Carbon

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Available online 11 July 2022

Junghwan Kim

^a

, Sungyong Kim

^a^,^b

, So Jeong Heo

^c

, Seo Gyun Kim

^a

, Nam-Ho You

^a

, Han Gi Chae

^c

, Hwan Chul Kim

^b^,^*

, Bon-Cheol Ku

^a^,^d^,^**

aCarbon Composite Materials Research Center, Korea Institute of Science and Technology, Wanju, South Korea

bDepartment of Organic Materials and Fiber Engineering, Jeonbuk National University, Jeonju, South Korea

cDepartment of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea

dDepartment of Nano Convergence, Jeonbuk National University, Jeonju, 54896, South Korea

A R T I C L E I N F O Keywords:

Polyimide nanocomposites Graphene oxide nanoribbon Carbon fiber

Mechanical properties Conductivity

A B S T R A C T

Polyimide/graphene oxide nanoribbon (PI/GONR) composite fibers were prepared by wet spinning. The unfolded GONRs by the intense drawing of fibers maximized their contribution to enhance mechanical and conductivity properties. The composite fibers were spun with an optimal drawing ratio of 18, followed by carbonization at 1200 and 1400 ^◦C, respectively. The PI/GONR (0.1 wt%) composite fibers carbonized at 1400 ^◦C have the highest tensile strength (2.12 ±0.45 GPa), which was 35% increase compared to PI-based carbon fibers that are carbonized at 1400 ^◦C. In addition, electrical (604 ±33 S/cm) and thermal conductivity (12 ±0.56 W/m⋅K) improved by 58% and 32%, compared to PI-based carbon fibers carbonized at 1400 ^◦C.

These values are comparable with 625 S/cm of electrical conductivity and 9.4 W/m⋅K of thermal conductivities of Toray carbon fiber (T700). Optimal drawing conditions can maximize orientation and packing density, resulting in maximum performance with a small amount of GONR (0.1 wt%). This significant improvement in macroscopic properties confirms wide possibilities for the manufacture of GONR composite fibers to acquire carbon fibers with excellent physical properties produced through the addition of GONR.

1. Introduction

Carbon fibers (CFs) are one of the most promising materials for a variety of applications including aerospace, automotive and lithium-ion battery [1–3]. The main precursors for carbon fibers are polyacrylonitrile (PAN) (~90%), pitch, and rayon [4–12]. PAN-derived CFs have a high tensile strength, but low modulus and conductivity compared to pitch-based CFs. Additionally, the manufacturing process needs stabilization step which requires cost-effective process due to its long-time process [13].

Polyimide (PI), one of the high-performance aromatic polymers containing diimide groups [14], is known to be a potential precursor for CFs, because of i) the simple evolution process of common inert atmo- spheric conditions for both imidization and carbonization, ii) the high carbon yield by high portion of aromatic groups compared to PAN-based CFs [15,16] and iii) versatility of molecular design for monomers [17].

PI-based CFs can be produced by wet-spinning process of poly(amic

acid) (PAA) as a precursor, followed by carbonization over 1000 ^◦C [14–16]. Recently, Xiao et al. reported thermal conductivity of PI-based graphite fiber by synthesis of PIs with changing monomer ratio of p-phenylenediamine(p-PDA)/4,4-oxydianiline(ODA) followed by graphitization at 2800 ^◦C [18]. Many research for PI-based nanocomposites using carbon nanotubes (CNT), and graphene oxide (GO) has also been conducted [19–26]. Jia et al. also reported polyimide/graphene nanosheet composite fibers via microwave-assisted imidization strategy [19]. However, most research was focused on improving properties of PIs film and fibers. Studies on CFs produced using PI-based composite fiber as a precursor that have high performance, such as electrical and thermal conductivity, have not been reported [27,28].

The electrical or thermal conductivity mechanism of highly conductive fillers in a moderately insulating matrix has been studied extensively for various applications [29]. The rule of mixtures predicts that the conductivity of a composite material can be determined by the

* Corresponding author.

** Corresponding author. Carbon Composite Materials Research Center, Korea Institute of Science and Technology, Wanju, South Korea.

E-mail addresses: [email protected] (H.C. Kim), [email protected] (B.-C. Ku).

https://doi.org/10.1016/j.carbon.2022.07.020

Received 1 April 2022; Received in revised form 27 June 2022; Accepted 10 July 2022

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intrinsic conductivities of its components and the ratio between them.

However, electrons and phonons travel through conductive material pathways with the highest conductivity rather than the entire bulk material [29,30]. Therefore, constructing a conductive network by finely dispersing and aligning conductive material in an insulating matrix can provide early conductivity percolation in a low concentration of conductive materials [30]. For CNT examples, Gau et al. measured various composites between polymers and multiwall carbon nanotubes (MWCNTs) [31]. They compared them with the values estimated by their percolation model from the theory of electron tunneling between adjacent multiwall nanotubes (MWNTs). The theoretical values corre- spond with the experimental data.

