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Hierarchical Polyphosphazene@Molybdenum Disulfide Hybrid Structure for Enhancing the Flame Retardancy and Mechanical Property of Epoxy Resins Xia Zhou, Shuilai Qiu, Weiyi Xing, Chandra Sekhar Reddy Gangireddy, Zhou Gui, and Yuan Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08878 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Hierarchical Polyphosphazene@Molybdenum Disulfide Hybrid Structure for Enhancing the Flame Retardancy and Mechanical Property of Epoxy Resins Xia Zhoua, Shuilai Qiu ab, Weiyi Xing a,*, Chandra Sekhar Reddy Gangireddya, Zhou Gui a,* and Yuan Hu a

a

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

96 Jinzhai Road, Hefei, Anhui 230026, P.R. China b

Department of Architecture and Civil Engineering, City University of Hong Kong, Tat

Chee Avenue, Kowloon, Hong Kong

Corresponding Authors * Zhou Gui. Fax/Tel: +86-551-63601669. E-mail: [email protected]. * Weiyi Xing. Fax/Tel: +86-551-63602353. E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract:

A novel polyphosphazene microspheres (PZS)@molybdenum disulfide

nanoflower (MoS2) hierarchical hybrid architecture was firstly synthesized, and applied for enhancing the mechanical performance and flame retardancy of epoxy resin (EP) via cooperative effect. Herein, using PZS microsphere as the template, a layer of MoS2 nanoflowers were anchored to PZS spheres via hydrothermal strategy. The well-designed PZS@MoS2 exhibits excellent fire retardancy and reinforcing effect. The obtained PZS@MoS2 significantly enhanced the flame retardant performance of EP composites, which can be proved by thermogravimetric and cone calorimeter results. For instance, the incorporation of 3 wt% PZS@MoS2 brought about a 41.3% maximum reduction in peak heat release rate and decrease 30.3% maximum in total heat release, accompanying higher graphitized char layer. With regards to mechanical property, the storage modulus of EP/[email protected] in the glassy state was dramatically increased to 22.4 GPa in comparison with that of pure EP (11.15 GPa). It is sensible to know that the improved flame retardant performance for EP composites is primarily assigned to a physical barrier effect of the MoS2 nanoflowers and polyphosphazene structure has an positive impact on promoting char formation in the condensed phase.

Keywords: Polyphosphazene, MoS2, epoxy resin, flame retardant, mechanical property


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1. Introduction Molybdenum disulfide (MoS2), a member of layered transition metal dichalcogenides family, has been a focal point of study ascribe to its fantabulous morphologies, high activity, excellent electronic and optical properties. It can be extensively applied in various areas such as catalysis,1 lubrication,2 electrochemistry and intercalation chemistry.3-4 Related to nanosized thickness and high surface area of various nanosheets, there have been some reports on MoS2 nanosheets as the candidates to heighten thermal properties, fire resistance and mechanical performances of polymer materials as well. Similar to layered nanofillers, like graphene,5-6 layered double hydroxides (LDH),7-8 polymer–layered silicate9 and montmorillonite (MMT),10 they have been also incorporated into polymeric materials to enhance their thermal stability and flame retardancy. For instance, CS/MoS2 nanocomposite membrane and melamine cyanurate/MoS2 hybrids were successfully fabricated, which significantly heightened the thermal properties and mechanical performances of polyamide 6.11-12 The layered MoS2 hybridized with modified graphene was successfully synthesized to decrease the fire hazards of epoxy composites as well.13 Furthermore, analogous to graphene, MoS2 nanosheets exhibit exceedingly high modulus over 300 GPa, which show remarkable potential as reinforcing agent for polymer materials.14-17 In this connections, numerous MoS2 nanostructures with various morphologies, such as nanospheres, nanoflowers and nanosheets, have been synthesized by different synthetic techniques including thermal decomposition,18 mechanical exfoliations,19 chemical vapor deposition (CVD),20 laser ablation,21 magnetron sputtering,22 and wet chemical synthesis.23-24 Except for the traditional syntheses of the MoS2 nanosheets, they can be acquired as a result of the intercalation of the metal ions (like Li+), and subsequently the bulk MoS2 sheets are exfoliated to few or a single layers by means of the hydrolysis of the metal ions.12,

25

Amongst the varieties of synthetic routes,

hydrothermal synthesis has been deemed to one of the most promising preparation techniques in virtue of its low cost, good crystallization of the formed product and high efficiency.



