Pre-shear induced anomalous distribution of β-form in injection molded iPP

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 β-form isotactic polypropylene (iPP) prepared by the MIM is discovered. That is, in the core region, crystallinity of β-form (Xβ) is unexpectedly equal to that in the intermediate region, while β-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 α-row-nuclei, which can act as nucleating sites for the growth of β-spherulites once the material is in-situ injected into the mold cavity.