Introduction

Polyacrylonitrile (PAN) is a semicrystalline thermoplast and the basis for a variety of products, e.g., membranes for ultrafiltration or fibers [1, 2]. The fiber industry uses PAN as material for textile applications, as hollow fibers for osmosis or—copolymerized with suitable monomers—as precursor fiber for the production of carbon fibers [2]. PAN fibers are the most important precursor material for the production of carbon fibers of high tensile strength [3, 4]. Due to the growing field of applications of carbon fiber-reinforced composite materials, the demand for carbon fibers is increasing and the optimization of fiber properties is in the focus of research [5,6,7]. Here, in particular the relationship between the parameters of the carbon fiber production process, the evolution of chemical and physical structure of the fibers during the thermal transformation from PAN to carbon and the resulting carbon fiber mechanical parameters are of high interest [8,9,10,11,12,13,14]. It is assumed that the prerequisite for a high-quality carbon fiber is a copolymerized PAN precursor fiber with optimally adapted chemical and physical structure, which is mainly defined by the polymerization and spinning process [15,16,17].

The PAN fiber can be formed using different types of spinning processes [2]. Wet spinning is the common production method for the manufacturing of tows with a large number of filaments [17]. It starts with the spinning of the PAN solution through a spinneret into a coagulation bath, followed by repeated steps of washing and stretching, and ends with the application of a textile finish agent and drying of the fibers [2, 18]. In this process, the coagulation is one of the most important steps determining the physical structure of the fiber. The parameters of the coagulation process, like chemical composition and temperature of the coagulation bath, control the formation of the micro- and nanostructure of the fiber [19, 20]. Depending on the conditions within the coagulation bath, a spinodal decomposition of the PAN polymer solution in two phases can occur [21, 22]. This results in a gel network, in which the polymer-rich phase forms the bulk of the fiber with the typical fibrillar substructure [23], while the solvent-rich phase results in voids [2, 24]. The fibrils, which typically have diameters of 5–400 nm, are dominantly oriented along the fiber axis [25, 26]. The fibrils consist of PAN molecule chains with a preferred orientation, resulting from the flow of the dilute PAN solution through the spinneret and the stretching of the thus resulting fiber [27, 28]. Long range order of the flexible PAN molecule chains occurs, which is described in literature by different models [29, 30]. The fringed-micelle model describes the formation of small crystallites by the partial alignment of several individual chains [31, 32]. Ref. [33] proposes that polymer chains fold several times to form crystallites. A planar crystal growth of such aligned chains results in a lamellar structure [34, 35]. Depending on the processing conditions, many polymers crystallize in complex lamellae-based structures, like spherulites or shish-kebab structures [35,36,37,38,39]. While spherulites consist of radially oriented lamellae, lamellae of a shish-kebab structure are oriented perpendicular with respect to centered parallel oriented molecular chains (shish) [36,37,38,39]. In these complex lamellae-based structures, the folding of the polymer chains within the lamellae is less organized than in polymer single crystals [38]. The folding of the polymer chain can be described by the so-called adjacent chain-folding model, where the molecule chains fold in a regular manner to form a lamella, or by the switchboard-model, where loops, cilia, and tie molecules are assumed to coexist [40,41,42]. These loops, cilia, tie molecules, and the irregular chain-folding on the fringes of the lamellae form amorphous areas around the crystallites, resulting in an overall semicrystalline material. The orientation of the PAN molecules relative to each other and along the fiber axis, which influences the mechanical properties of the fibers, increases by stretching the fibers during the spinning process [16, 17, 43].

The fibrillar substructure of the PAN fibers and the nanostructure of these fibrils are important for a fundamental understanding of PAN fiber properties, because they influence the properties of the resulting carbon fibers [44]. Microscopy of the surface of PAN fibers offers information about the diameter of surface fibrils and their orientation relative to the fiber axis, as well as of size and morphology of their nanostructures. Information about the volume structure of the PAN fibers can be obtained by fibril separation using an etching process, which also allows detailed investigation of the nanostructure of bulk fibrils [29, 45,46,47]. Fibril separation by ultrasonic etching applying a suitable solvent solution has become a widely accepted preparation method for different types of fibers [48,49,50,51,52,53,54]. Reference [29] for the first time applied an ultrasonic etching process to PAN fibers and described a fibril structure composed of periodically arranged transverse bands. However, the transmission electron microscope (TEM) image shown is of low resolution and, due to the shrinkage of the polymer fibril under the electron beam, diffuse. Wang et al. report a similar form of fibrils and describe the transverse bands as lamellae and the observed fibril morphology as shish-kebab structure without a visible shish [45, 46]. In addition to such fibrils with lamellar surface structure also fibrils with a smooth surface structure are found, which seem to be similar to fibrils investigated without ultrasonic etching process [45, 46].

