MD-DFT Calculations on Dissociative Absorption Configurations of FOX-7 on (001)- and (101)-Oriented Crystalline Parylene Protective Membranes

Crystalline poly-para-xylylene (parylene) has the potential for use as a protective membrane to delay the nucleation of explosives by separating the explosives and their decomposition products to decrease the explosive sensitivity. Here, molecular dynamics (MD) and density functional theory (DFT) techniques were used to calculate the dissociative adsorption configurations of 1,1-diamino-2,2-dinitroethylene (FOX-7) on (001)- and (101)-oriented crystalline parylene membranes. Based on the results of the calculations, this work demonstrates that the -NO2–π electrostatic interactions are the dominant passivation mechanism of FOX-7 on these oriented surfaces. FOX-7 can dissociatively adsorb on oriented parylene membranes due to the interactions between the LUMO of the toluene (or methyl) groups on parylene and the HOMO of the -NO2 (or -NH2) groups on FOX-7. The formation of a new intermolecular H-bond with the ONO group leads to FOX-7 decomposition via intramolecular C-NO2 bond fission and nitro-to-nitrite rearrangement. The most likely adsorption configurations are described in terms of the decomposition products, surface active groups of parylene, binding behaviors, and N charge transfer. Importantly, the (001)-oriented parylene AF8 membrane is promising for use as a protective membrane to passivate the high-energy -NO2 bonds during the dissociative adsorption of FOX-7. This study offers a new perspective on the development of protective membranes for explosives.


Introduction
The novel high-energy material 1,1-diamino-2,2-dinitroethylene (FOX-7, DADNE) was first synthesized in 1998 and instantly received much attention due to its high thermal stability and low impact and friction sensitivities [1].FOX-7 has a highly polarized carbon-carbon double bond, and the positive and negative charges on the carbons are stabilized by two amino and two nitro groups [2].It is a nitroenamine with a "push-pull" alkene structure [3].Under long-term storage conditions, these structures decompose due to the significant aging effects of temperature [4], pressure [5], and pH [6].NVT (Canonical Ensemble) and AIMD (Ab Initio Molecular Dynamics) simulations revealed that the thermal decomposition mechanism of FOX-7 involves three different initial decomposition steps, namely, C-NO 2 bond fission and intermolecular and intramolecular hydrogen transfer [7,8].The C-NO 2 bond fission mechanism was determined to be the most common route [9].Because this fission mechanism causes performance issues, particularly for FOX-7 gas and its decomposition products, this explosive requires the use of a protective membrane overcoat that inhibits the slow decomposition [10].For example, the oxygen atoms of the -NO 2 groups in FOX-7 partially or completely dissociate from the molecule to oxidize the matrix surface [11].The FOX-7 molecule decomposes into -ONO and -OC(NO 2 )=CN 2 H 4 , which have unsaturated bonds with high electrostatic potentials that can attract or repel the surface electrons of protective membranes [12].Hence, a dissociative adsorption mechanism is possible, depending on the particular surface sites involved [13].Therefore, the protective membranes required for this compound should have low sensitivity, good flexibility, and mechanical strength, and should be incompatible with organic molecules [14].
Recently, it was suggested that polymers can be coated on the surfaces of energetic materials (such as explosives, propellants, and pyrotechnics) to act as protective membranes [15].Surface crystalline polymers are well known to decrease explosive sensitivity and infiltration [16].Moreover, the explosive cannot crystallize on the surface of the protective membrane.These membranes can delay the nucleation of the explosive by inhibiting its volumetric expansion [17] and separating the explosive molecules from their decomposition products, hindering heat conduction [18].Of the various possible polymer membranes, pinhole-free poly-para-xylylene (parylene) and substituted parylene membranes have high stabilities, high chemical and electrical resistances, and low gas permeability [19,20].Substituted parylene membranes, such as poly-chloro-para-xylylene (parylene C), polytetrafluoro-para-xylylene (parylene F), α,α,α ,α -poly-tetrafluoro-para-xylylene (parylene AF4), and fluorinated poly-para-xylylene (parylene AF8) membranes, were shown to exhibit improved gas impermeability [21].The primary roles of these membranes are to reduce gas permeation and prevent interactions with organic compounds [22].The stabilities of fluoroelastomer-coated cyclotrimethylenetrinitramine (RDX) and cyclotetramethylene tetranitramine (HMX) were previously verified [23].The polar C-Cl and C-F bonds in parylene cause the formation of polymorphic structures [24], including those with (101) [25] and (001) [26] orientations.The exposed C-Cl and C-F bonds are different on the different surfaces [27], which improves the mitigation effects on the explosive thermolysis pathways [28].Consequently, in this study, based on previous studies of polymer-explosive systems that examined their interfacial structures, phase transitions, heat conduction barriers, etc., four parylene protective membranes were investigated for use as a barrier for FOX-7 and its decomposition products.
Currently, few experimental data are available to explain the efficiency of energetic compounds coated with parylene membranes.Moreover, the mechanism describing the interactions between FOX-7 (or its decomposition products) and parylene surfaces is unknown.To elucidate the fundamental issues related to the nitro-to-nitrite rearrangement of FOX-7 deposited on (101)-and (001)-oriented parylene membranes, this work presents an atomic-level description of the intermolecular hydrogen bonds (H-bonds) formed between the decomposition products of FOX-7 and parylene surfaces.Specifically, MD and DFT calculations were used to study the corresponding local chemical adsorption processes.

