Facile Construction of Flame-Retardant-Wrapped Molybdenum

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Facile Construction of Flame-Retardant-Wrapped Molybdenum Disulfide Nanosheets for Properties Enhancement of Thermoplastic Polyurethane Wei Cai, Jing Zhan, Xiaming Feng, Bihe Yuan, Jiajia Liu, Weizhao Hu, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 6, 2017

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Industrial & Engineering Chemistry Research

Facile Construction of Flame-Retardant-Wrapped Molybdenum Disulfide Nanosheets for Properties Enhancement of Thermoplastic Polyurethane Wei Caia, Jing Zhanb, Xiaming Fenga, Bihe Yuanc, Jiajia Liu a, Weizhao Hu*a, Yuan Hu*a a

State Key Laboratory of Fire Science, University of Science and Technology of China,

Hefei 230026, P. R. China b

School of Civil Engineering and Environmental Engineering, Anhui Xinhua

University, Hefei, Anhui 230088, P.R. China c

School of Resources and Environmental Engineering, Wuhan University of

Technology, Wuhan 430070, P. R. China

Corresponding Author. *E-mail: [email protected]; [email protected]; Fax/Tel: +86-551-63601664

Abstract A simple method of organically modified molybdenum disulfide (MoS2) was presented here for hindering the re-stack process of MoS2 nanosheets and enhancing interface interaction with polymer matrix. The obtained f-MoS2 was introduced into thermoplastic polyurethane (TPU) matrix for the properties enhancement. Incorporated f-MoS2 nanosheets resulted in dramatic suppression on fire hazards of TPU in terms of reduced peak heat-release rate (decreased by 45.4 %), low gas 1

ACS Paragon Plus Environment


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degradation yield, and formation of a compact and robust char layer. After adding 2.0 wt % f-MoS2, the tensile strength increased by 40.9 %. These significant reinforcements can be attributed to tripartite cooperative mechanism of the strong interface interaction between f-MoS2 and TPU matrix, the barrier effect of layered materials, and catalyst action of MoS2. The facile approach can provide a novel strategy to modify MoS2 nanosheets and extends its practical application in the field of polymer composites.

Keywords: molybdenum sulfide nanosheets, thermoplastic polyurethane composites, thermal stability, flame retardancy 1. Introduction Because of excellent abrasion resistance, good processability, high chemical stability and mechanical performance, thermoplastic polyurethane (TPU) has been used widely in many material fields, such as strain sensors,1 coatings2 and phase change materials.3 For example, Fan et al. prepared the steady conductive CNTs/TPU fibers for supervising large strains up to 400 % with high reversibility.4 Kinga et al. prepared polyurethane-based phase change materials and exhibited higher heat of phase transition.3 However, the application of TPU has been severely limited in many specific occasions, such as furniture, transportation facilities and household appliances, due to its intrinsic fire hazard. Therefore, it stimulates a lot of research about improving fire safety of TPU. For example, Wang et al. synthesized zinc hydroxystannate to reduce fire hazards of TPU with assistance of ammonium 2


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polyphosphate.5 Gavgani et al also enhanced the fire safety of polyurethane composites with a combination of reduced graphene and intumescent flame retardant.6 At present, most researches about improving fire safety of polyurethane are based on halogen-containing flame retardants, intumescent flame retardants and traditional inorganic fillers. Though enhancing authentically the fire safety of TPU, these solutions expose many problems also. For example, plenty of highly toxic and potentially carcinogenic halogenated furans and dioxins may generate during combustion, when halogen-containing flame retardants were incorporated into polymer matrix. Likewise, it can hardly avoid the high usage amount of intumescent flame retardant for achieving satisfactory effect on fire safety performance. And the mechanical properties of polymer composites usually worsen significantly when additional flame retardants are poor compatibility with polymer matrix.7 Given the fact mentioned above, it is necessary to find a high-efficiency flame retardant to enhance the fire safety of TPU without deteriorating the mechanical properties. Recently, nanostructured materials have shown a significant potential in enhancing the fire safety of polymer materials, and overcoming the drawbacks of above flame retardant approaches.8-10 Researchers throughout the world have synthesized numerous functionalized flame retardants base on graphene, layered double hydroxides and montmorillonite.8, 11-13 Even a small amount of these hybrids could greatly enhance the flame retardancy and mechanical properties of polymer composites, due to unique two dimensional structure which plays barrier effect. As a member of 2D layered nanomaterials, molybdenum disulfide (MoS2) has drawn 3



