In-Situ Synchrotron Study during Fiber Processing

We are investigating strain induced morphology development in fibers with a unique combination of modern insitu experiments and modeling techniques in order to understand the evolution of structure during the fiber formation process. This study is being carried out using several advanced experimental techniques, which have not been available in the U.S. to date. Sophisticated modeling approaches is also being developed to characterize and predict the structural changes in spinning/drawing to allow scientists to visualize the formation of molecular structures and superstructures in real time, enabling us to understand the effect of processing conditions on the evolution of fiber structure. Rather than studying the structure-property relationship using final products, i.e., “dead” fibers, our study is focusing on understanding the kinetics of structure development during the actual fiber forming process, which will have a tremendous impact on producing fibers with desired morphology and for designing fibers for unique high value end-uses. Both low and high speed spinning is being addressed. Evolution of fiber morphology for various conditions is characterized using simultaneous light scattering and birefringence measurements, high brilliance synchrotron X-rays (10 5 --10 8 times greater than typical in-laboratory source), integrated on-line experimental schemes (simultaneous small- and wide- angle X-ray scattering), advanced CCD X-ray detectors and unique image analysis software that is capable of separating crystal, amorphous and mesophase fractions in fibers and yielding corresponding shapes and dimensions. Background Semi-crystalline fibers combine regions of well organized molecules representing crystalline component and regions with no clearly definable molecular organization as amorphous component. Fibers have varying degrees of crystallinity depending on the chemical structure of the polymer and the nature of spinning and drawing involved in the manufacturing process. It has been understood that the properties and behavior of a fiber depends on the amount, size, and distribution of crystalline regions in the fiber as well as the orientation of molecules in the amorphous regions. Significant work has been done to understand this structure-property relationship that has provided meaningful insight into the science of fiber behavior. However, this knowledge, obtained from the final fiber products, do not provide us with critical insight into how the structure was formed (i.e., the particular the path through which the final morphology was achieved) and how the change in process conditions affect the evolution of this morphology. The final fiber morphology not only depends on the final state of strain induced by the process, it also depends on the thermodynamic path experienced by the fiber while reaching that final state. The information on the real-time evolution of fiber morphology can be extracted by studying its development using insitu measurements. In addition, the fundamental crystallization kinetics can be studied and modeled using networked polymers, as they exhibit significant amount of strain-induced crystallization that can be studied using scattering techniques in the laboratory when the morphology is developing under varying applied strains. These idealized experiments can be designed to provide realistic results on crystallization and morphology development since at the early stages of polymer crystallization crystalline regions can be considered to act as physical crosslinks, similar to that in cross-linked networked structures. Crystallization increases the intensity of scattered radiation from a polymer. By continuously measuring the scattering intensity while stretching a fiber one could identify several key aspects associated with crystallization and hence the evolution of morphology in the fiber. Both light scattering and x-ray scattering is being used for this purpose. It has been noticed that for the crystallization during the spinning process, small angle x-ray scattering (SAXS) intensity increases prior to wide-angle x-ray diffraction (WAXD). A combination of SAXS and WAXD can be applied to characterize the morphology development in fibers. However, using them online has been delayed until recently due to the unavailability of several modern developments, such as the availability of high brilliance x-rays and the software to extract information from synchrotron x-ray experimental data. Light scattering has also been used extensively in the past, although several fundament questions arise on the interpretation of the results when various scattering techniques are used together to characterize the morphology development in fibers of films. One observes an increase in light scattering when natural rubber is stretched to 100