Computational Details
The chain units of various parylene (parylene C, parylene F, parylene AF4, and parylene AF8) membranes were constructed using molecular mechanics (MM), and optimized by 200 ps NPT and NVT (NPT 100 ps before NVT 100 ps) molecular dynamics (MD) simulations at 298 K and 101 KPa using the COMPASS force field [21].In this work, gradually reduced size (GRS) methods were employed to optimize the weak interaction potentials, reducing the error in the van der Waals and electrostatic potentials between chain units, and regular arrangements of (101)-and (001)-oriented parylene membranes were constructed.These membranes were optimized and relaxed via 110 ps NPT annealing and 500 ps NPT and NVT MD simulations, and the T and P control method was set to Nose Q ratio with 0.01 timestep and Berend decay constant with 0.1 timestep.Short-range van der Waals interactions were calculated using an atom-based method.The Ewald+Group summa-tion method was routinely used to evaluate the long-range electrostatic interactions in reasonably small models [29].
The surface electrostatic energies of the crystalline parylene membranes were determined by density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional.Initially, several tests were performed to verify the accuracy of the method for bulk FOX-7 and its decomposition products on (101)-and (001)-oriented parylene membranes.Specifically, both the physical atomic distributions and chemical electron transfer processes were investigated.The atomic distances and total energies of the models of FOX-7 and its decomposition products on the parylene surfaces were tested for convergence [30,31].The atomic configurations were optimized by conjugate gradient minimization of the total energy via 500 ps NPT and NVT MD simulations [32].To elucidate the interactions between the dissociatively adsorbed FOX-7 molecules and the polymer surfaces, the electron localization functions (ELFs) were optimized [11].The corresponding kinetic energies were calculated by a spin-polarized GGA-PBE DFT method [33].Other areas of the model and the relative boundary area were simulated by MM using periodic boundary conditions [34].

