Adhesive force images

Sample. Three PS samples with different molecular weights and glass transition temperatures were investigated (Table 1). A blend of PMMA and PS was used to obtain adhesive force images. These polymers show phase separation due to their chemical difference. A PMMA with a molecular weight of 100000 g/mol and a glass transition temperature Tg of 109 was chosen as second component for the blend. Tg has been estimate by DSC measurements. 

The adhesive force images were performed by Pulsed Force Mode (PFM) using a modulation frequency of 1 kHz and an amplitude of 400 nm. Uncoated silicon cantilevers with 2.8 N/m were used as force sensors. The force values measured by PFM are recorded in volts. To convert this value into force values the following equation was used ... 

Figure 5. Scanning force images of a blend of PMMA and PS2 taken at room temperature and 67 °C. At 27 C the PS2 areas (marked by circles) have lower adhesive force (darker color) than the surrounding PMMA. Hheating causes significant change in the adhesive force images. The adhesive force histogram ( H) taken from the adhesive force image at 27 °C shows two pronounced peaks. In die AFH taken at 67 °C only one peak can be observed.

Figure 6. Scanning force images of a blend of 75 % PMMA and 25 % PSl 00 at room temperature and 97 °C. Adhesive force histograms taken from adhesive force images show two pronounced peaks at 27 °C and 97 C, which indicates no change in adhesive forces during heating. The PSIOO areas (marked by circles) have lower adhesives force ((terker) Aan the surrounding PMMA. Additional smaller droplets with a diameter of about 300 nm are visible.

Also of note in the adhesive force images, small droplets with a diameter of about 300 nm can be seen where the adhesive force appears to be different. This difference in adhesive force is due to a topography effect [20]. Heating the sample up to 97 °C, the adhesive force of the droplet structure decreases. A possible explanation for this may be that in these areas small droplets of PS are covered by PMMA. The spin-coated films are not in thermodynamical equilibrium but in a frozen state. Due to the evaporation of the solvent, it is also possible that PMMA remains at the surface because it less soluble in toluene than PS. Heating the sample close to its glass transition temperature increases the mobility of the chains and the PS droplet becomes visible by a second-phase separation step which is thermally induced. In this case the sample is in thermodynamic equlibrium. 

While the FDC has been mainly used for local probing of the surface adhesive force, images have been obtained using the PFM. This enables also the determina-... 

Figure 9. Friction force image (left, z-scale dark 0.0 V - bright 0.5 V) and adhesion force image (right, z-scale bright 1 nN - dark 60 nN) on PE lamellar crystal obtained in water. The contrast in the adhesion images is defined on the basis of the convention that attractive forces are negative.

Numerous AFM imaging techniques have been developed and commercialized to monitor topography, friction, mechanical response, capacitance, magnetic properties, etc. However, adhesion measurements require the tip to come into, and out of, contact to measure attractive and adhesion forces. Therefore, other than to select an analysis region, most imaging techniques are not useful for adhesion studies. Instead, measurements are necessarily based on force-displacement curve approaches. 

Figure 6.9 A schematic of the micromechanical force measurement (left) and video images of hydrate particles during each stage of the adhesive force measurement. (From Taylor, C.J., Adhesion Force between Hydrate Particles and Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, MS Thesis, Colorado School of Mines, Golden, CO (2006). With permission.)...

Note, that the surface and deformation forces are of the same order of magnitude. Therefore, surface forces should be as small as possible to minimise damaging and indentation of soft polymer samples. For example, sharp probes have a lower capillary attraction and adhesion forces, and therefore enable more gentle probing of a soft polymer than a blunt tip. A sharp tip can also be moved in and out of the contamination layer more readily than a blunt tip. This is particularly important for non-contact intermittent contact imaging described in Sect. 2.2.1. 

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