Synthesis of a novel graphene conjugated covalent organic framework nanohybrid for enhancing the flame retardancy and mechanical properties of epoxy resins through synergistic effect

Synthesis of a novel graphene conjugated covalent organic framework nanohybrid for enhancing the flame retardancy and mechanical properties of epoxy resins through synergistic effect

Composites Part B xxx (xxxx) xxx Contents lists available at ScienceDirect Composites Part B journal homepage: www.elsevier.com/locate/compositesb ...

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Composites Part B xxx (xxxx) xxx

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Synthesis of a novel graphene conjugated covalent organic framework nanohybrid for enhancing the flame retardancy and mechanical properties of epoxy resins through synergistic effect Yuling Xiao, Ziyu Jin, Lingxin He, Shicong Ma, Chenyu Wang, Xiaowei Mu **, Lei Song * State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230026, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Graphene conjugated covalent organic framework Synergistic effect Epoxy resin Flame retardant Mechanical performances

A novel graphene conjugated covalent organic framework ([email protected]) nanohybrid was synthesized by sol­ vothermal method and used to enhance the flame retardancy and mechanical performances of epoxy resins (EP) through synergistic effect firstly. It is deduced from thermogravimetric analysis and cone calorimeter results that the obtained [email protected] nanohybrid improves the flame retardancy of EP significantly. The time to peak heat release rate of EP increases by 17 s, the peak heat release rate and total heat release of EP decrease by 43.6 and 24.3% due to incorporation of 2 wt% [email protected] It is referred from thermogravimetric analysis-fourier trans­ form infrared spectrometry results that the release of toxic gases and combustible volatiles during pyrolysis is also suppressed. As for mechanical performance of EP nanocomposites, the storage modulus of EP/2 wt% [email protected] (34.66 GPa) in the glassy state increases by 23.8% compared with the neat epoxy (28.00 GPa). The possible mechanism for enhanced flame retardant and mechanical performances is proposed according to the test results. This work has opened up a new application for [email protected] nanohybrid in the field of flame-retardant polymers.

1. Introduction EP is one of the widely used thermosetting polymers due to its excellent toughness, dielectric properties, chemical stability, bond strength, dimensional stability and mold resistance, as well as low shrinkage rate and simple processing [1,2]. It has been widely applied as adhesives, anti-corrosion coatings, insulating coatings and laminates, etc [3]. However, EP is flammable and is liable to emitting a large vol­ ume of smoke and toxic gases during combustion, which will bring enormous fire hazards [4,5]. Thus, a series of compounds containing phosphorus, nitrogen, silicon and boron, etc. have been introduced to achieve halogen-free flame retardant EP using physical or chemical methods. However, substantial addition of flame retardant is required in the above method, which will destroy the thermal stability and me­ chanical properties of the matrix [6]. Therefore, a great deal of re­ searches and explorations have been made on flame-retardant EP. Nanofillers, such as montmorillonite [7], layered double hydroxides [3, 8], carbon nanotubes, graphene [9–12] and molybdenum disulfide (MoS2) [13,14] not only can work as flame retardant, but also do

contribute to the mechanical properties and thermal stability of EP. Simultaneously, it is an important issue to develop novel flame re­ tardants and study the synergistic effect between different additives to reduce the additives dosage and improve the flame retardant efficiency. Graphene with a unique two-dimensional carbon lamellar structure has attracted vast attention in the flame retardant fields in recent years [15–18]. When the polymer materials are exposed to open flame or high temperatures, the graphene nanosheets can function as a barrier to prevent oxygen from diffusing into the interior of the matrixs [6,19]. In addition, graphene with excellent thermal conductivity can improve heat transfer performance and avoid local overheating, making it diffi­ cult for fire to spread [20]. Besides, graphene with a high specific surface area (2630 m2/g) [16,21] makes it easier to adsorb organic volatiles generated during combustion. However, strong interaction between graphene nanosheets makes it easy to agglomeration when preparing polymer nanocomposites [22]. It has been reported that MoS2, silicon dioxide and DOPO, etc. have been used as a surface modifier of graphene [3,23–25]. The surface functionalization of graphene can effectively inhibit the agglomeration of graphene nanosheets and exert synergistic

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Mu), [email protected] (L. Song). https://doi.org/10.1016/j.compositesb.2019.107616 Received 29 July 2019; Received in revised form 17 October 2019; Accepted 20 November 2019 Available online 27 November 2019 1359-8368/© 2019 Published by Elsevier Ltd.

