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Influence of molecular structure on rheological and processing behaviour of molten polymers Kazatchkov, Igor B.

Abstract

The primary focus of this work was to study the effect of molecular characteristics of polymers on their rheological and processing behaviour. In particular, the melt fracture and wall slip phenomena have been extensively studied using various rheological tools and techniques. In order to study the effects of the two key molecular characteristics, the weight average molecular weight and the molecular weight distribution, two series of polyethylene resins have been specially selected. A new method of interpretation of the large-amplitude oscillatory shear (LAOS) data has been proposed. It provides an easier and more convenient way for the data analysis suitable for use as a non-linear viscoelastic polymer characterisation technique. In addition, this method has been shown to offer an alternative way for determination of the damping function. Furthermore, a criterion for melt fracture performance has been developed and it was found to be related to a characteristic relaxation time of polymers. Additional experiments have been carried out with two metallocene polyethylene resins, which exhibited unusually long start-up transients during their capillary extrusion. This phenomenon has been investigated in detail and the results are explained in terms of a qualitative molecular model based on the two mechanisms of slip, namely flow-induced chain detachment and disentanglement. A multimode dynamic slip model has also been developed and a procedure for the determination of its parameters is proposed. The model has been found to provide better predictions compared to a static and a single-mode slip models for the flow of molten polymers in transient experiments under slip conditions. Finally, a flow visualisation apparatus has been developed using a quartz capillary and interrupted laser beam. It allowed the measurement of the velocity profiles at high temperatures and shear stresses. Using this apparatus, the effect of processing aids on the flow profile and melt fracture behaviour of molten polypropylene has been studied.

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