Correlation of Elongational Fluid Properties to Fiber Diameter in

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Correlation of Elongational Fluid Properties to Fiber Diameter in Electrospinning of Softwood Kraft Lignin Solutions Ian Dallmeyer,† Frank Ko,‡ and John F. Kadla*,† †

Biomaterials Chemistry, Department of Wood Science, ‡Advanced Fibrous Materials, Department of Materials Engineering University of British Columbia, Vancouver, British Columbia V6T1Z4, Canada ABSTRACT: Viscoelastic properties of N,N-dimethylformamide (DMF) solutions of softwood kraft lignin (SKL) containing small amounts of poly(ethylene oxide) (PEO) were investigated. Of interest is the relationship between viscoelastic properties of the spinning solutions and their corresponding electrospinning behavior. Although it is well established that fiber diameter is critical in determining the material properties of nanofibers, lignin solutions have been observed to display poor electrospinnability in many instances. Thus, the motivation behind this work was to understand and exert control over the relevant fluid properties that control the fiber diameter of SKL/PEO fibers, The results of dynamic shear and capillary breakup extensional rheometry (CaBER) experiments indicated that SKL solutions were weakly elastic in shear and Newtonian in elongational flow. SKL solutions were not electrospinnable at concentrations of 25−45 wt % but form fibers at 50 wt % concentration. The addition of PEO to SKL solutions led to an increase in shear moduli and pronounced strain hardening in elongational flow. The characteristic time scales of tensile stress growth (λ) measured with CaBER were dependent on the SKL concentration, PEO concentration, and PEO molecular weight. In contrast to SKL solutions, SKL/PEO solutions are electrospinnable over the concentration range of 25−45 wt % SKL depending on the combination of SKL concentration, PEO concentration, and PEO molecular weight. Correlation between the fiber diameters obtained during electrospinning and the measured value of λ are discussed. lignin is a natural, renewable polymer. Lignin makes up ∼30% of the dry weight of wood and, in trees, provides structural rigidity, aids transport of water and metabolites, and provides protection against pathogens.10 Breaking woody biomass down to produce pulp and paper or biofuels requires separation of wood components through thermal, mechanical, or chemical means, of which kraft pulping is the predominant form.11,12 Lignin isolated during this process is typically burned as fuel during chemical recovery, but the potential also exists to isolate some lignin for conversion to value-added products, including CF.13−15 In recent years, our research group and others have investigated electrospinning of lignin for the production of nanofiber precursors for CF16−19 and other lignin-based materials.20−22 In particular, we have studied the beneficial effects of polyethylene oxide (PEO) addition on promoting the production of uniform fibers by electrospinning.23 The advantage of using PEO as a spinning aid in lignin electrospinning is that it can be applied to a variety of different types of lignins, which are known to display substantially variable properties depending on the plant source, method of isolation (e.g., kraft vs sulfite pulping of wood), and various postisolation modification or purification steps (e.g., sulfonation of kraft lignin to induce water solubility). Other researchers have succeeded in generating electrospun lignin fibers, but each study focused on a single type of lignin; hence,

1. INTRODUCTION Electrospinning is an extensively studied fiber spinning technique capable of drawing continuous filaments from polymer solutions using electrostatic forces.1−3 Electrospun fibers are distinguished by their nanometer-scale diameters, which differ by an order of magnitude or more compared to micrometer-scale fibers produced by other fiber spinning techniques such as melt spinning, wet spinning, and dry spinning. Among the numerous types of materials being explored in the electrospinning field, a particularly interesting area is the preparation of continuous carbon nanofibers.4 Of the many types of electrospun materials that can display enhanced properties, carbon fibers (CF) are a particularly interesting example because of their numerous potential applications as reinforcing fillers in composites, adsorbents, electrodes, catalyst supports, and tissue engineering scaffolds.4−8 A recent review summarizes a rather large body of research devoted to the preparation, characterization, and properties of carbon nanofibers obtained by electrospinning.4 A significant recent advance in this area was the demonstration that CF produced from electrospun polyacrylonitrile (PAN) displays impressive mechanical properties (tensile strength ∼7.3 GPa, tensile modulus ∼262 GPa) at a relatively low carbonization temperature (800 °C) when the fiber diameter decreased to 108 nm.9 The authors observed a correlation between the orientation of the carbon microstructure along the fiber axis and the mechanical properties of the fibers at diameters below 170 nm. These findings motivate further investigation into costeffective methods to produce carbon nanofibers by electrospinning. Lignin is currently being actively investigated as a lower-cost precursor for CF. In contrast to the petroleum-based PAN, © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2697

