Preparation of graphene/poly(p-phenylenebenzobisoxazole) composite fibers based on simultaneous zwitterion coating and chemical reduction of graphene oxide at room temperature

Abstract

Zwitterion-coated reduced graphene oxide (rGO) was used in fabricating graphene/PBO composite fibers via in situ polymerization. The zwitterionic 1 : 1 adducts were formed by two monomers of PBO and they could be crystallized using rGO sheets as templates. Covalent bonds could be formed between GO and 4,6-diaminoresorcinol dihydrochloride (DAR·2HCl, a monomer for PBO) with GO being reduced simultaneously at room temperature. The structure of rGO sheets was well characterized and a mechanism including nucleophilic addition and E1 dehydration was proposed. Zwitterions could precipitate out from water rapidly and yet dissolve in PPA easily. Thereby, rGO sheets could be homogeneously dispersed in the PBO matrix as zwitterions dissolve and polymerize. By adjusting the feeding of zwitterion-coated rGO sheets, various composites with 0.1–3.0 wt% GO sheets incorporated were obtained. With very low rGO sheet content, both the mechanical properties and thermal stability of rGO/PBO fibers increased remarkably.

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1. Introduction

Graphene, a single-layer platelet made up of sp²-hybridized carbon atoms, has been intensively explored in fabricating nanocomposites with high performances^1–5 and various functionalities for a range of applications^6–11 because of its exceptional properties. Chemical reduction of graphene oxide sheets enables low-cost and mass production of graphene nanosheets. As graphene oxide possess abundant oxygen-containing groups (e.g., hydroxyl, epoxy, ketone, carboxyl, etc.) for further functionalization,^12,13 it has been considered as an attractive filler for graphene-reinforced polymer nanocomposites. Graphene-filled polymer composites are commonly prepared by solution mixing, melt blending, and in situ polymerization.¹⁴ Mixing GO sheets and polymer solution is the most straightforward method to prepare graphene/polymer composites, which, unfortunately, is restricted by the limited solubility of GO in various kinds of solvents.¹⁵ Sonication has usually been applied to achieve metastable dispersions of graphene derivatives. Besides, high-speed shearing could also be used to mix graphene-based fillers and the polymer matrices. However, re-stacking and aggregation of graphene derivatives are unavoidable in the two physical mixing process mentioned above.¹⁶ Thus, surfactants or polymers¹⁷ are usually grafted onto GO sheets to improve the dispersibility, while the presence of such foreign stabilizers is generally undesirable for most applications.

Compared to graphene, densely oxidized regions and small amount of holes has been found throughout GO sheet.¹⁸ To restore the pi-conjugated structure of graphene, GO is usually converted back to graphene via chemical reduction or thermal annealing.¹⁹ Unfortunately, once reduced, there would be only few functional groups existing in rGO sheet, which might lead to weaker interfacial interaction between graphene sheet and the matrix. Poor filler-matrix interfaces would cause a stress concentration under external stresses, which sometimes lead to the breakage of the composites.³ Covalent bonding between two phases is needed to achieve favorable interfacial adhesions and facilitate the efficient load transfer.²⁰

So far, utilizing facile approaches to fabricate graphene-reinforced polymer composites remains quite challenging. Several key concerns of fabricating graphene/polymer composites include obtaining favorable dispersions of graphene derivatives, forming covalent bonding between graphene and matrices, reducing the introduction of foreign molecules, as well as applying environmentally friendly reducing agents and facile method to convert GO back to graphene.

