Epoxy composites with functionalized molybdenum disulfide nanoplatelet additives

Abstract

As a typical layered inorganic analogue of graphene, molybdenum disulfide (MoS₂) has gained intensive attention and become a research hotspot due to its unique two dimensional nanostructure and excellent properties. The enhancement in the thermal and mechanical properties of polymer/inorganic nanosheet composites depends strongly on their interfacial interaction. In this study, we exfoliated bulk MoS₂ into nanosheets which were subsequently functionalized using 3-mercaptopropyltriethoxysilane. The functionalized MoS₂ (f-MoS₂) were dispersed in epoxy polymers at loading fractions of up to 1% by weight via ultrasonication and three roll mills. We characterized the tensile, fracture and adhesive properties of the composite and show that f-MoS₂ nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (0.1–0.7% by weight). Our results show the potential of functionalized 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. The results indicate that the glass transition temperature increases significantly for the lower weight fraction composites, from 135 °C for the baseline (unfilled) epoxy to 146 °C at 0.7% f-MoS₂ loading. The apparent shear strength at 120 °C increases significantly for the lower weight fraction composites, from 13.8 MPa for the baseline (unfilled) epoxy to 24.9 MPa at 0.7% f-MoS₂ loading.

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1. Introduction

Over the past few years there has been great interest in trying to reinforce polymer materials with nanoscale fillers to improve their electronic, mechanical, optical, flexible and thermal properties^1–4 for a variety of engineering applications.^5–8 Two-dimensional (2D) layered nanomaterials exfoliated from bulk layered inorganic compounds,^9–12 such as graphene, molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), have motivated great attention owing to their applicability in fields ranging from catalysis,¹³ lubricants,¹⁴ super capacitors,^15–17 transistors¹⁸ and cathode materials for lithium batteries to biomedicine.¹⁹ As high band gap semiconductors, transition metal dichalcogenides (TMDCs) will not impart electrical conductivity to the polymer matrix while at the same time potentially improving mechanical properties such as elastic modulus, strength, toughness, wear, creep and fatigue resistance.^20,21 As is well known, the properties of inorganic nano-fillers/polymer composites depend strongly on two factors: one is the interaction between the nano-fillers and the polymer matrix; the other is the dispersion state of nano-fillers in the polymer matrix.^22,23 The exfoliation degree of bulk MoS₂ is very important to fabricate its polymer nanocomposites with effective reinforcement. The layered structure of MoS₂ enables easy intercalation of lithium ions (Li⁺). It is convenient to prepare the polymer nanocomposites by the intercalation of Li⁺ and then exfoliation to a single or few layers through the hydrolysis of the Li⁺.^24,25 Therefore, most of the MoS₂/polymer composites are focused on water-soluble polymers such as poly(vinyl alcohol),²⁶ poly(ethylene oxide),²⁷ and chitosan,²⁸ prepared through a solution blending method.

In this study, we exfoliated bulk MoS₂ into nanosheets which was subsequently functionalized using 3-mercaptopropyltriethoxysilane. We characterized the tensile and fracture and adhesive properties of the composite and show that f-MoS₂ nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions.

2. Experimental

2.1 Materials

Molybdenum disulfide, n-hexane and tetrahydrofuran were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The n-butyl lithium (2.2 M in hexane) was purchased from Alfa Aesar without further purification. Diglycidyl ether of Bisphenol-F type epoxy resin (NPEF-170) were purchased from Nan Ya Plastics Corporation. 3,3′-Dimethyl-4,4′-diamino-dicyclohexylmethane (DMDC) were purchased from BASF. 3-Mercaptopropyltriethoxysilane (KH580) were purchased from Nanjing Shuguang Chemical Group Co., Ltd. Deionized water is used for all experiments unless otherwise stated.

