Influence of Long Chain Branching and High Molecular Weight Components on Elongational and Shear Properties of Polyolefins

Abstract

The proposed study was designed to identify the influence of long chain branching and the presence of high molecular weight components during extensional and shear flow of polyolefins. Three types of Polyolefins including linear and linear-low density polythylenes were studied. In the first year the shear slow testing of five grades of polyethylene of commercial grade was carried out. Die swell characterization was carried out and the influence of molecular weight was detected. In the second year the flow birefringence and capillary pressure drop measurements were set up. The optical cell was connected to the screw extruder, optical bench was constructed and the flow of polarized light through the molten polymer through the slit die was successfully established. Flow birefringence measurements were carried out and photographs of the stress contours were taken. Application of the stress optical rule to the centerline streamline of the slit die revealed remarkable differences between three LPDEs which were selected to encompass both low, medium and high molecular weight polythylenes with broad molecular weight distributions and varying long chain branching. Numerical schemes for the prediction of principal stress differences were developed with a power law velocity profile and linear viscoelastic spectrum and damping function obtained from shear flow experiments. A preliminary comparison of experimental and numerical principal stress differences reveals that applicability of three non-linear and linear constitutive models. In the third year capillary pressure drop/extrudate swell measurements, rhemoteric characterization and flow birefringence measurements were carried out on three LDPEs with low to high molecular weight distributions and two m-LLDPEs with controlled long chain branching. The flow was modeled for shear flow and the combined and separate effect of molecular weight and level of branching was evaluated.