv fractionation had fewer impurities, higher content of phenolic hydroxyl groups, and more condensed structures as a result of extensive cleavage of aryl ether linkages. Higher organosolv severity improved the ability to spin fibers; in the case of switchgrass, only the high severity sample was spinnable. High severity also decreased thermostabilization time and increased tensile strength and modulus of carbon fibers. A decrease in the ratio of ether linkages to condensed units appears to be the main reason for faster stabilization. Switchgrass lignin had less thermal stability at low temperature that results in formation of volatiles, mainly due to the presence of ester-linked phenolic acids. These volatiles are more prevalent at low severity and prevent forming continuous fibers during spinning. As a result, pores forming on the surface of switchgrass fibers led to lower strength. Tuning the severity of fractionation is recommended as an easy method to change lignin characteristics, to find the proper severity range, and to produce lignin suitable for carbon fiber production. KEYWORDS: NMR spectroscopy, aryl ether linkages, hydroxyl groups, thermal decomposition, glass transition temperature, thermostabilization

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INTRODUCTION The high tensile strength, low density, high creep resistance, and good resistance to chemicals (in absence of oxidizing agents) make carbon fibers a good choice for composite materials. However, carbon fiber’s high price limits its use to specialty applications, such as aerospace and sporting goods. Polyacrylonitrile (PAN), the dominant precursor for making carbon fibers, contributes about 51% to the total manufacturing cost.1 Therefore, finding alternative precursors is a key target in reducing the overall cost of carbon fibers and expanding their use to more applications. Lignin is the second most abundant natural polymer in the plant cell wall, and the growth of the biorefining industry is expanding the availability of lignin as a chemical building block. Importantly, its low cost, high carbon content, and aromatic structure make it an attractive choice as a precursor for carbon fibers. Investigations of lignin as a precursor for carbon fibers started in the 1960s, first mainly using solution spinning, and later via melt-spinning.2−7 Electrospinning also has been commonly used to produce submicron carbon fibers.8−12 The history of efforts for producing lignin-based carbon fibers have been extensively discussed in review papers published by Frank et al. and Baker and Rials.1,7 Most research has explored © XXXX American Chemical Society

plasticizing lignin, chemically or through blending lignin with other polymers, for melt-spinning.3−6,13−15 Kadla et al. have different reports on efforts to melt-spin and produce ligninbased carbon fibers from organosolv lignin (Alcell), hardwood kraft, softwood kraft, and blends of kraft lignin with various synthetic polymers include poly(ethylene oxide) (PEO), polyethylene terephthalate (PET), and polypropylene (PP).3,5,16 Different methods for chemical modification of lignins, such as hydrogenolysis, phenolysis, and acetylation also have been tried to improve melt-spinning of lignin.4,13,14,17 Nordstrom et al. used fractionated softwood kraft lignin, fractionated hardwood kraft lignin, and a blend of fractionated hardwood kraft lignin (as plasticizer) with unfractionated softwood and hardwood kraft lignin for melt-spinning.15 Baker et al. used solvent extraction to improve spinnability of hardwood kraft lignin.6 Among the different technical lignins studied for making carbon fibers, organosolv lignin has demonstrated the most promise due to its high purity, melt characteristics, low ash, and sulfur content.3,13,14 These Received: August 1, 2016 Revised: September 2, 2016

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DOI: 10.1021/acssuschemeng.6b01828 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

(3 L of deionized water, room temperature, 12 h) to remove impurities (residual extractives and sugars). The final lignin was filtered through a paper filter and dried in a vacuum oven at 80 °C for 12 h. Lignin Analysis. The ash content and lignin content (acidsoluble plus acid-insoluble lignin) of the isolated samples was determined using standard methods.33,34 The carbon, hydrogen, and nitrogen content of each sample was measured in triplicate using a PerkinElmer 2400II CHNS/O combustion elemental analyzer. The lignin melt flow temperature (Tm) was determined optically using a Fisher-Johns melting point apparatus. Thermal decomposition of lignin samples was measured using a PerkinElmer Pyris 1 thermogravimetric analyzer. Thermogravimetric analyses were conducted in triplicate using 5 mg specimens, which were heated from 100 to 950 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere (10 mL min−1). Glass transition temperatures (Tg) were determined in triplicate on 2 mg samples of lignin using a PerkinElmer differential scanning calorimeter. Each specimen was heated at a rate of 500 °C min−1 under nitrogen (UHP, 20 mL min−1) to 140 °C and held at that temperature until the change in thermal energy from the sample was zero to expel any remaining moisture in the sample. The sample was then heated to 200 °C at a rate of 500 °C min−1 and immediately cooled at the same rate to erase the thermal history of the sample. A second DSC trace was obtained by heating from 25 to 220 °C at 20 °C min−1. This trace was used for calculation of Tg and the corresponding heat capacity (ΔCp). Infrared spectra were collected by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy on a PerkinElmer Spectrum One/Golden Gate ATR instrument, from five independent subsamples of each lignin. Each spectrum was averaged over 32 scans at a resolution of 4 cm−1 between 4000 and 600 cm−1. Samples for UV spectroscopy were prepared using the method of Scalbert et al.35 A lignin sample (10 mg) was dissolved in 25 mL of dioxane-water 9:1 (v/v), and a 3 mL aliquot was diluted to 25 mL with dioxane-water 1:1 (v/v). Absorbance between 200 and 350 nm was recorded on a Thermo Scientific UV−vis spectrometer. All NMR experiments were performed using a Varian 400-MR spectrometer operating at frequency of 399.78 MHz for proton and 100.54 MHz for carbon. For 2D (HSQC) spectroscopy, 100 mg of lignin were dissolved in 0.75 mL of DMSO-d6. NMR spectra were recorded at 25 °C using the (HC)bsgHSQCAD pulse program. The experiment used 32 transients and 512 time increments in the 13C dimension. A 90° pulse with a pulse delay of 1.5 s, an acquisition time of 0.15 s and a 1JCH of 147 Hz were employed. DMSO was used as an internal reference. Data processing and analysis were performed using Mnova version 8.1.2 (Mestrelab Research). HSQC crosssignals were assigned by correlation with literature databases.22−24,30,36−39 Semiquantitative analysis of volume integrals was performed and interunit linkages were calculated as a percentage of total side chains.39 In the aromatic region, the relative abundance of syringyl (S), guaiacyl (G), p-hydroxyphenyl (H), p-coumaryl (pCA), and feruyl (FA) units were estimated from their C2−H2 correlations.39 For quantitative 13C NMR spectroscopy, 200 mg of lignin was dissolved in 0.75 mL of DMSO-d6. An inverse-gated decoupling pulse sequence was used with a 90° pulse angle, 11 s relaxation delay, and an acquisition time of 1.41 s. A total of 18 000 scans were recorded. Quantitation of lignin’s hydroxyl groups was carried out with 31P NMR by initial derivatization with 100 μL of 2chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP).

