Molecular simulation research on the micro effect mechanism of interfacial properties of nano SiO2/meta-aramid fiber

The manufacturing requirements of aramid insulation paper is high, and Nomex paper. produced by DuPont Co., is representative of high performance aramid paper [1]. The full name of meta aramid is poly-m-phenyleneisophthalamide (PMIA), and the aramid fiber has excellent properties such as high strength, high modulus, high heat resistance and light weigh [2,3]. However, with the electirc power system reform continues to move forward [4] and the increase in the running voltage and capacity of transformers, the requirements of insulation performance and reliability of the aramid insulation paper become higher [5]. Therefore, it is urgent to carry out further research to improve the performance of aramid insulation paper.

The practice of adding nano SiO₂ particles to the polymer is widely used as a physical modification method [6-9]. The mechanical properties and electrical properties of the films were studied by T. Dong [10] through hybridization of thin films by using PI/SiO₂. The analysis of wide angle X diffraction showed that the introduction of SiO₂ destroyed the ordered degree of the molecular arrangement of the polyimide. The tensile test showed that the tensile strength and elongation at break of the hybrid films reached the maximum when the content of spherical nano SiO₂ was 3 wt%. Meanwhile, when the content of stick shaped nano SiO₂ was 1 wt%, the tensile strength and elongation at break of the hybrid films reached the maximum. The modification of low density polyethylene was carried out by doping nano SiO₂, and the LDPE/SiO₂ was analyzed by using molecular simulation technology [11]. The results showed that the addition of 5 nm and 7 nm nanoparticles improved glass transition temperature and thermal stability.

Molecular simulation, based on the first principle [12, 13] and as a rapid developing research method, is now widely used, as Stephen W. Watt et al. [14] used molecular simulation to research the melting temperature and glass transition temperature of myo-inositol and neo-inositol. The results showed that the glass transition temperature was 217 ℃ and 245 ℃ respectively, and a small change in the molecular structure of the two molecules led to a sharp rise in the melting point. Many other scholars also used molecular simulation technology to carry out a number of research projects [15-17]. The interfacial properties of nanoparticles/polymer composites are an important index to reflect the properties of nano modified polymer materials [18-22]. Because the microscopic mechanism of interface is on the molecular level, it is difficult for traditional experimental methods to meet the experimental requirements. Molecular simulation technology can provide a new path to overcome the shortcomings of these traditional methods.

Therefore, in this paper, for the first time, the interface model of PMIA amorphous region and SiO₂ is constructed by molecular simulation technology, and the interface properties of the model are studied by molecular dynamics simulation. The interaction is researched through the energy change of interface between SiO₂ particles and PMIA fibers, and the microscopic mechanism of SiO₂ combined with PMIA is researched by analyzing the combination of nano SiO₂ and meta-aramid fiber. Furthermore, by studying the change of the mean square displacement, the effects of temperature and SiO₂ lattice on the thermal stability of aramid fibers are studied. The conclusion of this paper will provide some references for further research on the modification of nano SiO₂ to the aramid insulation paper

3.1 Interaction of SiO₂/PMIA

After molecular dynamics, the interface model of the system is shown in Figure 3. Through comparing Figure 2 and Figure 3, after molecular dynamics, significant changes have taken place in the position of the PMIA fiber molecules. In the dynamics process, the PMIA fiber molecules gradually move to the surface of SiO₂, which means the interaction is relatively strong between the interface of PMIA fiber and SiO₂.

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Figure 3. The interfaces of models after molecular dynamics

The energy of the interface model includes non-bond interaction energy, bond interaction energy and binding energy, and the non-bond interaction energy plays a leading role in the total energy of the system. Non-bond interaction energy is composed of hydrogen bond energy, electrostatic energy and Van Der Waals force energy which includes the dispersive force energy and the repulsive force energy. The interaction energy between the interface of the PMIA amorphous region and SiO₂ can be expressed by the following formula:

${{E}_{Interaction}}={{E}_{Total}}-({{E}_{PMIA}}+{{E}_{Si{{O}_{2}}}})$         (1)

where E_Interaction is the interaction energy between the interface, E_Total is the total energy of the system, E_PMIA is the energy of the surface of the PMIA after removing SiO₂, E_SiO2 is the surface energy of SiO₂ after removing PMIA. The negative value of the interaction energy is the binding energy, and is expressed by formula (2), which indicates that the larger the value is, the easier it is to combine.

${{E}_{Binding}}=-{{E}_{Interaction}}$           (2)

The interaction between nanoparticles and polymer molecules leads to the formation of interfacial interactions, and this interaction is mainly reflected in Van Der Waals force, which is called Van Der Waals force energy, as shown in formula (3).

${{E}_{Vdw}}={{E}_{repulsive}}+{{E}_{dispersive}}$            (3)

Due to the limited space, here only gives the statistical average of energy of the interface structure of Models A, C and E with temperature changes, and the results are shown in Tables 1, 2 and 3.

