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Microwave Pre-oxidation for Polyacrylonitrile Precursor Coated with Nano-Carbon Black.

INTRODUCTION

Polyacrylonitrile (PAN) fiber has been considered as the predominant precursor for the production of high-performance carbon fibers (CFs) due to its ready availability, relatively low cost [1], high melting point, high carbon yield [2], and fast rate in pyrolysis without changing its basic structure [3]. Generally, the pre-oxidation as one of the main manufacturing stages for PAN-based CFs has a great effect on the final properties of CFs [4], and the PAN precursor fibers which pre-oxidized at the best conditions can produce a higher modulus CF [5]. The pre-oxidation is carried out usually at the temperature range of 180[degrees]C-300[degrees]C [6], although other researchers suggested that the optimum preoxidation requires higher than 300[degrees] C to insure better pre-oxidized structure [7]. The most important reaction involved during the pre-oxidation process is the cyclization, which is generated by nitrile group polymerization to form ladder polymer structure [8].

The relatively higher production cost of CF has limited its usage in industrial sectors [9]. In addition, the properties of CFs (in particular tensile strength) have remained only a fraction of the theoretical predictions. Various techniques, such as the application of magnetic field [10], and gamma irradiation [11] have been used to improve the strength of CFs [12], Kim et al. [13] heated the PAN precursor fibers under an atmospheric pressure plasma for 30 min, followed by the carbonization process, thus slightly increased the strength. Naskar et al. [14] exposed the PAN fibers to ultraviolet (UV) radiation for a few seconds prior to pre-oxidizing it at 320[degrees]C and resulted in better crosslinking and cyclization.

Nowadays microwave heating has become applicable in different industrial sectors instead of conventional heating because it is an effective way to accelerate the heating process. Microwave uses an electromagnetic energy which is composed of electric and magnetic fields in the frequency range of 300 MHz-300 GHz and can be used successfully to heat many dielectric materials. The main operating frequencies are 915 MHz and in the majority 2.450 GHz [15,16]. Microwave field infiltrates the material and works as an instant heating source at each point of the structure [17]. It can reduce processing times [18], increase product yields, and enhance product purities over that reported for conventionally processed experiments [19]. According to Kim et al. [20,21], carbonized CFs with microwave plasma system led to better mechanical properties and surface roughness, which can enhance the mechanical adhesion between resin and CFs and increase the mechanical properties of the CF-reinforced polymers.

The response of a material to microwave heating is defined by its dielectric loss tangent: tan [delta] = [epsilon]"/[epsilon]'. The dielectric constant ([epsilon]') determines how much of the incident energy is reflected and absorbed (i.e., the ability to store electrical energy in the material), while the dielectric loss factor ([epsilon]") measures the dissipation of electric energy in the form of heat within the material (the efficiency of the material in converting the electromagnetic energy into heat) [22]. When a microwave field is radiated into dielectric material whose charged particles cannot move freely, the electric charges produce dielectric polarization due to the displacement of charges from their average equilibrium position, and thus cause the material temperature to be raised. The dielectric polarization has two main forms involved in microwave heating; either dipolar polarization by reorientation of molecules which have permanent dipoles or interfacial polarization by accumulation of relatively mobile charges at grain/phase boundaries or surfaces. In case of carbon-based solids the interaction between microwaves and electrons is important, that the free electrons enable the material to absorb microwave energy and to be heated effectively by microwaves. The microwave heating of such materials is mainly explained by the interfacial polarization (so called Maxwell-Wagner-Sillars [MWS] polarization) [22,23]. Recently, nanostructure microwave absorber materials have attracted considerable attention because of their magical properties [24]. The low-cost carbon black (CB) nanoparticles consist of 97%-99% elemental carbon [25], which have good chemical stability, heat resistance, and electrical conductivity [26], it is widely used in coatings [27], reinforcing filler [28], and as an adsorbent material with dielectric loss tangent of 0.35-0.83 [22]. To the best of our knowledge there is no study relating to the use of CB in the PAN pre-oxidation via microwave heating.

