A ReaxFF-Based Molecular Dynamics Simulation of the Pyrolysis

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A ReaxFF-Based Molecular Dynamics Simulation of the Pyrolysis Mechanism for Polycarbonate Tong Zhao, Tan Li, Zhe Xin, Liang Zou, and Li Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03332 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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A ReaxFF-Based Molecular Dynamics Simulation of the Pyrolysis Mechanism for Polycarbonate Tong Zhao*, Tan Li, Zhe Xin, Liang Zou, Li Zhang School of Electrical Engineering, Shandong University, Shandong 250061, China

ABSTRACT: Polycarbonate (PC) is considered a promising substitute for insulating materials due to its excellent insulation and mechanical properties. The main cause for PC insulation aging is the breakage of chemical bonds at high temperatures. The reactive force field (ReaxFF) method is first employed in a molecular dynamics (MD) simulation of polycarbonate pyrolysis to elucidate the mechanism for thermal aging at the atomic level. The results show that the main reaction pathway for breakage of the polycarbonate main chain is C-O bond breakage of the terminal group or between PC monomers. CO2, CO, CH4, and H2 are the major products generated following polycarbonate pyrolysis. The formation mechanisms of these dominant products are detailed for the first time based on the simulation trajectories. The activation energy and pre-exponential factor extracted from the ReaxFF simulations show good agreement with experimental results. The fracturing of the main chain and the production of small molecule gases both decrease the degree of polymerization of the polycarbonate, resulting in thermal aging. These results are fully consistent with previous experimental results. This work demonstrates that ReaxFF simulation is a feasible and reliable method for elucidating detailed chemical reaction mechanisms in polycarbonate pyrolysis.

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1. Introduction Polycarbonate (PC) is a promising type of engineering plastic that shows good mechanical and electrical properties, with especially excellent impact resistance and electrical insulation properties. Consequently, many studies based on experimental analyses of PC and other polymers have been carried out for the exploration of new insulation materials. Tu et al. [1] and Li et al. [2] analyzed the internal insulation field in a transformer for which PC was used to replace cellulose insulation paper. Measurements of the dielectric constant, dielectric dissipation factor, and breakdown field strength in such studies showed PC to be a relatively ideal substitute for conventional insulation pressboards, but that a decrease in the polymerization degree can lead to insulation failure. However, thermal degradation can occur easily in PC during the molding process, resulting in changes in molecular chain structure and the mechanical and electrical properties. In its application, PC can gradually crack due to factors such as heat, electricity and chemical corrosion, which will decrease the insulation performance of PC, leading to insulation failure. Although the aging of insulation materials is a collective result of many factors, the life of the main insulation is chiefly determined by thermal aging [3]. The cleavage of the chemical bond is the main reason for PC pyrolysis. Therefore, a thorough understanding of the pyrolysis mechanism for PC is essential. Many studies have focused on experimental investigation of the mechanism for PC pyrolysis. Walczak [4] analyzed the glass transition temperature of PC by nuclear magnetic resonance spectroscopy (NMR). Jang [5] used thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) to analyze the products of PC pyrolysis in air and in N2. Most of the products formed under both conditions were found to be similar, with the main differences arising during the initial degradation stage. For the pyrolysis in air, the FTIR detected not only


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ester groups but also other carbonyl compounds including aldehydes and ketones. CO2 and H2O were produced throughout the process. However, the analysis based on TGA can only provide the total mass loss and the rate constant of the product formation, and, thus, cannot explain the reaction path for the PC pyrolysis process. Following an artificial aging experiment, fracture degradation of the side chain and terminal group was proposed as the main reaction pathway for the degradation of PC [6]. However, none of these experimental studies can explain the microscopic mechanism for PC pyrolysis at the atomic level. In recent years, molecular dynamics (MD)-assisted experimental simulation has become the third important scientific method (after laboratory and theoretical methods) for studying molecular structure, dynamic behavior, and physicochemical properties [7]. Established on the basis of experimental principles, MD simulation is a set of models and algorithms that can be used to calculate and analyze the dynamic behavior of a realistic molecular system. Tsai [8] studied the rotational unit of the PC system and compared the results with those obtained from the NMR method. The results of the two methods were basically consistent, and the carbonate linkages underwent a 180° rotation much more frequently than the phenylene ring. Zhang [9] established an MD model for PC with a density of 1.3 g·cm-3, and studied the role of PC in liquid and glassy states. However, reactive force field (ReaxFF) MD simulations for various processes such as the oxidation of toluene [10] and the pyrolysis of polyimide [11] show that the density does not affect the reaction mechanisms, but influences only the reaction rate, with the effect much weaker than that found for the influence of temperature. This is also true for the chemical reaction principle. Fan [12] studied the glass transition temperature of bisphenol-Apolycarbonate at the atomic level using a constant particle number, constant pressure, and constant temperature (the so-called NPT ensemble), and obtained three basic coordination modes


