Introduction

Carbon fibers are widely applied in aerospace, military, engineering, sports, and adsorption [1,2,3]. Due to the advantages of excellent mechanical properties and high carbon yield, over 90% of commercial carbon fibers are manufactured by polyacrylonitrile (PAN) precursor fibers [4,5,6]. At present, a series of high-strength and high-modulus carbon fibers have been launched successively. But their strength values only reach a small percentage of the theoretical values. Based on the previous researches, their mechanical performance depends on the quality of the precursor fibers [7, 8]. In order to prepare high-performance carbon fibers, there have been considerable efforts to explore the structure-properties relationship between precursor fibers and carbon fibers [9,10,11].

The physical structures (such as crystal structures, oriented chains, skin-core structures, and pore/void defects, et al.) of the PAN fibers would be successively transformed and inherited into the resultant carbon fibers during stabilization and carbonization process, which would control the properties of PAN based carbon fibers [12]. Lu et al. [13] found that the mechanical properties of carbon fibers could be tailored by adjusting the size and orientation of graphite crystallites and micro-voids. The prerequisite for high performance carbon fiber was regular graphitic structures parallel to the fiber axis, which was closely linked to the high regular and oriented molecular chains in precursor fibers [14,15,16]. The skin-core structures acted as structural inhomogeneity, formed and developed in spinning and thermal conversion process, which had a vital influence on the property of the carbon fibers [17,18,19]. Liu et al. [20] implied that the performance of the carbon fibers could be improved greatly if uniform carbon fibers were obtained. Furthermore, the strength of the carbon fibers was also limited by the size and content of the pore/void defects that were mainly inherited from the precursor fibers [21, 22]. Kunzmann et al. [23] revealed that the nanopores of PAN fibers were maintained along the stabilization and carbonization process and the mechanical properties of carbon fibers decreased with increasing effective pore area and pore aspect ratio. Wu et al. [24] summarized that the decrease of amorphous size might represent the reduction of defects, thus increasing the tensile strength. These results provided the effective ways for improving mechanical properties of carbon fibers through regulating the fiber microstructures.

To some degree, the microfibrils acted as important microstructural elements of the precursor fibers [25, 26]. The orientation and alignment of the microfibrils were closely related to the orientation and crystal structures of molecular chains of PAN fibers. Meanwhile, their packing density and stacking way also determined the distribution, shape and number of the pore/void defects [27]. On the other hand, Watt and Johnson [28] insisted that the reaction was limited in fibrils during the stabilization and carbonization process and new graphite microcrystals were comprised in these fibrils. Gong et al. [29] established new fibril structure models for the PAN fibers, stabilized fibers and carbon fibers, and revealed the certain inheritance relationship: from the PAN fibers microfibrils to the carbon fiber “microfibrils”. Perret and Ruland [30] and Diefendorf [31] established ribbon structure model for carbon fibers and insisted that ribbon fibrils were the structural units of carbon fibers. Therefore, it could be inferred that the microfibrils in precursor fibers were closely linked to the nano-strip structures of carbon fibers, which also provided the new strategies for regulating microstructures and increasing mechanical strength of carbon fibers from the point of view of fibril structures.

So far, there were few researches focused on the effect of microfibrils in precursor fibers on the microstructures and mechanical properties of the carbon fibers. Consequently, the systemic and comprehensive understanding of the structure-property relationship of precursor fibers and carbon fibers from the point of view of the fibril structures was necessary, which was also benefit to improve the mechanical strength of the carbon fibers.

In this work, the investigation of the microstructures and mechanical strength of PAN precursor fibers and carbon fibers was presented. Solution etching and ultrathin section technology were used to separate fibrils from the fiber samples. And scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) observations were carried out to characterize the fiber microstructure morphologies. Based on those experimental results, a new insight into structure-property correlation of precursor fibers and resultant carbon fibers was provided.

Experimental

Materials

The precursor fibers (PF) were prepared by the dry-jet wet spinning technology and the carbon fibers (CF) were obtained by thermal stabilization and carbonization of the precursor fibers. The precursor fibers were prepared by the coagulation process, washing, densifying and steam stretching, and the processing parameters of the precursor fiber PF1 and PF3 were set according the reference [32] and [33], respectively. The PF1 and PF3 were thermal stabilized in air atmosphere and carbonized in nitrogen atmosphere to prepare the corresponding carbon fiber CF1 and CF3. PF2 and CF2 were commercial products.

