Sulphonated polyisobutylene telechelic ionomers: 12. Solid-state mechanical properties

The solid-state mechanical properties of well defined sulphonated polyisobutylene telechelic ionomers are presented. Specifically, the effect of (1) molecular architecture, (2) molecular weight, (3) type of cation used for neutralization and (4) excess neutralizing agent has been investigated. In addition, the effect of moisture and ionic plasticizer on the stress-strain behaviour has also been studied.

These ionomers do not display the characteristic small-angle X-ray scattering (SAXS) peak, which is indicative of the presence of clusters, above a number-average molecular weight of about 10 000. However, below this molecular weight a weak shoulder is sometimes observed on the SAXS curve. The tri-arm species form a network structure at ambient temperatures which results in materials with good mechanical properties. The mechanical properties of the linear difunctional species are inferior to those of the three-arm star trifunctional species due to a less well developed network structure. The monofunctional species are very tacky at ambient temperatures and cannot be handled as solid materials. However, by their incorporation into the trifunctional systems they do serve as a model for ‘dangling ends’. As expected, these blends display significantly different properties than those possessed with the pure trifunctional species.

Addition of excess neutralizing agent significantly increases the high deformation properties with little effect on Young's modulus. A simple morphological model has been postulated in which it is suggested that the excess neutralizing agent resides at the ionic sites rather than being uniformly distributed throughout the matrix. Zinc-neutralized ionomers show stress-strain behaviour which is comparable to the potassium- and calcium-neutralized materials at ambient conditions, but the softening temperature is lower for the zinc neutralized material. Water absorption in these materials is relatively low. Addition of zinc stearate, an ionic plasticizer, facilitates melt processing by lowering the viscosity at high temperatures yet at ambient temperatures it crystallizes and acts as a reinforcing filler thus increasing Young's modulus.