IONIC AGGREGATION IN A BLOCK-COPOLYMER IONOMER

Small-angle X-ray scattering (SAXS) measurements of block copolymer ionomers based on lightly sulfonated poly(styrene-b-(ethylene-CO-butylene)-b-styrene) (29.8 % (by weight) styrene; M(n) = 50 000) revealed a unique three-phase microstructure consisting of ionic aggregates dispersed within the polystyrene domains of the block copolymer. Compression-molded specimens exhibited an ellipsoidal microstructure with a domain diameter of ca. 11-1 3 nm, while a well-ordered lamellar microstructure with 6.5-7.5-nm lamella thickness resulted from solution casting. The ionic domain size was restricted by the size of the polystyrene domains in which they were dispersed. An ionic aggregate size of ca. 2 nm and a closest approach distance of ca. 3.5 nm were obtained from a modified hard-sphere, interparticle interference model for the ionic domains. The ionic domain size was insensitive to sulfonation level, but the aggregates for the Na salts were larger than for the corresponding Zn salts. The block domain spacing in a block ionomer with a lamellar microstructure was independent of sulfonation level, which suggests that polymer chain dimensions are unperturbed by the presence of ionic interactions. The critical temperature for ionic aggregate dissociation in the block copolymer ionomer was lower than that for similar homopolymer ionomers due to perturbation by the block microstructure. The critical temperature for disappearance of the ionic microstructure was 235-250-degrees-C for the Zn salts; no ionic dissociation temperature was observed up to 300-degrees-C for alkali-metal salts.