Exploration of Mechanical and Thermal Properties of CTAB-Modified MoS 2 / LLDPE Composites Prepared by Melt Mixing

Molybdenum disulfide (MoS2) was functionalized by cetyltrimethylammonium bromide (CTAB) through a one-pot hydrothermal technique, and subsequently, linear low-density polyethylene (LLDPE) composites were prepared. The attachment of alkyl chains of CTAB onto the MoS2 surface was confirmed by Fourier transform infrared spectroscopy (FTIR). The enhanced mechanical properties of the composites relative to neat LLDPE revealed good compatibility between MoS2 and LLDPE. The improvement in thermomechanical properties further substantiated good interaction between MoS2 and LLDPE. The thermal stability of the prepared composites showed a small decrease in onset degradation temperature. Nevertheless, the char residue formation was found to be promoted in the presence of MoS2.


Introduction
The discovery of graphene and its subsequent surge in the academic and industrial spheres inspired researchers to excavate other 2-D materials, such as transition metal dichalcogenides, hexagonal boron nitride, carbon nitride (C 3 N 4 ), and so forth.[1].Among these, molybdenum disulfide (MoS 2 ), with a structure akin to graphene, has attracted significant attention in different fields, which arises mainly from its 2-D morphology, atomic thickness, and semiconductor characteristics.Thus, 2-D layered MoS 2 has been widely studied in the fields such as catalysis, supercapacitors, transistors, lithium ion batteries, and polymer nanocomposites [2][3][4][5][6][7][8][9].The 2-D layered MoS 2 possesses high modulus of ~300 GPa and has proven to be a very efficient filler in reinforcing polymer matrices [10,11].In recent times, MoS 2 has been used as a reinforcement filler in polymer matrices such as polyethylene, poly(vinylalcohol), epoxy, polystyrene, methyl methacrylate, chitosan, polyurethane, nylon, polyamide, methylcellulose, and so forth, and significant enhancements in mechanical, thermal, tribological, and fire resistance properties have been recorded with small weight fractions of MoS 2 [9,[11][12][13][14][15][16][17][18][19][20].Among the 2-D reinforcing filler, graphene has been widely used and has found wide acceptance in the scientific and industrial realms [21].Although graphene could produce mechanically robust and thermally stable high-performance composites, being electrically conducting in nature, it may also impart electrical conductivity to the host matrix, thereby restricting its uses in areas such as electronic packaging, electric motors, transmission lines, and so forth [11].In the aforementioned applications, it is necessary to enhance mechanical properties and thermal stability of the composite materials while preserving the insulating properties of the host polymer materials.
Because of its semiconductor characteristics, MoS 2 does not endow electrical conductivity to the host polymer matrix, so MoS 2 -based polymer composites may be befitting alternatives to conducting graphene-based composites.
The exciting properties of MoS 2 nanosheets are associated with its 2-D morphology and its atomic thickness.However, in order to transfer the intrinsic properties of MoS 2 into a polymer matrix, homogenous dispersion and distribution of MoS 2 is very crucial.The extent of interfacial interaction/adhesion between a polymer matrix and filler is strongly dependent on the degree of dispersion of filler particles [22][23][24].Anchoring organic moieties onto the surface of MoS 2 could be one of the plausible ways to improve the interaction, compatibility, and dispersion of MoS 2 in organic polymer systems.Hence, the preparation of mono-or multilayer MoS 2 and its functionalization is crucial to realize the full reinforcing potential.Many prior reports used functionalized MoS 2 nanosheets to reinforce the polymer matrix, and it has been observed that suitably tuned MoS 2 nanosheets could endow significant enhancement in mechanical, thermal, and other properties [11][12][13][14][15][16][17][18].Feng et al. used dodecanethiol as an antioxidant and surface modifier to produce defect-free MoS 2 by direct ultrasonication of bulk MoS 2 and prepared the polyethylene nanocomposites by incorporating defect-free modified MoS 2 [9].The study revealed improved mechanical and thermal properties upon incorporation of MoS 2 .Typically, solution-based exfoliation techniques are employed to generate MoS 2 nanosheets.The exfoliation-based technique consumes large amount of organic solvents and requires prolonged sonication, which may deform the MoS 2 structure and create defects.So, concurrent preparation and functionalization of MoS 2 through a one-pot method could be one of the effective ways to achieve defect-free MoS 2 sheets.The intent of this work is to shed light on the effect of functionalized MoS 2 sheets on the mechanical and thermal properties of linear low-density polyethylene (LLDPE) composites prepared through an industrially viable melt-mixing process.
Herein, the cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal synthesis of MoS 2 (denoted as CTAB-MoS 2 ) and the subsequent fabrication of CTAB-MoS 2 /LLDPE composites by melt mixing has been reported.It is expected that the CTAB surfactant shall be adsorbed electrostatically onto the surface of the anionic precursor (MoO 4 2− ) formed during the synthesis of MoS 2 and assist in the formation of mono-or multilayer MoS 2 particles, and additionally, help to inhibit the restacking or agglomeration of MoS 2 layers.LLDPE, one of the widely used thermoplastic polymers, has been selected as the polymer matrix.LLDPE has enormous applications, mainly for plastic films, tubes, and so forth, due to its excellent tear resistance and impact strength, large elongation, and excellent puncture resistance [25,26].However, pure LLDPE suffers from certain limitations such as low mechanical properties and low thermal resistance, amongst others.A melt-mixing technique was employed to fabricate the composites and it may be considered advantageous over solution mixing, as this technique is devoid of the use of hazardous solvents.

