Thermally stable organically modified clays based on 1,3-didecyl-2-methylimidazolium (IM2C10) and 1-hexadecyl-2,3-dimethyl-imidazolium (IMC16) were used to prepare poly(ethylene naphthalate) (PEN)/clay nanocomposites via a melt intercalation process. The clay dispersion in the resulting hybrids was studied by a combination of X-ray diffraction, polarizing optical microscopy, and transmission electron microscopy. It was found that IMC16 provided better compatibility between the PEN matrix and the clay than IM2C10, as evidenced by some intercalation of polymer achieved in the PEN/IMC16-MMT hybrid. The effects of clay on the crystal structure of PEN were investigated. It was found that both pristine MMT and imidazolium-treated MMT enhanced the formation of the beta-crystal phase under melt crystallization at 200 degrees C. At 180 degrees C, however, the imidazolium-treated MMT was found to favor the alpha-crystal form instead. The difference in clay-induced polymorphism behavior was attributed to conformational changes experienced by the clay modifiers as the crystallization temperature changes. (c) 2006 Wiley Periodicals, Inc.