Please cite this article as: Yuling Xiao, Composites Part B, https://doi.org/10.1016/j.compositesb.2019.107616

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Scheme 1. Synthesis of [email protected] nanohybrid.

flame-retardant effect. Covalent organic frameworks (COF), first reported in 2005 [26], is a type of two-dimensional or three-dimensional crystalline porous poly­ mers assembled from discrete pores and highly ordered organic building blocks [27–30]. Their crystal structure is completely maintained by strong bonds from light elements (C, N, H, B and O) [31]. The desirable properties of outstanding thermal stability, high specific surface area and the low density have offered the COF a variety of applications in gas adsorption, sensing, separation, catalyst, biosensor, optoelectronic de­ vice and more [32–34]. Meanwhile, the combination of functional components to obtain hybrids has attracted more and more attention on account of the combination of physical and chemical properties, multi-function of individual components and synergistic effect. Ac­ cording to previous reports [35], COF/graphene nanohybrid is used in the field of electrochemical capacitors, whereas its exploration for flame retardant properties of polymer matrix is rarely pursued. Herein, three-dimensional network polymer, namely COF, was pre­ pared by high-temperature condensation polymerization between the aldehyde group of terephthalaldehyde and the amine group of melamine in anhydrous dimethyl sulfoxide (DMSO) without catalyst. A novel graphene conjugated covalent organic framework ([email protected]) nano­ hybrid was fabricated by one-step reaction between the aldehyde groups in terephthalaldeyde and the amine group in NH2-graphene. For the first time, [email protected] nanohybrid was employed to improve the flame retardancy of EP matrix. The EP shows an enhanced flame retardancy, thermal stability and mechanical properties after the incorporation of [email protected] nanohybrid.

Fig. 1.

13

C NMR spectrum of COF.

2.2. Fabrication of graphite oxide (GO) and aminated graphene oxide (AGO) Firstly, GO was prepared by a modified Hummer’ method using graphite powder [36]. The as-prepared GO (0.5 g) was putted into DMF (500 ml) and sonicated for one day. Afterwards, NHS (1.7 g) and EDC�HCl (2.85 g) were charged into the above system as catalysts in an ice bath. After removing the ice bath, 1,3-diaminopropane (1 ml) was added to the system and stirred at room temperature for 12 h to obtain AGO. Whereafter, the product was filtered after washing with deionized water and anhydrous ethanol, and dried in a vacuum oven to give a black powder. 2.3. Synthesis of [email protected] nanohybrid The synthesis of the [email protected] nanohybrid based on 80 wt% AGO is depicted in Scheme 1. Firstly, AGO (1.25 g) was well dispersed in dry DMSO which has been removed water by nitrogen bubbling for 3 h at 100 � C. Melamine (0.12 g) and terephthalaldehyde (0.19 g) were added to the dispersion and then heated at 180 � C for 72 h under nitrogen atmosphere. The precipitates were filtered through a Buchner funnel before it cooling and washed three times with excess acetone, tetrahy­ drofuran and dichloromethane. The solid product was placed in a vac­ uum oven at 100 � C overnight to give a black fluffy powder in 52% yield. We also synthesized [email protected] nanohybrid with different AGO loading of 20 wt% and 50 wt% in the same way to determine appropriate addition ratio. [email protected] in the text refers to 80 wt% [email protected], while the characterization of 20 wt% [email protected] and 50 wt% [email protected] is described in the supplemental files.

2. Experimental section 2.1. Synthesis of COF Water was removed from the DMSO by nitrogen bubbling for 3 h at 100 � C before being used. A 500 ml three-necked flask equipped with a condenser, magnet, nitrogen inlet and outlet was charged with mel­ amine (9.07 g, 0.072 mol), terephthalaldehyde (14.47 g, 0.108 mol) and DMSO (450 ml). The three-neck flask was transferred to an oil bath at 180 � C and reacted for 72 h under nitrogen atmosphere. After that, the product was cooled to room temperature, filtered through a Buchner funnel and washed three times with excess acetone, tetrahydrofuran and dichloromethane. The solid product was placed in a vacuum oven at 120 � C overnight to give a pale yellow powder in 75% yield.

