Supercritical CO ₂ foaming of metallocene cyclic olefin copolymer: Processing window, cellular morphology, and functional properties

Metallocene Cyclic Olefin Copolymer foams (mCOC) were produced using supercritical CO ₂ (scCO ₂ ), and the effects of temperature, pressure, and impregnation time on morphology and properties were systematically investigated. The minimum relative density was achieved at 130°C, 13.8 MPa, and 300 min, producing a density as low as 0.089 g/cm ³ , corresponding to a relative density of 0.087 (ρ/ρ ₀ ), i.e., an estimated porosity of 91.2% and expansion ratio >10× calculated from density. SEM revealed average cell diameters tunable from 24 µm (110°C, 20.7 MPa, 300 min) to 295 µm (130°C, 13.8 MPa, 30 min), demonstrating the combined influence of thermal softening and supersaturation. Functional properties were strongly correlated with relative density (and density-derived porosity): the dielectric constant decreased from 1.90 in dense foams to 1.29 in highly porous foams. At the same time, impact strength declined from 2.39 kg·cm/cm to 0.69 kg·cm/cm. DSC and XRD confirmed that the Tg ≈ 140°C and amorphous structure were preserved, indicating excellent thermal stability. These findings highlight scCO ₂ foaming as a tunable, sustainable route to lightweight, low-dielectric-constant mCOC foams for potential applications in insulation, packaging, and electronics.