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Article

A New Sulfur-Containing Copolymer Created Through the Thermally Induced Radical Copolymerization of Elemental Sulfur with N2,N2-Diallylmelamine Comonomer for Potential CO2 Capture

by
Dharrinesh Narendiran
1,
Nurul Hazirah Sumadi
1,
Ali Shaan Manzoor Ghumman
2,
Noor Ashikin Mohamad
1,
Mohamed Mahmoud Nasef
1,3,*,
Amin Abbasi
4,* and
Rashid Shamsuddin
5
1
Department of Chemical and Environmental Engineering, Malaysia–Japan International Institute of Technology, Universiti Teknologi Malaysia, Kuala Lumpur 54100, Malaysia
2
Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak Darul Ridzuan, Malaysia
3
Advanced Materials Research Group, Center of Hydrogen Energy, Universiti Teknologi Malaysia, Kuala Lumpur 54100, Malaysia
4
PRISM Research Institute, Technological University of the Shannon: Midlands Midwest, Athlone Campus, University Road, N37 HD68 Athlone, Ireland
5
Department of Chemical Engineering, Faculty of Engineering, Islamic University of Madinah, Madinah 42351, Saudi Arabia
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(7), 362; https://doi.org/10.3390/jcs9070362
Submission received: 26 May 2025 / Revised: 3 July 2025 / Accepted: 9 July 2025 / Published: 11 July 2025
(This article belongs to the Special Issue Polymer Composites: Fiber Architecture, Interfacial Engineering, and Processing)
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Abstract

Sulfur-containing polymers are unique sustainable materials with promise for the development of various adsorbents for environmental remediation. However, they have not been explored for CO2 capture despite reports on its ability to decontaminate various aqueous pollutants. This study reports on the single-step synthesis of a diamine-functionalized sulfur-containing copolymer by the thermally induced radical copolymerization of N2,N2-Diallylmelamine (NDAM), a difunctional monomer, with sulfur and explores its use for CO2 capture. The influence of reaction parameters such as the weight ratios of sulfur to NDAM, reaction temperature, time, and the addition of a porogen on the properties of aminated copolymer was investigated. The resulting copolymers were characterized using FTIR, TGA, DSC, SEM, XRD, and BET surface area analyses. The incorporation of NDAM directly imparted amine functionality while stabilizing the polysulfide chains by crosslinking, leading to a thermoset copolymer with an amorphous structure. The addition of a NaCl particle porogen to the S/NDAM mixture generated a mesoporous structure, enabling the resulting copolymer to be tested for CO2 adsorption under varying pressures, leading to an adsorption capacity as high as 517 mg/g at 25 bar. This work not only promotes sustainable hybrid materials that advance green chemistry while aiding CO2 mitigation efforts but also adds value to the abundant amount of sulfur by-products from petroleum refineries.

