Study Of Rheological Properties Of Sulfonated Polystyrene At High Ion Contents

Ionomers are flexible polymers with a small amount of ionic groups covalently attached on the backbone.Due to the large polarity difference between the polymer and the ionic groups,the polar ionic groups tend to form nanoscale ionic aggregates,which serves as physical crosslinks,endowing the ionomers with excellent rheological and physical properties.The presence of ionic aggregates significantly changes the structure and dynamics of ionomers.Many of the observations can be explained by the classical restricted region model of Eisenberg[2].Despite the fact that the rheological study of ionomers at high ion contents(>10%)is of great important from both fundamental and practical points of view,the absence of rubbery behavior and the high glass transition temperatures in ionomers with high ion contents pose numerous challenges for accurate rheological measurements in experiment.To solve this problem and further understand the microstructure and molecular dynamics behavior at the high ion contents,we synthesized sulfonated polystyrene with cesium as counterion.Owing to the large size of cesium,the ion-ion distance was large and thus the association energy between ions was low,enabling us to measure the linear viscoelasticity of this series of ionomers up to the highest concentration of 19%.The main results are summarized below.For all the ionomer samples,ionic peaks were seen in X-ray scattering profiles,indicating the formation of ion aggregates.The glass transition temperature increased by degrees and the glass transition process widened with the increase of ionic content.The glassy and rubbery regions seen in the linear viscoelastic moduli merged into a broad process.We proposed a molecular picture to explain this transition:when the number of ions per chain becomes larger than that of Kuhn segments per chain,the strands between the two ionic groups became non-flexible and thus the ions could not dissociate independently,but instead exhibited coupled motion.This molecular picture differs from the Eisenberg's restricted region model that attributed the transition to the percolation of restricted regions.Going beyond the linear rheological study of ionomers at high ion contents,we further investigated the nonlinear rheological behavior of SPS-Cs ionomer with high ionic concentration through conducting the step strain followed by the stress relaxation,and the startup flow measurements.We found that the degree of damping weakened,and the melt edge fracture strengthened with increasing the ionic content.