We discovered that polyisobutylene (PIB)-based polyurethanes (PIB-PUs) containing minute amounts (0.5%) of chemically bound organically modified montmorillonite (OmMMT) surprisingly produce films exhibiting improved properties. The OmMMT was prepared by reacting sodium montmorillonite (Na+MMT-) with quaternary ammonium salts of a tertiary amine carrying a NH2 functionality. The positively charged quaternary amine group becomes electrostatically attached to negatively charged MMT layers and defoliates it, whereas the free NH2 group reacts with diisocyanates and acts as an additional chain extender. Thus, when OmMMT is added to a mixture of ingredients assembled for the synthesis of PIB-PUs, this modified clay becomes an integral part of the PU. Specifically, we found that the integration of 0.5% OmMMT to PIB-based PUs produces films with significantly enhanced tensile strength, elongation, toughness, creep, and stress relaxation relative to that of PIB-PUs. The findings were discussed and explained in terms of a proposed morphology for the nanocomposite. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4076-4087