The poor absorbability of PAN precursor to microwaves was considered as a limiting factor to stabilize it through microwave heating. The present study seeks to coat the PAN precursor with CB to improve their microwave absorbability toward conducting the pre-oxidation of the coated fibers (CB-PAN) through a continuous microwave oven which was designed especially for this purpose. In order to investigate the feasibility of the microwave preoxidation usage, the characterization of structure and properties of pre-oxidized CB-PAN fibers were carried out.

EXPERIMENTAL

Materials

Poly(acrylonitrile-co-itaconic acid-co-methyl methacrylate) (PAN-co-IA-cO-MMA) precursor fibers (98 wt% PAN, [M.sub.w]: 180,000 g/mol, 6 K filaments) were provided by Jilin Chemical Fiber Group Co., Ltd. CB (99.5%, 30 nm) was purchased from Shanghai Macklin Biochemical Co., Ltd. 3-Aminopropyltriethoxysilane (3APTES, 98%) used as the binder was supplied by Aladdin Industrial Corporation and Ethanol (99.80%) was supplied by Shanghai Yunli Economic and Trade Co., Ltd.

Preparation of Coatings. The coating bath suspension was prepared as follows: 5 mg/mL CB (30 nm) was added into ethanol/ deionized water (volume ratio = 90:10) mixture and then maintained by using bath ultrasonic device for 2 h at room temperature. To obtain coating layers at low temperature, a small amount (l%-2%) of a hydrolyzed binder (3APTES) was added drop wise under gentle stirring for 30 min. The cleaned PAN precursor fibers were dipped in the CB/binder suspension for three times. To ensure better exchange at every time the fibers were allowed to rest in the coating bath for 10 min and left out to dry for another 10 min. Finally, the CB-coated PAN (CB-PAN) fibers were rinsed out with water to remove the excess amount of CB, and then dried at room temperature for 24 h.

Pre-oxidation Process. The CB-PAN fibers were pre-oxidized under a continuous microwave furnace equipped with a tubular glass with gradient temperature between 100 and 170[degrees]C for different time. The system was divided into six heating zones operating at 2.45 GHz frequency. The furnace was equipped with a thermocouple to determine the inner temperature profile and an individual controller to adjust the microwave power and subsequently the temperature during the pre-oxidation process. Each zone has four keys and runs at kW power.

Measurements

Fourier Transfer Infrared Spectroscopy. Nicolet 8700 (USA) FTIR spectrometer was used to obtain Fourier transfer infrared (FTIR) spectra of pre-oxidized fibers. Analyses were performed by using KBr method over a spectral range of 4000-500 [cm.sup.-1].

Elemental Analysis. Vario EL III elemental analyzer (Germany) was employed to measure the element content in the pre-oxidized fibers. About 3 mg of dried samples ([less than or equal to]2 mm length) were used and the atomic percentage of carbon, hydrogen, nitrogen, and silicon were determined.

X-ray Diffraction Analysis. A Rigaku D/Max-2550 PC X-ray diffractometer (Rigaku Co., Japan) with Ni-filtered Cu K[alpha] radiation was used to measure the crystalline structure of pre-oxidized fibers with an accelerated voltage of 40 kV and a current of 200 mA, a scanning rate of 5[degrees]/min, and a scanning step of 0.02[degrees]. The raw data were plotted as received. The pre-oxidation extent Al, the crystallite size [L.sub.c] and planar spacing d of pre-oxidized fibers was calculated by aromatization index (AI) formula (Eq. 1), Scherrer formula (Eq. 2), and Bragg equation (Eq. 3) as follows:

AI = [I.sub.A]/[I.sub.A] + [I.sub.P] x 100% (1)

[L.sub.c] = K[lambda]/B cos[theta] (2)

n[lambda] = 2d sin[theta] (3)

where [I.sub.A] is the diffraction intensity of the aromatic structure around 2[theta] = 25.5[degrees], [I.sub.P] is the diffraction intensity of the peak around 2[theta] = 17[degrees], [lambda] = 0.154 nm is the wavelength of Cu K[alpha] radiation, [theta] is the Bragg angle, B is the full width at half the maximum intensity (FWHM), K is a constant, assigned as 0.89 and n = 1 is the first order reflection.

