Nonlinear elasticity and friction of liquid-crystalline polymer monolayers

Abstract

In the present paper we consider nonlinear elasticity and friction of grafted persistent chains, which are highly stretched in the normal to the surface direction due to orientational interactions. We examine the normal and the lateral forces both in equilibrium and under shear sliding when the monolayer is confined by a bare surface. We show that in the confined monolayer in equilibrium the tilted orientation of the director becomes stable. In the sliding regime the friction force passes through a maximum value. The additional normal force in the sliding regime, when the distance between the surfaces is fixed, is also considered. We show that this force is attractive for small velocities and becomes repulsive for high velocities after the friction force passes through the maximum value.

Nanotribological Properties of Unsymmetrical n-Dialkyl Sulfide Monolayers on Gold: Effect of Chain Length on Adhesion, Friction, and Imaging

“…The dialkyl sulfides used in our study fall in the same range of chain melting points (Table ) and the monolayers are believed to exhibit an amorphous state 8a. Moreover, the observed friction−velocity spectrum of unsymmetrical dialkyl sulfides most closely resembles the spectrum for unsymmetrical dialkylammonium surfactants reported in the literature. , …”

“…In all cases the friction force F fric increases linearly with the total load F load . This type of linear dependence has been observed in several other frictional studies of monolayers of various kinds. 2a,,,, This dependence can be described by an Amonton's-law-like equation: where μ resembles the friction coefficient and F load is the sum of the externally applied load F ext and the adhesion force F adh . The absolute friction forces increase with increasing chain length of the molecules on the substrate.…”

“…The first explanation is based on local phase transitions in the layers which are induced by flow, although the nature of these phase transitions is unclear . The second proposal is based on a characteristic relaxation time for molecular interdigitations. , In our opinion molecular interdigitation is one of the most plausible mechanisms for the friction behavior. Indeed, an increase of friction force with chain length supposes an increase of the contact area between the tip and the surface and thus a chain−chain interdigitation.…”

“…The critical velocity is found to shift upward in n -octane compared to ethanol (Figure , circles). The reason for this shift is probably the penetration ,, of n -octane molecules into the monolayer, thereby increasing the disorder and causing a phase transition, making the monolayer behave more fluidlike.…”

Lateral Force Microscopy Study of the Frictional Behavior of Self-Assembled Monolayers of Octadecyltrichlorosilane on Silicon/Silicon Dioxide Immersed in n-Alcohols

“…Greater insight into the observed frictional behavior may be gained with a molecular model of energy dissipation at the surface of the AFM tip. Subbotin and co-workers presented a mechanism for the energy dissipation caused by a hard surface shearing across an organic thin film, similar to the experimental setup in our LFM measurements. In this model, energy is dissipated as the ends of the alkyl chains adsorb to and desorb from the hard surface that contacts and travels across the upper surface of the SAM.…”