Structure–Property Relationships and Thermal Aging of EPDN/NBR Elastomer Networks Modified With Polydiene and Maleimide Additives

Structure–Property Relationships and Thermal Aging of EPDN/NBR Elastomer Networks Modified With Polydiene and Maleimide Additives | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Structure–Property Relationships and Thermal Aging of EPDN/NBR Elastomer Networks Modified With Polydiene and Maleimide Additives Elbrus Ahmedov, Sona Rzayeva This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8800335/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study investigates the structure–property relationships in binary elastomer systems based on ethylene–propylene diene copolymer (EPDN) and nitrile–butadiene rubber (NBR) modified with polydiene (PD) and dithiobis-maleimide (DTBFM) functional additives. The combined influence of reactive unsaturated rubber, polydiene modifiers and maleimide crosslinkers on crosslink density, gel fraction, rheological behavior, mechanical properties and thermal-oxidative stability was systematically analyzed for EPDN/NBR blends with component ratios of 80:20, 70:30 and 60:40. The results demonstrate that increasing NBR content significantly enhances radical-induced crosslinking due to the presence of carbon–carbon double bonds and polar nitrile groups, leading to the formation of a more developed spatial network. The introduction of polydiene and maleimide additives promotes the formation of hybrid elastomer networks, which is reflected in the increase of gel fraction, Mooney viscosity and crosslink density calculated using the Flory–Rehner approach. A clear correlation between network architecture and mechanical performance is established. An optimal balance between processability, mechanical strength and thermal-oxidative resistance is achieved at an EPDN/NBR ratio of 60:40, which exhibits the highest tensile strength, hardness and durability under thermal aging conditions. The obtained results indicate that targeted reactive modification using a combination of NBR, polydiene and maleimide additives represents an effective strategy for tailoring the network structure of EPDN-based elastomers. This approach enables the development of durable elastomer materials with controlled properties for demanding industrial and engineering applications without the use of conventional sulfur curing systems. EPDN/NBR blends elastomer networks structure–property relationships crosslink density thermal-oxidative aging mechanical durability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction In modern engineering and industrial applications, elastomeric materials are widely used as functional components in sealing systems, vibration-damping elements, flexible joints and protective coatings operating under combined thermal, mechanical and environmental loads. The long-term stability and durability of such materials are critically important, as elastomer components are often subjected to prolonged thermal exposure, cyclic deformation and aggressive service environments. Therefore, the development of elastomer systems with controlled network structure, enhanced thermal stability and predictable mechanical behavior remains an important task of applied polymer science. Ethylene–propylene diene copolymers (EPDN) are extensively employed in elastomer technology due to their excellent resistance to heat, ozone, moisture and ultraviolet radiation. These properties make EPDN-based materials attractive for applications requiring long-term environmental and thermal stability. However, the saturated nature of the EPDN macromolecular backbone limits its intrinsic chemical reactivity, which complicates the formation of dense and stable crosslinked networks without the use of highly active curing systems. As a result, EPDN-based elastomers may exhibit insufficient resistance to long-term thermal aging and cyclic mechanical loading. One effective strategy to overcome these limitations involves the incorporation of reactive elastomers containing unsaturated bonds and polar functional groups. In this context, nitrile–butadiene rubber (NBR) is of particular interest due to the presence of carbon–carbon double bonds and nitrile groups, which enhance radical reactivity, intermolecular interactions and resistance to oils and aggressive media. When blended with EPDN, NBR can act as a reactive structure-forming component, promoting the development of a spatial crosslinked network and improving the mechanical and thermal stability of elastomer materials. The performance of EPDN/NBR elastomer blends is largely governed by the architecture and density of the formed crosslinked network. Insufficient crosslinking may lead to creep, loss of mechanical integrity and accelerated aging, whereas excessive crosslinking can result in embrittlement and reduced fatigue resistance. Consequently, targeted regulation of network structure is essential to achieve an optimal balance between strength, elasticity and durability in elastomer systems intended for demanding service conditions. Functional modifiers capable of participating in radical reactions provide an effective route for controlling network formation in elastomer blends. Polydiene rubbers introduce additional unsaturated fragments that increase the concentration of reactive centers, while maleimide-based compounds are known to form rigid and thermally stable bridge-type crosslinks. The combined use of polydiene modifiers and maleimide crosslinkers enables the formation of hybrid elastomer networks consisting of flexible carbon–carbon crosslinks and rigid maleimide-bridged junctions, which can significantly enhance thermal endurance and structural stability under prolonged thermal and mechanical loading. Although the individual effects of unsaturated rubbers and maleimide compounds on elastomer properties have been reported in previous studies, a systematic structure–property analysis of EPDN/NBR systems simultaneously modified with polydiene and maleimide additives remains limited. In particular, the quantitative relationships between crosslink density, gel fraction, rheological characteristics and mechanical performance under thermo-oxidative aging conditions have not been sufficiently clarified. In this work, the structure–property relationships of EPDN/NBR elastomer blends modified with polydiene and dithiobis-maleimide systems are systematically investigated. Binary compositions with different EPDN/NBR ratios (80:20, 70:30 and 60:40) are analyzed to elucidate the role of reactive rubber content in spatial network formation. Special attention is paid to the influence of functional modifiers on crosslink density, gel fraction, mechanical strength and resistance to thermo-oxidative aging. The obtained results provide a scientific basis for the targeted design of durable elastomer materials with controlled properties for advanced engineering and industrial applications. 2. Materials and experimental methods (Expanded Springer Version) 2.1. Materials Ethylene–propylene diene copolymer (EPDN) and nitrile–butadiene rubber (NBR) were used as base elastomer components for the preparation of binary polymer systems. EPDN is characterized by high resistance to heat, ozone, ultraviolet radiation and environmental aging; however, its saturated macromolecular backbone limits its chemical reactivity and crosslinking ability without highly active curing systems. In contrast, NBR contains a significant amount of unsaturated double bonds and polar nitrile (–CN) groups, which provide high chemical activity and enhanced participation in radical reactions. Binary EPDN/NBR blends were prepared with mass ratios of 80:20, 70:30 and 60:40, which allowed a systematic investigation of the role of reactive rubber content on the kinetics of crosslinking, spatial network formation and mechanical performance. To regulate the spatial network structure and enhance the crosslinking efficiency, the following functional additives were introduced into the elastomer compositions: Polydiene rubber (PD) as an unsaturated reactive modifier increasing the number of available radical centers, Dithiobis-maleimide (DTBFM) as a thermally stable bridge-forming compound providing rigid heat-resistant crosslinks, Zinc oxide (ZnO) as a vulcanization activator and structural modifier, Technical carbon black (P324) as a reinforcing filler, Naphthenic acid ester as a plasticizing agent improving processability. All materials were of industrial grade and used without additional purification. 2.2. Preparation of elastomer blends Elastomer mixtures were prepared by mechanical mixing on a laboratory two-roll mill under controlled plasticization conditions. The initial blending of EPDN and NBR was performed at a temperature of 323 K and a roll rotation speed of 40 rpm. The total mixing time varied from 10 to 40 min, which made it possible to trace the development of network formation during plasticization. After initial homogenization of the rubber matrix, polydiene rubber was added, followed by dithiobis-maleimide, zinc oxide, carbon black and plasticizer. Each component was introduced sequentially with intermediate mixing to ensure uniform distribution in the polymer matrix. The total mixing time after the introduction of all additives was adjusted to obtain a visually homogeneous composition without agglomerates. To model thermo-oxidative degradation and intensify radical reactions, part of the samples was subjected to additional plasticization at an elevated temperature of 373 K and an increased roll speed of 102 rpm for up to 20 min. These conditions simulate aggressive processing and service environments and promote the formation of active radical centers in the polymer chains. 2.3. Vulcanization and thermal aging The prepared elastomer compositions were vulcanized in a laboratory hydraulic press using a compression molding technique. Prior to curing, the mixtures were conditioned at room temperature for 24 h to ensure stress relaxation and homogenization of internal structure after milling. Vulcanization was carried out at a temperature of 423 K under a pressure of 10 MPa for 40 min. These curing parameters were selected based on preliminary kinetic experiments and literature data to ensure complete development of the crosslinked network in both saturated and unsaturated elastomer phases. After vulcanization, the samples were slowly cooled to room temperature under pressure in order to minimize internal stresses and avoid post-curing deformation. Standard dumbbell-shaped specimens for mechanical testing and rectangular samples for swelling and gel analysis were cut from the molded plates. To evaluate the resistance of the crosslinked elastomer networks to thermo-oxidative degradation, accelerated thermal aging was performed. The vulcanized samples were placed in a forced-air circulation thermostat and aged at 373 K for 72 h. After aging, all mechanical, rheological and structural parameters were re-measured. The changes in tensile strength, elongation, hardness and elastic recovery were used to quantify the stability of the spatial network under prolonged thermal exposure. 2.4. Mechanical testing Mechanical characterization of the vulcanizates was performed using standardized testing procedures to ensure the reproducibility and reliability of the obtained results. Tensile strength and relative elongation at break were determined on a universal tensile testing machine in accordance with GOST 270 − 75. The tests were carried out at a constant crosshead speed under ambient conditions. For each composition, at least five parallel specimens were tested, and the average values were reported with standard deviation not exceeding 5%. Hardness was measured using the TM-2 method on flat vulcanized plates before and after thermal aging. The hardness values were expressed in conventional units and used as an indirect indicator of crosslink density and rigidity of the elastomer network. Rebound elasticity was determined after thermal aging in order to evaluate the ability of the material to recover its original shape after deformation. This parameter characterizes the elastic component of deformation and is highly sensitive to the mobility of polymer chains and the density of the crosslinked network. Dynamic endurance (fatigue resistance) was assessed under repeated cyclic stretching at room temperature after thermal aging. The number of deformation cycles sustained before the appearance of cracks or mechanical failure was recorded. This test allowed the evaluation of the resistance of the elastomer materials to long-term alternating mechanical loads, which is critical for sealing and vibration-isolating applications. 2.5. Rheological and structural characterization The rheological behavior of the elastomer systems was characterized by determining the Mooney viscosity at 100°C for 5 min, which served as a sensitive indicator of changes in macromolecular mobility, branching and crosslinking. The gel content was determined gravimetrically after extraction of the samples in benzene. The gel fraction was calculated as the ratio of the mass of the insoluble residue to the initial sample mass and expressed as a percentage. The intrinsic viscosity [η] of the sol fractions was measured using an Ubbelohde capillary viscometer in benzene solution at room temperature. The obtained values were used to evaluate changes in molecular weight and chain branching. The crosslink density (1/Mc) was calculated using the Flory–Rehner equation based on equilibrium swelling data [ 13 ]: \(\:\frac{1}{{M}_{c}}=-\frac{{\rho\:}}{{M}_{s}}\cdot\:\frac{\text{l}\text{n}(1-{V}_{r})+{V}_{r}+\chi\:{V}_{r}^{2}}{{V}_{r}^{1/3}-{V}_{1}/2}\) where \(\:\rho\:\) is the polymer density, \(\:{M}_{s}\) is the molar volume of the solvent, \(\:{V}_{r}\) is the polymer volume fraction in the swollen sample, and \(\:\chi\:\) is the polymer–solvent interaction parameter. 