Influence of Different Nanocellulose Additives on Processing and

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Article Cite This: ACS Omega 2019, 4, 9720−9730

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Influence of Different Nanocellulose Additives on Processing and Performance of PAN-Based Carbon Fibers Edward Jiang,*,† Maxime Maghe,‡ Nima Zohdi,‡,⊥ Nasim Amiralian,† Minoo Naebe,‡ Bronwyn Laycock,§ Bronwyn L. Fox,‡,∥ Darren J. Martin,† and Pratheep K. Annamalai*,† †

Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, St Lucia, QLD 4072, Australia Carbon Nexus, Deakin University, Institute for Frontier Materials, Geelong, VIC 3216, Australia § School of Chemical Engineering, The University of Queensland, Advanced Engineering Building, St Lucia, QLD 4072, Australia ∥ Manufacturing Futures Research Institute, Swinburne University of Technology, John St, Hawthorn, VIC 3122, Australia

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‡

S Supporting Information *

ABSTRACT: Nanocellulose, as a biobased versatile nanomaterial that can be derived with tailorable surface functionalities, dimensions, and morphologies, has considerable implications for modifying the rheology, mechanical reinforcement, and influencing the carbonization efficiency in the production of polyacrylonitrile (PAN)-based carbon fibers. Herein, we report the influence of three different nanocellulose types, varying in the derivatization method, source, and aspect ratio, on the mechanical properties and thermal transformations of solution-spun PAN/nanocellulose nanocomposite fibers into carbon fibers. The incorporation of 0.1 wt % nanocellulose into solution-spun PAN fibers led to a 7−19% increase in tensile modulus and 0−27% increase in tensile strength in the solution-spun fibers, compared to a control PAN fiber. These improvements varied depending on the nanocellulose type. After low-temperature carbonization at 1200 °C, improvements in the mechanical properties of the nanocellulose-reinforced carbon fibers, compared with a PAN fiber, were also observed. In contrast to the precursor fibers, the improvement % in the carbonized fibers was found to be dependent on the nanocellulose morphology and was linearly correlated with increasing aspect ratio of nanocellulose. For example, in carbon fibers with a cotton-derived low-aspect-ratio cellulose nanocrystal and spinifex-derived high-aspect-ratio CNC and nanofiber, up to 4, 87, and 172% improvements in tensile moduli were observed, respectively. Due to the processing methods used, the nanocellulose aspect ratio and crystallinity are inversely related, and as such, the increase in the carbon fiber mechanical properties was also related to a decrease in crystallinity of the nanocellulose reinforcers. Raman spectra and electron microscopy analysis suggest that mechanical improvement after carbonization is due to internal reinforcement by highly ordered regions surrounding the carbonized nanocellulose, within the turbostratic carbon fibers.

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INTRODUCTION The reinforcement of the polyacrylonitrile (PAN) precursor fiber using nanoadditives offers a unique strategy to improve the mechanical performance of a carbon fiber. Due to their high tensile modulus, single-wall carbon nanotubes have been investigated as a reinforcer;1−4 however, some of the drawbacks of single-wall carbon nanotubes (SWCNTs) include high cost of production (up to $750/g5) and dispersion challenges. Nanocellulose as a versatile nanomaterial with tailorable surface functionalities, dimensions, and morphologies can offer a potentially low-cost, biobased alternative reinforcer for PAN-based carbon fibers.6,7 Nanocellulose can be obtained with different surface chemistries, morphologies, and property profiles depending on the source, isolation methods, and processing conditions. Mostly, the rodlike or ricelike cellulose nanocrystals (CNCs) of low aspect ratio (10−50) and having a diameter in the range of 10−100 nm are isolated through common pulping and acid hydrolysis, © 2019 American Chemical Society

oxidation, enzymatic treatment, or high-intensity ultrasonic treatments,8,9 while the filament-like cellulose nanofibers (CNFs) of high aspect ratios (>100) are extracted from mild chemical pretreatments followed by mechanical shearing methods (high-pressure homogenization, milling, ultrasonication, microfluidization, double disc grinding, etc.).10−12 The individual differences in different nanocellulose types are particularly relevant in the context of polymer composites where highly stiff, high-aspect-ratio nanomaterials provide the greatest mechanical reinforcement.8,9 Therefore, when comparing cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), a balance between these variables is crucial, since CNCs tend to be more rigid, while CNF have a higher aspect ratio.10−12 Furthermore, given the inherently high oxygen Received: January 29, 2019 Accepted: May 22, 2019 Published: June 4, 2019 9720

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Table 1. Dimensions, Crystallinity, and Acidic Group Content of the Nanocellulose Types Studied nanocellulose type

length (nm)

width (nm)

aspect ratio

crystallinity index (%)

acidic group content (μmol/g)

cotton CNC (c-CNC) spinifex CNC (s-CNC) spinifex CNF (s-CNF)

195 ± 61 497 ± 106 1686 ± 591

17 ± 2 3.5 ± 1 3.5 ± 1

11 ± 5 144 ± 70 527 ± 190

69.2 50.0 43.8

50a 149a 82b

a

Indicates the presence of strong acid groups. bIndicates the presence of weak acid groups.

