Microwave assisted magnetite carbon dots for enhanced the thermal aging resistance,magnetic and dielectric properties of acrylonitrile butadiene rubber composites

Abstract

Magnetic carbon dots (Fe₃O₄/N–CQDs) were synthesized via a one-step microwave-assisted method using sugarcane bagasse as the carbon precursor. The synthesized materials were thoroughly characterized via thermogravimetric analysis (TGA/DTG), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) to examine their thermal behavior, nanoscale morphology, and phase composition. These Fe₃O₄/N–CQDs were incorporated into acrylonitrile butadiene rubber (NBR) at varying concentrations (0–11 phr), and the resulting composites were evaluated for their structural characteristics (FTIR, SEM), thermal stability, curing behavior, mechanical performance, swelling resistance, thermal aging resistance, as well as magnetic, and dielectric properties. Among the formulations studied, the composite containing 8 phr Fe₃O₄/N–CQDs showed the most balanced performance, with tensile strength increasing from 2.3 to 4.2 MPa and hardness from 54 to 65 Shore A. Post-aging analysis revealed improved tensile strength and reduced swelling, attributed to enhance crosslinking. The composites demonstrated superior thermal aging resistance under accelerated aging at 110°C for 10 days. Magnetic measurements indicated soft magnetic behavior, with saturated magnetization increasing from 29.05x 10⁻³ emu/g (0 phr) to 91.32 × 10⁻³ emu/g (11 phr) and coercivity below 120 G, while dielectric and conductivity studies showed frequency-dependent permittivity and conductivity consistent with interfacial polarization and hopping mechanisms. These results demonstrate that Fe₃O₄/N–CQDs can simultaneously enhance the thermal, mechanical, antiaging, magnetic, and dielectric properties of NBR compounds.

Keywords

Magnetite carbon quantum dots nitrile butadiene rubber rheological properties mechanical and structural characteristics magnetic and dielectric properties

Highlights

• Magnetic nitrogen-doped carbon quantum dots (Fe₃O₄/N–CQDs) were successfully synthesized from sugarcane bagasse via a green, sustainable route.

• Fe₃O₄/N–CQDs were incorporated into NBR rubber at different loadings (0–11 phr) to evaluate multifunctional performance.

• TEM and SEM revealed nanoscale dispersion and morphology, and FTIR confirmed chemical interactions within the NBR matrix.

• Fe₃O₄/N–CQDs significantly enhanced thermal aging resistance under 110°C for 10 days.

• Enhanced dielectric and magnetic properties indicate strong potential for antistatic, electrostatic discharge (ESD)-safe, and electromagnetic interference (EMI) shielding applications.

Introduction

The global accumulation of agricultural waste poses significant environmental and economic challenges, as crop processing generates vast quantities of byproducts that are often discarded or burned.^1,2 Sugarcane bagasse (SCB), the fibrous residue remaining after sugarcane stalks are crushed, is produced in millions of tons annually and remains largely unexploited.³ Its high carbon content, low ash content, and abundance make it an attractive feedstock for carbon-based nanomaterials, including carbon dots (CDs).^3,4 CDs are quasi-spherical, zero-dimensional nanoparticles (<10 nm) with a high surface-to-volume ratio and rich surface chemistry, including hydroxyl, carboxyl, carbonyl, and amine groups.^5–8 These features enable strong interactions with host matrices and facilitate doping or hybridization with inorganic nanoparticles.⁹

While CDs have been widely explored for optical applications such as sensing and imaging, recent attention has turned to their use as polymer nanofillers.^10,11 Their nanoscale dimensions and reactive surfaces allow uniform dispersion within hydrophilic polymer matrices, enhancing filler–matrix adhesion. This interfacial reinforcement in compatible matrices often translates into improvements in tensile strength, elasticity, thermal stability, and chemical resistance.¹⁰ Recent studies have demonstrated the potential of CDs as reinforcing agents in rubbers, including natural rubber, silicone rubber, and chloroprene rubber, where they promote crosslinking and impart multifunctional properties beyond conventional reinforcement.^12–15

In rubber chemistry, nanoscale fillers such as nano clay, nanosilica and carbon nanotubes are commonly used to enhance mechanical and thermal properties.^16,17 However, the growing interest in multifunctional rubbers has led researchers to explore alternative nanofillers, including CDs and metal or metal oxide/sulfide nanoparticles, which can impart additional functionalities.^10,16,18 While CDs primarily enhance mechanical strength, elasticity, thermal stability, and interfacial adhesion; and can also contribute to dielectric behavior¹⁹; metal and metal oxide nanoparticles provide complementary properties such as magnetic, dielectric, thermal, and electrical responsiveness.¹⁶ Metal nanoparticles with specific surface functionalities are particularly attractive because they are inexpensive, easy to prepare, and can achieve effects similar to more costly nanofillers like carbon nanotubes or graphene.¹⁶ By combining CDs with metal-based nanofillers, advanced rubber composites can be designed to simultaneously offer mechanical reinforcement and multifunctional performance.

Among metal oxides, magnetite (Fe₃O₄) nanoparticles are particularly attractive due to their strong magnetic response, high thermal stability, and ability to enhance dielectric and electrical properties.²⁰ Fe₃O₄ has been successfully incorporated into rubbers, including nitrile butadiene rubber (NBR), epoxidized natural rubber, and thermoplastic natural rubber, to simultaneously improve mechanical performance, thermal stability, magnetic behavior, and electrical conductivity.^21–24 Despite these advantages, the combination of CDs and Fe₃O₄ as nitrogen-doped magnetic carbon dots (Fe₃O₄/N–CQDs) has not been explored in rubber composites. To expand the functional scope of CDs, recent research has investigated their doping and hybridization with inorganic nanoparticles.⁹ Nitrogen doping introduces additional reactive sites on the CDs, improving their bonding with polymer chains, while attaching Fe₃O₄ nanoparticles to the CDs provides additional functionalities, including magnetic response, dielectric behavior, and electromagnetic wave absorption. Combining these two components into a single hybrid nanofiller offers a rational strategy to integrate nanoscale reinforcement with multifunctional properties.

Acrylonitrile–butadiene rubber (NBR) provides an ideal platform to evaluate such hybrid fillers. As a polar elastomer containing nitrile (–CN) groups, NBR exhibits excellent resistance to oils, fuels, and chemicals and shows good compatibility with polar and functional nanofillers.²⁵ Conventional NBR vulcanizates are widely employed in antistatic components, seals, and industrial rubber products²⁶; however, their performance is often constrained by susceptibility to thermo-oxidative aging, gradual mechanical deterioration during long-term service,^26,27 and limited magnetic or dielectric response. Enhancing these properties is therefore essential to extend the functionality and reliability of NBR-based materials for next-generation multifunctional rubber components.

In this context, Fe₃O₄/N–CQDs are expected to address these limitations by combining the reinforcing and interfacial advantages of nitrogen-doped CDs with the magnetic and dielectric functionalities of Fe₃O₄. In the present study, Fe₃O₄/N–CQDs were synthesized via a one-step microwave-assisted method using sugarcane bagasse as a sustainable carbon precursor, with FeCl₃.6H₂O and FeCl₂.4H₂O as iron sources. The hybrid nanofillers were incorporated into vulcanized NBR, and their effects on curing characteristics, mechanical properties, swelling behavior, thermal aging resistance, and magnetic and dielectric responses were systematically investigated. By integrating biomass-derived nanomaterials into NBR, this work provides a pathway toward multifunctional polymer composites with enhanced performance and reduced environmental impact.

