The Nanomechanical and Viscoelastic Responses of Ultrathin Polymer Films

非技术性总结：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。 目前的工作解决的纳米约束行为的材料，形成相关的使能技术的基础上的基本原理。解决的一组重要问题涉及纳米厚膜的工程性质（如刚度和屈服强度），这些纳米厚膜处于自由站立的形式，因此不能容易地测量。在这种材料中进行这种测量的唯一方法是PI实验室开发的气泡膨胀方法，该方法允许测试极少量的材料，特别是纳米薄的聚合物薄膜。这项工作研究了自由站立的聚合物膜的工程性质，特别强调在玻璃态的屈服行为。这些研究将是第一个提供薄膜厚度和温度依赖性的产量在自由站立的电影。此外，在自由站立的膜，在材料刚度的大的增强被观察到，最近，出现了相互矛盾的理论模型的硬化行为来解释这一现象。 这样的预测，当然是重要的纳米材料的设计和使用，目前的工作将建立这些理论的有效性范围。分子结构的影响也将被调查。最后，纳米气泡膨胀实验允许研究以前无法实现的新材料，因为它们的数量非常小。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，这种玻璃甚至比2000万年前的琥珀玻璃更稳定。这种高稳定性允许询问一个长期存在的问题，其解决方案是玻璃理论的基础，特别是如何在高级复合材料和粘合剂等重要领域的应用中对其行为进行长期预测。 技术总结：聚合物薄膜的行为仍然是一个激烈的研究领域，但大多数研究都局限于基板支撑的薄膜的情况下，即使研究表明，更大的影响发生在自由站立状态。本工作测试三个方面的自由站立的玻璃膜使用TTU气泡膨胀方法，并利用该方法的能力，使粘弹性测量的极少量的材料，研究的响应由物理气相沉积（PVD）制成的超稳定聚合物玻璃。解决的一组重要问题涉及工程性质，例如纳米厚膜的模量和屈服强度，其处于自由站立的形式，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气泡膨胀法，该方法允许测试极少量的材料，特别是纳米或纳米聚合物薄膜。这项工作研究了自由站立的聚合物膜的工程性质，特别强调在玻璃态的屈服行为。此外，在自由站立的薄膜，一个大的模量增强观察，最近，出现了相互矛盾的理论模型的硬化行为来解释这一现象。 这样的预测，当然是重要的纳米材料的设计和使用，目前的工作将建立这些理论的有效性范围。 支化聚合物已被证明表现出不同的纳米级行为从线性对应物约束后，在支撑层和TTU气泡膨胀方法将被用来检查的支化和未缠结的聚合物链长度的影响，对自由站立的粘弹性膜。最后，它仍然是有争议的动力学（弛豫时间或粘度）是否在玻璃形成液体，包括聚合物，在有限的温度下发散。PI的团队现在已经展示了第一种PVD超稳定聚合物玻璃，其可以用于以类似于先前使用2000万年前琥珀的工作的方式确定上限弛豫时间，但是在更大的温度“窗口”上，因为PVD聚合物具有比玻璃化转变温度低至少50 K的假想温度，并且PVD条件的优化提供了甚至更大的测试窗口的可能性。如果实验成功，它将提供进一步的实验数据，可以挑战玻璃形成系统的行为理论。

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.