Role of Structure and Dynamics of Molecules at the Interface in Controlling Friction, Adhesion and Adhesion Hysteresis

我们建议将表面敏感红外-可见和频产生光谱（SFG）与原位摩擦和粘附测量相结合。实验的动机是Chaudhury和Brown使用聚二甲基硅氧烷（PDMS）透镜和Israelachvili使用表面力仪的结果；他们发现了摩擦，附着滞后和界面上分子迁移率之间的关系。然而，没有关于聚合物/固体或隐藏界面的弛豫过程与体中的弛豫过程相比有何不同的信息。最近对聚合物薄膜玻璃化转变温度的实验表明，在空气界面处的弛豫可以比在本体处快几个数量级。聚合物链的弛豫时间在接触不同表面能的表面时会发生改变。此外，在移动或滑动界面处没有分子结构变化的直接信息。理解这些特性对于理解摩擦至关重要。最近，我们证明了SFG可以应用于研究弹性PDMS透镜与聚合物或固体衬底之间的接触界面。在这里，我们利用这种设计来同时测量两个滑动表面之间的结构变化，摩擦和粘附。此外，我们建议使用SFG测量界面处的松弛动力学，以了解迁移率与摩擦系数之间的关系。作为第5区总监，我在过去的5年里一直积极参与组织第5区5至12年级学生的科学日活动。除了这个外展活动，这个提案还为当地一所高中的两名高中生提供了研究机会，在研究生的指导下，在我的实验室工作。这些高中项目的研究可以作为其科学项目的一部分提交参加英特尔科学与工程博览会的竞赛。这项工作将与阿克伦大学现有的NSF-REU本科项目相结合。更广泛的影响：两个表面之间的摩擦在各种技术和生物应用中至关重要。认识和控制这些现象将对节约和利用宝贵的能源资源产生巨大的影响。由于缺乏对界面分子结构和性质的了解，这一领域的进展受到限制。这一提议提供了一种新的实验方法来探测在界面上发生的分子运动或变形，并研究其与摩擦和粘附的关系。这将广泛地影响表面、润滑剂和微或纳米致动器的设计。教育和外展活动以第5区的本地学校为对象，鼓励学生进行科学项目，并在地区、州和国际层面上进行比赛，以提高他们对科学的兴趣。

We propose to combine surface sensitive infrared-visible sum frequency generationspectroscopy (SFG) with insitu friction and adhesion measurements. The experimentsare motivated by results of Chaudhury and Brown using poly (dimethylsiloxane) (PDMS)lenses and Israelachvili using surface force apparatus; where they find a relationshipbetween friction, adhesion hysteresis and mobility of the molecules at the interface.However, there is no information on how different relaxation processes are atpolymer/solid or hidden interfaces in comparison to that in bulk. Recent experiments ofglass transition temperature of polymer thin films indicate that the relaxation at the airinterface can be several orders of magnitude faster than in the bulk. The relaxationtimes of polymer chains are expected to be altered in contact with surfaces of differentsurface energies. Furthermore, there is no direct information of the change in thestructure of molecules at moving or sliding interfaces. Understanding these properties iscrucial to understand friction. Recently, we demonstrated that SFG can be applied tostudy the contact interface between an elastomeric PDMS lens and a polymer or solidsubstrate. Here, we take advantage of this design to measure simultaneously thechanges in structure between two sliding surfaces, friction and adhesion. In addition, wepropose to measure the relaxation dynamics at interfaces using SFG to understand therelationship between mobility and friction coefficient. As a Director of District 5, I havebeen actively involved in organizing District 5 Science Day for students from K5-K12 forthe past five years. In addition to this outreach activity, this proposal offers researchopportunities to two high school students from a local high school to work duringsummer in my laboratory under the supervision of graduate students. The research ofthese high school projects can be submitted as a part of their science project forcompetition in the Intel Science and Engineering Fair. This effort will be integrated withthe NSF-REU undergraduate program already existing at The University of Akron. Broader Impact: Friction between two surfaces is of utmost importance in a variety oftechnological and biological applications. The understanding and control of thesephenomena will have a tremendous impact in conserving and using precious energyresources. The advances in this field are limited by the lack of understanding of thestructure and properties of molecules at the interface. This proposal provides a newexperimental method to probe molecular motion or deformation taking place at theinterface and studies its relationship to friction and adhesion. This will broadly impactour design of surfaces, lubricants and micro or nano-actuators. The education andoutreach activities target local schools in District 5 to promote an interest in science byencouraging students to undertake science projects and compete at district, state andinternational level.