The roles of alkali metal counter-ions of PFSA play in the formation of PVDF/PFSA-M hollow fiber membranes

Abstract

The aim of the present study was to thoroughly investigate the effect of alkali metal counter-ions (Li, Na, K and Rb) of PFSA on the formation, morphology and performances of polyvinylidenefluoride (PVDF)/perfluorosulfonic acid (PFSA-M) hollow fiber membranes. It is found that the homogeneous region in the ternary phase diagram of the PVDF–PFSA–M/NMP/H2O systems shrinks and the precipitation rate increases when varying the counter-ions of PFSA in the order of Li, Na and K. The double finger-like structure and the cavities can be evidently developed with increasing the radius of the counter-ions of the spun PVDF/PFSA-M membranes. The pure water permeability (JW) and mean effective pore size of the PVDF/PFSA-M membranes first increase from counter-ion Li, Na to K and then slightly decrease for Rb. In addition, the highest JW value of 96.5L M−2 H−1 bar−1 was obtained for the PVDF/PFSA-K membrane with a MWCO of 86kDa. XRD patterns and FTIR-ATR spectra verify the exclusive formation of β crystalline phase structure in the as-spun PVDF/PFSA-M membranes. The PVDF/PFSA-Li membrane has better Young's modulus and tensile strength than that of other PVDF/PFSA-M fibers.