Dynamic Processes at the Nanoscale in Polymer Systems

从有机电子器件和传感器到涂层和图案化，各种应用都依赖于聚合物在各种限制条件下的性质和性能，特别是薄膜几何形状。通常，聚合物链段和界面之间的相互作用会对链结构施加焓和熵惩罚，这些影响可能表现在链大小以外的长度尺度上的性质，高达数十纳米(纳米级)。已知的性质，如粘度、链扩散系数、D、玻璃化转变温度、玻璃化转变温度、混合物和共聚物的相分离温度以及形态不稳定性都表现出薄膜厚度的依赖性。到目前为止，对聚合物中这种尺寸依赖现象的全面理解仍然是难以捉摸的。上述情况为本提案审查的三个问题提供了依据。(1)一个目的是了解界面力如何影响链动力学，并了解链动力学与Tg在均聚物和可混溶聚合物-聚合物体系中表现出的膜厚依赖变化之间的关系。我们提出的实验策略在很大程度上是基于这样一个概念，即玻璃化转变是系统中潜在的动力学特征的表现。(2)我们还计划考察纳米粒子的浓度和空间排列对聚合物基纳米复合薄膜体系的形态稳定性、链扩散和平均玻璃化转变温度的影响。(3)研究了界面能量学和分子间相互作用对不稳定和亚稳态聚合物薄膜后期(粗化)结构演化的影响。智力优势：在简单的液体中，扩散和粘度之间的关系是在斯托克斯-爱因斯坦方程的背景下建立的。对于大体积长链聚合物熔体，Doi-Edwards理论描述了扩散和这些材料表现出的各种粘弹性过程之间的相互关系；这种温度依赖关系可以用WLF(或等价的Vogel-Fulcher)方程来理解。然而，在薄膜领域，情况尚不明朗。通过一系列实验，在基于模拟和理论的预测的指导下，考察了薄膜聚合物-聚合物混合物的厚度依赖量之间的关系。这一建议的第二部分致力于加深对聚合物/纳米颗粒相互作用对薄膜聚合物-纳米颗粒复合材料中玻璃化和动力学(链段动力学和形态不稳定性)的影响的理解。关于界面的影响以及尺寸相关性的问题尚未解决，尽管模拟提出了检查它们的策略。我们对第三个问题的目标是发展对界面能量学和分子间相互作用在形态不稳定和亚稳定的薄膜支持的聚合物薄膜后期粗化过程中的作用的理解。更广泛的影响：这是一个跨学科的计划，涉及不同的领域，从表面的物理化学(润湿和自组织)、薄膜的加工(形态稳定性、粘度、玻璃化转变)到二维粗化现象(这也是微电子薄膜加工中感兴趣的问题，是更广泛的普遍粗化现象的一部分)。虽然我们现在确实对如何通过共混等方法来调整本体聚合物的性能有了很大的了解，但我们对薄膜还处于一个不感兴趣的阶段。由于分子间力的作用，相变、自组装和各种形态变化的厚度依赖性是特有的。对这些问题的理解将有助于我们制定新的规则来设计或定制适用于各种应用的薄膜特性。显然，这项提案中涉及的问题具有科学和技术意义。参与这个项目的学生在从物理、材料、化学到工程的各个领域发展跨学科的背景。

A diverse range of applications, from organic electronic devices and sensors to coatingsand patterning, rely on the properties and performance of polymers under various conditions ofconfinement, particularly thin film geometries. Generally, interactions between polymer chainsegments and interfaces impose enthalpic and entropic penalties on the chain structure, and theseeffects may be manifested in properties at length scales beyond the size of a chain, up to tens ofnanometers (nano-scale). Properties such as the viscosity, chain diffusion coefficients, D,glass transition temperatures, Tg, phase separation temperatures of mixtures and copolymers, aswell as morphological instabilities are known to exhibit film thickness dependencies. To date, acomprehensive understanding of such size-dependent phenomena in polymers remains elusive. The foregoing provides the basis for three problems examined in this proposal. (1) Onegoal is to understand how interfacial forces influence the chain dynamics and to understand theconnections between chain dynamics and the film thickness dependent changes exhibited by theTg in homopolymer and in miscible polymer-polymer systems. The strategy of our proposedexperiments is predicated largely on the notion that the glass transition is a manifestation ofunderlying dynamical features in the system. (2) We also plan to examine how the concentrationand spatial organization of nanoparticles affect the morphological stability, chain diffusion andaverage Tg of polymer-based nanocomposite thin film systems. (3) Finally, we examine the roleof interfacial energetics and intermolecular interactions on the late-stage (coarsening) structuralevolution in unstable and metastable thin polymer films on substrates. Intellectual merit: In simple liquids, the relation between diffusion and viscosity isestablished within the context of the Stokes-Einstein equation. With regard to bulk long-chainpolymeric melts, the Doi-Edwards theory describes the interrelation between diffusion andvarious viscoelastic processes exhibited by these materials; the temperature dependence isunderstood in terms of the WLF (or equivalently the Vogel-Fulcher) equation. In thin films,however, the situation is unclear. Through a series of experiments, guided by predictions basedon simulations and theory, connections between the thickness dependent quantities are examinedfor thin film polymer-polymer mixtures. The second part of this proposal is devoted todeveloping an understanding of the influence of polymer/nanoparticle interactions on the glasstransition and on dynamics (segmental dynamics and morphological instabilities) in thin filmpolymer-nanoparticle nanocomposites. Questions regarding the effect of interfaces, coupled withsize-scale dependencies, are unresolved, though simulations suggest strategies to examine them.Our goal for the third problem, is the development of an understanding of the role of interfacialenergetics and intermolecular interactions on late-stage coarsening processes in morphologicallyunstable and metastable thin , supported, polymer films. Broader Impact: This is an interdisciplinary program cross-cutting different fields, fromphysical chemistry of surfaces (wetting and self-organization), processing of thin films(morphological stability, viscosity, glass transition) to two dimensional coarsening phenomena(an issue also of interest for the processing thin films for microelectronics and is part of thebroader ubiquitous phenomenon of coarsening). While it is true that we now understand a greatdeal about how to tailor properties of bulk polymers through blending etc. we are at a stage ofinfancy with regard to thin films. The thickness dependencies of the phase transitions, selfassemblyand various morphological changes that result from the actions of intermolecular forcesare endemic. An understanding of these issues will help us to develop new rules to design or totailor properties of thin films for various applications. Clearly, the questions addressed in thisproposal have scientific and technological implications. Students participating in this projectdevelop an interdisciplinary background in areas that range from physics, materials and chemistryto engineering.