The effect of doping graphene oxide on the structure and property of polyimide-based graphite fibre

State Key Laboratory of Chemical Resourc Fiber and Functional Polymers Ministry o Electrochemical Process and Technology Chemical Technology, Beijing, 100029, P. R Fax: +86-10-6443-4916; Tel: +86-139-1112-6 Key Laboratory of Carbon Materials, Institu Sciences, Taiyuan, 030001, P. R. China † Electronic supplementary information and GO/PAA bres. TG and DSC curves bres. TG and DSC curves of GO/PI com composite bres with different GO co composite graphite bre with 0.3 wt% temperature. Tensile modulus of graphi Thermal conductivity of GO/PI based c different GO content. TEM of the cros graphite ber with 2.0 wt% GO content. S Cite this: RSC Adv., 2017, 7, 56602


Introduction
With the increasing energy density of electronic devices, electronic devices are generating a large amount of heat when in use. If the heat does not conduct to the environment immediately, the temperature of electronic devices will increase, and this may even cause security issues. To conduct heat quickly, we need high thermal conductivity materials (TCMs). Metals have been used as traditional TCMs due to their high thermal conductivity; however, since they are heavy, inexible, and have relatively high thermal expansion coefficients, they are not ideal for cooling silicon-based electronic devices. 1 Carbon materials, due to their unique electron structures that have both sp 2 and sp 3 hybrid orbitals and unique phonon heat conduction mechanism, are very likely to produce high TCMs. 2 Among various carbon precursors, aromatic polyimide (PI) with different chemical structures can be easily graphitized at a relatively high temperature above 2200 C and changed into a highly crystalline graphite structure. 3,4 Graphite lms derived from PMDA-ODA-type PI lms can obtain an ultrahigh thermal conductivity of 1700 W m À1 K À1 in the plane direction. 5 Many researchers have studied the inuence of chemical structure, 6,7 drawing ratio, 8,9 and heat-treating 10 conditions on the performance of graphite lms. Various identication methods 11,12 have been used to illustrate the mechanism of pyrolysis strategy of polyimide lms, and the behaviour of polyimide lms has also been examined. 6,[13][14][15] PI bres are a class of high performance polymer bres, which are used mainly in heat resistance areas such as high temperature dust removal and ame-retardant protective clothing. Researchers have found that PI bres can be transformed into graphite bres with thermal conductivity higher than that of PAN-based carbon bres through simple heat treatment. 16, 17 Li Ang et al. found that both the degree of graphitization and thermal conductivity of PIbased graphite bres were improved with an increase in the heat treating temperature. 18,19 Zhang et al. studied the structure evolution of polyimide bres with different chemical structures during the carbonization process and discovered that the carbon yield and graphitization degree depended strongly on the chemical structure of the PI bre. 20 Graphene (GO)/polymer composites have been widely studied due to their excellent properties and high solubility of GO. 21,22 Graphite oxide (GO) 23 consists of a two-dimensional graphene sheets with a number of oxygen functional groups, such as hydroxyl, epoxide, and carbonyl groups, at its edges and basal planes. [23][24][25] Substituted with oxygen functional groups, GO dissolved well in water and other polar solvents such as DMF, DMAc etc. Via in situ polymerization, GO sheets can combine with the polymer chains due to van der Waals forces. 26 When the polymer chains attach to the large planer sheets of GO, the orientation of the composite material can be improved by optimizing the orientation of GO sheets. 1,24,27 Ting Huang et al. conducted detailed investigations to illustrate that an optimized orientation of GO in the composite material could enhance the performance and increase the thermal stability of the composite materials. 28 Chen minghua 29 found that the mechanical property, thermal conductivity, and electric conductivity clearly increased with the addition of a small amount of GO. Dong et al. 30 prepared GO/ PI composite bres with ODA-modied GO and found that the incorporation of graphene greatly improved the thermal stability and hydrophobic behaviour of the composite bre. Wang et al. 24 fabricated GO/PI composite lms via in situ polymerization and found that the Young's modulus of the GO/PI composite lms was 15 times greater and the tensile strength was 9 times greater than those of the pure PI lms.
Herein, we prepared graphite bres by doping different contents of GO in polyimide. Doping of GO improved the orientation of the GO/PI composite bre during the spinning process. The TG-DSC curves indicate that addition of GO can increase the thermal stability and reduce exothermic intensity during carbonization such that the preferred molecular orientation of carbon bres can be maintained. GO sheets acted as crystals during the carbonization and graphitization process and increased the degree of graphitization of the PI/GO-based graphite bre. Aer graphitization at 2800 C, the mechanical property and thermal conductivity improved. When 2.0 wt% GO was added, the thermal conductivity of the GO/PI composite graphite bres could reach 435 W m À1 K À1 , which was twice that of the pure PI-based graphite bre.
Preparation of the graphene oxide/polyamic acid (GO/PAA) solution As shown in Scheme 1, GO was prepared via the modied Hummers method and then dispersed in N 0 ,N-dimethylacetamide (DMAC) via ultrasonic dispersion. The GO/PAA solution was synthesized by dissolving 4,4-oxydianiline (4,4-ODA) in GO dispersion and then adding an equimolar amount of pyromellitic dianhydride (PMDA) gradually. The solution was then stirred at 0 C for 2 h under the atmosphere of pure nitrogen, having a concentration of 15 wt% in DMAc. The GO/PAA solution, as shown in Fig. 2(a), was degassed at a low temperature to remove the gas in the solution and obtain a homogeneous solution.
Preparation of the graphite oxide/polyimide (GO/PI) composite bre As shown in Scheme 2(a), the GO/PAA bres were fabricated by extruding the GO/PAA solution through the spinneret (single hole, 0.2 mm in diameter) using high-pressure nitrogen into a coagulation bath of 10% ethanol water. The temperature of the coagulation bath is in the range from 25 C to 40 C. The GO/PAA bres were obtained through a spinning roller. The pressure of nitrogen and the speed of the roller were controlled to decrease the diameter of the GO/PAA bres. The GO/PAA bres were tensioned and put in a vacuum environment to remove the solvent in the bres. The GO/PI bres were obtained by heating the GO/PAA bres at 100 C, 200 C, and 300 C, with each stage holding for one hour at a heating rate of 5 C per minute. 31,32 Carbonization and graphitization of the GO/PI composite bre The prepared GO/PI bres were put into corundum crucibles and covered by a graphite sheet to reduce the shrinkage of the bre. As shown in Scheme 2(b), the GO/PI bres were carbonized at 1200 C in high-purity nitrogen for 1 hour at a heating rate for 5 C per minute. Then, the GO/PI-based carbon bres were graphitized at 2800 C in high-purity argon for 1 hour at a heating rate of 10 C per minute to obtain GO/PI-based graphite bres.

