Non-linear Shear and Uniaxial Extensional Rheology of Polyether-Ester-Sulfonate Copolymer Ionomer Melts

We present unique nonlinear shear and extensional rheology data of unentan-gled amorphous polyester ionomers based on polyethers and sulphonated phthalates with sodium/ lithium counterions. Previous linear viscoelastic (LVE) measurements1 showed significant elasticity in these ionomers due to formation of strong ionic aggre-gates. These ionomer melts exhibit viscoelastic properties similar to well-entangled melts with an extended rubbery plateau. To evaluate the effects of nonlinear deforma-tion, the rheology of these ionomers was investigated using uniaxial extension and shear. The measurements were performed on an updated filament stretching rheome-ter and on a strain controlled shear rheometer equipped with a cone-partitioned-plate (CPP) setup. In extension, ionomer samples exhibited a decreasing strain hardening trend with increasing extension rates. 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 both fracture eventually. On the other hand, whereas PTMO-Na can-not be sheared due to slip at the wall, 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 offers a compro-mise of mechanical performance and ion conduction.