Substances and experimental techniques

Substances

• The sugarcane bagasse (SCB) was obtained from the Paper Industry Quena Company, Egypt.

• Nitrile rubber (NBR) was bought from Bayer AG Company (Germany) with CN content of 32% and specifc gravity 1.17 ± 0.005 g/cm³.

• N-cyclohexyl-2-benzothiazole sulphenamide (CBS), a light gray powder with a melting point of 95–100°C and a specific gravity of 1.27–1.31 at room temperature (25°C ± 1) provided from Sigma Aldrich.

• stearic acid and Zinc oxide as activators, with specific gravities of 5.55–5.61 and 0.90-0.97 at 15°C, respectively provided from Sigma Aldrich.

• The elemental sulfur as vulcanizing agent that was applied as a fine, pale yellow powder with a specific gravity of 2.04–2.06 at ambient temperature purchased from Sigma Aldrich.

Experimental techniques

Preparation of nitrogen doped carbon quantum dots

A homogeneous solution was initially formed by stirring a mixture of 30 mg SCB, 70 mg NaOH, and 2400 mg urea in 100 mL water for 30 min. The solution was then subjected to a freeze-thaw cycle (frozen overnight, defrosted the next day). Subsequently, the defrosted solution was treated with ultrasound for 2 min to ensure adequate de-clumping. For the final modification step, the solution was heated in a 700 W microwave for about 7 min.²⁸

Preparation of magnetite/nitrogen doped carbon quantum dots (magnetite N–CQDs)

Preparation of the magnetite N–CQDs composite was achieved through a microwave-assisted process, utilizing FeCl₃⋅6H₂O and FeCl₂⋅4H₂O as the iron sources. An aqueous solution of N–CQDs (1 g in 50 mL H₂O) was subjected to 5 min of sonication. Concurrently, an iron precursor solution (50 mL) containing 9.07 g Fe³⁺and 5.05 g Fe²⁺was prepared and introduced into the N–CQDs dispersion. 1 M NaOH was used to elevate the pH to 11−12. The mixture was then heated using a 700 W microwave for 10 min. Post-synthesis, the magnetite N–CQDs product was isolated magnetically. Purification involved washing the material with deionized water and absolute ethanol. The final product was obtained after drying in an oven at 60°C.²⁸

Preparation of NBR compounds

To create formulas for the rubber compounds being studied, Table 1 lists the rubber and its constituents. According to ASTM D3182, all rubber and its component parts were mixed in a two-roll mill with an outside diameter of 470 mm, a working distance of 300 mm, a slow roll speed of 24 rpm, and a friction ratio of 1:1.4. NBR compounds were made using two roll-mixing mills and varied magnetic carbon dot concentrations (0, 2, 5, 8, and 11 phr). The final compounds were then cured in a hydraulic press that was electrically heated to 152 ± 1°C and pressured to about 4 MPa for the ideal cure time (tc90).

Table 1.

Chemical substances used in NBR compounds.

Ingredients	Role	Content (phr)^a
Ingredients	Role	NBR-0	NBR-2	NBR-5	NBR-8	NBR-11
NBR rubber	Polymer	100	100	100	100	100
Stearic acid	Activators	2	2	2	2	2
Zinc oxide	Activators	4	4	4	4	4
CBS^b	Accelerators	1.2	1.2	1.2	1.2	1.2
Sulfur	Curing agent	2	2	2	2	2
Magnetic carbon dots (Fe₃O₄/N–CQDs)	Investigated material	0	2	5	8	11

^aPart per hundred parts of rubber.

^bN-cyclohexyl-2-benzothiazole sulphonamide.

Measurements and description

Fourier transforms infrared spectroscopy (FT-IR) analysis

FTIR spectra of the magnetic carbon dots were recorded using a Mattson 5000 FTIR spectrometer (Unicam, UK) employing the KBr pellet method in the range of 4000–400 cm⁻¹. ATR-FTIR measurements of the NBR samples were carried out using a VERTEX 80 FTIR spectrometer (Bruker Corporation, Germany) equipped with a platinum diamond crystal as the internal reflection element, over the same spectral range (4000–400 cm⁻¹).

Diffraction of X-ray powder (XRD)

X-ray diffraction (XRD) patterns were recorded using a Bruker D-8 Advance X-ray diffractometer (Germany) operated at 40 kV and 40 mA. Diffraction data were collected over a 2θ range of 5°–70° with a step size of 0.05°. Copper Kα radiation (λ = 1.5406 Å) was used to investigate the crystallinity of the samples.

Transmission electron microscope (TEM)

The JEM-2100F (JEOL, Japan) was utilized to capture TEM images at an accelerating voltage of 200 kV. The ethanol solution was put onto a carbon-coated copper grid and let to evaporate after several drops of the nanoparticle solution were diluted with one mL of ethanol.

Thermogravimetric data analysis

The instrument utilizedthe thermal gravimetric/differential thermogravimetric (TGA/DTG)was a TGA Q500 (TA Instruments, USA). It was evaluated by thermogravimetric analysis (TGA, USA). Using 0.1 mg thermo balance sensitivity, it was used to evaluate the thermal stability of powder filler and vulcanized rubber nanocomposite films. The samples were heated from 50°C to 1000°C at a rate of 10°C per minute while being exposed to nitrogen gas at a flow rate of 50 mL per minute. The differential curve was used to determine the samples weight loss at the proper temperature.

Rheology characteristics

The curing durations (Tc90, min), scorch times (Ts2, min), and cure rate indices (CRI, min⁻¹) at 162°C were measured using the TA instrument MDR 1 (Moving Die Rheometer), USA, in accordance with ASTM D2084 to ascertain the rheometric characteristics of rubber compounds.

Scanning electron microscopy (SEM)

Observing the surface morphology of things is possible using the scanelectron microscope (SEM), a type of electron microscopy. The Quanta FEG-250 was used to take SEM images of the samples in order to analyze the surface morphology of the produced NBR composites. The samples were covered with a very thin layer of gold, ranging in thickness from 5 to 10 nm, in around 6 min. The specimensmiddle cross-section was chosen to represent the overall morphology.

Mechanical properties evaluation

The tensile behavior of the NBR vulcanizates was assessed using a universal testing machine (Zwick Z010, Germany) following the ASTM D412 standard. Dumbbell-shaped specimens with a thickness of 1 mm were precision-cut from the compression-molded sheets using a Wallace die cutter (Model S6/1/6. A). The tests were conducted at room temperature under a constant crosshead speed of 500 mm/min to evaluate tensile strength, elongation at break, and modulus characteristics.

Shore A hardness measurements were performed using a digital durometer (Bareiss, Oberdischingen, Germany) following ASTM D2240. All measurements were taken at ambient conditions to ensure consistency and comparability across samples.

Measurements of swelling

They completed the equilibrium swelling Q test parts. For a full day, each specimen weighed between 0.1 and 0.2 g in a weighing bottle immersed in toluene to attain equilibrium swelling. Following weighing, the swollen samples were dried in an oven until their weight remained consistent. The samples are weighed to determine the final weight after being free of dissolved components. Utilizing the following formulas (1), the Q% was determined.

Q % = [(w - w ˋ) / w ˋ] x 100

(1)

W stands for the weight of the swollen sample and wˋ for the dry sample.

All of these tests were conducted at room temperature, and in accordance with ASTM D 471, the stated results were averaged from a minimum of five specimens.

Thermal oxidative aging

Aging of samples was carried out at 110 ± 1°C for 10 days in an air-circulating oven as ASTM D 572-04. The reported results were averaged from a minimum of five specimens.

