Collaborative Research: Nonlinear Mechanical Spectroscopy of Glassy Polymers to Probe Viscoelastic Constitutive Behavior

该奖项支持表征玻璃聚合物的非线性粘弹性行为的研究。了解玻璃聚合物的长期机械行为对于确保玻璃聚合物作为独立材料或高级复合材料的基体材料的耐久性至关重要，这些材料需要低密度和高比强度的材料，例如汽车，体育和航空航天工业。该项目将采取措施，利用一种新颖的非线性指纹识别方法，从根本上解决这一挑战，该方法有望为优质产品的开发带来重要进展，从而使美国工业和社会受益。过去，指纹识别方法已被用于表征聚合物流体。该奖项将该方法扩展到聚合物玻璃，既通过新颖的实验，又通过适当的非线性本构建模和验证。该项目的研究生将在这一前沿研究方面进行培训，其中包括随时间变化的材料的实验和计算力学，玻璃聚合物物理以及工程聚合物的结构-性能关系。该项目还将利用德克萨斯理工大学和德克萨斯大学达拉斯分校的STEM外展项目，激发K-12学生对科学和工程的兴趣。与固体聚合物的非线性粘弹性行为有关的基础知识将在这个项目中进行。这需要一个通用的方案来分类各种观察到的行为，并使用这些观察来挑战相关的本构方程。该研究将使用组合的振幅振荡剪切和机械光谱烧孔指纹方法来提供一系列玻璃聚合物的非线性粘弹性行为的指纹，并挑战它们与文献中几个主要模型的一组非线性本构律预测。这项工作不仅表明该方法可以应用于玻璃聚合物，而且通过选择检查具有不同程度非均质性的聚合物，通过β -弛豫强度估计，该方法可以区分不同类型的非线性行为，并且原则上确定动态非均质性的细节如何影响指纹和本构模型参数。这是聚合物非线性力学中结构-性能关系思想的重要进展。此外，通过在非体积保持条件下（即拉伸和压缩条件下）进行测量，以开发材料指纹作为所述变形几何形状的函数，并将其与剪切结果进行比较，该工作对目前用于非线性流体剪切变形的指纹范式进行了关键评估。

This award supports research to characterize the nonlinear viscoelastic behavior of glassy polymers. Understanding the long-term mechanical behavior of glassy polymers is critical for assurance of the durability of glassy polymers in their use as stand-alone materials, or as matrix materials for advanced composites, in applications that require low-density and high specific strength materials, such as in automotive, sports, and aerospace industries. This project will take steps to address the challenge from a fundamental perspective using a novel nonlinear fingerprinting methodology that promises to bring important advances for development of premier products, hence benefiting US industry and society. In the past, the fingerprinting methodology has been used to characterize polymeric fluids. This award extends the method to polymer glasses, both through novel experiments and through appropriate nonlinear constitutive modeling and validation. The graduate students on the project will be trained in this cutting edge research, which involves experimental and computational mechanics of time-dependent materials, glassy polymer physics, and structure-property relations for engineering polymers. The project will also leverage STEM outreach programs at both Texas Tech University and University of Texas Dallas to excite K-12 students about science and engineering.Fundamental knowledge related to the nonlinear viscoelastic behavior of solid polymers will be pursued in this project. This requires a general scheme to classify the variety of observed behaviors and use these observations to challenge the relevant constitutive equations. The research will use the combined Large Amplitude Oscillatory Shear and Mechanical Spectral Hole Burning fingerprinting methodology to provide fingerprints of the nonlinear viscoelastic behavior of a series of glassy polymers and to challenge them against a set of nonlinear constitutive law predictions from several leading models from the literature. Not only will the work show that the method can be applied to glassy polymers, but by choosing to examine polymers with different degrees of heterogeneity, as estimated by the strength of the beta-relaxation, the method can differentiate among different types of nonlinear behavior and, in principle, determine how the details of the dynamic heterogeneity impacts the fingerprints and the constitutive model parameters. This provides an important advance in ideas of structure-property relations in the nonlinear mechanics of polymers. Furthermore, the work provides a critical evaluation of the fingerprinting paradigm, currently developed for shearing deformations in nonlinear fluids, by making measurements in non-volume preserving conditions, viz., tension and compression to develop material fingerprints as a function of said deformation geometries and comparing them to the shearing results.