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When flow is applied on a semi-crystalline polymer melt, it can greatly impact how it crystallizes its final properties. Therefore, it is crucial to understand the basic mechanism of flow induced crystallization (FIC). Only then will it be possible to predict final properties from the type of resin and processing conditions. Polypropylene is one of the most widely used polymers. Its processing generally involves subjecting the melt to intense flow fields, so FIC commonly occurs.
This study investigates the influence of temperature of shear and crystallization on the FIC behavior of isotactic polypropylene (iPP). Temperature is one of the main controlled parameters in processing techniques such as injection and extrusion. Here, iPP is subjected to a range of shear stresses and shearing times at three different temperatures. Optical polarized microscopy displays differences in the morphology that forms after FIC at different temperatures. Real-time in-situ optical measurements track structure formation during and after shear. Capillary rheometry reveals changes in the rheological behavior of the polymer melt during flow. Finally, a depth-sectioning method is used to isolate the contribution to FIC of each sample layer at the different temperatures investigated.
It was found that temperature has a significant influence on how oriented structures form: the formation of a highly-oriented skin is facilitated at low temperature in terms of required critical shear stress. However, the boundary between the highly oriented skin and other types of morphologies is not as sharp as at higher temperatures due to the presence of other oriented structures (skin lines and sausages). Results also indicate that, at low temperature, the rapid rate of growth of kebabs already during the shear pulse can impact the formation of oriented precursors under strong flow conditions. Generally, the observed trends suggest that the effects of increasing temperature are comparable to the effects of increasing the ethylene content in a random copolymer of iPP previously observed in a prior study.
Adviser: Lucia Fernandez-Ballester