Development of Porous Polyacrylonitrile Composite Fibers: New Precursor Fibers with High Thermal Stability

Polyacrylonitrile (PAN) fibers with unique properties are becoming increasingly important as precursors for the fabrication of carbon fibers. Here, we suggest the preparation of porous PAN composite fibers to increase the homogeneity and thermal stability of the fibers. Based on the thermodynamics of polymer solutions, the ternary phase diagram of the PAN/H2O/Dimethylformamide (DMF) system has been modeled to introduce porosity in the fibers. Adding a conscious amount of water (4.1 wt.%) as a non-solvent to the PAN solution containing 1 wt.% of graphene oxide (GO), followed by wet spinning, has led to the preparation of porous composite fibers with high thermal stability and unique physicochemical properties. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) results elucidate that PAN/GO/H2O porous composite fibers have a higher thermal decomposition temperature, increased residual weight, reduced heat release rate, and higher crystallinity in comparison with the pristine PAN fibers, being a promising precursor for the development of high-performance carbon fibers. The results show a promising application window of the synthesized PAN fibers in electronic and electrochemical devices.


Introduction
Carbon fibers have been considered as promising materials for a large number of applications in the automotive industry, motorsports, and turbine blade manufacturing due to their light weight, stiffness, and extraordinary properties [1,2]. Polyacrylonitrile (PAN) fibers with high crystallinity, excellent thermal stability, and other remarkable characteristics are widely utilized as primary precursors in manufacturing high-performance carbon fibers [3][4][5].

Preparation of PAN Fibers
Four solutions were prepared for the wet spinning according to Table 1. For preparation of blank sample (dope A), a specific amount of PAN powder (14.4 gr) was added into 108 mL DMF to make an initial spinning dope for the wet spinning system, and it was stirred overnight at 45 • C. For dope B, H 2 O (4.8 mL) and DMF (103.2 mL) were mixed and stirred for 30 min at room temperature. Then, PAN (14.26 gr) was added to the solution and stirred overnight at 45 • C. In the case of dope C, GO (0.14 gr) was sonicated in DMF (108 mL) for 1 h to make a homogeneous dark solution. Subsequently, PAN was added to the solution and stirred overnight at 45 • C. Dope D was prepared by mixing defined amounts of H 2 O (4.8 mL), DMF (103.2 mL) and GO (0.14 gr) in the solution. Following this, PAN powder was swelled in the resultant solution and stirred overnight at 45 • C. It is to be noted that for the determination of cloud points, PAN/DMF solutions with different concentrations of 5, 8, 10, and 15 wt.% PAN was first prepared. Afterward, water was added slowly to the PAN/DMF solution till the initial solution turned turbid. The doped polymers were spun into the fibers using wet spinning system method. A metering pump (pump constant = 0.5 cm 3 /rev) extruded the dopes at same shear rate (16,000 1/s) into a 1-m-long coagulation bath containing tap water at 25 • C. The process was carried out through a single-hole spinneret with a diameter of 200 µm and L/D of 2. Cross-sectional images of as-spun fibers were obtained using a MIRA3 TESCAN microscope (Brno-Kohoutovice, Czech Republic). All samples were freeze-fractured in liquid nitrogen. Then, a thin layer of gold was coated on the fractured surfaces to be observed with FE-SEM.

High Resolution Transmission Electron Microscopy (HR-TEM)
The morphology of wet spun fibers was imaged by an HR-TEM (FEI Tecnai F20, Hillsboro, OR, USA) operated at 200 kV.

Fourier Transform Infrared Spectroscopy (FTIR)
FTIR spectra of the samples were obtained in a range between 500 and 4000 cm −1 using the KBr pellets on a Magna 550 Nicolet spectrometer (Waltham, MA, USA).

Differential Scanning Calorimetry (DSC)
DSC analysis was performed on a Mettler Toledo calorimeter (Strategy Tower 1, Singapore) through heating the samples from 25 • C to 400 • C at a rate of 10 • C/min under N 2 atmosphere.

Thermogravimetric Analysis (TGA)
In order to study thermal stability of the fibers, TGA was carried out using a BAHR thermal analyzer (model STA 503, Hüllhorst, Germany) at a heating rate of 10 • C/min in the temperature range of 25-800 • C under a constant flow of argon environment.