Graphene oxide nanoribbons (GONR) can be a proper alternative over carbon fillers for composite fibers. First, GONRs have a linear structure with high aspect ratio, whereas GOs have a laterally wide 2D structure [32–34]. Structural difference between GO and GONR stems from the structural features of precursors (CNT for GONR, graphite for GO) respectively. Graphene oxide nanoribbon (GONR) is the unzipped form of CNT [33,34]. In the preparation of GONR, CNT is unzipped longitudinally by a strong oxidizer, typically, potassium permanganate [33,35]. Owing to the high aspect ratio of CNT, GONR may be consid- ered as a pseudo-one-dimensional graphene oxide. Second, oxygen functional groups resulting from oxidative C–C bond cleavage produce high dispersibility in GONR. Meanwhile, other carbon fillers, such as CNT and graphene, are widely known to provide low dispersibility by strong intermolecular interaction. High dispersibility of GONR (C/O ratio =1.7) enables it to be dispersed at various organic solvents such as ethanol, N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) [32]. In addition, one dimensional structure of GONR is ad- vantageous for achieving liquid crystal phase and fine-packing structure, resulting in high orientation of composite fibers with polymers. Mean- while, GOs with a plate-like 2D structure have difficulties in their packing as well as orientation [36–39]. Assemblies with incomplete 2D structure have lower tensile strength due to stress concentration compared to 1D structured materials such as CNT fibers [40]. GONR can afford the same performance of graphene after reducing to graphene nanoribbon (GNR) by thermal or chemical treatment [41].

Herein, the integration of PI and GONR into composite fibers pro- vides many advantages. First, GONR and polyamic acid (PAA), a precursor of PI, both have functional groups that can participate in hydrogen bonding; thus, they are highly miscible in polar solvents. This contributed to the preparation of a homogeneous mixture, and well- distributed GONRs span across the composite fibers. Second, both require thermal treatment to be transformed into the highly conductive material. GONR has low electrical and thermal conductivity, thus requiring thermal reduction to be highly conductive GNR. Moreover, PAA requires thermal treatment to be transformed to PI or its carbonized form. Therefore, simple co-dispersing and spinning of GONR and PAA and subsequent thermal treatment could construct a finely distributed conductive pathway of graphene.

PI/GONR composites fibers with enhanced mechanical properties are prepared by wet-spinning process using a PAA/GONR solution in NMP, followed by imidization with heat-treatment. The properties of as- spun PI/GONR fibers were optimized by controlling the draw ratio (DR, length of wound fiber/length of extruded fiber) in wet-spinning process.

In addition, as-spun PI/GONR composite fibers were carbonized at 1200 and 1400 ^◦C. The composite fibers carbonized at 1400 ^◦C showed the highest tensile strength (2.12 ±0.45 GPa) when the GONR ratio was 0.1 wt% and the draw ratio was 18. Moreover, the addition of GONR leads to 58% and 32% improvement in electrical (604 ±33 S/cm) and thermal (11.89 ±0.65 W/m⋅K) conductivities, respectively, compared to the PI- based carbon fibers carbonized at 1400 ^◦C. In addition, the electrical conductivity of PI/GONR was superior to that of carbon fiber prepared from neat GONR [30], which emphasized the importance of alignment and dispersion of GONR.

2. Experimental section 2.1. Materials

MWCNT (JENOTUBE 8A) with a purity >98.5% was obtained from JEIO Co. Ltd., with a diameter of 5–7 nm and a length of 100–200 nm.

Potassium permanganate (KMnO4) was purchased from Sigma Aldrich.

NMP, concentrated sulfuric acid (conc. H2SO4) (98%) and concentrated phosphoric acid (conc. H3PO4) (85%) were purchased from Daejung Chemicals. The PAA varnish (viscosity of 5245 cP) of poly(pyromellitic dianhydride-co-4,4^′-oxydianiline), 9 wt% solid content and inherent viscosity of PAA, η_inh=0.174 dL/g which was measured at a concentration of 0.5 g/dL, was received from PI Advanced Materials Co. Ltd.

2.2. Synthesis of GONR

The preparation of GONR commenced after the modification and purification protocol. MWCNT (0.9 g) was poured into 216 mL of conc.

H₂SO₄(aq). The mixture was stirred for 1 h at 25 ^◦C. Thereafter, 24 mL of conc. H3PO4 (aq) was added in totality, followed by 15 min of stirring.

KMnO4 (6.3 g) was added slowly at 25 ^◦C. The mixture was stirred for 1 h at 25 ^◦C until a homogeneous mixture formed and heated to 75 ^◦C with frequent testing for the complete dissociation of KMnO4. After comple- tion of the reaction, the reaction mixture was cooled to 25 ^◦C and poured into ice (600 mL). The solid components were combined by filtration and dispersed in 4 M HCl (aq). Thereafter, the aggregated form was combined through centrifugation. The supernatant was discarded and re-dispersed in ethanol:ether (3:7, v/v). The supernatant was removed.