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for PC: positive rotation of the two benzene rings, negative rotation of the two benzene rings, and rotation of the carbonate groups. Currently, no good example exists for the molecular simulation of the PC pyrolysis mechanism. The quantum mechanics (QM) method for molecular simulation is widely used in the description of transition states, reactants, and reaction rates. However, because of the amount of calculation required and its use of a large number of molecular force fields, QM can only be used to simulate macromolecular systems and cannot be used to describe the formation and dissociation of bonds. The traditional MD method lacks two important forces: the van der Waals force and Coulomb interactions. Therefore, to overcome the shortcomings of QM and traditional MD, Van proposed a reaction MD method, called ReaxFF, to analyze complex chemical reactions [13]. The force field can be used to realize the free breakage and formation of chemical bonds between atoms and overcome the shortcomings of traditional MD, which cannot be used to simulate dynamic chemical reaction parameters. Moreover, ReaxFF parameters are derived from a combination of QM and experimental data, so accuracy is maintained to a certain extent. All possible reaction paths are considered in ReaxFF, with the simulation following the most thermodynamically favorable reaction path. Zhang [14] simulated the pyrolysis of unsaturated triglyceride using ReaxFF, with the results showing good agreement with experimental data. Kimberly [15] applied ReaxFF to the MD simulation of hydrocarbon oxidation, and qualitatively obtained the reaction rates for benzene and methane. Therefore, these successful applications have laid the foundation for the application of ReaxFF to PC pyrolysis. In this work, ReaxFF will be used to study the pyrolysis of PC for the first time. The detailed micro chemical reaction mechanism at the atomic level, micro reaction path, and products of the PC pyrolysis process will be elucidated. The paper is organized as follows. First, the basic


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concept and theory of ReaxFF are introduced, and simulation details are presented. Next, pyrolysis simulations of a single-molecule PC chain and a multi-molecule PC system at different high temperatures are reported. The initial cleavage mechanism of the reaction path, product distribution for the pyrolysis process, and the mechanism for the formation of the main gas products are all analyzed at the atomic level. Subsequently, kinetic analysis of PC pyrolysis is described, with a mechanism proposed for the thermal aging of PC at elevated temperatures. Finally, the results from the molecular simulation of PC pyrolysis are compared with experimental data to demonstrate the feasibility and reliability of the model calculations.

2. Simulation 2.1 Brief overview of the ReaxFF force field Based on bond order, bond distance, and the relationship between bond order and energy, ReaxFF can be used to determine the connection between any two atoms, thus, enabling characterization of the cleavage and formation of chemical bonds between atoms. The energy of the system can be expressed as follows [13]: E system = Ebond + E over + Eunder + E val + E pen + Etors + E conj + E vdWaals + ECoulomb

(1)

In Eq. (1), Esystem denotes the system energy; Ebond, which is used to calculate the bond energies from the corrected bond order, denotes the bond energy; however, the valence theory of bonding states that the total bond order of C should not exceed 4 and that of H should not exceed 1, which cannot be realized even after correction of the original bond orders; therefore, Eover and Eunder are added to denote terms for the atom over-coordination and under-coordination; Eval denotes the terms for the valence angle; Epen denotes the penalty energy, which is used to reproduce the stability of systems comprising two double bonds sharing one atom in a valence angle that is not included in Eval; Etors denotes the torsion angle energy; Econj denotes the contribution of