1. Introduction

In the past few decades, there has been increasingly more attention paid to sustainable development in industries, specifically seeking methods to utilize waste and by-products. This is to reduce the environmental burden and build a circular economy through the transformation of items that would otherwise be discarded into useful products [1]. For instance, elemental sulfur, a by-product of oil and gas refineries produced in millions of tons, is stored in huge stockpiles that are often poorly managed [2]. This leads to major threats to the environment through sulfur leaking into water bodies and being blown into the air as particulates by the wind, affecting human health and disturbing the ecosystem [3]. Thus, elemental sulfur has attracted recent interest as a sustainable feedstock for designing functional materials.
Sulfur-containing copolymers have recently emerged as a novel class of materials with immense potential for substituting petrochemical polymers and the development of advanced eco-friendly materials with tunable properties across various applications [4,5]. These innovative materials leverage the ability of sulfur with its cyclo-octasulfur ring to undergo S-S bond homolytic cleavage to form thiyl radicals (linear S8 atom with terminal bi-radicals) when heated to the molten state, enabling radical polymerization reactions with a variety of vinylic, allylic, and polyfunctional comonomers under solvent- and catalyst-free conditions [6]. This unique polymerization process, also known as inverse vulcanization, imparts exceptional properties to the polysulfides, including electrochemical [7], photocatalytic [8], optical (high refractive index and infrared light transmittance) [9,10,11], self-healing [12], antimicrobial [13,14], and anticorrosive [15] activities; thermo-responsivity [16]; control–release abilities [17]; oleophilicity [18]; the ability to plasticize polymers [19]; adhesion abilities [20]; interfacial modification abilities [21]; thermomechanical and flame retardancy [22]; and affinity to heavy metals [23,24,25] and boron [26]. This allowed for the transformation of oil and gas refinery by-products (i.e., sulfur) into versatile materials that can be designed to contribute to progress in diverse fields such as energy storage; camera, infrared, and thermal imaging applications; decorative plastics; photocatalysis; fertilizers; biomedicine and health care; wastewater treatment (removal of heavy metals and boron, and oil spill cleanup); and many more, as discussed in recent review articles [23,27,28,29,30].
The synthesis of sulfur-containing copolymers has been carried out using various types of alkene-containing comonomers originating from petroleum [27], fatty acids/triglycerides [31], and renewable sources [4,32]. Several petroleum-based vinylic comonomers have been copolymerized with sulfur using inverse vulcanization on many occasions. Styrene [33], vinylbenzyl chloride [34,35], and N-methylimidazole [36] were used to prepare sulfur-containing copolymers with distinctive properties dedicated to desired applications.
The reaction of sulfur with comonomers with diene or more, such as 1,3-diisopropenylbenzene [37,38], divinylbenzene [39,40], 1,3-diethynylbenzene [41], ethylene glycol dimethacrylate [42,43], dicyclopentadiene and 5-ethylidene-2-norbornene [44], and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane [45], forms stable crosslinked structures compared to otherwise unstable polysulfides. The diversity of selected crosslinkers aimed to impart appropriate thermal and mechanical properties that meet basic application requirements. Alkyl amines such as 4-vinylaniline [46] and oleylamine [47] were also reported for the preparation of sulfur-containing copolymers with amine pendent groups. Moreover, amine functional prepolymers were prepared from the reaction of elemental sulfur and phenylenediamine to be subsequently reacted with epoxides and other groups to form crosslinked sulfur-containing polymers [48]. Trialkyl amines were also used to promote time-efficient reactions as nucleophilic activators to prepare sulfur-rich copolymer at a low temperature of 110 °C and allow inverse vulcanization of the elemental sulfur with various unreactive crosslinkers [49]. However, there are no reports on the copolymerization of N2,N2-diallylmelamine (NDAM), also known as 2,4-Diamino-6-diallylamino-1,3,5-triazine, as a comonomer with elemental sulfur to form diamine functionalized sulfur-containing copolymer. The advantage of using NDAM as a comonomer is not only to stabilize the polysulfide by crosslinking as a result of the presence of divinyl groups [50] but also to impart affinity (selectivity) to CO2 because of the presence of di-amine groups in its molecular structure [51].
On the other hand, the continued reliance on fossil fuels as a primary energy source led to soaring CO2 emissions and subsequent climate change, underscoring the critical need for intensifying efforts to mitigate it by carbon capture and sequestration [52]. Several methods, including absorption with liquid amine, adsorption, membrane separation, and cryogenic distillation, are widely recognized strategies for the removal of CO2 from various streams. Of all, adsorption is a promising method for CO2 capture due to its versatility, ease of regeneration, scalability, and low energy requirement [53]. Polymeric materials with high performance offer promising adsorbents to improve CO2 capture [54]. However, further improvements in the cost-effectiveness and sustainability of these polymeric materials are essential to deploy them as alternatives to inorganic adsorbents.
Sulfur-containing copolymers were used for environmental applications, mainly wastewater treatment involving the decontamination of heavy metals [23], mercury [24], and boron [26] in addition to oil spill cleanup [18], organic compounds removal (caffeine) [38], and the extraction of gold from waste streams [25]. Despite these uses, the application of sulfur-containing copolymers for CO2 capture has not been explored, and this represents a great opportunity to extend their use in the treatment of pre- and post-combustion different gas streams. Moreover, the idea of using sulfur (a by-product of processing fossil fuel) to clean up gas streams from CO2 is truly circular and offers a novel solution to environmental problems.
To use sulfur-containing copolymers as effective adsorbents, it is essential to introduce porosity into their structure. This is typically achieved by adding a porogen into the raw materials during the synthesis reaction. NaCl was used to create porous structures in sulfur-containing copolymers, enabling their application in various adsorption processes. For instance, porous sulfur copolymers were used for the adsorption of gas-phase mercury [55] and the removal of palladium and mercury salts from water and soil [56]. Furthermore, porous sulfur copolymers synthesized using NaCl particles were evaluated for the adsorption of organic pollutants, such as caffeine [38]. In another study, a mixture of NaCl and urea served as a template to create pores in sulfur copolymers, which were subsequently used for the adsorption of gold from wastewater [25]. Another route involving supercritical CO2 was also used to generate porosity in sulfur-containing copolymers that were tested for mercury adsorption from aqueous solutions [57]. These examples highlight the versatility of porous sulfur-containing copolymers in addressing diverse adsorption challenges.
This study aimed to synthesize sulfur-containing copolymers with amine groups and explore their potential for CO2 capture. The copolymer was prepared by radical polymerization of sulfur with NDAM at molten temperatures with and without NaCl particles to yield an amine-containing crosslinked copolymer with high CO2 selectivity and adsorption capacity. The primary objective was to determine the best combination of reaction parameters such as S/NDAM ratio, temperature, and time. The thermal, structural, and physicochemical properties of the resulting copolymer were evaluated. The sulfur copolymer was made porous by the addition of NaCl particles of different contents, and its efficacy in CO2 adsorption was investigated under various conditions.

2. Experimental Section

2.1. Materials

Sulfur powder (assay ≥ 99.0%) was obtained from Sigma-Aldrich (St. Louis, MO, USA), while N2,N2-Diallylmelamine (NDAM) with a purity of ≥98.0% and an m.p. of 140 °C was supplied by Fischer Scientific (Hampton, NH, USA). NaCl fine particle Double Swallow salt (purity > 99.9%) was obtained from PJ Grocer Sdn Bhd (Petaling Jaya, Malaysia), and CO2 gas of 99.7% purity was utilized for the adsorption studies.

