Poiseuille/squeeze flow-induced crystallization in microinjection-compression molded isotactic polypropylene

Microinjection-compression molding was used to fabricate isotactic polypropylene part with microscale thickness. The combined effect of shear (up to the order of 105 s-1) and elongational deformations imposed by the Poiseuille (injection stage) and squeeze (compression stage) flow resulted in pronounced flow-induced crystallization under rapid quenching. Hierarchical crystalline morphology, characterized by two oriented layers, a transitional layer in between, and an isotropic core layer, was detected through the thickness in the upstream region initially filled in injection stage, whereas skin-core morphology appeared in the downstream region filled in subsequent compression stage. Under the molding conditions imposing sufficient strain rates, predominant shish-kebabs developed in the oriented layer. Furthermore, the oriented layer thicknesses, crystallinity, beta-form content, and melting behavior of molded parts, all of which were closely correlated with the calculated strain rates, as well as the location of inner oriented layer could be manipulated via varying the compression-related parameters. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013