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得到了很好的研究和理解，但对纳米薄膜中聚合物的对应问题知之甚少。人们普遍认为，在聚合物与空气相遇的自由表面上存在一层薄的可移动层。随着聚合物膜厚度的减小，这一可动层的影响随着表面积与体积比的增大而增大，并可能对膜的整体迁移率带来明显的增强。但对于高相对分子质量(MW)薄膜来说，这种情况很难协调，因为聚合物线圈的垂直尺寸超过了表面可动层的厚度，约为3 nm。具体地说，这需要表面链被高度拉伸，拉伸能量可以是每条链100KBT。Pi的猜想是，一种新的机制，包括聚合物链在衬底上的滑移和链展示的Rouse动力学，可能是由纳米限制和自由表面促进的，可以解释高相对分子质量薄膜的数据。至于两层模型，她推测它是可操作的，并与新机制竞争，并在低兆瓦制度下主导新机制。在该方案中，提出了实验来检验两层模型和新机制在不同的微波区域的有效性。主要目的是为了深入了解高相对分子质量薄膜表现出的粘度降低的原因。此外，该项目还将包括对不同聚合物薄膜系统的相应研究，以探索在聚苯乙烯薄膜粘度中观察到的新现象的共性。非技术摘要：许多现有的和新出现的技术应用都使用了聚合物纳米薄膜。其中包括润滑、粘合剂、生物医学技术和纳米电子技术等。粘度是一项重要的工程特性，它决定了薄膜的流动性，通常也决定了薄膜在热应力下的耐久性。虽然关于大样本中聚合物的粘度的科学已经很好地理解了，但对于纳米薄膜中受限的聚合物的粘度却知之甚少。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以及将受益于通过研究获得的新知识使聚合物材料能够更好地战略性设计或适应的技术领域产生影响。另一个重要影响是STEM领域的人员培训。该计划的参与者不仅将接受关于最先进的聚合物薄膜表征和加工技术的培训，还将获得大量机会在专业会议上发表演讲和张贴海报。将对两名研究生进行培训。此外，PI还将招收高中生参加该项目。在过去的六年里，她一直是波士顿大学科学与工程研究实习(BU Rise)项目的导师，该项目邀请全国的高中生与一名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.