A Novel Bismaleimide-Urea Resin: Breaking the Trade-off Between Strength, Toughness and Thermal Stability via Molecular Design

A Novel Bismaleimide-Urea Resin: Breaking the Trade-off Between Strength, Toughness and Thermal Stability via Molecular Design | 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 A Novel Bismaleimide-Urea Resin: Breaking the Trade-off Between Strength, Toughness and Thermal Stability via Molecular Design Kangbo Zhao, Mosha Cheng, Umut Bakhbergen, Sherif Araby, Sensen Han, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8750615/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 The intrinsic brittleness of bismaleimide (BMI) resins has long restricted their wider application in aerospace and automotive industry. Herein, a novel BMI-urea resin network that integrates flexible urea linkages into the rigid BMI matrix is developed. The developed BMI-urea polymer combines elasticity with high strength. Molecular dynamics simulations and AI-assisted predictions revealed that Michael addition dominates over BMI self-polymerization, enabling rational design of hybrid networks. The nanoscale phase-separated morphology endowed BMI-urea with ductility and toughness while preserving strength and thermal stability. Fracture analysis revealed the enhancement of mechanical properties via rigid BMI segments providing reinforcement while flexible urea segments dissipating energy and supressing crack propagation. The developed BMI-urea exhibited (i) tensile as high as 55 MPa – 120% improvement over diamine-urea; (ii) impact strength as high as 208 kJ/m 2 – 30% improvement over BMI; (iii) up to a 60-fold increase in elongation at break; (iv) preserved thermal stability above 290°C. Swelling resistance test in three different media showed a dependence of solvent uptake on soft segment content and microphase separation. The current study introduces, for the first time, an elastic BMI-urea resin that combines toughness, strength and thermal resistance – paving the way for advanced application is aerospace composites and structural adhesives. Bismaleimide Flexible urea segments Molecular dynamics Mechanical performance Thermal stability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Bismaleimide (BMI) is a heat-resistant thermosetting resin containing monomers that form through addition reactions to produce highly cross-linked network polyimides. They are featured by exceptional mechanical strength, enhanced thermal stability, excellent flame-retardant performance and low thermal expansion coefficient [ 1 ]. These features make BMI suitable for cutting edge applications including aerospace and automative industries. The key benefits stem from the reactive maleimide end groups, which enable thermal curing without the release of volatile by-products that create voids in highly cross-linked networks [ 2 ]. However, the resulting resins tend to be brittle because of their aromatic backbone and elevated cross-link density [ 3 ]. Therefore, broadening the application of BMI resins requires enhancing their toughness without compromising their other performance characteristics [ 4 ]. Michael addition copolymerization with aromatic diamines represents one of the simplest and most economical means of BMI resin synthesis [ 5 ]. Typically, the preparation of BMI involves formation of a maleamic acid intermediate, which then undergoes intramolecular cyclodehydration to form the characteristic imide ring [ 6 ]. Dels-Alder reaction with tuneable reversibility [ 7 ] as well as free-radical route for two step copolymerization [ 8 ] are alternative methods of BMI production [ 9 ]. The research on toughening the BMI is focused on strategies including synthesis of new classes of BMI monomers [ 10 ], allyl compound copolymerization [ 11 ], inorganic particle addition [ 12 ], diamine chain-extension [ 3 ] and elastomer incorporation [ 13 ]. Variations in the backbone architecture of BMI oligomers produce resins with distinct performance profiles. Bifunctional BMI monomers react twice via Michael addition reactions with difunctional nucleophiles, such as dithiols or diamines to form a step-growth, cross-linked network [ 10 ]. Hence, modulus, toughness and glass transition temperature of the BMI can be tailored by introducing soft aliphatic or ether segments between the rigid succinimide nodes [ 8 ]. Asymmetric BMI monomers were designed to improve the processability and reduce the brittleness of BMIs achieving resin films with elongation at break as high as 17.3% without significant changes in the maximum strength [ 14 ]. Trifunctional maleimide was used to form a micro-branched BMI resin achieving a 8.8% reduction in dielectric constant, and increase of 140% in bending and 149% in impact strengths [ 15 ]. Tunability of BMI properties is important for its various applications. Incorporation of 5 wt% of phosphate and phosphonate-bearing structure into BMI resin for flame retarding application increased its impact strength by 146.3% and 90.2%, respectively [ 16 ]. Fluorine BMI resin films developed for piezoelectric sensors exhibited outstanding mechanical performance ‒ tensile strength of 160–256 MPa and moduli of 3.55–5.29 GPa [ 17 ]. Cross-link density of the cured material has a significant effect on the performance of the polymer. It can be regulated by adjustment of oligomer’s molecular weight. Variation of allyl to imide ratio from 0.72 to 1.14 resulted in up to 35% improvement of impact strength in bio-derived allyl modified BMI resins [ 11 ]. Improvement of mechanical and thermal properties was reported for 50 mol% allylamine BMIs [ 18 ]. Another effective approach to improve mechanical properties in cured BMIs is the incorporation of flexible linkages into the network structure and increasing the distance between the two functional groups [ 3 ]. Enhanced thermomechanical properties of diamine-modified BMI resins was observed at different molar ratios by adjustment of the degree of cross-linking [ 9 ]. Toughening BMIs can be achieved via modification with rubber or thermoplastics. Cross-linking polyethylene glycol to a polymeric backbone via Diels-Alder reaction resulted in BMI with robust shape stability and mechanical strength [ 19 ]. Moreover, recyclability and malleable plasticity over multiple thermal cycle was achieved due to thermal reversibility of the cure reaction. Incorporating 15 wt% of eugenol allyl ether-grafted polysiloxane into BMI produced a thermoset with markedly improved toughness while maintaining comparable glass transition temperature and thermal stability [ 13 ]. Control of the phase-separation is one of the main challenges when employing rubber/thermoplastic components [ 20 ]. Hence, developing BMIs with excellent toughness while maintaining the inherent properties of the resin is still an ongoing topic of research. Tailoring the performance of the polymer is possible by modifying their microphase structure. Introducing urea moieties, for example, can improve interchain association [ 21 ]. This is attributed by the increased hydrogen bond density inherent to bidentate hydrogen bond in urea and “sacrificial” cross-links from them [ 22 ]. In addition, the reversible exchange of disulfide bonds in urea promotes elastomer softening [ 23 ]. Tuning urea-derived interactions in thermoset polymers is key to balancing their mechanical robustness. For example, at the optimal content of urea moieties in polyurethane-urea polymers resulted in 7-fold increase of Young’s modulus with 450% strain [ 24 ]. The toughening effect of urea can be employed when it is bonded to imides. A novel polyurethane-urea-imide coatings with improved adhesion performance were developed by employing the principle of introducing chain-separation segment and flexibility via urea moieties [ 21 ]. In the NCO-terminated poly(urethane-urea-imide) copolymers, the excess isocyanate is moisture-cured with diamine extenders. It formed urea linkages with embedded imide rings via hydrogen bonding [ 25 ]. Thus, hard-soft microphase separation is initiated resulting in the adhesive coatings with enhanced mechanical strength (16.7 MPa with 1097% stretchability) and thermal properties (3.87 W/mK). Inspired by the successful performance of imide-urea polymers, it is possible to graft urea linkages into the rigid BMI network. The resulting BMI-urea polymer can yield an interpenetrated hard-soft microstructure: stiff imide bear load while flexible diamine-urea dissipate energy and block crack propagation. As a result, toughening of BMI resin is achieved while maintaining its inherent properties. Since BMI-urea polymer was not previously reported, it is important to investigate not only its morphology and mechanical properties but comprehend its molecular structure. Motion characteristics of molecular chain segments, the evolution of internal structures and the system network formation can be investigated via molecular dynamics (MD) simulations. Microphase separation between rigid imide rings and diamine segments critically influences the network’s activation energy, mechanical response and water uptake of the resin [ 26 ]. Nevertheless, the atomistic mechanisms underlying these effects remain unclear given the limited amount of MD studies of BMI resins [ 27 ]. MD simulations can quantify activation energy barriers [ 28 ], predicting the likelihood of possible reaction scenarios. A detailed reactants and product models and MD simulation of their interactions and behaviour are needed to elucidate how diamine-urea-imide system tune energy barriers, enhance toughness, and regulate other properties of the BMI-urea polymer. In this study, isophorone diisocyanate (IPDI) and diamine-terminated poly(propylene glycol) are mixed at different molar ratios (2:1, 2.5:1 and 3:1), and added to isophorone diamine (IPDA) to produce diamine-urea as a spacer and softening agent for BMI. The morphology of resulting new BMI-urea resins is investigated along with their mechanical, thermal and water absorption behaviour. The MD simulations are used to comprehend the effect of diamine-urea modification on chain mobility and microphase separation, and the results are validated against experimental data. The study presents a pioneering systematic experimental and MD simulation approach to investigate the properties and performance of novel BMI-urea polymer. 2. Materials and methods 2.1. Materials and characterization Raw materials and characterization have been described in detail in Supporting Information. 2.2. Synthesis of BMI-urea Synthesis of BMI-urea was carried out in a two-step process as schematically presented in Fig. 1 . In the first step shown in Fig. 1 (a), diamine-urea prepolymer with different imide to J2000 molar ratios (2:1, 2.5:1, and 3:1) was synthesized. J2000 was placed in a vacuum oven at 90 ℃ to eliminate any residual moisture. IPDI was mixed with J2000; the mixture was stirred in an oil bath at 150 rpm using magnetic stirring. The oil bath temperature was gradually raised from room temperature to 80 ℃ and maintained for 1 h to facilitate the formation of the prepolymer. The IDPI/J2000 mixture was allowed to cool to room temperature before slowly adding 1.8 g of IPDA solution. The obtained mixture was stirred for 25 min. Diamine-urea prepolymers with IDPI to J2000 molar ratios 2:1, 2.5:1, and 3:1 was synthesized. The second step of the synthesis is illustrated in Fig. 1 (b). Diamine-urea prepolymers obtained in the first step were combined with BMI in a 100 ml three-necked flask at H 2 N– to C = C molar ration of 1:2. The mixture was mechanically stirred at 170 ℃ for approximately 1 h to achieve a homogeneous mixture. Three different BMI-urea polymers were obtained: BMI-urea 2/1 , BMI-urea 2.5/1, and BMI-urea 3/1 where subscripts correspond to IPDI to J2000 molar ratios (2:1, 2.5:1 and 3:1) in the first step of the synthesis. BMI-urea polymers were transferred into Teflon molds and cured at 170 ℃ for 10 hrs in a vacuum oven. 2.3. Molecular dynamics Molecular dynamics information has been described in detail in Supporting Information. 