Surface analysis force-distance curves

NR with standard recipe with 10 phr CB (NR 10) was prepared as the sample. The compound recipe is shown in Table 21.2. The sectioned surface by cryo-microtome was observed by AFM. The cantilever used in this smdy was made of Si3N4. The adhesion between probe tip and sample makes the situation complicated and it becomes impossible to apply mathematical analysis with the assumption of Hertzian contact in order to estimate Young s modulus from force-distance curve. Thus, aU the experiments were performed in distilled water. The selection of cantilever is another important factor to discuss the quantitative value of Young s modulus. The spring constant of 0.12 N m (nominal) was used, which was appropriate to deform at rubbery regions. The FV technique was employed as explained in Section 21.3.3. The maximum load was defined as the load corresponding to the set-point deflection. 

The atomic force microscope (AFM) is a promising device for the investigation of materials surface properties at the nanoscale. Precise analysis of adhesive and mechanical properties, and particularly of model polymer surfaces, can be achieved with a nanometer probe. This study distinguishes the different contributions (chemical and mechanical) included in an AFM force-distance curve in order to estabhsh relationships between interfacial tip-polymer interactions and the surface viscoelastic properties of the polymer. 

The simplest theory for the analysis of surface elasticity based on AFM force-distance curve measurement is Hertzian contact mechanics [Landau and Lifshitz, 1967], As shown schematically in Figure 3.1, a force-distance curve is a plot of the displacement, z, of the piezoelectric scanner normal to the specimen s surface as the horizontal axis and the cantilever deflection, A, as the vertical axis. Hertzian contact mechanics cannot treat adhesive force in principle. We need to make some effort to minimize the adhesive force in a practical experiment. Measurement in aqueous conditions is effective for polymeric materials with low water absorbability. A cantilever with a large spring constant also hides weak van der Waals forces. Figure 3.1a shows the... 

SFM also enables us to measure specific interaction forces between a small silicon tip and the surface. The pull-off forces between the tip and the surface estimated from Force vs Distance Curves (FDC) can be correlated to the adhesive interactions between tip and surface [9]. Recording of FDCs line-by-line allows us to image surface topography and adhesive surface properties simultaneously [10]. This technique has some disadvantages, like the requirement for a large amount of data acquisition and analysis, which have been alleviated by the invention of the Pulsed Force Mode (PFM). The PFM simplifies and accelerates the measurements of adhesive properties with high lateral resolution [11, 12]. 

Surface Force Measurements. This technique enables the measurement of the force (10 mN accuracy) versus distance (0.1-0.2 nm accuracy) between two curved mica surfaces. The forces between two solid surfaces across an aqueous solution are highly sensitive to the structure of the solid/liquid interfaces. When such surfaces are covered with adsorbed protein layers, then, the analysis of the force/distance profiles may reveal the formation of protein bridges between the two surfaces, the occurrence of steric interactions, or any possible protein conformation change. 

A critical point in the evaluation of AFM force curves is the determination of zero distance, meaning the point where the probe starts to touch the surface. While in the SFA, the analysis of the FECO pattern gives clear information on any gap remaining between the mica surfaces, there is no independent check for absolute distance in the AFM. This can lead to ambiguities either due to adsorbed layers or due to deformation of soft surfaces. One attempt to overcome titis limitation was the combination of AFM with TIRM [204] (see Section 3.4), where the scattering signal from a colloid probe was used for absolute determination of separation distance. 

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