November 3, 2013 January 15, 2014 January 22, 2014 January 22, 2014 dx.doi.org/10.1021/ie403724y | Ind. Eng. Chem. Res. 2014, 53, 2697−2705

Industrial & Engineering Chemistry Research

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Table 1. Fluid compositions, viscosity η, relaxation time λ, surface tension σ, and corresponding fiber diameters. - = too small to measure, *= incomplete fiber solidification during spinning, x = no fibers formed, n/a = not applicable SKL conc. (wt %) 25 25 25 25 25 30 30 30 30 30 35 35 35 35 35 40 40 40 40 40 45 45 45 45 45

PEO mol. wt. (g/mol) 1 1 5 5 1 1 5 5 1 1 5 5 1 1 5 5 1 1 5 5

n/a × 106 × 106 × 106 × 106 n/a × 106 × 106 × 106 × 106 n/a × 106 × 106 × 106 × 106 n/a × 106 × 106 × 106 × 106 n/a × 106 × 106 × 106 × 106

PEO conc. (wt %)

|η*(ω)| (mPa*s)

λ (ms)

σ (mN/m)

Mean Fiber Diameter (nm)

Standard deviation (nm)

0 0.1 0.2 0.1 0.2 0 0.1 0.2 0.1 0.2 0 0.1 0.2 0.1 0.2 0 0.1 0.2 0.1 0.2 0 0.1 0.2 0.1 0.2

19 24 28 27 32 45 61 70 63 72 116 177 200 168 194 448 546 523 539 586 1598 2023 2082 2007 2023

∼4 11 8 12 24 31 20 24 53 55 49 65 107 147 160 182 326 510

32 32 32 32 32 32 33 32 33 32 34 33 33 33 33 33 34 34 34 34 35 35 35 35 35

443 641 x 582 702 821 1010 x 895 1100 1401 1551 x 1482 1890 2176 2501 x 2658 2878 3261 *

65 79 n/a 84 83 99 149 n/a 98 138 231 338 n/a 229 410 494 420 n/a 440 502 350 *

between solution rheology and fiber diameter in lignin-based systems. Yu and co-workers showed that elasticity can exert a pronounced effect on fiber formation in electrospinning of PEO-based aqueous solutions.24 We were inspired by this study to investigate the elongational properties of lignin-based spinning solutions and the corresponding electrospinning behavior. Here, we extend upon our previous studies on steady shear rheometry with an investigation of the relationship between elongational properties of spinning solutions and the formation of lignin fibers by electrospinning. Two rheological techniques were used: small amplitude oscillatory shear to characterize the material response under shear deformation and capillary breakup extensional rheometry (CaBER) to determine the relaxation time under uniaxial elongational flow. The effect of PEO concentration and molecular weight as well as the effect of lignin concentration was studied by independently varying each of these three parameters and characterizing both rheology and electrospinning behavior under consistent conditions. Overall, it was observed that rheological studies provide a platform to quantify and compare the effects of changing different solution parameters on electrospinning behavior.

it is difficult to translate the findings related to one type of lignin to another. In addition, our group and others16,17,20 have reported that lignin solutions without other polymers added are prone to electrospray rather than fiber formation. In our previous work, this was true for SKL even at concentrations well above the apparent critical entanglement concentration determined from the change in slope on plots of specific (shear) viscosity vs SKL concentration.23 It is of interest to study mechanisms of promoting uniform fiber formation in lignin-based systems to aid the development of robust processing schemes for the production of advanced ligninbased materials. Here, it should be specified that the term “uniform fibers” refers to the absence of beads-on-string defects, which form due to the surface tension-driven Rayleigh instability and often occur when electrospinning polymers at low solution concentrations. Our assumption in this work is that uniform fibers of the smallest possible diameter are desirable because beads and variation of fiber diameter along its length reduce the potential to realize enhanced material properties through diameter reduction. Investigations on electrospinning of a variety of different technical lignins revealed that the fiber diameter is affected by the type of technical lignin used as well as the concentration of the PEO component.23 Because the fiber diameter is a critical in determining the properties of nanofibers, we sought to better understand the factors that most strongly influence the ability to produce smaller fibers. We initially attempted to identify correlations between the zero shear viscosity of the spinning solution and fiber diameter, but found that fibers could be obtained at a variety of different shear viscosities depending on the amount of PEO relative to lignin.23 These results led us to investigate other ways of understanding the relationship

