Nano-fiber/net structured PVA membrane: Effects of formic acid as solvent and crosslinking agent on solution properties and membrane morphological structures

A technique of electro-spinning/netting (ESN) has been investigated to prepare poly(vinyl alcohol) (PVA) nanofibrous membrane with unique morphological structure of nano-fiber/net (NFN). The prepared NFN membrane consists of both common electrospun nanofibers (with diameters in hundreds of nanometers) and two-dimensional nano-nets (with nanofibril diameters smaller than 50 nm). It is revealed that formic acid has dual effects (i.e., as solvent and crosslinking agent) on solution properties and membrane morphological structures, and systematic analyses on rheological properties of three different 10 wt% PVA solutions with solvents being water, formic acid, and their 1/1 (wt./wt.) mixture have been conducted. Results indicate that the molecular interactions (particularly hydrogen bonding) among water, formic acid, and PVA macromolecular chains, as well as the acetal formation (i.e., chemical crosslinking), have significant influences on macromolecular conformation, solution rheological properties, and morphological structure of membranes. The acetal formation, which leads to improved tensile strength of PVA NFN membrane, has been verified by FT-IR and DSC. Additionally, the PVA NFN membrane has been assembled into a gravity-driven filtration device; and the membrane exhibits high rejection ratio (> 99%) for polystyrene microspheres (with diameters of ~ 0.2 μm) and high water flux (3773 L/m2 ∙ h) under the pressure of 40 kPa.