Influence of the memory effect of a mesomorphic isotactic polypropylene melt on crystallization behavior

Abstract

In this study, the memory effect of a polymer is investigated with a mesomorphic isotactic polypropylene (iPP) melt as the initial state. It is found that the nucleation density and crystallization rate at low temperatures are strongly dependent on the initial melting temperature and melting time, indicating a strong memory effect. High melting temperatures decrease the nucleation density, while a short melting time causes a faster crystallization rate than in the melt with its thermal history removed. At 180 °C, it takes about 1 hour for the crystallization rate to be restored to the normal value found in the melt with erasure of its thermal history at 220 °C. Further experiments indicate that the increase in spherulite growth rate is mainly responsible for acceleration of the overall crystallization kinetics. Through comparison between meso crystallization and self-seeding crystallization, it is suggested that some ordered structures with higher thermal stability exist in the mesomorphic iPP melt. The high thermal stability of the ordered structure may be due to the random arrangement of helices of different tacticity. We propose that the ordered structure accelerates spherulite growth, however, long melting times at 180 °C can break down the ordered structure, leading to the formation of an ideal melt and restoration of the spherulite growth rate. This study indicates that a thermodynamically unstable ordered structure can survive in a supercooled melt for a long time and is involved in the crystallization process.