Low-temperature thermal stabilization of polyacrylontrile-based precursor fibers towards efficient preparation of carbon fibers with improved mechanical properties

Polymer 76 (2015) 131e139

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Low-temperature thermal stabilization of polyacrylontrile-based precursor fibers towards efficient preparation of carbon fibers with improved mechanical properties Xiaoyan Chai a, b, Hongwei Mi b, Caizhen Zhu b, Chuanxin He b, Jian Xu c, Xuechang Zhou b, **, Jianhong Liu b, * a b c

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China Shenzhen Key Laboratory of Functional Polymer, College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 May 2015 Received in revised form 5 August 2015 Accepted 24 August 2015 Available online 29 August 2015

This article describes a low-temperature thermal stabilization method, for the efficient preparation of polyacrylontrile (PAN)-based carbon fibers with improved mechanical properties. In this method, bundles of regular PAN precursor fibers were firstly heated and stored in air for 30 days at 120
C to cyclise the nitrile groups and oxidize the PAN backbone. A further stabilization above 200
C in air made the pretreated fibers fully stabilized. Structural changes of the as-made PAN fibers were observed by Fourier Transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, differential scanning calorimetry, thermogravimetry and dynamic mechanical analysis. Microvoid evolutions of the fibers during stabilization and carbonization process were studied by synchrotron small-angle X-ray scattering. Our results showed that the initiation of the cyclization and oxidation reaction at 120
C not only restricts the disorientation of PAN molecules but also reduces the pyrolysis of molecular chains at higher temperatures in the carbonization process. Hence, preferred orientation of crystallites and char yield increased. Moreover, microvoid defects were significantly reduced, leading to a significant improvement of the mechanical properties (a 16% increment in the tensile strength). © 2015 Elsevier Ltd. All rights reserved.

Keywords: Carbon fibers Low-temperature thermal stabilization Mechanical properties

1. Introduction It is estimated that the theoretical elastic modulus and tensile strength for a perfect graphite are approximately 1000 and 180 GPa, respectively [1]. However, there is still a large gap between the tensile strength of commercial carbon fibers (which is in the range of 1e9 GPa) and that of the theoretical value [2]. The defects in the microstructure of carbon fibers are the main restriction for performance improvement [1,2]. The theoretical char yield of a PAN precursor is 68%, however, it is lower than 50% in a typical commercial production process fibers [3]. This is due to weight loss of thermal chain scission along the polymer chains to form monomers and olimogers [4,5]. Suppression of weigh loss during stabilization and carbonization stage is expected to increase the char yield and

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhou), [email protected] (J. Liu). http://dx.doi.org/10.1016/j.polymer.2015.08.049 0032-3861/© 2015 Elsevier Ltd. All rights reserved.

tensile strength of carbon fibers [5,6]. High energy-consuming, complicated process and expensive facilities are generally required to obtain both high performance and high char yield for PAN-based CFs, leading to a barrier of cost and technology in large-scale production. Therefore, it is of great importance to develop, simple and reliable processes to fabricate high quality CFs. Notably, the preparation of carbon fibers includes multiple processes, such as spinning of the precursor fibers, stabilization (preoxidation), carbonization, and graphitization [6e9]. Among them, stabilization is one of the critical steps for the production of high quality PAN-based carbon fibers. Traditionally, stabilization is usually conducted in air at 180e300
C to form thermally-stable structures, which become infusible during hightemperature carbonization [10,11]. This is attributed to chemical reactions involved in the process, i.e., cyclization, dehydrogenation and oxidation, which gives rise to conjugated ladder structures [12,13]. It is recognized that oxidative chain pyrolysis compete with cyclization [4]. The extent of cyclization and crosslinking are limited to a certain level because some acrylontrile monomers and

