Mechanism for Viscosity Reduction in High Molecular Weight Polymer Nanometer Films

技术摘要：虽然聚合物在熔体中的动力学已经得到了很好的研究和理解，但对于限制在纳米膜中的聚合物的对应问题却知之甚少。 人们普遍认为，在聚合物与空气相遇的自由表面处存在薄的移动的层。 随着聚合物膜的厚度减小，该移动的层的影响随着表面积与体积比而增加，并且可以使膜整体的移动性明显增强。 但是对于高分子量（Mw）膜，这种情况难以调和，其中聚合物线圈的垂直尺寸超过表面移动的层的厚度，其为约3 nm。 具体地，这需要表面链被高度拉伸，拉伸能量可以是100 kBT/链。 PI的猜想是，一种新的机制，涉及滑动的聚合物链在基板上和展览的劳斯动力学的链，可能促进纳米限制和自由表面，可以解释的数据的高分子量膜。 至于双层模型，她推测它是可操作的，并与新机制竞争，并在低分子量区主导新机制。 在这个程序中，提出了实验来测试的有效性的两层模型和新的机制，在不同的Mw制度。 的主要目的是获得洞察的起源所表现出的高分子量膜的粘度降低。 此外，该项目将包括对不同的聚合物薄膜系统的对应研究，以探索在聚苯乙烯薄膜的粘度中观察到的新现象的普遍性。非技术摘要：许多现有的和新兴的技术应用采用聚合物纳米薄膜。 这些包括润滑，粘合剂，生物医学技术和纳米电子学等。粘度是一个重要的工程性质，它规定了薄膜的流体性，通常也决定了薄膜在热应力下的耐用性。 虽然关于大体积样品中聚合物粘度的科学已经很好地理解了，但对于纳米薄膜中聚合物的限制却知之甚少。 该计划旨在更好地了解一些聚合物纳米薄膜的粘度中发现的新特性。 它将对聚合物物理和材料科学等学术领域产生影响，并将受益于通过研究获得的新知识所实现的聚合物材料的更好的战略设计或适应性。 另一个重要影响是STEM领域人员的培训。 该计划的参与者不仅将接受有关聚合物薄膜最先进的表征和加工技术的培训，还将有充分的机会在专业会议上发表演讲和海报。 将培养两名研究生。 此外，PI将招募高中生参加该计划。 在过去的六年里，她一直是波士顿大学科学工程研究实习（BU RISE）计划的导师，该计划将全国高中生与教师导师一起进行研究，并将继续这样做。

TECHNICAL ABSTRACT:While the dynamics of polymers in a melt is well studied and understood, little is known about the counterpart problem for polymers confined in nanometer films. It is broadly believed that a thin mobile layer exists at the free surface where polymer meets the air. As the thickness of a polymer film decreases, the influence of this mobile layer increases with the surface-to-volume ratio and may bring about visible enhancement to the mobility of the film as a whole. But this picture is difficult to reconcile for high molecular weight (Mw) films, where the perpendicular dimension of the polymer coils exceeds the thickness of the surface mobile layer, which is about 3 nm. Specifically, this necessitates the surface chains to be highly stretched with a stretching energy that can be 100 kBT per chain. The PI's conjecture is that a new mechanism involving slippage of the polymer chains on the substrate and exhibition of Rouse dynamics by the chains, possibly facilitated by nano-confinement and the free surface, may explain the data of the high-Mw films. As for the two-layer model, she surmises that it is operative and in competition with the new mechanism, and dominates the new mechanism in the low-Mw regime. In this program, experiments are proposed to test the validity of the two-layer model and the new mechanism in different Mw regimes. The main purpose is to gain insight about the origin of the viscosity reduction exhibited by the high-Mw films. In addition, this project will include counterpart studies on a different polymer film system to explore the generality of the new phenomena observed in the viscosity of the polystyrene films.NON-TECHNICAL ABSTRACT:Many existing and emergent technological applications employ polymer nanometer films. These include lubrication, adhesives, biomedical technologies and nano-electronics, etc. Viscosity is an important engineering property that specifies how fluidic a film is and often also determines how durable a film would be under thermal stress. While the science about the viscosity of polymers in a bulk sample is well understood, little is known for that of polymers under confinement in nanometer films. This program aims for a better understanding of the new properties found in the viscosity of some polymer nanometer films. It will have an impact on academic fields as polymer physics and materials science, and technological fields that will benefit from better strategic design or adaptation of polymeric materials enabled by the new knowledge gained through the studies. Another important impact is the training of personnel in the STEM field. Participants of this program will not only receive training on the state-of-the-art characterization and processing techniques of polymer films, they will also be given ample opportunities to present talks and posters at professional meetings. Two graduate students will be trained. In addition, the PI will recruit high-school students to the program. In the past six years, she has been a mentor of the Boston University Research Internship in Science & Engineering (BU RISE) program that brings in high-school students nationwide to conduct research with a faculty mentor, and will continue to do so.