2.4. Synthesis of EP/[email protected] nanocomposites Briefly, the preparation of EP nanocomposites with 2 wt% [email protected]­ COF was depicted as below: [email protected] (0.9 g, 2 wt%) was added into acetone (100 ml) with mechanical stirring and sonicate for 2 h to make it well-dispersed. Then EP (36.21 g) was poured into the above dispersion with continuous mechanical stirring and sonication to obtain homoge­ neous mixtures. After that, the mixtures were heated to 100 � C in an oil bath until the acetone evaporated completely. Thereafter, pre-melted DDM (7.89 g) was added to the mixtures and stirred for 2 min. Finally, the EP/2 wt% [email protected] sample was pre-cured in an oven at 100 � C for 2 h and then post-cured at 150 � C for 2 h. After the curing 2

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Fig. 2. (a) FTIR spectra and (b) XRD patterns of COF, GO and [email protected]

reaction, the sample was naturally cooled to room temperature. Similar to the preparation procedure of EP/2 wt% [email protected], neat EP, EP/2 wt % COF and EP/2 wt% GO were also prepared as a contrast. All materials and characterization are listed in the Supplemental files.

and 286.9 eV, respectively [11]. Compared with GO, wide XPS scanning spectra of COF and [email protected] nanohybrid show a strong peak of N1s at 399.8 eV. In the meantime, the increase of the N1s peak and decrease of O1s peak can verify the formation of COF on AGO. As can be judged from Fig. 3(b) that the C1s peaks of GO in high-resolution XPS spectrum are deconvoluted into four forms of carbon, including C–C (284.6 eV), C–OH – O (288.1 eV). From Fig. 3(c), the (285.1 eV), C–O (286.9 eV) and C– peaks in C1s high-resolution XPS spectrum of COF correspond to C–C – N (287.6 eV) and C– – O (289.0 eV). (284.8 eV), C–N (285.7 eV), C– Meanwhile, there are mainly two types of nitrogen species in COF, – C) at 398.6 eV and which are assigned to 1,3,5-triazine units (C–N– amine (C–NH–C) moieties at 399.9 eV. In contrast to GO, the high-resolution XPS spectrum of [email protected] nanohybrid show the same types of nitrogen species and carbon species as COF, indicating the successfully formation of COF on AGO [31,35,43]. The morphology and micro-structure of COF, GO and [email protected] nanohybrid were exhibited in SEM and TEM results. It can be seen from Fig. 4 that GO is a typical pleated sheet with a size of several microns. As show in Fig. 4(b), the structure of COF is like three-dimensional net­ works with a good deal of clustered particles. Many porous structures can be clearly seen in Fig. 4(b), confirming that it is a polyporous polymer. As is clearly shown in Fig. 4(c,f) that [email protected] nanohybrid exhibits a morphology similar to GO rather than amorphous porous polymer when the condensation reaction between melamine and ter­ ephthalaldeyde is carried out on the surface of AGO. Neither separate porous polymer particles nor independent graphene sheets were observed in the TEM or SEM images, indicating that the majority of the monomers have been polymerized on the AGO surface. In order to understand the interfacial interaction between the addi­ tives and the EP resin, TEM was applied to study the microstructures and additive’s dispersion of the ultra-thin section surfaces of EP nano­ composites (Fig. 4(g–i)). After adding it to the EP matrix, the COF maintains its polyporous polymer structure of several hundred nano­ meters while the GO maintains its lamellar structure. Besides, only few individual COF and GO were observed in Fig. 4(i), indicating that the most of the COF still grow on the GO surface. All the additives are evenly dispersed in the EP.