The degree of crystallinity ([X.sub.c]) of the fibers was determined using the Hinrichen's method [29]:

[X.sub.c] = [[I.sub.c]/[I.sub.c] + [I.sub.a]] x 100% (4)

where [I.sub.c] is the integral of the peak corresponding to the crystalline phase of the polymer. [I.sub.a] is the integral of the peak corresponding to the amorphous phase of the polymer. The X-ray diffraction (XRD) data were fitted using the peak fit v4.12 software and then transferred to origin pro 8.5 software in order to obtain the peak parameters.

Thermal Analysis. Gravimetric thermal analysis (TGA) was carried out using a thermal analysis system discovery TGA (Q5000IR) instrument. About 0.3 mg of each sample was heated in nitrogen at a heating rate of 10 [degrees]C/min from 30[degrees]C to 600[degrees]C in a platinum crucible with nitrogen flow rate of 50 mL/min.

Differential scanning calorimetry (DSC) analysis was performed via high-temperature TA differential scanning calorimeter (Q20) instrument. About 0.5 mg of each sample was heated in nitrogen at 10 [degrees]C/min from 30[degrees]C to 450[degrees]C with nitrogen flow rate of 50 mL/min.

Mechanical Properties. A XQ-1 computerized mechanical tester was used to perform the mechanical properties test for the pre-oxidized fibers. The gauge length of 20 mm and crosshead speed of 10 mm/min were applied. At least 10 tests were carried out for each sample to calculate the average tensile strength and elongation at break.

Scanning Electron Microscopy Observation. The morphology of pre-oxidized fibers was studied using field emission scanning electron microscopy (SEM, JSM-5600LV, Japan). The accelerating voltage and magnifications used during the collection of the images were 1 kV and 5-10 pm, respectively. The surface of tested fibers was coated with gold to improve the quality of images.

RESULTS AND DISCUSSION

Changes of Chemical Structure

FTIR spectra of the PAN precursor and pre-oxidized samples are shown in Fig. 1 A. A strong absorption band of PAN precursor appeared at around 2240 [cm.sup.-1] which was assigned to nitrile groups C[equivalent to]N stretching vibration. The bands in the regions 2939-2870, 1454, and 1360 [cm.sup.-1] were assigned to the aliphatic C--H group vibrations of different modes in C[H.sub.3], C[H.sub.2], and CH. The overlapped bands in the region of 1270-1220 [cm.sub.-1] were assigned to the C--O group vibrations. The band at 1728 [cm.sup.-1] was attributed to the C=0 stretching vibration. The weak band at 1070 [cm.sup.-1] may be attributed to the stretching vibration of C--CN [30], The broad peak around 1600 [cm.sup.-1] was assigned to conjugated C=C [31], [(C=N).sub.n] structures [32], or to a combination of the two peaks [33-35]. The intensities of these peaks were used to calculate an internal indication of extent of reaction (EOR) which is defined as follows [36]

EOR = [I.sub.1600]/([I.sub.cn] + [I.sub.1600]) (5)