2.2 Synthesis of functionalized MoS₂ nanosheets

Li_xMoS₂ was prepared by the solvothermal methodology, similar to the preparation procedure of Li_xWS₂. In a typical experiment, 1.0 g bulk MoS₂ powder was soaked in the autoclave and 36 mL 0.5 M solution of n-butyl lithium in hexane was then added. The autoclave was tightly sealed and heated at 90 °C for 4 h. Following the intercalation by lithium, the MoS₂ sample was filtered and washed with anhydrous hexane, and then dried at 50 °C in vacuum oven. Exfoliation of MoS₂ into nanosheets was achieved via the rapid hydrolysis and ultrasonication of Li_xMoS₂. In a typical reaction, 0.5 g Li_xMoS₂ was hydrolysed in 1 L water, and ultrasonicated at ambient temperature for 4 h to produce a colloidal suspension of MoS₂ nanosheets. For the functionalization of MoS₂ nanosheets, 1.0 g of KH580 was added into 0.25 g of MoS₂ nanosheets dispersed in 500 mL of water. After sonication for 20 min and stirring overnight, excess KH580 was removed by filtration followed by several times of water washing. The obtained f-MoS₂ was dried in a vacuum oven at 50 °C for 12 hours.

2.3 Preparation of f-MoS₂/epoxy nanocomposites

Scheme 1 illustrates the procedure for preparing f-MoS₂/epoxy nanocomposites. The f-MoS₂/epoxy nanocomposites were prepared by the solvent blending method and grinding method, maintaining the nanofiller loading at 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.7 wt% and 1 wt%, respectively. First, the calculated f-MoS₂ nanosheets were re-dispersed in tetrahydrofuran and ultrasonicated at ambient temperature for 4 h. The epoxy resin was then added into the aforementioned dispersion and stirred under ultrasonic agitation for 6 h. The solvent was gradually removed through heating the mixture on a magnetic hot plate with stirring. After the f-MoS₂/epoxy mixture had cooled to room temperature, the mixture was grinded 3 times by three roll mills. Subsequently, a low-viscosity curing agent (DMDC) was added, and a planetary vacuum mixer (ARV-310, Thinky) was used to blend the f-MoS₂/epoxy slurry in a vacuum environment for 4 min at 1800 rpm. Finally, Teflon molds were used to cure the nanocomposite samples, cured for 2 h at 80 °C and post cured for 2 h at 150 °C.

Scheme 1 Schematic illustration of f-MoS₂/epoxy composite preparation route.

2.4 Characterization

Transmission electron microscopy (TEM; JEM-2100, Japan Electron Optics Laboratory Co., Ltd.) was used to investigate the morphologies of the exfoliated f-MoS₂ nanosheets. Atomic force microscopy (AFM) images of f-MoS₂ were obtained using a Being Nano-Instruments CSPM5500 in the tapping mode. The powder X-ray diffraction (XRD) patterns were recorded with a XRD-7000 Shimadzu X-ray diffractometer (Cu Kα a radiation and λ = 0.1542 nm), operating at 40 kV voltage and 30 mA current. Laser Raman spectroscopy measurements were performed using a XploRA PLUS Raman microscope (HORIBA Scientific) with an excitation provided in back-scattering geometry by a 532 nm argon laser line. Fourier transform infrared (FTIR) spectra were recorded on a BRUKER TENSOR27 FTIR spectrometer using the KBr disc method. All the samples were analyzed with the transmission mode and the wavenumber range was set from 400 to 4000 cm⁻¹. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 apparatus (TA Instruments Inc.) at a fixed frequency of 1 Hz in the temperature range from room temperature to 200 °C at a linear heating rate of 5 °C min⁻¹. Morphology of the sample after being gold-sputtered was studied by a JEOL JSM-IT300 scanning electron microscope (SEM). The specimens of nanocomposites were cryogenically fractured in liquid nitrogen first and then sputter-coated with a conductive layer. Energy dispersive X-ray analysis (EDAX) was performed using a AMETEK ELEMENT. Static tensile loading tests (the thickness is 3.2 ± 0.4 mm; The width is 10 ± 0.2 mm) and apparent shear strength tests (the thickness of the sheets is 1.62 ± 0.125 mm; the length of overlap is 12.7 ± 0.25 mm) were performed using an Instron 5982 universal testing system following the ASTM D638 standard and the ASTM D1002 standard, respectively. Impact resistance tests (sample size 80 ± 2 mm × 10 ± 0.5 mm × 4 ± 0.2 mm) were conducted using a PTM1000 material testing system (SHENZHEN SUNS TECHNOLOGY STOCK CO., LTD.) following the ISO 179 standard. Five parallels for each sample were tested and the average value was reported.

