Linear, nonlinear rheological properties and foamability of Surlyn partially neutralized with sodium counterion

Linear viscoelasticity (LVE), elongational rheological behavior, and foamability are examined for ionomer samples based on a commercial Surlyn sample: The Surlyn sample is dialyzed back into the parent acid copolymer and partially neutralized by sodium hydroxide to different degrees. The LVE pseudo-master curves shift to the lower frequency with an increase in the molar fraction of sodium, reflecting the increased activation energy for ionic dissociation. Incorporation of this energy into the sticky-double-reptation model gives good prediction of the LVE behavior. The degree of strain hardening under the extensional flow exhibits a non-monotonous change: the transient hardening first intensifies with increasing the ion content up to ∼50 mol%, and then weakens upon further increasing the ion content. The foamability of these samples in supercritical CO2 appears to be controlled by the flowability as well as the melt strength during the elongational flow. In particular, the foaming T window widens greatly when the strain-hardening has been strengthened by the physical network based on the ionic interaction.