Effect of Reduced Graphene Oxide on Curing, Mechanical, and Thermal Properties of Polymethylene Tetrasulfide

Abstract

Polymers have long been utilized in various industries due to their unique properties. Among the family of polymers, polysulfides are popular due to their strong adhesion and high resistance to fuels and solvents, and have been utilized in specific applications. In this study, polysulfide nanocomposites were prepared using methylene dichloride (MD), sodium tetrasulfide (Na₂S₄), and reduced graphene oxide (rGO) and then cured using a rheometer. Polymethylene tetrasulfide (PMTS) and nanocomposites were evaluated by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Also, the cured samples were evaluated using FTIR, XRD, SEM, thermogravimetric analysis (TGA), and tensile test. The results showed that PMTS has a completely amorphous structure. XRD and SEM results showed that with the addition of rGO, free sulfur accumulates in the matrix, which participates in the reaction during the curing process. The cured polymethylene tetrasulfide (CPMTS) and the cured nanocomposites have a completely amorphous structure. Also, the presence of rGO improved the final properties of the product.

1. Introduction

Sulfur is abundant on Earth and has been mined and studied for many years for various applications, but it is still at the center of research topics due to its extraordinary chemical versatility [1,2]. It can be said that sulfur is used in various applications, including the preparation of sulfuric acid, food additives, water treatment, fertilizers, detergents, dyes and pigments, paper production, medicinal, and polysulfide polymers [3,4]. Additionally, during the advent of batteries, sulfur played a crucial role [5]. As mentioned, the preparation of polysulfide polymers is one of the uses of sulfur. These polymers are popular due to their good adhesion and resistance to many solvents and UV radiation, are used in applications such as hoses, icephobic and superhydrophobic coatings, and are also very useful for removing heavy metals (due to their affinity for these metals) [6,7]. It can be said that the adhesives industry is very interested in polysulfides because they have self-healing properties thanks to their high sulfur content (due to dynamic covalent bonds) [8]. To improve their properties, nanofillers such as clay, graphene, carbon nanotubes, and silicon dioxide are used [9,10,11,12]. Pirayesh et al. [13] used ethylenediamine-modified graphene to co-cure epoxy-terminated polysulfide prepolymer. The polysulfide was prepared by the reaction of epichlorohydrin with Na₂S₄. The results showed that the thermal stability, hardness, and tensile properties improved with increasing graphene content up to 0.7 wt%. Kariminejad et al. [14] prepared polysulfide nanocomposites using ethylene dichloride, Na₂S₄, graphene oxide, and rGO. The results showed that the addition of nanosheets increased the thermal stability, and the rGO was also more appropriately dispersed in the matrix. Wang et al. [15] prepared an epoxy resin with a disulfide-containing curing agent through the condensation of dimethyl 3,3′-dithiodipropionate and polyether amine. The molar ratio of the reactants directly affected the mechanical performance and self-healing property, such that when the molar ratio of polyether amine to dimethyl 3,3′-dithiodipropionate was 2, the final sample had a final elongation of 795% and a repair efficiency of 98%.

There are different methods for preparing polymers, and it can be said that one of the good yet simple methods for preparing polysulfide polymers is interfacial polymerization, which is used in addition to preparing polymers for membranes, thin films, and fibers [16,17,18]. Briefly, this method involves two immiscible phases, with each monomer in one phase, and polymerization occurs at the interface between the two phases [19,20].

Herein, the aim was to prepare a flexible, amorphous polysulfide polymer rich in sulfur with minimal carbon. This was prepared by the reaction between MD and Na₂S₄. The rGO was also used to improve the properties of the polymer. Next, a series of additives were added to the nanocomposites, and the samples were cured by a rheometer. Subsequently, the samples were investigated in terms of morphology and structure, and their thermal resistance and mechanical properties were also evaluated.

2. Materials and Methods

2.1. Materials

MD, sulfur, N-cyclohexyl-2-benzothiazole sulfenamide (CBS), sodium hydroxide (NaOH), stearic acid, zinc oxide (ZnO), and other chemicals were purchased from Merck Chemicals (Darmstadt, Germany). Graphite and l-ascorbic acid were obtained from Sigma-Aldrich (St. Louis, MO, USA).

