MgCl2-supported Ziegler-Natta catalyst via an in situ polymerization

Accepted Manuscript Preparation and Properties of PE/MoS2 Nanocomposites with an ExfoliatedMoS2/MgCl2-supported Ziegler–Natta Catalyst via an in situ Polymerization He-Xin Zhang, Eun-Bin Ko, Jae-Hyeong Park, Young-Kwon Moon, Xue-Quan Zhang, Keun-Byoung Yoon PII: DOI: Reference:

S1359-835X(16)30379-7 http://dx.doi.org/10.1016/j.compositesa.2016.11.008 JCOMA 4480

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

19 August 2016 29 October 2016 5 November 2016

Please cite this article as: Zhang, H-X., Ko, E-B., Park, J-H., Moon, Y-K., Zhang, X-Q., Yoon, K-B., Preparation and Properties of PE/MoS2 Nanocomposites with an Exfoliated-MoS2/MgCl2-supported Ziegler–Natta Catalyst via an in situ Polymerization, Composites: Part A (2016), doi: http://dx.doi.org/10.1016/j.compositesa.2016.11.008

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation

and

Properties

of

PE/MoS2

Nanocomposites

with

an

Exfoliated-MoS2/MgCl2-supported Ziegler–Natta Catalyst via an in situ Polymerization

He-Xin Zhang,1,2 Eun-Bin Ko,1 Jae-Hyeong Park,1 Young-Kwon Moon,1 Xue-Quan Zhang*2 Keun-Byoung Yoon*1

1

Department of Polymer Science and Engineering, Kyungpook National University,

Korea 2

Key Lab. of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese

Academy of Science, China

Abstract Here, we report the first example of preparation of polyethylene (PE)/exfoliated-MoS2 nanocomposites

through

in-situ

Ziegler-Natta

polymerization.

The

exfoliated-MoS2/MgCl2-supported Ti-based Ziegler–Natta catalyst was synthesized via a facile coagglomeration method. The effects of MoS2 on the catalyst morphology and the ethylene polymerization behavior were examined. The resultant PE/MoS2 nanocomposites had a flake shape morphology, and the MoS2 fillers were well dispersed throughout the entire PE matrix. In addition, the thermal stability and mechanical properties of the PE base material were significantly enhanced by the introduction of a very small amount of the MoS2 filler (0.08 wt%). Therefore, this work provides a facile method to produce of high-performance PE.

1. Introduction Over the past decades, layered nanofiller-based polymer nanocomposites have attracted significant interest, especially those using the world’s most commonly used polymers, polyolefins. In the early years of polyolefin nanocomposite research, layered silicates and layered double hydroxides were the most widely studied fillers

[1-3]. Nowadays, graphene-based polyolefin nanocomposites are predominant in this field, because of their great potential and improved materials properties, such as excellent mechanical and thermal stabilities, flame resistance, and thermal and electrical conductivities [4-8]. Molybdenum disulfide (MoS2) has a structure similar to that of graphite. The basic MoS2 unit is composed of a molybdenum atom coordinated to six sulfur atoms. The structure consists of a layer of molybdenum atoms sandwiched between two layers of sulfur atoms. Each sulfur atom is coordinated to three molybdenum atoms within a single 2D layer of MoS2. The bulk material is formed of these 2D layers held together by van der Waals forces [9,10]. Hence, pristine MoS2 cannot be easily dispersed in a polymer matrix. In the past few decades, the intercalation of Li+ ions between MoS2 layers and their subsequent exfoliation by hydrolysis of the Li+ ions, yielding single or few exfoliated MoS2 layers (EMoS2), has been reported [9,10]. MoS2 can thus be converted into high aspect ratio reinforcement platelets. This intercalation technique provides an opportunity to disperse of MoS2 into polymers to obtain organic–inorganic polymer nanocomposites. Gui et al. [11] prepared poly(methyl methacrylate)/MoS2 nanocomposites via an in situ emulsion polymerization method, and the thermal stability of the poly(methyl methacrylate) improved remarkably. The same group also reported a poly(vinyl alcohol)/MoS2 nanocomposite prepared via a solution mixing method [12]. After mixing, the thermal stability, fire resistance, and mechanical properties of the polymer were significantly enhanced. Although the preparation of polar polymer/MoS2 nanocomposites has been achieved successfully, the synthesis of polyolefin/MoS2 nanocomposites with well-dispersed MoS2 fillers has proven more challenging, owing to the low solubility of polyolefins in common solvents. Moreover, as observed with graphene nanosheets, MoS2 nanosheets easily aggregate owing to van der Waals interactions between the individual MoS 2 layers. Therefore, we present a novel EMoS2/MgCl2-supported Ti-based Ziegler–Natta catalyst synthesized through a coagglomeration method. The aggregation of individual MoS2 layers is prevented by the solid-state Ziegler-Natta catalyst during

