Cellulose Nanofibers as Rheology Modifiers and ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cellulose Nanofibers as Rheology Modifiers and Enhancers of Carbonization Efficiency in Polyacrylonitrile Edward Jiang,† Nasim Amiralian,† Maxime Maghe,‡ Bronwyn Laycock,§ Eric McFarland,∥ Bronwyn Fox,⊥ Darren J. Martin,† and Pratheep K. Annamalai*,† †

Australian Institute for Bioengineering & Nanotechnology, The University of Queensland, Building 75, Corner of College Road and Cooper Road, St Lucia, Queensland, Australia, 4072 ‡ Carbon Nexus, Deakin University, 75 Pigdons Road, Geelong, Victoria, Australia, 3216 § School of Chemical Engineering, The University of Queensland, Advanced Engineering Building, St Lucia, Queensland, Australia, 4072 ∥ School of Chemical Engineering, University of California Santa Barbara, Engineering II Building, Santa Barbara, California 93106, United States ⊥ Factory of the Future, Swinburne University of Technology, John Street, Hawthorn, Victoria Australia, 3122 S Supporting Information *

ABSTRACT: The energy requirements for the production of high quality carbon fiber and other carbon-based materials made by carbonization is a key factor limiting the commercial application of these materials. With the aim of enhancing the carbonization efficiency, we have prepared polyacrylonitrile (PAN) based precursor materials doped with high aspect-ratio cellulose nanofibers (CNF) derived from Australian spinifex grass (T. pungens). This was achieved by systematically investigating the rheology and electrospinning properties of composite fibers of PAN and CNF prepared at various CNF concentration levels and subsequently stabilized and carbonized. The carbon properties were characterized by X-ray diffraction and Raman spectroscopy. Upon carbonization, the incorporation of CNF into the PAN precursor led to changes in the crystallite and graphitic structure of the carbon materials, and these changes found to be closely related to the CNF concentration. CNF loadings of 0.5−2 wt % resulted in spinnable solutions with well-ordered carbon structures exhibiting a reduced Raman D/G ratio and an increased [002] band intensity by XRD. These spinifex CNF additives highlight a new approach for enhancing the energy efficiency of the carbonization process for PAN-based precursors. KEYWORDS: Spinifex, Cellulose nanofibre, High aspect ratio, Nanocellulose carbonization, Carbon fiber, Electrospinning

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the fiber processing conditions,8−10 tailoring PAN comonomer compositions,11,12 as well as the synthesis of high molecular weight PAN by reversible addition−fragmentation chain transfer (RAFT) polymerization.13 The use of nanoparticles and nanofibers as reinforcing materials in the polymer precursor offers another potential route to improve mechanical properties and reduce energy costs. For carbon fiber applications, recent studies have found that the mechanical performance of wet spun polyacrylonitrile (PAN) precursor fibers and the resultant carbon fiber properties can be improved by the incorporation of carbon nanotubes (CNTs) at low weight fraction into the polymer precursor.14,15 However, currently, as well as being derived from nonrenewable

INTRODUCTION Lightweight and high strength carbon fiber composites have recently found considerable use in the automotive and aerospace sectors. However, due to the high cost of the petroleum-derived polyacrylonitrile (PAN) precursor, as well as the energy input required for the carbonization process, the uptake of industrial carbon fibers in other industries has been severely limited. As such, recent research in carbon materials has been largely driven by the need to reduce the cost and improve the sustainability of the manufacturing process. This includes renewed research interest in biobased precursors such as rayon,1 bioacrylonitrile,2 and lignin,3,4 as well as low-cost synthetic precursors such as polyethylene.5,6 Another approach to reducing energy costs involves methods of improving the mechanical performance of the carbonized fiber, where typically the mechanical performance scales with the maximum furnace temperature up to 2500 °C.7 This technology space has been investigated in a number of ways including the optimization of © 2017 American Chemical Society

Received: December 23, 2016 Revised: February 23, 2017 Published: February 27, 2017 3296