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enormous attention in various materials fields.14, 15 Similar to graphene, the hexagonal structure of MoS2 brings many excellent properties in semiconductor,16 catalytic,17 transistors18 and a solid lubricant.15 But, as far as we know, research work about the application of MoS2 on flame retardant field is relatively less than other layered nanomaterials, especially in improving fire safety of TPU. However, MoS2 owns higher thermal stability than other layered nanomaterials mentioned above and shows a better potential in flame retardant field due to stable layered structure. Given intrinsic semiconductor characteristic, MoS2 are applied in some special fields that require electrical insulation property and high dielectric constant of polymer composites. In addition, MoS2 can be used as smoke suppressor, due to inherent flame retarding effect. Specifically, various and abundant active sites exist on the surface of MoS2, including sulfur defect, vacancy, and edge sites. Previous literature has confirmed that MoS2 with active sites can integrate with gas molecules and smoke particles.19 These toxic volatiles and smoke particles are reduced and contribute to the formation of dense char layer, indicating the outstanding catalytic activity of MoS2. With that in mind, this work adopted molybdenum disulfide nanosheets to enhance fire safety and mechanical performance of TPU. So far, one of the most recognized technologies to obtain MoS2 nanosheets is to exfoliate 2D nanosheets from bulk materials with the lithium intercalation.20, 21 By the hydrolysis of lithium intercalated MoS2 bulk, the aqueous suspension containing dispersed MoS2 nanosheets is easily prepared. However, the re-stack into agglomeration of MoS2 nanosheets is highly susceptible to happen during the 4


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collection process from the aqueous phase. The agglomeration of MoS2 is difficult to disperse again in polymer matrix, thus destroying the processability in practical application of MoS2. In addition, the application of MoS2 on polymer composites is also confined, due to weak interface interaction between MoS2 and polymer matrix. When weak interface interaction is not able to facilitate the dispersion, MoS2 as a bulk material has a pronounced tendency to agglomerate in a polymer matrix.22 Thus, the re-stack to into agglomeration and weak interface interaction both create a side-effect about guaranteeing the homogeneous dispersions of MoS2 in polymers matrix. Specially, strong interface interaction can facilitate the delivery of load to robust nanomaterials from weak polymer chain and enhance the mechanical properties of polymer composites. Some literatures about the organic modification of MoS2 have been reported by our group.

23-25

However, these works all adopt small molecules to

decorate the surface of MoS2 nanosheets. It is widely accepted that MoS2 wrapped by polymer chain can result in stronger interface interaction with polymer matrix. On the basis of the discussion above, it is necessary to hinder the re-stack process of MoS2 nanosheets and enhance the interface interaction for maximizing MoS2 reinforcement potential in polymer composites. In this work, exfoliated MoS2 was firstly wrapped by branched polyethyleneimine; following flame retardant (DOPO) was used to modify the surface of PEI-MoS2 base on Kabachnik–Fields reaction. The polyethyleneimine (PEI) is positively charged in aqueous solutions. Thus chemically exfoliated MoS2 nanosheets were firstly wrapped with PEI in aqueous phase by electrostatic interaction. In consideration of hard segment part in TPU composition, 5



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9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was also used to modify MoS2 nanosheets and excepted to improve the interface interaction in term of enhanced π-π interaction. We anticipated that the addition of f-MoS2 nanosheets could significantly reinforce the thermal stability, fire safety and mechanical strength of TPU composites. This study will reveal the specific mechanism for the property enhancements and promote the development of polymer/MoS2 composites. 2. Experiment 2.1. Raw Materials Thermoplastic polyurethane (TPU, 1195A) was obtained from BASF Co. Ltd. Molybdenum disulfide (MoS2, purity ≈ 98%), branched polyethyleneimine (PEI, AP, Mw=ca. 70 000) and benzaldehyde (AP) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The n-butyl lithium (2.2 M in hexane) was purchased from Aladdin

Industrial

Corporation

without

further

purified.

