作者：Baobao Chang, Mancun Xie, Kun Dai, Guoqiang Zheng(通讯作者), Shiwei Wang, Chuntai Liu, Jingbo Chen, Chan
关键字：Isotactic polypropylene,b-Form,Precursors
论文来源：期刊
发表时间：2013年

On the laboratory scale, pre-shear induced orientation at high temperature can considerably
influence the subsequent crystallization, which has been intensively investigated by
various sophisticated methods. Unfortunately, during conventional polymer processing
(e.g., injection molding), the preservation of pre-shear induced orientation is a practical
challenge because oriented chains are prone to relax as far as possible. Motivated by the
studies on pre-shear induced crystallization, a Mixing-Injection Molding (MIM) machine
was designed and built to study pre-shear induced crystallization under practical processing
conditions. Its main features contain two aspects: (1) polymer composites or
blends can be simultaneously plasticized and melt mixed in the barrel by a rotating screw.
Logically, pre-shear is introduced to the homogeneously mixed melt; (2) the pre-sheared
melt can be in-situ injected into a mold once the mixing-plasticization process finishes. In
the present study, an unusual distribution of b-form isotactic polypropylene (iPP) prepared
by the MIM is discovered. That is, in the core region, crystallinity of b-form (Xb) is unexpectedly
equal to that in the intermediate region, while b-form is almost absent in the core
region of a conventional injection molded counterpart. Taking into account the main
features of MIM and the evidence of supplementary experiments, this interesting result
can be attributed to the fact that precursors can be induced by pre-shear. Such precursors
are most likely to be short a-row-nuclei, which can act as nucleating sites for the growth of
b-spherulites once the material is in-situ injected into the mold cavity.