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On

the other hand,

polyphosphazenes are

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one versatile category of

organic-inorganic hybrid materials based on a phosphorus–nitrogen (-P=N-) backbone, which possess very diverse properties due to the possibility of vast array of organic substituents.26-28 They can be split into three kinds of polymers, such as polyphosphazenes contained cyclophosphazene units in side chain or main chain and linear phosphatzene polymers.29 All of this polymers have been exhibiting self-extinction in flame tests and high limiting oxygen index (LOI) values. However, low output and high cost of the linear polyphosphenes indeed restrict their applications.29-30 However, the cyclotriphosphazenes have attracted considerable research interest, which possess the backbone unit of –P=N– and exhibit good flame retardancy.

Similar

to

that

of

graphene,

diamond,

fullerenes,

and

CNT,

cyclotriphosphazenes can also be characterized by layered, cubic, spherical, and tubular structures.27,

31-33

These synthesized micro- and nano-materials may exhibit

superior mechanical property, flame retardancy, and thermal stability. Till to date, a battery of polyphosphazene nanomaterials have been synthesized and conducted as reinforcing additive agents to enhance the thermal stability and flame-retardant properties of polymer materials. For instance, poly(phosphazene-co-bisphenol A)-coated boron nitride were successfully synthesized to strengthen the thermal property of epoxy resin.34 Polyphosphazene nanotubes wrapped with DOPO-based flame retardant were prepared as well, which showed a significant decrease in total heat release (THR), peak heat release rate (PHRR), and lower CO production of the epoxy polymer.35 But the hybrid of polyphosphazenes and MoS2 is rare to be studied for the flame resistant application. EP is widely used as one of the thermosetting materials, which possesses a substantial range of applications at casting materials, coating fields, adhesive, electrical and electronics, prominent structural strength and superior mechanical performance. However, the generation of a great quantity of toxic gas and dense smoke in the burning process and limitations of high flammability impose great restrictions on its application indeed.36-37 Therefore, numbers of methods have been studied to improve the flame-retardant properties of EP.35, 38-40 In this connection, the nanofillers, such as


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carbon nanofiber, carbon nanotube, nanoclay and MoS2 are considered to be potential additives for improving physical properties especially mechanical property and fire resistance synchronously at low loading.13, 40-42 In addition, it is desirable to achieve various fire retardant efficiencies in composites through the incorporation of different kinds of functional materials. For example, silica/polyphosphazene nanosphere hybrids were successfully fabricated for epoxy composites to enhance the thermal property.43 The MoS2 nanolayers grew on carbon nanotubes which awarded excellent fire resistance to the epoxy matrix.44 Since, MoS2 and polyphosphazenes play different important roles in improving flame retardancy in different combustion stages, the simultaneous integration of polyphosphazenes and MoS2 may unite the superiorities of physical barrier effect of MoS2 and thermal stability of polyphosphazenes. Therefore,

in

this

work,

the

polyphosphazene

microspheres,

using

hexachlorocyclotriphosphazene and 4,4′-sulfonyldiphenol as co-monomers, were synthesized via a polycondensation method followed by a layer of MoS2 nanoflowers which consisted of ultrathin nanosheets assembling on the surface of poly (cyclotriphosphazene-co-4,4′-sulfonyldiphenol)

(PZS)

microspheres

via

a

hydrothermal method. An highly stable hybrid architecture, PZS@MoS2 was obtained. The effect of the combination between PZS and MoS2 on flame retardant and mechanical performances of EP composites were studied.

