Butyl Rubber Nanocomposites with Monolayer MoS2 Additives: Structural Characteristics, Enhanced Mechanical, and Gas Barrier Properties

Emerging two-dimensional (2D) materialsm, such as molybdenum disulfide (MoS2), offer opportunities to tailor the mechanical and gas barrier properties of polymeric materials. In this study, MoS2 was exfoliated to monolayers by modification with ethanethiol and nonanethiol. The thicknesses of resulting MoS2 monolayers were 0.7 nm for MoS2-ethanethiol and 1.1 nm for MoS2-nonanethiol. MoS2 monolayers were added to chlorobutyl rubber to prepare MoS2-butyl rubber nanocomposites at concentrations of 0.5, 1, 3, and 5 phr. The tensile stress showed a maximum enhancement of about 30.7% for MoS2-ethanethiol-butyl rubber and 34.8% for MoS2-nonanethiol-butyl rubber when compared to pure chlorobutyl rubber. In addition, the gas barrier properties were increased by 53.5% in MoS2-ethanethiol-butyl rubber and 49.6% in MoS2-nonanethiol-butyl rubber. MoS2 nanosheets thus enhanced the mechanical and gas barrier properties of chlorobutyl rubber. The nanocomposites that are presented here may be used to manufacture pharmaceutical stoppers with high mechanical and gas barrier properties.


Introduction
Since the discovery of graphene, two-dimensional inorganic materials, such as MoS 2 , have attracted great attention. MoS 2 has a structure similar to that of graphite; two layers of sulfur and one layer of molybdenum atoms in a sandwiched structure make up its hexagonal crystal lattice structure. MoS 2 is unreactive, unaffected by both acids and oxygen, and has a low coefficient of friction due to weak van der Waals interactions between the layers. As such, it is widely used as a dry lubricant. In addition, MoS 2 can be exfoliated into nanolayers without the need for complex methods. MoS 2 nanosheets have previously been utilized in transistors [1], biomaterials [2], and nanocomposites [3], and can also be added to polymers as a filler material; because of the high band gap of MoS 2 , the electronic properties of the polymer matrices are not changed. A common reason to add fillers to polymers is to improve their mechanical properties. For example, polymer chains can interact with the nanosheet surfaces, resulting in reinforcements in all directions from the nanosheets. For the latter, it is important to fully exfoliate the two-dimensional inorganic materials to increase the surface area [4].
Many studies have reported the use of nanoscale fillers such as clay, reduced graphene oxide, and MoS 2 to improve the mechanical and gas barrier properties of polymer materials for a variety of applications. For example, the optimal mechanical or barrier properties were observed for exfoliated or intercalated polymer/clay nanocomposites, but using a high clay content of 5-10 wt % [5][6][7]. Clay applied in a bath for 24 h. After ultrasonication, the contents of the vials were allowed to settle, and exfoliated MoS 2 was obtained as the suspension.

Preparation of MoS 2 -butyl Rubber Nanocomposites
MoS 2 -butyl rubber nanocomposites were prepared with various MoS 2 concentrations 0.5, 1, 3, and 5 parts per hundreds of rubber (phr). The previously obtained MoS 2 nanosheets were mixed with chlorobutyl rubber and were dissolved in hexane under mechanical stirring for 1 h to achieve a homogenous mixture. The hexane was then evaporated and the samples thus obtained were dried at 100 • C in a vacuum oven for 12 h to completely remove the solvent. The samples were compounded by two-roll-mill with 20 phr silicon dioxide as a widely used filler for rubber to improve the wear resistance and also acts as a reinforcing agent and using 0.5 phr ethylenethiourea(2-mercaptoimidazoline) as the curing reagent. After compression molding at 185 • C at a pressure of 50 kgf/cm 2 for 10 min, MoS 2 -butyl rubber nanocomposite samples with dimensions of 15 cm × 15 cm and a 1-mm thickness were obtained.

Characterization
The morphologies of the MoS 2 nanosheets modified by ethanethiol and nonanethiol were observed using a Tecnai™ G2 F-20 (Philips, Amsterdam, Netherlands) transmission electron microscope (TEM). Raman spectra and Raman maps were obtained using an NRS5100 (JASCO, Tokyo, Japan) spectrometer. Cross-sectional images were obtained using a JSM-6500F (JEOL, Tokyo, Japan) scanning electron microscope (SEM); and, composite samples were cooled in liquid nitrogen and cut by a scalpel to prepare the samples for backscattered electron (BSE) imaging. Atomic force microscopy (AFM) was performed using a NX10 system (Park, Suwon, Korea). X-ray diffraction (XRD) was performed using a D8 SSS (Bruker, Billerica, MA, USA). UV-Vis spectra were obtained using a V-730 spectrometer (JASCO, Tokyo, Japan). Dynamic mechanical analysis was performed using a Q800 (TA Instruments, New Castle, DE, USA), while stress-strain curves were measured using a TS-2000 with a crosshead speed of 500 mm/min. The oxygen transmission rates were measured according to the ASTM D3985 standard using the OX-TRAN 2/61 (Mocon Inc., Minneapolis, MN, USA) at 23 • C and a relative humidity of 0%; film specimens of 5 cm in diameter and 1 mm in thickness were fixed between two chambers, and oxygen filled the upper chamber while nitrogen filled the lower chamber.