2.2. Synthesis of rGO

Briefly, graphite powder (2 g) was mixed with potassium permanganate (12 g) in sulfuric acid solution (200 mL) at 75 °C for 4 h; then, distilled water was added, and the reaction was continued for another 15 min. Finally, hydrogen peroxide solution was added, and washing was performed using hydrogen chloride and distilled water. The obtained graphite oxide was dispersed by sonication, l-ascorbic acid was added to it, and continuous stirring was carried out (4 h at 95 °C). Then, the pH of the solution was neutralized and the product was dried.

2.3. Synthesis of Na₂S₄, Polymer, and Nanocomposites

To prepare Na₂S₄, NaOH (16 g) was dissolved in distilled water (400 mL), and after raising the temperature to its boiling point, sulfur (25 g) was added and the reaction was continued for 1 h [6]. To prepare the polymer, 100 mL Na₂S₄ solution was added to a 250 mL four-necked round-bottom flask equipped with a stirrer, dropping funnel, condenser, and thermometer. Then, the flask was heated to 37 °C (with continuous stirring at 700 rpm) and 20 mL of MD was gradually added via a dropping funnel (during 1 h). The same procedure was used in the synthesis of nanocomposites, but before the addition of MD, rGO dispersed in distilled water (20 mL) by ultrasonication was added to the Na₂S₄ solution. The prepared solution contained 0.1, 0.5, and 1 mass% rGO, respectively.

2.4. Curing Process

Sulfur (2 phr), ZnO (5 phr), stearic acid (1.5 phr), and CBS (1 phr) were used to cure and prepare the samples. Also, the matrix content (PMTS/nanocomposite) was 100 phr. The samples were mixed at ambient temperature for about 20 min and then cured by rheometer at 140 °C.

2.5. The Measurements and Characterization

FT-IR was an Equinox 55 (Bruker, Billerica, MA, USA) spectrometer. The Xpert Pro MPD (Panalytical, Almelo, The Netherlands) diffractometer was used for XRD measurements. SEM images were provided using a Tescan VEGA-II (Brno, Czech Republic). The TGA properties were obtained using Perkin Elmer STA 6000 (Waltham, MA, USA), respectively. Also, a nitrogen atmosphere and a heating rate of 10 °C were used. Mechanical properties were measured at a speed of 500 mm/min (ASTM D412) [21] using a Hiwa 200 (Hiwa, Teheran, Iran) universal testing machine. Also, a Hielscher UP400S homogenizer (20 kHz, 400 W, Teltow, Germany) was used. Moreover, a laboratory-sized Rodolfo Comerio two-roll mill (Solbiate Olona, Italy) was used to compound the samples. The cure of the compounds was prepared using torque curves using a Hiwa 900 (Hiwa, Teheran, Iran) moving die rheometer according to ASTM D5289 [22] at 140 °C.