the preparation process. Following the in situ polymerization of ethylene, EMoS2 was well dispersed throughout the polymer matrix. Thus, this method is a facile way to produce polyethylene (PE)/MoS2 nanocomposites with well-dispersed MoS2 in the polymer matrix. In addition, the effects of EMoS2 on the catalyst performance and polymer properties were studied.

2. Experimental 2.1. Materials Molybdenum

disulfide

(MoS2,

~6

μm,

Sigma-Aldrich),

2-ethyl-1-hexanol

(EHA, >99.6%, Sigma-Aldrich), anhydrous magnesium chloride (MgCl2, >98%, Sigma-Aldrich), n-butyllithium (2.5 M in hexane, Sigma-Aldrich), triethylaluminum (TEA, 1.0 M in hexane, Sigma-Aldrich), and titanium tetrachloride (TiCl4, >99%, Sigma-Aldrich) were used as received. Polymerization-grade ethylene was provided by Korea Petrochemical Ind. Co., Ltd. (Korea). n-Hexane was distilled from sodium/benzophenone under N2 prior to use.

2.2. Exfoliation of MoS2 The exfoliation of MoS2 was carried out following a previously described method [10]. Typically, MoS2 (1 g) was placed in an autoclave, followed by addition of n-butyllithium (10 mL, 2.5 M in hexane solution). The autoclave was heated at 90 °C for 12 h under argon atmosphere. Then, the autoclave was cooled to room temperature and the product was filtered and washed with anhydrous hexane five times. Subsequently, the resultant MoS2 was vacuum dried and immersed in 1 L of distilled water with ultrasonication at ambient temperature for 4 h, yielding a colloidal suspension of exfoliated MoS2. The suspension was neutralized with dilute acid (1 M HCl). The product was washed five times (5 × 1 L) with distilled water and then vacuum freeze-dried to afford the exfoliated MoS2 powder.

2.3. Preparation of the MoS2/MgCl2-supported Ziegler–Natta Catalyst The

MoS2/MgCl2-supported

Ziegler–Natta

catalyst

was

prepared

using

a

coagglomeration method. Typically, 2 g of MgCl2 and 1 g of exfoliated MoS2 powder were added to 19.8 ml of EHA at room temperature under N2. The reaction medium was heated to 160 °C to obtain a homogeneous solution. After 2 h at 160 °C, the reaction solution was allowed to cool to 50 °C, and 50 mL of n-hexane was added. The suspension was ultrasonicated at 30 °C for 5 h. Then, 15 mL of TiCl4 was added dropwise to the reaction medium; some precipitation was observed during this process. After 2 h, the precipitate was filtered to remove the unreacted TiCl4, and a second portion of TiCl4 (20 mL) was charged into the reactor. The reaction proceeded to completion after stirring for 2 h at 30 °C. The reaction mixture was filtered, and the precipitate was washed several times with hot n-hexane and then redispersed in n-hexane. The resultant product was used as the catalyst for ethylene polymerization. For comparison, a MoS2-free catalyst (MgCl2/TiCl4) was also prepared by the same procedure.

