The trade-off between processability and performance in commercial ionomers

Strong ionic interactions between macromolecular chains improve physical and mechanical properties beyond those of the base polymer. Associating groups strongly affect the rheological properties and therefore the polymer processing window. Typically, there is a trade-off between mechanical properties and processability of associating polymers. From a molecular design perspective, we are interested in defining the chemical structure parameters that independently control mechanical properties and melt processability. In this work, we measure the effect of association strength on the mechanical properties of the solid, and the viscoelastic properties of the melt for poly-(ethylene-co-methacrylic acid) and its zinc ionomer. The linear and non-linear solid mechanics are dependent on the strength of associations. The linear viscoelasticity and thermal rheological complexity of the melt are also strongly dependent on association strength. We examine the non-linear flow properties via uniaxial extensional rheology. We show that the processing window for a given associating polymer is bounded by three timescales, namely the association lifetime, the bare sticker lifetime, and the longest relaxation time measured via small amplitude oscillatory shear. The presence of entanglements strongly impacts the magnitude of the longest relaxation time and has a positive impact on the processing window. These findings should increase our ability to design associating polymers for given processes.