It could been seen obviously from Fig. 1A that the intensity of the bands at 2243 [cm.sup.-1] decreased with the increasing of preoxidation time, while the overlap bands around 1631 [cm.sup.1] increased in intensity and shifted to 1600 [cm.sup.1], which was due to the generation of cyclic C=N, conjugated C=C, and N--H groups, originated from cyclization and dehydrogenation as a part of the oxidation reaction [37], The dehydrogenation also reduced the intensities of bands at 2923 [cm.sup.1] and 2855 [cm.sup.1] which were assigned to C--H group. The intensity of the out-of-plane bending of =C--H at 802 [cm.sup.1] slightly increased and shifted to 806 [cm.sup.1]. The carbonyl (C=0) stretching band located at 1728 [cm.sup.1] was weakened and shifted to a lower wavenumber, and it was only observed as a shoulder on the main conjugation absorption with the increase of pre-oxidation time, suggesting that the carbonyl group became a part of the conjugated structure [1,30]. Moreover, the absorption near 1260 [cm.sup.-1] for various C--O--C stretches can be attributed to crosslinking bonds [37]. These showed that various and complicated chemical structures were produced by the pre-oxidation process.

From Fig. IB, the EOR values started to increase gradually with the increase of heating time and reached a higher value when the pre-oxidation time was 200 min. From the above results we can concluded that FTIR analysis and the EOR values verified the occurrence of cyclization and crosslinking reactions during the microwave heating.

The chemical reaction also caused a change of the elemental compositions, and the taking-up of oxygen atoms into the fiber structure could significantly increase the oxygen content of the preoxidized fibers [38]. The pre-oxidation treatment was carried out until the oxygen content of the pre-oxidized fibers reached about 5% to 15% by weight, preferably 8% to 12% by weight. Less than about 5% oxygen content would result in great weight losses during the carbonization stage due to the formation of excessive amount of gases from the incompletely pre-oxidized central section of the fiber thereby leading to low carbon yield. When the oxygen content was more than 15%, excessive degradation of fiber surface layers would result in poor quality CF [39]. The effects of heating time on the oxygen content of pre-oxidized fibers showed some changes as given in Table 1. The oxygen content was about 9% to 12% by weight for samples pre-oxidized for 60-170 min, but exceeded 12% when the pre-oxidation time reached 200 min, this may occur due to higher pre-oxidation time. This result can be explained by the fact that, during pre-oxidation, the oxygen initiated the cyclization reaction by forming activated centers, but inhibited the reaction by increasing the activation energy.

XRD Analysis

XRD patterns of pre-oxidized fibers are shown in Fig. 2. It was noticed that the crystallinity can be recognized by the sharp diffraction peak at approximately 2[theta] = 17[degrees] assigned to the (100) crystallite plane and the weak diffraction peak at approximately 2[theta] = 29[degrees] assigned to the (110) crystallite plane [40], After microwave heating, the position of both peaks did not change, indicating the remaining crystallite structure. With increasing microwave heating time, the intensities of both diffraction peaks decreased. As we see, the gradual decrease in the crystallinity with the progress of pre-oxidation implied that the influence of microwave heating on PAN molecules mainly took place in amorphous regions and the crystallite regions began to be decreased. A new peak appeared at about 2[theta] = 25.5[degrees] corresponding to the (002) plane of the pre-graphitic structure (the ladder polymer) indicating the formation of new aromatic structure. Some crystalline parameters are listed in Table 2. The value of [L.sub.c] decreased with the prolonged heating time, while the aromatization index increased. The reduction in crystallite size and intensity of the (100) peak suggested a direct loss in the degree of lateral order and an increase in the disordered phase. It is possible to suggest that the higher pre-oxidation time leads to a more decrystallized structure. The aromatization index also increased significantly, but the planner spacing has not shown any effective change because the cyclization reaction mainly occurred in the amorphous region and then gradually transformed into the crystalline region.