3. Results and discussion 3.1. Characterization of COF, GO, and [email protected] The chemical structure of the COF was analyzed in detail by 13C solid state NMR spectroscopy (Fig. 1). The 13C NMR spectrum shows three resonances at 166, 128, and 53 ppm. The highest peak intensity in Fig. 1 – N bond in the is the peak at 166 ppm, which corresponds to the C– triazine ring of melamine. The chemical shift at 128 ppm was ascribed to the C–H aromatic carbons of benzene. The peak at 55 ppm corresponds to the -C–NH– bond of aminal structure formed by the addition of the amino groups with the aldehyde groups [37,38]. The structural information of COF, GO and [email protected] nanohybrid given by FTIR spectra (Fig. 2(a)) confirmed the successful fabrication of these three materials. The FTIR spectrum of GO exhibits some repre­ – O stretching sentative absorption peaks of oxygen functional groups: C– vibration of carboxyl groups (1728 cm 1), C–O stretching vibration of epoxy (1220 cm 1) and C–O stretching vibration of alkoxy (1053 cm 1). –C The peak at 1623 cm 1 is ascribed to the vibration of H2O or the C– stretching vibration of unoxidized graphite [39,40]. For the COF and [email protected] nanohybrid, the peaks at 3470 cm 1, 3420 cm 1 (NH2 stretching) and 1650 cm 1 (NH2 deformation) originate from the pri­ mary amine group of melamine, while 2870 cm 1 (C–H stretching) and – O stretching) for –CHO groups are disappeared or 1690 cm 1 (C– greatly weakened. Whereas, the obvious absorption peaks at 1550 cm 1 – N of the triazine ring, revealing the and 1460 cm 1 correspond to the C– melamine is successfully incorporated into the network. The presence of a broad and strong peak at 3400 cm 1 originates from N–H stretching vibration, as well as a peak at 1180 cm 1 belongs to the C–N stretching of secondary amine (–NH–), which proves a substantial amount of –NH–and –NH2 groups in COF and [email protected] nanohybrid [41,42]. The XRD patterns of GO, COF and [email protected] are shown in Fig. 2(b). The (002) sharp peak at 11.4� indicates an 0.760 nm interlayer distance of the GO. The weak peak at 42� is considered to the (100) reflection of GO. The COF shows a broad peak near 22� , originating from its amor­ phous structure. The XRD pattern of [email protected] shows only one broad peak around 24� , confirming that GO is successfully reduced and com­ bined with COF [31]. Fig. 3 displays the XPS information of COF, GO and [email protected] nanohybrid, which is applied to further characterize the elementary composition and bond state of the surface. Table S1 records the detailed elemental composition information. The wide XPS scanning spectrum of GO (Fig. 3(a)) exhibits the strong peaks of O1s and C1s located in 532.6

3.2. Thermal properties of EP and its nanocomposites The thermal stability and pyrolysis process of COF, GO and [email protected]­ COF nanohybrid were investigated by TGA under nitrogen atmosphere. The initial degradation temperature (T-5 wt%) and the maximum degra­ dation temperature (Tmax) are important parameters of thermal prop­ erties, which are defined by the temperature at 5 wt% mass loss and the temperature at maximum mass loss. It can be obviously seen from Fig. 5 that GO has undergone two major weightloss stages, the first stage (from room temperature to 110 � C) is derived from the evaporation of absor­ bed water and the second stage (190 � C–250 � C) is assigned to the 3

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Fig. 3. (a) XPS survey scan spectra of GO, COF and [email protected]; high-resolution C1s XPS spectra of (b) GO, (c) COF and (e) [email protected]; high-resolution N1s XPS spectra of (d) COF and (f) [email protected]

degradation of unstable oxygen-containing functional groups [44,45]. For the TGA curve of COF, the main weightloss from 400 to 480 � C is attributed to the decomposition of the acetal amine structure [46]. The amine and aldehyde groups decompose into ammonia and formic acid, leaving the triazine ring and the benzene ring. As the temperature in­ creases, the mass of the COF reposefully descends, leaving carbon skeleton [42]. TGA results demonstrate that [email protected] nanohybrid exhibits optimal thermal stability and the most char residues. The re­ sidual weight of COF, GO and [email protected] nanohybrid at 800 � C is calculated at 13.2, 47.9, and 73.2 wt%, respectively. Fig. 6 shows the TG/DTG curves for EP and its nanocomposites under

nitrogen and air atmosphere. Table S3 and Table S4 record detailed data of the TG/DTG curves. The thermal degradation behavior of EP nano­ composites is similar to neat epoxy, indicating that they have similar thermal degradation mechanism. Under nitrogen atmosphere, EP have only one phase of weight loss, corresponding to the degradation of the EP network structure. It can be intuitively seen from Table S2 that the COF, GO and [email protected] nanohybrid all catalyze the degradation of EP in advance, resulting in lower T-5 wt% and Tmax of EP. However, DTG curves indicate that the maximum mass loss rate of EP nanocomposites is all reduced, and EP/2 wt% [email protected] is reduced the most. Besides, the char residues of EP are increased by addition of COF, GO and 4

