(287b) Molecular Engineering of Nanoporous Hollow Fiber Membranes for NF and Bio-Molecular Separations

The molecular design of nanoporous membranes with desired morphology and selectivity has attracted significant interest over the past few decades. A major problem in their applications is the trade-off between sieving property and permeability. Here, we report the discovery of elongation-induced nano-pore evolution during the external stretching of a novel polyamide-imide (PAI) nanofiltration (NF) hollow fiber membrane in a dry-jet wet-spinning process that simultaneously leads to a decreased pore size but increased pure water permeability. The molecular weight cutoff, pore size and pore size distribution were finely tuned using this approach. The PAI hollow fiber membrane spun at 54.6 m/min has a mean pore radius of 0.46 nm and a narrow pore size distribution. The pure water permeability was achieved at 5.39 L m^-2 bar^-1 h^-1.

Perhaps we are the first group to discover the elongation-induced nano-pore evolution of the hollow fiber membranes by external stretching of the fibers during the spinning process. Both AFM and polarized FTIR were applied to verify this interesting phenomenon. AFM images demonstrate that a high take-up speed favors the ?spinodal decomposition? rather than the ?nucleation and growth? mechanism, which increases surface porosity and thus increases pure water permeability. AFM images also show that the high take-up speed may not only stretch the nascent nodules to form elongated fibers but also deform and elongate the pores beneath the skin surface to give narrower openings for enhanced solute rejection. Polarized FTIR verified an enhanced molecular orientation perpendicular to the spinning direction that may create fiber-like nodules and elongated pores with enhanced rejection.

The resultant NF membranes exhibit highly effective fractionation of the monovalent and divalent ions of NaCl/Na₂SO₄ binary salt solutions. We observed a negative rejection of Cl^- and a high rejection of SO₄^2- indicating the high separation efficiency of these two ions. The NF membranes also shows more than 99.5% rejection of glutathione even at neutral pH, offering the feasibility to recover, concentrate and purify this tripeptide. In short, the newly developed nanoporous hollow fiber membranes have the potential for both high volume and high selectivity in liquid based separations.