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oligomeric products are formed through thermal chain scission along the original polymer chain during the stabilization [4,14]. In addition, the dehydrogenation and dehydrocyanide reactions which lead to CN-substituted conjugated polyene structures along the backbone chain also participate in some extent followed by cyclization [1]. These structures are unable to withstand high temperature during carbonization. Various pretreatments have been employed to promote the cyclization and crosslinking. For example, pretreatments with plasma [15e17], Gamma ray [18,19], electron beam [20,21] and UV irradiation [22,23] have been reported to promote the intramolecular cyclization as well as the crosslinking of the nitrile groups. However, excessive irradiation could induce surface defects which had a negative impact on the mechanical properties of the resulted fibers. Qin et al. [24] proposed another pretreatment method, i.e., heating and stretching the PAN precursor fibers in nitrogen before full stabilization in air, and the mechanical properties of the carbon fibers were improved by about 10%, owing to the partial cyclization of the nitrile groups during the pretreatment in nitrogen. However, the whole stabilization process was conducted in a four-zone tube furnace by the positive stretching in nitrogen and six-zone tube furnace and by the negative stretching in air, respectively. The method is complicated and timeconsuming, and thus it is not suitable for large-scale industrial production. Herein, we for the first time report a low-temperature thermal stabilization method for the fabrication of PAN-based carbon fibers with improved mechanical properties and char yield without any complicated process and expensive equipments. In our study, bundles of PAN-based fibers were heated and stored in air at 120
C for couple of days, and cyclization in the amorphous phase and oxidation reactions occurred during the heat treatment. A further stabilization above 200
C in air made the pretreated fibers fully stabilized. To the best of our knowledge, this is the first report of preparing PAN carbon fibers with improved mechanical properties assisted by such a low-temperature thermal stabilization. It is generally reported that stabilization is usually conducted in air above 180
C since cyclization and crosslinking reactions are prominent above 180
C [7,8]. However, cyclization in the amorphous phase and oxidation reactions were favoured by prolonged heating (after 24 h) at 120
C in our study. The present method has several unique features. First of all, owing to the nitrile partial cyclization and oxidation reaction of PAN molecules at the lowtemperature stabilization step, the disorientation of PAN molecules is restricted to some extent during stabilization and carbonization processes. As a result, it reduces the pyrolysis of molecular chains and increases the char yield. Secondly, the preferred orientation of PAN molecules is retained which would results in less microvoid defects and better mechanical properties compared with their untreated counterparts. Furthermore, the low-temperature stabilization step is conducted in a batch method, without employing any complicated process or specially-designed facilities. Hence, it is very suitable for low-cost large-scale production. In addition, the restricted shrinkage and disorientation of PAN molecules require less stretching during stabilization process. As a result, the cost of equipment is reduced. Although the thermal stabilization time of 30 days is relatively long, the fibers are conducted in a batch method. It can be considered as lowertemperature energy consumption. Such kind of energy can be obtained from a wide variety of sources, for example, the by-product waste heat recycled from power generation plants. As proof-ofconcept, we successfully demonstrated the low-temperature thermal treatment of the PAN precursor fibers, followed by conventional stabilization and carbonization steps. And finally, we investigated the physical morphologies, chemical structures, and

tensile properties of the as-made PAN carbon fibers. 2. Experimental 2.1. Materials The PAN precursor fibers (named as P0) supplied by Yangzhou huitong company (China) were wet-spun from acrylonitrile together with 1 wt.% of itaconic acid (IA), and had 3000 filaments/ tow, an average diameter12 mm, a tensile strength of 595 MPa, a Young's modulus of 10.3 GPa, and an elongation of 10.9%. 2.2. Low-temperature thermal treatment of precursor fibers A bundle of precursor fibers (P0) were heated and stored in air at 120
C for 30 days, after which the color changed from white to yellow. The obtained fibers were denoted as P1. 2.3. Stabilization and carbonization Stabilization of PAN fibers (P1 and P0) was carried out in a furnace, which had four temperature zones of 200e230e250e270
C and stretching ratios of 3e1.5e1.5e1.5%. The whole stabilization process took about 40 min in an air atmosphere. Subsequently, these oxidized fibers were subjected to a low-temperature carbonization step in an atmosphere of 99.999% nitrogen from 500 to 600
C under a stretching ratio of 1.5e1.5%. Finally, high-temperature carbonization was carried out in an atmosphere of 99.999% nitrogen from 900 to 1300
C at a 2% shrinkage. 2.4. Characterization Differential scanning calorimetry (DSC) and thermogravimetry (TG) characterization of PAN fibers (P0 and P1) were carried out on a

Fig. 1. (a) Digital images of the PAN fibers P0 and P1 by the 120
C treatment in air. (b) Proposed reaction mechanism of the air-mediated, thermo-stabilization of PAN fibers.

