Rheology and morphology of a polypropylene/ethylene copolymer blend under channel flow

The objective of this thesis was to investigate rheological effects on the morphology of a polymer blend composed of 75 wt% Ziegler-Natta isotactic polypropylene and 25 wt% metallocene ethylene copolymer. This blend is a new class of thermoplastic polyolefin blends widely used in the automotive industry for making parts such as bumpers and fenders. Three steps were taken to meet the objective. First, viscoelastic properties of the blend components under shear were measured. Secondly, the blend was sheared in a microchannel at two processing temperatures. Above the equilibrium melting temperature of the polypropylene, shear-induced crystallization was found to be negligible. Below the melting temperature, on the other hand, crystallization was observed via the real time relative intensity and birefringence, ex situ polarized light microscopy and differential scanning calorimetry. Blend samples were obtained after different shear conditions, and were examined by laser scanning confocal fluorescence microscopy. Thirdly, the rheological and the morphological data were related by evaluating them in terms of distance from the channel centreline. It was found that droplet size is significantly influenced by droplet elasticity and coalescence, and that droplet size increases with elasticity ratio. As a result, and contrary to expectations, droplet size increased with distance from the centreline. With shear-induced crystallization, morphology depends on the interaction between rheology and crystallization. It appears that increasing droplet elasticity may lead to retraction of fibrous droplets, which would have been trapped by shear-induced crystallization.