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. 2025 Apr 3;10(14):14188–14198. doi: 10.1021/acsomega.4c11547

Positive Effect of Isotropic Components on the Elongation at Break of Mesophase Pitch-Based Carbon Fibers

Ruixiang Liu †, Huang Wu †, Gaoming Ye †, Kui Shi †, Dong Huang †,‡, Huafeng Quan †, Chong Ye †,‡,∥,*, Shipeng Zhu §, Zhen Fan §, Feng Qian ⊥, Jinshui Liu †,‡,∥
PMCID: PMC12004178  PMID: 40256551

Abstract

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Mesophase pitch (MP) is a discotic liquid crystal composed of various planar aromatic macromolecules characterized by high aromaticity, high carbon content, and ease of graphitization. During the melt-spinning process, MP is susceptible to shear-induced alignment in the spinneret, producing carbon fibers with large and highly oriented graphite layers. This contributes to a high tensile modulus, increased brittleness, and low elongation at break, thereby limiting their applications. To address these issues, this study prepared MP containing isotropic components via the thermal polycondensation method and investigated the mechanisms by which these isotropic components influence the mechanical and thermal properties of mesophase pitch-based carbon fibers (MPCFs). The results indicate that the isotropic components maintained the disordered graphite microcrystalline structure during heat treatment, suppressing the alignment and growth of graphite crystallites, which leads to a reduction in tensile modulus and thermal conductivity. However, the dispersed disordered graphite crystallites extend the crack propagation path, yielding carbon fibers with a higher tensile strength of 2.62 GPa, a moderate tensile modulus of 729 GPa, and an improved elongation at break by 24.6%. This work offers valuable theoretical insights into enhancing the elongation at break of MPCFs and broadening their application scenarios.

1. Introduction

Mesophase pitch-based carbon fibers (MPCFs) hold advantages such as lightweight, high tensile modulus, excellent thermal conductivity, and low coefficient of thermal expansion, making them applicable in various fields including aerospace, 3C consumer electronics, and sports equipment.1−7 However, compared to polyacrylonitrile (PAN)-based carbon fibers, MPCFs exhibit weaker tensile strength, higher brittleness, lower elongation at break, poorer weaving processability, and increased susceptibility to damage, which limit their use in complex applications.8−12 Therefore, improving the elongation at break of MPCFs is crucial for enhancing their processability and expanding their application scope.

The elongation at break of MPCFs is associated with both tensile strength and tensile modulus. To enhance the break elongation while maintaining the high modulus advantage of MPCFs, increasing tensile strength is an effective approach.13−17 M. Matsumoto investigated the effect of altering the mesh size (400 mesh) of the filter screen as well as the shape (circular, triangular, and rectangular) and dimensions of the storage tank during the melt-spinning process of coal-based mesophase pitch (MP). This result indicated that the distortion of pitch flows due to filtering and the noncircular shape of the reservoir refined the grain size and improved the tensile strength of MPCFs by 15.4% and elongation at break by 6.7%.18 Ogale focused on reducing the draw-down ratio (DDR) for enhancing fiber strength.19 A smaller DDR can facilitate intracrystalline graphitic-plane orientation and decrease defects in the intercrystalline regions along the fiber axis, leading to increased tensile strength. When the diameter was 10 μm, reducing the DDR from 58 to 32 raised the tensile strength from 1.5 to 1.8 GPa, though the tensile modulus also increased from 425 to 540 GPa, causing a 5.6% decrease in elongation at break. Lee et al. optimized the mechanical properties of carbon fibers by adjusting the initial temperature (Ti) during stabilization.20 Increasing Ti from 50 to 150 °C led to higher oxygen content in carbon fibers and improved the tensile strength from 2.35 to 2.8 GPa, with increased modulus from 410 to 620 GPa. However, the tensile modulus of carbon fibers increased more significantly than the tensile strength, giving rise to a 21.2% decrease in elongation at the break. Guo et al.21 and Ogale et al.22 explored a different approach by using MP mixed with nanoscale silicon powder and carbon black to produce MPCFs. Their research indicated that adding nanomaterials effectively altered the microstructure of the MPCFs and significantly improved their compressive properties. However, the structures of these nanomaterials differ evidently from the planar carbon network structure formed by MP molecules, which can easily introduce defects and thereby reduce the tensile strength of the carbon fibers.23