2.4. In situ Polymerization Polymerization was performed in a 300 mL glass reactor equipped with a magnetic stirring bar. The reactor was backfilled three times with N 2 and charged with the required amount of n-hexane. At 40 °C, the reaction solution was stirred under 1 atm of ethylene, and the co-catalyst (TEA) was added to the reactor. After the addition of co-catalyst, the catalyst was injected into the reactor, and polymerization was initiated under a continuous feed of ethylene. The ethylene pressure was kept constant throughout the polymerization process using a bubbler. After 0.5 h, the polymerization was terminated by adding a 10% HCl/methanol solution. The mixture was poured into methanol (500 mL) to precipitate the polymer, which was then dried under vacuum at 60 °C until a constant weight was obtained.

2.5. Characterization The Mg and Ti contents of the catalyst were determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES; PerkinElmer, Optima 7300DV).

Scanning electron microscopy (SEM) images were taken on a JEOL JSM-6380LV microscope. The morphology of the support and the catalyst was studied with a polarized optical microscope (POM, ANA-006, Leitz, Germany) equipped with a CCD camera. The X-ray diffraction (XRD) patterns were obtained on a Philips X-Pert PRO MRD diffractometer using Cu-Kα radiation. The melting temperature (Tm) of the obtained polymers was determined using differential scanning calorimetry (DSC; DSC131 evo, Setaram) at a heating rate of 10 °C/min. The samples were heated to 200 °C and held in the molten state for 3 min to eliminate the thermal history. The polymer melts were cooled to 30 °C at a rate of 10 °C/min. The melting point was determined in the second scan. The decomposition temperature was determined under a N2 atmosphere using thermogravimetric analysis (TGA; LABSYS evo, Setaram) at a programmed heating rate of 10 °C/min from 30 to 800 °C. The tensile mechanical properties of PE and the PE/MoS2 nanocomposites were determined using a universal testing machine (Instron M4465). The sample size for the tensile drawing experiments was 5.0 × 75.0 × 1.0 mm3, the sample gauge length was 25.0 mm, and the crosshead speed was 50.0 mm/min.

3. Results and Discussion

Scheme 1. Preparation of the MoS2/MgCl2-supported Ziegler–Natta catalyst and the PE/MoS2 nanocomposites.

The preparation of the exfoliated MoS2/MgCl2-supported Ziegler–Natta catalyst and the PE/MoS2 nanocomposites containing well-dispersed MoS2 fillers is illustrated in

Scheme 1. The MoS2/MgCl2-supported Ziegler–Natta catalyst was synthesized through a coagglomeration process in n-hexane. During this process, the surfaces of the exfoliated MoS2 sheets were covered by the MgCl2/TiCl4 catalyst, preventing the aggregation of the exfoliated MoS2 layers. In addition, the MoS2 fillers served as a template assisting the agglomeration of the MgCl2/TiCl4 catalyst onto the MoS2 surface affording a layered structure. The resultant MoS2/MgCl2-supported Ziegler–Natta catalyst was then used in the polymerization of ethylene to prepare the PE/MoS2 nanocomposites.

Figure 1. SEM images of (a, b) pristine MoS2, (c, d) exfoliated MoS2, and (e, f) the MoS2/MgCl2-supported Ziegler–Natta catalyst, at different magnifications.

The morphologies of pristine MoS2, exfoliated MoS2, and the MoS2/MgCl2/TiCl4 catalyst were investigated by SEM. The pristine MoS2 sample is composed of sheets with diameters in the range of several micrometers, as shown in Figure 1a and 1b. As shown in the high-magnification SEM image of pristine MoS2 (Figure 1b), the MoS2

fillers assembled into many thin, stacked MoS2 layers. Compared to the pristine MoS2, the exfoliated MoS2 exists as crumpled sheets, where some layers are separated from the rest (Figure 1c and 1d). These observations demonstrate the successful exfoliation of pristine MoS2 into graphene-like nanosheets. Furthermore, after the exfoliation of MoS2, some re-stacked MoS2 layers (formed during the vacuum drying process) remained, which was further confirmed by XRD analysis (Figure 2). In the case of the MoS2/MgCl2/TiCl4 catalyst, a stacked layered morphology was formed (Figure 1e and 1f), while the MgCl2-supported Ziegler–Natta catalyst (without MoS2) exhibited irregular particles (Figure S3, Supplementary Information). The MoS2 sheets were completely coated by MgCl2/TiCl4, which prevented the re-stacking of the exfoliated MoS2 layers.

