The present study aimed to investigate the effects of plasticization on the mechanical properties and morphology of poly(ethyl acrylate) ionomers neutralized with either Co2+ or Na+. In experiments, the dynamic mechanical properties of divalent Co2+-neutralized poly(ethyl acrylate) ionomers containing polar and non-polar plasticizers were compared with those of the monovalent Na+-neutralized ionomers. In the case of the ionomers plasticized with non-polar 4-decylaniline (4-DA), residing in non-ionic regions, the matrix and cluster T(g)s of the ionomer decreased with increasing 4-DA contents. The decreasing rates of the matrix and cluster T(g)s were found to be similar at 0.8 and 1.0 degrees C/(wt% of 4-DA) for the Co2+ and Na+ ionomers, respectively. The ionic modulus of the Co2+ ionomer changed only slightly with increasing 4-DA contents, but that of the Na+ ionomer decreased noticeably. In the SAXS study, it was observed that the un-plasticized Co2+ ionomer showed a strong small angle upturn and a very broad SAXS peak, indicating that the ionomer phase was compositionally heterogeneous. The plasticization of the Co2+ ionomer with 4-DA, however, induced a well-developed SAXS peak that was comparable to that of the un-plasticized Na+ ionomer. These results suggested that the addition of 4-DA to the Co2+ ionomer made the ionomer have more multiplets at a prevalent distance, leading to more clustering. In the case of the Co2+ ionomers plasticized with polar glycerol (Gly) that acted mainly as multiplet plasticizer, a very weak cluster glass transition, decreasing ionic modulus and only a well-developed small angle upturn were observed. These indicated that the addition of Gly to the Co2+ ionomer disrupted the multiplet formation, resulting in lower clustering. (c) 2005 Elsevier Ltd. All rights reserved.