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The molecules with higher tribochemical reactivity exhibited smaller activation volume, implying that less mechanical energy was required to initiate tribochemical reactions. 

Abstract Mechanical stress can directly activate chemical reactions by reducing the reaction energy barrier. A possible mechanism of such mechanochemical activation is structural deformation of the reactant species. However, the effect of deformation on the reaction energetics is unclear, especially, for shear stress-driven reactions. Here, we investigated shear stress-driven oligomerization reactions of cyclohexene on silica using a combination of reactive molecular dynamics simulations and ball-on-flat tribometer experiments. Both simulations and experiments captured an exponential increase in reaction yield with shear stress. Elemental analysis of ball-on-flat reaction products revealed the presence of oxygen in the polymers, a trend corroborated by the simulations, highlighting the critical role of surface oxygen atoms in oligomerization reactions. 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. 

Tribochemistry, which is another name for mechanochemistry driven by shear, deals with complex and dynamic interfacial processes that can lead to surface wear or formation of beneficial tribofilms. For better mechanistic understanding of these processes, we investigated the reactivity of tribopolymerization of organic molecules with different internal ring strain (methylcyclopentane, cyclohexane, and cyclohexene) on a stainless steel (SS) surface in inert (N2), oxidizing (O2), and reducing (H2) environments at room temperature. On the clean stainless steel surface, precursor molecules were found to physisorb with a broad range of molecular orientations. In inert and reducing environments, the strain-free cyclohexane showed the lowest tribochemical activity among the three molecules tested. Compared to the N2 environment, the tribochemical activity in H2 was suppressed. In the O2 environment, only cyclohexene produced tribofilms and methylcyclopentane while cyclohexane did not. When tribofilms were analyzed with Raman spectroscopy, the spectral features of diamond-like carbon (DLC) or amorphous carbon (a-C) were observed due to photochemical degradation of triboproducts. Based on infrared spectroscopy, tribofilms were found to be organic polymers containing oxygenated groups. Whenever polymeric tribrofilms were produced, wear volume was suppressed by orders of magnitudes but not completely to zero. These results support previously suggested mechanisms which involve surface oxygen as a reactant species in the tribopolymerization process. 

In the Raman analysis of tribofilms produced from organic precursors, the D- and G-band features are often observed, which resemble the characteristic bands of diamond-like carbon (DLC), amorphous carbon (a-C), or graphitic materials. This study reports experimental evidence that the D- and G-bands features in the Raman spectra of tribofilms could be generated by photochemical degradation of triboproducts due to the focused irradiation of laser beam during the Raman analysis, indicating that they are not unique to the genuine structure of the tribofilm produced via friction. This finding suggests that other complementary and non-destructive characterization is required to determine whether DLC, a-C, or graphitic species are produced tribochemically by frictional shear. 

Mechanochemical activation has created new opportunities for applications such as solvent-free chemical synthesis, polymer processing, and lubrication. However, mechanistic understanding of these processes is still limited because the mechanochemical response of a system is a complex function of many variables, including the direction of applied stress and the chemical features of the reactants in non-equilibrium conditions. Here, we studied shear-activated reactions of simple cyclic organic molecules to isolate the effect of chemical structure on reaction yield and pathway. Reactive molecular dynamics 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 mechanochemical activation of oxidative chemisorption and subsequent oligomerization reactions than either methylcyclopentane or cyclohexane. The oligomerization trend was consistent with shear-driven polymerization yield measured in ball-on-flat sliding experiments. Analysis of the simulations showed the distribution of carbon atom sites at which oxidative chemisorption occurred and identified the 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.