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conjugation effects to the molecular energy; EvdWaals denotes non-bonded van der Waals interactions, and ECoulomb denotes Coulomb interactions, which govern the interactions and attraction energies at short interatomic distances due to the Pauli principle orthogonalization and at long distances due to dispersion, respectively. In simulations, ReaxFF is used to continuously update the bond order between atoms as the atom position changes. 2.2 Simulation details for PC pyrolysis OQ2720 polycarbonate was studied in this work; its molecular structure is shown in Figure 1, where n is the degree of polymerization. Molecular models with different degrees of polymerization were simulated using Amsterdam density functional (ADF) software. The statistics showed that the polymerization degree did not change the initial pyrolysis law or main product types for the PC molecule. In addition, when the number of repeating units is 4, the solubility parameter (δ) of PC is 20.5, which is close to the experimental value (δ=20.3) [16]. Thus, considering computational power and efficiency, a PC molecule consisting of 4 repeat units was built for the simulation. The established and optimized PC molecular model is also shown in Figure 1.

CH3 O C O

O

C CH3

O C

O

O n

Figure 1. Molecular model and chemical formula for PC. Color code for every snapshot: carbon gray, hydrogen - white, and oxygen - red.


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MD in the ReaxFF module of ADF was used to simulate PC pyrolysis at elevated temperature. In reactive MD simulations, the typical time step for the motion of atoms and time taken for an elementary reaction is 0.1 fs and 0.1 ps respectively, which gives rise to the rate of reaction, but the direct simulation of the chemical reaction is too time-consuming [17]. So, the settings of high temperatures are favored in reactive MD simulations in order to promote sufficient atomic motion and molecular collisions, which can speed up the reaction and reduce the simulation time. Although this might lead to some uncertainty for the analysis of the chemical mechanism, sufficient ReaxFF MD simulations have previously demonstrated that simulation results acquired by elevation of the simulation temperature are in good agreement with experimental observations [7, 10, 11, 14-15, 18-19]. The simulations were carried out at temperatures ranging from 300 K4000 K each for a duration of 200 ps to determine the reaction temperatures and reaction time. As the simulation time increased beyond 70 ps, the products became relatively stable. Additionally, the results showed that changing the temperature altered the reaction rate and product content, but did not affect the final main product types, consistent with previous studies [7, 10, 11, 14-15, 18-19]. CO2, CO, CH4, C2H2, H2, and H2O were the main gas products, which is basically consistent with the results obtained for PC pyrolysis in N2 atmosphere [20]. Therefore, to improve the efficiency, a series of simulations were carried out at 2400 K, 2800 K, 3200 K, 3600 K, and 4000 K for 70 ps each with a time step of 0.1 fs. A small PC system, C77H66O15, including a single-chain model was established in a periodic cubic unit cell measuring 35 Å × 35 Å × 35 Å. The NPT (101.3 kPa, 298 K) ensemble was used to optimize the small PC system for 10 ps to ensure the stability of the structure, and then the simulation of the initial pyrolysis was performed. A PC model containing 8 chains was also established in a periodic cubic unit cell measuring 35 Å × 35 Å × 35 Å. The NPT (101.3 kPa,


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298 K) ensemble was also used for this model for 10 ps. Then, pyrolysis was simulated at various temperatures using a constant particle number, constant volume, and constant temperature (the so-called NVT ensemble). The status of the system was recorded every 100 fs.