Table 2. Mechanical Properties of the Multifilament Solution-Spun Precursor Fibers parameters

PAN

PAN/c-CNC

PAN/s-CNC

PAN/s-CNF

single filament diameter (μm) density (g/cm3) tensile modulus (GPa) break tension (MPa) elongation (%)

10.6 ± 0.4 1.18 5.9 ± 0.4 150 ± 10 44 ± 16

13.0 ± 1.8 1.18 6.7 ± 0.4 190 ± 10 21 ± 5

10.9 ± 0.3 1.18 7.0 ± 0.2 190 ± 10 19 ± 6

11.9 ± 0.7 1.18 6.3 ± 0.4 150 ± 2 22 ± 7

cellulose additives available, and to date, no work has been carried out to compare their effects on carbonization and mechanical performance of PAN-based carbon fibers. Hence, three different nanocellulose types of varying aspect ratio, morphology, crystallinity index, and acidic group content produced through either the mechanical or the typical acid hydrolysis method are benchmarked in this study, for their effect on carbonization and the properties of solution-spun PAN-based carbon fibers. For this, the low-aspect-ratio CNC and higher-aspect-ratio CNC were produced through common acid hydrolysis (digestion under acidic conditions, disintegration by ultrasonication and freeze-drying) from cotton and spinifex grass (Triodia pungens), respectively, and the highestaspect-ratio CNF was derived from spinifex grass through the mechanical method. As a departure from film cast carbon composites,29,30 herein, the wet-spinning technique was selected for a closer representation of the carbon fiber production process. This required the building of a custom solution-spinning unit, which is described in the Experimental Section (Figure S1a). Furthermore, to gain a deeper insight into the influence of nanocellulose on the mechanical and chemical transformations occurring throughout the heating process, the solution-spun fibers were tracked, through a scaled-down industrial carbon fiber production line (Figure S1b). These changes were studied as a function of the properties of the nanocellulose additives after low-temperature carbonization.

content of nanocellulose (10 oxygen atoms per cellulose monomer) and the fact that most cellulose pulp grades contain varying proportions of weak carboxylate groups,13 the oxygen released from cellulose pyrolysis14 in a nanocellulosereinforced PAN fiber may potentially influence the bulk oxidation and nitrile cyclization reactions during PAN thermal stabilization.15 This is because for carbon fiber-grade PAN fibers, oxygen-rich acidic co-monomers are often incorporated to control the rate of stabilization reactions.16,17 With surface functionalization chemistry such as TEMPO oxidation18 and sulfurization reactions,19,20 the nanocellulose surface can be modified to increase the oxygen content or to incorporate heat-controlled acid catalysts. From a fiber-spinning standpoint, there are numerous existing examples of nanocellulose wet-spun directly from concentrated hydrogels,21,22 as well as wet-spun polymer composite fibers utilizing the reinforcing additives at lower amounts typically between 1 and 10%.23−25 For instance, Akira Isogai’s group wet-spun a poly(vinyl alcohol) (PVA) composite with an addition of 1 wt % TEMPO-oxidized CNF, which yielded a fiber with a tensile modulus of 6 GPa, which increased to 57 GPa with stretching.24 The stiffening effects were attributed to the nanoreinforcement of the polymer matrix by the nanofibers facilitated by (1) the homogeneous dispersion of the individualized nanofibers without aggregation due to its high surface charge and (2) intermolecular compatibility between the TEMPO-oxidized CNF and the amorphous PVA matrix. Such effects would similarly be expected for PAN-based solution-spun fibers. In PAN-based composite fibers containing cellulose nanocrystals (CNCs), some studies have also demonstrated improvements in the tensile properties of the reinforced precursor fiber,26,27 from which Park et al. also demonstrated improvements in the performance of the carbon fiber after carbonization.27 A more recent study has even demonstrated that a CNC loading of up to 40 wt % can be achieved in a PAN/CNC carbon fiber, with mechanical properties comparable to those of a pure PAN fiber, highlighting the potential for more biobased carbon fibers.28 However, in all of these cases, the nanocellulose additives were limited to CNC without surface functionality (besides the natural cellulose −OH groups) and relatively low aspect ratios (10−40). This highlights the scope of enhanced reinforcement of PAN fibers using higher-aspect-ratio nanocelluloses. Overall, there are significant differences between the functionalities and morphologies of different types of nano-

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RESULTS AND DISCUSSION Nanocellulose Reinforcement of Multifilament Solution-Spun PAN Fibers. The key differences for each nanocellulose type used in this study are summarized in Table 1, and the detailed characterization including chemical analysis (Figure S2), electron microscopic images (Figure S3), and thermal decomposition behavior (Figure S4 and Table S1) of these nanocellulose types is provided in the Supporting Information. Four different multifilament fibers were produced by solution-spinning from the control PAN and the PAN/ nanocellulose solutions, each containing either cotton CNC (c-CNC), spinifex CNC (s-CNC), or spinifex CNF (s-CNF) at 0.1 wt %. The reinforcement potential of the various nanocellulose additives in the solution-spun fibers processed with the same conditions was investigated, and the mechanical properties are summarized in Table 2. 9721

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Figure 1. Effect of nanocellulose types on PAN rheology and fiber structure. Viscosity (η) vs shear rate (γ) plot in (a) 6% PAN solution and (b) 13% PAN solution (the inset shows power law fit), showing greater non-Newtonian behavior in the PAN/s-CNF solution compared to the PAN/cCNC, PAN/s-CNC, and blank PAN solutions. (c) Differential scanning calorimetry (DSC) curves showing the glass-transition (Tg) changes in the different multifilament fibers after (c) an initial heating cycle and (d) a second heating cycle.

