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The effective reinforcements of functionalized MoS2 nanosheets in polymer hybrid composites by sol-gel technique
Composites: Part A 94 (2017) 1–9
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The effective reinforcements of functionalized MoS2 nanosheets in polymer hybrid composites by sol-gel technique Keqing Zhou a,⇑, Rui Gao a, Zhou Gui b, Yuan Hu b a b
Faculty of Engineering, China University of Geosciences (Wuhan), 388 Lumo Road, Wuhan, Hubei 430074, PR China State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China
a r t i c l e
i n f o
Article history: Received 17 July 2016 Received in revised form 6 December 2016 Accepted 9 December 2016 Available online 10 December 2016 Keywords: MoS2 Functionalized Sol-gel method Polymer nanocomposites Reinforcement effect
a b s t r a c t In this work, siloxane group was grafted on the surface of exfoliated MoS2 nanosheets by chemical conjugation with 3-mercaptopropyl trimethoxysilane and then the functionalized MoS2 was incorporated into poly(vinyl alcohol) (PVA) matrix as reinforcements using sol-gel technique. The results of FTIR, TGA, XPS and TEM indicated that 3-mercaptopropyl trimethoxysilane was successfully grafted onto the surface of exfoliated MoS2 nanosheets. The functionalized MoS2 was well dispersed in PVA and no obvious aggregation of MoS2 nanosheets was observed. The incorporation of functionalized MoS2 nanosheets significantly enhanced the thermal properties, flame retardancy and mechanical properties of PVA films. The half weight loss degradation temperature of PVA films was increased from 298 to 416 °C and a 12 °C increment in the glass transition temperature was achieved with only 1.0 wt% functionalized MoS2. Furthermore, the peak heat release rate and the tensile strength of PVA films was decreased by 52.5% and improved by 98.4%, respectively, compared to that of neat PVA. These excellent reinforcements were mainly attributed to well dispersion of MoS2 nanosheets in the polymer matrix and strong interfacial adhesion between the two components. This work demonstrated herein will provide a promising route to fabricate MoS2-based polymer nanocomposites with excellent performances. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Poly(vinyl alcohol) (PVA), as one kind of water soluble and biocompatible polymer, has attracted intensive attention in various applications such as coating, textile sizing, and packaging films [1]. Nevertheless, insufficient mechanical, thermal stability and flammability properties of pure PVA have limited its wider applications in some fields. In the past few decades, polymer nanocomposites consist of polymer matrix and nanofillers have aroused tremendous interests due to the excellent performances of the obtained polymer materials, especially for the layered nanofillers. When the layered nanofillers are well dispersed in the polymer matrix, the incorporation of nanofillers with low loadings can trigger substantial improvements in mechanical properties, thermal stability, fire resistance, and gas permeability [2]. As well known, the typical layered nanofillers including MMT, LDH and graphene have been proven to impart effective reinforcement and flame retardancy with PVA matrix owing to their lamellar structure and high aspect ratio. Mallakpour et al. had prepared a coexistence
⇑ Corresponding author. E-mail address: [email protected] (K. Zhou). http://dx.doi.org/10.1016/j.compositesa.2016.12.010 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.
of exfoliated and intercalated Cloisite Na+/Val layers structure in the PVA matrix which improved the thermal stability property of the resulting films obviously [3]. Liu and his co-author had reported that the LDH nanoplates were uniformly dispersed in PVA matrix and the mechanical and thermal properties of the composites were improved obviously [4]. Bao et al. studied the influences of graphite oxide and graphene on the structure and properties of PVA nanocomposites contrastively and discussed the mechanism for the property enhancements [5]. As an emerging layered 2D nanomaterial, it has been reported recently that a monolayer of MoS2 has high surface areas, superb thermal stability and excellent mechanical properties, and show great potential as reinforcements for polymer materials [6]. Therefore, it is expected that the MoS2 can become a new generation of reinforcements to fabricate high performance polymer materials. 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 [7–10]. 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 [9,11–17]. However, as with graphene, complete
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utilization of MoS2 sheets in polymer nanocomposite inevitably depends on their dispersibility and sufficient compatibility in the polymer matrix. But actually, the exfoliated MoS2 nanosheets have a tendency to agglomerate and even restack in polymer matrices owing to their large specific surface area and van der Waals interactions, which results in the reduced efficiency for reinforcement [18]. In addition, the formation of a strong bonding between polymer matrix and nanofillers is another key factor to improve the properties of composites obviously. Consequently, the exfoliated MoS2 nanosheets are necessary to be functionalized before incorporation into polymer matrices. An effective way to overcome the agglomeration and enhance the compatibility and interfacial interactions between MoS2 nanosheets and polymer matrix is the chemical modification of exfoliated MoS2 nanosheets via noncovalent and covalent interaction. In our previous work, surfactant molecules, chitosan and melamine phosphate have been used to modify the exfoliated MoS2 nanosheets through noncovalent interaction, these modification methods are conducive to improve the compatibility and dispersibility of MoS2 nanosheets in polymer matrices [7,16]. However, there is no substantial improvement for the interfacial interactions between MoS2 nanosheets and polymer matrix. The feasible route to harnessing the poor dispersibility and interfacial interactions would be to incorporate exfoliated MoS2 nanosheets in polymer matrices via covalent interaction. The covalent functionalization of MoS2 nanosheets not only significantly improves the dispersion of MoS2 nanosheets, but also forms strong interfacial interactions with host polymer via covalent linkages [19]. Surface modification of exfoliated MoS2 nanosheets by covalent functionalization can provide active sites to form chemical bonds, acting as an ideal interface between the MoS2 and appropriate polymer matrices. However, different from graphene oxide, there are no functional groups on surface of MoS2 nanosheets, which makes it difficult to be decorated directly. Fortunately, some literatures have demonstrated that many defects of S atoms on MoS2 will be generated after being chemically