X. Chai et al. / Polymer 76 (2015) 131e139

Fig. 2. FTIR spectra of P0 and P1.

DSC Q200 (TA, USA) instrument and TGA Q50 thermal analyzer (TA, USA), respectively. The DSC analysis was performed at a heating rate of 10
C/min in the temperature range of 30e400
C in a nitrogen atmosphere (50 mL/min). The TG analysis was performed at a heating rate of 10
C/min in the temperature range of 30e800
C in a nitrogen atmosphere (50 mL/min). The dynamic mechanical analysis (DMA) was carried out with a Q 800 DMA apparatus (TA, USA) at a measurement frequency of 1 Hz and at a heating rate of 10
C/min from 50 to 280
C and then cool to ambient temperature slowly. Fourier transform infrared (FTIR) spectroscopy characterization was conducted by using an IRAffinity-1 FTIR spectrometer (Shimadzu, Japan) in 400e4000 cm1 range, 100 scans were collected at a resolution of 2 cm1. Density measurement was measured by the sink-float method. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALab 220I-XL instrument (Thermo, USA) with an Al Ka anode. X-ray diffraction

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(XRD) patterns were measured on a R-AXIS-RAPID diffractometer (Rigaku, Japan). The wavelength was 0.1540 nm. A bundle of fiber was arranged perpendicular to the incident ray. The value of interlayer spacing d was calculated by Bragg formula. The fullwidth at half maximum (FWHM) was calculated from diffraction peaks by LorentzianeGaussian fitting. The preferred orientation R of crystallites was measured by performing an azimuthal scan at the fixed Bragg position. The FWHM of the diffraction peak from the azimuthal scan was used to estimate the orientation degree of graphene planes [1,11,18]. Small angle X-ray scattering (SAXS) measurements were used to characterize microvoids in fibers. Length (L), average chord length (lp) in the cross section, the relative volume of microvoid (Vrel) and the parameter defining the preferred orientation (Beq) of microvoids in the fibers were obtained by calculation. SAXS experiments were performed at the beam line BL16B1 at Shanghai synchrotron radiation facility in shanghai, China, with the energy of 10 keV. The wavelength was 0.124 nm, with a sample-to-detector distance of 5200 mm. All XRD and SAXS patterns were corrected by the intensity of incident light, the thickness of fibers and background scattering from air. The mechanical properties of the CF samples were measured in accordance with ASTM D4018-99. The CF specimens were impregnated in an acetone solution of epoxy resin (the mass ratio of epoxy resin, acetone and triethylenetetramine was 10:20:1) for 2e3 min, then fixed on a stainless steel frame. The treated samples were cured in an oven at 120
C for 1 h. The mechanical properties of the cured CF samples were tested with a tensile-testing machine (Xin San Si CMT4304, China) at a crosshead speed of 10 mm/min. The gage length is 150 mm. In each case, ten samples were tested, and the average value was obtained. 3. Results and discussion As proof-of-concept, a bundle of regular PAN precursor fibers

Fig. 3. XPS spectra of P0 and P1. (a) C1s; (b) O1s; (c) N1s; (d) The de-convolutions of N1s spectra of fibers P1 using the LorentzianeGaussian curve fitting method.