Ternary Phase Diagram
The porosity within the wet spun fibers is accurately outlined using the thermodynamics and kinetics of liquid-liquid phase separation in the ternary mixture of PAN, solvent (DMF), and H 2 O (non-solvent). Achieving a correlation between PAN fiber morphology and initial H 2 O content is in no way trifling. To gain further intuition into phase behavior, the ternary phase diagram of PAN/H 2 O/DMF was constructed using the Flory-Huggins theory. Details of the model are presented in Note 1 in the Supporting Information. The calculated ternary phase diagram is shown in Figure 1. For clarity, the cloud points and tie lines are shown by the blue squares and black dashes, respectively. The obtained binodal fits the experimental cloud points excellently, confirming our approach.
In the next step, we aimed to determine the percentage of water related to the critical points. At such points, the polymer dopes move from the miscible region to the unstable region without passing through the metastable region, leading to spontaneous liquid-liquid demixing and the formation of an interpenetrating bicontinuous structure. Interestingly, the resulting as-spun fibers are homogeneous and have suitable properties [45]. On the In the next step, we aimed to determine the percentage of water related to the critical points. At such points, the polymer dopes move from the miscible region to the unstable region without passing through the metastable region, leading to spontaneous liquid-liquid demixing and the formation of an interpenetrating bicontinuous structure. Interestingly, the resulting as-spun fibers are homogeneous and have suitable properties [45]. On the other hand, a high percentage of non-solvent in the dope results in the formation of fibers with low porosity [46]. Thus, 4.1 wt.% of water as a non-solvent was added to the polymer dope, which is close to the critical point.

Morphological Analysis of PAN-Based Fibers
Cross-sectional FE-SEM images of different PAN fibers are illustrated in Figure 2. Pristine PAN fibers with a diameter of 277 µm have porosity and micrometric voids due to the presence of water as the coagulating agent in the coagulation bath during the wet spinning process [47]. FE-SEM images show that in the presence of H2O, its concentration enrichment lowered overall fiber porosity. In PAN/H2O fibers (d = 504 µm), a diminished thermodynamic affinity toward mass transfer between H2O and fiber precursor emerges, which decreases the numbers of micrometric channels compared to the pristine PAN fibers [46]. However, the number of finger-like channels with decreased width increased. PAN/GO composite fiber (d = 312 µm) has the same cross-sectional structure as pristine PAN fiber and no noticeable change in the channels or voids of fibers appeared. The crosssection FE-SEM image of PAN/GO/H2O (d = 319 µm) shows that the micrometric channels and holes protect the center of the fibers. Additionally, the number of finger-like channels enhances from the center to the outer surface of the fibers.
The HR-TEM image of the PAN/GO/H2O composite fiber and the corresponding SAED pattern are shown in Figure 3. The SAED pattern reveals continuous circles, originating from the crystalline nature of GO. The observed rings are related to the planes of GO with "d" spacing of 1.2 Å and 2.1 Å [48], suggesting the presence of GO inside the composite fiber.

Morphological Analysis of PAN-Based Fibers
Cross-sectional FE-SEM images of different PAN fibers are illustrated in Figure 2. Pristine PAN fibers with a diameter of 277 µm have porosity and micrometric voids due to the presence of water as the coagulating agent in the coagulation bath during the wet spinning process [47]. FE-SEM images show that in the presence of H 2 O, its concentration enrichment lowered overall fiber porosity. In PAN/H 2 O fibers (d = 504 µm), a diminished thermodynamic affinity toward mass transfer between H 2 O and fiber precursor emerges, which decreases the numbers of micrometric channels compared to the pristine PAN fibers [46]. However, the number of finger-like channels with decreased width increased. PAN/GO composite fiber (d = 312 µm) has the same cross-sectional structure as pristine PAN fiber and no noticeable change in the channels or voids of fibers appeared. The cross-section FE-SEM image of PAN/GO/H 2 O (d = 319 µm) shows that the micrometric channels and holes protect the center of the fibers. Additionally, the number of finger-like channels enhances from the center to the outer surface of the fibers.
The HR-TEM image of the PAN/GO/H 2 O composite fiber and the corresponding SAED pattern are shown in Figure 3. The SAED pattern reveals continuous circles, originating from the crystalline nature of GO. The observed rings are related to the planes of GO with "d" spacing of 1.2 Å and 2.1 Å [48], suggesting the presence of GO inside the composite fiber. Electron

