The present invention is a composition including an isobutylene-based copolymer and polybutene. The copolymers may be mixed with an exfoliating compound and clay, the entire composition forming a nanocomposite. The clay may or may not have an additional exfoliating treatment present prior to mixing with the interpolymer. The composition of the invention has improved air barrier properties and processing properties, and is suitable as an air barrier. One embodiment of the invention is an elastomeric composition including at least one random copolymer of at least a C.sub.4 to C.sub.7 isomonoolefin derived unit, at least one filler, and polybutene oil having a number average molecular weight greater than 400. The copolymer may be selected from a halogenated poly(isobutylene-co-p-methylstyrene), a halogenated star-branched butyl rubber, or a halogenated butyl rubber, and mixtures thereof.