Designed Local Segmental Motions and Their Interactions with Nonlinear Mechanical Deformation in Glassy Polymers

9971569是密歇根大学研究小组提出的这项研究的最终目标是发现和操纵聚合物分子的化学结构与其机械性能之间的基本关系。发现的过程可以分为两个步骤。在第一部分中，阐明了聚合物分子在固态时的本征热激发运动。在第二种情况下，确定了外应力对这些固有运动的影响。在之前由美国国家科学基金会资助的工作中，密歇根研究小组发现，部分聚合物分子如何旋转或扭曲的细节对关键的机械性能有关键影响。他们表明，如果聚合物主链的较长片段能够一起移动，那么聚合物更有可能是延展性的，而不是脆性的。然后，他们合成了一种聚酯-碳酸酯聚合物，其抗冲击断裂性能优于双酚A-聚碳酸酯，这是迄今为止所有玻璃聚合物中最坚韧的。这种合成的关键是在双酚A-聚碳酸酯链环己烯(C-C6)链中插入能够双稳态(椅子-船-椅子)构象转变的键。第二步是在目前的提议中进行的：发现内在流动性如何促进导致延性或脆性行为的应力的松弛。C-C6基团将被用来将各种刚性分子片段与有限的天然迁移率联系起来，这将允许测试c-C6的能力，使其成为为原本非常不能移动的片段提供流动性的“引擎”。由此产生的分子运动将通过固体核磁共振、动态机械光谱、介电松弛和正电子湮没进行监测。此外，合作运动将通过链内和链间的约束进行系统的修改，以改变在外加应力下分段重排的容易程度。这种效应可以通过正电子湮没寿命谱(PALS)跟踪链段相互滑动时形成的空穴的瞬变性质来揭示。PALS数据将与系统增加的大小和频率的振荡应力同步采集。通过这种方法，可以确定链运动的时间尺度及其随机械应力的变化。聚合物材料在医疗植入物、微电子、光盘、隐形飞机等各种应用中都是关键成分，更不用说主要的家用电器、汽车、民用飞机、桥梁和高速公路了。在这种应用中，聚合物的机械强度和耐久性都是极其重要的。然而，设计具有理想机械性能的聚合物仍然是一门艺术。本研究的成功结论应该允许合理设计具有高抗裂性的聚合物，同时保持其他所需的物理性能。最终，这将直接导致使用聚合物的更高性能、更耐用和更节能的工程系统。在对科学家进行聚合物教育时，传统的方法是教他们用化学方法合成聚合物，或者将聚合物用作材料。因此，培养成化学家的工业聚合物科学家和培养成材料工程师的工业聚合物科学家之间通常存在文化和语言鸿沟。这对聚合物的创新和成功应用造成了障碍。目前的提议要求学生学习聚合物科学的两个方面，从而为社会提供更多能够弥合这一差距的科学家。发展出的基本理解已经并将继续纳入到PI教授的研究生课程中。

9971569YeeThe ultimate goal of the research proposed by the group at the University of Michigan is to discover and manipulate the fundamental relationship between the chemical structure of polymer molecules and their mechanical properties. The process of discovery can be divided into two steps. In the first the intrinsic thermally excited motions that the polymer molecules are capable of in the solid state is clarified. In the second the effect of external stress on these intrinsic motions is determined. In previous NSF-supported work the Michigan group discovered that the specifics of how parts of polymeric molecules rotate or twist have critical effects on the key mechanical properties. They showed that if longer pieces of the main-chain of the polymer are able to move together then the polymer is more likely to be ductile rather that brittle. They then synthesized a polyester-carbonate polymer with impact fracture resistance superior to BPA-polycarbonate, heretofore the toughest of all glassy polymers. The key to this synthesis is the insertion into a BPA-polycarbonate chain cyclohexylene (c-C6) linkages capable of bi-stable (chair-boat-chair) conformational transitions. The second step is to be carried out in the present proposal: To discover how intrinsic mobility facilitates the relaxation of stresses that results in ductile or brittle behavior. C-C6 groups will be used to link various rigid molecular segments with limited native mobility, which will allow testing of the ability of c-C6 to be the "engine" for providing mobility to segments which are otherwise quite immobile. The resulting molecular motions will be monitored by solid state NMR, dynamic-mechanical spectroscopy, dielectric relaxation, and positronium annihilation. Furthermore, the cooperative motions will be systematically modified with intra-chain and inter-chain constraints to alter the ease of segmental rearrangement under applied stress. The effect can be revealed by following the transient nature of the holes formed when chain segments slip past each other by positronium annihilation lifetime spectroscopy (PALS). PALS data will be acquired synchronously with oscillating stress of systemically increasing magnitude and frequency. In this way the time scale of chain movement and its variation with mechanical stress can be determined.Polymeric materials are key constituents in applications as diverse as medical implants, microelectronics, compact disks, stealth aircraft, not to mention major household appliances, automobiles, civilian aircraft, bridges, and highways. In such applications, the mechanical strength and durability of the polymer are both extremely important. Yet designing polymers with desirable mechanical properties remains an art. Successful conclusion of the present research should allow the rational design of polymers with high resistance to fracture while maintaining other desirable physical properties. Ultimately, this should lead directly to higher performance, more durable and energy efficient engineering systems using polymers. In educating scientists about polymers the traditional approach has been to teach them either to chemically synthesize polymers, or to use polymers as materials. Consequently a culture and language gap typically exists between industrial polymer scientists trained as chemists and those as materials engineers. This creates barriers to innovation and successful application of polymers. The present proposal requires students to learn both aspects of polymer science thus providing society with more scientists that are able to bridge the gap. The fundamental understanding developed have been and will continue to be incorporated into graduate taught by the PI.