Mechanical properties were evaluated to assess changes in tensile strength, elasticity, and hardness due to thermal-induced crosslinking or degradation. Additionally, swelling tests were repeated to determine potential changes in solvent uptake behavior, reflecting alterations in crosslinking or network integrity caused by thermal aging. This approach enabled a detailed comparison of the thermal aging response between unfilled and (Fe₃O₄/N–CQDs) -filled NBR matrices, providing insights into the (Fe₃O₄/N–CQDs) role in thermal aging resistance.

The aging factor (Ka), which measures changes in mechanical properties, was calculated from the results of the tensile test using the following

Aging factor (Ka) = (T 1 \times E 1) / (T 2 \times E 2) \times 100 %

(2)

where T1 and T2 are the tensile strength of the aged and un-aged samples, respectively. E1 and E2 are the elongation at break of the aged and un-aged samples, respectively.^29,30

Magnetic properties

The magnetic characterization of NBR/Fe3O4/N–CQDs nanocomposites at room temperature was conducted using a Vibrating Sample Magnetometer (VSM, Lake Shore Model 7410, USA). Essential magnetic characteristics, such as saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc), were obtained from the hysteresis loops recorded under ambient conditions.

Dielectric spectroscopy technique

Dielectric and conductivity measurements were conducted using a high-resolution broadband impedance analyzer (Schlumberger Solartron 1260). The utilized AC electric field varied from 0.1 Hz to 1 MHz. Electromagnetic shielding was implemented on the sample holder to mitigate low-frequency noise. The impedance analyzer was interfaced with a PC using a GPIB cable IEE488 to facilitate automated measurements. Data was collected using LabVIEW. Calibration was conducted prior to sample readings to eliminate stray capacitance. The errors in the dielectric constant ε′ and the loss tangent tan δ are 1% and 3%, respectively. The sample temperature was regulated by a temperature controller equipped with a Pt 100 sensor. Desiccators loaded with silica gel maintained the samples' dryness. Subsequently, the sample was positioned in the measuring cell containing P₂O₅ until measurements were conducted.

Results and discussion

In this study, magnetic nitrogen-doped carbon quantum dots (Fe3O4/N–CQDs) were synthesized and characterized via FTIR, XRD, TGA, and TEM. These nanohybrids were then incorporated into an NBR matrix to develop multifunctional nanocomposites. The curing kinetics, mechanical performance, equilibrium swelling, and thermal stability of the resulting nanocomposites were investigated. Additionally, the thermal aging resistance, magnetic response, and dielectric behavior were evaluated to determine the influence of the filler on the resulting NBR composites.

Analysis of thermogravimetric data (TGA/DTG)

The thermal stability and decomposition behavior of the Fe₃O₄/N–CQDs were investigated by TGA/DTG analysis over the temperature range of 25–1000°C (Figure 1). The thermogram reveals a multistep degradation process characteristic of nitrogen-doped carbon quantum dots hybridized with magnetite nanoparticles. The first weight-loss stage occurs below 100°C, with a DTG maximum around 70–80°C, and is attributed to the removal of physically adsorbed moisture and residual volatile species associated with the hydrophilic surface of the carbon dots.⁸ The second degradation region appears as two partially overlapping DTG peaks between approximately 90 and 245°C, corresponding to the decomposition of labile oxygen- and nitrogen-containing surface functional groups, such as hydroxyl, carboxyl, carbonyl, and amine moieties introduced during nitrogen doping and carbonization. Similar behavior has been reported for N-doped graphene quantum dots and magnetite–CQD hybrid systems.^31,32

Figure 1.

TGA of magnetite Fe₃O₄/N-CQDs.

The third decomposition stage, extending from about 245 to 450°C with DTG maxima near 300 and 400°C, is attributed to the thermal degradation of more stable organic components and the partial breakdown of the carbonaceous framework of the quantum dots. This stage reflects the decomposition of condensed carbon structures and chemically bound functional groups.

A final broad weight-loss stage is observed above approximately 520°C and extends up to 980°C, which is associated with the gradual decomposition and oxidation of the carbon core, along with possible structural rearrangement or mineralization of the iron oxide phase. At these elevated temperatures, magnetite is expected to undergo partial oxidation or phase transformation toward more thermodynamically stable iron oxide phases (e.g., hematite), consistent with observations reported in the literature.³³

X-ray diffraction (XRD)

The X-ray diffraction (XRD) pattern of the Fe₃O₄/N–CQD nanocomposite is presented in Figure 2. A broad diffraction feature centered around 22–27° is observed, which is attributed to the presence of nitrogen-doped carbon dots. In addition, characteristic diffraction peaks appearing at 2θ ≈ 30.1°, 35.5°, 43.2°, 53.4°, 57.0°, and 62.6° correspond to the (220), (311), (400), (422), (511), and (440) crystallographic planes of cubic magnetite (Fe₃O₄), respectively. These reflections are consistent with the standard magnetite phase (JCPDS No. 01-076-7166), confirming the successful incorporation of Fe₃O₄ nanoparticles within the N–CQD structure.⁸ This result is in agreement with previous report.^34–36

Figure 2.

XRD of magnetite Fe₃O₄/N-CQDs.

Infrared fourier transforms spectroscopy (FTIR)

The FTIR spectrum of the magnetite N–CQDs is shown in Figure 3. A broad absorption band observed between 3191 cm⁻¹ and 3330 cm⁻¹ is assigned to O–H and N–H stretching vibrations, indicating the presence of multiple hydroxyl and amine functional groups on the NCQD surface.³⁷ The absorption band at 1621 cm⁻¹ is attributed to C = O stretching vibrations, whereas the band at 1409 cm⁻¹ corresponds to C = C stretching of the carbon framework. The band at 1064 cm⁻¹ is associated with O–C = O stretching, confirming the presence of carboxylate groups. The absorption at 865 cm⁻¹ is assigned to C–N stretching, providing evidence of nitrogen incorporation into the carbon dots. Finally, the characteristic absorption band at 655 cm⁻¹ corresponds to Fe–O stretching vibrations, confirming the successful incorporation of magnetite (Fe₃O₄) within the N–CQD structure.^8,38

Figure 3.

FTIR of magnetite Fe₃O₄/N-CQDs.

Morphological studies by using transmission electron microscopy (TEM)

Typical TEM images and diameter distributions of N-CQDs and Fe₃O₄/N-CQDs are shown in Figure 4. The TEM image of N-CQDs reveals uniformly spherical dots with diameters ranging from approximately 2.35 to 2.90 nm indicating good monodispersity and the absence of aggregation. These observations are consistent with previously reported HRTEM results, which showed spherical, homogeneously monodispersed CQDs with no noticeable aggregation and particle sizes in the range of 7–16 nm.³⁹ In contrast, for the Fe₃O₄/N-CQDs nanomaterial, the CQDs embedded between the platelets have diameters of about 0.74 to 0.97 nm, while the platelets themselves aggregate to sizes between 5.77 and 8.00 nm. Due to the small size and high dispersion of the N-CQDs, individual N-CQDs were not distinctly visible within the Fe₃O₄/N-CQDs sample. These observations, together with the FTIR and XRD results, confirm the successful synthesis of the Fe₃O₄/N-CQDs.⁸

Figure 4.

TEM image of N-CQDs and Fe₃O₄/N-CQDs.

Rheological studies

The rheological and curing characteristics of NBR compounds containing different loadings of Fe₃O₄/N-CQDs were evaluated using a moving-die rheometer at 162 ± 1°C, and the results are summarized in Table 2. Rheometric analysis provides essential information regarding processability, cure kinetics, and crosslink network development during vulcanization.

