Ternary epoxy nanocomposites with synergistic effects: Preparation, properties evaluation and structure analysis

Academic Editor: Yizhuo Gu Received: 13 December 2024 Revised: 5 January 2025 Accepted: 6 January 2025 Published: 10 January 2025 Citation: Suro´n, P.; Białkowska, A.; Bakar, M.; Hanulikova, B.; Masaˇr, M.; Kroisová, D. Ternary Epoxy Nanocomposites with Synergistic Effects: Preparation, Properties Evaluation and Structure Analysis. Polymers 2025,17, 158. https:// doi.org/10.3390/polym17020158 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Ternary Epoxy Nanocomposites with Synergistic Effects: Preparation, Properties Evaluation and Structure Analysis Patryk Suro´n 1,*, Anita Białkowska 1, Mohamed Bakar 1, Barbora Hanulikova 2, Milan Masaˇr 2 and Dora Kroisová 3 1Faculty of Applied Chemistry, University of Radom, 26-600 Radom, Poland; [email protected] (A.B.); [email protected] (M.B.) 2Centre of Polymer Systems, Tomas Bata University in Zlin, 760 01 Zlin, Czech Republic; [email protected] (B.H.); [email protected] (M.M.) 3Faculty of Mechanical Engineering, Technical University of Liberec, 461 17 Liberec, Czech Republic; [email protected] *Correspondence: sur[email protected] Abstract: The objective of the present work was to prepare hybrid epoxy composites with improved mechanical and thermal properties. The simultaneous use of two different modifiers in an epoxy resin was motivated by the expected occurrence of synergistic effects on the performance properties of the matrix. Such a hybrid composite can be used in more severe conditions and/or in broader application areas. Hybrid epoxy composites were prepared with polyurethane (PUR), Nanomer nanoclay and carbon nanotubes (CNT), followed by the evaluation of their mechanical and thermal properties. Synergistic improvements in mechanical properties of hybrid composites were observed for 0.5 wt% Nanomer and 1 wt% carbon nanotubes (CNT), 7.5 wt% PUR and 1 wt% CNT, and 5 wt% PUR and 1 wt% CNT, confirming the occurrence of synergistic effects as to the impact strength (IS) of the matrices, compared to binary systems. The toughening induced by CNT/Nanomer modifiers can be attributed to the specific interfacial interactions between the two nanoparticles, while in the case of CNT/PUR, it can be explained by the combined effects of flexible polymer chains and the specific arrangement of nanoparticles in epoxy systems. Spectroscopy analysis confirmed the occurrence of interaction between OH groups in the epoxy matrix with CNT and reactive groups of PUR. The fracture surface showed plastic deformations, with good dispersion of CNT, explaining the improved mechanical properties of the matrix composites. Keywords: epoxy hybrids; mechanical properties; synergism; thermal properties; structure and morphology 1. Introduction Hybrid polymer nanocomposites have received, in recent decades, considerable attention from both academics and industrialists because of the improved mechanical performance, thermal stability and barrier properties of these materials [ 1 – 10 ]. The unique properties of hybrid composites are the result of the interaction of integral components of the composition according to various strengthening mechanisms. This often leads to the synergism of properties in these hybrid materials. The phenomenon of exfoliation and/or intercalation of solid nanoclay nanoplatelets, the formation of a second phase by soft nanoparticles or the formation of a thermoplastic with possible grafting reactions with the matrix would explain the improvement in the toughness of the latter. Polymers 2025,17, 158 https://doi.org/10.3390/polym17020158 Polymers 2025,17, 158 2 of 19 These hybrid polymer composite materials can be prepared using various matrices and a large group of different modifiers such as solid microparticles, nanoparticles, liquid rubbers, plasticizers or thermoplastics. The choice of matrix in composites is dictated by its properties, the price and the ease of processing. Epoxy resin is one of the cheaper matrix materials, and it demonstrates good processing properties. However, its brittleness and low resistance to crack-propagation make its modification necessary. Modifiers have recently been widely used, either alone or combined with other modifiers, to prepare hybrid epoxy composites with improved mechanical and thermal properties. Recently, interesting reviews [ 11 – 17 ] and scientific works [ 18 – 26 ] have been conducted using two modifiers in order to obtain synergistic effects from the properties of epoxy hybrids, thus demonstrating the importance of the present investigation. Carbon nanotubes (CNTs) are known for their excellent stiffness and strength, and thus they have been considered as potential modifiers for various polymers. They have been combined with, among others, different engineering thermoplastics, nanoclays and graphene to form hybrid composites with epoxy resin. Polyetherimide (PEI) and polyether sulphone (PES) were combined with carbon nanotubes (CNTs) to prepare hybrid epoxy nanocomposites with synergistic toughening. Chen et al. [ 18 ] simultaneously used PEI and multi-walled carbon nanotubes modified with amine groups (NH 2 -MWCNTs) to improve the fracture toughness K C of diglycidyl ether