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was supplied by Shandong Mingshan Fine Chemical Industry Co. Ltd (Shandong, China). The solvents were all reagent grades and supported by Sinopharm Chemical Reagent Co. LtdS (Shanghai, China). 2.2. Preparation of LixMoS2 LixMoS2 was prepared by the solvothermal methodology. In a typical experiment, 1.0 g of MoS2 bulk was put into the autoclave and 10.0 mL of 2.2 M solution of n-butyl lithium in 80 mL hexane was added subsequently. The autoclave was heated at

6


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100 oC for 4 h. The product was collected by centrifugation, washed with anhydrous hexane, and then dried at 50 oC in a vacuum oven. 2.3. Preparation of Flame Retardant Functionalized MoS2 The MoS2 nanosheets were obtained through the hydrolysis and ultrasonication of LixMoS2 in deionized water. In a typical process, 1.0 g of LxMoS2 was added into 500 mL of deionized water and then exfoliated to single or few layers MoS2 with negatively charged via the rapid hydrolysis and ultrasonication process. Then the homogeneous suspension of MoS2 nanosheets was slowly dropped into the PEI solution (1 mM) with positive charged. After 4 h vigorous stir, the obtained PEI-MoS2 was collected by vacuum filtration and thoroughly washed with anhydrous ethanol to remove the needless PEI. Following, the PEI-MoS2 was dispersed again in anhydrous ethanol with assistance of a sonication bath for 2 h and vigorous stir. DOPO (1.6 g) and benzaldehyde (0.8 g) were introduced into the PEI-MoS2 solution and stirred at 60 oC for 6 h. The final product was collected by vacuum filtration and thoroughly washed with anhydrous ethanol and DMF. The obtained product was designated as f-MoS2 and f-PEI was also synthesized with same procedure at absence of MoS2 nanosheets. In addition, exfoliated MoS2 nanosheets were designated as unmodified MoS2, without further modification. Figure 1 illustrates the synthesis process of f-MoS2. 2.4. Preparation of f-MoS2/TPU Composites Briefly, TPU-based composites were fabricated by solvent blending and co-coagulation. Typically, f-MoS2 (0.5 g) was added into 100 mL DMF and got a 7



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homogeneous suspension with sonication for 1 h. Subsequently, 49.5 g TPU dissolved in DMF was poured into f-MoS2 dispersion with assistance of violent stirred for 2 h. Finally, the mixed solution was slowly poured into 1 L deionized water with slight stirring to get the flocculent production. The flocculate was hot-pressed at 190 oC and 10 MPa for 10 min into sheets with appropriate size. Another sample containing 2.0 wt % f-MoS2 was prepared by the same procedure. 2.5. Characterization X-ray diffraction (XRD) measurements were carried out with a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipped with a CuKα tube and Ni filter (λ = 0.1542 nm). Raman spectroscopy was performed with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA) with excitation provided in backscattering geometry by a 514.5 nm argon laser line. Morphologies of fractured surface and char residue were observed with a XL-30 ESEM scanning electron microscope (SEM) at an acceleration voltage of 10.0 kV. Fourier transform infrared (FTIR) spectroscopy was employed by a Nicolet 6700 spectrometer (Nicolet Instrument Company) in 500-4000 cm-1 scope. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded by a Nicolet 6700 spectrophotometer using 32 scans in the frequency region of 4000-400 cm-1.