DOI: 10.1021/acssuschemeng.6b03144 ACS Sustainable Chem. Eng. 2017, 5, 3296−3304

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ACS Sustainable Chemistry & Engineering

benchtop high-pressure homogenizer (Panda 2 K NS1001L, GEA Niro Soavi S.p.A, Italy) twice at 700 bar. The resulting CNF dispersions were then stirred overnight and freeze-dried. The composition of the resulting CNF after this delignification and bleaching protocol has been previously determined to contain: cellulose (55 wt %), hemicellulose (42 wt %), and lignin (3 wt %).24 Preparation of Electrospun PAN/CNF Nanocomposite Precursors. Prior to dispersion, to remove excess moisture, both the PAN powder and freeze-dried CNF samples were dried in a vacuum oven at 50 °C overnight. Composites of PAN and CNF (0− 10 wt % CNF wrt PAN) were dispersed in N,N-dimethylformamide (DMF) with a solids to solvent ratio between 6−10% w/v. PAN was first dissolved in DMF under constant stirring for 5 h at 75 °C until a clear and homogeneous solution was obtained, while the CNF was stirred in DMF separately before being slowly combined with the PAN/DMF solution. This mixture was then stirred for 2 h and ultrasonicated (Q500 Sonicator, QSonica, Newtown, United States) at an output of 500 W, frequency of 20 kHz, and an amplitude of 20% for 4 min under an ice bath, after which the stirring was continued for 24− 48 h at ambient lab conditions. Electrospun fibers were produced using an electrospinning setup consisting of an automatic syringe pump (single-infusion syringe pump, Cole-Parmer, Illonois, USA), a high voltage power supply (FC series, 1−60 kV, Glassman HV), and a ground stainless steel mesh collector positioned upright 15−20 cm from the tip of the syringe (G-16 PrecisionGlide needle, BD). The PAN/CNF mixtures (3−5 mL) were delivered to the syringe tip at a controlled flow rate (0.5−1.25 mL/h) under ambient lab conditions (25 °C, 1 atm, 50% RH). Electrospinning of mixtures was initiated by applying 15−25 kV in positive-ion mode from the power supply with the current set to 500) and high hemicellulose content and can be isolated using a low-energy procedure without the need for aggressive chemical pretreatments.24 Using this unique spinifex CNF, we have investigated the influence of CNF incorporation on the carbonization of CNF doped PAN precursors. By systematically studying the spinnability of the PAN/CNF dispersions at different concentrations, producing PAN/CNF composites fibers through electrospinning as a simple means of observing the carbonization behavior, we report the how the flexible, tough, and hemicellulose-containing CNF enhances the structural properties of carbon materials via a typical carbonization process without the need for additional heat energy. As a departure from the use of rigid reinforcers such as CNTs or CNCs, the spinifex CNF offers a sustainable functional nanoadditive with high flexibility (due to a very high amorphous hemicellulose content in spinifex CNFs24), high aspect-ratio, and a low production cost.

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EXPERIMENTAL SECTION

Materials. Mature spinifex grasses (T. pungens) were harvested from Camooweal, northwest Queensland, Australia. Glacial acetic acid (99.7%, Ajax Finechem), sodium hydroxide pellets (99.99%, Ajax Finechem), and sodium chlorite (80%, Sigma-Aldrich) were all used in the pulping process for the spinifex grass. Polyacrylonitrile (PAN) (99.5%, with methyl acrylate comonomer: 0.5%, Mw: 230 000 g·mol−1) from Goodfellow, UK, and N,N-dimethylformamide (99.8%, SigmaAldrich) were used as received. Production of Cellulose Nanofibers from Spinifex Grass (T. pungens). Cellulose nanofibers were isolated from T. pungens by delignification and bleaching of the grass, as using procedures described in earlier work.24 Briefly, this process involved a mild sodium hydroxide treatment of the harvested, washed, and grounded spinifex grasses (2% w/v NaOH, 10:1 solvent to grass ratio) at 80 °C to remove lignin. This was followed by a bleaching step (30:1 solvent to grass ratio) in a solution of sodium chlorite (1% w/v) at 70 °C at pH 4, after which the pulp was washed with deionized water at 60 °C to give a white pulp. Spinifex CNFs were obtained by passing a suspension of the bleached spinifex pulp in deionized water through a 3297


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Table 1. Summary of the Composition, Viscosity, and Conductivities of Electrospinnable PAN/Spinifex CNF Dispersions total solution concentration (w/v%)

CNF content (wt %)

viscosity (Pa·s) (at a shear rate of 5 s−1)

conductivity (μS/cm)