Exfoliation of MoS 2
Scheme 1 outlines the overall procedure for the preparation of the MoS 2 nanosheets and the production of MoS 2 -butyl rubber nanocomposites. The exfoliation of MoS 2 was achieved by bath ultrasonication of bulk MoS 2 powder in hexane. It has previously been reported that this exfoliation process can produce a number of structural defects, such as S vacancy defects [17,18]. Then, MoS 2 nanosheets can be modified with thiol ligands. Ethanethiol and nonanethiol were used as the surface modifiers in this study. The carbon chains of these two thiols were hypothesized to modify the surface of MoS 2 to enhance its compatibility with chlorobutyl rubber. The organic modification of the surface and robust nature of the modifiers ensured good dispersion and a dramatically enhanced properties of the polymer materials. The morphologies of MoS2 nanosheets modified by ethanethiol and nonanethiol are presented in TEM images (Figure 1a,b). The hexagonal structure of MoS2 modified by ethanethiol and nonanethiol was clearly visible in high-resolution TEM images (Figure 1c,d). It can be inferred that these MoS2 nanosheets were either several layers thick or monolayers, because the hexagonal lattice structure of MoS2 was visible. The latter indicates that the crystal structures of MoS2-ethanethiol and MoS2-nonanethiol were retained during ultrasonication [19]. Raman spectra were used to confirm the modification of the MoS2 nanosheet surfaces by ethanethiol and nonanethiol ( Figure 2). Peaks were seen at ~380 cm −1 (E 1 2g, in-plane vibrations) and ~410 cm −1 (A1g, out-of-plane vibrations), characteristic of the MoS2 trigonal structure. Peaks at ~680 and ~1100 cm −1 , which indicate carbon-sulfur (νcs) [20] and carbon-carbon bonds (νcc) [21], respectively, were noted for the modified MoS2. These results indicate that the surface of MoS2 was successfully modified by ethanethiol and nonanethiol. The morphologies of MoS 2 nanosheets modified by ethanethiol and nonanethiol are presented in TEM images (Figure 1a,b). The hexagonal structure of MoS 2 modified by ethanethiol and nonanethiol was clearly visible in high-resolution TEM images (Figure 1c,d). It can be inferred that these MoS 2 nanosheets were either several layers thick or monolayers, because the hexagonal lattice structure of MoS 2 was visible. The latter indicates that the crystal structures of MoS 2 -ethanethiol and MoS 2 -nonanethiol were retained during ultrasonication [19]. Raman spectra were used to confirm the modification of the MoS 2 nanosheet surfaces by ethanethiol and nonanethiol ( Figure 2). Peaks were seen at~380 cm −1 (E 1 2g , in-plane vibrations) and~410 cm −1 (A 1g , out-of-plane vibrations), characteristic of the MoS 2 trigonal structure. Peaks at~680 and~1100 cm −1 , which indicate carbon-sulfur (ν cs ) [20] and carbon-carbon bonds (ν cc ) [21], respectively, were noted for the modified MoS 2 . These results indicate that the surface of MoS 2 was successfully modified by ethanethiol and nonanethiol.  The thicknesses of the exfoliated nanosheets were monitored through AFM examination of the exfoliated samples. The thickness of bulk MoS2 was ~90-120 nm (Figure 3a), while that of MoS2ethanethiol was ~0.7 nm (Figure 3b) and that of MoS2-nonanethiol was ~1.1 nm (Figure 3c), values that correspond to that of ~0.65 nm in previous reports on the thickness of MoS2 monolayers [1]. The thicknesses obtained here being greater than the typical thickness of a single-layer MoS2 sheet may be attributed to thiol conjugation on the surface of MoS2 [22]. Blue-shifts of UV-Vis spectra are   The thicknesses of the exfoliated nanosheets were monitored through AFM examination of the exfoliated samples. The thickness of bulk MoS2 was ~90-120 nm (Figure 3a), while that of MoS2ethanethiol was ~0.7 nm (Figure 3b) and that of MoS2-nonanethiol was ~1.1 nm (Figure 3c), values that correspond to that of ~0.65 nm in previous reports on the thickness of MoS2 monolayers [1]. The thicknesses obtained here being greater than the typical thickness of a single-layer MoS2 sheet may be attributed to thiol conjugation on the surface of MoS2 [22]. Blue-shifts of UV-Vis spectra are may be attributed to thiol conjugation on the surface of MoS 2 [22]. Blue-shifts of UV-Vis spectra are dependent on changes in the band gap energy, which can be obtained from the wavelengths in UV-Vis spectra from the following equation: Band gap energy (E) = (hc)/λ (1) where hc is Planck's constant and λ is the wavelength. Bulk MoS 2 is an indirect semiconductor with a band gap of~1.2 eV, which increases to~1.8 and~1.9 eV for monolayers of MoS 2 [23,24]. To obtain the optimum parameters for exfoliation, the number of MoS 2 nanosheet layers was measured for various concentrations of ethanethiol and nonanethiol by UV-Vis spectra ( Figure 4). The MoS 2 -ethanethiol sample in Figure 4a shows a blue-shift from 697 to 688 nm. The latter wavelength of 688 nm corresponds to a band gap value of 1.80 eV. For MoS 2 -nonanethiol in Figure 4b, a blue-shift from 697 to 685 nm can be observed. The latter wavelength of 685 nm corresponds to a band gap value of 1.81 eV. The conditions to exfoliate MoS 2 into monolayer involved the addition of 0.5 mL of either ethanethiol or nonanethiol with 400 mg bulk MoS 2 powder into 20 mL hexane. The exfoliation efficiency for MoS 2 that was treated with nonanethiol was greater than that of MoS 2 treated with ethanethiol.
Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 dependent on changes in the band gap energy, which can be obtained from the wavelengths in UV-Vis spectra from the following equation: where hc is Planck's constant and λ is the wavelength. Bulk MoS2 is an indirect semiconductor with a band gap of ~1.2 eV, which increases to ~1.8 and ~1.9 eV for monolayers of MoS2 [23,24]. To obtain the optimum parameters for exfoliation, the number of MoS2 nanosheet layers was measured for various concentrations of ethanethiol and nonanethiol by UV-Vis spectra ( Figure 4). The MoS2ethanethiol sample in Figure 4a shows a blue-shift from 697 to 688 nm. The latter wavelength of 688 nm corresponds to a band gap value of 1.80 eV. For MoS2-nonanethiol in Figure 4b, a blue-shift from 697 to 685 nm can be observed. The latter wavelength of 685 nm corresponds to a band gap value of 1.81 eV. The conditions to exfoliate MoS2 into monolayer involved the addition of 0.5 mL of either ethanethiol or nonanethiol with 400 mg bulk MoS2 powder into 20 mL hexane. The exfoliation efficiency for MoS2 that was treated with nonanethiol was greater than that of MoS2 treated with ethanethiol.   dependent on changes in the band gap energy, which can be obtained from the wavelengths in UV-Vis spectra from the following equation: where hc is Planck's constant and λ is the wavelength. Bulk MoS2 is an indirect semiconductor with a band gap of ~1.2 eV, which increases to ~1.8 and ~1.9 eV for monolayers of MoS2 [23,24]. To obtain the optimum parameters for exfoliation, the number of MoS2 nanosheet layers was measured for various concentrations of ethanethiol and nonanethiol by UV-Vis spectra ( Figure 4). The MoS2ethanethiol sample in Figure 4a shows a blue-shift from 697 to 688 nm. The latter wavelength of 688 nm corresponds to a band gap value of 1.80 eV. For MoS2-nonanethiol in Figure 4b, a blue-shift from 697 to 685 nm can be observed. The latter wavelength of 685 nm corresponds to a band gap value of 1.81 eV. The conditions to exfoliate MoS2 into monolayer involved the addition of 0.5 mL of either ethanethiol or nonanethiol with 400 mg bulk MoS2 powder into 20 mL hexane. The exfoliation efficiency for MoS2 that was treated with nonanethiol was greater than that of MoS2 treated with ethanethiol.