Compared to nanosilicon powder and carbon black, graphene, a single-layer two-dimensional crystal composed of carbon atoms arranged in a honeycomb lattice through sp2 hybridization, has a planar structure similar to MP molecules. As a novel two-dimensional carbon material, graphene can tightly bond with MPCFs through π–π interactions during spinning and heat treatment, thereby regulating the structure and properties of the fibers.11,24−28 Given this, Ma et al. prepared MP-based composite carbon fibers by mixing AR pitch with various contents of chemically derived graphene nanosheets through crushing, ultrasonication, and stirring, followed by melt spinning, stabilization, carbonization, and graphitization.11 When the content of graphene was 0.1%, the thermal conductivity and tensile strength of the fibers peaked at 1322 W·m–1·K–1 and 2.12 GPa, respectively. Although the thermal conductivity was significantly improved, the mechanical properties remained relatively low and the elongation at break decreased from 0.44 to 0.35%. This reduction can be attributed to the graphene agglomeration driven by its high surface energy, making it insufficient to achieve uniform dispersion by simply mixing with MP. Additionally, the lack of oxygen-containing functional groups on the graphene nanosheets hindered the formation of chemical bonds with MPCFs during oxidation and subsequent heat treatment, leading to lower mechanical properties.24,29,30

It is well known that during the preparation of MP, the nucleation, growth, and development of mesophase spheres are accompanied by the absorption of isotropic matrix components.31,32 As the mesophase content continues to increase, the number of isotropic components decreases. Given the compositional and structural similarities between the two, they exhibit a competitive relationship. The isotropic components can be uniformly dispersed within the MP, effectively avoiding the technical bottleneck of poor dispersion that is often encountered in graphene modification. Moreover, the graphite crystallites derived from the isotropic components exhibit a typical disordered structure, which can inhibit the growth and orientation of the graphite crystallites derived from MP, thereby regulating the mechanical properties of the MPCFs.

In this study, MP containing isotropic components was prepared using the thermal polycondensation method and used as the precursor for MPCFs. The investigation focused on how isotropic components influence the mechanical and thermal properties of the MPCFs. This work provides significant theoretical reference values for improving the elongation at break of MPCFs and expanding their application scope.

2. Experimental Section

2.1. Materials

The petroleum-based MP used in this study was prepared from fluidized catalytic cracking (FCC) slurry oil as the raw material through a thermal polymerization process. The fundamental physical and chemical properties of FCC slurry oil are detailed as follows: the elemental composition includes carbon (91.12 wt %), hydrogen (6.98 wt %), nitrogen (0.53 wt %), and sulfur (0.75 wt %). Additionally, it possesses a density of 1143.3 kg/m3 and an ash content of less than 20 ppm. By adjusting the thermal polymerization process, 100% MP (MP-1) and MP containing isotropic components (MP-2 and MP-3) were prepared. The detailed thermal polymerization parameters are documented in Table S1. To clarify the mechanism of isotropic component involvement, the isotropic components in MP-2 were separated and enriched to form IP based on the density difference between isotropic and mesophase components. However, due to the difficulty in achieving complete separation based solely on density differences, approximately 5% mesophase remains dispersed in IP. The basic properties of the MP and IP are shown in Table 1. MP-1, MP-2, and MP-3 exhibit similar softening points and ash content.

