Block copolymer ionomers: I. Synthesis and properties of sulfonated poly(styrene- ethylene/butylene- styrene)
The styrene end-blocks of a poly(styrene-(ethylene-co-butylene)-styrene) (SEBS) triblock copolymer were sulphonated up to a concentration of 18 mol%. Sodium and zinc salts of the sulphonated polymers exhibited behaviour indicative of strong ionic interchain interactions, similar to what is observed with other ionomers. Dilute solution viscosities were enhanced upon sulphonation, the Tg of the styrene blocks was increased and the ionomers exhibited a rubbery plateau region above the styrene block Tg that was absent in the base resin. Whereas SEBS was insoluble in toluene and methanol mixed solvents, the sulphonated polymers were soluble. Moreover, the addition of the alcohol cosolvent significantly lowered the solution viscosity by solvating the interchain ionic interactions. The sulphonated polymers were thermal-oxidatively more stable and more hygroscopic than the unmodified polymers.