Linear Viscoelasticity and Dielectric Spectroscopy of Ionomer/Plasticizer Mixtures: A Transition from Ionomer to Polyelectrolyte

For ionomers, unfavorable interaction between highly polar ion pairs and the low polarity polymer medium leads to ion aggregation. In contrast, for polyelectrolytes, the counterions prefer solvation in the polar medium to leave the chain charged and accordingly stretched due to the charge repulsion. In this study, linear viscoelastic and dielectric properties of mixtures of two ionomers with high dielectric constant low volatility plasticizers were examined. The ionomer chains having bulky side chains are not entangled. Upon increasing the plasticizer content, the terminal relaxation is significantly accelerated due to two effects: (1) a plasticizing effect lowering the T_g and (2) a higher dielectric constant that softens the ionic interactions, leading to ionic dissociation into isolated pairs that further boosts the static dielectric constant at low frequency/long time. A model incorporating these two mechanisms and utilizing a dielectric constant ε_C, after the nonionic segmental α relaxation as the relevant dielectric constant for ion dissociation, predicts quantitatively the accelerated dynamics, as ionomers transition to polyelectrolytes on dilution.