| The crystal morphology of polymer is strongly dependent on molecular chains and processing conditions. The crystal morphology structure of plastic is forming under complex temperature and external force in the processing. The researches of polymer morphology forming, evolution, controlling and final structure are important to the basic theory of polymer processing and the development of polymer with high property, the compounding of function, low price and clean. In the past decades, several researchers have investigated the effects of shear conditions on the crystal morphology structure and the rheological behavior of polymer, but payed less attention to the effects of temperature. This paper is researching the effects of supercooled and nearby melting point extrusion to the morphology and structure of iPP, analysising the formation of cylindrites and P-iPP under different extrution temperatures,crystal temperatures and melting historis, building the forming models of cylindrites andβ-iPP.(1)The shear-induced samples were prepared by extruding the iPP melts nearby melting point through capillary die. The crystal structure and morphology of the shear-induced samples depended on the wall shear stress (σw) and extrusion temperature (Te), were characterized by polarized optical microscopy (POM), wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), differential scanning calorimeter (DSC), and Scanning Electron Microscope (SEM).At extremely lowσw (e.g.0.01 MPa,0.02MPa), the crystal morphology and structure of such sheared layer vary from mixed rich a-andβ-iPP cylindrites to the relative shorter iPP cylindrites, and finally to pure a-iPP spherulites, depending upon the increasing of Te, andβ-iPP crystallinity decreases with increasing Te. On the other hand, at a constant Te (180℃), the thickness of the shear-induced crystalline layer is found to decrease with the increasing ofσw. It is also found that P-iPP crystallinity decreases with increasingσw.According to the influences of different shear and thermal history on iPP crystal morphology as well as structure, a modified model is proposed for the growing of shear-induced crystallization of the iPP melts nearby melting point, which suggests that, there are some partially ordered molecular bundles in the quiescent iPP melts nearby melting point. When such partially ordered molecular bundles are above certain critical size (molecular weight), they can be oriented and form a-row nuclei.(2)Supercooled melt of iPP was extruded through capillary die. PLM, WAXD and DSC were used to investigate the effects of relatively weak wall shear stress (σw), extrusion temperature (Te) and crystallization temperature (Tc) on the structure and morphology of isotactic polypropylene (iPP). P-cylindrites crystals can be observed by PLM in the extruded specimen even at a lower wall shear stress (0.045 MPa), and the (3-iPP content increases with decreasing of extrusion temperature. Under a given Te of 150℃, the increase of wall shear stress positively influences theβ-iPP content. DSC and WAXD results indicate that the total crystallinity and the P-iPP content elevate when Tc is set from 105 to 125℃,other experimental parameters are kept on the same level.While Tc is above 125℃, the P-iPP content obviously decreases and the total crystallinity continues to increase. Based on the influences ofσw, Te and Tc on P-iPP crystal morphology and structure, a modified model is proposed to explain the growing of shear-inducedβ-iPP nucleation.
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