Structural Analysis of PAN-Based Fibers
FTIR spectra of the PAN-based fibers are represented in Figure 4a. The band in the range of 2850-2950 cm −1 is attributed to the stretching vibration modes of CH and CH 2 groups of PAN chains and the characteristic band at 2244 cm −1 is delineated to the stretching vibration of nitrile (C≡N). The absorption bands at 1631 cm −1 and 1732 cm −1 are related to C=C and C=O bonds, respectively [49]. Moreover, the C-O stretching band in the methyl acrylate monomer structure appears in the range of 1255-1090 cm −1 [50][51][52].
In the FTIR spectra of PAN/H 2 O and PAN/GO composite fibers, the intensity of bonds increased compared to that of the PAN/GO/H 2 O composite fiber, which could be related to the bond polarity. As illustrated in Figure 4b,

Structural Analysis of PAN-Based Fibers
FTIR spectra of the PAN-based fibers are represented in Figure 4a. The band in the range of 2850-2950 cm −1 is attributed to the stretching vibration modes of CH and CH2 groups of PAN chains and the characteristic band at 2244 cm −1 is delineated to the stretching vibration of nitrile (C≡N). The absorption bands at 1631 cm −1 and 1732 cm −1 are related to C=C and C=O bonds, respectively [49]. Moreover, the C-O stretching band in the methyl acrylate monomer structure appears in the range of 1255-1090 cm −1 [50][51][52]. In the FTIR spectra of PAN/H2O and PAN/GO composite fibers, the intensity of bonds increased compared to that of the PAN/GO/H2O composite fiber, which could be related to the bond polarity. As illustrated in Figure 4b, hydroxyl groups of H2O and GO can form H-bonding with the nitrile group of PAN chain that enhances the nitrile bond polarity. However, in the case of PAN/GO/H2O composite fiber, this polarity decreases since H2O has more affinity to interact with GO rather than PAN. XRD patterns of PAN-based fibers reveal a sharp and a small peak at 2θ angle of ~17° and ~27°, which are related to the crystalline and amorphous regions in the fibers, respectively ( Figure 5) [25,53]. Crystallinity of the pristine PAN fiber, PAN/H2O composite fiber, PAN/GO composite fiber, and PAN/GO/H2O composite fiber were obtained as 19.4%, 21.6%, 25.6%, and 26%, respectively, using the corresponding XRD patterns. The presence of GO with a layered structure during the composite fiber gelation process leads to higher crystallinity of the fiber as it can enforce the alignment of polymer chains [54]. Results suggest that the addition of a non-solvent along with filler resulted in the formation of PAN precursor fibers with augmented crystallinity, which can improve the graphite order and mechanical properties of the final carbon fibers [55,56]. XRD patterns of PAN-based fibers reveal a sharp and a small peak at 2θ angle of 17 • and~27 • , which are related to the crystalline and amorphous regions in the fibers, respectively ( Figure 5) [25,53]. Crystallinity of the pristine PAN fiber, PAN/H 2 O composite fiber, PAN/GO composite fiber, and PAN/GO/H 2 O composite fiber were obtained as 19.4%, 21.6%, 25.6%, and 26%, respectively, using the corresponding XRD patterns. The presence of GO with a layered structure during the composite fiber gelation process leads to higher crystallinity of the fiber as it can enforce the alignment of polymer chains [54]. Results suggest that the addition of a non-solvent along with filler resulted in the formation of PAN precursor fibers with augmented crystallinity, which can improve the graphite order and mechanical properties of the final carbon fibers [55,56].