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Fig. 4. SEM images of (a) GO, (b) COF and (c) [email protected]; TEM images of (d) GO, (e) COF and (f) [email protected]; TEM ultrathin section images of (g) EP/2 wt% GO, (h) EP/2 wt% COF, and (i) EP/2 wt% [email protected]

3.3. Fire hazards The cone calorimeter based on the oxygen-consumption method can accurately simulate the combustion behavior of materials in actual fires. Fig. 7 shows the HRR and total heat release (THR) curves as a function of burning time for the EP and its nanocomposites. Table S3 records the various parameters of the materials obtained in the cone calorimeter test. PHRR and THR are important parameters to characterize the fire risk of materials. Meanwhile, the fire spreading index (FGI) is the ratio of PHRR to time to PHRR (TTP). The smaller the FGI value is, the longer it takes to reach the PHRR, the lower the fire risk is. As illustrated in Fig. 7(a), the TTP of neat EP is 106 s, and its PHRR is as high as 1945.1 kW/m2. After the addition of 2 wt% COF, GO and [email protected] nanohybrid, the PHRR values of EP nanocomposites decrease by 15.8%, 36.7%, and 43.6% respectively. The THR values show the same downtrend as PHRR, with a maximum decline of 24.3% (EP/2 wt% [email protected]) compared with the neat EP (80.0 MJ/m2). Furthermore, the TTP of EP is also increased by the incorporation of COF, GO and [email protected] nanohybrid. The TTP of EP/2 wt% [email protected] increases by 17 s, while the value of FGI drops to 8.9 kW/(m2⋅s). The test results illuminate that [email protected] nanohybrid shows the lowest fire risk, indicating that there are synergistic effect between COF and GO on reducing PHRR, THR and FGI of EP. It is inferred from Fig. S4 that LOI values of EP/2 wt% [email protected] increase from 24.0 to 25.5%. It indicates that [email protected] has a better flame retardant effect on EP than COF or GO alone. To give prominence to the superiority of this work, the combustion properties of EP nanocomposites are compared with the published works. The influence of different modification methods on the flameretardant effect of GO is summarized in Table 1. It is distinct that [email protected] shows superior effect on reducing THR and PHRR. The re­ sults show the merits of [email protected] in the field of flame retardant.

Fig. 5. TGA curves of COF, GO and [email protected] under nitrogen atmosphere.

[email protected] nanohybrid. The char residues of EP/2 wt% [email protected] is 15.7 wt%, whereas the neat epoxy is only 12.9 wt%, indicating that [email protected] nanohybrid promotes the catalytic carbonization of epoxy resin. It is referred from Fig. 6 (d) that EP and its nanocomposites have three thermal degradation stages under air atmosphere, including the volatilization of small molecules, the decomposition of macromolecular chains and the oxidation of char residue [23]. As is summarized in Table S4 that the T-5 wt% of EP/2 wt% GO and EP/2 wt% COF decrease slightly since the degradation of GO and COF in advance. The char residues of EP nanocomposites is higher than neat EP because of the char forming effect of fillers. However, EP/2 wt% [email protected] shows the best thermal stability which is probably attributed to the “tortuous path” effect of GO or the synergistic effect between COF and GO [47]. Even­ tually it will reduce the release of combustible gas and heat-release rate (HRR).

3.4. Analysis of the pyrolytic gaseous products Pyrolytic gaseous products produced in the combustion process of the polymer matrix directly affect the formation and propagation of flame. The combustion process of the polymer matrix can be inhibited 5

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Fig. 6. (a,c) TGA and (b,d) DTG curves for EP and its nanocomposites.

Fig. 7. (a) HRR and (b) THR curves of EP and its nanocomposites.

by reducing the flammable evolved gas. TG-IR technology is extensively used to detect the evolution process of pyrolysis gas products, which is helpful to analyze the gas phase flame retardant mechanism of polymer. It is obvious that adding COF, GO and [email protected] nanohybrid can reduce the peak absorbance of the total gaseous products and extend the time to peak, as shown in Fig. 8(a). It indicates that these additives can delay the degradation of the polymer matrix and inhibit the generation of the gaseous products during pyrolysis. Furthermore, the combination of GO and COF exerts the most obvious effect on the reduction of the intensity absorption peak, showing a synergistic effect. The FTIR spec­ trum at the maximum degradation rate is shown in Fig. 8(b). The main