The product was dispersed in deionized water and freeze-dried, resulting in a brown free-flowing powder.

2.3. Preparation of dope for wet-spinning of PI/GONR

Various concentrations of dopes were prepared by the compositional mixing of the PAA varnish (9 wt% solid content) and the GONR dispersion in the NMP (1 wt% solid content). The dopes were prepared with various weight percentages, GONR/PAA (%, w/w) =0.05, 0.1, 1.0 and 2.5%. The final concentration of solid contents was adjusted to 7 wt

% by dilution with NMP. Dope was stirred for a minimum of 24 h and dispersed twice using a planetary centrifugal mixer (ARM-310, THINKY Co. Ltd.) for performing dispersion process. Dope was extruded using a glass syringe equipped with 28 G (diameter =0.18 mm) into a coagu- lation bath containing acetone:DI water =1:1 mixture. The washing process, with DI, and drawing process, were completed in the same 80 ^◦C water bath. The maximum draw ratio (DR) was 24 for the fibers with GONR (wt%) =0.05, 0.1 and 1.0, and 18 for the fibers with GONR (wt

%) =2.5%.

2.4. Carbonization of PI/GONR

PI/GONR fibers of fixed length, tightly attached to carbon sheets at both ends using carbon adhesive tape, were placed in a tube-furnace. As part of the imidization process, the fibers were heated at 10 ^◦C/min to 120, 220 and 350 ^◦C, respectively, with 1 h for stabilization at each target temperature. The entire process was conducted under an argon atmosphere. Thereafter, each sample was carbonized at 1200 and 1400 ^◦C respectively.

2.5. Specific strength and linear density

The specific strength and linear density were measured by a FAVIMAT+(Textechno). The 25 mm fibers were used for the tensile strength test. The strain rate was 2 mm/min, and the pre-tension was 2 cN. The linear density was measured by the resonance frequency obtained using a vibroscope [42]. The resonance frequency was measured while increasing the pre-tension from 1 to 2 cN at a rate of 2 mm/min.

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The linear density was estimated using Mersenne’s law as follows:

f=1 2l

̅̅̅̅

T μ

√

where f is the resonance frequency of the vibration, μ is the linear density (tex), T is the pre-tension applied to the fiber, and l is the gauge length. The specific strength and linear density were measured at least 30 times.

2.6. Characterization

X-ray photoelectron spectroscopy spectra were measured by Thermo

Scientific K-alpha (Thermo VG, USA) using monochromated Al Kα (1486.6 eV). Raman spectrum was collected on a Multidimensional Liquid-NIR Laser Raman Analyzer (inVia reflex) with excitation by an incident laser at 514.5 nm. A high-resolution transmission electron microscope (HR-TEM) analysis was conducted on a Titan G2 Cube 60–300 system operating at 80 kV. Thermogravimetric analysis (TGA) was conducted using a TGA Q50 instrument (TA instruments, USA) at a heating rate of 10 ^◦C/min under a N2 gas stream. The cross sections of the fibers were prepared by a focused ion beam (FIB) from a FEI-Helios (Scanning electron microscope) SEM. The electrical conductivity was measured using the four-point probe method with a probe station (MST- 4000A, MS Tech). The thermal conductivity was measured using a steady-state thermal bridge method [40]. The three-dimensional X-ray Fig. 1. (a) Schematic representation of prepared GONR, (b) Characterization of GONR (XPS, Raman spectrum and TEM image, respectively). (A colour version of this figure can be viewed online.)

Fig. 2. (a) Preparation of PI/GONR composite fibers, (b) TGA of GONR, (c) Schematic illustration of wet-spinning process. (A colour version of this figure can be viewed online.)

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microtomography (3D XRM) images were obtained using a ZEISS Xradia Ultra/Versa hybrid system at a power of 5 W and 50 keV.

3. Results and discussion

3.1. Preparation of PI/GONR composite fibers

The pseudo-one-dimensional structure of GONR is a special form of graphene “ideally” transformed by cutting a circular graphene sheet into a rod shape. These shapes can be easily arranged longitudinally along a fiber. Obtaining such a structure, using a bottom-up approach, is very challenging from an entropic point of view. Thus, typically, synthesis of GONR was performed via top-down approach, which utilizes a structure of CNT already containing the rolled-structure of the long graphene sheet. Therefore, typical preparation of GONR uses sequential unzipping with oxidative C–C bond cleavage. We followed the GONR preparation methodology [33] but excluding the purification step (Fig. 1a). The isolation of GONR from a reaction mixture containing over-oxidized short and irregular graphene is achieved through centrifugation. As GONR is heavy and stackable, centrifugation precipitated GONR and over-oxidized graphene is located in the supernatant.