3. Results and discussion 3.1 Initial reactions of PC Determining the initial PC pyrolysis mechanism is essential to clarify the insulation failure mechanism of PC. The single-chain PC molecule was simulated at a relatively low temperature of 2800 K by ReaxFF. Sixteen simulations of the initial reaction were carried out to avoid error. A bond was considered to have been broken for atoms that completely separated from each other and showed no re-bonding within a short time. By observation and analysis of the simulation process, it was possible to determine bond breaking for the single-chain PC molecule during initial pyrolysis, as shown in Figure 1. The theoretical bond energy of each bond is also shown in Figure 1. The C-O bond was the easiest to break, with a bond energy that was slightly lower than that of the C-C bond. The bond energy of the C-H bond was higher than that of both the C-O and C-C bond; therefore, it was harder to break. The C=O bond was very difficult to break because its bond energy was much greater than that of all the other three bonds. As the reaction proceeded, a C-O bond broke first with a probability of 100% due to its lower bond energy, leading to a change in the degree of polymerization for the PC molecule. As shown in Table 1, the breakage of C-O bonds was divided into two types: the C-O bond of the terminal group and between the carbonate monomers broke first with a probability of 68.75% and 31.25%, respectively. Both bond breaking paths caused the breakage of PC molecules, resulting in internal defects and aging. The breakage of C-O bonds occurred not only on the main chain, decreasing the polymerization degree but also on the side chain, generating the main gas


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products such as CH4 and H2. Montaudoa et al. proposed that the breakage of terminal chains and side chains were predominant in PC pyrolysis experiments [21-22]. Given the instability of the ester group in PC, the carbonate moiety in the terminal group is expected to degrade first, which is consistent with the simulation results obtained in this study. Analysis of the initial PC pyrolysis revealed that the C-O bonds of the carbonate group break first, fracturing the main chain followed by breaking of the C-C bonds.

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10+ CH4 CO CO2 H2O H2

80 70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

Figure 2. Distribution of major reactants and products at 3200 K. Table 1. The initial fracture types of a single PC molecule (Number is the simulation number). Type of breaking bonds Terminal group (68.75%) Single monomer (31.25%)

Number 3,4,8,9,10,11 12,13,14,15,16 1,2 5,6,7

Products C7H5O2+C70H61O13 C6H5O+C71H61O14 C22H19O4+C55H47O11 C38H34O7+C39H32O8


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0

5

10

15

20

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25

30

35

Figure 3. Proportion of each product at 70 ps and 3200 K. 3.2 Temperature-dependent pyrolysis of PC To gain a better understanding of the mechanism for PC pyrolysis, MD simulations for 70 ps of pyrolysis at 2400 K, 2800 K, 3200 K, 3600 K, and 4000 K were conducted for the PC model containing 8 chains. Analysis of the results at different temperatures showed that although the amount of the products differed slightly, the results and process of the reaction showed a certain similarity. Figure 2 shows the distribution of the major reactants and products in the pyrolysis process at 3200 K. C1-C9 represents the products containing 1-9 C atoms, and C10+ represents the products containing 10 or more C atoms. At the beginning of the simulation (0-15 ps), the PC molecules showed rapid splitting, forming large molecules of C10+. As the reaction continued, the C-O bonds and the C-C bonds broke, causing the larger molecules to break and produce smaller molecules. However, since the original PC molecule contained many benzene rings, considerable time was required to transform C10+ to C10-. In addition, crosslinking and rearrangement reactions occurred for C10- at high temperature [23]; therefore, the amount of C10+ continued to increase. Slightly later, with


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bond breakage, the formation of small molecules was an inevitable trend, leading to an increase in the number of C1-C9 molecules, especially C6+. Most of these molecules were benzene derivatives. A large amount of CH3· was produced, which was related to the fracturing of the side chain resulting from the breakage of the C-C bond. At approximately 15 ps, the quantities for most of the products entered a stationary period, with the amount of CO2 no longer changing greatly. The proportion of products at 3200 K after 70 ps is shown in Figure 3. The main gas products were CO2, CO, CH4, C2H2, H2, and H2O. This product composition is consistent with the report that CO2 production and H2O production are the most favorable reaction paths in the thermodynamics, as was proposed by Katajisto in [24]. The proportions of the gas products are slightly different from those reported previously [20]. Such differences are most likely due to secondary reactions caused by the high temperatures in the simulation or to the completely sealed simulation conditions with no possibility for the escape of gaseous products. Detailed statistics for the composition and quantity of the main gas products obtained from the simulations were collected and are shown in Table 2. As the temperature increased, the number of total fragments increased. Below a certain temperature, the number of CO2 molecules remained constant, and the number of CH4 molecules increased. In contrast, the numbers of both CO and H2 molecules increased constantly to high values. To explain these results, differences in the main gas products at different temperatures were analyzed. The evolution of CO2 and CO versus temperature is shown in Figure 4 and Figure 5, respectively. The amount of gas products increased over time. As the temperature increased, the initial formation rate of the two gases increased, which supported the hypothesis that increasing temperature can accelerate the reaction rate. When the temperature was below 3600 K, the amount of CO2 produced was much larger than that of CO; however, when the temperature reached 4000 K, the amount of CO exceeded