Materials
Sodium molybdate (Na 2 MoO 4 ) and thiourea (CH 4 N 2 S) were purchased from Merck, Mumbai, India.Cetyltrimethylammonium bromide (CTAB) was purchased from Himedia, Mumbai, India.LLDPE polymer pellets were supplied by Reliance Industries Limited, Jamnagar, Gujarat, India.All the reagents were of analytical grade and used as received without further purification.

CTAB Assisted Synthesis of MoS 2
The CTAB-embedded MoS 2 nanosheets were prepared by a facile hydrothermal route as reported in our earlier work [18].Typically, ~1.2 g of Na 2 MoO 4 •2H 2 O and 3.2 g of CH 4 N 2 S were dissolved in 60 mL of distilled water.After that, 3 g of CTAB was dissolved into the mixture under stirring to form a homogenous solution.The clear solution was then introduced into a 100 mL Teflon-lined stainless steel autoclave and maintained at 180 • C for 16 h.Then, the autoclave was allowed to cool to room temperature.The black precipitates so formed were collected by centrifugation, followed by washing with distilled water and absolute ethanol.The final product was dried in a vacuum oven at 80 • C for 12 h and designated as CTAB-MoS 2 .The pristine MoS 2 was prepared following the similar procedure, albeit without using CTAB.

Preparation of CTAB-MoS2/LLDPE Composites
LLDPE composites of different weight fractions of CTAB-MoS 2 were prepared by melt compounding on a Sigma melt mixer (S.C.Dey Co., Kolkata, India), at a processing temperature of 125 • C, screw speed of 60 rpm, and the mixing time of 15 min.The melt mixing was conducted by initially adding the polymer slowly into the mixing compartment.When the polymer melted completely, CTAB-MoS 2 was added to the molten polymer.The material obtained after melt mixing was cooled to room temperature and collected as lumps.It was then hot-pressed into a sheet of thickness ~2 mm on an electrically heated hydraulic press.The pallets were held for ~15 min in the hot-pressing machine at a temperature of 150 • C.After cooling to room temperature, the samples were cut into different dimensions from the sheet for performing various tests.Figure 1 shows the schematic of the functionalization of MoS 2 with CTAB and preparation of its composites with LLDPE matrix by melt mixing.
cool to room temperature.The black precipitates so formed were collected by centrifugation, followed by washing with distilled water and absolute ethanol.The final product was dried in a vacuum oven at 80 °C for 12 h and designated as CTAB-MoS2.The pristine MoS2 was prepared following the similar procedure, albeit without using CTAB.

Preparation of CTAB-MoS2/LLDPE Composites
LLDPE composites of different weight fractions of CTAB-MoS2 were prepared by melt compounding on a Sigma melt mixer (S.C.Dey Co., Kolkata, India), at a processing temperature of 125 °C, screw speed of 60 rpm, and the mixing time of 15 min.The melt mixing was conducted by initially adding the polymer slowly into the mixing compartment.When the polymer melted completely, CTAB-MoS2 was added to the molten polymer.The material obtained after melt mixing was cooled to room temperature and collected as lumps.It was then hot-pressed into a sheet of thickness ~2 mm on an electrically heated hydraulic press.The pallets were held for ~15 min in the hot-pressing machine at a temperature of 150 °C.After cooling to room temperature, the samples were cut into different dimensions from the sheet for performing various tests.Figure 1 shows the schematic of the functionalization of MoS2 with CTAB and preparation of its composites with LLDPE matrix by melt mixing.