characteristics enable high thermal mobility and lower glass transition temperatures (Tg), which make it fusible and meltspinnable (i. e., able to form fiber by melt-spinning).18 However, there is a lack of information about the effect of physicochemical properties of lignin, which vary by severity of pretreatment and botanical source, especially in the case of herbaceous plants, on lignin melt-spinning behavior and properties of carbon fibers. Severity of fractionation changes lignin properties;19,20 consequently, this can affect melt-spinning performance, conversion of fibers from green to carbonized, and final properties of carbon fibers. In the case of lignin from herbaceous plants, to the best of our knowledge, there is no report about producing carbon fibers to date. Differences in type and ratio of phenolic units in herbaceous plants, compared to softwoods and hardwoods, affects properties of lignin.21−23 These differences potentially can impact melt-spinning behavior and properties of resulting carbon fibers. Herbaceous plants, especially switchgrass, have gained significant interest in biofuel production and for using plant lignin in materials and chemicals to increase application of these high-productivity species.19,24−28 To understand the effect of severity of fractionation and botanical source of lignin on melt-spinning and properties of lignin fibers, we investigate the thermal properties and chemical structure of organosolv lignin from two botanical sources (hardwoods and herbaceous plants) and different severities. Isolated lignins were subsequently used for melt-spinning and producing lignin fibers. Lignin fibers were thermostabilized at different rates to study how lignin properties affect converting green to thermostabilized fibers. Finally, lignin fibers were carbonized, and mechanical properties were measured along with evaluating morphology. The results provide new information about the effect of lignin physicochemical characteristics on manufacturing lignin-based carbon fibers, especially in the case of herbaceous plants.

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EXPERIMENTAL SECTION Materials. Lignin samples were obtained from organosolv fractionation of Alamo switchgrass (knife milled to 2−5 cm) and pulp-grade yellow poplar chips (about 4 cm2 and thicknesses of 0.5−1 cm). Fractionation and lignin isolation was performed using a previously described process.29−31 In brief, the biomass sample was treated in a flow-through reactor with a 16:34:50 wt % mixture of methyl isobutyl ketone (MIBK), ethanol, and water in the presence of sulfuric acid (0.05 M) at a temperature of 140 or 160 °C for 120 min. The calculated severity at these temperatures was 1.91 (low severity, LS) and 2.50 (high severity, HS).32 The black liquor fraction containing dissolved lignin and hemicelluose was separated into a hemicellulose-rich aqueous phase and a lignin-rich organic phase by adding solid NaCl (10 g per 100 mL of deionized water in the initial solvent mixture) in a separatory funnel. After observation of phase separation between the aqueous and organic phases, the aqueous phase that rests at the bottom of the separatory funnel was drained and set aside. The organic phase was washed twice by adding 30% v/v deionized water to remove residual sugars and ethanol from the darker organic phase. Lignin from both phases was isolated by rotary evaporation. This is accomplished by removing solvent until the lignin percipitates in the case of aqueous phase and compelete evaporation of solvent in case of the organic phase. This is followed by trituration of the solid residue with diethyl ether (5× ∼ 200 mL) and final washing with deionized water B


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Table 1. Composition of Switchgrass (S) and Yellow Poplar (YP) Lignin Samples for Low (LS) and High (HS) Severity ASLa+AILb (%) ash (%) C (%) H (%) N (%) O (%)c C9 formulad a

YP-LS

YP-HS

S-LS

S-HS

93.7 0.07 63.0 6.12 0.25 30.6 C9H10.4O3.28

97.0 0.19 64.6 5.90 0.23 29.1 C9H9.79O3.04

91.3 0.10 62.3 6.01 0.93 30.7 C9H10.3O3.23

95.1 0.12 64.6 5.85 0.79 28.6 C9H9.71O2.99

Acid-soluble lignin. bAcid-insoluble lignin. cSubtracted from C, H, N, and ash. dNot corrected for impurities.

between lignin and carbohydrates, thus resulting in lignin with higher purity. Furthermore, elemental composition values are in the range of those reported for other technical and organosolv lignins.14,21,43,45,46 Nitrogen content in switchgrass lignin was higher than yellow poplar, due to higher protein-based contaminations in grasses.44,47 Strong covalent bonds between lignin and proteins, condensation reactions between proteinderived components and lignin, and possible physical trapping/ coprecipitation of protein-derived components are considered to be reasons for residual nitrogen in lignin, even after high severity fractionation.48,49 Carbon content is reduced by increased impurities, whereas more aromatic structure gives lignin higher carbon content compared to carbohydrate. Higher carbon content can increase carbon yield during conversion of lignin fibers to carbon fibers; therefore, it is expected that lignins from higher severity will have higher carbon yield. UV Spectroscopy. Lignin samples from high severity fractionations had more intense UV absorption compared to the lignin at low severity in both species (Figure 1), indicating

The derivatized samples (30 mg) were dissolved in 0.75 mL of pyridine and deuterated chloroform (1.6:1 v/v) and mixed with 100 μL of a solution of cyclohexanol (10 mg mL−1) and chromium(III) acetylacetonate (5 mg mL−1) as internal standard and relaxation agent, respectively.40,41 31P NMR spectra were acquired using an inverse-gated decoupling pulse sequence with a 90° pulse angle, 25 s relaxation delay, and 256 scans. Fiber Spinning and Processing. Melt spinning of the lignins was performed using a Haake MiniLab counter-rotating twin-screw extruder (Thermo Scientific). The extruder was modified with a 200 μm spinneret assembly with a heating band. The temperature of the extruder and spinneret was optimized for each lignin sample, and a 76 mm rotating cylinder was used to collect monofilament fibers. Oxidative thermostabilization of the fibers was performed by heating the samples to 250 °C under air at selected rates (0.05, 0.1, 0.2, and 0.5 °C min−1) and then holding the samples for 30 min at 250 °C in a Heratherm OGH60 oven (Thermo Scientific). The stabilized fibers were carbonized in a Lindberg/Blue M 25 mm tube furnace (Thermo Scientific) by heating from room temperature to 600 °C at a rate of 3 °C min−1, holding for 5 min at 600 °C, heating from 600 to 1000 °C at a rate of 5 °C min−1, and holding for 15 min at 1000 °C, under a nitrogen flow of 0.2 L min−1. Fiber morphology was determined on a Hitachi TM3000 tabletop microscope and LEO 1525 scanning electron microscopy (SEM). The tensile properties of carbon fibers were measured according to the ASTM standard (ASTM C1557−03) using an Instron 5943 single column tabletop testing system.42 Results were an average of 40 fibers per each sample.