Surface Structures of the (001)-and (101)-Oriented Crystalline Parylene Protective Membranes
The main parylene polymers used as protective membranes include parylene C ((C 8 H 7 Cl) n ), parylene F ((C 8 H 7 F) n ), parylene AF4 ((C 8 H 4 F 4 ) n ), and parylene AF8 ((C 8 F 8 ) n ) [21,29], which are shown in Figure 1b.This figure shows that the errors in the bond lengths and angles are less than 0.5%, which is in contrast to the literature data [35].The high-energy areas are the most likely sites for the condensation of adjacent chain units to construct low-energy parylene membranes (Figure 1c).Thus, the surface electrons in the -C-Cl (or -C-F), benzyl (π), or -C-F 2 (or -C-H 2 ) bonds of the parylene chain units tend to overlap.For example, the (101) and (001) orientations of parylene C were constructed by exploiting the coupling interactions between the π-π and -C-Cl-Cl-C-bonds, as shown in Figure 1a.The four optimized crystalline parylene configurations are given in Table 1.The calculated crystalline structures and densities are close to the experimental data with errors of less than 1% [21,35].The errors in the calculated T g values, which depend on the number and nature of the Cl and F atoms, are 4.4%, 6.5%, 8.7%, and 9.4%, which is in contrast to the literature data [21,[35][36][37][38][39][40][41].The experimental and computer data are tested by SEM and MD results, as shown in Refs.[21,[35][36][37][38][39][40][41].
The parylene surfaces are represented by slab models of these membranes with a 1 nm vacuum.Specifically, (101)-and (001)-oriented parylene membranes with two layers containing 1920 and 2880 atoms were used to investigate the adsorption of FOX-7 and its decomposition products.Typically, the electrons in the benzene π-bonds are the closest to the surface, as shown in Figure 2.Although the chlorine (Cl) and fluorine (F) atoms pull electrons towards themselves [22], the ortho-hydrogen (H) atoms in benzene have the highest electrostatic densities, as shown by the highest occupied molecular orbital (HOMO) analysis.They are the source of the high electrostatic energies (25-70 eV) of parylene C (or parylene F), as shown in Figure 3.However, replacing the H atoms on the benzyl groups with fluorine (F) atoms increases the total surface electron densities [21].This substitution results in the transfer of the active electrons to the benzyl groups and the formation of the lowest unoccupied molecular orbital (LUMO).The -CF 2 bonds in parylene AF4 and parylene AF8 have the highest electrostatic energies (350-385 eV).These surface potential energies correspond to positive charges, which facilitate the adsorption of the negatively charged FOX-7 (surface potential energy: −255-2235 eV).The parylene surfaces are represented by slab models of these membranes with a 1 nm vacuum.Specifically, (101)-and (001)-oriented parylene membranes with two layers containing 1920 and 2880 atoms were used to investigate the adsorption of FOX-7 and its decomposition products.Typically, the electrons in the benzene π-bonds are the closest to the surface, as shown in Figure 2.Although the chlorine (Cl) and fluorine (F) atoms pull electrons towards themselves [22], the ortho-hydrogen (H) atoms in benzene have the highest electrostatic densities, as shown by the highest occupied molecular orbital (HOMO) analysis.They are the source of the high electrostatic energies (25-70 eV) of parylene C (or parylene F), as shown in Figure 3.However, replacing the H atoms on the benzyl groups with fluorine (F) atoms increases the total surface electron densities [21].This substitution results in the transfer of the active electrons to the benzyl groups and the formation of the lowest unoccupied molecular orbital (LUMO).The -CF2 bonds in parylene AF4 and parylene AF8 have the highest electrostatic energies (350-385 eV).These

Adsorption Configurations of FOX-7 on the (001)-and (101)-Oriented Crystalline Pa Protective Membranes
After determining the potential adsorption sites on the (101)-and (001)-or parylene membranes, the possible adsorption configurations of the FOX-7 molec these parylene membranes were investigated, as illustrated in Figure 4.These con tions were obtained by defining the distributions of the O and H atoms.The interm lar H-bond lengths between the -NH2 and -NO2 groups in FOX-7 are approximat nm [12,42].The -NH2 and -NO2 bonds rearrange through interactions with adjacent functional groups, a process that was tested and verified by experiments and simu [3].On the surfaces of the FOX-7 molecules, the electrons are transferred between