Characterization
The functionalization of MoS2 was confirmed by Fourier transform infrared (FT-IR) spectra.FT-IR spectra were recorded with a Perkin Elmer RXI FT-IR (PerkinElmer, Inc., Waltham, MA, USA) in the range of 4000-400 cm −1 .The formation of MoS2, CTAB-MoS2, and exfoliation of functionalized MoS2 in LLDPE were confirmed by X-ray diffraction (XRD).XRD was performed in PANalytical (Model X pert PRO) at a scan rate of 0.106° s −1 in between the 2θ range of 5-60°.Field emission scanning electron microscopy (FE-SEM)was performed using ∑igma HD, Carl Zeiss, Germany, to study the morphology of the CTAB-MoS2/LLDP composite fracture sample.The dispersion of functionalized MoS2 in the LLDPE matrix was studied by transmission electron microscopy (TEM) with a JEOLTEM 2100 FS instrument (JEOL Ltd., Tokyo, Japan) at 200 kV.The ultrathin TEM sample of thickness ~70 nm of the composites was cut using a Leica Ultracut UCT (Leica Microsystems Inc., Buffalo Grove, IL, USA) ultramicrotome at room temperature.Microtomed composite samples were then collected over copper grids for TEM observation.The tensile measurements were performed using a Tinius Olsen h50KS (Tinius Olsen, Horsham, PA, USA) universal testing machine at 25 °C with a crosshead speed of 1 mm•min −1 according to ASTMD-638 standard.The sample dimension was 77 mm in length (working length 26 mm), 5 mm in width, and 1.5-2 mm in thickness.The values were taken as the average of three specimens.Dynamic mechanical analysis (DMA) was conducted in a Perkin Elmer DMA 8000 in the temperature range of −80 to 90 °C.The heating rate was 3 °C min −1 and the frequency and load strain were 1 Hz and 0.10 mm, respectively.Rectangular specimens of dimension 9mm × 9.6 mm × 2.2 mm were prepared for DMA measurements.Thermogravimetric

Characterization
The functionalization of MoS 2 was confirmed by Fourier transform infrared (FT-IR) spectra.FT-IR spectra were recorded with a Perkin Elmer RXI FT-IR (PerkinElmer, Inc., Waltham, MA, USA) in the range of 4000-400 cm −1 .The formation of MoS 2 , CTAB-MoS 2 , and exfoliation of functionalized MoS 2 in LLDPE were confirmed by X-ray diffraction (XRD).XRD was performed in PANalytical (Model X pert PRO) at a scan rate of 0.106 • s −1 in between the 2θ range of 5-60 • .Field emission scanning electron microscopy (FE-SEM)was performed using ∑igma HD, Carl Zeiss, Germany, to study the morphology of the CTAB-MoS 2 /LLDP composite fracture sample.The dispersion of functionalized MoS 2 in the LLDPE matrix was studied by transmission electron microscopy (TEM) with a JEOLTEM 2100 FS instrument (JEOL Ltd., Tokyo, Japan) at 200 kV.The ultrathin TEM sample of thickness ~70 nm of the composites was cut using a Leica Ultracut UCT (Leica Microsystems Inc., Buffalo Grove, IL, USA) ultramicrotome at room temperature.Microtomed composite samples were then collected over copper grids for TEM observation.The tensile measurements were performed using a Tinius Olsen h50KS (Tinius Olsen, Horsham, PA, USA) universal testing machine at 25 • C with a crosshead speed of 1 mm•min −1 according to ASTMD-638 standard.The sample dimension was 77 mm in length (working length 26 mm), 5 mm in width, and 1.5-2 mm in thickness.The values were taken as the average of three specimens.Dynamic mechanical analysis (DMA) was conducted in a Perkin Elmer DMA 8000 in the temperature range of −80 to 90 • C. The heating rate was 3 • C min −1 and the frequency and load strain were 1 Hz and 0.10 mm, respectively.Rectangular specimens of dimension 9mm × 9.6 mm × 2.2 mm were prepared for DMA measurements.Thermogravimetric analysis (TGA) was performed in a Jupiter STA 449 F1 (Netzsch, Germany) thermal analyser to study the thermal stability of the composites.The samples (~5.56 mg) were heated from 30 • C to 750 • C at a heating rate of 5 • C•min −1 in an air atmosphere.

Characterization of MoS 2 and CTAB-MoS 2
In order to confirm the functionalization of MoS 2 by long alkyl chain CTAB, FT-IR spectra analysis was performed.Figure 2a shows the FT-IR spectra of pure MoS 2 and CTAB-MoS 2 ; the two peaks observed for CTAB-MoS 2 at 2923 and 2853 cm −1 can be assigned to the C-H stretching vibrations of -CH 3 and -CH 2 groups present in CTAB, which are absent in pure MoS 2 prepared by a similar type of hydrothermal method.The appearance of peaks at 1459 and 712 cm −1 , attributed to the asymmetric and bending δ C-H vibrations of -CH 3 and -CH 2 , further validated the presence of long alkyl chains [27].To further confirm the modification of MoS 2 by CTAB, XRD was performed for CTAB-MoS 2 and pristine MoS 2 , and the corresponding reflection plane are shown in Figure 2b.For pristine MoS 2 , an intense reflection peak at 2θ = 14.48 • (corresponding to the interlayer distance of 0.62 nm) was observed, which was ascribed to the (002) reflection plane [18].In contrast, the (002) peak of CTAB-MoS 2 shifted to a lower 2θ (~7.5 • ).The shifting of the (002) peak towards a lower angle signifies that CTAB was successfully intercalated and attached on to the MoS 2 nanolayers, which led to enlargement of the interlayer spacing.The enlargement in interlayer spacing is considered to be advantageous for composite preparation as it would facilitate the intercalation of polymer within the gallery of 2D materials, thus leading to a robust interface.analysis (TGA) was performed in a Jupiter STA 449 F1 (Netzsch, Germany) thermal analyser to study the thermal stability of the composites.The samples (~5.56 mg) were heated from 30 °C to 750 °C at a heating rate of 5 °C•min −1 in an air atmosphere.