2. Experimental. 2.1. Materials. EP (DGEBA, E-44) was purchased from Anhui Jiangfeng Chemical Industry Co. Ltd. (China). Sodium molybdate (Na2MoO4•2H2O), triethylamine (TEA), L–cysteine, 4,4́-diaminodiphenylmethane (DDM), acetone, anhydrous ethanol and acetonitrile were provided by Sinopharm Chemical Reagent Co., Ltd. (China). 4,4′-sulfonyldiphenol (BPS) and hexachlorocyclotriphosphazene (HCCP) were provided by Aldrich Chemical Co. Ltd. (U.S.).



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2.2.Synthesis of PZS Microspheres with Active Hydroxyl Groups. First of all, HCCP (1.2 g, 3.46 mmol) and the given amount of BPS were dissolved each in 200 mL of acetonitrile and then transferred into a 500 mL three-necked flask. The system was stirred with ultrasonicating for 30 min. Then TEA (3.12 g, 30.9 mmol) was added to the above system. Continue the stirring under ultrasonication for an extra 4 h, meanwhile the temperature was mantained at 35 °C precisely. After that, the resultant product was obtained by centrifugation sequenced by a triple washing by de-ionized water and anhydrous ethanol. Finally the solid product was kept in a vacuum oven at 70 °C and dried for 12 h.

2.3. Fabrication of PZS@MoS2 Hybrid Architecture. MoS2 nanoflowers constituted of ultrathin nanosheets were assembled on PZS microspheres by a hydrothermal method as follows: firstly PZS was dissolved in de-ionized water by ultrasonication and then Na2MoO4•2H2O (1.2 g, 4.96 mmol) was added with stirring under ultrasonication. The pH of the solution was then adjusted to 6.5 sequenced by the addition of L–cysteine (3.3 g, 27.2 mmol). The mixture was stirred for 90 min and transferred to a 75 mL Teflon-lined autoclave with seal and heated in an oven at 180 °C for 48 h. Then the product was obtained by centrifugation, washed by anhydrous ethanol and de-ionized water for three times and finally kept the obtained PZS@MoS2 product in a vacuum oven at 80 °C for 12 h.

2.4. Preparation of EP/PZS@MoS2 Composites. The formulation technology of EP composite loading with 3.0 wt% PZS@MoS2 illustrated as below: 1.35 g of PZS@MoS2 was dissolved in 40 mL of acetone solution with continue ultrasonicating for 1 h. Subsequently, the corresponding 35.85 g of epoxy was dumped into the above system with continue mechanical stirring for further 3 h. Afterwards, the solvent was removed in a water bath at 90 °C for 6 h. After that, 7.80 g of DDM was dissolved and mixed into the related system by a vigorous mechanical stirring for 1 min. Finally, the EP/[email protected] sample was cured at 100 °C for 2 h and then post-cured at 150 °C for additional 2 h. After the curing


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process has been completed, the sample of EP/PZS@MoS2 (3.0 wt%) was cooled down to room temperature naturally. With regard to the fabrication of neat EP, EP/PZS (2.0 wt%), EP/MoS2 (2.0 wt%), and EP/PZS@MoS2 (2.0 wt%) composites, a process analogous to above was carried out exclude the variance of the additives and the formulations of the prepared EP composites are provied in Table S1.

