Ahmad, Naveed (2013) Relationship between Rheology and Molecular Structure of Innovative Crystalline Elastomers. [Tesi di dottorato]

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Abstract

The study of the rheology of polyolefins based on homogenous metallocenic catalyst has been mainly devoted to the understanding of material process ability. When used at a more advanced and sophisticated level, however, rheology is a useful tool to highlight the details of the polymer microstructure, such as the chemical stereo-regularity or the degree of chain branching. Rheology is also used to study the crystallization kinetics of the polymers and it gives more precise analysis than the conventional techniques like differential scanning calorimetry (DSC) when the crystallization kinetics are slow. When cooled below thermodynamic melting temperature, crystalline polymers undergo crystallization. The early stages of this process are characterized by the gradual change in the mechanical response of the material from the liquid to the solid, which is due to the microstructure evolution of the system. This is one of the great features of the rheological technique, which distinguishes it from the traditional DSC technique. In the present research work relationships between rheological parameters and molecular structure of syndiotactic polypropylenes (sPP) and poly-1butenes of different stereoregularity are explored by performing oscillatory shear experiments using ARES rheometer. The rheological response is found very sensitive to the degree of syndiotacticity of syndiotactic polypropylene, while in the case of poly-1butenes, it is also found dependent on the stereoregularity. Crystallization behavior of a series of syndiotactic polypropylenes of different degrees of syndiotacticity is investigated by performing both isothermal and non-isothermal crystallization tests using rheological and differential scanning calorimetric techniques. The aim is to investigate the effect of degree of syndiotacticity on the crystallization behavior of the syndiotactic polypropylene and to couple the rheological methods to more conventional techniques (such as Differential Scanning Calorimetry). Crystallization behavior is found strongly dependent on the degree of syndiotacticity of syndiotactic polypropylene. Good agreement is found between the results obtained by both the rheological and differential scanning calorimetric (DSC) methods. The effect of extensional flow on the crystallization kinetics of sPB is examined both in the melt and crystal phase by applying different extensional rates using sentmanat extensional rheometer (SER). Extensional flow is found to enhance the rate of crystallization in the crystal phase, which is further proved by the small angle X-Ray scattering experiment (SAX).

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