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that of CO2, with the difference between them becoming larger, indicating that the increase in temperature greatly promoted the generation of CO. At 3200 K, the amount of CO2 produced reached its highest value, and then decreased with increasing temperature. The main reason for these observations was that the formation mechanisms for CO2 and CO are similar. When the temperature was low, the energy available could be used to break only one C-O bond, which restrained the formation of CO. The specific mechanism for this process is discussed in the next section. Table 2. Detailed chemical compositions of small molecules observed at the end of 70 ps temperature-dependent PC simulations at temperatures ranging from 2400 to 4000 K. CO2

CO

CH4

H2

Total fragments

2400 K

35

2

2

2

82

2800 K

35

9

4

3

120

3200 K

36

22

14

10

245

3600 K

28

31

5

21

254

4000 K

11

65

2

45

311

40 2400K 2800K 3200K 3600K 4000K

35

30

25

20 15

10

5

0 0

10

20

30

40

50

60

70

Figure 4. Evolution over time of the number of CO2 molecules at different temperatures.


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70 2400K 2800K 3200K 3600K 4000K

60

50

40

30

20

10

0 0

10

20

30

40

50

60

70

Figure 5. Evolution over time of the number of CO molecules at different temperatures. 3.3 Formation mechanisms for the dominant products

Figure 6. Micro path for the formation of CO2 and CO. (a)-(e): (a) Carbonate group; (b) C-O bond of the carbonate group breaks; (c) C-O bond between the benzene and carbonate group breaks; (d) C-O bond on the other side of the molecule breaks, forming CO; (e) C-O bond


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breaks, forming CO2. The color code for every snapshot is carbon - gray, hydrogen - white, and oxygen – red. The broken bond is shown by the dashed line.

O C O O O C∗ ∗O O O C O∗ ∗ C O

O ∗ C ∗O O C∗ O C ∗O O Scheme 1. Chemical reaction pathway for the generation of CO2 and CO molecules. As discussed in the previous sections, the main products for PC pyrolysis were CO2, CO, CH4, and H2. To obtain a thorough understanding for the formation of these main products and elucidate the mechanism for PC pyrolysis, we analyzed the formation of CO2, CO, CH4, and H2 by labeling the atoms of the products during the pyrolysis process. Figure 6 shows the steps that lead to the formation of CO2 and CO, with the reaction process shown in Scheme 1. The main type of bond to break was the C-O bond. Different bond breakage positions led to different products. As shown in Figure 6 and Scheme 1, the C-O between the carbonate group and benzene or the C-O in the carbonate group broke at the first step. If the two C-O bonds of the carbonate group broke, CO was produced; otherwise, CO2 was produced. As


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shown in Scheme 1, C≡O was formed via the lone pair of electrons provided by the O atom. Following breakage of the benzene moiety, the C atom connected to the O atom also broke its bonds with the other C atoms leading to the formation of CO. However, this reaction appeared in the later stages of the process, and the probability for its occurrence was very low. The mechanism for the formation of CO2 and CO described here is basically the same as that described by Diepens [25] for the reaction process in PC photodegradation. In addition, Rivaton [26] proposed a thermal degradation mechanism for PC based on a comparison between photochemical and thermal degradation. The results of this analysis of the PC degradation mechanism under anaerobic conditions are consistent with our work, which, to some extent, supports the reliability of our simulation.

Figure 7. Micro path for the formation of CH4. (a)-(d): (a) Original bonds; (b) C-C bond breaks, forming CH3·; (c) C-H bond breaks, forming H·; (d) CH3· reacts with H· to form CH4. The color code for every snapshot is carbon - gray, hydrogen - white, and oxygen – red. The broken bond is shown by a dashed line.