spinning process, potentially leading to more oriented reinforcers, and higher observed fiber tensile values. By contrast, the PAN/s-CNF solution displayed far greater flow resistance and shear thinning as characterized by a higher initial viscosity at a low shear rate, followed by a rapid reduction in viscosity at higher shear. This shear thinning behavior in PAN/s-CNF solutions was observed in both 6 wt % (Figure 1a) and 13 wt % PAN/DMF solutions (Figure 1b). After fitting the curves for the 13 wt % solutions to a power law model, only the PAN/s-CNF solution showed shear thinning behavior where the flow behavior index, n, is below 1 (additional rheological data at 13 wt % is given in Appendix S3). This behavior is due to its lack of surface functionality and very high aspect ratio, which increase the number of fibril entanglements, as well as possible H-bond and van der Waals interactions with polymer chains.33 At higher shear rates, these fibrils are able to disentangle and overcome H-bonding forces, which leads to the reduction in viscosity. This suggests that a greater amount of initial shear force from the spinning process is required to enable flow-based unidirectional orientation of the s-CNF compared with the c-CNC and s-CNC along the axis of the spun fiber at low shear rates (which affects the mechanical properties of the composite fibers). The effect of the nanocellulose types on the thermal transitions of PAN/nanocellulose nanocomposite fibers was also investigated using differential scanning calorimetry (DSC) through two heating and cooling cycles between −10 and 160 °C (Figure 1c,d). These data show a higher glass-transition temperature for all fiber-spun samples (Tg: 122−128 °C) in the first heating cycle compared with the second heating cycle (Tg: ∼100 °C), which indicates the increased rigidity of the PAN molecular structure due to the fiber spinning process. In addition to this, the PAN fibers containing the nanocellulose additives also appear to have a slightly higher Tg (123−128 °C) compared with the blank PAN fiber (122 °C). Tracking the Influence of Different Nanocellulose Types on Spun Fiber Stabilization. As thermal stabilization is considered the most critical and energy-intensive step in the

The data indicate that the incorporation of the two types of CNCs led to an improvement in the modulus and strength (break tension) compared to the blank precursor fiber. The PAN/s-CNC fiber performed the best with a modulus improvement of 18% and strength improvement of 27% over the blank PAN fiber. An explanation for these effects is that the mechanical reinforcement of the PAN fiber by the nanocellulose helps dissipate the stress of a load on the fiber.31 Overall, the elongation % of the precursor fibers decreased by the incorporation of 0.1% nanocellulose additive and no statistically significant change in the elongation % was observed among nanocomposite fibers. With a modulus of 7.0 GPa and strength of 190 MPa, the PAN/s-CNC fiber also showed better performance than other previously published solution-spun PAN fiber precursors containing CNC at higher loadings up to 2 wt %.27 However, it is acknowledged that these values are still considered low compared to those of typical commercialgrade solution-spun PAN fibers (tensile modulus: ∼10 GPa, strength: ∼500 MPa32) since the fiber spinning technique was limited to what was achievable with our custom-built device (total draw ratio: 3.9) and the fibers were not drawn postspinning. The fibers were not drawn to maintain consistency in the four fiber variants, since drawability (stretchability) of each of them would be different. These values would be expected to increase upon drawing under suitable conditions with a commercial fiber spinning unit. Another consideration for the property differences between the precursors, particularly for the performance of the s-CNF (with no statistical difference in strength or modulus compared with the control PAN fiber), is the differences in the rheology of the PAN/nanocellulose solutions. The presence of strong surface charges on the CNCs increased its flowability in the polymer matrix in a polar solvent by electrostatic repulsion. This is indicated by the lower viscosity of the PAN/CNC solutions compared to that of the PAN/CNF solution at lower shear rates in a 6 wt % PAN solution (Figure 1a). As such, the PAN/CNC solutions will have greater flowability during the 9722