Aromatic heterocyclic polybenzobisazoles (PBZ) is a commercial family of materials with excellent mechanical and thermal properties, such as poly(benzimidazoles) (PBI), poly(benzothiazoles) (PBT) and poly(benzobisoxazoles) (PBO).^21–23 They exhibit great potential in the fields of aerospace and military industry. PBO polymeric materials could be used in fabricating high-performance fibers with the highest modulus and tensile strength among all commercial synthetic polymer fibers. In addition, they can also act as reinforcing fillers,²⁴ heat-resistant materials²⁴ and electron-transport films in a variety of photoelectric devices.²⁵

PBO is usually prepared in polyphosphoric acid (PPA) or methanesulfonic acid (MSA). In previous research, PBO has been synthesized in the presence of various nanoparticles.^26–30 For instance, carbon nanotubes (CNTs)/PBO composites have been manufactured to improve the mechanical properties and high-temperature resistance.^26,31 CNTs could be easily dispersed in strong acid solvent,^26,28,32 while neither GO sheets nor graphene sheets are soluble in them, making it quite difficult to prepare graphene/PBO composites.²⁸ In an pioneering attempt, graphene-based PBO composites were prepared by solution blending of modified-GO sheets and PBO in MSA under sonication, where MSA-soluble GO sheets were covalently functionalized with polyhydroxyamide. Then a 400 °C heat treatment was carried out to reduce modified GO to graphene. The above-mentioned solution blending method is designed for a film fabrication process and the preparation process is quite complicated. In addition, the introduction of surfactants or extra polymers might lead to the decrease of mechanical property and thermal stability.³³

Here we proposed an effective strategy to achieve GO reduction at room temperature and favorable dispersion of rGO sheets without introducing any foreign molecules. GO sheets are first modified by DAR·2HCl in its aqueous solution with simultaneous chemical reduction to graphene. Subsequently, disodium terephthalate solution (another monomer for PBO) was dropped into the reaction mixture to form zwitterions with DAR·2HCl. The zwitterionic 1 : 1 adducts crystallized immediately using DAR modified rGO sheets as 2D growth templates. The zwitterions could precipitate out from water rapidly and yet dissolve in PPA easily. Therefore, rGO sheets could be readily dispersed in PBO matrix as zwitterions dissolving and polymerizing in PPA. Our strategy is simple but effective to achieve chemical reduction, favorable dispersion, and interfacial adhesion of graphene sheets in PBO polymer matrix without introducing any extra reducing agents or stabilizers. Finally, we prepared continuous rGO/PBO fibers by dry-jet wet-spinning technique, and found that both the mechanical properties and thermal stability of PBO fibers are obviously increased.

In particular, the covalent bonds between GO and PBO has been assumed to be similar to CNT/PBO composite that covalent bonds could be formed between amino groups of DAR and carboxyl groups of oxidized CNT. However, the reaction with carboxyl groups would not lead to the reduction of GO. We thus think that the manufacture of CNT/PBO and rGO/PBO composites might be different in mechanism. This work has focused on the DAR·2HCl induced chemical reduction of GO and we first proposed a reduction mechanism including nucleophilic addition and E1 reaction on the basis of results of FTIR, XPS, and Raman spectra.

2. Experimental

2.1 Materials

DAR·2HCl were purchased from Zhejiang Dragon Chemical industry Co., China. Terephthalic acid (TPA), phosphorus pentoxide (P₂O₅) and PPA were purchased from Shanghai Reagents Company. MSA was purchased from Aladdin Chemistry Co., Ltd.

2.2 Preparation of GO aqueous suspension

GO single sheets were synthesized from natural graphite powder using a modified Hummers' method.³⁴ Briefly, graphite (10.0 g) and sodium nitrate (5.0 g) was added to concentrated sulfuric acid (460 ml) under stirring in ice-water bath, then potassium permanganate (60.0 g) was added, and the mixture was cooled to around 0 °C. Under vigorous agitation, the mixture was transferred to a 35 ± 1 °C water bath for 2 h and then kept at 95 °C for 15 min, forming a thick paste. Successively, 1000 ml of water was added, followed by a slow addition of 10 ml of hydrogen peroxide (30%). The mixture was filtered and washed with 5 wt% hydrochloric acid aqueous solution three times to remove metal ions. Then the filter cake was repeated water-washing and centrifugation until the pH value of the supernatant liquid was above 5.0. After sonication, the unexfoliated GO was removed by followed centrifugation at 5000 rpm.