Figure 2. XRD curves of (a) pristine MoS2, (b) exfoliated MoS2, and (c) MoS2/MgCl2-supported Ziegler–Natta catalyst.

To confirm the successful exfoliation of pristine MoS2, we carried out XRD analysis on the pristine MoS2, exfoliated MoS2, and MoS2/MgCl2-supported Ziegler–Natta catalyst. As shown in Figure 2, an intense reflection at 2θ = 14.3 (corresponding to an interlayer distance of 0.62 nm) is observed for pristine MoS2, which is attributed to the MoS2 (002) diffraction peak. After exfoliation, this (002) diffraction peak was

drastically reduced and a new peak at 7.8 (corresponding to an interlayer distance of 1.13 nm) was observed. This peak is attributed to the (001) diffraction peak of exfoliated-MoS2 [13]. This result further demonstrates the successful exfoliation of MoS2. Furthermore, the peak at 14.3 did not disappear completely, suggesting that some stacked MoS2 layers remained after exfoliation. The remaining stacked MoS2 sheets may correspond to single MoS2 layers that are re-assembled during the solvent removal process before analysis. As for the MoS2/MgCl2-supported Ziegler–Natta catalyst, interestingly, the (002) diffraction peak at 14.3 disappeared completely. This result indicates that the aggregation of exfoliated MoS2 layers was prevented by the MgCl2/TiCl4 surface covering.

Table 1. Elemental composition of the prepared catalysts Mg

Ti

Catalyst

MoS2 Ti/Mg

(wt%)

(wt%)

(wt%)

MgCl2/TiCl4

17.9

5.3

0.29

-

MoS2/MgCl2/TiCl4

15.8

4.2

0.27

16.6

The compositions of the resultant Ziegler–Natta catalysts in the absence and presence of MoS2 were further characterized by ICP analysis, and the results are presented in Table 1. The Mg and Ti content of the catalyst decreased with the introduction of MoS2 fillers, and 16.6 wt% MoS2 was detected. The Ti contents of the catalysts in the absence and presence of MoS2 were 5.3 wt% and 4.2 wt%, respectively. However, the Ti/Mg ratio barely changed on the introduction of MoS2 fillers. Therefore, the introduction of MoS2 did not affect the anchoring efficiency of TiCl4 on the MgCl2 support significantly.

Table

2.

Results

of

ethylene

polymerization

using

the

MgCl2-

and

MoS2/MgCl2-supported Ziegler–Natta catalysts Cat. Entry

Cat. (mg)

1

MoS2

Tm

Tc

Xc

(kg/mol-Ti•h)

(wt%)

(°C)

(°C)

(%)

50

100

261

-

136.0

116.9

63.1

2

20

200

480

0.08

136.3

117.3

63.6

3

50

100

250

0.16

136.1

117.7

68.7

100

50

130

0.29

136.7

117.4

70.0

5

200

25

66

0.57

136.5

118.0

74.8

6

400

13

31

1.23

136.3

118.3

75.1

4

MgCl2/TiCl4

Activity [Al]/[Ti]

MoS2/MgCl2/TiCl4

Polymerization conditions: 100 mL n-hexane, TEA cocatalyst, 0.5 h, 1 atm and 40 °C.

The ethylene polymerization behavior of the catalyst both in the absence and presence of MoS2 was evaluated after activation with the TEA cocatalyst. As shown in Table 2, the catalytic activity of MgCl2/TiCl4 catalyst was slightly higher than that with the MoS2/MgCl2/TiCl4 catalyst at the same catalyst weight (entry 1 vs. 3). The decrease in the catalytic activity can be attributed to the layered structure of MoS2, which functions as a physical barrier, retarding the diffusion of the ethylene into the catalyst core. By controlling the amount of catalyst, PE/MoS2 nanocomposites with a MoS2 content of 0.08–1.23 wt% were obtained.

Figure 3. Optical images of (a) PE and (b) the PE/MoS2 nanocomposite.