exfoliated, which shows that it is possible to modify the internal and perimeter edges of MoS2 using thiol ligand functionalization [20]. Zhou et al. have reported that carboxyl functional group decorated MoS2 nanosheets can be obtained by chemical conjugation of mercaptopropionic acid with chemically exfoliated MoS2 nanosheets [21]. Wang et al. chemically exfoliated bulk MoS2 into nanoplatelets, and functionalized using lipoic acid via disulfide conjugation. The lipoic acid functionalized MoS2 nanosheets offered carboxyl terminals as the initiator for in situ ring-opening polymerization of 3caprolactam to fabricate nylon-6 nanocomposites. The covalent functionalized MoS2 nanosheets dispersed well in nylon-6 matrix and induced significant thermal stabilization and mechanical reinforcement [22]. As far as we know, various kinds of siloxanes have been widely used as coupling agents between glass substrates and polymeric resins. The coupling process can be accomplished via the chemical reaction between the trialkoxy groups of silane molecules and the hydroxyl groups on the glass substrates. Recently, different types of silanes have been developed to act as multifunctional and crosslinking agents between graphene sheets and the polymeric substrate to produce functionalized graphene aerogels [23,24]. In addition, many literatures have reported that silane precursors are used to modify PVA by sol-gel reaction to form PVA/silica hybrid composites at a molecular level [25,26]. These hybrid composites usually exhibit good thermal stability, mechanical, separation properties and do little harms to the physical performance of polymers. Sol-gel chemistry has become one of the most exciting fields in the synthesis of novel functional nanomaterials and provides the possibility to form covalent bonds between the organic phases and inorganic phases [27]. However, to the best of our
knowledge, there are no literatures reported on the application of exfoliated MoS2 nanosheets as reinforcements in polymer composites by sol-gel method up to date. Herein, the PVA/functionalized MoS2 hybrid films are fabricated by the sol-gel method in aqueous solution, as shown in Fig. 1. Functionalization of MoS2 nanosheets with 3-mercaptopropyl trimethoxysilane (MTS) was completed by the ligand conjugation between the chemically exfoliated MoS2 nanosheets and the thiol groups of MTS. The MTS was selected as the ‘‘bridge’’ to covalently connect MoS2 nanosheets with PVA matrix. The covalent functionalization will improve the compatibility and interfacial interactions between PVA matrix and exfoliated MoS2 nanosheets, which leads to effective reinforcements in thermal, flame retardance and mechanical properties of the PVA films. 2. Experimental 2.1. Materials PVA (polymerization degree 1750 ± 50, CP), molybdenum disulfide (MoS2, AP), tetrahydrofuran (THF, AP) and n-hexane (AP) were purchased from the Sinopharm Chemical Reagent Co., Ltd. The nbutyl lithium (2.2 M in hexane) was purchased from Alfa Aesar without further purified. 3-mercaptopropyl trimethoxysilane (MTS) was purchased from J&K Chemical Ltd. All the other starting materials used in this work were of analytical grade and used without further purification. 2.2. Fabrication of MTS-functionalized MoS2 nanosheets The fabrication route of the MTS-functionalized MoS2 nanosheets was shown in Fig. 1a and b. The chemically exfoliated MoS2 nanosheets were prepared by lithium intercalated and ultrasonication hydrolysis method according to our previous work [9]. Then the colloidal suspension of chemically exfoliated MoS2 nanosheets was mixed with excess amount of MTS which were dissolved in THF under stirring for 8 h. The functionalized MoS2 (MoS2-MTS) was collected by centrifugation, several washing steps with THF and water. Most of the functionalized MoS2 were redispersed in water to form the suspension, and a small portion was dried at 60 °C for 24 h in vacuum oven for characterization. 2.3. Preparation of PVA/functionalized MoS2 hybrid films To prepare PVA/functionalized MoS2 hybrid films, PVA was first dissolved in deionized water with a concentration of 10 mg mL 1 at 95 °C in a three neck flask. The required MoS2-MTS aqueous suspension was dripped into the abovementioned PVA solution. Then, HCl solution was added to catalyze crosslink with stirring at 60 °C for 10 h. At last, the aqueous mixture was sonicated for 20 min and poured into telfon petridishes and heated to 40 °C in oven for approximately 24 h to form flat membranes. The obtained membranes were further heated at 60 °C for 24 h to remove residual water and cut into pieces for tests. The content of the MoS2-MTS in the PVA hybrid films was defined as 0.5 wt% and 1.0 wt%. In addition, a similar procedure was used to prepare pure PVA films. 2.4. Characterization Fourier transform infrared (FTIR) spectra were obtained with a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, Madison, WI). X-ray photoelectron spectroscopy (XPS) spectra were performed on VG ESCALB MK-II electron spectrometer. Transmission electron microscopy (TEM) images were obtained on a Hitachi model H-800 transmission electron microscope with an accelerat-
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Fig. 1. Synthesis route of functionalized MoS2 (a and b) and preparation procedure for PVA hybrid films (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ing voltage of 200 kV. In addition, TEM was also performed to observe the dispersion state of the MoS2-MTS in PVA films. Scanning electron microscopy (SEM) (JSM-6800F, JEOL) was used to observe the fracture surface structure of PVA and PVA hybrid films. The specimens were adhibited onto the copper plate, and then coated with gold/palladium alloy ready for imaging. Differential scanning calorimetry (DSC) was performed using a Q2000 DSC instrument (TA Instruments Inc.). About 5 mg samples were heated from 0 to 120 °C at a linear heating rate of 10 °C min 1, the temperature was kept at 120 °C for 10 min and then decreased from 120 to 0 °C at a linear rate of 10 °C min 1. This heating-cooling cycle was repeated, and the data obtained from the second heating section were plotted. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., New Castle, DE) from room temperature to 800 °C at a heating rate of 20 °C min 1 in air atmosphere. Microscale combustion colorimeter (MCC) was used to evaluate the flammability characteristics of PVA and PVA hybrid films according to ASTM D7309-07. The
tensile strength and elongation at break was measured with an electronic universal testing instrument according to GB13022-91 (MTS System Co., Ltd., China). The stretching rate was 50 mm min 1. Five parallel runs were performed for each sample to get the average. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 apparatus (TA Instruments Inc.) at a fixed frequency of 10 Hz in the temperature range from 40 to 150 °C at a linear heating rate of 5 °C min 1.