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PAN, c-CNC, s-CNC, and s-CNF fibers, respectively. Figure 2c indicates that the strength of the fibers was largely unchanged through the entire process. Both the initial reduction in elongation and increase in modulus have been attributed to the early formation of rigid interchain cyclic structures, as indicated by the FTIR data (Figure 3a,b). PAN is typically characterized by a nitrile absorption band at 2242 cm−1 due to a stretching mode about the triple bond. The intensity of this band drops through stabilization, as the nitriles react with adjacent groups to form cyclic carbon structures. These conjugated groups are characterized by an increase in the CC and CN bands at 1590 cm−1. From this information, the extent of cyclization (EOC) was calculated from the ratio of peak intensities of conjugated carbon and nitrile groups. This gives an indication of the progress of stabilization, and these data are summarized below in Figure 3b. Generally, the histogram shows a linearly increasing cyclization reaction for each of the different fibers from heating zones 1 to 4. The data shows a reduced EOC value for the PAN/nanocellulose fibers compared to the PAN fiber (6− 10% cf. 12% for PAN) after passing through zone 1. However, after further processing of the fibers through zones 2 and 3, the rate of cyclization for the nanocellulose composite fibers increased, leading to all of the fibers being fully stabilized after passing through zone 4 with the same EOC of approximately ∼70%. This suggests that even at a small loading of 0.1 wt % nanocellulose, it is found to be involved in reducing the rate of the initial cyclization reactions at lower stabilization temperatures (zone 1, 233 °C). Furthermore, this effect appears to be independent of the type or aspect ratio of the nanocellulose. This early inhibitive effect is similar to that observed for the stabilization of PAN fibers with SWCNTs,4 where steric interference of cyclization along the polymer backbone during the initial heating phases of stabilization has been observed. However, upon continued heat treatment, the remaining unreacted nitriles on the chain continue to cyclize. This information may have important implications for controlling and slowing the initial cyclization rate, which is typically the function of acidic co-monomers in PAN beneficial for preventing the uncontrolled overheating of the fiber.34 The density of all of the fibers also showed significant increases with stabilization treatment, particularly after zone 3 (Figure 3d). This late-stage increase in density is typical for the stabilization of PAN fibers, where sufficiently stabilized fibers have previously been reported to have mass densities fall in the range between 1.34 and 1.39.35 This rapid increase in density at higher stabilization temperatures has been previously implicated with increased oxygen uptake and exothermic oxidation reactions in the fiber.35,36 The DSC data (Figure 3c,d) also suggests that a greater amount of heat is released due to an exothermic reaction following zone 3, as shown by a sudden and steeper drop in enthalpy for the zone 4 fibers. This suggests that the oxidation reactions occur at higher temperatures during late-stage stabilization, while cyclization begins immediately at zone 1. The notion that the transformation of nitriles into cyclic structures is a kinetic prerequisite for the initiation of rapid oxygen uptake and fiber oxidation has also been proposed by other authors in PAN fiber kinetic studies.37,38 This is particularly interesting given that the nanocellulose composite fibers showed higher densification after zone 3−4 heat treatment compared to the neat PAN fiber. Accordingly, this may be related to the presence of

carbon fiber manufacturing process, the effect of the different nanocellulose additives on the mechanical and chemical changes through the stabilization process was also monitored. The solution-spun multifilament composite fibers were thermally stabilized by passing the fibers through a four-oven system of increasing temperature treatment (233, 245, 255, and 263 °C) under tension. The tensile properties of the various composite fibers were measured through each zone of the stabilization process and are shown in Figure 2, in addition to the change in density of the fibers.

Figure 2. Effect of nanocellulose types on the mechanical properties of the thermally stabilized solution-spun fibers: (a) elongation, (b) modulus, (c) break tension (strength), and (d) density. Zone temperatures from 1 to 4 are 233, 245, 255, and 263 °C, respectively. Error bars represent the standard deviation (n = 25).

Figure 2a shows a general decrease in elongation for all of the fibers as stabilization proceeds. For the PAN fiber, the elongation at break decreased rapidly from 44 to 6% through stabilization and from approximately 21 to 6% for the composite fibers. As the elongation of the fibers decreased with immediate exposure to the zone 1 oven, the modulus of all of the fibers also immediately increased (Figure 2b). Following zone 2−4 treatment, the modulus of the PAN fibers remained relatively constant, whereas the modulus of the composite fibers dropped by approximately ∼2 GPa. The total modulus change from the precursor fiber to zone 4 stabilized fiber for each fiber was a 33, 18, 18, and 21% increase for the 9723

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Figure 3. Effect of nanocellulose types on the chemical properties of the thermally stabilized solution-spun fibers. (a) Example FTIR spectra for the s-CNC fiber showing the increasing aromatic peak with decreasing nitrile through stabilization and (b) extent of cyclization (EOC) for each fiber through stabilization. Error bars are the standard deviation (n = 3). (c) Example DSC curves showing the change in reaction exotherms for the sCNC fiber through stabilization and (d) the change in enthalpy for each fiber through stabilization. Error bars represent the standard deviation (n = 3).

Figure 4. Effect of nanocellulose types on the mechanical and structural properties of the carbonized solution-spun fibers: (a) tensile modulus and (b) tensile strength. Error bars represent the standard deviation (n = 15). (c) Raman spectra and (d) Raman D/G ratios (error bars are standard deviation, n = 4) of the “as-obtained” carbon fiber samples indicating formation of carbon structure.

and 2.7 times increase in the modulus over the control PAN fiber, increasing with the increasing aspect ratio of the nanocellulose. The c-CNC reinforced carbon fiber surprisingly only showed marginal improvements in the tensile modulus (39.5 GPa) compared to the control, which was not statistically significant. Improvements in the tensile strength of the carbon fibers, on the other hand, trended up consistently with the increasing aspect ratio of the nanocellulose incorporated (PAN: 187 MPa, c-CNC: 307 MPa, s-CNC: 383 MPa, s-CNF: 593 MPa). On the other hand, the strains at

oxygen in cellulose and its release by degradation during stabilization heating.39 Effect of Nanocellulose Aspect Ratio on Carbon Fiber Properties. Figure 4a,b shows the mechanical properties of each of the different composite carbon fibers after carbonization at 1200 °C, with the representative stress−strain curves given in Figure S7. With tensile moduli of 71.1 and 103.4 GPa, stiffening effects were observed in both the s-CNC and s-CNF reinforced carbon fibers, respectively, compared to the control PAN fiber (38.0 GPa). This corresponds to a respective 1.9 9724

