Semicrystalline polymers constitute well over half of all the polymers produced worldwide. Their material properties depend sensitively on the thermal and flow history experienced during processing which strongly influences the kinetics of phase change and the morphology of the final crystalline microstructure. Therefore, it is of considerable interest to understand the mechanisms of flow effects on the rate and geometry of nucleation and crystal growth.
Microstructural development under the influence of flow is controlled by the interplay between melt relaxation processes and crystallization processes: the thermodynamic and kinetic aspects give rise to rich physics that are not well understood. This thesis elucidates key fundamentals of this process. We develop novel instrumentation that improves over prior approaches by examining the development of order at all the length scales of interest (in-situ rheo-optics, synchrotron small angle X-ray scattering (SAXS), and wide angle X-ray diffraction (WAXD), and ex-situ electron and optical microscopy); and by reducing the sample requirement by about three orders of magnitude, opening the way to study of model materials. We investigate a polydisperse, commercial Ziegler-Natta isotactic polypropylene (iPP) using the short term shearing protocol pioneered by the group of Janeschitz-Kriegl which imposes a well defined thermal and flow history on the polymer.
Rheo-optical investigations reveal that imposition of brief intervals of shear (less than a thousandth of the quiescent crystallization time) reduces the crystallization time by two orders of magnitude at a crystallization temperature of 141°C. Above a critical value of the shear stress, there is a transition to highly oriented growth with increase in shearing duration. This transition is correlated with changes in the transient behavior during flow and the semicrystalline morphology observed exsitu. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in-situ after cessation of flow correlates with development of a "skin-core" morphology observed ex-situ.
The transient structures that develop during flow are identified as oriented [alpha]-phase crystals by WAXD, and show an unexpected temperature dependence for their time of formation: with increase in temperature, they occur at shorter times after startup of flow. This very unusual temperature dependence is strikingly similar to that for rheological processes, and is in contrast to the exponential increase expected for crystallization time-scales. Thus, the transition to anisotropic nucleation in polymers subjected to flow follows a non-classical kinetic pathway controlled by the formation of a transient, highly oriented metastable melt state.
In-situ synchrotron SAXS and WAXD reveal that for shearing conditions that lead to anisotropic morphologies, crystals that are highly oriented in the flow direction develop during shear, templating the formation of crystallites after flow cessation. In the densely nucleated skin regions, ex-situ TEM shows lamellae growing radially from oriented central "shish" structures until they impinge to form the "shish-kebab" or row-nucleated structures. Under milder shear conditions, the rate of crystallization is gradual compared to strong shearing, and less oriented morphologies develop. Interestingly the ratio of parent to the crosshatched, epitaxial daughter lamellae for the oriented crystallites increases with increase in shearing time, imposed wall shear stress and temperature.
Our data suggests a mechanistic model for shear-enhanced crystallization: the rheologically-controlled formation of a critical anisotropic distribution of chain segments in the melt upon imposition of flow nucleates oriented crystallites. For intense shearing conditions, these line-nuclei are long and dense. Row nucleated structures develop from these line nuclei as lamellae grow radially to form fully impinged structures. For milder shearing conditions, lower nucleation densities lead to the development of less oriented structures
