Effect of non-solvent additives on the morphology and separation performance of poly(m-phenylene isophthalamide) (PMIA) hollow fiber nanofiltration membrane

Newly designed poly(m-phenylene isophthalamide) (PMIA) hollow fiber nanofiltration membranes were fabricated containing non-solvent additives, i.e., lithium chloride (LiCl), acetone and PVP by dry-jet wet spinning technology. The effects of non-solvent additives on the morphology and separation performance were investigated. The addition of non-solvent additives in the dopes enhanced the viscosity significantly. The membrane cross-sections were monitored by scanning electron microscopy (SEM). The results indicated that by increasing the concentration of non-solvent additives, the morphologies changed from finger-like to sponge-like, and the outer skin-layer thickness was slightly increased. The rejections to various salts and pure water permeability (PWP) of these membranes were determined. The mean pore size and molecular weight cut-off (MWCO) were estimated using the solute rejection method. The results showed that the salt rejection of the membranes decreased in the order of R(Na2SO4) > R(MgSO4) > R(NaCl) > R(MgCl2), and as the concentrations of non-solvent additives increased, the PWPs, mean pore sizes and MWCOs of the PMIA membranes all decreased. The atomic force microscope (AFM) experiments demonstrated that a smooth membrane surface was formed by adding non-solvent additives. Furthermore, the stress values at break, the elongation at break and the elastic modulus of the fibers increased as non-solvent additive contents increased.