3. Results and discussion

3.1 Characterization of MoS₂ nanosheets

To determine the average f-MoS₂ nanosheets thickness, we performed atomic force microscopy (AFM) scans as shown in Fig. 1a and b. The thickness of the f-MoS₂ nanosheets ranges from 1 to 6 nm, while the average thickness of the monolayer MoS₂ is about 1 nm according to the previous reports.^29,30 This indicates that the f-MoS₂ nanosheets in our study is composed of 1–6 layers of MoS₂. The morphologies of f-MoS₂ nanosheets (Fig. 1c) were investigated by TEM. The lateral dimensions of f-MoS₂ nanosheets is about 300–500 nm.

Fig. 1 AFM images (a) and thickness profiles (b) of f-MoS₂ nanosheets; TEM images of f-MoS₂ (c).

Raman spectra of the bulk MoS₂ (Fig. 2a) shows modes at 380.9 cm⁻¹ (the E¹_2g mode corresponding to in-plane vibrations) and at 408.6 cm⁻¹ (the A_1g mode that corresponds to out-of-plane vibrations) which are characteristic modes of the trigonal prismatic structure of MoS₂. For f-MoS₂, the E¹_2g peak shifts to a higher frequency and the A_1g peaks of MoS₂ is transferred to a lower frequency, indicating that f-MoS₂ are thinner than bulk MoS₂.^31–35

Fig. 2 (a) Raman spectrum of bulk MoS₂ and f-MoS₂ nanosheets; (b) Fourier transform infrared spectra of KH580, MoS₂ and f-MoS₂; (c) Powder X-ray diffraction patterns of MoS₂ and f-MoS₂.

To determine the conjugation of KH580 and MoS₂, Fourier transform infrared (FTIR) spectra were recorded as shown in Fig. 2b. Compared to pristine MoS₂, the spectra of f-MoS₂ exhibit characteristic absorption bands at 1297, 1170, 1080 and 784 cm⁻¹ which are attributed to the stretching vibration of C–S, C–C, Si–O and Si–C, respectively. These characteristic bands originate from KH580, which confirms the functionalization of KH580 on the MoS₂.

Fig. 2c shows the powder XRD profiles of bulk MoS₂ and f-MoS₂. The diffraction pattern of bulk MoS₂ showed intense and sharp peaks at 14.48, 32.78, 39.76, 49.96, and 56.32 (2 theta), which correspond to the (002), (100), (103), (105), and (110) planes, respectively. All the characteristics bands of bulk MoS₂ correspond well with the previous report. Post functionalization, the peaks corresponding to the (004), and (006) planes have disappeared. The (002) diffraction peak is still present at 14.4 for f-MoS₂, although the intensity of the peak has notably decreased. Low intensity peaks corresponding to the (100), (103), (105) and (008) planes can also be observed for f-MoS₂. These features are in good agreement with disordered restacking of the exfoliated sheets during the drying process. Additionally, the high quality exfoliated MoS₂ nanosheets reported by predecessors still displayed prominent diffraction peaks.³⁶