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(4 kg) was used in our study. In a typical experiment, the PAN precursor fibers (P0) were stored in an oven in air at 120
C for 30 days to conduct the low-temperature thermal stabilization process. Shrinkage is controlled by the fiber itself as it is wound and spool. As shown in Fig. 1a, an obvious and uniform change of the color was observed from white to yellow. Meanwhile, the as-prepared P1 fibers become insoluble in dimethyl sulfoxide (DMSO), while the original P0 fibers are soluble. Such change of the physical properties indicates that some structural changes in the PAN fibers occur during the thermal stabilization process. We proposed such a physical change and structural evolution were attributed to the cyclization and oxidation of PAN molecules chain as shown in Fig. 1b. It should be noted that we did not observe any significant change of the color of the PAN fibers when the incubation temperature was below 120
C even for more than 2 months, indicating no significant chemical reactions occur. For the temperatures above 130
C, we observed a similar color evolution of the fibers in a few days, however, an obvious shrinkage was observed simultaneously. It is believed that such a shrinkage would potentially generate some negative impacts on the uniformity as well as the performance of the resultant fibers [14]. The structural changes of PAN fibers P1 and P0 were first observed by FTIR spectroscopy. As shown in Fig. 2, importantly, two new bands at 1618 cm1 and 3300 cm1 were observed, attributing to the stretching vibration of C]N and NeH for the P1 fibers [25], respectively. This is probably due to the absence of eOH groups in IA unit (as shown in Fig. 1b), which produces extra electrons to make nucleophilic attack on the carbon atom of an adjacent nitrile group and initiate the intramolecular cyclization or intermolecular crosslinking reactions [26]. The nitrile band at 2240 cm1 doesn't decrease obviously, implying that the cyclization or crosslinking reactions progress only to a small extent [26,27]. In addition, the intensity of the peak at 1700 cm1 assigned to the C]O stretching vibration dramatically increases for P1 fibers, owing to the continuous oxygen uptake reactions in the presence of air [28]. It implies that cyclization and oxidation have been taken place during the thermal treatment of 120
C. To understand the change of chemical structures, XPS was then employed to characterize fibers P0 and P1. C1s, N1s, and O1s spectra of P0 and P1 are shown in Fig. 3. Peaks of the C1s, N1s, and O1s spectra become broad after the heat treatment, indicating the formation of new chemical bonds during the air-mediated thermal stabilization process [29]. As shown in Fig. 3c, the N1s spectrum of fibers P0 has a main peak corresponding to the C^N band at 399.6 eV. By the LorentzianeGaussian curve fitting method using the professional software of XPS speak 4.1 [30], the N1s peak of P1 in the XPS spectra was further de-convoluted into three peaks at 398.9, 399.6 and 400.7 eV, corresponding to C]N, C^N and NeH bands, respectively. This finding verifies the proposed structures as shown in Fig. 1b. We then characterized fibers P0 and P1 by using the dynamic mechanical analysis (DMA). Plots of Tan d versus temperatures were obtained before and after annealing. As shown in Fig. 4a, two main transitions in the range of 90e100
C and 140e170
C, namely aI and aII relaxations, were observed yet with some obvious differences for P0 and P1 fibers. So far, interpretation of two mechanical-loss peaks is still controversial. For instance, Okajima et al. [32,33] proposed that the higher-temperature transition aI and lower-temperature transition aII are attributed to the mobility in the ordered phase and the relaxation process in the amorphous phase. On the contrary, Minami et al. [34e36] proposed that aI is attributed to the mobility of structures in the glassy amorphous region, while aII is related to the process in the crystalline domains. Interestingly, herein, the relative intensity of the peak of the high temperature transition dramatically decreases after the thermal

Fig. 4. The loss tangent of P0 and P1 before (a) and after annealing (b).

Fig. 5. (a) DSC and (b) TG curves of P0 and P1, where the heating rate was 10
C/min in N2.

X. Chai et al. / Polymer 76 (2015) 131e139

treatment. The relative ratio of aI to aII becomes much smaller as compared with the control P0 fibers. This can be explained by considering the effect of the cyclization reaction during the low temperature thermal treatment. As a result, the chemical structure as well as the consequent mechanical-loss transition changes. Moreover, a new transition peak a0 III at a much higher temperature region was observed for P1 fiber, which is probably due to the relaxation of the partially-cyclized structure elements in the amorphous domains. Most interestingly, only a high-temperature transition peak was observed after the annealing for both PAN fibers (Fig. 5b). Notably, the molecular structures of PAN would undergo further cyclization reactions at the temperature above 280
C, forming more cyclized structures [7]. Herein, the partiallycyclized structures in P1 fibers had turned into closely-cyclized structures upon the first heating cycle during the DMA analysis. In this case, the a0 III transition corresponding to the closely-cyclized structures of the annealed P1 fibers should occur at a much higher temperature. Such temperature, however, should be far beyond the temperature range in as-mentioned DMA analysis, leading to the disappearance of the a0 III transition peak. In addition, the remaining transition peak for P1 fiber becomes significantly broader than that of P0, owing to more complex structural elements in the P1 fibers

