The dynamic mechanical properties and morphology of poly(styrene-co-3-sulfopropyl sodium-methacrylate) SSPMANa ionomers were investigated. It was found the increasing rate of ionic moduli of the SSPMANa ionomer was very low, and the cluster T-g of the ionomers remained more or less constant with increasing ion content. A well-developed SAXS peak was seen for low ion content SSPMANa ionomers and the peak position changed slightly with ion content. Thus, it was suggested that the presence of the alkyl ester side chains made the ion pairs form multiplets more easily at their prevalent distances, and the small-agglomerated multiplets were dispersed in the polymer matrix relatively evenly. The interpretation of ionic moduli using a number of theories implied that the multiplets and clusters acted as effective crosslinks and filler particles, respectively, and the size and shape of the clusters were irregular. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1043-1053