Introduction

Clay is widely utilized in road engineering1 and foundation construction2. Its loose structure leads to significant shrinkage and expansion, causing instability3. Traditional stabilization relies on mechanical compaction, which is noisy and slow4,5,6. In contrast, polymer-based chemical curing offers less curing time and enhanced stability7. Epoxy resins, essential thermosetting polymers in engineering, have found extensive applications in clay chemical curing8,9,10. Epoxy resins are low-molecular-weight prepolymers containing two or more epoxy groups11. Structurally, epoxy resins mainly include glycidyl ethers, glycidyl esters, glycidyl amines, aliphatic, and cycloaliphatic epoxy resins. Due to their excellent stability and curing properties, glycidyl ether epoxy resins like diglycidyl ether of bisphenol-A epoxy resins (DGEBA), diglycidyl ether 4,4’-dihydroxy diphenyl sulfone (DGEDDS), and Aliphatic epoxidation of olefin resin (AEOR) are often utilized as curing agents12,13,14,15. Most research on the curing of clay with polymers has primarily focused on experimental studies. The research on clay-polymer intercalation can be traced back to the 1980s16. Results in References17,18 indicate that laboratory-scale simulated aging tests show sand-fixing agent-poly and its composites enhance the compressive strength and wind erosion resistance of sand particles. The first study reporting the production of high-performance epoxy-clay nanocomposites was conducted by Kornmann et al.19. The results in References20,21 show that the macroscopic mechanical properties of epoxy-clay nanocomposites strongly depend on the type of nanocomposite structure. Current research has shown that most studies concentrate on macroscopic mechanical properties22. However, the nanoscale aspects of clay cured by epoxy resins are poorly studied, and the mechanisms remain unclear. The influence of nanoscale structure on adsorption behavior and mechanical properties is not well understood. Since the macroscopic properties of cured clay are largely determined by its nanoscale structure, investigating the nanoscale curing mechanisms of epoxy resins is crucial for their engineering applications.

Molecular dynamics simulation is one of the effective methods to reveal the nanoscale mechanisms of materials, offering a detailed perspective on atomic interactions23,24,25,26. It has been applied extensively in materials science to study the structural and mechanical properties of materials27,28. Molecular dynamics simulations have been utilized to quantitatively explore interactions between various materials at the nanoscale in previous studies10,29,30,31. These have provided theoretical foundations for understanding the binding mechanisms of surface-reactive groups, synthesis processes, and mechanical properties of materials. Therefore, employing molecular dynamics simulation to study the nanoscale mechanisms of clay cured by epoxy resins serves as a theoretical basis for determining the curing type and contents of curing agents.

Adsorption behavior is a crucial step in the process of clay cured by epoxy resins, determining the initial state of molecular motion within the process and serving as a key to understanding the curing mechanisms. Three essential parameters that delineate adsorption behavior are the quantity of interface hydrogen bonds, the adsorption energy and the radius of gyration. When in contact with soil particles, the polymer adheres to their surfaces due to electrostatic forces32, resulting in interparticle interactions, such as hydrogen bonds33, which substantially influence the effectiveness and performance of soil curing. Adsorption energy can be utilized to evaluate the strength and efficiency of interactions between various substances on mineral surfaces34. The radius of gyration is a parameter that describes the extent of a molecule’s expansion in space, providing insights into how the polymer adsorbs and unfolds on the soil surface35. In previous studies36,37,38, molecular dynamics simulation methods were employed to investigate the adsorption behavior between various organic materials and clay. These studies reveal that Interactions between substances are key factors affecting the stability and mechanical properties of clay composite materials. While the mentioned investigation on clay adsorption behavior mainly focuses on organic materials without epoxy groups, the nanoscale adsorption behavior in the field of clay cured by epoxy resins is still not understood. Therefore, conducting relevant research on the adsorption behavior of clay cured by epoxy resins is necessary.