Thermal Behavior

As the pre-oxidation time increases, the CB-PAN fibers undergo a change in color from white through shades of yellow and brown to black pre-oxidized fiber. The mechanism for coloration is not fully understood, however it is believed that the appearance of black color was due to the degradation and formation of ladder ring structure [41]. Figure 3A shows DSC curves of pre-oxidized CB-PAN fibers for 60-200 min. The relative parameters of thermal behavior for pre-oxidized CB-PAN fibers are listed in Table 3. It could be seen that peak temperature sequence in terms of magnitude was [T.sub.p] (60 min) < [T.sub.p] (90 min) < [T.sub.p] (120 min) < [T.sub.p] (150 min) < [T.sub.p] (170 min) < [T.sub.p] (200 min) below 300 [degrees]C. The exothermic peak shifted toward higher temperatures, broadened and weakened in intensity with the prolonged pre-oxidation time. The reduction of the exothermic peak area was attributed to the cyclization of the nitrile (C[equivalent to]N) groups presented in PAN precursor. Compared with precursor fibers, the heat of reaction decreased from 503.50 to 346.40 J/g for samples pre-oxidized for 60 min, and 249.40 J/g for samples pre-oxidized for 200 min. The higher the pre-oxidation time the lower the heat of reaction.

PAN fibers tend to degrade below about 350[degrees]C because they undergo certain exothermic reactions, which lead to the formation of heat-resistant ladder structure. It could be possible to say that the reaction under this environment could be either aromatization of the structure followed by dehydrogenation or the innermolecular crosslinking reactions, because there is sufficient hydrogen still left in the fiber structure that can be removed at higher temperature. Based on this analysis it is suggested that the cyclization of PAN molecules was initiated and upgraded with the progress of the pre-oxidation time.

The exothermic heat was used to calculate the aromatization index as shown in Table 4 according to the following formula:

Aromatization Index(%) = [DELTA][H.sub.0] - [DELTA]h/[DELTA][H.sub.0] X 100 (6)

where [DELTA][H.sub.0] is the exothermic heat of PAN precursor, and [DELTA]H is the exothermic heat of pre-oxidized fibers. As listed in Table 4, the calculated values of aromatization index show a significant increase, which was in good agreement with the cyclization parameter determined by XRD analysis. Although XRD aromatization index give higher values than DSC aromatization index especially when we consider the pre-oxidation times of 170 and 200 min. Thus can be explained by that the DSC aromatization index depends on [DELTA]H differences which seem to be low, indicating that the heat was released much slower for the longer preoxidation time interpreted the stability of fibers which preoxidized for a longer time. The XRD aromatization index seems to be overestimating the amount of the ladder structure for a longer pre-oxidation time [42].

Figure 3B shows the TGA curves of pre-oxidized CB-PAN fibers. In this curve, the first stage did not show great weight loss.

A sharp and significant weight loss occurred above 300[degrees]C. During the second stage, the rate of weight loss became rapid, which was mainly attributed to the dehydrogenation reaction and the evolution of hydrogen cyanide (HCN). In the third stage, 450-600[degrees]C, the rate of weight loss was quite steady. The total weight loss for the PAN precursor sample was 48%, while the samples which pre-oxidized for 200 min exhibited a smallest weight loss, 33%, and better stability suggesting that the structure changed from linear to ladder form by cyclization reactions. During the TG experiments, the fragmentation of chains occurred leading to weight loss, and the oxygen uptake reaction took place [43]. The decreases in weight loss values with the increase of pre-oxidation time meant that the stability of pre-oxidized CB-PAN fibers was promoted significantly, which may have the probability to enhance the carbon yield and the performance of resulting CFs.

Mechanical Properties of Pre-oxidized Fibers

Figure 4 displays the impacts of low-temperature microwave pre-oxidation time on the tensile and elongation at break values. The tensile strength of pre-oxidized fibers decreased with the increase of pre-oxidation time. A tensile value of ~0.39 and ~0.27 GPa was observed for the pre-oxidized CB-PAN fibers for 60 and 200 min, respectively. It was suggested that the reduction of the tensile strength was due to the formation of ladder structure of pre-oxidized fibers as a result of orientation disruption and crystalline structure by pre-oxidation, and to the loss in inter-chain cohesive energy as a result of cyclization reaction. This suggestion was also confirmed by FTIR and TGA results. On the other hand, a similar decreasing behavior was also noticed with the elongation at break values. The results show a continuous decrease from an initial value of 18% to 11% for the sample preoxidized for 200 min. This reduction was attributed to intermolecular crosslinking reactions leading to the network formation.