Table 2.

Rheological properties of the prepared NBR vulcanizates.

Properties	NBR-0	NBR-2	NBR-5	NBR-8	NBR-11
Minimum torque (ML), dN_m	0.56	0.64	0.64	0.67	0.65
Maximum torque (MH), dNm	11.19	11.67	11.67	11.69	11.26
Optimium cure time (t_C90), min	16.55	12.74	11.58	10.88	11.50
Scorch time t_S2, min	5.95	3.89	3.05	2.65	2.89
Cure rate index, (CRI, min^-1)	9.43	11.30	11.72	12.15	11.61

where dNm: Deci Newton m.

The minimum torque (ML), which reflects the viscosity and stiffness of the uncured rubber compound, shows a slight increase with the incorporation of Fe₃O₄/N-CQDs compared to neat NBR. This increase indicates enhanced filler–rubber interactions and restricted chain mobility due to the presence of well-dispersed nanofillers.⁴⁰ Similar behavior has been reported in carboxylated XNBR latex reinforced with carbon dots (CDs), where a decrease in the tan δ peak height (tan δmax) was observed with increasing CD concentration.¹⁵ The reduction in tan δmax reflects decreased polymer chain mobility during the dynamic transition process and is also attributed to strong polymer–filler interactions. In the present study, the highest ML value is observed for NBR-8, suggesting the formation of an interconnected filler–rubber network, whereas the slight reduction in ML at NBR-11 is likely due to partial aggregation of Fe₃O₄/N-CQDs, which reduces the effective interaction area between filler and polymer chains. The maximum torque (MH), which is directly related to the stiffness and crosslink density of the vulcanized rubber, increases progressively with increasing Fe₃O₄/N-CQDs content and reaches a maximum value of 11.69 dNm for NBR-8. This enhancement indicates improved crosslink density and reinforcement efficiency, arising from strong interfacial interactions between N-CQDs and the NBR matrix. The slight decrease in MH observed for NBR-11 is attributed to excessive filler loading causing aggregation, which hinders stress transfer and crosslink formation.

The scorch time (t_S2), representing the processing safety of the rubber compound, decreases significantly with increasing Fe₃O₄/N-CQDs loading. Compared with neat NBR, tS₂ decreases from 5.95 min to 2.65 min at NBR-8, indicating a faster onset of vulcanization. This behavior suggests that Fe₃O₄/N-CQDs act as cure activators, likely due to the presence of nitrogen-containing functional groups on the N-CQDs surface and the catalytic activity of Fe₃O₄, which facilitate sulfur crosslinking reactions.^41,42 In contrast to previously reported DVCR/N-CDs systems, where the t10 time was initially longer due to acidic surface groups requiring MgO to neutralize them before rapid vulcanization could proceed, in the Fe₃O₄/N-CQDs/NBR system the acidic groups interact with Fe₃O₄ and do not inhibit curing.⁴³ As a result, the vulcanization starts faster without an introduction period. At higher filler loading (NBR-11), a slight increase in t_S2 is observed, which may be attributed to partial nanofiller aggregation that limits the accessibility of active functional groups and reduces curing efficiency. Similarly, the optimum cure time (t_C90), defined as the time required to reach 90% of the final crosslink density, decreases markedly with increasing Fe₃O₄/N-CQDs content, from 16.55 min for neat NBR to 10.88 min for NBR-8, confirming accelerated curing kinetics. This behavior is consistent with the same mechanism observed for t_S2. A slight increase in t_C90 beyond 8 phr follows the same trend as tS₂, attributed to restricted polymer chain mobility and reduced availability of reactive sites due to excessive nanofiller loading.

The cure rate index (CRI), which quantitatively describes the curing speed, increases significantly with Fe₃O₄/N-CQDs addition and reaches a maximum value of 12.15 min⁻¹ at NBR-8. This trend confirms the enhanced vulcanization rate due the presence of nitrogen-containing functional groups on the N-CQDs surface and the catalytic activity of Fe₃O₄. The decrease in CRI at NBR-11 further supports the negative effect of excessive filler loading on curing efficiency.

From the rheological results, it has been demonstrated that Fe₃O₄/N-CQDs significantly influence the curing behavior of NBR by enhancing filler–rubber interactions, accelerating cure kinetics, and increasing crosslink density. However, beyond an optimal filler loading of 8 phr, the benefits diminish due to filler aggregation and restricted chain mobility.

Fourier transforms infrared spectroscopy for NBR and NBR/(Fe₃O₄/N-CQDs)

Figure 5 shows the FTIR of NBR and NBR/(Fe₃O₄/N-CQDs) composites. As observed from this figure that The FTIR spectrum of vulcanized NBR displayed the characteristic absorptions at ∼2237 cm⁻¹ (C≡N stretching), 2850–2919 cm⁻¹ (aliphatic C–H stretching), and CH₂ bending at ∼1440 and 1357 cm⁻¹, consistent with previous assignments.^44,45 Upon incorporation of (Fe₃O₄/N-CQDs), two notable spectral changes are observed. A new absorption band appears at 1596 cm⁻¹, which is attributed to the asymmetric stretching vibration of carboxylate (–COO^-) groups on the nitrogen-doped carbon dots.⁴⁶ In addition, a distinct band at approximately 696 cm⁻¹ corresponds to the Fe–O stretching vibration of magnetite, confirming the presence of Fe₃O₄ within the composite.⁸ Moreover, the broadening of the 3200–3500 cm⁻¹ regions indicates O–H/N–H stretching from CQD surface functionalities.³⁷ These modifications confirm the successful incorporation of (Fe₃O₄/N-CQDs) into the NBR matrix and suggest interfacial interactions between the polymer chains and the functional groups of the carbon dots.

Figure 5.

FTIR of NBR and NBR containing Fe₃O₄/N-CQDs.

Based on the FTIR results, multiple interfacial interactions are involved in the bonding between the NBR matrix and the Fe₃O₄/N-CQDs hybrid filler. The polar functional groups on the surface of the CQDs (–OH, –NH₂, and –COOH) interact with the nitrile (–C≡N) groups of NBR through hydrogen bonding and dipole–dipole interactions, enhancing the interfacial affinity between the filler and the rubber matrix.^46,47

Under vulcanization conditions, the elevated temperature and curing agents can induce partial deprotonation of surface carboxylic groups on the CQDs, enabling coordination with Fe₃O₄ nanoparticles.⁴⁶ This coordination is supported by the appearance of a new band at 1596 cm⁻¹, assigned to the asymmetric stretching of carboxylate (–COO^-) groups, and by the shift of the Fe–O stretching vibration from 655 cm⁻¹ in pristine Fe₃O₄/N-CQDs to 696 cm⁻¹ in the composite. Coordination between the CQDs and Fe₃O₄ stabilizes the hybrid filler structure, which in turn reinforces the physical interactions at the NBR–filler interface, improving adhesion and dispersion of the hybrid nanofiller. In addition, the flexible NBR chains can wrap around the Fe₃O₄/N-CQDs, providing mechanical interlocking and further enhancing the overall structural integrity of the composite.