Mechanical properties directly affect the stability and bearing capacity of clay and are crucial for assessing the effectiveness of clay curing. Mechanical parameters such as bulk modulus, shear modulus, and Young’s modulus characterize the response and deformation characteristics of soil structures under different types of stress and are key mechanical performance parameters to evaluate clay solidification effectiveness. Molecular dynamics simulation has been utilized to investigate the bulk modulus and shear modulus of clay composites under different environmental conditions in many previous studies39,40. However, due to differences in the study subjects, the obtained results cannot be directly applied to clay cured by epoxy resins. Moreover, the elastic constants of the nanoscale structure in clay cured by epoxy resins are lacking. Therefore, it is essential to analyze the mechanical properties of composite molecular systems comprising epoxy resins and clay to evaluate the curing effects of different epoxy resins.

Epoxy resins have been widely employed in curing clay. Relevant macroscopic and microscopic experiments have been investigated in previous studies41,42. However, the nanoscale curing mechanisms of clay from the atomic perspective are still not understood. Molecular models of epoxy resins and clay were constructed. Based on the optimized molecular models, the composite molecular systems between different types of epoxy resins and clay were constructed for molecular dynamics simulation. Through analyzing the quantity of interface hydrogen bonds and adsorption energy of composite molecular systems, a nanoscale perspective on the adsorption behavior between epoxy resins and clay was provided. Furthermore, calculations were conducted to analyze the mechanical properties of the composite molecular systems. This study aimed to construct molecular interface models of epoxy resins and clay and to investigate their curing mechanisms using molecular dynamics simulations. It provided essential insights for enhancing clay stability and performance, establishing a theoretical foundation for effective curing methods and optimal curing agent content.

Simulation method

Molecular models of epoxy resins

In this study, DGEBA, DGEDDS, and AEOR were utilized in the simulation. The molecular models of epoxy resins were constructed in Materials Studio software. Considering the practical engineering, the content of epoxy resin soil stabilizers is typically calculated by relative weight (the ratio of epoxy resin weight to clay weight)38. Therefore, the molecular weights of three epoxy resins are as close as possible while establishing the molecular models of epoxy resins. The parameters for the three epoxy resins are shown in Table 1. The structural formula is shown in Fig. 1a and molecular model of DGEBA is shown in Fig. 1b.

Table 1 Parameters for three epoxy resins.
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Fig. 1
figure 1

Structural formula and molecular model of DGEBA. (a) Structural formula; (b) molecular model.

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DGEDDS is a common type of glycidyl ether-type epoxy resin42. The substitution of the isopropyl group in DGEBA with the strongly polar sulfonyl group leads to the formation of DGDDS. The structural formula is shown in Fig. 2a and molecular model of DGEDDS is shown in Fig. 2b.

Fig. 2
figure 2

Structural formula and molecular model of DGEBS. (a) Structural formula; (b) molecular model.

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AEOR is characterized by a linear macromolecular structure, consisting of aliphatic chains in its molecular structure, devoid of benzene rings, cyclohexane rings, or heterocycles43. Its structure combines elements of both polybutadiene rubber and epoxy copolymer, resulting in its enhanced impact toughness and adhesive properties. The structural formula is shown in Fig. 3a and molecular model of AEOR is shown in Fig. 3b.

Fig. 3
figure 3

Structural formula and molecular model of AEOR. (a) Structural formula; (b) Molecular model.

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Geometry optimization was applied to the initial structures to obtain the geometry optimized configuration, as reported in reference44. This was done to ensure that the initial structures of the simulation system were stable and reasonable.

Molecular model of clay

In this study, kaolinite was utilized in the simulation, primarily composed of kaolinite minerals with the molecular formula [Si2O4][Al2O(OH)4]. It is a layered silicate clay mineral, constituting approximately 10 – 95% of kaolinite minerals45,46. Previous study has shown that a single-layer kaolinite crystal structure model effectively represents the interaction between the kaolinite surface and organic molecules33. Thus, a single-layer kaolinite crystal structure model was utilized in this study to construct the interface models between kaolinite surface and epoxy resins. The crystal structure model of the single-layer kaolinite (0 0 1) is shown in Fig. 4.

