Reevaluating CO2 solubility thresholds in PMMA nanocellular foam formation and its effect on cell structure

Nanocellular foams have recently received much interest due to their fascinating properties. Previous studies have shown that molecular weight and CO₂ solubility are critical in obtaining nanocellular foams from commercial-grade polymers, with accepted wisdom requiring solubility exceeding 30 wt% for nanocellular PMMA generation. This study challenges this assumption by demonstrating successful production of nanocellular PMMA at significantly lower solubility under moderate saturation conditions (13.79 MPa and 20 °C). Experiments with reagent-grade PMMA of 136 kg/mol weight-average molecular weight produced nanocellular PMMA with an average cell size of 140 nm and cell density of 1.72 × 10¹⁵ cells/cm³ at CO₂ solubility of 25.89 wt%. Various prediction models for glass transition temperature of CO₂-saturated PMMAs exhibited different accuracies between reagent-grade versus commercial-grade PMMAs. The relative density reached minimum values at foaming-saturation temperature difference of 60 °C to 80 °C. Increasing foaming temperature decreased solid skin thickness, while higher saturation temperature increased transition layer thickness.