2.2. Synthesis of Sulfur/NDAM Copolymers

The reaction started by placing sulfur (S) powder in a glass vial that was heated in a thermostatic oil bath with an adjustable magnetic stirrer (Zhejiang Lichen Instrument, Shaoxing, China) under continuous stirring. This was followed by the addition of NDAM powder to molten S under continuous stirring until the reaction mixture became homogeneous, with further heating to 145 °C. The weight ratio of S to NDAM was varied in the range of 50:50, 70:30, and 90:10 while keeping the reaction mixture weights to a constant value during all experiments. The molten mixtures were further heated to varying reaction temperatures, ranging from 155 to 175 °C. The reaction was allowed to proceed for different time periods in the range of 20–120 min.
A porous version of the S-containing copolymers was prepared by adding different weight ratios (12.5–50 wt%) of NaCl powder as a porogen to S/NDAM of 50:50 wt%, and the weight of the reaction mixture was kept constant in all trials. After reaction completion, the glass vials were left to cool down to room temperature, and the reaction mixtures were solidified into a dark brownish copolymer. The copolymers were washed four times with warm deionized water to remove the occluded NaCl porogen to obtain a porous structure, then dried to constant weight, and finally weighed. The sulfur-containing copolymers were stored in a tight container and kept under a desiccator for further use. The process for the reaction of S with NDAM is illustrated in Figure 1.

2.3. Characterization of Crosslinked Sulfur Copolymer

An FTIR analysis was carried out by a SHIMADZU IR Tracer-10 spectrometer (Kyoto, Japan) equipped with ATR. The spectra were obtained with 16 scans and a resolution of 4 cm−1 across a frequency range of 500–4000 cm−1. The FESEM analysis of the samples was conducted using a Zeiss SUPRA 55V microscope (Oberkochen, Germany). The sample thermal stability was evaluated using a PerkinElmer STA 6000 simultaneous thermal analyzer (Waltham, MA, USA) at a heating rate of 10 °C/min in a temperature range of 30–700 °C under N2 atmosphere. The melting behavior was analyzed by using a TA Instruments Q2000 thermal analyzer from TA Instruments Company (New Castle, DE, USA) in a N2 environment. The crystalline structure of the samples was examined by X-ray diffraction (XRD) using a PANalytical Empyrean analyzer (Almelo, The Netherlands) at Bragg’s angle in the range of 2–80°. The surface area and pore volume analyses were performed with a Quantachrome Corporation 4000E1 Brunauer–Emmett–Teller (BET) analyzer, USA. The samples were degassed at 80 °C for 4 h before analysis.

2.4. CO2 Adsorption Testing

The aminated sulfur-containing copolymer was tested for CO2 adsorption by placing 0.1 g of the sample inside the sample container, and CO2 adsorption equilibrium was assessed using a gravimetric sorption analyzer isoSORP®, supplied by Rubotherm-TA Instruments (New Castle, DE, USA), which is based on a magnetic suspended balance. Prior to measurement, the sample was pretreated at 80 °C for 2 h under vacuum to remove moisture and any impurities. This was followed by buoyancy measurements, which were conducted at 30 °C under vacuum conditions. Subsequently, the adsorption equilibrium measurements were performed at 30 °C under pressure ranging from vacuum to 25 bar, using pure CO2 at a 500 mL/min flow rate. This procedure was repeated for all the samples, and the sorption equilibrium was reached in 50 min. The pressure range of 1–25 bar was selected to evaluate the adsorbent’s potential for capturing CO2 from high-pressure flue gas streams, particularly in industrial applications such as cement plants and waste-to-energy facilities, where pressure swing adsorption (PSA) is commonly employed.

