Shear Effects in Complex Fluids Formed by Ionizable Polymers

The physics of complex fluids formed by ionizable polymers is controlled by two distinctive energy scales: electrostatic interactions of the ionic groups that drive the formation of clusters and van der Waals interactions, leading to the formation of unique assemblies, including micelles and gels. Understanding the response to shear of these complex fluids is imperative to their effective processing. Here, using atomistic molecular dynamics simulations, we probe the response to shear of sulfonated polystyrene melts (~ 2.5 * 10⁵ atoms and M_w=11 Kg/mol) below the entanglement length in the ionomer regime. The neat-melt results are compared with those obtained for swollen with THF (6, 20 Wt.%), where the constraints induced by electrostatic forces are modified. Upon shearing the neat melts, the shear viscosity decreases logarithmically with increasing shear rates, which is directly correlated with the breakup of the clusters. Upon the addition of THF, the clusters in the quiescent state become more elongated and their average size increases. Similar to the neat melt, the shear viscosity decreases with increasing shear rates, exhibiting a strong shear thinning for all systems. Surprisingly, increasing THF fraction lowers the shear viscosity for similar shear rates, which is attributed to cluster elongation.