Characterization of MoS 2 -butyl rubber Nanocomposites
XRD was performed to characterize the obtained layered-structure materials and partially evaluate the dispersion state of layered nanofillers in the polymer composites. XRD scans of the polymer nanocomposites showed a nanofiller peak and a shift to a lower 2θ or larger d-spacing value when compared to bulk MoS 2 . The peak shift indicates an expansion of the d-spacing of MoS 2 nanosheets; it was inferred that polymer chains had been intercalated in the MoS 2 nanosheets. For completely exfoliated layered nanofillers, no XRD peaks were expected for the nanocomposites, since they should not show regular spacing of the sheets [25].
The XRD patterns ( For MoS 2 -nonanethiol-butyl rubber, the peak was at 2θ = 14.36 • for the 0.5-phr sample, which indicates that the d-spacing of MoS 2 increased when MoS 2 nanosheets were inserted into the chlorobutyl rubber chains. The latter illustrates that, between the exfoliation and intercalation, the nanocomposites can be driven toward full exfoliation by decreasing the content of MoS 2 nanosheets. The greater shift at low concentrations indicates that nonanethiol is a more suitable modifier for MoS 2 exfoliation than ethanethiol.

Characterization of MoS2-butyl rubber Nanocomposites
XRD was performed to characterize the obtained layered-structure materials and partially evaluate the dispersion state of layered nanofillers in the polymer composites. XRD scans of the polymer nanocomposites showed a nanofiller peak and a shift to a lower 2θ or larger d-spacing value when compared to bulk MoS2. The peak shift indicates an expansion of the d-spacing of MoS2 nanosheets; it was inferred that polymer chains had been intercalated in the MoS2 nanosheets. For completely exfoliated layered nanofillers, no XRD peaks were expected for the nanocomposites, since they should not show regular spacing of the sheets [25].
The XRD patterns ( Figure 5) of the MoS2-butyl rubber nanocomposites confirm the intercalation of chlorobutyl rubber in the MoS2 nanosheet interlayers by showing a decrease in 2θ value as the concentration of MoS2 increased. The (002) peak of pure MoS2 was at 2θ = 14.44°, corresponding to a d-spacing value of 0.3088 nm. After adding MoS2 to chlorobutyl rubber, the 2θ peak of the (002) plane shifted to lower angles, associated with intercalation in nanocomposites. For MoS2-ethanethiol-butyl rubber, the peak at 2θ = 14.44° (d = 0.3088 nm) for 0 phr shifted to 2θ = 14.40° (d = 0.3097 nm), and 2θ = 14.38° (d = 0.3102 nm) for the samples with 3 and 5 phr MoS2, respectively. For MoS2-nonanethiolbutyl rubber, the peak was at 2θ = 14.36° for the 0.5-phr sample, which indicates that the d-spacing of MoS2 increased when MoS2 nanosheets were inserted into the chlorobutyl rubber chains. The latter illustrates that, between the exfoliation and intercalation, the nanocomposites can be driven toward full exfoliation by decreasing the content of MoS2 nanosheets. The greater shift at low concentrations indicates that nonanethiol is a more suitable modifier for MoS2 exfoliation than ethanethiol.  The typical Raman peaks for MoS2-butyl rubber nanocomposites are shown in Figure 7. The peaks at ~380 and ~410 cm−1 correspond to MoS2, while the peaks at ~720, ~820, ~910, and ~1080 cm−1 correspond to chlorobutyl rubber. Raman mapping (Figure 8) was used to further confirm the dispersion state of MoS2 nanosheets at different MoS2 concentrations. Figure 8 shows the intensity maps of the A1g peak (~410 cm −1 ) of MoS2 for nanocomposites with different concentrations of modified MoS2 nanosheets. The Raman mapping images correspond well with the SEM-BSE images The typical Raman peaks for MoS2-butyl rubber nanocomposites are shown in Figure 7. The peaks at~380 and~410 cm −1 correspond to MoS2, while the peaks at~720,~820,~910, and 1080 cm −1 correspond to chlorobutyl rubber. Raman mapping (Figure 8) was used to further confirm the dispersion state of MoS 2 nanosheets at different MoS 2 concentrations. Figure 8 shows the intensity maps of the A 1g peak (~410 cm −1 ) of MoS 2 for nanocomposites with different concentrations of modified MoS 2 nanosheets. The Raman mapping images correspond well with the SEM-BSE images (Figure 6). At low concentrations of MoS 2 nanosheets, their distribution was uniform, Polymers 2018, 10, 238 9 of 13 which implies homogeneous dispersion in chlorobutyl rubber. As the MoS 2 loading increased, however, agglomeration and clustering behavior of the MoS 2 was visible, illustrating poor dispersion. Nonetheless, due to their conjugation with ethanethiol or nonanethiol, MoS 2 nanosheets could disperse homogeneously in chlorobutyl rubber at low concentrations. As shown in Figure 6, MoS 2 -nonanethiol-butyl rubber had a more uniform appearance than MoS 2 -ethanethiol-butyl rubber; at 5 phr MoS 2 , in particular, the clustering for MoS 2 -ethanethiol-butyl rubber was more pronounced than for MoS 2 -nonanethiol-butyl rubber.
Polymers 2018, 10, x FOR PEER REVIEW 9 of 13 ( Figure 6). At low concentrations of MoS2 nanosheets, their distribution was uniform, which implies homogeneous dispersion in chlorobutyl rubber. As the MoS2 loading increased, however, agglomeration and clustering behavior of the MoS2 was visible, illustrating poor dispersion. Nonetheless, due to their conjugation with ethanethiol or nonanethiol, MoS2 nanosheets could disperse homogeneously in chlorobutyl rubber at low concentrations. As shown in Figure 6, MoS2nonanethiol-butyl rubber had a more uniform appearance than MoS2-ethanethiol-butyl rubber; at 5 phr MoS2, in particular, the clustering for MoS2-ethanethiol-butyl rubber was more pronounced than for MoS2-nonanethiol-butyl rubber.