3. Results and discussion 3.1. Characterization of functionalized MoS2 The attached organic functional groups on the exfoliated MoS2 nanosheets are first detected by FTIR. As can be seen in Fig. 2a, the bulk MoS2 almost has no any characteristic absorption peaks due to the absence of function groups on the surface of MoS2
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Fig. 2. FTIR spectrum (a) and TGA curves (b) of MoS2 and MoS2-MTS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanosheets [28]. However, some new characteristic bands including 2920 and 2850 cm 1 are observed clearly in the FTIR spectrum of the MoS2-MTS sample which are mainly attributed to the symmetric and asymmetric vibration of ACH2A groups, corresponding to the alkyl chains of the silane moieties on the surface of functionalized MoS2 [29]. Furthermore, the peak at 1080 cm 1, which is typical of the SiAO, further provides evidence to verify the successful functionalization of siloxane within the exfoliated MoS2 [30]. The presence of the MTS on the surface of the exfoliated MoS2 nanosheets is further confirmed by TGA analysis, as shown in Fig. 2b. The bulk MoS2 shows low weight loss between 550 and 700 °C, presumably due to the transformation of MoS2 to molybdenum oxide and sulfur oxide under air atmosphere in heating process [9]. In contrast, it can be observed clearly that the mass loss of MoS2-MTS is much larger than that of unmodified MoS2, which is mainly ascribed to the thermal decomposition of the vast attached silanes. The significantly decreased residues can be explained by the successful surface modification of MoS2 [31]. To provide further insight and clear evidence for the grafting of silanes on the surface of MoS2 nanosheets, the MoS2-MTS samples are characterized by XPS. According to the full survey spectra, as shown in Fig. 3a, the elements of Mo, S, C and O are found for all of the samples. It can be seen that the relative intensity of C1s and O1s binding energy at MoS2-MTS increases obviously, and the relative intensities of Mo and S binding energy at MoS2-MTS decrease obviously when compared to those of the MoS2, suggesting the successful attachment of silanes onto the surface of MoS2 nanosheets [18]. Moreover, a peak relating to Si2p binding energy appears, as depicted in Fig. 3a and b, further indicating the decoration of MTS on the surface of MoS2 nanosheets. XPS is further used to investigate the chemical states of Mo and S and to provide information for the interaction between the exfoliated MoS2 nanosheets and MTS. The high-resolution XPS spectra (Fig. 3c and d) show that the binding energies of Mo 3d 3/2, Mo 3d 5/2, S 2p 1/2 and S 2p 3/2 peaks in the pure MoS2 are located at 233.2, 230.0, 163.9 and 162.8 eV, respectively, suggesting that Mo4+ existed in the pure MoS2 [32]. For the MoS2-MTS, Mo 3d 3/2, Mo 3d 5/2, S 2p 1/2 and S 2p 3/2 peaks shift to 232.7, 229.5, 163.5 and 162.3 eV, respectively. The binding energy of Mo 3d 3/2, Mo 3d 5/2, S2p1/2 and S2p 3/2 of the MoS2-MTS samples all shift to the lower energy, indicated the presence of the strong interaction between the exfoliated MoS2 nanosheets and MTS [33]. Therefore, combined with the FTIR, TG and XPS results, it can infer that the exfoliated MoS2 nanosheets have been modified successfully by the silane coupling agent.
The micro-morphologies of the exfoliated MoS2 nanosheets and functionalized MoS2 are further characterized by TEM (Fig. 4). As shown in Fig. 4a, the MoS2 exhibits typical well-exfoliated structure with few-layer sheets and with a smooth surface. For the MoS2-MTS sample (Fig. 4b), it still keeps a similar-layered structure as that of unmodified MoS2 nanosheets, but presents a fairly rough surface. Moreover, some granules are emerged on the nanoplates, which correspond to the grafting of MTS [34]. These results in conjunction with the FTIR, TG and XPS results could be considered as indication of successfully functionalization. 3.2. Dispersion and interfacial interaction As is well known, the dispersion and interfacial interaction between nanofillers and polymer matrix play a key role in affecting the properties of polymer nanocomposites [35]. To obtain the information concerning the dispersion and interfacial interactions between the PVA matrix and functionalized MoS2 sheets, SEM images are employed to observe the micro-morphology of fracture surface of PVA and its hybrid films. As for the pure PVA in Fig. 5a and b, the fractured surface is relatively smooth. However, the fractured surface of PVA hybrid films with MoS2 loading of 1 wt% is much rougher than that of pure PVA, and no obvious pulled out and aggregation of MoS2 nanosheets can be observed (Fig. 5c and d), indicating the well dispersion of MoS2 nanosheets and formation of stronger interfacial interactions between functionalized MoS2 nanosheets and PVA matrix through covalent linkage by sol-gel technique. TEM observation is further employed to observe the dispersion state of functionalized MoS2 nanosheets within the PVA matrix. As can be observed from Fig. 6, the functionalized MoS2 sheets were dispersed well in PVA matrix without obvious aggregates and mainly shown with individual layer or intercalated structure. The well dispersion and strong interfacial adhesion can support a significant stress transfer and thus substantially reinforce the properties of polymer nanocomposites. 3.3. Thermal properties Glass transition temperature (Tg) is an important thermal parameter which is usually used to evaluate the segmental mobility of polymer chains. To verify the existence of the network and investigate its influence on the segmental motions in hybrid films, Tg of all the samples was tested and calculated by DSC. The DSC curves of the samples are presented in Fig. 7. A significant shift in the Tg of PVA hybrid films toward higher temperature can be
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Fig. 3. XPS spectra of pure MoS2 and MoS2-MTS (a), Si2p XPS spectrum of MoS2-MTS (b), high-resolution XPS spectra of Mo3d (c) and S2p peaks (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. TEM images of the exfoliated MoS2 (a) and MoS2-MTS (b).
observed after the incorporation of functionalized MoS2 nanosheets. The Tg value increases from 63 to 75 °C with the increase of functionalized MoS2 content to 1.0 wt%. The increase of Tg is ascribed to the great restriction of polymer’s chain motions, indicating the formation of network structure and good interfacial interaction between the functionalized MoS2 and PVA chains [36]. The thermal stability and thermal degradation behaviors of PVA and its hybrid films are evaluated by TGA under air atmosphere, as shown in Fig. 8. It can be observed that the thermal degradation process of pure PVA mainly consists of three stages based on the DTG profiles, which mainly correspond to the vaporization of the physically weak and chemically strong bound water molecules, the decomposition of the side-chain of the PVA, the cleavage
CAC backbone of the PVA [29], respectively. The onset degradation temperature at which 10% (T10%) weight loss occurs is decreased obviously, compared with neat PVA, which is mainly due to the degradation of the grafted silanes [37]. However, when 50% mass loss temperature (T50%) and maximum decomposition temperature (Tmax) are selected as a point of comparison, as the content of the MoS2 nanosheets increasing, the characteristic degradation temperatures of PVA hybrid films gradually shift to higher temperature. The T50% and Tmax for PVA hybrid films containing 1.0 wt% of the functionalized MoS2 are increased from 298 to 416 °C and 485 to 519 °C, respectively, which are mainly attributed to the cross linked network structure [36]. From the DTG curves (Fig. 8b), it can be clearly seen that the addition of 1.0 wt% functionalized
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Fig. 5. SEM images of the fractured surfaces for neat PVA (a and b) and PVA/1.0 wt% MoS2-MTS hybrid films (c and d).
Fig. 6. TEM image of the PVA hybrid films with 1.0 wt% MoS2-MTS.