Fourier transform infrared (FTIR) spectroscopy
Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet IS50 Series) was used to investigate the molecular structure and chemical bonding groups of the GO/PI composite bres with attenuated total reectance in the range from 4000 to 650 cm À1 .

Scanning electron microscopy (SEM)
An SEM (Zeiss Supra 55) operating at 20 kV was used to observe the surface and cross-section morphologies of the GO/PI-based graphite bre. The graphite bres were aligned along the same direction and embedded in epoxy resin. Then, the samples were put in liquid nitrogen and quickly broken into two pieces to investigate the morphology of the cross-section. All the samples were treated by gold-spray before observation.
Thermal gravimetric analysis (TG)/differential scanning calorimetry (DSC) The Thermal Sync Analyzer (NETZSCH STA 499C, German) was used to investigate the thermal stability of the GO/PAA bres and GO/PI bres. The TG/DSC curves were obtained by heating the samples from room temperature to 1000 C under a highpurity argon atmosphere at a heating rate of 5 C per minute. The calibrated baseline was obtained before sample measurement, and the samples were tested under the same conditions.

X-ray diffraction (XRD)
The crystalline structure of the graphite bre was examined by X-ray diffraction (XRD) using a diffractometer (X'Pert Pro, Analytical Co., Ltd). The diffracted intensity of Cu Ka radiation (k 0.1542 nm; 40 kV and 40 mA) was measured between 5 and 90 . We used 2q to represent the position of the (002) diffraction peak.
According to the Bragg's equation, we can obtain the average interlayer spacing of d 002 .
According to the Scherrer formula, the average size of the crystallite L a and the average crystallite thickness L c can be calculate as follows: B represents the full wave at half maximum (FWHM) of the d 002 peak.
According to the Mering-Maire empirical formula, the graphitization degree of graphite bres can be calculated as follows: Herein, the average interlayer spacing of the ideal graphite crystal is 0.3354 nm, and the average interlayer spacing of amorphous carbon is 0.344 nm.