Surface Structure and Cross-section Morphology

Figure 5A shows SEM images of PAN precursor and CB-PAN fibers. It is clearly evident in the SEM images that CB nanomaterial was sufficiently deposited onto the surface of PAN precursor, and the disordered grooves which appeared along the fibers' surface were attributed to the manufacturing technique which was the main reason for the instinct structure difference.

As shown in Fig. 5B no skin-core structures were visible in the pre-oxidized fibers indicating that the modification process had no obvious effects on the morphology of CB-PAN fibers during the pre-oxidation process. Similarly, no significant differences in the cross-section morphology could be observed between fibers that pre-oxidized for different time unless the pre-oxidation time reached 200 min, since the porosity and heterogeneity of preoxidized fibers slightly decreased resulting in more compact and homogeneous microstructures.

CONCLUSIONS

The aims of the present work are to verify the role of CB as microwave absorber in the PAN fiber pre-oxidation, as well as to study the structural evolution and the reaction pathway for CBPAN fibers during the microwave pre-oxidation. The cyclization and aromatization parameters, which calculated from XRD and DSC according to standard formulas, showed significant increase with the increase of heating time from 60 to 200 min. The tensile strength and elongation of the pre-oxidized fibers have also decreased according to the formation of ladder structure and network formation. In addition, the oxygen content for pre-oxidized samples is about 9%-12% by weight, suggesting that good quality CFs with high carbon yield will be obtained. It is believed that this kind of carbon-based material have great potential in promoting microwave pre-oxidation of CB-PAN fibers since initiating the cyclization reactions at lower temperature.
ABBREVIATIONS

3APTES        3-Aminopropyltriethoxysilane
AI            The pre-oxidation extent
CB            Carbon black
CB-PAN        Carbon black coated PAN fibers
CF            Carbon fiber
d             Planner spacing
DSC           Differential scanning calorimetry
EOR           Extent of reaction
FTIR           Fourier transfer infrared microscopy
[L.sub.c]     Crystallite size
PAN           Polyacrylonitrile
SEM           Scanning electron microscopy
TGA           Thermogravimetry
UV            Ultraviolet radiation
[X.sub.c]     Crystallinity
XRD           X-ray diffraction

LIST OF SYMBOLS

GHz             Microwave frequency
[I.sub.A]       Diffraction intensity of the aromatic structure
                around 2[theta] = 25.5[degrees],
[I.sub.P]       Diffraction intensity of the peak around 2[theta] =
                17[degrees],
[lambda] =
0.154 nm        Wavelength of Cu Ka radiation
[theta]         Bragg angle
B               Full width at half the maximum intensity (FWHM)
K               A constant, assigned as 0.89
n = 1           First order reflection
[I.sub.c]       Integral of the peak corresponding to the
                crystalline phase of the polymer
[I.sub.a]       Integral of the peak corresponding to the
                amorphous phase of the polymer
[DELTA]
[H.sub.0]       Exothermic heat of PAN precursor
[DELTAH         Exothermic heat of pre-oxidized fibers


ACKNOWLEDGMENT

Authors gratefully acknowledge the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials College of Materials Science and Engineering, Donghua University for providing research and analytical facilities.

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Tienah H. H. Elagib, Elwathig A. M. Hassan, Cheng Fan, Keqing Han, Muhuo Yu

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, China

Correspondence to: K. Han; e-mail: hankeqing@dhu.edu.cn and M. yu; e-mail: yumuhuo@dhu.edu.cn

Contract grant sponsor: Donghua University.