of bisphenol A (DGEBA). The toughening was explained by the crack-deflection and bridging properties of the plastic deformation of PEI, and the pulling forces of the carbon nanotubes. However, the use of PEI with carboxyl-functionalized multiwalled carbon nanotubes (COOH-MWCNTs) led to a synergistic effect on the fracture toughness of DGEBA which was attributed to the phase separation of PEI, good dispersion of CNTs in the matrix and active crack-energy dissipation [ 19 ]. A new method to reinforce epoxy resin was introduced by the use of MWCNT which was previously attached to exfoliated montmorillonite (MMT) [ 20 ]. Synergistic toughening of the epoxy resin was achieved at a MWCNT/MMT weight ratio of 0.1:1, due to strong interfacial adhesion between the MWCNT/MMT and the matrix, as well as the uniform stress distribution. Carbon nanotubes were also combined with polyetheretherketone (PEEK) and thermoplastic polyetherketone-cardo (PEK-C) to produce hybrid epoxy (EP). The results obtained showed significant improvements in the strength and toughness of the matrix [ 21 ], while the stronger interaction between PEEK/CNT and EP resulted in lower friction coefficients for the coatings [ 22 ]. However, the results of EP/CNT/PEK-C hybrid [ 23 ] confirmed that the fracture toughness was closely related to the two-phase structure formed between EP and PEK-C. Hyperbranched polymers (HBPs) and ultrahigh-molecular-weight polyethylene (UHMWPE) showed significant improvement in the properties of the epoxy matrix when combined with MWCNTs [ 24 , 25 ]. The impact strength, tensile strength, tensile modulus, fracture toughness, and glass transition temperature of the matrix were significantly increased compared to binary systems and pure resin [ 24 ]. However, the work of adhesion, tensile strength and tensile modulus of the epoxy matrix were enhanced by ~26%, ~67% and ~35%, respectively, with UHMWPE [25]. Other studies have focused on the preparation of ternary epoxy nanocomposites combining CNTs with polycarbonate [ 26 ] and CNTs with branched polyethyleneimine [ 27 ]. In both cases, the properties of the matrix were improved to the good dispersion of the modifiers and interfacial interactions. The concomitant addition of graphene oxide (GO) and CNTs resulted in improvements in the tensile strength, critical stress intensity factor (K C ), and critical strain energy release rate (G C ) of the hybrid composite, due to the good dispersion of the nanofillers in the Polymers 2025,17, 158 3 of 19 matrix [ 28 ]. However, Chatterjee et al. [ 29 ] confirmed that larger graphene nanoplatelets (GnPs)-based nanoparticles led to greater improvement in fracture toughness of the epoxy resin–modified CNTs (by ~75%, compared to the pristine matrix). Ghaleb et al. [ 30 ] confirmed that maximum improvements in the tensile and electrical properties of an epoxy matrix modified with a graphene nanopowder (GNP) and MWCNT hybrid nanocomposite was achieved with a GNP/MWCNT ratio of 0.1:0.4. The uniform dispersion of the nanoparticles in the matrix, combined with the alignment of the MWCNTs on the surface of the GNP, led to this improvement. A similar level of improvement was reached with samples containing 0.15–0.20 wt% CNTs/graphene oxide (at a 1:1 ratio), due to the interaction of the filler-layered structure with the polymer matrix [ 31 ]. Yue et al. [ 32 ] showed that CNTs coupled with graphene nanoplatelets (GnPs) at an 8:2 ratio led to a synergistic increase in the flexural properties of the epoxy matrix, which was caused by better dispersion of CNTs. Recently, it was confirmed that simultaneous addition of 0.17 wt% of amine-functionalized MWCNTs (NH 2 -MWCNTs) and 0.17 wt% of graphene nanoparticles resulted in the maximum improvements in fracture toughness (K C ) and tensile strength at a testing temperature of − 20 ◦ C [ 33 ]. The synergistic effects resulted from the uniform dispersion of the nanofillers and the strong adhesion between nanofillers and epoxy. The modifier NH 2 -MWCNT was found to be very useful in conjunction with amine-functionalized graphene with respect to synergistic effects on the tensile strength and thermal stability of the epoxy matrix [ 34 ]. Tangthana-Umrung et al. [ 35 ] obtained a positive enhancement of the fracture toughness. The synergistic toughening of the hybrid epoxy base was attributed to the improvements in crack deviation and bifurcation, which resulted in a shorter crack path. Carbon nanotubes were also combined with soft modifiers such as flexible polyurethane [36–38] or rubber particles [ 39 – 41 ] to improve the properties of the DGEBA matrix. Jia et al. [ 36 ] confirmed synergistic strengthening and toughening effects, specifically, the tensile, flexural and impact strengths, as well as fracture toughness (K C ), in an epoxy-grafted polyurethane (EP-PU) modified with NH 2 -MWCNTs. The thermal stability and fracture toughness of epoxy resin were significantly improved through the simultaneous use of functionalized carbon nanotubes and carboxyl-terminated butadiene acrylonitrile [ 39 ]. Morphology analysis revealed the association of the plastic deformation zone with rougher surfaces. Because polymeric materials are often used under different atmospheric conditions, Jen