8


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Transmission electron microscopy (TEM) was used to study the morphology of graphene with a JEOLJEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was executed with a TGA Q5000 IR thermogravimetric analyzer (TA Instruments, U.S.) at a heating rate of 20 oC min-1 in air and nitrogen atmosphere. Combustion test was performed on a cone calorimeter (Fire Testing Technology, UK) according to ISO 5660 standard procedures, with 100 × 100 × 3 mm3 specimens. Each specimen was exposed horizontally to 35 kW/m2 external heat flux. Tensile strength of the composites was recorded with an electronic universal testing instrument (MTS System Co., Ltd., China) at a crosshead speed of 100 mm/min. Thermogravimetric analysis-infrared spectrometry (TG-IR) was investigated with a TGA Q5000IR thermogravimetric analyzer linked to a Nicolet 6700 FTIR spectrophotometer from 20 to 700 °C at 20 °C min-1 (N2 atmosphere, flow rate of 30 mL min-1). X-ray photoelectron spectroscopy (XPS) test was performed to characterize the element of MoS2 nanosheets with a VG ESCALB MK-II electron spectrometer. The excitation source was an Al Kα line at 1486.6 eV.

3. Results and Discussion 3.1. Flame Retardant Wrapped MoS2 nanosheets For confirming the effect of organic modification on hindering the restack process, XRD patterns were used to research the crystal structures of the unmodified MoS2, 9



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PEI-MoS2 and f-MoS2. An intense characteristic peak at 14.48o is observed from XRD curve of unmodified MoS2 nanosheets inserted in Figure 2a, indicating the interlayer spacing of the (002) plane of pristine MoS2.26 This phenomenon confirms that the certain degree of restack is avoidless during the process of collecting exfoliated MoS2 nanosheets from solution phase. Compared to unmodified MoS2, it is easy to find that the (002) peak of PEI-MoS2 and f-MoS2 all shift to a lower position (2θ=8.99o), demonstrating

the

expansion

of

interlayer

space

induced

by

surface

functionalization.27 In addition, the intensity of peak at 2θ = 14.48o is visibly impaired in PEI-MoS2 and f-MoS2, especially the latter. The above analysis manifests that successful organic modification effectively restrains the restack process of f-MoS2 nanosheets. In order to obtain the morphological information, unmodified MoS2 and f-MoS2 were observed with TEM and the corresponding results are presented in Figure 2b and c. A thin 2D morphology is able to be observed in unmodified MoS2 and f-MoS2. It illustrates the process of functionalized modification does not destroy the layered structure of MoS2. Moreover, the energy dispersive X-ray (EDX) measurement is performed to research the element compositions of f-MoS2. The appearances of phosphorus and nitrogen elements confirm that MoS2 is successfully modified by PEI and DOPO. To confirm further the successful modification, FTIR spectra was used for investigating the unmodified MoS2, and PEI-MoS2 and f-MoS2. Compared with the FTIR spectra of unmodified MoS2, new absorption peaks at 2924 and 2853 cm-1 can 10


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be observed in the spectra of PEI-MoS2, which are attributed to the -CH2 stretching vibration of PEI.28 In addition, several representative absorption peaks at 3434 and 1630 cm-1 are due to the –NH2 and –NH of PEI.29, 30 These results indicate that PEI molecules adsorb stably on the surface of MoS2 nanosheets. Compared to PEI-MoS2, f-MoS2 presents a series of new absorption peaks. These peaks are attributed to the characteristic absorption of DOPO and benzaldehyde, at 1586 and 751 cm-1 attributed to phenyl structure, 1198 cm-1 attributed to P=O group.31, 32 As Figure 3b indicates, these same characteristic peaks are clearly observed in the FTIR spectra of f-PEI and f-MoS2. The above results verify the successfully organic modification of f-MoS2 nanosheets. In case of the organic modification of MoS2 nanosheets was processed favourably, the element compositions of f-MoS2 are distinct compared to those of the unmodified MoS2. Therefore, X-ray photoelectron spectroscopy (XPS) test was carried out to confirm the successful functionalization (Figure 3c). An interesting phenomenon can be found that the peak intensity corresponding to Mo 3p, Mo 3d and S 2p, gradually decrease. However, the content of O 1s presents a firstly decreasing trend and then increasing trend along with the modification process (Table 1). The changes of element content are accord with the designed process. Besides, nitrogen and phosphorus elements are simultaneously found in the XPS spectrum of f-MoS2 nanosheets (Figure 3d and e), which confirms the presence of f-PEI on the surface of MoS2 nanosheets.