Fig. 4
figure 4

Crystal structure model of single-layer kaolinite (0 0 1).

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Based on the kaolinite unit cell model, a supercell model was constructed, and its (0 0 1) surface was cleaved, followed by the creation of a vacuum slab with a thickness of 30 Å. The total atoms of kaolinite were 2040. Additionally, to protect the model structure against boundary conditions effects, periodic boundary conditions were utilized. This was done to ensure that interactions between molecules within the system were not affected by boundary molecules, thus preventing any unreasonable simulation results. The crystal structure molecular model of kaolinite is shown in Fig. 5.

Fig. 5
figure 5

Crystal structure model of kaolinite.

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Based on the models of the three epoxy resins and kaolinite that have been constructed, interface models between epoxy resins and kaolinite were established. The interface model formed between DGEBA and kaolinite was referred to as DGEBA-Kaolinite. Similarly, the interface model formed between DGEDDS or AEOR and kaolinite was referred to as DGEDDS-Kaolinite or AEOR-Kaolinite, respectively. Three interface models are shown in Fig. 6.

Fig. 6
figure 6

Interface models of epoxy resin and kaolinite. (a) DGEBA-Kaolinite, (b) DGEDDS-Kaolinite, (c) AEOR-Kaolinite.

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Simulation details

The molecular models of epoxy resins and kaolinite were constructed in Material Studio software. The simulation was carried out employing the Universal Force Field (UFF), a commonly employed classical force field within molecular dynamics simulations, relying on empirical potential functions for atomic interactions47. UFF is usually applied to describe atomic interactions in molecules and encompasses parameters for almost all elements in the periodic table. UFF is known for its versatility, as it can be effectively utilized for various organic and inorganic compounds35. Furthermore, it has the capability to predict the physical properties and chemical reactions of molecules based on their geometric structure and elemental composition48.

In this study, the canonical NVT ensemble was employed with 298 K temperature, and the total simulation duration lasted for 10 ns. The initial velocities of system atoms were randomly assigned in accordance with the Maxwell–Boltzmann distribution at the initial temperature. When the total energy of system converged, the system reached an equilibrium state. To prevent being confined within a singular potential well, three different epoxy resins were placed at least 5 Å above the initial kaolinite molecular model. At this position, the analysis of adsorption energies and interface hydrogen bonds for all configurations utilized the default values of the software (2.5 Å as the maximum hydrogen-receptor distance and 90° as the minimum donor-hydrogen-acceptor angle). The flowchart of main algorithm for simulation was shown in Fig. 7.

Fig. 7
figure 7

Flowchart of main algorithm of simulation.

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Results and discussion

Adsorption behavior analysis

Hydrogen bonds

The hydrogen bond is a special type of chemical bond that constitutes weak interaction between certain molecules, playing a significant role in adsorption processes. The quantity of hydrogen bonds formed between epoxy resins and the surface of kaolinite was calculated, as shown in Fig. 8.

Fig. 8
figure 8

Hydrogen bonds formed between epoxy resin and kaolinite surface (Hydrogen bonds are shown as light blue dashed lines). (a) DGEBA-Kaolinite; (b) DGEDDS-Kaolinite; (c) AEOR-Kaolinite.

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The results indicated that the quantity of hydrogen bonds within the lowest energy conformations of DGEBA-Kaolinite, DGEDDS-Kaolinite, and AEOR-Kaolinite was 7, 5, and 9, respectively. The quantity of interface hydrogen bonds formation was influenced by the reactant content. Therefore, to provide a more reasonable comparison of the adsorption efficiency of the three epoxy resins on kaolinite mineral surfaces, the concept of hydrogen bonds per unit molecular weight of the epoxy resin(\(N_{{{\text{unit}}}}\)) was calculated by Eq. (1), as reported in reference38:

$$N_{{{\text{unit}}}} = \frac{{N_{{{\text{surface}}}} }}{m}$$
(1)

where \(N_{{{\text{surface}}}}\) is the quantity of hydrogen bonds generated on kaolinite surface; \(m\) is molecular weight of the epoxy resin. The hydrogen bonds of DGEBA-Kaolinite, DGEDDS-Kaolinite, and AEOR-Kaolinite at unit molecular weight of epoxy resin are displayed in Table 2.

