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45101-G5
Friction of Self-Assembled, Polyaromatic Monolayers

Marina C. Ruths, University of Massachusetts Lowell

This project focused on fundamental aspects of friction of polyaromatic self-assembled monolayer structures (SAMs) at solid-liquid and solid-gas interfaces. The friction of aromatic and polyaromatic systems has thus far not been extensively studied despite their occurrence and importance for the natural lubricity of mineral oils and fuels.

Materials and Methods: SAMs of thiophenol, benzyl mercaptan, 2-napthalenethiol, 4-biphenylthiol, and p-terphenylthiol on template-stripped gold were studied in single-asperity contacts with an atomic force microscope (AFM) in friction mode. The AFM tips were functionalized with the same SAMs. The strength of adhesion (interfacial energy) was altered immersing the systems in ethanol (low adhesion) or in dry N₂ gas (higher adhesion). Recent contact mechanics models for thin, compliant elastic films confined between stiffer substrates were used to evaluate the data in systems with higher adhesion (in dry N₂) [Johnson, K.L.; Sridhar, I: J. Phys. D: Appl. Phys. 2001, 34, 683; Sridhar et al., Ibid. 2004, 37, 2886; Reedy, E. D., J. Mater. Res. 2006, 21, 2660. Ibid. 2007, 22, 2617].

Experimental results: In ethanol, the interfacial energy was low, gamma < 1 mJ/m², and a linear dependence of the friction force (F) on load (normal force, L), with F -> 0 at L -> 0 was found in all systems (not shown). The same F was found at low L for tip radii in the range R = 35–230 nm, i.e., F in systems with low adhesion did not depend on the contact area, which is consistent with previous observations [Ruths, M. J. Phys. Chem. B 2006, 110, 2209]. The friction coefficient decreased with increasing packing density of the SAM. F was also measured in dry N₂ gas (r.h. <1.5%), where the adhesion is higher, gamma = 35-40 mJ/m² (Figure 1). The shape of the F vs. L curves in Figure 1 are different from the linear F vs. L dependence observed in ethanol, which has also been observed previously for certain other aromatic and alkanethiol SAMs [Ruths, M., ibid.]. F depends on R under adhesive conditions, i.e., the data in Figure 1 shows a contact area-dependence.

Parameters from contact mechanics models: The radius of the contact area was comparable in size to the thickness of the monolayer, whose elastic modulus is 10-50 times lower than that of the confining surfaces. The contact mechanics models were applied to the data obtained in dry N₂. The solid curves in Figure 1 represent F = S_cA with the area A calculated using an effective film modulus of E_u = 3 GPa, giving a critical shear stress S_c = 450-500 MPa for thiophenol and ca 200 MPa for 4-biphenylthiol. These values suggest a lower friction in the 4-biphenylthiol system, which was also found when measuring linear friction F = mu*L in ethanol (mu = 1.2 and 1.0, respectively). All systems we studied have higher friction than close-packed alkanethiol SAMs.

Discussion: Low adhesion (in ethanol) resulted in a linear increase in friction force with load, i.e., F = mu*L, whereas higher adhesion (in N₂ gas, Figure 1) gave an apparent area-dependence of the form F = S_cA. Using recently developed contact mechanics models for confined thin films, we obtained S_c values at low load in the adhesive systems. S_c decreases with increasing packing density, i.e., the same trend is seen as for the friction coefficient in ethanol. The experimental data at the highest loads frequently agree less well with the models, i.e., with the linear dependence in ethanol (not shown) or with the solid curves in Figure 1. In some cases, a distinct onset of another region is seen as in Figure 1a&b (arrows). Measurements with tips with smaller radii show onsets of this divergence at lower loads [Ruths, M. ibid.]. In work on alkanethiol monolayers [Liu, G.-y.; Salmeron, M. Langmuir 1994, 10, 367], it has been suggested that this could be caused by a reversible displacement of molecules in the monolayer at a certain pressure. However, in our work, the values of F at high load are not the same as for a bare substrate, and it is therefore unlikely that the monolayer is completely removed from the contact.

This project provided one graduate student with the opportunity to develop expertise in surface modification and AFM characterization techniques. The results present a deeper understanding of the relationships between SAM structure, adhesive conditions, and friction response, which are long-standing research interests of the PI, and the approach is directly applicable to recently initiated projects on other types of SAMs and on spin-coated polymer films.

FIGURE 1. F vs. L measured in dry N₂ gas. (a) Thiophenol, R = 52 and 190 nm. (b) 4-Biphenylthiol, R = 62 nm. The curves are calculated according to the Reedy thin-coating DMT contact mechanics model with E_u = 3 GPa and film thicknesses h as in the Figure.

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