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Fig. 7. FTIR spectra of P0 and P1 stabilized at 200
C: (a) P0-200, (b) P1-200.

than those in P0. These observations also support our assumption that there are some partially-cyclized structure elements generated during the low-temperature thermal treatment. Importantly, the structural change of the PAN fibers during the thermal stabilization process is further confirmed by differential scanning calorimetry (DSC) characterization. DSC curves for heating fibers P0 and P1 in N2 at 10
C/min from 30 to 400
C were obtained. It is noted that the exothermic peak at temperatures around 250e300
C is attributed to cyclization reactions since no oxidative reactions occurred under the N2 atmosphere. As shown in Fig. 5a, there is only one sharp exothermic peak around 270
C for P0 fibers, causing a large amount of heat to be released at the same time, which results in the breakage of molecular chains and the defects in resultant carbon fibers [27]. In contrast to that of P0 fibers, there are two exothermic peaks of fibers P1. It is believed that reactions in the crystalline domain of the polymer lead to peak II in the DSC curves [14]. The exothermic peak I around 260
C, however, is attributed to the initiation of cyclization reactions in the amorphous domain. In this case, nitrile groups were partially cyclized during the thermo-treatment at 120
C, preferably in the amorphous domain. The presence of cyclized structures in the amorphous domain causes the peak shifting to a lower temperature, leading to a broad exothermic peak. Such a broad exothermic peak demonstrates significant impacts on the following stabilization and carbonization of PAN fibers at high temperatures. More importantly, higher char yield was achieved by the lowtemperature thermal stabilization. Fig. 5b shows the TGA curves of fibers P0 and P1 heated at 10
C/min from 30 to 800
C under N2 atmosphere. The curves can be roughly divided into three regions according to the rate of weight loss. The first region is up to about 270
C, where the weight loss is very small. In combination with the DSC results, it is known that there is only cyclization occurring in this step, so this reaction does not cause any weight loss theoretically. The second region is up to about 450
C. The rate of weight loss becomes relatively rapid, which is mainly due to the release of

Table 1 The extent of stabilization Es of P0 and P1 stabilized at different temperatures.


Fig. 6. FTIR spectra of P0 and P1 stabilized at different temperatures respectively.

Temperature ( C)

200

230

250

270

Es (P0) Es (P1)

1.01 0.76

1.98 1.62

2.96 2.98

3.54 3.78

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Table 2 The tensile properties of P0 and P1 with the increasing of temperature.


Temperature ( C)

200 230 250 270 500 600 1300

P0

P1

Tensile strength (GPa)

Tensile modulus (GPa)

Elongation %

Tensile strength (GPa)

Tensile modulus (GPa)