3. Results and Discussion

3.1. Selection of the Reaction Parameters of Sulfur with NDAM

The effect of reaction parameters, including the S to NDAM ratio (50:50, 70:30, and 90:10), temperature (155–175 °C), and reaction time (20–120 min), was investigated. The temperature increase was handled carefully to reduce the evolution of H2S gas, and this was monitored not only through determination of the copolymer conversion yield (%) and detection of weight loss but also by observing the changes in the color intensity after each experiment.
Figure 2 shows the effects of temperature and reaction time on the conversion yield of S/NDAM 50/50 copolymers. At all temperatures, the conversion yield of S and NDAM in the copolymer decreased as reaction time was prolonged, with such an effect more pronounced at higher temperatures. This is attributed to the increased evolution of H2S gas, a side reaction that intensifies with prolonged heating during inverse vulcanization [58]. The combination of higher temperatures and longer reaction times led to further reductions in conversion yield due to greater gas evolution. The highest conversion yield occurs at 160 °C for all reaction times, after which it declines up to 175 °C. Beyond 170 °C, the 20 min heated sample showed continued copolymer yield loss, while longer reaction durations imparted minimal change. This suggests that most volatile products were released between 160 and 170 °C and was accompanied by further blackening of an already brown copolymer. Based on these results, a temperature of 160 °C and a duration of 120 min were selected for further experiments to ensure high conversion of the reactant with minimal side reactions. It is noteworthy to mention that the level of decrease in the copolymer conversion yield as a result of gas evolution is rather small and falls in the range of 0.5 to 2.75%.
Figure 3 shows a comparison between the conversion yields of S/NDAM copolymers with different initial reactant ratios, and the reactions were conducted at 160 °C for 120 min. As the amount of sulfur increased, the conversion yield remarkably decreased up to 70 wt% S, beyond which it reached a plateau, suggesting that the increase in sulfur content promoted H2S evolution that lowered the conversion. Beyond this point, conversion yield becomes independent of the S/NDAM ratio. This behavior is likely due to changes in the C:H and C:S ratios after polymerization and the accompanied loss of H and S through H2S release, leading to a decrease in C:H and C:S ratios. Therefore, the best combination of parameters that was used to prepare porous samples using NaCl porogen was a weight ratio of 50:50 for S/NDAM, a temperature of 160 °C, and a reaction time of 120 min.
The obtained sulfur-containing copolymer with a diamine group was a brownish-black material, poly(S/NDAM), that was found to be insoluble in all tested solvents, including ethanol, acetone, chloroform, methylene chloride, toluene, DMSO, and DMF, regardless of S content in the copolymer. The insolubility of the copolymer most likely stemmed from the crosslinked network structure formed during the copolymerization of NDAM through its divinyl groups, rendering the obtained material insoluble in most common solvents [59,60,61].

3.2. Chemical Composition of Copolymer

An FTIR analysis was used to provide early evidence of successful reactions between the elemental S and NDAM. Figure 4 presents the FTIR spectra of all synthesized poly(S/NDAM) copolymers alongside elemental S and virgin NDAM as references. The FTIR spectrum of S exhibits a characteristic S–S bending peak at 465 cm−1. Considering the spectrum of the neat NDAM, the spectra of the developed copolymers with varying S concentrations demonstrated distinct peaks at 3311 and 3471 cm−1 corresponding to N–H stretching, while peaks at 1521, 1552 and 1408 cm−1 were assigned for the stretching vibration of C=N of planar triazine rings and the bending deformations of amino-azine. This was coupled with the appearance of the peak at 808 cm−1 resembling the out-of-plane triazine ring bending vibrations, confirming the presence of the triazine and amine functionalities in copolymers [62]. The weak peaks at 2937 and 3140 cm−1 represent the stretching vibration of aliphatic C-H originating from the vinyl group in the NDAM monomer. Notably, no peaks were observed in the 1600–1650 cm−1 range, which is typically associated with vinylic C=C stretching vibration of the vinyl group shown exactly at 1612 cm−1 in the neat NDAM spectrum, suggesting the successful reaction of S with NDAM. Furthermore, the appearance of a new peak at 615 cm−1 was attributed to the out-of-plane bending of C–H adjacent to C–S bonds that further supports the copolymer formation [63]. These findings are consistent with that in the literature [59,64] and confirm the successful reaction of S and NDAM forming a sulfur-containing copolymer.
The EDX analysis of the poly(S/NDAM) copolymer (Figure 5) obtained from the 50/50 wt% reactant ratio revealed that the copolymer sample contains 46.8% polysulfide, which is close to the initial S content in the feed mixture. The signals representing poly(S/NDAM) in the copolymer displayed a content of 53.2%, comprising 34.3, 16.8, and 2.1% for C, N, and O, respectively. The slight decrease in S content in the copolymer compared to its initial amount in the reaction mixture is probably due to a very minor release of H2S and slight decomposition in the copolymer during storage.

3.3. Surface Area and Pore Properties

The BET method was used to determine the surface area, pore volume, and mean pore diameter of poly(S/NDAM) copolymers with various porogen contents using N2 adsorption/desorption measurements, and the obtained results are summarized in Table 1. The increase in NaCl content led to an increase not only in the surface area of the samples but also in the mean pore diameters and total pore volume. The incorporation of NaCl allowed for the formation of mesoporous structures when the copolymer samples were washed with hot water after reaction completion, leading to the formation of mesopores. This structure will have an impact on the level of porosity increase and the accompanied increase in the surface area. These observations are consistent with that from studies reporting on developing S copolymers with increased surfaces using multifunctional monomers containing such porogen [25,55,65]. Moreover, the developed mesoporous structure is likely to affect the adsorption of isothermal behavior when the samples are tested for CO2 capture.
The morphological properties of poly(S/NDAM) copolymers were investigated after adding porogen with FESEM in comparison with the counterpart without porogen, as shown in Figure 6. Figure 6A,D demonstrate the intrinsically low porosity of the poly(S/NDAM) copolymer. When 25 W% of NaCl porogen was used, the sample showed a clearly defined porous structure (Figure 6B,E). Increasing the porogen content to 50 wt% further enhanced the pores’ density and distribution, although all samples retained nanoscale pore features (6C and 6F). These results are consistent with the BET analysis, which showed a progressive increase in surface area of the samples with an increase in the porogen content, confirming the mesoporous nature of the poly(S/NDAM) copolymers.

