The Crystalline Morphologies And Properties Of Isotactic Polypropylene In The Presence With The Flow Of Fabrication Processing And Poly(Ethylene Terephthalate) Microfibrils

This work is a foudational study with surpport from the scientific instrument foundation of China (50527301). In this thesis, The poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) microfibrillar blends have been prepared through a "slit die extrusion-hot stretch-quenching" process, subsequently, the corresponding injection molded parts have been fabricated by a new injection molding, named dynamic packing injection molding (DPIM). The crystalline morphologies and properties of iPP in the presence of a flow field and PET microfibrils have been studied by characterization of morphologies in the injection molded parts. The crystallization and morphology of oriented iPP melt in the presence of PET microfibrils have been also investigated by using the polarizing optical microscopy (POM) and differential scanning calorimetry (DSC), which gives further insight into the crystallization of iPP under a flow field of fabrication process. The main results have been presented as follows:1) The well-defined PET micro fibrils can be formed through the "slit die extrusion-hot stretch-quentching" process. With the increase of the hot stretch ratio (HSR), the diameter of PET microfibrils becomes more uniform; for the case of low hot stretch ratio, besides the existence of PET microfirils, a sheet-like phase morphology appears. 2) For the DIPM samples of iPP, a fibril-like crystalline morphology is formed in the interior of the parts, whereas the spherulites are formed in the conventional injection molded (CIM) parts. For the CIM samples of iPP, lamellae are oriented to the flow direction, which are composed of shish-kebab structure. The stretch has been imposed on the iPP melt during the formation of shish-kebab structure, and thus a more "stretched" shish-kebab structure formed, which leads to the increase of long spacing of iPP lamellae. When the dynamic shear force has been imposed on the iPP melt, the higher shear rate also exists in the interior region of injection molded parts,where the shish-kebab strucuture can form, and resulting in the distinct increase of iPP lamellae orientation degree and their long spacing.3) For the DIPM samples of PET/iPP blends, the effect of dynamic force on the kebab distribution is dominant, whereas the effect of PET dispersed phase is minor factor. And little difference of lamellae orientation degree exists in the three layers of the parts. The heterogenous nucleation of PET microfibril does not affect the long spacing of iPP, however, the addition of PET microfibrils leads to the decrease of shear rate due to the higher viscosity, which results in the decrease of the amount or dimension of shish-kebab strucuture and subsequently the decrease of long spacing of iPP. During the flow of iPP melt in the presence of PET microfibrillar network,the flow field will be redefined and becomes more homogenous, which is benefited to the achievement of more homogenous iPP molecular orientation distribution in injection molded parts.4) For the CIM samples of iPP, the shear rate in the intermediate layer can lead to the formation of alpha row nuclei, the phase transition of alpha- and beta- crystal happened between T_αβ and T_βα (i.e. 140-105℃), since the cooling rate is relative low in intermediate layer, the beta crystals have enough time to grow and form a certain amount which can be detected by one dimension-wide angle X-ray scattering (1D-WAXS). For the DIPM samples of iPP, the beta crystals can also form in the core layer due to the strong shear rate which also exists in the core region.Furthermore, the PET dispersed phase can suppress the growth of beta crystals.5) The DIPM can promote the orientation of iPP molecules and the formation of shish-kebab strucuture, resulting in the increase of the amount of tie chains between the neighboured lamellae, which leads to the distince increase of tensile strength,however, the orientation of iPP molecules will lead to the decrease of storage modulus (E'), which is not benefit to the improvement of heat distortion temperature, and no obvious relationship between PET and E' has been observed.6) The shish-kebab strucuture formed in the skin layer of injection molded parts has a higher melting point as well as the end temperature of melting, compared to the lamellae of normal spherulites. The shish-kebab structure also forms in the core region of DIPM samples, which leads to the higher end temperature of melting,whereas the melting point keeps unchanged due to the relative low shear rate.7) The fibrillar crystalline morphology will form during the crystallization of oriented iPP melt, which should be composed of lots of shish-kebab structure, not just one shish-kebab structure. In the iPP oriented melt, lots of row nuclei can be connected due to the entanglement of iPP molecules and induce the shish-kebab structure simultenously, which leads to a low length (c.a. hundres of micron).8) With the PET microfibrillar network, the row nuclei induced shish-kebab structure is also a dominant process, and PET microfibrils have also some nucleation for the oriented iPP molecules. During the nonisothermal crystallization of PET/iPP microfibrillar blend with low annealing temperature (170℃), PET can improve the crystallization of oriented molecules to some extent.