2.6. Infrared spectral analysis The molecular structure and chemical transformations occurring during plasticization, vulcanization and thermal aging were analyzed using Fourier-transform infrared (FTIR) spectroscopy. The spectra were recorded in the range of 700–3200 cm⁻¹ with a spectral resolution of 4 cm⁻¹. For FTIR analysis, thin films of the elastomer samples were prepared by dissolving the sol fraction in benzene, followed by solvent evaporation at room temperature. This approach ensured uniform film thickness and high spectral quality. Special attention was paid to the absorption bands in the region of 950–970 cm⁻¹, which correspond to trans-1,4-configurations and unsaturated C = C bonds actively involved in radical crosslinking reactions. The absorption bands associated with nitrile groups (–C ≡ N) near 2235–2240 cm⁻¹ were also analyzed, since these polar groups play an important role in intermolecular interactions and network stabilization. Changes in the intensity and shape of these characteristic bands were used to evaluate the degree of chemical modification, participation of NBR in crosslinking reactions, and the formation of thermally stable maleimide-bridged structures within the elastomer network. 3. Results and discussion 3.1. Effect of NBR content on gelation and Mooney viscosity The influence of nitrile–butadiene rubber (NBR) content on the formation of the spatial network in EPDN/NBR elastomer systems was evaluated using gel content measurements and Mooney viscosity analysis. These parameters are highly sensitive to changes in molecular mobility, chain branching and crosslink density and therefore provide reliable information on the kinetics of structure formation during plasticization. The evolution of the rheological behavior of the investigated elastomer systems during plasticization is illustrated in Fig. 1 . The figure shows the change in Mooney viscosity as a function of plasticization time for EPDN/NBR blends with different component ratios [ 1 , 2 ]. Figure 1 demonstrates the evolution of Mooney viscosity for EPDN/NBR blends with component ratios of 80:20, 70:30 and 60:40 as a function of plasticization time at 323 K. At the initial stage of mixing (10–15 min), all compositions exhibit relatively low viscosity values, indicating a predominance of chain disentanglement and mechanical softening of the polymer matrix. However, with increasing plasticization time, a pronounced growth of Mooney viscosity is observed, which reflects the development of radical-induced chain branching and the onset of network formation. The development of the insoluble three-dimensional network during plasticization is further evidenced by the evolution of gel content with time, as presented in Fig. 2 . The growth of the gel fraction directly reflects the progressive formation of a crosslinked spatial structure in the investigated EPDN/NBR systems [ 1 , 2 ]. As follows from Fig. 2 , the gel content in all investigated EPDN/NBR systems increases with plasticization time, confirming the progressive formation of an insoluble three-dimensional network. The highest rate of gel growth is observed for the 60:40 composition, which reflects the dominant role of NBR in radical-induced crosslinking. The 80:20 system exhibits the slowest gelation behavior due to the predominance of the saturated EPDN phase with limited radical activity. The intermediate behavior of the 70:30 blend indicates a transition region in which branching and crosslinking processes compete with partial thermo-mechanical degradation. The combined analysis of Figs. 1 and 2 clearly demonstrates that the increase in Mooney viscosity is directly associated with the growth of the gel fraction and the development of a continuous spatial network. The composition with an EPDN/NBR ratio of 60:40 shows the most intensive increase in Mooney viscosity over the entire plasticization interval. This behavior clearly indicates the dominant role of NBR as a reactive component providing unsaturated sites for radical reactions. In contrast, the composition with a low NBR content (80:20) demonstrates a significantly slower growth of viscosity, which is attributed to the saturated nature of the EPDN macromolecular backbone and its limited ability to participate in spontaneous crosslinking reactions under thermal-mechanical exposure. The 70:30 composition occupies an intermediate position and exhibits a non-monotonic viscosity behavior. At approximately 25–30 min of plasticization, this system shows a temporary viscosity maximum, which can be attributed to the formation of a transient branched structure followed by partial chain scission or rearrangement processes. This effect indicates the competition between radical branching and thermo-mechanical degradation reactions occurring simultaneously in the polymer system. The evolution of gel content confirms the trends observed in the Mooney viscosity data. As the NBR fraction increases, the gel content grows steadily, reaching its maximum values for the 60:40 composition. After 40 min of plasticization, the gel fraction in this system exceeds that of the 80:20 blend by more than 1.5 times. This result provides direct evidence of the accelerated formation of an insoluble three-dimensional network promoted by the unsaturated double bonds and polar nitrile groups of NBR. The enhanced gelation behavior of the 60:40 system is associated with the higher probability of radical initiation, chain propagation and intermolecular coupling reactions involving NBR macroradicals. Under thermo-mechanical action, the C = C bonds in NBR undergo homolytic cleavage, forming active radical centers that readily participate in intermolecular crosslinking. In contrast, EPDN, being a saturated copolymer, exhibits substantially lower radical activity under the same conditions. The combined analysis of Mooney viscosity and gel content clearly demonstrates that the EPDN/NBR ratio of 60:40 represents a critical concentration region, at which a continuous crosslinked network begins to dominate the system. Below this threshold, the polymer structure remains predominantly linear or weakly branched, while above it, a dense spatial network develops, leading to a sharp increase in viscosity and gel fraction. From a technological point of view, this behavior is of fundamental importance. On the one hand, the high viscosity and gel content of the 60:40 composition indicate the formation of a mechanically strong and thermally stable elastomer network. On the other hand, excessively rapid gelation at higher NBR contents could negatively affect processability. Therefore, the 60:40 ratio represents an optimal compromise between structural development and technological workability. Thus, the obtained results clearly confirm that NBR acts as a reactive structure-forming component in EPDN-based elastomer systems, and its concentration critically controls the kinetics of gelation, viscosity growth and the overall architecture of the spatial network. 3.2. Influence of polydiene and maleimide modifiers on crosslink density The effect of polydiene rubber (PD) and dithiobis-maleimide (DTBFM) on the spatial network formation in EPDN/NBR elastomer systems was evaluated through the analysis of crosslink density (1/Mc), intrinsic viscosity and gel content. These parameters directly reflect the intensity of intermolecular coupling reactions and the rigidity of the resulting three-dimensional network [ 3 , 6 ]. While the evolution of gel content reflects the qualitative development of the insoluble spatial network, a quantitative assessment of the network architecture requires the determination of crosslink density. Therefore, the effect of polydiene and maleimide modifiers on the crosslink density (1/Mc) of the EPDN/NBR systems is analyzed in Fig. 3 . For all investigated compositions, the introduction of PD and DTBFM leads to a pronounced increase in crosslink density compared to unmodified EPDN/NBR blends. This effect is especially significant for the 60:40 EPDN/NBR system, which already exhibits the highest radical activity due to the elevated content of unsaturated NBR chains. In this composition, the addition of PD provides supplementary unsaturated fragments that serve as additional reaction centers, while DTBFM acts as a multifunctional bridge-forming agent, promoting the formation of thermally stable intermolecular links. The calculated values of 1/Mc show a systematic increase with rising concentrations of PD and DTBFM. This indicates a progressive decrease in the average molecular weight between crosslinks and confirms the formation of a denser and more rigid spatial network. The increase in crosslink density is accompanied by a consistent growth of intrinsic viscosity [η], which reflects the increasing degree of chain branching and molecular integration in the sol fraction. Simultaneously, the gel content rises, confirming the transition of a larger fraction of the polymer matrix into an insoluble crosslinked state. The observed effect is explained by the synergistic radical mechanism involving all three key components of the system. Under thermo-mechanical and curing conditions, NBR macromolecules readily form radicals due to the presence of C = C double bonds. Polydiene fragments further increase the concentration of unsaturated sites, thereby intensifying chain propagation and intermolecular coupling. DTBFM molecules participate in radical addition reactions through the maleimide double bonds, forming rigid thermally stable bridges between elastomer chains. As a result, a mixed network structure consisting of C–C crosslinks and maleimide-bridged junctions is formed. A comparison of different EPDN/NBR ratios shows that the 80:20 system, despite the presence of PD and DTBFM, exhibits the lowest increase in crosslink density. This is due to the predominance of the saturated EPDN phase, which limits the overall concentration of radical-active sites. The 70:30 system demonstrates intermediate behavior, while the 60:40 composition clearly exhibits the most intensive network development, confirming the decisive role of the reactive NBR fraction in the efficiency of modification. Importantly, the increase in crosslink density induced by PD and DTBFM does not occur abruptly but develops in a controlled manner, allowing the formation of a mechanically strong yet technologically processable elastomer network. This is particularly important for industrial applications, where excessive crosslinking can lead to processing difficulties and brittleness of the final material. Thus, the obtained results demonstrate that polydiene and maleimide modifiers play a key role in regulating the architecture of the crosslinked network in EPDN/NBR systems, enabling targeted control of crosslink density through the synergistic interaction with reactive NBR chains. The highest efficiency of this modification route is achieved for the EPDN/NBR ratio of 60:40, which forms the structural basis for optimizing the mechanical and thermal performance of the elastomer material. 3.3. Mechanical properties as a function of crosslink density The variation of mechanical properties of the EPDN/NBR elastomer systems modified with polydiene and dithiobis-maleimide was analyzed in direct correlation with the crosslink density of the polymer network. Tensile strength, relative elongation at break and hardness were selected as the key indicators reflecting the balance between network rigidity and chain mobility [ 2 ]. The relationship between tensile strength and crosslink density for all investigated compositions is illustrated in Fig. 4 . A clear systematic relationship between crosslink density (1/Mc) and the mechanical behavior of the vulcanizates is observed for all investigated compositions. As the density of crosslinks increases, a pronounced growth of tensile strength is recorded. This effect is most significant for the EPDN/NBR = 60:40 composition, where the highest density of radical-active sites and the most intensive network formation were previously established (Sections 3.1 and 3.2 ). In this system, the introduction of polydiene and maleimide modifiers leads to a substantial increase in strength compared to unmodified blends, indicating the formation of an efficient load-bearing spatial network. The strengthening effect is associated with the reduction of the average molecular weight between crosslinks and the formation of rigid junction points, which limit the slippage of polymer chains under applied stress. The presence of maleimide-bridged crosslinks plays a particularly important role, since such junctions are characterized by enhanced thermal stability and mechanical rigidity. In contrast to strength, the relative elongation at break shows an inverse dependence on crosslink density. With increasing 1/Mc, a gradual decrease in elongation is observed for all compositions. This behavior reflects the restriction of macromolecular mobility caused by the densification of the spatial network. The most pronounced decrease in elongation is again observed for the 60:40 system, where the formation of a dense crosslinked structure most strongly limits chain extensibility. Nevertheless, despite the reduction in elongation, the modified systems retain sufficiently high deformation capability, indicating that the formed networks do not exhibit excessive brittleness. This confirms that the applied modification approach provides an optimal balance between rigidity and elasticity, which is essential for elastomeric applications under dynamic loads. The evolution of hardness closely follows the trend of tensile strength. As the crosslink density increases, hardness values rise monotonically, reflecting the increasing stiffness of the elastomer matrix. After thermal aging, the hardened state of the modified vulcanizates becomes even more pronounced, especially for compositions containing both polydiene and maleimide additives. This behavior confirms the formation of a thermally stable network structure resistant to segmental relaxation under elevated temperatures. A comparative analysis of the three EPDN/NBR ratios shows that the 80:20 composition, despite some increase in crosslink density after modification, exhibits the lowest strength and hardness and the highest elongation. This reflects the limited participation of the saturated EPDN chains in radical crosslinking. The 70:30 system demonstrates intermediate mechanical performance, while the 60:40 composition clearly exhibits the best combination of high strength, sufficient elasticity and enhanced hardness. From a structure–property perspective, these results clearly demonstrate that the mechanical performance of EPDN/NBR elastomers is governed primarily by the density and nature of the crosslinked network. The synergistic action of NBR, polydiene and maleimide modifiers allows the formation of a reinforced three-dimensional structure, in which the applied mechanical load is efficiently transferred through a system of rigid and thermally stable junction points. Thus, the obtained results confirm that targeted regulation of crosslink density using reactive modifiers represents an effective route for controlling the mechanical behavior of EPDN-based elastomer systems, providing a predictable transition from soft, highly extensible materials to mechanically robust and thermally stable elastomeric networks. 