Table 2 The quantity of hydrogen bonds formed between epoxy resins and the kaolinite surface per unit of relative molecular weight.
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The results showed that per molecular weight of AEOR formed the highest quantity among the three epoxy resins, surpassing DGEBA and DGEDDS by 18.2% and 59.1%, respectively. A higher quantity of hydrogen bonds contributed to the enhanced stability of the clay. DGEBA, DGEDDS, and AEOR molecules formed hydrogen bonds with the hydrogen atoms on the surface of kaolinite through their oxygen atoms. The hydrogen bond locations were concentrated at the terminal groups of DGEBA, DGEDDS, and AEOR, which typically contained hydroxyl (-OH) or ether (-O-) groups. Additionally, the oxygen atoms in the sulfonyl group (O = S = O) of DGEDDS, due to their high electronegativity, were prone to form strong hydrogen bonds with the hydrogen atoms on the surface of kaolinite. The chain-like structure of AEOR allowed it to cover a larger surface area and form more hydrogen bonds. Compared to rigid or cyclic molecules, chain-like AEOR were more easily conformed on the kaolinite surface, thereby increasing the area of their interaction with its surface. The flexibility of the molecular chains implied that AEOR molecules could better adjust themselves, allowing multiple functional groups to come closer to the hydrogen-bond-forming sites on the kaolinite surface, thereby increasing the number of hydrogen bonds.

Adsorption energy

In molecular dynamics simulation, interaction energy is a crucial physical quantity utilized to describe the binding strength between interacting molecules. Therefore, it could be utilized to assess the intensity of intermolecular interaction forces and the stability of composites34. Since the interaction energy in complexes tends to bring molecules closer together, the formation of complexes typically results in the release of energy, making the interaction energy a negative value. The reciprocal of interaction energy is referred to as adsorption energy, which is a relative measure utilized to compare the stability and strength of interactions between different systems, helping to study the effects of intermolecular interactions35,38. A larger adsorption energy indicates a stronger interaction and greater affinity between epoxy resins and kaolinite. In molecular dynamics simulations, force fields are commonly utilized to describe intermolecular interactions, which include bond energy terms, bond angle energy terms, and non-bonded interaction terms. The adsorption energies were calculated by Eq. (2), as reported in reference35:

$$E_{{\text{int}}} = E_{{{\text{total}}}} - \left( {E_{{{\text{epoxy}}}} { + }E_{{{\text{kao}}}} } \right) = - E_{{{\text{ads}}}}$$
(2)

where \(E_{{{\text{total}}}}\) represents the total energy of the entire system, containing all atoms, in \({\text{kcal}} \cdot {\text{mol}}^{ - 1}\);\(E_{{{\text{epoxy}}}}\) represents the configuration energy of epoxy resin molecules, in \({\text{kcal}} \cdot {\text{mol}}^{ - 1}\); \(E_{{{\text{kao}}}}\) is the energy associated with the kaolinite, in \({\text{kcal}} \cdot {\text{mol}}^{ - 1}\); \(E_{{{\text{ads}}}}\) represents the adsorption energy between epoxy resin and kaolinite, in \({\text{kcal}} \cdot {\text{mol}}^{ - 1}\), which dictates the energy necessary to disrupt the epoxy resin and kaolinite interaction. At the same time, the value also reflects the strength of adhesion at the interface and characterizes the adhesive strength between epoxy resins and kaolinite38. Figure 9 shows the variation curves of adsorption energies for the three interface models of epoxy resins and kaolinite, while Fig. 10 presents the corresponding adsorption energies.

Fig. 9
figure 9

Energies change of three interface models in 10 ns.

