Static deflection AFM

Static deflection AFM can be used to probe adhesion of polymer surfaces in air using soft cantilevers (k < 1 N/m). The measurements can be made in air under ambient conditions in the absence of plastic deformation. The calibrated pull-off force can be used in conjunction with JKR theory and an estimation of tip radius from electron microscopy to provide a local estimate of surface energy. The accuracy of the measurement depends on surface contamination, surface roughness (relative to the curvature of the tip), and tip shape uniformity. 

Static deflection AFM can be used to measure local mechanical properties of polymer surfaces, but only after consideration of the relative stiffness of the cantilever and the surface under study. Cantilevers with stiffness in excess of 50 N/m are necessary to indent materials with a bulk modulus in excess of 1 GPa (10 N/m ). Soft levers with a spring constant less than 1 N/m are sufficient to indent elastomers. Conventional staining techniques used in electron microscopy provide a viable way to harden unsaturated, hydrocarbon elastomers for imaging with soft cantilevers. Alternatively, low bulk modulus polymers (E < 1 MPa or 10 N/m ) require resonant imaging techniques such as Tapping Mode for direct imaging. 

Only for the PDMS elastomer is the stiffness of the surface on the order of the cantilever stiffness used to probe the mechanical properties. All other surfaces show negligible penetration. The sensitivity to measure mechanical properties of polymer surfaces using AFM under static deflection depends on the relative stiffness of the cantilever and surface (14). The soft (k contact mode cantilevers were not able to distinguish between materials with bulk modulus in excess of IGPa (2). 

The mean tip height is a little less than the amplitude of its oscillatory motion, so the tip comes into brief intermittent contact with the sample once every cycle. At 150 kHz, each oscillation takes 6.7 /zs. For a 1 Hz line scan rate with 512 points per line, each location on the surface experiences roughly 300 of these taps. The cantilever oscillation is affected by this interaction with the surface, and the signal for the feedback loop in regular IC-AFM is the amplitude of oscillation of the cantilever (Fig. 2.9 and Fig. 3.29). Because the cantilever is relatively stiff, and the mean distance from tip to sample surface is tens of nanometers, the mean static deflection of the cantilever is negligible. 

Cantilevers in AFM function as force transducers converting unknown force to measurable deflection. The value of the unknown force can then be expressed by Hookean mechanics following spring constant calibrations. In addition to static point loads, cantilevers can also be vibrated, e.g., by an oscillation piezo to which the fixed end of the beam is attached (or by other approaches). Excitation frequency, oscillation amplitude, and phase relationships are variables that govern dynamic tapping (intermittent contact) imaging. This problem will be discussed in the next section. 

AFM generally operates in three different modes contact, noncontact, and tapping mode. In the contact (also called static) mode, the tip is in contact with the sample surface. The force between the tip and the surface is kept constant during scanning by maintaining a constant deflection, which is used as the feedback signal. 

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