The effect of shear history on the isothermal crystallization behavior of poly(ethylene naphthalate) (PEN) was investigated by rheological and morphological measurements. Time sweep measurements of storage modulus (G＇) and dynamic viscosity (eta＇) were carried out on the molten PEN by Advanced Rheometric Expansion System (ARES) in the parallel-plate geometry at several different temperatures and frequencies, followed by structural analysis by differential scanning calorimeter (DSC), X-ray diffractometer, and polarizing microscopy for the shear-induced crystallized PEN specimens in the ARES measurements. The rate of isothermal crystallization of PEN was notably affected by temperature, while the shear rate has an important effect on the structures of the resultant crystals. At a constant shear rate, the rate of crystallization by shear-induced structuring mechanism was increased with lowering temperature over the temperature range 230-250 degrees C. The rate of crystallization was increased with increasing shear rate at a given temperature. An increase in shear rate increased both nucleation and number of crystallites. Further, it increased the content of the a-form crystal in the specimen. On the other hand, lower shear rate offered more favorable conditions for forming the p-form crystal. DSC analysis exhibited that the p-form crystal had higher melting temperature (T-m) than the alpha-form crystal. The wide angle X-ray diffraction (WAXD) patterns also ascertained that higher content of the a-form crystal was produced in the PEN specimen crystallized at higher frequency. (C) 2000 Elsevier Science Ltd. All rights reserved.