Characterization and Understanding of the Anomolous Diffusion Behavior in Polymer Ultra-thin Films

克里夫·亨德森/佐治亚州技术这项工作的主要目标是进一步描述聚合物超薄膜和聚合物在受限几何形状中的扩散行为，阐明这种行为的长度、尺度和大小，并对导致观察到的现象的机制形成自洽和全面的基本理解。来自实验和分子模拟的PI的初步结果表明，一个新的模型基于聚合物链迁移率和自由体积分布的变化，可以解释所有观察到的玻璃化转变、热膨胀系数、扩散和质子电导率的结果是一致的。结合起来，这两种协同机制可以导致到目前为止观察到的聚合物薄膜行为的丰富多样性。为了阐明这些薄膜现象的起源，已经组建了一个独特的团队，将实验表征与分子模拟相结合。克利福德·亨德森教授指导实验研究，在聚合物表征和聚合物薄膜方面拥有丰富的经验。彼得·卢多维斯教授在分子模拟方面拥有丰富的专业知识，L负责建模工作。这一提议的理论价值大致可以定义为：(1)表征超薄聚合物薄膜的扩散行为；(2)发展一个基本模型来解释受限几何条件下聚合物超薄膜和聚合物的物理化学性质。这项活动的更广泛的影响包括：(1)为克服微光刻技术中的一些障碍以造福微电子工业提供指导和机会，(2)为提高燃料电池和气体分离的聚合物膜性能的方法提供指导，(3)以协同混合建模和实验的方式教育本科生和研究生，以及(4)加强少数民族和中学的科学和工程教育。背景：目前，在从根本上了解受限系统(例如复合膜)中聚合物薄膜和聚合物的热力学和传输特性以及这些特性对聚合物类型、界面类型、膜厚，以及制备条件。薄膜和受限几何结构的聚合物是当今各种应用中的关键元素，包括半导体制造、生物医学和组织工程、工业气体分离和燃料电池。对聚合物薄膜的物理化学性质缺乏基本的了解，这对为这些应用而改进的材料和工艺的合理设计构成了障碍。最近的聚合物薄膜研究表明，当薄膜厚度减小到某一临界厚度以下时，各种聚合物性质都会偏离整体行为，该临界厚度取决于所关注的性质。到目前为止，还没有一个单一的理论能够充分解释所有观察到的薄膜行为，而且在许多情况下，观察到的薄膜性质的变化似乎彼此不一致。例如，已经观察到，当支撑型聚合物薄膜被涂覆在弱相互作用的衬底上时，其玻璃化转变温度(Tg)会降低，目前对此的解释是基于薄膜表面附近聚合物链迁移率的增加。另一方面，PIS最近的实验表明，小渗透分子在这种支撑薄膜中的扩散系数随着薄膜厚度的减小而急剧下降。简单的链迁移率论点不能解释这一点，实际上会预测相反的行为。PI的最新测量还表明，超薄聚合物薄膜的质子电导率并没有随着薄膜厚度的减小而表现出类似于气体渗透行为的降低。

Cliff Henderson / Georgia Inst. TechnologyThe overarching goals of this work are to further characterize the diffusion behavior of polymer ultra-thin films and polymers in confined geometries, elucidate the length scales and magnitudes of such behavior, and to develop a self consistent and comprehensive fundamental understanding of the mechanisms that are responsible for the observed phenomena. Preliminary results by the PIs from both experiment and molecular simulation suggest a novel model based on changes in both polymer chain mobility and free volume distribution that can explain all of the observed glass transition, coefficient of thermal expansion, diffusion, and proton conductivity results in a consistent manner. Combined, these two concerted mechanisms can lead to the rich diversity of polymer thin film behavior observed thus far. A unique team has been assembled to integrate experimental characterization with molecular simulation in order to elucidate the origin of these thin film phenomena. Professor Clifford Henderson directs the experimental studies and has extensive experience in polymer characterization and polymer thin films. Professor Peter Ludovice has extensive expertise in molecular simulation and l supervises the modeling work. The intellectual merits of the proposal can broadly be defined as: (1) characterizing the diffusion behavior of ultra-thin polymer films and (2) developing a fundamental model to explain the physiochemical properties of polymer ultra-thin films and polymers in confined geometries. The broader impacts of this activity include: (1) providing guidance on and opportunities for overcoming some of the roadblocks in microlithography to benefit the microelectronics industry, (2) providing guidance on methods to enhance polymer membrane performance for fuel cells and gas separations, (3) educating undergraduate and graduate students in a manner that synergistically blends modeling and experiment, and (4) enhancing minority and secondary school education in science and engineering.Background: Currently there are significant knowledge gaps in fundamentally understanding the thermodynamic and transport properties of polymer thin films and polymers in confined systems (e.g. composite membranes), and the dependence of these properties on polymer type, interface type, film thickness, and preparation conditions. Polymers in thin film and confined geometry configurations are a critical element today in a variety of applications including semiconductor manufacturing, biomedical and tissue engineering, industrial gas separations, and fuel cells. The lack of a fundamental understanding of the physiochemical properties of polymer thin films poses a roadblock to the rational design of improved materials and processes for these applications. Recent polymer film studies have shown that a wide variety of polymer properties deviate from bulk behavior as the film thickness decreases below some critical thickness that varies depending on the property of interest. No single theory proposed thus far can adequately explain all of the observed thin film behavior, and in many cases the observed changes in film properties appear to be inconsistent with one another. For example, it has been observed that the glass transition temperature (Tg) of supported polymer thin films can decrease when the film is coated on a weakly interacting substrate and current explanations of this are based on increases in polymer chain mobility near the film surfaces. On the other hand, recent experiments by the PIs have shown that the diffusion coefficient of small penetrant molecules in such supported thin films decreases dramatically as the film thickness decreases. The simple chain mobility argument does not explain this and would in fact predict an opposite behavior. Recent measurements by the PIs also suggest that the proton conductivity of ultra-thin polymer films does not exhibit reductions similar to the gaseous penetrant behavior as film thickness decreases.