Sample preparation

The precursor fiber tow was embedded in epoxy resin and cut into ultrathin section parallel to the fiber axis for HRTEM observation. Afterward, the ultrathin sections were etched by dimethysulfoxide (DMSO) solution at 75 oC (PF1-90%wt.-2 h, PF2-90%wt.-2 h, PF3-81%wt.-1 h). To analyze the fibril structures, the precursor fibers were processed by ultrasonic etching method (PF1-95%wt., PF2-92%wt., PF3-92%wt.) [34].

Characterization

The precursor fibers were needed to coat with the platinum for SEM observation (SU-70, Hitachi Co., Japan) with an accelerated voltage of 3 kV. And carbon fibers were directly characterized by SEM (accelerated voltage-15 kV). HRTEM measurements of the ultrathin sections and carbon fibers were carried out using a JEOL JEM-2100 at an accelerating voltage of 200 kV. X-ray diffraction (XRD, D/max-c, Rigaku Co., Japan) experiment was carried out to characterize the crystal structures of precursor fibers and carbon fibers [35]. Raman spectra was used to determine the order degree of graphitic structures of carbon fibers. The investigation of the mechanical strength was performed by a tensile testing machine (XQ-1, Donghua University, China) at room temperature and the details were listed in references [35, 36].

Results and discussion

SEM morphology of precursor fibers

Figure 1 shows SEM morphology of PAN precursor fibers. As shown in Fig. 1a1-c1, the fiber surface was smooth and there were no distinct surface defects, which brought little influence on the mechanical properties of precursor fibers.

After ultrasonic etching, the fibrils with different diameter were separated from the precursor fibers (Fig. 1a2-c2). The fibril strip was exfoliated from the surface of PF1. The fibril bundles with large diameter were obtained in PF2. However, a few of fibrils appeared in the crack of PF3 and there were no microfibrils carved out from the fiber surface, indicating that the microfibrils arranged closely and tightly in the fiber surface.

As shown in Fig. 1a3-c3, the fibrillar nodes existed in the cross-section of the PF1-PF3. The feature of the fibril structures was obvious and it also showed the ductile rupture. Especially for PF3, its interior microstructures were relatively dense. With respect to the fiber compactness, the fibrils were interconnected tightly and packed closely in PF3.

Fig. 1
figure 1

SEM morphology of PAN precursor fibers a PF1, b PF2, c PF3 (1 fiber surface, 2 fiber surface morphology after ultrasonic etching, 3 cross-section)

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Morphology of the ultrathin section of the precursor fibers

Figure 2 shows HRTEM morphology of the ultrathin section of PAN precursor fibers. As shown in Fig. 2a, the lamellae were vertical to the fiber axis and their shape was irregular in PF1. In PF2 (Fig. 2b), the microfibrils and lamellae co-existed. Compared with PF1, the lamellae were relatively small. However, PF3 exhibited different interior morphology, as shown in Fig. 2c. The lamellae were linked in the fiber transverse direction and their width was relatively large. And the inter-lamellae distance (parallel to fiber axis) was larger than that of PF1 and PF2. The thickness of lamellae from surface and core region of PF1-PF3 was 43.9/46.1 nm, 22.7/36.7 nm, and 47.0/49.3 nm, respectively. Furthermore, the difference of the skin-core structures was relatively obvious in PF1 and PF2.

Fig. 2
figure 2

HRTEM morphology of PAN precursor fibers ultrathin section a PF1, b PF2, c PF3 (1 low magnification, 2 surface region, 3 core region)

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The ultrathin sections of the precursor fibers were etched by DMSO solution and their morphologies were exhibited in Fig. 3. The fibril structures (Fig. 3a1-c1) and the crystal layers (Fig. 3a2-c2) were obtained. There were many grooves and spaces between the fibrils, which was caused by the dissolution of the amorphous materials between fibrils. The fibrils in precursor fibers were oriented along the fiber axis. The diameters of fibrils in PF1-PF3 (Fig. 3a1-c1) were 218 ± 63 nm, 185 ± 29 nm, and 176 ± 30 nm, respectively. The orientation degree of the fibrils in PF1 was good, but the inter-fibril distance was relatively larger. For PF2, the fibrils were not dense and there were some pore/voids existed in fibrils (as shown in blue rectangle of Fig. 3b1). As for PF3 (Fig. 3c1), the fibrils were compactness, and they arranged closely and tightly.

