A detailed insight of key reactive events related to oxidation and pyrolysis of hydrocarbon fuels further enhances our understanding of combustion chemistry. Though comprehensive kinetic models are available for smaller hydrocarbons (typically C or lower), developing and validating reaction mechanisms for larger hydrocarbons is a daunting task, due to the complexity of their reaction networks. The ReaxFF method provides an attractive computational method to obtain reaction kinetics for complex fuel and fuel mixtures, providing an accuracy approaching ab-initio-based methods but with a significantly lower computational expense. The development of the first ReaxFF combustion force field by Chenoweth et al. (CHO-2008 parameter set) in 2008 has opened new avenues for researchers to investigate combustion chemistry from the atomistic level. In this article, we seek to address two issues with the CHO-2008 ReaxFF description. While the CHO-2008 description has achieved significant popularity for studying large hydrocarbon combustion, it fails to accurately describe the chemistry of small hydrocarbon oxidation, especially conversion of CO from CO, which is highly relevant to syngas combustion. Additionally, the CHO-2008 description was obtained faster than expected H abstraction by O from hydrocarbons, thus underestimating the oxidation initiation temperature. In this study, we seek to systemically improve the CHO-2008 description and validate it for these cases. Additionally, our aim was to retain the accuracy of the 2008 description for larger hydrocarbons and provide similar quality results. Thus, we expanded the ReaxFF CHO-2008 DFT-based training set by including reactions and transition state structures relevant to the syngas and oxidation initiation pathways and retrained the parameters. To validate the quality of our force field, we performed high-temperature NVT-MD simulations to study oxidation and pyrolysis of four different hydrocarbon fuels, namely, syngas, methane, JP-10, and n-butylbenzene. Results obtained from syngas and methane oxidation simulation indicated that our redeveloped parameters (named as the CHO-2016 parameter set) has significantly improved the C chemistry predicted by ReaxFF and has solved the low-temperature oxidation initiation problem. Moreover, Arrhenius parameters of JP-10 decomposition and initiation mechanism pathways of n-butylbenzene pyrolysis obtained using the CHO-2016 parameter set are also in good agreement with both experimental and CHO-2008 simulation results. This demonstrated the transferability of the CHO-2016 description for a wide range of hydrocarbon chemistry.

During the carbonization process of raw polymer precursors, graphitic structures can evolve. The presence of these graphitic structures affects mechanical properties of the carbonized carbon fibers. To gain a better understanding of the chemistry behind the evolution of these structures, we performed atomistic scale simulations using the ReaxFF reactive force field. Three different polymers were considered as a precursor: idealized ladder PAN (polyacrylonitrile), a proposed oxidized PAN and PBO (poly(p-phenylene-2,6-benzobisoxazole)). We determined the underlying molecular details of polymers conversion into a carbon fiber structure. Since these are C/H/O/N-based polymers, we first developed an improved force field for C/H/O/N chemistry based on the Density Functional Theory (DFT) data with a particular focus on N2 formation kinetics and its interactions with polymer-associated radicals formed during the carbonization process. Then, using this improved force field, we performed atomistic scale simulations of the initial stage of the carbonization process for the considered polymers. Based on our simulation data we determined the molecular pathways for the formation of low-molecular weight gas-species, all-carbon rings crucial for further graphitic structures evolution and possible alignment of the evolved all-carbon 6-membered rings clusters.

Various methods have been developed to perform atomistic-scale simulations for the cross-linking of polymers. Most of these methods involve connecting the reactive sites of the monomers, but these typically do not capture the entire reaction process from the reactants to final products through transition states. Experimental time scales for cross-linking reactions in polymers range from minutes to hours, which are time scales that are inaccessible to atomistic-scale simulations. Because simulating reactions on realistic time scales is computationally expensive, in this investigation, an accelerated simulation method was developed within the ReaxFF reactive force field framework. In this method, the reactants are tracked until they reach a nonreactive configuration that provides a good starting point for a reactive event. Subsequently, the reactants are provided with a sufficient amount of energy-equivalent or slightly larger than their lowest-energy reaction barrier-to overcome the barrier for the cross-linking process and form desired products. This allows simulation of cross-linking at realistic, low temperatures, which helps to mimic chemical reactions and avoids unwanted high-temperature side reactions and still allows us to reject high-barrier events. It should be noted that not all accelerated events are successful as high local strain can lead to reaction rejections. The validity of the ReaxFF force field was tested for three different types of transition state, possibly for polymerization of epoxides, and good agreement with quantum mechanical methods was observed. The accelerated method was further implemented to study the cross-linking of diglycidyl ether of bisphenol F (bis F) and diethyltoluenediamine (DETDA), and a reasonably high percentage (82%) of cross-linking was obtained. The simulated cross-linked polymer was then tested for density, glass transition temperature, and modulus and found to be in good agreement with experiments. Results indicate that this newly developed accelerated simulation method in ReaxFF can be a useful tool to perform atomistic-scale simulations on polymerization processes that have a relatively high reaction barrier at a realistic, low temperature.

Irradiation of polymer films by a CO 2 infrared laser under ambient conditions converts the polymer into porous graphene or laser-induced graphene (LIG). Here, we simulate the formation of LIG from five different commercially available polymers using reactive molecular dynamics. We determined that the molecular structure of the parent polymer has a significant effect on the final graphitic structure. CO is liberated during the initial part of the LIG formation process when the polymer is converted into an amorphous structure, while H 2 is evolved steadily as the amorphous structure is converted to an ordered graphitic structure. The LIG structure has out-of-plane undulations and bends due to a significant number of 5-and 7-member carbon rings present throughout the structure. We find that the simulated molecular structure compares well with recent experimental observations from the literature. We also demonstrate that the yield of LIG is higher in inert conditions, compared to environments with oxygen. Polybenzimidazole-derived LIG has the highest surface area and yield among the five polymers examined. These findings provide knowledge of LIG formation mechanisms that can be leveraged for bulk LIG applications such as sensors, electrocatalysts, microfluidics, and targeted heating for welding polymers.

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