Linear viscoelastic and uniaxial extensional rheology of alkali metal neutralized sulfonated oligostyrene ionomer melts

The linear viscoelastic and nonlinear extensional behavior of melts of alkali metal salts of oligomeric sulfonated polystyrene (SPS) ionomers were characterized by dynamic shear and nonlinear, uniaxial extensional flow experiments. The oligomeric SPS had a weight-average molecular weight of 4000 g/mol, a polydispersity index of 1.06, and a degree of sulfonation of 6.5 mol %. The molecular weight was below the entanglement molecular weight of PS, so all rheological effects were due to association of the ionic dipoles and nanophase separation of the ionic species that provides a transient elastic network. The SPS salts exhibited linear viscoelastic properties similar to well-entangled polystyrene (PS) melts, with a distinct rubbery region that had a shear modulus comparable to that of high molecular weight PS. Time-temperature superposition failed as a consequence of overlapping relaxations for the terminal response of the chain and ion hopping of the ionic dipoles. Unlike entangled PS melts, the modulus of the ionomer increased with increasing extensional strain rate, and a maximum in the stress occurred at a relatively low Hencky strain that was nearly independent of strain rate. The maximum in the stress during stretching was attributed to a catastrophic failure of the physical ionic network. At sufficiently high stress, the chains can pull the ionic groups out of nanophase-separated ionic domains, which significantly disrupts the network microstructure.