Adsorption Configurations of FOX-7 on the (001)-and (101)-Oriented Crystalline Parylene Protective Membranes
After determining the potential adsorption sites on the (101)-and (001)-oriented parylene membranes, the possible adsorption configurations of the FOX-7 molecule on these parylene membranes were investigated, as illustrated in Figure 4.These configurations were obtained by defining the distributions of the O and H atoms.The intermolecular H-bond lengths between the -NH 2 and -NO 2 groups in FOX-7 are approximately 0.2 nm [12,42].The -NH 2 and -NO 2 bonds rearrange through interactions with adjacent active functional groups, a process that was tested and verified by experiments and simulations [3].On the surfaces of the FOX-7 molecules, the electrons are transferred between the O-O and H-O bonds through 2p 4 → 2p 4 and 1s 1 → 2p 4 orbital processes [10,11].The O-O bonds have high electron transfer ratios, that is, the O-O p → p orbitals have high electrostatic energies that facilitate electron transfer between the adjacent active groups [12].Figure 5a indicates that the -NO 2 groups on FOX-7 first bind to the π-bonds of benzene in (101)-oriented parylene C and F or to the benzyl groups in parylene AF4 and AF8.These electrostatic interactions cause the FOX-7 surface electrons to overlap with the parylene membranes, leading to the formation of new short-range intermolecular H-bonds.The adjacent -NH 2 groups continue to interact with the π-bonds to provide the missing electrons to the -NO 2 -benzene unit until the O-H bonds break [13] on the (101)-oriented parylene membranes because the HOMO on the -NO2 group of FOX-7 interacts with the LUMO on the -C-Cl group on the parylene benzene to lower the energy of the system, as shown by the HOMO-LUMO results presented in Figure 6.The electrostatic interactions are considered to be the dominant adsorption mechanism for FOX-7 on the (101)-oriented parylene membranes.The -NO2 bonds (FOX-7), which are the high-energy origin of detonation, are passivated at the two basic adsorption sites, that is, at the toluene and methyl groups, due to the electron transfer from the FOX-7 HOMO to the parylene LUMO [2,3].Simultaneously, the dissociative adsorption of FOX-7 is also possible and involves the breaking of the intramolecular H-bond in FOX-7 (-NO2-H2N-) and the formation of a new intermolecular H-bond, which has the smallest calculated bond length (0.2-0.24 nm) with the ONO group.Another useful tool for investigating the coupling interactions between the F active groups and the (101)-oriented parylene surfaces is the electron localization fu (ELF).Isosurfaces of the ELF can be calculated and used to explain bond and charge The relative energy differences and bond energies are discussed below and are sho Figure 7.The results of these calculations indicate that both the -NO2 and -NH2 gro FOX-7 are bound to toluene groups based on the lowest energy principle.The surfa (or -F) atoms of parylene preferentially capture the surface electrons of the be groups, which results in high-energy active electrons that can be transferred to th saturated -NH2 electron orbitals on FOX-7 [12].Then, the paired positive holes bind Another useful tool for investigating the coupling interactions between the FOX-7 active groups and the (101)-oriented parylene surfaces is the electron localization function (ELF).Isosurfaces of the ELF can be calculated and used to explain bond and charge data.The relative energy differences and bond energies are discussed below and are shown in Figure 7.The results of these calculations indicate that both the -NO 2 and -NH 2 groups of FOX-7 are bound to toluene groups based on the lowest energy principle.The surface -Cl (or -F) atoms of parylene preferentially capture the surface electrons of the benzene groups, which results in high-energy active electrons that can be transferred to the unsaturated -NH 2 electron orbitals on FOX-7 [12].Then, the paired positive holes bind to the negatively charged -NO 2 groups.In contrast to the electron-withdrawing nature of the -benzene-Cl(F) bonds, the -CF 2 -surface potentials homogenize and push the surface electrons of benzene [23].These low-energy sites are positively charged and thus preferentially bind to the negatively charged -NO 2 groups.The surface potentials at the negatively charged methyl bonds are relatively uniform, and these bonds interact with the positively charged -NH 2 bonds.A common tool for providing a semi-quantitative measure of the charge transfer is the Mulliken charge analysis in which the electronic charge is partitioned among the individual atoms, as shown in Figure 8.Although all the N atoms in the FOX-7 molecule have negative Mulliken charges on the parylene surfaces [43], the N atoms in both the -NH 2 and -NO 2 groups have highly negative charges (−2-0.2e) on the (101)-oriented parylene membranes.
Polymers 2024, 16, 438 8 of 15 charge transfer is the Mulliken charge analysis in which the electronic charge i tioned among the individual atoms, as shown in Figure 8.Although all the N atom FOX-7 molecule have negative Mulliken charges on the parylene surfaces [43], th oms in both the -NH2 and -NO2 groups have highly negative charges (−2-0.2e) (101)-oriented parylene membranes.In contrast to the observed interactions between both the -NH 2 and -NO 2 groups and the (101)-oriented parylene membranes during adsorption, active methyl groups with positive charges participate in the adsorption process on the (001)-oriented parylene membranes.Based on the lowest energy principle, the exposed paired positive holes on the benzenes can capture the active electrons of the -NO 2 bonds.The adsorption configura-Polymers 2024, 16, 438 9 of 15 tions of FOX-7 on the (001)-oriented parylene AF4 or AF8 membranes are similar to those on the (101)-oriented membranes.However, it should be noted that the -NO 2 surface electrons are transferred to the exposed low-energy orbitals on the toluene groups on the (001)-oriented parylene C and F membranes, as shown by the lowest energy results presented in Figure 7. Due to the coupling interaction between the high-and low-energy areas of adjacent -benzene-Cl(F) bonds, the exposed methyl groups do not have enough active electrons to break the H-bonds in the FOX-7 molecules.This conclusion is supported by the Mulliken charge results, which show that the positive Mulliken charges are mainly located on the N atoms of the -NO 2 groups (II2 on parylene C and F: ~0.11 e; II4 on parylene AF4 and AF8: 0.1-0.12e), as shown in Figure 8.Thus, the four-electron orbitals of the methyl C-H 2 2p 2 -1s 1 (or C-F 2 2p 2 -2p 5 ) groups mainly capture three surface electrons from the -NO 2 2p 3 -2p 4 orbitals on FOX-7 by forming sp electron orbitals, and the adsorption angles are 45 • based on the long-range half-full bonds (complex of seven-electron orbitals and five-electron orbitals (-NH 2 2p 3 -1s 1 )), as shown in Figure 5c,d.Because the high-energy -NO 2 bonds are passivated on the (001)-oriented parylene surfaces, these parylene materials can be used as protective membranes for FOX-7, which is in contrast to the (101)-oriented membranes.Furthermore, the (001)-oriented parylene AF8 is a potential protective membrane for FOX-7 because the binding energies of -NO 2 -benzene and -NO 2 -methyl are the lowest at ~0 eV and 2.5 eV, respectively.

