Viscoelastic Response of Ionizable Polymers Melts and Dense Suspensions: Role of Ionic Assemblies

Driven by their technological significance, numerous  efforts have probed the structure and dynamics of ionizable polymers, all pointing to the fact that their structure and dynamics are driven by the association of the ionic groups, which act as physical crosslinkers. These polymers form heterogenous media that consist of ionic and non-ionic regions that differ in their characteristics, where the interrelation between the response of the different domains determine their overall reaction to external perturbation. Here the viscoelastic response of polystyrene sulfonate melts and dense suspensions studied by non-equilibrium molecular dynamics simulations will be discussed. The polymer sulfonation levels were varied from the ionomer regime where the ionic clusters are prominently isolated to the polyelectrolyte regime where ionic networks are often formed.  The systems were exposed to both shear and elongational flows and the measured viscosity was correlated with chain conformation and the ionic cluster characteristics. With increasing shear rates, the ionic assemblies break up in both regimes and align with the flow direction, reducing the shear viscosity, though the chains are only slightly stretched. In extensional flow, the ionic clusters are dynamic, rapidly breaking and reforming.  For low suflonation fractions, the chains are stretched while at higher sulfonation fractions there is a very broad distribution of end-to-end distances of the chains. Molecular understanding that underlines the response of these model ionic polymers will be discussed.