Integrating Hollow Metal-Organic Frameworks with Supercritical CO₂: A Path to High-Performance Polystyrene Microcellular Foams

Polymer foams, particularly those prepared using supercritical carbon dioxide (Sc-CO₂), are extensively utilized in diverse applications like thermal insulation and packaging due to their lightweight and efficient properties. However, the effectiveness of Sc-CO₂ foaming is often limited by low cell density and large cell size, mainly resulting from insufficient nucleation control and inadequate gas management. In this study, hollow metal-organic frameworks (MOFs) (HZIF-8) were introduced as effective nucleating agents in Sc-CO₂-assisted polystyrene (PS) foaming. Owing to their unique hollow structure and high internal surface area, HZIF-8 significantly enhances gas adsorption and storage capacity, thereby improving nucleation efficiency during the foaming process. The results show that incorporating HZIF-8 into PS significantly increases the cell density (up to 1.4 × 10⁹ cells cm⁻³) and decreases the average cell size (to 8.6 μm) under optimal foaming conditions (13.8 MPa, 100 °C). Compared to conventional ZIF-8 nanoparticles, the hollow structure of HZIF-8 facilitates more efficient CO₂ uptake, leading to a finer microcellular morphology, more stable foam expansion, and enhanced mechanical properties. These results demonstrate the potential of hollow MOFs in creating high-performance microcellular foams, with promising applications in energy-efficient insulation, lightweight packaging, and functional materials.