The Nanomechanical and Viscoelastic Responses of Ultrathin Polymer Films

非技术摘要：为快速增长的纳米技术领域提供科学和工程解决方案的技术基础设施具有相当大的国家利益。本工作涉及构成相关使能技术基础的材料的纳米限制行为的基本原理。解决的一组重要问题与纳米厚膜的工程特性(如硬度和屈服强度)有关，这些膜是以自由站立的形式存在的，因此不容易测量。在这种材料中进行这种测量的唯一方法是在PI的实验室中开发的气泡充气方法，该方法允许测试极少量的材料，特别是纳米薄膜聚合物薄膜。这项工作研究了玻璃状态下自由站立聚合物薄膜的工程特性，特别强调了屈服行为。这些研究将首次提供自由站立薄膜中薄膜厚度和成品率与温度的关系。此外，在自由站立的薄膜中，观察到材料的刚度有很大的提高，最近出现了相互矛盾的关于硬化行为的理论模型来解释这种现象。这样的预测对纳米材料的设计和使用当然很重要，目前的工作将确定这些理论的有效性范围。此外，还将研究分子结构效应。最后，纳米气泡膨胀实验允许研究以前由于数量极少而无法实现的新材料。在这种情况下，研究人员将研究由物理气相沉积(PVD)制成的超稳定聚合物玻璃，这种玻璃甚至可以比2000万年前的琥珀玻璃更稳定。这种高度的稳定性使得人们可以询问一个长期存在的问题，这个问题的解决对于玻璃理论，特别是如何对其在先进复合材料和粘合剂等重要领域的应用中的行为进行长期预测是至关重要的。技术摘要：超薄聚合物薄膜的行为仍然是一个密集的研究领域，但大多数研究仅限于衬底支持的薄膜的情况，尽管研究表明在自由站立状态下会产生更大的影响。本文利用TTU气泡膨胀法对三个方面的自由站立超薄膜进行了测试，并利用该方法对极少量材料进行粘弹性测量的能力，研究了由物理气相沉积(PVD)制备的超稳聚合物玻璃的响应。解决的一组重要问题与工程特性有关，如纳米厚膜的弹性系数和屈服强度，这些膜处于可自由站立的形式，因此不易测量。在这种材料中进行这种测量的唯一方法是气泡膨胀法，它允许测试极少量的材料，特别是超薄或纳米聚合物薄膜。这项工作研究了玻璃状态下自由站立聚合物薄膜的工程特性，特别强调了屈服行为。此外，在自由站立的薄膜中，观察到了很大的模数增加，最近，关于硬化行为的相互矛盾的理论模型已经出现来解释这种现象。这样的预测对纳米材料的设计和使用当然很重要，目前的工作将确定这些理论的有效性范围。支化聚合物在支撑层上表现出不同于线性聚合物的纳米尺度行为，TTU气泡膨胀法将被用来研究支化和非缠绕聚合物链长对自由站立超薄膜粘弹性性质的影响。最后，包括聚合物在内的玻璃形成液体的动力学(松弛时间或粘度)是否在有限温度下发散仍然存在争议。PI的团队现在已经展示了第一个PVD超稳定聚合物玻璃，它可以用来确定上限弛豫时间，其方式类似于以前对2000万年前的琥珀所做的工作，但由于PVD聚合物的虚构温度至少比玻璃化转变温度低50K，因此可以在更大的温度窗口内进行测试，并且PVD条件的优化提供了更大测试窗口的可能性。如果实验成功，它将提供进一步的实验数据，挑战玻璃形成系统行为的理论。

NON-TECHNICAL SUMMARY:The technological infrastructure that provides science and engineering solutions to the rapidly growing nanotechnology area is of considerable national interest. The present work addresses fundamentals of the nanoconfinement behavior of materials that form the basis of the relevant enabling technologies. One important set of problems addressed relates to the engineering properties (such as stiffness and yield strength) of nanometer-thick films that are in freely standing form and, consequently, cannot be readily measured. The only method available for making such measurements in such materials is a bubble-inflation method developed in the PI's laboratory that allows testing of extremely small quantities of material, especially nanometer-thin polymer films. The work investigates the engineering properties of freely standing polymer films deep in the glassy state with particular emphasis on yield behavior. These studies will be the first to provide film thickness and temperature dependence of yield in freely standing films. Also, in the freely standing films, a large enhancement in the material stiffness is observed and, recently, conflicting theoretical models of the stiffening behavior have appeared to explain the phenomenon. Such predictions are, of course important to nanomaterial design and use, and the present work will establish the range of validity of these theories. Molecular architecture effects will also be investigated. Finally, the nanobubble inflation experiment permits investigations of novel materials that were previously unachievable due to their extremely small quantities. In this case, the investigators will study ultrastable polymer glasses made by physical vapor deposition (PVD) and that can be made more stable than even a 20 million year old amber glass. This high stability allows the interrogation of a long-standing question whose resolution is fundamental to theories of glasses and, in particular, how to make long-term predictions of their behavior in applications to important areas such as advanced composites and adhesives. TECHNICAL SUMMARY:The behavior of ultrathin polymer films remains an intense area of investigation, but most studies have been limited to the case of substrate-supported films even though studies suggest much larger effects occur in the freely standing state. The present work tests three aspects of freely standing ultrathin films using the TTU bubble inflation method and takes advantage of the method's capability of making viscoelastic measurements on extremely small quantities of material to study the response of an ultrastable polymer glass made by physical vapor deposition (PVD). One important set of problems addressed relates to the engineering properties, such as modulus and yield strength of nanometer thick films that are in freely standing form and, consequently, not readily measured. The only method available for making such measurements in such materials is a bubble inflation method that allows testing of extremely small quantities of material, especially ultrathin or nano-metric polymer films. The work investigates the engineering properties of freely standing polymer films deep in the glassy state with particular emphasis on yield behavior. Also, in freely standing films, a large modulus enhancement is observed and, recently, conflicting theoretical models of the stiffening behavior have appeared to explain the phenomenon. Such predictions are, of course important to nanomaterial design and use and the present work will establish the range of validity of these theories. Branched polymers have been shown to exhibit different nanoscale behavior from linear counterparts upon confinement on a supporting layer and the TTU bubble inflation method will be used to examine the effects of branching and unentangled polymer chain length on the viscoelastic properties of freely standing ultrathin films. Finally, it remains controversial whether or not the dynamics (relaxation time or viscosity) in glass-forming liquids, including polymers, diverge at a finite temperature. The PI's group has now demonstrated the first PVD ultrastable polymer glass that can be used to determine the upper bound relaxation times in a fashion similar to prior work with a 20 million year old amber but over a larger "window" of temperatures because the PVD polymer has a fictive temperature at least 50 K below the glass transition temperature, and optimization of the PVD conditions offers the possibility of an even larger testing window. Should the experiment be successful, it will provide further experimental data that can challenge theories of the behavior of glass-forming systems.