2. EXPERIMENTAL SECTION Materials. Softwood kraft lignin (Indulin-AT, Meadwestvaco, Glen Allen, VA, USA) was acid washed (dilute HCl, pH 2, 100 g/L) to exchange Na+ ions, washed with water, air-dried for 12 h, and oven-dried at 105 °C to constant weight. The dried lignin was then fractionated by sequential extraction with organic solvents according to a modification of an established procedure.25 Dried, acid-washed, unfractionated lignin (100 g) was first washed with methanol (1 L) by stirring for 30 min, 2698

dx.doi.org/10.1021/ie403724y | Ind. Eng. Chem. Res. 2014, 53, 2697−2705


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and the methanol-soluble fraction was filtered off. The undissolved material was air-dried and ground with a mortar and pestle, and the methanol wash was repeated. The methanol-insoluble material was then washed in the same manner as stated above with a 70/30 (v/v) mixture of methanol/dichloromethane (100 g/L). The soluble lignin from this step was concentrated and dried under reduced pressure on a rotary evaporator at 25−50 °C and is herein referred to as SKL. SKL was completely dried for 2 h at 0.1 Torr at 60 °C and stored in the dark under vacuum prior to use. PEO with a nominal viscosity average molecular weight (Mw) of 1 and 5 × 106 g/mol were obtained from Sigma-Aldrich and used as received. N,N′-dimethylformamide (DMF, ACS Reagent grade), methanol, and dichloromethane (ACS Reagent) were obtained from Fisher Scientific (Ottawa, ON) and used as received. Solution Preparation and Characterization. SKL solutions were prepared by adding dry lignin powder to the solvent. In the case of SKL/PEO solutions, PEO was first dissolved by briefly heating the solution at 80 °C for 10−15 min, and then SKL was added to the PEO solution. The mixtures were heated at 80 °C for 2 h with intermittent vortexing (1−2 min) every 30 min. After 2 h at 80 °C, the solutions were allowed to cool overnight (12−18 h) to ensure complete dissolution. The solutions were then reheated at 80 °C for 15−20 min, vortexed, and cooled to room temperature before spinning experiments or fluid characterization. The range of concentrations was 25−50 wt % SKL relative to the total mass of the solution prepared in 5 wt % increments. Four SKL/PEO solutions were prepared at each SKL concentration, where the PEO concentration was 0.1 or 0.2 wt % and the PEO molecular weight was 1 × 106 or 5 × 106 g/mol (Table 1). The solution densities (ρ) were calculated based on the mass fraction, density of polymers, and solvent obtained from an MSDS for alkali lignin (ρSKL = 1.3 g/mL). Surface tension was measured with a KSV Instruments (Linthicum Heights, MD) CAM-100 by image analysis of pendant droplets and calculation by the Young−Laplace equation. The dynamic shear moduli of SKL and SKL/PEO solutions were measured using a TA Instruments AR2000 controlled stress rheometer in stress sweep (1−1000 Pa, ω = 10 rad/s) and frequency sweep (oscillatory stress: 2 Pa) modes. The magnitude of the complex viscosity |η*(ω)| = √(G′2 + G″2) in the range ω = 1−100 rad/s was used to determine the shear viscosity, where G′ and G″ are the storage and loss moduli, respectively. In some cases, inertial effects resulted in scattered data at ω approaching 100 rad/s, in which case the higher ω data were not included in the calculation. A capillary breakup extensional rheometer (Haake CaBER 1, Thermo Scientific, Ottawa, ON) was used to characterize the elongational fluid properties. A vertical column of fluid was created by loading a sample between two horizontal circular plates (diameter = 4 mm) and rapidly raising the upper plate from 2 to 11.5 mm in 50 ms with a linear stretch profile. The midpoint diameter of the fluid column undergoing capillary thinning was measured after the upper plate came to a rest after the initial step strain. The elongational properties were determined from the data as discussed in the literature.24,26 Where applicable (for elastic solutions), the characteristic time scale of tensile stress growth in uniaxial elongational flow λ (herein referred to as the relaxation time) was obtained using the following equation to fit the portion of the thinning profile where exponential thinning was observed:

⎛ GD ⎞1/3 Dmid (t ) = D1⎜ 1 ⎟ e(−t /(3λ)) ⎝ 4σ ⎠

(1)

where Dmid(t) is the midpoint filament diameter as a function of time t, D1 is the initial filament midpoint diameter just after cessation of the upper plate, G is the elastic modulus, and σ is surface tension24,26 apparent transient elongational viscosity. ηe,app was estimated using the equation24,27 σ ηe,app = −d(D (t )) mid

dt

(2)