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RESULTS AND DISSCUSION Characterization of Lignin. Compositional and Elemental Analysis. High-purity lignin is desired for making carbon fiber; impurities can limit fusibility, resulting in poor melt-spinning performance of lignin, and can cause defects in carbon fibers.3 Based on compositional analysis results (Table 1), all lignin samples had low amounts of contaminants (below 9%), with the likely impurities being residual carbohydrates (particularly hemicelluloses), proteins, and ash. The values were comparable to, or even higher than (in case of high severity conditions), other technical and organosolv lignins.21,25,26,43−45 Ash content in particular was lower than kraft lignin and other organosolv lignins from herbaceous plants.25,26,43,3 Elemental analysis results are consistent with the compositional analysis results; carbon content increases and nitrogen content decreases with increasing severity (Table 1), indicating a higher purity of lignin using high severity fractionation. More generally, increasing pretreatment severity (time, temperature, and catalyst) leads to more extensive cleavage of linkages

Figure 1. UV spectra of lignin samples.

higher purity.35,50,51 Yellow poplar lignin exhibits a maximum at 278 nm, related to nonconjugated phenolic groups.35,50 This maximum is shifted to 285 nm in switchgrass and is correlated with a higher concentration of G units.52 Switchgrass lignin has a second maximum at about 320 nm, corresponding to conjugated phenolic groups in pCA and FA units,35,50 which make up a significant part of the phenolic units present in grasses cell walls.50,53−55 UV absorption in switchgrass lignins was higher than yellow poplar lignin due to the higher extinction coefficient of G and pCA units compared to S units.50 The UV results are consistent with chemical C


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ACS Sustainable Chemistry & Engineering Table 2. Assignment of Bands in Middle Infrared Spectra of Lignin Samples band position (cm−1) 3600−3100 2950−2840 1717 1665 1600 1510 1460 1423 1325 1267 1215 1167 1031 1120 915 830

assignment O−H stretching56,57 C−H stretching in methyl and methylene groups58,59 CO stretching in unconjugated ketones, carbonyl and ester groups; conjugated carboxylic acids and esters at around 1700 cm−1 (switchgrass lignin)58−61 conjugated p-substituted ketones58,60 aromatic skeletal vibration plus CO stretch58,59,62 aromatic skeletal vibration58,59,62 C−H deformation in methyl and methylene groups59,62 aromatic skeletal vibration coupled by C−H in plane deformation59,62 syringyl plus condensed guaiacyl ring breathing59,62 guaiacyl ring breathing plus CO stretch59,62 C−C, C−O, and CO stretching (G condensed > G etherified)59 CO stretch in conjugated ester groups59 aromatic in-plane deformation (G > S; characteristic of uncondensed G units), C−H deformation in primary alcohol and unconjugated CO stretch59,62 aromatic C−H in-plane deformation, typical for S units; secondary alcohol and CO stretch59,62 aromatic C−H out-of-plane deformation of S units59,62 C−H out-of-plane in position 2 and 6 of S units and all positions of H unit48,51

organosolv processing, lignin depolymerizes by acid-catalyzed cleavage of ether linkages (α and β-aryl ether bonds).64,65 Specifically, cleavage of β-aryl-ether bonds results in the formation of Hibbert’s ketones (Scheme 1).62,66 This reaction increases with severity, which is consistent with the stronger peak for unconjugated β-ketones at about 1710 cm−1 in YP-HS. Switchgrass lignins, similar to other lignins from Gramineae, exhibit a strong band in the CO range as a result of higher concentrations of phenolic acids (pCA and FA). pCAs are often acetylated at the γ-position of lignin side-chains (especially in S units) leading to naturally esterified lignin.41,67 These ester units can potentially plasticize lignin and increase its thermal mobility,13,14,61 which subsequently can improve its meltspinning performance. On the other hand, other factors such as stronger intermolecular hydrogen bonding interactions in switchgrass lignin and higher percentage of G and H units can potentially decrease its thermal mobility. The intensity of the carbonyl band did not show a pronounced change with increasing severity in switchgrass lignins. The IR spectra confirm that switchgrass lignin possesses an HGS type lignin, as indicated by absorptions at 1167 and 830 cm−1, while the maximum at 1120 cm−1 in yellow poplar lignins indicates a GS type lignin. The intensity of the band at 1031 cm−1 was stronger in lignins from lower severity fractionation, implying higher condensation in G units of lignins from higher severity. Switchgrass lignin’s HGS structure can also accelerate thermostabilization of fibers due to the higher tendency of G and H units to cross-link when compared to S units. The presence of more condensed units at higher severity can increase carbon yield during carbonization step. 31 P NMR Spectroscopy. The hydroxyl group content and distribution in lignin samples was determined by 31P NMR spectroscopy (Table 3). In both switchgrass and yellow poplar lignin, increasing severity resulted in an increase in phenolic hydroxyl groups and a decrease in aliphatic hydroxyl groups. The number of phenolic hydroxyl groups increases mainly as a result of greater scission of β−O−4′ interunit linkages at higher severity,19,68,69 along with possible demethylation of lignin.40,70 On the other hand, aliphatic OH groups decreased at increasing severity mainly as a result of cleaving terminal hydroxymethyl groups on side chains of phenylpropane units as formaldehyde,71 and elimination of OH groups on side chains due

composition analysis, showing that higher severity produces the higher purity lignin desired for making carbon fibers. IR Spectroscopy. The position and assignment of major IR bands are summarized in Table 2.56−62 The IR spectra show clear differences between lignins based on the species. Compared to a botanical source, the fractionation severity has a lower impact on lignin’s functional group profile. The IR spectra of all lignin samples had a broad O−H stretching band at about 3400 cm−1, indicating strong intermolecular hydrogen bonding (Figure 2).18,56−58 This band is more symmetrical in

Figure 2. IR spectra of lignin samples.

switchgrass lignin but has a pronounced shoulder at about 3500 cm−1 in yellow poplar lignin. This suggests formation of more intramolecular hydrogen bonds in yellow poplar lignin, whereas switchgrass lignin exhibits more intermolecular hydrogen bonding.56,57 The observation of greater intermolecular hydrogen bonding in switchgrass lignin is significant, as this has been correlated with restricted thermal mobility and fusibility of the lignin,57,63 which hinders melt-spinning to fibers. In yellow poplar lignins, the intensity of bands assigned to CO stretching (1717 and 1665 cm−1) were stronger in lignin from high severity fractionation, whereas the band at 1717 cm−1 is shifted slightly to a lower wavenumber (1710 cm−1). During D


