Engineering Defective UiO-66 Metal–Organic Frameworks as Efficient Polymer Foam Cell Nucleating Agents

Abstract

Nanocellular polymer foams are highly valuble for their lightweight nature, high strength, and unique nanostructures, which offer significant potential across a variety of applications. However, their production and practical use are limited by low cell nucleation efficiency and the requirement for extremely high-pressure foaming processes. In this study, we successfully fabricated poly(methyl methacrylate) (PMMA) nanocellular foams with a cell density on the order of 10¹² cells cm^–3 at a relatively low foaming pressure of 6.0 MPa. This achievement was made possible through the use of defective UiO-66 nanoparticles as nucleation enhancers, which demonstrated a high nucleation efficiency of 1.11. We synthesized four types of defective UiO-66 nanoparticles, each with varying levels of defects, and confirmed their efficacy as nucleators for PMMA nanofoams. The successful synthesis of these defective UiO-66 nanoparticles was verified using Fourier Transform Infrared Spectroscopy, X-ray Diffraction, Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy. The cell size and density of the PMMA nanocellular foams were characterized via SEM analysis. Our findings showed that the incorporation of defective UiO-66 nanoparticles markedly reduced the cell size of the PMMA foam, achieving a higher cell density. This improvement is attributed to a decrease in the free energy required for cell nucleation within nanocavities located at the matrix–nucleator interface. Consequently, the careful design of high-performance nucleating particles and the judicious choice of foam matrix components emerge as critical strategies in developing polymer cellular materials with nanoscale cell dimensions. These insights significantly advance the fabrication of polymer foams with enhanced thermal insulation properties and have broad implications for the field of cellular materials science. By optimizing nucleation mechanisms and material combinations, this work opens new avenues for the development of advanced cellular polymers tailored for applications requiring superior insulation or lightweight yet robust structures.