3.4. Structural Properties

Figure 7 depicts the powdered X-ray diffraction (XRD) diffractogram of elemental S and the poly(S/NDAM) copolymer. The crystalline nature of S is evident from the α-S8 crystalline peaks appearing in the diffractogram of S [66]. Whereas the copolymers prepared using 50 and 70 wt.% S showed an amorphous nature, with no or less intense peaks resembling the α-S8 crystalline structure from 20-30 2θ, the peaks representing α-S8 crystalline appeared in the diffractogram of the copolymer synthesized using 90 wt.% S, indicating the presence of unreacted S. These observations demonstrate that S has been successfully converted into a stable S-S backbone copolymer using NDAM, and no unreacted S is observed in the copolymer produced with 50 wt% S and very little in the 70 wt% S-containing copolymer. The amount of unreacted S increased with an increase in initial S concentration, which aligns with our earlier observations [59].

3.5. Thermal Properties

TGA is crucial for understanding the thermal stability and compositional properties of copolymers. By analyzing different percent compositions, TGA helps determine how each component affects the overall thermal behavior of the material. Figure 8 and Figure 9 present the TGA and DTG thermograms of poly(S/NDAM) copolymers together with virgin elemental S as a reference.
The poly(S/NDAM) copolymers, regardless of their percent composition, show similar thermal stability at around 160 °C, while virgin S starts to degrade above 200 °C. This trend is in line with that found in the literature, confirming the formation of a polymeric structure with S backbone [59,67]. Sulfur, due to its simple molecular structure, fully degrades at around 330 °C while the copolymers show remaining weights up to 1000 °C. While the 90/10 sample shows an almost plateau trend at higher temperatures, meaning that it has almost reached its final weight, the other two copolymer samples, i.e., 50/50 and 70/30, show a small but negative slope at 1000 °C, showing that their degradation will continue after this temperature for a while.
In S-containing polymers and based on the type of petroleum-based comonomer, there are usually two (sometimes three or more) degradation steps, consisting of a first sharp degradation of the S content, followed by degradation of the organic component [59,68]. The DTG graphs show similar onset temperatures for the second degradation step at 245, 244, and 252 °C for copolymer samples obtained from the 50/50, 70/30, and 90/10 (S/NDAM) combinations, respectively. This means the initial S content in the reaction feed does not alter the thermal stability of the copolymers. The accompanied decomposition onset is at 36, 48.2, and 67.7% of weight loss corresponding to S content, while the ideal values must be 50, 70, and 90%. The S content inside the samples is lower than that in the reaction feed, and this is most likely due to the difference in the reactivity of the reactants. Nevertheless, these results are very interesting since all resulting copolymers show a similar ratio of the actual S content in the polymers versus the initial S content in the reaction feed, i.e., 0.72, 0.69, and 0.75 for the 50/50, 70/30, and 90/10 combinations, expressing a mediocre but uniform S conversion rate for all the samples.
To further investigate the effect of different reaction times on the composition of the final product, different reaction times were generated by the important sample obtained from the 50/50 poly(S/NDAM) samples, and the TGA thermograms can be seen in Figure 10. The thermographs of the samples do not deviate from each other significantly. The small distortion at higher temperatures is most probably due to test errors and hence is negligible. These results suggest that the reaction reaches equilibrium in a very short time, i.e., 20 min.
DSC is essential for identifying unreacted elemental S in inverse vulcanized polymers. It detects crystalline S by measuring heat flow during phase transitions, pinpointing sulfur at the melting point. The DSC thermograms of the poly(S/NDAM) copolymers together with virgin S are presented in Figure 11. Sulfur shows two different melting points, attributed to its two different crystalline structures at around 106 and 117 °C, which are in excellent agreement with that in the literature [69,70]. On the other hand, S-containing polymers are, in general, amorphous in nature and hence are not expected to demonstrate any melting or cold crystallization points during a heating step [19,30]. In the case of poly(S/NDAM) copolymers made of 50/50, this expectation is satisfied, indicating a full conversion of elemental S into the polymeric structure; nonetheless, poly(S/NDAM) copolymers made of 70/30 and 90/10 show melting peaks with low intensity, corresponding to those of elemental S, suggesting the presence of some unreacted S inside the copolymer bulk. The addition of 10 wt% NDAM to 90 wt% S remarkably reduced the enthalpy of melting, which was further remarkably decreased by the increase in NDAM to 30 wt% in the copolymer. This behavior suggests that NDAM comonomer is capable of stabilizing polysulfide chains, and such stabilization is effective at a content close to 70 wt%. This trend can be comprehended based on previous reports indicating that in inverse vulcanized polymers, vinylic comonomers, despite their structures, are not capable of stabilizing a high percentage of polysulfide chains. This leads to the presence of some unreacted and/or some reacted but depolymerized polysulfide chains in the bulk of poly(S/NDAM) copolymers made using higher percentages of initial S loading [5,71].
The fully amorphous 50/50 and 70/30 copolymers show Tg values of 68 and 74 °C, respectively. This increase in the glass transition temperature by increasing the initial S content shows that more sulfur results in more entanglements in the structure of the copolymers, making them a bit more difficult to soften. It is necessary to mention that these polymers seem to have relatively high Tg values compared with some of the reported sulfur-containing polymers, making them suitable for applications with a higher range of temperature requirements [43,59,72]. The 90/10 poly(S/NDAM) copolymer, on the other hand, shows no Tg under the applied DSC thermal treatment conditions, and this is most probably due to the low sensitivity of the equipment in detecting the Tg of the polymeric chains in the presence of a higher percentage of unreacted S.

