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

探索研究(SGER)利用原子力显微镜中的位相成像测量聚合物纳米复合材料和分子厚润滑膜的粘弹性对比度在拟议的工作中，我们的目标是开发一种定量成像聚合物纳米复合材料轴承表面和由超薄膜润滑的表面的粘弹性性能的技术。这将是一项重要的技术进步，将提供一种测量和优化聚合物纳米复合材料的工具，以及一种测量润滑膜均匀性的工具。这样的工具对于MEMS和磁存储尤其重要。因此，这个项目具有非常高的潜在回报。尽管这是一个高风险的项目(因为这种技术以前从未被开发过)，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。利用原子力显微镜(AFM)对攻丝模式和力调制模式下的位相衬度进行了初步研究，结果表明存在不同的模式。不同的AFM悬臂梁/针尖振动幅度和针尖-样品表面分离距离会产生不同的相衬图像。我们认为，硬攻丝条件，即高悬臂/尖端振动幅度和低尖端-样品表面分离，可以产生强调粘弹性性质的相图，而粘弹性性质在聚合物中非常重要。在商用磁性颗粒磁带和含有硬质嵌入颗粒的聚合物薄膜中，发现了高相位对比度。正如预期的那样，在硬攻丝条件下，硬颗粒比周围的聚合物具有更低的相滞后。在这项拟议的研究中，我们将发展相衬测量技术。我们将使用几种成熟的AFM技术进行同步测量：摩擦力(基于材料之间润滑性差异的对比度)、力调制(基于硬度差异的对比度)和开尔文探头(基于表面电位差异的对比度)显微镜。这些应该提供了对这里正在开发的相位对比技术的结果的洞察。我们还寻求量化结果。我们将尝试将来自AFM的薄聚合物样品的相数据与通过另一种被称为动态机械分析(DMA)的成熟技术获得的这些样品的粘弹性数据相关联。在DMA中，薄聚合物样品受到拉伸/压缩应变循环，并测量相应的应力。然后求出了应力和应变之间的位相滞后。如果原子力显微镜真正测量的是粘弹性性质，那么应该有可能找到来自DMA的损耗正切和来自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性质。为此，我们将开发一个振动模型，该模型解释了样品的粘弹性引起的尖端和耗散之间的排斥力和吸引力。一个奖项将支持一名研究生和部分支持一名博士后研究员。这将提供一个招募少数群体或妇女并为其提供资金的机会。近年来，PI和他的部门一直在努力吸引这样的学生。该部定期邀请其他学校的学生，特别是妇女和少数族裔学生，并为他们提供旅费津贴，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。本科生也将参与这个项目。像这样的项目为本科生提供了在高科技实验室实习的机会。PI实验室过去的本科生实际上已经为技术期刊合著过文章。该项目将为PI的摩擦学课程和即将开发的纳米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.