Raman
A laser Raman spectrometer (LabRam HR800, Horiba Jobin Yvon Inc.) was used to investigate the structure of GO/PI-based graphite bres in the scanning range of 0-4000 cm À1 . The degree of graphitization can be represented by the value R, which is calculated by the formula R ¼ I D /I G . The smaller the R value, the higher the degree of graphitization.
Mechanical property of the graphite bre The mechanical property date of the composite carbon bres was obtained by the single bre testing method (YG001A, Taicang Instrument Corp. of China) according to the ASTM standard D3544-76 procedure. The average tensile strength and elongation at break were obtained by testing 50 single laments for each set.
Thermal conductivity of the graphite bre The Shangtai EC430 resistivity meter was used to measure the electrical resistivity of the composite carbon bre; for each sample, it was measured 30 times to obtain an average value. In our study, the thermal conductivity of GO/PI-based graphite bres was calculated according to the following empirical formula: 33 herein, l is the thermal conductivity (W m À1 K À1 ) of GFs, and r is the electrical conductivity (mU m À1 ) of GFs.

Results and discussion
Chemical structure and morphology of the PI bres and GO/PI composite bres As shown in Fig. 1, the two-adjacent peaks around 1718 cm À1 and 1776 cm À1 demonstrate the formation of an imide group and the ve-membered cyclic structure of polyimide, accompanied by the disappearance of amide groups at 1550 cm À1 . The sharp peak at 1373 cm À1 represents the stretching vibration peak of the C-N bond, and the sharp peak at 721 cm À1 represents the bending vibration of the C]O bond, which together indicate the formation of polyimide. The sharp peak at 1250 cm À1 shows the existence of an ether group in the molecular chains.
With an increase in the GO content in the composite bre, the intensity of the stretching vibration peak of OH group increased. The smaller peak at 1250 cm À1 represents the stretching vibration peak of the C-O-C bond, which indicates excellent exibility of the molecular chains. The peak at 1715 cm À1 represents the C-C]O bond, and the peak at 1500 cm À1 represents benzene. An increase in the intensity ratio of benzene bond and C-C]O bond indicates the reaction of GO and PI molecular chains. It can be inferred that the PI molecular chains are xed on the surface of the GO sheet due to p-p conjugation. The orientation of the GO sheet can be controlled to be along the bre axis during the spinning process. Moreover, the orientation of the PI molecular chains can be induced to be along the bre axis. Fig. 2 shows the surface and cross-sectional morphology of GO/PI-based graphite bres with different GO contents. As shown in the images (A and B), the surface of pure PI is quite smooth, and the cross-section images shows that it has several holes on it; this indicates the brittle fracture of the pure PIbased graphite bres. The images C and D show the surface and cross-sectional morphology of the 0.3 wt% GO/PI-based graphite bre. The magnied parts of the image C show several grooves on the surface of the bre along the bre axis; this indicates that addition of GO can improve the orientation of the graphite bre. From image D, it is clear that the surface of the 0.3 wt% GO/PI-based graphite bre is rough; thus, the mechanical quality of the composite bre is high. For the 2.0 wt% GO/PI composite graphite bres, the grooves on its surface become wider, and cross-sectional parts become rougher.
Thermal stability of the PI bre and GO/PI composite bre TG-DSC analysis was carried out under nitrogen for investigating the thermal stability of the PAA bres and GO/PAA bres. From room temperature to 150 C, both the PAA bres and GO/ PAA bres experienced a 5% weight loss, which was probably because of solvent evaporation inside the bres. From 150 C to 220 C, both the PAA bres and GO/PAA bres experienced an obvious weight loss of about 20-30%, which indicates that the PAA bres change into aromatic PI bres. Then, the PI bres experienced about 30% weight loss above 500 C, which indicates that the aromatic structure turns into amorphous carbon. Form 150 C to 700 C, the residual weight of the GO/PAA bres is higher than that of the PAA bres; this indicates that doping of GO improves the thermal stability of the GO/PAA bres. However, the residual weight of the GO/PAA bre was lower than that of the PAA bres above 700 C; this was mainly because doping of GO increased the decomposition rate of the GO/PAA bres.
PI bres are a kind of high-temperature resistant polymer bres, which are mainly used in a ame retardant area. The TG-DSC analysis of the PI bres and GO/PI bres was carried out in air. Fig. 3 demonstrates the thermal stability of the pure PI bres and GO/PI composite bres with different GO contents. As shown in Fig. 3, the 10% weight loss temperature of the GO/ PI composite bre with 1.0 wt% GO is 509 C, which is nearly 40 C higher than that of the pure PI bre. From Fig. 3, we can observe that the initial decomposition temperature of the GO/PI composite bre is higher than that of the pure PI bres; this indicates that GO doping can increase the thermal stability of the GO/PI bres. Fig. 4 shows the DSC curves of the PI bres and GO/PI composite bres.
From Fig. 4, it can be clearly observed that the GO/PI composite bre with 1 wt% GO has a exothermic peak temperature and lower exothermic peak intensity than that of the pure PI bre. The exothermic peak temperature of the GO/PI composite bre with 1 wt% GO added increases by 7% than that of the pure PI bre, and the exothermic intensity drops by 69.7%. It can be inferred that the addition of GO can effectively increase the thermal stability and decrease the exothermic heat of GO/PI bres during the carbonization process. The reason  why doping of GO can improve the thermal stability and decrease the exothermic heat can be inferred for the following two reasons. First, GO sheets have a strong interaction with PI molecular chains and may form a crosslinking structure, which greatly decreases the heat released during carbonization. Second, GO sheets can hinder the movement of the PI molecular chains; this increases the energy needed to break the PI molecular chains.
