A new amphiphilic diblock copolymer containing an ionomer segment, poly [(4-vinylbenzyl triethyl ammonium bromide)-co-(4-methylstyrene)-co-(4-bromomethylstyrene)]-b-polyisobutene [poly(4-VBTEAB)-b-PIB], was synthesized by the chemical modification of poly(4-methylstyrene)-b-polyisobutene [poly(4-MSt)-b-PIB]. First, the 4-methylstyrene moiety in poly(4-MSt)-b-PIB was brominated with azobisisobutyronitrile as an initiator at 60 degreesC in CCl4, and then the highly reactive benzyl bromide groups were ionized by a reaction with triethylamine in a toluene/isopropyl alcohol (80/20 v/v) mixture at about 85 degreesC to produce the ionomer diblock copolymer poly(40VBTEAB)-b-PIB. The solubility of the ionomer block copolymer was quite different from that of the corresponding poly [(4-methylstyrene)-co-(4-bromomethylstyrene)]-b-polyisobutene {poly [(4-MSt)-co-(4-BrMSt)]-b-PIB}. Transmission electron microscopy observations demonstrated that all three diblock copolymers had microphase-separation structures in which polyisobutene (PIB) domains existed in the continuous phase of the poly(4-methylstyrene) segment or its derivative segment matrix. Dynamic mechanical thermal analysis measurements showed that poly [(4-MSt)-co-(4-BrMSt)] -b-PIB had two glass-transition temperatures (T-g's), -56 degreesC for the PIB segment and 62 degreesC for the poly [(4-MSt)-co-(4-BrMSt)] domain, whereas poly(4-VBTEAB)-b-PIB showed one T-g at -8 degreesC of the PIB domain; T-g of the poly [(4-vinylbenzyl triethyl ammonium bromide)-co-(4-methylstyrene)-co-(4-bromomethylstyrene)] domain was not observable because of the strong ionic interactions resulting in a higher Tg and a retention of modulus up to 124 degreesC. (C) 2003 Wiley Periodicals, Inc.