In-Situ X-Ray Characterization of Fiber Structure During Melt Spinning

The properties of a polymer are strongly influenced by its morphology. In the case of fibers from semicrystalline polymers this consists of the degree of crystallinity, the spacing and alignment of the crystalline regions, and molecular orientation of the polymer chains in the amorphous regions. Information on crystallinity and orientation can be obtained from X-ray analysis. In-situ X-ray characterization of a polymer during the melt spinning process is a major source of information about the effects of material characteristics and processing conditions upon structure evolution along the spinline, and the final structure and properties of the end product. We have recently designed and installed an X-ray system capable of in-situ analysis during polymer melt spinning. To the best of our knowledge this system is unique in its capabilities for the simultaneous detection of wide angle and small angle X-ray scattering (WAXS and SAXS, respectively), its use of a conventional laboratory radiation source, its vertical mobility along the spinline, and its ability to simulate a semi-industrial environment. Setup, operation and demonstration of the capabilities of this system is presented herein as applied to the characterization of the melt spinning of isotactic poly(propylene). Crystallinity and crystalline orientation calculated from WAXS patterns, and lamellar long period calculated from SAXS patterns, were obtained during melt spinning of the polymer along the spinline. INTRODUCTION Polymers, particularly semi-crystalline polymers, can possess a wide range of properties depending on the processing conditions. Polymer fibers are commonly formed by melt spinning of semi-crystalline polymers, where their structure develops during the processes of spinning and of post drawing. Subsequent to exiting the spinneret, the fiber is cooled by heat transfer from the surface of the fiber to the surrounding air. Along the spinline, the polymer filament velocity increases from that at the face of the spinneret (the throughput rate) to that of the speed of the take-up winder. The fiber decreases in diameter until the point in the spinline where both the diameter and the velocity become constant. Orientation of the polymer chains increases due to elongational (spinline) stress on the filament. The primary contributor to this stress is the rheological force, which has its basis in the viscosity of the polymer. The decrease in entropy associated with uncoiling of the chains as they assume the oriented conformation contributes to this force. Inasmuch as viscous forces are inherently deformation ratedependent, the spinline stress increases with increase in spinning velocity; hence, we observed an increase in orientation with increase in spinning speed. Crystallization occurs in appropriate polymers during cooling; in the case of sufficiently high take-up speeds, crystallinity may be influenced by the speedinduced stress. In the case of fibers from crystallizable polymers, the parameters describing the internal structure of the fiber include the degree of crystallinity and crystallite orientation, the spacing and arrangement of the crystalline regions, and molecular orientation of the