The development of high-performance 6FDA-NDA/DABA/POSS/Ultem^® dual-layer hollow fibers for ethanol dehydration via pervaporation

A dual-layer hollow fiber membrane consisting of copoly(1,5-naphthalene/3,5-benzoic acid-2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropanedimide) (6FDA-NDA/DABA)/polyhedral oligomeric silsesquioxane (POSS) as the outer functional layer and polyetherimide (Ultem^®) as the inner support layer has been developed for ethanol hydration via pervaporation. By using lower dope flow rates and optimizing inner dope concentration, the newly developed hollow fiber membranes have desirable morphology consisting of a porous inner layer with low transport resistance and a macrovoid-free selective layer for a superior separation performance. Doppler broadening energy spectra (DBES) have been employed to analyze the depth profile of morphological evolution and examine the thickness of dense selective layer as a function of spinning conditions. Incorporating a small amount (2wt%) of POSS into the selective layer increases its free volume size and diffusion selectivity and hence improves the pervaporation performance of the hybrid membranes. The diffusion mechanism of permeate molecules through polymer chains containing POSS nanoparticles was proposed and verified via FTIR spectra, XRD curves, positron annihilation lifetime (PAL) characterization and vapor sorption tests. Compared to other hollow fiber membranes, the newly developed hollow fibers have a superior flux of 1.9kg/m²h and a comparable separation factor of 166 for ethanol dehydration. The work may open up a new perspective for the development of next-generation high-performance dual-layer hollow fiber membranes with a POSS embedded selective layer for biofuel separation.