Influence of temperature changes on crystallization of an ethylene–propylene random copolymer

Abstract

Unconventional procedures were used to crystallize an ethylene/propylene copolymer (E/P), with the aim of investigating the effect of temperature changes on crystallization kinetics and spherulitic morphology. The solidification process of specimens crystallizing under isothermal conditions was cyclically stopped for a while, by increasing the temperature, and afterward reestablished by cooling at the original temperature. The resulting morphology characterized by rings within spherulites was compared to that obtained by continuous isothermal crystallization. The different temperature dependencies of the thermal expansion coefficients of liquid and solid phases account for the presence of rings within spherulites grown in a temperature field.

Rebuilding growth mechanisms through visual observations

“…If a temperature change occurs occasionally, a black ring within spherulites will separate an external circular corona from an inner circle grown at a different temperature. Intermittent temperature changes produce concentring rings within each spherulite and, depending on the exact temperature variation with time, the distance between two successive rings may be constant or variable [3]. Polymer spherulites, indeed, are often banded, especially when developed under temperature jumps.…”

“…Rhythmic processes in both living and nonliving systems, despite remarkable differences, express the response of a steady state to chemical or physical perturbations. For instance, intermittent slowdown of the growth during polymer crystallization [3,5,8] is the most natural countermove to local superheating effects due to either heat release at the growth front and slow heat transfer by conduction. The heat of solidification developed at the growth front is hardly removed by conduction in low thermal conductivity substances as polymers, and the progress of liquid-solid transitions at low undercooling may undergo rhythmic temporary stops.…”