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ACS Sustainable Chemistry & Engineering Scheme 1. Possible Reactions of Lignin during Acid-Catalyzed Organosolv Fractionationa

(Route A) acidolytic cleavage of β-aryl ether linkages and formation of Hibbert’s ketones, (Route B) lignin−lignin condensation, (Route C) ethoxylation of benzylic position, (Route D) cleavage of β-aryl ether linkages and release of formaldehyde from Cγ, and (Route E) homolytic cleavage of β-aryl ether linkages and formation of stilbene (radical coupling).64−66,70,71,78,79.

a

Table 3. Hydroxyl Group Contents of Lignin Samples Obtained by Quantitative 31P NMR Spectroscopy (mmol g−1) phenolic OH sample ID

carboxylic acid OH (COOH)

p-hydroxyphenyl

C5-substituted

guaiacyl

syringyl

total phenolic OH

aliphatic OH

YP-LS YP-HS S-LS S-HS

0.04 0.04 0.18 0.17

0.03 0.06 0.49 0.60

0.23 0.38 0.19 0.34

0.56 0.70 0.62 0.71

1.46 2.56 0.35 0.55

2.28 3.70 1.65 2.20

2.69 1.49 2.84 2.05

to formation of β-1 linkages (stilbene structures) or Hibbert’s ketones.69,70 Ethoxylation of benzylic position (Scheme 1) also can decrease the number of aliphatic hydroxyl groups.72 pHydroxyphenyl OH groups are higher in grasses mainly due to the presence of pCA.41 Since hardwood lignins contain predominantly S and G lignin, the amount of p-hydroxyphenyl OH groups in yellow poplar lignin was lower than switchgrass. The number of hydroxyl groups in S-LS are close to values previously reported

for ethanol organosolv lignin.26 For yellow poplar lignins, the number and distribution of hydroxyl groups in YP-HS have more similarity to values reported for mixed hardwood organosolv (Alcell) lignin, especially in the case of aliphatic hydroxyl groups, although YP-HS had slightly higher number of phenolic hydroxyl groups.27 Yellow poplar has a higher content of S hydroxyl groups and total phenolic OH than switchgrass lignin, but aliphatic OH groups were greater in switchgrass lignins. Stronger intermolecular hydrogen bonds are more likely E


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3.3 ppm) also indicates formation of α-ethoxylated β−O−4′ linkages (Supporting Information, Figure S2).76,77 Ethoxylation has been suggested as a main reason for the lower Tg of organosolv lignin in comparison to kraft lignin, and will increase lignin’s thermal mobility and melt-spinning performance.18 While we have evidence for ethoxylation of the aromatic rings from our NMR studies, we currently have no evidence for the reaction of MIBK with lignin. Ongoing work is investigating the reactivity of MIBK under the fractionation conditions. Oxidation (α or β) is happening as a result of acidolytic cleavage of β−O−4′ linkages and formation of Hibbert’s ketones (Scheme 1).64−66,78,79 S units have a higher tendency than G units to undergo this oxidation,23 and this accounts for the greater proportion of this substructure in yellow poplar lignins. Phenylcoumaran (β−5′) units are synthesized from G and H units, whereas resinols (β−β′) are formed mainly from S units.23,80 Because switchgrass lignin is rich in G units while yellow poplar contains a higher number of S units, switchgrass exhibited a higher proportion of phenylcoumaran structures while yellow poplar is enriched with resinols. The increase in the relative abundance of resinol in yellow poplar and phenylcoumaran in switchgrass at higher severity is mainly due to higher cleavage of the more reactive β−O−4′ bonds and thus a reduction in their contribution to the overall number of substructural units identified. β−β and β−5′ structures may also form during condensation reactions that occur during fractionation, along with formation of stilbene through radical coupling (Scheme 1).70,78 Spirodienone units (β−1′/α−O−α′) were observed in yellow poplar but were not found in switchgrass lignin. A higher ratio of C−C linkages at higher severity indicates presence of more condensed structures, which can increase carbon yield during conversion of lignin fibers to carbon fibers. As expected, switchgrass lignin was highly acylated at Cγ, probably as the corresponding acetates or p-coumarates.39,74 Acylation decreased at higher severity, indicating cleavage of ester linkages between lignin and phenolic acids during fractionation. The S/G ratio calculated from integration of C−H pairs at the G2 and S2,6 positions increased with increasing severity. Overall, the intensity of cross-signals related to aromatic carbons decreased at higher severity (Supporting Information, Figure S3), possibly due to condensation reactions. This condensation is more prevalent with G units, resulting in a proportionally greater decrease in their signal and a concomitant increase in the overall S/G ratio at higher severity. Switchgrass samples contained a large proportion of phydroxycinnamate units which decreased at higher severity due to solvolysis of the ester bonds linking lignin and the phenolic acids. However, one must be cautious regarding the absolute percentage values. Integration of lignin end units such as pCA, FA, or H may be overestimated because of the rapid relaxation of the bulk polymer in comparison to terminal end units.22 Finally, anomeric and nonanomeric polysaccharide C−OH signals were observed in switchgrass spectra, but the intensity of these peaks decreased at higher severity (Supporting Information, Figure S3). Both phenolic acids and carbohydrates can decrease the thermal stability of lignin, which results in the formation of volatiles during melt-spinning and leading to fibers with voids and defects. In addition to 2D NMR spectroscopy, the structure of lignin samples was quantified using 13C NMR (Table 5). Assignments of peaks and spectral regions were made on the basis of literature correlations.37,70,80−85 The integral value of the region

to form between aliphatic hydroxyl groups, and intramolecular hydrogen bonding between phenolic hydroxyl groups limits the formation of intermolecular hydrogen bonds.57 IR spectra also indicate more intermolecular hydrogen bonding in switchgrass lignin compared to yellow poplar lignin, which can increase Tg and affect the fusibility of lignin.18,73 The amount of condensed phenolic OH groups increased slightly with increasing severity but carboxylic acid OH groups remained the same. The 31P results suggest that melt-spinning of switchgrass lignin, especially S-LS, will be more difficult due to the presence of a higher number of aliphatic hydroxyl groups and, consequently, stronger intermolecular hydrogen bonds. 2D and 13C NMR Spectroscopy. Integration of peaks in HSQC spectra of lignin represents information regarding the type and distribution of interunit linkages and aromatic units of lignins (Table 4).38,39,74,75 HSQC spectra and main correlation assignments are in the Supporting Information, and the main structures of lignins are illustrated in Scheme 2. Table 4. Lignin Structural Characteristics from Integration of 13C−1H Correlation Peaks in HSQC Spectra characteristics