Purified GONR was characterized. First, the C/O ratio was investigated using XPS to evaluate the degree of oxidation and found to be 1.7 (Fig. 1b). This value is relatively low considering a typical C/O ratio of graphene oxide (1.5–2.5). The low value is a result of (i) a high aspect ratio due to a relatively high portion of edges compared to the circular shape of graphene oxide, and (ii) newly-formed edges, which consist of oxygen functional groups that are products of oxidative C–C bond

cleavage through KMnO4 (mainly diol and dione). Next, the G/D ratio in the Raman spectrum of GONR (Fig. 1c) was measured to determine the effect of the oxidizing reaction condition on the crystallinity of the aromatic surface. We found I_G/I_D=0.96, which is significantly higher than the value of pristine MWCNT (IG/ID = 0.78). This abnormal enhancement of the G/D ratio was presumed to be due to the selection effect whereby the walls of CNT with low crystallinity were readily decomposed under harsh oxidizing conditions. The structure of GONR was investigated using TEM (Fig. 1d). The GONR strands showed ~6 nm of diameter and composed of 2–3 graphene sheets with small debris attached. Consequently, prepared GONR demonstrated a suitable degree of oxidation and crystallinity with a successfully unzipped and exfoli- ated structure.

Starting with the successful preparation of GONR, as described above, we incorporated GONR and PI into composite fibers (Fig. 2a).

The integration of PAA and GONR was expected to increase mechanical properties based on two main factors. First, a strong interaction of GONR and PAA is possible via hydrogen bonding between oxygen functional groups on GONR, and carboxylic acid or amide on PAA. Second, long GONRs can generate long-range load transfer with multiple distant PAA polymer chains, as shown in Scheme 1. It is known that intramolecular load transfer (green arrow) is more effective than intermolecular load transfer (red arrow). Thus, PI/GONR composite fiber with a longer green arrow and shorter red arrow would demonstrate superior mechanical properties.

The synergistic effects between PI and GONR were continued for carbonized PI/GONR composite fibers, but in different ways. GONR was thermally reduced to a graphene nanoribbon (GNR) below 200 ^◦C as Scheme 1.Effective load transfers of PAA/GONR composite fibers.

Fig. 3.SEM images of PI/GONR composite fibers. (A colour version of this figure can be viewed online.)

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shown in the TGA of GONR (Fig. 2b), resulting in loss of oxygen functional groups. Meanwhile, PAA was thermally transformed to PI above 350 ^◦C and carbonized above 1000 ^◦C [43]. As PI and its carbonized form consist of mainly aromatic groups, π-π interaction between GNR and PI (or its carbonized form) was predictable.

The PI/GONR composite fibers were fabricated using wet spinning techniques. The GONR dispersion in NMP was mixed with the PAA solution in NMP. The dope was prepared with various weight percentages of the GONR in PI (0.05, 0.1, 1.0 and 2.5 wt%) and wet-spun using a custom-designed spinning system, as shown in Fig. 2c. The dopes were coagulated in an acetone/DI water (1:1, v/v) bath and further drawn in a hot water bath (~90 ^◦C).

The surface morphologies of PI/GONR composite fibers were investigated using SEM as shown in Fig. 3. As the images were captured at the same magnification, a reduction of cross section is clear when comparing the carbonized and as-spun fibers. With a densely packed morphology, there are no voids, regardless of carbonization, both on the surface and in the cross section of the fibers. The densely packed morphology can result from hydrogen bonding of the hydrophilic functional groups present on the PAA or GONR, such as carboxylic acids or amides. During intensive drawing, the spontaneous hydrogen bonding maintains tight interfacial interactions and fills cracks similar to the self-healing process in hydrogels. We attempted to characterize the arrangement of GONR in PI/GONR composite fibers. However, indistinguishability between GONR and carbonized PI (or PAA) by aromatic groups in the polymer and deformation by slicing the TEM specimen made it difficult to track GONR in TEM monitoring visually.

3.2. Physical properties of PAA/GONR composite fibers and carbonized fibers

Fig. 4a displays the loading-elongation curves of PI/GONR as-spun composite fibers, and 1200 and 1400 ^◦C carbonized fibers. Their char- acteristics are depicted in Tables 1–3. The tensile strength of the fibers (Fig. 4b) demonstrated the highest value at GONR (wt%) = 0.1 regardless of carbonization. This indicated a trade-off between i) the role of props with small amount of GONR and ii) disturbance of crystallization with excess amount of GONR. Fig. 4c showed a similar trend with Fig. 4. (a) Representative loading-elongation curves, (b) Tensile strength, (c) Modulus of PI/GONR composite fibers. (DR =18). (A colour version of this figure can be viewed online.)

Table 1

Mechanical properties of the PAA/GONR composite fibers.

