The role of ionic clusters in the determination of the properties of partially neutralized ethylene-acrylic acid ionomers

[Display omitted] •The structure of ionomers is rather complicated.•Ionic species are forming multiplets which associate into clusters.•Mechanical properties are defined by crystalline structure and cluster formation.•A simple model can describe the dependence of modulus on structure of ionomers. Ionomers with different degree of neutralization were prepared from an ethylene-acrylic acid copolymer of 8.5 wt% acrylic acid content. The ionomers were characterized by Fourier transform infrared spectroscopy, small angle X-ray scattering, differential scanning calorimetry and dynamic mechanical thermal analysis measurements as well as rheology and tensile testing. The main relaxing species in ionomers are the methylene groups and the ethylene-acrylic acid copolymer segments. The mobility of these latter decreases upon neutralization and above a critical degree of neutralization a phase consisting of polyethylene segments attached to ionic clusters forms. The crystalline structure of the polymer contains primary and secondary crystallites. The amount and regularity, i.e. lamella thickness, of the latter decrease with increasing neutralization. Multiplets form from ionic species already at a small degree of neutralization and then they associate into clusters with increasing size as ion content increases. The mechanical properties of the ionomers are determined by their crystalline structure and cluster formation. Clusters increase strength and modulus, while decrease deformability. The combined effect of the two factors (crystallinity, cluster size) on stiffness was successfully described by a simple model.