Tensile Properties of MoS2-butyl Rubber Nanocomposites
The stress-strain curves ( Figure 9) for neat chlorobutyl rubber and MoS2-butyl rubber nanocomposites show that the tensile strength of the chlorobutyl rubber matrix increased upon MoS2 nanosheet loading. Furthermore, the elongation at break of MoS2-nonanethiol-butyl rubber was about 14.4% higher than that of MoS2-ethanethiol-butyl rubber. The maximum increase in tensile strength for MoS2-ethanethiol-butyl rubber was about 30.7% for a MoS2 content of 3 phr. In MoS2nonanethiol-butyl rubber, likewise, the tensile strength was increased by about 34.8% for 1 phr MoS2 as compared to that of the control sample. Therefore, the maximum increase in tensile strength was obtained for MoS2-nonanethiol-butyl rubber instead of MoS2-ethanethiol-butyl rubber. The  Figure 6). At low concentrations of MoS2 nanosheets, their distribution was uniform, which implies homogeneous dispersion in chlorobutyl rubber. As the MoS2 loading increased, however, agglomeration and clustering behavior of the MoS2 was visible, illustrating poor dispersion. Nonetheless, due to their conjugation with ethanethiol or nonanethiol, MoS2 nanosheets could disperse homogeneously in chlorobutyl rubber at low concentrations. As shown in Figure 6, MoS2nonanethiol-butyl rubber had a more uniform appearance than MoS2-ethanethiol-butyl rubber; at 5 phr MoS2, in particular, the clustering for MoS2-ethanethiol-butyl rubber was more pronounced than for MoS2-nonanethiol-butyl rubber.

Tensile Properties of MoS2-butyl Rubber Nanocomposites
The stress-strain curves (Figure 9) for neat chlorobutyl rubber and MoS2-butyl rubber nanocomposites show that the tensile strength of the chlorobutyl rubber matrix increased upon MoS2 nanosheet loading. Furthermore, the elongation at break of MoS2-nonanethiol-butyl rubber was about 14.4% higher than that of MoS2-ethanethiol-butyl rubber. The maximum increase in tensile strength for MoS2-ethanethiol-butyl rubber was about 30.7% for a MoS2 content of 3 phr. In MoS2nonanethiol-butyl rubber, likewise, the tensile strength was increased by about 34.8% for 1 phr MoS2 as compared to that of the control sample. Therefore, the maximum increase in tensile strength was obtained for MoS2-nonanethiol-butyl rubber instead of MoS2-ethanethiol-butyl rubber. The

Tensile Properties of MoS 2 -butyl Rubber Nanocomposites
The stress-strain curves (Figure 9) for neat chlorobutyl rubber and MoS 2 -butyl rubber nanocomposites show that the tensile strength of the chlorobutyl rubber matrix increased upon MoS 2 nanosheet loading. Furthermore, the elongation at break of MoS 2 -nonanethiol-butyl rubber was about 14.4% higher than that of MoS 2 -ethanethiol-butyl rubber. The maximum increase in tensile strength for MoS 2 -ethanethiol-butyl rubber was about 30.7% for a MoS 2 content of 3 phr. In MoS 2 -nonanethiol-butyl rubber, likewise, the tensile strength was increased by about 34.8% for 1 phr MoS 2 as compared to that of the control sample. Therefore, the maximum increase in tensile strength was obtained for MoS 2 -nonanethiol-butyl rubber instead of MoS 2 -ethanethiol-butyl rubber. The significant increase in tensile strength reached a peak at a loading of 3 phr for MoS 2 -ethanethiol-butyl rubber and of 1 phr for MoS 2 -nonanethiol-butyl rubber. At higher MoS 2 nanosheet contents, the tensile strength decreased again. The latter observations may be ascribed to the aggregation of MoS 2 nanosheets in the chlorobutyl rubber matrix, which is known to cause a decrease in tensile strength for rubber [26]. It is obvious from these results that MoS 2 nanosheets can significantly improve the strength of chlorobutyl rubber, possibly due to the high strength of MoS 2 nanosheets, better interactions between MoS 2 nanosheets and the polymer matrix, and/or a more uniform dispersion of MoS 2 nanosheets in the chlorobutyl rubber matrix due to abundant thiol groups on the MoS 2 nanosheet surfaces.
Polymers 2018, 10, x FOR PEER REVIEW 10 of 13 significant increase in tensile strength reached a peak at a loading of 3 phr for MoS2-ethanethiol-butyl rubber and of 1 phr for MoS2-nonanethiol-butyl rubber. At higher MoS2 nanosheet contents, the tensile strength decreased again. The latter observations may be ascribed to the aggregation of MoS2 nanosheets in the chlorobutyl rubber matrix, which is known to cause a decrease in tensile strength for rubber [26]. It is obvious from these results that MoS2 nanosheets can significantly improve the strength of chlorobutyl rubber, possibly due to the high strength of MoS2 nanosheets, better interactions between MoS2 nanosheets and the polymer matrix, and/or a more uniform dispersion of MoS2 nanosheets in the chlorobutyl rubber matrix due to abundant thiol groups on the MoS2 nanosheet surfaces.