According to the morphology replication characteristics of the supported Ziegler–Natta catalyst, the morphology of the final polymer can be controlled by modifying the morphology of the catalyst [14-16]. Therefore, the morphology of the resultant PE and PE/MoS2 nanocomposites directly mirror that of the catalyst. As shown in Figure 3, the particles of PE obtained using the MgCl2/TiCl4 catalyst are white and irregularly shaped, while those of PE/MoS2 nanocomposite have a flake shape (1–3 mm) with a homogeneous gray color. Additionally, no high-contrast black MoS2 sheets could be observed.

Figure 4. Optical micrographs of (a) PE and the PE/MoS2 nanocomposites with different MoS2 contents: (b) 0.08 wt%, (c) 0.16 wt%, (d) 0.29 wt%, (e) 0.57 wt%, and (f) 1.23 wt%.

To investigate the dispersion of exfoliated MoS2 in the PE matrix, the resultant PE and PE/MoS2 nanocomposites were hot-pressed into films. The films were observed under an optical microscope in transmission mode, and the obtained micrographs are shown in Figure 4. The MoS2 fillers are highly compatible with and homogeneously dispersed throughout the PE matrix. Interestingly, MoS2 could be clearly observed in the images of the PE/MoS2 nanocomposites, even at the lowest MoS2 filler content (0.08 wt%). On increasing the MoS2 feed, a larger quantity of the MoS2 nanofiller was observed in the PE matrix. Additionally, to confirm the absence of MoS2 aggregates in the PE/MoS2 nanocomposites, the PE/1.23 wt% MoS2 sample was characterized by XRD (Figure S4). No conspicuous diffraction peaks were observed, except the crystalline diffraction peaks of the PE; this result indicates that MoS2 aggregates were not present in the nanocomposites. We therefore expected the PE/MoS2 nanocomposites to exhibit improved thermal and mechanical properties. The effect of MoS2 on the crystallization of PE was examined by DSC, and typical DSC curves are shown in Figure S5 (Supplementary Information). As shown in Table 2, the Tm of the PE produced with the MoS2-free catalyst was 136.0 °C. On the introduction of MoS2 to the catalyst, the Tm of the produced PE/MoS2 nanocomposites changed little, although the degree of crystallinity (Xc) gradually increased with the MoS2 content. Compared to the neat PE sample, the non-isothermal crystallization peak temperature (Tc) gradually increased with the increasing MoS2 content in the PE/MoS2 nanocomposites, which demonstrates that the MoS2 fillers act as nucleating agents, inducing PE crystallization. This result is not surprising, and Naffakh et al. [17] reported a similar phenomenon for isotactic-PP/MoS2 nanocomposites produced by a melt mixing method. They found that the MoS 2 affected the crystallization of the nanocomposites remarkably, which was attributed to the nucleating effect of MoS2 on the monoclinic α-crystal form of PP. In addition, the DSC traces in Figure S5

(Supplementary Information) are smooth curves with relatively sharp endothermic peaks, reflecting the overall homogeneity of the PE/MoS2 nanocomposites.

Figure 5. TGA (a) and DTG (b) curves of PE and PE/MoS2 nanocomposites; (c) Td5%, Tdmax and Char yield vs MoS2 content

Thermal stability is a crucial property for polymeric materials because it is often the limiting factor in both polymer processing and end-use applications. Therefore, the thermal degradation of PE and the PE/MoS2 nanocomposites with different weight fractions of the MoS2 nanofiller was investigated by TGA under a N2 atmosphere. The results are shown in Figure 5. We found that the thermal stability of PE improved appreciably with the addition of the MoS2 nanofillers. As shown in Figure 5a, all the TGA curves indicate a single degradation process. Compared to the curve of neat PE, the thermal degradation temperatures of the composites linearly shifted to higher temperatures with increasing MoS2 filler content, suggesting a significant improvement in the thermal oxidation stability of PE. The degradation temperatures at 5 wt% loss (Td5%) and Tdmax values of all the composites are higher than those of pure PE. Upon incorporation of 0.08 wt%, 0.29 wt%, and 1.23 wt% MoS2 into the PE/MoS2 nanocomposites, the Td5% increased by about 5 °C, 20 °C, and 58 °C, respectively. Furthermore, on the addition of MoS2, the Tdmax of the PE/MoS2 nanocomposites increased to ~490 °C, which is ~35 °C higher than that of pure PE. These significant enhancements in the thermal stability of PE after the incorporation of MoS2 can be ascribed to the good dispersion of MoS2 in the PE matrix, which possibly acts as an insulator between the heat source and the polymer surface, where

