Investigating Fundamental Toughening Mechanisms in Nanocellular Foams

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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.