THERMALLY-INDUCED MICROSTRUCTURE TRANSITIONS IN A BLOCK-COPOLYMER IONOMER

Microphase transitions of a block copolymer ionomer based on lightly sulfonated poly(styrene-b-(ran-ethylene-co-butylene)-b-styrene) (SSEBS) were studied as a function of temperature by time-resolved small-angle X-ray scattering (SAXS). An ionic aggregate dissociation temperature, T-d, was observed for free acid and zinc salt derivatives of SSEBS. This represents the first time that an order-disorder transition has been observed for an ionomer, and it is attributed to a destabilization of the ionic microphase separation by the tendency of the blocks to mix at elevated temperatures. At relatively low sulfonation level (3 mol % of the styrene blocks), the order-disorder transition (ODT) of the block microstructure of Zn-SSEBS coincided with T-d. At intermediate sulfonation levels (8-12 mol %), dissociation of the ionic aggregates was followed by a mesophase-mesophase transition of the block microstructure where the texture changed from ellipsoidal to lamellar. For those materials, no ODT was observed below 300 degrees C. For the highest sulfonation level(18 mol %), an equilibrium microstructure was difficult to obtain because of the high melt viscosity. No evidence of the ODT was observed below the degradation temperature of the polymer. No microstructure transitions were observed for alkali metal salts due to much more robust ion-dipole interactions in those ionomers.