Preparation of Isotactic Polypropylene/Exfoliated MoS₂ Nanocomposites via In Situ Intercalative Polymerization

Abstract

In this research, a Ziegler–Natta catalyst intercalated MoS₂ was synthesized through the intercalation of a Grignard reagent into MoS₂ galleries, followed by the anchoring of TiCl₄. During propylene polymerization, the intercalated MoS₂ exfoliated in situ to form PP/exfoliated MoS₂ (EMoS₂) nanocomposites. The isotactic index values of the resultant PP/EMoS₂ nanocomposites were as high as 99%, varying from 98.1% to 99.0%. It was found that the incorporation of the EMoS₂ significantly improved the thermal stability and mechanical properties (tensile strength, modulus, and elongation at break) of PP. After introduction of EMoS₂, the maximum increases in T_d5% and T_dmax were 36.9 and 9.7 °C, respectively, relative to neat PP. After blending with commercial PP, the resultant nanocomposites increase in tensile strength and modulus up to 11.4% and 61.2% after 0.52 wt % EMoS₂ loading. Thus, this work provides a new way to produce high-performance PP.

1. Introduction

Polyolefin is the most widely used materials because of its excellent combination of chemical and physical properties, as well as having low production cost, superior processability, and good recyclability. However, for advanced applications, it is necessary to improve the performance of polyolefin in terms of its properties such as stiffness and rigidity in forming nanocomposites. Thus, the study of polyolefin nanocomposites has attracted considerable attention, because of their high potential as materials with improved properties, such as mechanical and thermal stability, flame resistance, and thermal and electrical conductivities.

Molybdenum disulfide (MoS₂) have enjoyed renewed interest owing to their crystallographic structure, which consists of covalently bonded S-Mo-S tri-layers in analogy to graphene [1,2,3,4,5]. According to the reports [6,7], MoS₂ monolayers exhibit more higher mechanical properties (breaking strength: ~23 GPa; Young modulus: ~300 GPa) than chemically reduced graphene. Thus, MoS₂ has a great potential to fabricate high-performance organic–inorganic polymer nanocomposites. Such as PVA, PS, PMMA, and chitosan have been successfully applied in nanocomposite fabrication with EMoS₂ through a solution mixing method. After incorporation of EMoS₂, the mechanical and thermal properties were significantly improved [8,9,10,11]. The uniform dispersion of EMoS₂ in the polymer matrix can be achieved by solution mixing, which is the ideal strategy, wherein these polymers are dissolved in common organic solvents. However, the solution mixing process is difficult and uneconomical in the case of polyolefin, since these polymers are soluble only in solvents like xylene and trichlorobenzene above 120 °C. In our previous reports [12,13], the PE/EMoS₂ nanocomposites were prepared through an in situ polymerization of ethylene using EMoS₂ containing Ziegler–Natta catalyst. The resulting nanocomposites exhibited enhanced thermal and mechanical properties than neat PE. However, the catalyst preparation required complicated procedures, including a long lithiation of MoS₂, violent exfoliation process, excessive amounts of water to remove lithium salt, a long vacuum freeze-drying process, and so on.

During the last decade, layered fillers (such as graphite and clay) have been successfully imbedded into polyolefins by in situ exfoliation method during olefin polymerization [14,15,16]. This approach is based on the intercalation of catalyst (Ziegler–Natta, metallocene, etc.) in the interlayers of the layered filler, followed by in situ polymerization. Due to the structural analogy to layered clay, a similar methodology can be applied to the synthesis of polyolefin/MoS₂ nanocomposites.

Therefore, in this research, a Ziegler–Natta catalyst intercalated MoS₂ was synthesized through the intercalation of a Grignard reagent into MoS₂ galleries, followed by the anchoring of TiCl₄. During the olefin polymerization, the bulk MoS₂ layers are predicted to be exfoliated in situ and dispersed in the polymer matrix (no need additional exfoliation process), producing polyolefin/exfoliated MoS₂ (EMoS₂) nanocomposites directly.