3. Results and discussion 3.1. Synthesis and morphology of BMI-urea The chemical reactions occurring during the BMI-urea synthesis process are schematically illustrated in Fig. 2 . The process starts by adding IPDI and J2000 which initiates the reaction of imide with isocyanate group. As shown in Fig. 2 (a&b) lone pair on the terminal -NH 2 of J2000 attacks the electrophilic carbon of one NCO group on IPDI producing a urea linkage (-NH-CO-NH-). One NCO of IPDI remains unreacted for sub sequential addition. Depending on the imide to J2000 molar ratio (2:1, 2.5:1, and 3:1) more isocyanate groups might be available in the system [ 29 ]. The reactivities of terminal amino groups at two ends of J2000 are different due to slightly different atomic partial charges [ 30 ]. This charge imbalance is caused by chain conformation, local environment and proximity of dipoles. Hence, one terminal amine group can carry slightly more negative electrostatic potential which makes it more nucleophilic and reactive [ 31 ]. Amino group with lower local steric hindrance should be the one not adjacent to a methyl-substituted propylene oxide unit. As a result of this reaction, NH 2 -terminated urea prepolymer is formed. The addition of IPDA (Fig. 2 (b&c)) leads to second nucleophilic attack of IPDA’s -NH 2 on the remaining -NCO from IPDI which forms the second urea linkage. Diamine-urea prepolymer is formed with a possible chemical structure illustrated in Fig. 2 (c). Addition of BMI to diamine-urea prepolymer initiates aza-Michael addition reaction where “Michael donor” comes from nucleophile diamine-urea and electron-poor N-C bond in BMI acts as “Michael acceptor” [ 32 ]. In the final curing step diamine-urea pre polymers is “Michael added” onto two maleimide C = C bonds of BMI at both ends of diamine-urea to produce the bis-adduct and crosslink [ 33 ]. At the same time, homopolymerization of BMI by free-radical addition across its two activated maleimide C = C bonds [ 6 , 10 ]. Elevated temperature of the curing adds a radical species to one of the maleimide C = C bonds generating carbon-centred radical on the succinimide ring. The newly formed radical on the succinimide adds into the C = C of a second BMI monomer which creates a new C-C bond between monomer units and regenerates a radical site at the terminus. The availability of the -NH from diamine-urea prepolymer (which, in turn, depends on the initial IPDI to J2000 molar ratio) determines the amount of BMI-urea formation and BMI homopolymerization. Homopolymerization of BMI will produce high-strength brittle BMI polymer, while increase of the IPDI to J2000 will increase the Michael addition reaction. Consequently, BMI-urea polymers with different amount of brittle homopolymerized BMI and more elastic BMI-urea are synthesized. The chemical structure of the reactants: diamine-urea and BMI and produced BMI-urea polymers was determined via FTIR given in Fig. 3 . The diamine-urea spectrum features a broad N-H stretching band at 3340 cm − 1 . It also shows strong aliphatic C-H stretches between 2960 and 2850 cm − 1 and an ether C-O-C band at 1180 − 1100 cm − 1 [ 29 ]. A residual NCO band appears at 2270 cm − 1 . The FTIR spectrum of the diamine-urea prepolymer confirms successful urea linkage formation. The complete disappearance of the 2270 cm − 1 -N = C=O stretch indicates full consumption of IPDI’s isocyanate groups during the first synthesis step. A broad N-H stretch at 3340 cm − 1 and a new C = O band at 1630 cm − 1 emerge, consistent with urea carbonyl formation and hydrogen-bonding [ 33 ]. The intensity of these urea-related bands scales with the IPDI:J2000 ratio. Strong aliphatic C-H stretches (2960 − 2850 cm − 1 ) and the ether C-O-C envelope (1180 − 1100 cm − 1 ) remain, reflecting the J2000 backbone structure. Neat BMI is featured by = C-H weak aryl stretches of maleimide ring between 3100 and 3000 cm − 1 two sharp imide C = O stretches at 1780 and 1710 cm − 1 and a prominent overtone at 1900 cm − 1 [ 5 , 10 ]. The maleimide C = C stretch emerges at 1630 cm − 1 . Ring deformation modes appear at 900 − 800 cm − 1 and 690n cm − 1 . After Michael addition and curing, the imide C = O bands at 1780 and 1710 cm − 1 persist [ 32 ]. The overtone at 1900 cm − 1 gradually vanishes with increase of IPDI content the “dilution” of imide content and hydrogen bond induced broadening [ 22 ]. The maleimide stretches at 1630 cm − 1 decrease in intensity as the IPDI to J2000 ratio increases. This confirms Michael addition onto the succinimide double bond, and partial homopolymerization of BMI [ 6 , 14 ]. The N-H stretching band at 3340 cm − 1 intensifies with growing IPDI ratio. The C-N stretching and N-H blending bands at 1390 and 1560 cm − 1 also intensify progressively. Hence, the controlled incorporation of more elastic diamine-urea into brittle BMI occurs. Ether C-O-C bands between 1180 and 1100 remain visible. Maleimide ring at 900 − 800 cm − 1 and 690 cm − 1 remain; however, they slightly decrease in intensity with increasing IPDI/J2000 ratio. In addition, =C-H aromatic stretching between 3100 − 3000 cm − 1 almost completely disappears in BMI-urea. Hence, the homopolymerization of BMI decreases with increase of IPDI/J2000 ratio. The structure of the synthesized BMI-urea was further confirmed by solid 13 C NMR. The results for diamine-urea are given in Fig. 3 (b). The successful formation of diamine urea is confirmed (i) strong and deshielded urea carbonyl (N-C = O) peak around 160 ppm; (ii) multiple ether (-CH 2 CH 2 O-) from Jeffamine D2000 at 70–75 ppm; (iii) CH 2 next to N (-CH 2 -NH-) possibly from IPDA or Jeffamine methylene adjacent to amines; (iv) -CH 3 terminal methyl on polyether or IPDI side chains. In addition, multiple peaks between 50 − 25 ppm from CH 3 /CH 2 in aliphatic chains and methine/methylene in IPDI ring are present in Fig. 3 (b). Four kinds of carbon in the BMI – maleimide carbonyl carbons C = O (165 ppm), from maleimide double bond C = C around 140 ppm, aromatic carbons forming a phenyl ring (125 ppm), and aliphatic linker carbons at around 30 ppm – are observed in NMR spectra of BMI in Fig. 3 (c). In the 13 C NMR spectrum of BMI-urea in Fig. 3 (d), the presence of distinct peaks at 172 and 160 ppm confirms the coexistence of imide and urea carbonyl groups, respectively. This indicates successful formation of the hybrid network. The broad signals at 70 − 75 ppm correspond to the polyether segments from J2000, validating the incorporation of flexible chains in the rigid BMI network. Additional peaks are observed in the 120–140 ppm region. They possibly arise from the aromatic and alkene carbons of the BMI unit. In addition, complex set of signals in the 20 − 60 ppm region reflect the aliphatic backbone. The increased complexity of the peaks around this region can also be attributed to crosslinked structure of the polymer and BMI self-polymerization. Hence, NMR analysis confirms that both Michael addition and BMI homopolymerization occur during the BMI-urea synthesis as it was illustrated in Fig. 2 (d). Thermogravimetric analysis (TGA) and derivative thermosgravimetery (DTG) in N 2 atmosphere were employed to evaluate the thermal stability of the synthesized BMI-urea resins in comparison with neat BMI and diamine-urea (Fig. 3 ). Diamine-urea exhibited the lowest thermal stability, with a single-step decomposition beginning around 280℃ and completing rapidly near 350℃ as can be observed from DTG plot in Fig. 3 (e2). In contrast, neat BMI demonstrated superior thermal resistance, with decomposition initiating above 400℃ and a high char yield, attributed to its highly crosslinked aromatic structure [ 32 , 34 ]. Low thermal stability of the diamine-urea is attributed to the absence of crosslinking in its linear oligomer structure [ 35 ], soft, flexible aliphatic backbone inherent from J200 and IPDI [ 36 ], and less thermal stability of urea (-NH-CO-NH-) linkages compared to more stable aromatic imide or maleimide structures found in BMI [ 33 , 37 ]. BMI-urea resins displayed intermediate thermal stability, reflecting the combined influence of rigid BMI segments and flexible, thermally less stable polyurea chains. All BMI-ureas underwent multi-step degradation as shown in Fig. 3 (e1), with an initial weight loss around 290–310℃, corresponding to partial cleavage of the urea C-N bonds [ 37 ]. A more significant degradation event occurred between 350–470℃, primarily due to the breakdown of BMI and J2000 segments [ 33 ]. The DTG profiles in Fig. 3 (e2) confirmed two overlapping degradation stages, with the intensity and position of peaks varying slightly with IPDI/J2000 ratio. Among the BMI-urea samples, BMI-urea 2/1 exhibited a slightly higher char yield, indicative of increased crosslinking density and graphitization resulting from the higher soft segment content. Conversely, BMI-urea 3/1 showed lower residue, correlating with reduced flexible content. These findings demonstrate that incorporating urea-based soft segments allows tunability of thermal decomposition behaviour while maintaining acceptable thermal resistance (> 290℃), suitable for high-performance thermosetting applications. To better understand the toughening effect of urea in BMI, AFM was used to analyse the cured resin. The AFM images (2D and 3D) of BMI-urea resins are given in Fig. 3 (f-h). They show that the BMI-urea resins has a phase that appears to be darker than the plane in 2D images as in Fig. 3 (f1–h1), and deeper in 3D images Fig. 3 (g1–g2)). The lighter phase can result from the protrusion from the plane while the darker phase can be caused by weaker interaction with the AFM tip. However, it suggests the nanoscale phase separation in the BMI-urea systems, driven by the presence of rigid BMI and soft urea segments in the polymer [ 38 ]. At a high IPDI/J2000 ratio (Fig. 3 (f1–f2)), the morphology is dominated by aggregated hard domains embedded in a limited soft matrix, yielding a rigid, brittle structure. As the IPDI content is decreased –Figure 3 (g1–g2) for BMI-urea 2.5/1 – the soft domains become more evenly distributed, and the phase interface appears less distinct, suggesting improved compatibility and enhanced energy dissipation. The BMI-urea 2/1 in Fig. 3 (h1–h2) exhibits the most extensive soft segment network with uniformly dispersed hard domains, forming a continuous structure. This architecture supports significant plastic deformation under stress, thus improving toughness. The progressive blurring of the phase boundaries and the nanoscale distribution of domains highlight strong interfacial interactions, likely due to covalent bonding via Michael addition. This well-regulated morphology is key to the synergistic balance of strength and elasticity in BMI-urea systems. The crystalline structure of BMI, diamine-urea and synthesized BMI-urea polymers was analysed by wide-angle XRD analysis at 2θ ranging from 10 to 50°. with the relevant findings depicted in Figure S2 (Section 2.1 , Supporting Information). 3.2. Mechanical properties Figure 4 presents the tensile properties of diamine-urea, BMI and BMI-urea resins (with three different IPDI/J2000 molar ratios: 2/1, 2.5/1 and 3/1). Figure 4 (a), stress-strain curves demonstrate that BMI exhibits a steep linear response with minimal strain, indicating high stiffness and brittleness. In contrast, diamine-urea displays extended strain capacity, confirming elastomeric behaviour due to flexible urea segments. Brittle behaviour of BMI and elastic strain for diamine-urea are inherent properties of both resin types [ 35 ]. Introducing urea to BMI shifts the BMI-urea from brittle to elastic failure, resulting in toughened BMI-urea resins where decrease of IPDI content and subsequent increase of urea content increases the elasticity. Figure 4 (b) shows that tensile strength increases with increase of IPDI content. However, tensile strength of all BMI-urea resins is about 2 times higher than strength of diamine-urea, and is in between 57 to 74% of the BMI’s tensile strength. Moreover, as shown in Fig. 4 (c), introducing urea into BMI polymer increases elongation at break to 60, 28 and 12 times for 2/1, 2.5/1 and 3/1 IPDI/J2000 molar ratios, respectively. Figure 4 (d) illustrates Young’s modulus of reactant and product polymers. BMI has the highest modulus of 3750MPa, while the modulus of diamine-urea is 2.8MPa. Increase of IPDI content increases the Young’s modulus from 7.4MPa for BMI-urea 2/1 to 69 MPa for BMI-urea 3/1 . Hence, the incorporation of diamine-urea into BMI led to a significant change in the BMI-ureas’ tensile behaviour. Synthesized BMI-urea resins became a tough material, exhibiting a ductile fracture in the tensile tests which can be observed from the “necking” in the stress-strain curve [ 9 , 39 ]. BMI-ureas contain the soft segment from diamine-urea and hard segments from BMI; diamine-ureas with flexible aliphatic ether bonds imparted flexibility to the BMI [ 33 , 37 ]. Moreover, as the content of IPDI increased the toughness of BMI-ureas increased linearly. This significant improvement was attributed not only to the higher content of flexible diamine-urea content, but also to the presence of the ordered/disordered hydrogen bonding network formed in diamine-urea chain segments [ 22 , 23 ]. Furthermore, from the FTIR graph in Fig. 3 the imide C = O stretch appears at approximately 1710 cm − 1 in BMI, indicating free carbonyl groups not involved in hydrogen bonding. In contrast, BMI-urea samples exhibit C = O peaks at lower wavenumber (around 1630 cm − 1 and 1560 cm − 1 ), along with intensified N-H bands at 3340 cm − 1 , confirming hydrogen bonding due to increased urea content [ 22 ]. Figure 3 shows that C = O peaks of all three BMI-ureas have approximately the same intensity which is reflected in similar tensile strength values of BMI-ureas. At the same time, the increase of intensity of the N-H bands for BMI-urea 3/1 resulted in significantly lower elongation at break for this type of BMI-urea where the diamine-urea, and consequently, soft segment content was the lowest. Figure 5 present the morphology and fracture behaviour of BMI and BMI-urea samples after tensile testing. The clear transition from brittle to ductile failure mode is observed with decreasing IPDI/J2000 ratio, i.e. hard segment content. As shown in the macroscopic images, neat BMI in Fig. 5 (a1) exhibits a smooth, flat fracture surface, characteristic to brittle failure. In contrast, BMI-urea samples shown in Fig. 5 (a2-a4) display progressively rougher and more deformed fracture surface indicative of plastic deformation and toughening. SEM images present in Fig. 5 (b1-b4) further support this trend. The neat BMI sample shows a featureless, glossy fracture surface typical of brittle cleavage [ 5 , 6 ], whereas BMI-ureas show clear ductile features such as fibrillated textures and elongated deformation structures [ 40 ]. The schematic diagrams in Fig. 5 (c&d) elucidate the underlying structural evolution responsible for this behaviour. In neat BMI shown in Fig. 5 (c1&d1), the highly crosslinked rigid aromatic segments form a dense and brittle network. Upon introducing diamine-urea (Fig. 5 (c2-c4&d2-d4)), flexible urea segments act as soft domain between the rigid BMI units, enabling stress redistribution and energy absorption under load. As IPDI content decreases, the proportion of soft segments increases, leading to more pronounced phase separation and elasticity. Since IPDI is a relatively rigid (hard) component in the diamine-urea chains, its reduction enhances chain mobility, promoting plastic deformation and fracture resistance [ 33 ]. This aligns with the tensile testing results, where the tensile strength and modulus were higher at high IPDI/J2000 ratio, while elongation at break decreased with increase of IPDI content. Impact strength of aluminum substrate and aluminum coated with BMI and BMI-urea resins is shown in Figure S3 (a). The neat substrate exhibited an impact strength of 154.64 kJ/m2, while the application of neat BMI coating resulted in only a marginal increase to 158.78 kJ/m2. This indicates limited improvement due to BMI’s inherent brittleness and inability to absorb impact energy [ 21 , 32 ]. with the relevant findings depicted in Figure S3 (Section 2.2 , Supporting Information). These results demonstrate that the construction of a BMI–urea semi-interpenetrating network via Michael addition copolymerization, combined with precise regulation of the soft-to-hard segment ratio, represents an effective strategy for significantly enhancing the toughness and impact resistance of BMI resins. The swelling behaviour of BMI and BMI-urea samples in 5wt% solution of strong acid, strong base and salt over 30 days were analysed, with the relevant findings depicted in Fig. S4 (Section 2.3 , Supporting Information). 