et al. [ 42 ] investigated the effects of temperature on the static tensile strength and fatigue resistance of epoxy/graphene/CNTs nanocomposites. The results showed that tensile strength and fatigue resistance decreased with increasing temperature. Hybrid epoxy nanocomposites can be used, among other applications, in the aviation and automotive industries because of the outstanding performance properties of these materials. These lightweight materials can also be applied in packaging, construction and coatings. However, hybrid nanocomposites based on tougher polymers could be used in applications requiring impact and deformation resistance. The purpose of the present study was to prepare hybrid epoxy composites utilizing either carbon nanotubes and polyurethane or montmorillonite, with the aims of improved mechanical and thermal properties. A synergistic toughening was expected as a result of the possible interactions between the modifiers and the epoxy matrix. 2. Experimental 2.1. Materials The following ingredients were used in the present work: Polymers 2025,17, 158 4 of 19 • Epoxy resin (Epidian 52 purchased from Sarzyna Co., Nowa Sarzyna, Poland), which has an epoxy number in the range of 0.510–0.550 mol/100 g and a viscosity between 400–800 mPa·s at 25 ◦C; • Triethylene tetramine (trade name Z1, from Sarzyna Co., Nowa Sarzyna, Poland), which was used as a curing agent; • Polyurethane prepolymer (Desmocap 12), produced by Bayer AG, Leverkusen, Germany ; • Carbon nanotubes, pyrolitically stripped platelets measuring D×L 100 nm ×20–200 µm , manufactured by Sigma Aldrich Co., St. Louis, MO, USA; • Nanomer I.28E, nanoclay modified with 25–30 wt% trimethyl stearyl ammonium, produced by Nanocor Inc. Copenhagen, Denmark. 2.2. Preparation of Samples 2.2.1. Epoxy-Based Composites with One Modifier Epoxy resin was mechanically mixed with different amounts of polyurethane (PUR) for 10 min. CNTs and Nanomer I.28E nanoparticles were mixed with the epoxy matrix using a mechanical stirrer and an ultrasonic stirrer. For the Nanomer I.28E, the epoxy composition was mixed for 10 min with a mechanical stirrer, and this was followed by mixing using an ultrasonic stirrer for 75 min at maximum amplitude of 270 µ m. The CNT was mixed for 10 min with a mechanical stirrer, followed by mixing with an ultrasonic stirrer for 8 h at a maximum amplitude of 270 µ m. Then, 14 phr of curing agent was added to each mixture and mixing continued for 5 min. The compositions were poured into metal molds, and then cured for 24 h at room temperature and post-cured for 3 h at 80 ◦C. 2.2.2. Hybrid Epoxy-Based Composites The following hybrid epoxy composites were prepared: PUR/CNT and Nanomer/CNT. The epoxy resin was mixed with the modifiers using a mechanical stirrer, followed by ultrasonic mixing as previously described. The mixing time and sonication amplitude were defined from the maximum impact strength of the tested nanocomposites. In order to obtain the desired mechanical properties, the ingredients were incorporated into the matrix in the following order: PUR–CNT–curing agent or CNT–nanoclay–curing agent. Finally, 14 phr of curing agent was added, with an additional 5 min of mixing performed before the materials were poured into metal molds. The curing and post-curing were carried out as above. A schematic presentation of the composite preparation is shown below. The scheme of obtaining hybrid nanocomposites is shown in Figure 1(below). Polymers 2025, 17, x FOR PEER REVIEW 5 of 20 Figure 1. Scheme for obtaining hybrid composites. 2.3. Evaluation of Mechanical and Thermal Properties The determination of flexural properties was carried out in accordance with the relevant standards at a room temperature, and four samples were used for each data point. In addition, error bars have been added in figures. Three-point Bending: The test was carried out on samples 10 cm long, 1 cm wide and 0.5 cm thick using a Zwick Roell machine, according to ISO-178-2019 [43]. The deformation rate was fixed at 5 mm/min. Charpy impact strength: The test was conducted with a Zwick Roell, on samples with the dimensions as described above and 1 mm of notch length, according to ISO-179-1:2023 [44]. The distance between the spans was 6 cm. Critical stress intensity factor (KC): Samples with dimensions and notch lengths identical to the samples used for strength were used for the test, which was carried out on a Zwick Roell device by means of ISO 13586:2018 [45]. The deformation rate was fixed at 5 mm/min. The parameter KC was calculated as follows: 𝐾𝐶=3𝑃.𝐿.𝑎1 2 ⁄ 2𝐵.