2.5. Characterization. X-ray diffraction (XRD) was conducted by using an X-ray diffractometer (Rigaku Co., Japan), equipped with a Ni filter (l= 0.1542 nm) and a Cu Kα tube. Thermogravimetric analysis (TGA) was conducted using a Q5000 thermo-analyzer instrument (TA Instruments Inc., USA), at a linear heating rate of 20 °C/min from room temperature to 800 °C under nitrogen atmospheres flow of 25 ml/min. X-Ray photoelectron spectroscopy (XPS) spectra were performed by using a VG ESCALAB MK-II electron spectrometer (V.G. Scientific Ltd., UK). A Nicolet 6700 spectrometer (Nicolet Instrument Co., USA) was served to carry out the Fourier transform infrared (FTIR) spectra test. Cone calorimeter carried out by ASTM E1354/ISO 5660 was performed to investigate the fire performance of polymer materials. Every specimen was exposed horizontally under a heat flux of 35 kW/m2. The fracture surface of EP and its composites and microstructures of the residual char obtained from the cone test were studied using high-resolution JEOL JSM-6700 field-emission scanning electron microscopy (FE-SEM). A JEM-2100F Transmission electron microscopy (Japan Electron Optics Laboratory Co., Ltd., Japan) with an accelerating voltage of 200 kV was conducted to obtain the Transmission electron microscopy (TEM) results. LABRAM-HR laser confocal microRaman spectrometer (Jobin Yvon Co., Ltd., France) measurements were carried out to evaluate the components and microstructure of the char residual of EP composites with an argon laser of 514.5 nm. The mechanical property of EP and its composites was studied by Dynamic mechanical analysis (DMA) (DMA Q800, TA Instruments Inc.) in the temperature range from room temperature to 250 °C with a heating rate of 5 °C/min at fixed frequency of 10 Hz.



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3. Results and Discussion. 3.1. Characterization of PZS@MoS2 The synthetic route and suggested structure of novel PZS@MoS2 microspheres are depicted in Scheme 1. The PZS microspheres were synthesized in an acetonitrile solution and with an excessive amount of TEA as an acid acceptor under the ultrasonic bath. Then the MoS2 nanoflowers were self-assembled on the surface of PZS spheres by the hydrothermal method. FE-SEM and TEM results of neat PZS and PZS@MoS2 are presented in Figure 1, which provide microstructure and morphology data of PZS and PZS@MoS2. It can be clearly found from Figure 1a and Figure 1b that the neat PZS microspheres exhibit a uniform sphere shape and their average diameter is about 1 µm. It can also be observed from Figure 1c-f that the PZS@MoS2 composite displays an interesting flower-like morphology in which the PZS microsphere acts as a template and the MoS2 nanosheets grow on the surface of PZS as an extra phase, like a layer of flowers. Besides, MoS2 nanoflowers are self-assembled by uniform dispersion of ultrathin nanosheets on the surface of PZS microspheres with an average diameter of about 200-250 nm. TEM and FE-SEM results clearly demonstrate that the PZS@MoS2 microspheres were successfully obtained. FTIR analysis indicates essential structural information of PZS and PZS@MoS2 hybrids (Figure 2a). As presented in Figure 2a, the absorption peaks at 1488 cm-1 and 1590 cm-1 are in correspondence with the phenylene group of BPS. And the typical absorptions at 1293 cm-1 and 1153 cm-1 belong to the O=S=O groups. The characteristic absorptions of P–N and P=N groups are centered at 883 cm-1 and 1186 cm-1 and the absorbance peak of Ar–O–P values at 941 cm-1. For the PZS@MoS2 hybrids, there are two distinctive absorptions centered at 937 cm-1 and 1100 cm-1 corresponding to the S–S and Mo–O bonds of MoS2, which indirectly reveals that the successful in situ growth of MoS2 nanoflowers on polyphosphazenes microspheres. Figure 2b shows the XPS information of PZS and PZS@MoS2 which are used to further clarify the chemical states and elements composition of the samples. The XPS survey spectrum of PZS (Figure 2b) presents that the sample contains C, N, P, S and O


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elements. Referring to the PZS@MoS2 hybrids, the sample exhibits an additional Mo 3d peak and the peak intensity of S 2p is increased, revealing that the MoS2 phase was formed on the surface of PZS spheres. The XRD patterns were performed to further investigate the chemical structure of PZS and PZS@MoS2. It can be clearly found from Figure 3a that the broad peak at a 2θ value of 15.0° is attributed to the reflection peak of amorphous polyphosphazenes. For as-prepared PZS@MoS2, the typical peaks indexed at 2θ =14°, 32°, 39.5° and 58° are in correspondence with the (002), (100), (103) and (110) crystal planes of the MoS2 with low crystallinity, indicating the formation of MoS2 crystalline phase on the surface of PZS.

