Polymer Engineering Faculty Research


On-Line Small angle and Wide angle X-ray diffraction Studies on Melt Spinning Polyvinylidene Fluoride Using Synchrotron Radiation

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On-line small-angle and wide-angle x-ray scattering experiments were performed during the melt spinning of polyvinylidene fluoride using the DESY synchrotron light source. In these studies, the melt-spinning apparatus consisting of a screw extruder, a metering pump, and a take-up motor system were assembled on two separate stepper-motor-driven platforms. To investigate the structure development during crystallization, the tape location at the desired distance from the die could be positioned at the beam level with synchronous vertical movement of extruder and take-up platforms. Small-angle and wide-angle x-ray patterns were taken simultaneously with a two-dimensional wire detector and one-dimensional wire detector. In a separate study, two-dimensional WAXS data were also taken under identical processing conditions to observe the off-equator diffraction behavior during the crystallization. The data obtained for a variety of take-up speeds generally indicate that SAXS d-spacings first appear large in the early stages of crystallization and gradually decrease along the spin-line (as the crystallization progresses). As the take-up speed increases, the crystallization onset position moves away from the die and d-spacings observed at the onset increase. In addition, the shape of the discrete scattering pattern starts as a meridional streak and converts to a teardrop shape with the tip of the pattern pointing toward the beam stop at the early stages of crystallization for high take-up speeds. This does not occur at low take-up speeds and is attributed to the differences in crystallization behavior which is spherulitic or sheaflike to row nucleated crystallites. Our studies also showed that SAXS patterns appear earlier than the wide-angle crystalline diffraction peaks indicating the SAXS technique to be more sensitive to the structural changes at these stages. An idealized model is developed to explain the observed patterns at high take-up speeds. This consists of two regimes. In regime 1, the periodic fluctuations due to the alternating crystalline and amorphous regions form along the regions where eventually the “shish” structures develop. In the second stage, a volume filling crystallization takes place between the existing crystallites along the shish structure and simultaneously radial overgrowth of crystallites (i.e., the “kebabs”) takes place. This causes the observed reduction in the average d-spacing.

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Journal of Polymer Science Part B: Polymer Physics





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