𝑤2∙ 𝑌 (1) where P represents the load-at-break, L the distance between the spans, a the notch length, w the sample width, B the sample thickness and Y a geometrical factor which depends on the a/w ratio. Thermogravimetric Analysis: The test was performed using a Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) in a nitrogen atmosphere, with a heating rate of 10 C/min. and a temperature profile of 25–800 °C. Differential scanning calorimetry (DSC): The test was performed on a 1 Star System calorimeter (Mettler Toledo Warszawa, Poland) under a nitrogen atmosphere and with a scanning rate of 10 C/min. 2.4. Evaluation of Structure and Morphology Analysis Fourier transform infrared spectroscopy (FTIR) was used to show the functional groups present in the samples. The test was carried out on the Nicolet 6700 spectrometer Figure 1. Scheme for obtaining hybrid composites. Polymers 2025,17, 158 5 of 19 2.3. Evaluation of Mechanical and Thermal Properties The determination of flexural properties was carried out in accordance with the relevant standards at a room temperature, and four samples were used for each data point. In addition, error bars have been added in figures. Three-point Bending: The test was carried out on samples 10 cm long, 1 cm wide and 0.5 cm thick using a Zwick Roell machine, according to ISO-178-2019 [ 43 ]. The deformation rate was fixed at 5 mm/min. Charpy impact strength: The test was conducted with a Zwick Roell, on samples with the dimensions as described above and 1 mm of notch length, according to ISO-179-1:2023 [ 44 ]. The distance between the spans was 6 cm. Critical stress intensity factor (K C ): Samples with dimensions and notch lengths identical to the samples used for strength were used for the test, which was carried out on a Zwick Roell device by means of ISO 13586:2018 [ 45 ]. The deformation rate was fixed at 5 mm/min. The parameter KCwas calculated as follows: KC= 3P·L·a1/2 2B·w2·Y(1) where Prepresents the load-at-break, Lthe distance between the spans, athe notch length, wthe sample width, Bthe sample thickness and Ya geometrical factor which depends on the a/w ratio. Thermogravimetric Analysis: The test was performed using a Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) in a nitrogen atmosphere, with a heating rate of 10 ◦C/min. and a temperature profile of 25–800 ◦C. Differential scanning calorimetry (DSC): The test was performed on a 1 Star System calorimeter (Mettler Toledo Warszawa, Poland) under a nitrogen atmosphere and with a scanning rate of 10 ◦C/min. 2.4. Evaluation of Structure and Morphology Analysis Fourier transform infrared spectroscopy (FTIR) was used to show the functional groups present in the samples. The test was carried out on the Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), mode ATR with diamond crystal, 64 scans, resolution 4 cm−1. The morphology of the samples was analyzed using a scanning electron microscope (SEM), the NovaNano SEM 450 microscope (The Netherlands, FEI company, Eindhoven, Th Netherlands). 3. Results and Discussion 3.1. Mechanical Properties Figure 2shows the effect of Nanomer nanoclay content on the impact strength (IS) of epoxy resin modified with 1 wt% carbon nanotubes (CNTs). It was observed that hybrid epoxy nanocomposite containing 0.5 wt% nanoclay and 1 wt% CNT showed a maximum increase in IS of approximately 70% and 20% above the IS of unmodified epoxy matrix and the nanocomposite with 1 wt% CNT. The IS of the ternary epoxy composite containing 0.5 wt% Nanomer and 1 wt% CNT (4.8 kJ/m 2 ) was superior to those of the binary epoxy based on 1 wt% CNT (4.0 kJ/m 2 ) and the sample based on 0.5 wt% Nanomer (2.7 kJ/m 2 ), confirming the occurrence of a synergistic effect. The significant improvement in IS can be attributed to the exceptional mechanical properties of CNTs as well as their sufficient and homogeneous dispersion in the polymer matrix. The additional parameters to take into account, and factors which could have contributed to the improvement, would be Polymers 2025,17, 158 6 of 19 the specific interfacial interactions between the modifiers with polymer matrix as well as the high degree of intercalation/exfoliation of the Nanomer in the matrix. Similar results were reported elsewhere with nanoclays [ 46 , 47 ]. Moreover, the impact strength decreased at higher nanoparticle contents, due most probably to the agglomeration of the nanoparticles, which constitute weak zones in the nanocomposites through which the resistance to crack propagation becomes rather weak and consequently, the IS shows a low value. Agglomeration of microparticles or nanoparticles is common at higher particleloading levels in different systems [48,49]. Polymers 2025, 17, x FOR PEER REVIEW 6 of 20 (Thermo Fisher Scientific, Waltham, MA, USA), mode ATR with diamond crystal, 64 scans, resolution 4 cm−1. The morphology of the samples was analyzed using a scanning electron microscope (SEM), the NovaNano SEM 450 microscope (The Netherlands, FEI company, Eindhoven, Th Netherlands). 3. Results and Discussion 3.1. Mechanical Properties Figure 2 shows the effect of Nanomer nanoclay content on the impact strength (IS) of epoxy resin modified with 1 wt% carbon nanotubes (CNTs). It was observed that hybrid epoxy nanocomposite containing 0.5 wt% nanoclay and 1 wt% CNT showed a maximum increase in IS of approximately 70% and 20% above the IS of unmodified epoxy matrix and the nanocomposite with 1 wt% CNT. The IS of the ternary epoxy composite containing 0.5 wt% Nanomer and 1 wt% CNT (4.8 kJ/m2) was superior to those of the binary epoxy based on 1 wt% CNT (4.0 kJ/m2) and the sample based on 0.5 wt% Nanomer (2.7 kJ/m2), confirming the occurrence of a synergistic effect. The significant improvement in IS can be attributed to the exceptional mechanical properties of CNTs as well as their sufficient and homogeneous dispersion in the polymer matrix. The additional parameters to take into