The dynamic mechanical properties of poly(styrene-co-Na tiglate) PSTANa, poly(styrene-co-Na methacrylate) PSMANa, and poly(styrene-co-Na acrylate) PSAANa ionomers were investigated to know the effect of the number of methyl groups of the acrylate unit on the mechanical properties and morphology of those ionomers. It was found that the ionic modulus of the PSTANa ionomers was much lower than that of the PSMANa ionomer but slightly lower than that of the PSAANa ionomers of comparable ionic unit content. The matrix and cluster T-g's of the PSTANa ionomers were also found to be lower than those of the PSMANa and PSAANa ionomers. The PSTANa ionomers showed a very weak, broad shoulder-like feature at very small angles of X-ray diffraction or scattering profiles. Thus, it was suggested that the presence of two methyl groups on the alpha and beta carbons of an acrylate unit might induce the high energy barrier for the bond rotation, which made the formation of the multiplets difficult; therefore, the PSTANa ionomers became less clustered materials than the PSMANa and PSAANa ionomers. (C) 2013 Elsevier Ltd. All rights reserved.