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

非技术综述：从轻质材料到柔性太阳能电池板，打开未来技术大门的材料经常呈现出“纳米结构”：它们由两个相互混合的微域组成，直径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。同时，第二个区域通常由微小的无机纳米颗粒组成--可以显著提高聚合物性能的刚性区域。在过去的十年里，科学家们发现了一些证据，表明在这些结构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粒子紧密地粘合在一起。这种被称为“不可逆吸附”的过程似乎会极大地改变这些材料的性能，有可能增强对高温的耐受性，改变渗透性，或许还会提高机械强度。然而，这种影响的原因--甚至为什么它会发生--仍然是未知的。甚至在更实际的情况下，人们对如何控制这种不可逆的吸附现象以获得下一代材料可能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计划和凝聚态物质和材料理论计划共同支持)将结合实验和计算机模拟，以了解这种吸附效应发生的原因，以及科学家和工程师如何控制它以优化材料性能。实验将采用纳米尺度的表征方法，其中定位于界面附近纳米尺度区域的荧光探针分子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运动和演化，从而有可能将材料性质的变化与分子结构和运动的潜在原因联系起来。总而言之，这些方法旨在提供所需的基本科学理解，以便能够更合理地设计这些材料，并与从基础设施到能源等经济部门相关。这项研究将与一项新的高中实习计划相结合，该计划将支持拓宽进入STEM专业的学生的渠道。技术摘要：在聚合物薄膜和纳米复合材料中，从聚合物熔体形成的不可逆吸附层可以显著改变主导这些材料整体性能的界面区域的属性。与聚合物从溶液中吸附不同，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，而从熔体(在没有这些因素的情况下)吸附的热力学机制仍然没有解决。此外，据报道，许多性质在吸附过程中共同进化，这对解释所有这些性质的吸附理论的发展提出了挑战。一个主要的挑战是在吸附层形成过程中以时间和空间分辨的方式探测近衬底和近粒子性质的演变的困难。为了克服这些挑战，这项工作将使用荧光实验来局部探测吸附过程中基质和颗粒附近的多种性质的演变。这些实验将与分子动力学模拟相结合，提供空间分辨率的洞察，了解链段堆积、链构象和聚合物动力学在吸附层形成过程中是如何共同演化的。协同实验和模拟将采用一种综合的方法，系统地探索层形成过程、孤立的吸附层的行为，以及吸附层的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矩阵。这一战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，并已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: 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.