Here, we investigate the fibrillar micro- and nanostructure of different types of PAN fibers, i.e., two PAN fibers originating from a technical production process and one textile PAN fiber. The fibers were separated into their fibrils by ultrasonic etching. The resulting microscale fibril morphologies were characterized by field emission scanning electron microscopy (FESEM). Nanoscale analysis of the different PAN fibrils was performed by high-resolution atomic force microscopy (AFM) in tapping mode and TEM. Based on this analysis, a model of the volume structure of PAN fibers is presented.

Materials and methods

Polyacrylonitrile fibers and fibril preparation

The fibril structures of three different types of PAN fibers (PAN A, PAN B and PAN C) are analyzed. PAN A and B are technical precursor fibers for the production of carbon fibers with high tensile strengths, and PAN C is a fiber for textile applications. PAN A results from a research project, PAN B is a precursor fiber by Bluestar and PAN C is a textile fiber by Dralon. To analyze the bulk structure, these fibers were separated into their fibrils by ultrasonic etching. To this end, the PAN fibers were cut into pieces with a length of 2–3 mm and were processed in dimethyl sulfoxide (DMSO) solution at a temperature of 75 °C in an ultrasonic bath (USB) (Bandelin Sonorex digitec, frequency of 35 kHz). The set temperature of 75 °C is below the glass transition temperatures of the fibers of 100–120 °C. The concentration of the DMSO solution and the ultrasonic treatment time necessary for fibril separation were found to depend on the PAN fiber type and are discussed in the experimental part of this paper. The resulting PAN dispersions were filtered using a TEM lacey carbon grid, washed with distilled water and dried at room temperature.

Scanning electron microscopy

The samples for FESEM measurements were coated with a thin platinum layer, to guarantee high electrical conductivity of the surface. A FESEM LEO Gemini 982 FEG from Zeiss AG at an acceleration voltage of 1 kV was used. TEM investigation of uncoated fibrils was performed using a JEOL JEM-2100F at an accelerating voltage of 200 keV.

Atomic force microscopy

AFM measurements were carried out using a Dimension Icon atomic force microscope (Bruker). They were performed using the tapping mode with super sharp EBD-SSS NCHR AFM probes from Nanotools with a tip radius of 2–3 nm at a scan rate of 0.5 Hz. The scanning area of the images was 1 µm × 1 µm (512 × 512 pixels). The images were processed and analyzed by the software Nanoscope Analysis v1.40.

X-ray diffraction

X-ray diffraction (XRD) investigations were performed at a Seifert 3003 PTS XRD diffractometer with copper anode. The crystallinity, the crystallite size and the orientation of the crystallites were determined according to Ref. [55].

Results

Microscale surface analysis of fibrils by FESEM

The FESEM investigation demonstrates that the technical PAN fiber A can be successfully separated into fibrils by ultrasonic etching. We find separated material with fragments of the original fiber still preserved after the USB etching. Some of the fibrils are partially connected to the surface of this fiber fragment and are flaking off in bundles (see Fig. 1a). These fibril bundles are of different thickness and are composed of a multitude of individual fibrils with a smooth surface, similar to those of Refs. [45, 46]. In addition, completely separated bundles of smooth fibrils are found. Figure 1b shows these individual fibrils in higher magnification. Evaluation of 1017 individual smooth fibrils results in an average diameter of 65 ± 36 nm, which is in good agreement with the size of smooth fibrils reported by Refs. [25, 45, 46].

