Phase contrast in periodic contact mode-AFM is determined by topographic, viscoelastic as well as adhesive properties of the samples. Image interpretation is not straightforward, unless the image acquisition procedure is previously well defined, making an adequate choice of the phase angle q between the wave applied to the driving piezo and the wave from the photodetector output. The effect of topography on phase contrast is minimum only if the two senoidal waves (probe oscillation and phododetector response) are ortogonal. By choosing theta = -90°, brighter domains in the image are more dissipative than darker domains, but the opposite is observed using theta = +90°. This rule is obtained from a simple analysis of the instrument operation, and it is experimentally verified with a sample prepared by partially coating a mica sheet with a natural rubber film cast from solution. Three different latex films were imaged and revealed different domain patterns: harder domains in a softer matrix are observed in a synthetic copolymer latex, while the natural rubber latex (NRL) has softer domains in a harder continuum, and a NRL-sodium polyphosphate composite film is formed by interpenetrating harder and softer domains.
SPM; phase contrast; hard and soft domains