3.4. Thermal-oxidative aging behavior and fatigue resistance The resistance of the modified EPDN/NBR elastomer systems to thermal-oxidative aging and cyclic mechanical loading was evaluated to assess the long-term stability of the formed crosslinked networks under conditions close to real service environments. The changes in tensile strength, relative elongation, hardness, rebound elasticity and dynamic endurance after accelerated aging at 373 K for 72 h were analyzed. The effect of thermal aging on the mechanical strength of the modified elastomer systems is illustrated in Fig. 5 . Thermal aging results demonstrate that all modified compositions exhibit significantly higher stability compared to unmodified EPDN/NBR blends. The most pronounced improvement is observed for the systems containing both polydiene and maleimide modifiers, which form a thermally stable network structure resistant to oxidative chain scission and post-curing relaxation [ 2 , 16 ]. The EPDN/NBR = 60:40 composition shows the highest resistance to thermal-oxidative degradation. After aging, this system retains a major part of its initial tensile strength and hardness, while the decrease in relative elongation remains moderate. This behavior directly confirms the formation of a dense and thermally stable spatial network stabilized by rigid maleimide bridges and reinforced by additional C–C crosslinks formed via polydiene and NBR fragments. In contrast, the 80:20 system, characterized by a lower initial crosslink density, exhibits more pronounced degradation effects after aging. A noticeable decrease in tensile strength and rebound elasticity is accompanied by an increase in brittleness, indicating partial destruction of the weakly developed polymer network dominated by the saturated EPDN phase. The 70:30 composition demonstrates intermediate aging behavior, reflecting the transitional nature of its structure, in which the contribution of radical-active NBR chains is already significant but still insufficient to provide the highest level of thermal stabilization. The observed thermal stability trends are directly correlated with the crosslink density and the nature of the formed junctions. In networks dominated by C–C and maleimide-bridged crosslinks, the probability of thermo-oxidative chain rupture is substantially reduced. The nitrile groups of NBR also contribute to the stabilization of the network by enhancing intermolecular interactions and reducing oxygen diffusion into the bulk of the material. The fatigue resistance of the modified elastomers was evaluated by dynamic endurance tests under repeated cyclic stretching after thermal aging. The results reveal a pronounced increase in the number of deformation cycles sustained before failure for all compositions modified with polydiene and maleimide additives. Once again, the maximum fatigue endurance is achieved for the 60:40 composition, which combines high crosslink density with sufficient chain mobility to dissipate mechanical energy under cyclic loading. The improvement in fatigue resistance is explained by the ability of the reinforced three-dimensional network to effectively redistribute local stresses and suppress microcrack initiation and propagation. The presence of thermally stable maleimide junctions plays a crucial role in maintaining network integrity under repeated deformation, while polydiene fragments enhance the toughness of the elastomer matrix. From an application-oriented perspective, the obtained results demonstrate that the synergistic modification of EPDN/NBR elastomers with polydiene and maleimide systems provides a substantial improvement in long-term thermal stability and fatigue durability, which is critically important for elastomer products operating under combined thermal and mechanical loads, such as seals, gaskets, cable insulation and vibration-damping elements. Thus, the thermal-oxidative aging and fatigue resistance results fully confirm the structure–property relationships established in Sections 3.1 – 3.3 and validate the effectiveness of the proposed reactive modification strategy for the development of durable high-performance elastomer materials. For a comprehensive comparison of the structural, rheological and mechanical performance of all investigated elastomer systems, the main experimental parameters are summarized in Table 1 . The table provides a unified overview of the effect of NBR content and the combined action of polydiene and maleimide modifiers on gel formation, crosslink density, mechanical strength and aging resistance of EPDN-based elastomers. Table 1 Comparative summary of structural, rheological and mechanical parameters of EPDN/NBR elastomer systems. Qualitative levels are used for comparative analysis. EPDN/NBR ratio Modification system Gel content (%) Mooney viscosity Crosslink density (1/Mc) Tensile strength Elongation at break Hardness Thermal aging resistance (373 K, 72 h) 80 : 20 none low low low low high low low 70 : 30 none medium medium medium medium medium medium medium 60 : 40 none high high high medium medium medium medium 80 : 20 PD + DTBFM medium medium medium medium medium medium medium 70 : 30 PD + DTBFM high high high high medium high high 60 : 40 PD + DTBFM maximum maximum maximum maximum optimal maximum maximum To provide an integrated visualization of the structure–property relationships for all investigated compositions, a mechanical–structural property map is presented in Fig. 6 . 4. Discussion The obtained experimental results clearly demonstrate that the structure–property relationships in the studied EPDN/NBR elastomer systems are governed by the synergistic interaction between three key factors: the concentration of reactive NBR chains, the presence of polydiene fragments, and the incorporation of maleimide bridge-forming agents. The combined action of these components determines the kinetics of radical processes, the architecture of the spatial network, and, consequently, the mechanical and thermal performance of the resulting elastomer materials. The role of nitrile–butadiene rubber is primarily associated with its high radical reactivity. The presence of unsaturated C = C bonds and polar nitrile groups in the NBR macromolecule provides a large number of potential sites for radical initiation and chain propagation under thermo-mechanical and curing conditions. The unified structure–property–performance relationship governing the behavior of the investigated elastomer systems is schematically illustrated in Fig. 7 . As illustrated in Fig. 7 , the mechanical and thermal performance of the investigated elastomer systems is governed by a hierarchical structure–property–performance relationship. The increase in NBR content and the introduction of polydiene and maleimide modifiers activate radical crosslinking, leading to the formation of a hybrid spatial network. The growth of crosslink density and gel fraction directly controls the strength, hardness and aging resistance of the materials. The optimal balance between network rigidity and chain mobility is achieved at the EPDN/NBR ratio of 60:40, which ensures superior long-term performance under combined thermal and mechanical loading. As shown in Section 3.1 , increasing the NBR content leads to a pronounced acceleration of gel formation and a sharp increase in Mooney viscosity, which indicates the transition from predominantly linear or weakly branched chains to a continuous three-dimensional crosslinked network. The existence of a critical concentration region at the EPDN/NBR ratio of 60:40 reflects the percolation-like character of network formation, when the number of reactive sites becomes sufficient for the development of a load-bearing spatial structure. The introduction of polydiene rubber further intensifies the radical mechanism by increasing the overall concentration of unsaturated fragments in the system. Polydiene chains act not only as additional sources of radical centers but also as flexible connecting segments that enhance the probability of intermolecular coupling reactions. As demonstrated in Section 3.2 , the addition of polydiene results in a systematic increase in crosslink density and intrinsic viscosity, confirming the formation of a denser and more highly branched network. At the same time, the flexible nature of polydiene fragments prevents excessive embrittlement of the material, allowing the preservation of sufficient chain mobility and energy dissipation capacity under mechanical loading. A special role in the formation of a thermally stable network is played by dithiobis-maleimide (DTBFM). Maleimide fragments participate in radical addition reactions through their highly reactive double bonds, forming rigid and thermally resistant intermolecular bridges. These junctions act as stable crosslinking nodes that suppress segmental relaxation and limit the reversibility of the network at elevated temperatures. The presence of such junctions explains the pronounced increase in hardness, tensile strength and thermal-oxidative stability observed for the modified systems (Sections 3.3 and 3.4 ). From a mechanistic viewpoint, the formation of the elastomer network in the studied systems can be described by a unified radical crosslinking model. Under thermo-mechanical and curing conditions, radical initiation occurs predominantly on unsaturated NBR and polydiene chains. The generated macroradicals undergo chain propagation through intermolecular coupling reactions, forming C–C crosslinks. Simultaneously, maleimide molecules participate in radical addition reactions, leading to the formation of rigid bridge-like junctions between elastomer chains. As a result, a hybrid network structure is formed, consisting of flexible C–C crosslinks and rigid maleimide-bridged nodes. This dual character of the network explains the unique combination of high strength, sufficient elasticity and enhanced thermal stability observed for the modified EPDN/NBR systems. The established correlations between crosslink density, gel content, viscosity and mechanical properties provide direct experimental confirmation of the classical structure–property concept for elastomers. An increase in crosslink density leads to a monotonic growth of tensile strength and hardness, accompanied by a gradual decrease in relative elongation. However, within the investigated modification range, the decrease in elongation does not result in catastrophic embrittlement, indicating that the applied modification strategy allows the formation of a mechanically robust yet sufficiently ductile network. The EPDN/NBR = 60:40 composition represents an optimal balance point where the density of active radical sites, the efficiency of crosslink formation and the preservation of chain mobility are harmonized. The thermal-oxidative aging results further highlight the decisive influence of the network architecture on long-term stability. In systems dominated by weak and sparse crosslinks, thermal exposure leads to accelerated chain scission, loss of strength and increased brittleness. In contrast, in networks reinforced by maleimide-bridged junctions and additional C–C crosslinks formed via polydiene and NBR fragments, the probability of thermo-oxidative rupture is significantly reduced. This explains the high retention of mechanical properties and enhanced fatigue resistance observed for the modified systems, particularly for the 60:40 composition. From a practical standpoint, the obtained results demonstrate that the proposed modification strategy allows the targeted design of EPDN-based elastomer materials with predictable and tunable performance characteristics. By regulating the content of NBR, polydiene and maleimide modifiers, it becomes possible to control the density and nature of the spatial network, thereby adjusting the balance between strength, elasticity, heat resistance and fatigue durability. Such materials are highly promising for applications requiring long-term mechanical reliability under combined thermal and cyclic loads, including sealing elements, vibration-damping components, and cable insulation systems. Thus, the discussion confirms that the observed improvements in mechanical and thermal performance are not random but are a direct consequence of the controlled radical modification of the elastomer network structure, fully consistent with the established structure–property relationships. The obtained results indicate that the improvement