Elongation %

0.53 0.44 0.32 0.28 0.78 0.91 3.1

12.6 14.3 16.4 18.5 25.3 38.7 205.2

11.2 11.4 11.8 12.1 4.34 3.74 1.53

0.59 0.5 0.42 0.36 0.91 1.1 3.6

12.3 14.1 16.9 20.2 28.3 43.5 219.8

11 11.4 11.9 12.4 4.52 3.81 1.65

volatile particles formed by random degradation of polymer chains. It is apparent that the P1 fibers show a much lower decreasing rate in weight loss as compared to the precursor fibers P0. In the final region, the rate of weight loss is relatively steady. Additionally, the char yield at 800
C [20] can also be seen in Fig. 5b. The char yield is 52% for P1 and 39% P0, suggesting an increase of 25%. This is due to the cyclization and oxidization reactions during the treatment of 120
C, which is essential to protect the macromolecular backbone from decomposition. Meanwhile, the formed cyclic structure is thermally-stable and might be able to withstand high temperatures during pyrolysis process [24]. In order to study the difference in the chemical and structure changes between this method and the early stages of the conventional stabilization process, the stabilization process of fibers P0 and P1 were studied by FT-IR. Fig. 6 shows FT-IR spectra of P0 and P1 stabilized from 200 to 270
C. Of particular interests are the bands at about 2240 cm1 assigned to stretching vibration of CN, the band at about 1730 cm1 assigned to C]O stretching vibration of IA comonomer, and the band at 1595 cm1 corresponding to combination vibrations of C]C and C]N stretching [25]. The assignments of other absorption bands are as follows [26,27]: 2925 cm1 (nCeH in CH2), 1450 cm1 (dCeH in CH2), 1360 cm1 (dCeH in CH) and 802 cm1 (dCeH in C]CeH). As shown in Fig. 6, the intensity of the band at 2240 cm1 decreases gradually with the increase of stabilization temperature for P0 and P1, while the intensity of the band at 1595 cm1 increases significantly, indicating the cyclization reactions which turn eCNe groups into eC]Ne structure have occurred. The intensities of bands at 2925 cm1 and 1450 cm1 decrease gradually with the increasing of stabilization temperature, indicating the dehydrogenation reactions which turn eCH2eCHe structure into eCH]Ce structure have occurred [27,28]. From the analysis of Fig. 6, it can be seen clearly the difference in the structure changes at early stages of the stabilization (200
C). Fig. 7 shows FTIR spectra of P0 and P1 stabilized at 200
C, respectively. We can see that the bands at 1595 cm1 and 802 cm1 of P0 corresponding to cyclization and dehydrogenation reactions are stronger than that of P1, indicating that cyclization and dehydrogenation reactions are more pronounced in P0 than P1 at 200
C. It is the reason that cyclization reactions proceed in the amorphous domain in the first place, and then spread to crystalline domain. The amorphous domain in P1 has partially cyclized during the heattreatment of 120
C, and the cyclized structure is easier to be oxidized, which imparts a negative effect on initiation of cyclization reaction in the crystalline zone. As a result, we can set a relative high initial temperature of stabilization, decreasing zone of stabilization and accelerating stabilization rate. In order to compare the degree of stabilization between P0 and P1, we define a parameter (Es) to evaluate the extent of stabilization [26,27,31].

where A1595 is the absorbance of C]N and C]C band and A2240 is the absorbance of CN. The extent of stabilization of P0 and P1 stabilized at different temperatures is listed in Table 1. The Es values increase with the increasing of stabilization temperature for both P0 and P1, indicating that high temperature is beneficial to increase the extent of stabilization. By contrast, The Es values of P1 are lower than P0 below 250
C, whereas higher values for P1 than that of P0 are observed when the temperature exceeds 250
C. It is further indicated that pretreatment of 120
C is beneficial to improve the stabilization of PAN fibers, further reducing pyrolysis under higher temperature at carbonization process. To demonstrate the impacts of low-temperature thermal stabilization on the mechanical properties, we conducted the stabilization and carbonization of P0 and P1 following typical conditions and measured the tensile properties at different stages. As shown in Table 2 and Fig. 8. The tensile strength of P0 and P1 fibers decreases along with the increase of temperatures in the stabilization process. Afterwards, The tensile strength increases abruptly with the increase of carbonization temperatures. After the treatment at 1300
C, the tensile strength of P1 carbon fibers increased to 3.6 GPa, while that of P0 carbon fibers is 3.1 GPa, suggesting an increase of 16%. Such an increment in the tensile strength of P1 carbon fibers is attributed to the higher preferred orientation and less microvoid defects as revealed by small-angle X-ray scattering characterization, which is discussed in the following section. The evolutions of microvoids in PAN fibers P0 and P1 during stabilization and carbonization were studied by small-angle X-ray scattering (SAXS). The SAXS two-dimensional scattering patterns of preoxidized (270
C) and carbonized (1300
C) fibers of P0 and P1 are shown in Fig. 9. The parameters of microvoids, such as length L, average chord length lp in the cross section, the relative volume of microvoid Vrel and the orientation angle Beq defining the preferred orientation of the microvoids in the fibers can be obtained from SAXS scattering patterns by the method developed by Ruland

Es ¼ A1595 =A2240 Fig. 8. Tensile strength of P0 and P1 during the stabilization and carbonization process.

