The development of the ionic microphase in sulfonated poly(ethylene-co-propylene-co-ethylidene norbornene) ionomers during physical aging above Tg

The effect of physical aging above Tg on ionic aggregation in the zinc salt of sulfonated poly(ethylene-co-propylene-co-ethylidene norbornene) ionomer was investigated by differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and dynamic mechanical thermal analysis (DMTA). The time-dependent development of a high temperature DSC endotherm and a high temperature tan δ relaxation were attributed to the dynamics of microphase separation of ionic aggregates. That conclusion was confirmed by the concurrent growth of a SAXS peak corresponding to the ionic microphase. The aging process at 25 °C occurred over a period of about 1 month, even though the ionomer was ca. 70 °C above Tg, where segmental mobility of the polymer is expected to be high. The kinetics of aging were dependent on temperature; a maximum in the rate of ionic aggregate formation occurred between 10 and 45 °C.