2. Materials and Methods

2.1. Materials

Molybdenum disulfide (MoS₂, ~6 μm), n-butylmagnesium chloride (BuMgCl, 2.0 M in THF), triethylaluminum (TEA, 1.0 M in hexane), diisobutylphthalate (DIBP), and titanium tetrachloride (TiCl₄, >99%) were purchased from Sigma-Aldrich (Seoul, Korea) and used as received. Cyclohexylmethyldimethoxysilane (CHMDMS) donor and polymerization-grade propylene were received from Korea Petrochemical Ind. Co., Ltd., Ulsan, Korea. n-Hexane was distilled from sodium/benzophenone under N₂ prior to use.

2.2. Preparation of MoS₂-MgCl-Supported Ziegler–Natta Catalysts

An autoclave was charged with MoS₂ (1 g), followed by addition of BuMgCl (20 mL), and was heated at 150 °C for 12 h under an argon atmosphere. Subsequently, the autoclave was cooled to room temperature, and the product was filtered and five times washed with anhydrous hexane. The resulting powder was suspended in n-hexane (200 mL) under ultrasonication for 30 min. Then DIBP (0.5 mL) was added, followed by dropwise addition of TiCl₄ (10 mL) to the suspension at 0 °C. After that, the temperature was slowly increased to 80 °C (2 °C/min) and the suspension was stirred for 4 h. The mixture was filtered to remove the unreacted TiCl₄, and the reactor was charged with fresh TiCl₄ (20 mL). The reaction was complete after stirring for 4 h at 80 °C. After filtering, the solid product was washed several times with hot n-hexane (60 °C). Then, the obtained powdery catalyst was dried under vacuum at 60 °C for 3 h. The Mg and Ti contents of the powdery catalyst were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES; MoS₂: 27.0 wt %, Mg: 9.6 wt %, Ti: 7.7 wt %). The catalyst was also synthesized without the addition of MoS₂ for comparison. For this, a similar procedure was followed to prepare the catalyst without including MoS₂ (Mg: 2.2 wt %, Ti: 13.9 wt %).

2.3. Propylene Polymerization

Propylene polymerization was performed in a three-neck glass reactor (300 mL). The reactor was thrice back-filled with N₂ and charged with 100 mL distilled n-hexane. The reaction solution was stirred at 40 °C under 1 bar of propylene for 5 min, followed by addition of the co-catalyst (TEA) and CHMDMS donor. Subsequently, the catalyst was added into the reactor, and polymerization was started under a continuous feed of propylene (1 bar). After 2 h polymerization, 10 mL HCl-methanol solution (10%) was added to the suspension to terminate to polymerization. The mixture was poured into large quantity of methanol (500 mL) to precipitate the polymer. The product was collected by filtration and washed with methanol. Then, the product was dried under vacuum at 60 °C until a constant weight was achieved.

2.4. Characterization

The Mg and Ti contents of the catalyst were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer, Optima 7300DV, Houston, TX, USA). Scanning electron microscopy (SEM) images were recorded on a JEOL JSM-6380LV microscope (Hitachi, Japan). The morphology of the support and catalyst was studied by polarized optical microscopy (POM, ANA-006, Leitz, Wetzlar, Germany) using a CCD camera. X-ray diffraction patterns were obtained on a Philips X-Pert PRO MRD (Philips, Holland) diffractometer equipped with a Cu-K_α radiation source.

The polymer product was fractionated by extraction with boiling n-heptane for 8 h to determine its isotactic index (I.I.), calculated as the weight percentage of n-heptane-insoluble polymer. The melting temperature (T_m) of the obtained polymer was determined using differential scanning calorimetry (DSC; DSC131evo, Setaram, Caluire, France) at a heating or cooling rate of 10 °C/min. The T_m was determined from the second scan. The decomposition temperature was determined under N₂ atmosphere by thermogravimetric analysis (TGA; Setaram Labsys evo instruments) employing a programmed heating rate of 10 °C/min from 30 to 800 °C. The tensile mechanical properties of polymers were measured with a universal testing machine (Instron M4465, Instron Corp., Norwood, MA, USA). The resulting sample (1.5 g) was melt-blended with commercial PP (4.5 g) using a twin-screw mixer (HAAKE MiniLab Micro Compounder, Karlsruhe, Germany) at 190 °C and 100 rpm for 5 min. Five 5.0 × 75.0 × 1.0 mm³ samples were used for the tensile drawing experiment. The sample gauge length was 40.0 mm, and the crosshead speed was 50.0 mm/min.

