Raman spectroscopy of individual poly(ethylene oxide) electrospun fibers: Effect of the collector on molecular orientation

Electrospun fibers are complex polymeric materials with a typical diameter of a few hundred nanometers that are often frozen in out-of-equilibrium states due to the fast solvent evaporation and high elongational forces during the process. Their molecular orientation, and the parameters affecting it, is a critical (although challenging) parameter to characterize because it allows improving and tuning many properties. In this paper, we characterize molecular orientation of poly(ethylene oxide) (PEO) fibers produced with the three most common collectors by applying our recently developed Raman spectroscopy methodologies to probe individual fibers. Results reveal an extremely high orientation in all PEO fibers and, surprisingly, the absence of any effect of the collector on molecular orientation. This contrasts with reports by several other research groups in the context of studies on bundles of fibers, as well as with our WAXD results on the same fibers. It is concluded that studies on bundles composed of thousands of imperfectly aligned fibers lead to a systematic underestimation of the orientation and to apparent results that prevent us from drawing clear conclusions on the molecular impact of processing parameters when they generate different levels of macroscopic alignment. This work highlights how Raman spectroscopy helps to overcome one of the major challenges in the field, the characterization of individual fibers, to reach a better understanding and control of electrospun fibers properties.