Crystalline structure of PI-based graphite bres and GO/PI composite graphite bres  Table 1, both the average crystallite size L a and the average crystallite thickness L c were increased with the increase of the GO content; this indicated that doping of GO could induce the growth of graphite crystallite. It can be inferred that GO with a large surface area acts as a crystal nucleus, which can induce the crystallization of graphite bres. As abovementioned, the PI molecular chains were attached to the graphene sheets and then graphitized to the graphene net during the graphitization process. As the process of graphitization carrys on, the layer of graphene sheets grows; this indicates the formation of graphite crystallites. The degree of graphitization increased as the GO content raised; this proved our speculation. The increase of graphitization could be conrmed by Raman spectroscopy.
Laser Raman spectrum was used to examine the crystallite structure of the graphite bre with different GO contents. For carbon materials, the D band is caused by the defects and disorder, whereas the G band is caused by the stretching of all sp 2 bonding atoms. For the graphite material, the value of R ¼ I D /I G is positive, related to the size of graphite crystallites, which can be used to represent the degree of graphitization. The lower the value of R, the higher the degree of graphitization. During the graphitization process, GO as a crystal nucleus could induce the formation of graphite crystallites. As shown in Fig. 6, the value of R increased with the addition of GO; this indicated the increase of the graphitization degree ( Table 2). Fig. 7 displays the orientation curves of the GO/PI-based carbon bres and graphite bres were obtained using XRD. Both the value of p and the value of f indicate the orientation degree of the graphite bre. With an increase in the heat treatment temperature from 1000 C to 2800 C, the value of p increases by 23.3%; this indicates that high temperature treatment can improve the orientation of the graphite bre. The orientation degree of the GO/PI-based graphite bre with 0.3 wt% GO added is higher than that of the pure PI bre; this indicates that GO can promote bre orientation.
The orientation of the GO/PI-based graphite bres was examined by XRD. A bunch of aligned graphite bres were xed   to the sample cell at both ends, with the bre length less than 5 mm. The samples were placed perpendicular to the X-ray, and the counter tube is placed at the maximum intensity of the (002) diffraction peak. Upon rotating the sample station in all directions, the reected intensity was determined to obtain the diffraction pattern with the highest intensity of the meridian. The orientation degree p can be calculated according to the value of the full width at half maximum (FWHM): Herein, H represents the value of FWHM, and Z represents the orientation angle. For the graphite bre, a certain angle between the normal direction of the microcrystalline plane and the bre orientation axis is called 4. The angle 4 is used to characterize the degree of orientation. The higher the value of this angle, the better the orientation.
As abovementioned, GO can promote the orientation of the as-spun PAA bres. The PI molecular chains were attached to the GO sheets according to the p-p interaction. The orientation of GO sheets was forced to be along the bre axis due to the strong shear force during the spinning process; this allowed the PI molecular chains to form along the bre axis at the same time. During the carbonization and graphitization process, GO acts as a crystal nucleus and can induce the proper arrangement of graphite microcrystals, thus improving the orientation of the graphite bre.
Mechanical property and thermal conductivity of the graphite bre Fig. 8 shows the tensile strength and breaking elongation of GO/ PI-based graphite bres with different GO contents. From Fig. 8, it is clear that the breaking elongation decreases with the addition of GO, and the tensile strength decreases beyond that at 0.3 wt% with the addition of GO. The tensile strength of 0.3 wt% GO-added graphite bre is 693.89 MPa, which increases by 67.7% as compared to that of the pure PI-based graphite bre, whereas the tensile strength of 0.5 wt% GO decreases by 37.2% as compared to that of 0.3 wt% GO. It can be inferred that the mechanical quality of the GO/PI composite graphite bre can be improved by adding a certain amount of GO. If too much GO is added, the mechanical properties of graphite bre will decrease since GO cannot disperse well in the bre. The over added GO will aggregate in the bre and form defects, which can limit the property of the bre. Moreover, with an increase in the GO content, the breaking elongation of the graphite bre decreases; this indicates the increase of tensile modules of graphite bre. Due to the improvement of bre orientation, the mechanical properties of the composite graphite bre was improved as well.
The electrical resistance and thermal conductivity of the mesophase carbon bre has an empirical equation, which is l ¼ 1261/r (l represents the thermal conductivity, and r represents the electric resistivity). A resistivity meter was used to measure the electrical resistance, and then, the thermal conductivity was calculated according to the equation. As shown in Fig. 9, the thermal conductivity increased with the addition of GO. The thermal conductivity of the GO/PI-based graphite bre with 2.0 wt% GO content is 435.5 W m À1 K À1 and is 81.2% higher than that of the pure PI-based graphite bre. The increased thermal conductivity of the composite graphite bre may due to two reasons: the rst reason is that the orientation along the   bre axis is improved during the spinning process and the beginning of carbonization process, another reason is that GO addition has induced the crystallization of the graphite bre. Still, we need to nd a balance between good mechanical property and high thermal conductivity. It can be reasonably believed that PI bres are an ideal carbon precursor to prepare graphite bre with high thermal conductivity.

