Poly , dynamic contact between

Figure 3. Dynamic contact between a silicon nitride SFM tip and a poly-vinylethylene sample, (a) Force curve, (b) Amplitude X, and phase a responses of the tip. (Adapted with permission fix>m reference 9. Copyri t 9 Tribology Letters.

FIGURE 17.15 Direct electrochemical contact between an incoming AFM-SECM probe and the redox Fc heads of nanometer-sized, flexible chains (such as PEG or DNA), end-grafted onto an electrode substrate. The Fc heads are alternatively oxidized at the tip and reduced back at the substrate. For clarity, the tip is not drawn to scale. The chains are pictured in the mushroom conformation. (Adapted with permission from Abbou, J., Anne, A., and Demaille, C., Probing the structure and dynamics of end-grafted flexible polymer chain layers by combined atomic force—Electrochemical microscopy. Cyclic voltammetry within nanometer-thick macromolecular poly(ethylene glycol) layers, J. Am. Chem. Soc., 126,10095-10108, 2004. Copyright 2004 American Chemical Society.)... 

The SEM was used to demonstrate and confirm these theoretical projections [306,309]. In the early study [306], molten drops of PE and PMMA were allowed to spread and solidify on a paper substrate prior to standard preparation for SEM. Later, poly(phenyl ether) (PPE) vacuum pump oil (Santovac 5-Monsanto Chemicals), which is not volatile, was used for dynamic studies. The PPE was fed through a hole in the sample stub, from outside the specimen chamber, and the wetting experiment was video tape recorded [309]. Mori et al. [310] used freshly cleaved mica surfaces as steps 60 nm in height inhibit wetting. Surface roughness has a major effect on the local contact angle between the liquid and the substrate of interest. 

In dynamic loading, the probe is initially out of contact with the surface. Most scanning force microscopes perform dynamic loading under displacement control of the sample (but not the probe), e.g., the distance Z(t) between the cantilever mount and sample is reduced at constant speed V,>,. The probe and sample are then brought into contact, the contact is loaded, and then unloaded until it ruptures. Figure 3a shows typical loading behavior for a viscoelastic substrate, poly(vinylethylene) (9). 

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