Study On The Induced Crystallization Of Isotactic Polypropylene

In this work, iPP fiber/matrix homogeneious composite andnucleating agents/iPP heterogeneous composites were prepared, andtheir interfacial supermolecular structuress were investigated. The iPPfibers were introduced into the iPP matrices at the supercooled moltenstate.Results show that the fiber introduction temperature andcrystallization temperature significantly affect the interfacialsupermolecular structure between the nucleation surface and iPPmatrix, and the influences were studied in details.When iPP fibers were introduced into the matrix at temperatureblow the fiber melting point, they remained in the solid state andexhibited rather high nucleation capacity towards the iPP matrix.Thenucleation behavior of iPP melt in the presence of its homogeneityfibers, DBS and NA11under various conditions was studied using anpolarized optical microscope equipped with a hot stage and thermalanalysis. The results show that the nucleation capacity of iPP fibers is stronger than that of NA11and DBS, due to better dimensionalmatching at lattice level. The iPP fibers have the same chemicalcomposition and crystalline structure with the α-phase iPP, and thusinduce the crystallization more efficiently than NA11and DBS, whoserepeated period does not exactly accord with structure of α-iPP. Thehigh-level dimensional matching is propitious to fast nucleationprocess, demonstrating that dimension matching is the control step ofheterogeneous nucleation for α-iPP isothermal crystallization.When iPP fibers were introduced at higher temperature, theinterfacial transcrystalline iPP was composed of more β-iPP thanα-modification. When the introduction temperature was173°C, nearthe fiber melting point, the introduced mainly β-modificationtranscrystallization layers were obtained. However, when fiberintroduction were conducted at even higher temperature, thetranscrystalline goes back to α-modification again. Adjusting the fiberrelaxation time at a fixed fiber introduction temperature obtainedsimilar conclusions. Therefore, it can be concluded that the nucleationof the β-modification was strongly related to the partially melting stateof the iPP fibers.The nucleation behavior of iPP in the presence of its partiallymelted homogeneity fibers, α nucleating agents (DBS and NA11) and βnucleating agents (NB328and calcium pimelate) under various conditions was compared using a polarized optical microscopeequipped with a hot stage and thermal analysis. Results show that theiPP fiber exhibited stronger nucleation capacity than the nucleatingagent NB328, but calcium pimelate shows the strongest nucleationcapacity because of its unique surface structure. And the iPP fiberexhibited stronger nucleation capacity than α nucleating agent DBSand NA11, demonstrating that dimensional matching is an importantfactor for nucleation and growth of iPP.Moreover, the crystallization temperature could also affect theinterfacial morphology of iPP induced by its own fiber and nucleatingagents. It is found in the experiments that the transcrystalline structuresof β-iPP can only generate below141°C. While the β nucleating agentNB328remained some nucleation capacity to β-iPP at the sametemperature, but it lose it at the crystallization temperature of143°C.