3. Results and Discussion

The preparation of the Ziegler–Natta catalyst intercalated MoS₂ (MoS₂-MgCl/TiCl₄) and PP/EMoS₂ nanocomposites with well-dispersed inorganic EMoS₂ filler is illustrated in Scheme 1. In the first step, the Grignard reagent (BuMgCl) was intercalated into the MoS₂ galleries and obtained MoS₂-MgCl support, and then treated with excess TiCl₄ to generate Mg/Ti catalyst species between MoS₂ layers. During the polymerization process, the layered MoS₂ will exfoliated in situ by the polymerization force arising from the propagation of polymer chain.

The morphologies of commercial MoS₂ and Ziegler–Natta catalyst intercalated MoS₂ were characterized by SEM and the images are given in Figure 1. It could be clearly seen that both MoS₂ and the Ziegler–Natta catalyst intercalated MoS₂ exhibited a sheet structure. Before intercalation, MoS₂ contained a large number of tightly stacked thin single MoS₂ layers. Interestingly, after intercalation of Ziegler–Natta catalyst into MoS₂ galleries, the tightly assembled thin MoS₂ layers became very loose, indicating the successful intercalation of catalyst into the interlayer spaces of MoS₂.

To confirm the successful catalyst intercalation into MoS₂ galleries, XRD analysis of pristine MoS₂ and the Ziegler–Natta catalyst intercalated MoS₂ was conducted. As shown in Figure 2, an intense reflection at 2 theta = 14.3° (corresponding to an interlayer distance of 0.62 nm) was observed for commercial MoS₂, attributable to the (002) plane. After treatment with the Grignard reagent and TiCl₄, the above characteristic peak became very weak, and a new strong peak at 2 theta = 7.7° (corresponding to an interlayer distance of 1.2 nm) appeared. This clearly indicated that the expansion of interlayer space is due to the successful intercalation of the Ziegler–Natta catalyst into MoS₂ galleries. Although the reflection peak at 2 theta = 14.3° could still be observed after intercalation, its intensity was drastically reduced compared to that of pristine MoS₂.

The propylene polymerization behaviors of the catalysts in the absence and presence of MoS₂ were evaluated after activation with the TEA co-catalyst. As shown in

Table 1. Results of propylene polymerization by the MoS₂-MgCl-supported Ziegler–Natta catalyst.

Entry	Cat.	Cat. (mg)	[Al]/[Ti]	Activity (g/mol-Ti·h)	EMoS2 (wt %)	I.I. (%)	Tm (°C)	Tc (°C)	Xc (%)
1	BuMgCl/TiCl4	200	50	500	-	-	159.6	116.7	43.2
2	MoS2-MgCl-TiCl4	50	200	20,000	0.4	98.1	160.6	118.2	50.8
3		100	100	13,000	0.7	98.5	161.4	119.9	55.9
4		200	50	7600	1.1	98.1	161.5	119.3	61.2
5		300	34	5000	1.7	98.2	161.8	119.5	57.0
6		400	25	4000	2.1	99.0	162.6	119.5	58.9

Polymerization conditions: 100 mL n-hexane, [CHMDMS]/[Al] = 0.2, TEA co-catalyst, 2 h, 1 atm, 40 °C.

, the catalyst produced only trace amounts of PP in absence of MoS₂, while the MoS₂-MgCl-TiCl₄ catalyst showed good activity towards propylene polymerization. By controlling the catalyst feed weight and [Al]/[Ti] ratio, PP/MoS₂ nanocomposites with a MoS₂ content of 0.4–2.1 wt % were obtained in this study. In the case of the MoS₂-MgCl-TiCl₄ catalyst, the isotactic index values of the resultant PP/EMoS₂ nanocomposites were as high as 99%, varying from 98.1% to 99.0%.