account, and factors which could have contributed to the improvement, would be the specific interfacial interactions between the modifiers with polymer matrix as well as the high degree of intercalation/exfoliation of the Nanomer in the matrix. Similar results were reported elsewhere with nanoclays [46,47]. Moreover, the impact strength decreased at higher nanoparticle contents, due most probably to the agglomeration of the nanoparticles, which constitute weak zones in the nanocomposites through which the resistance to crack propagation becomes rather weak and consequently, the IS shows a low value. Agglomeration of microparticles or nanoparticles is common at higher particle-loading levels in different systems [48,49]. Figure 2. Effect of Nanomer content on the impact strength of epoxy resin modified with 1 wt% CNT. The impact strength (IS) of epoxy resin modified with 1 wt% CNT is shown in Figure 3 as a function of the polyurethane (PUR) content. The maximum improvement in IS due to the synergistic effect was exhibited by the hybrid epoxy nanocomposite prepared with 7.5 wt% PUR and 1 wt% CNT. The improvement reached 77.5% and 85% compared to the nanocomposite containing 1 wt% CNT (without PUR) and epoxy with 7.5 wt% PUR Figure 2. Effect of Nanomer content on the impact strength of epoxy resin modified with 1 wt% CNT. The impact strength (IS) of epoxy resin modified with 1 wt% CNT is shown in Figure 3 as a function of the polyurethane (PUR) content. The maximum improvement in IS due to the synergistic effect was exhibited by the hybrid epoxy nanocomposite prepared with 7.5 wt% PUR and 1 wt% CNT. The improvement reached ~77.5% and ~85% compared to the nanocomposite containing 1 wt% CNT (without PUR) and epoxy with 7.5 wt% PUR (without CNT), respectively. Furthermore, it was demonstrated that the hybrid nanocomposite prepared with 5 wt% PUR and 1 wt% CNT had an IS of 6.7 kJ/m 2 , comprising a synergistic improvement in IS compared to both epoxy binary composites. The IS improvement may also result from the strong interactions between CNTs and the epoxy/PUR system. In addition, polyurethane may penetrate the porous structure of the CNT, which would explain the drastic enhancement in the IS of the epoxy nanocomposite. Similar results were presented in other studies with flexible polymeric chains and nanoclays [ 5 , 20 , 50 , 51 ]. It should be emphasized that the IS increased from 2.8 kJ/m 2 for pure epoxy resin to a maximum of 5.1 kJ/m 2 due to the addition of 2.5 wt% PUR, but only to 3.7 kJ/m 2 with 7.5 wt% PUR . The improvement in IS can be attributed to the formation of an interpenetrating polymer network combined with a grafting reaction between the reactive groups of the matrix (-OH) and the polymer modifier (-NCO) [15,52]. The obtained results confirmed the occurrence of a synergistic effect in the IS of the ternary epoxy nanocomposite containing 0.5 wt% Nanomer and 1 wt% carbon nanotubes (CNT). The IS of the hybrid nanocomposite (4.8 kJ/m 2 ) exceeded those of epoxy/CNT ( 4.0 kJ/m2 ) and epoxy/Nanomer (4.0 kJ/m 2 ) by 20% and 75%, respectively, confirming the occurrence of a synergistic toughening of the epoxy matrix. The best impact resistance of the epoxy nanocomposite was obtained when the CNT and Nanomer were used simultaneously rather than when they were used separately. Positive toughening of brittle epoxy resin may result from specific interfacial interactions between nanofillers as well as transfer of the applied stress from the matrix to the nanofillers. Polymers 2025,17, 158 7 of 19 Polymers 2025, 17, x FOR PEER REVIEW 7 of 20 (without CNT), respectively. Furthermore, it was demonstrated that the hybrid nanocomposite prepared with 5 wt% PUR and 1 wt% CNT had an IS of 6.7 kJ/m2, comprising a synergistic improvement in IS compared to both epoxy binary composites. The IS improvement may also result from the strong interactions between CNTs and the epoxy/PUR system. In addition, polyurethane may penetrate the porous structure of the CNT, which would explain the drastic enhancement in the IS of the epoxy nanocomposite. Similar results were presented in other studies with flexible polymeric chains and nanoclays [5,20,50,51]. It should be emphasized that the IS increased from 2.8 kJ/m2 for pure epoxy resin to a maximum of 5.1 kJ/m2 due to the addition of 2.5 wt% PUR, but only to 3.7 kJ/m2 with 7.5 wt% PUR. The improvement in IS can be attributed to the formation of an interpenetrating polymer network combined with a grafting reaction between the reactive groups of the matrix (-OH) and the polymer modifier (-NCO) [15,52]. Figure 3. Effect of polyurethane content on the impact strength (IS) of epoxy resin modified with 1 wt% CNT. The obtained results confirmed the occurrence of a synergistic effect in the IS of the ternary epoxy nanocomposite containing 0.5 wt% Nanomer and 1 wt% carbon nanotubes (CNT). The IS of the hybrid nanocomposite (4.8 kJ/m2) exceeded those of epoxy/CNT (4.0 kJ/m2) and epoxy/Nanomer (4.0 kJ/m2) by 20% and 75%, respectively, confirming the occurrence of a synergistic toughening of the epoxy matrix. The best impact resistance of the epoxy nanocomposite was obtained when the CNT and Nanomer were used simultaneously rather than when they were used