3.3. Molecular dynamics: precursors Molecular dynamics (MD) simulations were performed to gain a deeper insight into the reaction mechanisms at both atomic and electronic levels. The molecular electrostatic potential (MEP) visualizes the charge distribution; it is particularly important for evaluating potential intermolecular interactions, especially electrostatic ones [ 41 ]. Since molecular interactions typically favour electrostatic complementarity to minimize system energy, the analysis of atomic charges and electron cloud distributions of IPDI, J2000 and IPDA was critical. with the relevant findings depicted in Fig. S5 (Section 2.4, Supporting Information). Frontier molecular orbital (FMO) theory provides critical insight into the reaction mechanism by examining the interactions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the reactants [ 42 ]. Thereby, it offers a deeper understanding of their relative reactivity. The HOMO, LUMO and energy gaps for IPDI, J2000, and IPDA are given in Fig. 6 . In the reaction between IPDI and J2000/IPDA, the interaction between FMOs plays a crucial role in determining both the selectivity and efficiency of the reaction. In both J2000 and IPDA molecules, the nitrogen atoms of the amino groups (-NH₂) contain lone pair electrons that occupy the HOMO, which exhibits a high energy level and concentrated electron density demonstrating strong nucleophilicity. In contrast, the LUMO of the isocyanate group (-NCO) in the IPDI molecule is predominantly located on the carbon atom, with a lower energy level and increased electrophilicity. When the energy gap between the HOMO (-NH₂) and LUMO (-NCO) is small, electrons can efficiently transfer from the HOMO to the LUMO, leading to the formation of a stable transition state. This orbital interaction significantly lowers the activation energy of the reaction, allowing the nucleophilic addition to proceed efficiently under mild conditions (80℃). Moreover, due to the similar characteristics of the LUMO orbitals of the two -NCO groups in IPDI, the reaction exhibits excellent selectivity, predominantly producing a linear polyurea structure rather than branched or cross-linked byproducts. Figure 8 (d) illustrates the energy gaps of IPDI, IPDA, and J2000. The energy gap of J2000 is 0.234 and that of IPDA being 0.223. This suggests that IPDA exhibits higher reactivity than J2000. However, to obtain a flexible diamine-urea, it is essential for IPDI to react preferentially with J2000. This insight informed the sequence of reactant addition in the synthesis process. Specifically, IPDA should be introduced only after the reaction between IPDI and J2000 has concluded, to ensure the successful progression of the subsequent chain extension reaction. 3.4. Molecular dynamics analysis The atomic charges in molecules and molecular electron cloud distribution of BMI and diamine-urea with the relevant findings depicted in Fig. S6 (Section 2.4, Supporting Information). The ratio of J2000 to IPDI during the diamine-urea synthesis stage (Step 1) significantly influences the relative proportions of symmetric and asymmetric structures in the BMI-urea resin system, thereby controlling the final material properties [ 37 ]. According to chemical reaction stoichiometry, the presence of one of the reactants in excess ensures the limiting reactant is completely consumed [ 43 ]. In other words, the reaction continues at all available reactive sites. When the ratio of IPDI to J2000 is high (e.g., 3:1), the -NH₂ groups of J2000 are more likely to react with the both -NCO groups of IPDI, resulting in a symmetric double-ended graft structure – diamine-urea-I, unlike the route suggested in Fig. 2 (b). This symmetric structure promotes the formation of an orderly three-dimensional crosslinked network, which enhances both the mechanical strength and thermal stability of the material. As the J2000 proportion increases (e.g., when the ratio decreases to 2:1 or 2.5:1), the reaction gradually shifts towards an asymmetric structure. In this case, a single -NCO group in IPDI molecule is more reacts with -NH₂ groups of a J2000 molecule. A single-ended graft with an asymmetric structure – diamine-urea-II – is formed, following the synthesis route in Fig. 2 (b). The unreacted -NCO end group either remains unreacted or reacts with IPDA. The increased presence of asymmetric structures introduces additional flexible chain segments and free end groups, which reduce crosslink density but significantly improve the toughness and elongation at break of material. This is in tune with the mechanical testing results in Fig. 4 . It is important to note that the distribution of molecular electrostatic potential not only influences the primary reaction pathway but also dictates the extent of competing reactions. When the -NH₂ groups are abundant in the system, the electrostatic potential-driven Michael addition reaction dominates. However, once the -NH₂ groups are nearly depleted, the negatively charged C = C double bonds in the remaining BMI molecules engage in electrostatic interactions, triggering a self-polymerization reaction [ 44 ]. This differential reaction selectivity is directly manifested in the network structure of the final material: the Michael addition produces flexible, long-chain connections, whereas BMI self-polymerization results in rigid, tightly cross-linked structures. The results obtained from the thermal and mechanical properties testing. The HOMO, LUMO and energy gaps for BMI and diamine-ureas are given in Fig. 7 . The reaction between BMI and diamine-urea can be effectively explained using FMO theory. The HOMO of the -NH₂ group at the terminal position of the diamine-urea molecule overlaps with the LUMO (π* antibonding orbital) of the C = C double bond on the maleimide ring of the BMI molecule. This HOMO-LUMO interaction facilitates the Michael addition reaction, wherein the lone pair of electrons on the nitrogen atom attacks the C = C double bond, leading to the formation of a new C-N covalent bond. It is important to note that the self-polymerization of BMI molecules can occur through the interaction between the π orbitals (HOMO) of the C = C double bond and the π* orbitals (LUMO) of another BMI molecule [ 42 ]. However, due to the proximity of the HOMO energy level of diamine-urea to the LUMO energy level of BMI, the Michael addition reaction is more favourable both kinetically and thermodynamically. This theoretical prediction aligns closely with the experimental observation of reaction selectivity, which demonstrates that the Michael addition reaction predominates over BMI self-polymerization. Further calculations were performed to determine the HOMO, LUMO and energy gaps of the diamine-urea 2/1 , diamine-urea 2.5/1 and diamine-urea 3/1 systems. The energy gaps for these systems are presented in Fig. 9(d). The energy gap for diamine-urea 2/1 is 0.080, for diamine-urea 2.5/1 is 0.083, and for diamine-urea 3/1 is 0.084. No significant differences were observed in the energy gaps among the three systems, indicating that their reactivity during the reaction process is essentially identical. Figure S7 shows molecular dynamic simulations of BMI-urea systems with different compositions. In all three cases, BMI molecules interact with diamine-urea pre-polymers. By analysing the energy gaps of the three diamine-urea x/y systems, it was determined that their reaction activities during the reaction process are essentially identical. Therefore, it is necessary to calculate the interaction energies of the BMI/diamine-urea x/y systems to determine the performance differences between the various systems. The binding energies of three systems ‒ BMI/diamine-urea 2/1 , BMI/diamine-urea 2.5/1 , and BMI/diamine-urea 3/1 ‒ were calculated to predict their performance indicators after the reaction. The total energies for the BMI/diamine-urea 2/1 , BMI/diamine-urea 2.5/1 , and BMI/diamine-urea 3/1 systems were 2629779, 504109 kcal/mol, and − 3381263 kcal/mol, respectively as presented in Table 1 . The binding energy of the BMI-urea x/y system increases progressively with the increase of IPDI/J2000 ratio, indicating stronger molecular interactions in the BMI-urea 3/1 system. This increase is attributed to the higher proportion of IPDI in the IPDI/J2000 system suppressing the copolymerization of J2000. As a result, the diamine-urea molecular chains in BMI-urea 3/1 system are shorter compared to those in other systems, this indicates that the molecular structure is more stable. Consequently, the BMI-urea 3/1 system exhibits enhanced stiffness but reduced toughness. Table 1 Binding energy data of BMI/ diamine-urea x/y Systems Total energy (kcal/mol) Binding energy (kcal/mol) BMI-urea 2/1 2629779 E BMI 157778 372 E diamine−urea2/1 2472089 BMI-urea 2.5/1 504109 E BMI 123101 390 E diamine−urea2.5/1 381101 BMI-urea 3/1 3381263 E BMI 212378 405 E diamine−urea3/1 3168983 3.5. AI system-assisted prediction During the synthesis of BMI-urea resins, the microstructure is deterministically shaped by thermodynamic, kinetic and stoichiometric constraints [ 45 ]. Data-driven models, including temporal and graph neural networks, capture the dynamic interplay of Michael addition and imidization crosslinking reactions, with representative predictions summarized in Table S1 . Different AI models consistently predicted two dominant pathways in BMI-urea resin synthesis: the covalent BMI-urea link and BMI homopolymerization. Across methods, C = C peak attenuation near 1630 cm-1 in FTIR and imide (172 ppm) and urea (160 ppm) shown in NMR validated the coexistence of flexible and rigid domains. These results highlight the reliable capability of AI in capturing structural evolution and performance features during curing. Coupling MD with quantum chemical calculations with AI analysis further enables accurate prediction of structural evolution pathways during curing. AI analysis results in Fig. 8 shows that the primary -NCO group of IPDI is preferentially attacked by J2000 amines (~ 78%), and the IPDI/J2000 ratio dictates the dominant prepolymer structure, At 3:1 ratio, the symmetric double-head structures prevail (> 70%) resulting in a high tensile strength, while at 2:1, asymmetric single-head structures dominate (> 70%) leading to flexible polymer. At intermediate ration 2.5:1 a balanced mix that produces a network with stiffness and toughness is yielded; this is reflected in exceptional impact strength. Conclusion In this study, a novel bismaleimide (BMI)-urea polymer with tuneable mechanical and thermal properties were designed and synthesized by introducing flexible urea segments into the traditionally rigid BMI network. BMI-urea polymers with different IPDI/J2000 molar ratios were synthesized via a two-step procedure. FTIR and solid-state 13C NMR analyses confirmed the coexistence of urea linkages (-NH-CO-NH-) and imide rings, and revealed the competing reaction mechanisms of Michael addition and BMI self-polymerization. XRD and AFM analyses showed that incorporating diamine-urea disrupted the intrinsic crystalline structure of BMI resulting in microphase separation. As the IPDI/J2000 ratio decreased, the soft-phase regions became more continuous and uniformly distributed, whereas the hard-phase regions –BMI- were dispersed as reinforcing points. This microstructural arrangement represents a key factor driving the transition of the material from brittle to tough/ductile and thus the mechanical properties could be finely tuned by varying the IPDI/J2000 ratio. Increased IPDI content resulted in higher tensile strength and modulus, whereas greater J2000 content improved elongation. The BMI-urea resins exhibited a tensile strength as high as 55 MPa which is 120% higher than diamine-urea; an impact strength up to 208 kJ/m 2 – a 30% increase over BMI; and up to 60-fold increment in elongation at break. Impact test demonstrated that the BMI-urea2.5/1 sample exhibited the highest impact resistance; notably, it retained 71% of BMI’s tensile strength while achieving 12-fold higher elongation at break, demonstrating