6 6 6 6 6 6 8 8 8 8 8 10 10 10 10 10

0 0.5 1 2 5 10 0 0.5 1 2 5 0 0.5 1 2 5

0.23 0.26 0.32 0.37 0.80 1.54 1.15 1.08 1.20 1.51 1.12 3.27 3.35 3.59 2.67 2.91

42 39 38 38 38 37 49 49 48 47 46 55 53 53 49 50

EOC(%) =

(100 × 0.29Abs(1590)) ((Abs(2242) + (0.29Abs(1590))

concentrations up to 12% w/v were found to be electrospinnable, it was observed that PAN concentrations beyond this level led to increased sputtering and interruption of the spinning solution at the emitter tip (which occasionally produced droplets, rather than fibers) leading to the formation of a nonuniform fibrous mat. This was also the case for solutions containing CNF content greater than 5 wt % relative to PAN, where agglomerates of CNF appeared to disrupt the flow of the electrospinning solution. As such, the concentration range of PAN found to be optimal for electrospinning and selected for this study was between 6 and 10% w/v, while optimal CNF loadings were set between 0.5 and 5 wt %. As an exception, in the 6% w/v PAN solution, up to 10 wt % CNF was incorporated as this composition was also found to be spinnable. For CNF loadings greater than 5 wt %, alternative processing techniques, such as wet or dry spinning may be more suitable.27,28 The concentration of the dissolved PAN and the amount of CNF additive also affects the solution conductivity. This is an important consideration because the solution conductivity determines the rate at which charge moves within a solution during electrospinning, as well as the discharge of the collected nanofibers.29 It has also been observed that low conductive electrospinning solutions are more likely to form beaded fibers.30,31 Prior to electrospinning, the conductivity of each dispersion was measured (Table 1). These conductivity values show an increasing trend with increasing PAN concentration (42−55 μS/cm from 6−10% w/ v PAN/DMF), which is typical for PAN/DMF solutions.29 Conversely, the addition of the spinifex CNF led to a decrease in solution conductivity (e.g., 42 to 37 μS/cm from 0 to 10 wt % for the 6% w/v solutions). The reason for this is likely due to the reduction of dissolved PAN in substitution for the nonionic nanocellulose fibrils. Despite this factor, the effect of these differences on the electrospinnability of the solutions appeared to be negligible, given the order of magnitude of the conductivities was maintained, and all of the solutions in the specified range were still electrospinnable. From the rheology data, at a shear rate of 5 s−1, the various PAN/CNF dispersions (in the solid to solvent range between 6 and 10% w/v) were found to have spinnable viscosities in the range between 0.23− 3.59 Pa·s (Table 1). This is a viscosity range comparable to that of the behavior of pure PAN in DMF solutions.32 For

(1)

Where Abs(1590) and Abs(2242) are the measured absorbances for aromatic/CC and nitrile stretches, respectively. Once spun, the morphologies of the electrospun PAN/CNF mats and their respective carbonized products were imaged using a scanning electron microscope (SEM; JSM-6460LA, JEOL). These samples were iridium coated using a turbo-pumped sputter coater (Q150TS, Quorum Technologies) and imaged at 5 kV. The CNF dispersions in water were also imaged using transmission electron microscopy (JEOL 1011 TEM) at 100 kV. Raman spectroscopy was used to investigate the carbon structure and formation of graphitic crystallites. The spectra for the electrospun carbon fibers were obtained on a Renishaw inVia Raman microscope system equipped with a 514.5 nm laser. All spectra were taken with a 60 s acquisition over a range of 100−3500 cm−1 with a laser power of 1% to minimize sample degradation. XRD diffraction analysis was conducted on a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) with a 0.2 mm slit. Graphite-filtered Cu−Kα radiation was generated at 40 kV and 40 mA. The carbon samples were first crushed before being placed on the sample holder. All samples were scanned over a range of 2θ = 10−90° at a scan speed of 1.2 s/step. The mean crystallite thickness (Lc) and crystallite size (La) was also calculated for the carbon nanofibers using the Scherrer equation:

L(nlk) =

κλ β cos θ

(2)

where β is the fwhm in radians, λ is the wavelength of the radiation (1.54 Å), and θ is the scattering angle of the diffraction peak in the (nlk) plane, and K is the shape factor which is equal to 0.90 for crystallite thickness (Lc) determined from the [002] peak, and 1.84 for crystallite size (La) determined from the [100] peak.26

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RESULTS AND DISCUSSION Electrospinnability of PAN/Spinifex CNF Suspensions. The relative concentration of PAN dissolved in the DMF and the amounts of CNF incorporated are important factors that affect the solution viscosity and dispersion quality, which also have an influence on the solution “electrospinnability”. The criteria used to distinguish the electrospinnability of a precursor solution was whether it was able to be spun to give a uniform mat of nanofibers which was relatively free of droplets and defects, such as beads. It was found that PAN solutions below 6% w/v were too dilute for electrospinning and led to the formation of beads on the fiber structure. By contrast, while 3298


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The ability for the CNFs to flow and reorient appears be favored in lower concentration solutions, as solutions of a higher concentration of PAN may become too viscous to allow the orientation of the dispersed nanofibrils. This highlights that the orientability of a flexible high aspect-ratio nanofibril additive may be controlled by simply varying the concentration or the viscosity of the solution, at a given shear. Other studies have reported the rheological effects of the addition of low aspectratio CNC on a PAN solutions, where non-Newtonian shear thinning characteristics are also demonstrated.23,38 In these studies, the effects have been attributed to the interruption of CNC−CNC and CNC-PAN intermolecular bonding with increasing shear forces.23 However, such tests have not been performed with larger aspect-ratio flexible CNFs where it might be expected that a greater increase in viscosity be observed, since the intrinsic viscosities of aqueous CNF dispersions have also been shown to scale with aspect ratio.39 Effect of Spinifex CNF on PAN Stabilization. Due to the oxygen-rich nature of cellulose (five oxygen atoms per glucose monomer), it is expected that the incorporation of CNF into PAN will affect the thermochemical reactions of PAN during its stabilization. This is important since it has been shown that for carbon fiber production, the presence of oxygen plays a role in controlling the enthalpy of oxidation/cyclization reactions.40 This is typically achieved via a controlled air flow41,42 and the use of oxygen-rich comonomers.11,12 However, in this case, the incorporation of the CNFs will introduce a new source of oxygen homogeneously and internally throughout the fiber. The effect of CNF incorporation on the PAN stabilization reaction chemistry was investigated by comparing the FTIR spectra after the stabilization of the PAN/1%−CNF (from the 8% w/v precursor solution) electrospun nanofibers at 230 °C under nitrogen and air (Figure 2). The unstabilized PAN/1%−

commercial grade spinning solutions, a higher concentration is preferred to minimize solvent use and improve fiber tenacity.33 However, high concentration solutions may have viscosities that are too high to be readily extrudable at safe pressures.33,34 Molecular Interactions of PAN/CNF Dispersions. At the molecular level, the interactions between PAN−PAN and PAN−DMF are predominately due to polar forces from the polymer nitrile groups,35 while CNF and PAN−CNF interactions will be dominated by its capability of forming hydrogen bonding networks (due to the presence of alcohol functional groups).36,37 These molecular interactions play a significant role in determining the solution behavior and spinnability. Viscosity measurements for each composite matrix revealed non-Newtonian and shear thinning behavior in the PAN mixtures with different levels of CNF incorporation (Figure 1).

Figure 1. Viscosity (η) vs shear rate (γ) plots for PAN/CNF dispersions prepared at (a) 6%, (b) 8%, and (c) 10% w/v.

Figure 2. FTIR spectra of PAN/1%−CNF electrospun fibers that are (a) unstabilized control, (b) stabilized at 230 °C in nitrogen, and (c) stabilized at 230 °C in air indicating the conversion of nitrile groups in PAN to conjugated cyclic structure upon stabilization in air vs nitrogen.

This appears to be closely linked to the concentration of the PAN and the relative amount of CNF added. The solutions with the lowest PAN/DMF concentration (6% w/v) exhibited the greatest pseudoplastic shear thinning behavior, as indicated by an appreciable increase in viscosity with increasing CNF content at low shear rates. However, when compared with solutions of increasing PAN concentration (8−10% w/v), the viscosity modifying effect of the CNF diminished at the lower shear rates. This might be related to the molecular disentanglement of the CNF with increasing shear, which leads to more oriented fibrils and better flow of the solution.

