Deformation-induced changes in segmental dynamics of polymer glasses prepared by melt stretching and stress aging

非技术摘要：聚合物玻璃是重要的材料，因为它们的透明度高，将它们模制成特定形状所需的低能量输入，它们的硬度，以及它们的抗破碎能力。聚合物玻璃被用于各种应用，如先进的飞机复合材料和眼镜镜片。这项计划中的研究将调查聚合物玻璃在受到大的力时会有什么反应。特别是，这些实验将使用激光和显微镜来测量聚合物分子在聚合物玻璃变形时如何重新排列。目前的理论和计算机模拟已经将这种分子运动确定为一个关键的量，必须了解它才能确定聚合物玻璃是否会拉伸、变形或破裂。将使用的聚合物玻璃将以特殊的方式进行加工(高温拉伸或低温变形)，以严格检验这些模拟和理论。所提出的实验应该会导致更可靠的模型来预测聚合物玻璃的机械性能。这反过来可能会产生重大的经济影响，因为这些轻质材料具有广阔的潜力。参与这个项目的学生将学习如何表征聚合物玻璃并改善材料的性能。所有受此资助的人员都将参加一项计划，以增加来自代表不足群体的高素质学生进入美国技术概要中的博士课程：由于变形的非线性和玻璃的非平衡性质，聚合物玻璃的变形特性的预测具有极大的挑战性。在这里，建议使用光学光漂白方法来测量聚合物玻璃在变形过程中的链段迁移率，该方法提供了访问稀薄探针分子系综的旋转关联函数的途径。变形可以使聚合物玻璃的链段迁移率增加1000倍以上，了解这种增强的迁移率是准确模拟聚合物玻璃力学性能的关键要求。在拟议的工作中，将通过熔体淬火来制备聚合物玻璃，以便随后在变形期间的迁移率测量可以解开屈服和应变硬化对链段迁移率的影响。这项拟议的工作还将测试变形的聚合物玻璃是否会显示出过度老化的迹象，即由于施加应力而加速发生的物理老化。最后，将通过对剪切变形过程中的节段迁移率进行光学测量来检验拉伸变形结果的一般性。所提出的实验可以为聚合物玻璃变形的理论描述提供一个分子水平的检验。实验结果与理论和模拟结果相比较，可以加深对聚合物玻璃变形的基本认识。所有受此资助的人员都将参加一个项目，该项目旨在增加来自代表性不足群体的高素质学生进入美国的博士项目。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY:Polymer glasses are important materials due to their high transparency, the low energy input required for molding them into particular shapes, their stiffness, and their resistance to breakage. Polymer glasses are used in applications as diverse as advanced aircraft composites and eyeglass lenses. The planned research will investigate how polymer glasses respond when they are subjected to large forces. In particular, the experiments will use lasers and microscopes to measure how the polymer molecules rearrange when a polymer glass is deformed. Current theories and computer simulations have identified this molecular motion as a key quantity that must be understood to determine if a polymer glass will stretch, deform, or break. The polymer glasses to be used will be processed in special ways (stretched at high temperature or deformed at low temperature) to critically test these simulations and theories. The proposed experiments should lead to more reliable models for the prediction of the mechanical properties of polymer glasses. This in turn may have a significant economic impact because of the broad potential of these lightweight materials. The students involved in this project will learn how to characterize polymer glasses and improve the properties of materials. All personnel supported by this grant will participate in a program to increase the number of highly qualified students from underrepresented groups that enter Ph.D. programs in the U.S.TECHNICAL SUMMARY:The deformation properties of polymer glasses are extremely challenging to predict as a result of the nonlinear nature of the deformation and the nonequilibrium nature of the glass. Here it is proposed to measure segmental mobility in polymer glasses during deformation using an optical photobleaching method that provides access to the rotational correlation function of an ensemble of dilute probe molecules. Deformation can increase segmental mobility in polymer glasses by more than a factor of 1000, and an understanding of this enhanced mobility is a key requirement for accurate modeling of the mechanical properties of polymer glasses. For the proposed work, polymer glasses will be prepared by melt quenching so that subsequent mobility measurements during deformation can disentangle the influences of yield and strain hardening on segmental mobility. The proposed work will also test whether deformed polymer glasses can show indications of overaging, i.e., physical aging that occurs at an accelerated rate as a result of applied stress. Finally, the generality of the results obtained in tensile deformation will be tested by performing optical measurements of segmental mobility during shear deformation. The proposed experiments can provide a molecular-level test of theoretical descriptions of polymer glass deformation. The experimental results obtained, in comparison with theory and simulation, should lead to an enhanced fundamental understanding of the deformation of polymer glasses. All personnel supported by this grant will participate in a program to increase the number of highly qualified students from underrepresented groups that enter Ph.D. programs in the U.S..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.