Materials Contrast in AFM Imaging of Multicomponent Systems

Polymer-based multicomponent systems are abundant in many applications. The properties and performance of particulate-filled systems, such as elastomers and impact modified polymers, and also polymer blends, block copolymers, and fiber reinforced systems, depend to a large extent on the distribution of the components. Hence the local analysis of these distributions down to sub-100 nm length scales (dictated, e.g., by the size of primary filler particles) is of considerable significance. Materials contrast in several AFM approaches offers the possibility to address these issues directly at the surface of specimens or on bulk samples that have been prepared correspondingly. 

In addition to its capability of imaging the topography of polymer surfaces with virtually eliminated shear forces, intermittent contact (tapping) mode AFM, can also be useful to probe various surface properties, such as adhesive or surface mechanical properties. Thereby AFM can help to identify and quantify the abundance and distribution of the phases present in multicomponent systems. As shown already 

The phase angle shift can be used to obtain contrast due to local differences in energy dissipation as a consequence of different surface characteristics related to materials properties. These different properties allow one to differentiate materials with different adhesion [110] or widely different Young s moduli, if these differences are related to differences in energy dissipation [111-115]. Hence the amorphous and crystalline phases in semicrystalline polymers can be clearly differentiated, as discussed in Sect. 3.2, as well as different phases in polymer blends or filled systems (see below). As an example, we show in Fig. 3.52 an intermittent contact AFM phase image of a block copolymer thin film on silicon [116]. 

Compositional contrast, as well as modulus, can also be assessed by laterally resolved imaging of normal or lateral forces in the contact mode AFM. In the corresponding mapping [121] (see also Chap. 4), force-displacement curves are recorded for each pixel. Subsequently, the pull-off forces, as a measure for adhesion, and the indentation part of the loading curves, to extract/fit the elastic modulus, are evaluated for each pixel. In particular for adhesion mapping, the use of chemically functionalized AFM probe tips [122], has been shown to be a suitable approach to map chemical composition and functional group distributions down to the sub-50 nm scale [123]. The mapping of adhesion, friction, and surface mechanical properties will be treated in more detail in Chap. 4. 

4 A practical limitation for the approach in the field of polymers is the fact that shear forces, similar to contact mode, may cause sample damage or distortion of the underlying morphology. 

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