of mechanical performance and thermal-oxidative stability in the modified EPDN/NBR systems is primarily governed by changes in the architecture of the crosslinked network rather than by simple compositional effects. The introduction of nitrile–butadiene rubber increases the concentration of unsaturated sites and polar functional groups, which enhances radical reactivity during curing and promotes the formation of a more continuous spatial network. This effect is reflected in the increase in gel fraction and Mooney viscosity, confirming the transition from weakly branched structures to a more interconnected elastomer network. The role of polydiene rubber and dithiobis-maleimide modifiers appears to be synergistic. Polydiene components introduce additional unsaturated fragments that act as reactive bridges between macromolecular chains, facilitating network growth and improving stress distribution under mechanical loading. At the same time, dithiobis-maleimide participates in the formation of rigid, thermally stable bridge-type crosslinks, which restrict excessive chain mobility at elevated temperatures. As a result, a hybrid elastomer network is formed, combining flexible carbon–carbon crosslinks with rigid maleimide-bridged junctions. Such a network architecture effectively delays thermo-oxidative degradation and suppresses the development of irreversible deformation during prolonged thermal exposure. The balance between network density and chain mobility is a critical factor determining the durability of elastomer materials. The experimental results demonstrate that the EPDN/NBR ratio of 60:40 provides an optimal compromise between crosslink density, mechanical strength and elasticity. At lower NBR contents, the network remains insufficiently developed, leading to reduced resistance to thermal aging. In contrast, excessive crosslinking at higher reactive component concentrations may increase stiffness and limit fatigue resistance. The identified optimal composition ensures efficient energy dissipation under cyclic deformation while maintaining structural integrity during thermal aging, which is essential for elastomer materials intended for long-term industrial applications. Overall, the obtained structure–property relationships confirm that targeted reactive modification using a combination of nitrile–butadiene rubber, polydiene additives and maleimide crosslinkers represents an effective approach for tailoring the network architecture of EPDN-based elastomers. This strategy allows controlled regulation of mechanical durability and thermal stability without the need for complex chemical synthesis, which is particularly attractive for applied polymer engineering and industrial-scale elastomer production. 5. Conclusions A systematic investigation of the structure–property relationships in EPDN/NBR elastomer blends modified with polydiene and dithiobis-maleimide systems has been carried out. The obtained results allow the following conclusions to be drawn: Nitrile–butadiene rubber acts as a key reactive component in EPDN-based systems, controlling the kinetics of radical-induced spatial network formation. An increase in NBR content leads to a pronounced growth of gel fraction and Mooney viscosity, indicating the transition from weakly branched structures to a continuous three-dimensional crosslinked network. The introduction of polydiene rubber and maleimide modifiers provides effective regulation of crosslink density and network architecture. Polydiene increases the concentration of unsaturated reaction centers, while dithiobis-maleimide forms rigid and thermally stable bridge-type junctions. Their synergistic action results in the formation of a hybrid elastomer network consisting of flexible C–C crosslinks and rigid maleimide-bridged nodes. A clear quantitative correlation between crosslink density and mechanical properties has been demonstrated. An increase in crosslink density leads to a monotonic growth of tensile strength and hardness, accompanied by a controlled decrease in relative elongation, without inducing excessive brittleness. The EPDN/NBR ratio of 60:40 has been identified as the optimal composition, providing the best balance between processability, mechanical strength, elasticity and thermal-oxidative stability. This composition exhibits the highest resistance to thermal aging and superior fatigue durability under cyclic mechanical loading. The results of thermal-oxidative aging and dynamic endurance tests confirm that the formation of a reinforced spatial network significantly improves the long-term stability of elastomer materials under combined thermal and mechanical stresses. Overall, the obtained results demonstrate that targeted reactive modification of EPDN/NBR systems using polydiene and maleimide additives represents an efficient strategy for designing elastomer materials with predictable and tunable performance characteristics. The proposed approach opens prospects for the development of durable, heat-resistant and mechanically stable elastomer products for demanding industrial and engineering applications. Declarations Conflict of interest. The authors declare that they have no conflict of interest. Funding. The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. Author Contribution S.V.R. conceived and performed the experimental work and prepared the figures.E.N.A. performed data analysis and wrote the main manuscript text.Both authors discussed the results, contributed to manuscript editing, and approved the final version. Data Availability The experimental data supporting the findings of this study are available from the corresponding author upon reasonable request. References Ahmedova, E.N., Rzayeva, S.V.: The influence of NBR on the complex of technological properties of binary mixtures based on EPDN. Surf. Eng. Appl. Electrochem. 61 (6), 966–970 (2025). https://doi.org/10.3103/S1068375525701054 Ahmedova, E.N., Rzayeva, S.V.: The influence of the structure and composition of elastomer mixtures on the mechanical properties of SKEP-BNK vulcanizates. Surf. Eng. Appl. Electrochem. 61 (2), 261–265 (2025). https://doi.org/10.3103/S1068375525700152 Mamedova, S.M., Ahmedov, E.N., Rzayeva, S.V.: Modified ethylene propylene rubbers with unsaturated rubbers and low-molecular reactive compounds. Surf. Eng. Appl. Electrochem. 60 (6), 883–891 (2024). https://doi.org/10.3103/S1068375524700492 Ahmedov, E., Mammadov, N., Rzayeva, S.V.: The mechanism of electric discharge effect on the modification process of linear low-density polyethylene. Przegląd Elektrotechniczny. 99 (6), 208 (2023). https://doi.org/10.15199/48.2023.06.42 Ahmedov, E., Safiyev, E., Rzayeva, S., Mammadov, N., et al.: Obtaining a graft copolymer of polyethylene by electrodischarge synthesis. Przegląd Elektrotechniczny. 99 (11), 100 (2023). https://doi.org/10.15199/48.2023.11.17 Rzayeva, S.V.: Study of structural and mechanical properties of modified ethylene-propylene rubbers in the presence of 4,4′-dithiobis-p-phenylmaleimide. Vestnik Nauki. 5 (7), 104 (2022) Liyanaarachchi, L.A.D.A., Liyanage, N.M.V.K.: The effectiveness of a water-soluble synthetic acrylic polymer in enhancing reinforcing action of silica in carboxylated nitrile rubber latex. MERCon 2015, p. 240. (2015). https://doi.org/10.1109/MERCon.2015.7112352 Ramasinghe, R.L.P., Gannoruwa, G.K.B.M., Liyanage, N.M.V.K.: Use of surface-modified silica in reinforcing carboxylated nitrile rubber latex. MERCon 2016, p. 361. (2016). https://doi.org/10.1109/MERCon.2016.7480168 Maansilla, M.A., Marzocca, A.J., Macchi, C., Somoza, A.: Natural rubber/styrene-butadiene rubber blends prepared by solution mixing: Influence of vulcanization temperature using a Semi-EV sulfur curing system. Polym. Test. 63 , 150 (2017). https://doi.org/10.1016/j.polymertesting.2017.07.025 Lipińska, M., Imiela, M.: Morphology, rheology, and curing of EPR/HNBR blends reinforced by POSS and organoclay. Polym. Test. 75 , 26 (2019). https://doi.org/10.1016/j.polymertesting.2019.01.020 Mammadov, S.M., Rzayeva, S.A., Gojayeva, T.F., Garibov, A.A., et al.: Radiation-chemical structure of acrylonitrile butadiene rubber with copolymer vinyl chloride and vinyl acetate. Am. J. Polym. Sci. 3 (4), 76 (2013). https://doi.org/10.5923/j.ajps.20130304.03 Althues, H., Simon, P., Philipp, F., Kaskel, S.: Integration of zinc oxide nanoparticles into transparent poly(butanediol monoacrylate) via photopolymerisation. J. Nanosci. Nanotechnol. 6 (2), 409 (2006). https://doi.org/10.1166/jnn.2006.917 Flory, P.J., Rehner, J.: Statistical mechanics of cross-linked polymer networks. II. Swelling. J. Chem. Phys. 11 , 521–526 (1943) Mark, J.E.: Physical Properties of Polymers Handbook. Springer (2007) Dick, J.S.: Rubber Technology: Compounding and Testing for Performance. Hanser. (2001) Morton, M.: Rubber Technology, 3rd edn. Springer (1999) Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8800335","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":593933062,"identity":"7b0bb663-1799-41cb-ab6f-7625c332cffd","order_by":0,"name":"Elbrus Ahmedov","email":"","orcid":"","institution":"Azerbaijan State Oil and Industry University","correspondingAuthor":false,"prefix":"","firstName":"Elbrus","middleName":"","lastName":"Ahmedov","suffix":""},{"id":593933063,"identity":"c1f4ba12-4c53-4de1-852e-8dfffc5912f4","order_by":1,"name":"Sona 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19:23:06","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8800335/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8800335/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103050561,"identity":"d99c494b-4b3f-4ffc-a2ee-60030ae00fb4","added_by":"auto","created_at":"2026-02-20 07:50:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":35699,"visible":true,"origin":"","legend":"\u003cp\u003eMooney viscosity as a function of plasticization time for EPDN/NBR elastomer blends with different component ratios.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/531f2e4d6a8a8a5a733187be.png"},{"id":103041000,"identity":"ddcffc48-af1c-4ef4-a789-d5dc34368707","added_by":"auto","created_at":"2026-02-20 04:17:20","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":38032,"visible":true,"origin":"","legend":"\u003cp\u003eGel fraction versus plasticization time for EPDN/NBR elastomer systems with varying NBR content.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/c22ae7b5c3be61224af20682.png"},{"id":103050556,"identity":"866384e6-5054-4bbc-98c1-116eb0774e92","added_by":"auto","created_at":"2026-02-20 07:50:32","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":48230,"visible":true,"origin":"","legend":"\u003cp\u003eDependence of crosslink density on the EPDN/NBR ratio for elastomer blends modified with polydiene and dithiobis-maleimide additives.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/53e8168842cdcbdb5a365b4d.png"},{"id":103041003,"identity":"054dfe61-5559-4c1a-86a4-5347ae788709","added_by":"auto","created_at":"2026-02-20 04:17:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":42188,"visible":true,"origin":"","legend":"\u003cp\u003eTensile strength and elongation at break of EPDN/NBR elastomer blends as a function of composition.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/5723fa6e8c7c2faa6a4555d6.png"},{"id":103056578,"identity":"aad84f1e-484b-4ea9-b088-fe24ad230606","added_by":"auto","created_at":"2026-02-20 09:16:36","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":29506,"visible":true,"origin":"","legend":"\u003cp\u003eShore A hardness of EPDN/NBR elastomer blends with different component ratios.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/ec53115b0730f1592c537a32.png"},{"id":103050493,"identity":"9540ad62-ddef-43eb-b36e-4cdfb56eb7f8","added_by":"auto","created_at":"2026-02-20 07:50:17","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":34119,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in tensile strength of EPDN/NBR elastomer blends after thermal-oxidative aging.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/7a41d48bec8d71772b893b07.png"},{"id":103041004,"identity":"e171c796-c8ba-4e44-97f6-035dac064fe0","added_by":"auto","created_at":"2026-02-20 04:17:20","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":252017,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in elongation at break of EPDN/NBR elastomer blends after thermal-oxidative aging.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/f05eb98f01563fbd2b7f5e10.png"},{"id":103056992,"identity":"c346067a-a747-4df9-960f-089280fac3bf","added_by":"auto","created_at":"2026-02-20 09:27:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1180333,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/4edf5fea-9baf-48d4-92d6-85e605d85cea.pdf"},{"id":103041007,"identity":"7b666202-ce35-48fd-9b3b-d17ccb0ba6ad","added_by":"auto","created_at":"2026-02-20 04:17:20","extension":"tif","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":136054,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.tif","url":"https://assets-eu.researchsquare.com/files/rs-8800335/v1/3af71b0f2f188e3eb0b1f60f.tif"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eStructure–Property Relationships and Thermal Aging of EPDN/NBR Elastomer Networks Modified With Polydiene and Maleimide Additives\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn modern engineering and industrial applications, elastomeric materials are widely used as functional components in sealing systems, vibration-damping elements, flexible joints and protective coatings operating under combined thermal, mechanical and environmental loads. The long-term stability and durability of such materials are critically important, as elastomer components are often subjected to prolonged thermal exposure, cyclic deformation and aggressive service environments. Therefore, the development of elastomer systems with controlled network structure, enhanced thermal stability and predictable mechanical behavior remains an important task of applied polymer science.