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Figure 5. TEM images showing the carbon structure of the (a) PAN, (b) PAN/c-CNC, (c) PAN/s-CNC, (d) PAN/s-CNF carbon fibers respectively at low (left; a−d) and high magnification (right; e−h).

break for all of the carbon fibers were all in a similar range (0.9−1.2%), irrespective of the presence or absence of nanocellulose. The general improvement in the mechanical performance of the carbon fibers is attributed to an enhancement in the formation of a more ordered microstructure in the carbon fibers. Evidence for this can be seen by the change in the D/G ratio of the D and G bands in the Raman spectra for both the as-obtained carbon fiber samples (Figure 4c,d) and crushed samples (Figure S8). The D-band at ∼1340 cm−1 corresponds with sp3 disordered carbon structures, while the G-band located at ∼1590 cm−1 is related to a stretching mode of the sp2-hybridized carbon. The D/G ratio is thus a useful indicator

for the carbon microstructure and crystalline properties, which, in this case, shows that a drop in the Raman D/G ratio is observed with the increasing aspect ratio of the nanocellulose additive (Figure 4d). This suggests that the carbon fibers that contain increasingly higher-aspect-ratio nanocellulose have an increased proportion of structurally ordered carbon than disordered carbon. However, these data show that only the highest-aspect-ratio s-CNF composite carbon fiber demonstrated a significant improvement in the carbon properties compared to the control PAN fiber without any additive. These results are also corroborated by transmission electron microscopy (TEM) images of each composite carbon fiber, which show the 9725

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nanofillers during carbonization. This is because the tensile modulus of the nanocellulose additive itself is not fixed and will increase as it undergoes carbonization within the bulk fiber. Additionally, flexible reinforcers (e.g., low crystallinity, highaspect-ratio s-CNF) will become rigid upon carbonization, further increasing the modulus of the filler. Therefore, a highaspect-ratio CNF in a PAN fiber may experience a cumulative increase in its reinforcement potential during the carbonization process due to (1) the intrinsically high aspect ratio of the filler, followed by (2) an increasing modulus due the formation of a cyclic structure, which lead to the loss of flexibility and subsequent stiffening of the high-aspect-ratio nanofiber after carbonization. In other words, the flexible high-aspect-ratio reinforcer changes into a stiff high-aspect-ratio reinforcer upon carbonization, where nanoparticle stiffness and aspect ratio determine the reinforcement efficiency at fixed volumes according to Halpin−Tsai models.46 Such cumulative improvements in the reinforcement potential would thus not be observed during the carbonization of low-aspect-ratio CNC composites (since CNCs are rigid prior to carbonization) or a high-aspect-ratio SWCNT additive (since it also has rigid graphitic structure prior to carbonization). This behavior is shown diagrammatically in Figure 6 below.

increasing formation of ordered carbon layers in the nanocellulose composite carbon fibers, while the neat PAN carbon fiber retained a largely disordered structure (Figure 5). Generally, the size and quantity of a turbostratic structure in the composite fibers increased with the nanocellulose aspect ratio. The PAN/c-CNC fiber showed a largely disordered structure similar to the PAN standard fiber, though some turbostratic ordering is visible at the edges of the carbon material (d-spacing ∼0.34 nm), while the PAN/s-CNC fiber images showed an increasing turbostratic carbon structure throughout the sample with variable d-spacing (0.34−0.38 nm). Finally, the PAN/s-CNF fiber images indicate the presence of highly ordered lamellae of d-spacing 0.38 ± 0.01 nm. Interestingly, the interlayer spacing between the ordered lamellae of the PAN/s-CNF carbon fiber is significantly larger than that of PAN-based carbon fibers and conventional graphite (0.34 nm for natural graphite40 and PAN-based carbon fibers4,41). Unusually large carbon lamellae were also observed similarly in carbonized samples of freeze-dried s-CNF (d-spacing: 0.39 nm),42 which may indicate that it is a feature particular to the spinifex raw material. With the use of a custom-built fiber spinning unit and lowtemperature carbonization at 1200 °C, these fibers have not been optimized for commercial-grade production. As such, the mechanical properties of our carbon fibers are lower than commercial PAN-based carbon fibers43 and is not considered to be directly comparable. However, we believe that important new understandings into the potential effect of nanocellulose additives in PAN-based carbon fibers can still be gained by comparing within the dataset, and against the internal control PAN fiber. Factors for Enhanced Carbonization. In this study, we have shown the effect that the incorporation of different nanocellulose additives at a 0.1 wt % loading into solutionspun PAN fibers can have on the mechanical properties of the carbon fibers produced. The changes in the carbon fiber due to the nanocellulose incorporation are significant, while surprisingly, the physical and chemical effects of the nanocellulose on the solution-spun fiber or the fiber through stabilization appear to be minimal (most likely due to the low loadings). Clearly, the major role of the nanocellulose additive occurs during the carbonization stage of the manufacturing process. Previous work with electrospun carbon nanofibers reported that the incorporation of high-aspect-ratio s-CNF into electrospun carbon nanofibers at various loadings led to improvements in the carbonization efficiency (particularly at 0.5 wt %).44 However, those improvements were found to be related to the orientability of the CNF in the polymer matrix, which is determined by the concentration or amount of the CNF incorporated (0−10 wt %).44 In this case, at a lower CNF loading (0.1 wt %) and a higher PAN concentration (13 wt %), where the rheology and nanocellulose orientability effects are negligible (Figure 1), the data suggests that physical aspects, namely, the aspect ratio of the nanocellulose, have the greatest effect on this enhanced PAN carbonization behavior. These carbon improvements are also inversely correlated with the crystallinity of the nanocellulose additives, given that the retention of higher amorphous cellulose (hemicellulose) content enables the fibrillation of higher-aspect-ratio nanocelluloses,10,45 and these hemicelluloses are removed in the production of smaller CNCs. From a composite theory perspective, this effect may be related to changes in the relative reinforcement potential of the