Dynamic Mechanical Analysis of MoS2-butyl Rubber Nanocomposites
For MoS2-ethanethiol-butyl rubber, the storage modulus ( Figure 10a) is a measure of its stiffness and the elastic of material that means the ability to recover pristine shape, and it a little increased for all the MoS2-butyl rubber nanocomposites in rubbery region compared to pure chlorobutyl rubber but no significant increment in glassy region. In rubbery region, the nanocomposite containing 0.5 phr MoS2 nanosheets exhibited the highest modulus value. MoS2-nonanethiol-butyl rubber also showed an increase in the storage modulus (Figure 10b), with an increase in the content of MoS2 nanosheets, except for 0.5 phr, and reached the highest modulus value for 3 phr. These results indicate that MoS2 nanosheet incorporation into chlorobutyl rubber remarkably enhanced stiffness and had a significant reinforcing effect. This increase in storage modulus results from the intercalation of MoS2 nanosheets in chlorobutyl rubber and strong interactions between the chlorobutyl rubber polymer chain and MoS2 nanosheets. The mobility of the polymer chains in rubbery region was thus retarded by the MoS2 nanosheets, resulting in the higher storage modulus.

Dynamic Mechanical Analysis of MoS 2 -butyl Rubber Nanocomposites
For MoS 2 -ethanethiol-butyl rubber, the storage modulus ( Figure 10a) is a measure of its stiffness and the elastic of material that means the ability to recover pristine shape, and it a little increased for all the MoS 2 -butyl rubber nanocomposites in rubbery region compared to pure chlorobutyl rubber but no significant increment in glassy region. In rubbery region, the nanocomposite containing 0.5 phr MoS 2 nanosheets exhibited the highest modulus value. MoS 2 -nonanethiol-butyl rubber also showed an increase in the storage modulus (Figure 10b), with an increase in the content of MoS 2 nanosheets, except for 0.5 phr, and reached the highest modulus value for 3 phr. These results indicate that MoS 2 nanosheet incorporation into chlorobutyl rubber remarkably enhanced stiffness and had a significant reinforcing effect. This increase in storage modulus results from the intercalation of MoS 2 nanosheets in chlorobutyl rubber and strong interactions between the chlorobutyl rubber polymer chain and MoS 2 nanosheets. The mobility of the polymer chains in rubbery region was thus retarded by the MoS 2 nanosheets, resulting in the higher storage modulus.
Polymers 2018, 10, x FOR PEER REVIEW 10 of 13 significant increase in tensile strength reached a peak at a loading of 3 phr for MoS2-ethanethiol-butyl rubber and of 1 phr for MoS2-nonanethiol-butyl rubber. At higher MoS2 nanosheet contents, the tensile strength decreased again. The latter observations may be ascribed to the aggregation of MoS2 nanosheets in the chlorobutyl rubber matrix, which is known to cause a decrease in tensile strength for rubber [26]. It is obvious from these results that MoS2 nanosheets can significantly improve the strength of chlorobutyl rubber, possibly due to the high strength of MoS2 nanosheets, better interactions between MoS2 nanosheets and the polymer matrix, and/or a more uniform dispersion of MoS2 nanosheets in the chlorobutyl rubber matrix due to abundant thiol groups on the MoS2 nanosheet surfaces.

