Molecular friction dissipation and mode coupling in organic monolayers and polymer films

Abstract

The impact of thermally active molecular rotational and translational relaxation modes on the friction dissipation process involving smooth nano-asperity contacts has been studied by atomic force microscopy, using the widely known Eyring analysis and a recently introduced method, dubbed intrinsic friction analysis. Two distinctly different model systems, i.e., monolayers of octadecyl-phosphonic acid (ODPA) and thin films of poly(tert-butyl acrylate) (PtBA) were investigated regarding shear-rate critical dissipation phenomena originating from diverging mode coupling behaviors between the external shear perturbation and the internal molecular modes of relaxation. Rapidly (ODPA) versus slowly (PtBA) relaxing systems, in comparison to the sliding rate, revealed monotonous logarithmic and nonmonotonous spectral shear rate dependences, respectively. Shear coupled, enthalpic activation energies of 46 kJ∕mol for ODPA and of 35 and ∼65 kJ∕mol for PtBA (below and above the glass transition) were found that could be attributed to intrinsic modes of relaxations. Also, entropic energies involved in the cooperative backbone mobility of PtBA could be quantified, dwarfing the activation energy by more than a factor of five. This study provides (i) a material specific understanding of the molecular scale dissipation process in shear compliant substances, (ii) analyses of material intrinsic shear-rate mode coupling, shear coordination and energetics, (iii) a verification of Eyring's model applied to tribological systems toward material intrinsic specificity, and (iv) a valuable extension of the Eyring analysis for complex macromolecular systems that are slowly relaxing, and thus, exhibit shear-rate mode coupling.

Energy dissipation in atomic-scale friction

“…By plotting the friction force measured between an AFM tip and a polymer substrate as a function of logarithmic sliding velocity, one obtains a bell-shaped curve as shown in Fig. 9 if the shear rate matches the material intrinsic relaxation time [45].…”

Three-dimensionally bonded spongy graphene material with super compressive elasticity and near-zero Poisson’s ratio

“…Notably, the material also demonstrates rather stable viscoelastic temperature-and frequency-invariant stability, reversible deformation and fatigue resistance. DMA measurements of the sponge's viscoelastic properties in air and liquids show that the origin of sponge compressive elasticity is enthalpic, rather than the entropic elasticity found for ordinary rubbers 10,11 . As a consequence, in contrast to the conventional elastomers like silicone rubber, the sponge's storage and loss moduli depend relatively little on temperature ( À 50 to 300°C in air for Fig.…”

“…Even when the maximum (499%) strain is applied, the graphene sponge exhibits highly repeatable compression reversibility. Dynamic mechanical analysis (DMA) measurements demonstrate that the storage and loss moduli of our sponges are largely independent of temperature and frequency, indicating that the origin of elasticity is enthalpic (rather than entropic, as for ordinary rubbers) 10,11 . Furthermore, the graphene sponge that absorbs over 1,000 times its weight in pump oil, and reversibly discharges a similar amount during subsequent compression.…”

Dielectric relaxation properties of carboxylic acid-terminatedn-alkyl monolayers tethered to Si(1 1 1): dynamics of dipoles and gauche defects

“…Molecular-level insights into the dynamics of defect reorientation is also important to identify various channels of energy dissipation which govern tribological properties of functionalized surfaces [25][26][27].…”