The influence of MoS₂ and f-MoS₂ on the dynamic mechanical thermal behaviors of the epoxy composites was investigated by DMA. Storage modulus curves and loss angle tangent of epoxy and its nanocomposites are presented in Fig. 3. The storage modulus is a measure of the stiffness, the storage modulus (E′) of the f-MoS₂/epoxy composites decreased with the increase of the f-MoS₂ loading. In the case of f-MoS₂/epoxy-1.0, the storage modulus was about 7.31% lower than that of neat epoxy at 35 °C. Moreover, E′ was generally decreased by the addition of f-MoS₂ in all temperature ranges. Incorporation of nanofillers into a polymer matrix can also affect its glass transition temperature (T_g). The glass transition temperature is determined by the peak of tan delta curves. The results indicate that the T_g increases significantly for the lower weight fraction composites, from 135 °C for the baseline (unfilled) epoxy to 146 °C at 0.7% f-MoS₂ loading. For the higher weight loadings up to 1.0%, the T_g decreases to approximately that of the neat epoxy. Since the T_g is a thermal transition where polymer chain motion significantly increases, the increased T_g for the low weight fraction f-MoS₂ nanocomposites is indicative of decreased chain mobility caused by interfacial interactions between the epoxy chains and the surfaces of the well dispersed f-MoS₂ nanoplatelets additives.^37,38 Conversely, at the higher loading fractions, the f-MoS₂ nanoplatelets will likely form agglomerates, resulting in fewer interfacial interactions and the matrix mobility therefore returns toward the value of the neat epoxy.³⁹ The trend seen in the T_g response (Fig. 3) corresponds well to those seen for the high temperature apparent shear strength (Fig. 5b).

Fig. 3 Plots of storage modulus and tan delta versus temperature for neat epoxy and f-MoS₂/epoxy composites.

The typical stress–strain curves of the baseline epoxy and f-MoS₂/epoxy nanocomposites are shown in Fig. 4a. Fig. 4b compares the Young's modulus of the pristine epoxy and the nanocomposite samples, and Fig. 4c shows their ultimate tensile strength. The error bars represent the maximum and minimum values obtained. The results indicate that f-MoS₂ nanosheets are effective at increasing the elastic modulus and the tensile strength of the epoxy at small nanosheets loadings (0.1–0.7% by weight). The tensile modulus of MoS₂/epoxy composites increased from 2.97 to 3.62 GPa (approximately a 22% increase over neat epoxy), and the tensile strength increased from 84.3 to 90.7 MPa (approximately a 8% increase over neat epoxy) when the MoS₂ content increased from 0 to 0.7%. Moreover, as the MoS₂ content increased from 0.7 to 1%, the tensile modulus and tensile strength reduced to 2.93 GPa and 75.2 MPa, respectively.^40–42 There ported results are the average of 5 different tests for each f-MoS₂ nanosheets loading fraction.

Fig. 4 (a) Stress–strain curves of neat epoxy and f-MoS₂/epoxy nanocomposites with various loadings of f-MoS₂; (b) Young's modulus vs. f-MoS₂ nanosheets loading fraction; (c) ultimate tensile strength vs. f-MoS₂ nanosheets loading fraction; (d) impact intensity vs. f-MoS₂ nanosheets loading fraction.

As can be seen, the addition of f-MoS₂ nanosheets significantly improves the impact intensity of the epoxy matrix (Fig. 4d). It is strongly evident that even a small amount of f-MoS₂ nanosheets could significantly improve the mechanical strength. The maximum increase in impact intensity is 66% in the sample of f-MoS₂/epoxy-0.7. However, beyond 0.7% loading there is a reduction in performance and at 1.0% loading the impact intensity of the nanocomposite sample reduced to 13.3 kJ m⁻².

The reinforcing effects of the f-MoS₂ on the adhesive properties of the epoxy composites are summarized in Fig. 5a and b. The apparent shear strength of f-MoS₂/epoxy composites at room temperature increased from 26.5 to 31.4 MPa (approximately an 18% increase over neat epoxy), and the tensile shear strength at 120 °C increased from 13.8 to 24.9 MPa (approximately an 80% increase over neat epoxy) when the f-MoS₂ nanosheets content increased from 0 to 0.7%. However, beyond 0.7% loading there is a reduction in performance at room temperature and at 1% loading the performance of the nanocomposite sample is comparable to the baseline epoxy.^43,44

Fig. 5 (a) Apparent shear strength at room temperature vs. f-MoS₂ nanosheets loading fraction; (b) apparent shear strength at 120 °C vs. f-MoS₂ nanosheets loading fraction.

Moreover, as the f-MoS₂ nanosheets content increased from 0.7 to 0.9 and 1.0 wt%, the apparent shear strength of the f-MoS₂/epoxy composites at 120 °C turned to 22.9 and 18.2 MPa, respectively, but a reducing trend was observed on the apparent shear strength of the f-MoS₂/epoxy composites at 120 °C.