GONR

0 wt% 0.05 wt% 0.1 wt% 1.0 wt% 2.5 wt%

Force (cN) 2.79 ±

0.37 3.19 ±

0.22 2.97 ±

0.46 2.26 ±

0.33 2.05 ± 0.29 Lin. density (tex) 0.23 ±

0.01 0.23 ±

0.01 0.18 ±

0.01 0.22 ±

0.01 0.18 ± 0.01 Spec. strength

(cN/tex) 12.13 ±

1.53 15.21 ±

1.73 16.53 ±

1.63 11.93 ±

2.03 11.40 ± 1.99 Elongation (%) 18.06 ±

2.67 22.64 ±

3.06 23.22 ±

5.24 21.31 ±

2.57 15.10 ± 3.89 Spec. modulus

(cN/tex) 513 ±34 552 ±20 574 ±35 447 ±44 515 ±27

Density (g/cm³) 1.40 1.40 1.40 1.41 1.38

Table 2

Mechanical and physical properties of the carbonized PI/GONR composite fibers at 1200 ^◦C.

GONR

0 wt% 0.05 wt% 0.1 wt% 1.0 wt% 2.5 wt%

Force (cN) 8.10 ±

1.75 8.10 ±

1.12 11.64 ±

2.31 9.98 ±

1.91 9.81 ±

2.01 Lin. density (tex) 0.10 ±

0.01 0.11 ±

0.02 0.10 ±

0.01 0.09 ±

0.01 0.09 ± 0.02 Spec. strength

(cN/tex) 81 ±

6.81 97 ±

8.67 116 ±

7.87 110 ±

9.79 109 ± 8.22 Elongation (%) 1.22 ±

0.15 1.03 ±

0.15 1.53 ±

0.19 1.20 ±

0.36 1.22 ± 0.29 Spec. modulus

(cN/tex) 7,200 ±

500 7,600 ±

620 7,900 ±

700 7,800 ±

550 9,400 ± Density (g/cm³) 1.79 1.72 1.73 1.75 870 1.72 Elec. Cond. (S/

cm) 383 ±37 391 ±33 421 ±36 537 ±47 603 ±33

Table 3

Mechanical and physical properties of the carbonized PI/GONR composite fibers at 1400 ^◦C.

GONR

0 wt% 0.05 wt% 0.1 wt% 1.0 wt% 2.5 wt%

Force (cN) 9.1 ±

1.73 11.10 ±

3.23 12.11 ±

4.11 7.81 ±

2.14 6.69 ±

2.03 Lin. density (tex) 0.10 ±

0.02 0.10 ±

0.01 0.10 ±

0.01 0.09 ± 0.01 Spec. strength

(cN/tex) 93 ±

12.73 110 ±

20.05 121 ±

18.52 78 ±

8.24 74 ±9.17 Elongation (%) 1.29 ±

0.26 1.01 ±

0.22 1.23 ±

0.39 1.30 ±

0.11 0.86 ±

0.25 Spec. modulus

(cN/tex) 7,700 ±

860 8,400 ±

800 8,500 ±

750 7,900 ±

750 10,300 ± Density (g/cm³) 1.73 1.70 1.75 1.72 900 1.74 Elec. Cond. (S/

cm) 447 ±

33 448 ±36 520 ±37 527 ±

36 599 ±34

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tensile strength where the values, which first peaked at GONR (wt%) = 1.0, again increased at GONR (wt%) =2.5. The tensile strength (2.00 ± 0.09 GPa) of PI-based CFs with 0.1 wt% GONR was higher than that (1.82 ± 0.38 GPa) of PAN-based CFs with 1.0 wt% GONR [44], as indicated in Fig. 4b. Based on mechanical property profile of the PI/GONR composite fibers, when wt% of GONR is 0.1, GONR is well-distributed in the PI matrix without significant disturbance of crystallization. This comparison disclosed that our PI/GONR composite fiber achieved high tensile strength with relatively smaller amount of GONR. This difference may be attribute to strong π- π interaction of PI with reduced GNR because of aromatic groups and higher carbon yield of PI than PAN.

The electrical conductivity of carbonized samples was measured as shown in Fig. 5a. As GONR reduced to GNR with the graphene structure having high electrical conductivity, the increase in GONR content enables high electrical conductivity regardless of the carbonization temperature. Electrical conductivity shows the highest value at GONR (wt

%) =2.5 and 1200 ^◦C-carbonization and its value was 603 S/cm, but this value is slightly higher than the value of the 1400 ^◦C-carbonization sample (599 S/cm). The increase was 58% compared to the value at GONR (wt%) =0. The same trend was observed in the thermal conductivity graph as shown in Fig. 5b. Thermal conductivity increased with higher concentrations of GONR, as GONR facilitated the transport of phonon across the lattice which is composed of a long-range aromatic surface with attached carbonized PI. The highest peak is at GONR (wt%)

=2.5 for 11.89 W/m⋅K, which is 32% increase compared to the value without GONR content.