Figure 1
figure 1

FESEM images of the two different morphologies of PAN fibrils (a, b) type I: smooth fibrils (c, d) type II: fibrils with lamella-like microstructures

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In addition to the described smooth fibrils, a second type of fibril with a completely different, rough surface structure (Fig. 1c) is found. In agreement with the rough fibrils described by Refs. [29, 45, 46], the surface structure of these fibrils is characterized by thin lamella-like layers oriented perpendicular to the fibril axis. However, while the rough fibrils described by Refs. [29, 45, 46] have a lamellar, closely packed structure, the lamella-like layers described in this work are arranged more loosely. In Fig. 1d, this rough surface structure of three individual fibrils is shown in higher magnification. The image illustrates that the complete fibril is characterized by a fine porous network of thin layers, which are dominantly oriented perpendicular to the fibril axis. The thin layers are interconnected and show an average thickness of 28 ± 11 nm (evaluation of 126 layers). The average diameter of the complete rough fibrils amounts to 280 ± 115 nm (evaluation of 100 fibrils), which is significantly larger than the average diameter of the smooth fibrils.

Based on these distinctive differences, the described two types of fibrils are classified as type I (smooth fibril) and type II (rough fibril).

Figure 2 shows a region of the fiber characterized by fibrils of type II interpreted as different stages of the solvation process. The images indicate that the lamella-like microstructure of fibrils of type II is carved out during the etching procedure. Figure 2a shows the PAN fiber surface at an early state of the etching process. The fibrillar substructure is visible. At position 1 of Fig. 2a (shown in Fig. 2a1), small bright nodules exist, which have an irregularly elongated shape and are oriented perpendicular to the fiber axis. At position 2 (shown in Fig. 2a2), the bright nodules are carved out more distinctly and seem to stick out of the fiber surface. Additionally, round black areas become visible, which represent holes within the fiber surface presumably produced by the starting solvation process. These holes are arranged in lines along the fiber axis and are located between two fibrils. Apparently, the dissolution process in the region of Fig. 2a2 has proceeded slightly faster than in the region of Fig. 2a1.

Figure 2
figure 2

FESEM images of a PAN fiber surface at different dissolving states of ultrasonic etching, (a) surface at the beginning of etching (a 1 + a 2 at higher magnification), (b) surface at higher state of etching with a lamella-like framework (b 1 + b 2 at higher magnification)

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The FESEM image of Fig. 2b shows a PAN fiber at a presumably more advanced stage of etching. In contrast to Fig. 2a, the fiber surface is characterized by a much higher number of holes and only small regions of elongated nodules are left. Additionally, the lamella-like microstructure already known from Fig. 1d is found. The higher magnification image in Fig. 2b1 shows an area where these different stages of solvation coexist. Region 2 of Fig. 2b (shown in Fig. 2b2) is completely characterized by the lamellar microstructures, as found for completely separated rough fibrils of type II.

Based on the images of Fig. 2 the polymeric material in between the elongated nodules appears to be dissolved continuously by the proceeding etching process. Correspondingly, the nodules become more prominent, become thinner and develop the shape of fine layers, which are oriented perpendicular to the fiber axis. Due to the advanced solvation process holes develop.

It appears that the PAN material in between the lamella-like layers is dissolved faster by the DMSO solution than the layers themselves. As crystalline material is more resistant against solvation we assume, the layers consist of polymeric crystalline material, while the material in-between is amorphous in nature.

Nanoscale surface analysis of fibrils by AFM imaging

To obtain more detailed information on the surface morphology of the PAN fibrils on nanoscale, atomic force microscopy is used. For AFM, the tapping mode is an established method to analyze polymers. It has been shown by AFM that the surface of semicrystalline polymers has a nanoscale granular structure [56,57,58,59]. This granular structure is assumed to be caused by folding of polymer chains to small crystallites, separated by amorphous material in between [57, 60, 61].

Figure 3 shows AFM images of fibrils of type I and type II. For each fibril type, two different magnifications of AFM height images are shown. The white rectangles in Fig. 3a and c mark the areas of the corresponding high-magnification images in Fig. 3b and d, respectively. Figure 3a shows a fibril bundle of fibrils of type I. Individual fibrils of this bundle are marked by white arrows in Fig. 3b. In accordance with the previous FESEM investigations, fibrils of type I show a smooth surface morphology. The diameter of individual fibrils as determined by AFM amounts to 23 ± 7 nm (evaluation of 496 fibrils), which is smaller than that resulting from FESEM measurements. We suppose that due to insufficient contrast of the FESEM images several adjacent fibrils are falsely interpreted as one fibril. As AFM images provide a higher image contrast, the individual fibrils can be distinguished more clearly.

