Collaborative: Viscoelasticity of Nanoparticle Dispersed Polymer Melts: Experiment and Simulation

技术：将结合实验和理论来描述为什么在聚合物中添加纳米颗粒可以改善其机械性能。人们的注意力集中在两个方面：(A)颗粒大小的影响：现在公认的是，加入颗粒后，聚合物熔体的模数可以增加几个数量级。这种增强对于轮胎或引擎盖下的应用是至关重要的，因为未填充的聚合物太“软”，不能在这些环境中使用。由于这种模数增强被推测为作为颗粒尺寸(在10 nm尺寸范围内)的函数的最大值，因此显然存在一个最优的纳米颗粒尺寸来实现这一特性。证明这个假设的最大值的存在，并探索它的分子起源，是这项工作的重点。(B)佩恩效应：虽然通过添加纳米颗粒获得的模数增加对材料的某些静态特性至关重要，但这也会使纳米复合材料难以加工。佩恩效应，即材料的机械强度随着应变的增加而数量级下降，在不影响材料的最终使用行为的情况下绕过了这个问题。因此，理解佩恩效应的分子基础是这项工作的第二个相互关联的目标。具体地说，在这种情况下，结合聚合物层与颗粒之间的聚合物桥的作用将通过设计两者的相对比例不同的材料来严格检验。这项工作利用两个PI的独特能力，采用实验和模拟相结合的方式，研究完全分散的纳米颗粒复合材料的这些相互关联的流变行为。非技术性：塑料现在在许多环境中无处不在，比如在包装中。不太熟悉的是这些材料在结构应用(例如，建筑材料)或引擎盖下的使用，这些缺陷归因于这些材料相对较软的机械行为。一个正在进行的目标是改善聚合物的这一特定方面，人们推测添加纳米颗粒是实现这一目标的一种简单方法。这项工作将针对这一问题，系统地和批判性地评价纳米颗粒对塑料力学行为的作用。研究活动将与针对K-12、本科生和研究生的广泛教育和外联活动相结合。督导计划的目标是招收/挽留未获充分代表的少数族裔学生，攻读研究生及以上程度的科学/工程学科。特别是，已经与佛罗里达农工大学和格拉姆林州立大学(两者都是HBCU)发展了互动，目标是招收本科生进入该项目。PIS将继续与当地城市高中教师合作，目标是给学生，特别是高年级学生“动手”的研究体验。

TECHNICAL:Experiments and theory will be combined to delineate why adding nanoparticles to a polymer improves its mechanical properties. Attention is focused on two aspects: (a) Effect of Particle Size: It is now accepted that the modulus of a polymer melt can be increased by several orders of magnitude on the addition of particles. This reinforcement is critical to applications, e.g., tires or under the hood, since unfilled polymers are too "soft" to be used in these contexts. Since this modulus reinforcement is conjectured to go through a maximum as a function of particle size (in the 10 nm size scale), there is apparently an optimum nanoparticle size for this property. Proving the existence of this postulated maximum, and exploring its molecular origins, is a focus of this work. (b) The Payne Effect: While the increase in modulus achieved through the addition of nanoparticles is critical to certain quiescent properties of the material, this can also make the nanocomposites hard to process. The Payne effect, i.e., the orders-of-magnitude decrease of the mechanical strength of a material with increasing strain, circumvents this problem without compromising the materials' end use behavior. Understanding the molecular underpinnings of the Payne effect is then the second, interrelated goal of this work. In particular, the role of the bound polymer layer vs. polymer bridges between particles in this context will be critically examined by devising materials where the relative proportion of these two is varied. This work exploits the unique capabilities of the two PIs to study these interrelated aspects of the rheological behavior of nanocomposites with fully dispersed nanoparticles, using a combination of experiment and simulation. NON-TECHNICAL:Plastics are by now ubiquitous in many contexts, such as in packaging. Less familiar is the use of these materials in structural applications (e.g., building materials) or under the hood, with these deficiencies being attributed to the relative soft mechanical behavior of these materials. An ongoing goal has been to improve this particular aspect of polymers, and it has been conjectured that the addition of nanoparticles is one facile means of achieving this goal. This work will target this issue and systematically and critically evaluate the role of nanoparticles on the mechanical behavior of plastics. The research activities will be coupled to extensive education and outreach activities that target students at the K-12, undergraduate and graduate levels. The PIs will aim to recruit/retain underrepresented minority students into science/engineering disciplines at the graduate level and beyond. In particular, interactions have been developed with Florida A&M University and Grambling State University (both HBCUs) with the goal of recruiting undergraduates into the program. The PIs will continue to work with local city high school teachers with the goal of giving students, especially seniors, "hands-on"research experience.