Controlled Fabrication of Hydrophilic Nanofiber Membranes

Enhanced hydrophilicity is desirable for many membranes designed for liquid separation applications.  Surface modification techniques have been developed to make conventional membrane materials more hydrophilic.  However, most of these methods suffer from instability of the modification, low reproducibility, or high complexity.  In addition, gains in hydrophilicity may be modest.  In light of these challenges, a facile method for fabrication of hydrophilic electrospun mats was developed.  Stable nanofibers were electrospun from a solution of polysulfone (PSf) and polysulfone-block-poly(ethylene glycol) (PSf-b-PEG) in N,N-dimethylformamide.  The ratio of PSf-b-PEG to PSf homopolymer in the electrospinning solution was varied, along with the electrospinning parameters, to obtain nanofiber mats of differing composition and comparable morphology.  The nanofiber mats were then annealed in warm deionized water to drive the hydrophilic poly(ethylene glycol) (PEG) blocks toward the nanofiber surfaces.  The water-insoluble PSf block and PSf homopolymer act to anchor the hydrophilic PEG chain to the rest of the fiber, preventing its dissolution into aqueous environments.  As-spun and annealed fibers and fiber mats were characterized for morphological and material properties in SEM, TEM, XPS, and ATR-IR.  The water contact angle of dry mats was also measured before and after annealing.  Water contact angles decreased as annealing time increased, with dramatic drops from approximately 125° to less than 10° occurring with high PSf-b-PEG content and long (40 h) annealing times.   However, when annealing temperature or PSf-b-PEG content is too low, no change in contact angle occurs with annealing.  Insights into electrospun fiber formation gained from the investigation will be shared.  Potential applications of these hydrophilic membranes will also be discussed.