Growth habits and kinetics of crystallization of poly(ethylene 2,6-naphthalate) under isothermal and nonisothermal conditions

Abstract

Poly(ethylene 2,6-naphthalate) (PEN) can be formed into the glassy state by rapid quenching, or it can form semicrystalline structure by either annealing the amorphous precursor at elevated temperature or slow cooling from the melt. During isothermal crystallization, relatively high crystallinity develops, with a time dependence described by the Avrami equation with the exponent n = 2.5. The activation energy for isothermal crystallization was determined to be 60 kcal/mol. The modified Avrami equation proposed by Velisaris was found to describe the primary and secondary crystallization reasonably well. For nonisothermal studies, PEN was crystallized by cooling at rates ranging from 1°C/min to 5°C/min from the melt. The models proposed by Ozawa and Nakamura were used to describe the nonisothermal crystallization kinetics of PEN. The exponent n and rate constant k of the nonisothermal crystallization determined by Ozawa analysis showed good agreement with those of isothermal crystallization. The Nakamura model predicted the development of relative crystallinity reasonably well. The crystal growth habit during isothermal crystallization was studied using an optical microscope under crossed polars. The dominant morphology was found to be spherulitic, indicating regime II growth. When PEN crystallized at high temperatures (245°C–265°C), elliptical-shaped hedritelike crystals were grown. These structures are formed under regime I kinetics. Crystals grown by continuous cooling showed highly distorted, banded spherulitic structure.