Electrospinning. Electrospinning experiments were conducted as previously described23 with a flat-tip 21G needle as a spinneret, operating voltage of 15 kV, and spinneret-tocollector distance of 20 cm. Fiber appearance and morphology was determined using an Olympus BX41 Microscope or Hitachi S3000N SEM using gold-coated samples and accelerating voltage of 5 kV. Fiber diameters were measured from SEM images using ImageJ analysis software (NIST) counting a minimum of N = 100 fiber diameters per sample.

3. RESULTS AND DISCUSSION Shear Rheometry of SKL Solutions. The dynamic storage (G′) and loss (G″) moduli were measured as a function of oscillatory stress amplitude σo (σ = σosin(ωt)) and frequency ω to investigate the linear viscoelastic regime of SKL and SKL/ PEO solutions. In the limit of linear viscoelasticity, all of the solutions can be considered to be weakly elastic under shear deformation, because G″ ≫ G′ by roughly an order of magnitude. Figure 1a shows representative stress sweep data for 40 wt % SKL/DMF solutions with different combinations of PEO concentration and molecular weight.

Figure 1. (a) Stress sweep and (b) frequency sweep data for SKL and SKL/PEO DMF solutions with SKL concentration = 40 wt %. Filled symbols are G′ (Pa), unfilled symbols are G″, and unfilled symbols connected by solid lines represent |η*(ω)| (Pa s). ● = SKL; ⧫ = PEO 0.1 wt %, 106 g/mol; ■ = PEO 0.2 wt %, 106 g/mol; ▲ = PEO 0.1 wt %, 5 × 106 g/mol; ▼ = PEO 0.2 wt %, 5 × 106 g/mol.

In the stress sweeps shown in Figure 1a, G″ was constant as a function of stress for SKL without PEO, but displayed a decrease with increasing stress in SKL/PEO solutions. G′ increased in SKL/PEO solutions compared to SKL solutions, but G′ was always less than G″. The stress sweeps indicate that there is a weak elastic network in the SKL solutions. This elastic network becomes noticeably stronger with the addition of PEO. A significant increase in G′ was observed despite the small amount of PEO relative to SKL: less than 0.6 - 0.8% by mass. The increase in G′ may be due to interactions between PEO and SKL, such as hydrogen bonding28 which mutually influences molecular mobility. Regardless, it appears that the 2699



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elastic network is disrupted during shear deformation, as the value of G′ drops off above 10 Pa. From the stress sweeps, a stress of 2 Pa was considered to be within the linear viscoelastic regime and used in the subsequent frequency sweeps. Frequency sweep data was used to calculate the magnitude of the complex viscosity |η*(ω)|, shown in Table 1 for all of the SKL and SKL/PEO solutions. The values of |η*(ω)| varied over nearly 3 orders of magnitude from 20 mPa*s at 25% SKL to 1600 mPa*s for 45% SKL concentration. Figure 1b shows G′(ω), G″(ω) and |η*(ω)| for 40 wt % SKL solutions with different PEO concentration and molecular weight. Interestingly, a clear dependence of G′ was observed as a function of PEO molecular weight and concentration, indicating that the elasticity of the solutions is increased by PEO addition and depends on PEO concentration and molecular weight. This observation was consistent with the results of stress sweeps. Here it should be emphasized that the addition of PEO led to an increase in |η*(ω)| of roughly 35−70% when comparing SKL vs SKL/PEO solutions with the same SKL concentration. On the other hand, solutions differing in SKL concentration by 5 wt % displayed larger differences in |η*(ω)|, where |η*(ω)| increased by a factor of roughly 3−4 when increasing SKL concentration by 5 wt %. Thus, the increase in |η*(ω)| due to increasing SKL concentration was consistently larger than that due to PEO addition. For example, all SKL/PEO solutions with SKL concentration of 35% exhibited |η*(ω)| of 100 − 200 mPa*s, while SKL solutions at 40% without PEO exhibited |η*(ω)| of 400 − 600 mPa*s. Each set of 5 solutions with a constant SKL concentration therefore represented a set within a specific range of |η*(ω)| as shown in Table 1. Elongational rheometry of SKL solutions. The viscoelastocapillary thinning process was investigated with CaBER to explore the elongational behavior of SKL and SKL/PEO solutions. Figure 2 displays the thinning profile of different SKL and SKL/PEO solutions. Time is on the abscissa with t = 0 corresponding to the time at which the upper plate stopped moving, and Dmid(t) was then recorded by the laser micrometer. The thinning profiles are plotted as the measured filament diameter Dmid(t) normalized by the initial filament diameter D1 at t = 0 just after the imposed step strain. In general, SKL solutions at or below 35% concentration could not be measured using the CaBER due to breakup occurring before the end of the initial step. It has been reported that there is a limiting viscosity below which the breakup process cannot be accurately measured with CaBER.26 Regardless of this limitation, a good qualitative picture of the breakup of SKL solutions was obtained by measuring at higher SKL concentrations of 40, 45, and 50 wt %. It was observed that the filament thinning profiles were linear in time, consistent with the behavior of a Newtonian fluid as shown in Figure 2a.29 The elongational viscosity of these fluids is constant in time with a value that increases with increasing SKL concentration. In theory, eq 2 could be used to obtain the apparent elongational viscosity ηe,app, and the, or Trouton ratio, (ηe,app/ η0) where η0 denotes zero-shear viscosity. For purely Newtonian fluids, ηe,app = 3η0 (Trouton ratio =3). However, we found that it was rather difficult to obtain reproducible CaBER data of SKL solutions compared to SKL/PEO, which made it difficult to obtain a consistent slope to obtain an apparent elongational viscosity. It was confirmed that the only reproducible profiles decreased linearly in time at 40, 45, and 50 wt %. Some artifacts were occasionally observed in the data at 45 and 50 wt %. Anomalous artifacts can be interpreted by

