Effect of shearing on crystallization behavior of poly(ethylene nephthalate)

The effect of shear history on the isothermal crystallization behavior of p oly(ethylene naphthalate) (PEN) was investigated by rheological and morphol ogical measurements. Time sweep measurements of storage modulus (G') and dy namic viscosity (eta') were carried out on the molten PEN by Advanced Rheom etric Expansion System (ARES) in the parallel-plate geometry at several dif ferent temperatures and frequencies, followed by structural analysis by dif ferential scanning calorimeter (DSC), X-ray diffractometer, and polarizing microscopy for the shear-induced crystallized PEN specimens in the ARES mea surements. The rate of isothermal crystallization of PEN was notably affect ed by temperature, while the shear rate has an important effect on the stru ctures of the resultant crystals. At a constant shear rate, the rate of cry stallization by shear-induced structuring mechanism was increased with lowe ring temperature over the temperature range 230-250 degrees C. The rate of crystallization was increased with increasing shear rate at a given tempera ture. An increase in shear rate increased both nucleation and number of cry stallites. Further, it increased the content of the a-form crystal in the s pecimen. On the other hand, lower shear rate offered more favorable conditi ons 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. T he wide angle X-ray diffraction (WAXD) patterns also ascertained that highe r content of the a-form crystal was produced in the PEN specimen crystalliz ed at higher frequency. (C) 2000 Elsevier Science Ltd. All rights reserved.