Mechanical properties of NBR vulcanizates reinforced with magnetic carbon dots (Fe₃O₄/N-CQDs)

The mechanical properties of NBR vulcanizates as a function of Fe₃O₄/N-CQD loading, including tensile strength, elongation at break, and modulus at 50, 100, 200, and 300% strain, are summarized in Table 3. Incorporation of Fe₃O₄/N-CQDs results in a noticeable improvement in tensile strength and modulus up to 8 phr. The tensile strength increases from 2.3 MPa for neat NBR to 4.95 MPa for NBR-8, while the modulus at all measured strains also shows a moderate increase. This improvement can be attributed to enhanced filler–matrix interactions, where the well-dispersed Fe₃O₄/N-CQDs restrict polymer chain mobility and facilitate more efficient stress transfer from the NBR matrix to the nanofiller.⁴⁰ Beyond 8 phr, at 11 phr, the tensile strength and modulus decrease slightly. This behavior is attributed to partial agglomeration of the Fe₃O₄/N-CQDs, which reduces the effective surface area for polymer–filler interactions and creates local heterogeneities in the matrix. These agglomerates act as rigid inclusions that limit the uniform deformation of the polymer, leading to decreased reinforcement efficiency. A similar trend has been reported for chloroprene rubber reinforced with nitrogen-doped carbon nanodots (N-CDs), where excessive filler loading led to reduced mechanical performance due to agglomeration effects.⁴³

Table 3.

Mechanical properties of NBR vulcanizates loaded with different concentrations of magnetic carbon dot (Fe₃O₄/N-CQD).

Samples	Tensile strength (MPa)	Elongation at break (%)	Modulus, 50% (MPa)	Modulus, 100% (MPa)	Modulus, 200% (MPa)	Modulus, 300% (MPa)	Hardness, shore A
NBR-0	2.3	673	0.641	0.951	1.254	1.487	54
NBR-2	3.0	470	0.722	1.055	1.365	1.642	56
NBR-5	3.9	427	0.788	1.123	1.464	1.767	61
NBR-8	4.2	423	0.933	1.254	1.634	2.034	65
NBR-11	3.5	409	0.884	1.198	1.556	1.851	68

Elongation at break shows a decreasing trend with increasing Fe₃O₄/N-CQD content. The reduction in elongation reflects the increased rigidity of the vulcanizates due to filler incorporation, which restricts polymer chain mobility. Consistent behavior has been reported in polymer nanocomposites reinforced with graphene oxide nanosheets (GOn) and ferromagnetic iron oxide (Fe₃O₄) nanoparticles, where increasing filler content led to reduced elongation due to constrained chain motion and increased stiffness in PVDF/SBR-based hybrid systems.⁴⁰

These results indicate that moderate loading of Fe₃O₄/N-CQDs (up to 8 phr) provides optimal reinforcement, improving tensile strength and modulus. Excessive filler loading leads to agglomeration, reduced polymer–filler interactions, and limited stress transfer, which diminish the mechanical benefits.

Hardness of NBR/Fe₃O₄/N-CQD vulcanizates

The hardness of NBR vulcanizates increases steadily with Fe₃O₄/N-CQD loading, from 54 Shore A for neat NBR to 68 Shore A at 11 phr (Table 3). This continuous increase reflects the addition of rigid nanofillers, which restrict local polymer chain mobility and enhance resistance to surface deformation.⁴⁸

It was also noticed that this trend differs from the modulus and tensile strength, which showed an increase up to 8 phr and slightly decrease at higher loadings. This divergence can be explained by the fact that hardness is primarily a measure of local surface rigidity, whereas modulus and tensile strength depend on efficient stress transfer across the bulk material.⁴⁹ At high filler content, partial aggregation of Fe₃O₄/N-CQDs reduces bulk stress transfer, lowering modulus and tensile strength, but the surface stiffness, and thus hardness, continues to rise due to the overall filler content.

Swelling behavior of NBR/Fe₃O₄/N-CQD vulcanizates

The effect of Fe₃O₄/N-CQD concentration on the equilibrium swelling behavior of NBR composites in toluene is presented in Table 4. A gradual decrease in equilibrium swelling is observed with increasing Fe₃O₄/N-CQD content up to 8 phr, followed by a slight increase at higher loading. This reduction indicates a restriction in solvent uptake. The decrease in swelling can be attributed to the presence of nonswellable Fe₃O₄/N-CQDs within the rubber matrix, which effectively reduce the volume fraction of swellable rubber and limit chain mobility.^50,51 In addition, enhanced filler–rubber interactions can lead to an increase in the effective crosslink density and hinder the diffusion of toluene molecules into the network.¹² At higher Fe₃O₄/N-CQD loadings, a slight increase in swelling is observed, likely due to partial agglomeration of the fillers, which allows greater solvent penetration into the network.⁵²

Table 4.

Equilibrium swelling of NBR vulcanizates loaded with different concentrations of magnetic carbon dot (Fe₃O₄/N-CQD).

Samples	Equilibrium swelling, %
NBR-0	188
NBR-2	176
NBR-5	173
NBR-8	169
NBR-11	170

The crosslinking density of NBR and NBR/Fe₃O₄/N–CQDs nanocomposites was quantitatively determined from equilibrium swelling measurements in toluene for 24 h at 25 ± 1°C and calculated using the Flory–Rehner equation.¹² The crosslink density (v) and the molecular weight (g/mol) between crosslinks (Mc) were calculated as follows:

v = \frac{1}{2 Mc}

(3)

Mc = - ρ VsVr 13 [\ln (1 - Vr) + Vr + χ Vr 2]

(4)

where Vr is the volume fraction of swollen rubber that can be obtained from the mass and density of rubber samples and the solvent (Vr = 1/1 + Q), ρ is the density of rubbers (ρ NBR is 1.17 g/cm³), versus the molar volume of the solvent (toluene) = 106.35 cm³/mol, and χ is the interaction parameter of NBR rubber, which is 0.39.

All swelling experiments were conducted under identical conditions (24 h at 25 ± 1°C) to ensure consistent comparisons, and the calculated values of v and Mc are summarized in Table 5. It was observed that up to 8 phr of Fe₃O₄/N-CQDs, the crosslinking density of NBR vulcanizates increased while the molecular weight between crosslinks (Mc) decreased. This behavior is attributed to strong interactions between the NBR chains and the Fe₃O₄/N-CQDs, which restrict polymer chain mobility and promote the formation of a more effective physical and chemical network within the matrix.^40,43 At 11 phr, a slight decrease in crosslinking density and an increase in Mc were observed, indicating that partial agglomeration of Fe₃O₄/N-CQDs creates heterogeneous regions where the polymer chains are less constrained. In these less-restricted areas, solvent penetration is easier, resulting in the observed increase in Mc.

Table 5.

Swelling properties of NBR vulcanizates loaded with different concentrations of Fe₃O₄/N–CQDs.

Conc.	Qm	VR	1/2Mc	Mc
NBR-0	188	0.00529101	7.33027E-08	6,821,033.072
NBR-2	176	0.00564972	8.19159E-08	6,103,819.539
NBR-5	173	0.00574713	8.43236E-08	5,929,539.949
NBR-8	169	0.00588235	8.77142E-08	5,700,330.844
NBR-11	170	0.00584795	8.68464E-08	5,757,292.832

SEM images of NBR samples

Figure 6(a)–(c) shows the fracture surface morphology of pristine NBR, NBR/(Fe₃O₄/N-CQDs) at 8 phr, and NBR/(Fe₃O₄/N-CQDs) at 11 phr at low and higher magnifications. The SEM micrograph of pristine NBR (Figure 6(a)) shows a relatively smooth surface without obvious defects, except for a few asperity pits and original processed textures, which is typical of unfilled rubber matrices and indicates limited resistance to crack propagation.^46,53

Figure 6.

SEM of (a and b) NBR, (c and d) NBR/(Fe₃O₄/N-CQDs) (8 phr)and (e and f) NBR/(Fe₃O₄/N-CQDs) (11 phr) at low and high magnification.

