The Crystallization Behavior Of Metallocene Polyethylene Blends And Morphological Study

Crystallization Behaviors and Morphology of Metallocene Polyethylenes and Their BlendsDepartment of Polymer Science & Materials, Sichuan UniversityAuthor: Ya PengSuperviser: Qiang FuCollaborating with Philips Company, we have obtained sets of metallocene polyethylenes with narrow molecular weight distribution and uniform chain branch content. The effect of the molecular weight and branch content on the crystallization behavior and morphology of the metallocene polyethylene has been investigated by WAXD, DSC, and PLM.The WAXD results indicate that the orthorhombic crystal structure is always obtained for all the samples in spite of molecular weight and branch chain content. The unit cell of crystal expands as the molecular weight or the content of branch chain increase. The morphology observed by the PLM shows that size of spherulite and the width of the circle band of high molecular weight or branch chain content decrease. The multi-endothermic peaks induced by stepwise annealing treatment via DSC show that metallocene polyethylene possesses at least the intramolecular heterogeneity even if by cross fractionation.Metallocene polyethylene (m-LLDPE) was also blended with LDPE, HDPE, PS, EPDM and iPP in the weight proportion of 80 and 20. The morphology and miscibility of these blends were studied by DSC, WAXD, IR, PLM, DMA and ARES.m-LLDPE+LDPE, m-LLDPE+HDPE and m-LLDPE+EPDM form the circle band spherulites, as seen by PLM, whose width of band is smaller than that for pure metallocene polyethylene. No crystallite was observed in the completely immiscible m-LLDPE+PS. The crystallite of the two components in the m-LLDPE+iPP becomesmaller and no complete sperulite is observed. WAXD results indicate the crystal structure does not change when metallocene polyethylene mixed with other polyolefins.The results of isothermal crystallization kinetics show that the half time of crystallization of metallocene polyethylene hardly change in adding LDPE. EPDM and HDPE, and the half time of crystallization of m-LLDPE+HDPE is less than that of pure m-LLDPE due to the acceleration of crystallization of the m-LLDPE by HDPE. It is interesting that the half time of crystallization greatly decreases when m-LLDPE+iPP is melted at 15000 at which iPP is not entirely melted, but the half time of crystallization greatly aggrandize at 175 0C at which iPP is completely melted.In order to understand the effect on crystallization kinetics of m-LLDPE by LDPE, HDPE and iPP, the comparison experiments have been done. One is the unmixed samples, the other is mixed sample. Compared the un-mixed blends with mixed blends, it is found that metallocene polyethylene and LLDPE are miscible in some degree and the former acts as nucleation agent in the latter's crystallization. The miscibility of metallocene polyethylene and HDPE leads to form the co-crystal. The only one peak in the DSC graph of the un-mixed iPP / metallocene polyethylene, the same with mixed m-LLDPE+iPP, shows the interfacial effect on the crystallization kinetics.DMA results show the value of tan 6 decrease when adding the rigid PS and increase when adding other polyolefins. ARES results reveal that there exists some miscibility between HDPE, LDPE, EPDM and metallocene polyethylene. But the "effect of the terminal region" reveals the immiscibility between the iPP and metallocene polyethylene.