Mauritz - Atomic Force Microscopy

Atomic Force Microscopy

Atomic force microscopy (AFM), or scanning probe microscopy (SPM), is becoming increasingly important in polymer characterization with regard to both surface topography and surface mechanical properties. The instrument used by Mauritz Research is a Digital Instruments Dimension 3000 AFM. The instrument is capable of both contact and non-contact modes of operation, including lateral force imaging, force modulation imaging and tapping mode imaging. Of particular interest in determining topography and phase morphology in polymer films is tapping mode AFM. In tapping mode, the silicon probe tip oscillates at its resonance frequency as it rasters across the sample surface, experiencing only intermittent contact with the surface. Below is a summary of the various types of images that are collected in tapping mode.

Tapping Mode Data

Figure 1: Three Types of Data Collected in Tapping Mode

Explaination of Three Types of Tapping Mode Data

Height Data: The vertical position of the probe tip is monitored by noting changes in the length of the z-axis on the xyz scanning piezo tube. Input voltage to the scanning piezo tube is proportional to the length of the tube. The change in the z-axis is plotted as a topographical map of the sample surface. Height data is a good measure of the height of surface features but does not show distinct edges of these features.

Phase Data: This type of imaging monitors the change in phase offset, or phase angle, of the input drive signal [to the drive piezo] with respect to the phase offset of the oscillating cantilever. The phase of the drive signal (i) is compared to the phase of the cantilever response signal (ii) on the photo diode detector. See (2) in the above figure. The phase offset between the two signals is defined as zero for the cantilever oscillating freely in air. As the probe tip engages the sample surface, the phase offset of the oscillating cantilever changes by some angle with respect to the phase offset of the input drive signal. As regions of differing elasticity are encountered on the sample surface, the phase angle between the two signals changes. These changes in phase offset are due to differing amounts of damping experienced by the probe tip as it rasters across the sample surface. These differences are plotted as the so-called 'phase image'.

Amplitude Data: The amplitude of the cantilever is monitored by the photo diode detector. The RMS value of the laser signal on the y-axis of the detector is recorded for each of the 512 segments on a given raster of the probe tip. These values are plotted as an amplitude map of the sample surface. Amplitude images tend to show edges of surface features well.

Figure 2 shows AFM micrographs of freeze fractured styrene-isobutylene-styrene a-b-a block copolymer film. BCP is 21 wt % styrene. Extent of sulfonation of styrene blocks = 6.1%. BCP in the acid form. Film melt pressed at 136 C. Digital Insruments D3000 with 125 mM cantilever in tapping mode used.

Figure 2: AFM Tapping Mode Images


Figure 3: Tapping mode image of cryo-microtomed block face of TBMA form BCPI/SiO2 composite. BCPI Mw = 76,900, 18 mole % polystyrene, sulfonated to 16 mole %, sol-gel reacted for one hour in DMAc, inorganic uptake of 6.5%. DMAc was used as the solvent medium for the in situ sol-gel reaction of TEOS within the swollen BCPI film.

Written by: David Reuschle


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Questions or comments about this page can be mailed to Dr. Ken Mauritz at Kenneth.Mauritz@usm.edu