Carbon fibers display several advantages like high strength to weight ratio but the high costs prohibit more widespread use. Thus, it is important to seek novel feedstock and processing techniques to make them cheaper. Carbon fibers synthesized from coal-tar and petroleum pitch display excellent mechanical properties, especially the elastic modulus. Processes used to generate them are complicated and involves numerous parameters, making both optimization and understanding of key mechanisms challenging. Computational modeling can provide a useful tool for providing fundamental insight into the chemistry-structure-property correlations, as well as for determining which parameters play the most important roles in determining the mechanical properties of carbon fibers. In this talk, I will discuss our recent work employing molecular dynamics simulations with a reaxFF force field to simulate the formation of fibers from precursor pitch molecules, focusing in particular on the impact of aliphatic and aromatic hydrogen atoms on the moduli. In addition to the stabilization and carbonization steps, the simulations capture the graphitization process which leads to the fibers with the highest moduli. The elastic moduli of the simulated carbon fibers match closely with the experimental values. Our results indicate that density and alignment of the fibers are critical in determining moduli while the size and shape of the initial molecules play a less important role. Further, we also show that theoretically increasing density corresponds to an increased sp<sup>3</sup> fraction that also positively correlates with the moduli, opening up an alternative path for high modulus fibers other than graphitization. The higher sp<sup>3</sup> fraction also leads to better compressive performance with implications on alternative applications for carbon fibers.

Atomistic modeling of carbon fibers made from pitch: Insight into elastic modulus

ACS Fall 2021

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