Figure 3
figure 3

AFM height images of smooth fibrils of type I (a, b) and of rough fibrils of type II (c, d)

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Figure 3c shows several individual fibrils of type II marked with white arrows. They show the typical lamella-like microstructure described above. The average diameter amounts to 461 ± 162 nm (evaluation of 11 fibrils), which is slightly higher but coincides within the margins of error with the value determined from FESEM images. Again, the lamella-like layers are oriented perpendicular to the fibril axis. In Fig. 3d, the lamellar structures are shown in higher magnification. Three lamella-like layers are marked by black arrows. The surfaces of the individual lamella-like layers are not smooth. Each layer is individually shaped and composed of granules with widths of around 20–80 nm. We interpret these granular structures as small polymer crystallites, which are not perfectly aligned relative to each other. Our AFM results are in accordance with the interpretation that lamella-like microstructures of fibrils of type II are crystalline.

Nanoscale bulk analysis of fibrils by TEM

To obtain information about the volume structure of the fibrils and to analyze the structural differences of the two types of fibrils, the samples were further analyzed by TEM. The TEM investigations shown in Fig. 4 support the FESEM and AFM results that two types of fibrils exist. In Fig. 4a, smooth fibrils of type I are shown, which are characterized by a low TEM contrast. The fibrils show an average diameter of 20 ± 11 nm (evaluation of 45 fibrils), which is similar to that determined by AFM measurements. Their volume structure is homogenous with no further substructures. The TEM image is typical for amorphous materials.

Figure 4
figure 4

TEM images of a smooth (type I) and b lamella-like fibrils (type II)

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TEM images of fibrils of type II (Fig. 4b) show a complex substructure. In the center of the fibrils, which have an average diameter of 264 ± 102 nm (evaluation of 14 fibrils), a long string is found which is oriented along the fibril axis (in Fig. 4b from top left to bottom right). The string is clearly resolved by TEM, which, due to the transparency of the polymeric material for the electron beam, shows the projection of the complete fibril volume. In contrast, this string was not observed in the FESEM and AFM images, since here only the surface structure of the fibrils is resolved. To the best of our knowledge, this is the first time that such kind of string is observed within PAN fibrils. These strings exhibit an average diameter of 26 ± 17 nm. Perpendicular to the string, filigree lamella-like substructures are clearly visible, which are characterized by a thickness of 10 ± 4 nm (evaluation of 72 lamella-like layers) and seem to surround the string. The lamella-like substructures were detected already by FESEM and AFM analysis, where they are responsible for the rough surface morphology of the fibrils of type II. The thickness of these lamella-like structures appears smaller in the TEM measurements, because only material with sufficiently high TEM contrast was considered for the thickness measurements. A high TEM contrast can be due to density differences, thickness differences or variations in crystallinity and crystallite orientation. We assume that the lamella-like structures of the rough fibrils are crystalline. This is consistent with the results described above, namely the etching behavior of the fibrils and the granular structure of the lamella-like layers.

The dimensions of the observed fibril substructures as measured by TEM are summarized in the diagram of Fig. 5. The smooth fibrils of type I have the smallest diameter of 20 ± 11 nm, which is similar to the diameter of the string of type II fibrils of 26 ± 17 nm. The diameter of the fibrils of type II amounts to 264 ± 102 nm. Their lamellar-like layers exhibit a thickness of only 10 ± 4 nm and arrange perpendicular to the fibril axis.

Figure 5
figure 5

Dimensions of two types of PAN fibrils and their substructures

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Comparison of fibril morphology of technical and textile PAN fibers

All investigations described so far were performed on the technical PAN fiber A. To elucidate the influence of the PAN fiber type on the fibril morphology, additionally two other PAN fiber types (PAN B and PAN C) were analyzed. PAN B is also a technical PAN fiber, while PAN C is used for textile applications. The character of the three PAN fiber types is defined by their crystalline properties, which were analyzed by XRD. The qualitative results of the XRD analysis are shown in Table 1. The three PAN fiber types differ in their crystallinity, crystallite size, and crystallite orientation. The crystallinity, the crystallite size, and the orientation of the PAN crystallites decreases in the following order: PAN A > PAN B > PAN C.

