Practical Adhesion Mapping

Practical adhesion mapping can be performed in the so-called force-volume (FV) mode. In this mode, f-d curves are acquired for each pixel. Thus, information is obtained on attractive forces before the tip contacts the surface, indentation in the contact region (see Sect. 4.3), adhesive interactions, and the dissipated energy (as area under the f-d curve, compare Fig. 4.3). 

In the FV approach the deflection data are shced and sorted in deflection values that are larger and smaller than some arbitrarily chosen value. Hence, this mode displays the deflection values with respect to a reference point, which is different from true pull-off forces. For instance, f-d curves displaying a large hysteresis may show a strongly negative (deflection = force) value in the FV image, which may erroneously imply large pull-off forces (for an illustrative example see hands-on example 38). 

The data shown below, in Fig. 4.4, illustrate the richness of local information that can be extracted. On a micropattemed surface of organic molecules exposing hydrophobic -CH3 and hydrophilic -COOH groups at the film surface (prepared by microcontact printing [23]), the border region of the printed pattern was investigated in AFM-FV measurements. F-d curves were acquired pixel-per-pixel and provided information of local tip-surface interactions as shown in panels a and b. Here, the difference in pull-off force between the two chemically distinct regions and the tip is clearly pronounced. 

Advanced off-fine analysis software can help to access the statistical and the lateral distribution of the pull-off forces (panels c and d, respectively). The corresponding data, e.g., magnitude of the pull-off force, displayed in a 2D adhesion map (Fig. 4.4d) show that the border region is indeed characterized by a gradually changing, mixed chemical composition. The corresponding composition gradient can thus be mapped. Similarly, the statistically treated data yield a bimodal distribution of the pull-off forces (Fig. 4.4c). 

4 Polymer Surface and Interface Properties and (Dynamic) Processes 

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If we set out to unravel surface chemical functionalities with high spatial resolution down to atomic detail, we also encounter various practical (technical) problems. It is fair to say that the technique development for direct space analysis (again, we exclude Fourier space methods) is still lagging much behind. Chemical force microscopy can be considered as a first step in the direction of a true description of surface chemical functionalities with high spatial resolution in polymers, primarily based on the chemically sensitive analysis of AFM data via adhesion mapping. At this point the detailed theory for force spectroscopy is not developed beyond the description of London forces. The consideration of the effect of polar functional groups in force spectroscopy (similar to difficulties with solubihty parameter and surface tension approaches for polar forces, as well as specific interactions) is still in its infancy. Instead, one must still rely on continuiun contact mechanics to couple measured forces and surface free energies. 

In some AFM adhesion measurements, integrated silicon or silicon nitride sharp tips are used instead of spherical particles. With integrated tips, the lateral distribution of the surface tension of a sample can be mapped or even imaged with a resolution of roughly 10 nm 172-751. It is, however, difficult to obtain quantitative results because the size and shape of the tip in the 1-10 nm regime is unknown and practically impossible to determine 176-78). 

Nanomechanical mapping has been applied to several material systems to date, as introduced in Section 3.3. However, in these applications we adopted Hertzian theory and argued only elastic modulus, and therefore the analyses were subject to many restrictions. More seriously, practical measurements must be performed under appropriate conditions to avoid other complex interactions, such as adhesion and viscoelasticity, and to obtain precise and correct results. Measurement in an aqueous environment to avoid adhesion effects is a possible example, where we can suppress the water capillary effect, which is unavoidable, and the major contribution to the adhesion force under ambient conditions. 

The usability of IS methods to predict adhesion in metallized plastics has been discussed. A method has been presented, allowing low-cost, fast simulation of the impedance map in non-nomogeneous systems. The results of simulation runs carried out on systems modelling the actual geometry of practical devices have been reported, and limits and potentials of the method nave been discussed. The technique qualifies for interrogation of modified interfaces, and a remarkable sensitivity to diffusivity gradients is expected. 

As the performance of the composite is profoxmdly dominated by the micromechanical deformation process, its knowledge and control are critical for the improvement of composite properties. The effect of particle characteristics and interfacial adhesion on the micromechanical deformation processes in PP-wood composites was investigated by Renner et al. [7]. They proposed a failure map as well as the practical results and considered the influence of matrix characteristics on deformation and failure in PP-natural fiber composites in other research [24]. Hietala et al. [78] studied the effect of chemical pre-treatment and moisture content of wood chips on the wood particle aspect ratio during the processing and mechanical properties of WPCs. The use of pretreated wood chips enhanced the flexural properties of the wood chip-PP composites. Moreover, the use of undried wood chips compared to dried one can improve and reduce the flexural strength and flexural modulus, respectively. On the other hand, they concluded that the use of pretreated and undried wood chips lead to the highest aspect ratio after compounding. The effect of composition and the incorporation... 

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