As shown in Fig. 3a2-c2, the crystal layers were stacked vertical to the fiber axis. According to our previous results [37], the crystal layers were mainly consisted of the folded chains. Compared with the PF1 and PF3, the crystal layers in PF2 arranged loosely and their regularity was poor.

For SEM images of fibril structures (Fig. 3a3-c3), their diameters were 186 ± 46 nm, 212 ± 36 nm, and 199 ± 41 nm, respectively. The fibrils aligned along the fiber axis in PF1, but the grooves between fibrils were obvious. This result indicated that the packing density of the fibrils in PF1 was relatively small. However, the most of fibrils in PF3 were oriented parallel to the fiber axis, and they were stacked closely and orderly (Fig. 3c3). Consequently, the order and packing density of the fibrils in PF3 was relatively perfect.

Fig. 3
figure 3

Fibril morphology of the PAN precursor fiber ultrathin section after solution etching a PF1, b PF2, c PF3 (1 HRTEM image of fibrils, 2 HRTEM image of the crystal layers, 3 SEM image of fibrils)

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Crystal structures of precursor fibers

Figure 4 shows XRD property of PAN precursor fibers. The diffraction peak at 2θ ≈ 17o represented the (100) crystal plane. Among the precursor fibers, the PF3 had sharper diffraction peak than other samples. And there was a small diffraction peak at about 2θ ≈ 29o, indicating that the crystal structures of the precursor fiber were relatively perfect. As shown in Fig. 4b, the crystal sizes of PF1-PF3 were 8.64 nm, 8.60 nm and 8.49 nm, respectively. And their crystallinity was 56.3%, 63.0% and 64.8%, respectively. Consequently, PF3 possessed more perfect crystal structures.

Fig. 4
figure 4

XRD property of PAN precursor fibers a XRD patterns, b XRD parameters

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Mechanical properties of precursor fibers

Figure 5 shows mechanical properties of PAN precursor fibers. As shown in Fig. 5a, PF1-PF3 possessed similar stress-strain curves. And the elongation at break of PF3 was 10.3%, larger than that of other precursor fibers. As exhibited in Fig. 5b, the tensile strength of PF1-PF3 was 684 MPa, 712 MPa and 711 MPa, respectively. And their tensile modulus was 11.2GPa, 10.5GPa and 11.9GPa, respectively. Consequently, PF3 had better mechanical strength.

According to the previous works [34, 38, 39], the homogenous, compact, well-oriented and closely interlinked fibrils served as the self-reinforcing elements to efficiently improve fiber tensile strength, and microfibril alignment parallel to fiber axis could reinforce the fiber mechanical properties. Based on the analysis of the fibril structures of precursor fibers, PF3 possessed ordered and compactness fibrils, and most of fibrils aligned parallel to fiber axis. Furthermore, the inter-fibril distance was also small, and the packing density of the microfibrils was relatively large. Consequently, PF3 exhibited better mechanical strength.

Fig. 5
figure 5

Mechanical properties of PAN precursor fibers a stress-strain curves, b tensile strength and tensile modulus

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Microstructure of resultant carbon fibers

Figure 6 shows the microstructures of PAN based carbon fibers. The fiber surface was smooth and there were no obvious surface defects for all of carbon fibers (Fig. 6a), which was consistent with the surface morphology of the precursor fibers. Compared with the morphologies of the cross-section of precursor fibers, the size of the gradual-like nodes was relatively small and the interior microstructures of the carbon fibers were denser (Fig. 6b-c). It also meant that the fibrils in carbon fibers arranged more tightly and closely, and it was hard to distinguish the morphology of the individual fibril. Especially for CF3, the gradual-like nodes stacked very tightly and closely, and its homogeneous and compactness were also better than that of other carbon fibers. It suggested that the compactness of the carbon fibers was closely related to the precursor fibers.

Fig. 6
figure 6

SEM images of PAN based carbon fibers a surface morphology, and cross-section morphology b CF1, c CF2, d CF3

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Figure 7 shows HRTEM morphologies of carbon fibers. According to SAED patterns (Fig. 7a), the crystallites were preferentially oriented parallel to fiber axis. And the pattern of CF3 was sharper than those of CF1 and CF2, suggesting that it had better crystallite. As shown in Fig. 7b-d, the nano-strip crystallites were denoted by green arrows, and nano-pores and amorphous carbon [21] were marked by blue dashed loops. Compared with CF1 and CF2, the width and length of the nano-strips were relatively large in CF3 and their regularity and continuity were also better. Consequently, CF3 possessed better crystallinity and more perfect nano-strip structures than other carbon fibers.

