Continuous polyacrylonitrile nanofiber yarns: preparation and dry-drawing treatment for carbon nanofiber production

Abstract

Precursor fibers with diameters in nanometer scale and highly aligned polymer chains in fibers are highly promising for the preparation of high-performance carbon nanofibers, but are challenging to make. In this study, we demonstrate for the first time that a carbon nanofiber precursor can be prepared by the electrospinning of polyacrylonitrile into a nanofiber yarn and by the subsequent drawing treatment of the yarn in dry conditions. The yarn shows excellent drawing performance, which can be drawn evenly up to 6 times of its original length without breaking. The drawing treatment improves the yarn and fiber uniformity, polymer chain orientation within the fibers, as well as yarn tension and modules, but shows decreased yarn and fiber diameter and elongation at break. The drawing temperature and force show influences on the drawing behavior. The highest strength and modules (362 ± 37 MPa and 9.2 ± 1.4 GPa, respectively) are found on the yarn drawn by 5 times its length, which increased by 800% and 1800% when compared to the as-spun yarn. Through un-optimized stabilization and carbonization treatments, we further demonstrate that the carbonized nanofiber yarn shows comparable tensile properties as the commercial carbon fibers. Electrospun nanofiber yarns may form next generation precursors for making high performance carbon fibers.

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Introduction

High-performance carbon fibers (HPCFs) are characterized by high tensile strength (2–7 GPa) and high modulus (228–392 GPa) as well as very light weight.¹ They are widely used as structural materials where a high strength-to-weight ratio is required, such as aerospace,^2,3 wind turbine⁴ and automobiles.⁵ Recent increase in the demand for HPCFs has led to the active development of carbon fiber production technology.

HPCFs can be derived from polyacrylonitrile (PAN) or pitch. Although pitch derived carbon fibers exhibit very high modulus, they are more rigid and have low strain level, typically below 2%.⁶ A significant proportion of HPCFs is made from PAN.^6,7 To form HPCFs, PAN precursor fibers are prepared normally by a wet spinning technique, and then subjected to a series of treatments, including drawing, stabilization and carbonization. Uniaxial drawing makes polymer chains highly orientated within the precursor fibers, minimizing the defects per unit length in the precursor fiber and moreover, reducing fiber diameter from tens of to a few micrometers. The drawn fibers are then stabilized (also known as oxidized) by heating at temperatures around 250–500 °C in air to become infusible.^8,9 Further carbonizing at higher temperatures (e.g. 600–1600 °C) in an inert environment results in HPCFs.¹⁰

T-1000 carbon fibers with tensile strengths as high as 7.07 GPa have been available commercially; however,⁷ the ultimate tensile strength of HPCFs does not exceed 25% of the theoretically estimated value.^6,8 The lower tensile strength than the theoretical prediction originates primarily from structural defects generated in the carbon fiber production process.^11,12 For example, fiber coagulation and associated solvent diffusion between the polymer and coagulant during wet spinning may cause the phase separation and formation of voids, cracks, or cavities within the fibers.^11,13,14 Core–sheath structure may occur when the inner core of fiber is oxidized incompletely during the stabilization step, and carbon fibers prepared from such a core–shell structure have poor mechanical properties due to the burning off of the core in the carbonization process.^15,16

Reducing fiber diameter is an effective solution to minimize structure defects, thus improving mechanical properties. It also facilitates the stabilization of precursor fibers, thus diminishing the formation of core–shell structures. A good example is T-1000 carbon fibers, the diameter (5.0 μm) of which is just 2.7 micron less than T-300 (7.7 μm); however, the tensile strength (7.07 GPa) is 2.97 GPa greater (see the tensile property of other HPCFs in Fig. S1†).¹⁷ It is expected that carbon nanofibers (CNFs) will deliver tremendous improvement in the mechanical properties because of the small diameter, which is several orders of magnitude smaller than the existing carbon fibers.¹⁶ Precursor fibers prepared without involving a coagulation process avoids solvent diffusion and associated defect formation.