MoS2 decreases the maximum mass loss rate (the peak of DTG curves) of the PVA hybrid films, suggesting that functionalized MoS2 nanosheets act as an effective barrier to prevent the permeation of oxygen and the escape of volatile degradation products during the thermal degradation process. In conclusion, DSC and TGA results indicate that the thermal properties of the PVA hybrid films are effectively improved by incorporating functionalized MoS2, which can be attributed to the physical barrier effect of functionalized MoS2 nanosheets and the enhanced interaction between functionalized MoS2 and PVA matrix [29]. 3.4. Flammability As well known, 2D layered nanofillers or silicon-containing compounds are usually used to improve the flame retardancy of polymers. Consequently, functionalized MoS2 nanosheets by grafting silane are expected to impart the PVA with fire resistance. Fig. 9
Fig. 7. DSC curves of pure PVA and PVA hybrid films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
shows MCC results of neat PVA and its hybrid films. The peak of heat release rate (PHRR) is one of the most important parameters to evaluate flammability, and a low value of PHRR is an indication of low flammability. Compared with pure PVA, the PVA hybrid films achieve significant improvements in flame resistance. The PHRR of all the PVA hybrid films are lower than that of pure PVA and gradually decrease as the content of the functionalized MoS2 increases. Meanwhile, the temperatures for PHRR are increased obviously. When the concentration of functionalized MoS2 reaches 1.0 wt%, the PHRR of PVA hybrid films decrease apparently, falling by 52.5%, compared with that of pure PVA. The abovementioned results confirm that the incorporation of functionalized MoS2 into PVA matrix by sol-gel method can improve the flame retardancy of PVA hybrid films obviously. The possible mechanisms for enhanced
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Fig. 8. TGA and DTG curves of pure PVA and PVA hybrid films in air atmosphere. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. HRR curves of pure PVA and PVA hybrid films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
thermal properties and flame retardancy are proposed as follows: firstly, functionalized MoS2 nanosheets create a barrier effect on the surface of the PVA, which could slow down mass transfer between gas and condensed phases and prevent the underlying materials from further combustion [16]; secondly, the crosslinked network structure in the hybrid films is conducive to improve the thermal stability. The cross-linked network caused by the sol-gel method can effectively induce the mobility restriction of polymer chains, thus retarding the thermal degradation of hybrid films [38]; thirdly, the immigration of silicon to the surface during the combustion and carbonization also plays an important role, which protects the char layer from thermo-degradation, enhances the stability of the char layer, and prevents the release of volatile products from the matrix [39]. 3.5. Mechanical properties According to previous research work, the mechanical properties of the PVA nanocomposites are improved obviously by incorporation of 2D layered nanofillers such as graphene and LDH [4,5]. Based on the well dispersion of the functionalized MoS2 in PVA matrix, and strong interfacial interaction between functionalized
MoS2 and PVA matrix, it is speculated that the addition of the functionalized MoS2 nanosheets is conducive to improve the mechanical properties of the PVA hybrid films. The typical stress-strain curves for PVA and its hybrid composites are presented in Fig. 10. The tensile strength of all the PVA hybrid films is larger than that of pure PVA and gradually increases as the content of the functionalized MoS2 increases. With loading of 1.0 wt% functionalized MoS2, the tensile strength of PVA hybrid films increases by 98.4%, compared with that of the pure PVA. The obvious increment of the tensile strength is mainly attributed to the well dispersion of functionalized MoS2 nanosheets and strong interfacial interactions between the functionalized MoS2 and PVA matrix. Similar results have been observed for PVA nanocomposites with graphene as nanofillers in reported literature [29]. The elongation at break of the hybrid films gradually decreases with the addition of functionalized MoS2, which is mainly due to the lamellar barrier effect of the nanoflakes restricting the segmental motion of the polymer chains in the nanocomposites [34]. In addition, DMA is further carried out to evaluate the mechanical properties of the PVA hybrid films. The storage modulus and loss tangent curves are shown in Fig. 11. The storage modulus is a measure of the stiffness and all the PVA hybrid films containing MoS2-MTS exhibit much higher storage modulus than pure PVA at the full temperature range investigated. Especially, the storage modulus of the PVA/1.0% MoS2-MTS composite is about 36% higher compared to that of the PVA at 40 °C. The glass transition temperature (Tg) is determined from the peak temperature of tan delta curves. As expected, an obvious increase in Tg values for PVA hybrid films is achieved. The pure PVA displays a Tg of 92 °C, while for the composite film with 1.0 wt% MoS2-MTS, the Tg is significantly increased to 105 °C, which indicates a strong confinement effect of MoS2MTS to the PVA chains. In addition, it is of great significance to study the mobility of polymer chains inside the fillers and also how the presences of nanofillers affect the mobility of the surrounding polymer chains. DMA can be used to study the mobility of polymer chains in composites. During the glass transition, the long range polymer chain gains mobility and this dissipates a great amount of energy through viscous movement. In Fig. 11b, the tan delta peak value decreases with addition of MoS2-MTS. Any depression in the tan delta indicates the reduction of the number of the mobile chains during the glass transition, indicating the restriction of polymer chains in the obtained PVA hybrid films. The DMA results further prove the strong interfacial adhesion between the MoS2-MTS and PVA matrix.
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Fig. 10. Typical stress-strain curves of pure PVA and PVA hybrid films (a) and tensile strength and elongation at break of pure PVA and PVA hybrid films as a function of MoS2MTS loadings (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. DMA curves of pure PVA and PVA hybrid films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
To highlight the advantages of the sol-gel method reported in this work, the thermal properties, flame retardancy and mechanical properties of PVA nanocomposites in terms of T50%, PHRR and tensile strength values in present study are compared with the results reported in our previous work. The increment of the T50% and tensile strength values and the decrement of the PHRR values in present work are much larger than those of previous research work when the addition volume is equal [9]. Obviously, the reinforcement efficiency of the functionalized MoS2 in PVA hybrid films which are prepared by the sol-gel method in this work is more excellent than the simple solvent blending method in our previous work. The effective reinforcements are mainly attributed to well dispersion of functionalized MoS2 nanosheets in the polymer matrix and strong interfacial adhesion between the two components which can efficiently transfer the load from the weak polymer chains to the robust MoS2 nanosheets.
The SEM and TEM images indicated that the functionalized MoS2 was well dispersed in PVA and formed strong interfacial adhesion with PVA matrix due to the organosiloxane with active end-group grafted on the surface of exfoliated MoS2 nanosheets. The incorporation of functionalized MoS2 with a relatively low concentration induced a significant reinforced effect on the thermal, flame retardance and mechanical properties of the PVA hybrid films. With the functionalized MoS2 volume content of 1.0%, T50% was increased from 298 to 416 °C and a 12 °C increment in the Tg was also achieved, compared to pure PVA. Furthermore, the PHRR and the tensile strength of PVA hybrid films was decreased by 52.5% and improved by 98.4%, respectively. These remarkably enhanced properties were mainly attributed to the well dispersion of the functionalized MoS2 and the formation of strong interfacial adhesion between functionalized MoS2 and PVA matrix. The sol-gel method presented herein will provide a promising approach to fabricate MoS2-based polymer hybrid composites with excellent performances.
4. Conclusion Acknowledgement In present study, siloxane group was successfully grafted on the surface of exfoliated MoS2 nanosheets by chemical conjugation with 3-mercaptopropyl trimethoxysilane and then the functionalized MoS2 was incorporated into PVA matrix by sol-gel method.
This work was supported by the Fundamental Research Funds for the Central Universities (G1323521619), China University of Geosciences (Wuhan) (CUG160607).