Nitro-to-Nitrite Rearrangement of the Decomposition Products of FOX-7 on the Parylene Membranes
According to Rashkeev et al. [12], FOX-7 molecules decompose via C-NO 2 bond fission and nitro-to-nitrite rearrangement, and the oxygen atoms in the -NO 2 groups of FOX-7 partially or completely dissociate to produce (NH 2 )(NO 2 )C=C(NH 2 )O and ONO.Under continuous aging by temperature [4], pressure [5], and pH [6], the -N-O-and -C-O-bonds in the (NH 2 )(NO 2 )C=C(NH 2 )O product rearrange in the same manner as those in FOX-7.The oxygen atoms in the -NO 2 groups of (a) (NH 2 )(NO 2 )C=C(NH 2 )O partially or completely dissociate to give (b) NH(NH 2 )C=CO and (c) ONO.To analyze the active adsorption sites on the parylene membranes, the probable configurations of the decomposition products of FOX-7 were determined.The probable configurations of the decomposition products on the active adsorption sites (A: methyl (M) and toluene (T) groups) of the parylene membranes are presented as (a1) (NH 2 )(NO 2 )C=C(NH 2 )O and (a2) A-(NH 2 )(NO 2 )C=C(NH 2 )OH in Figure 9; (b1) NH(NH 2 )C=CO, (b2) A-NH(NH 2 )C=CO and (b3) (NH 2 ) 2 C=CO in Figure 10; and (c1) HON=O and (c2) O=NO-A in Figure 11.Considering the effects of the active adsorption sites (A: methyl and toluene groups) and orientations of the parylene membranes, the most likely dissociative adsorption processes for the decomposition products of FOX-7 were analyzed based on the lowest binding energy principle.The -NH-and -O=N-Obonds in the decomposition products have unsaturated orbitals and thus high electrostatic potentials that attract the surface electrons of the exposed toluene and methyl groups of the (101)-and (001)-oriented parylene membranes [11], as illustrated in Figure 12.Thus, active electrons are transferred from the highest occupied molecular orbitals (HOMOs) on the -NH-and -O=N-O-groups of the decomposition products to the lowest unoccupied molecular orbitals (LUMOs) on the active adsorption sites on the parylene surfaces [13].Therefore, the high-energy -NO 2 bonds in the decomposition products of FOX-7 are passivated by the toluene and methyl groups, which is consistent with the results for FOX-7.In contrast to the observed interactions with the toluene π groups, the h bridized methyl (-C-F2) p 4 (2p 2 -2p 5 2) orbitals on the (001)-oriented membranes teract with the half-full p 4 (N-H 2p 3 -1s 1 ) and approximately half-full p 3 (O-N= 2p 4 ) orbitals on the NH(NH2)C=CO and ONO products, respectively.These supported by the low energy results for both the -NO-π and -NH-π interact shown in Figure 13.The half-full p 4 and approximately half-full p 3 orbitals o In contrast to the observed interactions with the toluene π groups, the half-full hybridized methyl (-C-F 2 ) p 4 (2p 2 -2p 5  2 ) orbitals on the (001)-oriented membranes mainly interact with the half-full p 4 (N-H 2p 3 -1s 1 ) and approximately half-full p 3 (O-N=O 2p 4 -2p 3 -2p 4 ) orbitals on the NH(NH 2 )C=CO and ONO products, respectively.These results are supported by the low energy results for both the -NO-π and -NH-π interactions (V), as shown in Figure 13.The half-full p 4 and approximately half-full p 3 orbitals of the -NH 2 and -NO 2 bonds, respectively, bind to the active electron-hole sites on the toluene π groups.It should be noted that the positive Mulliken charges (0.09-0.145 e) on the N atoms become negative ((NH 2 )(NO 2 )C=C(NH 2 )O: −1.31-0.25 e; NH(NH 2 )C=CO: −0.34-0.04e; ONO: −0.36-0.02e), as shown in the Supporting Information (Figure S1).The N 2p 3 electrons are transferred from the positively charged -NO 2p 3 -2p 4 orbitals to the negatively charged -NH 2p 3 -1s 1 (or neighboring O 2p 4 ) orbitals through the electronic transitions of the intramolecular -N-C-N-(or -O-N=O) bonds.Consequently, the enhanced -NH orbital strength induced by the (001)-oriented parylene surfaces can delay the nucleation of the two neighboring decomposition products of FOX-7 and inhibit the nitro-to-nitrite rearrangement by hindering intermolecular and intramolecular hydrogen transfer [7].Therefore, the (001)-oriented parylene AF8 membrane, which has the lowest bond energy, is proposed to be a potential protective membrane for inhibiting FOX-7 decomposition pathways.

