Development of a Layer Model for the Dynamics of Polymer Films

技术概述：人们早就观察到，许多聚合物薄膜的玻璃化转变温度随着薄膜厚度的减小而降低。最常被引用的模型是层模型，它假设在薄膜的自由表面存在一个高度可移动的层，并导致薄膜整体动力学的降低。目前尚不清楚表面移动层是如何引起所观察到的玻璃化转变温度变化的。在最近的一项实验中，测量了硅支撑的短链聚苯乙烯薄膜粘度的温度依赖性，发现它与实验玻璃化转变温度具有很好的一致性。更重要的是，数据被发现完全由两层模型负责，假设薄膜的总流动流动性是表面流动层和底部块状层流动流动性的总和。在该实验中，只研究了单分子量硅上的聚苯乙烯薄膜。在本项目中，将进行一系列实验，以探索层模型在更大范围的聚合物分子量和具有不同聚合物-衬底相互作用的聚合物薄膜中的有效性。众所周知，分子量对聚合物的动力学有很大的影响。具体来说，聚合物是否纠缠会带来质的不同动力学行为。同时，在某些聚合物薄膜体系中，不同的聚合物-衬底相互作用导致了玻璃化转变温度随着膜厚的减小而升高。该计划的目标是开发一个包含层模型，该模型可以有效地预测各种薄膜系统的粘度和玻璃化转变温度。非技术概要：玻璃化转变温度是聚合物在无数技术应用中的重要物理性质。它决定了材料何时软化并最终失去保持其形状的能力。在过去的15年中积累的大量证据表明，当聚合物被制成纳米厚薄膜时，这种性质会大大偏离体的性质。在这个程序中，将开发一个物理模型来描述这种现象。如果成功，该模型将能够战略性地设计纳米聚合物薄膜的玻璃化转变温度，从而大大简化聚合物在纳米级应用中的适应性。该计划的主要影响将是在STEM领域产生新知识和培训人员。该项目的参与者不仅将接受最先进的表征技术和纳米聚合物薄膜加工方面的培训，而且由于PI与许多国际团体之间的持续合作，他们还将接触到国际环境。将培训两名研究生。PI还将积极招收大学生和高中生。她在吸引大三学生参与她的研究方面有着良好的记录。

TECHNICAL SUMMARY:It has long been observed that the glass transition temperature of a large variety of polymer films decreases with decreasing film thickness. The most frequently cited model, the layer model, assumes that a highly mobile layer exists at the free surface of the films and engenders a reduction in the overall dynamics of the films. It is still not known how the surface mobile layer brings about the observed change in the glass transition temperature. In a recent experiment, the temperature dependence of the viscosity of short-chain polystyrene films supported by silicon was measured, and found to display a good consistency with the experimental glass transition temperature. More importantly, the data was found to be fully accountable by a two-layer model assuming the total flow mobility of the film to be the sum of the flow mobility of a surface mobile layer and a bottom, bulk-like layer. In that experiment, only polystyrene films on silicon with a single molecular weight was studied. In this program, a series of experiments will be carried out to explore the validity of the layer model for a wider range of polymer molecular weights and polymer films with different polymer-substrate interactions. Molecular weight is well-known to have a strong effect on the dynamics of polymers. Specifically, it can bring about qualitatively different dynamical behaviors depending on whether the polymer is entangled or not. At the same time, different polymer-substrate interactions have been attributed to be the cause of the increase in the glass transition temperature with decreasing film thickness observed in some polymer film systems. The goal of this program is to develop an encompassing layer model that would allow an effective way of predicting the viscosity and glass transition temperature of a broad range of thin film systems.NON-TECHNICAL SUMMARY:Glass transition temperature is an important physical property of polymers in a myriad of technological applications. It determines when the material softens and eventually loses the ability to hold its shape. Ample evidence accumulated over the past 15 years shows that this property can deviate substantially from that of the bulk when the polymer is made into nanometer thick films. In this program, a physical model will be developed to describe the phenomenon. If successful, the model will enable strategic design of the glass transition temperature of nanometer polymer films, and thereby significantly streamline the adaptation of polymers in nano-scale applications. The major impact of this program would be in the generation of new knowledge and training of personnel in the STEM field. Participants of this program will not only receive training on the state-of-the-art characterization techniques and processing of nanometer polymer films, they will also be exposed to an international environment due to the on-going collaborations between the PI and a number of international groups. Two graduate students will be trained. The PI will also actively recruit undergraduates and high-school students to the program. She has a strong record of engaging junior students in her research.