CNF fibers are characterized by a strong nitrile absorption band at 2242 cm−1, which reduces in intensity as the nitrile functional groups react with one another to form conjugated cyclic carbon structures indicated by an increase in the CC band at 1590 cm−1. Using eq 1, the extent of the cyclization (EOC) reaction, and thus the progress of PAN stabilization, was calculated from the ratio of peak intensities for conjugated carbon vs nitrile 3299


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ACS Sustainable Chemistry & Engineering groups. These are summarized below (Table 2). These data show that in an oxygen-rich atmosphere, the presence of the Table 2. Extent of Cyclization Reaction (EOC) for Electrospun PAN and PAN/1%−CNF Fibers Stabilized under Air and Nitrogen for 120 min at 230 °C stabilization atmosphere air

nitrogen

precursor

mean dev

mean dev

PAN PAN/1%−CNF

90.1 ± 0.3% 79.8 ± 0.6%

86.4 ± 0.3% 85.6 ± 0.6%

CNF at 1 wt % reduces the extent of cyclization of PAN by 10.3%, while in nitrogen atmosphere, there is no significant change in the stabilization of PAN due to the presence of CNF. A possible explanation for this dramatic difference may be due to the formation of inhibitive byproducts from the competitive oxidation of cellulose in the air atmosphere. It has been reported that the rate of thermal degradation of cellulose is increased in an air atmosphere,43,44 and lowtemperature oxidative degradation of cellulose has been shown to lead to early glycosidic bond scission and the formation of byproducts such as carbon monoxide, and dioxide at temperatures below 230 °C, reactions which do not occur in nitrogen atmosphere.43 Contrastingly, in an inert nitrogen atmosphere, the CNF is stable until after 250 °C (see Figure S1 in the Supporting Information for TGA data). However, it was expected that the release of oxygen-containing groups from the CNF during stabilization would promote the cyclization of PAN under a nitrogen atmosphere. A potential explanation for why this was not the case may be that oxygen from the CNF may have been liberated entirely as water vapor, rather than oxygen gas during stabilization. Water can be liberated from cellulose during heating through surface desorption, dehydration of alcohol groups, or eliminated during glycosidic cleavage.45 As such, since water is also a byproduct of dehydrogenation reactions in PAN,46 it is likely that water released by the CNF has no effect on the progress of PAN cyclization, as indicated by similar EOC values for the two precursors when stabilized in nitrogen. This finding led to the decision to conduct all subsequent stabilization reactions under nitrogen, rather than air. Morphological Aspects. Figure 3a shows the TEM image of the spinifex CNF from the aqueous suspension, which have a diameter of 6.15 ± 1.4 nm. However, it is difficult to accurately measure the length of the individual CNF due to twisting and entanglements as well as random in-plane orientations. In spite of this, the TEM image clearly indicates that the length of spinifex CNFs is at least several microns. Figure 3b shows the morphology of the freeze-dried CNFs, which through the freeze-drying process, the long and thin CNFs have self-assembled into larger sheetlike lamellar structures. These lamellar structures are formed due to increased hydrogen bonding and van der Waals attraction between individual fibrils during lyophilization.47 Upon ultrasonic-assisted dispersion of the freeze-dried CNF, the hydrogen bonds in the lamellar structure are broken down and the individual fibrils are resuspended. After electrospinning the PAN/CNF composites, all of the fibers were stabilized and carbonized by heating to 230 and 1200 °C under nitrogen. The carbonization process has a significant effect on both the carbon

Figure 3. Electron microscopic images showing the morphology of (a) CNF from aqueous suspension (b) freeze-dried CNF, and (c−h) the electrospun PAN/CNF fibers before (left) and after carbonization (right) respectively, for the composite precursors from 6% (c and d), 8% (e and f), 10% (g and h) PAN/CNF solutions.