combustion occurs. In addition, the char yield of the PE/MoS2 nanocomposites was significantly higher than that of the virgin PE (0.3 wt% at 600 °C). Therefore, MoS2 catalyzes the formation of char from PE during the thermal degradation process. The char yields of PE and the PE/MoS2 nanocomposites containing 0.08 wt%, 0.29 wt%, and 1.23 wt% MoS2 were 0.3 wt%, 3.1 wt%, 3.9 wt%, and 7.9 wt%, respectively. These results are not surprising because molybdenum catalyzes the formation of char in polymers, and sulfur improves the flame retardancy of polymers [12,18]. Additionally, layered nanofillers such as montmorillonite and graphene have been reported to improve the thermal stability of polymers owing to a physical barrier effect, which retards the diffusion of the degradation products, gases, and heat.

Table 3. Mechanical properties of PE and the PE/MoS2 nanocomposites with different MoS2 contents MoS2

Tensile

Elongation Modulus

content

Strength

at break (MPa)

Neat PE

(wt%)

(MPa)

(%)

-

20.0±1

515±20

720±50

0.08

25.4±1

835±30

1200±80

0.16

26.6±1

980±40

1400±80

0.29

30.2±1

960±40

840±40

0.57

27.3±1

1090±40

660±30

1.23

31.3±1

1100±50

580±30

PE/MoS2 nanocomposites

The mechanical properties of PE and the PE/MoS2 nanocomposites with different MoS2 contents are presented in Table 3. Clearly, the tensile strength, modulus, and elongation at break values of the resultant PE nanocomposites were significantly enhanced, even at very low MoS2 nanofiller loadings. As the MoS2 loading from 0 to 1.23 wt%, the tensile strength and modulus increased from 20.0 MPa and 515 MPa to 31.3 MPa and 1100 MPa, respectively. Interestingly, the elongation at break value of

resultant PE/MoS2 nanocomposites also increased at relatively low MoS2 loadings (0.08~0.29 wt%). Hu et al. [19], reported that the largest increases in tensile strength and modulus were 16.8 % and 37.5 % for PE/MoS2 nanocomposites that were prepared by a solution mixing method, significantly lower than the increases found by us (56.5% and 114%, respectively). Additionally, the mechanical properties of PE/MoS2 nanocomposites are not less than those of graphene and graphene oxide reinforced PE nanocomposites reported previously [6-8]. The improved mechanical properties of PE/MoS2 nanocomposites prepared by in situ polymerization method can be attributed to the excellent dispersion of MoS2 fillers throughout the PE matrix. These results indicate that the PE/MoS2 nanocomposites obtained by in situ polymerization with the MoS2/MgCl2/TiCl4 catalyst prepared by a coagglomeration method have a remarkable stiffness/toughness balance.

4. Conclusions A novel MoS2/MgCl2-supported Ti-based Ziegler–Natta catalyst was prepared through the coagglomeration of MgCl2 and MoS2 in hexane. We successfully fabricated PE/MoS2 nanocomposites with well-dispersed MoS2 nanofillers by in situ polymerization. The resultant PE/MoS2 nanocomposites displayed enhanced thermal stability compared to that of PE obtained from the MoS2-free catalyst system. In addition, the mechanical properties of PE were enhanced significantly. The maximum increases in tensile strength, modulus, and elongation at break value are 56.5%, 114%, and 94%, respectively. Hence, this study provides a facile method to produce high-performance PE with good thermal stability and excellent stiffness/toughness balance.

References 1 Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003; 28(11):1539-1641. 2

Giannelis

EP.

1996;8(1):29-35.

Polymer

layered

silicate

nanocomposites.