2.3 Synthesis of zwitterion-coated graphene oxide (GO–DAR–TPA)

468 ml GO aqueous (0.5 g L⁻¹) was placed in a 1000 ml three-necked reaction flask equipped with mechanical stirrer and a nitrogen gas inlet/outlet. 0.1 mol DAR·2HCl were dissolved in deionized water under nitrogen protection, and then dropped into the GO aqueous. The reaction solution was kept stirring at ambient temperature for 4 h, and the brownish solution gradually turned into a black mixture. Followed that, disodium terephthalate (0.1 mol) solution was dropped into the reaction system, and the mixture precipitated out as a gray aggregation. Finally, this product was isolated by vacuum filtration, washed with water and dried at 80 °C in vacuum oven for 12 h. Finally, zwitterions with 0.76 wt% GO sheets incorporated were obtained.

2.4 Copolymerization of PBO with GO–DAR–TPA

A typical procedure to prepare graphene/PBO composites with 0.2 wt% GO sheets containing was depicted as follows: 29.7 g PPA, 0.05 g SnCl₂·H₂O, and 17.0 g DAR·2HCl (0.08 mol) were loaded into a 500 ml three-neck round-bottom flask. The reaction mixture underwent dehydrochlorination at 90 °C for 10 h. Subsequently, 6.12 g zwitterion-coated GO sheets were added into the reaction system followed with mechanical stirring for 1 h. 13.3 g TPA (0.08 mol) and 19.5 g P₂O₅ was then added into the mixture. Followed that, a typical PBO condensation step was carried out at 140 °C for 10 h, 160 °C for 8 h and 180 °C for 8 h, respectively. By adjusting the feed weight percentage of zwitterion-coated GO sheets, the containing of GO sheets ranged from 0.1 wt% to 3.0 wt% with respect to the polymer concentrations in the polymerization.

2.5 Fabrication of graphene–PBO copolymer fibers

The fibers of composites were fabricated using a custom-designed spinning apparatus. The key spinning conditions were as follows: spinneret diameter, 0.2 mm; air gap, 700 mm; spinning temperature, 180 °C; coagulation bath, dilute phosphoric acid. The drawn fibers were washed in running water and subsequently dried under vacuum at 115 °C for 2 h.

2.6 Characterizations and measurements

Fourier transformed infrared (FTIR) spectra were recorded on a Nicolet 6700 spectrometer with a diamond attenuated total reflection accessory. Raman spectra were recorded on SENTERRA Micro Raman Spectrometer (Bruker Instruments, Germany) at 532 nm laser excitation. Thermogravimetric analysis (TGA) were performed on a STA 409 PC analyzer (NETZSCH, Germany). The testing was carried out at a heating rate of 10 °C min⁻¹ from 30 to 1000 °C under nitrogen. The standard uncertainty of weight loss temperature is ±1 °C. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10⁻¹⁰ mbar. Typically the hydrocarbon C1s line at 284.8 eV from adventitious carbon is used for energy referencing. X-ray powder diffraction (XRD) was performed at room temperature on a Rigaku Model D/max-2B diffractometer using Cu/Kα radiation (λ = 0.154 nm) at a generator voltage of 20 kV and a generator current of 200 mA. Testing data were collected from 3 to 80° at a scanning rate of 3° min⁻¹. Sample dispersed in water was deposited onto a standard TEM-grid. The PBO composite samples were prepared by Focused Ion beam technique and transmission electron microscopy (TEM) images were obtained on a JEM-2100F microscope (operated at 200 kV). The microscopic analysis of the fracture surfaces of composite samples were sputtered with gold for 120 s and observed under a scanning electron microscope (SEM, HITACHI S-4800). The intrinsic viscosity of PBO composites was measured using a ubbelohde viscometer at 30 °C. Typically, 0.01 g sample of rGO/PBO composites was dissolved in 25 ml MSA, then measurement was carried out after the solution was vacuum filtered using a mixed cellulose ester membrane with 0.22 μm pores. The fiber diameter was measured with an optical microscope (equipped with CCD, CAMERAL). The mechanical property test was based on ASTM D3379-75 (Reapproved 1989), and every single fiber was randomly selected. Stress–strain curves for the as-spun fibers were collected on a universal tensile machine (Instron series 3365) using 20.0 mm gauge length at a strain rate of 1.0 mm per min. For each sample, 20 single fibers were tested and averaged mechanical properties were reported.

