(215a) Thermal Transitions and Physical Foaming of an Ethylene/Acrylic Acid/n-Butyl Acrylate Ionomer with Carbon Dioxide

Ionomers are copolymers in which one of the monomers is ionizable. Olefin based ionomers are typically formed from copolymerization of ethylene with an unsaturated carboxylic acid. The acid groups are then neutralized with metal salts to form an ionic complex. The ionic groups are hydrophilic and form small microphase separated ionic nanodomains known as ion cores, ionic multiplets, or clusters which act as physical crosslinks. Mobility of the polymer chains that are covalently connected to these ionic domains become hindered in the vicinity of these ion clusters. Ionic domains improve mechanical properties such as toughness or melt viscosity and may also lead to an increase in the glass transition temperature of the polymer. The influence of carbon dioxide on the thermomechanical transitions of ionomers and their modifications in carbon dioxide is thus of interest in foaming of these materials. In this presentation, we will discuss the thermal transitions and the physical foaming of an ethylene /acrylic acid /n-butyl acrylate ionomer with carbon dioxide. The structure of this ionomer and the presence of polyethylene crystalline domains; ionic-cores; ion-poor amorphous domains, and ion-rich amorphous domains are schematically illustrated in the figure below.

Foaming experiments were conducted by saturation at 100, 120, 140, and 160 ℃ 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 and /or temperature as assessed by HP-TBA (High Pressure Torsional Braid Analysis). The HP-TBA analyses were carried out by conducting (a) temperature scans holding pressure constant, and (b) pressure scans holding temperature constant. The relaxation processes displayed in these HP-TBA analyses will be discussed in terms of the transitions in the ionomer that are associated with the devitrification of ion-poor and ion-rich domains, melting of secondary or primary crystalline domains, and dissociation of ionic bonds.