The highest electrical and thermal conductivities showed the comparable values with commercial PAN-derived CF products of Toray, T700. The electrical and thermal conductivities of T700 are 625 S/cm and 9.4 W/m⋅K, respectively. However, the pristine GNR fiber treated at 1500 ^◦C have an electrical conductivity of 285 S/cm, which is significantly lower than that of the PI-based carbon fibers (383 S/cm and 447 S/cm when treated at 1200 ^◦C and 1400 ^◦C, respectively) [35]. It is

counterintuitive that incorporating a small amount of GNR into carbon fiber yields higher conductivity of the composite fibers than the PI-based carbon fibers, considering the rule of mixtures.

We postulated that these abnormally high conductivities originated from the electric wire structure of PI/GONR, with highly conductive metal inside and a somewhat insulating outer layer [29]. This structure can be achieved by intense drawing after carbonization. First, coiled GONR is randomly distributed in dope for PI/GONR composite fibers. In the fiber-drawing process, unfolded GONR span across the fiber longitudinally, surrounded by PAA. After carbonization, reduced GONR with graphene structure becomes highly electrical and thermal conductive when surrounded by carbonized PI, which has enhanced electrical and thermal conductivities. As electrons move along the highest conductive pathway, GNR would be the major pathway.

The electrical and thermal conductivities of carbonized PI/GONR fibers demonstrated percolation points at GONR (wt%) = 0.1. This means that there is a minimum amount of GNR throughout the matrix to barely construct a conducting pathway with DR =18. Therefore, the fiber with GONR (wt%) =0.1 is acceptable to investigate the effect of drawing on electrical and thermal conductivities. Fine distribution of GONR in the fiber with GONR (wt%) =0.1, described by the mechanical property profile also support the selection, as mentioned in Fig. 4b and c.

3.3. Effects of draw ratio on physical properties of PI/GONR composite fibers

Fiber drawing is a critical element in the manufacturing of mechanically strong fibers. A high draw ratio could provide an unfolding coiled structure of polymer chains inside fibers. In the case of the polymer containing a large portion of aromatic groups along the polymer backbone, unfolded polymers could readily interact with each other via π-π interaction using parallel orientation. Further, the needle in wet- spinning process can develop a pre-ordered state by driving a longitudinal orientation with shear force. Thus, GONRs exist as a linear shape in Fig. 5. (a) Electrical and (b) Thermal conductivities of carbonized PI/GONR composite fibers at 1200 ^◦C. (A colour version of this figure can be viewed online.)

Scheme 2. Alignment of GONR in polymer matrix by intensive drawing and expected electron transfer mechanism in the drawn composite.

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a longitudinal direction to the fibers in the same way with the polymer chains in the drawn fiber. These unfolded GONRs can span across the whole fiber, thus maximizing the interaction with PAA and electron- transfer along the fiber as shown in Scheme 2. Conductive pathway is constructed by electron-transfer process through delocalized aromatic surfaces of GONR (green arrow) and hampered by less-conducting electron tunneling process between GONRs (red arrow). Furthermore, orientation factors of GONR along the fiber axis were calculated from the peaks of 2θ =20^◦in azimuthal scan (Fig. S1) using the wide-angle X- ray scattering (WAXS) study. The GONR orientation factors were nearly unchanged at draw ratio 6 and 12, then, sharply increased from DR =12 to 24, indicating that alignment of GONR along the fiber axis is performed following drawing the coiled PAA chain. Therefore, we investigated the effect of high DR on the physical properties of PAA/GONR composite fibers and their carbonized form.

First, the effect of DR on the mechanical properties of PAA/GONR composite fibers was explored, and a maximum DR of 24 was confirmed.

PAA/GONR composite fibers spun with a DR = 6, 12, 18 and 24, respectively, were prepared. The carbonized fibers were manufactured at 1200 ^◦C. Fig. 6a shows the dramatic decrease of elongation from DR

=18 to DR =24 for the sample of GONR (wt%) =0.1. This indicated

saturated drawing of the fiber due to hampered mobility of the matrix by a strong interaction between unfolded GONR and PAA [45]. However, for the precursor sample without GONR, a significant decrease of elongation was not found, owing to a large intrinsic elongation of the polymer. This unfolding mechanism was cross-checked by measuring the tensile strength of the precursor and the composite fibers (Fig. 6b). In contrast to the slower increase of specific strength for the precursor with an increase in DR, the sample of GONR (wt%) =0.1 showed a sharp increase up to DR =18. The degree of interaction between GONR and PAA increases as they become aligned more parallel in the longitudinal direction by unfolding of GONR during intensive drawing [45,46].

Morphologies of these fibers were also investigated using SEM (Fig. 6d). For the case of DR =6, many voids were detected in the cross section, regardless of carbonization. The number of voids decreased for the fibers of DR =12 and became negligibly small in the fibers of DR = 18. The value increased in the case of DR =24. This trend was supported by the density data of the fibers (Fig. 6c). Density data also demonstrated a maximum density of voids at DR =18. Because the same porosity trend is observed in the SEM images and density profiles even after carbonization, it is assumed that the voids are preserved after carbonization.