Electrostatic spinning, also known as electrospinning, offers a unique opportunity to prepare ultrafine precursor fibers without involving any coagulation process. It involves drawing a polymer solution under a strong electric field to form dry filaments. Without coagulation, fibers are formed by the fast evaporation of the solvent from a polymer solution jet during electrospinning. Electrospinning has been used to prepare PAN fibers, which have a diameter typically of about hundreds of nanometers.¹⁸ Several studies have reported the production of carbon fibers from electrospun PAN nanofibers.^19,20 A single CNF was reported to have a bending modulus of 63 GPa and a fracture strength of 0.64 GPa with a failure probability of 63%.²¹ Separately, the tensile strength and elastic modulus of single CNFs were reported to be 3.5 ± 0.6 GPa and 172 ± 40 GPa, respectively.²² Short CNF bundles prepared from electrospun nanofiber mats showed a tensile strength of 986 MPa (ref. 23) and a modulus of 58 GPa.²⁴ The lower tensile strength than conventional carbon fibers was attributed to the un-optimized processing conditions.^20,24

With the conventional technique to produce carbon fibers, the drawing of precursor fibers before stabilization plays a key role in improving the polymer molecular orientation and crystallinity, as well as lowering the defects within fibers,^25–28 which determine the final mechanical properties.⁸ PAN is a semi-crystal polymer with a glass transition temperature (T_g) in the range of 72–150 °C.^29–31 Above T_g, PAN fibers become plastic and can be drawn to a large ratio. The conventional PAN precursor fibers are drawn in either an aqueous solution or a steam environment at an elevated temperature (e.g. 40–170 °C).^32–34 However, the drawing treatment of electrospun nanofibers for carbon fiber production has not been reported widely in previous studies.²⁶

One of the difficulties in the drawing treatment of electrospun nanofibers comes from the randomly-orientated fibrous structure. In most of the cases, nanofibers are electrospun into a nonwoven fiber mat, and it is difficult to stretch such a thin, low strength, two-dimensional fiber web uniformly. Electrospinning nanofibers into yarns, i.e., continuous nanofiber bundles with an interlocked fibrous structure, offers a promising solution to drawing nanofibers effectively and continuously.

Recently, considerable efforts have been devoted to directly electrospin nanofiber yarns. Nanofiber yarns have been electrospun using liquid bath,^35–38 solid surface collector^39–41 or rotary funnel,^42,43 In our previous study, we have developed an effective technique to electrospin highly-twisted continuous nanofiber yarns.⁴³ Using poly(vinylidene fluoride-co-hexafluoropropene) as a model polymer, we examined the effect of working parameters on yarn dimension, twist structure and tensile property. The yarns showed better mechanical property than nonwoven mats.

Herein, we report the preparation of continuous PAN nanofiber yarns and the effect of a post-electrospinning drawing treatment on the fiber/yarn dimension and tensile property. We found that PAN nanofiber yarns can be drawn up to 6 times its original length under a hot, dry condition without using an aqueous solution and steam. Drawing treatment reduced the fiber diameter, but considerably improves fiber uniformity and alignment. The yarn tensile strength and modulus can be increased by 800% and 1800%, respectively, after the drawing treatment. We further demonstrated that the carbon nanofibers made of the post-drawn PAN nanofiber yarn have a tensile strength of 1.12 ± 0.18 GPa. To the best of our knowledge, it is the first report of the drawing treatment of PAN nanofiber yarns and the effect on fiber mechanical properties.

Experimental

Materials

Polyacrylonitrile (PAN, M_w 150 000 g mol⁻¹) and N,N′-dimethylformide (DMF) from Sigma-Aldrich were used as received. PAN solution (10 wt%) was prepared by dissolving PAN in DMF at 80 °C.

Electrospinning nanofiber yarns

A purpose-made electrospinning system was used for producing PAN nanofiber yarns. A back metal electrode (diameter 8 cm, thickness 2 mm) was added to each electrospinning nozzle, and two power supplies, one having a positive electrode (ES30P, Gamma High Voltage Research) and another negative (ES30N, Gamma High Voltage Research), were used to drive the two electrospinning nozzles separately. The two electrospinning nozzles were placed at 60 degrees to the funnel axis on the opposite side of the funnel. Nozzle to funnel distance was set at 25 cm. To prepare a nanofiber yarn, both the positively and negatively charged nanofibers were electrospun and deposited to the large end of the funnel to form a thin nanofiber web. The winding rate and funnel rotating speed were set at 0.32 m min⁻¹ and 245 rpm, respectively. The nanofiber web was then manually deformed to be a 3D cone, such that a nanofiber yarn could be stretched from the apex of the cone. The rotating speed of the funnel was adjusted so that a fibrous cone is formed stably on the funnel. A continuous PAN nanofiber yarn can be winded for hours without breaking.

