Viscoelastic Behavior of Low Molecular Weight Sulfonated Polystyrene Ionomers

Date of Completion

January 2012


Chemistry, Polymer|Engineering, Materials Science




Ionomers are those hydrophobic polymers having small amounts of bonded ionic groups. The introduction of the ionic groups into polymer chain produces large changes in the physical, mechanical and rheological properties of the parent polymer. Characterization of the effect of the ionic interactions on the rheology is complicated by the difficulty in separating effects due to molecular entanglements and the ionic interactions. In this study, low molecular weight (Mw=4000) sulfonated polystyrene (SPS) was used to study the dynamic and steady shear rheology of SPS ionomers. The polymer chain length used was far below the entanglement molecular weight of polystyrene and effects of molecular entanglements will be absent. Any polymer chain entanglements or lengthening behavior on the melt rheology should be due to the ionic interactions. ^ Random SPS ionomers with two sulfonation levels were examined, 2.5 and 4.8 mol%, which corresponded, respectively, to one and two sulfonate groups per chain on average. The metal counterions was varied across the alkali metal series of the periodic table. Morphology of the ionomer was characterized by using small angle x-ray scattering (SAXS) analysis, and dynamic and steady shear measurements were performed to investigate rheological behavior of the ionomers. ^ Glass transition temperatures of the ionomers increased with increasing ion concentration but were insensitive to cation used. The scattering peak in SAXS indicates the existence of the nanophase separated ionic clusters. The strong ionic nanophase persist up to very high temperatures and is not sensitive to the external stress. Time-temperature superposition (TTS) of G' worked reasonably well while TTS of G" failed for most ionomers. Ionic interactions increased the terminal relaxation time of the melts as much as seven orders of magnitude greater than the unentangled PS melt. The zero shear viscosity and first normal stress coefficients scaled with cq/a, where c was the concentration of the ionic groups, q was the charge of the cation, and a was the cation radius. The flow activation energy of the ionomers was similar to that of high molecular weight PS and the calculated molecular weight between "entanglements" (Me) of the SPS4.8 ionomers was the same as for PS. However, SPS2.5 ionomers contain more inactive chains that do not contribute to the melt elasticity and lead to the higher Me value. ^ All ionomers exhibit Newtonian flow behavior at low shear rates and shear thinning behavior were observed for SPS2.5 ionomers. However, shear thickening were found for LiSPS2.5 and NaSPS2.5 under lower temperatures and up to some critical shear rate. The shear thickening is believed to be due to the mechanism that the interchain association of the chains increases as they are deformed in the shear flow. The larger complexes that formed at higher shear rates lead to higher viscosities. ^