Fibril Aggregates and Oriented Lamellae Developed in Electrospun Nanofibers of Poly(vinyl alcohol)

Fibrils are validated to be the basic structural units of electrospun nanofibers; an individual nanofiber is composed of several fibril bundles (FBs), and each FB is an aggregate of many fibrils. In this study, highly aligned nanofibers of poly(vinyl alcohol) (PVA) were obtained by a rotating drum-wire collector during electrospinning. Subjected to a stepwise heating protocol until 250 °C, the structural evolution of the aligned nanofibers at different temperatures (T_a) was investigated by using synchrotron 2D small-angle X-ray scattering (SAXS) with a beam diameter of 0.5 mm. It was found that in the T_a range of 30–100 °C, a barely seen scattering peak associated with the lamellar structure was detected on the meridian, and a strong streak pattern was seen on the equator. The equatorial streak may originate from (1) the scattering of the individual fibers as a whole, (2) the fibrils and FBs within the fibers, and (3) the longitudinal microvoids as well as fasciated domains between FBs within the fibers. At T_a ≥ 120 °C, the lamella-related SAXS peak became more discernible and attained the maximum intensity at 190 °C, followed by diminishing and eventually vanishing at 220 °C due to lamellar melting. In the T_a range of 120–210 °C, the long period of the lamellar stacks increased from 7.1 to 14.4 nm, and the lamellar thickness also increased from 4.3 to 8.3 nm. Remarkably, Herman’s orientation factor of the lamellar normal in the accessible T_a range of 140–200 °C remained constant at 0.77, plausibly due to confined lamellar growth in the fibrils (or FBs). The highly oriented lamellar stacks may also suggest that oriented crystallization is involved in the fibrils, which contain paracrystals with large distortions in the crystal lattice, resulting in highly oriented amorphous chains. Despite the reduction of the equatorial intensity at high T_a, the streak pattern was still preserved at 220–250 °C; this is because nearly all of the heated fibers did not fuse to flatten but rather were thermally degraded and cross-linked to maintain the fiber shape. Judging from the strong streak intensity retained at 250 °C, scattering source (1) plays the dominant role in the equatorial streak. Filling the macrovoids between PVA fibers with the nonsolvent of 1-propanol effectively reduced the contribution from scattering source (1) to uncover the hidden intensity profile associated with the fibrils. As the aligned fiber mats were wetted with a water solvent, the equatorial intensities dramatically reduced and the streak scattering associated with the undissolved fibrils and FBs was obtained. High-resolution transmission electron microscopy was applied to evidence the existence of fibrils and FBs within PVA single fibers, the precursors of which are the phase-separated structures of liquid strings developed in the spinning jet due to flow-induced phase separation.