β-crystal in the iPP melt encapsulated by transcrystallinity and spherulites: effect of molecular weight
writer：Yanyan Liang, Guoqiang Zheng (通訊作者), Shuangyang Liu, Kun Dai, Chuntai Liu, Jingbo Chen, Changyu Shen
keywords：transcrystallinity
source：期刊
Issue time：2013年
The crystallization in isotactic polypropylene
(iPP) melt inclusions, encapsulated by transcrystallinity
and spherulites, was investigated by polarized light
microscope. Generally, owing to the proceeding of crystallization
in melt inclusions, a negative pressure will be
built up. Up to a critical value, the negative pressure is
released by the formation of cavitation in melt. Unexpectedly,
b-crystals with different morphologies emerge at
different locations in the melt inclusions. For low molecular
weight iPP, only smaller b-crystals around bubbles are
discernable. Turn to higher molecular weight iPP, apart
from the smaller b-crystals, fan-shaped ones can be
developed under suitable conditions (e.g., lower than
131 C) at the growth front of spherulites and transcrystallinity.
Considering the characteristics of molecular
weight and morphology, it is proposed that b-crystals are
induced by different mechanisms. That is, smaller b-crystals
are resulted from the stress caused by the appearance of
bubbles while the fan-shaped ones are derived due to the
temperature gradient during the release of negative
pressure.