Table 1. Basic Properties of MP-1, MP-2, MP-3, and IP.

samples softening point (°C) ash (ppm) mesophase content (%)
MP-1 284.4 <20 100
MP-2 283.8 <20 ∼93
MP-3 282.8 <20 ∼85
IP 187.4 <20 ∼5
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2.2. Preparation of MPCFs

Melt spinning was conducted by using MP-1, MP-2, and MP-3 as raw materials. The melt spinning apparatus (spinneret diameter D = 0.2 mm, length-to-diameter ratio L/D = 3) and the spinning process followed previously established work.33 To ensure consistent flow conditions for MP-1, MP-2, and MP-3 in the spinning channel and spinneret micropores, spinning was performed at the same viscosity, corresponding to spinning temperatures of 318, 313, and 311 °C, respectively. The winding speed was controlled at 170 m/min, producing as-spun fibers with diameters of 17 μm. The as-spun fibers were stabilized in air by heating from room temperature to 290 °C at 0.5 °C/min. Subsequently, carbonization was carried out in nitrogen by heating to 1500 °C at 5 °C/min, yielding carbon fibers designated as MPCF-1 and MPCF-2. Finally, they were graphitized at 2800 °C in an argon atmosphere, producing the graphite fibers (labeled as MPGF-1 and MPGF-2, respectively).

The IP was melt-spun using a nitrogen-pressurized single-hole spinning machine (D = 0.2 mm, L/D = 3) at a spinning temperature of 220 °C. The carbonization and graphitization processes were performed as described previously, with the resulting carbon fibers designated as IPCF and IPGF, respectively. This approach was employed to investigate the influence mechanism of isotropic components on the mechanical and thermal properties of carbon fibers.

2.3. Characterization

The SP of pitches was measured utilizing a droplet softening point apparatus (DP 70, Mettler Toledo) at a heating rate of 2 °C/min. The ash content of the pitches was determined according to the SH/T 0422-2000 standard. The optical texture of the pitches was observed using a polarized light microscope (BX-53P, Olympus), and the mesophase content was calculated using Image-Pro Plus software followed by GB/T 38396-2019. The thermal decomposition behavior of the pitches was analyzed using a simultaneous thermal analyzer (TA Instruments SDT 650) under isothermal conditions (heating from room temperature to 330 °C at a rate of 2 °C/min and holding for 3 h) and variable temperature mode (heating from room temperature to 1000 °C at 5 °C/min). The viscosity–temperature curves of the pitches were detected utilizing a rotational rheometer (Anton Paar MCR102) at a constant shear rate of 10 s–1 in a nitrogen atmosphere. The group compositions of pitch fractions, including n-heptane soluble (HS), n-heptane insoluble-toluene soluble (HI-TS), toluene insoluble-quinoline soluble (TI-QS), and quinoline insoluble (QI), were analyzed using a Soxhlet extractor.

The molecular weight (Mn) of the pitches was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS, Bruker Daltonics) with tetrahydrofuran as the solvent. The average Mn was calculated using the following equation34:

2.3. 1

where Mi is the ion mass, and Ii is the relative ion intensity. The cross-sectional microstructures of MPCFs were observed using a scanning electron microscope (SEM, Tescan MAIA3). The crystal structure of carbon fibers was analyzed using a high-resolution transmission electron microscope (HRTEM, Titan G2 60-300). Powder diffraction characterization of carbon fibers was conducted using an X-ray diffractometer (XRD, Rigaku MiniFlex600) with Cu Kα radiation (λ = 0.15406 nm), and the interlayer spacing (d002) was calculated using Bragg’s equation, and the degree of graphitization (g) was determined based on the formula g = [(0.3440 – d002)/(0.3440–0.3354)] × 100%.10 An X-ray diffractometer (D/Max-2550PC) was used to perform equatorial, meridional, and azimuthal scans of carbon fibers. The crystallite height (Lc) was calculated from the (002) diffraction peak of the equatorial scan, and the crystallite width (La) was calculated from the (100) diffraction peak of the meridional scan. Specifically, the misorientation angle (Z) was obtained from the fwhm of the azimuthal scan of the (002) diffraction peak, where a smaller Z indicates a higher degree of graphite lamellae alignment.35 The microstructure defects of GF were analyzed using Raman spectroscopy (LabRam HR Evolution, Horiba Scientific), and the ID/IG ratio was calculated from the Raman spectrum. A higher ID/IG ratio indicates a higher degree of disorder in the graphite structure of carbon fibers.36