With the incorporation of 8 phr Fe₃O₄/N-CQDs (Figure 6(b)), the fracture surface becomes significantly rougher and more irregular. This behavior is analogous to that reported for NR reinforced with hybrid CDs/MoS₂ fillers, where the introduction of a low filler content promoted a more uniform mixed-filler system.⁴⁶

At higher filler loading (11 phr, Figure 6(c)), localized platelet-like structures and micro-scale agglomerates are observed on the fracture surface, particularly at higher magnification. This behavior closely resembles that reported in CDs/MoS₂-reinforced NR at elevated filler contents, where excessive filler loading led to deteriorated dispersion and aggregation.⁴⁶ This result is consistent with the slight decline in mechanical performance observed at higher filler concentrations.

Thermal analysis

Figure 7(a) and (b) presents the TGA and DTG curves of neat NBR and NBR filled with Fe₃O₄/N-CQDs. Both samples exhibit a single dominant thermal degradation step, which is characteristic of NBR and is mainly associated with the decomposition of the polybutadiene backbone and acrylonitrile segments. Compared with neat NBR, the Fe₃O₄/N-CQDs–filled NBR shows a noticeable shift of the main degradation region toward higher temperatures, indicating a delay in the thermal decomposition process. This improvement in thermal stability can be attributed to the presence of thermally stable inorganic Fe₃O₄ nanoparticles and the strong interfacial interactions between N-CQDs and the NBR matrix. Similar behavior has been reported in PVDF/SBR nanocomposites reinforced with Fe₃O₄ and graphene oxide, where the incorporation of inorganic nanofillers increased the onset degradation temperature and shifted the maximum degradation temperature (Tmax) to higher values due to their barrier effect and high intrinsic thermal stability.^37,40

Figure 7.

TGA (a) and DTG (b) curves of neat NBR and NBR filled with Fe₃O₄/N-CQDs.

The DTG curves further support this observation, as the maximum degradation rate of the composite shifts slightly toward higher temperatures compared to neat NBR, confirming a slower degradation kinetics. This trend is consistent with previous reports on Fe₃O₄-based polymer nanocomposites.^40,53

In addition, the char yield of the Fe₃O₄/N-CQDs/NBR composite (∼30%) is significantly higher than that of neat NBR (∼24%). The increased residue content is attributed to the inorganic Fe₃O₄ component and the carbonaceous structure of the CQDs, which promote char formation and enhance thermal resistance at elevated temperatures. Similar increases in char yield have been observed in Fe₃O₄@GO-reinforced polymer systems.^37,40

Thermal aging resistance

Thermal aging refers to the progressive degradation of polymeric materials resulting from prolonged exposure to elevated temperatures, often in the presence of oxygen. In elastomers such as nitrile butadiene rubber (NBR), which contain unsaturated bonds, thermal-oxidative aging can induce chain scission, oxidative reactions, and undesirable changes in crosslink structure. These processes typically lead to deterioration in mechanical properties, reduced elasticity, and ultimately premature material failure. Consequently, improving the resistance of rubber composites to thermal aging is critical for extending their service life.^12,53

Mechanical properties after aging

The thermal aging behavior of NBR compounds was evaluated by aging the samples at 110 ± 1°C for 10 days. The variations in tensile strength, elongation at break, and hardness after aging are presented in Figure 8(a)–(c). The aging behavior of Fe₃O₄/N-CQD–filled NBR was systematically compared with that of unfilled NBR (NBR-0). For neat NBR, a significant deterioration in mechanical properties was observed after aging. The tensile strength decreased from 2.3 MPa to 1.5 MPa, corresponding to a loss of approximately 35% of its initial strength, which can be attributed to thermal oxidative degradation and chain scission. In contrast, all Fe₃O₄/N-CQD–filled NBR samples exhibited markedly improved tensile strength retention after aging. It was also noticed that the NBR-8 sample showed the highest tensile strength after aging, reaching 5.9 MPa, indicating superior resistance to thermal aging compared to unfilled NBR. The improved aging resistance of Fe₃O₄/N-CQD–filled NBR can be attributed to several synergistic effects. The hybrid nanofillers act as effective barriers against thermal and oxidative degradation by restricting oxygen diffusion and limiting polymer chain mobility. In addition, carbon dots are known to possess radical-scavenging capability, which can suppress thermo-oxidative chain scission by neutralizing free radicals generated during aging, thereby retarding degradation of the rubber network.^12,53,54

Figure 8.

Physico-mechanical properties of NBR vulcanizates before and after aging: (a) tensile strength; (b) elongation at break; (c) Hardness.

Elongation at break decreased for all samples after aging, reflecting increased stiffness caused by degradation of polysulfidic cross-links and the subsequent post-curing process.⁵² For NBR-0, elongation at break dropped sharply from 673% to 205%, indicating a severe loss of elasticity due to dominant oxidative degradation. Hardness measurements further support these observations, as the hardness of all samples increased after aging. This increase is primarily attributed to the transformation of thermally unstable polysulfide cross-links into shorter mono- and disulfide cross-links, which are more resistant to heat and oxidation and result in a stiffer and more compact rubber network.⁵² In addition, the polar functional groups on Fe₃O₄/N-CQDs (–OH, –COOH, –C = O) can form hydrogen bonds with oxygenated species generated in the aged NBR. These hydrogen bonds act as temporary physical crosslinks that restrict chain mobility and complement the chemical network, further stabilizing the rubber structure.⁵² That is why Fe₃O₄/N-CQD–filled NBR samples exhibited higher Shore A hardness values than neat NBR.

From these results it was found that the incorporation of Fe₃O₄/N-CQDs significantly enhances the thermal aging resistance of NBR by improving tensile strength retention, maintaining reasonable ductility, and increasing hardness.

The aging factor

The product of tensile strength and elongation at break is commonly used as an indicator of a material’s ability to absorb mechanical energy during deformation prior to fracture.⁵⁵ Based on this concept, the aging factor (K_a) of the NBR vulcanizates was calculated to evaluate the overall retention of mechanical performance after thermal aging, and the results are represented in Figure 9.

Figure 9.

Values of aging factor after thermal oxidative aging.

As shown in Figure 9, the aging factor of all NBR compounds increased after aging, indicating structural changes in the rubber network during prolonged thermal exposure. The incorporation of Fe₃O₄/N-CQDs significantly enhanced the aging factor compared to unfilled NBR. With increasing filler content, K_a increased gradually and reached a maximum at 8 phr, after which a slight decrease was observed at higher loading levels. For neat NBR (NBR-0), the aging factor increased from 19.37 before aging to 61.08 after 10 days of aging. It was also noticed that the NBR filled with 8 phr Fe₃O₄/N-CQDs exhibited the highest aging factor after aging, demonstrating superior retention of mechanical energy absorption capability.⁵⁵

The higher K_a values observed for Fe₃O₄/N-CQD–filled NBR indicate improved resistance to thermal aging. This enhancement is attributed to the synergistic effects of the hybrid nanofillers, including improved interfacial interactions, restriction of polymer chain mobility, and suppression of thermo-oxidative degradation. Although thermal aging generally leads to deterioration of rubber mechanical properties, the presence of CQDs helps maintain higher tensile strength and elongation at break, resulting in a higher aging factor compared to unfilled NBR.

Swelling properties after aging

The effect of thermal-oxidative aging on the swelling ratio (Q%) of NBR composites before and after aging is presented in Figure 10. For neat NBR (NBR-0), Q% increased from 188% to 194% after aging at 110°C, indicating the predominance of chain-scission processes that progressively weaken the rubber network under thermal-oxidative conditions.

