Structure evolution of electrospun nanofibers probed by in-situ synchrotron X-ray scattering

To characterize the evolution of crystalline structure during heating of electrospun nanofibers is a challenging task due to the fine fiber diameter. In-situ X-ray scattering with synchrotron radiation sources are essentially powerful to trace the subtle structural variation. In this study, aligned syndiotactic polypropylene (sPP) nanofibers with an average diameter of 180 nm were obtained by means of high-temperature electrospinning of an 8 wt.% sPP/ortho-dichlorobenzene solution. The as-spun fibers were characterized to consist of the antichiral form I (11 %), mesophase (27 %) and amorphous phase (62 %), in the absence of isochiral form II. The aligned fibers were subjected to a progressive heating (either stepwisely or continuously at a rate of 4 °C/min) to fiber melting. During heating, structure evolution of the sPP chains was investigated by means of in-situ two-dimensional wide-angle and small-angle X-ray scattering. Upon heating, melting and phase transformation of the mesophase were detected; at 90 °C the processes were completed, giving rise to the annealed fibers of 27 % mixed form I/II and 73 % amorphous phase. In addition to directly producing amorphous phase, heating of the mesophase was accompanied with another two phase transitions, i.e. the solid-solid meso → II phase transition at 50–80 °C, as well as the formation of form I at 80–110 °C. The former is likely due to the chain reorganization in the mesomorphic solid, and the latter is associated with the crystallization of amorphous chains at high temperatures. These events were in accordance with the heating curve obtained from differential scanning calorimetry, which exhibited a small endotherm centered at T₁ for the melting of mesophase, followed by a shallow exotherm centered at T₂ associated with the formation of form I/II crystallites. Both T₁ and T₂ were found to increase with increasing heating rates. However, the melting enthalpy and the double melting temperatures of the sPP nanofibers are relatively independent of the heating rate applied (4 ~ 40 °C/min).