DFT and QSAR studies of PTFE/ZnO/SiO2 nanocomposite

Polytetrafluoroethylene (PTFE) is one of the most significant fluoropolymers, and one of the most recent initiatives is to increase its performance by using metal oxides (MOs). Consequently, the surface modifications of PTFE with two metal oxides (MOs), SiO2 and ZnO, individually and as a mixture of the two MOs, were modeled using density functional theory (DFT). The B3LYPL/LANL2DZ model was used in the studies conducted to follow up the changes in electronic properties. The total dipole moment (TDM) and HOMO/LUMO band gap energy (∆E) of PTFE, which were 0.000 Debye and 8.517 eV respectively, were enhanced to 13.008 Debye and 0.690 eV in the case of PTFE/4ZnO/4SiO2. Moreover, with increasing nano filler (PTFE/8ZnO/8SiO2), TDM changed to 10.605 Debye and ∆E decreased to 0.273 eV leading to further improvement in the electronic properties. The molecular electrostatic potential (MESP) and quantitative structure activity relationship (QSAR) studies revealed that surface modification of PTFE with ZnO and SiO2 increased its electrical and thermal stability. The improved PTFE/ZnO/SiO2 composite can, therefore, be used as a self-cleaning layer for astronaut suits based on the findings of relatively high mobility, minimal reactivity to the surrounding environment, and thermal stability.


Results and discussion
Building PTFE model structures. A model of molecules simulating PTFE coated with MOs to promote hydrophobicity, anticorrosion, and self-cleaning qualities 53 . MOs including ZnO and SiO 2 have been suggested as coating layers because of their anticorrosion and self-cleaning properties 54,55 . Consequently, the model of the smallest set of PTFE chemical units, representing a PTFE polymer chain, is designed to interact with ZnO and SiO 2 both individually and combined. The interaction of PTFE with MOs takes place via the oxygen atom of the MO 26,56 . Because PTFE interacts chemically via its active sides, and since it has four equal active sides according to the chemical formula C 2 F 4 ; therefore, any fluorine (F-) atom can interact with other chemical structures. As indicated in Fig. 1a,b, the model of the smallest unit representing the two MOs (ZnO and SiO 2 ) and the PTFE polymer chain, was made up of four units of C 2 F 4 that were designed to interact with the suggested two MOs. First, the PTFE chain was designed to interact with four units of ZnO and four units of SiO 2 , separately, coated on one side, as shown in Fig. 1c,d, respectively. After that, the PTFE chain is designed to interact with a combination of four units of ZnO and four units of SiO 2 covered layer by layer as shown in Fig. 1e. Figure 1f depicts the PTFE chain's model structure as it interacts with a combination of four units of SiO 2 and four units of ZnO covered layer by layer on its surface. Figure 1g depicts the final model structure for the PTFE chain in interaction with a single mixed layer of four ZnO and four SiO 2 units. Increasing the quantity of nanoparticles on the polymer's surface has a significant impact on the electrical properties of the polymer's matrix 57 . Following that, the PTFE chain is next expected to be coated from both sides, as previously done with four units of ZnO and four units of SiO 2 individually and combined, as shown in Fig. 2. The electrical characteristics of PTFE/MOs were then examined for the proposed interaction mechanisms by studying the calculated TDM, ΔE and MESP maps. Figure 3 illustrates the HOMO/LUMO orbital distribution of PTFE and its interactions with 4ZnO, 4SiO 2 , and their hybrids. The HOMO/LUMO orbital distribution of the four PTFE chains is demonstrated in Fig. 3a to be spread across the chain. When PTFE interacted with ZnO and SiO 2 on one side, the HOMO/LUMO orbitals were rearranged as in Fig. 3b-f for all interaction cases and localized around the MO atoms. HOMO/LUMO orbital distribution reflects the effect of MO on orbital distribution, which in turn reflects on band gap energy changes. TDM and ΔE were also determined for the different forms of interactions. TDM improved from 0.000 for pure PTFE to 16 www.nature.com/scientificreports/ Figure 4 shows the calculated HOMO/LUMO orbital distribution of PTFE interaction with 8 ZnO, 8 SiO 2 and a combination of the two MOs. In these cases of interactions, as shown by HOMO/LUMO orbitals rearranged and localized around the MO on one side only, at the top and/or down the PTFE chain (at the top in Fig. 4a,b,d,e but down in the 4c case).The TDM and ΔE were determined for all the studied structures and are listed in Table 2. As shown in the table, TDM improved from 0.000 corresponding to pure PTFE to 32.934, 0.867, 7.844, 10.605 and 6.963 Debye for PTFE/8ZnO, PTFE/8SiO 2 , PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO, and PTFE/(8ZnO&8SiO 2 ), respectively. While ΔE was observed to decrease from 8.517 eV for pure PTFE to 0.163, 3.253, 0.273, 0.860, and 0.368 eV for PTFE/8ZnO, PTFE/8SiO 2 , PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO, and PTFE/(8ZnO&8SiO 2 ), respectively. The highest value of TDM and the lowest value of ΔE were reported for PTFE/8ZnO and PTFE/8ZnO/8SiO 2 as an indication of the interactions that enhanced the electrical characteristics of the PTFE the most. From all the results, the most enhanced PTFE structure interacted with 4 unit's MOs    www.nature.com/scientificreports/ the richest charge area, the colour difference represented as blue refers to the poorest charge region, and the colour difference described as green represents zero electrostatic potential. The strongest potential is commonly found in red regions, whereas the weakest potential is found in blue regions 59 . MESP mapping was calculated for all studied structures at the same level of theory. Figure 5 shows the MESP for PTFE/4ZnO, PTFE/4SiO 2 , PTFE/4ZnO/4SiO 2 , PTFE/4SiO 2 /4ZnO, PTFE/(4ZnO&4SiO 2 ), PTFE/8ZnO, PTFE/8SiO 2 , PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO and PTFE/(8ZnO&8SiO 2 ), which displayed a map for the interaction status of nucleophilicity. Figure 5a shows the MESP map for all considered PTFE, ZnO and SiO 2 interactions colored with intermediate colors between orange and yellow, with a plane for the PTFE chain and perpendicular in the case of MOs, which represents less electrostatic repulsion. The MESP results revealed that these structures were exceptionally stable, with the MESP surface appearing in yellow, and there was still no chance of interfering with others, and representing more chemical equilibrium. Figure 5b-k demonstrate the interaction of PTFE with MOs. The red colour spread on the up and down terminals of the polymer, indicating that PTFE's reactivity increased, and MOs enhanced PTFE's active sides. When PTFE interacted with 4SiO 2 , 4ZnO/4SiO 2 , 4SiO 2 /4ZnO, (4ZnO&4SiO 2 ), 8SiO 2 , 8ZnO/8SiO 2 , 8SiO 2 /8ZnO, and (8ZnO&8SiO 2 ), low-potential red regions were localized mainly around the oxygen atom of MO. Whereas when PTFE interacted with 4ZnO and 8ZnO, the red regions were spread across the polymer and increased on the other side of it. These results of MESP are in good agreement with the results of TDM and ΔE. As a result, PTFE's electrical properties improved, and it may now be employed in a variety of fields of applications, such as a corrosion-inhibiting layer for astronaut suits.