Figure 2. Representative thinning profiles of SKL and SKL/PEO solutions. a) SKL solutions without PEO and varying SKL conc.: ○ = SKL 40 wt %, □ = SKL 45 wt %, ◊ = SKL 50 wt %, b) SKL/PEO solutions with PEO conc. = 0.2 wt %, PEO Mw = 1 × 106 g/mol and varying SKL conc.: ◊ = SKL 30 wt %, □ = SKL 35 wt % Δ = SKL 40 wt %, ▽ =SKL 45 wt %. c) SKL/PEO solutions with SKL conc. = 40 wt %, PEO Mw = 1 × 106 g/mol and varying PEO conc.: ◊ = PEO 0.1 wt %, □ = PEO 0.2 wt %. d) SKL/PEO solutions with SKL conc. = 40 wt %, PEO conc. = 0.1 wt %, and varying PEO Mw; ◊ = 1 × 106 g/mol, □ = 5 × 106 g/mol.

remembering that CaBER only measures the filament diameter at a given plane which is presumed to capture the midpoint diameter of the thinning fluid column. However, bulges, gravitational sagging, or undissolved particles passing through the measuring plane appear in the data as increases in the value of Dmid(t).27 Sagging can be prevented by careful selection of experimental conditions, and observed by photographing the entire fluid column as discussed elsewhere.26 We can not rule out the possibility that some aggregates or inhomogeneity could exist in concentrated SKL solutions, since kraft lignin is known to display colloidal or associative behavior.30,31 These results emphasize the importance of conducting parallel experiments in both shear and extension to better understand the rheology of solutions used in electrospinning, as viscosities of SKL solutions are difficult to measure in extension but can readily be measured in shear. The addition of PEO generally resulted in a deviation of the thinning behavior from linear to exponential in time. Figure 2b, 2c and 2d show representative thinning profiles of SKL/PEO solutions when either the SKL concentration, PEO concentration, or PEO molecular weight, respectively, were varied independently. In contrast to the SKL solutions, it was relatively easy to obtain CaBER measurements for the SKL/ PEO solutions, as their elasticity allowed them to form stable fluid columns. SKL concentrations as low as 30 wt % with PEO produced consistent thinning behavior. Solutions with higher SKL concentration, higher PEO concentration, and higher PEO molecular weight produced pronounced exponential thinning behavior, longer filament lifetimes, and higher values of λ, as reported in Table 1. Exponential thinning behavior is a 2700