To understand the dispersion levels of and f-MoS₂ in epoxy resin, the freeze-fractured surface microstructures (Fig. 6a–f) of epoxy nanocomposites were investigated by SEM. According to the previous studies, the fracture roughness of the polymer nanocomposites reflects the dispersion level and interfacial interaction to some degree.^45,46 It can be observed in Fig. 6a that the neat epoxy displays a smooth fracture surface. It is clear that the fracture surface roughness of the f-MoS₂/epoxy nanocomposites increase with increasing addition of f-MoS₂.^47–49 In addition, it is observed that numerous MoS₂ agglomerates are pulled out of 1.0 wt% f-MoS₂/epoxy nanocomposite. Nevertheless, few MoS₂ agglomerates emerge in the SEM images of 0.7 wt% f-MoS₂/epoxy nanocomposite because most of the f-MoS₂ nanosheets are uniformly embedded into the EP matrix.

Fig. 6 Scanning electron microscopy images of the fracture surfaces of neat EP (a), 0.3 wt% f-MoS₂/epoxy (b), 0.7 wt% f-MoS₂/epoxy (c and e) and 1.0 wt% f-MoS₂/epoxy (d and f) nanocomposites.

To further verify the dispersion states of bulk MoS₂ and f-MoS₂ in an epoxy matrix, the molybdenum and sulfur element distribution mapping of the bulk MoS₂/epoxy-0.7 and f-MoS₂/epoxy-0.7 is provided in Fig. 7. In short, f-MoS₂ nanosheets exhibit better dispersion in the epoxy matrix and compatibility with the matrix than those of bulk MoS₂ because of the thinner structure and stronger interfacial interaction.^50,51 Undoubtedly, the resultant homogeneous dispersion of f-MoS₂ sheets in nanocomposites is beneficial to their reinforced performances.

Fig. 7 Energy dispersive X-ray analysis images of the molybdenum element distribution mapping of 0.7 wt% f-MoS₂/epoxy (a) and 0.7 wt% bulk MoS₂/epoxy (b) nanocomposites, the sulfur element distribution mapping of 0.7 wt% f-MoS₂/epoxy (c) and 0.7 wt% bulk MoS₂/epoxy (d) nanocomposites.

Based on the results aforementioned, the high-efficiency reinforcement might be probably attributed to the uniform dispersion of the f-MoS₂ in the epoxy matrix (as observed in the EDAX images of Fig. 7a and c), as well as to the formation of strong interactive interfaces between f-MoS₂ and the epoxy matrix (as indicated in the SEM images of Fig. 6b–f). In the case of f-MoS₂/epoxy nanocomposites, the exfoliated f-MoS₂ nanosheets with a high elastic modulus served as the reinforcement rather than the bulk MoS₂ particles. The polymer chains adsorbed on the nanosheet surfaces are substantial due to the strong interfacial interactions, which increased the effective volume fraction of the reinforcement. The expansion of the effective volume fraction is presented to the continuous phase with consequences for adsorption and immobilization,^52,53 which provided a high capacity for transferring the stress from the epoxy matrix to f-MoS₂ nanosheets.

4. Conclusions

In summary, functionalized MoS₂ nanosheets were synthesized from bulk MoS₂ via chemical exfoliation and subsequent surface modification with 3-mercaptopropyltriethoxysilane. The functionalized MoS₂ were dispersed in epoxy polymers at loading fractions of up to wt 1% by ultrasonication and three roll mills. The dispersion states of f-MoS₂ in an epoxy matrix was much better than bulk MoS₂ by EDAX and SEM. With only 0.7 wt% of f-MoS₂, a 22% increase in tensile elastic modulus, a 66% increase in impact intensity, an 80% increase in tensile shear strength at 120 °C and an 11 °C increment in glass transition temperature of the resultant f-MoS₂/epoxy nanocomposite were attained. The superior reinforcing efficiency of f-MoS₂ can be attributed to the good dispersion of MoS₂ sheets and the strong interfacial interaction with the polymer matrix enabled by surface functionalization of MoS₂.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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