YP-LS

YP-HS

S-LS

S-HS

33.1 3.90 23.1 9.44 24.8 5.69 0.04

18.2 6.67 4.53 0.00 58.1 12.5 0.24

54.8 0 20.2 24.7 0.26 0 27.4

41.4 0 19.7 37.0 1.86 0 20.1

75.7 24.3 0 3.12

85.5 14.5 0 5.90

41.0 55.5 3.47 0.74

48.3 47.6 4.06 1.01

0 0 0

0 0 0

37.6 14.3 2.63

32.6 13.7 2.39

a

lignin interunit linkages (%) β-O-4′ aryl ethers (A/A′) α-oxidized β-O-4′ aryl ethers (Aox) α-ethoxylated β-O-4′ aryl ethers (Aet) phenylcoumarans (B) resinols (C) spirodienones (D) lignin side chain γ-acylationb (%) lignin aromatic units S (%) G (%) H (%) S/G ratio p-hydroxycinnamatesc (%) p-coumarates ferulates pCA/F ratio a

Percentage of interunit linkages involved. bRatio of Cγ acylated (A′) and hydroxylated (A) β -O-4′ aryl ethers. cPercentage of lignin content (H+G+S).

Clear differences are present in the proportion of various interunit linkages in lignin samples and are correlated to both the botanical source and fractionation severity. β−O−4′ linkages were the dominant units in most samples, and their proportion decreased with increasing fractionation severity indicating greater depolymerization (Scheme 1). α-Ethoxylated β−O−4′ linkages were observed in the spectra of both switchgrass and yellow poplar lignin, whereas α-oxidized β-O4′ aryl ethers were observed only in yellow poplar lignins (Table 4; Supporting Information, Figure S2). Samples at low severity still have large portion of aryl ether linkages. These linkages increase the thermal mobility of lignin and decrease the Tg,63 but on the other hand, they can increase time required for thermostabilization of lignin fibers and decrease final carbon yield of fibers. The ethanol used during organosolv fractionation can also serve as a nucleophile, adding benzyl carbocations formed during cleavage of α ether bonds and leading to ethoxylation of the benzylic position (Scheme 1).18,65 The methylene peak in side chain region (δC/δH 64.0/ F


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ACS Sustainable Chemistry & Engineering Scheme 2. Main Structures Present in Ligninsa

(A) β-O-4′ aryl ethers, (A′) Cγ acylated β-O-4′ aryl ether, (Aox) α-oxidized β-O-4′ aryl ether, (Aet) α-ethoxylated β-O-4′ aryl ether, (B) phenylcoumaran, (C) resinol, (D) spirodienone, (E) stilbene, (pCA) p-coumarate, (F) ferulate, (S) syringyl unit, (Sox) α-oxidized syringyl unit, (G) guaiacyl unit, and (H) p-hydroxyphenyl unit. a

The aromatic region of lignin 13C NMR spectra can be divided into methine (124−102 ppm), condensed (141−124 ppm), and oxygenated (160−141 ppm) aromatic carbons (Supporting Information, Figure S4).37 The protonated aromatic region contains C2 and C6 methine carbons and any protonated G5 carbons, while the condensed aromatic region contains the C1 side chain position and the C5 position from condensed aromatic ring linkages. The oxygenated aromatic region contains mainly C3,4 in G units, and the C3,5 and 4−O− 5′ structures present in S units.37,70,82 Increased fractionation severity resulted in a decrease in the amount of protonated and oxygenated aromatic carbons and an increase in the number of condensed aromatic carbons (Table 5). An increase in number of condensed carbons is consistent with the formation of more C−C linkages between aromatic units during fragmentation and/or cleavage of aryl ether linkages (Scheme 1); this also further correlated with a decrease in the number of protonated carbons. The degree of condensation was calculated by subtracting the integrated value of protonated aromatic carbons (CAr−H) from the theoretical values of these units.37,82 The degree of condensation increased with increasing severity (Table 5), confirming observations from HSQC spectra. These results also agree with the elemental composition of lignins, which display an increase in carbon content and decrease in hydrogen content at high severity. Lignin with a more condensed structure (C−C linkages) can potentially undergo a faster stabilization during fiber formation as a result of having fewer aryl ether linkages. Therefore, lignins from higher severity fractionation can decrease the time required for conversion of lignin fibers to carbon fibers. In addition, G units also can undergo faster condensation reactions, due to having more unsubstituted carbon on the aromatic rings. Switchgrass lignin, with a higher percentage of G units, can potentially have faster stabilization rate than yellow poplar lignin. Higher carbon content of lignins with more condensed structure indicates higher carbon yield after conversion of lignin fibers to carbon fibers. Thermal Properties. The TGA curves of lignin samples indicate that their decomposition is mainly affected by their botanical source, rather than severity of fractionation (Figure 3; Table 6). Thermal decomposition started with a slight mass

Table 5. Lignin Structures Determined by 13C NMR Spectroscopy structure (per Ar) aromatic methine carbons (CAr−H) oxygenated aromatic carbons (CAr−O) aromatic C−C structure (CAr−C) methoxyl content p-coumarates ferulates S:G ratio degree of condensation (%)

YP-LS

YP-HS

S-LS

S-HS

2.63 2.88 0.49 1.99 0 0 3.00 25

2.00 2.19 1.81 1.81 0 0 1.21 95

2.39 2.99 1.35 1.79 0.18 0.12 0.90 48

1.65 2.44 2.31 1.43 0.07 0.09 0.64 95

between 160 and 103 ppm was set as the reference and equivalent to 6 aromatic carbons after subtracting the contribution of integral values from the four vinyl carbons of cinnamaldehyde and cinnamyl alcohol, which were observed mainly in switchgrass lignins.81 The absence of strong peaks in the region between 102−90 ppm, especially for yellow poplar, indicates very few carbohydrate impurities in the lignin samples (Supporting Information, Figure S4).70 The methoxyl groups slightly decreased in both feedstocks at high severity, which could be due to the decrease in S/G ratio, and implies some demethylation/demethoxylation of lignins at high severity.70 The S/G ratio calculated from 13C NMR was close to that from the 2D NMR spectra in samples at low severity. However, the S/G ratio calculated from the 13C NMR decreased at high severity, in contrast to the results from HSQC measurements. This is due to condensation in lignins at high severity and the inability to observe the quaternary carbons in condensed aromatics, which are mainly G units, in HSQC spectra. However, these units can contribute, to some extent, to the 13 C NMR spectra. It is expected that lignin at higher severity possess a lower S/G ratio. It has been shown that S units, due to fewer C−C linkages, are less extensively bonded and are extracted faster and easier during fractionation, whereas extraction of G units requires a longer time and higher severity.52 Similar to what was observed in the HSQC data, 13C data indicated that the amount of pCA and FA units in switchgrass lignin decreased at high severity. G