X. Chai et al. / Polymer 76 (2015) 131e139

137

microvoids lp can be obtained via Eqs. (2) and (3) by fitting the theoretical to experimental sI(s, p/2) curves.

. s2 B2p=2 ðsÞ ¼ 1 L2 þ s2 B2eq

(1)

L2 2 Iðs; p=2Þfnr2m qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2ffijFD j ðsÞ 1 þ sLBeq

(2)

Fig. 9. Two-dimensional scattering patterns obtained at the center of the PAN, preoxidized (270
C) and carbonized (1300
C) fibers: (a) P0 and (b) P1 fibers.

Table 3 The parameters of microviods of P0 and P1.

P0 P1

L(nm)

Beq (
)

lp(nm)

Vrel

18.9 17.5

14.2 14.9

6.3 6.1

1 0.99

[37,38]. Ruland's method has been successfully used to analyze microvoids in carbon fibers [39e41] and preoxidized PAN fibers [42,43]. According to the reported calculation method by Ruland [40], the length of the microvoids L, the orientation angle Beq defining the preferred orientation of the microvoids can be obtained from the intercept and slope of s2~s2B2p/2(s) plot in Eq. (1). Where Bp/2(s) is the integration breadth along azimuthal scan. Inserting the values of L and Beq obtained from Eq. (1) into Eq. (2), the number of microvoids n and the average chord length in the cross-section of

l2p

 2 i3 2 1 þ 2plp s =

jFD j2 ðsÞfh

(3)

The relative volume of microvoids Vrel can be evaluated approximately by VfnLl2p (the relative total volume Vrel is the ratio of the microvoids volume content in the samples to the precursor fibers P0). The parameters of microviods in P0 and P1 are shown in Table 3. We can see that heating at 120
C for 30 days does not have much effect on microvoids. However, when the heating temperature is beyond 200
C, it showed much more difference. The evolution of length L, orientation angle Beq, average chord length lp and relative volume Vrel during the stabilization and carbonization process of P0 and P1 are showed in Fig. 10. We can find that for both P0 and P1, the values of L, Beq, and lp increase continuously and reach a maximum at the temperature around 250
C during the process of stabilization. It is due to the imperfect packing of the chains during the cyclization, which results in a sequential increase of L and Vrel. The temperature around 250
C is a key point for the evolution of microvoids in fibers during stabilization. In addition, the values of L, Beq, lp and Vrel start to decrease after 270
C, which may be due to the intermolecular crosslinking leads to rearrangement and compactness of the chains. This results in less microvoid defects and higher tensile strength in the carbonized fibers. In addition, when the temperature exceeds 600
C, L, Beq, lp and

Fig. 10. The evolution of length L, orientation angle Beq, average chord length lp and relative microvoid volume Vrel during the stabilization and carbonization process of P0 and P1.

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Fig. 11. The two dimension XRD patterns of precursor fibers and preoxidized fibers (PF) and carbon fibers (CF).

Vrel of P1 are smaller than those of P0, indicating the size and volume of microvoids in P1 are smaller than that in P0. Meanwhile, the orientation of microvoids is improved, and the cyclized structures formed during the stabilization at 120
C restrains disorientation. It is reasonable to believe that the higher tensile strength of fibers P1 is attributed to less microvoid defects and higher preferred of microvoids produced during the stabilization and carbonization. The two dimension XRD patterns and intensity distribution of precursor fibers and preoxidized fibers (PF) and carbon fibers (CF) are shown in Figs. 11 and 12. It is showed that for both P0 and P1, the peak intensity of (100) crystalline plane gradually decreased and the peak intensity of (002) crystalline plane increased during the stabilization and carbonization. It is due to the evolution of linear PAN structure to cyclized and graphitized structure [44,45]. Comparing the profile between fibers P1 and P0, it is noted that the peak intensity of (100) crystal plane in P1 becomes stronger and sharper than that of P0. It is due to the further crystallization in PAN polymers during the thermo-treatment at 120
C. However, after stabilization, it is showed that the (100) diffraction peak represented linear structure of P1ePF disappeared while a weak peak of (100) present in P0ePF. It indicates that the extent of stabilization in P1ePF is higher than that of P0ePF. The preferred orientation R of crystallites is determined by FWHM of the diffraction profile from azimuthal scan on (100)

