Graphitic materials such as carbon fibers play a critical role in enabling a wide range of high-performance applications ranging from nuclear to aerospace industries due to their high stiffness, negligible creep, chemical inertness, and high temperature stability (&gt;2000 °C). Unlocking the potential of such complex carbon materials requires a robust understanding of how the chemistry and structure of its constituent molecules evolve and crosslink, to achieve a tailorable set of functional properties. 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. In this talk, I will discuss our recent work on isolating the effect of methyl groups on crosslinking reactions and evaluating the impact on the elastic modulus by considering three monodisperse pyrene-based systems with increasing fraction of methyl groups. Utilizing our previously developed framework of molecular dynamics simulations with ReaxFF to generate carbon fibers and evaluate their elastic modulus, we find that increasing the fraction of methyl functional groups can: a) almost double the number of crosslinking bonds formed, b) ensure a larger fraction of the crosslinked bonds are oriented in the direction of tensile stress, and c) result in a higher degree of alignment of the precursor molecules along the direction of tensile stress, resulting in enhanced tensile modulus. Additionally, we were able to demonstrate via experiments that precursor molecules with higher fraction of methyl groups resulted in a higher alignment of precursor molecules in the resultant product. Moreover, increased methyl groups result in lower spread of molecular alignment which may lead to smaller variation in tensile modulus and more consistent properties in carbon materials derived from methyl rich precursors.

Effect of methyl groups on crosslinking reactions in carbon-based systems

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