Summary and Conclusions
In conclusion, the dissociative adsorption of FOX-7 on (101)-and (001)-oriented crystalline parylene C, F, AF4, and AF8 membranes was demonstrated.Based on the data provided by this study, it can be concluded that FOX-7 adsorbs on the oriented parylene membranes via -NO 2 -π electrostatic interactions, which appear to be the dominant passivation mechanism of FOX-7.FOX-7 molecules lie flat on the (101)-oriented parylene membranes due to both -NO 2 -π and -NH 2 -π electrostatic interactions, and the -NO 2 groups mainly bind to the exposed toluene (or methyl) groups on the (001)-oriented parylene membranes.FOX-7 dissociates into (NH 2 )(NO 2 )C=C(NH 2 )O, NH(NH 2 )C=CO, and ONO via C-NO 2 bond fission and nitro-to-nitrite rearrangement.The high-energy -NO 2 bonds of (NH 2 )(NO 2 )C=C(NH 2 )O and ONO are passivated by the exposed toluene and methyl groups.In particular, the positively charged -NO 2p 3 -2p 4 orbitals become negatively charged -NH 2p 3 -1s 1 (or neighboring O 2p 4 ) orbitals through intramolecular -N-C-N-(or -O-N=O) electronic transitions on the (001)-oriented membranes.Based on the ELF and Mulliken charge analysis results, the (001)-oriented parylene AF8 membrane is proposed to be a potential protective membrane for passivating the high-energy -NO 2 bonds in FOX-7 by a dissociative adsorption process.
The results of this work are insufficient for analyzing the effects of oriented parylene membranes on FOX-7 nucleation and thermolysis mitigation.Further investigations will be focused on the complexation of two FOX-7 molecules and their decomposition products under long-term storage or extreme conditions, such as widely fluctuating temperatures or high pressures.However, this study provides useful information for determining the dissociative processes of FOX-7 on crystalline parylene surfaces and understanding the interactions between explosive products and protective membranes.