Table 1 Relative degree of crystallinity, crystallite size, and crystallite orientation of the three types of PAN fibers
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The fibers PAN B and PAN C were prepared in accordance with PAN A in DMSO solution in the USB to separate them into their fibrils. The resulting material was analyzed using FESEM. A test series with different concentrations of the DMSO solution was performed to identify the optimal parameters for each PAN fiber type. Table 2 summarizes the DMSO concentrations used. The values highlighted in italics are concentrations, which are too low for the respective type of fiber and did not result in fibril separation. In contrast, the concentrations highlighted in bold are too high and resulted in complete dissolution of the fibers. The optimal concentrations for the separation of the respective fibers into their fibrils are the remaining ones which are underlined.

Table 2 DMSO concentration used for ultrasonic etching of the three types of PAN fibers
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A clear correlation between the crystalline properties and the dissolution behavior is observed. The higher the crystallinity, the crystallite size and the crystallite orientation of the fiber, the higher the concentration of DMSO necessary to separate the fibers into their fibrils. From the data, it cannot be decided which of the crystallite properties determines the solvation behavior.

For all three types of fibers, both smooth and rough fibrils are found. However, the ratio of these two fibril types depends on the fiber character. Samples of PAN B and C show less rough fibrils of type II than samples of PAN A. The amount of fibrils of type II decreases in the order PAN A > PAN B > PAN C, i.e., the crystallinity is correlated to the number of type II fibrils.

Discussion

Based on the microscopic investigations, we suggest a structural model of the PAN fiber where the two types of fibrils coexist. Figure 6 shows schematically the structural relation between the type I and type II fibrils.

Figure 6
figure 6

Schematic model of the fibril structures of PAN fibers and the differences between the two fibril types

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Type I fibrils have a smooth surface and a homogeneous volume structure. Such fibrils are described in literature [27]. They are formed by stretched polymer chains, which are oriented preferentially parallel to each other. Stretching of the polymer chains and their orientation along the fibril and fiber axis can occur during the spinning and stretching process of the PAN fibers [27, 62]. Although no crystalline regions were detected, the smooth fibrils show a higher resistance to the solution process than the surrounding amorphous material. We assume that this resistance results from the increased intermolecular forces between the parallel-aligned molecule chains.

In contrast, type II fibrils are characterized by lamella-like layers arranged perpendicular to a central string. In literature, this arrangement is referred to as shish-kebab structure and is typical for polymer crystallization under shear flow stress or under uniaxial tension [37, 63]. Such a structure is also found in other polymer fibers, e.g., polyethylene or polypropylene fibers [64,65,66]. Its existence is attributed to shear conditions during the spinning process of such fibers. The molecular chains of the polymer are stretched, arrange parallel to each other and form shish structures, similar to type I fibrils [67]. Growth of the kebab structure takes place at those parts of the shish, where the polymer chains are particularly well oriented [62, 68]. These well-aligned areas of the shish represent nuclei for crystallization of the kebab structures and define the direction of their growth perpendicular to the direction of the shish [35, 62, 68]. The kebab structures in turn consist of the sheet-like arrangement of polymer crystallites formed from folded molecular chains. We identify these kebab structures with our lamella-like layers and the shish structure with our strings within the type II fibrils. The lamella-like layers with a granular surface structure, as detected by AFM, are interpreted as a result of the agglomeration of small polymer crystallites, which are not perfectly oriented with respect to each other and connected by amorphous regions [69].

Conclusion

Technical and textile PAN fibers were separated into their fibrils by ultrasonic etching. In all types of PAN fibers, two different fibril morphologies were found. On the one hand, fibrils with smooth surface and homogeneous, amorphous volume structure exist (type I). On the other hand, fibrils with the typical shish-kebab structure are found (type II), characterized by crystalline lamella-like layers oriented perpendicular to a centered shish. As the diameter of this shish is similar to the diameter of the smooth fibrils, the smooth fibril is assumed to be the nucleus for crystallization of lamella-like structures and, therefore, the formation of type II fibrils. The growth of lamella-like crystalline layers occurs probably at well-ordered surface regions of smooth fibrils. This results in the coexistence of smooth and rough fibrils within one PAN fiber. With increasing crystallinity of the PAN fibers, the relative amount of shish-kebab fibrils of type II increases, thus representing an indicator for the character of the PAN fibers.