Drawing treatment

Batch yarn drawing was carried out in an oven. Firstly, 10 pieces of nanofiber yarns (with gauge length 20 mm), with both ends, were mounted onto the opposite sides on a paper frame using double side tapes. The paper frame was then cut on the adjacent sides to the yarn mounting edges to release the load to the yarns. Nanofiber yarns were drawn under a constant load (calculated based on optimum value from DMA results) at 140 °C. One edge of the paper frame was fixed at the top grate of the oven and another was applied with the weight load. Drawn by the gravity, the yarn stopped at a pre-set length (drawing ratio). The post drawn nanofiber yarns were then cooled to room temperature for various characterizations.

Stabilization and carbonization

The stabilization treatment of nanofiber yarns was carried out under constant load (0.5 cN) at 250 °C for 4 hours in an oven. The yarn was clamped at one end and hanged on top grate of the oven, and the other end was clamped and applied with the load. Carbonization was conducted in a tube furnace at 800 °C for 1 hour without tension. Nitrogen was purged initially and a constant flow rate was throughout the carbonization process.

Characterizations

The morphology of the nanofibers and yarns was observed under a scanning electron microscope (Jeol, Neoscope SEM). Fiber and yarn diameters were measured based on SEM images using an image analysis software (ImagePro+ 4.5, Media Cybernetics Co.). Glass transition temperature was measured using the Q800 Dynamic Mechanical Analyzer (DMA) (TA instruments) under a vibration frequency of 10 Hz and a heating rate of 5 °C min⁻¹ from 40 °C to 200 °C. The thermal mechanical properties were examined using two methods: (1) constant displacement (400 μm min⁻¹) at holding temperatures and (2) constant force at elevating temperature from 40 °C to 300 °C (heating rate, 1 °C min⁻¹). The molecular orientation in the nanofiber was characterized using a Fourier transform infrared spectroscope (FTIR) (Brucker Vertex 70) at 64 scan rate. A polarized IR lens was used to obtain polarized IR beam. Tensile properties were examined using FAVIMAT single fiber tester (Textechno). Yarn samples were tested with the gauge length of 20 mm and a crosshead speed of 2 mm min⁻¹. The cross-sectional area of nanofiber yarns was calculated from the denier of the yarns and density of PAN.⁴⁴ Young's modulus and strain at break were the average value of 5 repeated measurements.

Results and discussion

Fig. 1a and b illustrate the apparatus for electrospinning of PAN nanofiber yarns. Oppositely charged nanofibers were prepared from two electrospinning spinnerets, and a fibrous cone was then formed on a metal funnel collector. A fiber bundle was drawn continuously from the apex of the fibrous cone, and twists were inserted simultaneously through the rotation of the funnel collector. PAN nanofiber yarn was collected with a winder. Fig. 1c shows a fibrous cone formed on the funnel collector, and nanofiber yarn collected on a spool is shown in Fig. 1d.

Fig. 1 (a) Schematic illustration of a yarn electrospinning setup, (b) image of the fibrous cone, (c) nanofiber yarn collected on a spool, (d) image of the actual setup and yarn electrospinning process.

Fig. 2a shows the typical morphology of the nanofiber yarns prepared. A good proportion of fibers aligned in an angle along the yarn axis. Fluffy fiber coils and curled fibers were also found on the yarn surface. The yarn had a diameter of 279 ± 30 μm and the PAN fibers were 812 ± 312 nm in diameter.

Fig. 2 SEM images of nanofiber yarns and fiber, the histogram of fiber diameter distributions and angles of nanofibers along the yarn axis.

It was noted that the twist level in the as-spun nanofiber yarn was lower in comparison to our previously reported PVDF-HFP nanofiber yarns,⁴³ although the setup used for making the two nanofiber yarns was very similar. The different twist feature was attributed to the difference in yarn electrospinning conditions. In the present study, PAN nanofiber yarn was prepared at a low funnel speed (245 rpm). Increasing the funnel speed can increase the twist level. However, PAN nanofiber yarn was hard to prepare at higher funnel speed because PAN nanofibers are brittle, and nanofiber yarn produced was easy to break at higher funnel speed.