Fig. 7
figure 7

HRTEM images of PAN based carbon fibers a SAED patterns b CF1, c CF2, d CF3

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Figure 8a shows XRD patterns of PAN based carbon fibers. Compared with precursor fibers (Fig. 4), these diffraction peaks (2θ ≈ 17o and 29o) disappeared and new diffraction peak of (002) crystal plane at 2θ ≈ 25o appeared. The new peak attributed to the formation of graphite structures. According to the intensity and peak width of (002) diffraction peak, the carbon crystal was not perfect and there were a lot of irregular structures in carbon fibers. The results also indicated that the order regions of the linear PAN molecules were destroyed and converted into the turbostratic graphite structures during the thermal stabilization and carbonization process. Among the carbon fibers, CF3 possessed better crystallinity. Furthermore, the intensity of the diffraction peak (CF1 < CF2 < CF3) was consistent with the change tendency of the crystallinity of the precursor fibers.

As shown in Fig. 8b, the ratio of ID/IG of CF1-CF3 were 0.918, 0.917 and 0.915, respectively, indicated that CF3 possessed highly-ordered graphitic structures. Consequently, it also meant that the precursor fibers with perfect crystal structures were convenient to prepare the carbon fibers with better crystallinity and ordered graphitic structures.

Fig. 8
figure 8

a XRD patterns and b Raman spectra of PAN based carbon fibers

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Mechanical strength of carbon fibers

Figure 9 shows the mechanical properties of PAN based carbon fibers. The tensile strength of CF1-CF3 was 4.68GPa, 4.80GPa, and 5.05GPa, respectively. For carbon fibers, their mechanical strength was determined by the microstructures (crystal structures, graphitic structures, nano-strips and defects). The variation of mechanical strength values of carbon fibers was accordance with microstructural features. The carbon fibers with good crystallinity and ordered graphitic structures exhibited excellent mechanical strength. Furthermore, it also proved that high-quality precursor fibers were the prerequisite for manufacturing the high-performance carbon fibers.

Fig. 9
figure 9

Mechanical properties of PAN based carbon fibers

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Structure-property correlation of PAN precursor fibers and resultant carbon fibers

From the point of the inheritance relationship of the microfibril structures, the regular and crystallized molecules in the microfibrils of precursor fibers would be converted into the nano-strips and turbostratic graphite structures of carbon fibers [28, 29]. It was inferred that the fibrils in precursor fiber were closely related to the nano-strips in carbon fibers. The microfibrils with well-orientation, perfect crystallization and regular packing were conducive to prepare the carbon fibers with good crystallinity and ordered graphitic structures, which played a critical role to determine their mechanical properties.

On the other hand, the microfibrils arranged closely and tightly. As a result, the content and volume of the pore defects decreased. After thermal stabilization and carbonization process, the pore/void defects between or within microfibrils developed into the nano-defects between nano-strip, and the number of the pore defects of the resultant carbon fibers was also small [23]. According to Griffith’ theory [24], reducing the pore defects of carbon fibers was an important strategy for increasing their tensile strength.

Therefore, in order to control the mechanical properties of the carbon fibers, it was an effective way to regulating the microfibril structures of precursor fibers: promoting the microfibrils oriented and aligned along the fiber axis, improving their homogeneity and compactness, and increasing their crystallinity and packing density.

Conclusions

In this study, the microstructures and mechanical properties of PAN precursor fibers with different fibril morphology and corresponding carbon fibers were investigated. The fibrils oriented along the fiber axis and crystal layers were separated from the precursor fibers. And the precursor fibers with large crystallinity, well-oriented fibrils and high packing density exhibited excellent mechanical performance. Meanwhile, the nano-strips were observed in carbon fibers. The carbon fibers with perfect graphitic structures and nano-strip structures showed large tensile strength. Consequently, the precursor fibers with ordered, compact and closely packing fibrils were the prerequisite for preparing the high-performance carbon fibers with perfect nano-strip and highly-ordered graphitic structures. Furthermore, a new insight into structure-property correlation of PAN precursor fibers and resultant carbon fibers from the point of view of fibril structures was gleaned, which also provided a new strategy for improving the mechanical properties of precursor fibers and resultant carbon fibers through regulating the fibril structures.