structure and the morphology of the precursor fibers which appears to be influenced by the spinifex CNF. The changes in the morphology of the electrospun PAN/CNF composite fibers before and after carbonization are illustrated in the SEM images (Figure 3c−h). It can be seen that the diameter of the electrospun PAN/CNF precursor fibers increases with increasing solids concentration (293 nm at 6% to 976 nm at 10%), which may be attributed to viscosity increases in the solution dope when electrospun under the same regime (voltage, flow rate, collector distance; see Figure S2 in the Supporting Information for diameter histograms). When carbonized, the diameters of these fibers were all similar (299−377 nm) regardless of the precursor−which suggests greater fiber shrinkage in the 8 and 10% fibers. Both of these observations are characteristic for normal electrospun PAN fibers and their carbonization.31,48 The images also largely show the retention of the fibrous structure after carbonization; however a higher breakage of individual fibers at higher PAN concentrations is apparent. Structural Properties of PAN/CNF Electrospun Carbon Fibers. The development of the carbon microstructure after thermal treatment can be observed using both Raman spectroscopy and XRD. From the Raman spectra, each carbonized sample shows two prominent peaks corresponding to the G and D bands, which have been deconvoluted using a Gaussian−Lorrentzian fitting (Origin 8.6) (Figure 4). These two bands confirm the formation of carbon from the various PAN/CNF precursors, where the G band at approximately 1595 cm−1 is associated with sp2 vibrations of an ordered graphite crystal, while the position of the D band at 3300


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indicator of graphitic order). Particularly evident is the general correlation between the minima and maxima values when comparing the change in D/G ratio and relative XRD intensity, at a given level of CNF content. From the Raman spectral data, a decrease in D/G ratio due to the addition of the CNF was observed at certain CNF concentrations (5 wt % at 6% w/v/ PAN and between 0.5 and 2 wt % at 8−10% w/v PAN). This appears to be related to an increase in sp2 hybridized carbon− carbon bonds over that of sp3 due to the addition of the CNF. Similarly, a corresponding increase in [002] XRD peak intensity with CNF incorporation is observed at certain CNF content levels, which also highlights improvements to the carbon crystallinity. These changes are also corroborated with similar trends in crystallite thickness (Lc) and crystallite size (La) which were calculated using the Scherrer equation (See Figure S3 in the Supporting Information). In light of recent literature discussion over the validity of the inverse relationship between Raman D/G ratio and crystallite size (La) for small carbon crystals 2 nm. This suggests that for these electrospun carbon fibers, an inverse linear relationship between D/G and La still holds. From these data, an optimum concentration of PAN and CNF additive content can be determined for maximizing the carbon properties of an electrospun PAN fiber mat enhanced with a CNF additive. For instance, at a dope concentration of 8% w/v, based on the minimum of the D/G and the maximum of the [002] intensity plots, the ideal amount of CNF additive would be approximately 0.5 wt %. An excess of CNF incorporation beyond this optimum appears to affect crystallite formation negatively. Generally for PAN-based carbon fibers, an increase of ordered graphite crystals (typically characterized by a reduction in D/G ratio and an increase in crystallite size) is obtained by increasing the carbonization temperature9,53 under tension, which leads to an improvement in mechanical performance.7,54 As such, given all the samples were carbonized at the same temperature of 1200 °C, the incorporation of CNF into the PAN precursor may have implications for improving the mechanical performance of PAN-based precursors without the additional heat energy typically required for graphitic structure formation. In other words, the efficiency of carbonization appears to have been improved by the incorporation of CNF generally between 0.5 and 2 wt % loading, depending on the PAN concentration. The side-by-side comparative plot shows the pronounced effect that the addition of this high aspect-ratio CNF to PAN can have on the properties of the carbon material produced (Figure 5). Although it is uncertain what the mechanism for this improvement in carbon crystallinity and graphitic order due to the incorporation of CNF is, a possible explanation may involve an induced-orientation effect, where the CNF may participate in orienting the PAN polymer chains during the spinning process, leading to improvements in structural order. This might be supported by considering the effect of the dope concentration on the graphitic order of the electrospun carbon fibers (Figure 5). For example, it can be determined that as the dope concentration increased from 6% to 10%, the optimum CNF amount required to maximize the graphitic order (corresponding to a minimum in D/G value) generally decreased from 2−5 wt % to 0.5−1 wt %. This is supported by earlier rheology results, where more of the high aspect-ratio CNF appeared to be more orientable at lower dope concentrations and viscosities. With greater CNF orientation,

Figure 4. Example Raman spectra showing the Gaussian−Lorrentzian deconvolution profiles of the D and G bands for (a) PAN control (8% w/v) and (b) PAN/0.5% CNF (8% w/v).