Dynamic Mechanical Analysis of MoS2-butyl Rubber Nanocomposites
For MoS2-ethanethiol-butyl rubber, the storage modulus ( Figure 10a) is a measure of its stiffness and the elastic of material that means the ability to recover pristine shape, and it a little increased for all the MoS2-butyl rubber nanocomposites in rubbery region compared to pure chlorobutyl rubber but no significant increment in glassy region. In rubbery region, the nanocomposite containing 0.5 phr MoS2 nanosheets exhibited the highest modulus value. MoS2-nonanethiol-butyl rubber also showed an increase in the storage modulus (Figure 10b), with an increase in the content of MoS2 nanosheets, except for 0.5 phr, and reached the highest modulus value for 3 phr. These results indicate that MoS2 nanosheet incorporation into chlorobutyl rubber remarkably enhanced stiffness and had a significant reinforcing effect. This increase in storage modulus results from the intercalation of MoS2 nanosheets in chlorobutyl rubber and strong interactions between the chlorobutyl rubber polymer chain and MoS2 nanosheets. The mobility of the polymer chains in rubbery region was thus retarded by the MoS2 nanosheets, resulting in the higher storage modulus.   Figure 10a. For all of the samples of MoS 2 -ethanethiol-butyl rubber, shifts to lower temperatures were observed when compared to the 0 phr sample. MoS 2 intercalated in chlorobutyl rubber may act as a lubricant, which leads to lowering of the glass transition temperature [27]. The tan(δ) values of MoS 2 -nonanethiol-butyl rubber are shown in Figure 11b; similar shifts to lower temperatures can be seen, again indicating intercalation of MoS 2 nanosheets in the chlorobutyl rubber. The barrier effect of the nano-flakes restricting the motion of the polymer chains in the nanocomposites can be ascribed to the MoS 2 nanosheets.
Polymers 2018, 10, x FOR PEER REVIEW 11 of 13 The tan(δ) values of MoS2-ethanethiol-butyl rubber are shown in Figure 10a. For all of the samples of MoS2-ethanethiol-butyl rubber, shifts to lower temperatures were observed when compared to the 0 phr sample. MoS2 intercalated in chlorobutyl rubber may act as a lubricant, which leads to lowering of the glass transition temperature [27]. The tan(δ) values of MoS2-nonanethiolbutyl rubber are shown in Figure 11b; similar shifts to lower temperatures can be seen, again indicating intercalation of MoS2 nanosheets in the chlorobutyl rubber. The barrier effect of the nanoflakes restricting the motion of the polymer chains in the nanocomposites can be ascribed to the MoS2 nanosheets.

Gas Barrier Properties of MoS2-butyl Rubber Nanocomposites
The barrier properties of polymers can be significantly altered by including sufficient inorganic platelets to alter the path of gas molecules (Scheme 2) [4]. The oxygen transmission rate (OTR) ( Table 1) of each MoS2-butyl rubber nanocomposite was measured at 25 °C using the method outlined by ASTM D3985. When compared to that of pure chlorobutyl rubber, the OTR of MoS2-ethanethiolbutyl rubber decreased dramatically to 42.3 cc/m 2 -day at the MoS2 nanosheet concentration of 0.5 phr. The OTR of MoS2-nonanethiol-butyl rubber decreased to 47.2 cc/m 2 -day at 0.5 phr, and thereafter decreased slowly at higher concentrations. The barrier performance for all MoS2-butyl rubber nanocomposites could be improved markedly by the application of a small amount of organicmodified MoS2. Moreover, there was little difference between the gas barriers of MoS2-ethanethiolbutyl rubber and MoS2-nonanethiol-butyl rubber, since the surface areas of MoS2-ethanethiol and MoS2-nonanethiol nanosheets were too small to retard the pathway of gas molecules. There are two reasons behind the enhancement of the gas barrier properties of the MoS2-butyl rubber nanocomposites. First, MoS2 nanosheets form tortuous pathways in chlorobutyl rubber, which retard the progress of gas molecules through the composite. Secondly, the diffusion coefficient of the gas molecules decreases because MoS2 nanosheets strongly restrict the motion of the polymer chains [7].