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The amount of f-PEI modified to MoS2 nanosheets is of vital importance for f-MoS2 to reinforce the performances of composites. Therefore, TGA is performed to research the composition of PEI-MoS2 and f-MoS2. As Figure 3f exhibits, unmodified MoS2 owns a high thermal stability which maintains quality up to 700 oC in N2 atmosphere. In contrast, PEI-MoS2 and f-MoS2 all present a weight loss from 150 to 400 oC. According to the weight loss in the TGA curves, the amount of f-PEI modified to MoS2 nanosheets is calculated to be approximately 22.1 wt %, implying a high degree of functionalized modification. Remarkably, f-MoS2 presents a higher weight loss than that of PEI-MoS2. The phenomenon illustrates that the incorporated DOPO does not destroy the electrostatic interaction between the PEI and MoS2. 3.2. Characterization of TPU/f-MoS2 Composites The fracture morphologies of composites were characterized by SEM, for investigating the dispersion and interfacial interaction between f-MoS2 and TPU matrix. A wave-like and relatively smooth fracture surface is observed in pure TPU (Figure 4a). The similar fracture surfaces are also obtained in TPU composites (Figure 4b and c). Though the similar micro-morphologies can be found in TPU composites, the significant differences between TPU and its composites are clearly visible. Obviously, irregular protuberances are presented in fracture surface of TPU composites, which imply the increasing roughness. A vital phenomenon is acquired that the boundary of wave-like fracture surface becomes more apparent and clear. These observations together indicate the strong interfacial interaction between f-MoS2 and TPU matrix, which is attributed to the π-π action exist between the benzene rings 12


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of the f-MoS2 and the hard segment of TPU.33 In addition, no apparent agglomerates are found in the fractured surface of TPU composites. And it is noteworthy that many light white spot distributes randomly on the fractured surface of TPU-2.0 (Figure 4c and d), demonstrating that the f-MoS2 nanosheets exhibit excellent dispersibility in the TPU matrix. ATR-FTIR spectrum was used to further evaluation of the interface interaction between nanofiller and polymer matrix. ATR-FTIR spectra of pure TPU and TPU-2.0 have two distinguished bands: a main peak centered at 1732 and a relatively small shoulder around 1700 cm-1. The peak at 1732 cm-1 is associated with -C=O groups that are “free” (non-hydrogen bonded) and the shift to 1700 cm-1 results from hydrogen bonding with f-MoS2.34 For 2.0 wt % sample, the intensity ratio between hydrogen-bonded

and

“free”

carbonyl

domain

is

obviously

altered

and

hydrogen-bonded carbonyl is dominant (Figure 5). This phenomenon illustrates that hydrogen bonding also enhances the interfacial interaction between f-MoS2 and TPU matrix. 3.3. Properties of TPU/f-MoS2 Composites Due to thermal stability is an important property for flame retardant composites, TPU and its composites were investigated by TGA under air and nitrogen atmosphere. The TGA curves and data of TPU and its composites are shown in Figure 6 and Table 2. Three-stage decomposition process of pure TPU is observed in air atmosphere, which corresponds to the degradation of the principal TPU chains, the further degradation of polyols and isocyanates and oxidation of the char residue.5 Compared 13