\u003c/p\u003e \u003cp\u003eEthylene\u0026ndash;propylene diene copolymers (EPDN) are extensively employed in elastomer technology due to their excellent resistance to heat, ozone, moisture and ultraviolet radiation. These properties make EPDN-based materials attractive for applications requiring long-term environmental and thermal stability. However, the saturated nature of the EPDN macromolecular backbone limits its intrinsic chemical reactivity, which complicates the formation of dense and stable crosslinked networks without the use of highly active curing systems. As a result, EPDN-based elastomers may exhibit insufficient resistance to long-term thermal aging and cyclic mechanical loading.\u003c/p\u003e \u003cp\u003eOne effective strategy to overcome these limitations involves the incorporation of reactive elastomers containing unsaturated bonds and polar functional groups. In this context, nitrile\u0026ndash;butadiene rubber (NBR) is of particular interest due to the presence of carbon\u0026ndash;carbon double bonds and nitrile groups, which enhance radical reactivity, intermolecular interactions and resistance to oils and aggressive media. When blended with EPDN, NBR can act as a reactive structure-forming component, promoting the development of a spatial crosslinked network and improving the mechanical and thermal stability of elastomer materials.\u003c/p\u003e \u003cp\u003eThe performance of EPDN/NBR elastomer blends is largely governed by the architecture and density of the formed crosslinked network. Insufficient crosslinking may lead to creep, loss of mechanical integrity and accelerated aging, whereas excessive crosslinking can result in embrittlement and reduced fatigue resistance. Consequently, targeted regulation of network structure is essential to achieve an optimal balance between strength, elasticity and durability in elastomer systems intended for demanding service conditions.\u003c/p\u003e \u003cp\u003eFunctional modifiers capable of participating in radical reactions provide an effective route for controlling network formation in elastomer blends. Polydiene rubbers introduce additional unsaturated fragments that increase the concentration of reactive centers, while maleimide-based compounds are known to form rigid and thermally stable bridge-type crosslinks. The combined use of polydiene modifiers and maleimide crosslinkers enables the formation of hybrid elastomer networks consisting of flexible carbon\u0026ndash;carbon crosslinks and rigid maleimide-bridged junctions, which can significantly enhance thermal endurance and structural stability under prolonged thermal and mechanical loading.\u003c/p\u003e \u003cp\u003eAlthough the individual effects of unsaturated rubbers and maleimide compounds on elastomer properties have been reported in previous studies, a systematic structure\u0026ndash;property analysis of EPDN/NBR systems simultaneously modified with polydiene and maleimide additives remains limited. In particular, the quantitative relationships between crosslink density, gel fraction, rheological characteristics and mechanical performance under thermo-oxidative aging conditions have not been sufficiently clarified.\u003c/p\u003e \u003cp\u003eIn this work, the structure\u0026ndash;property relationships of EPDN/NBR elastomer blends modified with polydiene and dithiobis-maleimide systems are systematically investigated. Binary compositions with different EPDN/NBR ratios (80:20, 70:30 and 60:40) are analyzed to elucidate the role of reactive rubber content in spatial network formation. Special attention is paid to the influence of functional modifiers on crosslink density, gel fraction, mechanical strength and resistance to thermo-oxidative aging. The obtained results provide a scientific basis for the targeted design of durable elastomer materials with controlled properties for advanced engineering and industrial applications.\u003c/p\u003e"},{"header":"2. Materials and experimental methods (Expanded Springer Version)","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eEthylene\u0026ndash;propylene diene copolymer (EPDN) and nitrile\u0026ndash;butadiene rubber (NBR) were used as base elastomer components for the preparation of binary polymer systems. EPDN is characterized by high resistance to heat, ozone, ultraviolet radiation and environmental aging; however, its saturated macromolecular backbone limits its chemical reactivity and crosslinking ability without highly active curing systems. In contrast, NBR contains a significant amount of unsaturated double bonds and polar nitrile (\u0026ndash;CN) groups, which provide high chemical activity and enhanced participation in radical reactions.\u003c/p\u003e \u003cp\u003eBinary EPDN/NBR blends were prepared with mass ratios of 80:20, 70:30 and 60:40, which allowed a systematic investigation of the role of reactive rubber content on the kinetics of crosslinking, spatial network formation and mechanical performance.\u003c/p\u003e \u003cp\u003eTo regulate the spatial network structure and enhance the crosslinking efficiency, the following functional additives were introduced into the elastomer compositions:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003ePolydiene rubber (PD) as an unsaturated reactive modifier increasing the number of available radical centers,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eDithiobis-maleimide (DTBFM) as a thermally stable bridge-forming compound providing rigid heat-resistant crosslinks,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eZinc oxide (ZnO) as a vulcanization activator and structural modifier,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eTechnical carbon black (P324) as a reinforcing filler,\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eNaphthenic acid ester as a plasticizing agent improving processability.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eAll materials were of industrial grade and used without additional purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Preparation of elastomer blends\u003c/h2\u003e \u003cp\u003eElastomer mixtures were prepared by mechanical mixing on a laboratory two-roll mill under controlled plasticization conditions. The initial blending of EPDN and NBR was performed at a temperature of 323 K and a roll rotation speed of 40 rpm. The total mixing time varied from 10 to 40 min, which made it possible to trace the development of network formation during plasticization.\u003c/p\u003e \u003cp\u003eAfter initial homogenization of the rubber matrix, polydiene rubber was added, followed by dithiobis-maleimide, zinc oxide, carbon black and plasticizer. Each component was introduced sequentially with intermediate mixing to ensure uniform distribution in the polymer matrix. The total mixing time after the introduction of all additives was adjusted to obtain a visually homogeneous composition without agglomerates.\u003c/p\u003e \u003cp\u003eTo model thermo-oxidative degradation and intensify radical reactions, part of the samples was subjected to additional plasticization at an elevated temperature of 373 K and an increased roll speed of 102 rpm for up to 20 min. These conditions simulate aggressive processing and service environments and promote the formation of active radical centers in the polymer chains.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Vulcanization and thermal aging\u003c/h2\u003e \u003cp\u003eThe prepared elastomer compositions were vulcanized in a laboratory hydraulic press using a compression molding technique. Prior to curing, the mixtures were conditioned at room temperature for 24 h to ensure stress relaxation and homogenization of internal structure after milling. Vulcanization was carried out at a temperature of 423 K under a pressure of 10 MPa for 40 min. These curing parameters were selected based on preliminary kinetic experiments and literature data to ensure complete development of the crosslinked network in both saturated and unsaturated elastomer phases.\u003c/p\u003e \u003cp\u003eAfter vulcanization, the samples were slowly cooled to room temperature under pressure in order to minimize internal stresses and avoid post-curing deformation. Standard dumbbell-shaped specimens for mechanical testing and rectangular samples for swelling and gel analysis were cut from the molded plates.\u003c/p\u003e \u003cp\u003eTo evaluate the resistance of the crosslinked elastomer networks to thermo-oxidative degradation, accelerated thermal aging was performed. The vulcanized samples were placed in a forced-air circulation thermostat and aged at 373 K for 72 h. After aging, all mechanical, rheological and structural parameters were re-measured. The changes in tensile strength, elongation, hardness and elastic recovery were used to quantify the stability of the spatial network under prolonged thermal exposure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Mechanical testing\u003c/h2\u003e \u003cp\u003eMechanical characterization of the vulcanizates was performed using standardized testing procedures to ensure the reproducibility and reliability of the obtained results.\u003c/p\u003e \u003cp\u003eTensile strength and relative elongation at break were determined on a universal tensile testing machine in accordance with GOST 270\u0026thinsp;\u0026minus;\u0026thinsp;75. The tests were carried out at a constant crosshead speed under ambient conditions. For each composition, at least five parallel specimens were tested, and the average values were reported with standard deviation not exceeding 5%.\u003c/p\u003e \u003cp\u003eHardness was measured using the TM-2 method on flat vulcanized plates before and after thermal aging. The hardness values were expressed in conventional units and used as an indirect indicator of crosslink density and rigidity of the elastomer network.\u003c/p\u003e \u003cp\u003eRebound elasticity was determined after thermal aging in order to evaluate the ability of the material to recover its original shape after deformation. This parameter characterizes the elastic component of deformation and is highly sensitive to the mobility of polymer chains and the density of the crosslinked network.\u003c/p\u003e \u003cp\u003eDynamic endurance (fatigue resistance) was assessed under repeated cyclic stretching at room temperature after thermal aging. The number of deformation cycles sustained before the appearance of cracks or mechanical failure was recorded. This test allowed the evaluation of the resistance of the elastomer materials to long-term alternating mechanical loads, which is critical for sealing and vibration-isolating applications.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Rheological and structural characterization\u003c/h2\u003e \u003cp\u003eThe rheological behavior of the elastomer systems was characterized by determining the Mooney viscosity at 100\u0026deg;C for 5 min, which served as a sensitive indicator of changes in macromolecular mobility, branching and crosslinking.\u003c/p\u003e \u003cp\u003eThe gel content was determined gravimetrically after extraction of the samples in benzene. The gel fraction was calculated as the ratio of the mass of the insoluble residue to the initial sample mass and expressed as a percentage.\u003c/p\u003e \u003cp\u003eThe intrinsic viscosity [η] of the sol fractions was measured using an Ubbelohde capillary viscometer in benzene solution at room temperature. The obtained values were used to evaluate changes in molecular weight and chain branching.\u003c/p\u003e \u003cp\u003eThe crosslink density (1/Mc) was calculated using the Flory\u0026ndash;Rehner equation based on equilibrium swelling data [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{1}{{M}_{c}}=-\\frac{{\\rho\\:}}{{M}_{s}}\\cdot\\:\\frac{\\text{l}\\text{n}(1-{V}_{r})+{V}_{r}+\\chi\\:{V}_{r}^{2}}{{V}_{r}^{1/3}-{V}_{1}/2}\\)\u003c/span\u003e \u003c/span\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:\\)\u003c/span\u003e\u003c/span\u003e is the polymer density, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{M}_{s}\\)\u003c/span\u003e\u003c/span\u003e is the molar volume of the solvent, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{r}\\)\u003c/span\u003e\u003c/span\u003e is the polymer volume fraction in the swollen sample, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\chi\\:\\)\u003c/span\u003e\u003c/span\u003e is the polymer\u0026ndash;solvent interaction parameter.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6. Infrared spectral analysis\u003c/h2\u003e \u003cp\u003eThe molecular structure and chemical transformations occurring during plasticization, vulcanization and thermal aging were analyzed using Fourier-transform infrared (FTIR) spectroscopy. The spectra were recorded in the range of 700\u0026ndash;3200 cm⁻\u0026sup1; with a spectral resolution of 4 cm⁻\u0026sup1;.\u003c/p\u003e \u003cp\u003eFor FTIR analysis, thin films of the elastomer samples were prepared by dissolving the sol fraction in benzene, followed by solvent evaporation at room temperature. This approach ensured uniform film thickness and high spectral quality.\u003c/p\u003e \u003cp\u003eSpecial attention was paid to the absorption bands in the region of 950\u0026ndash;970 cm⁻\u0026sup1;, which correspond to trans-1,4-configurations and unsaturated C\u0026thinsp;=\u0026thinsp;C bonds actively involved in radical crosslinking reactions. The absorption bands associated with nitrile groups (\u0026ndash;C\u0026thinsp;\u0026equiv;\u0026thinsp;N) near 2235\u0026ndash;2240 cm⁻\u0026sup1; were also analyzed, since these polar groups play an important role in intermolecular interactions and network stabilization.\u003c/p\u003e \u003cp\u003eChanges in the intensity and shape of these characteristic bands were used to evaluate the degree of chemical modification, participation of NBR in crosslinking reactions, and the formation of thermally stable maleimide-bridged structures within the elastomer network.