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In Situ X-ray Scattering Studies of Flow-Induced Crystallization in Polymer Melts under Uniaxial Extensional Flow

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Polymers permeate almost all facets of modern life. For end use applications, these materials are typically processed into products at elevated temperatures under which molten polymers are subjected to flow. Particular interest lies in the flow-induced crystallization behavior of polymer melts under extensional flow, which is a flow type dominant in many important polymer processing operations but has received less attention due to challenges associated with producing a well-defined extensional flow. This thesis presents a systematic study on flow-induced crystallization of polymer melts under uniaxial extensional flow that aims to quantify the effect of flow on the subsequent crystallization process. The work described here utilizes a custom instrumentation that consists of a Sentmanat Extensional Rheometer housed in a convection oven designed to facilitate x-ray access. Simultaneous extensional rheometry and in situ x-ray scattering measurements are performed to characterize the nonlinear rheology and to elucidate the underlying molecular mechanism of the complicated phenomenon. Two different flow protocols are employed to monitor the isothermal crystallization behavior during flow inception and following flow cessation. Extensional flow-induced crystallization experiments are performed on a wide variety of semi-crystalline polymers. In situ structural characterization during flow-induced crystallization of poly(lactic acid) under extensional flow is reported for the first time in literature. Unusual effects of flow are observed in the crystallization behavior under intermediate flow conditions, but the overall crystallization kinetics are associated with the orientation distribution of crystallites. Experiments designed to produce similar enhancements in nucleation under combinations of various extension rates and Hencky strains are performed on poly(1-butene) to directly test the underlying hypothesis in existing flow-induced crystallization models that molecular orientation leads to enhanced nucleation. The impact of crystallization initiated during as well as following the application of flow on the induced microstructure during isothermal crystallization is explored using low-density polyethylene and high-density polyethylene. A wide range of lamellar-scale morphologies is observed across various semi-crystalline polymers. The comprehensive experimental studies coordinated with computational modeling have the potential to positively contribute to the rational design of polymer processing operations.

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