Plasticization of biphasic ionomers by chemically identical, short, monofunctional oligomers

Blends of carboxylated polystyrene ionomers containing 7 mol% of ionic units and carboxylate-terminated, monofunctional, monodisperse styrene oligomers of MW 800 were investigated by dynamic mechanical analysis and small angle X-ray scattering(SAXS). Both Cs and Ba carboxylate salts were studied,as well as blends of the Cs oligomer with homopolystyrene. The cluster phase is preferentially plasticized by the oligomers for both salts.; However, whereas the nonclustered phase of the Cs system is only a Little more plasticized than that of the Ba system, the cluster phase in the Cs system undergoes dramatic plasticization, the cluster transition disappearing at 5 wt % oligomer content. Simultaneously, the SAXS ''ionomer peak'' disappears. A peak reappears at 20 wt % oligomer content, suggesting the development of a new morphology, proposed to be micelles or ''inverted multiplets''. The difference between the two systems is attributed to the much stronger electrostatic interactions in the Ba system; this is analyzed in terms of elementary ion units. It is concluded that, whereas the nature of the nonpolar tail is relatively unimportant, the nature and strength of the electrostatic interactions involved can be of critical importance to the plasticization effects in blends of ionomers with short, monofunctional oligomers.