3. Results and Discussion

Figure 1 shows the FT-IR results of PMTRS and rGO. As shown in the FT-IR spectrum of PMTRS, peaks at around 668, 809, 1013, 1166, 1365, 2544, and 2954 cm⁻¹ correspond to the C-S vibrations, C-H in-plane bending vibrations, C-C stretching vibrations, -CH₂ stretching vibrations, C-H in-plane scissoring vibrations, stretching vibrations of S-H groups, and C-H symmetric stretching vibrations, respectively [3,6]. Also, peaks at around 1646 and 3386 cm⁻¹ are related to the -OH stretching vibration [6]. Also, in the rGO spectrum, the peaks at around 1123, 1384–1579, and 2854–2923 cm⁻¹ correspond to the C-O vibrations, C=C vibrations, and sp³ C-H bonding, respectively [23,24]. Also, the peak at around 3432 cm⁻¹ is attributed to symmetric vibrations of -OH units [6]. Figure 2 shows the FT-IR spectrum of nanocomposites. The results obtained show the peaks of the functional groups in PMTRS and the bands related to the vibrational mode of rGO. But the intensity of the peak related to the S-H groups in the sample containing 0.1% rGO is greatly reduced, and this peak disappeared in the other two samples (see Figure 2b). Also, the intensity of the peak related to C-S groups in all nanocomposite samples reduced by almost half (see Figure 2c). The rGO has a polyhedral construction around its sheets; in fact, this is due to the many defects and porous structures on the surface of the rGO sheet, which lead to a high surface area; this structure can host sulfur, and the presence of under-coordinated carbon atoms at the edges can cause interactions between sulfur and carbon atoms [25,26]. In general, in the polymerization of polysulfide polymers, the yield increases as the number of carbon atoms increases [3,6]. In fact, the polymerization of monomers with one carbon atom gives the lowest yield [3]. It seems that sulfur tends to interact more with rGO and participate less in the polymerization reaction. In fact, the presence of rGO seems to have partially prevented the polymerization, and there is a lot of sulfur (free sulfur) at the edges of rGO and the matrix. The XRD pattern (see Figure 3) shows that PMTRS has a broad peak around 22.5°, which is observed in most polysulfide polymers and is related to the amorphous nature of polysulfide rubber chains [6]. Sulfur content is important in determining the amount of crystallinity because disulfide bonds can affect the chain unfolding process by reducing conformational chain entropy [3]. Hence, crystallinity decreases with increasing sulfur content. Also, in the XRD pattern of rGO, a peak at 2θ (°) = 28.3° is observed. The XRD patterns of the nanocomposites show that with increasing rGO content in the samples, peaks appear at around 2θ (°) = 28–48°, to the point where the intensity of the peaks increases clearly in the sample containing 0.5% rGO. These peaks are related to free sulfur in the samples. These results confirm the topics mentioned in the FT-IR section. This behavior is related to the presence of sulfur on the edges of rGO. Probably, the presence of sulfur on rGO prevented the interaction of PMTRS macromolecules and the interface region on the surface of rGO. It can be said that the observed peaks are related to free sulfur in the matrix.

The morphology of the samples is shown in Figure 4. The surface of the PMTRS fracture area is smooth; however, upon rGO loading, a significant change in morphology was observed. In the sample containing 0.1% rGO, sulfur crystals appeared, some of which have rGO on their surface. Additionally, in certain areas of the matrix, a layer of sulfur can be observed on the rGO sheets. By increasing the content of rGO to 0.3%, the presence of sulfur crystals increased, and to some extent, rGO sheets tended to surround sulfur crystals (see Figure 4e,f). In the sample containing 0.5% rGO, the morphology was completely different from the previous two samples. On the surface of the matrix, lumps are observed, which are composed of rGO and sulfur crystals. This shows the amazing tendency for sulfur crystals to be surrounded by rGO sheets. However, free sulfur (not surrounded by rGO) is still observed in the matrix (see the yellow arrow in Figure 4g). The SEM images confirm the results for the FTIR and XRD sections.

The results of the sample curing are shown in Figure 5 and

Table 1. Cure characteristics of prepared compounds.

Compounds	Cure Temperature (°C)	ML (μNm)	MH (μNm)	tscorch (Min)	t10 (Min)	t50 (Min)	t90 (Min)	tcure (Min)
PMTS	140	319	929	10.1	12.6	22.4	32.2	34.6
PMTS/0.1% rGO	140	322	940	10.8	13.3	23.2	33.1	35.5
PMTS/0.3% rGO	140	331	965	12.9	15.5	25.8	36.2	38.7
PMTS/0.5% rGO	140	339	980	15.3	17.8	27.8	37.8	40.3

Note: ML: minimum torque, MH: maximum torque.