Polymers 2023 , 15 Figure 1 .
Figure 1.Chain polymerization configurations (a), chemical formulas (b), potential energy and crystalline models (c) of parylene membranes.The color code is the following: grey, C; green, Cl; blue, F and white H. Blue area of parylene chains, low energy area; and yellow area, high energy area.

Figure 1 .Figure 2 .
Figure 1.Chain polymerization configurations (a), chemical formulas (b), potential energy and crystalline models (c) of parylene membranes.The color code is the following: grey, C; green, Cl; blue, F and white H. Blue area of parylene chains, low energy area; and yellow area, high energy area.Polymers 2023, 15, x FOR PEER REVIEW 5 o

Figure 2 .
Figure 2. Illustrations of frontier orbitals (Highest Occupied Molecular Orbital: HOMO; Lowest Unoccupied Molecular Orbital: LUMO) of FOX-7, (101)-and (001)-oriented parylene F surfaces.The color code is the following: grey, C; green, Cl; blue, F and white H. Blue area of parylene chains, low energy area; and yellow area, high energy area.

Figure 2 .
Figure 2. Illustrations of frontier orbitals (Highest Occupied Molecular Orbital: HOMO; Low occupied Molecular Orbital: LUMO) of FOX-7, (101)-and (001)-oriented parylene F surfac color code is the following: grey, C; green, Cl; blue, F and white H. Blue area of parylene chai energy area; and yellow area, high energy area.
. The corresponding energy differences (∆E) between the adsorption models and single systems are the lowest (parylene C: −11 kcal•mol −1 ; parylene F: −130 kcal•mol −1 ; parylene AF4: −7.5 kcal•mol −1 ; parylene AF8: −10 kcal•mol −1 ), as shown in Figure 5b.Thus, the FOX-7 molecules lie flat on the (101)-oriented parylene membranes because the HOMO on the -NO 2 group of FOX-7 interacts with the LUMO on the -C-Cl group on the parylene benzene to lower the energy of the system, as shown by the HOMO-LUMO results presented in Figure 6.The electrostatic interactions are considered to be the dominant adsorption mechanism for FOX-7 on the (101)-oriented parylene membranes.The -NO 2 bonds (FOX-7), which are the high-energy origin of detonation, are passivated at the two basic adsorption sites, that is, at the toluene and methyl Polymers 2024, 16, 438 6 of 15groups, due to the electron transfer from the FOX-7 HOMO to the parylene LUMO[2,3].Simultaneously, the dissociative adsorption of FOX-7 is also possible and involves the breaking of the intramolecular H-bond in FOX-7 (-NO 2 -H 2 N-) and the formation of a new intermolecular H-bond, which has the smallest calculated bond length (0.2-0.24 nm) with the ONO group.

Figure 5 .
Figure 5. (a,c) The -NO 2 groups on FOX-7 bind to the groups in parylene; (b,d) The corresponding energy differences (∆E) between the adsorption models and single systems.

Figure 7 .
Figure 7. Binding energies (eV) of various probable configurations of FOX-7 adsorbed on (101)-and (001)-oriented parylene membranes.In the figure, "M" refers to the methyl group, "T" refers to the toluene group, and "O" and "H" refer to the -NO 2 and -NH 2 functional groups of FOX-7, respectively.