The mechanical properties of MPCFs were tested by using a universal testing machine (BOYI 2025-010ST, Guangzhou Rocinst Co., Ltd.) in accordance with ASTM D 4018–17. Multifilament tensile tests were conducted with a load capacity of 10 kN, a crosshead speed of 15 mm/min, and a gauge length of 150 mm. The tests were performed at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10%. The reported tensile properties represent the average results from at least 10 specimens. The axial resistivity of carbon fibers was measured utilizing the four-probe method, and the thermal conductivity (λ) was calculated based on the empirical formula λ = 440,000/(100ρ + 258) – 295.37

3. Results and Discussion

3.1. Effects of Isotropic Components on the Properties of MP

The polarized micrographs of the pitches are shown in Figure 1. MP-1 consists of 100% mesophase content with a wide-domain texture, while MP-2 contains 93% mesophase content with isotropic components dispersed as small spheres (3 μm in diameter). Furthermore, MP-2 was oxidized by using the same stabilization procedure applied to the as-spun fibers. Polarized optical microscopy was then conducted on the MP-2 both before and after oxidation (Figure S1). The findings show that after oxidation, the isotropic components remain uniformly dispersed throughout the MP. MP-3 displays a wide-domain texture with 85% mesophase content, and the isotropic components are still dispersed as spheres with diameters ranging from 3 to 6 μm. Based on the above analysis, we have provided possible schematic illustrations for MP-1, MP-2, and MP-3 (Figure S2). The IP isolated and enriched from MP-2 still contains 5% mesophase components, which are dispersed as small spheres within the IP.

Figure 1.

Figure 1

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Polarized micrograph of (a) MP-1, (b) MP-2, (c) MP-3, and (d) IP.

A rotational rheometer and a simultaneous thermal analyzer were employed to investigate the flow behavior and thermal decomposition of MP-1, MP-2, MP-3, and IP, as shown in Figure 2. As illustrated in Figure 2a, the viscosity of MP-1, MP-2, and MP-3 decreases sharply and then stabilizes with an increase of temperature. MP-1 consistently exhibits higher viscosity than MP-2 and MP-3 at the same temperature, indicating that the isotropic components reduce the overall viscosity of the system. Figure 2b demonstrates the flow behavior of IP within the temperature range of 185–250 °C. Similar to MP, the viscosity of IP initially decreases sharply with increasing temperature and then stabilizes, approaching 0 Pa·s at 220 °C. This indicates that the viscosity of IP is significantly lower than that of MP, effectively acting as a “viscosity reducer” in MP. Figure 2c depicts the TG curves of MP-1, MP-2, MP-3, and IP under heating and isothermal conditions. During the heating phase (0–150 min), MP-1, MP-2, and MP-3 show only slight weight loss, while IP exhibits significant weight loss starting at 130 min, primarily due to the volatilization of light components and the polycondensation and decomposition of aromatic compounds.38 During the isothermal phase (150–330 min), MP-1, MP-2, and MP-3 show significant weight loss, dominated by the pyrolyzed components including 1,12-benzopyrene, o-phthalate esters, dibutyl phthalate, and benzoic acid.33,39 The weight loss of MP-2 and MP-3 is slightly higher than that of MP-1, mainly due to the continued pyrolysis of the less heat-resistant IP component. The weight loss of MP-1, MP-2, MP-3, and IP is 2.68, 2.95, 3.20, and 17.92%, respectively. The higher weight loss of IP can be attributed to its lower aromatic condensation degree and higher fraction of low-molecular-weight components, which are more susceptible to thermal degradation. Additionally, the presence of more aliphatic and heteroatom-containing structures in IP leads to increased volatilization and decomposition at lower temperatures.31Figure 2d further analyzes the TG curves of MP-1, MP-2, MP-3, and IP under variable temperature conditions, which can be divided into three stages. The first stage (RT −300 °C, stable phase) shows minimal weight loss as the temperature is insufficient for significant volatilization of small molecules, resulting in a nearly horizontal TG curve.40 The second stage (300–500 °C, rapid decomposition phase) involves a sharp increase in weight loss, due to the rapid volatilization and polycondensation of small molecules and the cleavage of large aromatic structures.41 During this phase, MP-1, MP-2, MP-3, and IP begin to show significant weight loss around 300 °C. The weight loss rate of IP is much higher than that of MP. The thermal weight loss trends of MP-1, MP-2, and MP-3 are similar, with MP-3 exhibiting higher weight loss than MP-2 and MP-2 higher than MP-1. In the third stage (500–1000 °C, slow decomposition phase), the weight loss trend gradually slows down as the temperature continues to rise. Ultimately, the weight loss is 30.24% for MP-1, 30.67% for MP-2, 31.61% for MP-3, and 47.51% for IP, indicating that the isotropic components are more prone to volatilization and decomposition during heating, whereas the condensed polycyclic aromatic structures in MP exhibit higher stability at elevated temperatures.1

Figure 2.