separately. Positive toughening of brittle epoxy resin may result from specific interfacial interactions between nanofillers as well as transfer of the applied stress from the matrix to the nanofillers. Figure 4 shows the effect of polyurethane (PUR) content on the stress intensity factor (KC) of an epoxy matrix containing 1 wt% CNT (a) and the KC of epoxy resin containing 1 wt% CNT and different amounts of Nanomer nanoclay. It can be seen that the addition of 3.75–7.5 wt% PUR did not affect the values of the KC factor, most probably due to the flexibilization of the epoxy achieved by the polymeric modifier. Furthermore, the incorporation of Nanomer nanoparticles did not lead to a hybrid nanocomposite with higher KC values compared to the binary epoxy nanocomposite containing 1 wt% CNT. These results are contrary to those for impact strength (Figures 2 and 3), although both IS and KC evaluate the resistance of a material to crack propagation. Figure 3. Effect of polyurethane content on the impact strength (IS) of epoxy resin modified with 1 wt% CNT. Figure 4shows the effect of polyurethane (PUR) content on the stress intensity factor (K C ) of an epoxy matrix containing 1 wt% CNT (a) and the K C of epoxy resin containing 1 wt% CNT and different amounts of Nanomer nanoclay. It can be seen that the addition of 3.75–7.5 wt% PUR did not affect the values of the K C factor, most probably due to the flexibilization of the epoxy achieved by the polymeric modifier. Furthermore, the incorporation of Nanomer nanoparticles did not lead to a hybrid nanocomposite with higher K C values compared to the binary epoxy nanocomposite containing 1 wt% CNT. These results are contrary to those for impact strength (Figures 2and 3), although both IS and KCevaluate the resistance of a material to crack propagation. Polymers 2025, 17, x FOR PEER REVIEW 8 of 20 Figure 4. Stress intensity factor (KC) of epoxy matrix modified with 1 wt% CNT, as a function of polyurethane content (a) and Nanomer content (b). As the KC did not demonstrate any improvement in the resistance to crack propagation in the tested samples, the fracture energy, which takes into account the contribution of stress and deformation, will be used instead. The effect of Nanomer I.28E nanoclay content on the fracture energy of epoxy resin containing 1 wt% CNT is presented in Figure 5. The fracture energy was determined from the area under the load–deflection curve obtained during the crack propagation test and the evaluation of the critical stress intensity factor (KC). The energy required to fracture the hybrid epoxy prepared with 1 wt% CNT and 0.5 wt% Nanomer is more than 15% higher than that required for the epoxy nanocomposite containing 1 wt% CNT and about 75% compared to the epoxy matrix with 0.5 wt% Nanomer. In addition, the fracture energy value of the hybrid nanocomposite was higher than that of the pure matrix by more than 37.5%, thus demonstrating the toughening effects of the two incorporated modifiers. The positive effect of the simultaneous use of Nanomer nanoclay and carbon nanotubes is clearly confirmed, and the synergistic effect compared to binary nanocomposites may come from the interfacial interaction between the nanoparticles, but is also due to that between the latter and the matrix. Figure 5. Fracture energy of epoxy resin modified with 1 wt% CNT as function of Nanomer nanoclay content. Figure 4. Stress intensity factor (K C ) of epoxy matrix modified with 1 wt% CNT, as a function of polyurethane content (a) and Nanomer content (b). As the K C did not demonstrate any improvement in the resistance to crack propagation in the tested samples, the fracture energy, which takes into account the contribution of stress and deformation, will be used instead. The effect of Nanomer I.28E nanoclay content on the fracture energy of epoxy resin containing 1 wt% CNT is presented in Figure 5. The fracture energy was determined from the area under the load–deflection curve obtained during the crack propagation test and the evaluation of the critical stress intensity factor (K C ). The energy required to fracture the hybrid epoxy prepared with 1 wt% CNT and 0.5 wt% Nanomer is more than ~15% higher than that required for the epoxy nanocomposite containing 1 wt% CNT and about ~75% Polymers 2025,17, 158 8 of 19 compared to the epoxy matrix with 0.5 wt% Nanomer. In addition, the fracture energy value of the hybrid nanocomposite was higher than that of the pure matrix by more than 37.5%, thus demonstrating the toughening effects of the two incorporated modifiers. The positive effect of the simultaneous use of Nanomer nanoclay and carbon nanotubes is clearly confirmed, and the synergistic effect compared to binary nanocomposites may come from the interfacial interaction between the nanoparticles, but is also due to that between the latter and the matrix. Polymers 2025, 17, x FOR PEER REVIEW 8 of 20 Figure 4. Stress intensity factor (KC) of epoxy matrix modified with 1 wt% CNT, as a function of polyurethane content (a) and Nanomer content (b). As the KC did not demonstrate any improvement in the resistance to crack propagation in the tested samples, the fracture energy, which takes into account the contribution of stress and deformation, will be used