Adv

Mater

3 Lee DH, Kim HS, Yoon KB, Min KE, Seo KH, Noh SK. Polyethylene/MMT nanocomposites prepared by in situ polymerization using supported catalyst systems. Sci Technol Adv Mat 2005;6(5):457-462. 4 Huang YJ, Qin YW, Zhou Y, Niu H, Yu ZZ, Dong JY. Polypropylene/graphene oxide nanocomposites prepared by in situ Ziegler-Natta polymerization. Chem Mater 2010;22(13):4096-4102. 5 Huang Y, Qin Y, Wang N, Zhou Y, Niu H, Dong JY, Hu J, Wang Y. Reduction of graphite oxide with a grignard reagent for facile in situ preparation of electrically conductive

polyolefin/graphene

nanocomposites.

Macromol

Chem

Phys

2012;213(7):720-728. 6 Zhang HX, Park JH, Ko EB, Moon YK, Lee DH, Hu YM, Zhang XQ, Yoon KB. Comparison of the properties of graphene- and graphene oxide-based polyethylene nanocomposites prepared by an in situ polymerization method. RSC Adv 2016;6(77):73013-73019. 7 Zhang HX, Bae MG, Park JH, Ko EB, Lee DH, Zhang XQ, Yoon KB, Effects of GO oxidation degree on GO/BuMgCl-supported Ti-based Ziegler–Natta catalyst performance and nanocomposite properties. RSC Adv 2016;6(25):20734-20740. 8 Zhang HX, Hu YM, Lee DH, Yoon KB, Zhang XQ. Preparation of PE/GO nanocomposites using in situ polymerization over an efficient, thermally stable GO-supported V-based catalyst. RSC Adv 2016;6(32):26553-26558. 9 Benavente E, Ana MAS, Mendizábal F, González G. Intercalation chemistry of molybdenum disulfide. Coord Chem Rev 2002;224(1-2):87-109. 10 Joensen P, Frindt R, Morrison SR. Single-layer MoS2. Mater Res Bull 1986;21(4):457-461. 11 Zhou K, Liu J, Zhang Q, Shi Y, Jiang S, Hu Y, Gui Z. Facile preparation of poly(methyl methacrylate)/MoS2 nanocomposites via in situ emulsion polymerization. Mater Lett 2014;126(1):159-161. 12 Zhou K, Jiang S, Bao C, Song L, Wang B, Tang G, Hu Y, Gui Z. Preparation of poly(vinyl alcohol) nanocomposites with molybdenum disulfide (MoS 2): structural characteristics and markedly enhanced properties. RSC Adv 2012;2(31):11695-11703.

13 Golub AS, Rupasov DP, Lenenko ND, Novikov YuN. Modification of molybdenum disulfide (2H-MoS2) and synthesis of its interaction compounds. Russ J Inorg Chem 2010; 55(8):1166-1171. 14 Kashiwa N. The discovery and progress of MgCl2-supported TiCl4 catalysts. J Polym Sci Part A: Polym Chem 2004; 42(1):1-8. 15 Cai HG, Zhang CY, Chen B, Zhang XQ, Zhang HX. Preparation and morphology of polyethylene/clay nanocomposite with novel MgCl2/montmorillonite(MMT) bi-supported Ziegler-Natta catalyst. J Nanosci Nanotechnol 2012;12(9):7296-7300. 16 Wang S, Feng N, Zheng J, Yoon KB, Lee DH, Qu MJ, Zhang XQ, Zhang HX. Preparation

of

polyethylene/lignin

nanocomposites

from

hollow

spherical

lignin-supported vanadium-based Ziegler–Natta catalyst. Polym Adv Technol 2016; 24(10):1351-1354. 17 Naffakh M, Marco C, Gómez-Fatou M, Isothermal crystallization kinetics of novel isotactic polypropylene/MoS2 inorganic nanotube nanocomposites. J Phys Chem B 2011;115(10):2248-2255. 18 Tang T, Chen XC, Meng XY, Chen H, Ding YP. Synthesis of multiwalled carbon nanotubes by catalytic combustion of polypropylene. Angew Chem Int Ed 2005;44(10):1517-1520. 19 Feng X, Wen P, Cheng Y, Liu L, Tai Q, Hu Y, Liew KM. Defect-free MoS2 nanosheets:advanced nanofillers for polymer nanocomposites. Compos Part A Appl Sci Manuf 2016;81:61-68.