In order to investigate the dispersion of EMoS₂ in the PP matrix, the resultant PP and PP/EMoS₂ nanocomposites were hot-pressed into films. These films were observed in a transparent mode using an optical microscope; the obtained micrographs are listed in Figure 3. The MoS₂ sheets were homogeneously dispersed in the PP matrix. Interestingly, although only 0.4 wt % of the EMoS₂ filler was added, it could be clearly observed in the PP/EMoS₂ nanocomposites. Larger amounts of the EMoS₂ nanofiller could be observed in the PP matrix at increased EMoS₂ feed ratios. Good dispersion of EMoS₂ layers in the PP matrix without aggregation could also be observed. Dispersion of the latter filler in the PP/EMoS₂ nanocomposites was further investigated by XRD and TEM, with the spectra shown in Figure 4. The PP/EMoS₂ nanocomposites displayed diffraction peaks at 2 theta = 14.0°, 16.8°, 18.5°, 21.0°, and 21.8°, associated with the (110), (040), (130), (131), and (301) planes of PP, respectively. The peaks due to intercalation (7.7° and 11.0°) disappeared completely. The disappearance of these peaks is ascribed to the complete exfoliation of the intercalated EMoS₂ by the chain propagation force of propylene polymerization. No conspicuous diffraction peaks were observed in addition to the ones of crystalline PP, indicating that no obvious stacking of EMoS₂ sheets occurs in the PP/EMoS₂ nanocomposites and that the stacked MoS₂ sheets of the catalyst are completely exfoliated. Figure 4b is a TEM image of the PP with 2.1 wt % EMoS₂ loading, showing that MoS₂ is well exfoliated in PP matrix. Therefore, we expected the PP/EMoS₂ nanocomposites to exhibit good thermal and mechanical properties.

The effects of EMoS₂ on the melting temperature, degree of crystallinity of PP are listed in Table 1 and the DSC curves are given in Figure 5. The degree of crystallinity was measured by means of DSC, using the ratio of fusion enthalpies of product to 100% crystalline PP. As shown in Table 1, the melting temperature (T_m) of PP produced using the MoS₂-free catalyst was 159.6 °C. Introduction of EMoS₂ raised the T_m of PP/EMoS₂ nanocomposites, which could be ascribed to the restricted polymer chain motion (due to the interaction between the EMoS₂ filler and the PP matrix) and the higher isotactic index of PP obtained using the MoS₂-MgCl-TiCl₄ catalyst [17]. Compared to neat PP, the non-isothermal crystallization peak temperature (T_c) gradually increased with increasing EMoS₂ content in PP/EMoS₂ nanocomposites, which demonstrates that the above filler can act as a nucleating agent to induce PP crystallization. It is not surprising that Naffakh et al. [18] also reported a similar phenomenon for PP/MoS₂ nanocomposites produced by melt mixing method.

The thermal degradation of neat PP and PP/EMoS₂ nanocomposites with different EMoS₂ contents was confirmed by TGA under N₂ atmosphere (

Table 2. Effect of EMoS₂ content on the thermal stabilities of PP/EMoS₂ nanocomposites.

EMoS2 Content (wt %)	Td5% (°C)	Tdmax (°C)	Char Yield (wt %)
-	315.1	435.3	0.7
0.4	336.7	445.0	4.2
0.7	345.2	440.6	4.9
1.1	347.0	441.5	4.2
1.7	350.5	438.3	5.8
2.1	352.0	440.3	8.9

