MORPHOLOGY AND PROPERTIES OF MIXED ANION IONOMERS

A series of model polyurethane ionomers containing varying concentrations of both sulphonate and carboxylate anions is examined. Tensile testing results demonstrate that the greater acid strength of the sulphonate anions causes the ultimate tensile properties of the ionomer to regularly increase as sulphonate content increases. However, the small-strain moduli of two mixed carboxylate/sulphonate ionomers were shown to vary from the trends predicted from strict acid strength arguments; the two mixed-anion ionomers have higher Young's moduli than the fully sulphonated ionomer. D.s.c., d.m.t.a. and SAXS analysis indicates that the modulus enhancement in the mixed-anion ionomer arises from a combination of factors, including aggregate packing, ability to immobilize non-ionic chain segments (ionic anchoring) and degree of phase separation. The onset of flow temperature in d.m.t.a. is seen to decrease in parallel with increasing carboxylate content, unlike the tensile properties. Thus, the concept of mixed anion derivatization is shown to fulfil a primary goal in the ionomer field: development of a method to increase ease of processability while retaining strong tensile properties.