Conclusions
The inuences of addition of GO on the structure and properties of a GO/PI-based graphite bre were carefully studied. In our study, we prepared graphite bres with different GO contents and used different methods, such as FTIR spectroscopy, TG-DSC, XRD, and Raman spectroscopy, to characterize graphite bres. Through a series of identication tests, it was found that the addition of GO can improve the orientation along the bre axis and increase the degree of graphitization; thus, we can obtain a graphite bre with a high mechanical property and high thermal conductivity. According to the TG-DSC curves, a GO/PI-based graphite bre with 1 wt% GO addition has a 7% higher exothermic peak temperature and 69.7% lower exothermic peak intensity than the pure PI bre; this indicates that doping of GO can improve the thermal stability and therefore maintain the preferred orientation of the graphite bre. XRD and Raman curves indicate that doping of GO can increase the degree of graphitization of the graphite bre because GO sheets act as a nucleus crystal during the graphitization process. GO can improve the orientation of the GO/PI composite bres during the spinning process and maintain the preferred orientation during the beginning process of carbonization; thus, we can fabricate a graphite bre with higher mechanical properties and thermal conductivity.
Although the increase of graphitization degree can increase the thermal conductivity of the graphite bre, it will decrease the mechanical properties of the graphite bre. It is better to have a balance between mechanical properties and thermal conductivity, and we found that 0.3 wt% addition amount of GO is a better choice. In this study, we introduced a new approach to prepare graphite bres with high thermal conductivity from PI bres, and these bres have potential application in heat management areas.

Conflicts of interest
There are no conicts of interest to declare.