Table 1. Statistical average of energy of Model A (1 cal = 4.1868 J)

Table 2. Statistical average of energy of Model C

Table 3. Statistical average of energy of Model E

From the Tables 1, 2 and 3, we can see that E_Total of system are negative, which shows that the system is stable. The energy values are close to each other, and the fluctuation range is small with an increase in the temperature, which indicates that the influence of temperature on the interfacial interaction between the PMIA amorphous region and the SiO₂ is small. With an increase in temperature, each energy value increased in addition to the binding energy. This is because the kinetic energy increased with the increase in temperature. Furthermore, due to lower temperature, the combination was closer, and the interaction was stronger. So, the smaller the negative value of the interaction energy was, the greater the binding energy was. On the contrary, the higher the temperature was, the greater the kinetic energy of the molecule was, the less the combination degree was, the weaker the interaction was, and the less the interaction energy was, the smaller the binding energy was. Throughout the system, because the number of SiO₂ atoms is large and the number of PMIA is small, the proportion of E_SiO2 is larger, and it has a greater influence on E_Total. E_Revulsive is positive, and E_Dispersive is negative, so the dispersive force energy plays a lead role in Van Der Waals force energy.

In order to study the effect of interface size on the interfacial characteristics of the PMIA amorphous region and SiO₂ under the same temperature, 90℃ is used as an example, and the results are shown in Table 4.

Table 4. Statistical average of energy of different models at 90℃

In order to observe the changes of energy with the change of the size of lattice, according to the results of Table 4, Figure 4 and Fig. 5 are drawn:

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Figure 4. E_Total and E_SiO2 change with the size of lattice

Figure 4 shows that the trend of E_Total and E_SiO2 is the same, and with an increase in the lattice size of SiO₂, the two increase along the negative direction. At 90℃, the superlattice of Model E has increased by about 295.75% over that of the superlattice of Model A along the negative direction. 

Table 4 and Figure 5 show that with an increase in lattice size, E_Vdw, E_Dispersive and E_Interaction all have a significant increase trend along the negative direction. The dispersive force effect is the dominant effect on the interfacial interaction between the PMIA amorphous region and SiO₂ of non-bond interaction. With an increase in the lattice size, the interfacial interaction energy increases along the negative direction, which indicates that the interface between PMIA and SiO₂ is enhanced, and it is beneficial for the nano SiO₂ particles doping into PMIA fibers. Overall, the interfacial interaction energy is negative, the binding energy is positive, and with an increase in SiO₂ surface, the binding energy of the two has an increasing tendency, which is beneficial for the stable combination of SiO₂ particles and PMIA fibers. Moreover, the interfacial interaction between MPIA molecules and nano SiO₂ is related to the size of the interface. The binding energy of Model C is the largest, and its interfacial combination effect is the best.

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Figure 5. E_Vdw, E_Dispersive, E_Interaction, E_Interaction, E_Bindingof interface change with the lattice

3.2 Hydrogen bonds

The hydrogen bond network formed between molecules of polymer is beneficial to the stability of the molecular structure, and determines the structure of the molecule to a certain extent, such as melting point, boiling point, etc. [26].

A strong non-bond interaction formed between H atoms with large electronegativity and adjacent atoms with large electronegativity is referred to as a hydrogen bond. Its definition is divided into the geometric principle and the energy criterion [27]. The definition of hydrogen bond is used in this paper.

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Figure 6. Hydrogen bonds network, 1 represents the MPIA intramolecular hydrogen bonds, 2 represents the intermolecular hydrogen bonds

The maximum distance of acceptor atoms and hydrogen atoms is 3 Å, the minimum value of the angle of donor atoms, hydrogen atoms and acceptor atoms is 90^。.

The interface Model A is used as an example of the hydrogen bonding. Through the dynamics, the hydrogen bonds of the model of aramid fiber and SiO₂ are shown in Figure 6. In order make the figure clearer, the background color is black.

As shown in Figure 6, the intramolecular hydrogen bonds are formed in the N, H and O atoms of aramid fiber, and the interfacial hydrogen bonds are formed between the hydrogen bonds H atoms of aramid fiber and O atoms of SiO₂. Compared to the interfacial hydrogen bonds, the number of intramolecular hydrogen bonds in aramid fiber is large. This is because the spacing of H atoms to O and N atoms is small, which easily meets the necessary conditions for the formation of hydrogen bonds, thus making the probability of forming hydrogen bonds high. The hydrogen bonding between aramid fiber and SiO₂ is beneficial to the bonding between aramid fiber and SiO₂.

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Figure 7. The MSD of different models at different temperatures

In this paper, molecular dynamics simulations are carried out to study the interfacial interaction in micro and nanoscale between the amorphous region of PMIA and SiO₂, and the characteristics of the interface between the two and the effect of the lattice size on the thermal stability of PMIA are studied. Firstly, through the visual interface of MS software, a model of PMIA and SiO₂ is constructed. Then through the relaxation process, the model is energy optimized. Finally, the molecular dynamics simulation is carried out. Through the relationship between the interfacial energy and lattice, and the analysis of hydrogen bond interaction and the mean square displacement, the following conclusions are obtained.

1) A stable interface can be formed between MPIA molecules and SiO₂, with the dispersion force as the main reason for the formation of the interface between the two. The binding energy of Model C is the largest, and its interfacial combination effect is the best.

2) In the bonding process of nano SiO₂ and aramid fiber, the hydrogen bond is formed between the H atoms of the aramid fiber and the O atoms of SiO₂, which is beneficial for the combination of aramid fiber and SiO₂.

3) With an increase in temperature, the chain motion of PMIA increases, and the thermal stability of PMIA decreases. Among the interface models in this paper, the thermal stability of Model C is the best.

This paper provides a solid reference for further research on the modification of nano SiO₂ particles to the meta-aramid insulation paper.