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

探索性研究小额资助（SGER）在原子力显微镜中使用相位成像测量聚合物纳米复合材料和分子厚润滑膜中的粘弹性对比度在拟议的工作中，我们的目标是开发一种技术，用于定量成像聚合物纳米复合材料轴承表面和润滑膜表面的粘弹性。 这将是一个重要的技术进步，将提供一个工具，用于测量和优化的聚合物纳米复合材料和工具，用于测量润滑油膜的均匀性。 这种工具对于MEMS和磁存储特别重要。 所以这个项目有很高的潜在回报。 虽然这是一个高风险的项目（因为以前从未开发过这种技术），但我们在初步实验中看到了有希望的结果。 然而，还有更多的工作要做。相位对比的初步研究，使用原子力显微镜（AFM）在轻敲和力调制模式表明，存在不同的制度。 不同的AFM悬臂梁/针尖振动幅度和针尖-样品表面分离距离给出不同的相衬图像。 我们认为，硬轻敲条件，即高悬臂/尖端振动振幅和低尖端样品表面分离，得到强调粘弹性的相位图像，这是重要的聚合物。 在商用磁性颗粒带和具有硬嵌入颗粒的聚合物膜中，发现了高的相衬度。正如预期的那样，在硬攻丝条件下，硬颗粒比周围的聚合物提供更低的相位滞后。在拟议的研究中，我们将开发的技术与相衬测量。 我们将使用几种成熟的AFM技术进行同时测量：摩擦力（基于材料之间润滑性差异的对比），力调制（基于刚度差异的对比）和Kelvin探针（基于表面电位差的对比）显微镜。 这些应该提供洞察的结果，相位对比技术，正在开发这里。我们也寻求量化的结果。 我们将试图从原子力显微镜薄聚合物样品的粘弹性数据获得这些样品的另一种成熟的技术称为动态力学分析（DMA）的相位数据。 在DMA中，使薄聚合物样品经受拉伸/压缩应变循环，并测量相应的应力。 如果AFM真正测量的是粘弹性，那么应该可以找到DMA的损耗角正切和AFM的相位滞后之间的相关性。 为了量化，有必要了解接触的物理特性。 为此，我们将开发一个振动模型，该模型可以解释尖端和样品粘弹性引起的耗散之间的排斥力和吸引力。一个奖项将支持一名研究生，并部分支持一名博士后研究员。 这将提供一个招募和资助少数群体或妇女的机会。 近年来，PI和他的部门一直在努力吸引这些学生。 该部定期邀请其他学校的学生，特别是妇女和少数民族学生，并为他们提供旅费赠款，以吸引他们攻读研究生课程;我们在这方面取得了一些成功。 本科生也将参与该项目。 像这样的项目为本科生提供了在高科技实验室动手的机会。 PI实验室过去的本科生实际上已经为技术期刊合著了文章。 该项目将为PI的摩擦学课程和即将开发的纳米级课程提供额外的材料。

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.