Collaborative Research: Integrated experiments and simulations to understand the mechanism and consequences of polymer adsorption in films and nanocomposites

非技术总结：从轻质材料到柔性太阳能电池板，为未来技术打开大门的材料经常展示“纳米结构”：它们由两个精细混合的畴组成，只有数百到数千个原子。在许多情况下，其中一个领域由聚合物组成，聚合物是塑料、橡胶和许多生物材料的长分子链。与此同时，第二畴通常由微小的无机纳米颗粒组成——刚性区域，可以显著提高聚合物的性能。在过去的十年里，科学家们发现了证据，表明在这些区域之间的界面上发生了一些奇怪的事情：聚合物分子在分子水平上与颗粒紧密地“粘在一起”。这一过程被称为“不可逆吸附”，它似乎极大地改变了这些材料的性质，有可能增加对高温的耐受性，改变渗透性，也许还能提高机械强度。然而，这种影响的原因——甚至为什么会发生——仍然未知。更实际的是，人们对如何控制这种不可逆吸附现象以获得下一代材料的最佳性能知之甚少。这个合作项目（由材料研究部的聚合物项目和凝聚态物质和材料理论项目共同支持）将结合实验和计算机模拟来理解为什么会发生这种吸附效应，以及科学家和工程师如何控制它来优化材料性能。实验将采用纳米级表征方法，其中荧光探针分子定位于界面附近的纳米级区域，报告吸附层的性质及其形成方式。在超级计算机上进行的分子模拟将放大到分子尺度，以了解分子在不可逆吸附过程中如何移动和进化，从而有可能将材料特性的变化与分子结构和运动的潜在原因联系起来。总之，这些方法旨在提供基本的科学理解，以实现这些材料的更合理的工程和设计，并与从基础设施到能源的经济部门相关。这项研究将与一项新的高中实习计划相结合，该计划将支持扩大学生进入STEM专业的渠道。技术概述：在聚合物薄膜和纳米复合材料中，聚合物熔体形成的不可逆吸附层可以显著改变界面域的性质，而界面域的性质决定了这些材料的整体性能。聚合物从溶液中吸附是由溶剂和聚合物之间的能量不匹配和熵大小不对称共同驱动的，而聚合物从熔体中吸附的热力学机制（这些因素不存在）仍未得到解决。此外，据报道，许多性质在吸附过程中共同进化，挑战了吸附理论的发展。在吸附层形成过程中，以时间和空间分辨的方式探测近基质和近颗粒性质的演变是一个核心挑战。为了克服这些挑战，本工作将采用荧光实验来局部探测吸附过程中底物和颗粒附近多种性质的演变。这些实验将与分子动力学模拟相结合，将为在吸附层形成过程中，段填料、链构象和聚合物动力学如何共同演化提供空间解析的见解。协同实验和模拟将采用综合方法系统地探索层的形成过程，孤立吸附层的行为，以及吸附层存在对材料性能的最终影响，所有这些都跨越关键控制变量的矩阵。该策略将建立对多种机制如何相互作用以驱动吸附层形成并调节其对聚合物性能的影响的理解。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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: From lightweight materials to flexible solar panels, the materials opening the door to tomorrow’s technologies frequently exhibit “nanostructure”: they are comprised of two finely intermixed domains only hundreds to thousands of atoms across. In many cases, one of these domains consists of polymers, which are long chains of molecules that plastics, rubber, and many biological materials are made of. In parallel, the second domain often consists of tiny inorganic nanoparticles – rigid regions that can dramatically enhance the polymers’ properties. Over the past decade, scientists have found evidence that something strange happens at the interfaces between these domains: the polymer molecules become tightly ‘glued’ to the particles at the molecular level. This process, known as “irreversible adsorption”, seems to dramatically alter these materials’ properties, with the potential to imbue tolerance of higher temperatures, to alter permeability, and perhaps to enhance mechanical strength. However, the cause of this effect – or even why it should occur at all – remains unknown. Even more practically, there is little understanding of how to control this irreversible adsorption phenomenon in order to obtain the best possible properties for next-generation materials. This collaborative project (co-supported by the Polymers Program and the Condensed Matter and Materials Theory Program in the Division of Materials Research) will combine experiments and computer simulations to understand why this adsorption effect occurs and how scientists and engineers can control it to optimize material properties. Experiments will employ a nanoscale characterization method wherein fluorescent probe molecules, localized to the nanoscale domain near the interface, report on the properties of the adsorbed layer and how it forms. Molecular simulations performed on supercomputers will zoom in to the molecular scale to understand how molecules move and evolve during irreversible adsorption, making it possible to link changes in material properties with underlying causes in molecular structure and motion. Together, these approaches aim to provide the fundamental scientific understanding needed to enable more rational engineering and design of these materials, with relevance to economic sectors ranging from infrastructure to energy. This research will be coupled with a new high-school internship program that will support broadening the pipeline of students moving into STEM professions.TECHNICAL SUMMARY: In polymer films and nanocomposites, the formation of an irreversibly adsorbed layer from the polymer melt can dramatically alter the properties of the interfacial domains that dominate the overall properties of these materials. Unlike in polymer adsorption from solution, which is driven by a combination of an energetic mismatch and an entropic size asymmetry between solvent and polymer, the thermodynamic mechanism of adsorption from the melt (where these factors are absent) remains unresolved. Moreover, numerous properties are reported to co-evolve during adsorption, challenging the development of a theory of adsorption accounting for all of them. A central challenge has been the difficulty of probing the evolution of near-substrate and near-particle properties in a temporally and spatially resolved manner during adsorbed layer formation. To overcome these challenges, this work will employ fluorescence experiments to locally probe the evolution of multiple properties near substrates and particles during adsorption. These experiments will be combined with molecular dynamics simulations that will provide spatially resolved insight into how segmental packing, chain conformations, and polymer dynamics co-evolve during adsorbed layer formation. Synergistic experiments and simulations will take an integrated approach to systematically probe the layer formation process, the behavior of isolated adsorbed layers, and the ultimate impact of the adsorbed layer presence on material properties, all across a matrix of key controlling variables. This strategy will establish an understanding of how multiple mechanisms may interact to drive adsorbed layer formation and mediate its impact on polymer properties..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.