approximately 1380 cm−1 is associated with amorphous-phase sp3 carbon bonds. The ratio of the intensity of the two peaks (D/G ratio) gives an indication of the degree of order of the graphitic crystallites of the carbon material; a lower value typically corresponds with higher graphitic order. The D/G ratio for each electrospun carbon fiber sample from the various composite precursors was calculated and summarized in Figure 5a−c as a function of CNF additive amount. The XRD data also suggest the successful formation of carbon from the composite precursors as indicated by the [002], [100/101], and [110] reflection peaks typical for graphitic materials (Figure 6a).49 By comparing the sharpness of the [002] peak at 2θ = 24.7° (d-spacing = 3.564 Å) in the XRD spectra, the relative crystallite differences between the various composite precursors can be compared. For carbon fibers and other carbon materials, this broad peak has been shown to increase in intensity and become narrower with an increase in carbonization temperature.50,51 In this case, it is apparent that the electrospun carbon fibers differed in the carbon crystallinity with varying levels of CNF content. The intensity of the peak at 2θ = 24.7° for each carbon sample was plotted as a function of CNF amount for comparison with the Raman data and is shown in Figure 5d−f. The side-by-side comparison of these two indices highlight, to an extent, the relationship between increases in XRD peak intensity (an indicator of the degree of graphite crystallinity) with a decrease in carbon D/G ratio in the Raman spectra (an 3301


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Figure 5. Effect of spinifex CNF on the carbon properties of the electrospun PAN/CNF composite precursors. The plots show the change of the Raman D/G ratio with CNF content for the electrospun carbon fibers prepared from spinning solutions at (a) 6%, (b) 8%, and (c) 10% w/v, while the change in XRD intensity with CNF is given for (d) 6%, (e) 8%, and (f) 10% w/v precursors.

by oriented CNF is likely to be similar to those factors that increase the viscosity of the electrospinning dope: hydrogen bonding and polar interactions between PAN-CNF. This induced-orientation mechanism draws parallels with some of those proposed for carbon nanotube reinforcement of PAN composites and fibers, which involve the alignment of molecular PAN chains in the vicinity of the CNTs via interphase templating.55,56 Future studies should involve the stretching of the fibers to prevent relaxation of the polymer chains and ensure that any induced orientation due to the CNF is maintained through the carbonization process.

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CONCLUSIONS In summary, we have demonstrated that the addition of high aspect ratio CNF into PAN composites has a significant effect on the solution rheology, as well as the carbonization efficiency of an electrospun precursor. From the rheological measurements, it was found that the arrangement of the CNF in solution via electrospinning is highly dependent on the PAN concentration and applied shear force. After electrospinning, it was also observed that the electrospun composite fibers were better stabilized under a nitrogen atmosphere, than air at 230 °C which is not typically the case for traditional PAN-based carbon precursors. At certain compositions of the composite PAN/spinifex precursor fiber, the electrospun carbon fibers produced by heating to 1200 °C were found to show a reduction in D/G ratio and a corresponding improvement in

Figure 6. Representative XRD curves for the electrospun carbon fibers prepared from PAN precursors (8% w/v) with increasing levels of spinifex CNF.

an increase in PAN polymer orientation may also result. This is important, since for wet-spun carbon fibers, the higher molecular orientation of the PAN precursor leads to improvements in the fiber properties upon carbonization.33 The mechanism for this induced orientation of the PAN polymer 3302


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XRD crystallite structure, when compared with a control PAN without any additive incorporation. This highlights the significant effect that the incorporation of a high aspect-ratio CNF has on the carbon formation reactions during the carbonization process, which may have implications for reducing the energy costs of the carbonization process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03144. TGA curves for the spinifex CNF, CNF/PAN composite precursor and stabilized precursor. Average fiber diameters for the electrospun PAN/CNF fibers and the electrospun carbon fibers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pratheep K. Annamalai: 0000-0002-7284-0813 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from Australian Research Council (under ARC Discovery Grant No. DP150101846). They also acknowledge the Aboriginal collaborator, Dugalunji Aboriginal Corporation in Camooweal, for project collaboration and the supply of grass samples. The authors acknowledge the facilities and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland.

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REFERENCES

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