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continuous spinning or results in the formation of pores within the fibers. It is expected S-LS produces more volatiles during melt-spinning due to its higher impurities, aliphatic hydroxyl groups, and phenolic acids. The peak temperature of DTG curves (maximum rate of mass loss) happens at around 370 °C (Table 6). Thermal decomposition after 300 °C involves scission of interunit linkages between lignin monomers, leading to the release of monomeric phenols, and includes cleavage of methyl−aryl ether bonds resulting in methanol formation.87,88,92 This decomposition continues by cracking of methoxyl groups, decomposition, and condensation of aromatic rings (charring) at temperatures higher than 400 °C.87,88,92 The faster maximum decomposition rate observed for yellow poplar lignins, especially YP-LS at temperatures between 300 and 400 °C, could be due to the higher number of S units. G units have a higher tendency to condense during thermal degradation and can decrease the mass loss.87 13C NMR data confirmed YP-LS has the highest S/G ratio and the least degree of condensation. The YP-HS had the lowest mass loss due to its already greater percentage of condensed structures. YP-HS also had the highest thermal stability at a temperature of 500 °C and the highest final char yield. YP-LS had the lowest yield but was close to switchgrass lignins. In addition to the S/G ratio, other parameters such as the amount of hydroxyl, methoxyl, and carboxyl groups also affect the char yield.87 Increasing the number of these functional groups as a source of volatiles results in a decrease in char yield.87,90 As observed in the NMR data, the high methoxyl content of YP-LS is another reason for its low char yield. It has been shown that acetylation results in a decrease in char yield due to an increase in the release of small molecules.87 Switchgrass lignin has a lower char yield than YPHS due to its acylated structure and a high number of aliphatic hydroxyl and carboxylic acid groups. The higher char yield and thermal stability of YP-HS corresponds to a higher carbon content combined with lower contaminants, lower aliphatic hydroxyl content, and higher degree of condensation compared to the other lignins. Higher char yield potentially can result in a higher yield of carbon fiber after carbonization of lignin fibers, which is an advantage for reducing manufacturing cost of carbon fiber. The Tg of yellow poplar lignin samples increased slightly with increasing severity, whereas the Tg for switchgrass lignins significantly decreased with increasing severity (Table 6). The transition from a glassy to a rubbery state is an extremely important parameter as it determines the processing temperature for extruding lignin into fibers. This temperature needs to be low enough to produce fibers without inducing the decomposition discussed previously. However, the Tg also needs to be high enough to render the fibers infusible during oxidative stabilization in a reasonable amount of time. Factors that influence Tg include free volume and interactions among polymer chains, molecular chain length and stiffness, and the presence of contaminants.73,88,93,94 Lignin sample YP-LS had the lowest Tg, and as revealed by NMR analysis, its high alkyl aryl ether linkage content makes it less condensed (lower chain stiffness). More condensed structures reduce thermal mobility and free volume thereby increasing Tg. As discussed earlier, hydroxyl groups also participate in intermolecular interaction, reducing thermal mobility and increasing the Tg.18,73 YP-HS’s higher concentration of condensed units and hydroxyl groups compared to YP-LS was expected to increase its Tg. However, its Tg was only slightly higher than YP-HS, indicating that

Figure 3. Thermal decomposition of lignin samples.

Table 6. Thermal Properties of Lignin Samplesa Tg (°C) delta Cp (J g−1 °C−1) Tm (°C) Td (°C) DTG peak temperature (°C) DTG peak value (% min−1) mass at 300 °C (%) mass at 400 °C (%) mass at 500 °C (%) residual char (%)

YP-LS

YP-HS

S-LS

S-HS

115 0.37 144 260 372 −5.01 89.1 51.8 41.3 33.1

117 0.42 150 256 376 −4.86 90.2 57.7 46.3 37.4

128 0.29 156 247 373 −3.71 86.0 56.3 43.9 34.1

118 0.39 148 251 376 −4.01 87.1 56.2 43.7 34.1

a Tg: Glass transition temperature; Td: thermal decomposition temperature (5% weight loss temperature); Tm: melting temperature.

loss between 120 and 180 °C, which was assigned to the loss of bound water (Figure 3).86 This mass loss was more pronounced with YP-HS, which correlates with its higher content of hydroxyl groups and, thus, a greater ability to adsorb water via hydrogen bonding. The primary decomposition started at about 200 °C, and derivative TGA curves have shoulders at about 240 °C, especially for switchgrass lignins (Figure 3). This mass loss at temperatures between 200 and 300 °C can be due to decomposition of aliphatic groups, especially the Cγ terminal hydroxymethyl groups and carboxyl groups, plus volatilization of low-molecular-weight phenols.63,87−89 Decomposition of aliphatic hydroxyl groups and Cγ also leads to the production of volatile compounds such as water and formaldehyde.87,88,90 The presence of phenolic acids (pCA and FA), which are mostly esterified to Cγ, can also decrease the thermal stability of switchgrass lignins in this range. Residual hemicellulose may also contribute to the mass loss over a similar temperature range (200−300 °C).91,92 The HSQC spectra of switchgrass lignins showed the presence of residual carbohydrates, but the contaminants in switchgrass lignins were just slightly higher than that of yellow poplar lignins, indicating a low contribution of carbohydrate to the mass loss in this region. In addition, switchgrass lignins had a lower thermal decomposition temperature (Td), which confirms a higher percentage of less thermally stable compounds in this range (Table 6). Having higher Td is important for melt-spinning, because samples with lower Td produce more volatiles during melt-spinning, which interrupts H