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CH 3 C CH3

CH 2 ∗ ∗ H C ∗ ∗

CH3

CH 2 C CH 4

Scheme 2. Chemical reaction pathway for the generation of CH4 molecules. As the product with the third highest content, CH4 has been found in many pyrolysis experiments [20]. Though the path for CH4 formation was found to involve no chain breaking, CH4 production can also cause insulation failure. This formation path was also determined by labeling the atoms in CH4. Figure 7 shows the steps leading to the formation of CH4, with the reaction process for CH4 shown in Scheme 2. The formation paths mainly involved the breakage of C-H and C-C bonds. The C-C bond between a methyl and the main chain broke first, forming CH3·. Then, a C-H bond broke, forming H·. Next, CH3· and H· combined to form CH4. This process also generated H2, whose formation mechanism was similar to that of CH4. The analysis above provides the reaction paths for CO2, CO, CH4, and H2. The gas products form holes on the surface and in the interior of the material, resulting in insulation failure [27]. Therefore, although free radicals such as CH3· and H· are difficult to detect by current experimental methods, the above descriptions of radical-related pyrolysis reactions are reasonable compared with observations from practical pyrolysis experiments, which suggests


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that the ReaxFF MD simulation is a promising method for studying the radical-dominated pyrolysis process of PC. 3.4 Kinetic analysis of PC pyrolysis To examine the reliability of the simulation, we investigated the kinetic properties of PC pyrolysis at high temperatures in the range 2400 K-4000 K. Time evolution profiles for PC pyrolysis were used to study the first-order kinetics of PC pyrolysis. The use of the consumption rate of reactants to study the first-order kinetics of pyrolysis has been thoroughly reported by extensive studies [19, 28]. The kinetic model used in such studies assumed that PC was fully transformed to its products. In this study, the concentration of the PC reactant was simply replaced by the molecular number of PC. The rate constant k at each constant temperature T was then calculated from a linear fitting of the molecular number Nt against the simulation time t, as shown by Eq. (2). The symbol N0 denotes the original molecular number of PC, which, in this case, was 8. ln N t − ln N 0 = kt

(2)

The Napierian logarithm of the rate constant k (i.e., lnk) was then applied to a linear fitting against the reciprocal of constant temperature T (i.e., 1/T) based on the Arrhenius expression in Eq. (3) to calculate the activation energy (Ea) and pre-exponential factor (A). The symbol R in Eq. (3) is the molar gas constant, which has an approximate value of 8.314. ln k = ln A − Ea RT

(3)

The rate constant k was calculated several times by several simulations, and the average value at each temperature was used to fit the Arrhenius expression. As shown in Figure 8, the fitted slope and Y-intercept were -15.12 ×103 K·s-1 and 23.06 s-1, respectively. Therefore, the activation energy Ea and the pre-exponential factor A were 125.7 kJ·mol-1 and 1.03 ×1010 s-1,


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respectively. In [29], the activation energy for random scissions of PC and the pre-exponential factor A was calculated to be 98.9 kJ·mol-1 and 3.88 ×109 s-1, respectively. In [30], the range of the activation energy was determined to be 140-160 kJ·mol-1. In [31], application of classical Kissinger and Ozawa methods was used to obtain an average Ea value of 142 kJ·mol-1. Considering that the high temperature in this paper ranged from 2400 K to 4000 K, which is different to that used in the practical experiments, the Arrhenius parameters calculated here are in close agreement with the experimental results.

19 18.5 18 17.5 17 0.25

0.3

0.35

0.4

Figure 8. Fitted Napierian logarithm of the rate constant k versus inverse temperature T obtained from 70 ps temperature-dependent simulations of PC pyrolysis at 2400 K-4000 K. 3.5 Analysis of insulation failure due to PC pyrolysis As proposed in [3], thermal aging can finally lead to insulation failure. According to the above analysis, at high temperature, the breakage of C-O bonds occurred first, decreasing the polymerization degree, followed by the formation of small gas products such as CO2, CO, CH4, and H2, which can decrease the level of insulation. When PC is used for the insulation of