and Figure 6). Compared with neat PP, the thermal stability of PP was greatly improved by the incorporation of EMoS₂ nanofillers. As shown in Figure 6, all TGA curves corresponded to a single degradation process and the thermal degradation curves were shifted to a higher-temperature region with increasing EMoS₂ content, implying an improved thermal stability of PP. Table 2 shows that the degradation temperatures of all nanocomposites at 5 wt % loss (T_d5%) are higher than that of pure PP. For 0.4, 1.1, and 2.1 wt % EMoS₂ content in the PP/EMoS₂ nanocomposites, the T_d5% increased by 21.6, 31.9, and 36.9 °C, respectively, relative to neat PP. With regard to 0.4 wt % EMoS₂ loading, the temperature at the maximum degradation rate increased by 10 °C compared to neat PP. This significant enhancement of PP thermal stability following the incorporation of EMoS₂ could be ascribed to the good dispersion of MoS₂ in the PP matrix. This enhancement of polymer thermal stability upon incorporation of the EMoS₂ filler has already been reported by both ourselves and other groups [8,9,10,12,13]. The char yield of neat PP is 0.7 wt % at 600 °C, while those of PP/EMoS₂ nanocomposites are all higher, being in the range of 4.2–8.9 wt %. These results are not surprising, since Mo is a transition metal and can catalyze polymer char formation, while sulfur can improve flame retardancy [19,20]. Considering the above results, it is believable that the introduction of inorganic components into organic polymers, such as PP, can improve their thermal stabilities on the basis of the fact that EMoS₂ fillers have good thermal stability due to the heat insulation effect of the EMoS₂ layers and to the mass transport barrier to the volatile products generated during decomposition.

The mechanical properties of PP/EMoS₂ nanocomposites with 0.4, 1.1, and 2.1 wt % EMoS₂ fillers were studied by melt-blending with commercial PP (1:3, mass ratio). The mechanical properties were summarized in

Table 3. Mechanical properties of PP and PP/EMoS₂ nanocomposites with various MoS₂ contents.

Samples	EMoS2 Content (wt %)	Tensile Strength (MPa)	Modulus (MPa)	Elongation at Break (%)
Neat PP	-	35.2 ± 2	850 ± 80	7 ± 3
PP/EMoS2 nanocomposites	0.10	37.4 ± 2	960 ± 80	14 ± 4
	0.28	36.7 ± 2	1140 ± 100	13 ± 4
	0.52	39.2 ± 2	1370 ± 120	13 ± 4

and the stress–strain curves were given in Figure 7. The tensile strength, modulus, and elongation at break values of PP/EMoS₂ nanocomposites were significantly enhanced even at very low loadings of the EMoS₂ nanofiller. At an EMoS₂ loading of 0.10 wt %, the tensile modulus and tensile strength increased from 35.2 MPa and 850 MPa to 37.4 MPa and 960 MPa, respectively. When the EMoS₂ loading increased to 0.52 wt %, the tensile strength of the PP/EMoS₂ nanocomposites reached 39.2 MPa and 1370 MPa, while the elongation at break values also increased. The improved mechanical properties of PP/EMoS₂ nanocomposites prepared by in situ polymerization method could be attributed to the good dispersion of EMoS₂ fillers throughout the PP matrix. These results indicate that the PP/EMoS₂ nanocomposites obtained by in situ polymerization using the MoS₂-MgCl/TiCl₄ catalyst provide a facile approach to efficient reinforcement of polyolefins.

4. Conclusions

A novel MoS₂-MgCl-supported Ti-based Ziegler–Natta catalyst was successfully synthesized via intercalation of the latter catalyst into MoS₂ galleries. PP/EMoS₂ nanocomposites with well-dispersed EMoS₂ nanofillers were successfully fabricated using an in situ exfoliation method during the propylene polymerization, displaying enhanced thermal stability compared to neat PP. After introduction of EMoS₂, the maximum increases in T_d5% and T_dmax were 36.9 and 9.7 °C, respectively, relative to neat PP. The resultant products were used as a masterbatch to reinforce the commercial PP through a melting blending method. The obtained PP/EMoS₂ nanocomposites exhibited a significant improvement in mechanical properties, even at very small EMoS₂ loadings. After blending with commercial PP, the resultant nanocomposites increased in tensile strength and modulus up to 11.4% and 61.2% after 0.52 wt % EMoS₂ loading. Thus, this work provides a facile approach to the production of high-performance PP.