Disclosed is an ethylene-propylene copolymer having isotactic polypropylene crystallinity compounded with a halobutyl rubber in an amount effective to improve one or more of aged tensile strength retention, fatigue resistance, and dimensional stability. Also disclosed is an elastomer composition wherein the copolymer can have at least 60 wt % propylene-derived units and at least 6 wt % ethylene-derived units, and a tire inner tube or tire innerliner composition comprising from 1 to 10 phr copolymer. Also disclosed is a vulcanizate of the elastomer composition, and articles, tire innerliners, tires, and tire inner tubes comprising the vulcanizate. Further disclosed are processes for making molded articles, tire inner tubes and tires.