Effect Of Initial Crystalline Structure Of Polypropylene On Pressure Induced Flow-Processing

Pressure-Induced-Flow processing is a new solid molding process. It can significantly improve the mechanical properties of materials by changing their internal morphology and structure. In this paper, we controlled the initial crystalline structure of i-PP by adding various contents of nucleating agent (Talc, nano-scaled) and then discussed the influence on PIF-processing.The initial crystallinity and crystal form of the material characterizated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) changed little, while the spherulite size showed large differences proved by polarizing optical microscope (POM). The mechanical performances of the samples including the impact strength and the tensile strength were simultaneously improved after PIF-processing. The impact strength arrived at 32.3 KJ/m2 and the tensile strength reached 75 MPa after PIF-processing.Various methods of characterization such as, field emission scanning electron microscope (F-TEM), scanning electron microscope (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), dynamic mechanical analyzer (DMA) and small angle X ray scattering (SAXS) were used to study the microscopic morphologies and the mechanisms of molecular relaxation. It indicated the effect of initial crystalline structure of polypropylene on PIF-processing from the "structure-properties" point. The main research work and a series of consequence were as follows:(1) The deformation and orientation of spherulites during PIF-processing were believed to form the hierarchically ordered layered structures. The morphology and the thickness of the layers were strongly depended on the contents of the NA. Then we initially confirmed that the highly stratified morphologies characteristic of "brick and mortar" model would form, which is also contributed to the differences of the mechanical properties. (2) Many phenomena, such as the improvement of the onset melting temperature (Tonset), the melting temperature (Tm), the glass-transition temperature (Tg), the intensity of the a relaxation peak, occurred after PIF-processing. It was implied that the special morphology that hindered movement of some molecules. And the bigger the initial spherulite size is the greater increment of the thermal and dynamic mechanical properties would receive. It was further confirmed that the initial spherulite size of i-PP could directly affect the micromorphology and the microstructure after PIF-processing.(3) The internal lamellar thickness of PP after PIF (characterized by long period) characterized by SAXS two-dimensional scatter picture did not change greatly, but the lamellae deflection occured obviously. The lamellae slipped, deflected and twisted during the PIF-processing and oriented along LD,70℃and Indirection, at last.