3.6. Evaluation of Aminated Sulfur-Containing Copolymer as Adsorbent for CO2 Adsorption

3.6.1. Effect of Sulfur Content

The effect of S/NDAM ratio (without porogen) on the CO2 adsorption capacity of the aminated S-containing copolymer at a pressure of 1 bar is presented in Figure 12. The CO2 adsorption capacity was found to increase gradually with the decrease in S content in the copolymer due to the incorporation of more amine sites into the material with the increase in NDAM content. However, the overall adsorption values remained low, and this is likely due to the copolymer’s bulky and dense structure blocking access to many amine sites. To overcome this problem, NaCl was introduced to S and NDAM reactants at varying amounts as a porogen to enable the formation of a porous structure in the obtained copolymer.

3.6.2. Effect of Porogen Content on CO2 Adsorption Capacity

Figure 13 shows the effect of porogen content on the adsorption capacity of S-containing copolymers. The CO2 adsorption capacity remarkably improved with the addition of NaCl porogen. For instance, it increased from 4.5 to 19.0 mg/g with the variation in NaCl amounts from 0 to 50 wt%. It was reported that the CO2 adsorption capacity of mesoporous materials is highly dependent on the micropore volume and micropore surface area [73]. Thus, this behavior can be attributed to the increase in the copolymer’s surface area and mean pore diameter, leading to greater access to the amine sites, facilitating gas–solid interactions, and boosting the adsorption capacity. The level of adsorption capacity of the present poly(S/NDAM) copolymer adsorbent (50 wt% NaCl) is on par with that of silica gel grafted with a 3-aminopropyl group [74], mesoporous SBA-15 modified with ethylene diamine [75], and mesoporous silica loaded with pentaethylenehexamine [76].
To understand the adsorption behavior of the poly(S/NDAM) copolymer, it is important to recognize that CO2 acts as a Lewis acid, and its adsorption is affected by the chemically stable, electron-withdrawing triazine rings and the Lewis basic diamine functionalities within the adsorbent copolymer. These features promote a chemisorption mechanism through acid–base interactions [77].
Furthermore, the adsorption of CO2 under dry conditions most likely led to the formation of triazine-ammonium carbamate, which occurs when a CO2 molecule is adsorbed on a closely spaced pair of amine groups [78]. The process for the adsorption of CO2 on the present diamine-containing adsorbent and subsequent carbamate formation is illustrated in Figure 14.
It is noteworthy to mention that unlike amine physically impregnated adsorbents, which were tested for low-pressure uses and reviewed elsewhere [79], the diamine in the present adsorbent (originating from NDAM) that was chemically bonded to the polysulfide chains is mostly suitable for CO2 capture in high-pressure applications involving the PSA process.

3.6.3. Effect of Pressure on CO2 Adsorption

The effect of pressure on the CO2 adsorption capacity of the S copolymer composed of 50/50 S/NDAM made porous by adding 50% NaCl is presented in Figure 15. As shown, the adsorption capacity increases slowly at low pressures and exponentially at higher pressures (increased from 4 to 517 mg/g at 25 bar) without reaching a saturation plateau, reflecting an adsorption isotherm close to type III according to the IUPAC classification [80,81]. This trend reflects the mesoporous structure present in the poly(S/NDAM) copolymer that was likely accompanied by an initially limited interaction between the CO2 and solid adsorbent material caused by steric hindrance. The adsorption capacity subsequently increased with the pressure rise due to the enhancement of gas penetration into the mesoporous structure and subsequent improvement in the access to the internal amine sites. This was likely accompanied by a transition to physisorption, forming multilayer adsorption at higher pressures. The pressure-dependent adsorption demonstrated in this study is consistent with the CO2 adsorption result of Si-MCM-41 under varying pressure [73,82].
The CO2 adsorption capacity of the poly(S/NDAM) copolymer was compared with several chemically modified aminated adsorbents possessing mesoporous structures, as shown in Table 2. The present adsorbent exhibited a CO2 adsorption capacity on par with that of mesoporous materials such as meso-silica modified with methyl diethanolamine (MDEA) and SBA-15 modified with 3-aminopropyltrimethoxysilane (APTMS), both tested at 1 bar and 30 °C while the adsorption was primarily driven by chemisorption. This behavior contrasts with that of other adsorbents such as MCM-48 modified with silane and diethanolamine (EDA), and silica gel modified with APTMS/TEPA both of which attained high surface areas and exhibited enhanced adsorption (at 1 bar and 70 °C) by combining chemisorption and physisorption. However, the current adsorbent showed a high CO2 adsorption capacity of 519 mg/g at 25 bar, indicating its strong potential for high-pressure applications.