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with pure TPU, the initial decomposition process of TPU composites is accelerated and the advanced tendency is more obvious with increasing f-MoS2 content. When the addition of f-MoS2 reaches 2.0 wt %, the temperatures at 5.0 wt % weight loss (T5%) is reduced by 10.8 oC compared to that of pure TPU. Consistent with previous literature,35 it is attributed to the high thermal conductivity of f-MoS2, which improves the delivery of heat. However, the maximum weight loss rate of TPU-2.0 for the first decomposition stages is decreased by 25.2 % (Figure 6b). Besides, the temperatures corresponding to 50.0 wt% weight loss (T50%) of TPU-2.0 is obviously increased by 34.3 oC. Though the oxidation of the char residue is promoted in the presence of f-MoS2, the char residue at 700 oC is still increased from 1.6 % to 3.8 % for TPU-2.0. Similarly, TPU composites present a lower initial decomposition temperature under nitrogen atmosphere. The residual percentage at 700 oC of TPU-2.0 is about 8.1 wt %, which is higher than that of pure TPU (5.0 wt %). The enhanced thermal stability can be ascribed to homogeneous dispersion of f-MoS2 nanosheets, which plays a role as barrier to prevent the delivery of small gaseous molecules. Moreover, f-MoS2 may also act as catalyst to promote the formation of the char residue during degradation. 3.4. Fire Hazards Analysis In previous work, the fire safety of polymer can be enhanced with the addition of layered montmorillonite, LDH and graphene.36-38 As a member of 2D layered material famliy, MoS2 is also expected as effective flame retardant to reduce the fire hazards of TPU. Therefore, cone calorimeter test accepted as a universal instrument was carried out to confirm this hypothesis. Heat release rate (HRR) and total heat release (THR) 14


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are the key factors for evaluating fire safety. The HRR and THR curves of TPU and its composites are presented in Figure 7a and b. Pure TPU shows a poignant HRR plot and a high pHRR value of 1241 kW/m2. With increasing the content of f-MoS2,the samples show a gradually decreasing pHRR values. With 2.0 wt % f-MoS2 in TPU matrix, a remarkable decline in pHRR value (663 kW/m2) can be obtained. In addition, the TPU composites present advanced HRR curves, which is constent with the TGA test.39 Moreover, significant decrease can be observed in THR curves, which the THR value of TPU-2.0 (55.7 kJ/g) is considerably reduced compared to that of pure TPU (74.6 kJ/g). These results indicate that the incorporation of f-MoS2 obviously enhances the fire safety of TPU composites. 3.5. Flame Retardant Mechanism In consideration of a thorough understanding of flame retardant mechanism is contributed to establish the thorough fire safety theory. Multifarious test means were carried out to research the gas phase and condensed phase mechanism. TG-FTIR test possesses the ability to track the production process of gas volatiles during thermal degradation process. FTIR spectra obtained at the maximum evolution rate during the thermal degradation of pure TPU and TPU-2.0 are presented in Figure 8a. There is no obvious difference in the two spectra, indicating incorporating f-MoS2 do not change the types of primary degradation products. The absorbance intensity is normalized with the total sample mass. The total absorbance intensity of TPU-2.0 is effectively reduced (Figure 8b), implying the suppressed release of gas decomposition products. It can be observed that the absorbance intensity of the pyrolysis products for TPU-2.0 15



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is lower than that of pure TPU (Figure 9), including isocyanate (2275 cm-1), aromatic compounds (1608 cm-1), CO2 (2354 cm-1), and ethers (1112 cm-1). The decrease of isocyanate, aromatic compounds and ethers can effectively enhance the fire safety, due to the fuel role of organic gas pyrolysis products during combustion. In addition, the assumption is reasonable that the smoke production and toxic gas release are significantly hindered, due to the volatiles easily gather into hydrocarbons and generate smoke particles. Digital photos and SEM photographs of residue char of TPU and TPU-2.0 after cone calorimeter test are presented in Figure 10a and b. Sparse and fragile char is left for pure TPU (Figure 10a), implying the char residue of pure TPU is not able to protect the bottom polymer during combustion.40 Obviously, the char residue yield of TPU-2.0 is significantly increased and continuously structural char can be obtained (Figure 10b). Further, the microstructure of char residue is analyzed based on the SEM photographs. Plentiful cavity can be observed in the residue char of pure TPU. However, TPU-2.0 presents an integrated and compact char structure (Figure 10d). Previous literatures have confirm that the compact and robust char structure enhance the fire safety of TPU composites.41, 42 In order to investigate the composition of char residue, XPS analysis was carried out to determine the kind of carbon element. For the characteristic C 1s spectrum, a sign peak for C-O and C=O are observed in Figure 11a. In remarkable contrast, the content of C-O and C=O in the C 1s spectrum of TPU-2.0 is much lower than that of TPU, implying a higher graphitization degree in TPU-2.0.39 More intuitive evidences 16


