Synthesis and characterization of Cu–MoS₂ hybrids and their influence on the thermal behavior of polyvinyl chloride composites

Abstract

In this work, Cu–MoS₂ hybrids were synthesized by a facile wet chemical method. The structure and morphology of the synthesized hybrids were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy measurements. Subsequently, the hybrids were introduced into a polyvinyl chloride matrix to act as a thermal stabilizer. The hybrids were well dispersed in the matrix and no obvious aggregation was observed. The obtained nanocomposites exhibited significant improvements in thermal stability and inhibited the release of pyrolysis products effectively, which were mainly attributed to the enhancement of char formation, a physical barrier effect of the MoS₂ nanosheets and an inhibition effect of the Cu nanoparticles.

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1. Introduction

Poly(vinyl chloride) (PVC), as a typical commodity polymer, has been widely used in many fields due to its excellent processability, good mechanical properties, inherent flame retardancy and low production cost.¹ However, the main drawback of PVC is its low thermal stability. The thermal degradation of PVC involves an autocatalytic dehydrochlorination reaction, which may produce undesirable discoloration and deterioration of mechanical properties.^2,3 In addition, another challenge for PVC is how to efficiently inhibit the generation of black smoke upon combustion.⁴ Therefore, it is of great significance to explore appropriate thermal stabilizers which are environmentally friendly and highly efficient at restraining the thermal degradation of PVC.

As is well known, the introduction of small amounts of two dimensional (2D) layered nanofillers can inhibit the thermal degradation of polymer materials effectively.⁵ This property enhancement is mainly attributed to the good barrier effect and low heat conductivity of 2D nanomaterials.⁶ Typical 2D layered nanofillers such as layered double hydroxides (LDHs) and montmorillonite (MMT) have been widely used to enhance the thermal stability of PVC composites in previous work.⁷ In addition, it has found that copper containing compounds might be the most effective class of smoke suppressants for PVC.⁸ Moreover, Liu et al. has investigated the influence of copper containing LDHs on thermal and smoke behavior of PVC. The results indicated that the combination of copper and LDHs can improve the thermal stability and smoke suppression property of PVC composites effectively.⁹

As an emerging 2D layered nanomaterial, the exfoliated MoS₂ nanosheets exhibit good thermal insulating property and high thermal stability (initial thermal temperature is over 500 °C), which make it a promising candidate for thermal property application in polymers. In the past few years, some reports have demonstrated that incorporation of exfoliated MoS₂ nanosheets at extremely low loading can endow polymer matrices with prominent thermal and mechanical properties.^10–13 In addition, MoS₂ or its derivatives have been reported as flame retardant nanoadditives to improve the flame retardancy of various polymers such as PVA, PS, PMMA, EP and TPEE.^12,14–20 However, to the best of our knowledge, few researchers have investigated the effect of MoS₂ on the thermal behavior of PVC. Additionally, it is worth noting that the excellent performances are only achieved when MoS₂ nanosheets are uniformly dispersed in polymer matrix.¹⁹ But actually, the exfoliated MoS₂ nanosheets are inclined to agglomerate and even restack in polymer matrices owing to their large specific surface area and van der Waals interactions.²¹

Recently, polymer nanocomposites based on hybrid nanomaterials which comprise two or more components with different properties, have been proven to be a more attractive strategy, because they usually offer unexpected properties. Moreover, it can be expected that the agglomeration of the nanoparticles and weak interfacial interaction problems in polymer nanocomposites could be solved simultaneously by using such functional hybrid nanofillers.²² Previous work in our group has reported that Cu/graphene hybrids can effectively inhibit the generation of smoke gas.²³ Moreover, it is well known that the exfoliated MoS₂ nanosheets have a large specific surface area and abundant active edges so that they can act as a perfect support for nanoparticle decoration.²⁴ From the perspective of materials design, the decoration of nanoparticles not only suppresses the agglomeration and restacking of MoS₂ nanosheets, but also endows MoS₂ nanosheets with new physical and chemical properties. Inspired by this, it provides us a novel strategy to decorate copper nanoparticles on the surface of exfoliated MoS₂ nanosheets, the well-dispersed Cu nanoparticles can serve as a nanospacer to inhibit or decrease of serious agglomeration and restacking of MoS₂ nanosheets, which is beneficial for the uniform dispersion of MoS₂ in the polymer matrix. Therefore, it is interesting to investigate how the combination of the two conceptions, which means the synthesis of copper nanoparticles decorated MoS₂ hybrids, will actually affect the thermal behavior of PVC.

Herein, the Cu–MoS₂ hybrids are synthesized by a facile and efficient wet chemical method and then incorporated into the PVC matrix through solvent blending. The thermal degradation behavior of PVC composites is investigated via TGA and TG-IR. It is anticipated that loading Cu nanoparticles on the MoS₂ nanosheets can inhibit the emission of HCl and prevent the MoS₂ nanosheets from restacking, thus resulting in enhanced thermal stability and reduced pyrolysis products of PVC. This work attempts to provide a promising strategy for constructing a new class of environmental friendly thermal stabilizers with efficient smoke suppression effect for PVC composites.