Characterization of MoS2and CTAB-MoS2
In order to confirm the functionalization of MoS2 by long alkyl chain CTAB, FT-IR spectra analysis was performed.Figure 2a shows the FT-IR spectra of pure MoS2 and CTAB-MoS2; the two peaks observed for CTAB-MoS2 at 2923 and 2853 cm −1 can be assigned to the C-H stretching vibrations of -CH3 and -CH2 groups present in CTAB, which are absent in pure MoS2 prepared by a similar type of hydrothermal method.The appearance of peaks at 1459 and 712 cm −1 , attributed to the asymmetric and bending δC-H vibrations of -CH3 and -CH2, further validated the presence of long alkyl chains [27].To further confirm the modification of MoS2 by CTAB, XRD was performed for CTAB-MoS2 and pristine MoS2, and the corresponding reflection plane are shown in Figure 2b.For pristine MoS2, an intense reflection peak at 2θ = 14.48° (corresponding to the interlayer distance of 0.62 nm) was observed, which was ascribed to the (002) reflection plane [18].In contrast, the (002) peak of CTAB-MoS2 shifted to a lower 2θ (~7.5°).The shifting of the (002) peak towards a lower angle signifies that CTAB was successfully intercalated and attached on to the MoS2 nanolayers, which led to enlargement of the interlayer spacing.The enlargement in interlayer spacing is considered to be advantageous for composite preparation as it would facilitate the intercalation of polymer within the gallery of 2D materials, thus leading to a robust interface.Figure 3a,b depicts the overview of the morphologies of pristine MoS 2 and CTAB-MoS 2 .Figure 3a shows the highly ordered tightly stacked FE-SEM image of hydrothermally prepared pristine MoS 2 .Upon functionlization, the highly ordered structure of MoS 2 was found to be disturbed as shown in Figure 3b.The red arrows (Figure 3b) point to the separated layers of MoS 2 particles.It was expected that CTAB assisted in the growth of multilayer MoS 2 particles.

Stress-Strain Behaviour Analysis
The optimum reinforcing potential of the layered structured filler can only be realized if efficient stress transfer takes place across the polymer-filler interface.Thus, homogenous dispersion in tandem with strong interfacial interaction between the filler and polymer matrix should be ensured.Tensile testing was carried out to evaluate the effect of CTAB-MoS2 on the mechanical properties of LLDPE. Figure 4a displays the typical stress-strain curves of pure LLDPE and its composites with different content of CTAB-MoS2.The composites showed enhanced tensile strength and a maximum improvement of 35% in tensile strength containing 2 wt % of CTAB-MoS2 loading.The improvement in tensile strength revealed the good reinforcing competency of MoS2.The modification of MoS2 with the moiety bearing long alkyl chains improved its good interfacial interaction with the LLDPE matrix, which in turn leads to the creation of strong junction cohesion suitable for efficient stress transfer across the interface.It is assumed that the long alkyl chains of the surfactant attached to MoS2 might have developed hydrophobic affinity with LLDPE matrix, leading to the robust interface.The Young's modulus for various LLDPE/CTAB-MoS2 composites derived from stress-strain curves are shown in Figure 4b.The LLDPE/CTAB-MoS2 composites containing 2 wt % CTAB-MoS2 showed highest improvement among the studied composites.An improvement of ~43.5% was recorded against neat LLDP, which was ascribed to the stiffness and physical structure of MoS2 particles.

Stress-Strain Behaviour Analysis
The optimum reinforcing potential of the layered structured filler can only be realized if efficient stress transfer takes place across the polymer-filler interface.Thus, homogenous dispersion in tandem with strong interfacial interaction between the filler and polymer matrix should be ensured.Tensile testing was carried out to evaluate the effect of CTAB-MoS 2 on the mechanical properties of LLDPE. Figure 4a displays the typical stress-strain curves of pure LLDPE and its composites with different content of CTAB-MoS 2 .The composites showed enhanced tensile strength and a maximum improvement of 35% in tensile strength containing 2 wt % of CTAB-MoS 2 loading.The improvement in tensile strength revealed the good reinforcing competency of MoS 2 .The modification of MoS 2 with the moiety bearing long alkyl chains improved its good interfacial interaction with the LLDPE matrix, which in turn leads to the creation of strong junction cohesion suitable for efficient stress transfer across the interface.It is assumed that the long alkyl chains of the surfactant attached to MoS 2 might have developed hydrophobic affinity with LLDPE matrix, leading to the robust interface.The Young's modulus for various LLDPE/CTAB-MoS 2 composites derived from stress-strain curves are shown in Figure 4b.The LLDPE/CTAB-MoS 2 composites containing 2 wt % CTAB-MoS 2 showed highest improvement among the studied composites.An improvement of ~43.5% was recorded against neat LLDP, which was ascribed to the stiffness and physical structure of MoS 2 particles.

