Fibril Aggregates\nand Oriented Lamellae Developed\nin Electrospun Nanofibers of Poly(vinyl alcohol)

Fibrils are validated\nto be the basic structural units of electrospun\nnanofibers; an individual nanofiber is composed of several fibril\nbundles (FBs), and each FB is an aggregate of many fibrils. In this\nstudy, highly aligned nanofibers of poly(vinyl alcohol) (PVA) were\nobtained by a rotating drum-wire collector during electrospinning.\nSubjected to a stepwise heating protocol until 250 °C, the structural\nevolution of the aligned nanofibers at different temperatures (<i>T</i><sub>a</sub>) was investigated by using synchrotron 2D\nsmall-angle X-ray scattering (SAXS) with a beam diameter of 0.5 mm.\nIt was found that in the <i>T</i><sub>a</sub> range of 30–100\n°C, a barely seen scattering peak associated with the lamellar\nstructure was detected on the meridian, and a strong streak pattern\nwas seen on the equator. The equatorial streak may originate from\n(1) the scattering of the individual fibers as a whole, (2) the fibrils\nand FBs within the fibers, and (3) the longitudinal microvoids as\nwell as fasciated domains between FBs within the fibers. At <i>T</i><sub>a</sub> ≥ 120 °C, the lamella-related\nSAXS peak became more discernible and attained the maximum intensity\nat 190 °C, followed by diminishing and eventually vanishing at\n220 °C due to lamellar melting. In the <i>T</i><sub>a</sub> range of 120–210 °C, the long period of the lamellar\nstacks increased from 7.1 to 14.4 nm, and the lamellar thickness also\nincreased from 4.3 to 8.3 nm. Remarkably, Herman’s orientation\nfactor of the lamellar normal in the accessible <i>T</i><sub>a</sub> range of 140–200 °C remained constant at\n0.77, plausibly due to confined lamellar growth in the fibrils (or\nFBs). The highly oriented lamellar stacks may also suggest that oriented\ncrystallization is involved in the fibrils, which contain paracrystals\nwith large distortions in the crystal lattice, resulting in highly\noriented amorphous chains. Despite the reduction of the equatorial\nintensity at high <i>T</i><sub>a</sub>, the streak pattern\nwas still preserved at 220–250 °C; this is because nearly\nall of the heated fibers did not fuse to flatten but rather were thermally\ndegraded and cross-linked to maintain the fiber shape. Judging from\nthe strong streak intensity retained at 250 °C, scattering source\n(1) plays the dominant role in the equatorial streak. Filling the\nmacrovoids between PVA fibers with the nonsolvent of 1-propanol effectively\nreduced the contribution from scattering source (1) to uncover the\nhidden intensity profile associated with the fibrils. As the aligned\nfiber mats were wetted with a water solvent, the equatorial intensities\ndramatically reduced and the streak scattering associated with the\nundissolved fibrils and FBs was obtained. High-resolution transmission\nelectron microscopy was applied to evidence\nthe existence of fibrils and FBs within PVA single fibers, the precursors\nof which are the phase-separated structures of liquid strings developed\nin the spinning jet due to flow-induced phase separation.