New polyisobutylene-based model elastomeric ionomers. VIII. Thermal-mechanical analysis

A series of well-characterized telechelic polyisobutylene-based sulfonated metal-neutralized ionomers have been studied using thermal-mechanical analysis (TMA). These ionomers serve as models in the sense that the ionic groups are located exclusively at the chain ends and hence the ionomeric character is well defined. The chemical parameters varied are (i) molecular weight, (ii) molecular architecture (linear or triarm products), (iii) addition of excess neutralizing agent, and (iv) type of cation. The effect of the above parameters on the glass transition temperature and the softening temperature (after the rubbery plateau region) is presented. It is observed that the glass transition temperature is only slightly affected by the above parameters due to the very low ionic content in these ionomers (< 2 mol %). In the case of the triarm ionomer with excess neutralizing agent, the softening temperature following the rubbery plateau is much higher than that of the linear difunctional species. Linear monofunctional species do not show a rubbery plateau behavior and readily flow above their T_g in the absence or presence of excess neutralizing agent. The excess salt is most likely located at the ionic sites rather than being uniformly distributed throughout the matrix. Zinc-neutralized ionomers were found to have the lowest softening temperatures as compared to the corresponding calcium and potassium-neutralized ionomers. The covalent character of zinc is believed to be primarily responsible for this behavior. Thermal stability of these metalneutralized ionomers is not significantly different from the sulfonated hydrocarbon precursor polymer. However, the unneutralized acid precursor polymers start to discolor at relatively lower temperatures, thereby suggesting poorer thermal stability.