Structure-Property Relationships in Elastomeric Carboxy-Telechelic Polyisoprene Ionomers Neutralized with Divalent Cations

A series of narrow molecular weight distribution carboxy-telechelic vinyl polyisoprenes of molecular weight 8000, neutralized with Ca²⁺, Sr²⁺, Ni²⁺, Zn²⁺, and Cd²⁺, are examined by several techniques. The small-strain tensile moduli of these materials are found to be significantly higher than predicted on the basis of the contribution of ionic aggregates acting as physical cross-links and fillers, with the Ca²⁺ and Sr²⁺ ionomers having the highest moduli. The modulus enhancement is attributed to entanglements, in the form of interlocking loops formed when both ends of a telechelic chain reside in the same aggregate. Small-angle X-ray scattering reveals that the Ca²⁺ and Sr²⁺ materials have larger aggregates and therefore more trapped entanglements and higher moduli. Strain-hardening behavior is observed for the Ca²⁺ and Ni²⁺ telechelics, but not in the other three materials. Extended X-ray absorption fine structure spectroscopy shows that these two materials have more ordered ionic microdomains, making ion hopping more difficult. The Ca²⁺ and Ni²⁺ telechelics strain-harden and break when the interlocking loops pull taut, whereas stressed entanglements in the other telechelics can relax by pulling ionic chain ends out of the aggregates.