Flow-Induced Phase Separation and Subsequent Crystallization Leading to Shish-Kebab-like Structures in Polyoxymethylene Electrospinning Jets

High Resolution Image Download MS PowerPoint Slide Polyoxymethylene (POM) solutions at various concentrations (ϕ = 1–5 wt %) were electrospun using volatile hexafluoroisopropanol (HFIP) as the solvent. Beaded fibers were observed at ϕ ≤ 4 wt %, and bead-free fibers were successfully obtained at ϕ = 5 wt %. Regardless of the concentration, the as-spun fibers constantly exhibited highly irregular porous surfaces and internal interconnected pores aligned along the fiber axis. We propose that this surface and internal porosity is not solely driven by rapid solvent evaporation and instead results from a sequence of dynamic processes, predominantly flow-induced phase separation (FIPS) occurring within the jet. First, FIPS occurs within the straight jet segment, generating hierarchical string-like structures of varying diameters (categorized as strings, small strings, and fine strings ). These hierarchical strings represent the polymer-rich phase, surrounded by a polymer-lean matrix. The fine strings consist of highly oriented chains with emanating subchains, which serve as nucleation sites to recruit polymer chains from the matrix for lamellar growth. This FIPS-triggered crystallization subsequently produces overgrown lamellae at the string periphery. The existence of these hierarchical strings decorated with lamellar “kebabs” was validated through the cryogenic quenching of the spinning jet in liquid nitrogen, followed by freeze-drying. These shish-kebab-like assemblies eventually split and evolve into nanofibers of various diameters upon complete solvent removal. The isolated thin lamellae at the fiber surface collapse as the HFIP solvent evaporates, resulting in the observed irregular porous topography. The interconnected internal pores represent the “negative space” left by the removal of solvent-rich domains, which were trapped between the fibril bundles that evolved from the hierarchical strings. Despite being a multistage process, this FIPS-triggered crystallization occurs with remarkable speed─approximately 3 ms─as the fluid elements travel from the cone apex to the end of the straight jet, preceding the onset of whipping instability.