Stress-Strain Behaviour Analysis
The optimum reinforcing potential of the layered structured filler can only be realized if efficient stress transfer takes place across the polymer-filler interface.Thus, homogenous dispersion in tandem with strong interfacial interaction between the filler and polymer matrix should be ensured.Tensile testing was carried out to evaluate the effect of CTAB-MoS2 on the mechanical properties of LLDPE. Figure 4a displays the typical stress-strain curves of pure LLDPE and its composites with different content of CTAB-MoS2.The composites showed enhanced tensile strength and a maximum improvement of 35% in tensile strength containing 2 wt % of CTAB-MoS2 loading.The improvement in tensile strength revealed the good reinforcing competency of MoS2.The modification of MoS2 with the moiety bearing long alkyl chains improved its good interfacial interaction with the LLDPE matrix, which in turn leads to the creation of strong junction cohesion suitable for efficient stress transfer across the interface.It is assumed that the long alkyl chains of the surfactant attached to MoS2 might have developed hydrophobic affinity with LLDPE matrix, leading to the robust interface.The Young's modulus for various LLDPE/CTAB-MoS2 composites derived from stress-strain curves are shown in Figure 4b.The LLDPE/CTAB-MoS2 composites containing 2 wt % CTAB-MoS2 showed highest improvement among the studied composites.An improvement of ~43.5% was recorded against neat LLDP, which was ascribed to the stiffness and physical structure of MoS2 particles.

Morphological and Structural Investigation
FE-SEM images of the tensile fractured sample were used to study the variations in the micro-morphology of the fracture surface of pure LLDPE and its composite with CTAB-MoS 2 .The different-magnification images of the fractured morphology of pure LLDPE and composites have been presented in Figure 5a-d.Subtle investigation of the FE-SEM images of pure LLDPE and composites exhibited variations in their surface morphologies.The neat LLDPE showed a comparatively smooth fracture surface, which signifies that the failure of the material took place at lower stress.On the contrary, the fracture surface of LLDPE composites displayed rough surfaces, which indicates improved resistance of the materials against load.The appearance of a pulled thread-like morphology confirms the strong interfacial interactions between MoS 2 and the LLDPE matrix, which accounts for the improved mechanical properties.
exhibited variations in their surface morphologies.The neat LLDPE showed a comparatively smooth fracture surface, which signifies that the failure of the material took place at lower stress.On the contrary, the fracture surface of LLDPE composites displayed rough surfaces, which indicates improved resistance of the materials against load.The appearance of a pulled thread-like morphology confirms the strong interfacial interactions between MoS2 and the LLDPE matrix, which accounts for the improved mechanical properties.X-ray diffraction is helpful to comprehend the degree of exfoliation of the filler in the polymer matrix.Figure 6 shows the X-ray diffraction patterns of pure LLDPE and CTAB-MoS2/LLDPE composites.The XRD pattern of pure LLDPE exhibited two main peaks at 2θ~21.49° and 23.75°, corresponding to the (110) and (200) planes, respectively [28].As the loading of the CTAB-MoS2 increased, the peaks at 2θ~21.49° and 23.75° for LLDPE were found to increase.However, no peaks corresponding to MoS2 were observed, thus indicating the complete exfoliation and distribution of MoS2 in the LLDPE composites.TEM micrographs of the composites demonstrate the intricate nature of the morphology of the dispersed MoS2 nanosheets.As can be seen from the TEM micrograph, complete exfoliations of MoS2 nanosheets are hardly observed.However, the diffused nature of the MoS2 nanosheets is observed in Figure 7. X-ray diffraction is helpful to comprehend the degree of exfoliation of the filler in the polymer matrix.Figure 6 shows the X-ray diffraction patterns of pure LLDPE and CTAB-MoS 2 /LLDPE composites.The XRD pattern of pure LLDPE exhibited two main peaks at 2θ~21.49• and 23.75 • , corresponding to the (110) and (200) planes, respectively [28].As the loading of the CTAB-MoS 2 increased, the peaks at 2θ~21.49• and 23.75 • for LLDPE were found to increase.However, no peaks corresponding to MoS 2 were observed, thus indicating the complete exfoliation and distribution of MoS 2 in the LLDPE composites.TEM micrographs of the composites demonstrate the intricate nature of the morphology of the dispersed MoS 2 nanosheets.As can be seen from the TEM micrograph, complete exfoliations of MoS 2 nanosheets are hardly observed.However, the diffused nature of the MoS 2 nanosheets is observed in Figure 7.The regions with highly concentrated MoS2 nanosheets are distinguished from the regions of low concentration of MoS2 nanosheets.The phase separation may also be due to the high stress induced by the diamond knife during sample preparation by ultramicrotomy [29].The TEM images did not reveal the sheet-like morphology of MoS2, which may be due to the low concentration of exfoliated MoS2 nanosheets.However, the dark areas are certainly generated due to the small thickness of debonded MoS2 nanosheets.