instead. The effect of Nanomer I.28E nanoclay content on the fracture energy of epoxy resin containing 1 wt% CNT is presented in Figure 5. The fracture energy was determined from the area under the load–deflection curve obtained during the crack propagation test and the evaluation of the critical stress intensity factor (KC). The energy required to fracture the hybrid epoxy prepared with 1 wt% CNT and 0.5 wt% Nanomer is more than 15% higher than that required for the epoxy nanocomposite containing 1 wt% CNT and about 75% compared to the epoxy matrix with 0.5 wt% Nanomer. In addition, the fracture energy value of the hybrid nanocomposite was higher than that of the pure matrix by more than 37.5%, thus demonstrating the toughening effects of the two incorporated modifiers. The positive effect of the simultaneous use of Nanomer nanoclay and carbon nanotubes is clearly confirmed, and the synergistic effect compared to binary nanocomposites may come from the interfacial interaction between the nanoparticles, but is also due to that between the latter and the matrix. Figure 5. Fracture energy of epoxy resin modified with 1 wt% CNT as function of Nanomer nanoclay content. Figure 5. Fracture energy of epoxy resin modified with 1 wt% CNT as function of Nanomer nanoclay content. The fracture energy of epoxy resin containing 1 wt% CNT is shown in Figure 6 as a function of polyurethane (PUR) content. The energy increased from the 4 kJ/m2 of the unmodified epoxy matrix to 7.2 kJ/m 2 for the hybrid epoxy nanocomposite based on 7.5 wt% PU and 1 wt% CNT, which represents an 80% improvement in energy. Nevertheless, a significant energy improvement of ~60% was obtained with 5 wt% PU/1 wt% CNT hybrid. Polymers 2025, 17, x FOR PEER REVIEW 9 of 20 The fracture energy of epoxy resin containing 1 wt% CNT is shown in Figure 6 as a function of polyurethane (PUR) content. The energy increased from the 4 kJ/m2 of the unmodified epoxy matrix to 7.2 kJ/m2 for the hybrid epoxy nanocomposite based on 7.5 wt% PU and 1 wt% CNT, which represents an 80% improvement in energy. Nevertheless, a significant energy improvement of  60% was obtained with 5 wt% PU/ 1 wt% CNT hybrid. The fracture energy enhancement might be attributed to the presence of the flexible chain of the polymeric modifier (PUR), with which more free volume is created, leading to a more flexible hybrid nanocomposite sample, and a higher energy is required to break the sample. Similar results were reported with epoxy/polyurethane-based nanocomposites [36,53,54]. Figure 6. Fracture energy of epoxy resin modified with 1 wt% CNT as function of polyurethane content. Table 1 presents the values of the critical stress intensity factor (KC) and the fracture energy of epoxy/CNT nanocomposites modified with different amounts of PUR ( Table 1a) and Nanomer (Table 1b). It can be seen that the incorporation of polyurethane did not affect the fracture toughness parameter (KC), most probably due to the flexibilization of the epoxy matrix and the associated lower value of stress-at-break. However, the fracture energy, which is calculated from the contributions of the stress and the relative elongation, is a more appropriate metric to use to define the resistance to crack propagation in the samples. The brittle fracture energy is nothing other than the GC for materials obeying the law of linear elastic fracture mechanics or the JC for materials whose behavior is nonlinear [55]. A similar trend was demonstrated by the KC of the epoxy/CNT/Nanomer ternary nanocomposites (Table 1b). Table 1. Values of critical stress intensity factor (KC) and brittle fracture energy of epoxy/CNT nanocomposites based on polyurethane content and (a) Nanomer nanoclay content (b). PUR Content (wt%) KC MPa.m0.5 Brittle Fracture Energy (kJ/m2) Nanomer Content (wt%) KC MPa.m0.5 Brittle Fracture Energy (kJ/m2) 0/0 2.8 4.0 0/0 2.5 4.0 0 3.2 4.8 0 3.5 4.8 3.75% 3.3 6.2 0.5 3.3 5.5 5% 2.9 6.4 1 2.2 2.3 7.50% 3.2 7.2 2 2.3 2.4 10% 1.8 2.7 a b Figure 6. Fracture energy of epoxy resin modified with 1 wt% CNT as function of polyurethane content. The fracture energy enhancement might be attributed to the presence of the flexible chain of the polymeric modifier (PUR), with which more free volume is created, leading to a more flexible hybrid nanocomposite sample, and a higher energy is required to break the sample. Similar results were reported with epoxy/polyurethane-based nanocomposites [36,53,54]. Table 1presents the values of the critical stress intensity factor (K C ) and the fracture energy of epoxy/CNT nanocomposites modified with different amounts of PUR (Table 1a) Polymers 2025,17, 158 9 of 19 and Nanomer (Table 1b). It can be seen that the incorporation of polyurethane did not affect the fracture toughness parameter (K C ), most probably due to the flexibilization of the epoxy matrix and the associated lower value of stress-at-break. However, the fracture energy, which is calculated from the contributions of the stress and the relative elongation, is a more appropriate metric to use to define the resistance to crack propagation in the samples. The brittle fracture energy is nothing other than the G C for materials obeying the law of linear elastic fracture mechanics or the J C for materials