Figure 6. Halpin−Tsai diagram highlighting the proposed origin of the enhanced carbonization effects leading to improved mechanical reinforcement (Ec/Em) in the carbon nanocomposites. Flexible highaspect-ratio CNF is compared with more rigid low-aspect-ratio CNCs. Although high-aspect-ratio CNFs demonstrate lower reinforcement potential in the precursor composite than more rigid CNFs, CNFs have greater reinforcement potential in carbonized materials due to a gain in stiffness from carbonization.

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CONCLUSIONS Nanocellulose is commonly utilized as a “green” additive in polymer composites as a reinforcer according to the rule of mixtures to improve the mechanical properties of the composite. This can involve incorporating a large volume (up to 40 wt %) of rigid/stiff cellulose nanocrystals. In this study, three different nanocellulose types were studied in wetspun PAN nanocomposite fibers through a stabilization and carbonization heat treatment process. We found that PAN composite fibers with nanocellulose additives at low loadings (0.1 wt %), when carbonized, can demonstrate significant mechanical performance improvements comparable to reinforced fibers with far greater nanoparticle loading, depending on the type of nanocellulose. These effects on the carbon fibers were correlated with increasing nanocellulose aspect ratio and decreasing crystallinity, while no such correlations were observed in the precursor fibers. The improvements have been isolated to occur predominately during the carbonization stage of the process, and Raman spectra and TEM analysis 9726

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Preparation of CNC from Cotton by Acid Hydrolysis. To prepare CNC from cotton, Whatman filter paper (no. 1) was initially soaked in water (0.02 g/mL) for approximately 5 days until a dispersed pulp was formed. This pulp was then treated with concentrated sulfuric acid (35% v/v) at 50 °C for 3.5 h while being stirred. After cooling to room temperature, the excess acid was extracted by decanting the dispersion after centrifugation at 4750 rpm for 20 min. This centrifugation and decanting process was repeated four times until the pH of the mixture was approximately 5−6. The CNC was then rinsed and dialyzed with deionized water for 7 days until the pH reached a value of approximately pH 7, before being resuspended in water and ultrasonicated for 1.5 h at 20% amplitude in an ice bath. These purified CNCs were then freeze-dried. Nanocellulose Characterization. Transmission electron microscopy (TEM) at 80 kV (Hitachi HT770) was used to study the morphology of the various nanocellulose types. Samples were prepared by dropping a dilute suspension of the nanocellulose onto a glow-discharged copper TEM grid, which was allowed to dry before being stained with uranyl acetate (2% w/v) for 10 min. The crystallinity index (CI) for each nanocellulose type was determined by X-ray diffraction (XRD) (Bruker D8 Advance XRD). Graphite-filtered Cu Kα radiation was generated at 40 kV and 40 mA. All samples were scanned over a range of 2θ = 10−90° at a scan rate of 1.2 s/step. The acidic group content in the different nanocellulose types was quantified by conductometric titration according to the Scandinavian pulp, paper, and board standard method (SCAN-CM 65:0213) under standard lab conditions without variation. Solution Spinning. Solution-spinning solutions were prepared from four different PAN mixtures, each containing a different nanocellulose type (at 0.1 wt %): cotton CNC, spinifex CNC, and spinifex CNF, which were also compared to a blank PAN solution without any additive. The 0.1 wt % loading was selected for the investigation, as preliminary experiments found that higher loadings reduced the fiber elongation (Table S2). These dispersions were prepared by first dissolving PAN in DMF (13 wt %) with constant stirring at 70 °C for 5 h until a transparent and homogeneous solution was obtained. Then, in a separate vessel, the nanocellulose variants were added to DMF at the appropriate quantity and allowed to swell and disperse for 12 h, while stirring under ambient lab conditions. These nanocellulose suspensions were then ultrasonicated (Q500 Sonicator, QSonica) at an output of 500 W, a frequency of 20 kHz, and an amplitude of 20% for 4 min under an ice bath, after which they were combined with the dissolved PAN solution. This mixture was then stirred for 24 h under ambient lab conditions and allowed to stand for 3 h to degas before being spun. All composite PAN/nanocellulose and blank PAN dopes were wet-spun under the same conditions, by pumping each room temperature dope at a rate of 4 mL/min through a spinneret (200 holes, 50 μm) submerged into a deionized water/DMF coagulation bath (1:1) under standard lab conditions (25 °C, 50% RH) (Figure S1). After coagulation, the extruded filaments were guided over a winding rotor (DR = 2.0) into a washing bath (100% water, room temperature, DR = 1.1), followed by a stretching bath (100% water, 75 °C, DR = 1.8), and finally collected on a rotating spool (Figure S1). The total draw ratio for the fibers using this custom spinning setup was 3.9. These fibers were dried during the spinning process by brief exposure to hot air.