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Effect of NBR content on gelation and Mooney viscosity\u003c/h2\u003e \u003cp\u003eThe influence of nitrile\u0026ndash;butadiene rubber (NBR) content on the formation of the spatial network in EPDN/NBR elastomer systems was evaluated using gel content measurements and Mooney viscosity analysis. These parameters are highly sensitive to changes in molecular mobility, chain branching and crosslink density and therefore provide reliable information on the kinetics of structure formation during plasticization.\u003c/p\u003e \u003cp\u003eThe evolution of the rheological behavior of the investigated elastomer systems during plasticization is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The figure shows the change in Mooney viscosity as a function of plasticization time for EPDN/NBR blends with different component ratios [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e demonstrates the evolution of Mooney viscosity for EPDN/NBR blends with component ratios of 80:20, 70:30 and 60:40 as a function of plasticization time at 323 K. At the initial stage of mixing (10\u0026ndash;15 min), all compositions exhibit relatively low viscosity values, indicating a predominance of chain disentanglement and mechanical softening of the polymer matrix. However, with increasing plasticization time, a pronounced growth of Mooney viscosity is observed, which reflects the development of radical-induced chain branching and the onset of network formation.\u003c/p\u003e \u003cp\u003eThe development of the insoluble three-dimensional network during plasticization is further evidenced by the evolution of gel content with time, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The growth of the gel fraction directly reflects the progressive formation of a crosslinked spatial structure in the investigated EPDN/NBR systems [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs follows from Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the gel content in all investigated EPDN/NBR systems increases with plasticization time, confirming the progressive formation of an insoluble three-dimensional network. The highest rate of gel growth is observed for the 60:40 composition, which reflects the dominant role of NBR in radical-induced crosslinking. The 80:20 system exhibits the slowest gelation behavior due to the predominance of the saturated EPDN phase with limited radical activity. The intermediate behavior of the 70:30 blend indicates a transition region in which branching and crosslinking processes compete with partial thermo-mechanical degradation. The combined analysis of Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e clearly demonstrates that the increase in Mooney viscosity is directly associated with the growth of the gel fraction and the development of a continuous spatial network.\u003c/p\u003e \u003cp\u003eThe composition with an EPDN/NBR ratio of 60:40 shows the most intensive increase in Mooney viscosity over the entire plasticization interval. This behavior clearly indicates the dominant role of NBR as a reactive component providing unsaturated sites for radical reactions. In contrast, the composition with a low NBR content (80:20) demonstrates a significantly slower growth of viscosity, which is attributed to the saturated nature of the EPDN macromolecular backbone and its limited ability to participate in spontaneous crosslinking reactions under thermal-mechanical exposure.\u003c/p\u003e \u003cp\u003eThe 70:30 composition occupies an intermediate position and exhibits a non-monotonic viscosity behavior. At approximately 25\u0026ndash;30 min of plasticization, this system shows a temporary viscosity maximum, which can be attributed to the formation of a transient branched structure followed by partial chain scission or rearrangement processes. This effect indicates the competition between radical branching and thermo-mechanical degradation reactions occurring simultaneously in the polymer system.\u003c/p\u003e \u003cp\u003eThe evolution of gel content confirms the trends observed in the Mooney viscosity data. As the NBR fraction increases, the gel content grows steadily, reaching its maximum values for the 60:40 composition. After 40 min of plasticization, the gel fraction in this system exceeds that of the 80:20 blend by more than 1.5 times. This result provides direct evidence of the accelerated formation of an insoluble three-dimensional network promoted by the unsaturated double bonds and polar nitrile groups of NBR.\u003c/p\u003e \u003cp\u003eThe enhanced gelation behavior of the 60:40 system is associated with the higher probability of radical initiation, chain propagation and intermolecular coupling reactions involving NBR macroradicals. Under thermo-mechanical action, the C\u0026thinsp;=\u0026thinsp;C bonds in NBR undergo homolytic cleavage, forming active radical centers that readily participate in intermolecular crosslinking. In contrast, EPDN, being a saturated copolymer, exhibits substantially lower radical activity under the same conditions.\u003c/p\u003e \u003cp\u003eThe combined analysis of Mooney viscosity and gel content clearly demonstrates that the EPDN/NBR ratio of 60:40 represents a critical concentration region, at which a continuous crosslinked network begins to dominate the system. Below this threshold, the polymer structure remains predominantly linear or weakly branched, while above it, a dense spatial network develops, leading to a sharp increase in viscosity and gel fraction.\u003c/p\u003e \u003cp\u003eFrom a technological point of view, this behavior is of fundamental importance. On the one hand, the high viscosity and gel content of the 60:40 composition indicate the formation of a mechanically strong and thermally stable elastomer network. On the other hand, excessively rapid gelation at higher NBR contents could negatively affect processability. Therefore, the 60:40 ratio represents an optimal compromise between structural development and technological workability.\u003c/p\u003e \u003cp\u003eThus, the obtained results clearly confirm that NBR acts as a reactive structure-forming component in EPDN-based elastomer systems, and its concentration critically controls the kinetics of gelation, viscosity growth and the overall architecture of the spatial network.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Influence of polydiene and maleimide modifiers on crosslink density\u003c/h2\u003e \u003cp\u003eThe effect of polydiene rubber (PD) and dithiobis-maleimide (DTBFM) on the spatial network formation in EPDN/NBR elastomer systems was evaluated through the analysis of crosslink density (1/Mc), intrinsic viscosity and gel content. These parameters directly reflect the intensity of intermolecular coupling reactions and the rigidity of the resulting three-dimensional network [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhile the evolution of gel content reflects the qualitative development of the insoluble spatial network, a quantitative assessment of the network architecture requires the determination of crosslink density. Therefore, the effect of polydiene and maleimide modifiers on the crosslink density (1/Mc) of the EPDN/NBR systems is analyzed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor all investigated compositions, the introduction of PD and DTBFM leads to a pronounced increase in crosslink density compared to unmodified EPDN/NBR blends. This effect is especially significant for the 60:40 EPDN/NBR system, which already exhibits the highest radical activity due to the elevated content of unsaturated NBR chains. In this composition, the addition of PD provides supplementary unsaturated fragments that serve as additional reaction centers, while DTBFM acts as a multifunctional bridge-forming agent, promoting the formation of thermally stable intermolecular links.\u003c/p\u003e \u003cp\u003eThe calculated values of 1/Mc show a systematic increase with rising concentrations of PD and DTBFM. This indicates a progressive decrease in the average molecular weight between crosslinks and confirms the formation of a denser and more rigid spatial network. The increase in crosslink density is accompanied by a consistent growth of intrinsic viscosity [η], which reflects the increasing degree of chain branching and molecular integration in the sol fraction. Simultaneously, the gel content rises, confirming the transition of a larger fraction of the polymer matrix into an insoluble crosslinked state.\u003c/p\u003e \u003cp\u003eThe observed effect is explained by the synergistic radical mechanism involving all three key components of the system. Under thermo-mechanical and curing conditions, NBR macromolecules readily form radicals due to the presence of C\u0026thinsp;=\u0026thinsp;C double bonds. Polydiene fragments further increase the concentration of unsaturated sites, thereby intensifying chain propagation and intermolecular coupling. DTBFM molecules participate in radical addition reactions through the maleimide double bonds, forming rigid thermally stable bridges between elastomer chains. As a result, a mixed network structure consisting of C\u0026ndash;C crosslinks and maleimide-bridged junctions is formed.\u003c/p\u003e \u003cp\u003eA comparison of different EPDN/NBR ratios shows that the 80:20 system, despite the presence of PD and DTBFM, exhibits the lowest increase in crosslink density. This is due to the predominance of the saturated EPDN phase, which limits the overall concentration of radical-active sites. The 70:30 system demonstrates intermediate behavior, while the 60:40 composition clearly exhibits the most intensive network development, confirming the decisive role of the reactive NBR fraction in the efficiency of modification.\u003c/p\u003e \u003cp\u003eImportantly, the increase in crosslink density induced by PD and DTBFM does not occur abruptly but develops in a controlled manner, allowing the formation of a mechanically strong yet technologically processable elastomer network. This is particularly important for industrial applications, where excessive crosslinking can lead to processing difficulties and brittleness of the final material.\u003c/p\u003e \u003cp\u003eThus, the obtained results demonstrate that polydiene and maleimide modifiers play a key role in regulating the architecture of the crosslinked network in EPDN/NBR systems, enabling targeted control of crosslink density through the synergistic interaction with reactive NBR chains. The highest efficiency of this modification route is achieved for the EPDN/NBR ratio of 60:40, which forms the structural basis for optimizing the mechanical and thermal performance of the elastomer material.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Mechanical properties as a function of crosslink density\u003c/h2\u003e \u003cp\u003eThe variation of mechanical properties of the EPDN/NBR elastomer systems modified with polydiene and dithiobis-maleimide was analyzed in direct correlation with the crosslink density of the polymer network. Tensile strength, relative elongation at break and hardness were selected as the key indicators reflecting the balance between network rigidity and chain mobility [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The relationship between tensile strength and crosslink density for all investigated compositions is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA clear systematic relationship between crosslink density (1/Mc) and the mechanical behavior of the vulcanizates is observed for all investigated compositions. As the density of crosslinks increases, a pronounced growth of tensile strength is recorded. This effect is most significant for the EPDN/NBR\u0026thinsp;=\u0026thinsp;60:40 composition, where the highest density of radical-active sites and the most intensive network formation were previously established (Sections \u003cspan refid=\"Sec10\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e and \u003cspan refid=\"Sec11\" class=\"InternalRef\"\u003e3.2\u003c/span\u003e). In this system, the introduction of polydiene and maleimide modifiers leads to a substantial increase in strength compared to unmodified blends, indicating the formation of an efficient load-bearing spatial network.\u003c/p\u003e \u003cp\u003eThe strengthening effect is associated with the reduction of the average molecular weight between crosslinks and the formation of rigid junction points, which limit the slippage of polymer chains under applied stress. The presence of maleimide-bridged crosslinks plays a particularly important role, since such junctions are characterized by enhanced thermal stability and mechanical rigidity.\u003c/p\u003e \u003cp\u003eIn contrast to strength, the relative elongation at break shows an inverse dependence on crosslink density. With increasing 1/Mc, a gradual decrease in elongation is observed for all compositions. This behavior reflects the restriction of macromolecular mobility caused by the densification of the spatial network. The most pronounced decrease in elongation is again observed for the 60:40 system, where the formation of a dense crosslinked structure most strongly limits chain extensibility.\u003c/p\u003e \u003cp\u003eNevertheless, despite the reduction in elongation, the modified systems retain sufficiently high deformation capability, indicating that the formed networks do not exhibit excessive brittleness. This confirms that the applied modification approach provides an optimal balance between rigidity and elasticity, which is essential for elastomeric applications under dynamic loads.