Dynamic Mechanical Analysis
DMA is a powerful investigation technique to study the interfacial interaction between the filler material and polymer matrix.DMA studies are sensitive to filler matrix interaction and the segmental motion of the polymer chains at the filler-matrix interfaces.The DMA were studied to quantify the mobility of the polymeric segments and to understand the interphase properties of pure epoxy and its composites with CTAB-MoS2.The results investigated in terms of storage modulus (E'), loss modulus (E"), and tan δ (loss factor) are shown in Figure 8a, Figure 8b, and Figure 8c, respectively.The dynamic mechanical properties of neat LLDPE and its composites were evaluated within the temperature range of −80 °C to 90°C. Figure 7a shows the temperature dependence of the storage modulus, E', and the corresponding values are summarized in Table 1.It quantifies the energy storing ability of the material under load.The E' value of the composites was found to be higher than those of neat LLDPE, irrespective of the range of temperature under study.The maximum enhancement of ~79% in the storage modulus of LLDPE/CTAB-MoS2 composites was recorded at 2 wt % of CTAB-MoS2.This indicated improved stiffness and restricted mobility of the polymeric chains.The significantly improved modulus suggested obstruction in the movements of polymer chains by The regions with highly concentrated MoS 2 nanosheets are distinguished from the regions of low concentration of MoS 2 nanosheets.The phase separation may also be due to the high stress induced by the diamond knife during sample preparation by ultramicrotomy [29].The TEM images did not reveal the sheet-like morphology of MoS 2 , which may be due to the low concentration of exfoliated MoS 2 nanosheets.However, the dark areas are certainly generated due to the small thickness of debonded MoS 2 nanosheets.The regions with highly concentrated MoS2 nanosheets are distinguished from the regions of low concentration of MoS2 nanosheets.The phase separation may also be due to the high stress induced by the diamond knife during sample preparation by ultramicrotomy [29].The TEM images did not reveal the sheet-like morphology of MoS2, which may be due to the low concentration of exfoliated MoS2 nanosheets.However, the dark areas are certainly generated due to the small thickness of debonded MoS2 nanosheets.

Dynamic Mechanical Analysis
DMA is a powerful investigation technique to study the interfacial interaction between the filler material and polymer matrix.DMA studies are sensitive to filler matrix interaction and the segmental motion of the polymer chains at the filler-matrix interfaces.The DMA were studied to quantify the mobility of the polymeric segments and to understand the interphase properties of pure epoxy and its composites with CTAB-MoS2.The results investigated in terms of storage modulus (E'), loss modulus (E"), and tan δ (loss factor) are shown in Figure 8a, Figure 8b, and Figure 8c, respectively.The dynamic mechanical properties of neat LLDPE and its composites were evaluated within the temperature range of −80 °C to 90°C. Figure 7a shows the temperature dependence of the storage modulus, E', and the corresponding values are summarized in Table 1.It quantifies the energy storing ability of the material under load.The E' value of the composites was found to be higher than those of neat LLDPE, irrespective of the range of temperature under study.The maximum enhancement of ~79% in the storage modulus of LLDPE/CTAB-MoS2 composites was recorded at 2 wt % of CTAB-MoS2.This indicated improved stiffness and restricted mobility of the polymeric chains.The significantly improved modulus suggested obstruction in the movements of polymer chains by

