Study On The Rheological Constitutive Theory And The Relationship Between Mechanical Properties And Sub-Cluster Parameters Of Metallocene Polyethylenes And Their Alloys

The development of polyolefin industry lies in catalysts. With the birth of metallocene catalysts, a technology revolution has taken place in the polyolefin industry since 1990s, which will have an important impact on the polyolefin products.Metallocene polyethylenes ( mPE ) are the earliest industrialized products catalyzed by metallocene catalyst in the field of olefin polymerization. The so called metallocene polyethylenes have two types named as polyolefin plastomers ( POP ) and polyolefin elastomers ( POE ). The study based on these two kinds of mPE included three parts:The first part was to study the structure, morphology and properties of mPE. In this part, the chain structure, crystalline structure, shear and extensional rheology, and mechanical properties were studied by means of a series of modern analysis technologies such as GPC, FTIR, WAXD, DSC, 13C-NMR, rheometer, and tensile tester and so on. Based on all these experimental data, some important results were obtained. First, mPE had uniform distribution of comonomers and narrower distribution of molecular weight. The branching structure of mPE had an important effect on their density and crystallinity, and mPE with very low density and good tensile properties could be obtained by introducing long chain branches onto the molecular structure of mPE. Both during isothermal and non-isothermal crystallization, mPE were apt to nucleate homogeneously. During the isothermal crystallization, the crystalline rate of mPE was lower than that of common PE. During the non-isothermal crystallization, compared with common PE, the cooling rate had less impact on the crystallization rate of mPE. Second, the sensitivity of the melt viscosity of mPE to the shear rate was poor, and the introduction of long chain branches onto the molecular structure of mPE could improve the dependence of their melt viscosity on both shear rate and temperature. In the extensional rheometric tests, the occurrence of phenomena of 'strain hardening1relied on the structure of linear long chains, and mPE didn't show the behavior of "strain hardening', which implied that the melt strength of mPE was poor. Third, compared with common PE, mPE exhibited better mechanical properties. When their tensile strain exceeded 300%, their tensile stress rose abruptly and this meant that the phenomena of 'strain hardening' occurred. By observing the morphology of fracture surface obtained from the tensile tests, it was also found that the tensile yielding was mainly due to melting and re-crystallization. Overall, although the properties of mPE were excellent, their shear and extensional rheology still needed improving.The second part was to study the rheology and mechanical properties of the mPE/LDPE blends in order to find some effective ways to improve the processibility of mPE. By adding a little amount of LDPE into mPE systems, both their shear and extensional rheolpgical properties could be improved effectively. On one hand, their melt viscosity was decreased a lot and the dependence of their melt viscosity on the shear rate was also increased. On the other hand, phenomena of 'strain hardening' was observed obviously in the elongational flow of mPE/LDPE blends. In addition, with the increase of LDPE content, the tensile strength of mPE/LDPE blends decreased remarkably, which implied that when LDPE was used to improve the processibility of mPE, 10 percent or lower was the preferable content of LDPE.Based on the data of dynamic shear rheometric tests, a constitutive equation was also proposed, which could characterize the dependence of the melt viscosity of PE and its blends on both shear rate and temperature. It was also found that the value of a in the constitutive equation had an important effect on the sensitivity of melt viscosity of PE and its blends to the shear rate, which was not only related to the effect of long chains, but also could exhibited the tendency of 'strain hardening' of PE and its blends occurred in the elongational flow. With the increase of a value, the sensitivity...