Thermal Behavior of PAN-Based Fibers
DSC thermograms of the fibers are demonstrated in Figure 6 and their relevant parameters have been summarized in Table 2. For the pristine PAN fibers, an exothermic peak temperature (Tp) appeared at 308.4 °C, related to the typical cyclization reaction of PAN [57,58]. The PAN/H2O, PAN/GO, and PAN/GO/H2O composite fibers showed peak temperatures at 297.6, 295.1, and 294.6 °C, respectively. Due to the addition of H2O to the PAN fiber, the TP decreased by 10.75 °C compared to the pristine PAN fiber. Additionally, for the PAN/GO composite fiber, a decline of 13.31 °C was observed for the exothermic peak temperature since acidic functional groups can decline the cyclization temperature and the released heat by ionic interactions between GO and PAN [59]. Interestingly, the most change in the exothermic peak temperature occurs in the presence of both H2O and GO. Results illustrate that the incorporation of H2O and GO in the PAN fibers decreases the required activation energy for initiation cyclization reaction, reducing Tp [60]. In addition, such a reduction in Tp can help avoid extreme heat eruptions and obtain a homogeneous stabilized structure in the prepared composite fibers [61]. The exothermic rate parameter (ΔH/ΔT) was also calculated for each sample from the DSC thermograms (Table 2). It was found that the parameter decreased by 31.2% and 49.2% for PAN/H2O and PAN/GO composite fiber, respectively, in comparison with pristine PAN fiber. However, a significant reduction (51.6%) was obtained in ΔH/ΔT when both H2O and GO were introduced into the PAN fibers, confirming that our integrated technique was successful in diminishing the amount of released heat in less time for the fiber. As a consequence, the as-prepared, modified PAN-based composite fiber had a homogeneous stabilized structure with less decomposition of polymer chains and high graphitic order, which are vital for manufacturing high-performance carbon fibers [43,62].

Thermal Behavior of PAN-Based Fibers
DSC thermograms of the fibers are demonstrated in Figure 6 and their relevant parameters have been summarized in Table 2. For the pristine PAN fibers, an exothermic peak temperature (T p ) appeared at 308.4 • C, related to the typical cyclization reaction of PAN [57,58]. The PAN/H 2 O, PAN/GO, and PAN/GO/H 2 O composite fibers showed peak temperatures at 297.6, 295.1, and 294.6 • C, respectively. Due to the addition of H 2 O to the PAN fiber, the T P decreased by 10.75 • C compared to the pristine PAN fiber. Additionally, for the PAN/GO composite fiber, a decline of 13.31 • C was observed for the exothermic peak temperature since acidic functional groups can decline the cyclization temperature and the released heat by ionic interactions between GO and PAN [59]. Interestingly, the most change in the exothermic peak temperature occurs in the presence of both H 2 O and GO. Results illustrate that the incorporation of H 2 O and GO in the PAN fibers decreases the required activation energy for initiation cyclization reaction, reducing T p [60]. In addition, such a reduction in T p can help avoid extreme heat eruptions and obtain a homogeneous stabilized structure in the prepared composite fibers [61]. The exothermic rate parameter (∆H/∆T) was also calculated for each sample from the DSC thermograms (Table 2). It was found that the parameter decreased by 31.2% and 49.2% for PAN/H 2 O and PAN/GO composite fiber, respectively, in comparison with pristine PAN fiber. However, a significant reduction (51.6%) was obtained in ∆H/∆T when both H 2 O and GO were introduced into the PAN fibers, confirming that our integrated technique was successful in diminishing the amount of released heat in less time for the fiber. As a consequence, the as-prepared, modified PAN-based composite fiber had a homogeneous stabilized structure with less decomposition of polymer chains and high graphitic order, which are vital for manufacturing high-performance carbon fibers [43,62].   Figure 7a and the corresponding parameters are listed in Table 3. For all samples, the first main weight loss starts around 50 °C, which can be attributed to the loss of water molecules and low molecular weigh oligomers [63]. DTG curves show a thermal decomposition temperature (Td) of 299.4 °C for pristine PAN fibers (Figure 7b). Through the addition of water and nanofiller to the fibers, the temperature reached 302.6 and 302.8 °C for PAN/H2O fiber and PAN/GO com posite fibers, respectively. However, Td increased to a high temperature of 307.1 °C for PAN/GO/H2O composite fibers. It is worth noting that decomposition, which happens above ~300 °C, is assigned to backbone degradation of the pristine PAN fibers [41]. Thus the PAN backbone decomposition temperature was notably enhanced for the PAN/GO/H2O composite fibers. The layered structures of GO may function as cross-bond ing points and contribute to a vital interaction between polymer chains and nanofillers resulting in a highly intensified thermal stability [64]. Moreover, the weight loss in Td was calculated as 10.1%, 9.5%, 5.1%, and 2.2% for pristine PAN fibers, PAN/H2O fibers PAN/GO composite fibers, and PAN/GO/H2O composite fibers, respectively, demonstrat ing high thermal stability of the PAN/GO/H2O composite fibers among other samples [42] The residual weight percentages of the samples were also attained at 800 °C ignoring the weight of evaporated H2O in the water-containing fibers. This parameter reached 33.8% for the pristine PAN fiber; however, it increased to 35.0%, 47.2%, and 48.2% for PAN/H2O fiber, PAN/GO composite fibers, and PAN/GO/H2O composite fibers, respectively. High residual weight percentages of the modified PAN/GO/H2O composite fibers originate   TGA curves of the PAN-based fibers are observed in Figure 7a and the corresponding parameters are listed in Table 3. For all samples, the first main weight loss starts around 50 • C, which can be attributed to the loss of water molecules and low molecular weight oligomers [63]. DTG curves show a thermal decomposition temperature (T d ) of 299.4 • C for pristine PAN fibers (Figure 7b). Through the addition of water and nanofiller to the fibers, the temperature reached 302.6 and 302.8 • C for PAN/H 2 O fiber and PAN/GO composite fibers, respectively. However, T d increased to a high temperature of 307.1 • C for PAN/GO/H 2 O composite fibers. It is worth noting that decomposition, which happens above~300 • C, is assigned to backbone degradation of the pristine PAN fibers [41]. Thus, the PAN backbone decomposition temperature was notably enhanced for the PAN/GO/H 2 O composite fibers. The layered structures of GO may function as crossbonding points and contribute to a vital interaction between polymer chains and nanofillers, resulting in a highly intensified thermal stability [64]. Moreover, the weight loss in T d was calculated as 10.1%, 9.5%, 5.1%, and 2.2% for pristine PAN (Table S3). As a consequence, the promising approach presented here is an effective strategy to produce modified precursor PAN fibers with unique properties, which helps the fabrication of high-performance carbon fibers.

