Dynamic Processes at the Nanoscale in Polymer Systems

各种各样的应用，从有机电子器件和传感器到涂层和图案，都依赖于聚合物在各种限制条件下的特性和性能，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的惩罚，这些影响可能在超过链尺寸的长度尺度上表现出来，高达几十纳米（纳米尺度）。众所周知，粘度、链扩散系数、D、玻璃化转变温度、Tg、混合物和共聚物的相分离温度以及形态不稳定性都与薄膜厚度有关。到目前为止，对聚合物中这种依赖尺寸的现象的全面理解仍然是难以捉摸的。上述情况为本建议所审查的三个问题提供了基础。(1)一个目标是了解界面力如何影响链动力学，并了解链动力学与均聚物和混相聚合物-聚合物体系中tg所表现出的膜厚度依赖变化之间的联系。我们提出的实验策略主要基于玻璃化转变是系统中潜在动力学特征的表现这一概念。(2)我们还计划研究纳米颗粒的浓度和空间组织如何影响聚合物基纳米复合薄膜系统的形态稳定性、链扩散和平均Tg。(3)最后，我们研究了界面能量学和分子间相互作用在基底上不稳定和亚稳定聚合物薄膜后期（粗化）结构演变中的作用。智力优势：在简单液体中，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起来的。对于大块长链聚合物熔体，Doi-Edwards理论描述了这些材料所表现出的扩散和各种粘弹性过程之间的相互关系；温度依赖性是根据WLF（或等价的Vogel-Fulcher）方程来理解的。然而，在薄膜行业，情况并不明朗。通过一系列实验，在基于模拟和理论的预测指导下，对薄膜聚合物-聚合物混合物的厚度依赖量之间的联系进行了研究。本提案的第二部分致力于了解聚合物/纳米颗粒相互作用对薄膜聚合物-纳米颗粒纳米复合材料的玻璃化转变和动力学（片段动力学和形态不稳定性）的影响。关于接口影响的问题，加上大规模的依赖关系，还没有解决，尽管模拟提供了检查它们的策略。我们的第三个问题的目标是理解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支撑聚合物薄膜后期粗化过程中的作用。更广泛的影响：这是一个跨学科的项目，横跨不同的领域，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感兴趣的问题，是广泛普遍的粗化现象的一部分）。虽然我们现在确实对如何通过混合等方式调整大块聚合物的特性有了很大的了解，但关于薄膜，我们还处于一个想象的阶段。分子间力作用导致的相变、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各种应用的薄膜特性。显然，本提案中所讨论的问题具有科学和技术意义。参与该项目的学生将在物理、材料、化学和工程等领域发展跨学科背景。

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.