indicate that the origin of these improvements involves internal reinforcement of the carbon fiber by increasingly ordered carbon layers facilitated by the carbonized nanocellulose. The mechanical and chemical changes in the four different nanocomposite fibers were also tracked through the multistage thermal stabilization process. While it was observed that the presence of nanocellulose can slow the initial cyclization reactions during stabilization and increase the fiber density after thermal stabilization, overall, the addition of nanocellulose of varying aspect ratios and surface chemistries, surprisingly, showed relatively little effect on the stabilization of the fibers. Ultimately, the potential implications of these results may include easier polymer composite processability (since high amounts of nanofillers can negatively affect the rheology and spinning of fibers) and lower-cost additives (due to lower amounts of nanofiller and avoidance of acid hydrolysis processes) for carbon applications. However, continued work is still necessary to demonstrate these effects for commercial fibers with carbon fiber-grade PAN, commercially relevant solution-spinning techniques, and industry know-how.

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EXPERIMENTAL SECTION Materials. The cellulose nanofiber, “s-CNF”, and cellulose nanocrystal, “s-CNC”, were isolated from spinifex pulp by homogenization and acid hydrolysis, respectively, while cottonbased CNCs, “c-CNCs”, were obtained from Advantec filter paper (no. 1). Glacial acetic acid (99.7%, Ajax Finechem), sulfuric acid (98%, RCI Labscan), sodium hydroxide pellets (99.9%, Ajax Finechem), and sodium chlorite (80%, SigmaAldrich) were all used as received in the pulping and bleaching processes. Composite fibers were prepared from dispersions of different nanocellulose types in solutions of PAN (AN: 99.5%, methyl acrylate comonomer: 0.5%, Mw: 230 000 g/mol; Goodfellow) dissolved in anhydrous N,N-dimethylformamide (DMF) (99.8%, Sigma-Aldrich). Preparation of CNF from Spinifex. As described in earlier work,45 the CNFs were isolated from spinifex grass by a low-cost pulping, bleaching, and homogenization process. Briefly, this involved harvesting, washing, and grounding the spinifex grass, after which the ground was treated with sodium hydroxide (2% w/v NaOH) at 80 °C to remove lignin. This was followed by a bleaching step in sodium chlorite solution (1% w/v) at 70 °C, after which the pulp was washed with hot water. Spinifex CNFs were obtained by passing a suspension of the bleached spinifex pulp in deionized water through a benchtop high-pressure homogenizer (Panda 2 K NS1001L, GEA Niro Soavi S.p.A, Italy) at 700 bar for two passes. The resulting CNF dispersions were then exchanged into DMF by centrifugation (10 000 rpm) and decanting five times. Preparation of CNC from Spinifex by Acid Hydrolysis. To obtain spinifex CNC,10 the amorphous components of the bleached spinifex pulp were digested by stirring in sulfuric acid solution (40% v/v) at 45 °C for 3 h. The excess aqueous acid and dissolved amorphous components were extracted by decanting the dispersion after centrifuging at 4750 rpm for 20 min. The spinifex fibers were re-dispersed in water, and the centrifugation and decanting process was repeated five times. The remaining treated fibers were then rinsed and dialyzed with deionized water for 7 days until the fiber dispersion reached a pH of approximately pH 7, before being resuspended in water and ultrasonicated for 20 min at 25% amplitude in an ice bath. These purified CNCs were then freeze-dried. 9727

DOI: 10.1021/acsomega.9b00266 ACS Omega 2019, 4, 9720−9730

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Article

Prior to spinning, the rheology of the solutions was measured at both PAN concentrations of 6 and 13 wt % in DMF and 0.1 wt % nanocellulose relative to PAN. The viscosity was measured using a stress-controlled rheometer (AR 1500EX, TA Instruments) as a function of shear rate in the range of 0.08−100 s−1 (geometry: 40 mm steel plate, 500 μm gap) at 25 °C. For the 13 wt % solution, the data at shear rates between 0.12 and 0.5 s−1 were fit to a power law equation η = Kγ n − 1