DOI 10.1002/pen.24943

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. (A) FTIR spectra of (a) PAN precursor and pre-oxidized CB-PAN fibers at a temperature of 100- 170[degrees]C for (b) 60 min, (c) 90 min, (d) 120 min, (e) 150 min, (f) 170 min, and (g) 200 min. (B) EOR obtained from spectra of preoxidized CB-PAN fibers. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. XRD patterns of (a) PAN precursor and pre-oxidized CB-PAN fibers at a temperature of 100-170[degrees]C for (b) 60 min, (c) 90 min, (d) 120 min, (e) 150 min, (f) 170 min, and (g) 200 min. [Color figure can be viewed at wileyonlmelibrary.com]

Caption: FIG. 3. (A) DSC curves, (B) TGA curves, in a nitrogen atmosphere of (a) PAN precursor and pre-oxidized CB-PAN fibers at a temperature of 100-170[degrees]C for, (b) 60 min, (c) 90 min, (d) 120 min, (e) 150 min, (f) 170 min, and (g) 200 min. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Tensile strength and elongation at break of pre-oxidized CB-PAN Fibers [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 5. (A) SEM images of (a) PAN precursor and (b) CB-coated PAN fibers. (B). SEM images of fracture morphologies of (a) PAN precursor and pre-oxidized CB-PAN fibers at a temperature of 100-170[degrees]C for, (b) 60 min, (c) 90 min, (d) 120 min, (e) 150 min, (f) 170 min, and (g) 200 min.
TABLE 1. Elemental content for pre-oxidized CB-PAN fibers.

                           Element content (%)

Pre-oxidation time (min)     C      H       N       O      Si

60                         59.40   5.56   20.48   11.21   3.35
90                         59.94   5.56   21.36   10.35   2.79
120                        58.90   5.77   21.28   10.10   3.95
150                        60.70   5.57   21.68   9.48    2.57
170                        60.20   5.33   21.29   10.34   2.84
200                        59.22   5.00   20.43   12.83   2.52

                           Molar

Pre-oxidation time (min)    C      H       N      O      Si

60                         4.95   5.56   1.46    0.70   0.24
90                         5.00   5.56   1.53    0.65   0.20
120                        4.91   5.77   1.52    0.63   0.28
150                        5.06   5.57   1.54    0.59   0.18
170                        5.02   5.33   1.52    0.65   0.20
200                        4.94   5.00   1.46    0.80   0.18

TABLE 2. Data obtained from XRD patterns of pre-oxidized
CB-PAN fibers.

Pre-oxidation   2[theta] = 17[degrees] peak      Crystallite stack
time (min)               FWHM (deg)             height [L.sub.c] (nm)

60                          1.77                        4.50
90                          1.83                        4.34
120                         2.10                        3.78
150                         2.20                        3.61
170                         2.93                        2.71
200                         3.02                        2.62

Pre-oxidation      Planner       Aromatization    Crystallinity
time (min)      spacing d (nm)     index (%)      ([X.sub.c] %)

60                    --               --               --
90                    --               --               --
120                  0.35            33.55            66.45
150                  0.33            59.02            40.06
170                  0.34            68.94            31.06
200                  0.36            70.26            29.74

TABLE 3. Some parameters derived from DSC data for
pre-oxidized CB-PAN fibers.


Pre-oxidation    [T.sub.p]([degrees]C)    [DELTA][H.sub.c]
time (min)            in [N.sub.2]        (J/g) in [N.sub.2]

0                        281.07                 503.50
60                       283.77                 346.40
90                       286.27                 297.30
120                      286.88                 293.10
150                      288.27                 282.00
170                      293.51                 257.67
200                      297.41                 249.40

TABLE 4. Comparison of indexes obtained from DSC and XRD.

Pre-oxidation   DSC aromatization    XRD aromatization
time (min)            index                index

60                    31.20                  --
90                    40.95                  --
120                   41.79                33.55
150                   43.99                59.02
170                   48.82                68.94
200                   50.47                70.26
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Author:Elagib, Tienah H.H.; Hassan, Elwathig A.M.; Fan, Cheng; Han, Keqing; Yu, Muhuo
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2019
Words:5318
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