Surface Chemistry Dependence of Mechanochemical Reaction of Adsorbed Molecules—An Experimental Study on Tribopolymerization of α-Pinene on Metal, Metal Oxide, and Carbon Surfaces

Abstract

Mechanochemical reactions between adsorbate molecules sheared at tribological interfaces can induce association of adsorbed molecules, forming oligomeric and polymeric products often called tribopolymers). This study revealed the role or effect of surface chemistry of the solid substrate in mechanochemical polymerization reactions. As a model reactant, α-pinene was chosen because it was known to readily form tribopolymers at the sliding interface of stainless steel under vapor-phase lubrication conditions. Eight different substrate materials were tested-palladium, nickel, copper, stainless steel, gold, silicon oxide, aluminum oxide, and diamond-like carbon (DLC). All metal substrates and DLC were initially covered with surface oxide species formed naturally in air or during the oxidative sample cleaning. It was found that the tribopolymerization yield of α-pinene is much higher on the substrates that can chemisorb α-pinene, compared to the ones on which only physisorption occurs. From the load dependence of the tribopolymerization yield, it was found that the surfaces capable of chemisorption give a smaller critical activation volume for the mechanochemical reaction, compared to the ones capable of physisorption only. On the basis of these observations and infrared spectroscopy analyses of the adsorbed molecules and the produced polymers, it was concluded that the mechanochemical reaction mechanisms might be different between chemically reactive and inert surfaces and that the chemical reactivity of the substrate surface greatly influences the tribochemical polymerization reactions of adsorbed molecules.

Activation Volume in Shear-Driven Chemical Reactions

“…[66] For the same molecule, the ∆𝑉𝑉 * also varies depending on the reactivity of the substrate on which the molecule is being sheared. [67] The reason for the differences or similarities between activation volumes may vary depending on the physical meaning of ∆𝑉𝑉 * itself. Reprinted with permission from Ref.…”

Mechanochemical Association Reaction of Interfacial Molecules Driven by Shear

“…[24][25][26] Intuitively, the molecules chemically anchored to the sliding surface would experience more mechanical shear than the molecules slipping within the interface. 27 However, the exact roles of surface chemistry and mechanical shear need to be unraveled for a deeper understanding of mechanochemistry.…”

Dependence of Tribological Performance and Tribopolymerization on the Surface Binding Strength of Selected Cycloalkane-Carboxylic Acid Additives

“…Reports of tripolymerization can be dated back to the 1950s [5][6][7], in which tribopolymer deposit was generated on metal contacts in the presence of hydrocarbons, while others discussed the mechanistic aspect of tribopolymerization and its application to improve tribological performance [8][9][10]. More recently, Kim et al [11][12][13] reported the use of vapor phase lubrication using alcohols and α-pinene that ultimately led to the formation of tribopolymers. Our group [14,15] described the use of cyclopropanecarboxylic acid (CPCa) as a model additive for tribopolymerization, which Qiang Ma and Arman Mohammad Khan have contributed equally to this work.…”