. The results show that with the presence and increase in rGO content, the ML and MH increase, which is due to the increased resistance of macromolecules to mobility [12]. As expected, the longest curing time is for PMTS/0.5% rGO because it has a higher graphene content. For better understanding, the curing times are shown in Figure 6. As can be seen, the curves are not linear with increasing curing time; in fact, the graphs for PMTS/0.5% rGO and PMTS/0.3% rGO move away from PMTS/0.1% rGO after t₅₀. Also, the difference in curing time between PMTS/0.5% rGO and PMTS/0.3% rGO samples is 1.6 min, but this time is 3.2 min for PMTS/0.3% rGO and PMTS/0.1% rGO samples. The rGO can act as a thermal shield for the matrix and, on the other hand, increase the thermal conductivity of the sample [23,27]. This change in behavior in the samples could be due to both the increase in rGO content and the free sulfur present in the samples due to the presence of rGO. As previously mentioned, free sulfur increased with increasing rGO content in the samples (see XRD and SEM discussion), and in the PMTS/0.5% rGO and PMTS/0.3% rGO samples, rGO somehow covered them; therefore, it is likely that with the start of the curing process, rGO prevents the necessary heat from reaching them, but with increasing curing time, the necessary heat from rGO reaches the free sulfur and they start to react.

Figure 7 shows the XRD patterns of the cured samples. In all samples, the broad peak at around 22.5° is still observed, but in the nanocomposites, the peaks related to free sulfur have disappeared, which is due to their participation in the cured reaction.

Figure 8 shows SEM images of CPMTS and CPMTS/0.5% rGO samples. It is observed that after curing, the fracture surface is very rough (for both samples). This change is due to the cross-linking that occurs after curing, which increases the sample’s resistance to fracture in liquid nitrogen [12].

Figure 9 shows the thermal degradation performance of the cured samples. The thermal decomposition temperatures of CPMTS, CPMTS/0.1% rGO, CPMTS/0.3% rGO, and CPMTS/0.5% rGO were about 310, 325, 335, and 342 °C, respectively. In the literature, the thermal decomposition temperature for PMTS is reported to be 262 °C [28], but it is observed that after the curing process, due to the formation of cross-links, this temperature increased to 310 °C. Also, in nanocomposites, the presence of rGO increased the thermal resistance due to the formation of interfacial interactions.

Table 2. Mechanical characteristics of cured samples.

Samples	Elongation at Break (%)	Strength (MPa)	Elastic Modulus (MPa)
CPMTS	284.4 ± 1	2.1 ± 0.1	40.2 ± 1
CPMTS/0.1% rGO	280.3 ± 1	2.3 ± 0.1	43.4 ± 1
CPMTS/0.3% rGO	274.5 ± 1	2.7 ± 0.1	47.1 ± 1
CPMTS/0.5% rGO	264.4 ± 1	3.3 ± 0.1	52.4 ± 1

reports the results of tensile tests of the samples. Non-cross-linked polysulfide polymers have poor tensile strength because they have weak molecular interactions, so they must undergo a curing process to improve tensile properties [12]. Typically, polysulfide polymers contain high amounts of sulfur, and this sulfur content plays a very decisive role in the behavior of the polymer [6]. In fact, S-S groups are very flexible, and as their content increases, the flexibility of the polymer increases [3,6]. The results in Table 2 show that with the addition of rGO to the matrix and also with increasing rGO content, the elastic modulus and strength increased, which is due to proper interaction between rGO and the matrix. Also, a significant difference is observed in the properties when comparing the CPMTS/0.3% rGO and CPMTS/0.5% rGO samples, which could be due to the cross-links created by the free sulfur (see the XRD and SEM sections of uncured samples) present in the matrix.

4. Conclusions

In summary, polysulfide nanocomposites based on MD, Na₂S₄, and rGO were prepared and cured. The results showed that with the presence and increase in rGO in the matrix, the content of free sulfur in the matrix also increased, and after the curing process, free sulfur was absentin the matrix due to participation in the reaction. The evaluation of the curing process showed that with increasing rGO in the matrix, the ML and MH increased. TGA results of the cured samples showed that adding 0.5% rGO to the matrix increased the degradation temperature to 32 °C. Also, the highest strength and elastic modulus were found for the sample containing 0.5% rGO. According to the results obtained from the rheometer and TGA, it can be said that rGO can both act as a thermal shield for the matrix and, on the other hand, increase the thermal conductivity of the sample.