3. Results and discussion

3.1 Characterization of GO and rGO–DAR

FTIR spectrum was firstly used to characterized the structure of GO sheets. In the FTIR spectrum (Fig. 1a), several characteristic peaks are observed: a broad O–H stretching vibrations at around 3300 cm⁻¹, the C=O stretching vibrations of carbonyl and carboxyl vibrations at 1720 cm⁻¹, the C=C skeletal stretching vibrations at 1624 cm⁻¹ and the C–O stretching peaks of epoxy and alkoxy at 1225 cm⁻¹ and 1050 cm⁻¹. The XRD pattern of GO sheets show a typical reflection peak at 2θ = 10.58°, corresponding to an interlayer spacing of about 0.83 nm (Fig. 1c). These data suggest the natural graphite has been successfully oxidized during the chemical modification process. The TEM image in Fig. 1b shows wrinkled and paper-like sheets, suggesting that the bulk graphite has been exfoliated into individually dispersed GO single layers after oxidation and sonication.

Fig. 1 (a) FTIR spectra of GO and rGO–DAR, (b) TEM image of GO sheets, (c) XRD patterns of GO, DAR·2HCl and rGO–DAR, (d) SEM image of rGO–DAR sheets.

The procedure for preparing GO nanosheets reinforced PBO fibers is presented in Scheme 1 and Fig. 2. After DAR·2HCl was dropped into GO dispersion, the reaction mixture gradually turn black. The black color of the suspension may suggest restoration of conjugated planes for the reduction of GO to rGO.^35,36 SEM images reveal that the reduced GO is forming a disordered solid (Fig. 1d).³⁷ Further evidence for depressed aggregation in reduced GO sheets are demonstrated by their XRD patterns, a straight line with no apparent diffraction peaks is obtained (Fig. 1c). This data indicates that the rGO–DAR sheets remain separated in the resulting precipitation.

Scheme 1 Manufacturing process of rGO/PBO composite fibers.

Fig. 2 Structures of GO sheet, rGO–DAR sheet, rGO–DAR–TPA and rGO–PBO.