Figure 10.

Equilibrium swelling of NBR vulcanizates before and after aging.

In contrast, the (Fe₃O₄/N-CQD)-filled composite (NBR-8) exhibited a pronounced decrease in Q%, from 180% before aging to 153% after aging. This reduction suggests that the nano-(Fe₃O₄/N-CQD) hybrid acts as an anti-aging additive by promoting post-curing and additional crosslink formation during aging, resulting in a denser and more stable network structure.^12,53–55 Consequently, this network resists solvent uptake, resulting in the marked decrease in swelling observed in the aged composites.

Magnetic properties

The room-temperature magnetic characterization of NBR/Fe3O4/N–CQDs nanocomposites is presented in Figure 11. It exhibits a clear dependence on filler loading, as evidenced by the hysteresis loops and the summarized magnetic properties. Saturation magnetization (Ms) increases from 29.05x 10⁻³ emu/g at 0 phr to 91.32 × 10⁻³ emu/g at 11 phr, indicating the appropriate addition of magnetic fillers and an enhancement in magnetic response. This aligns with existing knowledge regarding Fe₃O₄-based polymer nanocomposites.⁵⁶ The coercivity (Hci) values exhibit a non-monotonic trend. The pristine matrix exhibits a notably high coercivity of 3546.3 G, likely attributed to residual magnetic impurities or anisotropy inherent to the matrix. The filled samples exhibit soft magnetic behavior, characterized by a Hci below 120 G, consistent with the effects of nanoscale magnetite dispersion.⁵⁷ The remnant magnetization (Mr) and squareness ratio (Mr/Ms) remain low across all samples, indicating super paramagnetic-like properties suitable for applications requiring rapid magnetic reversibility and minimal residual magnetization.⁵⁸ The more pronounced hysteresis loops observed with 11 phr loading indicate stronger magnetic interactions and a greater magnetic moment density. Slight loop asymmetries and negative coercivity values may result from internal stress or measurement errors. The findings suggest that NBR/Fe3O4/N–CQDs composites are suitable for flexible magnetic and magneto-responsive applications, and EMI shielding.⁵⁹

Figure 11.

Magnetization curves of NBR/Fe₃O₄/N–CQDs nanocomposites.

Dielectric properties

The incorporation of Fe₃O₄/N-CQDs into a rubber matrix significantly alters the composite’s dielectric and conduction properties, as evidenced by the frequency-dependent results presented in Figure 12(a)–(d). The hybrid filler, composed of magnetite nanoparticles (Fe₃O₄) encased in nitrogen-doped carbon quantum dots (N-CQDs), exhibits a distinctive combination of magnetic, conductive, and polar attributes that interact with the polymer matrix at various levels.

Figure 12.

Frequency-dependent dielectric properties of NBR rubber filled with Fe3O4/N-CQDs at varying concentrations (0,2, 5, 8, and 11 phr), measured at 25°C over the range 0.1 Hz to 1 MHz. (a) Real part of permittivity (ε′), (b) dielectric loss (ε″), (c) loss tangent (tanδ), and (d) Cole–Cole plot of electric modulus (M″ vs M′).

The real part of permittivity (ε′) signifies a material’s ability to retain electrical energy when subjected to an electric field. Figure 12(a) demonstrates that ε′ reduces with rising frequency as the loading of Fe3O4/N-CQDs increases, owing to dipolar orientation lag at higher frequencies.⁶⁰ The considerable rise in ε′ at low frequencies, with increased Fe3O4/N-CQDs loading, is attributed to interfacial polarization, which occurs when the conductivity and permittivity of the rubber matrix and Fe₃O₄/N-CQDs are distinctive. The accumulation of charges at the interfaces is referred to as the Maxwell–Wagner–Sillars (MWS) effect.^61,62 The N-CQDs incorporate supplementary polar functional groups, including –NH and –OH, which enhance the number of dipolar sites and improve the dielectric response. The nanoscale arrangement of Fe₃O₄/N-CQDs improves the strength of microcapacitive networks, therefore elevating ε′. The observed saturation and loss in ε′ at high frequencies indicate that the polarization mechanisms are unable to sufficiently adapt to the rapid oscillations of the electric field, resulting in a decrease in permittivity.

Figure 12(b) illustrates the imaginary component of permittivity (ε″), which quantifies the energy dissipated via conduction and relaxation processes. It is obvious that the values of ε″ decreases as the frequency increases. However, as the frequency increases, the time available for dipoles to reposition is insufficient. Consequently, their impact on dielectric loss ε″ diminishes, resulting in a reduction of ε″.⁶⁰ On the other hand, the rubber composites with higher Fe₃O₄/N-CQDs content exhibited increased ε″ values, attributable to interfacial polarization and possibly hopping conduction among adjacent Fe₃O₄/N-CQDs. The Fe₃O₄component may generate localized magnetic and conductive regions that facilitate the movement of charge carriers in the presence of an electric field. This increases the dielectric loss, particularly in composites with extensive filler networks.^63,64 In heterogeneous systems like rubber composites, charge carriers accumulate at interfaces due to differences in conductivity and permittivity across phases. This accumulation contributes to ε″. At elevated frequencies, the carriers cannot migrate or aggregate effectively, hence hindering interfacial polarization and further reducing ε″.⁶⁵

The loss tangent (tanδ), shown in Figure 12(c), indicates the efficiency of energy dissipation relative to energy storage. For all samples, tanδ decreases with increasing frequency, consistent with the observation that dielectric losses diminish at higher frequencies.⁶⁰ tanδ exhibits considerable values at low frequencies due to ohmic conduction losses and electrode polarization effects. However, it subsequently reaches a minimum in the mid-frequency range, specifically between 10³ and 10⁴ Hz.⁶⁶ The observed increase at elevated frequencies may suggest the onset of additional relaxation processes or the high-frequency limit of the primary relaxation.⁵² In addition tanδ increases with the increasing concentration of Fe₃O₄/N-CQDs at low frequencies. The incorporation of Fe₃O₄/N-CQDs enhances energy dissipation, attributed to several factors: the restricted mobility of polymer chains at filler surfaces, increased friction between these surfaces, and the presence of polar and conductive sites that facilitate polymer relaxation. The broadening of tanδ peaks with increasing Fe₃O₄/N-CQDs content indicates a variety of relaxation times, suggesting complex interactions between the filler and the matrix.⁶⁷

The electric modulus formalism employed in Figure 12(d), where M* = 1/ε*, is highly effective for examining materials with elevated conductivity, as it eliminates electrode polarization artifacts and emphasizes bulk relaxation phenomena.⁶⁸ Figure 12(d) shows a Cole–Cole plot of electric modulus, with M″ on the y-axis and M′ on the x-axis. This model emphasizes relaxation dynamics by reducing the effects of electrode polarization. The semicircular arcs you see are common in dielectric relaxation processes. The 0phr sample has the broadest and most symmetrical arc. The arcs get thinner and more twisted as the content of Fe₃O₄/N-CQDs increases. Moreover, the diameter of the semi-circles decreased with increasing Fe₃O₄/N-CQDs content. This behavior is attributed to the increased conductivity of the rubber composites resulting from a rise in charge carriers. However, there are a lot of relaxation centers, such as filler-matrix interfaces, polar groups, and conductive domains. This leads to a large range of relaxation times and more variety.⁶⁹

Electrical conductivity

The frequency dependence of alternating current conductivity (σ_ac) can be explained by Jonscher’s power law equation (5);⁶⁰

σ_{ac} = σ_{dc} + A ω^{s}

(5)

where ω (=2πf) denotes angular frequency. A and s are constants, while σ_acdenotes the frequency-dependent conductivity assessed under an alternating current field. σ_dc denotes the direct current conductivity. Figure 13(a) illustrates the relationship between σ_ac and the frequency of the applied field. In the low frequency range, it is evident that the conductivity increases extremely slowly or stays the same, but in the high frequency range, it increases quickly. However, in the low frequency region, DC- conductivity (σ_dc) predominates, manifesting as plateau-like behavior in samples with high Fe₃O₄/N-CQD loading compared to other counterparts. At low frequencies and low concentrations of Fe₃O₄/N-CQD, conductivity decreases as frequency diminishes due to the electrode polarization (EP) effect.^52,66 In contrast, conductivity increases at high frequencies, following the universal power law σ_ac = Aωⁿ. n -values are between 0.5 and 1 are indicative of an electrical hopping mechanism.⁷⁰

Figure 13.

