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Negative effect of stretching on the development of β‐phase in β‐nucleated isotactic polypropylene
Authors:Guoqiang Zheng  Songwei Li  Xiaoli Zhang  Chuntai Liu  Kun Dai  Jingbo Chen  Qian Li  Xiangfang Peng  Changyu Shen
Affiliation:1. College of Materials Science and Engineering, Key Laboratory of Advanced Materials Processing and Mold of Ministry of Education, Zhengzhou University, Zhengzhou 450002, PR China;2. Key Laboratory of Polymer Processing Engineering, Ministry of Education, Guangzhou 510641, PR China
Abstract:On the premise that shear in the slit die of an extruder was minimized as far as possible, β‐nucleated isotactic polypropylene (iPP) was extruded. Simultaneously, once the extrudate (in the melt state) left the die exit, it was stretched at various stretching rates (SRs). For iPP with a low content of β‐nucleating agent (β‐NA), the crystallinity of β‐phase (Xβ) initially increases with increasing SR, and then decreases slightly with further increase in SR. However, for iPP containing a higher content of β‐NA, with increasing SR, Xβ decreases monotonically, indicating a negative effect of SR on β‐phase formation. Small‐angle X‐ray scattering and polarized optical microscopy experiments reveal that, when SR is less than 30 cm min?1, the increasing amount of row nuclei induced by increasing SR is mainly responsible for the increase of Xβ. In contrast, when SR exceeds 30 cm min?1, the overgrowth of shish structures unexpectedly restrains the development of β‐phase, and spatial confinement is considered as a better explanation for the suppression of β‐phase. Copyright © 2011 Society of Chemical Industry
Keywords:β  ‐nucleating agent  β  ‐crystal  orientation  stretching
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