Sample Fiber
Electron. Mater. 2021, 2, FOR PEER REVIEW 9 from high thermal stability of the sample [42,65]. Additionally, it can be seen that compared to previous studies, notable results have been obtained in our study (Table S3). As a consequence, the promising approach presented here is an effective strategy to produce modified precursor PAN fibers with unique properties, which helps the fabrication of high-performance carbon fibers.

Conclusions
In summary, we have prepared PAN precursor porous composite fibers by simultaneously adding nanofiller (GO) and non-solvent (H2O), which are profoundly modified in thermal stability and properties. The ternary phase diagram of the PAN/H2O/DMF system has been modeled using the Flory-Huggins theory to determine the non-solvent content and tailor porosity in the fibers. DSC results demonstrate higher crystallinity content and a more homogeneous stabilized structure of PAN/GO/H2O composite fiber compared to pristine PAN fibers due to its reduced heat release rate. Our modification technique also results in the thermal stability of porous composite fibers confirmed by TGA. Thermal decomposition temperature and residual weight of PAN/GO/H2O composite fibers has increased in comparison with pristine PAN fibers, giving a suitable precursor for the production of high-performance carbon fibers.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Note S1: including additional descriptions for 1. Phase diagram of PAN/H2O/DMF ternary system, 2. Critical point, 3. H2O-DMF binary system, 4. DMF-PAN binary system, 5. H2O -PAN binary system, 6. Algorithm description, Table S1: Molar absorption constant at 16 °C and Table S2: Chemical structure and calculated solubility parameter. Table S3: An overview on the properties of the PAN fibers prepared in our work and previous studies.

Conclusions
In summary, we have prepared PAN precursor porous composite fibers by simultaneously adding nanofiller (GO) and non-solvent (H 2 O), which are profoundly modified in thermal stability and properties. The ternary phase diagram of the PAN/H 2 O/DMF system has been modeled using the Flory-Huggins theory to determine the non-solvent content and tailor porosity in the fibers. DSC results demonstrate higher crystallinity content and a more homogeneous stabilized structure of PAN/GO/H 2 O composite fiber compared to pristine PAN fibers due to its reduced heat release rate. Our modification technique also results in the thermal stability of porous composite fibers confirmed by TGA. Thermal decomposition temperature and residual weight of PAN/GO/H 2 O composite fibers has increased in comparison with pristine PAN fibers, giving a suitable precursor for the production of high-performance carbon fibers.