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
The effective reinforcements of functionalized MoS2 nanosheets in polymer hybrid composites by sol-gel technique Keqing Zhou a,⇑, Rui Gao a, Zhou Gui b, Yuan Hu b a b
Faculty of Engineering, China University of Geosciences (Wuhan), 388 Lumo Road, Wuhan, Hubei 430074, PR China State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China
a r t i c l e
i n f o
Article history: Received 17 July 2016 Received in revised form 6 December 2016 Accepted 9 December 2016 Available online 10 December 2016 Keywords: MoS2 Functionalized Sol-gel method Polymer nanocomposites Reinforcement effect
a b s t r a c t In this work, siloxane group was grafted on the surface of exfoliated MoS2 nanosheets by chemical conjugation with 3-mercaptopropyl trimethoxysilane and then the functionalized MoS2 was incorporated into poly(vinyl alcohol) (PVA) matrix as reinforcements using sol-gel technique. The results of FTIR, TGA, XPS and TEM indicated that 3-mercaptopropyl trimethoxysilane was successfully grafted onto the surface of exfoliated MoS2 nanosheets. The functionalized MoS2 was well dispersed in PVA and no obvious aggregation of MoS2 nanosheets was observed. The incorporation of functionalized MoS2 nanosheets significantly enhanced the thermal properties, flame retardancy and mechanical properties of PVA films. The half weight loss degradation temperature of PVA films was increased from 298 to 416 °C and a 12 °C increment in the glass transition temperature was achieved with only 1.0 wt% functionalized MoS2. Furthermore, the peak heat release rate and the tensile strength of PVA films was decreased by 52.5% and improved by 98.4%, respectively, compared to that of neat PVA. These excellent reinforcements were mainly attributed to well dispersion of MoS2 nanosheets in the polymer matrix and strong interfacial adhesion between the two components. This work demonstrated herein will provide a promising route to fabricate MoS2-based polymer nanocomposites with excellent performances. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Poly(vinyl alcohol) (PVA), as one kind of water soluble and biocompatible polymer, has attracted intensive attention in various applications such as coating, textile sizing, and packaging films [1]. Nevertheless, insufficient mechanical, thermal stability and flammability properties of pure PVA have limited its wider applications in some fields. In the past few decades, polymer nanocomposites consist of polymer matrix and nanofillers have aroused tremendous interests due to the excellent performances of the obtained polymer materials, especially for the layered nanofillers. When the layered nanofillers are well dispersed in the polymer matrix, the incorporation of nanofillers with low loadings can trigger substantial improvements in mechanical properties, thermal stability, fire resistance, and gas permeability [2]. As well known, the typical layered nanofillers including MMT, LDH and graphene have been proven to impart effective reinforcement and flame retardancy with PVA matrix owing to their lamellar structure and high aspect ratio. Mallakpour et al. had prepared a coexistence
⇑ Corresponding author. E-mail address: [email protected] (K. Zhou). http://dx.doi.org/10.1016/j.compositesa.2016.12.010 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.
of exfoliated and intercalated Cloisite Na+/Val layers structure in the PVA matrix which improved the thermal stability property of the resulting films obviously [3]. Liu and his co-author had reported that the LDH nanoplates were uniformly dispersed in PVA matrix and the mechanical and thermal properties of the composites were improved obviously [4]. Bao et al. studied the influences of graphite oxide and graphene on the structure and properties of PVA nanocomposites contrastively and discussed the mechanism for the property enhancements [5]. As an emerging layered 2D nanomaterial, it has been reported recently that a monolayer of MoS2 has high surface areas, superb thermal stability and excellent mechanical properties, and show great potential as reinforcements for polymer materials [6]. Therefore, it is expected that the MoS2 can become a new generation of reinforcements to fabricate high performance polymer materials. 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 [7–10]. 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 [9,11–17]. However, as with graphene, complete
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utilization of MoS2 sheets in polymer nanocomposite inevitably depends on their dispersibility and sufficient compatibility in the polymer matrix. But actually, the exfoliated MoS2 nanosheets have a tendency to agglomerate and even restack in polymer matrices owing to their large specific surface area and van der Waals interactions, which results in the reduced efficiency for reinforcement [18]. In addition, the formation of a strong bonding between polymer matrix and nanofillers is another key factor to improve the properties of composites obviously. Consequently, the exfoliated MoS2 nanosheets are necessary to be functionalized before incorporation into polymer matrices. An effective way to overcome the agglomeration and enhance the compatibility and interfacial interactions between MoS2 nanosheets and polymer matrix is the chemical modification of exfoliated MoS2 nanosheets via noncovalent and covalent interaction. In our previous work, surfactant molecules, chitosan and melamine phosphate have been used to modify the exfoliated MoS2 nanosheets through noncovalent interaction, these modification methods are conducive to improve the compatibility and dispersibility of MoS2 nanosheets in polymer matrices [7,16]. However, there is no substantial improvement for the interfacial interactions between MoS2 nanosheets and polymer matrix. The feasible route to harnessing the poor dispersibility and interfacial interactions would be to incorporate exfoliated MoS2 nanosheets in polymer matrices via covalent interaction. The covalent functionalization of MoS2 nanosheets not only significantly improves the dispersion of MoS2 nanosheets, but also forms strong interfacial interactions with host polymer via covalent linkages [19]. Surface modification of exfoliated MoS2 nanosheets by covalent functionalization can provide active sites to form chemical bonds, acting as an ideal interface between the MoS2 and appropriate polymer matrices. However, different from graphene oxide, there are no functional groups on surface of MoS2 nanosheets, which makes it difficult to be decorated directly. Fortunately, some literatures have demonstrated that many defects of S atoms on MoS2 will be generated after being chemically exfoliated, which shows that it is possible to modify the internal and perimeter edges of MoS2 using thiol