HOMO/LUMO orbital distribution.
Quantitative structure activity relationship (QSAR). Table 3 defines QSAR descriptors trying to describe PTFE relationships with MOs as PTFE/4ZnO, PTFE/4SiO 2 , PTFE/4ZnO/4SiO 2 , PTFE/4SiO 2 /4ZnO, PTFE/(4ZnO&4SiO 2 ), PTFE/8ZnO, PTFE/8SiO 2 , PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO and PTFE/ (8ZnO&8SiO 2 ). Descriptors are summarized as total energy (TE) as Kcal/mol, heat formation (HF) as Kcal/mol, ionization potential (IP) as eV, log P, polarizability as A 3 , molar refractive (MR) and molecular weight (MR) as au. Firstly, TE is stated to describe the stability of the system, and that reducing TE values takes the structure toward stability 60  found to be the most stable and most probable structure. As well, HF is a significant thermal descriptor that defines the energy produced in the form of heat, as the atoms that exist at potentially infinite distances are linked and form a molecule 61 , even though HF may be clarified through the difference observed in the enthalpy during the formation of a single mole of a substance from its components. This occurs in its natural and full balance under the atmospheric characteristics of a particular temperature. For PTFE, HF was equal to − 1570.772 kcal/mol. In the case of PTFE interaction with MOs, the calculated HF for PTFE/4ZnO, PTFE/4SiO 2 , PTFE/4ZnO/4SiO 2 , PTFE/4SiO 2 /4ZnO, and PTFE/(4ZnO&4SiO 2 ) were changed to − 1826.101 , − 1963.831, − 2457.839, − 2598.762, and − 2266.677 kcal/mol. While in the case of PTFE/8ZnO, PTFE/8SiO 2 , PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO and PTFE/(8ZnO&8SiO 2 ), HF changed to be equal to − 1871.705, − 259 7.869, − 3128.116, − 2372.734, and − 3300.601 kcal/mol, respectively. Accordingly, the most probable structure to be formed, which needed a low energy value for formation, was PTFE/8ZnO/8SiO 2 and PTFE/(8ZnO&8SiO 2 ).
Another important descriptor is IP, which is defined as the energy required for the material to be ionized. The IP described by Dewar and Morita using the following equation: IP = a + bq + cq, that a, b is the variational parameter defined as a 2 + b 2 = 1, charge of an atom in a molecule (q), and electron density of an atom in a molecule (C) 62,63 . IP was calculated for all PTFE models of interaction. The importance of IP value is that it reflects the reactivity of the studied structures. The IP value is inversely proportional to the compound's reactivity, which means that the reactivity of a certain chemical compound increases as the IP value decreases 64   The PTFE/8ZnO/8SiO 2 structure had the lowest reactive structure with the surrounding environment and the most thermally enhanced structure. The chemical structure was described by the logarithm of the partition coefficient (log P). Accordingly, the log p value for a compound is the logarithm (base 10) of the partition coefficient (p), which is defined as the ratio of the compounds' organic to aqueous phase concentration as in the equation 65  66 . All proposed models recorded a positive log P, which is an indication that the structures are hydrophobic and have not affected the surrounding environment. The lowest value of log P reflects a more polar and increase in hydrophobicity of the compound, which confirms the ability to act as a self-cleaning material. Self-cleaning surfaces have sparked significant interest in industrial applications, particularly in the aerospace industry. Self-cleaning super hydrophobic coatings, such as silicones, fluorocarbons, organic materials, and inorganic materials, are sensitive to the accumulation of ice, water, and other contaminants, as well as having a hard, wear-resistant, and phobic coating for an aerodynamic surface to improve de-icing properties via low-pressure plasma vapour deposition technologies. So, enhancing the hydrophobicity of the studying material makes it a promising material to be used as a self-cleaning surface, which is an important application in the aerospace field 67,68 . The lowest values of log P were recorded for PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO and PTFE/(8ZnO&8SiO 2 ). Consequently, polarizability is a basic property that determines how the chemical formula can be polarized in response to varying forces. Representing the responsiveness of the structural factors affecting the volume, the molar refractor is a descriptor which can specify the overall polarization of the mole 69 . The greater the molar refractor, the greater the stability of the structures, which were recorded for PTFE/8ZnO/8SiO 2 , PTFE/8SiO 2 /8ZnO, and PTFE/(8ZnO&8SiO 2 ).
In summary, the obtained results regarding the most expected interaction between PTFE and MOs, suggested a coating of ZnO with SiO 2 on PTFE, particularly PTFE/8ZnO/8SiO 2 layer by layer. PTFE/8ZnO/8SiO 2 is the most probable way of interaction based on its physical, chemical, and thermal stability. These enhancements serve as a corrosion-inhibiting and self-cleaning layer for astronaut suits owing to its lower response with the surroundings and its higher polarity and hydrophobic nature.