pyrolysis products of EP are hydroxyl-containing compounds like water and/or phenol (3650 cm 1), hydrocarbons (3100-2800 cm 1), CO2 (2360 cm 1), CO (2190 cm 1), carbonyl-containing compounds (1740 cm 1) and aromatics (1610 and 1510 cm 1) [11,48]. The spectra of EP nanocomposites are same except for adsorption peak intensity, indi­ cating that they generate similar gaseous products during pyrolysis. In order to specifically elaborate the variation of pyrolytic gaseous products for EP and its nanocomposites, the curves of absorbance are shown in Fig. 9, including:(a) aromatics, (b) carbonyl-containing com­ pounds, (c) CO and (d) hydrocarbons. CO is the main toxic gas released in the combustion process of EP, which will seriously increase the fire 6

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risk. In Fig. 9(c), the addition of COF and GO can reduce the absorption intensity of CO, probably arised from the adsorption and blocking effect of GO and COF. Due to the synergistic effect of GO and COF, [email protected] nanohybrid has a superior inhibition effect on the release of CO. Combustible volatiles, including aromatics, hydrocarbons and carbonylcontaining compounds, exhibit a similar downward trend to CO, further demonstrating the synergistic effect of GO and COF on reducing pyrol­ ysis gas products. The reduced combustible volatiles will be converted to char residues with a barrier effect in condensed phase, as demonstrated by increased char yield in TGA results. Increased char residues and reduced combustible volatiles both contribute to the decrease of PHRR and THR during combustion, as confirmed by cone results. Therefore, the addition of [email protected] nanohybrid reduces the most important fire

Table 1 Combustion properties of EP nanocomposites in the literature. Additives

Reduced THR (%)

Reduced PHRR (%)

References

[email protected] wt% FRs-rGO-5 wt% SiO2-GNS-2 wt% PPGO-2 wt% DOPO-VTES-GO-3.6 wt % MoS2/GNS-2 wt% RGO-LDH-2 wt% FRGO-4 wt%

24,3 – 7.9 22 41.3

43.6 35 29.5 42 45.4

This work [12] [52] [45] [44]

25.3 13.6 30.2

45.8 37.9 37.7

[23] [53] [11]

Fig. 8. TG-IR results of EP and its nanocomposites; (a) Gram-Schmidt curves; (b) IR spectra at the maximum decomposition rate.

Fig. 9. Comparison of the absorbance of pyrolysis products for EP and its nanocomposites. 7

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Fig. 10. Digital photos and SEM images of the char residues. (a,e) EP, (b,f) EP/2 wt% COF, (c,g) EP/2 wt% GO and (d,h) EP/2 wt% [email protected]

Fig. 11. Raman curves of char residues. (a) EP, (b) EP/2 wt% COF, (c) EP/2 wt% GO and (d) EP/2 wt% [email protected]

hazards of EP including heat, smoke and toxic gas.

related to the flame retardancy of EP. Digital photos of char residues after cone test are shown in Fig. 10(a–d), the neat EP shows few frag­ mentary char residues. The char residues of EP/2 wt% COF and EP/2 wt % GO are more compared with the neat EP, however, they are untightness and incomplete. It is evident that more continuous and compact char residues are formed during combustion of EP/2 wt% [email protected] Meanwhile, the SEM assist us to observe the microstructure

3.5. Analysis of char residues of EP and its nanocomposites The research of morphologies, structures and properties of char residues is of great help to analyze of the flame retardant mechanism. The weight, density and graphitization degree of char residues are 8

