Poly(vinyl alcohol) Fibrils with Highly Oriented Amorphous Chains Developed in Electrospun Nanofibers

Aligned poly(vinyl alcohol) (PVA) fibers with diameters of about 180 ± 22 nm were obtained by using a wire–drum collector during electrospinning of aqueous solutions of PVA with 99% hydrolysis. The fiber orientation factor, defined by the Herman’s orientation function, was about 0.95 with respect to the collecting direction. Subjected to a stepwise heating protocol, the crystal orientation and crystallinity in the aligned fibers at different annealing temperatures (T_a) were determined by using Fourier transform infrared spectroscopy (FTIR) and synchrotron 2D wide-angle X-ray diffraction (WAXD). Upon heating, FTIR results disclosed the desorption of residual water molecules at T_a < 100 °C and an increase in crystal orientation at elevated temperatures as-revealed by the decreasing dichroism of the crystal-related band at 1143 cm^–1 until fiber melting, followed by thermal degradation at T_a ≳ 190 °C. The enhanced crystal orientation along the fiber axis at high T_a (≳100 °C) was also validated by the detailed analyses of the 2D WAXD patterns to extract the Herman’s orientation factor of the reciprocal lattice vectors of the (101/101̅) crystal planes from which the chain orientation factor in the crystalline phase was derived to dramatically increase from 0.57 at 35 °C to reach the highest value of 0.88 at 200 °C. Meanwhile, the fiber crystallinity was also increased upon heating from 31% at 35 °C to 35% at 100 °C and further up to 37% at 180 °C. The peculiar two-step enhancement in the crystal orientation and fiber crystallinity at elevated temperatures is attributed to the presence of the fibrils comprising the fiber, which are composed of highly oriented amorphous chains as well as the loosely packed amorphous chains existing in between the oriented crystalline domains. The presence of fibrils and fiber bundles (superstructures of the fibrils) was validated by atomic force microscopy as well as scanning and transmission electron microscopy. The highly oriented amorphous chains were disclosed by the anomalous meridional X-ray diffraction with a unique spacing of 2.44 Å, which is associated with the spacing along the chain axis in the closely packed and aligned chains with zigzag conformation. These extended amorphous chains may form strong intrachain and interchain hydrogen bonds, which firmly hold the fiber integrity for restricting orientation relaxation of chain molecules upon heating, therefore, to readily proceed the chain reorganization and develop oriented crystals directly at high T_a. The unique features of fibrils existing in the as-spun PVA fibers are relevant to the flow-induced phase separation of the electrospinning jet to produce phase-separated structure of strings with various widths. A discussion on the structures evolved from the liquid strings to the solid fibrils and subsequent heating effect is provided.