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ACS Sustainable Chemistry & Engineering multiple factors influence the Tg. In this case, the possible lower molecular weight of YP-HS arising from greater cleavage of interunit linkages at higher severity and elimination of Cγ (lower aliphatic hydroxyl groups) may play a role. As mentioned before, aliphatic hydroxyl groups at the Cγ position form strong hydrogen bonds that reduce thermal mobility and increases Tg.18,57,63 The Tg of lignin samples is lower than reported data for softwood kraft lignin and higher than previously reported hardwood organosolv lignins.46 S-LS has the highest Tg among all samples (Table 6). The high Tg in this sample is likely caused by the its slightly higher contaminants (Table 1) and more importantly, its high content of aliphatic hydroxyl groups (Table 3). S-HS had a Tg similar to YP-HS, representing its higher thermal mobility, mainly due to its lower aliphatic hydroxyl group content. The acylated structure of switchgrass lignin is expected to increase its thermal mobility and decrease Tg. These ester groups can reduce hydrogen bonding and plasticize the lignin.61 However, the higher Tg results from residual carbohydrates and protein-based impurities (observed in HSQC NMR and the high nitrogen content of switchgrass lignins) along with a greater aliphatic hydroxyl group concentration (more intermolecular hydrogen bonding in IR spectra). Another important parameter is the S/G ratio, which is lower in switchgrass lignins. G units tend to condense and cross-link more readily, which decreases thermal mobility. On the other hand, S units have more methoxyl groups, which prevent cross-linking and increase the free volume, preventing chain entanglement and leading to higher mobility and lower Tg.95 Switchgrass samples possess the lowest change in heat capacity while poplar samples from higher severity had the highest. A higher heat capacity change can indicate increased thermal mobility leading to fusible materials. However, it may not be a good indicator of the differences between lignins as heat capacity increases with increasing Tg.96 All lignin samples had clear softening and zero shear flow (Tm) (Table 6). The trend of Tm was similar to Tg, as expected. It needs to be considered that as an amorphous material as lignin does not melt but turns into a viscous liquid at high molecular mobility above its Tg. Thermal mobility and fusibility of lignin are very important to the melt-spinning process and sufficient thermal mobility and melt flow is needed for this process. Higher Tm and lower Td of S-LS make this sample more exposed to producing volatiles during melt-spinning process. Spinning and Fiber Properties. Lignin samples were used directly for melt-spinning without any additional pretreatment. Although in practice a thermal pretreatment for devolatilization has been recommended,3 this step was eliminated to prevent any effect of additional thermal history on lignin structure. The extruder and spinneret temperatures were 180 and 185 °C, respectively, for both yellow poplar lignins. The S-HS was spun at extruder and spinneret temperatures of 190 and 195 °C, respectively. The S-LS was not able to produce a fiber spinning at temperatures ranging between 180 and 190 °C. Yellow poplar lignins showed very good spinnability (ability to form fiber and collect them continuously on a spool) by continuously spinning for about 40 min. The spinnability of yellow poplar lignin was even better in case of the high severity sample, making it possible to carry out spinning at a much faster winding speed. The winding speed was set at 30 m min−1 for yellow poplar lignins, although it was possible to spin YPHS at winding speed of up to 145 m min−1. Spinning fibers

from S-HS lignin was possible but not continuous, having filament breaks every few minutes. S-HS lignin required a lower winding speed (18 m min−1) and exhibited very low melt strength. Despite the Tg and melt temperature of S-HS lignin being close to those from yellow poplar lignins, melt-spinning was possible only at higher temperatures (190 °C compared to 180 °C for yellow poplar), which indicated that the sample had a higher melt viscosity.3 S-LS turned to foamy, short filaments that were very brittle and exhibited almost no melt strength for stretching and winding on the spool. The main problems related to spinning of switchgrass lignins appear to be foaming and a higher melt viscosity during extrusion. Foaming is due to emission of volatiles, and TGA results showed a lower Td for switchgrass lignins and higher mass loss at low temperatures. Impurities, especially for S-LS with higher impurities, which also have higher Tg and lower ΔCp, which requires a higher melt-spinning temperature, can contribute to volatiles and limit the melt flow of switchgrass lignins. In addition, low severity lignins had a higher content of aliphatic OH groups that also can increase intermolecular interaction and limit thermal mobility.18,73 High numbers of G units in switchgrass lignin, relative to yellow poplar, also promote condensation reactions and decrease thermal mobility, as the S/G ratio was lowest for low severity switchgrass. For both lignin feedstocks, a higher severity fractionation resulted in improved fiber spinning due to factors such as reduced amount of aliphatic hydroxyl groups, lower contaminants, and higher cleavage of β−O−4′ interunit linkages, which increase thermal mobility. YP-HS, which had the lowest number of aliphatic OH groups, extensive cleavage of aryl ether linkages, and a high number of S units (compared to switchgrass lignins), demonstrated the best spinning performance. S-LS, which was not spinnable, had the highest Tg and lowest ΔCp (thermal mobility), the highest number of aliphatic hydroxyl groups, the lowest thermal stability at low temperature, a high content of thermolabile ester linked units, and the lowest S/G ratio. Lignin fibers were oxidatively thermostabilized at different rates prior to carbonization (Table 7). Thermostabiliziation Table 7. Thermostabilization Behavior of Lignin Fibers

converts fusible fiber to infusible fiber through cross-linking and condensation. Oxygen infused into the lignin fiber from air in a convection oven and oxygen in lignin molecules both contribute in this process to form carbonyl and carboxyl groups that cross-links (anhydride and ester linkages) lignin macromolecules.97,98 However, this process also involves oxygen loss through dehydration and formation of CO2.3,14,97 Therefore, both mass loss and mass gain occur simultaneously during thermostabilization. Other reactions during thermostabilization include oxidation of alkyl chains, a decrease in aryl ether linkages, and demethoxylation.97,98 The stabilization yield was higher at faster stabilization rates for both yellow poplar and switchgrass lignin fibers (Table 8). Mass is lost in the form of H2O, CO2, and CO with extensive I


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further mass loss. Therefore, both switchgrass and yellow poplar lignin have a higher yield at faster stabilization rates. Thermostabilization needs to be slow enough to maintain the Tg below the thermostabilization temperature and prevent fiber fusion as the temperature ramps up. Sample YP-HS, because of its more condensed structure and lower number of aryl ether linkages compared to YP-LS, did not exhibit severe fusion even at the fastest heating rate (Figure 4). It has been shown previously that β−O−4′ interunit linkages cleave extensively during thermostabilization while most C−C linkages are stable.97,102 β−O−4′ linkages also increase the thermal mobility of lignin, and therefore, slower stabilization is required to prevent fusing, while allowing oxidation and cleavage of these linkages. Cleavage of β−O−4′ linkages can result in the formation of new C−C linkages between C5 and Cβ.100,103 SHS had the best performance during thermostabilization. This is likely due to its high content of G units, which have

Table 8. Thermostabilization and Carbonization Yield of Lignin Fibers Thermostabilized at Different Ratesa,b heating rate (°C min−1) source

0.05

0. 1

0. 2

0.5

YP-LS YP-HS S-HS

30.1 (66.7) 32.8 (72.1) 34.7 (72.9)