ligand functionalization [20]. Zhou et al. have reported that carboxyl functional group decorated MoS2 nanosheets can be obtained by chemical conjugation of mercaptopropionic acid with chemically exfoliated MoS2 nanosheets [21]. Wang et al. chemically exfoliated bulk MoS2 into nanoplatelets, and functionalized using lipoic acid via disulfide conjugation. The lipoic acid functionalized MoS2 nanosheets offered carboxyl terminals as the initiator for in situ ring-opening polymerization of 3caprolactam to fabricate nylon-6 nanocomposites. The covalent functionalized MoS2 nanosheets dispersed well in nylon-6 matrix and induced significant thermal stabilization and mechanical reinforcement [22]. As far as we know, various kinds of siloxanes have been widely used as coupling agents between glass substrates and polymeric resins. The coupling process can be accomplished via the chemical reaction between the trialkoxy groups of silane molecules and the hydroxyl groups on the glass substrates. Recently, different types of silanes have been developed to act as multifunctional and crosslinking agents between graphene sheets and the polymeric substrate to produce functionalized graphene aerogels [23,24]. In addition, many literatures have reported that silane precursors are used to modify PVA by sol-gel reaction to form PVA/silica hybrid composites at a molecular level [25,26]. These hybrid composites usually exhibit good thermal stability, mechanical, separation properties and do little harms to the physical performance of polymers. Sol-gel chemistry has become one of the most exciting fields in the synthesis of novel functional nanomaterials and provides the possibility to form covalent bonds between the organic phases and inorganic phases [27]. However, to the best of our
knowledge, there are no literatures reported on the application of exfoliated MoS2 nanosheets as reinforcements in polymer composites by sol-gel method up to date. Herein, the PVA/functionalized MoS2 hybrid films are fabricated by the sol-gel method in aqueous solution, as shown in Fig. 1. Functionalization of MoS2 nanosheets with 3-mercaptopropyl trimethoxysilane (MTS) was completed by the ligand conjugation between the chemically exfoliated MoS2 nanosheets and the thiol groups of MTS. The MTS was selected as the ‘‘bridge’’ to covalently connect MoS2 nanosheets with PVA matrix. The covalent functionalization will improve the compatibility and interfacial interactions between PVA matrix and exfoliated MoS2 nanosheets, which leads to effective reinforcements in thermal, flame retardance and mechanical properties of the PVA films. 2. Experimental 2.1. Materials PVA (polymerization degree 1750 ± 50, CP), molybdenum disulfide (MoS2, AP), tetrahydrofuran (THF, AP) and n-hexane (AP) were purchased from the Sinopharm Chemical Reagent Co., Ltd. The nbutyl lithium (2.2 M in hexane) was purchased from Alfa Aesar without further purified. 3-mercaptopropyl trimethoxysilane (MTS) was purchased from J&K Chemical Ltd. All the other starting materials used in this work were of analytical grade and used without further purification. 2.2. Fabrication of MTS-functionalized MoS2 nanosheets The fabrication route of the MTS-functionalized MoS2 nanosheets was shown in Fig. 1a and b. The chemically exfoliated MoS2 nanosheets were prepared by lithium intercalated and ultrasonication hydrolysis method according to our previous work [9]. Then the colloidal suspension of chemically exfoliated MoS2 nanosheets was mixed with excess amount of MTS which were dissolved in THF under stirring for 8 h. The functionalized MoS2 (MoS2-MTS) was collected by centrifugation, several washing steps with THF and water. Most of the functionalized MoS2 were redispersed in water to form the suspension, and a small portion was dried at 60 °C for 24 h in vacuum oven for characterization. 2.3. Preparation of PVA/functionalized MoS2 hybrid films To prepare PVA/functionalized MoS2 hybrid films, PVA was first dissolved in deionized water with a concentration of 10 mg mL 1 at 95 °C in a three neck flask. The required MoS2-MTS aqueous suspension was dripped into the abovementioned PVA solution. Then, HCl solution was added to catalyze crosslink with stirring at 60 °C for 10 h. At last, the aqueous mixture was sonicated for 20 min and poured into telfon petridishes and heated to 40 °C in oven for approximately 24 h to form flat membranes. The obtained membranes were further heated at 60 °C for 24 h to remove residual water and cut into pieces for tests. The content of the MoS2-MTS in the PVA hybrid films was defined as 0.5 wt% and 1.0 wt%. In addition, a similar procedure was used to prepare pure PVA films. 2.4. Characterization Fourier transform infrared (FTIR) spectra were obtained with a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, Madison, WI). X-ray photoelectron spectroscopy (XPS) spectra were performed on VG ESCALB MK-II electron spectrometer. Transmission electron microscopy (TEM) images were obtained on a Hitachi model H-800 transmission electron microscope with an accelerat-
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Fig. 1. Synthesis route of functionalized MoS2 (a and b) and preparation procedure for PVA hybrid films (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ing voltage of 200 kV. In addition, TEM was also performed to observe the dispersion state of the MoS2-MTS in PVA films. Scanning electron microscopy (SEM) (JSM-6800F, JEOL) was used to observe the fracture surface structure of PVA and PVA hybrid films. The specimens were adhibited onto the copper plate, and then coated with gold/palladium alloy ready for imaging. Differential scanning calorimetry (DSC) was performed using a Q2000 DSC instrument (TA Instruments Inc.). About 5 mg samples were heated from 0 to 120 °C at a linear heating rate of 10 °C min 1, the temperature was kept at 120 °C for 10 min and then decreased from 120 to 0 °C at a linear rate of 10 °C min 1. This heating-cooling cycle was repeated, and the data obtained from the second heating section were plotted. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., New Castle, DE) from room temperature to 800 °C at a heating rate of 20 °C min 1 in air atmosphere. Microscale combustion colorimeter (MCC) was used to evaluate the flammability characteristics of PVA and PVA hybrid films according to ASTM D7309-07. The
tensile strength and elongation at break was measured with an electronic universal testing instrument according to GB13022-91 (MTS System Co., Ltd., China). The stretching rate was 50 mm min 1. Five parallel runs were performed for each sample to get the average. Dynamic mechanical analysis (DMA) was performed using a DMA Q800 apparatus (TA Instruments Inc.) at a fixed frequency of 10 Hz in the temperature range from 40 to 150 °C at a linear heating rate of 5 °C min 1.
