Metallocene Polypropylene Crystallization Properties

Three polypropylenes with various isotacticities, prepared with metallocene catalyst at different polymerization temperatures, and four copolymers of propylene with hexene and octene respectively in various comonomer content were studied in the present thesis for their crystallization behavior.First, nonisothermal crystallization of the homopolypropylenes was systematically studied by treating under three different routes : quick , slow or stepwise cooling. Multiple melting peaks are observed for the polypropylene with lowest isotacticity, after stepwise crystallization, indicating that crystals formed by propylene sequences of different lengths segregate each other and defects are rejected out of the crystal lattice. Under other crystallization condition only single melting peak is observed. As a result, whether structure defects are includes in the crystal lattice strongly depends on crystallization conditions.Secondly, isothermal crystallization of the homopolypropylenes was studied. WAXD result shows that the relative content of y crystals in polypropylenes increases as isotacticity decreases. It is found that y crystal overcomes a crystal and becomes predominant in the polypropylene of low isotacticity and more regioerrors . More y crystals are also formed at higher crystallization temperatures. The isothermal crystallization kinetics was studied. The metallocene-based polypropylenes with different isotacticities have similar Avrami exponents, but y crystal-predominant polypropylene has a smaller crystallization rate constant and a weaker dependence on crystallization temperature. As the content of y crystal increases, double melting peaks become more evident. Equilibrium melting temperatures were derived from Hoffman-Weeks analysis and very close equilibrium melting temperatures were obtained, 185.5癈 and 184.0癈, for two metallocene-based polypropylenes containing major a crystals, and 182.5癈 for the polypropylene with predominant y crystals.As for the polypropylene copolymers with hexene and octene, the distribution of commoners, was studied by treating the samples through methods of stepwise cooling. The isothermal crystallization kinetics, meltng traces and crystal morphology were also studied. With the analysis of the comonomer's effect, the relations between the defects structure and polyproplyene's crystallization behavior were understood better.