Dynamic Mechanical Analysis
DMA is a powerful investigation technique to study the interfacial interaction between the filler material and polymer matrix.DMA studies are sensitive to filler matrix interaction and the segmental motion of the polymer chains at the filler-matrix interfaces.The DMA were studied to quantify the mobility of the polymeric segments and to understand the interphase properties of pure epoxy and its composites with CTAB-MoS 2 .The results investigated in terms of storage modulus (E'), loss modulus (E"), and tan δ (loss factor) are shown in Figure 8a, Figure 8b, and Figure 8c, respectively.The dynamic mechanical properties of neat LLDPE and its composites were evaluated within the temperature range of −80 • C to 90 • C. Figure 7a shows the temperature dependence of the storage modulus, E', and the corresponding values are summarized in Table 1.It quantifies the energy storing ability of the material under load.The E' value of the composites was found to be higher than those of neat LLDPE, irrespective of the range of temperature under study.The maximum enhancement of ~79% in the storage modulus of LLDPE/CTAB-MoS 2 composites was recorded at 2 wt % of CTAB-MoS 2 .This indicated improved stiffness and restricted mobility of the polymeric chains.The significantly improved modulus suggested obstruction in the movements of polymer chains by firmly embedded CTAB-MoS 2 .Furthermore, the improvement of the storage modulus in the solid state can be correlated with the improved mechanical properties and impact strength of the composites.It means that incorporation of CTAB-MoS 2 enhanced the impact resistance of LLDPE composites.It was assumed that the presence of the hydrophobic long alkyl chains in CTAB-MoS 2 played a crucial role in enhancing the compatibility of MoS 2 within the LLDPE matrix, which ameliorated the interaction between the two constituents, thereby inhibiting the segmental motion of the polymer chains.Figure 8b shows the variation of tan δ (E"/E') and the loss modulus (E") of the CTAB-MoS 2 / LLDPE composites with temperature.The glass transition temperature (T g ) of the polymer composite system can be deduced from the peak position of the tan δ (E"/E') curve.However, at times, it becomes difficult to derive the exact maxima value due to the elusive nature of the curves [30].In such instances, the peak position of the E" curve could be assumed as the probable T g of the polymer matrix in the composites.The E'' value signifies the quantity of unrecoverable energy dissipation per cycle, and it was found to increase upon the incorporation of CTAB-MoS 2 , which may be ascribed to the increase in friction between the MoS 2 and LLDPE matrix.
firmly embedded CTAB-MoS2.Furthermore, the improvement of the storage modulus in the solid state can be correlated with the improved mechanical properties and impact strength of the composites.It means that incorporation of CTAB-MoS2 enhanced the impact resistance of LLDPE composites.It was assumed that the presence of the hydrophobic long alkyl chains in CTAB-MoS2 played a crucial role in enhancing the compatibility of MoS2 within the LLDPE matrix, which ameliorated the interaction between the two constituents, thereby inhibiting the segmental motion of the polymer chains.Figure 8b shows the variation of tan δ (E"/E') and the loss modulus (E") of the CTAB-MoS2/LLDPE composites with temperature.The glass transition temperature (Tg) of the polymer composite system can be deduced from the peak position of the tan δ (E"/E') curve.However, at times, it becomes difficult to derive the exact maxima value due to the elusive nature of the curves [30].In such instances, the peak position of the E" curve could be assumed as the probable Tg of the polymer matrix in the composites.The E'' value signifies the quantity of unrecoverable energy dissipation per cycle, and it was found to increase upon the incorporation of CTAB-MoS2, which may be ascribed to the increase in friction between the MoS2 and LLDPE matrix.

Thermogravimetric Analysis
The thermal degradation behaviour of LLDPE and its composites were investigated by TGA.The TGA thermograms of LLDPE and CTAB-MoS2/LLDPE composites performed under air atmosphere are shown in Figure 9.The initial decomposition of CTAB-MoS2/LLDPE composites,

Thermogravimetric Analysis
The thermal degradation behaviour of LLDPE and its composites were investigated by TGA.The TGA thermograms of LLDPE and CTAB-MoS 2 /LLDPE composites performed under air atmosphere are shown in Figure 9.The initial decomposition of CTAB-MoS 2 /LLDPE composites, which was evaluated from the temperatures corresponding to 5 and 10 wt % weight loss (T-5% and T-10% ) was found to be decreased as compared to pure LLDPE.The onset degradation temperature of the composites was further found to decrease with the increased in the loading of CTAB-MoS 2 .When the content of CTAB-MoS 2 was increased to 3.0 wt %, the temperature corresponding to 5.0 wt % mass loss (T-5% ) was decreased by ~20 • C as compared to that of pure LLDPE.The reduction in onset degradation temperature could be ascribed to the decomposition of the CTAB and to the high thermal conductivity of MoS 2 , which could have assisted in the easy delivery of heat.The char formation during thermal decomposition of the materials is a crucial parameter to judge the materials' competency in real-life applications.The char residue was found to be promoted in the presence of CTAB-MoS 2 ; the char residue until 750 • C for all the composites showed an increase as compared to pure LLDPE (as shown in the inset of Figure 9).Thus, CTAB-MoS 2 could have acted as a catalyst to promote the formation of the char residue during degradation [16,31].In addition, the presence of inorganic MoS 2 contributed to the formation of char residue, due to its nondegradable nature.The formation of a char is expected to impede the out-diffusion of the decomposed products by forming a protective layer, which consequently promotes the thermal stability of the composites.
J. Compos.Sci.2018, 2, x 9 of 11 10%) was found to be decreased as compared to pure LLDPE.The onset degradation temperature of the composites was further found to decrease with the increased in the loading of CTAB-MoS2.When the content of CTAB-MoS2 was increased to 3.0 wt %, the temperature corresponding to 5.0 wt % mass loss (T-5%) was decreased by ~20 °C as compared to that of pure LLDPE.The reduction in onset degradation temperature could be ascribed to the decomposition of the CTAB and to the high thermal conductivity of MoS2, which could have assisted in the easy delivery of heat.The char formation during thermal decomposition of the materials is a crucial parameter to judge the materials' competency in real-life applications.The char residue was found to be promoted in the presence of CTAB-MoS2; the char residue until 750 °C for all the composites showed an increase as compared to pure LLDPE (as shown in the inset of Figure 9).Thus, CTAB-MoS2 could have acted as a catalyst to promote the formation of the char residue during degradation [16,31].In addition, the presence of inorganic MoS2 contributed to the formation of char residue, due to its nondegradable nature.The formation of a char is expected to impede the out-diffusion of the decomposed products by forming a protective layer, which consequently promotes the thermal stability of the composites.

