SGER: Use of Phase Imaging in Atomic Force Microscopy for Measurement of Viscoelastic Contrast in Polymer Nanocomposites and Molecularly-Thick Lubricant Films

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

Small Grant for Exploratory Research (SGER) Use of Phase Imaging in Atomic Force Microscopy for Measurement of Viscoelastic Contrast in Polymer Nanocomposites and Molecularly-Thick Lubricant Films In the proposed work our goal is the development of a technique for quantitative imaging of viscoelastic properties across polymer nanocomposite bearing surfaces and surfaces lubricated with ultrathin films. This would be an important technological advancement that would provide a tool for measurement and optimization of polymer nanocomposites and a tool for measurement of lubricant film uniformity. Such a tool would be particularly important for MEMS and magnetic storage. So this project has a very high potential payoff. Although this is a high risk project (because such a technique has never been developed before), we have seen promising results in our preliminary experiments. Much more work must be carried out however. Preliminary studies of phase contrast, using an atomic force microscope (AFM) in both tapping and force modulation modes have shown that different regimes exist. Different AFM cantilever/tip vibration amplitudes and tip-sample surface separation distances give different phase contrast images. We believe that hard tapping conditions, i.e. high cantilever/tip vibration amplitude and low tip-sample surface separation, give phase images that emphasize viscoelastic properties, which are important in polymers. In commercial magnetic particulate tapes and polymer films with hard embedded particles, high phase contrast is found. The hard particles give lower phase lag than the surrounding polymer at hard tapping conditions, as expected. In the proposed research we will develop the technique with phase contrast measurements. We will make simultaneous measurements using several well-established AFM techniques: friction force (for contrast based on differences in lubricity among materials), force modulation (for contrast based on stiffness differences), and Kelvin probe (for contrast based on surface potential differences) microscopies. These should provide insight into the results of the phase contrast technique that is being developed here. We also seek to quantify results. We will attempt to relate phase data from the AFM for thin polymer samples to viscoelasticity data obtained for these samples by another proven technique known as dynamic mechanical analysis (DMA). In the DMA thin polymer samples are subjected to tension/compression strain cycles and the corresponding stress is measured. The phase lag between the stress and strain is then found. If the AFM is truly measuring viscoelastic properties then it should be possible to find a correlation between loss tangent from the DMA and phase lag from the AFM. In order to quantify, it will be necessary to understand the physics of the contact. To this end we will develop a vibration model that accounts for the repulsive and attractive forces between the tip and the dissipation caused by the viscoelasticity of the sample. An award would support one graduate student and partially support one postdoctoral researcher. This would provide an opportunity to recruit and fund a minority or woman. The PI and his department have made efforts to attract such students in recent years. The department regularly invites, and provides travel grants for, students, particularly women and minorities, from other schools to attract them for graduate studies; we have had some success in this. Undergraduate students would work on the project as well. Projects like these provide hands-on opportunities for undergraduate students in a high-tech lab. Past undergraduates in the PI's lab have actually coauthored articles for technical journals. This project will provide additional material for the PI's course on tribology and in soon-to-be-developed nanoscale courses.