At room temperature, PAN fibers often have a low strain level, while the chain mobility within the fiber increases considerably at a temperature above the T_g. However, when temperature is higher than 250 °C, degradation takes place.⁴⁵ In our study, the temperature for drawing PAN fiber yarn was chosen above the T_g but lower than the degradation temperature. Before the drawing treatment, the T_g of PAN nanofiber yarn was examined using DMA and DSC. Fig. 3a shows the storage modulus (E′)–temperature curve. The maximum storage modulus indicated that T_g was 105 °C (see DSC result in Fig. S2†), which is consistent with the previous report.⁴⁶

Fig. 3 (a) Storage modulus (E′)–temperature curve of PAN nanofiber yarns (heating rate 5 °C min⁻¹, frequency vibration 10 Hz), (b) the strain and breaking stress with temperature (yarn was drawn under constant displacement 400 μm min⁻¹), (c) strain–temperature curves of nanofiber yarns drawn under different forces, (d) strain rate–temperature curves of nanofiber yarns drawn with 3.0 cN, 5.0 cN and 7.5 cN force (heating rate for (c and d) is 1 °C min⁻¹).

Fig. 3b shows the effect of drawing temperature on the tensile strength and strain at break of nanofiber yarn. At 30 °C, the yarn can only be drawn up to a strain of 69% before breaking. Below 90 °C, the strain increased gradually with increasing temperature, whereas a larger increase of the strain at break with increasing the temperature took place in the temperature range from the T_g to 140 °C. At a higher temperature, the strain then reduced rapidly. At T_g, the strain was about 330%, whereas the yarn showed the highest strain value (543%) at 140 °C. The tensile strength decreased linearly with increasing drawing temperature.

Fig. 3c shows the strain change of a nanofiber yarn at a constant tension when it is heated up at a constant rate (1 °C min⁻¹). The tension force affected the maximum temperature that the yarn can be heated upto. At 2.0 cN, the nanofiber yarn could be drawn continuously until the temperature reached 250 °C. Higher drawing force between 2.0 cN and 10.0 cN led to reduction in the upper temperature limit to 180 °C. Further increasing the drawing force led to a considerable reduction of the temperature range. The tension force also affected the maximum strain. A low tension force led to a small strain value. With increasing the force, the maximum strain increased. At 7.5 cN, the strain reached the maximum value (530%), suggesting the nanofiber yarn can be drawn at the largest drawing ratio.

A two-stage strain response with temperature increase was observed when the yarn was drawn at a force of 3.0–7.5 cN. The 1st stage took place between T_g and 130 °C, and the 2nd started at around 140 °C until the upper temperature limit. The low temperature stage stemmed from the movement of short polymer chain segments under stretching, while higher temperature was required to move longer chain segments. Such a two-stage strain change suggested that elongation rate changed with temperature when the yarn was drawn at a constant force.

Fig. 3d shows elongation rate change during heating a nanofiber yarn (at rate 1 °C min⁻¹) and a constant tension force. At 3.0 cN, maximum elongation rate (7% min⁻¹) occurred at around 112 °C, which is in the 1st stage elongation. The elongation rate then reduced gradually to a minimal value (4% min⁻¹) at around 128 °C, and finally increased monotonously (up to 150% min⁻¹) until the yarn broke at 145 °C. At higher tension force, the maximum elongation rate moved to a lower temperature. However, the elongation rate at the upper temperature limit was little affected. This result reflects the competition of molecule movement and chain relaxation under an external tension force. At a small force (e.g. lower than 3.0 cN), polymer chains cannot be drawn to move unless the temperature is above the T_g. The gentle elongation allows sufficient chain movement with temperature increase. Increasing the force facilitates the chain movement. However, the temperature upper limit is not affected much by variation of drawing force. In this case, the chain movement is not accelerated unless the force is high enough. When the tension force is above 10.0 cN, the tension is so strong that the yarn breaks before it is fully stretched. A force (e.g. 3.0–7.5 cN) between the two extreme states leads to an initially accelerated stretching at a relatively low temperature followed by further fully stretching a higher temperature, showing a two-stage change in elongation rate. Drawing under a suitable force at a temperature close to 140 °C is effective to elongate the PAN nanofiber yarn to a high strain value.

Fig. 2 also shows the yarn morphology after the drawing treatment. Herein, drawing ratio (i.e. ratio between the elongated and the initial yarn lengths) was employed to represent the elongation length. At a higher drawing ratio, the yarns became more compact and the curled fibers became straightened. Yarn and fiber diameters were both decreased after drawing treatment. When the drawing ratio was 3 times, the average yarn and fiber diameters reduced to 91 ± 7 μm and 408 ± 141 nm, respectively. Higher drawing ratio, such as 6 times, further reduced the yarn and fiber diameters to 64 ± 5 μm and 336 ± 142 nm.