4. Conclusions

Sulfur-containing copolymers with diamine groups were successfully prepared by thermally induced radical copolymerization of elemental S with difunctional NDAM. The reaction parameters, including S and NDAM ratio, temperature, and reaction time, were varied to identify the best parameter combination for achieving maximum copolymerization. The presence of triazine in the copolymer was evident from the peaks at 1521, 1552, and 1408 cm−1 corresponding to C=N stretching in the planar triazine rings and bending of the amino-azine groups, coupled with the peak at 808 cm−1 resembling out-of-plane bending of the triazine rings, as revealed by FTIR. The EDX analysis of the poly(S/NDAM) copolymer proved the presence of a composition close to S in the feed ratio. The incorporation of NaCl allowed the formation of a mesoporous structure and imparted a higher surface area, as suggested by the BET and FESEM analyses. The XRD analysis demonstrated the complete conversion of elemental S and NDAM of 50/50 wt% in the copolymer with a stable S-S backbone copolymer. The poly(S/NDAM) copolymers demonstrated similar thermal stability at around 160 °C, regardless of their percent ratio, according to the TGA analysis. The DSC results revealed that the crosslinked poly(S/NDAM) copolymer is an amorphous material with a Tg of 68 °C at a 50/50 reactant ratio. The obtained copolymer was insoluble in the tested solvents. The addition of NaCl porogen led to the formation of a mesoporous structure that allowed the obtained copolymer to be used as an adsorbent, showing remarkably enhanced CO2 adsorption capacity from 4 to 517 mg/g with a pressure increase from 1 to 25 bar, suggesting its suitability for CO2 capture. The adsorption isotherm suggested that CO2 is adsorbed by chemisorption at low pressure, which was accompanied by physisorption when the pressure was increased. More ongoing work is being carried out to further investigate the CO2 adsorption isothermal and kinetic behaviors. This study not only promotes a green chemistry approach involving waste sulfur utilization for CO2 capture but also contributes to environmental sustainability and circular economy advancements.

Author Contributions

This study was conceived and designed by M.M.N. Material preparation and data collection were performed by D.N. and N.H.S. The data analysis was performed by A.A., A.S.M.G., N.A.M., R.S., and M.M.N. The first draft of the manuscript was written by M.M.N. and A.A. Previous versions of the manuscript were reviewed by A.S.M.G., N.A.M., R.S., and M.M.N. All authors have read and agreed to the published version of the manuscript.

