Surface Capillary Waves on Polymer Films Studied by Atomic Force Microscopy

技术摘要：近十年来，理解薄膜中聚合物的动力学一直是聚合物物理学中最具挑战性的问题之一。特别是，已经有许多实验和理论尝试来解释当薄膜厚度减小到纳米范围时，观察到的某些聚合物薄膜的玻璃化转变温度急剧降低~60K的原因。一个困难来自于这样一个事实，即玻璃化转变只代表表征聚合物薄膜全部动态行为的动态松弛曲线的一个参数；聚合物动态曲线通常表现出的Vogel-Tamman-Fucher标度需要三个参数。然而，对10 nm厚度范围内的聚合物薄膜的动态测量很少，这是由于这些薄膜中含有少量的聚合物材料。人们注意到，这些薄膜中的许多在长时间加热时会破裂。在这个项目中，将发展一种新的方法，以原子力显微镜为基础，辅以掠入射小角X射线散射，在1~100 S时间范围内探测薄膜的表面结构及其动力学，这与玻璃化转变直接相关，对于厚度为2~10 nm的聚合物薄膜，在1~100 S范围内的动力学很少被探索。该方法是良性的，涉及到监测由于固有分子作用力(如范德华作用力和表面张力)和随机热力而导致的聚合物膜表面结构(或聚合物膜上的毛细波)的时间演变，由此可以确定聚合物膜的粘度。非技术概述：聚合物薄膜的应用无处不在，包括用于微电子和微型机电系统(MEMS)制造的保护涂层、粘合剂、电子封装和光致抗蚀剂薄膜等。该计划的结果将有助于更好地了解薄膜中聚合物的材料特性，这些材料经常被发现与本体材料有显著不同。很好地理解这些发现的原因将使我们能够准确地预想出在特定的薄膜应用中使用哪种聚合物材料。该项目为研究生提供了一个极好的培训基础，让他们参与到具有学术挑战性和技术重要性的聚合物薄膜问题中，同时也为原子力显微镜提供了新的应用，这是材料研究中一项迅速发展的技术。该计划将通过波士顿大学的本科生研究机会计划(UROP)积极招收本科生，特别是代表人数不足的女学生。PI在让本科生和女学生参与她的研究方面有着很好的记录。

TECHNICAL SUMMARY: Understanding the dynamics of polymers confined in thin films with thicknesses comparable to the radius of gyration of the polymer chains has been one of the most challenging problems of polymer physics in the recent decade. In particular, there have been numerous experimental and theoretical attempts to understand the cause for the observed precipitously reduction in the glass transition temperature of some polymer films by ~ 60 K when the film thickness is decreased to the nanometer range. One difficulty stems from the fact that the glass transition only represents one parameter of the dynamic relaxation curve characterizing the full dynamic behavior of a polymer film; the Vogel-Tamman-Fucher scaling generally exhibited by the dynamic curves of polymers requires three parameters. However, dynamic measurements of polymer films in the 10 nm thickness range are scarce, attributable to the small amount of polymer material contained in these films. It is noticed that many of these films would rupture when subjected to prolonged heating. In this program, a new method based on atomic force microscopy, and complemented with grazing incidence small-angle x-ray scattering, will be developed to probe the surface structure of the films and their dynamics in the 1 ~ 100 s time range, directly relevant to the glass transition, for polymer films with thicknesses from 2 to 10 nm - a regime that has been seldom explored for the dynamics in the 1 ~ 100 s range. The method is benign and involves monitoring of the time evolution of the surface structure of (or capillary waves on) a polymer film due to the innate molecular forces (such as van der Waals interaction forces and surface tension) and the stochastic thermal forces from which the viscosity of the polymer film can be determined. NON-TECHNICAL SUMMARY: Applications of polymer films are ubiquitous including protective coatings, adhesives, electronic packaging, and photoresist films for microelectronics and micro-electro-mechanical systems (MEMS) fabrication, etc. Results of this program will lead to better understanding of the materials properties of polymers in thin films, which have frequently been found to differ noticeably from those of the bulk. A good understanding on the cause of these findings would allow us to accurately premeditate which polymer materials to use for a given thin film application. This program provides an excellent training ground for graduate students to be involved in the academically challenging and technologically important problem of polymer thin films, and at the same time, novel applications of atomic force microscopy, which is a rapidly developing technology in materials research. This program will actively recruit undergraduates - especially the underrepresented female students - to the program through Boston University's Undergraduate Research Opportunities Program (UROP). The PI has a strong record of getting undergraduates and female students participating in her research.