Influence of Tacticity on the Rheology of Syndiotactic Polypropylene

The development of new, metallocenes and post-metallocenes catalysts has allowed in recent years the formulation of new classes of crystalline polyolefines where the typical mechanical properties of crystalline materials (high rigidity and high stress to break) are associated with those of an elastomer (high deformability, high ductility). For these reasons, crystalline elastomers constitute a class of very promising innovative materials for applications in different technological areas. The study of the rheology of polyolefins based on homogenous metallocenic catalyst has been mainly devoted to the understanding material process performance. When used at a more advanced and sophisticated level, however, rheology is an useful tool to highlight the details of the polymer microstructure, such as the chemical stereo-regularity or the degree of chain branching. In this work, we have examined the relation between rheology and molecular structure (tacticity) of a series of syndiotactic polypropylenes (sPP) with different degrees of tacticity. Small-amplitude oscillatory rheometry tests were performed in the melt phase using an ARES rheometer. In this way, the plateau modulus, and consequently the molecular weight between entanglements could be calculated by different methods. The results are discussed in terms of their dependence upon the degree of tacticity of the polymer chain