3.2. Thermal Properties of EP Composites. The thermal behavior of PZS and PZS@MoS2 were analyzed using TGA test under nitrogen. The onset degradation temperature (Tonset) and the maximum degradation temperature (Tmax) are important indexes of thermal properties, which are defined by the temperatures of the 5 wt% mass loss and maximum mass loss of materials, respectively. From Figure 3b, it can be found that pure PZS displays only one-phase weight loss in nitrogen atmosphere and the Tonset of PZS is about 470 °C accompanying by the mass of residual char of 56 wt% at 800 °C which indicates that PZS presents excellent thermal stability. As for PZS@MoS2, the Tonset of the hybrids in comparison to the pure PZS microspheres is slightly decreased, however, the char residue of PZS@MoS2 reaches about 80 wt % which is increased by approximately 43% compared to the PZS. Thermal performances of EP composites were analyzed using TGA tests in nitrogen to explore the influence of PZS@MoS2 upon the thermal stability of EP. As shown in Figure 4a, all of the composites demonstrate similar thermal degradation behaviors in comparison with that of pure EP matrix. The thermal degradation process of neat EP and its composites contains merely one phase, which is primarily owing to the degradation of the macromolecular chains during combustion. After the incorporation of PZS and MoS2 into EP, both of their Tonset show slight reductions



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compared to that of neat EP revealing that the catalytic effect of PZS and MoS2 accelerates the thermal degradation of EP composites. The addition of 2 wt% PZS@MoS2 hybrids brings about slightly higher char residue in comparison with EP, EP/PZS2.0, and EP/MoS22.0 composites, which reaches up to 22.14 wt%, whereas that of EP/[email protected] composite values at 23.01 wt%. A protective shield is provided by the char residues, which can remarkably retard the mass and heat transfer and decelerate the heat release rate during burning. From the mentioned analyses, it is demonstrated that the PZS@MoS2 hybrids are able to extremely heighten the fire resistance of EP composites. In Figure 4b, the value of DTG peak of the EP/[email protected] sample significantly decreases in comparison with that of pristine EP. The detailed data of TGA results are presented in Table S2. This improvement in thermal property can be assigned to the high aspect ratio of MoS2 nanoflowers, which may serve as a physical barrier to stem the oxygen supply from the environment to the bulk and suppress the emission of combustible gas generated in the thermal degradation process. To understand the interface interaction between the PZS@MoS2 hybrids and EP resin, FE-SEM was used to study the microstructures of freeze-fractured surfaces of EP composites (Figure 5). To a certain extent, the fractured surface roughness of polymer matrices materials gives expression to the interfacial interaction and dispersion level. It’s clear seen that the pure EP presents a smoother surface (Figure 5a), whereas the fracture surfaces of EP composites are much rougher (Figure 5b-d). Several pulled out PZS microspheres and the remaining holes can be obviously observed from Figure 5b. In addition, numerous MoS2 particles are agglomerated and drew out of the EP/MoS22.0 composite. However, the agglomerates of PZS@MoS2 particles of EP/[email protected] sample was evidently fewer (Figure 5d), which indicates the stronger interfacial interaction and better dispersion of EP/[email protected] composites.