Evidently, a shortage of solvent-squeeze and arrangement owing to Fig. 6. (a) Elongation of the precursor without GONR and PAA/GONR composite fibers with DR =6, 12, 18 and 24, (b) Specific strength of the precursor without GONR and PAA/GONR composite fibers with DR =6, 12, 18 and 24, (c) Cross section of PAA/GONR and carbonized PI/GONR composite fibers with DR =6, 12, 18 and 24 (*different magnifications (X 3000) due to oversize cross section), (d) Density of PAA/GONR and carbonized PI/GONR at 1200 ^◦C with DR =6, 12, 18 and 24.

(A colour version of this figure can be viewed online.)

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inefficient draw leads to voids inside the fibers [45]. However, despite the maximum degree of orientation, a large portion of voids in the maximum drawn fibers are generated regardless of carbonization. A rational assumption is that CNT and PI had different tensile modulus,

hampering the structural integrity during the drawing process at high DR [45].

The void volume fraction is a major factor for mechanical properties as voids are defects that increase the stress concentration of the cross section [45]. During the drawing process, pores can be generated by either deficient or excessive drawing. To elucidate the relationship between mechanical strength and porosity, the porosity of the fibers with various draw ratios was quantified using XRM, as shown in Fig. 7. XRM is a powerful tool to visualize and quantify voids embedded in fibers, which have been widely used in recent studies [47–49]. The pores of each sample were visualized by golden spots, as shown in Fig. 7a. The porosity decreased from DR =6 to DR =18 and increased again to DR = 24. The porosity was quantified and compared to the trend in tensile strength, as displayed in Fig. 7b. The pore volume of the fiber with DR = 18 showed the lowest value of 0.1%. This indicates that the fiber with DR =18 has a very compact structure and optimum draw ratio. This trend shows an exact inverse relationship with the trend of tensile strength. Hence, minimizing the void volume fraction by controlling the

Fig. 7. (a) Visualization of pore images using X-ray computed tomography (CT) for 3D-scanning of carbonized PI/GONR composite fibers with a draw ratio of 6, 12, 18 and 24, (b) The pore volume of carbonized PI/GONR composite fibers with a draw ratio of 6, 12, 18 and 24, which is calculated from X-ray CT above.

The tensile strength of the fibers is shown together for comparison. (A colour version of this figure can be viewed online.)

Fig. 8. (a) Loading-elongation curves of the PI/GONR composite fibers with DR =6, 12, 18 and 24 and carbonized fibers of DR =18, (b) Tensile strength, and (c) Modulus of PI/GONR composite fibers with DR =6, 12, 18 and 24. (A colour version of this figure can be viewed online.)

Table 4

Mechanical properties of the PAA/GONR (0.1%) composite fibers according to the draw ratio.

Draw Ratio

6 12 18 24

Force (cN) 5.53 ±

0.97 3.63 ±

0.82 2.97 ±

0.88 2.41 ±

0.91 Linear density (tex) 0.78 ±

0.04 0.39 ±

0.05 0.18 ±

0.02 0.15 ±

0.01 Specific strength (cN/

tex) 7.09 ±

0.97 9.32 ±

1.32 16.53 ±

1.68 16.07 ±

2.61 Elongation (%) 12.20 ±

3.92 12.32 ±

2.26 23.23 ±

6.24 4.14 ±

0.42 Specific modulus (cN/

tex) 360 ±25 480 ±30 570 ±35 700 ±60

Density (g/cm³) 0.86 1.13 1.40 1.23

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draw ratio can be a key factor in improving mechanical properties.

Fig. 8a displays loading-elongation curves of PI/GONR composite fibers with DR =6, 12, 18 and 24, respectively, and their details are depicted in Tables 4–6. The tensile strength of the fibers (Fig. 8b) demonstrated the highest peak at DR =18 regardless of carbonization.

The tensile modulus of the fibers (Fig. 8c) also had its peak at DR =18.

This data suggests the DR =18 sample had the highest mechanical strength. Despite the transition of the matrix (PAA to carbonized PI) and

the filler (GONR to GNR), the trend continued after carbonization at 1200 ^◦C. This indicates that the void volume fraction is critical to mechanical properties, regardless of the components.

The electrical conductivity of carbonized PI/GONR fibers was measured as shown in Fig. 9. As mentioned above, the degree of unfolding of GONR increased as DR increased. The electrical conductivity increased up to DR =18 as the unfolded GONRs could create a longer conducting pathway and integrated and finely stacked structure.

Since the fiber of DR =24 had fully extended GONR, no more increase of electrical conductivity was observed. The increase in electrical conductivity by intensive drawing is significantly high at ~200 S/cm (twice the value when DR =6), even though the components did not change.