an optimal balance between rigidity and toughness. All BMI-urea samples exhibited high thermal stability with initial decomposition temperature within range 290 − 310 ℃. Although the decomposition temperature is lower than that of BMI (> 400 ℃), these temperatures remain significantly higher than those of many conventional polymers. This confirms their suitability for high-temperature performance applications. Swelling resistance analysis in acidic, alkaline and saline environments confirmed that solvent uptake can be tailored through soft segment content and microphase separation. Molecular dynamics simulations elucidated the origins of performance variations at the atomic level. Analyses of electrostatic potential and frontier molecular orbitals clarified the distinct reactivity of the terminal amino groups of J2000 and the reaction mechanisms between IPDI and J2000/IPDA. Binding energy calculations revealed that the interaction strength between BMI and diamine-urea decreased in the order BMI-urea3/1 BMI-urea2.5/1 and BMI-urea2/1. This trend aligns with experimental measurements in modulus and strength; this theoretically validating the role of IPDI as a rigid component in enhancing intermolecular interactions and material stiffness. These findings highlight a straightforward approach to adjust strength, toughness and thermal stability in BMI via adding diamine-urea, offering practical solution for their use in demanding environments Declarations Conflict of interest: The authors declare no conflict of interests. Funding: The authors extend their gratitude to Scientific Compass www.shiyanjia.com for providing invaluable assistance with the SEM analysis. This work was supported by the following funding sources: The Universities of Liaoning province (LJ212510143025); The Liaoning Province Science and Technology Joint Program (Key R&D Program Project) (2025JH2/101800288); Faculty Development Competitive Research Grant Program, Nazarbayev University, Kazakhstan (040225FD4725) Author Contribution K.Z. wrote the main manuscript text, M.S. and U.B. Validation, S.A. Review & Editing, S.H.and H.Y. Data Curation,Q.S.suggest the method, All authors reviewed the manuscript. 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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-8750615","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":584221095,"identity":"0dcff928-9ac5-4214-9346-64bf30152b0b","order_by":0,"name":"Kangbo Zhao","email":"","orcid":"","institution":"Shenyang Aerospace University","correspondingAuthor":false,"prefix":"","firstName":"Kangbo","middleName":"","lastName":"Zhao","suffix":""},{"id":584221096,"identity":"90ef18ce-fd00-407c-be73-31f661bb491b","order_by":1,"name":"Mosha Cheng","email":"","orcid":"","institution":"China Medical University","correspondingAuthor":false,"prefix":"","firstName":"Mosha","middleName":"","lastName":"Cheng","suffix":""},{"id":584221098,"identity":"d2616657-460a-4592-a37c-86b880cb5d7e","order_by":2,"name":"Umut Bakhbergen","email":"","orcid":"","institution":"Nazarbayev University","correspondingAuthor":false,"prefix":"","firstName":"Umut","middleName":"","lastName":"Bakhbergen","suffix":""},{"id":584221099,"identity":"133a1938-09cb-4c12-b058-0fb8407d1195","order_by":3,"name":"Sherif Araby","email":"","orcid":"","institution":"Nazarbayev University","correspondingAuthor":false,"prefix":"","firstName":"Sherif","middleName":"","lastName":"Araby","suffix":""},{"id":584221100,"identity":"25e0b642-9078-4a6d-91a9-43669028eb76","order_by":4,"name":"Sensen Han","email":"","orcid":"","institution":"Shenyang Aerospace University","correspondingAuthor":false,"prefix":"","firstName":"Sensen","middleName":"","lastName":"Han","suffix":""},{"id":584221101,"identity":"aa388bd0-62d3-4007-ae81-8d1fcca2fb71","order_by":5,"name":"Haochen Yuan","email":"","orcid":"","institution":"Shenyang Aerospace University","correspondingAuthor":false,"prefix":"","firstName":"Haochen","middleName":"","lastName":"Yuan","suffix":""},{"id":584221102,"identity":"fdccb6fb-6f61-4b60-bb27-1916a7b0a2d7","order_by":6,"name":"Qingshi Meng","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA90lEQVRIiWNgGAWjYNACAyCWYGx8DOYwMzcQraXZGMxiZiRGCwhIMLBJQzQT0KLb3nv4xZuCO3bzZze3VRdU/Inmbwdq+VGxDacWszPn0iznGDxL3nDnYNvtGWcMcmccZmxg7DlzG7eWGzlmxjwGh5MNJBLbbvO2GeQ2ALUwM7bh0XL/DUSL/IzEtmKQlvkEtdzgMX4M1GLHcCOxjRmkZQNBLWdyzBjnGBxOMLiR2CzNc8Y4dyNQy0G8fjl+xvjDmz+H7eVnpD/8zFMhlzvv/OGDD35U4NYCBGwSPAwMiQ3IQgfwqQcC5g9ALfYEFI2CUTAKRsFIBgA+gV1d0oAgaAAAAABJRU5ErkJggg==","orcid":"","institution":"Shenyang Aerospace University","correspondingAuthor":true,"prefix":"","firstName":"Qingshi","middleName":"","lastName":"Meng","suffix":""}],"badges":[],"createdAt":"2026-01-31 14:25:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8750615/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8750615/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101766692,"identity":"044042d1-a761-4dd6-a1ec-e4ba6fca93c4","added_by":"auto","created_at":"2026-02-03 12:12:25","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":205369,"visible":true,"origin":"","legend":"\u003cp\u003eProcess of BMI-urea synthesis from J2000, isophorone diisocyanate (IPDI), isophorone diamine (IPDA) and bismaleimide (BMI): (a) Step 1 – diamine-urea prepolymer; (b) Step 2 – BMI-urea polymer.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/bec51ea4ad997a19f7307368.png"},{"id":101766704,"identity":"a2ceab61-3cb1-499f-8911-366c4bdb10c3","added_by":"auto","created_at":"2026-02-03 12:12:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":160387,"visible":true,"origin":"","legend":"\u003cp\u003eSchematics of BMI-urea synthesis process: (a) Step 1-1: IPDI reaction with J2000; (b) Step 1-2: IPDA addition; (c) Step-2: BMI reaction with diamine-urea prepolymer; (d) Curing of BMI-urea: Michael addition and homopolymerization.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/baf296fa7e7170d2ba02aede.png"},{"id":101766674,"identity":"bb2780f2-830b-45fd-bb6a-e68574a83c9e","added_by":"auto","created_at":"2026-02-03 12:12:17","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":914711,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR of diamine-urea, BMI, and BMI-urea\u003csub\u003e2/1\u003c/sub\u003e, BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e, and BMI-urea\u003csub\u003e3/1\u003c/sub\u003e; Solid \u003csup\u003e13\u003c/sup\u003eC NMR spectra of (b) diamine-urea; (c) BMI; (d) BMI-urea; (e1) TGA; (e2) DTG curves of diamine-urea, BMI, and BMI-urea resins; AFM images of BMI-urea resins: (f) BMI-urea\u003csub\u003e3/1\u003c/sub\u003e; (g) BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e; (h) BMI-urea\u003csub\u003e2/1\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/a95ccfdfe95a13c17ce3d87b.jpeg"},{"id":101766670,"identity":"e6b01391-5778-411b-8ee1-62abc8ba17de","added_by":"auto","created_at":"2026-02-03 12:12:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":136750,"visible":true,"origin":"","legend":"\u003cp\u003eMechanical properties of diamine-urea, BMI and BMI-ureas: (a) stress-strain diagram; (b) tensile strength; (c) elongation at break; (d) Young's modulus.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/051fe5239891c6d968b39998.png"},{"id":101766710,"identity":"b41c0baa-0d8c-49cf-81b1-26ef4fe4560e","added_by":"auto","created_at":"2026-02-03 12:12:38","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":687337,"visible":true,"origin":"","legend":"\u003cp\u003eFracture surface of tensile test samples: (a) electronically amplified images – (a1) BMI; (a2) BMI-urea\u003csub\u003e3/1\u003c/sub\u003e; (a3) BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e; (a4) BMI-urea\u003csub\u003e2/1\u003c/sub\u003e; (b) SEM images – (b1) BMI; (b2) BMI-urea\u003csub\u003e3/1\u003c/sub\u003e; (b3) BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e; (b4) BMI-urea\u003csub\u003e2/1\u003c/sub\u003e; schematic view: (c) before the fracture – (c1) BMI; (c2) BMI-urea\u003csub\u003e3/1\u003c/sub\u003e; (c3) BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e; (c4) BMI-urea\u003csub\u003e2/1\u003c/sub\u003e; (d) after the fracture – (d1) BMI; (d2) BMI-urea\u003csub\u003e3/1\u003c/sub\u003e; (d3) BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e; (d4) BMI-urea\u003csub\u003e2/1\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/d4aae5a2a7aba1edfdb3c1bb.png"},{"id":101766678,"identity":"b610ec53-e99f-416d-9019-385c01c6fe1e","added_by":"auto","created_at":"2026-02-03 12:12:22","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":696488,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Frontline electronic orbitals and (b) energy level information of IPDI, IPDA and J2000.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/6ff73815ad47c73a724ad212.jpeg"},{"id":101766679,"identity":"3c91d1e2-bda2-40cf-ba80-9e987f605ddd","added_by":"auto","created_at":"2026-02-03 12:12:23","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":573236,"visible":true,"origin":"","legend":"\u003cp\u003e(a-c) Frontline electronic orbitals; and (b) energy level information of BMI and diamine-urea\u003csub\u003ex/y\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/fa6363b9225b14250f849ec0.jpeg"},{"id":101766695,"identity":"ea5e95d8-c344-4e6a-a128-4aefd968fcd8","added_by":"auto","created_at":"2026-02-03 12:12:31","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":129273,"visible":true,"origin":"","legend":"\u003cp\u003eVarious AI engines predicting structural evolution and probability distribution during the synthesis of BMI-urea resin.\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/bfddf96087e2d33c3cd9e2e4.jpeg"},{"id":103621577,"identity":"5053ea00-20ee-470b-ab2a-1c3d890de50a","added_by":"auto","created_at":"2026-02-27 18:25:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4283937,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/f187a335-ee5c-4dd7-b049-897b5b0c04ac.pdf"},{"id":101766671,"identity":"70438286-4d07-4989-ab8b-00cbfa97e26b","added_by":"auto","created_at":"2026-02-03 12:12:16","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":3788992,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterialforonlinepublicationonly.docx","url":"https://assets-eu.researchsquare.com/files/rs-8750615/v1/cc208ab5bb95f0950e28eae6.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"A Novel Bismaleimide-Urea Resin: Breaking the Trade-off Between Strength, Toughness and Thermal Stability via Molecular Design","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eBismaleimide (BMI) is a heat-resistant thermosetting resin containing monomers that form through addition reactions to produce highly cross-linked network polyimides. They are featured by exceptional mechanical strength, enhanced thermal stability, excellent flame-retardant performance and low thermal expansion coefficient [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. These features make BMI suitable for cutting edge applications including aerospace and automative industries. The key benefits stem from the reactive maleimide end groups, which enable thermal curing without the release of volatile by-products that create voids in highly cross-linked networks [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, the resulting resins tend to be brittle because of their aromatic backbone and elevated cross-link density [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Therefore, broadening the application of BMI resins requires enhancing their toughness without compromising their other performance characteristics [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMichael addition copolymerization with aromatic diamines represents one of the simplest and most economical means of BMI resin synthesis [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Typically, the preparation of BMI involves formation of a maleamic acid intermediate, which then undergoes intramolecular cyclodehydration to form the characteristic imide ring [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Dels-Alder reaction with tuneable reversibility [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] as well as free-radical route for two step copolymerization [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] are alternative methods of BMI production [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The research on toughening the BMI is focused on strategies including synthesis of new classes of BMI monomers [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], allyl compound copolymerization [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], inorganic particle addition [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], diamine chain-extension [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] and elastomer incorporation [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eVariations in the backbone architecture of BMI oligomers produce resins with distinct performance profiles. Bifunctional BMI monomers react twice via Michael addition reactions with difunctional nucleophiles, such as dithiols or diamines to form a step-growth, cross-linked network [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Hence, modulus, toughness and glass transition temperature of the BMI can be tailored by introducing soft aliphatic or ether segments between the rigid succinimide nodes [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Asymmetric BMI monomers were designed to improve the processability and reduce the brittleness of BMIs achieving resin films with elongation at break as high as 17.3% without significant changes in the maximum strength [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Trifunctional