Friction of Polyaromatic Thiol Monolayers in Adhesive and Nonadhesive Contacts

Abstract

We have used friction force microscopy to study the effects of adhesion on the boundary friction of self-assembled monolayers of the aromatic compounds thiophenol, p-phenylthiophenol, p-terphenyl thiol, 2-naphthalenethiol, and benzyl mercaptan on gold. To control the adhesion between the monolayer-covered tip and substrate, the friction measurements were made in dry N(2) gas or in ethanol. At low loads, low adhesion (in ethanol) resulted in a linear dependence of the friction force on load (i.e., F = muL) whereas higher adhesion between the same monolayers (in N(2)) gave an apparent area-dependent friction. The friction in the adhesive systems was well described by F = S(c)A with the contact area, A, calculated for a thin, linearly elastic film confined between rigid substrates using the thin-coating contact mechanics (TCCM) model in a transition regime between its DMT- and JKR-like limits. With increasing packing density of the monolayers, a systematic decrease was found in the friction coefficient (mu) obtained in ethanol and the critical shear stress (S(c)) obtained in N(2). To describe these aromatic monolayers with the extended TCCM model, a higher Young's modulus was neeeded than for fatty acid monolayers of similar packing density.

Nanomanufacturing : nano-structured materials made layer-by-layer.

“…1. This includes friction data for a glass tip interacting with model lubricant silane and alkanethiol selfassembled monolayers (Burns, Houston et al 1999), a silicon AFM tip sliding over an OTS (octadecyltrichlorosilane) self-assembled monolayer-coated silicon substrate (Reedy, Starr et al 2005), and an AFM tip coated with a variety of self-assembled aromatic compounds sliding along a similarly coated substrate (Yang and Ruths 2009). These results suggest that the friction response defined by Eq.…”

The Effects of Interface Structure and Polymerization on the Friction of Model Self-Assembled Monolayers

“…However, because packing density is also affected by chain length in alkanethiol SAMs, it was difficult to separate chain length from packing density effects. Several experiments were designed to separate the effects of chain length from packing density [11][12][13][14][15][16][17][18]. One experiment utilized SAMs composed of spiroalkanedithiols [11], as a unique way to have the same packing of the anchor group but different packing density at the sliding interface.…”

Odd–Even Effects in the Friction of Self-Assembled Monolayers of Phenyl-Terminated Alkanethiols in Contacts of Different Adhesion Strengths

“…Aromatic compounds exhibit potentially useful electronic and optical properties [10], and their stiffness is of practical use for forming end-functionalized monolayers where the orientation of the end-group is unaffected by the gauche defects found in alkane-based systems [10,11]. The orientation and close-packing in the aromatic systems are affected by the stiffness of the molecules, and it has been shown that even the introduction of a single -CH 2 -group between the aromatic moiety and the group anchored to the surface enables a better packing of the resulting monolayer [12,13], and correspondingly diminished friction [8,[14][15][16].…”

“…Despite these important applications, there is remarkably limited information available on the molecular-level lubricating properties of aromatic compounds [7,20]. To this end, we have examined simple aromatic monolayers [8,14,15] and a series of polyaromatic thiol-based monolayers [16] that are highly rigid and give a relatively high friction. In this work, we use atomic force microscopy (AFM) to study a series of phenyl-terminated alkanethiol self-assembled monolayers [21] with the same end-group functionality but higher molecular packing than the simple aromatic thiols studied previously.…”

“…Friction in single-asperity contacts is commonly observed to depend on the strength of adhesion [8,16,[22][23][24][25][26][27], therefore the measurements were done in ethanol, where the adhesion (attraction due to van der Waals forces) is low, and in dry N 2 gas, where the adhesion is higher. As in several of these previous studies [16,23], we observe here also different functional forms of the friction force F versus load (normal force) L in adhesive versus non-adhesive systems.…”