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Fig. 10
figure 10

Adsorption energies of the three epoxy resins and kaolinite.

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The adsorption energies of three interface models of epoxy resins and kaolinite stabilized within 4.0 ns and were in a fluctuating state around equilibrium. AEOR showed the largest fluctuations in energy compared to DGEBA and DGEDDS. This could indicate more dynamic interactions on the kaolinite surface, due to the flexibility of AEOR’s chain structure, allowing it to adapt more dynamically to the kaolinite surface. The results of adsorption energies for DGEBA, DGEDDS, and AEOR with kaolinite were 92.59, 98.25, and 116.87 \({\text{kcal}} \cdot {\text{mol}}^{ - 1}\), respectively. The results indicated that AEOR exhibited the highest adsorption energy with kaolinite. This suggested the strongest affinity between AEOR and kaolinite among the three epoxy resins, which was consistent with the hydrogen bond formation results.

Although DGEBA formed more hydrogen bonds with the kaolinite surface than DGEDDS, DGEDDS exhibited greater adsorption energy. This is due to the higher electronegativity of the oxygen atoms in the sulfonyl group (O = S = O) of DGEDDS. These highly polar sulfonyl groups could interact strongly with the kaolinite surface through electrostatics. The relatively high surface energy of DGEDDS indicates a strong interaction force with kaolinite, resulting in stronger hydrogen bonds with the kaolinite surface hydrogen atoms and increased adsorption energy. This enhanced intermolecular interaction makes DGEDDS more stable on the surface.

Radius of gyration

The spatial dimensions of a molecule could be defined through the parameter known as its radius of gyration (\(R_{g}\)), which was calculated by Eq. (3). The \(R_{g}\) of DGEBA, DGEDDS, and AEOR on the kaolinite surface in the lowest energy conformations are shown in Fig. 11.

$$R_{g} = \sqrt {\frac{{\sum\nolimits_{i} {\left\| {r_{i} } \right\|^{2} m_{i} } }}{{\sum\nolimits_{i} {m_{i} } }}}$$
(3)

where \(m_{i}\) is the mass of the atom \(i\), \(r_{i}\) is considered alongside the position of the atom \(i\), in relation to the center of mass of the molecule.

Fig. 11
figure 11

The radius of gyration of DGEBA,DGEDDS and AEOR at the kaolinite surface. (a) DGEBA; (b) DGEDDS; (c) AEOR.

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At the beginning of the simulation, DGEBA approached and adsorbed onto the surface of kaolinite rapidly. This caused the conformation to quickly transition from an extended state to a compact state, resulting in the \(R_{g}\) decreasing sharply from nearly 1.30 to 0.70 nm. After approximately 2 ns, the \(R_{g}\) gradually stabilized, with the amplitude of fluctuation decreasing and remaining between 0.50 and 0.60 nm. This stable state indicated that DGEBA had predominantly adsorbed onto the kaolinite surface and formed a relatively stable conformation.

DGEDDS rapidly approached and partially adsorbed onto the kaolinite surface within the first 1 ns of the simulation, forming a more compact conformation. Consequently, the \(R_{g}\) decreased sharply within the first 1 ns. After 2 ns, the adsorption state of DGEDDS on the kaolinite surface tended to stabilize, with the \(R_{g}\) remaining in the range of 1.03 to 1.08 nm. Compared to DGEBA, the stabilized \(R_{g}\) for DGEDDS on the kaolinite surface was larger, indicating that its adsorption efficiency was slightly lower than that of DGEBA.

Compared to DGEBA and DGEDDS, AEOR, due to its chain-like structure, exhibited higher conformational flexibility and a larger contact area. This characteristic allowed AEOR to maintain a certain degree of conformational freedom and undergo frequent conformational changes during the adsorption process with kaolinite, resulting in significant fluctuations in the \(R_{g}\).