3.3. Fire Performance of EP Composites. Cone calorimeter is the most vital instrument to measure the flammable properties of numerous polymeric materials during combustion. The Heat release rate (HRR) and


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Total heat release (THR) vs time curves of the EP composites are shown in Figure 6. The PHRR and THR values for the EP with a loading of 2 wt% PZS are slightly decreased by 12.0% and 18.5%, respectively. In addition, the introduction of 2 wt% of MoS2 into EP further reduces the PHRR and THR values and leads to 31.8% and 19.5% reductions in PHRR and THR, respectively. Furthermore, the PHRR and THR of EP/[email protected] are decreased by 37.5% and 27.0% respectively, in comparison to pristine EP matrix. The incorporation of 3 wt% PZS@MoS2 into EP brings about 41.3% and 30.3% reductions in PHRR and THR respectively, indicating that the best fire retardancy among the rest of EP composites. The significant reduction in the fire potential of EP/PZS@MoS2 is mainly assigned due to the physical barrier effect of MoS2 nanoflowers and catalytic carbonization effect of PZS, which inhibit the mass and heat transfer between gas and condensed phases, thereby depressing the further combustion of the sublayer of polymer matrices.11, 45

3.4. Analysis of Char Residues and Flame Retardant Mechanism. In order to intensively study the flame-resistance mechanism in gas phase and/or condensed phase, the microstructure of the residual char of EP composites collected after cone tests were studied by SEM (Figure 7). Figure 7a-d present macro-morphologies of the residue chars of selected samples by using a digital camera. The neat EP is completely burned with a part of the char residue remained after cone test, while the EP/PZS2.0 and EP/MoS22.0 composites remain a little more residues after combustion. In regard of the EP/[email protected] composite, a more compact and continuous char surface is formed, and the amount of char residue is significantly increased. Furthermore, the microstructures of external char residues of neat EP, EP/PZS2.0 and EP/MoS22.0, EP/[email protected] composites were investigated by FE-SEM presented in Figure 7e-h. The densities of the holes of the EP/PZS2.0 and EP/MoS22.0 composites are faintly lower compared with pure EP. As for the EP/[email protected] sample, a compressed and continuous char surface is obtained during combustion, and there is a higher density of whole char with fewer holes. Therefore, the residue char consisted of a more cohesive and compressed layer is



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formed which can restrain the conductivity of mass and heat transfer between vapor and condensed phases, thereby enhancing the flame retardancy. The graphitization degree of char residues is a crucial factor for enhancing the fire retardancy of polymer materials. The specific components and microstructure of the residual char were evaluated using the Raman spectroscopy (Figure 8a-d). It’s can be clearly found that all of the spectra display D and G bands valued at about 1366 and 1596 cm−1 respectively, indicating that the similar char structures of these composites. The G peak is concordant with the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline, nevertheless the D peak gives expression to the glassy carbons or disordered graphite. The ratio of accumulated intensity of the D and G bands (ID/IG) could be defined as the degree of graphitization of the char residues. As is known to us all, the lower ratio of ID/IG, the higher graphitization degree of the resultant char. The ID/IG of pure EP values at 2.96, nevertheless, the EP/PZS2.0 and EP/MoS22.0 possess lower value of 2.82 and 2.9, respectively. However, EP/[email protected] has the lowest ID/IG value of 2.80, indicating the formation of char with highest graphitization degree, which can play a role of physical barrier to insulate combustible gas and heat transfer between the underlying matrix and flame.

3.5. Mechanical Properties of EP Composites. The mentioned analyses have indicated that the cooperative effect of enhancing the fire resistance of EP through the combination of PZS microspheres and MoS2 nanoflowers. The layered structure of MoS2 is similar to that of graphene and montmorillonite (MMT), the previous works have reported that the graphene or MMT can significantly enhance mechanical properties of polymer materials by transferring load across the nanolayer–polymer interfaces. On the other hand, both dispersion state of nanoadditives and interfacial interaction between fillers and polymer matrices have a crucial impact upon the mechanical performances of polymer composites.46-49 As for the mechanical behaviors of the neat EP and its composites, DMA was conducted as a function of the temperature to assess the thermal stability and mechanical performance. Storage modulus and loss angle tangent vs temperature curves of the EP composites are


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presented in Figure 9. The storage modulus is a performance index of the stiffness. As can be observed from Figure 9a, compared to that of neat EP (11.15 GPa), the storage modulus of EP/PZS2.0 and EP/MoS22.0 increases by 17.2% and 51.9% respectively in the glassy state (T

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