Exploration of Mechanical and Thermal Properties of CTAB-Modified MoS₂/LLDPE Composites Prepared by Melt Mixing

Abstract

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.

1. 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 (C₃N₄), and so forth. [1]. Among these, molybdenum disulfide (MoS₂), 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 MoS₂ 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 MoS₂ possesses high modulus of ~300 GPa and has proven to be a very efficient filler in reinforcing polymer matrices [10,11]. In recent times, MoS₂ 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 MoS₂ [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, MoS₂ does not endow electrical conductivity to the host polymer matrix, so MoS₂-based polymer composites may be befitting alternatives to conducting graphene-based composites.

The exciting properties of MoS₂ nanosheets are associated with its 2-D morphology and its atomic thickness. However, in order to transfer the intrinsic properties of MoS₂ into a polymer matrix, homogenous dispersion and distribution of MoS₂ 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 MoS₂ could be one of the plausible ways to improve the interaction, compatibility, and dispersion of MoS₂ in organic polymer systems. Hence, the preparation of mono- or multilayer MoS₂ and its functionalization is crucial to realize the full reinforcing potential. Many prior reports used functionalized MoS₂ nanosheets to reinforce the polymer matrix, and it has been observed that suitably tuned MoS₂ 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 MoS₂ by direct ultrasonication of bulk MoS₂ and prepared the polyethylene nanocomposites by incorporating defect-free modified MoS₂ [9]. The study revealed improved mechanical and thermal properties upon incorporation of MoS₂. Typically, solution-based exfoliation techniques are employed to generate MoS₂ nanosheets. The exfoliation-based technique consumes large amount of organic solvents and requires prolonged sonication, which may deform the MoS₂ structure and create defects. So, concurrent preparation and functionalization of MoS₂ through a one-pot method could be one of the effective ways to achieve defect-free MoS₂ sheets. The intent of this work is to shed light on the effect of functionalized MoS₂ 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 MoS₂ (denoted as CTAB–MoS₂) and the subsequent fabrication of CTAB–MoS₂/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 (MoO₄²⁻) formed during the synthesis of MoS₂ and assist in the formation of mono- or multilayer MoS₂ particles, and additionally, help to inhibit the restacking or agglomeration of MoS₂ 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.

2. Materials and Methods

2.1. Materials

Sodium molybdate (Na₂MoO₄) and thiourea (CH₄N₂S) 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.

2.2. CTAB Assisted Synthesis of MoS₂

The CTAB-embedded MoS₂ nanosheets were prepared by a facile hydrothermal route as reported in our earlier work [18]. Typically, ~1.2 g of Na₂MoO₄∙2H₂O and 3.2 g of CH₄N₂S 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–MoS₂. The pristine MoS₂ was prepared following the similar procedure, albeit without using CTAB.

2.3. 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.

2.4. 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 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.

3. Results and Discussion

3.1. 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 δ_C–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 MoS₂ and CTAB–MoS₂. Figure 3a shows 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 Figure 3b. The red arrows (Figure 3b) point to the separated layers of MoS₂ particles. It was expected that CTAB assisted in the growth of multilayer MoS₂ particles.

3.2. 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.

3.3. 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–MoS₂. 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 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.

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.

3.4. 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. Storage modulus, E’, of neat LLDPE and its composites at different temperatures.

Sample Code	E’ (MPa) −78 °C	E’ (MPa) −50 °C	E’ (MPa) 5 °C	E’ (MPa) 60 °C
Pure LLDPE	1560	1291	632	250
0.75 wt %	2444	2117	1059	401
2 wt %	2797	2362	1167	451
3 wt %	1345	2025	979	371

. 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 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 (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–MoS₂, which may be ascribed to the increase in friction between the MoS₂ and LLDPE matrix.

3.5. 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, 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–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.

4. 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.