maleimide was used to form a micro-branched BMI resin achieving a 8.8% reduction in dielectric constant, and increase of 140% in bending and 149% in impact strengths [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTunability of BMI properties is important for its various applications. Incorporation of 5 wt% of phosphate and phosphonate-bearing structure into BMI resin for flame retarding application increased its impact strength by 146.3% and 90.2%, respectively [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Fluorine BMI resin films developed for piezoelectric sensors exhibited outstanding mechanical performance ‒ tensile strength of 160\u0026ndash;256 MPa and moduli of 3.55\u0026ndash;5.29 GPa [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Cross-link density of the cured material has a significant effect on the performance of the polymer. It can be regulated by adjustment of oligomer\u0026rsquo;s molecular weight. Variation of allyl to imide ratio from 0.72 to 1.14 resulted in up to 35% improvement of impact strength in bio-derived allyl modified BMI resins [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Improvement of mechanical and thermal properties was reported for 50 mol% allylamine BMIs [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAnother effective approach to improve mechanical properties in cured BMIs is the incorporation of flexible linkages into the network structure and increasing the distance between the two functional groups [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Enhanced thermomechanical properties of diamine-modified BMI resins was observed at different molar ratios by adjustment of the degree of cross-linking [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Toughening BMIs can be achieved via modification with rubber or thermoplastics. Cross-linking polyethylene glycol to a polymeric backbone via Diels-Alder reaction resulted in BMI with robust shape stability and mechanical strength [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Moreover, recyclability and malleable plasticity over multiple thermal cycle was achieved due to thermal reversibility of the cure reaction. Incorporating 15 wt% of eugenol allyl ether-grafted polysiloxane into BMI produced a thermoset with markedly improved toughness while maintaining comparable glass transition temperature and thermal stability [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Control of the phase-separation is one of the main challenges when employing rubber/thermoplastic components [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Hence, developing BMIs with excellent toughness while maintaining the inherent properties of the resin is still an ongoing topic of research.\u003c/p\u003e \u003cp\u003eTailoring the performance of the polymer is possible by modifying their microphase structure. Introducing urea moieties, for example, can improve interchain association [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. This is attributed by the increased hydrogen bond density inherent to bidentate hydrogen bond in urea and \u0026ldquo;sacrificial\u0026rdquo; cross-links from them [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. In addition, the reversible exchange of disulfide bonds in urea promotes elastomer softening [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Tuning urea-derived interactions in thermoset polymers is key to balancing their mechanical robustness. For example, at the optimal content of urea moieties in polyurethane-urea polymers resulted in 7-fold increase of Young\u0026rsquo;s modulus with 450% strain [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe toughening effect of urea can be employed when it is bonded to imides. A novel polyurethane-urea-imide coatings with improved adhesion performance were developed by employing the principle of introducing chain-separation segment and flexibility via urea moieties [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In the NCO-terminated poly(urethane-urea-imide) copolymers, the excess isocyanate is moisture-cured with diamine extenders. It formed urea linkages with embedded imide rings via hydrogen bonding [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Thus, hard-soft microphase separation is initiated resulting in the adhesive coatings with enhanced mechanical strength (16.7 MPa with 1097% stretchability) and thermal properties (3.87 W/mK). Inspired by the successful performance of imide-urea polymers, it is possible to graft urea linkages into the rigid BMI network. The resulting BMI-urea polymer can yield an interpenetrated hard-soft microstructure: stiff imide bear load while flexible diamine-urea dissipate energy and block crack propagation. As a result, toughening of BMI resin is achieved while maintaining its inherent properties.\u003c/p\u003e \u003cp\u003eSince BMI-urea polymer was not previously reported, it is important to investigate not only its morphology and mechanical properties but comprehend its molecular structure. Motion characteristics of molecular chain segments, the evolution of internal structures and the system network formation can be investigated via molecular dynamics (MD) simulations. Microphase separation between rigid imide rings and diamine segments critically influences the network\u0026rsquo;s activation energy, mechanical response and water uptake of the resin [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Nevertheless, the atomistic mechanisms underlying these effects remain unclear given the limited amount of MD studies of BMI resins [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. MD simulations can quantify activation energy barriers [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], predicting the likelihood of possible reaction scenarios. A detailed reactants and product models and MD simulation of their interactions and behaviour are needed to elucidate how diamine-urea-imide system tune energy barriers, enhance toughness, and regulate other properties of the BMI-urea polymer.\u003c/p\u003e \u003cp\u003eIn this study, isophorone diisocyanate (IPDI) and diamine-terminated poly(propylene glycol) are mixed at different molar ratios (2:1, 2.5:1 and 3:1), and added to isophorone diamine (IPDA) to produce diamine-urea as a spacer and softening agent for BMI. The morphology of resulting new BMI-urea resins is investigated along with their mechanical, thermal and water absorption behaviour. The MD simulations are used to comprehend the effect of diamine-urea modification on chain mobility and microphase separation, and the results are validated against experimental data. The study presents a pioneering systematic experimental and MD simulation approach to investigate the properties and performance of novel BMI-urea polymer.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials and characterization\u003c/h2\u003e \u003cp\u003eRaw materials and characterization have been described in detail in Supporting Information.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Synthesis of BMI-urea\u003c/h2\u003e \u003cp\u003eSynthesis of BMI-urea was carried out in a two-step process as schematically presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In the first step shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a), diamine-urea prepolymer with different imide to J2000 molar ratios (2:1, 2.5:1, and 3:1) was synthesized. J2000 was placed in a vacuum oven at 90 ℃ to eliminate any residual moisture. IPDI was mixed with J2000; the mixture was stirred in an oil bath at 150 rpm using magnetic stirring. The oil bath temperature was gradually raised from room temperature to 80 ℃ and maintained for 1 h to facilitate the formation of the prepolymer. The IDPI/J2000 mixture was allowed to cool to room temperature before slowly adding 1.8 g of IPDA solution. The obtained mixture was stirred for 25 min. Diamine-urea prepolymers with IDPI to J2000 molar ratios 2:1, 2.5:1, and 3:1 was synthesized.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe second step of the synthesis is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). Diamine-urea prepolymers obtained in the first step were combined with BMI in a 100 ml three-necked flask at H\u003csub\u003e2\u003c/sub\u003eN\u0026ndash; to C\u0026thinsp;=\u0026thinsp;C molar ration of 1:2. The mixture was mechanically stirred at 170 ℃ for approximately 1 h to achieve a homogeneous mixture. Three different BMI-urea polymers were obtained: BMI-urea\u003csub\u003e2/1\u003c/sub\u003e, BMI-urea\u003csub\u003e2.5/1,\u003c/sub\u003e and BMI-urea\u003csub\u003e3/1\u003c/sub\u003e where subscripts correspond to IPDI to J2000 molar ratios (2:1, 2.5:1 and 3:1) in the first step of the synthesis. BMI-urea polymers were transferred into Teflon molds and cured at 170 ℃ for 10 hrs in a vacuum oven.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Molecular dynamics\u003c/h2\u003e \u003cp\u003eMolecular dynamics information has been described in detail in Supporting Information.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Synthesis and morphology of BMI-urea\u003c/h2\u003e \u003cp\u003eThe chemical reactions occurring during the BMI-urea synthesis process are schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The process starts by adding IPDI and J2000 which initiates the reaction of imide with isocyanate group. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a\u0026amp;b) lone pair on the terminal -NH\u003csub\u003e2\u003c/sub\u003e of J2000 attacks the electrophilic carbon of one NCO group on IPDI producing a urea linkage (-NH-CO-NH-). One NCO of IPDI remains unreacted for sub sequential addition. Depending on the imide to J2000 molar ratio (2:1, 2.5:1, and 3:1) more isocyanate groups might be available in the system [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The reactivities of terminal amino groups at two ends of J2000 are different due to slightly different atomic partial charges [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. This charge imbalance is caused by chain conformation, local environment and proximity of dipoles. Hence, one terminal amine group can carry slightly more negative electrostatic potential which makes it more nucleophilic and reactive [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Amino group with lower local steric hindrance should be the one not adjacent to a methyl-substituted propylene oxide unit. As a result of this reaction, NH\u003csub\u003e2\u003c/sub\u003e-terminated urea prepolymer is formed. The addition of IPDA (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b\u0026amp;c)) leads to second nucleophilic attack of IPDA\u0026rsquo;s -NH\u003csub\u003e2\u003c/sub\u003e on the remaining -NCO from IPDI which forms the second urea linkage. Diamine-urea prepolymer is formed with a possible chemical structure illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAddition of BMI to diamine-urea prepolymer initiates aza-Michael addition reaction where \u0026ldquo;Michael donor\u0026rdquo; comes from nucleophile diamine-urea and electron-poor N-C bond in BMI acts as \u0026ldquo;Michael acceptor\u0026rdquo; [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. In the final curing step diamine-urea pre polymers is \u0026ldquo;Michael added\u0026rdquo; onto two maleimide C\u0026thinsp;=\u0026thinsp;C bonds of BMI at both ends of diamine-urea to produce the bis-adduct and crosslink [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. At the same time, homopolymerization of BMI by free-radical addition across its two activated maleimide C\u0026thinsp;=\u0026thinsp;C bonds [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Elevated temperature of the curing adds a radical species to one of the maleimide C\u0026thinsp;=\u0026thinsp;C bonds generating carbon-centred radical on the succinimide ring. The newly formed radical on the succinimide adds into the C\u0026thinsp;=\u0026thinsp;C of a second BMI monomer which creates a new C-C bond between monomer units and regenerates a radical site at the terminus. The availability of the -NH from diamine-urea prepolymer (which, in turn, depends on the initial IPDI to J2000 molar ratio) determines the amount of BMI-urea formation and BMI homopolymerization. Homopolymerization of BMI will produce high-strength brittle BMI polymer, while increase of the IPDI to J2000 will increase the Michael addition reaction. Consequently, BMI-urea polymers with different amount of brittle homopolymerized BMI and more elastic BMI-urea are synthesized.\u003c/p\u003e \u003cp\u003eThe chemical structure of the reactants: diamine-urea and BMI and produced BMI-urea polymers was determined via FTIR given in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The diamine-urea spectrum features a broad N-H stretching band at 3340 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. It also shows strong aliphatic C-H stretches between 2960 and 2850 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and an ether C-O-C band at 1180\u0026thinsp;\u0026minus;\u0026thinsp;1100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. A residual NCO band appears at 2270 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The FTIR spectrum of the diamine-urea prepolymer confirms successful urea linkage formation. The complete disappearance of the 2270 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e -N\u0026thinsp;=\u0026thinsp;C=O stretch indicates full consumption of IPDI\u0026rsquo;s isocyanate groups during the first synthesis step. A broad N-H stretch at 3340 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a new C\u0026thinsp;=\u0026thinsp;O band at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e emerge, consistent with urea carbonyl formation and hydrogen-bonding [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The intensity of these urea-related bands scales with the IPDI:J2000 ratio. Strong aliphatic C-H stretches (2960\u0026thinsp;\u0026minus;\u0026thinsp;2850 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and the ether C-O-C envelope (1180\u0026thinsp;\u0026minus;\u0026thinsp;1100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) remain, reflecting the J2000 backbone structure.