2. Materials and methods

All reagents were analytical reagents and used as received without further purification. The schematic illustration for the fabrication of Cu–MoS₂ hybrids was presented in Fig. 1. The typical process included two steps: (1) the preparation of exfoliated MoS₂ nanosheets from bulk MoS₂ through the hydrolysis and ultrasonication of Li_xMoS₂ in deionized water¹⁴ and (2) chemical reduction of the copper acetate by hydrazine to generate Cu–MoS₂ hybrids. Briefly, 0.2 g of Li_xMoS₂ was hydrolyzed in 300 mL distilled water, and ultrasonicated at room temperature for 4 h to obtain a colloidal suspension of MoS₂ layers. Then 1.5 mmol of copper acetate was dissolved in the above suspension at room temperature. Finally, excessive hydrazine hydrate was dripped into the solution slowly and further heated at 100 °C for 2 h. The as-prepared products were separated by centrifugation and washed with deionized water for several times and then dried in a vacuum at 40 °C. PVC composites with 0.5 wt% Cu–MoS₂ hybrids were prepared by a solvent blending method.²⁵

Fig. 1 Schematic illustration of Cu–MoS₂ hybrids and PVC composites formation.

XRD patterns were performed with a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipped with a Cu-Kα tube and Ni filter (λ = 0.1542 nm). X-ray photoelectron spectroscopy (XPS) spectra were performed on VG ESCALB MK-II electron spectrometer. TEM images were obtained on a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 keV. Scanning electron microscopy (SEM) (JSM-6800F, JEOL) was used to observe the fracture surface structure of PVC nanocomposites. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., New Castle, DE) under N₂ flow of 60 mL min⁻¹. In each case, the samples were heated from room temperature to 800 °C at a linear heating rate 20 °C min⁻¹. The thermogravimetric analysis/infrared spectrometry (TG-IR) of the samples was performed using a TGA Q5000 IR thermogravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer.

3. Results and discussion

Fig. 2 shows the XRD patterns of the exfoliated MoS₂ and Cu–MoS₂ hybrids. It can be seen that the exfoliated MoS₂ shows a characteristic reflection peak centered at 2θ = 14.4°, corresponding to the (002) plane. For the Cu–MoS₂ hybrids, the diffraction patterns located at 2θ = 43.3° and 50.5° are attributed to the (111) and (200) crystalline planes of the face centered-cubic Cu, respectively (JCPDS 85-1326).²³ In addition, the intensity of the diffraction peak for (002) (MoS₂) in the Cu–MoS₂ hybrids decreases, indicating that the face-to-face stacking is broken due to the deposition of Cu nanoparticles on the surface of MoS₂ nanosheets.²⁶

Fig. 2 XRD patterns of exfoliated MoS₂ and Cu–MoS₂ hybrids.

XPS analysis (Fig. 3) is further performed to reveal the chemical composition of the obtained Cu–MoS₂ hybrids. The entire XPS spectrum (Fig. 3a) shows the peaks of Cu, Mo, and S derived from the Cu–MoS₂ hybrids. As shown in Fig. 3b, the two peaks at 229.1 eV and 232.5 eV can be attributed to the doublet of Mo 3d_5/2 and Mo 3d_3/2. The bonding energy for S 2p in Fig. 3c is 162.2 eV, which are in good agreement with the results of a previous report on MoS₂ nanosheets.²⁷ In Fig. 3d, the XPS peak corresponding to Cu 2p_3/2 appeared at 932.1 eV which indicated the formation of Cu⁰ nanoparticles. However, the peaks around 943 and 963 eV are mainly attributed to the Cu²⁺ satellite peak which can be evidently observed in copper(II) oxide.²⁸ The results suggested that the surface of Cu nanoparticles was partially oxidized. The XPS results further indicate the successful preparation of Cu–MoS₂ hybrids.

Fig. 3 XPS analysis results of Cu–MoS₂ hybrids.

The morphology and microstructure of the exfoliated MoS₂ nanosheets and Cu–MoS₂ hybrids are characterized by TEM (Fig. 4a and b). As observed in Fig. 4a, the exfoliated MoS₂ nanosheets exhibit a typical two dimensional nanoplatelet shape. As for the Cu–MoS₂ hybrids (Fig. 4b), the Cu nanoparticles are decorated on the surface of MoS₂ nanosheets successfully without obvious aggregation, which inhibit the restacking of MoS₂ nanosheets effectively. The EDX spectra (inset in Fig. 4b) further confirm that the Cu–MoS₂ hybrids are successfully fabricated.

Fig. 4 TEM images of exfoliated MoS₂ (a) and Cu–MoS₂ hybrids (b).