Gas Barrier Properties of MoS 2 -butyl Rubber Nanocomposites
The barrier properties of polymers can be significantly altered by including sufficient inorganic platelets to alter the path of gas molecules (Scheme 2) [4]. The oxygen transmission rate (OTR) ( Table 1) of each MoS 2 -butyl rubber nanocomposite was measured at 25 • C using the method outlined by ASTM D3985. When compared to that of pure chlorobutyl rubber, the OTR of MoS 2 -ethanethiol-butyl rubber decreased dramatically to 42.3 cc/m 2 -day at the MoS 2 nanosheet concentration of 0.5 phr. The OTR of MoS 2 -nonanethiol-butyl rubber decreased to 47.2 cc/m 2 -day at 0.5 phr, and thereafter decreased slowly at higher concentrations. The barrier performance for all MoS 2 -butyl rubber nanocomposites could be improved markedly by the application of a small amount of organic-modified MoS 2 . Moreover, there was little difference between the gas barriers of MoS 2 -ethanethiol-butyl rubber and MoS 2 -nonanethiol-butyl rubber, since the surface areas of MoS 2 -ethanethiol and MoS 2 -nonanethiol nanosheets were too small to retard the pathway of gas molecules. There are two reasons behind the enhancement of the gas barrier properties of the MoS 2 -butyl rubber nanocomposites. First, MoS 2 nanosheets form tortuous pathways in chlorobutyl rubber, which retard the progress of gas molecules through the composite. Secondly, the diffusion coefficient of the gas molecules decreases because MoS 2 nanosheets strongly restrict the motion of the polymer chains [7]. The tan(δ) values of MoS2-ethanethiol-butyl rubber are shown in Figure 10a. For all of the samples of MoS2-ethanethiol-butyl rubber, shifts to lower temperatures were observed when compared to the 0 phr sample. MoS2 intercalated in chlorobutyl rubber may act as a lubricant, which leads to lowering of the glass transition temperature [27]. The tan(δ) values of MoS2-nonanethiolbutyl rubber are shown in Figure 11b; similar shifts to lower temperatures can be seen, again indicating intercalation of MoS2 nanosheets in the chlorobutyl rubber. The barrier effect of the nanoflakes restricting the motion of the polymer chains in the nanocomposites can be ascribed to the MoS2 nanosheets.

Gas Barrier Properties of MoS2-butyl Rubber Nanocomposites
The barrier properties of polymers can be significantly altered by including sufficient inorganic platelets to alter the path of gas molecules (Scheme 2) [4]. The oxygen transmission rate (OTR) ( Table 1) of each MoS2-butyl rubber nanocomposite was measured at 25 °C using the method outlined by ASTM D3985. When compared to that of pure chlorobutyl rubber, the OTR of MoS2-ethanethiolbutyl rubber decreased dramatically to 42.3 cc/m 2 -day at the MoS2 nanosheet concentration of 0.5 phr. The OTR of MoS2-nonanethiol-butyl rubber decreased to 47.2 cc/m 2 -day at 0.5 phr, and thereafter decreased slowly at higher concentrations. The barrier performance for all MoS2-butyl rubber nanocomposites could be improved markedly by the application of a small amount of organicmodified MoS2. Moreover, there was little difference between the gas barriers of MoS2-ethanethiolbutyl rubber and MoS2-nonanethiol-butyl rubber, since the surface areas of MoS2-ethanethiol and MoS2-nonanethiol nanosheets were too small to retard the pathway of gas molecules. There are two reasons behind the enhancement of the gas barrier properties of the MoS2-butyl rubber nanocomposites. First, MoS2 nanosheets form tortuous pathways in chlorobutyl rubber, which retard the progress of gas molecules through the composite. Secondly, the diffusion coefficient of the gas molecules decreases because MoS2 nanosheets strongly restrict the motion of the polymer chains [7]. Scheme 2. Barrier to permeation imposed by nanoparticles embedded in a polymeric matrix. Scheme 2. Barrier to permeation imposed by nanoparticles embedded in a polymeric matrix.

Conclusions
In conclusion, we have demonstrated that MoS 2 nanosheets are an excellent filler material to enhance the tensile properties of chlorobutyl rubber. Ethanethiol and nonanethiol played an important role in modifying the surface of MoS 2 nanosheets. Using thiol modification of nanosheets helped to obtain MoS 2 monolayers with a thickness of~0.8-1 nm, a key feature of MoS 2 nanosheets intercalated in chlorobutyl rubber. The obtained MoS 2 nanosheets were dispersed homogeneously in chlorobutyl rubber due to the thiol ligands modifying MoS 2 to enable greater affinity between MoS 2 and chlorobutyl rubber. Due to the high stiffness of the MoS 2 nanosheets, MoS 2 improved the mechanical properties of chlorobutyl rubber in tensile test, but not significantly in storage modulus. On the other hand, the gas barrier was improved dramatically, although similarly for MoS 2 -ethanethioland MoS 2 -nonanethiol-butyl rubber. These results offer new opportunities utilizing nanocomposites of polymers and MoS 2 . Controlling the dimensions of MoS 2 nanosheets remains a challenge. Therefore, improved techniques are necessary to produce MoS 2 nanosheets of appropriate sizes, which can then achieve their full potential in polymer nanocomposites.