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characteristic of elastic fluids.26,27,29 The exponential character of the thinning process in elastic fluids is believed to originate from elastic stress generated by uncoiling and alignment of long, linear polymer chains into an extended conformation due to the strong character of the elongational flow. The elastic stress in the fluid column grows to balance the increasing capillary pressure, which increases as the fluid filament decreases in diameter. Elasticity can be recognized macroscopically in the laboratory and is often referred to as “spinnability.” Analyzing the CaBER data in terms of apparent elongational viscosity using eq 2, it can be seen that ηe,app increases exponentially with time during exponential thinning. Figure 3a

decrease in Dmid(t) was observed (Figure 3c). This data scatter could be due to instability in the fluid column and/or resolution limitations of the detector so it is not clear if this region can be considered representative of the actual fluid behavior. If the expected linear decrease in diameter for the steady state is assumed, then linear regression can be employed to extract an estimate of the steady elongational viscosity. However, the somewhat low R2 values indicate the measurement is not quantitative. Nevertheless, a rough estimate of the steady elongational viscosity based on linear regression demonstrated that the Trouton ratio of SKL/PEO solutions exceeds 100 in some cases, well above the Newtonian value of 3. We suggest based on these observations that strain hardening is key in determining the morphology of electrospun SKL/PEO fibers, as shown elsewhere in electrospinning of aqueous PEO solutions.24 During exponential thinning we can estimate the transient value of ηe,app using eq 2. However, Stelter et al. pointed out that the transient elongational viscosity is not a fluid property, and is not suitable for describing elongational flow behavior.32 On the other hand comparing the time scales of viscoelastic stress growth, λ, provides an alternative means to compare the elastic properties of the spinning solutions24,32,33The values of λ obtained from CaBER are tabulated in Table 1. Generally, higher values of λ were observed with higher SKL concentration, PEO concentration, or PEO molecular weight. This observation is consistent with the literature reports on model systems where increasing the solvent viscosity, polymer concentration, or polymer molecular weight increased the value of λ.24,26,32−35 The observation that higher molecular weight increased λ can be explained in terms of the relative contributions of different relaxation modes to the tensile stress in the fluid. While it is known that real fluids exhibit a spectrum of relaxation times due to polydispersity and other molecular features, it has been shown that capillary thinning is governed by the longest relaxation time, since the tensile stress contributions due to all other modes relax away at earlier times during thinning.36 The longest relaxation time corresponds to the unraveling of the longest chains in solution, which in this case should be the PEO. The fact that the values of λ increase as molecular weight increases is indicative of the slower process of unraveling longer chains. The most interesting part of these measurements is that they reveal that λ varies over a wide range of values upon slightly changing the PEO component concentration or molecular weight even though it is only a small fraction (less than 1%) of the polymer in solution. Although it is not currently known to what extent, the chemical compatibility and miscibility of lignin and PEO may influence the extent to which the minor PEO component can exert such a strong influence on the system rheology. Ability of SKL and SKL/PEO Solutions to Form Fibers during Electrospinning. Electrospinning experiments were carried out using the same SKL and SKL/PEO solutions discussed above to investigate the correlation between the ability to form fibers and the rheological parameters |η*(ω)| and λ. Representative images of the resulting fibers are shown in Figure 4. SKL/PEO solutions were capable of forming beaded fibers at relatively low SKL concentration, as low as 25 wt % SKL, whereas all SKL solutions below 50 wt % only electrosprayed. Most of the SKL/PEO solutions at SKL concentrations of 30 wt % and higher formed bead-free fibers, with the exception of the 30 wt % SKL solution containing 0.1 wt % PEO (Mw = 106 g/mol), which formed beaded fibers.

Figure 3. a) Transient elongational viscosity of SKL/PEO solutions with SKL conc. = 40 wt %. b) Semilog plot of thinning profiles of SKL/PEO solutions with SKL conc. = 40 wt %. c) Region of CaBER data close to filament breakup showing data scatter at small filament diameters. ◊ = PEO 0.1 wt %, 106 g/mol, □ = PEO 0.2 wt %, 106 g/ mol, Δ = PEO 0.1 wt %, 5 × 106 g/mol, ▽ = PEO 0.2 wt %, 5 × 106 g/mol.

shows a comparison of ηe,app between four different SKL/PEO solutions with the same SKL concentration (40 wt %) and different combinations of PEO concentration and molecular weight. In order to calculate ηe,app eqs 1 and 2 were applied to the intermediate region of the data corresponding to elastocapillary balance.26 Figure 3a shows that the concentration and molecular weight of PEO in solution have a strong effect on the values of ηe,app even though the values of |η*(ω)| are very similar, in the range of 500−600 mPa*s (Table 1). Figure 3a shows that the SKL/PEO solution with 0.2 wt % PEO, Mw = 5 × 106 g/mol reaches a ηe,app value over 100 Pa*s during thinning, while the other solutions at SKL concentration of 40 wt % deviate from exponential thinning at lower ηe,app values. However, it is difficult to interpret the data in terms of elongational viscosities because the steady elongational viscosity is reached at very small filament diameters where measurement by the CaBER micrometer is less accurate. The thinning behavior does appear to deviate from exponential thinning near breakup (Figure 3b, 3c) as reported elsewhere,32,33 but when focused on the region of the data near breakup a stepwise 2701



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Figure 4. SEM images of fibers electrospun from solutions with different compositions (SKL wt %/PEO wt %/PEO Mw g/mol). Scale bar = 10 μm.