\u003c/p\u003e \u003cp\u003eThe evolution of hardness closely follows the trend of tensile strength. As the crosslink density increases, hardness values rise monotonically, reflecting the increasing stiffness of the elastomer matrix. After thermal aging, the hardened state of the modified vulcanizates becomes even more pronounced, especially for compositions containing both polydiene and maleimide additives. This behavior confirms the formation of a thermally stable network structure resistant to segmental relaxation under elevated temperatures.\u003c/p\u003e \u003cp\u003eA comparative analysis of the three EPDN/NBR ratios shows that the 80:20 composition, despite some increase in crosslink density after modification, exhibits the lowest strength and hardness and the highest elongation. This reflects the limited participation of the saturated EPDN chains in radical crosslinking. The 70:30 system demonstrates intermediate mechanical performance, while the 60:40 composition clearly exhibits the best combination of high strength, sufficient elasticity and enhanced hardness.\u003c/p\u003e \u003cp\u003eFrom a structure\u0026ndash;property perspective, these results clearly demonstrate that the mechanical performance of EPDN/NBR elastomers is governed primarily by the density and nature of the crosslinked network. The synergistic action of NBR, polydiene and maleimide modifiers allows the formation of a reinforced three-dimensional structure, in which the applied mechanical load is efficiently transferred through a system of rigid and thermally stable junction points.\u003c/p\u003e \u003cp\u003eThus, the obtained results confirm that targeted regulation of crosslink density using reactive modifiers represents an effective route for controlling the mechanical behavior of EPDN-based elastomer systems, providing a predictable transition from soft, highly extensible materials to mechanically robust and thermally stable elastomeric networks.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Thermal-oxidative aging behavior and fatigue resistance\u003c/h2\u003e \u003cp\u003eThe resistance of the modified EPDN/NBR elastomer systems to thermal-oxidative aging and cyclic mechanical loading was evaluated to assess the long-term stability of the formed crosslinked networks under conditions close to real service environments. The changes in tensile strength, relative elongation, hardness, rebound elasticity and dynamic endurance after accelerated aging at 373 K for 72 h were analyzed. The effect of thermal aging on the mechanical strength of the modified elastomer systems is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThermal aging results demonstrate that all modified compositions exhibit significantly higher stability compared to unmodified EPDN/NBR blends. The most pronounced improvement is observed for the systems containing both polydiene and maleimide modifiers, which form a thermally stable network structure resistant to oxidative chain scission and post-curing relaxation [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe EPDN/NBR\u0026thinsp;=\u0026thinsp;60:40 composition shows the highest resistance to thermal-oxidative degradation. After aging, this system retains a major part of its initial tensile strength and hardness, while the decrease in relative elongation remains moderate. This behavior directly confirms the formation of a dense and thermally stable spatial network stabilized by rigid maleimide bridges and reinforced by additional C\u0026ndash;C crosslinks formed via polydiene and NBR fragments.\u003c/p\u003e \u003cp\u003eIn contrast, the 80:20 system, characterized by a lower initial crosslink density, exhibits more pronounced degradation effects after aging. A noticeable decrease in tensile strength and rebound elasticity is accompanied by an increase in brittleness, indicating partial destruction of the weakly developed polymer network dominated by the saturated EPDN phase.\u003c/p\u003e \u003cp\u003eThe 70:30 composition demonstrates intermediate aging behavior, reflecting the transitional nature of its structure, in which the contribution of radical-active NBR chains is already significant but still insufficient to provide the highest level of thermal stabilization.\u003c/p\u003e \u003cp\u003eThe observed thermal stability trends are directly correlated with the crosslink density and the nature of the formed junctions. In networks dominated by C\u0026ndash;C and maleimide-bridged crosslinks, the probability of thermo-oxidative chain rupture is substantially reduced. The nitrile groups of NBR also contribute to the stabilization of the network by enhancing intermolecular interactions and reducing oxygen diffusion into the bulk of the material.\u003c/p\u003e \u003cp\u003eThe fatigue resistance of the modified elastomers was evaluated by dynamic endurance tests under repeated cyclic stretching after thermal aging. The results reveal a pronounced increase in the number of deformation cycles sustained before failure for all compositions modified with polydiene and maleimide additives. Once again, the maximum fatigue endurance is achieved for the 60:40 composition, which combines high crosslink density with sufficient chain mobility to dissipate mechanical energy under cyclic loading.\u003c/p\u003e \u003cp\u003eThe improvement in fatigue resistance is explained by the ability of the reinforced three-dimensional network to effectively redistribute local stresses and suppress microcrack initiation and propagation. The presence of thermally stable maleimide junctions plays a crucial role in maintaining network integrity under repeated deformation, while polydiene fragments enhance the toughness of the elastomer matrix.\u003c/p\u003e \u003cp\u003eFrom an application-oriented perspective, the obtained results demonstrate that the synergistic modification of EPDN/NBR elastomers with polydiene and maleimide systems provides a substantial improvement in long-term thermal stability and fatigue durability, which is critically important for elastomer products operating under combined thermal and mechanical loads, such as seals, gaskets, cable insulation and vibration-damping elements.\u003c/p\u003e \u003cp\u003eThus, the thermal-oxidative aging and fatigue resistance results fully confirm the structure\u0026ndash;property relationships established in Sections \u003cspan refid=\"Sec10\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Sec12\" class=\"InternalRef\"\u003e3.3\u003c/span\u003e and validate the effectiveness of the proposed reactive modification strategy for the development of durable high-performance elastomer materials.\u003c/p\u003e \u003cp\u003eFor a comprehensive comparison of the structural, rheological and mechanical performance of all investigated elastomer systems, the main experimental parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The table provides a unified overview of the effect of NBR content and the combined action of polydiene and maleimide modifiers on gel formation, crosslink density, mechanical strength and aging resistance of EPDN-based elastomers.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparative summary of structural, rheological and mechanical parameters of EPDN/NBR elastomer systems. Qualitative levels are used for comparative analysis.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEPDN/NBR ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eModification system\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eGel content (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMooney viscosity\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCrosslink density (1/Mc)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTensile strength\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eElongation at break\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHardness\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eThermal aging resistance (373 K, 72 h)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e80 : 20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003enone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003elow\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003elow\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003elow\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003elow\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003elow\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003elow\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e70 : 30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003enone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e60 : 40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003enone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e80 : 20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePD\u0026thinsp;+\u0026thinsp;DTBFM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e70 : 30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePD\u0026thinsp;+\u0026thinsp;DTBFM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003emedium\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003ehigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003e60 : 40\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003ePD\u0026thinsp;+\u0026thinsp;DTBFM\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003emaximum\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003emaximum\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003emaximum\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003emaximum\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eoptimal\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cb\u003emaximum\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003cb\u003emaximum\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo provide an integrated visualization of the structure\u0026ndash;property relationships for all investigated compositions, a mechanical\u0026ndash;structural property map is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe obtained experimental results clearly demonstrate that the structure\u0026ndash;property relationships in the studied EPDN/NBR elastomer systems are governed by the synergistic interaction between three key factors: the concentration of reactive NBR chains, the presence of polydiene fragments, and the incorporation of maleimide bridge-forming agents. The combined action of these components determines the kinetics of radical processes, the architecture of the spatial network, and, consequently, the mechanical and thermal performance of the resulting elastomer materials.\u003c/p\u003e \u003cp\u003eThe role of nitrile\u0026ndash;butadiene rubber is primarily associated with its high radical reactivity. The presence of unsaturated C\u0026thinsp;=\u0026thinsp;C bonds and polar nitrile groups in the NBR macromolecule provides a large number of potential sites for radical initiation and chain propagation under thermo-mechanical and curing conditions.\u003c/p\u003e \u003cp\u003eThe unified structure\u0026ndash;property\u0026ndash;performance relationship governing the behavior of the investigated elastomer systems is schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the mechanical and thermal performance of the investigated elastomer systems is governed by a hierarchical structure\u0026ndash;property\u0026ndash;performance relationship. The increase in NBR content and the introduction of polydiene and maleimide modifiers activate radical crosslinking, leading to the formation of a hybrid spatial network. The growth of crosslink density and gel fraction directly controls the strength, hardness and aging resistance of the materials. The optimal balance between network rigidity and chain mobility is achieved at the EPDN/NBR ratio of 60:40, which ensures superior long-term performance under combined thermal and mechanical loading.\u003c/p\u003e \u003cp\u003eAs shown in Section \u003cspan refid=\"Sec10\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e, increasing the NBR content leads to a pronounced acceleration of gel formation and a sharp increase in Mooney viscosity, which indicates the transition from predominantly linear or weakly branched chains to a continuous three-dimensional crosslinked network. The existence of a critical concentration region at the EPDN/NBR ratio of 60:40 reflects the percolation-like character of network formation, when the number of reactive sites becomes sufficient for the development of a load-bearing spatial structure.\u003c/p\u003e \u003cp\u003eThe introduction of polydiene rubber further intensifies the radical mechanism by increasing the overall concentration of unsaturated fragments in the system. Polydiene chains act not only as additional sources of radical centers but also as flexible connecting segments that enhance the probability of intermolecular coupling reactions. As demonstrated in Section \u003cspan refid=\"Sec11\" class=\"InternalRef\"\u003e3.2\u003c/span\u003e, the addition of polydiene results in a systematic increase in crosslink density and intrinsic viscosity, confirming the formation of a denser and more highly branched network. At the same time, the flexible nature of polydiene fragments prevents excessive embrittlement of the material, allowing the preservation of sufficient chain mobility and energy dissipation capacity under mechanical loading.