(373f) Molecular Energy Dissipation in Organic Compliant Viscous Thin Film Materials

One of the enduring challenges in tribology (i.e., the study of friction) is the molecular description of energy dissipation in frictional contact zones if complex material systems, such as polymers and solid organic lubricants, are involved. Thereby, tribological sliding processes involving solids have been viewed as activated processes, and have been modeled in two ways, i.e., based on (a) fixed barrier models for shear induction (e.g. Prandtl-Tomlinson model), and (b) reaction kinetic models (e.g. Eyring model) for rigid solids and compliant viscous materials, respectively. This paper expands the view of reaction kinetic models applied to solid organic lubricants and polymers (e.g., octadecylphosphonic acid monolayers covalently bound to hafnium oxide substrates, polystyrene and poly(tert-butyl acrylate)) by investigating the enthalpic and entropic components of the apparent activation energy close to and far from the material intrinsic submolecular relaxation times. Involving an intrinsic friction analysis (IFA) that is based on scanning probe methods, see figure below, we will investigate the impact of material intrinsic relaxation modes on energy dissipation expressed by process parameters, such as the friction coefficient and the applied load. We will illustrate how an excessive load beyond the load capacity of the top surface material layer can lead to a shift in the molecular mode responsible for energy dissipation, and thus, significantly impact the friction coefficient. Also, temperature induced molecular relaxations have been found to affect both the enthalpic and entropic energy contributions to the apparent activation energy, as well as tribological process parameters. Interestingly, the entropic energy component, which is cooperative in nature, is found to rise up to 70% of the total energy barrier in polymer melts close to the glass transition temperature. Finally, this paper will also address the shortcomings of the Eyring model and compare it to IFA and dielectric spectroscopy results for polymer systems.