(a) The frequency dependence of ac NBR rubber filled with Fe₃O₄/N-CQDs at varying concentrations (0,2, 5, 8, and 11 phr), measured at 25°C and (b) Varation of conductivity with Fe₃O₄/N-CQDs loading.

Figure 13(b) shows how the DC conductivity (σ_dc) changes with Fe₃O₄/N-CQD loading. As the concentration of Fe₃O₄/N-CQD doping rose, a significant rise in conductivity was noted, indicated by the percolation threshold. The direct current conductivity (σ_dc) values of the Fe3O4/N-CQD composites are found between 3 × 10⁻⁹ and 6 × 10⁻⁷ S/cm, placing these materials in the semiconducting to weakly conductive category. The defined conductivity range is advantageous for applications requiring antistatic characteristics or safeguarding against electrostatic discharge (ESD). Materials with σ_dc values under 10⁻⁵ S/cm are classified as dissipative, allowing them to neutralize static charges effectively while inhibiting significant current flow. The conductivity measurements demonstrate that the Fe3O4/N-CQD composites effectively reduce electrostatic buildup on surfaces. This characteristic makes them suitable for applications in packaging sensitive electronic components, antistatic coatings, and environments intended to be ESD-safe. The relatively low conductivity suggests that these composites pose minimal risk of short-circuiting in electronic assemblies, while permitting regulated charge dissipation. The observed frequency-dependent behavior, marked by reduced conductivity at lower frequencies due to electrode polarization (EP), supports the presence of a hopping conduction mechanism.^70,71 The observed behavior corresponds with the disordered structure and nanoscale interfaces in the composite, which facilitate localized charge transport.⁷² The capacity to modify σ_dc through compositional or structural alterations offers increased adaptability for tailoring these materials for specific electrostatic control applications.

Conclusion

In this study, a multifunctional Fe₃O₄/N–CQD hybrid nanofiller was successfully synthesized from sugarcane bagasse using a rapid microwave-assisted method. FTIR and XRD analyses confirmed the formation of Fe₃O₄ and nitrogen-doped carbon dots, while SEM revealed the nanoscale morphology of the hybrid filler. The Fe₃O₄/N–CQDs were incorporated into an NBR matrix at loadings ranging from 0 to 11 phr. Rheological results showed that the nanofiller significantly influenced the curing behavior of NBR by accelerating cure kinetics and increasing crosslink density, with optimal effects at 8 phr; higher loadings reduced benefits due to filler aggregation and restricted polymer chain mobility. FTIR analysis further confirmed successful incorporation of the filler into the NBR matrix.

Thermogravimetric analysis indicated enhanced thermal stability for Fe₃O₄/N–CQD-filled NBR, as the main degradation region shifted to higher temperatures, attributed to thermally stable Fe₃O₄ nanoparticles and strong interfacial interactions with N–CQDs. Mechanical testing demonstrated that moderate filler loading improved tensile strength and modulus, while excessive loading led to agglomeration and reduced stress transfer, as confirmed by SEM. Thermal aging studies showed improved tensile strength retention, maintained ductility, and increased hardness. Magnetic measurements revealed soft magnetic behavior, with saturation magnetization increasing from 29.05x 10⁻³ to 91.32 × 10⁻³ emu/g and coercivity below 120 G. Dielectric and conductivity analyses indicated frequency-dependent permittivity and hopping conduction governed by interfacial polarization.

Based on these results, Fe₃O₄/N–CQDs derived from agricultural waste can act as an effective multifunctional nanofiller, simultaneously enhancing curing behavior, mechanical performance, thermal stability, aging resistance, magnetic properties, and dielectric behavior of NBR.

Footnotes

Acknowledgements

The author appreciates the National Research Centre for supporting.

Author contributions

Hebat-Allah S. Tohamy: Investigation & Methodology, Software, Conceptualization, Validation, Formal Analysis, Writing – Original Draft, and Writing – Review & Editing. Heba Kandil: Methodology, Data Curation, Investigation, Formal Analysis, Validation, Writing – Original Draft and Writing – Review & Editing. Azza A. Ward: Data Curation, Validation, Investigation, Visualization, Formal Analysis, and Writing – Review & Editing. Doaa E. El Nashar: Supervision, Resources, Conceptualization, Investigation, Formal Analysis, Writing – Original Draft, and Writing – Review & Editing.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

ORCID iD

Doaa E El Nashar

Data Availability Statement

All data is presented in the figures, graphics and tables in the context of the manuscript.*

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Microwave assisted magnetite carbon dots for enhanced the thermal aging resistance,magnetic and dielectric properties of acrylonitrile butadiene rubber composites

Abstract

Keywords

Highlights

Introduction

Substances and experimental techniques

Substances

Experimental techniques

Preparation of nitrogen doped carbon quantum dots

Preparation of magnetite/nitrogen doped carbon quantum dots (magnetite N–CQDs)

Preparation of NBR compounds

Measurements and description

Fourier transforms infrared spectroscopy (FT-IR) analysis

Diffraction of X-ray powder (XRD)

Transmission electron microscope (TEM)

Thermogravimetric data analysis

Rheology characteristics

Scanning electron microscopy (SEM)

Mechanical properties evaluation

Measurements of swelling

Thermal oxidative aging

Magnetic properties

Dielectric spectroscopy technique

Results and discussion

Analysis of thermogravimetric data (TGA/DTG)

X-ray diffraction (XRD)

Infrared fourier transforms spectroscopy (FTIR)

Morphological studies by using transmission electron microscopy (TEM)

Rheological studies

Fourier transforms infrared spectroscopy for NBR and NBR/(Fe3O4/N-CQDs)

Mechanical properties of NBR vulcanizates reinforced with magnetic carbon dots (Fe3O4/N-CQDs)

Hardness of NBR/Fe3O4/N-CQD vulcanizates

Swelling behavior of NBR/Fe3O4/N-CQD vulcanizates

SEM images of NBR samples

Thermal analysis

Thermal aging resistance

Mechanical properties after aging

The aging factor

Swelling properties after aging

Magnetic properties

Dielectric properties

Electrical conductivity

Conclusion

Footnotes

Acknowledgements

Author contributions

Declaration of conflicting interests

Funding

ORCID iD

Data Availability Statement

References

Fourier transforms infrared spectroscopy for NBR and NBR/(Fe₃O₄/N-CQDs)

Mechanical properties of NBR vulcanizates reinforced with magnetic carbon dots (Fe₃O₄/N-CQDs)

Hardness of NBR/Fe₃O₄/N-CQD vulcanizates

Swelling behavior of NBR/Fe₃O₄/N-CQD vulcanizates