\u003c/p\u003e \u003cp\u003eNeat BMI is featured by =\u0026thinsp;C-H weak aryl stretches of maleimide ring between 3100 and 3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e two sharp imide C\u0026thinsp;=\u0026thinsp;O stretches at 1780 and 1710 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a prominent overtone at 1900 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The maleimide C\u0026thinsp;=\u0026thinsp;C stretch emerges at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Ring deformation modes appear at 900\u0026thinsp;\u0026minus;\u0026thinsp;800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 690n cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. After Michael addition and curing, the imide C\u0026thinsp;=\u0026thinsp;O bands at 1780 and 1710 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e persist [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The overtone at 1900 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e gradually vanishes with increase of IPDI content the \u0026ldquo;dilution\u0026rdquo; of imide content and hydrogen bond induced broadening [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The maleimide stretches at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e decrease in intensity as the IPDI to J2000 ratio increases. This confirms Michael addition onto the succinimide double bond, and partial homopolymerization of BMI [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The N-H stretching band at 3340 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e intensifies with growing IPDI ratio. The C-N stretching and N-H blending bands at 1390 and 1560 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e also intensify progressively. Hence, the controlled incorporation of more elastic diamine-urea into brittle BMI occurs. Ether C-O-C bands between 1180 and 1100 remain visible. Maleimide ring at 900\u0026thinsp;\u0026minus;\u0026thinsp;800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 690 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e remain; however, they slightly decrease in intensity with increasing IPDI/J2000 ratio. In addition, =C-H aromatic stretching between 3100\u0026thinsp;\u0026minus;\u0026thinsp;3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e almost completely disappears in BMI-urea. Hence, the homopolymerization of BMI decreases with increase of IPDI/J2000 ratio.\u003c/p\u003e \u003cp\u003eThe structure of the synthesized BMI-urea was further confirmed by solid \u003csup\u003e13\u003c/sup\u003eC NMR. The results for diamine-urea are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). The successful formation of diamine urea is confirmed (i) strong and deshielded urea carbonyl (N-C\u0026thinsp;=\u0026thinsp;O) peak around 160 ppm; (ii) multiple ether (-CH\u003csub\u003e2\u003c/sub\u003eCH\u003csub\u003e2\u003c/sub\u003eO-) from Jeffamine D2000 at 70\u0026ndash;75 ppm; (iii) CH\u003csub\u003e2\u003c/sub\u003e next to N (-CH\u003csub\u003e2\u003c/sub\u003e-NH-) possibly from IPDA or Jeffamine methylene adjacent to amines; (iv) -CH\u003csub\u003e3\u003c/sub\u003e terminal methyl on polyether or IPDI side chains. In addition, multiple peaks between 50\u0026thinsp;\u0026minus;\u0026thinsp;25 ppm from CH\u003csub\u003e3\u003c/sub\u003e/CH\u003csub\u003e2\u003c/sub\u003e in aliphatic chains and methine/methylene in IPDI ring are present in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). Four kinds of carbon in the BMI \u0026ndash; maleimide carbonyl carbons C\u0026thinsp;=\u0026thinsp;O (165 ppm), from maleimide double bond C\u0026thinsp;=\u0026thinsp;C around 140 ppm, aromatic carbons forming a phenyl ring (125 ppm), and aliphatic linker carbons at around 30 ppm \u0026ndash; are observed in NMR spectra of BMI in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c). In the \u003csup\u003e13\u003c/sup\u003eC NMR spectrum of BMI-urea in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d), the presence of distinct peaks at 172 and 160 ppm confirms the coexistence of imide and urea carbonyl groups, respectively. This indicates successful formation of the hybrid network. The broad signals at 70\u0026thinsp;\u0026minus;\u0026thinsp;75 ppm correspond to the polyether segments from J2000, validating the incorporation of flexible chains in the rigid BMI network. Additional peaks are observed in the 120\u0026ndash;140 ppm region. They possibly arise from the aromatic and alkene carbons of the BMI unit. In addition, complex set of signals in the 20\u0026thinsp;\u0026minus;\u0026thinsp;60 ppm region reflect the aliphatic backbone. The increased complexity of the peaks around this region can also be attributed to crosslinked structure of the polymer and BMI self-polymerization. Hence, NMR analysis confirms that both Michael addition and BMI homopolymerization occur during the BMI-urea synthesis as it was illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThermogravimetric analysis (TGA) and derivative thermosgravimetery (DTG) in N\u003csub\u003e2\u003c/sub\u003e atmosphere were employed to evaluate the thermal stability of the synthesized BMI-urea resins in comparison with neat BMI and diamine-urea (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Diamine-urea exhibited the lowest thermal stability, with a single-step decomposition beginning around 280℃ and completing rapidly near 350℃ as can be observed from DTG plot in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(e2). In contrast, neat BMI demonstrated superior thermal resistance, with decomposition initiating above 400℃ and a high char yield, attributed to its highly crosslinked aromatic structure [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Low thermal stability of the diamine-urea is attributed to the absence of crosslinking in its linear oligomer structure [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], soft, flexible aliphatic backbone inherent from J200 and IPDI [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], and less thermal stability of urea (-NH-CO-NH-) linkages compared to more stable aromatic imide or maleimide structures found in BMI [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBMI-urea resins displayed intermediate thermal stability, reflecting the combined influence of rigid BMI segments and flexible, thermally less stable polyurea chains. All BMI-ureas underwent multi-step degradation as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(e1), with an initial weight loss around 290\u0026ndash;310℃, corresponding to partial cleavage of the urea C-N bonds [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. A more significant degradation event occurred between 350\u0026ndash;470℃, primarily due to the breakdown of BMI and J2000 segments [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The DTG profiles in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(e2) confirmed two overlapping degradation stages, with the intensity and position of peaks varying slightly with IPDI/J2000 ratio.\u003c/p\u003e \u003cp\u003eAmong the BMI-urea samples, BMI-urea\u003csub\u003e2/1\u003c/sub\u003e exhibited a slightly higher char yield, indicative of increased crosslinking density and graphitization resulting from the higher soft segment content. Conversely, BMI-urea\u003csub\u003e3/1\u003c/sub\u003e showed lower residue, correlating with reduced flexible content. These findings demonstrate that incorporating urea-based soft segments allows tunability of thermal decomposition behaviour while maintaining acceptable thermal resistance (\u0026gt;\u0026thinsp;290℃), suitable for high-performance thermosetting applications.\u003c/p\u003e \u003cp\u003eTo better understand the toughening effect of urea in BMI, AFM was used to analyse the cured resin. The AFM images (2D and 3D) of BMI-urea resins are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(f-h). They show that the BMI-urea resins has a phase that appears to be darker than the plane in 2D images as in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (f1\u0026ndash;h1), and deeper in 3D images Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(g1\u0026ndash;g2)). The lighter phase can result from the protrusion from the plane while the darker phase can be caused by weaker interaction with the AFM tip. However, it suggests the nanoscale phase separation in the BMI-urea systems, driven by the presence of rigid BMI and soft urea segments in the polymer [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. At a high IPDI/J2000 ratio (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(f1\u0026ndash;f2)), the morphology is dominated by aggregated hard domains embedded in a limited soft matrix, yielding a rigid, brittle structure. As the IPDI content is decreased \u0026ndash;Figure 3 (g1\u0026ndash;g2) for BMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e \u0026ndash; the soft domains become more evenly distributed, and the phase interface appears less distinct, suggesting improved compatibility and enhanced energy dissipation. The BMI-urea\u003csub\u003e2/1\u003c/sub\u003e in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(h1\u0026ndash;h2) exhibits the most extensive soft segment network with uniformly dispersed hard domains, forming a continuous structure. This architecture supports significant plastic deformation under stress, thus improving toughness. The progressive blurring of the phase boundaries and the nanoscale distribution of domains highlight strong interfacial interactions, likely due to covalent bonding via Michael addition. This well-regulated morphology is key to the synergistic balance of strength and elasticity in BMI-urea systems.\u003c/p\u003e \u003cp\u003eThe crystalline structure of BMI, diamine-urea and synthesized BMI-urea polymers was analysed by wide-angle XRD analysis at 2θ ranging from 10 to 50\u0026deg;. with the relevant findings depicted in Figure S2 (Section \u003cspan refid=\"Sec3\" class=\"InternalRef\"\u003e2.1\u003c/span\u003e, Supporting Information).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Mechanical properties\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents the tensile properties of diamine-urea, BMI and BMI-urea resins (with three different IPDI/J2000 molar ratios: 2/1, 2.5/1 and 3/1). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), stress-strain curves demonstrate that BMI exhibits a steep linear response with minimal strain, indicating high stiffness and brittleness. In contrast, diamine-urea displays extended strain capacity, confirming elastomeric behaviour due to flexible urea segments. Brittle behaviour of BMI and elastic strain for diamine-urea are inherent properties of both resin types [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Introducing urea to BMI shifts the BMI-urea from brittle to elastic failure, resulting in toughened BMI-urea resins where decrease of IPDI content and subsequent increase of urea content increases the elasticity. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) shows that tensile strength increases with increase of IPDI content. However, tensile strength of all BMI-urea resins is about 2 times higher than strength of diamine-urea, and is in between 57 to 74% of the BMI\u0026rsquo;s tensile strength. Moreover, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c), introducing urea into BMI polymer increases elongation at break to 60, 28 and 12 times for 2/1, 2.5/1 and 3/1 IPDI/J2000 molar ratios, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d) illustrates Young\u0026rsquo;s modulus of reactant and product polymers. BMI has the highest modulus of 3750MPa, while the modulus of diamine-urea is 2.8MPa. Increase of IPDI content increases the Young\u0026rsquo;s modulus from 7.4MPa for BMI-urea\u003csub\u003e2/1\u003c/sub\u003e to 69 MPa for BMI-urea\u003csub\u003e3/1\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eHence, the incorporation of diamine-urea into BMI led to a significant change in the BMI-ureas\u0026rsquo; tensile behaviour. Synthesized BMI-urea resins became a tough material, exhibiting a ductile fracture in the tensile tests which can be observed from the \u0026ldquo;necking\u0026rdquo; in the stress-strain curve [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. BMI-ureas contain the soft segment from diamine-urea and hard segments from BMI; diamine-ureas with flexible aliphatic ether bonds imparted flexibility to the BMI [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Moreover, as the content of IPDI increased the toughness of BMI-ureas increased linearly. This significant improvement was attributed not only to the higher content of flexible diamine-urea content, but also to the presence of the ordered/disordered hydrogen bonding network formed in diamine-urea chain segments [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Furthermore, from the FTIR graph in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e the imide C\u0026thinsp;=\u0026thinsp;O stretch appears at approximately 1710 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in BMI, indicating free carbonyl groups not involved in hydrogen bonding. In contrast, BMI-urea samples exhibit C\u0026thinsp;=\u0026thinsp;O peaks at lower wavenumber (around 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1560 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), along with intensified N-H bands at 3340 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, confirming hydrogen bonding due to increased urea content [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows that C\u0026thinsp;=\u0026thinsp;O peaks of all three BMI-ureas have approximately the same intensity which is reflected in similar tensile strength values of BMI-ureas. At the same time, the increase of intensity of the N-H bands for BMI-urea\u003csub\u003e3/1\u003c/sub\u003e resulted in significantly lower elongation at break for this type of BMI-urea where the diamine-urea, and consequently, soft segment content was the lowest.