In order to characterize changes in the functional groups before and after the reduction, FTIR spectra were collected and shown in Fig. 1a. After DAR functionalization, the disappearance of peaks at around 1050, 1225, 1720, and 3300 cm⁻¹ suggest that the oxygen-containing groups of GO were almost entirely removed in rGO–DAR.³⁸ To further validate the chemical reduction of GO, we used XPS to characterize GO and rGO–DAR. In the XPS spectrum of GO (Fig. 3a), two obvious peaks are observed at 286.2 eV (C1s) and 533.2 eV (O1s), and no peak of nitrogen element is detected. For the sample of rGO–DAR, strong signal of N1s is found at 400.2 eV besides the signals of C1s and O1s, indicating the existing of nitrogen element originated from rGO–DAR.³⁹ The C1s XPS spectrum of GO (Fig. 3c) could be curved into five main components with binding energies at 284.8, 285.6, 286.7, 287.3, and 288.8 eV, which are attributed to the aromatic C, C–O, C–O–C, C=O and O–C=O species, respectively.³⁷ In the C1s XPS spectrum of rGO–DAR (Fig. 3d), the intensities of peaks of C–O, C–O–C, and C=O functional groups decreased dramatically, revealing that most oxygen-containing functional groups were removed. In the N1s XPS spectrum of rGO–DAR (Fig. 3b), two peak components with binding energies at 399.7 and 400.8 are assigned to C–N and NH₃⁺ species (Fig. 3 and S1†), which confirms the covalent bonding between GO sheets with DAR molecules. Furthermore, according to the result of the XPS spectra, the atomic ratio of C/N in rGO–DAR is 11.4, which suggest that every 16.8 carbon atoms would share one DAR molecular. The intensity ratio of the D- to the G-band (I_D/I_G) in Raman spectrum usually reveals the change of the electronic conjugation state.⁴⁰ As depicted in Fig. 4, with DAR grafted onto GO sheets, the I_D/I_G ratio of rGO–DAR did increase notably. This change suggests an increase of number of sp²-hybridized domains.⁴¹ In other words, the conjugated graphene network is partly re-established. These data confirms that GO sheets were grafted and chemically reduced by DAR at room temperature, as Fig. 2 shows.

Fig. 3 (a) XPS spectra, (b) N1s XPS spectrum of rGO–DAR, C1s XPS spectrum of (c) GO and (d) rGO–DAR.

Fig. 4 Raman spectra of GO and rGO–DAR.

The covalent bonds between GO and PBO has been assumed to be similar to CNT/PBO composite. It's worth noting that carboxyl is the only oxygen-containing functionality located on oxidized CNT,⁴² while GO contains mostly hydroxyl and epoxy functionalities and small amount of carboxyl groups.^18,43 In particular, the reaction with carboxyl groups would not lead to the reduction of GO. We thus think that the manufacture of CNT/PBO and rGO/PBO composites might be different in mechanism.

Although acidic amine derivatives such as amino acids⁴⁴ and dopamine^45,46 has been using as reducing agents for the reduction of GO, the chemical reduction mechanism remains ambiguous.¹⁹ It's well accepted that epoxy and hydroxyl groups are main oxygen-containing functional groups attached to GO sheets.^47,48 Epoxy groups could be readily converted into hydroxyl groups via nucleophilic additions especially by acid catalysis.^19,44,49 The covalent C–N bond in XPS spectra might be assigned to the epoxide opening with DAR molecule (Fig. 5b). Notice that the formation of hydroxyl groups originated from epoxy groups would not result in oxygen removal and the deoxygenation process also result in significant recovery of the sp² carbon sites.¹⁹ The core issue in the reduction of GO, therefore, is how to explain the removal of –OH moieties and the simultaneous restoration of C=C groups.

Fig. 5 A proposed reaction pathway for the reduction of GO with DAR: (a) E1 reaction of hydroxyl groups, (b) nucleophilic addition and E1 dehydration of epoxy.

According to previous studies, the –OH bond is generally considered to be removed via nucleophilic substitution or thermal reduction.⁵⁰ Nevertheless, –OH is a poor leaving group that could hardly be substituted or removed except it is protonated to –⁺OH₂, a very good leaving group. Considering the acidity of DAR·2HCl, the reduction of GO is highly speculated to undergo a E1 reaction, as is presented in Fig. 5: –OH is first converted to its protonated form, then a H₂O molecule is eliminated irreversibly giving an carbocation intermediate. Finally, the adjacent C–H bond is cleaved readily and rapidly to produce a C=C bond (Fig. 5a). While epoxy is first attacked by DAR molecule and turn to –⁺OH₂ via ring-opening reaction under acid catalysis. Then –⁺OH₂ is eliminated via an E1 reaction (Fig. 5b). The covalent bonding between GO and PBO originates from the ring-opening reaction of epoxy at room temperature. Although the exact mechanism requires experimental or computational verification, it is very probable, that the proposed acid-catalyzed mechanism might be extensively applicable in the reduction of GO with reducing agents such as amino acids, HI acid,⁵¹ ascorbic acid,⁵² and dopamine.