Figure 2

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(a, b) Viscosity-temperature curves of MP-1, MP-2, MP-3, and IP, (c) pyrolysis behavior of MP-1, MP-2, MP-3, and IP in constant temperature mode, and (d) pyrolysis behavior of MP-1, MP-2, MP-3, and IP in variable temperature mode.

Based on the previous analysis and the principle of “like dissolves like”, the group compositions of MP and IP were analyzed using a Soxhlet extractor, as depicted in Figure 3a,b. The results reveal that MP-1, MP-2, and MP-3 have a significantly higher QI content compared to IP, while the heat-sensitive small molecule HS content is notably lower in MP-1, MP-2, and MP-3 than in IP. This indicates that MP is predominantly composed of large polycyclic aromatic hydrocarbons with higher average molecular weights.42,43 Additionally, MP-2 and MP-3 exhibit an increase in HS and TI-QS content and a decrease in QI content, attributed to the contribution of the IP component. The molecular weight distribution of the tetrahydrofuran-soluble components of the pitches was analyzed by time-of-flight mass spectrometry, and the Mn was calculated, as shown in Figure 3c,d. The results indicate that the molecular weights of MP-1, MP-2, and MP-3 are primarily distributed between 300–800 m/z, whereas IP has broader molecular weight distribution ranging from 300 to 1300 m/z. The peak distributions of MP-1, MP-2, and MP-3 are quite similar, with lower Mn of 531.19, 568.22, and 627.12 compared to 814.19 for IP. The histogram in Figure 3d further shows that MP-1 has a much higher proportion of molecules in 400–700 m/z compared to IP and slightly higher than MP-2 and MP-3. Conversely, IP exhibits a significantly higher proportion in the 700–1500 m/z range compared to MP, with MP-3 showing a higher proportion than MP-2 and MP-2 higher than MP-1. The reason for the lower Mn of MP compared to that of IP is attributed to the insolubility of large polycyclic aromatic hydrocarbons in tetrahydrofuran.

Figure 3.

Figure 3

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(a) Schematic diagram of the separation of group components, (b) group component content of MP-1, MP-2, MP-3, and IP, (c) TOF-mass spectrometry of MP-1, MP-2, MP-3, and IP, and (d) the molecular weight distribution of MP-1, MP-2, MP-3, and IP.

3.2. Impact of Isotropic Components on the Structure and Properties of MPCFs

After the role of the isotropic components in MP was clarified, the four pitches were subjected to melt-spinning to verify their spinnability. The SEM images of the as-spun fibers are shown in Figure 4. During the melt-spinning process, MP-1 and MP-2 exhibited good drawability, allowing continuous filament collection at the corresponding winding speeds with a relatively uniform diameter distribution (Figure 4a,b). Specifically, as-spun fibers derived from MP-1 and MP-2 exhibited mean diameters of 16.83 and 17.14 μm, respectively, with coefficients of variation (CVs) of approximately 5.9 and 6.0%, indicating minimal variability in fiber dimensions. However, MP-3 demonstrated poor drawability during the melt-spinning process, leading to frequent filament breakage and an inability to successfully collect as-spun fibers with the target diameter. The fiber diameter distribution for MP-3 exhibited marked heterogeneity (Figure 4c), with a mean diameter of 62.69 μm and a CV of approximately 36.3%, reflecting significant variability. Due to the significant diameter variation of fibers spun from MP-3, it was excluded from further analysis and characterization. To investigate the mechanism of how isotropic components affect the fiber properties, the fiber was spun from IP using a nitrogen-pressurized single-hole spinning machine, and the process exhibited stable conditions. However, due to the inherent limitation of this spinning method, which does not allow precise control over the extrusion flow rate, the fiber diameter (21.34 μm) was larger than the target diameter (17 μm), with a standard deviation of 1.20 μm.