Mechanical properties analysis

Mechanical property simulations and analysis were performed on the optimized configurations. Under constant temperature \(T\), the relationship between the elastic stiffness coefficient \(C_{ijkl}\), stress \(\sigma_{ij}\), and strain \(\varepsilon_{kl}\) could be described by Eq. (4), as reported in reference35:

$$C_{ijkl} = \left. {\frac{{\partial \sigma_{ij} }}{{\partial \varepsilon_{kl} }}} \right|_{{T,\varepsilon_{kl} }} = \frac{1}{{V_{0} }}\left. {\frac{{\partial^{2} A}}{{\partial \varepsilon_{ij} \partial \varepsilon_{kl} }}} \right|_{{T,\varepsilon_{ij} ,\varepsilon_{kl} }}$$
(4)

where \(V_{0}\) represents the volume of the simulation cell the undeformed configuration, in \({\text{m}}^{{3}}\), and \(A\) represents the Helmholtz free energy, in \(J\).

Based on the elastic tensor, the Voigt -Reuss-Hill (VRH) approximation40,49 was utilized to calculate the bulk modulus and shear modulus of the systems in this study. Assuming each component has the same strain as the polycrystal, Voigt50 denotes the polycrystal’s bulk modulus (\(K_{V}\)) and shear modulus (\(G_{V}\)) as Eqs. (5) and (6). Reuss51, on the other hand, assumes the same stress and denotes the polycrystal’s bulk modulus (\(K_{R}\)) and shear modulus (\(G_{R}\)) as Eqs. (7) and (8). Hill49 indicates that the Voigt and Reuss models can serve as the respective upper and lower bounds for the average elastic modulus in polycrystalline materials. Consequently, the Eqs. (9) and (10) define the bulk modulus (\(K\)) and shear modulus (\(G\)) as the arithmetic averages of the Voigt and Reuss models.

$$K_{V} = \frac{1}{9}\left[ {C_{11} + C_{33} + 2(C_{21} + C_{13} + C_{23} )} \right]$$
(5)
$$G_{V} = \frac{1}{15}\left[ {C_{11} + C_{22} + C_{33} + 2(C_{44} + C_{55} + C_{66} ) - (C_{12} + C_{13} + C_{23} )} \right]$$
(6)
$$K_{R} = \left[ {S_{11} + S_{22} + S_{33} + 2(S_{12} + S_{13} + S_{23} )} \right]^{ - 1}$$
(7)
$$G_{R} = 15\left[ {4(S_{11} + S_{22} + S_{33} - S_{12} - S_{13} - S_{23} ) + 3(S_{44} + S_{55} + S_{66} )} \right]^{ - 1}$$
(8)
$$K = \frac{{K_{V} + K_{R} }}{2}$$
(9)
$$G = \frac{{G_{V} + G_{R} }}{2}$$
(10)

where subscripts \(V\) and \(R\) are the Reuss and Voigt averages, respectively. \(S_{ij}\) is the elastic compliance matrix components obtained from the inverse of the elastic stiffness matrix, in \(m^{2} \cdot N\).

The calculated results of the bulk modulus and shear modulus of different interface models are shown in Table 3. The mechanical properties of clay have been investigated in previous studies52,53. The simulated bulk modulus in the previous study ranges from 38.00 to 80.00 GPa under different conditions52, and the bulk modulus in this study fell within the range of 47.00 to 50.00 GPa, showing good agreement with the results of the previous study. Furthermore, the simulated bulk modulus of kaolinite in this study was 47.42 GPa, which was consistent with the experimental bulk modulus value of well-crystallized kaolinite reported in previous studies, which is 47.90 ± 8.00 GPa53. The values of the shear modulus obtained in this study are close to those in previous study52, which ranges from 18.00 to 42.00 GPa.