Considering that pristine GNR fiber has an electrical conductivity of 285 S/cm, the value at DR =24, electrical conductivity of carbonized PI/

GNR composite fiber, 428 S/cm supports the explanation that unfolded GNR facilitate conducting pathways. In the study of pristine GNR fibers, intensive drawing was difficult due to their inherent low elongation properties. Thus, sufficient drawing to fully align the GNRs was not achieved. On the contrary, we used the PAA matrix that has (i) high tolerance toward intensive drawing, (ii) high miscibility with GNR precursor, and (iii) the possibility to be carbonized under a common range of temperatures to thermally reduce GONR to GNR. Therefore, the high electrical conductivity was achieved with a very small amount of the carbon filler, enhancing the full capabilities of the filler.

The synergistic effects of controlling alignment by intensive drawing and adopting PAA matrix provided high physical properties as shown in Fig. 10. Most of graphene fibers used graphene oxide as a precursor, which have little effect of intensive drawing, displaying low tensile strengths (≤1 GPa) (Fig. 10a) and electrical conductivity (≤100 S/cm).

High electrical conductivity of the carbonized PI/GONR fiber (0.1 wt%

GONR), which is achieved with relatively small amount of graphene content compared to other graphene fibers in Fig. 10b (5–15 wt%), is comparable to that of T700. This means that it is important to align the filler material in the longitudinal direction, providing an economical strategy for obtaining the desired properties of composite fibers.

4. Conclusion

In conclusion, PI/GONR composites fibers with a relatively small amount of GONR (0.1%) were fabricated for the enhancement of their physical properties. GONR was prepared through the optimization of reaction conditions including temperature and amount of oxidizer. PI/

GONR composite fibers were prepared by wet spinning using a polar

Density (g/cm³) 1.24 1.33 1.73 1.34

Table 6

Mechanical properties of the PI/GONR (0.1%) composite fibers carbonized at 1400 ^◦C according to the draw ratio.

Draw Ratio

6 12 18 24

Force (cN) 16.82 ±

3.81 12.46 ±

3.33 12.12 ±

4.19 7.49 ±2.13

Linear density (tex) 0.31 ±

0.03 0.20 ±

0.04 0.10 ±0.01 0.07 ±0.01 Specific strength

(cN/tex) 54.26 ±

8.46 62.31 ±

9.79 121.22 ±

18.52 107.13 ±

10.70 Elongation (%) 1.12 ±

0.37 1.06 ±

0.43 1.23 ±0.39 1.02 ±0.19 Specific modulus

(cN/tex) 4,700 ±

430 5,700 ±

430 8,500 ±750 9,600 ± 1000

Fig. 9. Electrical conductivity of carbonized PI/GONR composite fibers at 1200 ^◦C with DR =6, 12, 18 and 24. (GONR/PI =0.1% (wt/wt)). (A colour version of this figure can be viewed online.)

Fig. 10.Comparison of (a) tensile strengths and (b) electrical conductivities of carbonized PI/GONR composite fibers and reported graphene fibers [25,35,39, 44,50–58]. (A colour version of this figure can be viewed online.)

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organic solvent, NMP, followed by carbonization at 1200 and 1400 ^◦C, respectively. An optimization process of drawing process disclosed that the unfolding of GONR was crucial for the physical properties, and an optimal DR was 18. The 278% and 222% increases in tensile strength for as-spun and carbonized fibers at 1200 ^◦C were achieved, compared to the fiber with DR =6. The tensile strength of PI/GONR composite fiber revealed the maximum properties at 0.1% GONR for both as-spun (232

±7 MPa) and carbonized samples (2.12 ±0.45 GPa). The electrical and thermal conductivity increased continuously with increased GONR contents. The electrical and thermal conductivity increased 58% and 32%, respectively, compared to the PI-based carbon fibers carbonized at 1200 ^◦C. These high conductivities may be originated from fully- unfolded and well-oriented GONR, which can develop a conducting pathway spanning a longitudinal distance of fiber with enough drawing.

We expect that further enhancement of physical properties can be achieved by i) design and synthesis of new monomers, ii) GONR with enhanced crystallinity and high aspect ratio, and iii) graphitization of the PI fibers for improving thermal conductivity. These tasks are un- derway in our laboratory and will be reported.

CRediT authorship contribution statement

Junghwan Kim: Methodology, Writing – original draft. Sungyong Kim: Methodology. So Jeong Heo: Methodology. Seo Gyun Kim:

Methodology. Nam-Ho You: Conceptualization. Han Gi Chae: Valida- tion, Supervision. Hwan Chul Kim: Investigation, Supervision. Bon- Cheol Ku: Investigation, Conceptualization, Supervision, Writing – review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by grants from the Open Research Program (2E31902) and K-Lab of the Korea Institute of Science and Technology (KIST) and the grant (20011577) from Ministry of Trade, Industry and Energy (MOTIE) in Korea.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.carbon.2022.07.020.

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