Funding

M.M. Nasef wishes to acknowledge the financial support from the UTM R&D fund under cost center R.K130000.7743.4J758.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Process for the preparation of the crosslinked copolymer by the reaction of S with NDAM.
Figure 1. Process for the preparation of the crosslinked copolymer by the reaction of S with NDAM.
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Figure 2. Effects of temperature and reaction time on the conversion yield of the copolymer at a S/NDAM ratio of 50:50.
Figure 2. Effects of temperature and reaction time on the conversion yield of the copolymer at a S/NDAM ratio of 50:50.
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Figure 3. Effect of different initial reactant ratios on the conversion yield of S/NDAM copolymers at 160 °C for 120 min.
Figure 3. Effect of different initial reactant ratios on the conversion yield of S/NDAM copolymers at 160 °C for 120 min.
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Figure 4. FTIR spectra of virgin S and developed poly(S/NDAM) copolymers.
Figure 4. FTIR spectra of virgin S and developed poly(S/NDAM) copolymers.
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Figure 5. EDX analysis of poly(S/NDAM) copolymer obtained at 50/50 wt%.
Figure 5. EDX analysis of poly(S/NDAM) copolymer obtained at 50/50 wt%.
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Figure 6. SEM images of the poly(S/NDAM) copolymer at 50/50 (w/w) (left, 5.00 Kx and right, 30 Kx) with no porogen (A,D), 25 wt% NaCl porogen (B,E), and 50 wt% NaCl porogen (C,F).
Figure 6. SEM images of the poly(S/NDAM) copolymer at 50/50 (w/w) (left, 5.00 Kx and right, 30 Kx) with no porogen (A,D), 25 wt% NaCl porogen (B,E), and 50 wt% NaCl porogen (C,F).
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Figure 7. p-XRD diffractogram of virgin S and poly(S/NDAM) copolymers with varying S concentrations.
Figure 7. p-XRD diffractogram of virgin S and poly(S/NDAM) copolymers with varying S concentrations.
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Figure 8. TGA thermographs of elemental S and poly(S/NDAM) copolymers with different initial percent compositions.
Figure 8. TGA thermographs of elemental S and poly(S/NDAM) copolymers with different initial percent compositions.
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Figure 9. DTG thermographs of elemental S and poly(S/NDAM) copolymers with different initial percent compositions.
Figure 9. DTG thermographs of elemental S and poly(S/NDAM) copolymers with different initial percent compositions.
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Figure 10. TGA thermographs of poly(S/NDAM) copolymers made of 50/50 composition at different reaction times.
Figure 10. TGA thermographs of poly(S/NDAM) copolymers made of 50/50 composition at different reaction times.
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Figure 11. DSC thermographs of elemental S and S/NDAM copolymers with different initial percent compositions.
Figure 11. DSC thermographs of elemental S and S/NDAM copolymers with different initial percent compositions.
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Figure 12. Variation in the adsorption capacity of NDAM/S copolymers with S/NDAM ratio content percent in the mixture. The pressure is 1 bar, and the temperature is 25 °C.
Figure 12. Variation in the adsorption capacity of NDAM/S copolymers with S/NDAM ratio content percent in the mixture. The pressure is 1 bar, and the temperature is 25 °C.
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Figure 13. Variation in the adsorption capacity of amine-containing sulfur copolymers with porogen content percent in the mixture. The S/NDAM ratio is 50/50, the pressure is 1 bar, and the temperature is 25 °C.
Figure 13. Variation in the adsorption capacity of amine-containing sulfur copolymers with porogen content percent in the mixture. The S/NDAM ratio is 50/50, the pressure is 1 bar, and the temperature is 25 °C.
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Figure 14. Process for the formation of carbamate due to CO2 adsorption on diamine-containing poly(S/NDAM) adsorbent in the absence of water.
Figure 14. Process for the formation of carbamate due to CO2 adsorption on diamine-containing poly(S/NDAM) adsorbent in the absence of water.
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Figure 15. Variation in CO2 adsorption capacity of amine-containing sulfur copolymer. The S/NDAM ratio is 50/50, and the weight percent of porogen is 50 wt%.
Figure 15. Variation in CO2 adsorption capacity of amine-containing sulfur copolymer. The S/NDAM ratio is 50/50, and the weight percent of porogen is 50 wt%.
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Table 1. Surface area, pore volume, and mean pore diameter of poly(S/NDAM) copolymers with various porogen contents.
Table 1. Surface area, pore volume, and mean pore diameter of poly(S/NDAM) copolymers with various porogen contents.
NaCl Content
(wt%)		Total Pore Volume
×103 (cm3/g)		Means Pore Diameter
(nm)		Surface Area (m2/g)
0.06.52.783.18
12.56.73.894.56
25.07.14.847.63
37.57.44.897.92
50.07.55.299.62
Table 2. Comparison of CO2 adsorption capacity of some chemically modified amine-containing adsorbents with mesoporous structures.
Table 2. Comparison of CO2 adsorption capacity of some chemically modified amine-containing adsorbents with mesoporous structures.
Adsorbent Materials Surface Area (m2/g)CO2 Capacity (mg/g)Mechanism of AdsorptionRefs
MCM 41 modified with TEPA and DEA on133.0132 (1 bar, 70 °C)Chemisorption/physisorption[83]
Mesoporous silica gel modified with APTS/TEPA75.0162 mg/g (1 bar, 70 °C)Chemisorption/physisorption[84]
Mesosilica MCM-48 modified with silane and EDA 419129 (1 bar, 30 °C)Chemisorption/physisorption[85]
Mesosilica gel modified with MDEA441.015.8 mg/g (1 bar, 70 °C)Chemisorption/physisorption[86]
Mesosilica (SBA-15) modified with APTMS17024.0 (at 1 bar, 30 °C)Chemisorption[87]
Poly(S/NDAM) copolymer
50/50 (S/NDAM)		9.6219.0
(at 1 bar, 30 °C)		ChemisorptionThis study
Poly(S/NDAM) copolymer
50/50 (S/NDAM)		9.62517.0
At 25 bar, 30 °C		ChemisorptionThis study
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Narendiran, D.; Sumadi, N.H.; Manzoor Ghumman, A.S.; Mohamad, N.A.; Nasef, M.M.; Abbasi, A.; Shamsuddin, R. A New Sulfur-Containing Copolymer Created Through the Thermally Induced Radical Copolymerization of Elemental Sulfur with N2,N2-Diallylmelamine Comonomer for Potential CO2 Capture. J. Compos. Sci. 2025, 9, 362. https://doi.org/10.3390/jcs9070362

AMA Style

Narendiran D, Sumadi NH, Manzoor Ghumman AS, Mohamad NA, Nasef MM, Abbasi A, Shamsuddin R. A New Sulfur-Containing Copolymer Created Through the Thermally Induced Radical Copolymerization of Elemental Sulfur with N2,N2-Diallylmelamine Comonomer for Potential CO2 Capture. Journal of Composites Science. 2025; 9(7):362. https://doi.org/10.3390/jcs9070362

Chicago/Turabian Style

Narendiran, Dharrinesh, Nurul Hazirah Sumadi, Ali Shaan Manzoor Ghumman, Noor Ashikin Mohamad, Mohamed Mahmoud Nasef, Amin Abbasi, and Rashid Shamsuddin. 2025. "A New Sulfur-Containing Copolymer Created Through the Thermally Induced Radical Copolymerization of Elemental Sulfur with N2,N2-Diallylmelamine Comonomer for Potential CO2 Capture" Journal of Composites Science 9, no. 7: 362. https://doi.org/10.3390/jcs9070362

APA Style

Narendiran, D., Sumadi, N. H., Manzoor Ghumman, A. S., Mohamad, N. A., Nasef, M. M., Abbasi, A., & Shamsuddin, R. (2025). A New Sulfur-Containing Copolymer Created Through the Thermally Induced Radical Copolymerization of Elemental Sulfur with N2,N2-Diallylmelamine Comonomer for Potential CO2 Capture. Journal of Composites Science, 9(7), 362. https://doi.org/10.3390/jcs9070362

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