Table 3 Bulk modulus and shear modulus of interface models.
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The results of the Hill values of bulk modulus and shear modulus demonstrated that the addition of all three epoxy resins increased the compressive and shear deformation resistance of kaolinite. Consistent with the results of hydrogen bonds and adsorption energy, the bulk modulus of AEOR-Kaolinite was the highest among the four systems, being 4.93% higher than that of kaolinite. At the same time, the shear modulus of AEOR-Kaolinite was the highest among the four systems, and it was 4.80% higher than that of kaolinite. Hydrogen bonds are able to enhance the bonding force between epoxy resin and kaolinite, making the interface more stable. High adsorption energy indicates strong adsorption and interaction between the epoxy resin and kaolinite, which contributes to the improvement of the interface’s stability and mechanical properties.

$$E = \frac{9G}{{3 + G/K}}$$
(11)

Young’s modulus (\(E\)) is one of the important parameters describing the mechanical properties of materials and is utilized to assess the stiffness of materials for linear elastic deformation when subjected to a force. Based on the calculations of bulk modulus (\(K\)) and shear modulus (\(G\)) as obtained above, Young’s modulus (\(E\)) can be defined by Eq. (11), as reported in reference49. The calculated results of the Young’s modulus of different interface models are shown in Table 4.

Table 4 Young’s modulus of interface models.
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The results indicated that three epoxy resins all effectively improved the Young’s modulus of kaolinite. Among the epoxy resins, DGEBA exhibited relatively weaker bonding with kaolinite. However, due to the structural characteristics and molecular arrangement of the interface in the Z-direction, a significant enhancement in the Young’s modulus in the Z-direction was still observed. DGEDDS, with its greater number of hydrogen bonds and moderate adsorption energy, showed stronger interface bonding, leading to improvements in Young’s modulus in the X and Y directions, while the increase in the Z-direction was slightly smaller compared to DGEBA. AEOR, due to its chain-like structure and the highest number of hydrogen bonds formed, demonstrated the strongest interface bonding and the highest adsorption energy. This resulted in a significant increase in the Young’s modulus in all directions, with the greatest enhancement observed in the Z-direction, which was related to its strong interface bonding and favorable molecular arrangement. Consistent with the trend in Young’s modulus improvement, the higher bulk modulus and shear modulus reflected the enhanced overall rigidity and resistance to deformation of the material after adsorption. In terms of mechanical properties, AEOR exhibited the most significant improvement among the three epoxy resins.

Conclusions

This study investigated the nanoscale models of DGEBA-Kaolinite, DGEDDS-Kaolinite, and AEOR-Kaolinite through molecular dynamics simulation, aiming to elucidate the curing mechanisms of kaolinite cured by epoxy resins. The primary findings were summarized as follows.

  1. 1.

    Compared to DGEBA and DGEDDS, AEOR, due to its chain-like structure, exhibited higher conformational flexibility and a larger contact area, resulting in significant fluctuations in the radius of gyration. This characteristic allowed AEOR to maintain a certain degree of conformational freedom, undergo frequent conformational changes, and form more hydrogen bonds on the surface of kaolinite.

  2. 2.

    AEOR had the highest adsorption energy on the surface of kaolinite among the three epoxy resins, including DGEBA, DGEDDS and AEOR, indicating the strongest affinity. DGEDDS showed higher electronegativity in its sulfonyl group oxygen atoms, leading to the formation of stronger hydrogen bonds with kaolinite, thereby increasing adsorption energy.

  3. 3.

    The three epoxy resins enhanced the bulk and shear modulus of kaolinite, with AEOR showing the most significant improvement in both. Hydrogen bonds strengthen the bonding force between epoxy resin and kaolinite, stabilizing the interface. High adsorption energy indicates strong interaction, improving interface stability and mechanical properties.

  4. 4.

    The three epoxy resins improved Young’s modulus of kaolinite. DGEDDS had more hydrogen bonds and moderate adsorption energy, enhancing Young’s modulus in X and Y directions more than in the Z direction compared to DGEBA. AEOR, with its chain-like structure and the most hydrogen bonds, resulted in the strongest interface bonding and highest adsorption energy, increasing Young’s modulus in all directions, especially in the Z direction. Higher bulk modulus and shear modulus indicated increased rigidity and resistance to deformation after adsorption. AEOR had the most significant improvement in mechanical properties among the resins.