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are supported in Figure 11c. The Ca/Cox (Ca, aliphatic and aromatic carbons and Cox, oxidized carbons) value in TPU-2.0 is higher than that of pure TPU, indicating a higher graphitization degree in char residue of TPU-2.0. After a cone calorimeter test, many floriform crystals are observed on the char residue of TPU-2.0. For determining what this crystals are, XRD analysis was used to survey the char residue (Figure 11d). The XRD pattern exhibits a broad and weak diffraction peak in 23o, indicating the formation of graphitized char. In addition, the visible peaks at 11.4o, 32.9o, 33.8o, 39.7o, 44.3o and 49.9o are ascribed to the diffraction of MoO3,43 generating from the catalytic oxidation of f-MoS2 with the presence of oxygen-containing pyrolysis products (C-O-C).35 Based on the above analyses, a possible flame retardant mechanism of f-MoS2 in TPU matrix is speculated in Figure 12. Firstly, f-MoS2 distributes homogeneously in TPU matrix by strong interface interaction, due to the embrace of flame retardant. Therefore, f-MoS2 favourably performs a barrier effect and causes a labyrinth-like construction. The special construction increases the difficulty of the delivery of degradation product to combustion source, with a flexural and endless road. And the layered structure of f-MoS2 also results in the high-efficiency physisorptions and chemisorptions of the pyrolysis products generated from the TPU matrix. With assistance of the catalytic action of MoS2 nanosheets and the inherent effect of flame retardant, the reactive oxygen species degraded from the oxygen-containing pyrolysis products are reduced into aliphatic or aromatic carbons and f-MoS2 itself is oxidized into MoO3.35 The transferred aliphatic and aromatic carbons form a compact and 17



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robust char residue and reduce the provision of fuel which degradation products usually play. It can be concluded that the well-disperse of f-MoS2 caused by organic modification is the base of the excellent flame retardancy, thus catalytic effect and layered structure of MoS2 have optimal effects on the enhanced fire safety. 3.6. Mechanical Property In view of the established high interface interaction between f-MoS2 and TPU matrix, the mechanical property of TPU composites was evaluated by tensile test. As shown in Figure 13, the reinforcing effect of f-MoS2 is prominent: TPU-2.0 achieves the maximum augment in the tensile strength (40.9 %). As a typical elastomer, pure TPU presents a weak stress with a high elongation at break. With the addition of 1.0 and 2.0 wt % f-MoS2, TPU composites present an improved mechanical property with the increasing filler content. When the strain achieves 300 %, the stress of TPU-2.0 is increased by 407.5%. The enhanced mechanical property results from the higher load transfer efficiency between f-MoS2 sheets and TPU matrix with strong interface interaction. These are the hydrogen bond and conjugation effects formed between flame retardant and TPU matrix cause the strong interface interaction.1, 22, 36

Conclusions In this work, flame retardant wrapped MoS2 nanosheets were prepared by the combination of PEI and DOPO and characterized by multiple tests. SEM photographs of fracture morphologies confirmed the well-dispersed f-MoS2 in TPU matrix and strong interface interaction between f-MoS2 and TPU matrix. The TPU/f-MoS2 composites exhibited an obvious enhancement in thermal stability, fire safety and 18


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mechanical property. From TGA results and the cone calorimeter test, the thermal stability and fire safety of TPU composites were effectively improved with the suppressive second degradation process and less heat release rate, respectively. With the combination of various methods, the mechanism of improved fire safety was put: based on the embrace of flame retardant, f-MoS2 well dispersed in TPU matrix and strongly winded with the molecule chain of TPU. Thus, the layered structure and catalyze action of f-MoS2 gain maximum effect in suppressing the fire hazard of TPU. This facile approach provides a new route to modify MoS2 nanosheets and broads the MoS2 application on the field of polymer composites.

