Real-Time Polarized Raman Spectroscopy of an Electrospun Polymer Nanofiber John F. Rabolt, University of Delaware, DMR Shown in Figure 1 is the first real-time, polarized Raman spectra obtained during electrospinning of an atactic PS nanofiber at the tip of the Taylor cone and at various distances along the unperturbed jet. The spectra were normalized to the 1004 cm -1 band for comparison. The normalized intensity of the 623 cm -1 band in the YY polarization is not significantly different from the intensity in ZZ polarization at the location close to the tip. However, moving further down from the tip, at 5 mm distance, we note that the intensity of the 623 cm -1 band in the YY polarization is slightly higher than the intensity in the ZZ polarization. This intensity difference increases further and is significant at 9 mm and 14 mm below the tip. The difference in the intensities for ZZ and YY polarizations clearly indicate that anisotropy of the a-PS chains is induced in the electrospinning process. The aromatic ring of a-PS is aligned with the electric field in the jet region during the process. Thus, in addition to macroscopic changes such as thinning of the jet and a decrease in fiber diameter, there is also significant molecular orientation due to high elongation caused by the electric field induced whipping action. Thus the polarized Raman experiments successfully measured the onset of chain orientation in the jet.. Figure 1. Real-time Polarized Raman spectra of atactic poly(styrene) (PS) during electrospinning. The numbers indicate the distance below the Taylor cone from which the spectra were recorded. The YY and ZZ designations indicate the polarization direction of the incident laser and the direction of the analyzer of the Raman scattered radiation. For an isotropic fiber/film, sample ZZ=YY. Conversely, if ZZ is not equal to YY, this indicates molecular orientation in the fiber

Real-Time Polarized Raman Spectroscopy of an Electrospun Polymer Nanofiber John F. Rabolt, University of Delaware, DMR The broader implications of inducing molecular orientation in electrospun polymer nanofibers during the electrospinning process is that it provides another engineering parameter that can be used to optimize the mechanical (and, sometimes, the electrical) properties of the fibers. High strength polymeric fibers play an important role in a wide variety of markets and uses, ranging from ballistic vests to high strength cabling to safety and protection. For example, Kevlar , the first high performance fiber, was developed in the 1960s by DuPont scientists. Other high performance commercial fibers (Spectra  and Dynema  ) followed with Zylon , the fiber made from the rigid rod polymer developed by the Air Force in the 1970s, finally being commercialized by Toyobo in the late 90s. The high value of these microfibers derives from specific mechanical properties, which start with the chemical architecture, develop through the fiber spinning process and often are enhanced by a post spinning thermal treatment. The initial choices of the polymer and the processing conditions for manufacturing of a high strength fiber are complex and often empirical. An improved understanding of the structure and processing that result in development of optimum mechanical properties will eventually increase the variety of available high strength polymer nanofibers and allow refinement of the electrospinning process so as to determine the extent of molecular orientation that must be achieved near the Taylor cone and the optimal working distance needed to insure that the collected dry nanofibers retain that molecular orientation when collected. The development of new processes for existing materials that can be used for production at a lower cost will benefit the polymer producers and those derivative industries such as filtration, textiles and tissue engineering. This rational approach to new materials and process development requires a fundamental understanding of the molecular level structure, the molecular orientation, and the morphology of electrospun nanofibers. The students and postdocs who carried out this research have moved on to careers at Schott Glass, Conoco-Phillips and the Navy Research Laboratory during the past year.