Conclusion
DFT calculations of PTFE chains modified with 4 and 8 units of nano-MOs including ZnO, SiO 2 individually and as a hybrid were subjected to enhanced chemical, physical, and thermal stability. The B3LYPL/LANL2DZ was used to evaluate TDM, ΔE and MESP for PTFE polymer and its interaction with suggested MOs model structures. For the studied structures, QSAR descriptors were calculated using MO-G at the PM6 level of theory to investigate electronic properties as well as thermal, physical, and chemical stability. The TDM and ΔE results for the interaction of PTFE with MOs indicated that the electronic properties of PTFE were improved by increasing the number of MOs units. Furthermore, introducing the two MOs layer by layer improves and keeps the PTFE polymer chain stable. Electronic property calculations showed that the most enhanced PTFE structure, as interacted with 4 units of MOs, was that for PTFE/4ZnO/4SiO 2 . For the interaction of PTFE with 8 units of MOs, PTFE/8ZnO/8SiO 2 layer by layer presented the best electronic properties results. The MESP maps also confirmed that the studied structure PTFE/8ZnO/8SiO 2 shows enhancement and redistribution of the charge on the polymer surface. The results of MESP are in good agreement with the results of TDM and ΔE. Furthermore, QSAR data indicated that coating PTFE as PTFE/8ZnO/8SiO 2 layer by layer improved electronic and thermal stability, and hydrophobicity properties. Correlating the results, it can be concluded that the modified PTFE with ZnO and SiO 2 layer by layer has innovative features such as thermal, chemical and physical stability with little sensitivity to surrounding materials, which might be employed as an anti-corrosion and self-cleaning layer for astronaut suits.

P = Concentration in Organic Concentration in Aqueous
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Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.