The invention provides a method for increasing the viscosity of halogenated (brominated) elastomeric copolymers of a C.sub.4 to C.sub.7 isomonoolefin (isobutylene) and a para-alkylstryrene (p-methylstyrene) by mixing the copolymer with a silica or clay particulate filler which has been contacted with an aminosilane containing at least one C.sub.1 to C.sub.4 alkoxy group and at least one primary, secondary or tertiary amine group. The resulting elastomer compositions are used to prepare thermoplastic elastomer blend compositions, containing more finely dispersed elastomers which results in compositions having improved mechanical properties.