To evaluate the dispersion state of Cu–MoS₂ hybrids in PVC nanocomposites, the morphologies of the fractured surfaces are observed by SEM (Fig. 5). For pure PVC, it has a uniform surface morphology revealing a rather smooth surface (Fig. 5a and b). Compared to the pure PVC, the fractured surface of the PVC composites shows considerably different fractographic features with a relative rough surface, as shown in Fig. 5c and d. Moreover, it can be observed that the Cu–MoS₂ hybrids are dispersed uniformly and embedded in the PVC matrix without obvious aggregation. The homogeneous distribution of Cu–MoS₂ hybrids across PVC matrix can increase the interfacial area between the matrix and the nanosheets, which is important for the following property improvements.

Fig. 5 SEM images of the fractured surfaces for neat PVC (a and b) and PVC/0.5% Cu–MoS₂ composites (c and d).

The thermal degradation behavior of PVC and its composites is investigated by TG and TG-IR. TG-DTG profiles of neat PVC and its composites under nitrogen atmosphere are plotted in Fig. 6. As can be observed, the PVC/Cu–MoS₂ composites present similar degradation behaviors to that of neat PVC, consisting of three stages referred to the DTG profiles, corresponding to the vaporization of small molecules, dehydrochlorination of the polymer, resulting in the formation of conjugated double bonds that break during the third step, respectively.²⁵ However, it is clearly observed that the addition of Cu–MoS₂ hybrids can inhibit the emission of hydrogen chloride effectively in the second degradation stage (240–350 °C), which is good agreement with previous work.²⁹ In comparison with those of neat PVC, the onset degradation temperature of the PVC composites with 0.5% Cu–MoS₂ hybrids is almost the same. On the contrary, T_−50% and T_max of the PVC nanocomposites is enhanced obviously, indicating that the presence of Cu–MoS₂ hybrids defers the thermal degradation of PVC. Moreover, it can be obtained from DTG profiles that the maximum mass loss rate (the peak of DTG curve) of the PVC composites with Cu–MoS₂ hybrids is lower than that of neat PVC, suggesting that MoS₂ nanosheets function as an effective mass transport barrier to inhibit the mass loss during the thermal degradation process.¹⁹ In addition, the incorporation of Cu–MoS₂ hybrids exhibits higher char residues compared to neat PVC. When 0.5% Cu–MoS₂ hybrids are added, the residues increase significantly by 120 wt% in comparison to the pure PVC, owning to the catalytic carbonization effect of the Cu–MoS₂ hybrids. The formation of high char residues decreases the release of combustible gases and provides a protective shield of mass and heat transfer.

Fig. 6 TGA and DTG curve of neat PVC and PVC composites under nitrogen atmosphere.

To further investigate the influence of Cu–MoS₂ hybrids on the evolved gaseous volatiles during pyrolysis, the volatile components released from PVC and its composites are monitored by TG-IR. The gaseous decomposition products of the PVC materials are mainly composed of hydrocarbons (2950 cm⁻¹), HCl (2798 cm⁻¹), CO₂ (2360 cm⁻¹), aromatic compounds (690 cm⁻¹) and water (4000–3500 and 1300–1800 cm⁻¹).²⁵ To investigate the differences between PVC and its composites, the Gram–Schmidt (GS) curves as well as pyrolysis products vs. time are shown in Fig. 7a–d. It can be clearly observed that the absorbance of total release of the decomposition products from PVC/0.5% Cu–MoS₂ composites is much lower than that from virgin PVC. In order to provide a more clear comparison, the intensity of typical gaseous organic volatiles such as HCl, hydrocarbons and aromatic compounds of neat PVC and its composites are represented in Fig. 7b–d. It can be obviously observed that the addition of Cu–MoS₂ hybrids reduces the evolution of all gaseous organic volatiles significantly which is mainly attributed to the enhancement of char formation and the physical barrier effect of the MoS₂ nanosheets and inhibition effect of Cu nanoparticles. The reduced amount of the organic volatiles further leads to the inhibition of smoke.¹⁹ All in all, these results suggest that incorporating Cu–MoS₂ hybrids into PVC would retard the thermal degradation of PVC molecular chains and inhibit the emission of toxic gaseous products, organic volatiles and smoke.

Fig. 7 Gram–Schmidt curves (a) and intensity of characteristic peaks for pyrolysis products of neat PVC and PVC/0.5% Cu–MoS₂ composites (b–d).

4. Conclusion

In present study, Cu–MoS₂ hybrids were synthesized by a facile wet chemical method and its structure was confirmed by XRD and XPS. A morphological study showed that Cu nanoparticles were homogeneously anchored on the MoS₂ nanosheets, which effectively prevented the MoS₂ nanosheets from restacking. SEM results demonstrated that the Cu–MoS₂ hybrids were homogenously dispersed in PVC matrix without obvious aggregation structure. The thermal stability of the PVC composites was significantly improved and the release of combustible gas and toxic HCl was effectively restrained, which was mainly attributed to the inhibition effect of Cu nanoparticles, physical barrier effect and charring effect of Cu–MoS₂ hybrids.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG160607).

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