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enhanced by the synergistic effect between GO and COF. The [email protected] nanohybrid shows a layer structure similar to graphene. It has been reported that layered compounds such as MoS2, graphene and mont­ morillonite (MMT) can effectively transfer load at the nanolayerpolymer interface, thus enhancing the mechanical properties of poly­ mer materials [51]. Simultaneously, the dispersion status of additives and the interfacial interaction between additives and polymer matrix can significantly affect the mechanical properties of nanocomposites. Herein, DMA is adopted to evaluate the relation between mechanical properties and temperature of EP and its nanocomposites. The temperature-dependent curves of storage modulus (a) and loss angle tangent Tanδ (b) were plotted in Fig. 12. The value of the storage modulus is closely related to the stiffness. It can be seen from Fig. 12(a) that the storage modulus of neat EP is only 28.00 GPa in the glassy state (T < Tg). The storage modulus of EP/2 wt% COF (29.07 GPa) increases by 3.7%, mainly owing to the large number of aromatic ring structures in the COF molecular chains. The movement of EP molecular chains is restricted by the rigid structure to a certain extent, thereby the storage modulus is enhanced. The storage modulus of EP/2 wt% GO (31.72 GPa) increases by 13.29%, mainly originating from the excellent mechanical enhancement effect of GO. Furthermore, EP/2 wt% [email protected] (34.66 GPa) shows the most significant increase in storage modulus, with an increase of 23.77%. It is arised from the synergistic enhancement effect between GO and COF. In Fig. 12(b), the peak temperature is considered to be the glass transition temperature (Tg), reflecting the relaxation of the polymer chain. In Fig. 12(b), Tg of neat EP is 172.7 � C. The increased Tg of EP/2 wt% COF (175.8 � C) may be attributed to nano-enhancement effect [4]. However, Tg of EP/2 wt% [email protected] (171.8 � C) and EP/2 wt % GO (166.6 � C) decreases, which may be caused by partial aggregation of additives according to the morphological results of EP and its nano­ composites (Fig. 4).

Scheme 2. Schematic illustration for flame-retardant mechanism.

and composition of external char residues (Fig. 10(a–d)). Fewer and smaller holes were observed on the char residue’s surface of EP/2 wt% COF and EP/2 wt% GO compared with that of neat EP. The addition of 2 wt% [email protected] nanohybrid into EP not only leads to an increase in char residues but also do contribute to formation of a continuous and dense char layers which function as an excellent physical barrier and inhibit the exchange of mass, heat and oxygen. Fig. 11 depicts the Raman spectra of char residues. It is obvious that all of the spectra display two strong peaks at approximately 1360 cm 1 and 1600 cm 1, corresponding to the D and G bands, respectively. The G band is generated by the E2g type vibration in the graphite lattice plane, while the D band is derived from the first-order Raman scattering of sp3 hybrid carbon atoms. The ratio of the integral area of D and G bands (ID/ IG) is used to evaluate the graphitization degree of char residues [49,50]. The ID/IG value of neat EP is 2.90. However, ID/IG value of EP/2 wt% [email protected] is 2.60, indicating its char residues with the highest graph­ itization degree. On the ground of aforesaid discussions, the possible flame-retardant mechanism of [email protected] nanohybrid in EP is speculated in Scheme 2. In the combustion process, [email protected] nanohybrid can promote the carbonization of EP which not only leads to a mass addition in char residues but also makes char layers more compact and graphitized. Compact char layers with high graphitization degree show excellent barrier effect on inhibiting the exchange of mass, heat and oxygen and the elimination of pyrolysis gases and smoke. Increased char residues and reduced combustible volatiles both contribute to the decrease of PHRR and THR and the enhancement of LOI. Therefore, the addition of [email protected] nanohybrid reduces the most important fire hazards of EP including heat, smoke and toxic gas.

4. Conclusions In this work, three-dimensional network polymer, namely COF, was prepared by high-temperature condensation polymerization between the aldehyde group of terephthalaldehyde and the amine group of melamine in anhydrous DMSO without catalyst. It exhibited incon­ spicuous advantage in flame retardant application. [email protected] nano­ hybrid was obtained by providing AGO as a reactive 2D template for the controlled polymerization of melamine and terephthalaldehyde. The results of XPS, XRD, and TEM demonstrated that COF successfully grows on AGO template. The well-designed [email protected] nanohybrid was applied to enhance the properties of EP through synergistic effect. The excellent physical blocking effect and thermal stability of [email protected] nanohybrid was helpful to reduce the fire hazards of EP including heat, smoke and toxic gas. At the same time, enhanced mechanical properties of EP were attributed to the load transfer of lamellar structure. This work

3.6. Mechanical properties of EP and its nanocomposites The above analyses indicate that the fire resistance of EP can be

Fig. 12. DMA curves of EP and its nanocomposites. (a) storage modulus and (b) tanδ as a function of temperature. 9

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has opened up a new application for [email protected] nanohybrid in the field of flame-retardant polymers.

