Nonlinear Shear and Uniaxial Extensional Rheology of Polyether-ester-sulfonate Copolymer Ionomer Melts

We present unique nonlinear shear and extensional rheology data of unentangled amorphous polyester ionomers based on polyethers and sulphonated phthalates with sodium/lithium counterions. Previous linear viscoelastic measurements showed significant elasticity in these ionomers due to the formation of strong ionic aggregates. These ionomer melts exhibit viscoelastic properties similar to well-entangled melts with an extended rubbery plateau. To evaluate the effects of nonlinear deformation, the rheology of these ionomers was investigated using uniaxial extension and shear. The measurements were performed on a filament stretching rheometer and on a strain controlled rotational rheometer equipped with a cone-partitioned-plate setup. In extension, ionomer samples exhibited a decreasing strain hardening trend with increasing extension rates. At the same Weissenberg number, the same strain hardening was observed for different counterions. The presence of high solvating poly(ethylene oxide), PEO, along the backbone in the coionomer with poly(tetramethylene glycol), PTMO, increases the maximum Hencky strain at fracture, thus adding ductility to the brittle PTMO-Na ionomer. As a result, the coionomer deforms much more compared to PTMO-Na, but eventually, both fracture. On the other hand, whereas PTMO-Na cannot be sheared due to wall slip, the coionomer deforms in shear and eventually suffers from edge fracture instabilities. From the above, a picture emerges suggesting that PEO coionomers enhance ductility, make fracture smoother and offer a compromise of mechanical performance and ion conduction.