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e present the morphology and fracture behaviour of BMI and BMI-urea samples after tensile testing. The clear transition from brittle to ductile failure mode is observed with decreasing IPDI/J2000 ratio, i.e. hard segment content. As shown in the macroscopic images, neat BMI in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a1) exhibits a smooth, flat fracture surface, characteristic to brittle failure. In contrast, BMI-urea samples shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a2-a4) display progressively rougher and more deformed fracture surface indicative of plastic deformation and toughening. SEM images present in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b1-b4) further support this trend. The neat BMI sample shows a featureless, glossy fracture surface typical of brittle cleavage [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], whereas BMI-ureas show clear ductile features such as fibrillated textures and elongated deformation structures [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. The schematic diagrams in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c\u0026amp;d) elucidate the underlying structural evolution responsible for this behaviour. In neat BMI shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c1\u0026amp;d1), the highly crosslinked rigid aromatic segments form a dense and brittle network. Upon introducing diamine-urea (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c2-c4\u0026amp;d2-d4)), flexible urea segments act as soft domain between the rigid BMI units, enabling stress redistribution and energy absorption under load. As IPDI content decreases, the proportion of soft segments increases, leading to more pronounced phase separation and elasticity. Since IPDI is a relatively rigid (hard) component in the diamine-urea chains, its reduction enhances chain mobility, promoting plastic deformation and fracture resistance [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. This aligns with the tensile testing results, where the tensile strength and modulus were higher at high IPDI/J2000 ratio, while elongation at break decreased with increase of IPDI content.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eImpact strength of aluminum substrate and aluminum coated with BMI and BMI-urea resins is shown in Figure S3 (a). The neat substrate exhibited an impact strength of 154.64 kJ/m2, while the application of neat BMI coating resulted in only a marginal increase to 158.78 kJ/m2. This indicates limited improvement due to BMI\u0026rsquo;s inherent brittleness and inability to absorb impact energy [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. with the relevant findings depicted in Figure S3 (Section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e2.2\u003c/span\u003e, Supporting Information). These results demonstrate that the construction of a BMI\u0026ndash;urea semi-interpenetrating network via Michael addition copolymerization, combined with precise regulation of the soft-to-hard segment ratio, represents an effective strategy for significantly enhancing the toughness and impact resistance of BMI resins.\u003c/p\u003e \u003cp\u003eThe swelling behaviour of BMI and BMI-urea samples in 5wt% solution of strong acid, strong base and salt over 30 days were analysed, with the relevant findings depicted in Fig. S4 (Section \u003cspan refid=\"Sec5\" class=\"InternalRef\"\u003e2.3\u003c/span\u003e, Supporting Information).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Molecular dynamics: precursors\u003c/h2\u003e \u003cp\u003eMolecular dynamics (MD) simulations were performed to gain a deeper insight into the reaction mechanisms at both atomic and electronic levels. The molecular electrostatic potential (MEP) visualizes the charge distribution; it is particularly important for evaluating potential intermolecular interactions, especially electrostatic ones [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Since molecular interactions typically favour electrostatic complementarity to minimize system energy, the analysis of atomic charges and electron cloud distributions of IPDI, J2000 and IPDA was critical. with the relevant findings depicted in Fig. S5 (Section 2.4, Supporting Information).\u003c/p\u003e \u003cp\u003eFrontier molecular orbital (FMO) theory provides critical insight into the reaction mechanism by examining the interactions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the reactants [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Thereby, it offers a deeper understanding of their relative reactivity. The HOMO, LUMO and energy gaps for IPDI, J2000, and IPDA are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. In the reaction between IPDI and J2000/IPDA, the interaction between FMOs plays a crucial role in determining both the selectivity and efficiency of the reaction. In both J2000 and IPDA molecules, the nitrogen atoms of the amino groups (-NH₂) contain lone pair electrons that occupy the HOMO, which exhibits a high energy level and concentrated electron density demonstrating strong nucleophilicity. In contrast, the LUMO of the isocyanate group (-NCO) in the IPDI molecule is predominantly located on the carbon atom, with a lower energy level and increased electrophilicity. When the energy gap between the HOMO (-NH₂) and LUMO (-NCO) is small, electrons can efficiently transfer from the HOMO to the LUMO, leading to the formation of a stable transition state. This orbital interaction significantly lowers the activation energy of the reaction, allowing the nucleophilic addition to proceed efficiently under mild conditions (80℃). Moreover, due to the similar characteristics of the LUMO orbitals of the two -NCO groups in IPDI, the reaction exhibits excellent selectivity, predominantly producing a linear polyurea structure rather than branched or cross-linked byproducts.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(d) illustrates the energy gaps of IPDI, IPDA, and J2000. The energy gap of J2000 is 0.234 and that of IPDA being 0.223. This suggests that IPDA exhibits higher reactivity than J2000. However, to obtain a flexible diamine-urea, it is essential for IPDI to react preferentially with J2000. This insight informed the sequence of reactant addition in the synthesis process. Specifically, IPDA should be introduced only after the reaction between IPDI and J2000 has concluded, to ensure the successful progression of the subsequent chain extension reaction.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Molecular dynamics analysis\u003c/h2\u003e \u003cp\u003eThe atomic charges in molecules and molecular electron cloud distribution of BMI and diamine-urea with the relevant findings depicted in Fig. S6 (Section 2.4, Supporting Information).\u003c/p\u003e \u003cp\u003eThe ratio of J2000 to IPDI during the diamine-urea synthesis stage (Step 1) significantly influences the relative proportions of symmetric and asymmetric structures in the BMI-urea resin system, thereby controlling the final material properties [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. According to chemical reaction stoichiometry, the presence of one of the reactants in excess ensures the limiting reactant is completely consumed [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. In other words, the reaction continues at all available reactive sites. When the ratio of IPDI to J2000 is high (e.g., 3:1), the -NH₂ groups of J2000 are more likely to react with the both -NCO groups of IPDI, resulting in a symmetric double-ended graft structure \u0026ndash; diamine-urea-I, unlike the route suggested in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b). This symmetric structure promotes the formation of an orderly three-dimensional crosslinked network, which enhances both the mechanical strength and thermal stability of the material. As the J2000 proportion increases (e.g., when the ratio decreases to 2:1 or 2.5:1), the reaction gradually shifts towards an asymmetric structure. In this case, a single -NCO group in IPDI molecule is more reacts with -NH₂ groups of a J2000 molecule. A single-ended graft with an asymmetric structure \u0026ndash; diamine-urea-II \u0026ndash; is formed, following the synthesis route in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b). The unreacted -NCO end group either remains unreacted or reacts with IPDA. The increased presence of asymmetric structures introduces additional flexible chain segments and free end groups, which reduce crosslink density but significantly improve the toughness and elongation at break of material. This is in tune with the mechanical testing results in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eIt is important to note that the distribution of molecular electrostatic potential not only influences the primary reaction pathway but also dictates the extent of competing reactions. When the -NH₂ groups are abundant in the system, the electrostatic potential-driven Michael addition reaction dominates. However, once the -NH₂ groups are nearly depleted, the negatively charged C\u0026thinsp;=\u0026thinsp;C double bonds in the remaining BMI molecules engage in electrostatic interactions, triggering a self-polymerization reaction [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. This differential reaction selectivity is directly manifested in the network structure of the final material: the Michael addition produces flexible, long-chain connections, whereas BMI self-polymerization results in rigid, tightly cross-linked structures. The results obtained from the thermal and mechanical properties testing.\u003c/p\u003e \u003cp\u003eThe HOMO, LUMO and energy gaps for BMI and diamine-ureas are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The reaction between BMI and diamine-urea can be effectively explained using FMO theory. The HOMO of the -NH₂ group at the terminal position of the diamine-urea molecule overlaps with the LUMO (π* antibonding orbital) of the C\u0026thinsp;=\u0026thinsp;C double bond on the maleimide ring of the BMI molecule. This HOMO-LUMO interaction facilitates the Michael addition reaction, wherein the lone pair of electrons on the nitrogen atom attacks the C\u0026thinsp;=\u0026thinsp;C double bond, leading to the formation of a new C-N covalent bond. It is important to note that the self-polymerization of BMI molecules can occur through the interaction between the π orbitals (HOMO) of the C\u0026thinsp;=\u0026thinsp;C double bond and the π* orbitals (LUMO) of another BMI molecule [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. However, due to the proximity of the HOMO energy level of diamine-urea to the LUMO energy level of BMI, the Michael addition reaction is more favourable both kinetically and thermodynamically. This theoretical prediction aligns closely with the experimental observation of reaction selectivity, which demonstrates that the Michael addition reaction predominates over BMI self-polymerization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurther calculations were performed to determine the HOMO, LUMO and energy gaps of the diamine-urea\u003csub\u003e2/1\u003c/sub\u003e, diamine-urea\u003csub\u003e2.5/1\u003c/sub\u003e and diamine-urea\u003csub\u003e3/1\u003c/sub\u003e systems. The energy gaps for these systems are presented in Fig.\u0026nbsp;9(d). The energy gap for diamine-urea\u003csub\u003e2/1\u003c/sub\u003e is 0.080, for diamine-urea\u003csub\u003e2.5/1\u003c/sub\u003e is 0.083, and for diamine-urea\u003csub\u003e3/1\u003c/sub\u003e is 0.084. No significant differences were observed in the energy gaps among the three systems, indicating that their reactivity during the reaction process is essentially identical.\u003c/p\u003e \u003cp\u003eFigure S7 shows molecular dynamic simulations of BMI-urea systems with different compositions. In all three cases, BMI molecules interact with diamine-urea pre-polymers.