whose behavior is nonlinear [ 55 ]. A similar trend was demonstrated by the K C of the epoxy/CNT/Nanomer ternary nanocomposites (Table 1b). Table 1. Values of critical stress intensity factor (K C ) and brittle fracture energy of epoxy/CNT nanocomposites based on polyurethane content and (a) Nanomer nanoclay content (b). PUR Content (wt%) KC MPa.m0.5 Brittle Fracture Energy (kJ/m2) Nanomer Content (wt%) KC MPa.m0.5 Brittle Fracture Energy (kJ/m2) 0/0 2.8 4.0 0/0 2.5 4.0 0 3.2 4.8 0 3.5 4.8 3.75% 3.3 6.2 0.5 3.3 5.5 5% 2.9 6.4 1 2.2 2.3 7.50% 3.2 7.2 22.3 2.4 10% 1.8 2.7 a b Table 2presents the values of flexural strength and flexural energy-to-break of epoxy resin modified with 1 wt% CNT and different amounts of Nanomer (Table 2a) or polyurethane content (Table 2b). The addition of 0.5 − 2 wt% Nanomer 3.75 − 10 wt% did not induce any improvement in flexural strength compared to the epoxy/1 wt% CNT system. Similar results were reported in other studies [ 56 – 58 ]. This was attributed to the stress concentration in particle agglomerates and/or the formation of micro-voids, which act as weak points in the composites. Moreover, poor interactions or a lack of interactions between nanoparticles and a matrix may contribute to decrease in flexural strength. Interestingly, the trend in flexural strength is similar to that of the K C parameter (Table 1) and can therefore be used to explain the decrease in the latter. Indeed, as expressed in Equation (1), the K C which was evaluated under flexure is directly proportional to the applied flexural load. Table 2. Flexural strength and flexural and flexural energy-to-break of hybrid epoxy resin containing 1 wt% CNT and different amounts of Nanomer (a) or polyurethane (b). Nanomer Content (wt%) Flexural Strength (MPa) Flexural Energy to Break (kJ/m2) Polyurethane Content (wt%) Flexural Strength (MPa) Flexural Energy to Break (kJ/m2) 0/0 73.0 17.5 0/0 73.0 17.5 0 106.0 14.5 0 70.0 14.5 0,5 94.9 15. 8 3.75 71.0 14.1 1 74.7 6.3 5 67.9 10.6 268.4 4.4 7.5 71.7 18.6 10 42.0 5.9 a b The flexural strain-at-break of the epoxy resin filled with 1 wt% CNT is shown in Figure 7as a function of the Nanomer nanoclay content. The strain-at-break increased Polymers 2025,17, 158 16 of 19 4. Conclusions The following conclusions were drawn from the results obtained: • This work confirmed the successful preparation of epoxy hybrid composites with improved mechanical properties: impact strength and brittle fracture energy were significantly increased compared to pure resin and binary systems. Indeed, the following synergistic effects have been obtained: - The impact strengths of epoxy hybrid composites containing 1 wt% CNT and 5 wt% or 7.5 wt% of polyurethane and 0.5 wt% Nanomer and 1 wt% CNT increased by approximately 140%, 155%, and 70%, respectively; - The fracture energy for hybrid nanocomposites modified with PUR/CNT and Nanomer/CNT systems increased by approximately 80% and 50%, respectively; - The significantly improved mechanical and thermal properties of epoxy hybrids can be attributed to the uniform dispersion of modifiers in the matrix and the interfacial interactions between the ingredients. It is essential to create as many interactions as possible between the matrix and the modifiers, and within the latter, to eliminate agglomerates in order to generate the optimal mechanical properties of the hybrids produced. • The addition of polyurethane and nanomodifiers increased the thermal stability of epoxy composites. The addition of flexible polyurethane chains increased the glass transition temperature as well as the softening point and the temperature range of use of the epoxy nanocomposites containing nanofillers. • Interactions between the epoxy matrix and the added modifiers through the O-H groups of the epoxy resin with PUR and CNT were confirmed. • Hybrid epoxy composites with improved performance properties can be safely used in harsh environments or used as advanced composite materials in the aerospace and construction industries. Author Contributions: Conceptualization, P.S.; Methodology, M.M.; Formal analysis, P.S.; Investigation, P.S., A.B., M.B., B.H., M.M. and D.K.; Data curation, P.S. and B.H.; Writing – original draft, M.B.; Writing – review & editing, A.B.; Visualization, D.K.; Supervision, A.B. All authors have read and agreed to the published version of the manuscript. Funding: The authors B.H. and M.M. from the Centre of Polymer Systems acknowledge the support from the Ministry of Education, Youth and Sports of the Czech Republic within the project DKRVO (RP/CPS/2024–28/007). Institutional Review Board Statement: Not applicable. Data Availability Statement: The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. References 1. Szeluga, U.; Kumanek, B.; Trzebicka, B. Synergy in hybrid polymer/nanocarbon composites. A review. Compos. Part A Appl. Sci. Manuf. 2015,73, 204–231. [CrossRef] 2. Safdari, M.; Al-Haik, M.S. A review on polymeric nanocomposites: Effect of hybridization and synergy on electrical properties. In Carbon-Based Polymer Nanocomposites for Environmental and Energy Applications; Elsevier: Amsterdam, The Netherlands, 2018; pp. 113–146. 3. Kumar, A.; Sharma, K.; Dixit, A.R. 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