Overcoming Artifacts: Polarized Raman of Individual Electrospun Fibers Under Immersion

Molecular orientation is a key structural characteristic that allows optimizing the properties and performance of polymer films and fibers. Polarized Raman spectroscopy is among the most widely used techniques to quantify orientation thanks to its high level of chemical selectivity and high spatial resolution, which is especially needed when probing micro- and nanofibers. The cylindrical shape of nanofibers can nevertheless produce aberrations that prevent the accurate determination of molecular orientation when using conventional methodologies, as illustrated in the important case of polyacrylonitrile (PAN) nanofibers. Herein, we demonstrate that immersion of PAN nanofibers in a refractive index-matching medium can alleviate or eliminate these geometry-induced artifacts. While nanofibers measured in air can produce inaccurate or even physically impossible order parameters, results obtained under immersion show a strong agreement with calibrated data. This approach opens the door to accurate Raman orientation measurements in some of the most challenging samples, such as thin electrospun nanofibers.