3. Results and discussion 3.1. Characterization of functionalized MoS2 The attached organic functional groups on the exfoliated MoS2 nanosheets are first detected by FTIR. As can be seen in Fig. 2a, the bulk MoS2 almost has no any characteristic absorption peaks due to the absence of function groups on the surface of MoS2
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Fig. 2. FTIR spectrum (a) and TGA curves (b) of MoS2 and MoS2-MTS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanosheets [28]. However, some new characteristic bands including 2920 and 2850 cm 1 are observed clearly in the FTIR spectrum of the MoS2-MTS sample which are mainly attributed to the symmetric and asymmetric vibration of ACH2A groups, corresponding to the alkyl chains of the silane moieties on the surface of functionalized MoS2 [29]. Furthermore, the peak at 1080 cm 1, which is typical of the SiAO, further provides evidence to verify the successful functionalization of siloxane within the exfoliated MoS2 [30]. The presence of the MTS on the surface of the exfoliated MoS2 nanosheets is further confirmed by TGA analysis, as shown in Fig. 2b. The bulk MoS2 shows low weight loss between 550 and 700 °C, presumably due to the transformation of MoS2 to molybdenum oxide and sulfur oxide under air atmosphere in heating process [9]. In contrast, it can be observed clearly that the mass loss of MoS2-MTS is much larger than that of unmodified MoS2, which is mainly ascribed to the thermal decomposition of the vast attached silanes. The significantly decreased residues can be explained by the successful surface modification of MoS2 [31]. To provide further insight and clear evidence for the grafting of silanes on the surface of MoS2 nanosheets, the MoS2-MTS samples are characterized by XPS. According to the full survey spectra, as shown in Fig. 3a, the elements of Mo, S, C and O are found for all of the samples. It can be seen that the relative intensity of C1s and O1s binding energy at MoS2-MTS increases obviously, and the relative intensities of Mo and S binding energy at MoS2-MTS decrease obviously when compared to those of the MoS2, suggesting the successful attachment of silanes onto the surface of MoS2 nanosheets [18]. Moreover, a peak relating to Si2p binding energy appears, as depicted in Fig. 3a and b, further indicating the decoration of MTS on the surface of MoS2 nanosheets. XPS is further used to investigate the chemical states of Mo and S and to provide information for the interaction between the exfoliated MoS2 nanosheets and MTS. The high-resolution XPS spectra (Fig. 3c and d) show that the binding energies of Mo 3d 3/2, Mo 3d 5/2, S 2p 1/2 and S 2p 3/2 peaks in the pure MoS2 are located at 233.2, 230.0, 163.9 and 162.8 eV, respectively, suggesting that Mo4+ existed in the pure MoS2 [32]. For the MoS2-MTS, Mo 3d 3/2, Mo 3d 5/2, S 2p 1/2 and S 2p 3/2 peaks shift to 232.7, 229.5, 163.5 and 162.3 eV, respectively. The binding energy of Mo 3d 3/2, Mo 3d 5/2, S2p1/2 and S2p 3/2 of the MoS2-MTS samples all shift to the lower energy, indicated the presence of the strong interaction between the exfoliated MoS2 nanosheets and MTS [33]. Therefore, combined with the FTIR, TG and XPS results, it can infer that the exfoliated MoS2 nanosheets have been modified successfully by the silane coupling agent.
The micro-morphologies of the exfoliated MoS2 nanosheets and functionalized MoS2 are further characterized by TEM (Fig. 4). As shown in Fig. 4a, the MoS2 exhibits typical well-exfoliated structure with few-layer sheets and with a smooth surface. For the MoS2-MTS sample (Fig. 4b), it still keeps a similar-layered structure as that of unmodified MoS2 nanosheets, but presents a fairly rough surface. Moreover, some granules are emerged on the nanoplates, which correspond to the grafting of MTS [34]. These results in conjunction with the FTIR, TG and XPS results could be considered as indication of successfully functionalization. 3.2. Dispersion and interfacial interaction As is well known, the dispersion and interfacial interaction between nanofillers and polymer matrix play a key role in affecting the properties of polymer nanocomposites [35]. To obtain the information concerning the dispersion and interfacial interactions between the PVA matrix and functionalized MoS2 sheets, SEM images are employed to observe the micro-morphology of fracture surface of PVA and its hybrid films. As for the pure PVA in Fig. 5a and b, the fractured surface is relatively smooth. However, the fractured surface of PVA hybrid films with MoS2 loading of 1 wt% is much rougher than that of pure PVA, and no obvious pulled out and aggregation of MoS2 nanosheets can be observed (Fig. 5c and d), indicating the well dispersion of MoS2 nanosheets and formation of stronger interfacial interactions between functionalized MoS2 nanosheets and PVA matrix through covalent linkage by sol-gel technique. TEM observation is further employed to observe the dispersion state of functionalized MoS2 nanosheets within the PVA matrix. As can be observed from Fig. 6, the functionalized MoS2 sheets were dispersed well in PVA matrix without obvious aggregates and mainly shown with individual layer or intercalated structure. The well dispersion and strong interfacial adhesion can support a significant stress transfer and thus substantially reinforce the properties of polymer nanocomposites. 3.3. Thermal properties Glass transition temperature (Tg) is an important thermal parameter which is usually used to evaluate the segmental mobility of polymer chains. To verify the existence of the network and investigate its influence on the segmental motions in hybrid films, Tg of all the samples was tested and calculated by DSC. The DSC curves of the samples are presented in Fig. 7. A significant shift in the Tg of PVA hybrid films toward higher temperature can be
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Fig. 3. XPS spectra of pure MoS2 and MoS2-MTS (a), Si2p XPS spectrum of MoS2-MTS (b), high-resolution XPS spectra of Mo3d (c) and S2p peaks (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. TEM images of the exfoliated MoS2 (a) and MoS2-MTS (b).
observed after the incorporation of functionalized MoS2 nanosheets. The Tg value increases from 63 to 75 °C with the increase of functionalized MoS2 content to 1.0 wt%. The increase of Tg is ascribed to the great restriction of polymer’s chain motions, indicating the formation of network structure and good interfacial interaction between the functionalized MoS2 and PVA chains [36]. The thermal stability and thermal degradation behaviors of PVA and its hybrid films are evaluated by TGA under air atmosphere, as shown in Fig. 8. It can be observed that the thermal degradation process of pure PVA mainly consists of three stages based on the DTG profiles, which mainly correspond to the vaporization of the physically weak and chemically strong bound water molecules, the decomposition of the side-chain of the PVA, the cleavage
CAC backbone of the PVA [29], respectively. The onset degradation temperature at which 10% (T10%) weight loss occurs is decreased obviously, compared with neat PVA, which is mainly due to the degradation of the grafted silanes [37]. However, when 50% mass loss temperature (T50%) and maximum decomposition temperature (Tmax) are selected as a point of comparison, as the content of the MoS2 nanosheets increasing, the characteristic degradation temperatures of PVA hybrid films gradually shift to higher temperature. The T50% and Tmax for PVA hybrid films containing 1.0 wt% of the functionalized MoS2 are increased from 298 to 416 °C and 485 to 519 °C, respectively, which are mainly attributed to the cross linked network structure [36]. From the DTG curves (Fig. 8b), it can be clearly seen that the addition of 1.0 wt% functionalized
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Fig. 5. SEM images of the fractured surfaces for neat PVA (a and b) and PVA/1.0 wt% MoS2-MTS hybrid films (c and d).
Fig. 6. TEM image of the PVA hybrid films with 1.0 wt% MoS2-MTS.
