Investigating Fundamental Toughening Mechanisms in Nanocellular Foams

该奖项支持的研究将结合纳米、微观和宏观尺度的数值和实验研究，以揭示利用纳米级气泡制造更坚固、更轻重量泡沫的机制。聚合物泡沫的轻重量和吸能能力使其成为飞机复合材料板和头盔等防护设备的理想应用，但它们面临一个根本问题：减轻重量会大大降低其强度和韧性。最近的研究表明，当泡沫的孔径与传统泡沫相比减小~1000倍时，其韧性可以显著提高，有时甚至超过体材。这项研究将通过在纳米和微观尺度上研究材料，然后利用这些基本知识建立模型，预测和再现泡沫在宏观尺度上的物理性质，从而深入了解这一意想不到的现象。这项工作将对聚合物在防弹装甲、合成革和防撕裂农业地膜中的先进应用产生深远的影响。该项目还支持建立纳米材料与结构工程（NEMS）项目的推广工作，该项目的重点是将社区大学中有学术天赋的低收入学生引入一个为期一个月的项目，向他们教授纳米材料，并激励他们从事工程方面的职业。该项目的具体目标是全面了解分子和纳米尺度结构如何与材料尺寸效应耦合，从而影响纳米结构聚合物的宏观性质。泡沫的韧性被认为与其细胞大小的平方根成比例，这意味着纳米孔的韧性会降低，但这一理论与最近对纳米细胞泡沫的实验不一致。为了深入了解这一现象，pi将使用原位纳米力学实验以及粗粒度分子动力学来表征细胞水平和分子水平的塑性机制。这些将与微极有限元模型相结合，以阐明在宏观尺度上发生在纳米细胞泡沫中的断裂过程。该项目旨在回答以下基本问题：1)纳米孔隙度和纳米约束如何影响分子水平的塑性，以及2)孔隙大小和结构如何影响断裂过程区大小以提高韧性。通过这项研究，pi将开发一种结合实验和数值的多尺度方法来揭示跨长度尺度的韧性力学。这将导致坚韧、轻质的商用热塑性塑料和纳米复合材料等新兴工程材料的新发展。该奖项反映了美国国家科学基金会的法定使命，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。

This award supports research that will combine numerical and experimental investigations at the nano-, micro- and macroscale to uncover the mechanisms for creating tougher, lighter-weight foams using nano-sized bubbles. The light weight and energy-absorbing capacity of polymer foams makes them ideal for applications in aircraft composite panels and protective equipment like helmets, but they suffer from a fundamental problem: reducing their weight greatly reduces their strength and toughness. Recent research has shown that when the pore sizes of foams are reduced by ~1000x compared to the pores in traditional foams, their toughness can significantly increase, sometimes even exceeding that of the bulk material. This research will provide deep insight into this unexpected phenomenon by studying materials at the nano- and microscale and then using that fundamental knowledge to build models that predict and reproduce the physical properties of the foams at the macroscale. This work will have profound implications for advanced applications of polymers in bulletproof armor, synthetic leather, and tear-proof agricultural mulch films. This project also supports outreach efforts to create a Nano-Engineering of Materials and Structures (NEMS) program focused on bringing academically talented, low-income students from community colleges into a month-long program to teach them about nanomaterials and inspire them to pursue a career in engineering. The specific goal of this project is to provide a comprehensive understanding of how molecular and nanoscale architecture can couple with material size-effects to influence the macroscale properties of a nanostructured polymer. The toughness of a foam is thought to scale with the square root of its cell size, meaning nanopores would have a reduced toughness, but this theory is inconsistent with recent experiments on nanocellular foams. To provide insight into this phenomenon, the PIs will use in-situ nanomechanical experiments along with coarse grained molecular dynamics to characterize cell-level and molecular-level plasticity mechanisms. These will be incorporated with micropolar finite element models to elucidate the fracture processes occurring in nanocellular foams at the macroscale. This project seeks to answer fundamental questions of: 1) how nanoscale porosity and nanoconfinement affect molecular level plasticity, and 2) how pore-size and architecture affect fracture process zone sizes to improve toughness. Through this research, the PIs will develop a combined experimental and numerical multiscale approach to reveal the mechanics of toughness across length scales. This will lead to novel developments in tough, lightweight commercial thermoplastics and emerging engineering materials like nanocomposites.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.