3.2 Synthesis of zwitterion-coated rGO and rGO/PBO composites

By anchoring or grafting of molecules or polymers onto rGO sheets, aggregation could be depressed because of steric hindrance or electrostatic repulsion. While the presence of such foreign stabilizers is generally undesirable. In this context, zwitterionic 1 : 1 adducts were formed by directly solution mixing of two monomers of PBO (Fig. S2–S4†). Generally, neat DAR–TPA zwitterions would exist as rectangular parallelepiped crystals (Fig. 6a) and might be used to prepare PBO in phosphoric acid by a solution polycondensation.⁵³ With dropping disodium terephthalate into rGO–DAR suspension, the newly generated zwitterions crystallized immediately using rGO sheets as templates and grew to thin plates (Fig. 6b). RGO sheets are coated with crystals and separated from each other. The zwitterionic adducts could precipitate out from water rapidly and yet dissolve in PPA easily. Therefore, zwitterion-coated rGO sheets could be dispersed in PBO matrix as zwitterions dissolved and in situ polymerized in PPA.

Fig. 6 SEM images of (a) neat DAR–TPA zwitterions, (b) zwitterion-coated rGO sheets.

By a commonly PBO polycondensation process, rGO/PBO composites with 0.1–3.0 wt% graphene containing were obtained (see their FTIR spectra in Fig. S5† and intrinsic viscosities in Fig. S6†). The morphologies of cross-section of neat PBO and rGO/PBO composites are presented in Fig. 7. The cross-section of neat PBO is featureless (Fig. 7a). The bright regions in SEM images of rGO/PBO composites are attributed to rGO sheets because of their high conductivity.² It's obviously observed that rGO sheets are well distributed in PBO matrices (Fig. 7f) and their interfaces are combined closely (Fig. 7c). As a contrast, graphene/PBO composite was also prepared by high-speed mixing of graphene and PBO polymer solution at the beginning of polymerization. Most graphene sheets remain in an aggregated state after polymerization (Fig. S7†). Meanwhile, some gaps could be found between graphene sheets and PBO matrix (Fig. 7b and S7†), indicating the week interactions between two phases. The improved interface interactions in this context could be attributed to covalent bonding between rGO and DAR molecules.

Fig. 7 SEM images of cross-section from (a) neat PBO, (b) graphene/PBO composite prepared by mechanical mixing, and rGO/PBO composites with (c) 0.2 wt% GO incorporated and (d) 3.0 wt% GO incorporated. TEM images of (e) neat PBO and (f) 0.3 wt% GO/PBO.

3.3 Characterization of rGO/PBO composite fibers

RGO/PBO composites with intrinsic viscosity ranging from 24.8 to 25.8 dL g⁻¹ were prepared in pilot test for spinning. Shiny green rGO/PBO fibers were fabricated using a typical dry-jet wet-spinning technique. Digital photographs of PBO and rGO/PBO fibers are shown in Fig. 8. The diameter of fibers fabricated in our process is about 20 μm and the fibers are macroscopically homogenous and smooth. Moreover, there are no visible cracks or microfibrils on the surfaces of rGO/PBO fibers (Fig. 8d). XRD patterns of neat PBO and rGO/PBO fibers are presented in Fig. 9. The diffraction peaks located at 2θ = 16.0° and 27.6° are corresponding to (200) and (010) reflections in highly oriented PBO fiber.²⁶ The XRD patterns of rGO/PBO composite fibers do not show any specific distinction compared with that of PBO fibers, indicating identical PBO orientation in the four samples. Furthermore, the apparent crystal size (L_hkl) of PBO could be computed using Scherrer's equation L_hkl = Kλ/B cos θ_hkl. The apparent crystal sizes are obtained from the most prominent diffraction peaks (200) at 16.0° in the equatorial direction. Compared to 7.82 nm for PBO fibers, rGO/PBO fibers show slightly higher apparent crystal sizes of 8.17, 8.50 and 8.94 nm for 0.1, 0.2 and 0.3 wt% GO loading, respectively. The introduction of rGO sheets might enlarge the crystal size of PBO.

