UC Merced

Shear-driven chemical reactions play a critical role in many manufacturing, tribological, and synthesis processes. However, mechanistic understanding of such reactions is still limited due to the inherent difficulty in studying shear-driven reactions using experimental techniques as these reactions often happen at a sliding interface. This dissertation aims to use reactive molecular dynamics (MD) to further the current understanding of shear-activation. First, we investigated the individual effects of heat, normal stress, and shear stress on reaction pathways using reactive MD simulations of α-pinene molecules on silica. Results identified shear stress as the key driver of oligomerization reactions under sliding conditions. Normal stress alone was ineffective in inducing any reactions and oligomerization could be driven thermally only at very high temperatures. Analysis of the reaction pathways showed that shear can activate multiple reaction mechanisms that are not accessible thermally. Calculations of bond lengths and dihedral angles revealed that such activations are associated with physical deformation of reacting species. Next, we studied shear-activated reactions of simple cyclic organic molecules to isolate the effect of chemical structure on reaction yield and pathway. Reactive MD simulations were used to model methylcyclopentane, cyclohexane, and cyclohexene subject to pressure and shear stress between silica surfaces. Cyclohexene was found to be more susceptible to undergo shear-activated oxidative chemisorption and subsequent oligomerization reactions than methylcyclopentane and cyclohexane. The oligomerization trend was consistent with shear-driven polymerization yield measured in ball-on-flat sliding experiments. Analysis of the reactants in simulations identified the C=C double bond in cyclohexene as being the origin of its shear susceptibility. Lastly, the most common reaction pathways for association were identified, providing insight into how the chemical structures of the precursor molecules determined their response to mechanochemical activation. Finally, we investigated the effect of molecular deformation on lowering the reaction energy barrier by studying the shear stress-driven oligomerization reactions of cyclohexene on silica using reactive MD simulations. Simulations captured an exponential increase in reaction yield with shear stress and highlighted the role of surface oxygen atoms in driving the reactions. Trends from simulations were corroborated by ball-on-flat tribometer experiments and elemental analysis of ball-on-flat reaction products. Structural analysis of the reacting molecules in simulations indicated the reactants were deformed just before a reaction occurred. Quantitative evidence of shear-induced deformation was established by comparing bond lengths in cyclohexene molecules in equilibrium and prior to reactions. Nudged elastic band calculations showed that the deformation had a small effect on the transition state energy but notably increased the reactant state energy, ultimately leading to a reduction in the energy barrier. Finally, a quantitative relationship was developed between molecular deformation and energy barrier reduction by mechanical stress. The findings from this dissertation on the activation mechanisms underlying shear-driven mechanochemical reactions provide critical insights that can guide design of materials and processes with optimized and potentially tunable shear-induced reactions.