Use of phase imaging in atomic force microscopy for measurement of viscoelastic contrast in polymer nanocomposites and molecularly thick lubricant films.

Phase contrast microscopy, using an atomic force microscope, is used to detect and quantify changes in composition across polymer nanocomposites and molecularly thick lubricated surfaces. The technique takes advantage of the contrast in viscoelastic (viscous energy dissipation) properties of the different materials across the surface. Some materials, especially polymers, are found to display viscoelastic behavior. For such materials, the strain response lags the stress by a phase angle that is characteristic of the material. In tapping (or intermittent contact) mode, phase angle contrast is found to be highly dependent on vibration amplitude and mean tip-to-sample distance (setpoint). Phase angle contrast seems to be a stronger function of viscoelastic properties at relatively high vibration amplitude and low mean tip-to-sample distance. In this regime the effects of sample deformation, and thus viscoelastic properties, are dominant. In these contrast images, low phase angle corresponds to materials with low viscoelastic properties. This technique was used to find fairly reproducible phase angle contrast for polyethylene terephthalate (PET) films with embedded ceramic particles, metal particle (MP) magnetic tape, and Si(100) with a nonuniform Z-15 lubricant film. Very little correlation is found between phase angle images and friction force images for PET films with embedded ceramic particles and MP tape; phase angle images give information that cannot be obtained from topography or friction images. A numerical vibration model verifies that viscoelastic properties are dominant for high vibration amplitude and low mean tip-to-sample distance. For these conditions, the model also verifies that low phase angle corresponds to low viscoelastic properties.