Physical Foaming of an Ethylene/Acrylic Acid/n-Butyl Acrylate Ionomer with Carbon Dioxide

Thermal transitions of and the physical foaming of an ethylene/acrylic acid/n-butyl acrylate ionomer with carbon dioxide have been explored. Foaming experiments were conducted using 1 mm thick films of the polymer by saturation at 100, 120, 140, and 160 °C at 120, 140, 160, 180, 200, and 220 bar in CO2 followed by rapid depressurization. The foam morphologies that develop have been mapped along the rigidity reduction path of the polymer in CO2 as a function of pressure as assessed by high-pressure torsional braid analysis (HP-TBA). The HP-TBA analyses were carried out by conducting constant temperature–pressure scans. The relaxation processes displayed in these HP-TBA analyses are interpreted in terms of plausible transitions in the ionomer that may be attributed to the devitrification of ion-poor and ion-rich domains, melting of secondary or primary crystalline domains, and dissociation of ionic bonds. Insights for the selection of a rational foaming window are provided along with information on foam morphology.