Figure 4.

Figure 4

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SEM images of as-spun fibers of (a) MP-1, (b) MP-2, (c) MP-3, and (d) IP.

The composition and structural differences in MP greatly impact the structure and properties of the MPCFs. To analyze the role and evolution of isotropic components during the fiber preparation, the cross-sectional morphology diagram of MPCFs is investigated, as depicted in Figure 5. MPCF-1 generally exhibits a standard circular cross-sectional structure (Figure 5b). Some fibers display a split structure with a cleavage angle of 90° (Figure 5c). After graphitization, MPGF-1 shows a more clearly defined and orderly arrangement of graphite layers, as evidenced by the split-radial structure, with the cleavage angle increasing to approximately 135° (Figure 5f). Notably, 33.3% of the observed structures in MPGF-1 are cleaved. This proportion was determined by analyzing eight SEM images acquired at 1000× magnification and calculating the ratio of fibers exhibiting cleaved structures to the total fiber count. Because cleaved structures are indicative of high graphite lamellae orientation, this result demonstrates that the graphite layers in MPGF-1 are well-aligned and regularly stacked, corresponding to an enhanced degree of graphitization.44 MPCF-2 shows a standard circular cross-sectional structure similar to that of MPCF-1. However, the microstructure of MPGF-2 exhibits a smaller cleavage angle of 119° and a lower proportion of split-radial structures, accounting for 9.1%. This suggests that the trace isotropic components present in MP-2 have a significant effect on the structure, partially inhibiting the high orientation of graphite crystallites and consequently reducing the occurrence of cleavages. In contrast, IPCF maintains a standard circular cross-sectional structure with a typical “glassy-like” appearance. After graphitization, the microstructure of IPGF closely resembles that of IPCF, indicating that the isotropic components preserved the disordered graphite crystallite structure during the heat treatment, which significantly impacts the overall properties of the carbon fibers.

Figure 5.

Figure 5

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SEM images of the cross section of fibers: (a–c) MPCF-1, (d–f) MPGF-1, (g–i) MPCF-2, (j–l) MPGF-2, (m, n) IPCF, and (o, p) IPGF.

To further elucidate the influence of isotropic components on the microstructure of MPGF-1 and MPGF-2, XRD and laser Raman spectroscopy were employed to analyze it, with the results shown in Figure 6 and Table 2. Due to the substantial morphological differences between IPGF and MPGF-1/MPGF-2, the XRD and Raman spectroscopy results for IPGF are provided in the Supporting Information (Figure S3). As shown in Figure 6a,b, the equatorial and meridional scans reveal prominent (002) and (101) diffraction peaks for MPGF-1 and MPGF-2, indicating a well-developed graphite microcrystalline structure.45 In the equatorial scan spectra, two prominent diffraction peaks are observed: one located at a larger 2θ angle, referred to the (002)G peak, representing the ordered regions within the graphite microcrystals; the other located at a smaller 2θ angle, resourced from the (002)T peak, signifying the disordered stacking regions within the graphite layers.46 By calculating the area ratio of the (002)G peak, the order index (OI = A(002)G/(A(002)G + A(002)T))47 for MPGF-1 and MPGF-2 is determined to be 71.53 and 67.11%, indicating that higher alignment of graphite crystallites formed in MPGF-1. The orientation angle of the graphite layers relative to deviating from the fiber axis was obtained from the azimuthal scan of the (002) peak in Figure 6c, yielding Z values of 9.18° for MPGF-1 and 12.07° for MPGF-2, suggesting that MPGF-1 exhibits better axial alignment of the graphite crystallites. The powder diffraction patterns (Figure 6d) show a distinct (002) diffraction peak near 26.3°, from which the g calculated to be 73.65% for MPGF-1 and 70.73% for MPGF-2, further confirming that MPGF-1 has a higher graphitization degree.35 Moreover, Figure 6e presents the Raman spectra results of GFs, where the disorder degree of the fibers quantified by the ID/IG value is lower for MPGF-1 (0.14) compared to that for MPGF-2 (0.16), consistent with the XRD results. In summary, compared with MPGF-1, MPGF-2 exhibits smaller coherence lengths (La = 46.57, Lc = 9.44 nm) and lower axial orientation (Z = 12.07°). This suggests that during the thermal processing of the fibers, the isotropic components inhibit the preferential orientation of mesophase pitch molecules, resulting in a reduced degree of graphite microcrystalline development. Compared to MPGF-1 and MPGF-2, IPGF exhibits smaller microcrystalline parameters, and its graphitization degree and orientation degree are far lower than those of the two aforementioned fibers. This further confirms the existence of the disordered structure of the isotropic components.

