Ionomer dynamics and the sticky Rouse model

Linear viscoelastic (LVE) and dielectric relaxation spectroscopic (DRS) responses were examined for polyester ionomers based on polyethers and sulfonated phthalates with sodium counterions. For these ionomers, LVE shows both glassy and polymer relaxations increasingly delayed with increase of ionic content. DRS shows an α-relaxation process associated with the glass transition, followed by a ∼100× slower α₂-relaxation process associated with ion rearrangements, before electrode polarization. A detailed comparison between LVE and DRS reveals that the α₂ relaxation in DRS corresponds to a characteristic modulus of kT per ionic group, strongly suggesting that the molecular origin of the α₂ relaxation is the dissociation/association of ion pairs from/into the ionic clusters. Based on this molecular picture, we can predict satisfactorily the LVE of the samples using a sticky Rouse model by setting the α₂ relaxation time as the lifetime of ionic associations. For ionomers based on poly(ethylene oxide), the association energy is 8-13 kJ/mol, causing only a short delay of the terminal relaxation beyond the delay in the glassy relaxation. In contrast, for ionomers based on the weaker solvating poly(tetramethylene oxide), the delay spans nine decades of frequency, analogous to a highly entangled polymer, yielding an association energy of 58 kJ/mol.