31.4 (67.9) 36.4 (82.4) 40.3 (83.6)

34.0 (75.5) 38.8 (84.8) 41.5 (84.7)

36.4 (79.5) 40.9 (87.3) 43.1 (88.7)

a Number in parentheses are thermostabilization yield. bValues are in percentage.

weight loss occurring at higher temperatures or longer thermostabilization times.11,97,99,100 Oxidation starts at fiber surfaces forming an oxidized layer that limits further diffusion of oxygen into the inner part of the fiber and mass loss.100,101 Stabilization for longer periods of time increases oxygen diffusion and induces oxidation reactions in the fiber leading to

Figure 4. SEM images of carbon fibers: (left) thermostabilized at heating rate of 0.05 °C min−1 and (right) thermostabilized at heating rate of 0.5 °C min−1. J


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ACS Sustainable Chemistry & Engineering Table 9. Mechanical Properties of Carbon Fibersa

a

source

stabilization rate (°C/min)

YP-LS YP-HS YP-HS YP-HS S-HS S-HS S-HS

0.05 0.05 0.1 0.2 0.05 0.1 0.2

diameter (μm) 17.8 17.1 21.2 21.5 25.8 32.5 36.5

tensile strength (MPa)

(2.60) (1.59) (1.77) (2.09) (2.21) (2.66) (2.95)

346 (73.0) 544 (96.0) 418 (81.9) 402 (102) 370 (73.9) 244 (66.8) 204 (68.5)

tensile modulus (GPa) 32.9 36.5 35.4 34.8 34.7 31.7 31.5

(4.13) (2.81) (4.08) (4.36) (3.03) (3.17) (3.66)

strain at break (%) 0.74 1.30 1.05 1.04 1.01 0.85 0.82

(0.20) (0.12) (0.17) (0.22) (0.17) (0.12) (0.09)

Standard deviations are shown in parentheses.

units, can potentially have a more linear molecular structure. On the other hand, S substitution at C3 and C5 limits branching, which can increases the thermostabilization time. Lignin with a more linear structure can increase chance of having oriented carbon structure and enhanced mechanical properties of carbon fibers.

unoccupied C5 sites and can undergo more facile cross-linking and condensation than S units. The carbonization yields have a similar trend to stabilization yield and were lowest for low severity yellow poplar lignin (Table 8). Carbonization reactions occur by elimination of methoxyl, carbonyl, and carboxyl groups along with crosslinking between aryl carbons and elimination of noncarbon atoms.98 Lignin can form turbostratic and graphitic carbon structures during carbonization.104,105 However, the lack of orientation, presence of voids, and heterogeneities in chemical structure limit the tensile strength of lignin carbon fibers.104−107 The existence of a preferred orientation at the beginning of carbonization is necessary for formation of oriented carbon structures.7 Lignin, due to its nonlinear and 3D structure, has less chance to form a linear fiber structure, especially in the case of technical lignin, which undergoes severe depolymerization during the extraction process. SEM images reveal significant morphological differences between switchgrass and yellow poplar carbon fibers (Figure 4). Switchgrass fibers, regardless of the thermostabilization rate, had larger diameters than yellow poplar fibers, indicating less stretching during extrusion and potentially lower molecular alignment. Different size pores, which are due to the presence of compounds with less thermal stability and volatiles that do not participate in cross-linking reactions, were observed on surface of switchgrass carbon fibers. Pores are initiated during melt-spinning, thermostabilization, and/or carbonization. Nevertheless, the final carbonization yield was higher for switchgrass lignin than yellow poplar lignins, which may indicate these volatiles are a small fraction (mainly ester linked phenolic acids) of the total lignin mass, as discussed previously. It has been shown that esterification of lignin can increase the number of pores and the surface area of carbon fibers.108 Tensile strength and modulus of the carbon fibers decreased by increasing the thermostabilization rate (Table 9). This could be due to incomplete thermostabilization and oxidation leading to fusion and defects in carbon fibers. The larger diameter of carbon fibers thermostabilized at a faster rate negatively affects the mechanical properties of the fibers. The correlation between fiber diameter and mechanical properties of carbon fibers has been shown previously as the fiber develops a shell core structure.3,109,110 In the case of yellow poplar lignins, carbon fibers made from high severity fractions had better mechanical properties (Table 9). Better thermostabilization performance, more condensed structure, higher carbon yield, fewer impurities, and higher carbon content contribute to improved mechanical properties. YP-LS, fused even at the lowest heating rate, had the poorest mechanical properties in the selected rates. YP-HS had the highest mechanical properties at all selected thermostabilization rates, mainly due to its defect free structure. Yellow poplar lignin, due to presence of more S

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CONCLUSIONS The changes in chemical structure and properties of lignin, derived from differences in severity of pretreatment and botanical source of lignin can significantly change spinnability and properties of carbon fibers made from lignin. In the same species, higher severity resulted in lignin with fewer impurities, more cleavage of ether linkages, more condensed structure, lower number of aliphatic hydroxyl groups, and more phenolic OH groups. Such lignin demonstrated better performance in melt-spinning and thermostabilization processes, in the experimental severity range. Carbon fibers made from lignins extracted at higher severity also presented higher tensile strength and modulus, which mainly came from better morphological properties of fibers (fewer defects). Switchgrass lignin was spinnable when extracted at high severity, which leads to a lower number aliphatic hydroxyl groups and phenolic acids as well as a more depolymerized structure. Ester-linked phenolic acids and aliphatic hydroxyl groups seem to be the sources of high levels of volatiles in switchgrass lignin, which cause difficulty in spinning and the formation of pores in the final carbonized fibers. Aliphatic hydroxyl groups also form strong intermolecular hydrogen bonds, limiting the fusibility of switchgrass lignins. Switchgrass lignin had faster stabilization compared to yellow poplar lignin, due to presence of more G units. Organosolv lignin extracted from yellow poplar at high severity presents the best mechanical properties. Decomposition of Cγ, carboxylic acids and volatilization of pCA and FA units could be the reason for the formation of volatiles and foaming, which is prevalent in lignin extracted from switchgrass.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01828. 31 P NMR, 2D NMR, and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS This research was supported by grants from the South Eastern Regional Sun Grant Center and Agriculture and Food Research Initiative Competitive Grant no. 2013-67021-21178 from the USDA National Institute of Food and Agriculture. The authors thank Dr. Darren Baker at Innventia AB, Sweden for his assistance at the onset of the research, and Dr. Stephen Young at The University of Tennessee, Civil and Environmental Engineering Department for assisting with SEM Images.

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