Figure 5. SEM image of fibers electrospun from 50 wt % SKL solution without PEO. (a) Purified SKL. (b) SKL without purification. Scale bar = 20 μm.

Interestingly, the 25 wt % SKL solutions containing 5 × 106 g/ mol PEO at 0.2 wt % formed a nearly beadless fiber, demonstrating that higher concentration and molecular weight of PEO could compensate the destabilizing effect of the lower SKL concentration. These results demonstrate that at a given SKL concentration, increasing the PEO molecular weight or PEO concentration above a certain threshold promotes the transition from beaded fiber formation to uniform fibers. To connect spinnability with the rheological characterization, we can also calculate a Deborah number, De, by dividing the measured λ values with the Rayleigh breakup time, tR.26 If we take the characteristic length scale r0 = 0.8 mm to approximate the radius of the electrified jet as reported by Yu et al.,24 we estimate that the transition from electrospray to beaded fiber formation roughly corresponds to a De > 1. This result indicates that when λ exceeds the tR, breakup into droplets is suppressed by elastic stress on the jet. In the absence of PEO,

the solutions are Newtonian and the viscous stresses are insufficient to stabilize the jet over an SKL concentration range of 25−45 wt %. Interestingly, we were also able to obtain fibers from solutions without PEO at 50 wt % SKL using a slightly lower flow rate (0.01−0.02 mL/min) and higher applied potential (20 kV) compared to SKL/PEO solutions. SEM images of the ∼1200 nm diameter fibers produced from a 50% SKL solution are shown in Figure 5. Although we previously observed that fiber formation began to occur around 50 wt % for unfractionated SKL,23 those fibers were not uniform, ranging in fiber diameter from less than 100 to over 1000 nm with numerous droplets and beads. The increased uniformity of the fibers obtained in the present work is likely a result of the fact that the SKL sample was purif ied by extraction with organic solvents, whereas the SKL in previous work was unfractionated. We suggest that the extraction improved the electrospinnability 2702



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SEM image in Figure 4f. As the λ of the spinning solution increases, a deviation from fibers with cylindrical cross sections was observed. Figure 4f shows what appears to be a flattened and twisted fiber instead of a round cylindrical fiber observed at lower λ. Flattened morphology led to a broader diameter distribution because in analyzing the SEM images, flat fibers essentially had two dimensions corresponding to a shorter and longer radial dimension. Flattened morphology may be due to incomplete solvent evaporation from the core of the fiber coupled with skin formation at the jet surface and collapse of the walls of the solid sheath around the liquid core. It should be noted that even considering only fibers with cylindrical cross sections, the width of the diameter distribution was still larger for solutions with higher λ. Although all the factors influencing the width of the diameter distribution are not clear, the elongational rheology of the spinning solution is clearly related to the mean fiber diameter and width of the diameter distribution. An important goal for future work is to leverage our understanding of the fiber formation process to reduce the diameter further to produce true lignin nanofibers with diameters on the order of 100 nm. Although diameters on the order of 100 nm are highly desirable from an applications perspective, it is now known that nonwoven fabrics consisting of larger (micrometer-sized) diameter lignin-based carbon fibers may still be viable candidates for nonstructural applications such as lithium ion battery anodes.37 These exciting findings suggest that even micrometer or submicrometer fibers could be applicable in the real world even if they do not approach the impressive mechanical properties of true carbon nanofibers produced from PAN.9 The current interest in applications of lignin fibers as well as the potential to realize enhanced properties through diameter reduction motivate further research on the production of lignin nanofibers by electrospinning. The results reported here emphasize the importance of striking a balance between shear viscosity and elasticity to generate smaller diameter fibers while preventing bead formation. The approach presented here should provide a good basis for future studies aimed at reducing the fiber diameter and controlling the diameter distribution as well as measuring the effect of fiber diameter on material properties, where precise control over the fiber diameter is needed. Finally, it should be noted that measurements of λ using CaBER represents a rare case where a single, rapidly measurable parameter can clearly be correlated with the processing behavior for a lignin-based fluid undergoing a complex electrohydrodynamical deformation. Technical lignins have highly complex molecular structures. Structural complexity and heterogeneity implies a general lack of predictable, reproducible processing behavior. This is a limiting factor for the processing of lignin as a carbon fiber or other renewable material precursor. We have discussed electrospinning as an example where blending relatively small amounts of PEO can be used to overcome a lack of lignin processability, and a rapidly measurable rheological parameter, λ, can be correlated with both the ability to form a fiber and the fiber diameter. We hope that these findings may help guide future efforts aimed at reducing the diameters of lignin fibers to a point where enhanced material properties might be realized.

