Formation of Oriented Precursors in Flow-Induced Polymer Crystallization: Experimental Methods and Model Materials

Abstract

This thesis presents new insights into the early events of formation of oriented precursors in flow-induced crystallization of polymers, specifically isotactic polypropylene. Experimental approaches are developed to follow the creation of thread-like precursors during flow. The use of model bimodal polymers provides insight on the role of long chains in the mechanism of formation of oriented precursors. The addition of very long chains (3500 kg/mol) at low concentration ( The phenomenological effects of flow on polymer crystallization have been known for decades, manifested dramatically in most processing techniques due to the high stresses imposed onto the polymer melt. Processing flows can accelerate the kinetics of crystallization by orders of magnitude, and can induce the formation of highly oriented crystallites that, in turn, impact the final material properties in the solid state. The formation of oriented thread-like precursors is at the heart of these effects of flow on polymer crystallization; however, the fundamental mechanisms underlying their development remain elusive. This lack of understanding frustrates the formulation of a predictive model that relates the polymer molecular characteristics and the imposed processing conditions to the ensuing crystallization kinetics, the final morphology, and hence, the ultimate material properties. Here, we develop experimental approaches that provide insight into the physics of formation of the oriented precursors, which identify the essential elements required in a truly predictive model of flow-induced crystallization. In this work, we build on experimental capabilities of imposing well-defined flow and thermal histories onto a polymer melt, and of utilizing small quantities of material so that model polymers can be investigated, which allows us to isolate the effect of specific molecular characteristics and flow conditions. Our apparatus provides us with real-time measurements that probe a range of shear stresses throughout a slit flow channel; thus, we develop a depth sectioning as a strategy to isolate the contribution to the real-time signal that arises from a specific level of shear stress. This method is of utmost importance since the formation of thread-like precursors depends strongly on stress. To separate the development of oriented precursors during flow from the growth of oriented crystallites on them, we develop an experimental approach, the temperature-jump, inspired by classical nucleation studies. We use a small concentration of ultra-high molecular weight isotactic polypropylene in a matrix of shorter chains to examine the role of long chains in the creation of thread-like precursors. The use of such high molecular weight chains has revealed a richer behavior than could be observed in earlier studies, indicating that there are two stages in thread formation, kick-off and propagation, and that the stress requirement for the first step is more stringent than for the second. The data are consistent with the hypothesis that the interaction of long chains with the tip of a shish creates a local orientation that is not found elsewhere in the flowing melt. Finally, we combine the two experimental approaches to perform measurements that capture the development of the threads during flow. For intermediate shearing times, our results are well described by the most promising model currently available, the strain model, and lay the groundwork for determining the velocity of propagation of threads at different shearing stresses. Also, it suggests that some modifications to the recoverable strain model should be included to correctly capture kick-off and saturation of the formation of threads. The experimental tools described here can be extended to other model materials, for example, to expose the effects of long chain length > 3500 kg/mol and of the stereo-regularity of the long chains. A larger parameter space can be surveyed in the future to provide additional data to test predictive models that connect molecular characteristics of a resin to structure formation under processing conditions.