Figure 6.

Figure 6

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XRD diffraction patterns and Raman spectra of carbon fibers: (a) equatorial scan, (b) meridional scan, (c) azimuthal scan on (002) crystal face, (d) powder diffraction, (e) Raman spectra, and (f) the OI and Z of carbon fibers.

Table 2. Crystalline Parameters of Graphite Fibers.

samples d002 (nm) Lc (nm) La (nm) g (%)a Z (°)b OI (%)c
MPGF-1 0.3379 10.35 47.04 73.65 9.18 71.53
MPGF-2 0.3382 9.44 46.57 70.73 12.07 67.11
IPGF 0.3413 4.23 9.76 56.04 40.65  
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a

g: the degree of graphitization, g = [(0.3440 – d002)/(0.3440–0.3354)] × 100%.

b

Z: the misorientation angle, fwhm of the diffraction profile from the azimuthal scan of the (002) peak.

c

OI: Order Index = A(002)G/(A(002)G + A(002)T); A(002)G: area of (002)G peak; A(002)T: area of (002)T peak.

The crystal structure characteristics of carbon fibers were analyzed using HRTEM, as shown in Figure 7. The lattice fringes of MPGF-1 exhibit a regular and orderly arrangement along the axis, while the lattice fringes of MPGF-2 are generally orderly arranged but with significant local twists in some areas. In contrast, IPGF shows a more disordered pattern with twisted, staggered, and disordered arrangements, reflecting the typical structure inherited from isotropic microcrystal disordered arrangements during heat treatment. The microcrystals of isotropic components during the liquid-phase and solid-phase carbonization processes inhibit the preferential alignment of mesophase pitch molecules, thereby restricting the growth and development of the graphite crystallites. Additionally, the Lc for MPGF-2 is 10.72 nm, which is significantly smaller than the 14.08 nm of MPGF-1. This observation is consistent with the XRD results and underscores the role of isotropic components in impeding the orientation and growth of the structure of graphite crystallites.

Figure 7.

Figure 7

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HRTEM images of carbon fibers: (a, d) MPGF-1, (b, e) MPGF-2, and (c, f) IPGF.

Building on the significant microstructural differences between MPGF-1 and MPGF-2, we further investigated the impact of these differences on the mechanical and thermal properties of the carbon fibers, as depicted in Figure 8. The mechanical properties of IPGF were determined using single-filament mechanical testing, and its mechanical properties and thermal conductivity are presented in Table S2. As can be seen from Figure 8a,b, it is evident that MPCF-2 and MPGF-2 exhibit higher tensile strength and elongation at break compared to MPCF-1 and MPGF-1, while maintaining a moderate tensile modulus and thermal conductivity. Specifically, MPGF-2 demonstrates a comprehensive performance with a tensile strength of 2.62 GPa, a tensile modulus of 729 GPa, a thermal conductivity of 640 W·m–1·K–1, and an elongation at break of 0.36% (an increase of 24.6% over MPGF-1). The improvement in tensile strength and elongation at the break is primarily attributed to the dispersed distribution of disordered graphite crystallites, which effectively extends the crack propagation path. On the other hand, the reduction in tensile modulus and thermal conductivity is mainly due to the isotropic components retaining the disordered graphite crystallite structure during the heat treatment process, thereby inhibiting the orientation and growth of the graphite crystallites in the carbon fibers.