crystal plane of precursor fibers P0 and P1, and (002) crystal plane of preoxidized fibers (PF) and carbon fibers (CF), as shown in Fig. 13. The results are listed in Table 4. The precursor and preoxidized fibers of P1 and P0 showed the same density, while the density of P1eCF is slightly higher than that of P0eCF. It is due to the higher char yield and less microvoids defect in fibers of P1eCF. It is much more effective to reduce the negative effects of disorientation on the cyclization by releasing residual stress prior to the stabilization rather than during stabilization [46]. During the thermo-treatment of 120
C, crystallization of PAN chain and the cyclized and oxidized structure formed during 120
C will result in recovery of PAN chain. In addition, the residual stress that generated during the spinning process of PAN precursor fibers will be released when fibers are heated up to a temperature higher than their glass transition temperature, which gives rise to the disorientation of PAN chains. As a result, the preferred orientation R of crystallites of fibers P1 is smaller than that of P0. When they are stabilized at higher temperature high than 200
C, the further cyclization and crosslinking reactions destroy the orientation of PAN chain. Comparing the data of P1eCF to P0eCF, it is obvious that the preferred orientation R of crystallites of P1eCF is larger than that of P0eCF, while the interlayer spacing d of P1eCF is smaller than that of P0eCF. It is reasonable to believe that the cyclized and oxidized structure formed during the thermo-treatment at 120
C

Fig. 12. XRD intensity distribution of precursor fibers and preoxidized fibers (PF) and carbon fibers (CF).

X. Chai et al. / Polymer 76 (2015) 131e139

139

Fig. 13. Azimuthal scan on (100) crystal plane of precursor fibers. and (002) crystal plane of preoxidized fibers (PF) and carbon fibers (CF).

References

Table 4 Densities and crystallite parameters of different fibers. Sample

Density (g/cm3)

d (nm)

FWHM (
)

R (%)

P0 P0ePF P0eCF P1 P1ePF P1eCF

1.16 1.38 1.80 1.16 1.38 1.81

0.5241 0.5365 0.3568 0.5258 0.5358 0.3552

20.69 41.86 38.38 25.90 41.20 37.50

88.5 76.7 78.2 85.6 77.1 78.7

restricted the disorientation of crystallites during stabilization and carbonization processes. Hence, the higher preferred orientation R of crystallites results in better mechanical properties.

4. Conclusion In conclusion, we have reported a low-temperature thermal stabilization process for assisting the preparation of PAN-based carbon fibers with improved mechanical properties. Significantly, this study for the first time reports the initial cyclization of the nitrile groups and oxidation of PAN by thermal stabilization in air at 120
C, a much lower temperature compared to that involved in the traditional stabilization process. Owing to the partial cyclization and oxidation reactions at low temperatures, the disorientation of PAN molecules is restricted to some extent during stabilization and carbonization processes. Hence, preferred orientation of crystallites and char yield of the resulted carbon fibers increased. As a result, the tensile strength of carbon fibers from the low temperature thermal stabilized fibers increased from 3.1 GPa to 3.6 GPa. This finding not only leads to a better understanding of the cyclization and oxidation reactions in PAN fibers at low temperatures and their impacts on the mechanical properties of the resultant carbon fibers, but also provides a low-cost path for the production of carbon fibers without using any complicated and expensive facilities. Despite a relative long incubation period (e.g. 30 days) required in the current proof-of-concept demonstration, we believe that such a low-temperature thermal stabilization process can be directly applied to produce carbon fibers with improved property at large scale.

Acknowledgments This work was supported by grants from the National Basic Research Programs (973Programs) (Grant Nos. 2011CB605603 and 2011CB605605).

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