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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Acknowledgements The work was financially supported by the National Key Research and Development Program of China (2017YFC0805904), National Natural Science Foundation of China (51761135113), Fundamental Research Funds for the Central Universities (WK2320000041) and China Postdoctoral Special Funding (2019TQ0309). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107616. References [1] Li Z, Gonz� alez AJ, Heeralal VB, Wang D-Y. Covalent assembly of MCM-41 nanospheres on graphene oxide for improving fire retardancy and mechanical property of epoxy resin. Compos B Eng 2018;138:101–12. [2] Guo W, Yu B, Yuan Y, Song L, Hu Y. In situ preparation of reduced graphene oxide/ DOPO-based phosphonamidate hybrids towards high-performance epoxy nanocomposites. Compos B Eng 2017;123:154–64. [3] Liu S, Fang Z, Yan H, Chevali VS, Wang H. Synergistic flame retardancy effect of graphene nanosheets and traditional retardants on epoxy resin. Compos Appl Sci Manuf 2016;89:26–32. [4] Zhou X, Qiu S, Xing W, Gangireddy CSR, Gui Z, Hu Y. Hierarchical [email protected] disulfide hybrid structure for enhancing the flame retardancy and mechanical property of epoxy resins. ACS Appl Mater Interfaces 2017;9(34):29147–56. [5] Gong J, Li Y, Chen Y, Li J, Wang X, Jiang J, et al. Approximate analytical solutions for transient mass flux and ignition time of solid combustibles exposed to timevarying heat flux. Fuel 2018;211:676–87. [6] Wang Z, Wei P, Qian Y, Liu J. The synthesis of a novel graphene-based inorganic–organic hybrid flame retardant and its application in epoxy resin. Compos B Eng 2014;60:341–9. [7] Qin JY, Zhang WC, Yang RJ. Intercalation process in the preparation of 9,10dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-montmorillonite nanocompounds and their application in epoxy resins. Mater Des 2019:178. [8] Wang X, Zhou S, Xing W, Yu B, Feng X, Song L, et al. Self-assembly of Ni–Fe layered double hydroxide/graphene hybrids for reducing fire hazard in epoxy composites. J Mater Chem 2013;1(13). [9] Liu S, Chevali VS, Xu Z, Hui D, Wang H. A review of extending performance of epoxy resins using carbon nanomaterials. Compos B Eng 2018;136:197–214. [10] Yan W, Yu J, Zhang M, Wang T, Wen C, Qin S, et al. Effect of multiwalled carbon nanotubes and phenethyl-bridged DOPO derivative on flame retardancy of epoxy resin. J Polym Res 2018;25(3). [11] Yu B, Shi Y, Yuan B, Qiu S, Xing W, Hu W, et al. Enhanced thermal and flame retardant properties of flame-retardant-wrapped graphene/epoxy resin nanocomposites. J Mater Chem 2015;3(15):8034–44. [12] Qian X, Song L, Yu B, Wang B, Yuan B, Shi Y, et al. Novel organic–inorganic flame retardants containing exfoliated graphene: preparation and their performance on the flame retardancy of epoxy resins. J Mater Chem 2013;1(23). [13] Wang D, Song L, Zhou K, Yu X, Hu Y, Wang J. Anomalous nano-barrier effects of ultrathin molybdenum disulfide nanosheets for improving the flame retardance of polymer nanocomposites. J Mater Chem 2015;3(27):14307–17. [14] Zhou K, Liu J, Shi Y, Jiang S, Wang D, Hu Y, et al. MoS2 nanolayers grown on carbon nanotubes: an advanced reinforcement for epoxy composites. ACS Appl Mater Interfaces 2015;7(11):6070–81. [15] Sang B, Li Z-w, Li X-h, Yu L-g, Zhang Z-j. Graphene-based flame retardants: a review. J Mater Sci 2016;51(18):8271–95. [16] Kausar A, Anwar Z, Muhammad B. Overview of nonflammability characteristics of graphene and graphene oxide-based polymeric composite and essential flame retardancy techniques. Polym Plast Technol Eng 2016;56(5):488–505. [17] Liu S, Yan H, Fang Z, Wang H. Effect of graphene nanosheets on morphology, thermal stability and flame retardancy of epoxy resin. Compos Sci Technol 2014; 90:40–7. [18] Cai W, Wang J, Pan Y, Guo W, Mu X, Feng X, et al. Mussel-inspired functionalization of electrochemically exfoliated graphene: based on selfpolymerization of dopamine and its suppression effect on the fire hazards and smoke toxicity of thermoplastic polyurethane. J Hazard Mater 2018;352:57–69.

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