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transformers (or other electrical equipment), small gas products following pyrolysis gather in PC micropores, leading to an increase in the relatively low dielectric constant, which increases the local electric field intensity. When the local electric field intensity exceeds the breakdown field strength, partial discharge occurs in the micropore, leading to insulation failure. At the same time, the pressure in the micropore increases instantly, resulting in tears in the PC, further destroying the molecular structure. To experimentally investigate such properties, PC material was aged for 300 days at 130 °C [2]. The original PC material was smooth and compact as measured by scanning electronic microscopy (SEM) at 10 000 times magnification [2]. However, in the aged PC material, different degrees of tear block cracks and holes appeared on the surface of the sample, leading to insulation failure. In practical application, small gas products and benzene rings can be measured by mass spectrometry, infrared spectrum analysis, and other methods. Therefore, the results obtained by ReaxFF can be applied in engineering practice as new microscopic characteristic quantities or characteristic structures for the detection of PC pyrolysis.

4. Conclusions ReaxFF is used to simulate the reaction of PC molecules. The mechanism for PC pyrolysis is revealed at the atomic level. The initial cleavage of PC molecules was investigated based on a 70 ps simulation of the pyrolysis for a single-chain PC molecule. The breakage of C-O bonds between terminal groups and between PC monomers is determined to be the primary cause of the main chain scission. In addition, the reaction occurs mainly between a carbonate group and benzene or within a carbonate group.


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By simulating the pyrolysis of a multi-molecule PC system at various temperatures, the content of all products was obtained, with the main gas products identified as follows: CO2, CO, CH4, H2, and H2O. In addition, the formation pathways for the products were also analyzed. The activation energy Ea and the pre-exponential factor A in the ReaxFF simulations were calculated to be 125.7 kJ·mol-1 and 1.03 ×1010 s-1, respectively, which is both in agreement with experimental results and useful for the characterization of the pyrolysis properties of PC. Relating the simulation results to experimental analysis shows that the breakage of the PC main chain decreases the polymerization degree and that the gas products form holes both on the surface and in the interior of the material. Both these effects destroy the structure of the PC, leading to thermal aging. A mechanism for insulation failure due to thermal aging is proposed. Therefore, we envisage that the insulation failure of PC can be detected by mass spectrometry, as well as other physical or chemical methods, based on the use of the microscopic characteristics provided by the ReaxFF simulations. The micro formation process for small PC products is highlighted based on the molecular simulation. The breakage and formation of chemical bonds, recombination of free radicals, distribution of important products, and the pathways of formation can all be observed. This molecular simulation method can be used to not only calculate the microscopic parameters of the insulating materials but also analyze the microscopic mechanisms for various complex phenomena involved in insulation failure. Thus, the relationship between the microstructure and the macroscopic properties of insulating materials can be established to explore novel examples for the severity of pyrolysis, which is very important for the analysis of insulation failure and diagnosis of transformer insulation materials. This molecular simulation method can also be used


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to facilitate study of the modification of insulating materials from the perspective of molecular design. AUTHOR INFORMATION

Corresponding Author *Tel.: +86-531-81696129. E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (Grant 11675095) and the Fundamental Research Funds of Shandong University (Grant 2017JC012). ABBREVIATIONS PC, polycarbonate; ReaxFF, reactive force field; MD, molecular dynamic; NMR, nuclear magnetic resonance; TGA, thermal gravimetric analysis; FTIR, Fourier transform infrared spectroscopy; NPT, constant particle number, constant pressure, and constant temperature; QM, quantum mechanics; ADF, Amsterdam density functional; NVT, constant particle number, constant volume, and constant temperature; SEM, scanning electronic microscopy. REFERENCES 1. Y. P. Tu, W. Z. Sun and C. P. Yue, Trans. CES, 2013, 28, 7-13. 2. F. P. Li, W. Wang and K. Liu, Insulat. Mater., 2015, 48, 20-29. 3. R. J. Liao, L. J. Yang, H. B. Zheng, Trans. China Electrotech. Soc., 2012, 27, 1-12.


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A ReaxFF-Based Molecular Dynamics Simulation of the Pyrolysis

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