Acknowledgement The research grants were supported by National High-tech R&D program (2016YFB0302104), the National Natural Science Foundation of China (Grant Nos. 51303167 and 51603200) and the Fundamental Research Funds for the Central Universities (Grant Nos. WK2320000027 and WK2320000037).

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Table and Figure Captions Table 1 Quantitative comparison of elements detected from XPS spectra (in at. %) Table 2 TGA data of TPU and its composites. Figure 1 Schematic for preparation of f-MoS2 with combination of the electrostatic interaction and Kabachnik–Fields reaction. Figure 2 (a) XRD patterns of unmodified MoS2, PEI-MoS2 and f-MoS2; TEM images of (b) unmodified MoS2 and (c) f-MoS2; (d) EDX result of f-MoS2. Figure 3 (a) FTIR spectra of unmodified MoS2, PEI- MoS2 and f-MoS2; (b) FTIR spectra of f-PEI and f-MoS2; (c) XPS survey scans of unmodified MoS2, PEI- MoS2 and f-MoS2; (d) N 1s and (e) P 2p spectra of f-MoS2; (f) TGA of unmodified MoS2, PEI- MoS2, f-MoS2 and f-PEI. Figure 4 SEM images of the fractured surfaces of (a) pure TPU, (b) TPU-1.0 and (c and d) TPU-2.0. Figure 5 ATR FTIR spectra of pure TPU and TPU-2.0 Figure 6 TGA and DTG curves of TPU and its composites under (a and b) air and (c and d) nitrogen atmosphere; Figure 7 (a) HRR and (b) THR versus time curves of TPU and its composites. Figure 8 (a) FTIR spectra of the pyrolysis gaseous products emitted from TPU and TPU-2.0 at the maximum degradation rate; (b) Total absorbance of pyrolysis products for TPU and TPU-2.0 versus time Figure 9 Absorbance of pyrolysis products for TPU and TPU-2.0 versus time: (a) ether, (b) –NCO, (c) aromatic compounds and (d) CO2. 27



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Figure 10 Photos of the chars of (a) pure TPU and (b) TPU-2.0; SEM micrographs of residual char after cone calorimetry test for (c) pure TPU and (d) TPU-2.0 composite. Figure 11 High-resolution spectra of C 1s regions of the char residues of (a) pure TPU and (b) TPU-2.0; (c) The characteristic data of XPS for pure TPU and TPU-2.0; (d) XRD pattern of the char residues of TPU-2.0 after cone calorimetry test. Figure 12 Scheme of proposed flame-retardant mechanism for f-MoS2 in TPU composites. Figure 13 (a) Stress-strain curves of TPU and its composites; (b) Tensile results for TPU and its composites.

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Sample Unmodified MoS2 PEI-MoS2 f-MoS2

C (at. %) 33.84

O (at. %) 15.41

S (at. %) 35.15

Mo (at. %) 15.6

N (at. %)

P (at. %)

42.29 59.78

14.67 16.01

17.11 8.32

7.97 4.1

17.98 9.98

1.84

Table 1

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Air Sample

T5% (oC)

Pure TPU

302.3

TPU-1.0 TPU-2.0

T50% (oC)

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Nitrogen

Char residue at 700 oC (wt %)

T5% (oC)

355.9

1.6

317.1

368.4

5.0

293.1

393.7

2.4

306.2

367.2

6.7

291.5

390.2

3.8

302.0

363.0

8.1

Table 2

30


T50% (oC)

Char residue at 700 oC (wt %)

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Figure 13

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For Table of Contents Only

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Facile Construction of Flame-Retardant-Wrapped Molybdenum

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