Nonlinear Rheology of Lightly Sulfonated Polystyrene Ionomers

The effects of nonlinear deformation on the melt rheology of low molecular weight lightly sulfonated polystyrene ionomers (SPS) were investigated with dynamic, steady shear, and transient shear experiments. Changes in the viscosity and elasticity of the ionomers that occurred in large deformation flows were explained in terms of changes in the nanodomain microstructure of the ionomers. Large strains (or strain rates) significantly reduced the elasticity that resulted from a physical network produced by the ionic nanodomain structure. Recovery of the viscoelastic properties was rapid once the strain (strain rate) was removed, A three-region viscosity vs shear rate flow curve was observed, and the different regions were explained in terms of changes in the microstructure of the ionomer. Increasing the sulfonation level or the Coulomb energy of the ion-pair generally shifted the flow curve to higher shear rates. Shear flows produced no stress overshoot behavior upon start-up of the flow and the stress growth was relatively rapid even for low shear rates. In general, while the nanodomain microstructure produced high elasticity of the ionomers, the nonlinear rheological behavior of SPS differed significantly from that produced by chain entanglements.