\u003c/p\u003e \u003cp\u003eBy analysing the energy gaps of the three diamine-urea\u003csub\u003ex/y\u003c/sub\u003e systems, it was determined that their reaction activities during the reaction process are essentially identical. Therefore, it is necessary to calculate the interaction energies of the BMI/diamine-urea\u003csub\u003ex/y\u003c/sub\u003e systems to determine the performance differences between the various systems. The binding energies of three systems ‒ BMI/diamine-urea\u003csub\u003e2/1\u003c/sub\u003e, BMI/diamine-urea\u003csub\u003e2.5/1\u003c/sub\u003e, and BMI/diamine-urea\u003csub\u003e3/1\u003c/sub\u003e ‒ were calculated to predict their performance indicators after the reaction. The total energies for the BMI/diamine-urea\u003csub\u003e2/1\u003c/sub\u003e, BMI/diamine-urea\u003csub\u003e2.5/1\u003c/sub\u003e, and BMI/diamine-urea\u003csub\u003e3/1\u003c/sub\u003e systems were 2629779, 504109 kcal/mol, and \u0026minus;\u0026thinsp;3381263 kcal/mol, respectively as presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The binding energy of the BMI-urea\u003csub\u003ex/y\u003c/sub\u003e system increases progressively with the increase of IPDI/J2000 ratio, indicating stronger molecular interactions in the BMI-urea\u003csub\u003e3/1\u003c/sub\u003e system. This increase is attributed to the higher proportion of IPDI in the IPDI/J2000 system suppressing the copolymerization of J2000. As a result, the diamine-urea molecular chains in BMI-urea\u003csub\u003e3/1\u003c/sub\u003e system are shorter compared to those in other systems, this indicates that the molecular structure is more stable. Consequently, the BMI-urea\u003csub\u003e3/1\u003c/sub\u003e system exhibits enhanced stiffness but reduced toughness.\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\u003eBinding energy data of BMI/ diamine-urea x/y\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSystems\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTotal energy (kcal/mol)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBinding energy (kcal/mol)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBMI-urea\u003csub\u003e2/1\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e2629779\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003csub\u003eBMI\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e157778\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e372\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003csub\u003ediamine\u0026minus;urea2/1\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2472089\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBMI-urea\u003csub\u003e2.5/1\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e504109\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003csub\u003eBMI\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123101\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e390\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003csub\u003ediamine\u0026minus;urea2.5/1\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e381101\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBMI-urea\u003csub\u003e3/1\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e3381263\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003csub\u003eBMI\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e212378\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e405\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE\u003csub\u003ediamine\u0026minus;urea3/1\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3168983\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5. AI system-assisted prediction\u003c/h2\u003e \u003cp\u003eDuring the synthesis of BMI-urea resins, the microstructure is deterministically shaped by thermodynamic, kinetic and stoichiometric constraints [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Data-driven models, including temporal and graph neural networks, capture the dynamic interplay of Michael addition and imidization crosslinking reactions, with representative predictions summarized in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. Different AI models consistently predicted two dominant pathways in BMI-urea resin synthesis: the covalent BMI-urea link and BMI homopolymerization. Across methods, C\u0026thinsp;=\u0026thinsp;C peak attenuation near 1630 cm-1 in FTIR and imide (172 ppm) and urea (160 ppm) shown in NMR validated the coexistence of flexible and rigid domains. These results highlight the reliable capability of AI in capturing structural evolution and performance features during curing.\u003c/p\u003e \u003cp\u003eCoupling MD with quantum chemical calculations with AI analysis further enables accurate prediction of structural evolution pathways during curing. AI analysis results in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows that the primary -NCO group of IPDI is preferentially attacked by J2000 amines (~\u0026thinsp;78%), and the IPDI/J2000 ratio dictates the dominant prepolymer structure, At 3:1 ratio, the symmetric double-head structures prevail (\u0026gt;\u0026thinsp;70%) resulting in a high tensile strength, while at 2:1, asymmetric single-head structures dominate (\u0026gt;\u0026thinsp;70%) leading to flexible polymer. At intermediate ration 2.5:1 a balanced mix that produces a network with stiffness and toughness is yielded; this is reflected in exceptional impact strength.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, a novel bismaleimide (BMI)-urea polymer with tuneable mechanical and thermal properties were designed and synthesized by introducing flexible urea segments into the traditionally rigid BMI network. BMI-urea polymers with different IPDI/J2000 molar ratios were synthesized via a two-step procedure. FTIR and solid-state 13C NMR analyses confirmed the coexistence of urea linkages (-NH-CO-NH-) and imide rings, and revealed the competing reaction mechanisms of Michael addition and BMI self-polymerization. XRD and AFM analyses showed that incorporating diamine-urea disrupted the intrinsic crystalline structure of BMI resulting in microphase separation. As the IPDI/J2000 ratio decreased, the soft-phase regions became more continuous and uniformly distributed, whereas the hard-phase regions \u0026ndash;BMI- were dispersed as reinforcing points. This microstructural arrangement represents a key factor driving the transition of the material from brittle to tough/ductile and thus the mechanical properties could be finely tuned by varying the IPDI/J2000 ratio. Increased IPDI content resulted in higher tensile strength and modulus, whereas greater J2000 content improved elongation. The BMI-urea resins exhibited a tensile strength as high as 55 MPa which is 120% higher than diamine-urea; an impact strength up to 208 kJ/m\u003csup\u003e2\u003c/sup\u003e \u0026ndash; a 30% increase over BMI; and up to 60-fold increment in elongation at break. Impact test demonstrated that the BMI-urea2.5/1 sample exhibited the highest impact resistance; notably, it retained 71% of BMI\u0026rsquo;s tensile strength while achieving 12-fold higher elongation at break, demonstrating an optimal balance between rigidity and toughness. All BMI-urea samples exhibited high thermal stability with initial decomposition temperature within range 290 \u0026minus;\u0026thinsp;310 ℃. Although the decomposition temperature is lower than that of BMI (\u0026gt;\u0026thinsp;400 ℃), these temperatures remain significantly higher than those of many conventional polymers. This confirms their suitability for high-temperature performance applications. Swelling resistance analysis in acidic, alkaline and saline environments confirmed that solvent uptake can be tailored through soft segment content and microphase separation. Molecular dynamics simulations elucidated the origins of performance variations at the atomic level. Analyses of electrostatic potential and frontier molecular orbitals clarified the distinct reactivity of the terminal amino groups of J2000 and the reaction mechanisms between IPDI and J2000/IPDA. Binding energy calculations revealed that the interaction strength between BMI and diamine-urea decreased in the order BMI-urea3/1 BMI-urea2.5/1 and BMI-urea2/1. This trend aligns with experimental measurements in modulus and strength; this theoretically validating the role of IPDI as a rigid component in enhancing intermolecular interactions and material stiffness. These findings highlight a straightforward approach to adjust strength, toughness and thermal stability in BMI via adding diamine-urea, offering practical solution for their use in demanding environments\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003e\u003cstrong\u003eConflict of interest:\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThe authors declare no conflict of interests.\u003c/p\u003e\n\u003ch2\u003eFunding:\u003c/h2\u003e\n\u003cp\u003eThe authors extend their gratitude to Scientific Compass \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ewww.shiyanjia.com\u003c/span\u003e\u003c/span\u003e for providing invaluable assistance with the SEM analysis. This work was supported by the following funding sources: The Universities of Liaoning province (LJ212510143025); The Liaoning Province Science and Technology Joint Program (Key R\u0026amp;D Program Project) (2025JH2/101800288); Faculty Development Competitive Research Grant Program, Nazarbayev University, Kazakhstan (040225FD4725)\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eK.Z. wrote the main manuscript text, M.S. and U.B. Validation, S.A. Review \u0026amp; Editing, S.H.and H.Y. Data Curation,Q.S.suggest the method, All authors reviewed the manuscript.\u003c/p\u003e\n\u003ch2\u003eData Availability\u003c/h2\u003e\n\u003cp\u003eAll data generated or analysed during this study are included in this published article (and its supplementary information files).The data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eFan, Z., et al., \u003cem\u003eUnderstanding of the modified bismaleimide composite with multifunctional phthalonitrile and its outstanding mechanical properties.\u003c/em\u003e Polymer, 2024/12/17. \u003cstrong\u003e315\u003c/strong\u003e. http://doi.org/10.1016/j.polymer.2024.127784\u003c/li\u003e\n\u003cli\u003eBibiao, J., et al., \u003cem\u003eSynthesis and properties of novel polybismaleimide oligomers.\u003c/em\u003e European Polymer Journal, 2001/03/01. \u003cstrong\u003e37\u003c/strong\u003e(3). http://doi.org/10.1016/S0014-3057(00)00147-6\u003c/li\u003e\n\u003cli\u003eSava, M., et al., \u003cem\u003eSynthesis and Characterization of Some Bismaleimides Containing Ether Groups in the Backbone.\u003c/em\u003e Macromolecular Chemistry and Physics, 2001/08/01. \u003cstrong\u003e202\u003c/strong\u003e(12). http://doi.org/10.1002/1521-3935(20010801)202:12\u0026lt;2601::AID-MACP2601\u0026gt;3.0.CO;2-Y\u003c/li\u003e\n\u003cli\u003eShah, D., et al., \u003cem\u003eEffect of Nanoparticle Mobility on Toughness of Polymer Nanocomposites.\u003c/em\u003e Advanced Materials, 2005. \u003cstrong\u003e17\u003c/strong\u003e(5): p. 525-528. http://doi.org/https://doi.org/10.1002/adma.200400984\u003c/li\u003e\n\u003cli\u003eCai, H., et al., \u003cem\u003eCharacterization of Mechanical, Electrical and Thermal Properties of Bismaleimide Resins Based on Different Branched Structures.\u003c/em\u003e Polymers 2023, Vol. 15, Page 592, 2023-01-24. \u003cstrong\u003e15\u003c/strong\u003e(3). http://doi.org/10.3390/polym15030592\u003c/li\u003e\n\u003cli\u003eHsiao, C.-C., J.-J. 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Shekgola, \u003cem\u003eUtilising artificial intelligence chatbots for conducting research in the fifth industrial revolution.\u003c/em\u003e Journal of Infrastructure, Policy and Development, 2025. \u003cstrong\u003e9\u003c/strong\u003e(3): p. 11334-11334 D3 - 2025-08-15. http://doi.org/10.24294/jipd11334\u003c/li\u003e\n\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":"Bismaleimide, Flexible urea segments, Molecular dynamics, Mechanical performance, Thermal stability","lastPublishedDoi":"10.21203/rs.3.rs-8750615/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8750615/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe intrinsic brittleness of bismaleimide (BMI) resins has long restricted their wider application in aerospace and automotive industry. Herein, a novel BMI-urea resin network that integrates flexible urea linkages into the rigid BMI matrix is developed. The developed BMI-urea polymer combines elasticity with high strength. Molecular dynamics simulations and AI-assisted predictions revealed that Michael addition dominates over BMI self-polymerization, enabling rational design of hybrid networks. The nanoscale phase-separated morphology endowed BMI-urea with ductility and toughness while preserving strength and thermal stability. Fracture analysis revealed the enhancement of mechanical properties \u003cem\u003evia\u003c/em\u003e rigid BMI segments providing reinforcement while flexible urea segments dissipating energy and supressing crack propagation. The developed BMI-urea exhibited (i) tensile as high as 55 MPa \u0026ndash; 120% improvement over diamine-urea; (ii) impact strength as high as 208 kJ/m\u003csup\u003e2\u003c/sup\u003e \u0026ndash; 30% improvement over BMI; (iii) up to a 60-fold increase in elongation at break; (iv) preserved thermal stability above 290\u0026deg;C. Swelling resistance test in three different media showed a dependence of solvent uptake on soft segment content and microphase separation. The current study introduces, for the first time, an elastic BMI-urea resin that combines toughness, strength and thermal resistance \u0026ndash; paving the way for advanced application is aerospace composites and structural adhesives.\u003c/p\u003e","manuscriptTitle":"A Novel Bismaleimide-Urea Resin: Breaking the Trade-off Between Strength, Toughness and Thermal Stability via Molecular Design","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-03 12:11:17","doi":"10.21203/rs.3.rs-8750615/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":"7b317a90-bf8c-4cf4-8ae5-61ab80cc401b","owner":[],"postedDate":"February 3rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-27T18:23:56+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-03 12:11:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8750615","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8750615","identity":"rs-8750615","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}