Conclusions
Motivated by the wide array of applications of graphene, in this study, 2-D layered structure MoS2 was explored as a filler for a LLDPE matrix.Surfactant CTAB was used to assist the preparation of exfoliated MoS2 and also as modifier.CTAB-MoS2/LLDPE composites were prepared following an industrially viable and environment friendly melt-mixing technique.FT-IR spectra analysis was used to confirm the embedment of CTAB on the surface of MoS2.The presence of long alkyl chains helped to generate strong junction cohesion, which was reflected by the improvement of mechanical properties.The tensile strength of the composites was found to improve by ~35% at 2 wt % of CTAB-MoS2 loadings.The substantial improvement in the storage modulus value confirmed the role of CTAB in improving the interfacial interaction of MoS2 within the LLDPE matrix.A maximum improvement of ~79% in the storage modulus at the loading of 2 wt % was recorded.The improved storage modulus signified the superior interfacial interaction between the modified MoS2 and LLDPE matrix.The thermal stability of the CTAB-MoS2/LLDPE composites investigated by TGA showed a small decrease in the onset degradation temperature.Nevertheless, the char residue formation was found to improve in the presence of MoS2.Although the improvements in the end properties of the polymer composites might be lessened when processed by melt mixing, as compared to solution mixing, this simple strategy may be promising for developing mechanically strong composites.

Conclusions
Motivated by the wide array of applications of graphene, in this study, 2-D layered structure MoS 2 was explored as a filler for a LLDPE matrix.Surfactant CTAB was used to assist the preparation of exfoliated MoS 2 and also as modifier.CTAB-MoS 2 /LLDPE composites were prepared following an industrially viable and environment friendly melt-mixing technique.FT-IR spectra analysis was used to confirm the embedment of CTAB on the surface of MoS 2 .The presence of long alkyl chains helped to generate strong junction cohesion, which was reflected by the improvement of mechanical properties.The tensile strength of the composites was found to improve by ~35% at 2 wt % of CTAB-MoS 2 loadings.The substantial improvement in the storage modulus value confirmed the role of CTAB in improving the interfacial interaction of MoS 2 within the LLDPE matrix.A maximum improvement of ~79% in the storage modulus at the loading of 2 wt % was recorded.The improved storage modulus signified the superior interfacial interaction between the modified MoS 2 and LLDPE matrix.The thermal stability of the CTAB-MoS 2 /LLDPE composites investigated by TGA showed a small decrease in the onset degradation temperature.Nevertheless, the char residue formation was found to improve in the presence of MoS 2 .Although the improvements in the end properties of the polymer composites might be lessened when processed by melt mixing, as compared to solution mixing, this simple strategy may be promising for developing mechanically strong composites.

Figure 1 .
Figure 1.Schematic for the functionalization of MoS2 by cetyltrimethylammonium bromide (CTAB) and preparation of linear low-density polyethylene (LLDPE) composites by melt mixing.

Figure 1 .
Figure 1.Schematic for the functionalization of MoS 2 by cetyltrimethylammonium bromide (CTAB) and preparation of linear low-density polyethylene (LLDPE) composites by melt mixing.

Figure
Figure3a,b depicts the overview of the morphologies of pristine MoS2 and CTAB-MoS2.Figure3ashows the highly ordered tightly stacked FE-SEM image of hydrothermally prepared pristine MoS2.Upon functionlization, the highly ordered structure of MoS2 was found to be disturbed as shown in Figure3b.The red arrows (Figure3b) point to the separated layers of MoS2 particles.It was expected that CTAB assisted in the growth of multilayer MoS2 particles.

Figure 4 .
Figure 4. Stress-strain curve of neat LLDPE and its composites with CTAB-MoS2 at various loadings (a) and variation of Young's modulus with weight percentage of CTAB-MoS2 (b).
FE-SEM images of the tensile fractured sample were used to study the variations in the micromorphology of the fracture surface of pure LLDPE and its composite with CTAB-MoS2.The differentmagnification images of the fractured morphology of pure LLDPE and composites have been presented in Figure5a-d.Subtle investigation of the FE-SEM images of pure LLDPE and composites

Figure 4 .
Figure 4. Stress-strain curve of neat LLDPE and its composites with CTAB-MoS2 at various loadings (a) and variation of Young's modulus with weight percentage of CTAB-MoS2 (b).
FE-SEM images of the tensile fractured sample were used to study the variations in the micromorphology of the fracture surface of pure LLDPE and its composite with CTAB-MoS2.The differentmagnification images of the fractured morphology of pure LLDPE and composites have been presented in Figure5a-d.Subtle investigation of the FE-SEM images of pure LLDPE and composites

Figure 4 .
Figure 4. Stress-strain curve of neat LLDPE and its composites with CTAB-MoS 2 at various loadings (a) and variation of Young's modulus with weight percentage of CTAB-MoS 2 (b).

Table 1 .
Storage modulus, E', of neat LLDPE and its composites at different temperatures.

Table 1 .
Storage modulus, E', of neat LLDPE and its composites at different temperatures.