Figure 8.

Figure 8

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(a) Mechanical properties of MPCF-1 and MPCF-2, (b) mechanical properties of MPGF-1 and MPGF-2, (c) thermal conductivity of MPGF-1 and MPGF-2, and (d) five-star chart of MPGF-1 and MPGF-2.

3.3. Evolution Mechanism of Isotropic Components and Their Impact on Properties

Based on the above analysis, Figure 9 illustrates the evolution of trace amounts of dispersed isotropic components during preparation of fibers and the impact on the mechanical and thermal properties of the carbon fibers. During the melt spinning process, the isotropic components acting as “viscosity reducers” and cospin with MP molecules remain dispersed, discontinuous phases in the as-spun fibers. During subsequent stabilization, carbonization, and graphitization, the microcrystals retain the disordered structural features inherent to the isotropic components. This disordered arrangement limits the development and growth of the highly oriented graphite crystallites that form from the liquid crystallite of MP, leading to smaller crystallite dimensions and a restricted growth effect. Moreover, π–π interactions, which typically facilitate the stacking and alignment of aromatic molecules, are disrupted by these disordered regions. As a result, the axial alignment of the graphite crystallites is reduced.48−50

Figure 9.

Figure 9

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Evolution mechanism of isotropic components during preparation of MPCFs and their impact on the properties.

The subtle change in the microstructure of the carbon fibers significantly impacts their mechanical and thermal properties. The increase in tensile strength and elongation at break is primarily ascribed to the disordered graphite crystallites dispersed throughout the carbon fibers, which extend the crack propagation path.51 Consistent with prior studies in composites, polymers, and biomaterials where the incorporation of disordered or softer regions enhances toughness, the extended crack path is a consequence of the underlying microstructural disorder.52−55 The reduction in tensile modulus and thermal conductivity is mainly attributed to the retention of the disordered graphite crystallite structure by the isotropic components during heat treatment, which suppresses the orientation and growth of the graphite crystallites.56

Consequently, the isotropic components play a crucial role by influencing the overall performance of the carbon fibers, particularly in enhancing the elongation at break through inhibiting the orientation and growth of graphite crystallites. The advantage of this modification strategy lies in the structural similarity between the isotropic and mesophase components, which circumvents the compatibility issues often encountered with external nanoparticle additives, thereby rendering this approach more feasible for industrial-scale production.

4. Conclusions

In this study, MP with different contents of isotropic components was used as the precursor to prepare MPCFs, and the evolution of isotropic components during the preparation process and their influence on the mechanical and thermal properties of MPCFs were investigated. The main conclusions can be drawn as follows: (1) the disordered graphite crystallite structure derived from the isotropic components inhibited the growth and development of the highly oriented graphite structure from the mesophase pitch molecules, leading to smaller graphite crystallite sizes. (2) The disordered structure disrupted the highly oriented graphite structure derived from MP through π–π interactions, resulting in a lower degree of graphite crystallite orientation parallel to the fiber axis. This structural disorder extended the crack propagation path, ultimately yielding MPGF-2 with a tensile strength of 2.62 GPa, a moderate tensile modulus of 729 GPa, a thermal conductivity of 640 W·m–1·K–1, and a 24.6% increase in elongation at break compared with MPGF-1. This study provides valuable theoretical insights for enhancing the elongation at break of MPCFs and broadening their applications.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (52202037, U21B2067, and U24B2027), the Scientific and Technological Talent Lifting Project for Hunan Province (2022TJ-N11), Research Project of Advanced Functional Composites Technology Key Laboratory (HTKJ2023KL703001), the science and technology innovation Program of Hunan Province (2023RC3110), and Hunan Province major scientific and technological research project (2023ZJ1040).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c11547.

  • Detailed experimental data records, data analysis methods, and image files (PDF)

Author Contributions

# R.L. and H.W. contributed equally to the work.

The authors declare no competing financial interest.

Supplementary Material

ao4c11547_si_001.pdf (341.8KB, pdf)

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Supplementary Materials

ao4c11547_si_001.pdf (341.8KB, pdf)

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