In addition to the decrease in the diameter, its distribution became narrower after the drawing treatment (Fig. 2). Without stretching, PAN fibers within the as-spun yarn had a wide diameter distribution, in a range between 300 nm to 1.6 μm. After the drawing treatment (drawing ratio, 6 times), the fiber diameter range changed to 100–900 nm. Higher drawing ratio did not further narrow the fiber diameter range much, except that the average fiber diameter decreased.

After the drawing treatment, the fiber alignment angle along the yarn axis (also referred to as alignment angle) decreased (see Fig. 2). For non-drawn yarn, the fibers had an alignment angle of 0–60°. After drawing for 2 times of the yarn length, the alignment angle reduced significantly to 30°. Higher drawing ratio led to a high alignment of fibers with an alignment angle as small as 15°.

The effect of drawing treatment on polymer chain orientation within PAN fibers was also examined by measuring the nitrile group vibration band (C≡N, peak wavenumber 2244 cm⁻¹) in polarized FTIR. Chain-orientation factor (f) was calculated according to the following equations:^47,48

where D is the dichroic ratio of C≡N vibration peak intensity (A) under parallel (∥) and perpendicular (⊥) IR beams. D₀ is the dichroic ratio of the polymer with perfect orientation.⁴⁹ Fig. 4a shows the D value at different beam angle. The drawing treatment increased the D value, indicating that the drawing process facilitated the PAN molecule to orientate along the fiber length. The effect of drawing ratio on chain orientation is shown in Fig. 4b. The f value increased with increasing drawing ratio. The highest f, 0.63, was found on the nanofiber yarn drawn to 6 times of its length. The f at 0 and 1, respectively, indicates random and perfectly orientated states.

Fig. 4 (a) Dichroic ratio at different beam angles, (b) effect of drawing ratio on orientation factor, (c) strain–stress curves of nanofiber yarns under different drawing ratios, (d) effect of drawing ratio on yarn tensile strength and Young's modulus.

Fig. 4c and d show the stress–strain curves and effect of drawing ratio on the tensile strength and Young's modulus of nanofiber yarns. With increasing drawing ratio, both the tensile strength and the Young's modulus increased until the drawing ratio reached 5 times. The 5-times drawn yarns had a tensile strength and a Young's modulus of 362 ± 37 MPa and 9.2 ± 1.4 GPa, respectively, which were more than 800% and 1800% those of as-spun yarns. In addition, the drawing treatment significantly decreased the strain level. This was attributed to the increased nanofiber alignment and polymer chain after the drawing treatment.

To prove the feasibility of forming carbon nanofibers from nanofiber yarn, a PAN nanofiber yarn after 5 times drawing treatment was subjected to stabilization and carbonization treatments. Fig. 5a shows the SEM images of the carbonized nanofiber yarn, which has similar morphology to the precursor yarn. After carbonation, the nanofiber and yarn diameters changed to 44.29 ± 0.09 μm and 190.02 ± 31.79 nm, respectively. The stress–strain curve of the carbon nanofiber yarn showed some elongation and its modulus changed from 3 GPa to 40 GPa when the strain changed 1–2.5% (Fig. 5b), respectively. The tensile strength was 1.12 ± 0.18 GPa. Such a relatively low tensile strength value was attributed to the un-optimized stabilization/carbonization condition and nanofiber structure. A systematic study will be conducted to make high strength carbon nanofibers based on PAN nanofiber yarns in future.

Fig. 5 (a) SEM images of PAN nanofiber yarn after stabilization and carbonization treatments (inset shows the cross section of carbon nanofibers), (b) stress–strain curve of resultant carbon nanofiber yarn.

Conclusions

We have demonstrated that a continuous PAN nanofiber yarn can be prepared directly by an electrospinning technique. The yarn shows excellent drawing performance at a dry condition. The drawing treatment improves the yarn and fiber uniformity, fiber alignment, polymer chain orientation, and yarn tensile strength, but decreases the yarn and fiber diameters and elongation at break. Drawing temperature and force show influences on yarn drawing behavior. The nanofiber yarns after stabilization and carbonization treatments maintain the fibrous morphology, and the carbonized nanofiber yarns show comparable tensile properties to the commercial carbon fibers. Electrospun nanofiber yarns may form a next generation precursor for making high performance carbon fibers.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The relationship between strain at break and tensile strength of carbon fibers. DSC results of PAN power and nanofiber yarns. See DOI: 10.1039/c4ra16247a

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