\u003c/p\u003e \u003cp\u003eA special role in the formation of a thermally stable network is played by dithiobis-maleimide (DTBFM). Maleimide fragments participate in radical addition reactions through their highly reactive double bonds, forming rigid and thermally resistant intermolecular bridges. These junctions act as stable crosslinking nodes that suppress segmental relaxation and limit the reversibility of the network at elevated temperatures. The presence of such junctions explains the pronounced increase in hardness, tensile strength and thermal-oxidative stability observed for the modified systems (Sections \u003cspan refid=\"Sec12\" class=\"InternalRef\"\u003e3.3\u003c/span\u003e and \u003cspan refid=\"Sec13\" class=\"InternalRef\"\u003e3.4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFrom a mechanistic viewpoint, the formation of the elastomer network in the studied systems can be described by a unified radical crosslinking model. Under thermo-mechanical and curing conditions, radical initiation occurs predominantly on unsaturated NBR and polydiene chains. The generated macroradicals undergo chain propagation through intermolecular coupling reactions, forming C\u0026ndash;C crosslinks. Simultaneously, maleimide molecules participate in radical addition reactions, leading to the formation of rigid bridge-like junctions between elastomer chains. As a result, a hybrid network structure is formed, consisting of flexible C\u0026ndash;C crosslinks and rigid maleimide-bridged nodes. This dual character of the network explains the unique combination of high strength, sufficient elasticity and enhanced thermal stability observed for the modified EPDN/NBR systems.\u003c/p\u003e \u003cp\u003eThe established correlations between crosslink density, gel content, viscosity and mechanical properties provide direct experimental confirmation of the classical structure\u0026ndash;property concept for elastomers. An increase in crosslink density leads to a monotonic growth of tensile strength and hardness, accompanied by a gradual decrease in relative elongation. However, within the investigated modification range, the decrease in elongation does not result in catastrophic embrittlement, indicating that the applied modification strategy allows the formation of a mechanically robust yet sufficiently ductile network. The EPDN/NBR\u0026thinsp;=\u0026thinsp;60:40 composition represents an optimal balance point where the density of active radical sites, the efficiency of crosslink formation and the preservation of chain mobility are harmonized.\u003c/p\u003e \u003cp\u003eThe thermal-oxidative aging results further highlight the decisive influence of the network architecture on long-term stability. In systems dominated by weak and sparse crosslinks, thermal exposure leads to accelerated chain scission, loss of strength and increased brittleness. In contrast, in networks reinforced by maleimide-bridged junctions and additional C\u0026ndash;C crosslinks formed via polydiene and NBR fragments, the probability of thermo-oxidative rupture is significantly reduced. This explains the high retention of mechanical properties and enhanced fatigue resistance observed for the modified systems, particularly for the 60:40 composition.\u003c/p\u003e \u003cp\u003eFrom a practical standpoint, the obtained results demonstrate that the proposed modification strategy allows the targeted design of EPDN-based elastomer materials with predictable and tunable performance characteristics. By regulating the content of NBR, polydiene and maleimide modifiers, it becomes possible to control the density and nature of the spatial network, thereby adjusting the balance between strength, elasticity, heat resistance and fatigue durability. Such materials are highly promising for applications requiring long-term mechanical reliability under combined thermal and cyclic loads, including sealing elements, vibration-damping components, and cable insulation systems.\u003c/p\u003e \u003cp\u003eThus, the discussion confirms that the observed improvements in mechanical and thermal performance are not random but are a direct consequence of the controlled radical modification of the elastomer network structure, fully consistent with the established structure\u0026ndash;property relationships.\u003c/p\u003e \u003cp\u003eThe obtained results indicate that the improvement of mechanical performance and thermal-oxidative stability in the modified EPDN/NBR systems is primarily governed by changes in the architecture of the crosslinked network rather than by simple compositional effects. The introduction of nitrile\u0026ndash;butadiene rubber increases the concentration of unsaturated sites and polar functional groups, which enhances radical reactivity during curing and promotes the formation of a more continuous spatial network. This effect is reflected in the increase in gel fraction and Mooney viscosity, confirming the transition from weakly branched structures to a more interconnected elastomer network.\u003c/p\u003e \u003cp\u003eThe role of polydiene rubber and dithiobis-maleimide modifiers appears to be synergistic. Polydiene components introduce additional unsaturated fragments that act as reactive bridges between macromolecular chains, facilitating network growth and improving stress distribution under mechanical loading. At the same time, dithiobis-maleimide participates in the formation of rigid, thermally stable bridge-type crosslinks, which restrict excessive chain mobility at elevated temperatures. As a result, a hybrid elastomer network is formed, combining flexible carbon\u0026ndash;carbon crosslinks with rigid maleimide-bridged junctions. Such a network architecture effectively delays thermo-oxidative degradation and suppresses the development of irreversible deformation during prolonged thermal exposure.\u003c/p\u003e \u003cp\u003eThe balance between network density and chain mobility is a critical factor determining the durability of elastomer materials. The experimental results demonstrate that the EPDN/NBR ratio of 60:40 provides an optimal compromise between crosslink density, mechanical strength and elasticity. At lower NBR contents, the network remains insufficiently developed, leading to reduced resistance to thermal aging. In contrast, excessive crosslinking at higher reactive component concentrations may increase stiffness and limit fatigue resistance. The identified optimal composition ensures efficient energy dissipation under cyclic deformation while maintaining structural integrity during thermal aging, which is essential for elastomer materials intended for long-term industrial applications.\u003c/p\u003e \u003cp\u003eOverall, the obtained structure\u0026ndash;property relationships confirm that targeted reactive modification using a combination of nitrile\u0026ndash;butadiene rubber, polydiene additives and maleimide crosslinkers represents an effective approach for tailoring the network architecture of EPDN-based elastomers. This strategy allows controlled regulation of mechanical durability and thermal stability without the need for complex chemical synthesis, which is particularly attractive for applied polymer engineering and industrial-scale elastomer production.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eA systematic investigation of the structure\u0026ndash;property relationships in EPDN/NBR elastomer blends modified with polydiene and dithiobis-maleimide systems has been carried out. The obtained results allow the following conclusions to be drawn:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eNitrile\u0026ndash;butadiene rubber acts as a key reactive component in EPDN-based systems, controlling the kinetics of radical-induced spatial network formation. An increase in NBR content leads to a pronounced growth of gel fraction and Mooney viscosity, indicating the transition from weakly branched structures to a continuous three-dimensional crosslinked network.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe introduction of polydiene rubber and maleimide modifiers provides effective regulation of crosslink density and network architecture. Polydiene increases the concentration of unsaturated reaction centers, while dithiobis-maleimide forms rigid and thermally stable bridge-type junctions. Their synergistic action results in the formation of a hybrid elastomer network consisting of flexible C\u0026ndash;C crosslinks and rigid maleimide-bridged nodes.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eA clear quantitative correlation between crosslink density and mechanical properties has been demonstrated. An increase in crosslink density leads to a monotonic growth of tensile strength and hardness, accompanied by a controlled decrease in relative elongation, without inducing excessive brittleness.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe EPDN/NBR ratio of 60:40 has been identified as the optimal composition, providing the best balance between processability, mechanical strength, elasticity and thermal-oxidative stability. This composition exhibits the highest resistance to thermal aging and superior fatigue durability under cyclic mechanical loading.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe results of thermal-oxidative aging and dynamic endurance tests confirm that the formation of a reinforced spatial network significantly improves the long-term stability of elastomer materials under combined thermal and mechanical stresses.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eOverall, the obtained results demonstrate that targeted reactive modification of EPDN/NBR systems using polydiene and maleimide additives represents an efficient strategy for designing elastomer materials with predictable and tunable performance characteristics. The proposed approach opens prospects for the development of durable, heat-resistant and mechanically stable elastomer products for demanding industrial and engineering applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eConflict of interest.\u003c/h2\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003ch2\u003eFunding.\u003c/h2\u003e\n\u003cp\u003eThe authors declare that no funds, grants, or other support were received during the preparation of this manuscript.\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eS.V.R. conceived and performed the experimental work and prepared the figures.E.N.A. performed data analysis and wrote the main manuscript text.Both authors discussed the results, contributed to manuscript editing, and approved the final version.\u003c/p\u003e\n\u003ch2\u003eData Availability\u003c/h2\u003e\n\u003cp\u003eThe experimental data supporting the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAhmedova, E.N., Rzayeva, S.V.: The influence of NBR on the complex of technological properties of binary mixtures based on EPDN. Surf. Eng. Appl. Electrochem. \u003cb\u003e61\u003c/b\u003e(6), 966\u0026ndash;970 (2025). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3103/S1068375525701054\u003c/span\u003e\u003cspan address=\"10.3103/S1068375525701054\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAhmedova, E.N., Rzayeva, S.V.: The influence of the structure and composition of elastomer mixtures on the mechanical properties of SKEP-BNK vulcanizates. Surf. Eng. Appl. Electrochem. \u003cb\u003e61\u003c/b\u003e(2), 261\u0026ndash;265 (2025). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3103/S1068375525700152\u003c/span\u003e\u003cspan address=\"10.3103/S1068375525700152\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMamedova, S.M., Ahmedov, E.N., Rzayeva, S.V.: Modified ethylene propylene rubbers with unsaturated rubbers and low-molecular reactive compounds. Surf. Eng. Appl. 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Springer (1999)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"EPDN/NBR blends, elastomer networks, structure–property relationships, crosslink density, thermal-oxidative aging, mechanical durability","lastPublishedDoi":"10.21203/rs.3.rs-8800335/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8800335/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigates the structure\u0026ndash;property relationships in binary elastomer systems based on ethylene\u0026ndash;propylene diene copolymer (EPDN) and nitrile\u0026ndash;butadiene rubber (NBR) modified with polydiene (PD) and dithiobis-maleimide (DTBFM) functional additives. The combined influence of reactive unsaturated rubber, polydiene modifiers and maleimide crosslinkers on crosslink density, gel fraction, rheological behavior, mechanical properties and thermal-oxidative stability was systematically analyzed for EPDN/NBR blends with component ratios of 80:20, 70:30 and 60:40. The results demonstrate that increasing NBR content significantly enhances radical-induced crosslinking due to the presence of carbon\u0026ndash;carbon double bonds and polar nitrile groups, leading to the formation of a more developed spatial network. The introduction of polydiene and maleimide additives promotes the formation of hybrid elastomer networks, which is reflected in the increase of gel fraction, Mooney viscosity and crosslink density calculated using the Flory\u0026ndash;Rehner approach. A clear correlation between network architecture and mechanical performance is established. An optimal balance between processability, mechanical strength and thermal-oxidative resistance is achieved at an EPDN/NBR ratio of 60:40, which exhibits the highest tensile strength, hardness and durability under thermal aging conditions. The obtained results indicate that targeted reactive modification using a combination of NBR, polydiene and maleimide additives represents an effective strategy for tailoring the network structure of EPDN-based elastomers. This approach enables the development of durable elastomer materials with controlled properties for demanding industrial and engineering applications without the use of conventional sulfur curing systems.\u003c/p\u003e","manuscriptTitle":"Structure–Property Relationships and Thermal Aging of EPDN/NBR Elastomer Networks Modified With Polydiene and Maleimide Additives","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-20 04:17:15","doi":"10.21203/rs.3.rs-8800335/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"28908a97-c66f-48e9-9766-920532dc075a","owner":[],"postedDate":"February 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-20T04:17:15+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-20 04:17:15","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8800335","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8800335","identity":"rs-8800335","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}