by eliminating lower molecular weight compounds and insoluble high molecular weight fragments,25 which could destabilize the electrospinning jet. This result is also interesting because, as shown in Figure 2a, the SKL 50 wt % solution behaved as a Newtonian fluid in capillary thinning studies. This observation shows that multiple mechanisms could succeed in stabilizing the electrospinning jet. In SKL/PEO solutions, elastic stresses provide additional stability, whereas in the absence of PEO, a higher SKL concentration is required to electrospin because stretched SKL solutions do not appear to strain harden. In any event, the jet must maintain a fiber shape as it is elongated rapidly by the electric field and must remain stable long enough for solidification to occur due to solvent evaporation before the Rayleigh instability causes the formation of droplets. Based on previous work24 and our results, it is reasonable to conclude that PEO provides stability to the electrospinning of lignin at least partially through induced strain hardening under elongational flow. Correlation of Relaxation Time with Fiber Diameter. We also attempted to generate a unified picture of the effect of λ on the diameter of electrospun fibers by plotting λ vs fiber diameter, plotted in Figure 6. Figure 6 shows that λ was

Figure 6. Mean fiber diameter vs λ for SKL/PEO fibers.

strongly correlated with the diameters of fiber obtained by electrospinning. On the basis of this curve, we conclude that using CaBER to measure λ is an effective method to compare the effect of using a certain combination of SKL concentration, PEO concentration, and PEO molecular weight in the spinning solution on fiber diameter obtained by electrospinning. The diameter (d) vs λ data could be fitted using an exponential of the form d = A + B(1 − e−kλ)

where A = 363.66, B = 3021.99, k = 0.0091051. Although the physical significance of this relationship is not yet clear because it is empirical, it provides a basis for further investigation of the relationship between fiber diameter and elongational rheology. The fitting parameters A, B, and k were obtained by estimating A and B as the approximate diameter as λ → 0 (∼350 nm) and the asymptotic value of d (∼3200 nm) obtained at high λ, respectively. The parameter k was guessed, and a solver program was used to obtain the best combination of the three parameters by minimizing the sum of the squared difference between the measured and calculated values. It can also be seen that the standard deviation of the measured diameters (error bars in Figure 6) increases with increasing λ. Part of the explanation for this can be found in the

4. CONCLUSION The results obtained using CaBER showed that SKL solutions exhibit linear Newtonian-like behavior in capillary thinning. 2703



Article

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The addition of 0.4−0.8% PEO (relative to SKL) changed the solutions to non-Newtonian strain-hardening fluids, as indicated by exponential thinning. λ was observed to depend on the concentrations of SKL and PEO as well as the PEO molecular weight in SKL/PEO solutions. Solutions displaying λ above ∼12 ms showed a corresponding transition in the electrospinning behavior from beaded to bead-free fibers. The fiber diameter positively correlated with λ, indicating that the increased elasticity resisted thinning of the jet, resulting in larger fibers. SKL/PEO solutions with lower |η*(ω)| produced smaller diameters and more uniform diameter distributions. Interestingly, it was also observed that relatively high concentration (50 wt %) SKL solutions with Newtonian elongational behavior were also capable of forming fibers, suggesting either viscous or elastic stress can stabilize electrospinning of lignin. To the best of our knowledge, this is the first report on the elongational fluid properties of SKL or SKL/PEO blend solutions for use in electrospinning. The results of this study suggest that shear and elongational rheometry measurements provide a good basis for further studies on controlling the electrospinning behavior of other technical lignins.

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AUTHOR INFORMATION

Corresponding Author

*J. F. Kadla. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Correlation of Elongational Fluid Properties to Fiber Diameter in

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