MoS2 decreases the maximum mass loss rate (the peak of DTG curves) of the PVA hybrid films, suggesting that functionalized MoS2 nanosheets act as an effective barrier to prevent the permeation of oxygen and the escape of volatile degradation products during the thermal degradation process. In conclusion, DSC and TGA results indicate that the thermal properties of the PVA hybrid films are effectively improved by incorporating functionalized MoS2, which can be attributed to the physical barrier effect of functionalized MoS2 nanosheets and the enhanced interaction between functionalized MoS2 and PVA matrix [29]. 3.4. Flammability As well known, 2D layered nanofillers or silicon-containing compounds are usually used to improve the flame retardancy of polymers. Consequently, functionalized MoS2 nanosheets by grafting silane are expected to impart the PVA with fire resistance. Fig. 9
Fig. 7. DSC curves of pure PVA and PVA hybrid films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
shows MCC results of neat PVA and its hybrid films. The peak of heat release rate (PHRR) is one of the most important parameters to evaluate flammability, and a low value of PHRR is an indication of low flammability. Compared with pure PVA, the PVA hybrid films achieve significant improvements in flame resistance. The PHRR of all the PVA hybrid films are lower than that of pure PVA and gradually decrease as the content of the functionalized MoS2 increases. Meanwhile, the temperatures for PHRR are increased obviously. When the concentration of functionalized MoS2 reaches 1.0 wt%, the PHRR of PVA hybrid films decrease apparently, falling by 52.5%, compared with that of pure PVA. The abovementioned results confirm that the incorporation of functionalized MoS2 into PVA matrix by sol-gel method can improve the flame retardancy of PVA hybrid films obviously. The possible mechanisms for enhanced
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Fig. 8. TGA and DTG curves of pure PVA and PVA hybrid films in air atmosphere. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. HRR curves of pure PVA and PVA hybrid films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
thermal properties and flame retardancy are proposed as follows: firstly, functionalized MoS2 nanosheets create a barrier effect on the surface of the PVA, which could slow down mass transfer between gas and condensed phases and prevent the underlying materials from further combustion [16]; secondly, the crosslinked network structure in the hybrid films is conducive to improve the thermal stability. The cross-linked network caused by the sol-gel method can effectively induce the mobility restriction of polymer chains, thus retarding the thermal degradation of hybrid films [38]; thirdly, the immigration of silicon to the surface during the combustion and carbonization also plays an important role, which protects the char layer from thermo-degradation, enhances the stability of the char layer, and prevents the release of volatile products from the matrix [39]. 3.5. Mechanical properties According to previous research work, the mechanical properties of the PVA nanocomposites are improved obviously by incorporation of 2D layered nanofillers such as graphene and LDH [4,5]. Based on the well dispersion of the functionalized MoS2 in PVA matrix, and strong interfacial interaction between functionalized
MoS2 and PVA matrix, it is speculated that the addition of the functionalized MoS2 nanosheets is conducive to improve the mechanical properties of the PVA hybrid films. The typical stress-strain curves for PVA and its hybrid composites are presented in Fig. 10. The tensile strength of all the PVA hybrid films is larger than that of pure PVA and gradually increases as the content of the functionalized MoS2 increases. With loading of 1.0 wt% functionalized MoS2, the tensile strength of PVA hybrid films increases by 98.4%, compared with that of the pure PVA. The obvious increment of the tensile strength is mainly attributed to the well dispersion of functionalized MoS2 nanosheets and strong interfacial interactions between the functionalized MoS2 and PVA matrix. Similar results have been observed for PVA nanocomposites with graphene as nanofillers in reported literature [29]. The elongation at break of the hybrid films gradually decreases with the addition of functionalized MoS2, which is mainly due to the lamellar barrier effect of the nanoflakes restricting the segmental motion of the polymer chains in the nanocomposites [34]. In addition, DMA is further carried out to evaluate the mechanical properties of the PVA hybrid films. The storage modulus and loss tangent curves are shown in Fig. 11. The storage modulus is a measure of the stiffness and all the PVA hybrid films containing MoS2-MTS exhibit much higher storage modulus than pure PVA at the full temperature range investigated. Especially, the storage modulus of the PVA/1.0% MoS2-MTS composite is about 36% higher compared to that of the PVA at 40 °C. The glass transition temperature (Tg) is determined from the peak temperature of tan delta curves. As expected, an obvious increase in Tg values for PVA hybrid films is achieved. The pure PVA displays a Tg of 92 °C, while for the composite film with 1.0 wt% MoS2-MTS, the Tg is significantly increased to 105 °C, which indicates a strong confinement effect of MoS2MTS to the PVA chains. In addition, it is of great significance to study the mobility of polymer chains inside the fillers and also how the presences of nanofillers affect the mobility of the surrounding polymer chains. DMA can be used to study the mobility of polymer chains in composites. During the glass transition, the long range polymer chain gains mobility and this dissipates a great amount of energy through viscous movement. In Fig. 11b, the tan delta peak value decreases with addition of MoS2-MTS. Any depression in the tan delta indicates the reduction of the number of the mobile chains during the glass transition, indicating the restriction of polymer chains in the obtained PVA hybrid films. The DMA results further prove the strong interfacial adhesion between the MoS2-MTS and PVA matrix.
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Fig. 10. Typical stress-strain curves of pure PVA and PVA hybrid films (a) and tensile strength and elongation at break of pure PVA and PVA hybrid films as a function of MoS2MTS loadings (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. DMA curves of pure PVA and PVA hybrid films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
To highlight the advantages of the sol-gel method reported in this work, the thermal properties, flame retardancy and mechanical properties of PVA nanocomposites in terms of T50%, PHRR and tensile strength values in present study are compared with the results reported in our previous work. The increment of the T50% and tensile strength values and the decrement of the PHRR values in present work are much larger than those of previous research work when the addition volume is equal [9]. Obviously, the reinforcement efficiency of the functionalized MoS2 in PVA hybrid films which are prepared by the sol-gel method in this work is more excellent than the simple solvent blending method in our previous work. The effective reinforcements are mainly attributed to well dispersion of functionalized MoS2 nanosheets in the polymer matrix and strong interfacial adhesion between the two components which can efficiently transfer the load from the weak polymer chains to the robust MoS2 nanosheets.
The SEM and TEM images indicated that the functionalized MoS2 was well dispersed in PVA and formed strong interfacial adhesion with PVA matrix due to the organosiloxane with active end-group grafted on the surface of exfoliated MoS2 nanosheets. The incorporation of functionalized MoS2 with a relatively low concentration induced a significant reinforced effect on the thermal, flame retardance and mechanical properties of the PVA hybrid films. With the functionalized MoS2 volume content of 1.0%, T50% was increased from 298 to 416 °C and a 12 °C increment in the Tg was also achieved, compared to pure PVA. Furthermore, the PHRR and the tensile strength of PVA hybrid films was decreased by 52.5% and improved by 98.4%, respectively. These remarkably enhanced properties were mainly attributed to the well dispersion of the functionalized MoS2 and the formation of strong interfacial adhesion between functionalized MoS2 and PVA matrix. The sol-gel method presented herein will provide a promising approach to fabricate MoS2-based polymer hybrid composites with excellent performances.
4. Conclusion Acknowledgement In present study, siloxane group was successfully grafted on the surface of exfoliated MoS2 nanosheets by chemical conjugation with 3-mercaptopropyl trimethoxysilane and then the functionalized MoS2 was incorporated into PVA matrix by sol-gel method.
This work was supported by the Fundamental Research Funds for the Central Universities (G1323521619), China University of Geosciences (Wuhan) (CUG160607).
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