Rheological and solid-state physical properties of blends containing high-density polyethylene (HDPE) and a polyampholyte derivative (PE-g-PA) are assessed along with their onium ion-exchanged montmorillonite clay (NR4+-MM) nanocomposites. Strong deviations from the log-additivity rule of zero-shear viscosity, combined with synergistic behavior in tensile moduli, are consistent with a multi-phase blend morphology. While this affects clay dispersion in filled blends, PE-g-PA/HDPE based nanocomposites are shown to exhibit a favorable balance between material stiffness and ductility.