where Abs (1590) and Abs (2242) are the measured absorbance peaks for the CN and CN functional groups, respectively. The thermal transitions of stabilized (or partially stabilized) fibers were also measured using a differential scanning calorimeter (DSC, Mettler Toledo STAR DSC 1). Samples (3 mg) were placed into vented aluminum pans and heated from 40 to 400 °C, at a heating rate of 20 °C/min in a nitrogen environment. The residual enthalpy for each fiber through the stabilization process was calculated from the integral, from baseline to baseline, for the exothermic peak at approximately 315 °C. Fiber Carbonization. The stabilized fibers were carbonized in a batch process without tension using a single-zone tube furnace (CTF wire wound, 1300 °C, Carbolite Gero) by heating to 1200 °C at a rate of 20 °C/min in an aluminum oxide tube in a nitrogen environment. The maximum temperature of 1200 °C was held for 30 min before allowing the samples to cool to room temperature. The mechanical properties of the carbon fibers were measured under ambient lab conditions using an Instron model 5543 universal testing machine equipped with a 5 N load cell. A single filament of each fiber was attached to a square paper frame, which was used as a mount for the samples onto the testing grips. The diameter of each fiber was determined using a light microscope (Olympus BX51) prior to testing. The tests were performed with a gauge length of 20 mm and a crosshead speed of 5 mm/ min. Each sample was tested 15 times, and the modulus was determined from the slope of the initial stress−strain curve. All Raman spectra for the carbon fiber samples were recorded over a wavenumber range of 200−3500 cm−1, using a Raman spectrometer (Renishaw inVia) with a 514 nm laser using a laser power of 10% at a 30 s acquisition. Spectra were also recorded on both the as-obtained fiber and then “crushed or ground” samples. Raman D/G ratios were calculated from the ratio of the intensities of the D-band (1380 cm−1) and G-band (1595 cm−1). The carbon microstructure was imaged by TEM (JEM2100, 200 kV). Samples were prepared by grinding the carbon fibers and dispersing the ground powder in ethanol, followed by deposition onto a holey carbon grid.

(1)

where η is the viscosity, K is the flow consistency index, γ is the shear rate, and n is the flow behavior index. DSC was conducted using Mettler Toledo DSC 1 Star with a sample heating range between −10 and 160 °C at a heating rate of 10 °C/min and a cooling rate of 5 °C/min. Each sample was heated and cooled in a nitrogen environment for two cycles to observe the thermal history. Fiber Stabilization. Each PAN/nanocellulose fiber was stabilized using center-to-ends carbon fiber oxidation ovens (Despatch Industries) consisting of four independent heating zones with controlled air flow (Figure S1b). The fibers were passed through each zone (4 m in length) three times, in an oxidative atmosphere (air), with each zone set to gradually increasing temperatures of 233, 245, 255, and 263 °C, respectively. The speed of the stabilization line was run at 20 m/h, leading to a residence time of 36 min per zone or 144 min for complete stabilization. Due to the small tow size of the spun multifilament fibers, the fibers were tied to a “carrier fiber” made of carbon fiber (12 k tow) to enable passage of the spun fibers through the ovens under constant tension. The tension on this carrier fiber was set as 2500 cN, with a measured draw ratio for each zone being DR = 0.2. The tensile properties of the precursor and stabilized fibers were measured using a Favimat + Robot 2 single fiber tester (Textechno H. Stein) with a 210 cN load cell. The linear density of the fibers was determined using a gauge length of 20 mm and a pretension of 1.5 cN/tex, while the force−elongation curves were collected at a test speed of 2.0 mm/min from a pretension of 0.5 cN/tex. All force−elongation data were determined for 25 replicate measurements on individual filaments loaded onto a magazine. A density column unit (Davenport 2-COL) fitted with a temperature controller (Julabo F12) was used to measure the mass density of the precursor and oxidized fiber samples at 23 °C in accordance with ASTM D1505-10: Standard Test Method for Density of Plastics by the Density-Gradient technique. The column was filled with calcium nitrate and distilled water with a density testing range between 1.170 and 1.415 g/cm3. To study the effect of the various additives on the progress of stabilization reactions of PAN, the chemical changes through the stabilization process were monitored using a Fourier transform infrared (FTIR) spectrometer (Bruker Lumos) equipped with a germanium crystal. Each sample was measured using attenuated total reflectance mode from 800 to 4000 cm−1 with 128 scans and a resolution of 4 cm−1. The extent of cyclization (EOC) for PAN was then calculated using the following equation37 EOC (%) =

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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/acsomega.9b00266. Schematic of the fiber production process; nanocellulose characterization; TEM images of CNF; thermal degradation differences between CNFs; change in fiber properties with concentration of CNF; rheological fitting data for 13 wt % solutions; DSC comparison of different precursor fibers; SEM images of stabilized fibers; stress− strain curve for carbonized fibers; and Raman spectra of crushed fiber samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.J.). *E-mail: [email protected] (P.K.A.).

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

ORCID

Nasim Amiralian: 0000-0003-2624-2561 Bronwyn Laycock: 0000-0002-0251-844X

(2) 9728

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Pratheep K. Annamalai: 0000-0002-7284-0813 Present Address ⊥

Western Sydney University, Penrith, NSW 2751, Australia (N.Z.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Australian Research Council (under ARC Discovery Grant No. DP150101846), as well as travel support from the Australian Nanotechnology Network and the Australian Institute for Bioengineering and Nanotechnology. The authors would also like to thank Dr Azam Oroumei from Deakin University for her advice on the solution-spinning technique, as well as Steve Atkiss and his production team at CarbonNexus (Deakin University) for their assistance and support on the thermal processing of the fibers. Finally, the authors also wish to acknowledge the technical assistance of Rohit R. Gaddam and Dr Mun Soo of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, the University of Queensland. P.K.A. and N.A. thank the Queensland Government for the Advance Queensland Research Fellowships. The authors are thankful to the Dugalunji Aboriginal Corporation (DAC) for supporting the research through Federal Indigenous Advancement Strategy (IAS) funding, sourcing the spinifex grass used in this study and other in-kind support. The authors also acknowledge that any commercial proceeds resulting from the spinifex research are shared between the University of Queensland and DAC in accordance with the Spinifex Umbrella Research Agreement (SURA).

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