Fig. 8 Photographs of (a) PBO fibers and (b) rGO/PBO fibers, SEM images of (c) neat PBO fibers and (d) rGO/PBO fibers.

Fig. 9 XRD patterns of neat PBO and rGO/PBO fibers.

Given the excellent intrinsic strength of rGO^54,55 and the existence of covalent bonding at rGO/PBO interfaces, it's expected that the mechanical properties of PBO fiber would be enhanced. The mechanical properties of PBO and rGO/PBO fibers are shown in Table 1 (see their stress–strain curve in Fig. S8†). Neat PBO fibers perform a relatively high tensile strength of 3.50 GPa and a Young's modulus of 130 GPa. Visibly increase of tensile strength and Young's modulus occurs after rGO was incorporated. For instance, the tensile strength significantly increases 0.76 GPa with only 0.2 wt% rGO incorporated and the Young's modulus corresponding increases by 20.7%.

Table 1 Mechanical properties of the neat PBO and rGO/PBO composites fibers

Sample	Intrinsic viscosity (dL g^−1)	Tensile strength (GPa)	Young's modulus (GPa)	Elongation at break (%)
PBO	25.6	3.50 ± 0.38	135 ± 13	2.64 ± 0.21
0.1 wt% GO/PBO	25.1	4.02 ± 0.41	155 ± 17	2.51 ± 0.26
0.2 wt% GO/PBO	24.8	4.26 ± 0.47	163 ± 19	2.65 ± 0.23
0.3 wt% GO/PBO	25.8	4.32 ± 0.44	167 ± 18	2.72 ± 0.25

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As a heat-resistant material, the thermal stability is another major concern of PBO. Fig. 10 illustrates the TGA traces of PBO composites. All the samples exhibit excellent thermal resistance. The onset temperature of neat PBO fibers is around 600 °C, it shifts to 626, 659, and 681 °C with 0.1%, 0.2% and 0.3% GO sheets loaded, respectively. The homogeneous dispersed rGO sheets and their close combination with the matrix suppressed the chain mobility⁵⁶ of PBO. During decomposition, rGO nanosheets might form a jammed network structure in the PBO matrix and retard the decomposition process.⁵⁷ Thereby, the improvement in thermal stability of PBO fibers is achieved.

Fig. 10 TGA curves of neat PBO and rGO/PBO composites.

4. Conclusions

The rGO/PBO composite fibers were successfully produced by a facile approach in PPA. In this case, zwitterionic 1 : 1 adducts were formed by two monomers of PBO. The zwitterions could crystallize using rGO sheets as templates and separate rGO sheets from each other. The reaction between DAR and GO were well characterized and a reduction mechanism including nucleophilic addition and E1 reaction was proposed. RGO sheets could be well dispersed in PBO matrix as zwitterions dissolved and polymerized in PPA. The abundant covalent bonds provide enhancing filler-matrix adhesion between graphene sheets and PBO matrix. Furthermore, continuous rGO/PBO composite fibers were fabricated by a dry-jet wet-spinning process. The mechanical properties and thermal stability of PBO composite fibers were obviously improved with very low containing of GO sheets. This work opens a new scenario for chemical reduction of graphene oxide and provides an effectively method to fabricate graphene-based nanocomposites.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 50803067 and 51373186).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Reduction pathway of GO, FTIR, XRD figures of zwitterions, SEM images of neat zwitterions and cross-section of rGO/PBO composites, intrinsic viscosities and FTIR figures of as-spun rGO/PBO fibers. See DOI: 10.1039/c5ra18551c

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